Requirements for IP Version 4 Routers
RFC 1812

Network Working Group F. Baker, Editor
Request for Comments: 1812 Cisco Systems
Obsoletes: 1716, 1009 June 1995
Category: Standards Track
 Requirements for IP Version 4 Routers
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
PREFACE
 This document is an updated version of RFC 1716, the historical
 Router Requirements document. That RFC preserved the significant
 work that went into the working group, but failed to adequately
 describe current technology for the IESG to consider it a current
 standard.
 The current editor had been asked to bring the document up to date,
 so that it is useful as a procurement specification and a guide to
 implementors. In this, he stands squarely on the shoulders of those
 who have gone before him, and depends largely on expert contributors
 for text. Any credit is theirs; the errors are his.
 The content and form of this document are due, in large part, to the
 working group's chair, and document's original editor and author:
 Philip Almquist. It is also largely due to the efforts of its
 previous editor, Frank Kastenholz. Without their efforts, this
 document would not exist.
Table of Contents
 1. INTRODUCTION ........................................ 6
 1.1 Reading this Document .............................. 8
 1.1.1 Organization ..................................... 8
 1.1.2 Requirements ..................................... 9
 1.1.3 Compliance ....................................... 10
 1.2 Relationships to Other Standards ................... 11
 1.3 General Considerations ............................. 12
 1.3.1 Continuing Internet Evolution .................... 12
 1.3.2 Robustness Principle ............................. 13
 1.3.3 Error Logging .................................... 14
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 1.3.4 Configuration .................................... 14
 1.4 Algorithms ......................................... 16
 2. INTERNET ARCHITECTURE ............................... 16
 2.1 Introduction ....................................... 16
 2.2 Elements of the Architecture ....................... 17
 2.2.1 Protocol Layering ................................ 17
 2.2.2 Networks ......................................... 19
 2.2.3 Routers .......................................... 20
 2.2.4 Autonomous Systems ............................... 21
 2.2.5 Addressing Architecture .......................... 21
 2.2.5.1 Classical IP Addressing Architecture ........... 21
 2.2.5.2 Classless Inter Domain Routing (CIDR) .......... 23
 2.2.6 IP Multicasting .................................. 24
 2.2.7 Unnumbered Lines and Networks Prefixes ........... 25
 2.2.8 Notable Oddities ................................. 26
 2.2.8.1 Embedded Routers ............................... 26
 2.2.8.2 Transparent Routers ............................ 27
 2.3 Router Characteristics ............................. 28
 2.4 Architectural Assumptions .......................... 31
 3. LINK LAYER .......................................... 32
 3.1 INTRODUCTION ....................................... 32
 3.2 LINK/INTERNET LAYER INTERFACE ...................... 33
 3.3 SPECIFIC ISSUES .................................... 34
 3.3.1 Trailer Encapsulation ............................ 34
 3.3.2 Address Resolution Protocol - ARP ................ 34
 3.3.3 Ethernet and 802.3 Coexistence ................... 35
 3.3.4 Maximum Transmission Unit - MTU .................. 35
 3.3.5 Point-to-Point Protocol - PPP .................... 35
 3.3.5.1 Introduction ................................... 36
 3.3.5.2 Link Control Protocol (LCP) Options ............ 36
 3.3.5.3 IP Control Protocol (IPCP) Options ............. 38
 3.3.6 Interface Testing ................................ 38
 4. INTERNET LAYER - PROTOCOLS .......................... 39
 4.1 INTRODUCTION ....................................... 39
 4.2 INTERNET PROTOCOL - IP ............................. 39
 4.2.1 INTRODUCTION ..................................... 39
 4.2.2 PROTOCOL WALK-THROUGH ............................ 40
 4.2.2.1 Options: RFC 791 Section 3.2 ................... 40
 4.2.2.2 Addresses in Options: RFC 791 Section 3.1 ...... 42
 4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 ..... 43
 4.2.2.4 Type of Service: RFC 791 Section 3.1 ........... 44
 4.2.2.5 Header Checksum: RFC 791 Section 3.1 ........... 44
 4.2.2.6 Unrecognized Header Options: RFC 791,
 Section 3.1 .................................... 44
 4.2.2.7 Fragmentation: RFC 791 Section 3.2 ............. 45
 4.2.2.8 Reassembly: RFC 791 Section 3.2 ................ 46
 4.2.2.9 Time to Live: RFC 791 Section 3.2 .............. 46
 4.2.2.10 Multi-subnet Broadcasts: RFC 922 .............. 47
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 4.2.2.11 Addressing: RFC 791 Section 3.2 ............... 47
 4.2.3 SPECIFIC ISSUES .................................. 50
 4.2.3.1 IP Broadcast Addresses ......................... 50
 4.2.3.2 IP Multicasting ................................ 50
 4.2.3.3 Path MTU Discovery ............................. 51
 4.2.3.4 Subnetting ..................................... 51
 4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ........... 52
 4.3.1 INTRODUCTION ..................................... 52
 4.3.2 GENERAL ISSUES ................................... 53
 4.3.2.1 Unknown Message Types .......................... 53
 4.3.2.2 ICMP Message TTL ............................... 53
 4.3.2.3 Original Message Header ........................ 53
 4.3.2.4 ICMP Message Source Address .................... 53
 4.3.2.5 TOS and Precedence ............................. 54
 4.3.2.6 Source Route ................................... 54
 4.3.2.7 When Not to Send ICMP Errors ................... 55
 4.3.2.8 Rate Limiting .................................. 56
 4.3.3 SPECIFIC ISSUES .................................. 56
 4.3.3.1 Destination Unreachable ........................ 56
 4.3.3.2 Redirect ....................................... 57
 4.3.3.3 Source Quench .................................. 57
 4.3.3.4 Time Exceeded .................................. 58
 4.3.3.5 Parameter Problem .............................. 58
 4.3.3.6 Echo Request/Reply ............................. 58
 4.3.3.7 Information Request/Reply ...................... 59
 4.3.3.8 Timestamp and Timestamp Reply .................. 59
 4.3.3.9 Address Mask Request/Reply ..................... 61
 4.3.3.10 Router Advertisement and Solicitations ........ 62
 4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .......... 62
 5. INTERNET LAYER - FORWARDING ......................... 63
 5.1 INTRODUCTION ....................................... 63
 5.2 FORWARDING WALK-THROUGH ............................ 63
 5.2.1 Forwarding Algorithm ............................. 63
 5.2.1.1 General ........................................ 64
 5.2.1.2 Unicast ........................................ 64
 5.2.1.3 Multicast ...................................... 65
 5.2.2 IP Header Validation ............................. 67
 5.2.3 Local Delivery Decision .......................... 69
 5.2.4 Determining the Next Hop Address ................. 71
 5.2.4.1 IP Destination Address ......................... 72
 5.2.4.2 Local/Remote Decision .......................... 72
 5.2.4.3 Next Hop Address ............................... 74
 5.2.4.4 Administrative Preference ...................... 77
 5.2.4.5 Load Splitting ................................. 79
 5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ....... 79
 5.2.6 Fragmentation and Reassembly: RFC-791,
 Section 3.2 ...................................... 80
 5.2.7 Internet Control Message Protocol - ICMP ......... 80
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 5.2.7.1 Destination Unreachable ........................ 80
 5.2.7.2 Redirect ....................................... 82
 5.2.7.3 Time Exceeded .................................. 84
 5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........ 84
 5.3 SPECIFIC ISSUES .................................... 85
 5.3.1 Time to Live (TTL) ............................... 85
 5.3.2 Type of Service (TOS) ............................ 86
 5.3.3 IP Precedence .................................... 87
 5.3.3.1 Precedence-Ordered Queue Service ............... 88
 5.3.3.2 Lower Layer Precedence Mappings ................ 89
 5.3.3.3 Precedence Handling For All Routers ............ 90
 5.3.4 Forwarding of Link Layer Broadcasts .............. 92
 5.3.5 Forwarding of Internet Layer Broadcasts .......... 92
 5.3.5.1 Limited Broadcasts ............................. 93
 5.3.5.2 Directed Broadcasts ............................ 93
 5.3.5.3 All-subnets-directed Broadcasts ................ 94
 5.3.5.4 Subnet-directed Broadcasts .................... 94
 5.3.6 Congestion Control ............................... 94
 5.3.7 Martian Address Filtering ........................ 96
 5.3.8 Source Address Validation ........................ 97
 5.3.9 Packet Filtering and Access Lists ................ 97
 5.3.10 Multicast Routing ............................... 98
 5.3.11 Controls on Forwarding .......................... 98
 5.3.12 State Changes ................................... 99
 5.3.12.1 When a Router Ceases Forwarding ............... 99
 5.3.12.2 When a Router Starts Forwarding ............... 100
 5.3.12.3 When an Interface Fails or is Disabled ........ 100
 5.3.12.4 When an Interface is Enabled .................. 100
 5.3.13 IP Options ...................................... 101
 5.3.13.1 Unrecognized Options .......................... 101
 5.3.13.2 Security Option ............................... 101
 5.3.13.3 Stream Identifier Option ...................... 101
 5.3.13.4 Source Route Options .......................... 101
 5.3.13.5 Record Route Option ........................... 102
 5.3.13.6 Timestamp Option .............................. 102
 6. TRANSPORT LAYER ..................................... 103
 6.1 USER DATAGRAM PROTOCOL - UDP ....................... 103
 6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................ 104
 7. APPLICATION LAYER - ROUTING PROTOCOLS ............... 106
 7.1 INTRODUCTION ....................................... 106
 7.1.1 Routing Security Considerations .................. 106
 7.1.2 Precedence ....................................... 107
 7.1.3 Message Validation ............................... 107
 7.2 INTERIOR GATEWAY PROTOCOLS ......................... 107
 7.2.1 INTRODUCTION ..................................... 107
 7.2.2 OPEN SHORTEST PATH FIRST - OSPF .................. 108
 7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -
 DUAL IS-IS ....................................... 108
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 7.3 EXTERIOR GATEWAY PROTOCOLS ........................ 109
 7.3.1 INTRODUCTION .................................... 109
 7.3.2 BORDER GATEWAY PROTOCOL - BGP .................... 109
 7.3.2.1 Introduction ................................... 109
 7.3.2.2 Protocol Walk-through .......................... 110
 7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
 .................................................. 110
 7.4 STATIC ROUTING ..................................... 111
 7.5 FILTERING OF ROUTING INFORMATION ................... 112
 7.5.1 Route Validation ................................. 113
 7.5.2 Basic Route Filtering ............................ 113
 7.5.3 Advanced Route Filtering ......................... 114
 7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........ 114
 8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
 ..................................................... 115
 8.1 The Simple Network Management Protocol - SNMP ...... 115
 8.1.1 SNMP Protocol Elements ........................... 115
 8.2 Community Table .................................... 116
 8.3 Standard MIBS ...................................... 118
 8.4 Vendor Specific MIBS ............................... 119
 8.5 Saving Changes ..................................... 120
 9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ......... 120
 9.1 BOOTP .............................................. 120
 9.1.1 Introduction ..................................... 120
 9.1.2 BOOTP Relay Agents ............................... 121
 10. OPERATIONS AND MAINTENANCE ......................... 122
 10.1 Introduction ...................................... 122
 10.2 Router Initialization ............................. 123
 10.2.1 Minimum Router Configuration .................... 123
 10.2.2 Address and Prefix Initialization ............... 124
 10.2.3 Network Booting using BOOTP and TFTP ............ 125
 10.3 Operation and Maintenance ......................... 126
 10.3.1 Introduction .................................... 126
 10.3.2 Out Of Band Access .............................. 127
 10.3.2 Router O&M Functions ............................ 127
 10.3.2.1 Maintenance - Hardware Diagnosis .............. 127
 10.3.2.2 Control - Dumping and Rebooting ............... 127
 10.3.2.3 Control - Configuring the Router .............. 128
 10.3.2.4 Net Booting of System Software ................ 128
 10.3.2.5 Detecting and responding to misconfiguration
 ............................................... 129
 10.3.2.6 Minimizing Disruption ......................... 130
 10.3.2.7 Control - Troubleshooting Problems ............ 130
 10.4 Security Considerations ........................... 131
 10.4.1 Auditing and Audit Trails ....................... 131
 10.4.2 Configuration Control ........................... 132
 11. REFERENCES ......................................... 133
 APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ...... 145
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 APPENDIX B. GLOSSARY ................................... 146
 APPENDIX C. FUTURE DIRECTIONS .......................... 152
 APPENDIX D. Multicast Routing Protocols ................ 154
 D.1 Introduction ....................................... 154
 D.2 Distance Vector Multicast Routing Protocol -
 DVMRP .............................................. 154
 D.3 Multicast Extensions to OSPF - MOSPF ............... 154
 D.4 Protocol Independent Multicast - PIM ............... 155
 APPENDIX E Additional Next-Hop Selection Algorithms
 ................................................... 155
 E.1. Some Historical Perspective ....................... 155
 E.2. Additional Pruning Rules .......................... 157
 E.3 Some Route Lookup Algorithms ....................... 159
 E.3.1 The Revised Classic Algorithm .................... 159
 E.3.2 The Variant Router Requirements Algorithm ........ 160
 E.3.3 The OSPF Algorithm ............................... 160
 E.3.4 The Integrated IS-IS Algorithm ................... 162
 Security Considerations ................................ 163
 APPENDIX F: HISTORICAL ROUTING PROTOCOLS ............... 164
 F.1 EXTERIOR GATEWAY PROTOCOL - EGP .................... 164
 F.1.1 Introduction ..................................... 164
 F.1.2 Protocol Walk-through ............................ 165
 F.2 ROUTING INFORMATION PROTOCOL - RIP ................. 167
 F.2.1 Introduction ..................................... 167
 F.2.2 Protocol Walk-Through ............................ 167
 F.2.3 Specific Issues .................................. 172
 F.3 GATEWAY TO GATEWAY PROTOCOL - GGP .................. 173
 Acknowledgments ........................................ 173
 Editor's Address ....................................... 175
1. INTRODUCTION
 This memo replaces for RFC 1716, "Requirements for Internet Gateways"
 ([INTRO:1]).
 This memo defines and discusses requirements for devices that perform
 the network layer forwarding function of the Internet protocol suite.
 The Internet community usually refers to such devices as IP routers or
 simply routers; The OSI community refers to such devices as
 intermediate systems. Many older Internet documents refer to these
 devices as gateways, a name which more recently has largely passed out
 of favor to avoid confusion with application gateways.
 An IP router can be distinguished from other sorts of packet switching
 devices in that a router examines the IP protocol header as part of
 the switching process. It generally removes the Link Layer header a
 message was received with, modifies the IP header, and replaces the
 Link Layer header for retransmission.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 The authors of this memo recognize, as should its readers, that many
 routers support more than one protocol. Support for multiple protocol
 suites will be required in increasingly large parts of the Internet in
 the future. This memo, however, does not attempt to specify Internet
 requirements for protocol suites other than TCP/IP.
 This document enumerates standard protocols that a router connected to
 the Internet must use, and it incorporates by reference the RFCs and
 other documents describing the current specifications for these
 protocols. It corrects errors in the referenced documents and adds
 additional discussion and guidance for an implementor.
 For each protocol, this memo also contains an explicit set of
 requirements, recommendations, and options. The reader must
 understand that the list of requirements in this memo is incomplete by
 itself. The complete set of requirements for an Internet protocol
 router is primarily defined in the standard protocol specification
 documents, with the corrections, amendments, and supplements contained
 in this memo.
 This memo should be read in conjunction with the Requirements for
 Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
 routers must both be capable of originating IP datagrams and receiving
 IP datagrams destined for them. The major distinction between
 Internet hosts and routers is that routers implement forwarding
 algorithms, while Internet hosts do not require forwarding
 capabilities. Any Internet host acting as a router must adhere to the
 requirements contained in this memo.
 The goal of open system interconnection dictates that routers must
 function correctly as Internet hosts when necessary. To achieve this,
 this memo provides guidelines for such instances. For simplification
 and ease of document updates, this memo tries to avoid overlapping
 discussions of host requirements with [INTRO:2] and [INTRO:3] and
 incorporates the relevant requirements of those documents by
 reference. In some cases the requirements stated in [INTRO:2] and
 [INTRO:3] are superseded by this document.
 A good-faith implementation of the protocols produced after careful
 reading of the RFCs should differ from the requirements of this memo
 in only minor ways. Producing such an implementation often requires
 some interaction with the Internet technical community, and must
 follow good communications software engineering practices. In many
 cases, the requirements in this document are already stated or implied
 in the standard protocol documents, so that their inclusion here is,
 in a sense, redundant. They were included because some past
 implementation has made the wrong choice, causing problems of
 interoperability, performance, and/or robustness.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 This memo includes discussion and explanation of many of the
 requirements and recommendations. A simple list of requirements would
 be dangerous, because:
 o Some required features are more important than others, and some
 features are optional.
 o Some features are critical in some applications of routers but
 irrelevant in others.
 o There may be valid reasons why particular vendor products that are
 designed for restricted contexts might choose to use different
 specifications.
 However, the specifications of this memo must be followed to meet the
 general goal of arbitrary router interoperation across the diversity
 and complexity of the Internet. Although most current implementations
 fail to meet these requirements in various ways, some minor and some
 major, this specification is the ideal towards which we need to move.
 These requirements are based on the current level of Internet
 architecture. This memo will be updated as required to provide
 additional clarifications or to include additional information in
 those areas in which specifications are still evolving.
1.1 Reading this Document
1.1.1 Organization
 This memo emulates the layered organization used by [INTRO:2] and
 [INTRO:3]. Thus, Chapter 2 describes the layers found in the Internet
 architecture. Chapter 3 covers the Link Layer. Chapters 4 and 5 are
 concerned with the Internet Layer protocols and forwarding algorithms.
 Chapter 6 covers the Transport Layer. Upper layer protocols are
 divided among Chapters 7, 8, and 9. Chapter 7 discusses the protocols
 which routers use to exchange routing information with each other.
 Chapter 8 discusses network management. Chapter 9 discusses other
 upper layer protocols. The final chapter covers operations and
 maintenance features. This organization was chosen for simplicity,
 clarity, and consistency with the Host Requirements RFCs. Appendices
 to this memo include a bibliography, a glossary, and some conjectures
 about future directions of router standards.
 In describing the requirements, we assume that an implementation
 strictly mirrors the layering of the protocols. However, strict
 layering is an imperfect model, both for the protocol suite and for
 recommended implementation approaches. Protocols in different layers
 interact in complex and sometimes subtle ways, and particular
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 functions often involve multiple layers. There are many design
 choices in an implementation, many of which involve creative breaking
 of strict layering. Every implementor is urged to read [INTRO:4] and
 [INTRO:5].
 Each major section of this memo is organized into the following
 subsections:
 (1) Introduction
 (2) Protocol Walk-Through - considers the protocol specification
 documents section-by-section, correcting errors, stating
 requirements that may be ambiguous or ill-defined, and providing
 further clarification or explanation.
 (3) Specific Issues - discusses protocol design and implementation
 issues that were not included in the walk-through.
 Under many of the individual topics in this memo, there is
 parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
 material is intended to give a justification, clarification or
 explanation to the preceding requirements text. The implementation
 material contains suggested approaches that an implementor may want to
 consider. The DISCUSSION and IMPLEMENTATION sections are not part of
 the standard.
1.1.2 Requirements
 In this memo, the words that are used to define the significance of
 each particular requirement are capitalized. These words are:
 o MUST
 This word means that the item is an absolute requirement of the
 specification. Violation of such a requirement is a fundamental
 error; there is no case where it is justified.
 o MUST IMPLEMENT
 This phrase means that this specification requires that the item be
 implemented, but does not require that it be enabled by default.
 o MUST NOT
 This phrase means that the item is an absolute prohibition of the
 specification.
 o SHOULD
 This word means that there may exist valid reasons in particular
 circumstances to ignore this item, but the full implications should
 be understood and the case carefully weighed before choosing a
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 different course.
 o SHOULD IMPLEMENT
 This phrase is similar in meaning to SHOULD, but is used when we
 recommend that a particular feature be provided but does not
 necessarily recommend that it be enabled by default.
 o SHOULD NOT
 This phrase means that there may exist valid reasons in particular
 circumstances when the described behavior is acceptable or even
 useful. Even so, the full implications should be understood and
 the case carefully weighed before implementing any behavior
 described with this label.
 o MAY
 This word means that this item is truly optional. One vendor may
 choose to include the item because a particular marketplace
 requires it or because it enhances the product, for example;
 another vendor may omit the same item.
1.1.3 Compliance
 Some requirements are applicable to all routers. Other requirements
 are applicable only to those which implement particular features or
 protocols. In the following paragraphs, relevant refers to the union
 of the requirements applicable to all routers and the set of
 requirements applicable to a particular router because of the set of
 features and protocols it has implemented.
 Note that not all Relevant requirements are stated directly in this
 memo. Various parts of this memo incorporate by reference sections of
 the Host Requirements specification, [INTRO:2] and [INTRO:3]. For
 purposes of determining compliance with this memo, it does not matter
 whether a Relevant requirement is stated directly in this memo or
 merely incorporated by reference from one of those documents.
 An implementation is said to be conditionally compliant if it
 satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
 requirements. An implementation is said to be unconditionally
 compliant if it is conditionally compliant and also satisfies all the
 Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An
 implementation is not compliant if it is not conditionally compliant
 (i.e., it fails to satisfy one or more of the Relevant MUST, MUST
 IMPLEMENT, or MUST NOT requirements).
 This specification occasionally indicates that an implementation
 SHOULD implement a management variable, and that it SHOULD have a
 certain default value. An unconditionally compliant implementation
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 implements the default behavior, and if there are other implemented
 behaviors implements the variable. A conditionally compliant
 implementation clearly documents what the default setting of the
 variable is or, in the absence of the implementation of a variable,
 may be construed to be. An implementation that both fails to
 implement the variable and chooses a different behavior is not
 compliant.
 For any of the SHOULD and SHOULD NOT requirements, a router may
 provide a configuration option that will cause the router to act other
 than as specified by the requirement. Having such a configuration
 option does not void a router's claim to unconditional compliance if
 the option has a default setting, and that setting causes the router
 to operate in the required manner.
 Likewise, routers may provide, except where explicitly prohibited by
 this memo, options which cause them to violate MUST or MUST NOT
 requirements. A router that provides such options is compliant
 (either fully or conditionally) if and only if each such option has a
 default setting that causes the router to conform to the requirements
 of this memo. Please note that the authors of this memo, although
 aware of market realities, strongly recommend against provision of
 such options. Requirements are labeled MUST or MUST NOT because
 experts in the field have judged them to be particularly important to
 interoperability or proper functioning in the Internet. Vendors
 should weigh carefully the customer support costs of providing options
 that violate those rules.
 Of course, this memo is not a complete specification of an IP router,
 but rather is closer to what in the OSI world is called a profile.
 For example, this memo requires that a number of protocols be
 implemented. Although most of the contents of their protocol
 specifications are not repeated in this memo, implementors are
 nonetheless required to implement the protocols according to those
 specifications.
1.2 Relationships to Other Standards
 There are several reference documents of interest in checking the
 status of protocol specifications and standardization:
 o INTERNET OFFICIAL PROTOCOL STANDARDS
 This document describes the Internet standards process and lists
 the standards status of the protocols. As of this writing, the
 current version of this document is STD 1, RFC 1780, [ARCH:7].
 This document is periodically re-issued. You should always
 consult an RFC repository and use the latest version of this
 document.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 o Assigned Numbers
 This document lists the assigned values of the parameters used in
 the various protocols. For example, it lists IP protocol codes,
 TCP port numbers, Telnet Option Codes, ARP hardware types, and
 Terminal Type names. As of this writing, the current version of
 this document is STD 2, RFC 1700, [INTRO:7]. This document is
 periodically re-issued. You should always consult an RFC
 repository and use the latest version of this document.
 o Host Requirements
 This pair of documents reviews the specifications that apply to
 hosts and supplies guidance and clarification for any
 ambiguities. Note that these requirements also apply to routers,
 except where otherwise specified in this memo. As of this
 writing, the current versions of these documents are RFC 1122 and
 RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].
 o Router Requirements (formerly Gateway Requirements)
 This memo.
 Note that these documents are revised and updated at different times;
 in case of differences between these documents, the most recent must
 prevail.
 These and other Internet protocol documents may be obtained from the:
 The InterNIC
 DS.INTERNIC.NET
 InterNIC Directory and Database Service
 info@internic.net
 +1-908-668-6587
 URL: http://ds.internic.net/
1.3 General Considerations
 There are several important lessons that vendors of Internet software
 have learned and which a new vendor should consider seriously.
1.3.1 Continuing Internet Evolution
 The enormous growth of the Internet has revealed problems of
 management and scaling in a large datagram based packet communication
 system. These problems are being addressed, and as a result there
 will be continuing evolution of the specifications described in this
 memo. New routing protocols, algorithms, and architectures are
 constantly being developed. New internet layer protocols, and
 modifications to existing protocols, are also constantly being
 devised. Routers play a crucial role in the Internet, and the number
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 of routers deployed in the Internet is much smaller than the number
 of hosts. Vendors should therefore expect that router standards will
 continue to evolve much more quickly than host standards. These
 changes will be carefully planned and controlled since there is
 extensive participation in this planning by the vendors and by the
 organizations responsible for operation of the networks.
 Development, evolution, and revision are characteristic of computer
 network protocols today, and this situation will persist for some
 years. A vendor who develops computer communications software for
 the Internet protocol suite (or any other protocol suite!) and then
 fails to maintain and update that software for changing
 specifications is going to leave a trail of unhappy customers. The
 Internet is a large communication network, and the users are in
 constant contact through it. Experience has shown that knowledge of
 deficiencies in vendor software propagates quickly through the
 Internet technical community.
1.3.2 Robustness Principle
 At every layer of the protocols, there is a general rule (from
 [TRANS:2] by Jon Postel) whose application can lead to enormous
 benefits in robustness and interoperability:
 Be conservative in what you do,
 be liberal in what you accept from others.
 Software should be written to deal with every conceivable error, no
 matter how unlikely. Eventually a packet will come in with that
 particular combination of errors and attributes, and unless the
 software is prepared, chaos can ensue. It is best to assume that the
 network is filled with malevolent entities that will send packets
 designed to have the worst possible effect. This assumption will
 lead to suitably protective design. The most serious problems in the
 Internet have been caused by unforeseen mechanisms triggered by low
 probability events; mere human malice would never have taken so
 devious a course!
 Adaptability to change must be designed into all levels of router
 software. As a simple example, consider a protocol specification
 that contains an enumeration of values for a particular header field
 - e.g., a type field, a port number, or an error code; this
 enumeration must be assumed to be incomplete. If the protocol
 specification defines four possible error codes, the software must
 not break when a fifth code is defined. An undefined code might be
 logged, but it must not cause a failure.
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 The second part of the principal is almost as important: software on
 hosts or other routers may contain deficiencies that make it unwise
 to exploit legal but obscure protocol features. It is unwise to
 stray far from the obvious and simple, lest untoward effects result
 elsewhere. A corollary of this is watch out for misbehaving hosts;
 router software should be prepared to survive in the presence of
 misbehaving hosts. An important function of routers in the Internet
 is to limit the amount of disruption such hosts can inflict on the
 shared communication facility.
1.3.3 Error Logging
 The Internet includes a great variety of systems, each implementing
 many protocols and protocol layers, and some of these contain bugs
 and misguided features in their Internet protocol software. As a
 result of complexity, diversity, and distribution of function, the
 diagnosis of problems is often very difficult.
 Problem diagnosis will be aided if routers include a carefully
 designed facility for logging erroneous or strange events. It is
 important to include as much diagnostic information as possible when
 an error is logged. In particular, it is often useful to record the
 header(s) of a packet that caused an error. However, care must be
 taken to ensure that error logging does not consume prohibitive
 amounts of resources or otherwise interfere with the operation of the
 router.
 There is a tendency for abnormal but harmless protocol events to
 overflow error logging files; this can be avoided by using a circular
 log, or by enabling logging only while diagnosing a known failure.
 It may be useful to filter and count duplicate successive messages.
 One strategy that seems to work well is to both:
 o Always count abnormalities and make such counts accessible through
 the management protocol (see Chapter 8); and
 o Allow the logging of a great variety of events to be selectively
 enabled. For example, it might useful to be able to log
 everything or to log everything for host X.
 This topic is further discussed in [MGT:5].
1.3.4 Configuration
 In an ideal world, routers would be easy to configure, and perhaps
 even entirely self-configuring. However, practical experience in the
 real world suggests that this is an impossible goal, and that many
 attempts by vendors to make configuration easy actually cause
 customers more grief than they prevent. As an extreme example, a
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 router designed to come up and start routing packets without
 requiring any configuration information at all would almost certainly
 choose some incorrect parameter, possibly causing serious problems on
 any networks unfortunate enough to be connected to it.
 Often this memo requires that a parameter be a configurable option.
 There are several reasons for this. In a few cases there currently
 is some uncertainty or disagreement about the best value and it may
 be necessary to update the recommended value in the future. In other
 cases, the value really depends on external factors - e.g., the
 distribution of its communication load, or the speeds and topology of
 nearby networks - and self-tuning algorithms are unavailable and may
 be insufficient. In some cases, configurability is needed because of
 administrative requirements.
 Finally, some configuration options are required to communicate with
 obsolete or incorrect implementations of the protocols, distributed
 without sources, that persist in many parts of the Internet. To make
 correct systems coexist with these faulty systems, administrators
 must occasionally misconfigure the correct systems. This problem
 will correct itself gradually as the faulty systems are retired, but
 cannot be ignored by vendors.
 When we say that a parameter must be configurable, we do not intend
 to require that its value be explicitly read from a configuration
 file at every boot time. For many parameters, there is one value
 that is appropriate for all but the most unusual situations. In such
 cases, it is quite reasonable that the parameter default to that
 value if not explicitly set.
 This memo requires a particular value for such defaults in some
 cases. The choice of default is a sensitive issue when the
 configuration item controls accommodation of existing, faulty,
 systems. If the Internet is to converge successfully to complete
 interoperability, the default values built into implementations must
 implement the official protocol, not misconfigurations to accommodate
 faulty implementations. Although marketing considerations have led
 some vendors to choose misconfiguration defaults, we urge vendors to
 choose defaults that will conform to the standard.
 Finally, we note that a vendor needs to provide adequate
 documentation on all configuration parameters, their limits and
 effects.
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1.4 Algorithms
 In several places in this memo, specific algorithms that a router
 ought to follow are specified. These algorithms are not, per se,
 required of the router. A router need not implement each algorithm
 as it is written in this document. Rather, an implementation must
 present a behavior to the external world that is the same as a
 strict, literal, implementation of the specified algorithm.
 Algorithms are described in a manner that differs from the way a good
 implementor would implement them. For expository purposes, a style
 that emphasizes conciseness, clarity, and independence from
 implementation details has been chosen. A good implementor will
 choose algorithms and implementation methods that produce the same
 results as these algorithms, but may be more efficient or less
 general.
 We note that the art of efficient router implementation is outside
 the scope of this memo.
2. INTERNET ARCHITECTURE
 This chapter does not contain any requirements. However, it does
 contain useful background information on the general architecture of
 the Internet and of routers.
 General background and discussion on the Internet architecture and
 supporting protocol suite can be found in the DDN Protocol Handbook
 [ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
 [ARCH:4]. The Internet architecture and protocols are also covered
 in an ever-growing number of textbooks, such as [ARCH:5] and
 [ARCH:6].
2.1 Introduction
 The Internet system consists of a number of interconnected packet
 networks supporting communication among host computers using the
 Internet protocols. These protocols include the Internet Protocol
 (IP), the Internet Control Message Protocol (ICMP), the Internet
 Group Management Protocol (IGMP), and a variety transport and
 application protocols that depend upon them. As was described in
 Section [1.2], the Internet Engineering Steering Group periodically
 releases an Official Protocols memo listing all the Internet
 protocols.
 All Internet protocols use IP as the basic data transport mechanism.
 IP is a datagram, or connectionless, internetwork service and
 includes provision for addressing, type-of-service specification,
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 fragmentation and reassembly, and security. ICMP and IGMP are
 considered integral parts of IP, although they are architecturally
 layered upon IP. ICMP provides error reporting, flow control,
 first-hop router redirection, and other maintenance and control
 functions. IGMP provides the mechanisms by which hosts and routers
 can join and leave IP multicast groups.
 Reliable data delivery is provided in the Internet protocol suite by
 Transport Layer protocols such as the Transmission Control Protocol
 (TCP), which provides end-end retransmission, resequencing and
 connection control. Transport Layer connectionless service is
 provided by the User Datagram Protocol (UDP).
2.2 Elements of the Architecture
2.2.1 Protocol Layering
 To communicate using the Internet system, a host must implement the
 layered set of protocols comprising the Internet protocol suite. A
 host typically must implement at least one protocol from each layer.
 The protocol layers used in the Internet architecture are as follows
 [ARCH:7]:
 o Application Layer
 The Application Layer is the top layer of the Internet protocol
 suite. The Internet suite does not further subdivide the
 Application Layer, although some application layer protocols do
 contain some internal sub-layering. The application layer of the
 Internet suite essentially combines the functions of the top two
 layers - Presentation and Application - of the OSI Reference Model
 [ARCH:8]. The Application Layer in the Internet protocol suite
 also includes some of the function relegated to the Session Layer
 in the OSI Reference Model.
 We distinguish two categories of application layer protocols: user
 protocols that provide service directly to users, and support
 protocols that provide common system functions. The most common
 Internet user protocols are:
 - Telnet (remote login)
 - FTP (file transfer)
 - SMTP (electronic mail delivery)
 There are a number of other standardized user protocols and many
 private user protocols.
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 Support protocols, used for host name mapping, booting, and
 management include SNMP, BOOTP, TFTP, the Domain Name System (DNS)
 protocol, and a variety of routing protocols.
 Application Layer protocols relevant to routers are discussed in
 chapters 7, 8, and 9 of this memo.
 o Transport Layer
 The Transport Layer provides end-to-end communication services.
 This layer is roughly equivalent to the Transport Layer in the OSI
 Reference Model, except that it also incorporates some of OSI's
 Session Layer establishment and destruction functions.
 There are two primary Transport Layer protocols at present:
 - Transmission Control Protocol (TCP)
 - User Datagram Protocol (UDP)
 TCP is a reliable connection-oriented transport service that
 provides end-to-end reliability, resequencing, and flow control.
 UDP is a connectionless (datagram) transport service. Other
 transport protocols have been developed by the research community,
 and the set of official Internet transport protocols may be
 expanded in the future.
 Transport Layer protocols relevant to routers are discussed in
 Chapter 6.
 o Internet Layer
 All Internet transport protocols use the Internet Protocol (IP) to
 carry data from source host to destination host. IP is a
 connectionless or datagram internetwork service, providing no
 end-to-end delivery guarantees. IP datagrams may arrive at the
 destination host damaged, duplicated, out of order, or not at all.
 The layers above IP are responsible for reliable delivery service
 when it is required. The IP protocol includes provision for
 addressing, type-of-service specification, fragmentation and
 reassembly, and security.
 The datagram or connectionless nature of IP is a fundamental and
 characteristic feature of the Internet architecture.
 The Internet Control Message Protocol (ICMP) is a control protocol
 that is considered to be an integral part of IP, although it is
 architecturally layered upon IP - it uses IP to carry its data
 end-to-end. ICMP provides error reporting, congestion reporting,
 and first-hop router redirection.
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 The Internet Group Management Protocol (IGMP) is an Internet layer
 protocol used for establishing dynamic host groups for IP
 multicasting.
 The Internet layer protocols IP, ICMP, and IGMP are discussed in
 chapter 4.
 o Link Layer
 To communicate on a directly connected network, a host must
 implement the communication protocol used to interface to that
 network. We call this a Link Layer protocol.
 Some older Internet documents refer to this layer as the Network
 Layer, but it is not the same as the Network Layer in the OSI
 Reference Model.
 This layer contains everything below the Internet Layer and above
 the Physical Layer (which is the media connectivity, normally
 electrical or optical, which encodes and transports messages).
 Its responsibility is the correct delivery of messages, among
 which it does not differentiate.
 Protocols in this Layer are generally outside the scope of
 Internet standardization; the Internet (intentionally) uses
 existing standards whenever possible. Thus, Internet Link Layer
 standards usually address only address resolution and rules for
 transmitting IP packets over specific Link Layer protocols.
 Internet Link Layer standards are discussed in chapter 3.
2.2.2 Networks
 The constituent networks of the Internet system are required to
 provide only packet (connectionless) transport. According to the IP
 service specification, datagrams can be delivered out of order, be
 lost or duplicated, and/or contain errors.
 For reasonable performance of the protocols that use IP (e.g., TCP),
 the loss rate of the network should be very low. In networks
 providing connection-oriented service, the extra reliability provided
 by virtual circuits enhances the end-end robustness of the system,
 but is not necessary for Internet operation.
 Constituent networks may generally be divided into two classes:
 o Local-Area Networks (LANs)
 LANs may have a variety of designs. LANs normally cover a small
 geographical area (e.g., a single building or plant site) and
 provide high bandwidth with low delays. LANs may be passive
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 (similar to Ethernet) or they may be active (such as ATM).
 o Wide-Area Networks (WANs)
 Geographically dispersed hosts and LANs are interconnected by
 wide-area networks, also called long-haul networks. These
 networks may have a complex internal structure of lines and
 packet-switches, or they may be as simple as point-to-point
 lines.
2.2.3 Routers
 In the Internet model, constituent networks are connected together by
 IP datagram forwarders which are called routers or IP routers. In
 this document, every use of the term router is equivalent to IP
 router. Many older Internet documents refer to routers as gateways.
 Historically, routers have been realized with packet-switching
 software executing on a general-purpose CPU. However, as custom
 hardware development becomes cheaper and as higher throughput is
 required, special purpose hardware is becoming increasingly common.
 This specification applies to routers regardless of how they are
 implemented.
 A router connects to two or more logical interfaces, represented by
 IP subnets or unnumbered point to point lines (discussed in section
 [2.2.7]). Thus, it has at least one physical interface. Forwarding
 an IP datagram generally requires the router to choose the address
 and relevant interface of the next-hop router or (for the final hop)
 the destination host. This choice, called relaying or forwarding
 depends upon a route database within the router. The route database
 is also called a routing table or forwarding table. The term
 "router" derives from the process of building this route database;
 routing protocols and configuration interact in a process called
 routing.
 The routing database should be maintained dynamically to reflect the
 current topology of the Internet system. A router normally
 accomplishes this by participating in distributed routing and
 reachability algorithms with other routers.
 Routers provide datagram transport only, and they seek to minimize
 the state information necessary to sustain this service in the
 interest of routing flexibility and robustness.
 Packet switching devices may also operate at the Link Layer; such
 devices are usually called bridges. Network segments that are
 connected by bridges share the same IP network prefix forming a
 single IP subnet. These other devices are outside the scope of this
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 document.
2.2.4 Autonomous Systems
 An Autonomous System (AS) is a connected segment of a network
 topology that consists of a collection of subnetworks (with hosts
 attached) interconnected by a set of routes. The subnetworks and the
 routers are expected to be under the control of a single operations
 and maintenance (O&M) organization. Within an AS routers may use one
 or more interior routing protocols, and sometimes several sets of
 metrics. An AS is expected to present to other ASs an appearence of
 a coherent interior routing plan, and a consistent picture of the
 destinations reachable through the AS. An AS is identified by an
 Autonomous System number.
 The concept of an AS plays an important role in the Internet routing
 (see Section 7.1).
2.2.5 Addressing Architecture
 An IP datagram carries 32-bit source and destination addresses, each
 of which is partitioned into two parts - a constituent network prefix
 and a host number on that network. Symbolically:
 IP-address ::= { <Network-prefix>, <Host-number> }
 To finally deliver the datagram, the last router in its path must map
 the Host-number (or rest) part of an IP address to the host's Link
 Layer address.
2.2.5.1 Classical IP Addressing Architecture
 Although well documented elsewhere [INTERNET:2], it is useful to
 describe the historical use of the network prefix. The language
 developed to describe it is used in this and other documents and
 permeates the thinking behind many protocols.
 The simplest classical network prefix is the Class A, B, C, D, or E
 network prefix. These address ranges are discriminated by observing
 the values of the most significant bits of the address, and break the
 address into simple prefix and host number fields. This is described
 in [INTERNET:18]. In short, the classification is:
 0xxx - Class A - general purpose unicast addresses with standard
 8 bit prefix
 10xx - Class B - general purpose unicast addresses with standard
 16 bit prefix
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 110x - Class C - general purpose unicast addresses with standard
 24 bit prefix
 1110 - Class D - IP Multicast Addresses - 28 bit prefix, non-
 aggregatable
 1111 - Class E - reserved for experimental use
 This simple notion has been extended by the concept of subnets.
 These were introduced to allow arbitrary complexity of interconnected
 LAN structures within an organization, while insulating the Internet
 system against explosive growth in assigned network prefixes and
 routing complexity. Subnets provide a multi-level hierarchical
 routing structure for the Internet system. The subnet extension,
 described in [INTERNET:2], is a required part of the Internet
 architecture. The basic idea is to partition the <Host-number> field
 into two parts: a subnet number, and a true host number on that
 subnet:
 IP-address ::=
 { <Network-number>, <Subnet-number>, <Host-number> }
 The interconnected physical networks within an organization use the
 same network prefix but different subnet numbers. The distinction
 between the subnets of such a subnetted network is not normally
 visible outside of that network. Thus, routing in the rest of the
 Internet uses only the <Network-prefix> part of the IP destination
 address. Routers outside the network treat <Network-prefix> and
 <Host-number> together as an uninterpreted rest part of the 32-bit IP
 address. Within the subnetted network, the routers use the extended
 network prefix:
 { <Network-number>, <Subnet-number> }
 The bit positions containing this extended network number have
 historically been indicated by a 32-bit mask called the subnet mask.
 The <Subnet-number> bits SHOULD be contiguous and fall between the
 <Network-number> and the <Host-number> fields. More up to date
 protocols do not refer to a subnet mask, but to a prefix length; the
 "prefix" portion of an address is that which would be selected by a
 subnet mask whose most significant bits are all ones and the rest are
 zeroes. The length of the prefix equals the number of ones in the
 subnet mask. This document assumes that all subnet masks are
 expressible as prefix lengths.
 The inventors of the subnet mechanism presumed that each piece of an
 organization's network would have only a single subnet number. In
 practice, it has often proven necessary or useful to have several
 subnets share a single physical cable. For this reason, routers
 should be capable of configuring multiple subnets on the same
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 physical interfaces, and treat them (from a routing or forwarding
 perspective) as though they were distinct physical interfaces.
2.2.5.2 Classless Inter Domain Routing (CIDR)
 The explosive growth of the Internet has forced a review of address
 assignment policies. The traditional uses of general purpose (Class
 A, B, and C) networks have been modified to achieve better use of
 IP's 32-bit address space. Classless Inter Domain Routing (CIDR)
 [INTERNET:15] is a method currently being deployed in the Internet
 backbones to achieve this added efficiency. CIDR depends on
 deploying and routing to arbitrarily sized networks. In this model,
 hosts and routers make no assumptions about the use of addressing in
 the internet. The Class D (IP Multicast) and Class E (Experimental)
 address spaces are preserved, although this is primarily an
 assignment policy.
 By definition, CIDR comprises three elements:
 o topologically significant address assignment,
 o routing protocols that are capable of aggregating network layer
 reachability information, and
 o consistent forwarding algorithm ("longest match").
 The use of networks and subnets is now historical, although the
 language used to describe them remains in current use. They have
 been replaced by the more tractable concept of a network prefix. A
 network prefix is, by definition, a contiguous set of bits at the
 more significant end of the address that defines a set of systems;
 host numbers select among those systems. There is no requirement
 that all the internet use network prefixes uniformly. To collapse
 routing information, it is useful to divide the internet into
 addressing domains. Within such a domain, detailed information is
 available about constituent networks; outside it, only the common
 network prefix is advertised.
 The classical IP addressing architecture used addresses and subnet
 masks to discriminate the host number from the network prefix. With
 network prefixes, it is sufficient to indicate the number of bits in
 the prefix. Both representations are in common use. Architecturally
 correct subnet masks are capable of being represented using the
 prefix length description. They comprise that subset of all possible
 bits patterns that have
 o a contiguous string of ones at the more significant end,
 o a contiguous string of zeros at the less significant end, and
 o no intervening bits.
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 Routers SHOULD always treat a route as a network prefix, and SHOULD
 reject configuration and routing information inconsistent with that
 model.
 IP-address ::= { <Network-prefix>, <Host-number> }
 An effect of the use of CIDR is that the set of destinations
 associated with address prefixes in the routing table may exhibit
 subset relationship. A route describing a smaller set of
 destinations (a longer prefix) is said to be more specific than a
 route describing a larger set of destinations (a shorter prefix);
 similarly, a route describing a larger set of destinations (a shorter
 prefix) is said to be less specific than a route describing a smaller
 set of destinations (a longer prefix). Routers must use the most
 specific matching route (the longest matching network prefix) when
 forwarding traffic.
2.2.6 IP Multicasting
 IP multicasting is an extension of Link Layer multicast to IP
 internets. Using IP multicasts, a single datagram can be addressed
 to multiple hosts without sending it to all. In the extended case,
 these hosts may reside in different address domains. This collection
 of hosts is called a multicast group. Each multicast group is
 represented as a Class D IP address. An IP datagram sent to the
 group is to be delivered to each group member with the same best-
 effort delivery as that provided for unicast IP traffic. The sender
 of the datagram does not itself need to be a member of the
 destination group.
 The semantics of IP multicast group membership are defined in
 [INTERNET:4]. That document describes how hosts and routers join and
 leave multicast groups. It also defines a protocol, the Internet
 Group Management Protocol (IGMP), that monitors IP multicast group
 membership.
 Forwarding of IP multicast datagrams is accomplished either through
 static routing information or via a multicast routing protocol.
 Devices that forward IP multicast datagrams are called multicast
 routers. They may or may not also forward IP unicasts. Multicast
 datagrams are forwarded on the basis of both their source and
 destination addresses. Forwarding of IP multicast packets is
 described in more detail in Section [5.2.1]. Appendix D discusses
 multicast routing protocols.
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2.2.7 Unnumbered Lines and Networks Prefixes
 Traditionally, each network interface on an IP host or router has its
 own IP address. This can cause inefficient use of the scarce IP
 address space, since it forces allocation of an IP network prefix to
 every point-to-point link.
 To solve this problem, a number of people have proposed and
 implemented the concept of unnumbered point to point lines. An
 unnumbered point to point line does not have any network prefix
 associated with it. As a consequence, the network interfaces
 connected to an unnumbered point to point line do not have IP
 addresses.
 Because the IP architecture has traditionally assumed that all
 interfaces had IP addresses, these unnumbered interfaces cause some
 interesting dilemmas. For example, some IP options (e.g., Record
 Route) specify that a router must insert the interface address into
 the option, but an unnumbered interface has no IP address. Even more
 fundamental (as we shall see in chapter 5) is that routes contain the
 IP address of the next hop router. A router expects that this IP
 address will be on an IP (sub)net to which the router is connected.
 That assumption is of course violated if the only connection is an
 unnumbered point to point line.
 To get around these difficulties, two schemes have been conceived.
 The first scheme says that two routers connected by an unnumbered
 point to point line are not really two routers at all, but rather two
 half-routers that together make up a single virtual router. The
 unnumbered point to point line is essentially considered to be an
 internal bus in the virtual router. The two halves of the virtual
 router must coordinate their activities in such a way that they act
 exactly like a single router.
 This scheme fits in well with the IP architecture, but suffers from
 two important drawbacks. The first is that, although it handles the
 common case of a single unnumbered point to point line, it is not
 readily extensible to handle the case of a mesh of routers and
 unnumbered point to point lines. The second drawback is that the
 interactions between the half routers are necessarily complex and are
 not standardized, effectively precluding the connection of equipment
 from different vendors using unnumbered point to point lines.
 Because of these drawbacks, this memo has adopted an alternate
 scheme, which has been invented multiple times but which is probably
 originally attributable to Phil Karn. In this scheme, a router that
 has unnumbered point to point lines also has a special IP address,
 called a router-id in this memo. The router-id is one of the
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 router's IP addresses (a router is required to have at least one IP
 address). This router-id is used as if it is the IP address of all
 unnumbered interfaces.
2.2.8 Notable Oddities
2.2.8.1 Embedded Routers
 A router may be a stand-alone computer system, dedicated to its IP
 router functions. Alternatively, it is possible to embed router
 functions within a host operating system that supports connections to
 two or more networks. The best-known example of an operating system
 with embedded router code is the Berkeley BSD system. The embedded
 router feature seems to make building a network easy, but it has a
 number of hidden pitfalls:
 (1) If a host has only a single constituent-network interface, it
 should not act as a router.
 For example, hosts with embedded router code that gratuitously
 forward broadcast packets or datagrams on the same net often
 cause packet avalanches.
 (2) If a (multihomed) host acts as a router, it is subject to the
 requirements for routers contained in this document.
 For example, the routing protocol issues and the router control
 and monitoring problems are as hard and important for embedded
 routers as for stand-alone routers.
 Internet router requirements and specifications may change
 independently of operating system changes. An administration
 that operates an embedded router in the Internet is strongly
 advised to maintain and update the router code. This might
 require router source code.
 (3) When a host executes embedded router code, it becomes part of the
 Internet infrastructure. Thus, errors in software or
 configuration can hinder communication between other hosts. As
 a consequence, the host administrator must lose some autonomy.
 In many circumstances, a host administrator will need to disable
 router code embedded in the operating system. For this reason,
 it should be straightforward to disable embedded router
 functionality.
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 (4) When a host running embedded router code is concurrently used for
 other services, the Operation and Maintenance requirements for
 the two modes of use may conflict.
 For example, router O&M will in many cases be performed remotely
 by an operations center; this may require privileged system
 access that the host administrator would not normally want to
 distribute.
2.2.8.2 Transparent Routers
 There are two basic models for interconnecting local-area networks
 and wide-area (or long-haul) networks in the Internet. In the first,
 the local-area network is assigned a network prefix and all routers
 in the Internet must know how to route to that network. In the
 second, the local-area network shares (a small part of) the address
 space of the wide-area network. Routers that support this second
 model are called address sharing routers or transparent routers. The
 focus of this memo is on routers that support the first model, but
 this is not intended to exclude the use of transparent routers.
 The basic idea of a transparent router is that the hosts on the
 local-area network behind such a router share the address space of
 the wide-area network in front of the router. In certain situations
 this is a very useful approach and the limitations do not present
 significant drawbacks.
 The words in front and behind indicate one of the limitations of this
 approach: this model of interconnection is suitable only for a
 geographically (and topologically) limited stub environment. It
 requires that there be some form of logical addressing in the network
 level addressing of the wide-area network. IP addresses in the local
 environment map to a few (usually one) physical address in the wide-
 area network. This mapping occurs in a way consistent with the { IP
 address <-> network address } mapping used throughout the wide-area
 network.
 Multihoming is possible on one wide-area network, but may present
 routing problems if the interfaces are geographically or
 topologically separated. Multihoming on two (or more) wide-area
 networks is a problem due to the confusion of addresses.
 The behavior that hosts see from other hosts in what is apparently
 the same network may differ if the transparent router cannot fully
 emulate the normal wide-area network service. For example, the
 ARPANET used a Link Layer protocol that provided a Destination Dead
 indication in response to an attempt to send to a host that was off-
 line. However, if there were a transparent router between the
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 ARPANET and an Ethernet, a host on the ARPANET would not receive a
 Destination Dead indication for Ethernet hosts.
2.3 Router Characteristics
 An Internet router performs the following functions:
 (1) Conforms to specific Internet protocols specified in this
 document, including the Internet Protocol (IP), Internet Control
 Message Protocol (ICMP), and others as necessary.
 (2) Interfaces to two or more packet networks. For each connected
 network the router must implement the functions required by that
 network. These functions typically include:
 o Encapsulating and decapsulating the IP datagrams with the
 connected network framing (e.g., an Ethernet header and
 checksum),
 o Sending and receiving IP datagrams up to the maximum size
 supported by that network, this size is the network's Maximum
 Transmission Unit or MTU,
 o Translating the IP destination address into an appropriate
 network-level address for the connected network (e.g., an
 Ethernet hardware address), if needed, and
 o Responding to network flow control and error indications, if
 any.
 See chapter 3 (Link Layer).
 (3) Receives and forwards Internet datagrams. Important issues in
 this process are buffer management, congestion control, and
 fairness.
 o Recognizes error conditions and generates ICMP error and
 information messages as required.
 o Drops datagrams whose time-to-live fields have reached zero.
 o Fragments datagrams when necessary to fit into the MTU of the
 next network.
 See chapter 4 (Internet Layer - Protocols) and chapter 5
 (Internet Layer - Forwarding) for more information.
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 (4) Chooses a next-hop destination for each IP datagram, based on the
 information in its routing database. See chapter 5 (Internet
 Layer - Forwarding) for more information.
 (5) (Usually) supports an interior gateway protocol (IGP) to carry
 out distributed routing and reachability algorithms with the
 other routers in the same autonomous system. In addition, some
 routers will need to support an exterior gateway protocol (EGP)
 to exchange topological information with other autonomous
 systems. See chapter 7 (Application Layer - Routing Protocols)
 for more information.
 (6) Provides network management and system support facilities,
 including loading, debugging, status reporting, exception
 reporting and control. See chapter 8 (Application Layer -
 Network Management Protocols) and chapter 10 (Operation and
 Maintenance) for more information.
 A router vendor will have many choices on power, complexity, and
 features for a particular router product. It may be helpful to
 observe that the Internet system is neither homogeneous nor fully
 connected. For reasons of technology and geography it is growing
 into a global interconnect system plus a fringe of LANs around the
 edge. More and more these fringe LANs are becoming richly
 interconnected, thus making them less out on the fringe and more
 demanding on router requirements.
 o The global interconnect system is composed of a number of wide-area
 networks to which are attached routers of several Autonomous
 Systems (AS); there are relatively few hosts connected directly to
 the system.
 o Most hosts are connected to LANs. Many organizations have clusters
 of LANs interconnected by local routers. Each such cluster is
 connected by routers at one or more points into the global
 interconnect system. If it is connected at only one point, a LAN
 is known as a stub network.
 Routers in the global interconnect system generally require:
 o Advanced Routing and Forwarding Algorithms
 These routers need routing algorithms that are highly dynamic,
 impose minimal processing and communication burdens, and offer
 type-of-service routing. Congestion is still not a completely
 resolved issue (see Section [5.3.6]). Improvements in these areas
 are expected, as the research community is actively working on
 these issues.
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 o High Availability
 These routers need to be highly reliable, providing 24 hours a
 day, 7 days a week service. Equipment and software faults can
 have a wide-spread (sometimes global) effect. In case of failure,
 they must recover quickly. In any environment, a router must be
 highly robust and able to operate, possibly in a degraded state,
 under conditions of extreme congestion or failure of network
 resources.
 o Advanced O&M Features
 Internet routers normally operate in an unattended mode. They
 will typically be operated remotely from a centralized monitoring
 center. They need to provide sophisticated means for monitoring
 and measuring traffic and other events and for diagnosing faults.
 o High Performance
 Long-haul lines in the Internet today are most frequently full
 duplex 56 KBPS, DS1 (1.544 Mbps), or DS3 (45 Mbps) speeds. LANs,
 which are half duplex multiaccess media, are typically Ethernet
 (10Mbps) and, to a lesser degree, FDDI (100Mbps). However,
 network media technology is constantly advancing and higher speeds
 are likely in the future.
 The requirements for routers used in the LAN fringe (e.g., campus
 networks) depend greatly on the demands of the local networks. These
 may be high or medium-performance devices, probably competitively
 procured from several different vendors and operated by an internal
 organization (e.g., a campus computing center). The design of these
 routers should emphasize low average latency and good burst
 performance, together with delay and type-of-service sensitive
 resource management. In this environment there may be less formal
 O&M but it will not be less important. The need for the routing
 mechanism to be highly dynamic will become more important as networks
 become more complex and interconnected. Users will demand more out
 of their local connections because of the speed of the global
 interconnects.
 As networks have grown, and as more networks have become old enough
 that they are phasing out older equipment, it has become increasingly
 imperative that routers interoperate with routers from other vendors.
 Even though the Internet system is not fully interconnected, many
 parts of the system need to have redundant connectivity. Rich
 connectivity allows reliable service despite failures of
 communication lines and routers, and it can also improve service by
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 shortening Internet paths and by providing additional capacity.
 Unfortunately, this richer topology can make it much more difficult
 to choose the best path to a particular destination.
2.4 Architectural Assumptions
 The current Internet architecture is based on a set of assumptions
 about the communication system. The assumptions most relevant to
 routers are as follows:
 o The Internet is a network of networks.
 Each host is directly connected to some particular network(s); its
 connection to the Internet is only conceptual. Two hosts on the
 same network communicate with each other using the same set of
 protocols that they would use to communicate with hosts on distant
 networks.
 o Routers do not keep connection state information.
 To improve the robustness of the communication system, routers are
 designed to be stateless, forwarding each IP packet independently
 of other packets. As a result, redundant paths can be exploited
 to provide robust service in spite of failures of intervening
 routers and networks.
 All state information required for end-to-end flow control and
 reliability is implemented in the hosts, in the transport layer or
 in application programs. All connection control information is
 thus co-located with the end points of the communication, so it
 will be lost only if an end point fails. Routers control message
 flow only indirectly, by dropping packets or increasing network
 delay.
 Note that future protocol developments may well end up putting
 some more state into routers. This is especially likely for
 multicast routing, resource reservation, and flow based
 forwarding.
 o Routing complexity should be in the routers.
 Routing is a complex and difficult problem, and ought to be
 performed by the routers, not the hosts. An important objective
 is to insulate host software from changes caused by the inevitable
 evolution of the Internet routing architecture.
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 o The system must tolerate wide network variation.
 A basic objective of the Internet design is to tolerate a wide
 range of network characteristics - e.g., bandwidth, delay, packet
 loss, packet reordering, and maximum packet size. Another
 objective is robustness against failure of individual networks,
 routers, and hosts, using whatever bandwidth is still available.
 Finally, the goal is full open system interconnection: an Internet
 router must be able to interoperate robustly and effectively with
 any other router or Internet host, across diverse Internet paths.
 Sometimes implementors have designed for less ambitious goals.
 For example, the LAN environment is typically much more benign
 than the Internet as a whole; LANs have low packet loss and delay
 and do not reorder packets. Some vendors have fielded
 implementations that are adequate for a simple LAN environment,
 but work badly for general interoperation. The vendor justifies
 such a product as being economical within the restricted LAN
 market. However, isolated LANs seldom stay isolated for long.
 They are soon connected to each other, to organization-wide
 internets, and eventually to the global Internet system. In the
 end, neither the customer nor the vendor is served by incomplete
 or substandard routers.
 The requirements in this document are designed for a full-function
 router. It is intended that fully compliant routers will be
 usable in almost any part of the Internet.
3. LINK LAYER
 Although [INTRO:1] covers Link Layer standards (IP over various link
 layers, ARP, etc.), this document anticipates that Link-Layer
 material will be covered in a separate Link Layer Requirements
 document. A Link-Layer Requirements document would be applicable to
 both hosts and routers. Thus, this document will not obsolete the
 parts of [INTRO:1] that deal with link-layer issues.
3.1 INTRODUCTION
 Routers have essentially the same Link Layer protocol requirements as
 other sorts of Internet systems. These requirements are given in
 chapter 3 of Requirements for Internet Gateways [INTRO:1]. A router
 MUST comply with its requirements and SHOULD comply with its
 recommendations. Since some of the material in that document has
 become somewhat dated, some additional requirements and explanations
 are included below.
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 DISCUSSION
 It is expected that the Internet community will produce a
 Requirements for Internet Link Layer standard which will supersede
 both this chapter and the chapter entitled "INTERNET LAYER
 PROTOCOLS" in [INTRO:1].
3.2 LINK/INTERNET LAYER INTERFACE
 This document does not attempt to specify the interface between the
 Link Layer and the upper layers. However, note well that other parts
 of this document, particularly chapter 5, require various sorts of
 information to be passed across this layer boundary.
 This section uses the following definitions:
 o Source physical address
 The source physical address is the Link Layer address of the host
 or router from which the packet was received.
 o Destination physical address
 The destination physical address is the Link Layer address to
 which the packet was sent.
 The information that must pass from the Link Layer to the
 Internetwork Layer for each received packet is:
 (1) The IP packet [5.2.2],
 (2) The length of the data portion (i.e., not including the Link-
 Layer framing) of the Link Layer frame [5.2.2],
 (3) The identity of the physical interface from which the IP packet
 was received [5.2.3], and
 (4) The classification of the packet's destination physical address
 as a Link Layer unicast, broadcast, or multicast [4.3.2],
 [5.3.4].
 In addition, the Link Layer also should provide:
 (5) The source physical address.
 The information that must pass from the Internetwork Layer to the
 Link Layer for each transmitted packet is:
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 (1) The IP packet [5.2.1]
 (2) The length of the IP packet [5.2.1]
 (3) The destination physical interface [5.2.1]
 (4) The next hop IP address [5.2.1]
 In addition, the Internetwork Layer also should provide:
 (5) The Link Layer priority value [5.3.3.2]
 The Link Layer must also notify the Internetwork Layer if the packet
 to be transmitted causes a Link Layer precedence-related error
 [5.3.3.3].
3.3 SPECIFIC ISSUES
3.3.1 Trailer Encapsulation
 Routers that can connect to ten megabit Ethernets MAY be able to
 receive and forward Ethernet packets encapsulated using the trailer
 encapsulation described in [LINK:1]. However, a router SHOULD NOT
 originate trailer encapsulated packets. A router MUST NOT originate
 trailer encapsulated packets without first verifying, using the
 mechanism described in [INTRO:2], that the immediate destination of
 the packet is willing and able to accept trailer-encapsulated
 packets. A router SHOULD NOT agree (using these mechanisms) to
 accept trailer-encapsulated packets.
3.3.2 Address Resolution Protocol - ARP
 Routers that implement ARP MUST be compliant and SHOULD be
 unconditionally compliant with the requirements in [INTRO:2].
 The link layer MUST NOT report a Destination Unreachable error to IP
 solely because there is no ARP cache entry for a destination; it
 SHOULD queue up to a small number of datagrams breifly while
 performing the ARP request/reply sequence, and reply that the
 destination is unreachable to one of the queued datagrams only when
 this proves fruitless.
 A router MUST not believe any ARP reply that claims that the Link
 Layer address of another host or router is a broadcast or multicast
 address.
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3.3.3 Ethernet and 802.3 Coexistence
 Routers that can connect to ten megabit Ethernets MUST be compliant
 and SHOULD be unconditionally compliant with the Ethernet
 requirements of [INTRO:2].
3.3.4 Maximum Transmission Unit - MTU
 The MTU of each logical interface MUST be configurable within the
 range of legal MTUs for the interface.
 Many Link Layer protocols define a maximum frame size that may be
 sent. In such cases, a router MUST NOT allow an MTU to be set which
 would allow sending of frames larger than those allowed by the Link
 Layer protocol. However, a router SHOULD be willing to receive a
 packet as large as the maximum frame size even if that is larger than
 the MTU.
 DISCUSSION
 Note that this is a stricter requirement than imposed on hosts by
 [INTRO:2], which requires that the MTU of each physical interface
 be configurable.
 If a network is using an MTU smaller than the maximum frame size
 for the Link Layer, a router may receive packets larger than the
 MTU from misconfigured and incompletely initialized hosts. The
 Robustness Principle indicates that the router should successfully
 receive these packets if possible.
3.3.5 Point-to-Point Protocol - PPP
 Contrary to [INTRO:1], the Internet does have a standard point to
 point line protocol: the Point-to-Point Protocol (PPP), defined in
 [LINK:2], [LINK:3], [LINK:4], and [LINK:5].
 A point to point interface is any interface that is designed to send
 data over a point to point line. Such interfaces include telephone,
 leased, dedicated or direct lines (either 2 or 4 wire), and may use
 point to point channels or virtual circuits of multiplexed interfaces
 such as ISDN. They normally use a standardized modem or bit serial
 interface (such as RS-232, RS-449 or V.35), using either synchronous
 or asynchronous clocking. Multiplexed interfaces often have special
 physical interfaces.
 A general purpose serial interface uses the same physical media as a
 point to point line, but supports the use of link layer networks as
 well as point to point connectivity. Link layer networks (such as
 X.25 or Frame Relay) use an alternative IP link layer specification.
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 Routers that implement point to point or general purpose serial
 interfaces MUST IMPLEMENT PPP.
 PPP MUST be supported on all general purpose serial interfaces on a
 router. The router MAY allow the line to be configured to use point
 to point line protocols other than PPP. Point to point interfaces
 SHOULD either default to using PPP when enabled or require
 configuration of the link layer protocol before being enabled.
 General purpose serial interfaces SHOULD require configuration of the
 link layer protocol before being enabled.
3.3.5.1 Introduction
 This section provides guidelines to router implementors so that they
 can ensure interoperability with other routers using PPP over either
 synchronous or asynchronous links.
 It is critical that an implementor understand the semantics of the
 option negotiation mechanism. Options are a means for a local device
 to indicate to a remote peer what the local device will accept from
 the remote peer, not what it wishes to send. It is up to the remote
 peer to decide what is most convenient to send within the confines of
 the set of options that the local device has stated that it can
 accept. Therefore it is perfectly acceptable and normal for a remote
 peer to ACK all the options indicated in an LCP Configuration Request
 (CR) even if the remote peer does not support any of those options.
 Again, the options are simply a mechanism for either device to
 indicate to its peer what it will accept, not necessarily what it
 will send.
3.3.5.2 Link Control Protocol (LCP) Options
 The PPP Link Control Protocol (LCP) offers a number of options that
 may be negotiated. These options include (among others) address and
 control field compression, protocol field compression, asynchronous
 character map, Maximum Receive Unit (MRU), Link Quality Monitoring
 (LQM), magic number (for loopback detection), Password Authentication
 Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP),
 and the 32-bit Frame Check Sequence (FCS).
 A router MAY use address/control field compression on either
 synchronous or asynchronous links. A router MAY use protocol field
 compression on either synchronous or asynchronous links. A router
 that indicates that it can accept these compressions MUST be able to
 accept uncompressed PPP header information also.
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 DISCUSSION
 These options control the appearance of the PPP header. Normally
 the PPP header consists of the address, the control field, and the
 protocol field. The address, on a point to point line, is 0xFF,
 indicating "broadcast". The control field is 0x03, indicating
 "Unnumbered Information." The Protocol Identifier is a two byte
 value indicating the contents of the data area of the frame. If a
 system negotiates address and control field compression it
 indicates to its peer that it will accept PPP frames that have or
 do not have these fields at the front of the header. It does not
 indicate that it will be sending frames with these fields removed.
 Protocol field compression, when negotiated, indicates that the
 system is willing to receive protocol fields compressed to one
 byte when this is legal. There is no requirement that the sender
 do so.
 Use of address/control field compression is inconsistent with the
 use of numbered mode (reliable) PPP.
 IMPLEMENTATION
 Some hardware does not deal well with variable length header
 information. In those cases it makes most sense for the remote
 peer to send the full PPP header. Implementations may ensure this
 by not sending the address/control field and protocol field
 compression options to the remote peer. Even if the remote peer
 has indicated an ability to receive compressed headers there is no
 requirement for the local router to send compressed headers.
 A router MUST negotiate the Asynchronous Control Character Map (ACCM)
 for asynchronous PPP links, but SHOULD NOT negotiate the ACCM for
 synchronous links. If a router receives an attempt to negotiate the
 ACCM over a synchronous link, it MUST ACKnowledge the option and then
 ignore it.
 DISCUSSION
 There are implementations that offer both synchronous and
 asynchronous modes of operation and may use the same code to
 implement the option negotiation. In this situation it is
 possible that one end or the other may send the ACCM option on a
 synchronous link.
 A router SHOULD properly negotiate the maximum receive unit (MRU).
 Even if a system negotiates an MRU smaller than 1,500 bytes, it MUST
 be able to receive a 1,500 byte frame.
 A router SHOULD negotiate and enable the link quality monitoring
 (LQM) option.
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 DISCUSSION
 This memo does not specify a policy for deciding whether the
 link's quality is adequate. However, it is important (see Section
 [3.3.6]) that a router disable failed links.
 A router SHOULD implement and negotiate the magic number option for
 loopback detection.
 A router MAY support the authentication options (PAP - Password
 Authentication Protocol, and/or CHAP - Challenge Handshake
 Authentication Protocol).
 A router MUST support 16-bit CRC frame check sequence (FCS) and MAY
 support the 32-bit CRC.
3.3.5.3 IP Control Protocol (IPCP) Options
 A router MAY offer to perform IP address negotiation. A router MUST
 accept a refusal (REJect) to perform IP address negotiation from the
 peer.
 Routers operating at link speeds of 19,200 BPS or less SHOULD
 implement and offer to perform Van Jacobson header compression.
 Routers that implement VJ compression SHOULD implement an
 administrative control enabling or disabling it.
3.3.6 Interface Testing
 A router MUST have a mechanism to allow routing software to determine
 whether a physical interface is available to send packets or not; on
 multiplexed interfaces where permanent virtual circuits are opened
 for limited sets of neighbors, the router must also be able to
 determine whether the virtual circuits are viable. A router SHOULD
 have a mechanism to allow routing software to judge the quality of a
 physical interface. A router MUST have a mechanism for informing the
 routing software when a physical interface becomes available or
 unavailable to send packets because of administrative action. A
 router MUST have a mechanism for informing the routing software when
 it detects a Link level interface has become available or
 unavailable, for any reason.
 DISCUSSION
 It is crucial that routers have workable mechanisms for
 determining that their network connections are functioning
 properly. Failure to detect link loss, or failure to take the
 proper actions when a problem is detected, can lead to black
 holes.
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 The mechanisms available for detecting problems with network
 connections vary considerably, depending on the Link Layer
 protocols in use and the interface hardware. The intent is to
 maximize the capability to detect failures within the Link-Layer
 constraints.
4. INTERNET LAYER - PROTOCOLS
4.1 INTRODUCTION
 This chapter and chapter 5 discuss the protocols used at the Internet
 Layer: IP, ICMP, and IGMP. Since forwarding is obviously a crucial
 topic in a document discussing routers, chapter 5 limits itself to
 the aspects of the protocols that directly relate to forwarding. The
 current chapter contains the remainder of the discussion of the
 Internet Layer protocols.
4.2 INTERNET PROTOCOL - IP
4.2.1 INTRODUCTION
 Routers MUST implement the IP protocol, as defined by [INTERNET:1].
 They MUST also implement its mandatory extensions: subnets (defined
 in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and
 Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).
 Router implementors need not consider compliance with the section of
 [INTRO:2] entitled "Internet Protocol -- IP," as that section is
 entirely duplicated or superseded in this document. A router MUST be
 compliant, and SHOULD be unconditionally compliant, with the
 requirements of the section entitled "SPECIFIC ISSUES" relating to IP
 in [INTRO:2].
 In the following, the action specified in certain cases is to
 silently discard a received datagram. This means that the datagram
 will be discarded without further processing and that the router will
 not send any ICMP error message (see Section [4.3]) as a result.
 However, for diagnosis of problems a router SHOULD provide the
 capability of logging the error (see Section [1.3.3]), including the
 contents of the silently discarded datagram, and SHOULD count
 datagrams discarded.
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4.2.2 PROTOCOL WALK-THROUGH
 RFC 791 [INTERNET:1] is the specification for the Internet Protocol.
4.2.2.1 Options: RFC 791 Section 3.2
 In datagrams received by the router itself, the IP layer MUST
 interpret IP options that it understands and preserve the rest
 unchanged for use by higher layer protocols.
 Higher layer protocols may require the ability to set IP options in
 datagrams they send or examine IP options in datagrams they receive.
 Later sections of this document discuss specific IP option support
 required by higher layer protocols.
 DISCUSSION
 Neither this memo nor [INTRO:2] define the order in which a
 receiver must process multiple options in the same IP header.
 Hosts and routers originating datagrams containing multiple
 options must be aware that this introduces an ambiguity in the
 meaning of certain options when combined with a source-route
 option.
 Here are the requirements for specific IP options:
 (a) Security Option
 Some environments require the Security option in every packet
 originated or received. Routers SHOULD IMPLEMENT the revised
 security option described in [INTERNET:5].
 DISCUSSION
 Note that the security options described in [INTERNET:1] and RFC
 1038 ([INTERNET:16]) are obsolete.
 (b) Stream Identifier Option
 This option is obsolete; routers SHOULD NOT place this option
 in a datagram that the router originates. This option MUST be
 ignored in datagrams received by the router.
 (c) Source Route Options
 A router MUST be able to act as the final destination of a
 source route. If a router receives a packet containing a
 completed source route, the packet has reached its final
 destination. In such an option, the pointer points beyond the
 last field and the destination address in the IP header
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 addresses the router. The option as received (the recorded
 route) MUST be passed up to the transport layer (or to ICMP
 message processing).
 In the general case, a correct response to a source-routed
 datagram traverses the same route. A router MUST provide a
 means whereby transport protocols and applications can reverse
 the source route in a received datagram. This reversed source
 route MUST be inserted into datagrams they originate (see
 [INTRO:2] for details) when the router is unaware of policy
 constraints. However, if the router is policy aware, it MAY
 select another path.
 Some applications in the router MAY require that the user be
 able to enter a source route.
 A router MUST NOT originate a datagram containing multiple
 source route options. What a router should do if asked to
 forward a packet containing multiple source route options is
 described in Section [5.2.4.1].
 When a source route option is created (which would happen when
 the router is originating a source routed datagram or is
 inserting a source route option as a result of a special
 filter), it MUST be correctly formed even if it is being
 created by reversing a recorded route that erroneously includes
 the source host (see case (B) in the discussion below).
 DISCUSSION
 Suppose a source routed datagram is to be routed from source _?S to
 destination D via routers G1, G2, Gn. Source S constructs a
 datagram with G1's IP address as its destination address, and a
 source route option to get the datagram the rest of the way to its
 destination. However, there is an ambiguity in the specification
 over whether the source route option in a datagram sent out by S
 should be (A) or (B):
 (A): {>>G2, G3, ... Gn, D} <--- CORRECT
 (B): {S, >>G2, G3, ... Gn, D} <---- WRONG
 (where >> represents the pointer). If (A) is sent, the datagram
 received at D will contain the option: {G1, G2, ... Gn >>}, with S
 and D as the IP source and destination addresses. If (B) were
 sent, the datagram received at D would again contain S and D as
 the same IP source and destination addresses, but the option would
 be: {S, G1, ...Gn >>}; i.e., the originating host would be the
 first hop in the route.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 (d) Record Route Option
 Routers MAY support the Record Route option in datagrams
 originated by the router.
 (e) Timestamp Option
 Routers MAY support the timestamp option in datagrams
 originated by the router. The following rules apply:
 o When originating a datagram containing a Timestamp Option, a
 router MUST record a timestamp in the option if
 - Its Internet address fields are not pre-specified or
 - Its first pre-specified address is the IP address of the
 logical interface over which the datagram is being sent
 (or the router's router-id if the datagram is being sent
 over an unnumbered interface).
 o If the router itself receives a datagram containing a
 Timestamp Option, the router MUST insert the current time
 into the Timestamp Option (if there is space in the option
 to do so) before passing the option to the transport layer
 or to ICMP for processing. If space is not present, the
 router MUST increment the Overflow Count in the option.
 o A timestamp value MUST follow the rules defined in [INTRO:2].
 IMPLEMENTATION
 To maximize the utility of the timestamps contained in the
 timestamp option, the timestamp inserted should be, as nearly as
 practical, the time at which the packet arrived at the router.
 For datagrams originated by the router, the timestamp inserted
 should be, as nearly as practical, the time at which the datagram
 was passed to the Link Layer for transmission.
 The timestamp option permits the use of a non-standard time clock,
 but the use of a non-synchronized clock limits the utility of the
 time stamp. Therefore, routers are well advised to implement the
 Network Time Protocol for the purpose of synchronizing their
 clocks.
4.2.2.2 Addresses in Options: RFC 791 Section 3.1
 Routers are called upon to insert their address into Record Route,
 Strict Source and Record Route, Loose Source and Record Route, or
 Timestamp Options. When a router inserts its address into such an
 option, it MUST use the IP address of the logical interface on which
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 the packet is being sent. Where this rule cannot be obeyed because
 the output interface has no IP address (i.e., is an unnumbered
 interface), the router MUST instead insert its router-id. The
 router's router-id is one of the router's IP addresses. The Router
 ID may be specified on a system basis or on a per-link basis. Which
 of the router's addresses is used as the router-id MUST NOT change
 (even across reboots) unless changed by the network manager.
 Relevant management changes include reconfiguration of the router
 such that the IP address used as the router-id ceases to be one of
 the router's IP addresses. Routers with multiple unnumbered
 interfaces MAY have multiple router-id's. Each unnumbered interface
 MUST be associated with a particular router-id. This association
 MUST NOT change (even across reboots) without reconfiguration of the
 router.
 DISCUSSION
 This specification does not allow for routers that do not have at
 least one IP address. We do not view this as a serious
 limitation, since a router needs an IP address to meet the
 manageability requirements of Chapter [8] even if the router is
 connected only to point-to-point links.
 IMPLEMENTATION
 One possible method of choosing the router-id that fulfills this
 requirement is to use the numerically smallest (or greatest) IP
 address (treating the address as a 32-bit integer) that is
 assigned to the router.
4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1
 The IP header contains two reserved bits: one in the Type of Service
 byte and the other in the Flags field. A router MUST NOT set either
 of these bits to one in datagrams originated by the router. A router
 MUST NOT drop (refuse to receive or forward) a packet merely because
 one or more of these reserved bits has a non-zero value; i.e., the
 router MUST NOT check the values of thes bits.
 DISCUSSION
 Future revisions to the IP protocol may make use of these unused
 bits. These rules are intended to ensure that these revisions can
 be deployed without having to simultaneously upgrade all routers
 in the Internet.
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4.2.2.4 Type of Service: RFC 791 Section 3.1
 The Type-of-Service byte in the IP header is divided into three
 sections: the Precedence field (high-order 3 bits), a field that is
 customarily called Type of Service or TOS (next 4 bits), and a
 reserved bit (the low order bit).
 Rules governing the reserved bit were described in Section [4.2.2.3].
 A more extensive discussion of the TOS field and its use can be found
 in [ROUTE:11].
 The description of the IP Precedence field is superseded by Section
 [5.3.3]. RFC 795, Service Mappings, is obsolete and SHOULD NOT be
 implemented.
4.2.2.5 Header Checksum: RFC 791 Section 3.1
 As stated in Section [5.2.2], a router MUST verify the IP checksum of
 any packet that is received, and MUST discard messages containing
 invalid checksums. The router MUST NOT provide a means to disable
 this checksum verification.
 A router MAY use incremental IP header checksum updating when the
 only change to the IP header is the time to live. This will reduce
 the possibility of undetected corruption of the IP header by the
 router. See [INTERNET:6] for a discussion of incrementally updating
 the checksum.
 IMPLEMENTATION
 A more extensive description of the IP checksum, including
 extensive implementation hints, can be found in [INTERNET:6] and
 [INTERNET:7].
4.2.2.6 Unrecognized Header Options: RFC 791 Section 3.1
 A router MUST ignore IP options which it does not recognize. A
 corollary of this requirement is that a router MUST implement the End
 of Option List option and the No Operation option, since neither
 contains an explicit length.
 DISCUSSION
 All future IP options will include an explicit length.
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4.2.2.7 Fragmentation: RFC 791 Section 3.2
 Fragmentation, as described in [INTERNET:1], MUST be supported by a
 router.
 When a router fragments an IP datagram, it SHOULD minimize the number
 of fragments. When a router fragments an IP datagram, it SHOULD send
 the fragments in order. A fragmentation method that may generate one
 IP fragment that is significantly smaller than the other MAY cause
 the first IP fragment to be the smaller one.
 DISCUSSION
 There are several fragmentation techniques in common use in the
 Internet. One involves splitting the IP datagram into IP
 fragments with the first being MTU sized, and the others being
 approximately the same size, smaller than the MTU. The reason for
 this is twofold. The first IP fragment in the sequence will be
 the effective MTU of the current path between the hosts, and the
 following IP fragments are sized to minimize the further
 fragmentation of the IP datagram. Another technique is to split
 the IP datagram into MTU sized IP fragments, with the last
 fragment being the only one smaller, as described in [INTERNET:1].
 A common trick used by some implementations of TCP/IP is to
 fragment an IP datagram into IP fragments that are no larger than
 576 bytes when the IP datagram is to travel through a router.
 This is intended to allow the resulting IP fragments to pass the
 rest of the path without further fragmentation. This would,
 though, create more of a load on the destination host, since it
 would have a larger number of IP fragments to reassemble into one
 IP datagram. It would also not be efficient on networks where the
 MTU only changes once and stays much larger than 576 bytes.
 Examples include LAN networks such as an IEEE 802.5 network with a
 MTU of 2048 or an Ethernet network with an MTU of 1500).
 One other fragmentation technique discussed was splitting the IP
 datagram into approximately equal sized IP fragments, with the
 size less than or equal to the next hop network's MTU. This is
 intended to minimize the number of fragments that would result
 from additional fragmentation further down the path, and assure
 equal delay for each fragment.
 Routers SHOULD generate the least possible number of IP fragments.
 Work with slow machines leads us to believe that if it is
 necessary to fragment messages, sending the small IP fragment
 first maximizes the chance of a host with a slow interface of
 receiving all the fragments.
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4.2.2.8 Reassembly: RFC 791 Section 3.2
 As specified in the corresponding section of [INTRO:2], a router MUST
 support reassembly of datagrams that it delivers to itself.
4.2.2.9 Time to Live: RFC 791 Section 3.2
 Time to Live (TTL) handling for packets originated or received by the
 router is governed by [INTRO:2]; this section changes none of its
 stipulations. However, since the remainder of the IP Protocol
 section of [INTRO:2] is rewritten, this section is as well.
 Note in particular that a router MUST NOT check the TTL of a packet
 except when forwarding it.
 A router MUST NOT originate or forward a datagram with a Time-to-Live
 (TTL) value of zero.
 A router MUST NOT discard a datagram just because it was received
 with TTL equal to zero or one; if it is to the router and otherwise
 valid, the router MUST attempt to receive it.
 On messages the router originates, the IP layer MUST provide a means
 for the transport layer to set the TTL field of every datagram that
 is sent. When a fixed TTL value is used, it MUST be configurable.
 The number SHOULD exceed the typical internet diameter, and current
 wisdom suggests that it should exceed twice the internet diameter to
 allow for growth. Current suggested values are normally posted in
 the Assigned Numbers RFC. The TTL field has two functions: limit the
 lifetime of TCP segments (see RFC 793 [TCP:1], p. 28), and terminate
 Internet routing loops. Although TTL is a time in seconds, it also
 has some attributes of a hop-count, since each router is required to
 reduce the TTL field by at least one.
 TTL expiration is intended to cause datagrams to be discarded by
 routers, but not by the destination host. Hosts that act as routers
 by forwarding datagrams must therefore follow the router's rules for
 TTL.
 A higher-layer protocol may want to set the TTL in order to implement
 an "expanding scope" search for some Internet resource. This is used
 by some diagnostic tools, and is expected to be useful for locating
 the "nearest" server of a given class using IP multicasting, for
 example. A particular transport protocol may also want to specify
 its own TTL bound on maximum datagram lifetime.
 A fixed default value must be at least big enough for the Internet
 "diameter," i.e., the longest possible path. A reasonable value is
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 about twice the diameter, to allow for continued Internet growth. As
 of this writing, messages crossing the United States frequently
 traverse 15 to 20 routers; this argues for a default TTL value in
 excess of 40, and 64 is a common value.
4.2.2.10 Multi-subnet Broadcasts: RFC 922
 All-subnets broadcasts (called multi-subnet broadcasts in
 [INTERNET:3]) have been deprecated. See Section [5.3.5.3].
4.2.2.11 Addressing: RFC 791 Section 3.2
 As noted in 2.2.5.1, there are now five classes of IP addresses:
 Class A through Class E. Class D addresses are used for IP
 multicasting [INTERNET:4], while Class E addresses are reserved for
 experimental use. The distinction between Class A, B, and C
 addresses is no longer important; they are used as generalized
 unicast network prefixes with only historical interest in their
 class.
 An IP multicast address is a 28-bit logical address that stands for a
 group of hosts, and may be either permanent or transient. Permanent
 multicast addresses are allocated by the Internet Assigned Number
 Authority [INTRO:7], while transient addresses may be allocated
 dynamically to transient groups. Group membership is determined
 dynamically using IGMP [INTERNET:4].
 We now summarize the important special cases for general purpose
 unicast IP addresses, using the following notation for an IP address:
 { <Network-prefix>, <Host-number> }
 and the notation -1 for a field that contains all 1 bits and the
 notation 0 for a field that contains all 0 bits.
 (a) { 0, 0 }
 This host on this network. It MUST NOT be used as a source
 address by routers, except the router MAY use this as a source
 address as part of an initialization procedure (e.g., if the
 router is using BOOTP to load its configuration information).
 Incoming datagrams with a source address of { 0, 0 } which are
 received for local delivery (see Section [5.2.3]), MUST be
 accepted if the router implements the associated protocol and
 that protocol clearly defines appropriate action to be taken.
 Otherwise, a router MUST silently discard any locally-delivered
 datagram whose source address is { 0, 0 }.
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 DISCUSSION
 Some protocols define specific actions to take in response to a
 received datagram whose source address is { 0, 0 }. Two examples
 are BOOTP and ICMP Mask Request. The proper operation of these
 protocols often depends on the ability to receive datagrams whose
 source address is { 0, 0 }. For most protocols, however, it is
 best to ignore datagrams having a source address of { 0, 0 } since
 they were probably generated by a misconfigured host or router.
 Thus, if a router knows how to deal with a given datagram having a
 { 0, 0 } source address, the router MUST accept it. Otherwise,
 the router MUST discard it.
 See also Section [4.2.3.1] for a non-standard use of { 0, 0 }.
 (b) { 0, <Host-number> }
 Specified host on this network. It MUST NOT be sent by routers
 except that the router MAY use this as a source address as part
 of an initialization procedure by which the it learns its own
 IP address.
 (c) { -1, -1 }
 Limited broadcast. It MUST NOT be used as a source address.
 A datagram with this destination address will be received by
 every host and router on the connected physical network, but
 will not be forwarded outside that network.
 (d) { <Network-prefix>, -1 }
 Directed Broadcast - a broadcast directed to the specified
 network prefix. It MUST NOT be used as a source address. A
 router MAY originate Network Directed Broadcast packets. A
 router MUST receive Network Directed Broadcast packets; however
 a router MAY have a configuration option to prevent reception
 of these packets. Such an option MUST default to allowing
 reception.
 (e) { 127, <any> }
 Internal host loopback address. Addresses of this form MUST
 NOT appear outside a host.
 The <Network-prefix> is administratively assigned so that its value
 will be unique in the routing domain to which the device is
 connected.
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 IP addresses are not permitted to have the value 0 or -1 for the
 <Host-number> or <Network-prefix> fields except in the special cases
 listed above. This implies that each of these fields will be at
 least two bits long.
 DISCUSSION
 Previous versions of this document also noted that subnet numbers
 must be neither 0 nor -1, and must be at least two bits in length.
 In a CIDR world, the subnet number is clearly an extension of the
 network prefix and cannot be interpreted without the remainder of
 the prefix. This restriction of subnet numbers is therefore
 meaningless in view of CIDR and may be safely ignored.
 For further discussion of broadcast addresses, see Section [4.2.3.1].
 When a router originates any datagram, the IP source address MUST be
 one of its own IP addresses (but not a broadcast or multicast
 address). The only exception is during initialization.
 For most purposes, a datagram addressed to a broadcast or multicast
 destination is processed as if it had been addressed to one of the
 router's IP addresses; that is to say:
 o A router MUST receive and process normally any packets with a
 broadcast destination address.
 o A router MUST receive and process normally any packets sent to a
 multicast destination address that the router has asked to
 receive.
 The term specific-destination address means the equivalent local IP
 address of the host. The specific-destination address is defined to
 be the destination address in the IP header unless the header
 contains a broadcast or multicast address, in which case the
 specific-destination is an IP address assigned to the physical
 interface on which the datagram arrived.
 A router MUST silently discard any received datagram containing an IP
 source address that is invalid by the rules of this section. This
 validation could be done either by the IP layer or (when appropriate)
 by each protocol in the transport layer. As with any datagram a
 router discards, the datagram discard SHOULD be counted.
 DISCUSSION
 A misaddressed datagram might be caused by a Link Layer broadcast
 of a unicast datagram or by another router or host that is
 confused or misconfigured.
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4.2.3 SPECIFIC ISSUES
4.2.3.1 IP Broadcast Addresses
 For historical reasons, there are a number of IP addresses (some
 standard and some not) which are used to indicate that an IP packet
 is an IP broadcast. A router
 (1) MUST treat as IP broadcasts packets addressed to 255.255.255.255
 or { <Network-prefix>, -1 }.
 (2) SHOULD silently discard on receipt (i.e., do not even deliver to
 applications in the router) any packet addressed to 0.0.0.0 or {
 <Network-prefix>, 0 }. If these packets are not silently
 discarded, they MUST be treated as IP broadcasts (see Section
 [5.3.5]). There MAY be a configuration option to allow receipt
 of these packets. This option SHOULD default to discarding
 them.
 (3) SHOULD (by default) use the limited broadcast address
 (255.255.255.255) when originating an IP broadcast destined for
 a connected (sub)network (except when sending an ICMP Address
 Mask Reply, as discussed in Section [4.3.3.9]). A router MUST
 receive limited broadcasts.
 (4) SHOULD NOT originate datagrams addressed to 0.0.0.0 or {
 <Network-prefix>, 0 }. There MAY be a configuration option to
 allow generation of these packets (instead of using the relevant
 1s format broadcast). This option SHOULD default to not
 generating them.
 DISCUSSION
 In the second bullet, the router obviously cannot recognize
 addresses of the form { <Network-prefix>, 0 } if the router has no
 interface to that network prefix. In that case, the rules of the
 second bullet do not apply because, from the point of view of the
 router, the packet is not an IP broadcast packet.
4.2.3.2 IP Multicasting
 An IP router SHOULD satisfy the Host Requirements with respect to IP
 multicasting, as specified in [INTRO:2]. An IP router SHOULD support
 local IP multicasting on all connected networks. When a mapping from
 IP multicast addresses to link-layer addresses has been specified
 (see the various IP-over-xxx specifications), it SHOULD use that
 mapping, and MAY be configurable to use the link layer broadcast
 instead. On point-to-point links and all other interfaces,
 multicasts are encapsulated as link layer broadcasts. Support for
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 local IP multicasting includes originating multicast datagrams,
 joining multicast groups and receiving multicast datagrams, and
 leaving multicast groups. This implies support for all of
 [INTERNET:4] including IGMP (see Section [4.4]).
 DISCUSSION
 Although [INTERNET:4] is entitled Host Extensions for IP
 Multicasting, it applies to all IP systems, both hosts and
 routers. In particular, since routers may join multicast groups,
 it is correct for them to perform the host part of IGMP, reporting
 their group memberships to any multicast routers that may be
 present on their attached networks (whether or not they themselves
 are multicast routers).
 Some router protocols may specifically require support for IP
 multicasting (e.g., OSPF [ROUTE:1]), or may recommend it (e.g.,
 ICMP Router Discovery [INTERNET:13]).
4.2.3.3 Path MTU Discovery
 To eliminate fragmentation or minimize it, it is desirable to know
 what is the path MTU along the path from the source to destination.
 The path MTU is the minimum of the MTUs of each hop in the path.
 [INTERNET:14] describes a technique for dynamically discovering the
 maximum transmission unit (MTU) of an arbitrary internet path. For a
 path that passes through a router that does not support
 [INTERNET:14], this technique might not discover the correct Path
 MTU, but it will always choose a Path MTU as accurate as, and in many
 cases more accurate than, the Path MTU that would be chosen by older
 techniques or the current practice.
 When a router is originating an IP datagram, it SHOULD use the scheme
 described in [INTERNET:14] to limit the datagram's size. If the
 router's route to the datagram's destination was learned from a
 routing protocol that provides Path MTU information, the scheme
 described in [INTERNET:14] is still used, but the Path MTU
 information from the routing protocol SHOULD be used as the initial
 guess as to the Path MTU and also as an upper bound on the Path MTU.
4.2.3.4 Subnetting
 Under certain circumstances, it may be desirable to support subnets
 of a particular network being interconnected only through a path that
 is not part of the subnetted network. This is known as discontiguous
 subnetwork support.
 Routers MUST support discontiguous subnetworks.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 IMPLEMENTATION
 In classical IP networks, this was very difficult to achieve; in
 CIDR networks, it is a natural by-product. Therefore, a router
 SHOULD NOT make assumptions about subnet architecture, but SHOULD
 treat each route as a generalized network prefix.
 DISCUSSION The Internet has been growing at a tremendous rate of
 late. This has been placing severe strains on the IP addressing
 technology. A major factor in this strain is the strict IP
 Address class boundaries. These make it difficult to efficiently
 size network prefixes to their networks and aggregate several
 network prefixes into a single route advertisement. By
 eliminating the strict class boundaries of the IP address and
 treating each route as a generalized network prefix, these strains
 may be greatly reduced.
 The technology for currently doing this is Classless Inter Domain
 Routing (CIDR) [INTERNET:15].
 For similar reasons, an address block associated with a given network
 prefix could be subdivided into subblocks of different sizes, so that
 the network prefixes associated with the subblocks would have
 different length. For example, within a block whose network prefix
 is 8 bits long, one subblock may have a 16 bit network prefix,
 another may have an 18 bit network prefix, and a third a 14 bit
 network prefix.
 Routers MUST support variable length network prefixes in both their
 interface configurations and their routing databases.
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP
4.3.1 INTRODUCTION
 ICMP is an auxiliary protocol, which provides routing, diagnostic and
 error functionality for IP. It is described in [INTERNET:8]. A
 router MUST support ICMP.
 ICMP messages are grouped in two classes that are discussed in the
 following sections:
 ICMP error messages:
 Destination Unreachable Section 4.3.3.1
 Redirect Section 4.3.3.2
 Source Quench Section 4.3.3.3
 Time Exceeded Section 4.3.3.4
 Parameter Problem Section 4.3.3.5
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 ICMP query messages:
 Echo Section 4.3.3.6
 Information Section 4.3.3.7
 Timestamp Section 4.3.3.8
 Address Mask Section 4.3.3.9
 Router Discovery Section 4.3.3.10
 General ICMP requirements and discussion are in the next section.
4.3.2 GENERAL ISSUES
4.3.2.1 Unknown Message Types
 If an ICMP message of unknown type is received, it MUST be passed to
 the ICMP user interface (if the router has one) or silently discarded
 (if the router does not have one).
4.3.2.2 ICMP Message TTL
 When originating an ICMP message, the router MUST initialize the TTL.
 The TTL for ICMP responses must not be taken from the packet that
 triggered the response.
4.3.2.3 Original Message Header
 Historically, every ICMP error message has included the Internet
 header and at least the first 8 data bytes of the datagram that
 triggered the error. This is no longer adequate, due to the use of
 IP-in-IP tunneling and other technologies. Therefore, the ICMP
 datagram SHOULD contain as much of the original datagram as possible
 without the length of the ICMP datagram exceeding 576 bytes. The
 returned IP header (and user data) MUST be identical to that which
 was received, except that the router is not required to undo any
 modifications to the IP header that are normally performed in
 forwarding that were performed before the error was detected (e.g.,
 decrementing the TTL, or updating options). Note that the
 requirements of Section [4.3.3.5] supersede this requirement in some
 cases (i.e., for a Parameter Problem message, if the problem is in a
 modified field, the router must undo the modification). See Section
 [4.3.3.5]).
4.3.2.4 ICMP Message Source Address
 Except where this document specifies otherwise, the IP source address
 in an ICMP message originated by the router MUST be one of the IP
 addresses associated with the physical interface over which the ICMP
 message is transmitted. If the interface has no IP addresses
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 associated with it, the router's router-id (see Section [5.2.5]) is
 used instead.
4.3.2.5 TOS and Precedence
 ICMP error messages SHOULD have their TOS bits set to the same value
 as the TOS bits in the packet that provoked the sending of the ICMP
 error message, unless setting them to that value would cause the ICMP
 error message to be immediately discarded because it could not be
 routed to its destination. Otherwise, ICMP error messages MUST be
 sent with a normal (i.e., zero) TOS. An ICMP reply message SHOULD
 have its TOS bits set to the same value as the TOS bits in the ICMP
 request that provoked the reply.
 ICMP Source Quench error messages, if sent at all, MUST have their IP
 Precedence field set to the same value as the IP Precedence field in
 the packet that provoked the sending of the ICMP Source Quench
 message. All other ICMP error messages (Destination Unreachable,
 Redirect, Time Exceeded, and Parameter Problem) SHOULD have their
 precedence value set to 6 (INTERNETWORK CONTROL) or 7 (NETWORK
 CONTROL). The IP Precedence value for these error messages MAY be
 settable.
 An ICMP reply message MUST have its IP Precedence field set to the
 same value as the IP Precedence field in the ICMP request that
 provoked the reply.
4.3.2.6 Source Route
 If the packet which provokes the sending of an ICMP error message
 contains a source route option, the ICMP error message SHOULD also
 contain a source route option of the same type (strict or loose),
 created by reversing the portion before the pointer of the route
 recorded in the source route option of the original packet UNLESS the
 ICMP error message is an ICMP Parameter Problem complaining about a
 source route option in the original packet, or unless the router is
 aware of policy that would prevent the delivery of the ICMP error
 message.
 DISCUSSION
 In environments which use the U.S. Department of Defense security
 option (defined in [INTERNET:5]), ICMP messages may need to
 include a security option. Detailed information on this topic
 should be available from the Defense Communications Agency.
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4.3.2.7 When Not to Send ICMP Errors
 An ICMP error message MUST NOT be sent as the result of receiving:
 o An ICMP error message, or
 o A packet which fails the IP header validation tests described in
 Section [5.2.2] (except where that section specifically permits
 the sending of an ICMP error message), or
 o A packet destined to an IP broadcast or IP multicast address, or
 o A packet sent as a Link Layer broadcast or multicast, or
 o A packet whose source address has a network prefix of zero or is an
 invalid source address (as defined in Section [5.3.7]), or
 o Any fragment of a datagram other then the first fragment (i.e., a
 packet for which the fragment offset in the IP header is nonzero).
 Furthermore, an ICMP error message MUST NOT be sent in any case where
 this memo states that a packet is to be silently discarded.
 NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT
 ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.
 DISCUSSION
 These rules aim to prevent the broadcast storms that have resulted
 from routers or hosts returning ICMP error messages in response to
 broadcast packets. For example, a broadcast UDP packet to a non-
 existent port could trigger a flood of ICMP Destination
 Unreachable datagrams from all devices that do not have a client
 for that destination port. On a large Ethernet, the resulting
 collisions can render the network useless for a second or more.
 Every packet that is broadcast on the connected network should
 have a valid IP broadcast address as its IP destination (see
 Section [5.3.4] and [INTRO:2]). However, some devices violate
 this rule. To be certain to detect broadcast packets, therefore,
 routers are required to check for a link-layer broadcast as well
 as an IP-layer address.
 IMPLEMENTATION+ This requires that the link layer inform the IP layer
 when a link-layer broadcast packet has been received; see Section
 [3.1].
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4.3.2.8 Rate Limiting
 A router which sends ICMP Source Quench messages MUST be able to
 limit the rate at which the messages can be generated. A router
 SHOULD also be able to limit the rate at which it sends other sorts
 of ICMP error messages (Destination Unreachable, Redirect, Time
 Exceeded, Parameter Problem). The rate limit parameters SHOULD be
 settable as part of the configuration of the router. How the limits
 are applied (e.g., per router or per interface) is left to the
 implementor's discretion.
 DISCUSSION
 Two problems for a router sending ICMP error message are:
 (1) The consumption of bandwidth on the reverse path, and
 (2) The use of router resources (e.g., memory, CPU time)
 To help solve these problems a router can limit the frequency with
 which it generates ICMP error messages. For similar reasons, a
 router may limit the frequency at which some other sorts of
 messages, such as ICMP Echo Replies, are generated.
 IMPLEMENTATION
 Various mechanisms have been used or proposed for limiting the
 rate at which ICMP messages are sent:
 (1) Count-based - for example, send an ICMP error message for
 every N dropped packets overall or per given source host.
 This mechanism might be appropriate for ICMP Source Quench,
 if used, but probably not for other types of ICMP messages.
 (2) Timer-based - for example, send an ICMP error message to a
 given source host or overall at most once per T milliseconds.
 (3) Bandwidth-based - for example, limit the rate at which ICMP
 messages are sent over a particular interface to some
 fraction of the attached network's bandwidth.
4.3.3 SPECIFIC ISSUES
4.3.3.1 Destination Unreachable
 If a router cannot forward a packet because it has no routes at all
 (including no default route) to the destination specified in the
 packet, then the router MUST generate a Destination Unreachable, Code
 0 (Network Unreachable) ICMP message. If the router does have routes
 to the destination network specified in the packet but the TOS
 specified for the routes is neither the default TOS (0000) nor the
 TOS of the packet that the router is attempting to route, then the
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 router MUST generate a Destination Unreachable, Code 11 (Network
 Unreachable for TOS) ICMP message.
 If a packet is to be forwarded to a host on a network that is
 directly connected to the router (i.e., the router is the last-hop
 router) and the router has ascertained that there is no path to the
 destination host then the router MUST generate a Destination
 Unreachable, Code 1 (Host Unreachable) ICMP message. If a packet is
 to be forwarded to a host that is on a network that is directly
 connected to the router and the router cannot forward the packet
 because no route to the destination has a TOS that is either equal to
 the TOS requested in the packet or is the default TOS (0000) then the
 router MUST generate a Destination Unreachable, Code 12 (Host
 Unreachable for TOS) ICMP message.
 DISCUSSION
 The intent is that a router generates the "generic" host/network
 unreachable if it has no path at all (including default routes) to
 the destination. If the router has one or more paths to the
 destination, but none of those paths have an acceptable TOS, then
 the router generates the "unreachable for TOS" message.
4.3.3.2 Redirect
 The ICMP Redirect message is generated to inform a local host that it
 should use a different next hop router for certain traffic.
 Contrary to [INTRO:2], a router MAY ignore ICMP Redirects when
 choosing a path for a packet originated by the router if the router
 is running a routing protocol or if forwarding is enabled on the
 router and on the interface over which the packet is being sent.
4.3.3.3 Source Quench
 A router SHOULD NOT originate ICMP Source Quench messages. As
 specified in Section [4.3.2], a router that does originate Source
 Quench messages MUST be able to limit the rate at which they are
 generated.
 DISCUSSION
 Research seems to suggest that Source Quench consumes network
 bandwidth but is an ineffective (and unfair) antidote to
 congestion. See, for example, [INTERNET:9] and [INTERNET:10].
 Section [5.3.6] discusses the current thinking on how routers
 ought to deal with overload and network congestion.
 A router MAY ignore any ICMP Source Quench messages it receives.
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 DISCUSSION
 A router itself may receive a Source Quench as the result of
 originating a packet sent to another router or host. Such
 datagrams might be, e.g., an EGP update sent to another router, or
 a telnet stream sent to a host. A mechanism has been proposed
 ([INTERNET:11], [INTERNET:12]) to make the IP layer respond
 directly to Source Quench by controlling the rate at which packets
 are sent, however, this proposal is currently experimental and not
 currently recommended.
4.3.3.4 Time Exceeded
 When a router is forwarding a packet and the TTL field of the packet
 is reduced to 0, the requirements of section [5.2.3.8] apply.
 When the router is reassembling a packet that is destined for the
 router, it is acting as an Internet host. [INTRO:2]'s reassembly
 requirements therefore apply.
 When the router receives (i.e., is destined for the router) a Time
 Exceeded message, it MUST comply with [INTRO:2].
4.3.3.5 Parameter Problem
 A router MUST generate a Parameter Problem message for any error not
 specifically covered by another ICMP message. The IP header field or
 IP option including the byte indicated by the pointer field MUST be
 included unchanged in the IP header returned with this ICMP message.
 Section [4.3.2] defines an exception to this requirement.
 A new variant of the Parameter Problem message was defined in
 [INTRO:2]:
 Code 1 = required option is missing.
 DISCUSSION
 This variant is currently in use in the military community for a
 missing security option.
4.3.3.6 Echo Request/Reply
 A router MUST implement an ICMP Echo server function that receives
 Echo Requests sent to the router, and sends corresponding Echo
 Replies. A router MUST be prepared to receive, reassemble and echo
 an ICMP Echo Request datagram at least as the maximum of 576 and the
 MTUs of all the connected networks.
 The Echo server function MAY choose not to respond to ICMP echo
 requests addressed to IP broadcast or IP multicast addresses.
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 A router SHOULD have a configuration option that, if enabled, causes
 the router to silently ignore all ICMP echo requests; if provided,
 this option MUST default to allowing responses.
 DISCUSSION
 The neutral provision about responding to broadcast and multicast
 Echo Requests derives from [INTRO:2]'s "Echo Request/Reply"
 section.
 As stated in Section [10.3.3], a router MUST also implement a
 user/application-layer interface for sending an Echo Request and
 receiving an Echo Reply, for diagnostic purposes. All ICMP Echo
 Reply messages MUST be passed to this interface.
 The IP source address in an ICMP Echo Reply MUST be the same as the
 specific-destination address of the corresponding ICMP Echo Request
 message.
 Data received in an ICMP Echo Request MUST be entirely included in
 the resulting Echo Reply.
 If a Record Route and/or Timestamp option is received in an ICMP Echo
 Request, this option (these options) SHOULD be updated to include the
 current router and included in the IP header of the Echo Reply
 message, without truncation. Thus, the recorded route will be for
 the entire round trip.
 If a Source Route option is received in an ICMP Echo Request, the
 return route MUST be reversed and used as a Source Route option for
 the Echo Reply message, unless the router is aware of policy that
 would prevent the delivery of the message.
4.3.3.7 Information Request/Reply
 A router SHOULD NOT originate or respond to these messages.
 DISCUSSION
 The Information Request/Reply pair was intended to support self-
 configuring systems such as diskless workstations, to allow them
 to discover their IP network prefixes at boot time. However,
 these messages are now obsolete. The RARP and BOOTP protocols
 provide better mechanisms for a host to discover its own IP
 address.
4.3.3.8 Timestamp and Timestamp Reply
 A router MAY implement Timestamp and Timestamp Reply. If they are
 implemented then:
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 o The ICMP Timestamp server function MUST return a Timestamp Reply to
 every Timestamp message that is received. It SHOULD be designed
 for minimum variability in delay.
 o An ICMP Timestamp Request message to an IP broadcast or IP
 multicast address MAY be silently discarded.
 o The IP source address in an ICMP Timestamp Reply MUST be the same
 as the specific-destination address of the corresponding Timestamp
 Request message.
 o If a Source Route option is received in an ICMP Timestamp Request,
 the return route MUST be reversed and used as a Source Route
 option for the Timestamp Reply message, unless the router is aware
 of policy that would prevent the delivery of the message.
 o If a Record Route and/or Timestamp option is received in a
 Timestamp Request, this (these) option(s) SHOULD be updated to
 include the current router and included in the IP header of the
 Timestamp Reply message.
 o If the router provides an application-layer interface for sending
 Timestamp Request messages then incoming Timestamp Reply messages
 MUST be passed up to the ICMP user interface.
 The preferred form for a timestamp value (the standard value) is
 milliseconds since midnight, Universal Time. However, it may be
 difficult to provide this value with millisecond resolution. For
 example, many systems use clocks that update only at line frequency,
 50 or 60 times per second. Therefore, some latitude is allowed in a
 standard value:
 (a) A standard value MUST be updated at least 16 times per second
 (i.e., at most the six low-order bits of the value may be
 undefined).
 (b) The accuracy of a standard value MUST approximate that of
 operator-set CPU clocks, i.e., correct within a few minutes.
 IMPLEMENTATION
 To meet the second condition, a router may need to query some time
 server when the router is booted or restarted. It is recommended
 that the UDP Time Server Protocol be used for this purpose. A
 more advanced implementation would use the Network Time Protocol
 (NTP) to achieve nearly millisecond clock synchronization;
 however, this is not required.
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4.3.3.9 Address Mask Request/Reply
 A router MUST implement support for receiving ICMP Address Mask
 Request messages and responding with ICMP Address Mask Reply
 messages. These messages are defined in [INTERNET:2].
 A router SHOULD have a configuration option for each logical
 interface specifying whether the router is allowed to answer Address
 Mask Requests for that interface; this option MUST default to
 allowing responses. A router MUST NOT respond to an Address Mask
 Request before the router knows the correct address mask.
 A router MUST NOT respond to an Address Mask Request that has a
 source address of 0.0.0.0 and which arrives on a physical interface
 that has associated with it multiple logical interfaces and the
 address masks for those interfaces are not all the same.
 A router SHOULD examine all ICMP Address Mask Replies that it
 receives to determine whether the information it contains matches the
 router's knowledge of the address mask. If the ICMP Address Mask
 Reply appears to be in error, the router SHOULD log the address mask
 and the sender's IP address. A router MUST NOT use the contents of
 an ICMP Address Mask Reply to determine the correct address mask.
 Because hosts may not be able to learn the address mask if a router
 is down when the host boots up, a router MAY broadcast a gratuitous
 ICMP Address Mask Reply on each of its logical interfaces after it
 has configured its own address masks. However, this feature can be
 dangerous in environments that use variable length address masks.
 Therefore, if this feature is implemented, gratuitous Address Mask
 Replies MUST NOT be broadcast over any logical interface(s) which
 either:
 o Are not configured to send gratuitous Address Mask Replies. Each
 logical interface MUST have a configuration parameter controlling
 this, and that parameter MUST default to not sending the
 gratuitous Address Mask Replies.
 o Share subsuming (but not identical) network prefixes and physical
 interface.
 The { <Network-prefix>, -1 } form of the IP broadcast address MUST be
 used for broadcast Address Mask Replies.
 DISCUSSION
 The ability to disable sending Address Mask Replies by routers is
 required at a few sites that intentionally lie to their hosts
 about the address mask. The need for this is expected to go away
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 as more and more hosts become compliant with the Host Requirements
 standards.
 The reason for both the second bullet above and the requirement
 about which IP broadcast address to use is to prevent problems
 when multiple IP network prefixes are in use on the same physical
 network.
4.3.3.10 Router Advertisement and Solicitations
 An IP router MUST support the router part of the ICMP Router
 Discovery Protocol [INTERNET:13] on all connected networks on which
 the router supports either IP multicast or IP broadcast addressing.
 The implementation MUST include all the configuration variables
 specified for routers, with the specified defaults.
 DISCUSSION
 Routers are not required to implement the host part of the ICMP
 Router Discovery Protocol, but might find it useful for operation
 while IP forwarding is disabled (i.e., when operating as a host).
 DISCUSSION We note that it is quite common for hosts to use RIP
 Version 1 as the router discovery protocol. Such hosts listen to
 RIP traffic and use and use information extracted from that
 traffic to discover routers and to make decisions as to which
 router to use as a first-hop router for a given destination.
 While this behavior is discouraged, it is still common and
 implementors should be aware of it.
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP
 IGMP [INTERNET:4] is a protocol used between hosts and multicast
 routers on a single physical network to establish hosts' membership
 in particular multicast groups. Multicast routers use this
 information, in conjunction with a multicast routing protocol, to
 support IP multicast forwarding across the Internet.
 A router SHOULD implement the host part of IGMP.
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5. INTERNET LAYER - FORWARDING
5.1 INTRODUCTION
 This section describes the process of forwarding packets.
5.2 FORWARDING WALK-THROUGH
 There is no separate specification of the forwarding function in IP.
 Instead, forwarding is covered by the protocol specifications for the
 internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3],
 [INTERNET:8], and [ROUTE:11]).
5.2.1 Forwarding Algorithm
 Since none of the primary protocol documents describe the forwarding
 algorithm in any detail, we present it here. This is just a general
 outline, and omits important details, such as handling of congestion,
 that are dealt with in later sections.
 It is not required that an implementation follow exactly the
 algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3].
 Much of the challenge of writing router software is to maximize the
 rate at which the router can forward packets while still achieving
 the same effect of the algorithm. Details of how to do that are
 beyond the scope of this document, in part because they are heavily
 dependent on the architecture of the router. Instead, we merely
 point out the order dependencies among the steps:
 (1) A router MUST verify the IP header, as described in section
 [5.2.2], before performing any actions based on the contents of
 the header. This allows the router to detect and discard bad
 packets before the expenditure of other resources.
 (2) Processing of certain IP options requires that the router insert
 its IP address into the option. As noted in Section [5.2.4],
 the address inserted MUST be the address of the logical
 interface on which the packet is sent or the router's router-id
 if the packet is sent over an unnumbered interface. Thus,
 processing of these options cannot be completed until after the
 output interface is chosen.
 (3) The router cannot check and decrement the TTL before checking
 whether the packet should be delivered to the router itself, for
 reasons mentioned in Section [4.2.2.9].
 (4) More generally, when a packet is delivered locally to the router,
 its IP header MUST NOT be modified in any way (except that a
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 router may be required to insert a timestamp into any Timestamp
 options in the IP header). Thus, before the router determines
 whether the packet is to be delivered locally to the router, it
 cannot update the IP header in any way that it is not prepared
 to undo.
5.2.1.1 General
 This section covers the general forwarding algorithm. This algorithm
 applies to all forms of packets to be forwarded: unicast, multicast,
 and broadcast.
 (1) The router receives the IP packet (plus additional information
 about it, as described in Section [3.1]) from the Link Layer.
 (2) The router validates the IP header, as described in Section
 [5.2.2]. Note that IP reassembly is not done, except on IP
 fragments to be queued for local delivery in step (4).
 (3) The router performs most of the processing of any IP options. As
 described in Section [5.2.4], some IP options require additional
 processing after the routing decision has been made.
 (4) The router examines the destination IP address of the IP
 datagram, as described in Section [5.2.3], to determine how it
 should continue to process the IP datagram. There are three
 possibilities:
 o The IP datagram is destined for the router, and should be
 queued for local delivery, doing reassembly if needed.
 o The IP datagram is not destined for the router, and should be
 queued for forwarding.
 o The IP datagram should be queued for forwarding, but (a copy)
 must also be queued for local delivery.
5.2.1.2 Unicast
 Since the local delivery case is well covered by [INTRO:2], the
 following assumes that the IP datagram was queued for forwarding. If
 the destination is an IP unicast address:
 (5) The forwarder determines the next hop IP address for the packet,
 usually by looking up the packet's destination in the router's
 routing table. This procedure is described in more detail in
 Section [5.2.4]. This procedure also decides which network
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 interface should be used to send the packet.
 (6) The forwarder verifies that forwarding the packet is permitted.
 The source and destination addresses should be valid, as
 described in Section [5.3.7] and Section [5.3.4] If the router
 supports administrative constraints on forwarding, such as those
 described in Section [5.3.9], those constraints must be
 satisfied.
 (7) The forwarder decrements (by at least one) and checks the
 packet's TTL, as described in Section [5.3.1].
 (8) The forwarder performs any IP option processing that could not be
 completed in step 3.
 (9) The forwarder performs any necessary IP fragmentation, as
 described in Section [4.2.2.7]. Since this step occurs after
 outbound interface selection (step 5), all fragments of the same
 datagram will be transmitted out the same interface.
 (10) The forwarder determines the Link Layer address of the packet's
 next hop. The mechanisms for doing this are Link Layer-
 dependent (see chapter 3).
 (11) The forwarder encapsulates the IP datagram (or each of the
 fragments thereof) in an appropriate Link Layer frame and queues
 it for output on the interface selected in step 5.
 (12) The forwarder sends an ICMP redirect if necessary, as described
 in Section [4.3.3.2].
5.2.1.3 Multicast
 If the destination is an IP multicast, the following steps are taken.
 Note that the main differences between the forwarding of IP unicasts
 and the forwarding of IP multicasts are
 o IP multicasts are usually forwarded based on both the datagram's
 source and destination IP addresses,
 o IP multicast uses an expanding ring search,
 o IP multicasts are forwarded as Link Level multicasts, and
 o ICMP errors are never sent in response to IP multicast datagrams.
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 Note that the forwarding of IP multicasts is still somewhat
 experimental. As a result, the algorithm presented below is not
 mandatory, and is provided as an example only.
 (5a) Based on the IP source and destination addresses found in the
 datagram header, the router determines whether the datagram has
 been received on the proper interface for forwarding. If not,
 the datagram is dropped silently. The method for determining
 the proper receiving interface depends on the multicast routing
 algorithm(s) in use. In one of the simplest algorithms, reverse
 path forwarding (RPF), the proper interface is the one that
 would be used to forward unicasts back to the datagram source.
 (6a) Based on the IP source and destination addresses found in the
 datagram header, the router determines the datagram's outgoing
 interfaces. To implement IP multicast's expanding ring search
 (see [INTERNET:4]) a minimum TTL value is specified for each
 outgoing interface. A copy of the multicast datagram is
 forwarded out each outgoing interface whose minimum TTL value is
 less than or equal to the TTL value in the datagram header, by
 separately applying the remaining steps on each such interface.
 (7a) The router decrements the packet's TTL by one.
 (8a) The forwarder performs any IP option processing that could not
 be completed in step (3).
 (9a) The forwarder performs any necessary IP fragmentation, as
 described in Section [4.2.2.7].
 (10a) The forwarder determines the Link Layer address to use in the
 Link Level encapsulation. The mechanisms for doing this are
 Link Layer-dependent. On LANs a Link Level multicast or
 broadcast is selected, as an algorithmic translation of the
 datagrams' IP multicast address. See the various IP-over-xxx
 specifications for more details.
 (11a) The forwarder encapsulates the packet (or each of the fragments
 thereof) in an appropriate Link Layer frame and queues it for
 output on the appropriate interface.
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5.2.2 IP Header Validation
 Before a router can process any IP packet, it MUST perform a the
 following basic validity checks on the packet's IP header to ensure
 that the header is meaningful. If the packet fails any of the
 following tests, it MUST be silently discarded, and the error SHOULD
 be logged.
 (1) The packet length reported by the Link Layer must be large enough
 to hold the minimum length legal IP datagram (20 bytes).
 (2) The IP checksum must be correct.
 (3) The IP version number must be 4. If the version number is not 4
 then the packet may be another version of IP, such as IPng or
 ST-II.
 (4) The IP header length field must be large enough to hold the
 minimum length legal IP datagram (20 bytes = 5 words).
 (5) The IP total length field must be large enough to hold the IP
 datagram header, whose length is specified in the IP header
 length field.
 A router MUST NOT have a configuration option that allows disabling
 any of these tests.
 If the packet passes the second and third tests, the IP header length
 field is at least 4, and both the IP total length field and the
 packet length reported by the Link Layer are at least 16 then,
 despite the above rule, the router MAY respond with an ICMP Parameter
 Problem message, whose pointer points at the IP header length field
 (if it failed the fourth test) or the IP total length field (if it
 failed the fifth test). However, it still MUST discard the packet
 and still SHOULD log the error.
 These rules (and this entire document) apply only to version 4 of the
 Internet Protocol. These rules should not be construed as
 prohibiting routers from supporting other versions of IP.
 Furthermore, if a router can truly classify a packet as being some
 other version of IP then it ought not treat that packet as an error
 packet within the context of this memo.
 IMPLEMENTATION
 It is desirable for purposes of error reporting, though not always
 entirely possible, to determine why a header was invalid. There
 are four possible reasons:
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 o The Link Layer truncated the IP header
 o The datagram is using a version of IP other than the standard
 one (version 4).
 o The IP header has been corrupted in transit.
 o The sender generated an illegal IP header.
 It is probably desirable to perform the checks in the order
 listed, since we believe that this ordering is most likely to
 correctly categorize the cause of the error. For purposes of
 error reporting, it may also be desirable to check if a packet
 that fails these tests has an IP version number indicating IPng or
 ST-II; these should be handled according to their respective
 specifications.
 Additionally, the router SHOULD verify that the packet length
 reported by the Link Layer is at least as large as the IP total
 length recorded in the packet's IP header. If it appears that the
 packet has been truncated, the packet MUST be discarded, the error
 SHOULD be logged, and the router SHOULD respond with an ICMP
 Parameter Problem message whose pointer points at the IP total length
 field.
 DISCUSSION
 Because any higher layer protocol that concerns itself with data
 corruption will detect truncation of the packet data when it
 reaches its final destination, it is not absolutely necessary for
 routers to perform the check suggested above to maintain protocol
 correctness. However, by making this check a router can simplify
 considerably the task of determining which hop in the path is
 truncating the packets. It will also reduce the expenditure of
 resources down-stream from the router in that down-stream systems
 will not need to deal with the packet.
 Finally, if the destination address in the IP header is not one of
 the addresses of the router, the router SHOULD verify that the packet
 does not contain a Strict Source and Record Route option. If a
 packet fails this test (if it contains a strict source route option),
 the router SHOULD log the error and SHOULD respond with an ICMP
 Parameter Problem error with the pointer pointing at the offending
 packet's IP destination address.
 DISCUSSION
 Some people might suggest that the router should respond with a
 Bad Source Route message instead of a Parameter Problem message.
 However, when a packet fails this test, it usually indicates a
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 protocol error by the previous hop router, whereas Bad Source
 Route would suggest that the source host had requested a
 nonexistent or broken path through the network.
5.2.3 Local Delivery Decision
 When a router receives an IP packet, it must decide whether the
 packet is addressed to the router (and should be delivered locally)
 or the packet is addressed to another system (and should be handled
 by the forwarder). There is also a hybrid case, where certain IP
 broadcasts and IP multicasts are both delivered locally and
 forwarded. A router MUST determine which of the these three cases
 applies using the following rules.
 o An unexpired source route option is one whose pointer value does
 not point past the last entry in the source route. If the packet
 contains an unexpired source route option, the pointer in the
 option is advanced until either the pointer does point past the
 last address in the option or else the next address is not one of
 the router's own addresses. In the latter (normal) case, the
 packet is forwarded (and not delivered locally) regardless of the
 rules below.
 o The packet is delivered locally and not considered for forwarding
 in the following cases:
 - The packet's destination address exactly matches one of the
 router's IP addresses,
 - The packet's destination address is a limited broadcast address
 ({-1, -1}), or
 - The packet's destination is an IP multicast address which is
 never forwarded (such as 224.0.0.1 or 224.0.0.2) and (at least)
 one of the logical interfaces associated with the physical
 interface on which the packet arrived is a member of the
 destination multicast group.
 o The packet is passed to the forwarder AND delivered locally in the
 following cases:
 - The packet's destination address is an IP broadcast address that
 addresses at least one of the router's logical interfaces but
 does not address any of the logical interfaces associated with
 the physical interface on which the packet arrived
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 - The packet's destination is an IP multicast address which is
 permitted to be forwarded (unlike 224.0.0.1 and 224.0.0.2) and
 (at least) one of the logical interfaces associated with the
 physical interface on which the packet arrived is a member of
 the destination multicast group.
 o The packet is delivered locally if the packet's destination address
 is an IP broadcast address (other than a limited broadcast
 address) that addresses at least one of the logical interfaces
 associated with the physical interface on which the packet
 arrived. The packet is ALSO passed to the forwarder unless the
 link on which the packet arrived uses an IP encapsulation that
 does not encapsulate broadcasts differently than unicasts (e.g.,
 by using different Link Layer destination addresses).
 o The packet is passed to the forwarder in all other cases.
 DISCUSSION
 The purpose of the requirement in the last sentence of the fourth
 bullet is to deal with a directed broadcast to another network
 prefix on the same physical cable. Normally, this works as
 expected: the sender sends the broadcast to the router as a Link
 Layer unicast. The router notes that it arrived as a unicast, and
 therefore must be destined for a different network prefix than the
 sender sent it on. Therefore, the router can safely send it as a
 Link Layer broadcast out the same (physical) interface over which
 it arrived. However, if the router can't tell whether the packet
 was received as a Link Layer unicast, the sentence ensures that
 the router does the safe but wrong thing rather than the unsafe
 but right thing.
 IMPLEMENTATION
 As described in Section [5.3.4], packets received as Link Layer
 broadcasts are generally not forwarded. It may be advantageous to
 avoid passing to the forwarder packets it would later discard
 because of the rules in that section.
 Some Link Layers (either because of the hardware or because of
 special code in the drivers) can deliver to the router copies of
 all Link Layer broadcasts and multicasts it transmits. Use of
 this feature can simplify the implementation of cases where a
 packet has to both be passed to the forwarder and delivered
 locally, since forwarding the packet will automatically cause the
 router to receive a copy of the packet that it can then deliver
 locally. One must use care in these circumstances to prevent
 treating a received loop-back packet as a normal packet that was
 received (and then being subject to the rules of forwarding,
 etc.).
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 Even without such a Link Layer, it is of course hardly necessary
 to make a copy of an entire packet to queue it both for forwarding
 and for local delivery, though care must be taken with fragments,
 since reassembly is performed on locally delivered packets but not
 on forwarded packets. One simple scheme is to associate a flag
 with each packet on the router's output queue that indicates
 whether it should be queued for local delivery after it has been
 sent.
5.2.4 Determining the Next Hop Address
 When a router is going to forward a packet, it must determine whether
 it can send it directly to its destination, or whether it needs to
 pass it through another router. If the latter, it needs to determine
 which router to use. This section explains how these determinations
 are made.
 This section makes use of the following definitions:
 o LSRR - IP Loose Source and Record Route option
 o SSRR - IP Strict Source and Record Route option
 o Source Route Option - an LSRR or an SSRR
 o Ultimate Destination Address - where the packet is being sent to:
 the last address in the source route of a source-routed packet, or
 the destination address in the IP header of a non-source-routed
 packet
 o Adjacent - reachable without going through any IP routers
 o Next Hop Address - the IP address of the adjacent host or router to
 which the packet should be sent next
 o IP Destination Address - the ultimate destination address, except
 in source routed packets, where it is the next address specified
 in the source route
 o Immediate Destination - the node, System, router, end-system, or
 whatever that is addressed by the IP Destination Address.
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5.2.4.1 IP Destination Address
 If:
 o the destination address in the IP header is one of the addresses of
 the router,
 o the packet contains a Source Route Option, and
 o the pointer in the Source Route Option does not point past the end
 of the option,
 then the next IP Destination Address is the address pointed at by the
 pointer in that option. If:
 o the destination address in the IP header is one of the addresses of
 the router,
 o the packet contains a Source Route Option, and
 o the pointer in the Source Route Option points past the end of the
 option,
 then the message is addressed to the system analyzing the message.
 A router MUST use the IP Destination Address, not the Ultimate
 Destination Address (the last address in the source route option),
 when determining how to handle a packet.
 It is an error for more than one source route option to appear in a
 datagram. If it receives such a datagram, it SHOULD discard the
 packet and reply with an ICMP Parameter Problem message whose pointer
 points at the beginning of the second source route option.
5.2.4.2 Local/Remote Decision
 After it has been determined that the IP packet needs to be forwarded
 according to the rules specified in Section [5.2.3], the following
 algorithm MUST be used to determine if the Immediate Destination is
 directly accessible (see [INTERNET:2]).
 (1) For each network interface that has not been assigned any IP
 address (the unnumbered lines as described in Section [2.2.7]),
 compare the router-id of the other end of the line to the IP
 Destination Address. If they are exactly equal, the packet can
 be transmitted through this interface.
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 DISCUSSION
 In other words, the router or host at the remote end of the line
 is the destination of the packet or is the next step in the source
 route of a source routed packet.
 (2) If no network interface has been selected in the first step, for
 each IP address assigned to the router:
 (a) isolate the network prefix used by the interface.
 IMPLEMENTATION
 The result of this operation will usually have been computed and
 saved during initialization.
 (b) Isolate the corresponding set of bits from the IP Destination
 Address of the packet.
 (c) Compare the resulting network prefixes. If they are equal to
 each other, the packet can be transmitted through the
 corresponding network interface.
 (3) If the destination was neither the router-id of a neighbor on an
 unnumbered interface nor a member of a directly connected network
 prefix, the IP Destination is accessible only through some other
 router. The selection of the router and the next hop IP address
 is described in Section [5.2.4.3]. In the case of a host that is
 not also a router, this may be the configured default router.
 Ongoing work in the IETF [ARCH:9, NRHP] considers some cases such as
 when multiple IP (sub)networks are overlaid on the same link layer
 network. Barring policy restrictions, hosts and routers using a
 common link layer network can directly communicate even if they are
 not in the same IP (sub)network, if there is adequate information
 present. The Next Hop Routing Protocol (NHRP) enables IP entities to
 determine the "optimal" link layer address to be used to traverse
 such a link layer network towards a remote destination.
 (4) If the selected "next hop" is reachable through an interface
 configured to use NHRP, then the following additional steps apply:
 (a) Compare the IP Destination Address to the destination addresses
 in the NHRP cache. If the address is in the cache, then send
 the datagram to the corresponding cached link layer address.
 (b) If the address is not in the cache, then construct an NHRP
 request packet containing the IP Destination Address. This
 message is sent to the NHRP server configured for that
 interface. This may be a logically separate process or entity
 in the router itself.
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 (c) The NHRP server will respond with the proper link layer address
 to use to transmit the datagram and subsequent datagrams to the
 same destination. The system MAY transmit the datagram(s) to
 the traditional "next hop" router while awaiting the NHRP reply.
5.2.4.3 Next Hop Address
 EDITORS+COMMENTS
 The router applies the algorithm in the previous section to
 determine if the IP Destination Address is adjacent. If so, the
 next hop address is the same as the IP Destination Address.
 Otherwise, the packet must be forwarded through another router to
 reach its Immediate Destination. The selection of this router is
 the topic of this section.
 If the packet contains an SSRR, the router MUST discard the packet
 and reply with an ICMP Bad Source Route error. Otherwise, the
 router looks up the IP Destination Address in its routing table to
 determine an appropriate next hop address.
 DISCUSSION
 Per the IP specification, a Strict Source Route must specify a
 sequence of nodes through which the packet must traverse; the
 packet must go from one node of the source route to the next,
 traversing intermediate networks only. Thus, if the router is not
 adjacent to the next step of the source route, the source route
 can not be fulfilled. Therefore, the router rejects such with an
 ICMP Bad Source Route error.
 The goal of the next-hop selection process is to examine the entries
 in the router's Forwarding Information Base (FIB) and select the best
 route (if there is one) for the packet from those available in the
 FIB.
 Conceptually, any route lookup algorithm starts out with a set of
 candidate routes that consists of the entire contents of the FIB.
 The algorithm consists of a series of steps that discard routes from
 the set. These steps are referred to as Pruning Rules. Normally,
 when the algorithm terminates there is exactly one route remaining in
 the set. If the set ever becomes empty, the packet is discarded
 because the destination is unreachable. It is also possible for the
 algorithm to terminate when more than one route remains in the set.
 In this case, the router may arbitrarily discard all but one of them,
 or may perform "load-splitting" by choosing whichever of the routes
 has been least recently used.
 With the exception of rule 3 (Weak TOS), a router MUST use the
 following Pruning Rules when selecting a next hop for a packet. If a
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 router does consider TOS when making next-hop decisions, the Rule 3
 must be applied in the order indicated below. These rules MUST be
 (conceptually) applied to the FIB in the order that they are
 presented. (For some historical perspective, additional pruning
 rules, and other common algorithms in use, see Appendix E.)
 DISCUSSION
 Rule 3 is optional in that Section [5.3.2] says that a router only
 SHOULD consider TOS when making forwarding decisions.
 (1) Basic Match
 This rule discards any routes to destinations other than the
 IP Destination Address of the packet. For example, if a
 packet's IP Destination Address is 10.144.2.5, this step
 would discard a route to net 128.12.0.0/16 but would retain
 any routes to the network prefixes 10.0.0.0/8 and
 10.144.0.0/16, and any default routes.
 More precisely, we assume that each route has a destination
 attribute, called route.dest and a corresponding prefix
 length, called route.length, to specify which bits of
 route.dest are significant. The IP Destination Address of
 the packet being forwarded is ip.dest. This rule discards
 all routes from the set of candidates except those for which
 the most significant route.length bits of route.dest and
 ip.dest are equal.
 For example, if a packet's IP Destination Address is
 10.144.2.5 and there are network prefixes 10.144.1.0/24,
 10.144.2.0/24, and 10.144.3.0/24, this rule would keep only
 10.144.2.0/24; it is the only route whose prefix has the same
 value as the corresponding bits in the IP Destination Address
 of the packet.
 (2) Longest Match
 Longest Match is a refinement of Basic Match, described
 above. After performing Basic Match pruning, the algorithm
 examines the remaining routes to determine which among them
 have the largest route.length values. All except these are
 discarded.
 For example, if a packet's IP Destination Address is
 10.144.2.5 and there are network prefixes 10.144.2.0/24,
 10.144.0.0/16, and 10.0.0.0/8, then this rule would keep only
 the first (10.144.2.0/24) because its prefix length is
 longest.
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 (3) Weak TOS
 Each route has a type of service attribute, called route.tos,
 whose possible values are assumed to be identical to those
 used in the TOS field of the IP header. Routing protocols
 that distribute TOS information fill in route.tos
 appropriately in routes they add to the FIB; routes from
 other routing protocols are treated as if they have the
 default TOS (0000). The TOS field in the IP header of the
 packet being routed is called ip.tos.
 The set of candidate routes is examined to determine if it
 contains any routes for which route.tos = ip.tos. If so, all
 routes except those for which route.tos = ip.tos are
 discarded. If not, all routes except those for which
 route.tos = 0000 are discarded from the set of candidate
 routes.
 Additional discussion of routing based on Weak TOS may be
 found in [ROUTE:11].
 DISCUSSION
 The effect of this rule is to select only those routes that have a
 TOS that matches the TOS requested in the packet. If no such
 routes exist then routes with the default TOS are considered.
 Routes with a non-default TOS that is not the TOS requested in the
 packet are never used, even if such routes are the only available
 routes that go to the packet's destination.
 (4) Best Metric
 Each route has a metric attribute, called route.metric, and a
 routing domain identifier, called route.domain. Each member
 of the set of candidate routes is compared with each other
 member of the set. If route.domain is equal for the two
 routes and route.metric is strictly inferior for one when
 compared with the other, then the one with the inferior metric
 is discarded from the set. The determination of inferior is
 usually by a simple arithmetic comparison, though some
 protocols may have structured metrics requiring more complex
 comparisons.
 (5) Vendor Policy
 Vendor Policy is sort of a catch-all to make up for the fact
 that the previously listed rules are often inadequate to
 choose from the possible routes. Vendor Policy pruning rules
 are extremely vendor-specific. See section [5.2.4.4].
 This algorithm has two distinct disadvantages. Presumably, a
 router implementor might develop techniques to deal with these
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 disadvantages and make them a part of the Vendor Policy pruning
 rule.
 (1) IS-IS and OSPF route classes are not directly handled.
 (2) Path properties other than type of service (e.g., MTU) are
 ignored.
 It is also worth noting a deficiency in the way that TOS is
 supported: routing protocols that support TOS are implicitly
 preferred when forwarding packets that have non-zero TOS values.
 The Basic Match and Longest Match pruning rules generalize the
 treatment of a number of particular types of routes. These routes
 are selected in the following, decreasing, order of preference:
 (1) Host Route: This is a route to a specific end system.
 (2) Hierarchical Network Prefix Routes: This is a route to a
 particular network prefix. Note that the FIB may contain
 several routes to network prefixes that subsume each other
 (one prefix is the other prefix with additional bits). These
 are selected in order of decreasing prefix length.
 (5) Default Route: This is a route to all networks for which there
 are no explicit routes. It is by definition the route whose
 prefix length is zero.
 If, after application of the pruning rules, the set of routes is
 empty (i.e., no routes were found), the packet MUST be discarded
 and an appropriate ICMP error generated (ICMP Bad Source Route if
 the IP Destination Address came from a source route option;
 otherwise, whichever of ICMP Destination Host Unreachable or
 Destination Network Unreachable is appropriate, as described in
 Section [4.3.3.1]).
5.2.4.4 Administrative Preference
 One suggested mechanism for the Vendor Policy Pruning Rule is to
 use administrative preference, which is a simple prioritization
 algorithm. The idea is to manually prioritize the routes that one
 might need to select among.
 Each route has associated with it a preference value, based on
 various attributes of the route (specific mechanisms for assignment
 of preference values are suggested below). This preference value
 is an integer in the range [0..255], with zero being the most
 preferred and 254 being the least preferred. 255 is a special
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 value that means that the route should never be used. The first
 step in the Vendor Policy pruning rule discards all but the most
 preferable routes (and always discards routes whose preference
 value is 255).
 This policy is not safe in that it can easily be misused to create
 routing loops. Since no protocol ensures that the preferences
 configured for a router is consistent with the preferences
 configured in its neighbors, network managers must exercise care in
 configuring preferences.
 o Address Match
 It is useful to be able to assign a single preference value to
 all routes (learned from the same routing domain) to any of a
 specified set of destinations, where the set of destinations is
 all destinations that match a specified network prefix.
 o Route Class
 For routing protocols which maintain the distinction, it is
 useful to be able to assign a single preference value to all
 routes (learned from the same routing domain) which have a
 particular route class (intra-area, inter-area, external with
 internal metrics, or external with external metrics).
 o Interface
 It is useful to be able to assign a single preference value to
 all routes (learned from a particular routing domain) that would
 cause packets to be routed out a particular logical interface on
 the router (logical interfaces generally map one-to-one onto the
 router's network interfaces, except that any network interface
 that has multiple IP addresses will have multiple logical
 interfaces associated with it).
 o Source router
 It is useful to be able to assign a single preference value to
 all routes (learned from the same routing domain) that were
 learned from any of a set of routers, where the set of routers
 are those whose updates have a source address that match a
 specified network prefix.
 o Originating AS
 For routing protocols which provide the information, it is
 useful to be able to assign a single preference value to all
 routes (learned from a particular routing domain) which
 originated in another particular routing domain. For BGP
 routes, the originating AS is the first AS listed in the route's
 AS_PATH attribute. For OSPF external routes, the originating AS
 may be considered to be the low order 16 bits of the route's
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 external route tag if the tag's Automatic bit is set and the
 tag's Path Length is not equal to 3.
 o External route tag
 It is useful to be able to assign a single preference value to
 all OSPF external routes (learned from the same routing domain)
 whose external route tags match any of a list of specified
 values. Because the external route tag may contain a structured
 value, it may be useful to provide the ability to match
 particular subfields of the tag.
 o AS path
 It may be useful to be able to assign a single preference value
 to all BGP routes (learned from the same routing domain) whose
 AS path "matches" any of a set of specified values. It is not
 yet clear exactly what kinds of matches are most useful. A
 simple option would be to allow matching of all routes for which
 a particular AS number appears (or alternatively, does not
 appear) anywhere in the route's AS_PATH attribute. A more
 general but somewhat more difficult alternative would be to
 allow matching all routes for which the AS path matches a
 specified regular expression.
5.2.4.5 Load Splitting
 At the end of the Next-hop selection process, multiple routes may
 still remain. A router has several options when this occurs. It
 may arbitrarily discard some of the routes. It may reduce the
 number of candidate routes by comparing metrics of routes from
 routing domains that are not considered equivalent. It may retain
 more than one route and employ a load-splitting mechanism to divide
 traffic among them. Perhaps the only thing that can be said about
 the relative merits of the options is that load-splitting is useful
 in some situations but not in others, so a wise implementor who
 implements load-splitting will also provide a way for the network
 manager to disable it.
5.2.5 Unused IP Header Bits: RFC-791 Section 3.1
 The IP header contains several reserved bits, in the Type of
 Service field and in the Flags field. Routers MUST NOT drop
 packets merely because one or more of these reserved bits has a
 non-zero value.
 Routers MUST ignore and MUST pass through unchanged the values of
 these reserved bits. If a router fragments a packet, it MUST copy
 these bits into each fragment.
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 DISCUSSION
 Future revisions to the IP protocol may make use of these unused
 bits. These rules are intended to ensure that these revisions can
 be deployed without having to simultaneously upgrade all routers
 in the Internet.
5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2
 As was discussed in Section [4.2.2.7], a router MUST support IP
 fragmentation.
 A router MUST NOT reassemble any datagram before forwarding it.
 DISCUSSION
 A few people have suggested that there might be some topologies
 where reassembly of transit datagrams by routers might improve
 performance. The fact that fragments may take different paths to
 the destination precludes safe use of such a feature.
 Nothing in this section should be construed to control or limit
 fragmentation or reassembly performed as a link layer function by
 the router.
 Similarly, if an IP datagram is encapsulated in another IP
 datagram (e.g., it is tunnelled), that datagram is in turn
 fragmented, the fragments must be reassembled in order to forward
 the original datagram. This section does not preclude this.
5.2.7 Internet Control Message Protocol - ICMP
 General requirements for ICMP were discussed in Section [4.3]. This
 section discusses ICMP messages that are sent only by routers.
5.2.7.1 Destination Unreachable
 The ICMP Destination Unreachable message is sent by a router in
 response to a packet which it cannot forward because the destination
 (or next hop) is unreachable or a service is unavailable. Examples
 of such cases include a message addressed to a host which is not
 there and therefore does not respond to ARP requests, and messages
 addressed to network prefixes for which the router has no valid
 route.
 A router MUST be able to generate ICMP Destination Unreachable
 messages and SHOULD choose a response code that most closely matches
 the reason the message is being generated.
 The following codes are defined in [INTERNET:8] and [INTRO:2]:
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 0 = Network Unreachable - generated by a router if a forwarding path
 (route) to the destination network is not available;
 1 = Host Unreachable - generated by a router if a forwarding path
 (route) to the destination host on a directly connected network
 is not available (does not respond to ARP);
 2 = Protocol Unreachable - generated if the transport protocol
 designated in a datagram is not supported in the transport layer
 of the final destination;
 3 = Port Unreachable - generated if the designated transport protocol
 (e.g., UDP) is unable to demultiplex the datagram in the
 transport layer of the final destination but has no protocol
 mechanism to inform the sender;
 4 = Fragmentation Needed and DF Set - generated if a router needs to
 fragment a datagram but cannot since the DF flag is set;
 5 = Source Route Failed - generated if a router cannot forward a
 packet to the next hop in a source route option;
 6 = Destination Network Unknown - This code SHOULD NOT be generated
 since it would imply on the part of the router that the
 destination network does not exist (net unreachable code 0
 SHOULD be used in place of code 6);
 7 = Destination Host Unknown - generated only when a router can
 determine (from link layer advice) that the destination host
 does not exist;
 11 = Network Unreachable For Type Of Service - generated by a router
 if a forwarding path (route) to the destination network with the
 requested or default TOS is not available;
 12 = Host Unreachable For Type Of Service - generated if a router
 cannot forward a packet because its route(s) to the destination
 do not match either the TOS requested in the datagram or the
 default TOS (0).
 The following additional codes are hereby defined:
 13 = Communication Administratively Prohibited - generated if a
 router cannot forward a packet due to administrative filtering;
 14 = Host Precedence Violation. Sent by the first hop router to a
 host to indicate that a requested precedence is not permitted
 for the particular combination of source/destination host or
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 network, upper layer protocol, and source/destination port;
 15 = Precedence cutoff in effect. The network operators have imposed
 a minimum level of precedence required for operation, the
 datagram was sent with a precedence below this level;
 NOTE: [INTRO:2] defined Code 8 for source host isolated. Routers
 SHOULD NOT generate Code 8; whichever of Codes 0 (Network
 Unreachable) and 1 (Host Unreachable) is appropriate SHOULD be used
 instead. [INTRO:2] also defined Code 9 for communication with
 destination network administratively prohibited and Code 10 for
 communication with destination host administratively prohibited.
 These codes were intended for use by end-to-end encryption devices
 used by U.S military agencies. Routers SHOULD use the newly defined
 Code 13 (Communication Administratively Prohibited) if they
 administratively filter packets.
 Routers MAY have a configuration option that causes Code 13
 (Communication Administratively Prohibited) messages not to be
 generated. When this option is enabled, no ICMP error message is
 sent in response to a packet that is dropped because its forwarding
 is administratively prohibited.
 Similarly, routers MAY have a configuration option that causes Code
 14 (Host Precedence Violation) and Code 15 (Precedence Cutoff in
 Effect) messages not to be generated. When this option is enabled,
 no ICMP error message is sent in response to a packet that is dropped
 because of a precedence violation.
 Routers MUST use Host Unreachable or Destination Host Unknown codes
 whenever other hosts on the same destination network might be
 reachable; otherwise, the source host may erroneously conclude that
 all hosts on the network are unreachable, and that may not be the
 case.
 [INTERNET:14] describes a slight modification the form of Destination
 Unreachable messages containing Code 4 (Fragmentation needed and DF
 set). A router MUST use this modified form when originating Code 4
 Destination Unreachable messages.
5.2.7.2 Redirect
 The ICMP Redirect message is generated to inform a local host the it
 should use a different next hop router for a certain class of
 traffic.
 Routers MUST NOT generate the Redirect for Network or Redirect for
 Network and Type of Service messages (Codes 0 and 2) specified in
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 [INTERNET:8]. Routers MUST be able to generate the Redirect for Host
 message (Code 1) and SHOULD be able to generate the Redirect for Type
 of Service and Host message (Code 3) specified in [INTERNET:8].
 DISCUSSION
 If the directly connected network is not subnetted (in the
 classical sense), a router can normally generate a network
 Redirect that applies to all hosts on a specified remote network.
 Using a network rather than a host Redirect may economize slightly
 on network traffic and on host routing table storage. However,
 the savings are not significant, and subnets create an ambiguity
 about the subnet mask to be used to interpret a network Redirect.
 In a CIDR environment, it is difficult to specify precisely the
 cases in which network Redirects can be used. Therefore, routers
 must send only host (or host and type of service) Redirects.
 A Code 3 (Redirect for Host and Type of Service) message is generated
 when the packet provoking the redirect has a destination for which
 the path chosen by the router would depend (in part) on the TOS
 requested.
 Routers that can generate Code 3 redirects (Host and Type of Service)
 MUST have a configuration option (which defaults to on) to enable
 Code 1 (Host) redirects to be substituted for Code 3 redirects. A
 router MUST send a Code 1 Redirect in place of a Code 3 Redirect if
 it has been configured to do so.
 If a router is not able to generate Code 3 Redirects then it MUST
 generate Code 1 Redirects in situations where a Code 3 Redirect is
 called for.
 Routers MUST NOT generate a Redirect Message unless all the following
 conditions are met:
 o The packet is being forwarded out the same physical interface that
 it was received from,
 o The IP source address in the packet is on the same Logical IP
 (sub)network as the next-hop IP address, and
 o The packet does not contain an IP source route option.
 The source address used in the ICMP Redirect MUST belong to the same
 logical (sub)net as the destination address.
 A router using a routing protocol (other than static routes) MUST NOT
 consider paths learned from ICMP Redirects when forwarding a packet.
 If a router is not using a routing protocol, a router MAY have a
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 configuration that, if set, allows the router to consider routes
 learned through ICMP Redirects when forwarding packets.
 DISCUSSION
 ICMP Redirect is a mechanism for routers to convey routing
 information to hosts. Routers use other mechanisms to learn
 routing information, and therefore have no reason to obey
 redirects. Believing a redirect which contradicted the router's
 other information would likely create routing loops.
 On the other hand, when a router is not acting as a router, it
 MUST comply with the behavior required of a host.
5.2.7.3 Time Exceeded
 A router MUST generate a Time Exceeded message Code 0 (In Transit)
 when it discards a packet due to an expired TTL field. A router MAY
 have a per-interface option to disable origination of these messages
 on that interface, but that option MUST default to allowing the
 messages to be originated.
5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP
 IGMP [INTERNET:4] is a protocol used between hosts and multicast
 routers on a single physical network to establish hosts' membership
 in particular multicast groups. Multicast routers use this
 information, in conjunction with a multicast routing protocol, to
 support IP multicast forwarding across the Internet.
 A router SHOULD implement the multicast router part of IGMP.
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5.3 SPECIFIC ISSUES
5.3.1 Time to Live (TTL)
 The Time-to-Live (TTL) field of the IP header is defined to be a
 timer limiting the lifetime of a datagram. It is an 8-bit field and
 the units are seconds. Each router (or other module) that handles a
 packet MUST decrement the TTL by at least one, even if the elapsed
 time was much less than a second. Since this is very often the case,
 the TTL is effectively a hop count limit on how far a datagram can
 propagate through the Internet.
 When a router forwards a packet, it MUST reduce the TTL by at least
 one. If it holds a packet for more than one second, it MAY decrement
 the TTL by one for each second.
 If the TTL is reduced to zero (or less), the packet MUST be
 discarded, and if the destination is not a multicast address the
 router MUST send an ICMP Time Exceeded message, Code 0 (TTL Exceeded
 in Transit) message to the source. Note that a router MUST NOT
 discard an IP unicast or broadcast packet with a non-zero TTL merely
 because it can predict that another router on the path to the
 packet's final destination will decrement the TTL to zero. However,
 a router MAY do so for IP multicasts, in order to more efficiently
 implement IP multicast's expanding ring search algorithm (see
 [INTERNET:4]).
 DISCUSSION
 The IP TTL is used, somewhat schizophrenically, as both a hop
 count limit and a time limit. Its hop count function is critical
 to ensuring that routing problems can't melt down the network by
 causing packets to loop infinitely in the network. The time limit
 function is used by transport protocols such as TCP to ensure
 reliable data transfer. Many current implementations treat TTL as
 a pure hop count, and in parts of the Internet community there is
 a strong sentiment that the time limit function should instead be
 performed by the transport protocols that need it.
 In this specification, we have reluctantly decided to follow the
 strong belief among the router vendors that the time limit
 function should be optional. They argued that implementation of
 the time limit function is difficult enough that it is currently
 not generally done. They further pointed to the lack of
 documented cases where this shortcut has caused TCP to corrupt
 data (of course, we would expect the problems created to be rare
 and difficult to reproduce, so the lack of documented cases
 provides little reassurance that there haven't been a number of
 undocumented cases).
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 IP multicast notions such as the expanding ring search may not
 work as expected unless the TTL is treated as a pure hop count.
 The same thing is somewhat true of traceroute.
 ICMP Time Exceeded messages are required because the traceroute
 diagnostic tool depends on them.
 Thus, the tradeoff is between severely crippling, if not
 eliminating, two very useful tools and avoiding a very rare and
 transient data transport problem that may not occur at all. We
 have chosen to preserve the tools.
5.3.2 Type of Service (TOS)
 The Type-of-Service byte in the IP header is divided into three
 sections: the Precedence field (high-order 3 bits), a field that
 is customarily called Type of Service or "TOS (next 4 bits), and a
 reserved bit (the low order bit). Rules governing the reserved
 bit were described in Section [4.2.2.3]. The Precedence field
 will be discussed in Section [5.3.3]. A more extensive discussion
 of the TOS field and its use can be found in [ROUTE:11].
 A router SHOULD consider the TOS field in a packet's IP header
 when deciding how to forward it. The remainder of this section
 describes the rules that apply to routers that conform to this
 requirement.
 A router MUST maintain a TOS value for each route in its routing
 table. Routes learned through a routing protocol that does not
 support TOS MUST be assigned a TOS of zero (the default TOS).
 To choose a route to a destination, a router MUST use an algorithm
 equivalent to the following:
 (1) The router locates in its routing table all available routes
 to the destination (see Section [5.2.4]).
 (2) If there are none, the router drops the packet because the
 destination is unreachable. See section [5.2.4].
 (3) If one or more of those routes have a TOS that exactly matches
 the TOS specified in the packet, the router chooses the route
 with the best metric.
 (4) Otherwise, the router repeats the above step, except looking
 at routes whose TOS is zero.
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 (5) If no route was chosen above, the router drops the packet
 because the destination is unreachable. The router returns
 an ICMP Destination Unreachable error specifying the
 appropriate code: either Network Unreachable with Type of
 Service (code 11) or Host Unreachable with Type of Service
 (code 12).
 DISCUSSION
 Although TOS has been little used in the past, its use by hosts is
 now mandated by the Requirements for Internet Hosts RFCs
 ([INTRO:2] and [INTRO:3]). Support for TOS in routers may become
 a MUST in the future, but is a SHOULD for now until we get more
 experience with it and can better judge both its benefits and its
 costs.
 Various people have proposed that TOS should affect other aspects
 of the forwarding function. For example:
 (1) A router could place packets that have the Low Delay bit set
 ahead of other packets in its output queues.
 (2) a router is forced to discard packets, it could try to avoid
 discarding those which have the High Reliability bit set.
 These ideas have been explored in more detail in [INTERNET:17] but
 we don't yet have enough experience with such schemes to make
 requirements in this area.
5.3.3 IP Precedence
 This section specifies requirements and guidelines for appropriate
 processing of the IP Precedence field in routers. Precedence is a
 scheme for allocating resources in the network based on the
 relative importance of different traffic flows. The IP
 specification defines specific values to be used in this field for
 various types of traffic.
 The basic mechanisms for precedence processing in a router are
 preferential resource allocation, including both precedence-
 ordered queue service and precedence-based congestion control, and
 selection of Link Layer priority features. The router also
 selects the IP precedence for routing, management and control
 traffic it originates. For a more extensive discussion of IP
 Precedence and its implementation see [FORWARD:6].
 Precedence-ordered queue service, as discussed in this section,
 includes but is not limited to the queue for the forwarding
 process and queues for outgoing links. It is intended that a
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 router supporting precedence should also use the precedence
 indication at whatever points in its processing are concerned with
 allocation of finite resources, such as packet buffers or Link
 Layer connections. The set of such points is implementation-
 dependent.
 DISCUSSION
 Although the Precedence field was originally provided for use in
 DOD systems where large traffic surges or major damage to the
 network are viewed as inherent threats, it has useful applications
 for many non-military IP networks. Although the traffic handling
 capacity of networks has grown greatly in recent years, the
 traffic generating ability of the users has also grown, and
 network overload conditions still occur at times. Since IP-based
 routing and management protocols have become more critical to the
 successful operation of the Internet, overloads present two
 additional risks to the network:
 (1) High delays may result in routing protocol packets being lost.
 This may cause the routing protocol to falsely deduce a
 topology change and propagate this false information to other
 routers. Not only can this cause routes to oscillate, but an
 extra processing burden may be placed on other routers.
 (2) High delays may interfere with the use of network management
 tools to analyze and perhaps correct or relieve the problem
 in the network that caused the overload condition to occur.
 Implementation and appropriate use of the Precedence mechanism
 alleviates both of these problems.
5.3.3.1 Precedence-Ordered Queue Service
 Routers SHOULD implement precedence-ordered queue service.
 Precedence-ordered queue service means that when a packet is selected
 for output on a (logical) link, the packet of highest precedence that
 has been queued for that link is sent. Routers that implement
 precedence-ordered queue service MUST also have a configuration
 option to suppress precedence-ordered queue service in the Internet
 Layer.
 Any router MAY implement other policy-based throughput management
 procedures that result in other than strict precedence ordering, but
 it MUST be configurable to suppress them (i.e., use strict ordering).
 As detailed in Section [5.3.6], routers that implement precedence-
 ordered queue service discard low precedence packets before
 discarding high precedence packets for congestion control purposes.
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 Preemption (interruption of processing or transmission of a packet)
 is not envisioned as a function of the Internet Layer. Some
 protocols at other layers may provide preemption features.
5.3.3.2 Lower Layer Precedence Mappings
 Routers that implement precedence-ordered queuing MUST IMPLEMENT, and
 other routers SHOULD IMPLEMENT, Lower Layer Precedence Mapping.
 A router that implements Lower Layer Precedence Mapping:
 o MUST be able to map IP Precedence to Link Layer priority mechanisms
 for link layers that have such a feature defined.
 o MUST have a configuration option to select the Link Layer's default
 priority treatment for all IP traffic
 o SHOULD be able to configure specific nonstandard mappings of IP
 precedence values to Link Layer priority values for each
 interface.
 DISCUSSION
 Some research questions the workability of the priority features
 of some Link Layer protocols, and some networks may have faulty
 implementations of the link layer priority mechanism. It seems
 prudent to provide an escape mechanism in case such problems show
 up in a network.
 On the other hand, there are proposals to use novel queuing
 strategies to implement special services such as multimedia
 bandwidth reservation or low-delay service. Special services and
 queuing strategies to support them are current research subjects
 and are in the process of standardization.
 Implementors may wish to consider that correct link layer mapping
 of IP precedence is required by DOD policy for TCP/IP systems used
 on DOD networks. Since these requirements are intended to
 encourage (but not force) the use of precedence features in the
 hope of providing better Internet service to all users, routers
 supporting precedence-ordered queue service should default to
 maintaining strict precedence ordering regardless of the type of
 service requested.
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5.3.3.3 Precedence Handling For All Routers
 A router (whether or not it employs precedence-ordered queue
 service):
 (1) MUST accept and process incoming traffic of all precedence levels
 normally, unless it has been administratively configured to do
 otherwise.
 (2) MAY implement a validation filter to administratively restrict
 the use of precedence levels by particular traffic sources. If
 provided, this filter MUST NOT filter out or cut off the
 following sorts of ICMP error messages: Destination Unreachable,
 Redirect, Time Exceeded, and Parameter Problem. If this filter
 is provided, the procedures required for packet filtering by
 addresses are required for this filter also.
 DISCUSSION
 Precedence filtering should be applicable to specific
 source/destination IP Address pairs, specific protocols, specific
 ports, and so on.
 An ICMP Destination Unreachable message with code 14 SHOULD be sent
 when a packet is dropped by the validation filter, unless this has
 been suppressed by configuration choice.
 (3) MAY implement a cutoff function that allows the router to be set
 to refuse or drop traffic with precedence below a specified
 level. This function may be activated by management actions or
 by some implementation dependent heuristics, but there MUST be a
 configuration option to disable any heuristic mechanism that
 operates without human intervention. An ICMP Destination
 Unreachable message with code 15 SHOULD be sent when a packet is
 dropped by the cutoff function, unless this has been suppressed
 by configuration choice.
 A router MUST NOT refuse to forward datagrams with IP precedence
 of 6 (Internetwork Control) or 7 (Network Control) solely due to
 precedence cutoff. However, other criteria may be used in
 conjunction with precedence cutoff to filter high precedence
 traffic.
 DISCUSSION
 Unrestricted precedence cutoff could result in an unintentional
 cutoff of routing and control traffic. In the general case, host
 traffic should be restricted to a value of 5 (CRITIC/ECP) or
 below; this is not a requirement and may not be correct in certain
 systems.
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 (4) MUST NOT change precedence settings on packets it did not
 originate.
 (5) SHOULD be able to configure distinct precedence values to be used
 for each routing or management protocol supported (except for
 those protocols, such as OSPF, which specify which precedence
 value must be used).
 (6) MAY be able to configure routing or management traffic precedence
 values independently for each peer address.
 (7) MUST respond appropriately to Link Layer precedence-related error
 indications where provided. An ICMP Destination Unreachable
 message with code 15 SHOULD be sent when a packet is dropped
 because a link cannot accept it due to a precedence-related
 condition, unless this has been suppressed by configuration
 choice.
 DISCUSSION
 The precedence cutoff mechanism described in (3) is somewhat
 controversial. Depending on the topological location of the area
 affected by the cutoff, transit traffic may be directed by routing
 protocols into the area of the cutoff, where it will be dropped.
 This is only a problem if another path that is unaffected by the
 cutoff exists between the communicating points. Proposed ways of
 avoiding this problem include providing some minimum bandwidth to
 all precedence levels even under overload conditions, or
 propagating cutoff information in routing protocols. In the
 absence of a widely accepted (and implemented) solution to this
 problem, great caution is recommended in activating cutoff
 mechanisms in transit networks.
 A transport layer relay could legitimately provide the function
 prohibited by (4) above. Changing precedence levels may cause
 subtle interactions with TCP and perhaps other protocols; a
 correct design is a non-trivial task.
 The intent of (5) and (6) (and the discussion of IP Precedence in
 ICMP messages in Section [4.3.2]) is that the IP precedence bits
 should be appropriately set, whether or not this router acts upon
 those bits in any other way. We expect that in the future
 specifications for routing protocols and network management
 protocols will specify how the IP Precedence should be set for
 messages sent by those protocols.
 The appropriate response for (7) depends on the link layer
 protocol in use. Typically, the router should stop trying to send
 offensive traffic to that destination for some period of time, and
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 should return an ICMP Destination Unreachable message with code 15
 (service not available for precedence requested) to the traffic
 source. It also should not try to reestablish a preempted Link
 Layer connection for some time.
5.3.4 Forwarding of Link Layer Broadcasts
 The encapsulation of IP packets in most Link Layer protocols (except
 PPP) allows a receiver to distinguish broadcasts and multicasts from
 unicasts simply by examining the Link Layer protocol headers (most
 commonly, the Link Layer destination address). The rules in this
 section that refer to Link Layer broadcasts apply only to Link Layer
 protocols that allow broadcasts to be distinguished; likewise, the
 rules that refer to Link Layer multicasts apply only to Link Layer
 protocols that allow multicasts to be distinguished.
 A router MUST NOT forward any packet that the router received as a
 Link Layer broadcast, unless it is directed to an IP Multicast
 address. In this latter case, one would presume that link layer
 broadcast was used due to the lack of an effective multicast service.
 A router MUST NOT forward any packet which the router received as a
 Link Layer multicast unless the packet's destination address is an IP
 multicast address.
 A router SHOULD silently discard a packet that is received via a Link
 Layer broadcast but does not specify an IP multicast or IP broadcast
 destination address.
 When a router sends a packet as a Link Layer broadcast, the IP
 destination address MUST be a legal IP broadcast or IP multicast
 address.
5.3.5 Forwarding of Internet Layer Broadcasts
 There are two major types of IP broadcast addresses; limited
 broadcast and directed broadcast. In addition, there are three
 subtypes of directed broadcast: a broadcast directed to a specified
 network prefix, a broadcast directed to a specified subnetwork, and a
 broadcast directed to all subnets of a specified network.
 Classification by a router of a broadcast into one of these
 categories depends on the broadcast address and on the router's
 understanding (if any) of the subnet structure of the destination
 network. The same broadcast will be classified differently by
 different routers.
 A limited IP broadcast address is defined to be all-ones: { -1, -1 }
 or 255.255.255.255.
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 A network-prefix-directed broadcast is composed of the network prefix
 of the IP address with a local part of all-ones or { <Network-
 prefix>, -1 }. For example, a Class A net broadcast address is
 net.255.255.255, a Class B net broadcast address is net.net.255.255
 and a Class C net broadcast address is net.net.net.255 where net is a
 byte of the network address.
 The all-subnets-directed-broadcast is not well defined in a CIDR
 environment, and was deprecated in version 1 of this memo.
 As was described in Section [4.2.3.1], a router may encounter certain
 non-standard IP broadcast addresses:
 o 0.0.0.0 is an obsolete form of the limited broadcast address
 o { <Network-prefix>, 0 } is an obsolete form of a network-prefix-
 directed broadcast address.
 As was described in that section, packets addressed to any of these
 addresses SHOULD be silently discarded, but if they are not, they
 MUST be treated according to the same rules that apply to packets
 addressed to the non-obsolete forms of the broadcast addresses
 described above. These rules are described in the next few sections.
5.3.5.1 Limited Broadcasts
 Limited broadcasts MUST NOT be forwarded. Limited broadcasts MUST
 NOT be discarded. Limited broadcasts MAY be sent and SHOULD be sent
 instead of directed broadcasts where limited broadcasts will suffice.
 DISCUSSION
 Some routers contain UDP servers which function by resending the
 requests (as unicasts or directed broadcasts) to other servers.
 This requirement should not be interpreted as prohibiting such
 servers. Note, however, that such servers can easily cause packet
 looping if misconfigured. Thus, providers of such servers would
 probably be well advised to document their setup carefully and to
 consider carefully the TTL on packets that are sent.
5.3.5.2 Directed Broadcasts
 A router MUST classify as network-prefix-directed broadcasts all
 valid, directed broadcasts destined for a remote network or an
 attached nonsubnetted network. Note that in view of CIDR, such
 appear to be host addresses within the network prefix; we preclude
 inspection of the host part of such network prefixes. Given a route
 and no overriding policy, then, a router MUST forward network-
 prefix-directed broadcasts. Network-Prefix-Directed broadcasts MAY
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 be sent.
 A router MAY have an option to disable receiving network-prefix-
 directed broadcasts on an interface and MUST have an option to
 disable forwarding network-prefix-directed broadcasts. These options
 MUST default to permit receiving and forwarding network-prefix-
 directed broadcasts.
 DISCUSSION
 There has been some debate about forwarding or not forwarding
 directed broadcasts. In this memo we have made the forwarding
 decision depend on the router's knowledge of the destination
 network prefix. Routers cannot determine that a message is
 unicast or directed broadcast apart from this knowledge. The
 decision to forward or not forward the message is by definition
 only possible in the last hop router.
5.3.5.3 All-subnets-directed Broadcasts
 The first version of this memo described an algorithm for
 distributing a directed broadcast to all the subnets of a classical
 network number. This algorithm was stated to be "broken," and
 certain failure cases were specified.
 In a CIDR routing domain, wherein classical IP network numbers are
 meaningless, the concept of an all-subnets-directed-broadcast is also
 meaningless. To the knowledge of the working group, the facility was
 never implemented or deployed, and is now relegated to the dustbin of
 history.
5.3.5.4 Subnet-directed Broadcasts
 The first version of this memo spelled out procedures for dealing
 with subnet-directed-broadcasts. In a CIDR routing domain, these are
 indistinguishable from net-drected-broadcasts. The two are therefore
 treated together in section [5.3.5.2 Directed Broadcasts], and should
 be viewed as network-prefix directed broadcasts.
5.3.6 Congestion Control
 Congestion in a network is loosely defined as a condition where
 demand for resources (usually bandwidth or CPU time) exceeds
 capacity. Congestion avoidance tries to prevent demand from
 exceeding capacity, while congestion recovery tries to restore an
 operative state. It is possible for a router to contribute to both
 of these mechanisms. A great deal of effort has been spent studying
 the problem. The reader is encouraged to read [FORWARD:2] for a
 survey of the work. Important papers on the subject include
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 [FORWARD:3], [FORWARD:4], [FORWARD:5], [FORWARD:10], [FORWARD:11],
 [FORWARD:12], [FORWARD:13], [FORWARD:14], and [INTERNET:10], among
 others.
 The amount of storage that router should have available to handle
 peak instantaneous demand when hosts use reasonable congestion
 policies, such as described in [FORWARD:5], is a function of the
 product of the bandwidth of the link times the path delay of the
 flows using the link, and therefore storage should increase as this
 Bandwidth*Delay product increases. The exact function relating
 storage capacity to probability of discard is not known.
 When a router receives a packet beyond its storage capacity it must
 (by definition, not by decree) discard it or some other packet or
 packets. Which packet to discard is the subject of much study but,
 unfortunately, little agreement so far. The best wisdom to date
 suggests discarding a packet from the data stream most heavily using
 the link. However, a number of additional factors may be relevant,
 including the precedence of the traffic, active bandwidth
 reservation, and the complexity associated with selecting that
 packet.
 A router MAY discard the packet it has just received; this is the
 simplest but not the best policy. Ideally, the router should select
 a packet from one of the sessions most heavily abusing the link,
 given that the applicable Quality of Service policy permits this. A
 recommended policy in datagram environments using FIFO queues is to
 discard a packet randomly selected from the queue (see [FORWARD:5]).
 An equivalent algorithm in routers using fair queues is to discard
 from the longest queue or that using the greatest virtual time (see
 [FORWARD:13]). A router MAY use these algorithms to determine which
 packet to discard.
 If a router implements a discard policy (such as Random Drop) under
 which it chooses a packet to discard from a pool of eligible packets:
 o If precedence-ordered queue service (described in Section
 [5.3.3.1]) is implemented and enabled, the router MUST NOT discard
 a packet whose IP precedence is higher than that of a packet that
 is not discarded.
 o A router MAY protect packets whose IP headers request the maximize
 reliability TOS, except where doing so would be in violation of
 the previous rule.
 o A router MAY protect fragmented IP packets, on the theory that
 dropping a fragment of a datagram may increase congestion by
 causing all fragments of the datagram to be retransmitted by the
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 source.
 o To help prevent routing perturbations or disruption of management
 functions, the router MAY protect packets used for routing
 control, link control, or network management from being discarded.
 Dedicated routers (i.e., routers that are not also general purpose
 hosts, terminal servers, etc.) can achieve an approximation of
 this rule by protecting packets whose source or destination is the
 router itself.
 Advanced methods of congestion control include a notion of fairness,
 so that the 'user' that is penalized by losing a packet is the one
 that contributed the most to the congestion. No matter what
 mechanism is implemented to deal with bandwidth congestion control,
 it is important that the CPU effort expended be sufficiently small
 that the router is not driven into CPU congestion also.
 As described in Section [4.3.3.3], this document recommends that a
 router SHOULD NOT send a Source Quench to the sender of the packet
 that it is discarding. ICMP Source Quench is a very weak mechanism,
 so it is not necessary for a router to send it, and host software
 should not use it exclusively as an indicator of congestion.
5.3.7 Martian Address Filtering
 An IP source address is invalid if it is a special IP address, as
 defined in 4.2.2.11 or 5.3.7, or is not a unicast address.
 An IP destination address is invalid if it is among those defined as
 illegal destinations in 4.2.3.1, or is a Class E address (except
 255.255.255.255).
 A router SHOULD NOT forward any packet that has an invalid IP source
 address or a source address on network 0. A router SHOULD NOT
 forward, except over a loopback interface, any packet that has a
 source address on network 127. A router MAY have a switch that
 allows the network manager to disable these checks. If such a switch
 is provided, it MUST default to performing the checks.
 A router SHOULD NOT forward any packet that has an invalid IP
 destination address or a destination address on network 0. A router
 SHOULD NOT forward, except over a loopback interface, any packet that
 has a destination address on network 127. A router MAY have a switch
 that allows the network manager to disable these checks. If such a
 switch is provided, it MUST default to performing the checks.
 If a router discards a packet because of these rules, it SHOULD log
 at least the IP source address, the IP destination address, and, if
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 the problem was with the source address, the physical interface on
 which the packet was received and the Link Layer address of the host
 or router from which the packet was received.
5.3.8 Source Address Validation
 A router SHOULD IMPLEMENT the ability to filter traffic based on a
 comparison of the source address of a packet and the forwarding table
 for a logical interface on which the packet was received. If this
 filtering is enabled, the router MUST silently discard a packet if
 the interface on which the packet was received is not the interface
 on which a packet would be forwarded to reach the address contained
 in the source address. In simpler terms, if a router wouldn't route
 a packet containing this address through a particular interface, it
 shouldn't believe the address if it appears as a source address in a
 packet read from this interface.
 If this feature is implemented, it MUST be disabled by default.
 DISCUSSION
 This feature can provide useful security improvements in some
 situations, but can erroneously discard valid packets in
 situations where paths are asymmetric.
5.3.9 Packet Filtering and Access Lists
 As a means of providing security and/or limiting traffic through
 portions of a network a router SHOULD provide the ability to
 selectively forward (or filter) packets. If this capability is
 provided, filtering of packets SHOULD be configurable either to
 forward all packets or to selectively forward them based upon the
 source and destination prefixes, and MAY filter on other message
 attributes. Each source and destination address SHOULD allow
 specification of an arbitrary prefix length.
 DISCUSSION
 This feature can provide a measure of privacy, where systems
 outside a boundary are not permitted to exchange certain protocols
 with systems inside the boundary, or are limited as to which
 systems they may communicate with. It can also help prevent
 certain classes of security breach, wherein a system outside a
 boundary masquerades as a system inside the boundary and mimics a
 session with it.
 If supported, a router SHOULD be configurable to allow one of an
 o Include list - specification of a list of message definitions to be
 forwarded, or an
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 o Exclude list - specification of a list of message definitions NOT
 to be forwarded.
 A "message definition", in this context, specifies the source and
 destination network prefix, and may include other identifying
 information such as IP Protocol Type or TCP port number.
 A router MAY provide a configuration switch that allows a choice
 between specifying an include or an exclude list, or other equivalent
 controls.
 A value matching any address (e.g., a keyword any, an address with a
 mask of all 0's, or a network prefix whose length is zero) MUST be
 allowed as a source and/or destination address.
 In addition to address pairs, the router MAY allow any combination of
 transport and/or application protocol and source and destination
 ports to be specified.
 The router MUST allow packets to be silently discarded (i.e.,
 discarded without an ICMP error message being sent).
 The router SHOULD allow an appropriate ICMP unreachable message to be
 sent when a packet is discarded. The ICMP message SHOULD specify
 Communication Administratively Prohibited (code 13) as the reason for
 the destination being unreachable.
 The router SHOULD allow the sending of ICMP destination unreachable
 messages (code 13) to be configured for each combination of address
 pairs, protocol types, and ports it allows to be specified.
 The router SHOULD count and SHOULD allow selective logging of packets
 not forwarded.
5.3.10 Multicast Routing
 An IP router SHOULD support forwarding of IP multicast packets, based
 either on static multicast routes or on routes dynamically determined
 by a multicast routing protocol (e.g., DVMRP [ROUTE:9]). A router
 that forwards IP multicast packets is called a multicast router.
5.3.11 Controls on Forwarding
 For each physical interface, a router SHOULD have a configuration
 option that specifies whether forwarding is enabled on that
 interface. When forwarding on an interface is disabled, the router:
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 o MUST silently discard any packets which are received on that
 interface but are not addressed to the router
 o MUST NOT send packets out that interface, except for datagrams
 originated by the router
 o MUST NOT announce via any routing protocols the availability of
 paths through the interface
 DISCUSSION
 This feature allows the network manager to essentially turn off an
 interface but leaves it accessible for network management.
 Ideally, this control would apply to logical rather than physical
 interfaces. It cannot, because there is no known way for a router
 to determine which logical interface a packet arrived absent a
 one-to-one correspondence between logical and physical interfaces.
5.3.12 State Changes
 During router operation, interfaces may fail or be manually disabled,
 or may become available for use by the router. Similarly, forwarding
 may be disabled for a particular interface or for the entire router
 or may be (re)enabled. While such transitions are (usually)
 uncommon, it is important that routers handle them correctly.
5.3.12.1 When a Router Ceases Forwarding
 When a router ceases forwarding it MUST stop advertising all routes,
 except for third party routes. It MAY continue to receive and use
 routes from other routers in its routing domains. If the forwarding
 database is retained, the router MUST NOT cease timing the routes in
 the forwarding database. If routes that have been received from
 other routers are remembered, the router MUST NOT cease timing the
 routes that it has remembered. It MUST discard any routes whose
 timers expire while forwarding is disabled, just as it would do if
 forwarding were enabled.
 DISCUSSION
 When a router ceases forwarding, it essentially ceases being a
 router. It is still a host, and must follow all of the
 requirements of Host Requirements [INTRO:2]. The router may still
 be a passive member of one or more routing domains, however. As
 such, it is allowed to maintain its forwarding database by
 listening to other routers in its routing domain. It may not,
 however, advertise any of the routes in its forwarding database,
 since it itself is doing no forwarding. The only exception to
 this rule is when the router is advertising a route that uses only
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 some other router, but which this router has been asked to
 advertise.
 A router MAY send ICMP destination unreachable (host unreachable)
 messages to the senders of packets that it is unable to forward. It
 SHOULD NOT send ICMP redirect messages.
 DISCUSSION
 Note that sending an ICMP destination unreachable (host
 unreachable) is a router action. This message should not be sent
 by hosts. This exception to the rules for hosts is allowed so
 that packets may be rerouted in the shortest possible time, and so
 that black holes are avoided.
5.3.12.2 When a Router Starts Forwarding
 When a router begins forwarding, it SHOULD expedite the sending of
 new routing information to all routers with which it normally
 exchanges routing information.
5.3.12.3 When an Interface Fails or is Disabled
 If an interface fails or is disabled a router MUST remove and stop
 advertising all routes in its forwarding database that make use of
 that interface. It MUST disable all static routes that make use of
 that interface. If other routes to the same destination and TOS are
 learned or remembered by the router, the router MUST choose the best
 alternate, and add it to its forwarding database. The router SHOULD
 send ICMP destination unreachable or ICMP redirect messages, as
 appropriate, in reply to all packets that it is unable to forward due
 to the interface being unavailable.
5.3.12.4 When an Interface is Enabled
 If an interface that had not been available becomes available, a
 router MUST reenable any static routes that use that interface. If
 routes that would use that interface are learned by the router, then
 these routes MUST be evaluated along with all the other learned
 routes, and the router MUST make a decision as to which routes should
 be placed in the forwarding database. The implementor is referred to
 Chapter [7], Application Layer - Routing Protocols for further
 information on how this decision is made.
 A router SHOULD expedite the sending of new routing information to
 all routers with which it normally exchanges routing information.
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5.3.13 IP Options
 Several options, such as Record Route and Timestamp, contain slots
 into which a router inserts its address when forwarding the packet.
 However, each such option has a finite number of slots, and therefore
 a router may find that there is not free slot into which it can
 insert its address. No requirement listed below should be construed
 as requiring a router to insert its address into an option that has
 no remaining slot to insert it into. Section [5.2.5] discusses how a
 router must choose which of its addresses to insert into an option.
5.3.13.1 Unrecognized Options Unrecognized IP options in forwarded
 packets MUST be passed through unchanged.
5.3.13.2 Security Option
 Some environments require the Security option in every packet; such a
 requirement is outside the scope of this document and the IP standard
 specification. Note, however, that the security options described in
 [INTERNET:1] and [INTERNET:16] are obsolete. Routers SHOULD
 IMPLEMENT the revised security option described in [INTERNET:5].
 DISCUSSION
 Routers intended for use in networks with multiple security levels
 should support packet filtering based on IPSO (RFC-1108) labels.
 To implement this support, the router would need to permit the
 router administrator to configure both a lower sensitivity limit
 (e.g. Unclassified) and an upper sensitivity limit (e.g. Secret)
 on each interface. It is commonly but not always the case that
 the two limits are the same (e.g. a single-level interface).
 Packets caught by an IPSO filter as being out of range should be
 silently dropped and a counter should note the number of packets
 dropped because of out of range IPSO labels.
5.3.13.3 Stream Identifier Option
 This option is obsolete. If the Stream Identifier option is present
 in a packet forwarded by the router, the option MUST be ignored and
 passed through unchanged.
5.3.13.4 Source Route Options
 A router MUST implement support for source route options in forwarded
 packets. A router MAY implement a configuration option that, when
 enabled, causes all source-routed packets to be discarded. However,
 such an option MUST NOT be enabled by default.
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 DISCUSSION
 The ability to source route datagrams through the Internet is
 important to various network diagnostic tools. However, source
 routing may be used to bypass administrative and security controls
 within a network. Specifically, those cases where manipulation of
 routing tables is used to provide administrative separation in
 lieu of other methods such as packet filtering may be vulnerable
 through source routed packets.
 EDITORS+COMMENTS
 Packet filtering can be defeated by source routing as well, if it
 is applied in any router except one on the final leg of the source
 routed path. Neither route nor packet filters constitute a
 complete solution for security.
5.3.13.5 Record Route Option
 Routers MUST support the Record Route option in forwarded packets.
 A router MAY provide a configuration option that, if enabled, will
 cause the router to ignore (i.e., pass through unchanged) Record
 Route options in forwarded packets. If provided, such an option MUST
 default to enabling the record-route. This option should not affect
 the processing of Record Route options in datagrams received by the
 router itself (in particular, Record Route options in ICMP echo
 requests will still be processed according to Section [4.3.3.6]).
 DISCUSSION
 There are some people who believe that Record Route is a security
 problem because it discloses information about the topology of the
 network. Thus, this document allows it to be disabled.
5.3.13.6 Timestamp Option
 Routers MUST support the timestamp option in forwarded packets. A
 timestamp value MUST follow the rules given [INTRO:2].
 If the flags field = 3 (timestamp and prespecified address), the
 router MUST add its timestamp if the next prespecified address
 matches any of the router's IP addresses. It is not necessary that
 the prespecified address be either the address of the interface on
 which the packet arrived or the address of the interface over which
 it will be sent.
 IMPLEMENTATION
 To maximize the utility of the timestamps contained in the
 timestamp option, it is suggested that the timestamp inserted be,
 as nearly as practical, the time at which the packet arrived at
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 the router. For datagrams originated by the router, the timestamp
 inserted should be, as nearly as practical, the time at which the
 datagram was passed to the network layer for transmission.
 A router MAY provide a configuration option which, if enabled, will
 cause the router to ignore (i.e., pass through unchanged) Timestamp
 options in forwarded datagrams when the flag word is set to zero
 (timestamps only) or one (timestamp and registering IP address). If
 provided, such an option MUST default to off (that is, the router
 does not ignore the timestamp). This option should not affect the
 processing of Timestamp options in datagrams received by the router
 itself (in particular, a router will insert timestamps into Timestamp
 options in datagrams received by the router, and Timestamp options in
 ICMP echo requests will still be processed according to Section
 [4.3.3.6]).
 DISCUSSION
 Like the Record Route option, the Timestamp option can reveal
 information about a network's topology. Some people consider this
 to be a security concern.
6. TRANSPORT LAYER
 A router is not required to implement any Transport Layer protocols
 except those required to support Application Layer protocols
 supported by the router. In practice, this means that most routers
 implement both the Transmission Control Protocol (TCP) and the User
 Datagram Protocol (UDP).
6.1 USER DATAGRAM PROTOCOL - UDP
 The User Datagram Protocol (UDP) is specified in [TRANS:1].
 A router that implements UDP MUST be compliant, and SHOULD be
 unconditionally compliant, with the requirements of [INTRO:2], except
 that:
 o This specification does not specify the interfaces between the
 various protocol layers. Thus, a router's interfaces need not
 comply with [INTRO:2], except where compliance is required for
 proper functioning of Application Layer protocols supported by the
 router.
 o Contrary to [INTRO:2], an application SHOULD NOT disable generation
 of UDP checksums.
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 DISCUSSION
 Although a particular application protocol may require that UDP
 datagrams it receives must contain a UDP checksum, there is no
 general requirement that received UDP datagrams contain UDP
 checksums. Of course, if a UDP checksum is present in a received
 datagram, the checksum must be verified and the datagram discarded
 if the checksum is incorrect.
6.2 TRANSMISSION CONTROL PROTOCOL - TCP
 The Transmission Control Protocol (TCP) is specified in [TRANS:2].
 A router that implements TCP MUST be compliant, and SHOULD be
 unconditionally compliant, with the requirements of [INTRO:2], except
 that:
 o This specification does not specify the interfaces between the
 various protocol layers. Thus, a router need not comply with the
 following requirements of [INTRO:2] (except of course where
 compliance is required for proper functioning of Application Layer
 protocols supported by the router):
 Use of Push: RFC-793 Section 2.8:
 Passing a received PSH flag to the application layer is now
 OPTIONAL.
 Urgent Pointer: RFC-793 Section 3.1:
 A TCP MUST inform the application layer asynchronously
 whenever it receives an Urgent pointer and there was
 previously no pending urgent data, or whenever the Urgent
 pointer advances in the data stream. There MUST be a way for
 the application to learn how much urgent data remains to be
 read from the connection, or at least to determine whether or
 not more urgent data remains to be read.
 TCP Connection Failures:
 An application MUST be able to set the value for R2 for a
 particular connection. For example, an interactive
 application might set R2 to ``infinity,'' giving the user
 control over when to disconnect.
 TCP Multihoming:
 If an application on a multihomed host does not specify the
 local IP address when actively opening a TCP connection, then
 the TCP MUST ask the IP layer to select a local IP address
 before sending the (first) SYN. See the function
 GET_SRCADDR() in Section 3.4.
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 IP Options:
 An application MUST be able to specify a source route when it
 actively opens a TCP connection, and this MUST take
 precedence over a source route received in a datagram.
 o For similar reasons, a router need not comply with any of the
 requirements of [INTRO:2].
 o The requirements concerning the Maximum Segment Size Option in
 [INTRO:2] are amended as follows: a router that implements the
 host portion of MTU discovery (discussed in Section [4.2.3.3] of
 this memo) uses 536 as the default value of SendMSS only if the
 path MTU is unknown; if the path MTU is known, the default value
 for SendMSS is the path MTU - 40.
 o The requirements concerning the Maximum Segment Size Option in
 [INTRO:2] are amended as follows: ICMP Destination Unreachable
 codes 11 and 12 are additional soft error conditions. Therefore,
 these message MUST NOT cause TCP to abort a connection.
 DISCUSSION
 It should particularly be noted that a TCP implementation in a
 router must conform to the following requirements of [INTRO:2]:
 o Providing a configurable TTL. [Time to Live: RFC-793 Section
 3.9]
 o Providing an interface to configure keep-alive behavior, if
 keep-alives are used at all. [TCP Keep-Alives]
 o Providing an error reporting mechanism, and the ability to
 manage it. [Asynchronous Reports]
 o Specifying type of service. [Type-of-Service]
 The general paradigm applied is that if a particular interface is
 visible outside the router, then all requirements for the
 interface must be followed. For example, if a router provides a
 telnet function, then it will be generating traffic, likely to be
 routed in the external networks. Therefore, it must be able to
 set the type of service correctly or else the telnet traffic may
 not get through.
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7. APPLICATION LAYER - ROUTING PROTOCOLS
7.1 INTRODUCTION
 For technical, managerial, and sometimes political reasons, the
 Internet routing system consists of two components - interior routing
 and exterior routing. The concept of an Autonomous System (AS), as
 define in Section 2.2.4 of this document, plays a key role in
 separating interior from an exterior routing, as this concept allows
 to deliniate the set of routers where a change from interior to
 exterior routing occurs. An IP datagram may have to traverse the
 routers of two or more Autonomous Systems to reach its destination,
 and the Autonomous Systems must provide each other with topology
 information to allow such forwarding. Interior gateway protocols
 (IGPs) are used to distribute routing information within an AS (i.e.,
 intra-AS routing). Exterior gateway protocols are used to exchange
 routing information among ASs (i.e., inter-AS routing).
7.1.1 Routing Security Considerations
 Routing is one of the few places where the Robustness Principle (be
 liberal in what you accept) does not apply. Routers should be
 relatively suspicious in accepting routing data from other routing
 systems.
 A router SHOULD provide the ability to rank routing information
 sources from most trustworthy to least trustworthy and to accept
 routing information about any particular destination from the most
 trustworthy sources first. This was implicit in the original
 core/stub autonomous system routing model using EGP and various
 interior routing protocols. It is even more important with the
 demise of a central, trusted core.
 A router SHOULD provide a mechanism to filter out obviously invalid
 routes (such as those for net 127).
 Routers MUST NOT by default redistribute routing data they do not
 themselves use, trust or otherwise consider valid. In rare cases, it
 may be necessary to redistribute suspicious information, but this
 should only happen under direct intercession by some human agency.
 Routers must be at least a little paranoid about accepting routing
 data from anyone, and must be especially careful when they distribute
 routing information provided to them by another party. See below for
 specific guidelines.
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7.1.2 Precedence
 Except where the specification for a particular routing protocol
 specifies otherwise, a router SHOULD set the IP Precedence value for
 IP datagrams carrying routing traffic it originates to 6
 (INTERNETWORK CONTROL).
 DISCUSSION
 Routing traffic with VERY FEW exceptions should be the highest
 precedence traffic on any network. If a system's routing traffic
 can't get through, chances are nothing else will.
7.1.3 Message Validation
 Peer-to-peer authentication involves several tests. The application
 of message passwords and explicit acceptable neighbor lists has in
 the past improved the robustness of the route database. Routers
 SHOULD IMPLEMENT management controls that enable explicit listing of
 valid routing neighbors. Routers SHOULD IMPLEMENT peer-to-peer
 authentication for those routing protocols that support them.
 Routers SHOULD validate routing neighbors based on their source
 address and the interface a message is received on; neighbors in a
 directly attached subnet SHOULD be restricted to communicate with the
 router via the interface that subnet is posited on or via unnumbered
 interfaces. Messages received on other interfaces SHOULD be silently
 discarded.
 DISCUSSION
 Security breaches and numerous routing problems are avoided by
 this basic testing.
7.2 INTERIOR GATEWAY PROTOCOLS
7.2.1 INTRODUCTION
 An Interior Gateway Protocol (IGP) is used to distribute routing
 information between the various routers in a particular AS.
 Independent of the algorithm used to implement a particular IGP, it
 should perform the following functions:
 (1) Respond quickly to changes in the internal topology of an AS
 (2) Provide a mechanism such that circuit flapping does not cause
 continuous routing updates
 (3) Provide quick convergence to loop-free routing
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 (4) Utilize minimal bandwidth
 (5) Provide equal cost routes to enable load-splitting
 (6) Provide a means for authentication of routing updates
 Current IGPs used in the internet today are characterized as either
 being based on a distance-vector or a link-state algorithm.
 Several IGPs are detailed in this section, including those most
 commonly used and some recently developed protocols that may be
 widely used in the future. Numerous other protocols intended for use
 in intra-AS routing exist in the Internet community.
 A router that implements any routing protocol (other than static
 routes) MUST IMPLEMENT OSPF (see Section [7.2.2]). A router MAY
 implement additional IGPs.
7.2.2 OPEN SHORTEST PATH FIRST - OSPF
 Shortest Path First (SPF) based routing protocols are a class of
 link-state algorithms that are based on the shortest-path algorithm
 of Dijkstra. Although SPF based algorithms have been around since
 the inception of the ARPANET, it is only recently that they have
 achieved popularity both inside both the IP and the OSI communities.
 In an SPF based system, each router obtains the entire topology
 database through a process known as flooding. Flooding insures a
 reliable transfer of the information. Each router then runs the SPF
 algorithm on its database to build the IP routing table. The OSPF
 routing protocol is an implementation of an SPF algorithm. The
 current version, OSPF version 2, is specified in [ROUTE:1]. Note
 that RFC-1131, which describes OSPF version 1, is obsolete.
 Note that to comply with Section [8.3] of this memo, a router that
 implements OSPF MUST implement the OSPF MIB [MGT:14].
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS
 The American National Standards Institute (ANSI) X3S3.3 committee has
 defined an intra-domain routing protocol. This protocol is titled
 Intermediate System to Intermediate System Routeing Exchange
 Protocol.
 Its application to an IP network has been defined in [ROUTE:2], and
 is referred to as Dual IS-IS (or sometimes as Integrated IS-IS).
 IS-IS is based on a link-state (SPF) routing algorithm and shares all
 the advantages for this class of protocols.
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7.3 EXTERIOR GATEWAY PROTOCOLS
7.3.1 INTRODUCTION
 Exterior Gateway Protocols are utilized for inter-Autonomous System
 routing to exchange reachability information for a set of networks
 internal to a particular autonomous system to a neighboring
 autonomous system.
 The area of inter-AS routing is a current topic of research inside
 the Internet Engineering Task Force. The Exterior Gateway Protocol
 (EGP) described in Section [Appendix F.1] has traditionally been the
 inter-AS protocol of choice, but is now historical. The Border
 Gateway Protocol (BGP) eliminates many of the restrictions and
 limitations of EGP, and is therefore growing rapidly in popularity.
 A router is not required to implement any inter-AS routing protocol.
 However, if a router does implement EGP it also MUST IMPLEMENT BGP.
 Although it was not designed as an exterior gateway protocol, RIP
 (described in Section [7.2.4]) is sometimes used for inter-AS
 routing.
7.3.2 BORDER GATEWAY PROTOCOL - BGP
7.3.2.1 Introduction
 The Border Gateway Protocol (BGP-4) is an inter-AS routing protocol
 that exchanges network reachability information with other BGP
 speakers. The information for a network includes the complete list
 of ASs that traffic must transit to reach that network. This
 information can then be used to insure loop-free paths. This
 information is sufficient to construct a graph of AS connectivity
 from which routing loops may be pruned and some policy decisions at
 the AS level may be enforced.
 BGP is defined by [ROUTE:4]. [ROUTE:5] specifies the proper usage of
 BGP in the Internet, and provides some useful implementation hints
 and guidelines. [ROUTE:12] and [ROUTE:13] provide additional useful
 information.
 To comply with Section [8.3] of this memo, a router that implements
 BGP is required to implement the BGP MIB [MGT:15].
 To characterize the set of policy decisions that can be enforced
 using BGP, one must focus on the rule that an AS advertises to its
 neighbor ASs only those routes that it itself uses. This rule
 reflects the hop-by-hop routing paradigm generally used throughout
 the current Internet. Note that some policies cannot be supported by
 the hop-by-hop routing paradigm and thus require techniques such as
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 source routing to enforce. For example, BGP does not enable one AS
 to send traffic to a neighbor AS intending that traffic take a
 different route from that taken by traffic originating in the
 neighbor AS. On the other hand, BGP can support any policy
 conforming to the hop-by-hop routing paradigm.
 Implementors of BGP are strongly encouraged to follow the
 recommendations outlined in Section 6 of [ROUTE:5].
7.3.2.2 Protocol Walk-through
 While BGP provides support for quite complex routing policies (as an
 example see Section 4.2 in [ROUTE:5]), it is not required for all BGP
 implementors to support such policies. At a minimum, however, a BGP
 implementation:
 (1) SHOULD allow an AS to control announcements of the BGP learned
 routes to adjacent AS's. Implementations SHOULD support such
 control with at least the granularity of a single network.
 Implementations SHOULD also support such control with the
 granularity of an autonomous system, where the autonomous system
 may be either the autonomous system that originated the route,
 or the autonomous system that advertised the route to the local
 system (adjacent autonomous system).
 (2) SHOULD allow an AS to prefer a particular path to a destination
 (when more than one path is available). Such function SHOULD be
 implemented by allowing system administrator to assign weights
 to Autonomous Systems, and making route selection process to
 select a route with the lowest weight (where weight of a route
 is defined as a sum of weights of all AS's in the AS_PATH path
 attribute associated with that route).
 (3) SHOULD allow an AS to ignore routes with certain AS's in the
 AS_PATH path attribute. Such function can be implemented by
 using technique outlined in (2), and by assigning infinity as
 weights for such AS's. The route selection process must ignore
 routes that have weight equal to infinity.
7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
 It is possible to exchange routing information between two autonomous
 systems or routing domains without using a standard exterior routing
 protocol between two separate, standard interior routing protocols.
 The most common way of doing this is to run both interior protocols
 independently in one of the border routers with an exchange of route
 information between the two processes.
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 As with the exchange of information from an EGP to an IGP, without
 appropriate controls these exchanges of routing information between
 two IGPs in a single router are subject to creation of routing loops.
7.4 STATIC ROUTING
 Static routing provides a means of explicitly defining the next hop
 from a router for a particular destination. A router SHOULD provide
 a means for defining a static route to a destination, where the
 destination is defined by a network prefix. The mechanism SHOULD
 also allow for a metric to be specified for each static route.
 A router that supports a dynamic routing protocol MUST allow static
 routes to be defined with any metric valid for the routing protocol
 used. The router MUST provide the ability for the user to specify a
 list of static routes that may or may not be propagated through the
 routing protocol. In addition, a router SHOULD support the following
 additional information if it supports a routing protocol that could
 make use of the information. They are:
 o TOS,
 o Subnet Mask, or
 o Prefix Length, or
 o A metric specific to a given routing protocol that can import the
 route.
 DISCUSSION
 We intend that one needs to support only the things useful to the
 given routing protocol. The need for TOS should not require the
 vendor to implement the other parts if they are not used.
 Whether a router prefers a static route over a dynamic route (or
 vice versa) or whether the associated metrics are used to choose
 between conflicting static and dynamic routes SHOULD be
 configurable for each static route.
 A router MUST allow a metric to be assigned to a static route for
 each routing domain that it supports. Each such metric MUST be
 explicitly assigned to a specific routing domain. For example:
 route 10.0.0.0/8 via 192.0.2.3 rip metric 3
 route 10.21.0.0/16 via 192.0.2.4 ospf inter-area metric 27
 route 10.22.0.0/16 via 192.0.2.5 egp 123 metric 99
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 DISCUSSION
 It has been suggested that, ideally, static routes should have
 preference values rather than metrics (since metrics can only be
 compared with metrics of other routes in the same routing domain,
 the metric of a static route could only be compared with metrics
 of other static routes). This is contrary to some current
 implementations, where static routes really do have metrics, and
 those metrics are used to determine whether a particular dynamic
 route overrides the static route to the same destination. Thus,
 this document uses the term metric rather than preference.
 This technique essentially makes the static route into a RIP
 route, or an OSPF route (or whatever, depending on the domain of
 the metric). Thus, the route lookup algorithm of that domain
 applies. However, this is NOT route leaking, in that coercing a
 static route into a dynamic routing domain does not authorize the
 router to redistribute the route into the dynamic routing domain.
 For static routes not put into a specific routing domain, the
 route lookup algorithm is:
 (1) Basic match
 (2) Longest match
 (3) Weak TOS (if TOS supported)
 (4) Best metric (where metric are implementation-defined)
 The last step may not be necessary, but it's useful in the case
 where you want to have a primary static route over one interface
 and a secondary static route over an alternate interface, with
 failover to the alternate path if the interface for the primary
 route fails.
7.5 FILTERING OF ROUTING INFORMATION
 Each router within a network makes forwarding decisions based upon
 information contained within its forwarding database. In a simple
 network the contents of the database may be configured statically.
 As the network grows more complex, the need for dynamic updating of
 the forwarding database becomes critical to the efficient operation
 of the network.
 If the data flow through a network is to be as efficient as possible,
 it is necessary to provide a mechanism for controlling the
 propagation of the information a router uses to build its forwarding
 database. This control takes the form of choosing which sources of
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 routing information should be trusted and selecting which pieces of
 the information to believe. The resulting forwarding database is a
 filtered version of the available routing information.
 In addition to efficiency, controlling the propagation of routing
 information can reduce instability by preventing the spread of
 incorrect or bad routing information.
 In some cases local policy may require that complete routing
 information not be widely propagated.
 These filtering requirements apply only to non-SPF-based protocols
 (and therefore not at all to routers which don't implement any
 distance vector protocols).
7.5.1 Route Validation
 A router SHOULD log as an error any routing update advertising a
 route that violates the specifications of this memo, unless the
 routing protocol from which the update was received uses those values
 to encode special routes (such as default routes).
7.5.2 Basic Route Filtering
 Filtering of routing information allows control of paths used by a
 router to forward packets it receives. A router should be selective
 in which sources of routing information it listens to and what routes
 it believes. Therefore, a router MUST provide the ability to
 specify:
 o On which logical interfaces routing information will be accepted
 and which routes will be accepted from each logical interface.
 o Whether all routes or only a default route is advertised on a
 logical interface.
 Some routing protocols do not recognize logical interfaces as a
 source of routing information. In such cases the router MUST provide
 the ability to specify
 o from which other routers routing information will be accepted.
 For example, assume a router connecting one or more leaf networks to
 the main portion or backbone of a larger network. Since each of the
 leaf networks has only one path in and out, the router can simply
 send a default route to them. It advertises the leaf networks to the
 main network.
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7.5.3 Advanced Route Filtering
 As the topology of a network grows more complex, the need for more
 complex route filtering arises. Therefore, a router SHOULD provide
 the ability to specify independently for each routing protocol:
 o Which logical interfaces or routers routing information (routes)
 will be accepted from and which routes will be believed from each
 other router or logical interface,
 o Which routes will be sent via which logical interface(s), and
 o Which routers routing information will be sent to, if this is
 supported by the routing protocol in use.
 In many situations it is desirable to assign a reliability ordering
 to routing information received from another router instead of the
 simple believe or don't believe choice listed in the first bullet
 above. A router MAY provide the ability to specify:
 o A reliability or preference to be assigned to each route received.
 A route with higher reliability will be chosen over one with lower
 reliability regardless of the routing metric associated with each
 route.
 If a router supports assignment of preferences, the router MUST NOT
 propagate any routes it does not prefer as first party information.
 If the routing protocol being used to propagate the routes does not
 support distinguishing between first and third party information, the
 router MUST NOT propagate any routes it does not prefer.
 DISCUSSION
 For example, assume a router receives a route to network C from
 router R and a route to the same network from router S. If router
 R is considered more reliable than router S traffic destined for
 network C will be forwarded to router R regardless of the route
 received from router S.
 Routing information for routes which the router does not use (router
 S in the above example) MUST NOT be passed to any other router.
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE
 Routers MUST be able to exchange routing information between separate
 IP interior routing protocols, if independent IP routing processes
 can run in the same router. Routers MUST provide some mechanism for
 avoiding routing loops when routers are configured for bi-directional
 exchange of routing information between two separate interior routing
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 processes. Routers MUST provide some priority mechanism for choosing
 routes from independent routing processes. Routers SHOULD provide
 administrative control of IGP-IGP exchange when used across
 administrative boundaries.
 Routers SHOULD provide some mechanism for translating or transforming
 metrics on a per network basis. Routers (or routing protocols) MAY
 allow for global preference of exterior routes imported into an IGP.
 DISCUSSION
 Different IGPs use different metrics, requiring some translation
 technique when introducing information from one protocol into
 another protocol with a different form of metric. Some IGPs can
 run multiple instances within the same router or set of routers.
 In this case metric information can be preserved exactly or
 translated.
 There are at least two techniques for translation between
 different routing processes. The static (or reachability)
 approach uses the existence of a route advertisement in one IGP to
 generate a route advertisement in the other IGP with a given
 metric. The translation or tabular approach uses the metric in
 one IGP to create a metric in the other IGP through use of either
 a function (such as adding a constant) or a table lookup.
 Bi-directional exchange of routing information is dangerous
 without control mechanisms to limit feedback. This is the same
 problem that distance vector routing protocols must address with
 the split horizon technique and that EGP addresses with the
 third-party rule. Routing loops can be avoided explicitly through
 use of tables or lists of permitted/denied routes or implicitly
 through use of a split horizon rule, a no-third-party rule, or a
 route tagging mechanism. Vendors are encouraged to use implicit
 techniques where possible to make administration easier for
 network operators.
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
 Note that this chapter supersedes any requirements stated under
 "REMOTE MANAGEMENT" in [INTRO:3].
8.1 The Simple Network Management Protocol - SNMP
8.1.1 SNMP Protocol Elements
 Routers MUST be manageable by SNMP [MGT:3]. The SNMP MUST operate
 using UDP/IP as its transport and network protocols. Others MAY be
 supported (e.g., see [MGT:25, MGT:26, MGT:27, and MGT:28]). SNMP
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 management operations MUST operate as if the SNMP was implemented on
 the router itself. Specifically, management operations MUST be
 effected by sending SNMP management requests to any of the IP
 addresses assigned to any of the router's interfaces. The actual
 management operation may be performed either by the router or by a
 proxy for the router.
 DISCUSSION
 This wording is intended to allow management either by proxy,
 where the proxy device responds to SNMP packets that have one of
 the router's IP addresses in the packets destination address
 field, or the SNMP is implemented directly in the router itself
 and receives packets and responds to them in the proper manner.
 It is important that management operations can be sent to one of
 the router's IP Addresses. In diagnosing network problems the
 only thing identifying the router that is available may be one of
 the router's IP address; obtained perhaps by looking through
 another router's routing table.
 All SNMP operations (get, get-next, get-response, set, and trap) MUST
 be implemented.
 Routers MUST provide a mechanism for rate-limiting the generation of
 SNMP trap messages. Routers MAY provide this mechanism through the
 algorithms for asynchronous alert management described in [MGT:5].
 DISCUSSION
 Although there is general agreement about the need to rate-limit
 traps, there is not yet consensus on how this is best achieved.
 The reference cited is considered experimental.
8.2 Community Table
 For the purposes of this specification, we assume that there is an
 abstract `community table' in the router. This table contains
 several entries, each entry for a specific community and containing
 the parameters necessary to completely define the attributes of that
 community. The actual implementation method of the abstract
 community table is, of course, implementation specific.
 A router's community table MUST allow for at least one entry and
 SHOULD allow for at least two entries.
 DISCUSSION
 A community table with zero capacity is useless. It means that
 the router will not recognize any communities and, therefore, all
 SNMP operations will be rejected.
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 Therefore, one entry is the minimal useful size of the table.
 Having two entries allows one entry to be limited to read-only
 access while the other would have write capabilities.
 Routers MUST allow the user to manually (i.e., without using SNMP)
 examine, add, delete and change entries in the SNMP community table.
 The user MUST be able to set the community name or construct a MIB
 view. The user MUST be able to configure communities as read-only
 (i.e., they do not allow SETs) or read-write (i.e., they do allow
 SETs).
 The user MUST be able to define at least one IP address to which
 notifications are sent for each community or MIB view, if traps are
 used. These addresses SHOULD be definable on a community or MIB view
 basis. It SHOULD be possible to enable or disable notifications on a
 community or MIB view basis.
 A router SHOULD provide the ability to specify a list of valid
 network managers for any particular community. If enabled, a router
 MUST validate the source address of the SNMP datagram against the
 list and MUST discard the datagram if its address does not appear.
 If the datagram is discarded the router MUST take all actions
 appropriate to an SNMP authentication failure.
 DISCUSSION
 This is a rather limited authentication system, but coupled with
 various forms of packet filtering may provide some small measure
 of increased security.
 The community table MUST be saved in non-volatile storage.
 The initial state of the community table SHOULD contain one entry,
 with the community name string public and read-only access. The
 default state of this entry MUST NOT send traps. If it is
 implemented, then this entry MUST remain in the community table until
 the administrator changes it or deletes it.
 DISCUSSION
 By default, traps are not sent to this community. Trap PDUs are
 sent to unicast IP addresses. This address must be configured
 into the router in some manner. Before the configuration occurs,
 there is no such address, so to whom should the trap be sent?
 Therefore trap sending to the public community defaults to be
 disabled. This can, of course, be changed by an administrative
 operation once the router is operational.
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8.3 Standard MIBS
 All MIBS relevant to a router's configuration are to be implemented.
 To wit:
 o The System, Interface, IP, ICMP, and UDP groups of MIB-II [MGT:2]
 MUST be implemented.
 o The Interface Extensions MIB [MGT:18] MUST be implemented.
 o The IP Forwarding Table MIB [MGT:20] MUST be implemented.
 o If the router implements TCP (e.g., for Telnet) then the TCP group
 of MIB-II [MGT:2] MUST be implemented.
 o If the router implements EGP then the EGP group of MIB-II [MGT:2]
 MUST be implemented.
 o If the router supports OSPF then the OSPF MIB [MGT:14] MUST be
 implemented.
 o If the router supports BGP then the BGP MIB [MGT:15] MUST be
 implemented.
 o If the router has Ethernet, 802.3, or StarLan interfaces then the
 Ethernet-Like MIB [MGT:6] MUST be implemented.
 o If the router has 802.4 interfaces then the 802.4 MIB [MGT:7] MUST
 be implemented.
 o If the router has 802.5 interfaces then the 802.5 MIB [MGT:8] MUST
 be implemented.
 o If the router has FDDI interfaces that implement ANSI SMT 7.3 then
 the FDDI MIB [MGT:9] MUST be implemented.
 o If the router has FDDI interfaces that implement ANSI SMT 6.2 then
 the FDDI MIB [MGT:29] MUST be implemented.
 o If the router has interfaces that use V.24 signalling, such as RS-
 232, V.10, V.11, V.35, V.36, or RS-422/423/449, then the RS-232
 [MGT:10] MIB MUST be implemented.
 o If the router has T1/DS1 interfaces then the T1/DS1 MIB [MGT:16]
 MUST be implemented.
 o If the router has T3/DS3 interfaces then the T3/DS3 MIB [MGT:17]
 MUST be implemented.
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 o If the router has SMDS interfaces then the SMDS Interface Protocol
 MIB [MGT:19] MUST be implemented.
 o If the router supports PPP over any of its interfaces then the PPP
 MIBs [MGT:11], [MGT:12], and [MGT:13] MUST be implemented.
 o If the router supports RIP Version 2 then the RIP Version 2 MIB
 [MGT:21] MUST be implemented.
 o If the router supports X.25 over any of its interfaces then the
 X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be implemented.
8.4 Vendor Specific MIBS
 The Internet Standard and Experimental MIBs do not cover the entire
 range of statistical, state, configuration and control information
 that may be available in a network element. This information is,
 nevertheless, extremely useful. Vendors of routers (and other
 network devices) generally have developed MIB extensions that cover
 this information. These MIB extensions are called Vendor Specific
 MIBs.
 The Vendor Specific MIB for the router MUST provide access to all
 statistical, state, configuration, and control information that is
 not available through the Standard and Experimental MIBs that have
 been implemented. This information MUST be available for both
 monitoring and control operations.
 DISCUSSION
 The intent of this requirement is to provide the ability to do
 anything on the router through SNMP that can be done through a
 console, and vice versa. A certain minimal amount of
 configuration is necessary before SNMP can operate (e.g., the
 router must have an IP address). This initial configuration can
 not be done through SNMP. However, once the initial configuration
 is done, full capabilities ought to be available through network
 management.
 The vendor SHOULD make available the specifications for all Vendor
 Specific MIB variables. These specifications MUST conform to the SMI
 [MGT:1] and the descriptions MUST be in the form specified in
 [MGT:4].
 DISCUSSION
 Making the Vendor Specific MIB available to the user is necessary.
 Without this information the users would not be able to configure
 their network management systems to be able to access the Vendor
 Specific parameters. These parameters would then be useless.
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 ne 2 The format of the MIB specification is also specified.
 Parsers that read MIB specifications and generate the needed
 tables for the network management station are available. These
 parsers generally understand only the standard MIB specification
 format.
8.5 Saving Changes
 Parameters altered by SNMP MAY be saved to non-volatile storage.
 DISCUSSION
 Reasons why this requirement is a MAY:
 o The exact physical nature of non-volatile storage is not
 specified in this document. Hence, parameters may be saved in
 NVRAM/EEPROM, local floppy or hard disk, or in some TFTP file
 server or BOOTP server, etc. Suppose that this information is
 in a file that is retrieved through TFTP. In that case, a
 change made to a configuration parameter on the router would
 need to be propagated back to the file server holding the
 configuration file. Alternatively, the SNMP operation would
 need to be directed to the file server, and then the change
 somehow propagated to the router. The answer to this problem
 does not seem obvious.
 This also places more requirements on the host holding the
 configuration information than just having an available TFTP
 server, so much more that its probably unsafe for a vendor to
 assume that any potential customer will have a suitable host
 available.
 o The timing of committing changed parameters to non-volatile
 storage is still an issue for debate. Some prefer to commit
 all changes immediately. Others prefer to commit changes to
 non-volatile storage only upon an explicit command.
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS
 For all additional application protocols that a router implements,
 the router MUST be compliant and SHOULD be unconditionally compliant
 with the relevant requirements of [INTRO:3].
9.1 BOOTP
9.1.1 Introduction
 The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol that allows
 a booting host to configure itself dynamically and without user
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 supervision. BOOTP provides a means to notify a host of its assigned
 IP address, the IP address of a boot server host, and the name of a
 file to be loaded into memory and executed ([APPL:1]). Other
 configuration information such as the local prefix length or subnet
 mask, the local time offset, the addresses of default routers, and
 the addresses of various Internet servers can also be communicated to
 a host using BOOTP ([APPL:2]).
9.1.2 BOOTP Relay Agents
 In many cases, BOOTP clients and their associated BOOTP server(s) do
 not reside on the same IP (sub)network. In such cases, a third-party
 agent is required to transfer BOOTP messages between clients and
 servers. Such an agent was originally referred to as a BOOTP
 forwarding agent. However, to avoid confusion with the IP forwarding
 function of a router, the name BOOTP relay agent has been adopted
 instead.
 DISCUSSION
 A BOOTP relay agent performs a task that is distinct from a
 router's normal IP forwarding function. While a router normally
 switches IP datagrams between networks more-or-less transparently,
 a BOOTP relay agent may more properly be thought to receive BOOTP
 messages as a final destination and then generate new BOOTP
 messages as a result. One should resist the notion of simply
 forwarding a BOOTP message straight through like a regular packet.
 This relay-agent functionality is most conveniently located in the
 routers that interconnect the clients and servers (although it may
 alternatively be located in a host that is directly connected to the
 client (sub)net).
 A router MAY provide BOOTP relay-agent capability. If it does, it
 MUST conform to the specifications in [APPL:3].
 Section [5.2.3] discussed the circumstances under which a packet is
 delivered locally (to the router). All locally delivered UDP
 messages whose UDP destination port number is BOOTPS (67) are
 considered for special processing by the router's logical BOOTP relay
 agent.
 Sections [4.2.2.11] and [5.3.7] discussed invalid IP source
 addresses. According to these rules, a router must not forward any
 received datagram whose IP source address is 0.0.0.0. However,
 routers that support a BOOTP relay agent MUST accept for local
 delivery to the relay agent BOOTREQUEST messages whose IP source
 address is 0.0.0.0.
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10. OPERATIONS AND MAINTENANCE
 This chapter supersedes any requirements of [INTRO:3] relating to
 "Extensions to the IP Module."
 Facilities to support operation and maintenance (O&M) activities form
 an essential part of any router implementation. Although these
 functions do not seem to relate directly to interoperability, they
 are essential to the network manager who must make the router
 interoperate and must track down problems when it doesn't. This
 chapter also includes some discussion of router initialization and of
 facilities to assist network managers in securing and accounting for
 their networks.
10.1 Introduction
 The following kinds of activities are included under router O&M:
 o Diagnosing hardware problems in the router's processor, in its
 network interfaces, or in its connected networks, modems, or
 communication lines.
 o Installing new hardware
 o Installing new software.
 o Restarting or rebooting the router after a crash.
 o Configuring (or reconfiguring) the router.
 o Detecting and diagnosing Internet problems such as congestion,
 routing loops, bad IP addresses, black holes, packet avalanches,
 and misbehaved hosts.
 o Changing network topology, either temporarily (e.g., to bypass a
 communication line problem) or permanently.
 o Monitoring the status and performance of the routers and the
 connected networks.
 o Collecting traffic statistics for use in (Inter-)network planning.
 o Coordinating the above activities with appropriate vendors and
 telecommunications specialists.
 Routers and their connected communication lines are often operated as
 a system by a centralized O&M organization. This organization may
 maintain a (Inter-)network operation center, or NOC, to carry out its
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 O&M functions. It is essential that routers support remote control
 and monitoring from such a NOC through an Internet path, since
 routers might not be connected to the same network as their NOC.
 Since a network failure may temporarily preclude network access, many
 NOCs insist that routers be accessible for network management through
 an alternative means, often dial-up modems attached to console ports
 on the routers.
 Since an IP packet traversing an internet will often use routers
 under the control of more than one NOC, Internet problem diagnosis
 will often involve cooperation of personnel of more than one NOC. In
 some cases, the same router may need to be monitored by more than one
 NOC, but only if necessary, because excessive monitoring could impact
 a router's performance.
 The tools available for monitoring at a NOC may cover a wide range of
 sophistication. Current implementations include multi-window,
 dynamic displays of the entire router system. The use of AI
 techniques for automatic problem diagnosis is proposed for the
 future.
 Router O&M facilities discussed here are only a part of the large and
 difficult problem of Internet management. These problems encompass
 not only multiple management organizations, but also multiple
 protocol layers. For example, at the current stage of evolution of
 the Internet architecture, there is a strong coupling between host
 TCP implementations and eventual IP-level congestion in the router
 system [OPER:1]. Therefore, diagnosis of congestion problems will
 sometimes require the monitoring of TCP statistics in hosts. There
 are currently a number of R&D efforts in progress in the area of
 Internet management and more specifically router O&M. These R&D
 efforts have already produced standards for router O&M. This is also
 an area in which vendor creativity can make a significant
 contribution.
10.2 Router Initialization
10.2.1 Minimum Router Configuration
 There exists a minimum set of conditions that must be satisfied
 before a router may forward packets. A router MUST NOT enable
 forwarding on any physical interface unless either:
 (1) The router knows the IP address and associated subnet mask or
 network prefix length of at least one logical interface
 associated with that physical interface, or
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 (2) The router knows that the interface is an unnumbered interface
 and knows its router-id.
 These parameters MUST be explicitly configured:
 o A router MUST NOT use factory-configured default values for its IP
 addresses, prefix lengths, or router-id, and
 o A router MUST NOT assume that an unconfigured interface is an
 unnumbered interface.
 DISCUSSION
 There have been instances in which routers have been shipped with
 vendor-installed default addresses for interfaces. In a few
 cases, this has resulted in routers advertising these default
 addresses into active networks.
10.2.2 Address and Prefix Initialization
 A router MUST allow its IP addresses and their address masks or
 prefix lengths to be statically configured and saved in non-volatile
 storage.
 A router MAY obtain its IP addresses and their corresponding address
 masks dynamically as a side effect of the system initialization
 process (see Section 10.2.3]);
 If the dynamic method is provided, the choice of method to be used in
 a particular router MUST be configurable.
 As was described in Section [4.2.2.11], IP addresses are not
 permitted to have the value 0 or -1 in the <Host-number> or
 <Network-prefix> fields. Therefore, a router SHOULD NOT allow an IP
 address or address mask to be set to a value that would make any of
 the these fields above have the value zero or -1.
 DISCUSSION
 It is possible using arbitrary address masks to create situations
 in which routing is ambiguous (i.e., two routes with different but
 equally specific subnet masks match a particular destination
 address). This is one of the strongest arguments for the use of
 network prefixes, and the reason the use of discontiguous subnet
 masks is not permitted.
 A router SHOULD make the following checks on any address mask it
 installs:
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 o The mask is neither all ones nor all zeroes (the prefix length is
 neither zero nor 32).
 o The bits which correspond to the network prefix part of the address
 are all set to 1.
 o The bits that correspond to the network prefix are contiguous.
 DISCUSSION
 The masks associated with routes are also sometimes called subnet
 masks, this test should not be applied to them.
10.2.3 Network Booting using BOOTP and TFTP
 There has been much discussion of how routers can and should be
 booted from the network. These discussions have revolved around
 BOOTP and TFTP. Currently, there are routers that boot with TFTP
 from the network. There is no reason that BOOTP could not be used
 for locating the server that the boot image should be loaded from.
 BOOTP is a protocol used to boot end systems, and requires some
 stretching to accommodate its use with routers. If a router is using
 BOOTP to locate the current boot host, it should send a BOOTP Request
 with its hardware address for its first interface, or, if it has been
 previously configured otherwise, with either another interface's
 hardware address, or another number to put in the hardware address
 field of the BOOTP packet. This is to allow routers without hardware
 addresses (like synchronous line only routers) to use BOOTP for
 bootload discovery. TFTP can then be used to retrieve the image
 found in the BOOTP Reply. If there are no configured interfaces or
 numbers to use, a router MAY cycle through the interface hardware
 addresses it has until a match is found by the BOOTP server.
 A router SHOULD IMPLEMENT the ability to store parameters learned
 through BOOTP into local non-volatile storage. A router MAY
 implement the ability to store a system image loaded over the network
 into local stable storage.
 A router MAY have a facility to allow a remote user to request that
 the router get a new boot image. Differentiation should be made
 between getting the new boot image from one of three locations: the
 one included in the request, from the last boot image server, and
 using BOOTP to locate a server.
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10.3 Operation and Maintenance
10.3.1 Introduction
 There is a range of possible models for performing O&M functions on a
 router. At one extreme is the local-only model, under which the O&M
 functions can only be executed locally (e.g., from a terminal plugged
 into the router machine). At the other extreme, the fully remote
 model allows only an absolute minimum of functions to be performed
 locally (e.g., forcing a boot), with most O&M being done remotely
 from the NOC. There are intermediate models, such as one in which
 NOC personnel can log into the router as a host, using the Telnet
 protocol, to perform functions that can also be invoked locally. The
 local-only model may be adequate in a few router installations, but
 remote operation from a NOC is normally required, and therefore
 remote O&M provisions are required for most routers.
 Remote O&M functions may be exercised through a control agent
 (program). In the direct approach, the router would support remote
 O&M functions directly from the NOC using standard Internet protocols
 (e.g., SNMP, UDP or TCP); in the indirect approach, the control agent
 would support these protocols and control the router itself using
 proprietary protocols. The direct approach is preferred, although
 either approach is acceptable. The use of specialized host hardware
 and/or software requiring significant additional investment is
 discouraged; nevertheless, some vendors may elect to provide the
 control agent as an integrated part of the network in which the
 routers are a part. If this is the case, it is required that a means
 be available to operate the control agent from a remote site using
 Internet protocols and paths and with equivalent functionality with
 respect to a local agent terminal.
 It is desirable that a control agent and any other NOC software tools
 that a vendor provides operate as user programs in a standard
 operating system. The use of the standard Internet protocols UDP and
 TCP for communicating with the routers should facilitate this.
 Remote router monitoring and (especially) remote router control
 present important access control problems that must be addressed.
 Care must also be taken to ensure control of the use of router
 resources for these functions. It is not desirable to let router
 monitoring take more than some limited fraction of the router CPU
 time, for example. On the other hand, O&M functions must receive
 priority so they can be exercised when the router is congested, since
 often that is when O&M is most needed.
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10.3.2 Out Of Band Access
 Routers MUST support Out-Of-Band (OOB) access. OOB access SHOULD
 provide the same functionality as in-band access. This access SHOULD
 implement access controls, to prevent unauthorized access.
 DISCUSSION
 This Out-Of-Band access will allow the NOC a way to access
 isolated routers during times when network access is not
 available.
 Out-Of-Band access is an important management tool for the network
 administrator. It allows the access of equipment independent of
 the network connections. There are many ways to achieve this
 access. Whichever one is used it is important that the access is
 independent of the network connections. An example of Out-Of-Band
 access would be a serial port connected to a modem that provides
 dial up access to the router.
 It is important that the OOB access provides the same
 functionality as in-band access. In-band access, or accessing
 equipment through the existing network connection, is limiting,
 because most of the time, administrators need to reach equipment
 to figure out why it is unreachable. In band access is still very
 important for configuring a router, and for troubleshooting more
 subtle problems.
10.3.2 Router O&M Functions
10.3.2.1 Maintenance - Hardware Diagnosis
 Each router SHOULD operate as a stand-alone device for the purposes
 of local hardware maintenance. Means SHOULD be available to run
 diagnostic programs at the router site using only on-site tools. A
 router SHOULD be able to run diagnostics in case of a fault. For
 suggested hardware and software diagnostics see Section [10.3.3].
10.3.2.2 Control - Dumping and Rebooting
 A router MUST include both in-band and out-of-band mechanisms to
 allow the network manager to reload, stop, and restart the router. A
 router SHOULD also contain a mechanism (such as a watchdog timer)
 which will reboot the router automatically if it hangs due to a
 software or hardware fault.
 A router SHOULD IMPLEMENT a mechanism for dumping the contents of a
 router's memory (and/or other state useful for vendor debugging after
 a crash), and either saving them on a stable storage device local to
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 the router or saving them on another host via an up-line dump
 mechanism such as TFTP (see [OPER:2], [INTRO:3]).
10.3.2.3 Control - Configuring the Router
 Every router has configuration parameters that may need to be set.
 It SHOULD be possible to update the parameters without rebooting the
 router; at worst, a restart MAY be required. There may be cases when
 it is not possible to change parameters without rebooting the router
 (for instance, changing the IP address of an interface). In these
 cases, care should be taken to minimize disruption to the router and
 the surrounding network.
 There SHOULD be a way to configure the router over the network either
 manually or automatically. A router SHOULD be able to upload or
 download its parameters from a host or another router. A means
 SHOULD be provided, either as an application program or a router
 function, to convert between the parameter format and a human-
 editable format. A router SHOULD have some sort of stable storage
 for its configuration. A router SHOULD NOT believe protocols such as
 RARP, ICMP Address Mask Reply, and MAY not believe BOOTP.
 DISCUSSION
 It is necessary to note here that in the future RARP, ICMP Address
 Mask Reply, BOOTP and other mechanisms may be needed to allow a
 router to auto-configure. Although routers may in the future be
 able to configure automatically, the intent here is to discourage
 this practice in a production environment until auto-configuration
 has been tested more thoroughly. The intent is NOT to discourage
 auto-configuration all together. In cases where a router is
 expected to get its configuration automatically it may be wise to
 allow the router to believe these things as it comes up and then
 ignore them after it has gotten its configuration.
10.3.2.4 Net Booting of System Software
 A router SHOULD keep its system image in local non-volatile
 storage such as PROM, NVRAM, or disk. It MAY also be able to load
 its system software over the network from a host or another
 router.
 A router that can keep its system image in local non-volatile
 storage MAY be configurable to boot its system image over the
 network. A router that offers this option SHOULD be configurable
 to boot the system image in its non-volatile local storage if it
 is unable to boot its system image over the network.
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 DISCUSSION
 It is important that the router be able to come up and run on its
 own. NVRAM may be a particular solution for routers used in large
 networks, since changing PROMs can be quite time consuming for a
 network manager responsible for numerous or geographically
 dispersed routers. It is important to be able to netboot the
 system image because there should be an easy way for a router to
 get a bug fix or new feature more quickly than getting PROMs
 installed. Also if the router has NVRAM instead of PROMs, it will
 netboot the image and then put it in NVRAM.
 Routers SHOULD perform some basic consistency check on any image
 loaded, to detect and perhaps prevent incorrect images.
 A router MAY also be able to distinguish between different
 configurations based on which software it is running. If
 configuration commands change from one software version to another,
 it would be helpful if the router could use the configuration that
 was compatible with the software.
10.3.2.5 Detecting and responding to misconfiguration
 There MUST be mechanisms for detecting and responding to
 misconfigurations. If a command is executed incorrectly, the router
 SHOULD give an error message. The router SHOULD NOT accept a poorly
 formed command as if it were correct.
 DISCUSSION
 There are cases where it is not possible to detect errors: the
 command is correctly formed, but incorrect with respect to the
 network. This may be detected by the router, but may not be
 possible.
 Another form of misconfiguration is misconfiguration of the network
 to which the router is attached. A router MAY detect
 misconfigurations in the network. The router MAY log these findings
 to a file, either on the router or a host, so that the network
 manager will see that there are possible problems on the network.
 DISCUSSION
 Examples of such misconfigurations might be another router with
 the same address as the one in question or a router with the wrong
 address mask. If a router detects such problems it is probably
 not the best idea for the router to try to fix the situation.
 That could cause more harm than good.
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10.3.2.6 Minimizing Disruption
 Changing the configuration of a router SHOULD have minimal affect on
 the network. Routing tables SHOULD NOT be unnecessarily flushed when
 a simple change is made to the router. If a router is running
 several routing protocols, stopping one routing protocol SHOULD NOT
 disrupt other routing protocols, except in the case where one network
 is learned by more than one routing protocol.
 DISCUSSION
 It is the goal of a network manager to run a network so that users
 of the network get the best connectivity possible. Reloading a
 router for simple configuration changes can cause disruptions in
 routing and ultimately cause disruptions to the network and its
 users. If routing tables are unnecessarily flushed, for instance,
 the default route will be lost as well as specific routes to sites
 within the network. This sort of disruption will cause
 significant downtime for the users. It is the purpose of this
 section to point out that whenever possible, these disruptions
 should be avoided.
10.3.2.7 Control - Troubleshooting Problems
 (1) A router MUST provide in-band network access, but (except as
 required by Section [8.2]) for security considerations this
 access SHOULD be disabled by default. Vendors MUST document
 the default state of any in-band access. This access SHOULD
 implement access controls, to prevent unauthorized access.
 DISCUSSION
 In-band access primarily refers to access through the normal
 network protocols that may or may not affect the permanent
 operational state of the router. This includes, but is not
 limited to Telnet/RLOGIN console access and SNMP operations.
 This was a point of contention between the operational out of the
 box and secure out of The box contingents. Any automagic access
 to the router may introduce insecurities, but it may be more
 important for the customer to have a router that is accessible
 over the network as soon as it is plugged in. At least one vendor
 supplies routers without any external console access and depends
 on being able to access the router through the network to complete
 its configuration.
 It is the vendors call whether in-band access is enabled by
 default; but it is also the vendor's responsibility to make its
 customers aware of possible insecurities.
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 (2) A router MUST provide the ability to initiate an ICMP echo.
 The following options SHOULD be implemented:
 o Choice of data patterns
 o Choice of packet size
 o Record route
 and the following additional options MAY be implemented:
 o Loose source route
 o Strict source route
 o Timestamps
 (3) A router SHOULD provide the ability to initiate a traceroute.
 If traceroute is provided, then the 3rd party traceroute
 SHOULD be implemented.
 Each of the above three facilities (if implemented) SHOULD have
 access restrictions placed on it to prevent its abuse by unauthorized
 persons.
10.4 Security Considerations
10.4.1 Auditing and Audit Trails
 Auditing and billing are the bane of the network operator, but are
 the two features most requested by those in charge of network
 security and those who are responsible for paying the bills. In the
 context of security, auditing is desirable if it helps you keep your
 network working and protects your resources from abuse, without
 costing you more than those resources are worth.
 (1) Configuration Changes
 Router SHOULD provide a method for auditing a configuration
 change of a router, even if it's something as simple as
 recording the operator's initials and time of change.
 DISCUSSION
 Configuration change logging (who made a configuration change,
 what was changed, and when) is very useful, especially when
 traffic is suddenly routed through Alaska on its way across town.
 So is the ability to revert to a previous configuration.
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 (2) Packet Accounting
 Vendors should strongly consider providing a system for
 tracking traffic levels between pairs of hosts or networks.
 A mechanism for limiting the collection of this information
 to specific pairs of hosts or networks is also strongly
 encouraged.
 DISCUSSION
 A host traffic matrix as described above can give the network
 operator a glimpse of traffic trends not apparent from other
 statistics. It can also identify hosts or networks that are
 probing the structure of the attached networks - e.g., a single
 external host that tries to send packets to every IP address in
 the network address range for a connected network.
 (3) Security Auditing
 Routers MUST provide a method for auditing security related
 failures or violations to include:
 o Authorization Failures: bad passwords, invalid SNMP
 communities, invalid authorization tokens,
 o Violations of Policy Controls: Prohibited Source Routes,
 Filtered Destinations, and
 o Authorization Approvals: good passwords - Telnet in-band
 access, console access.
 Routers MUST provide a method of limiting or disabling such
 auditing but auditing SHOULD be on by default. Possible
 methods for auditing include listing violations to a console
 if present, logging or counting them internally, or logging
 them to a remote security server through the SNMP trap
 mechanism or the Unix logging mechanism as appropriate. A
 router MUST implement at least one of these reporting
 mechanisms - it MAY implement more than one.
10.4.2 Configuration Control
 A vendor has a responsibility to use good configuration control
 practices in the creation of the software/firmware loads for their
 routers. In particular, if a vendor makes updates and loads
 available for retrieval over the Internet, the vendor should also
 provide a way for the customer to confirm the load is a valid one,
 perhaps by the verification of a checksum over the load.
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 DISCUSSION
 Many vendors currently provide short notice updates of their
 software products through the Internet. This a good trend and
 should be encouraged, but provides a point of vulnerability in the
 configuration control process.
 If a vendor provides the ability for the customer to change the
 configuration parameters of a router remotely, for example through a
 Telnet session, the ability to do so SHOULD be configurable and
 SHOULD default to off. The router SHOULD require valid
 authentication before permitting remote reconfiguration. This
 authentication procedure SHOULD NOT transmit the authentication
 secret over the network. For example, if telnet is implemented, the
 vendor SHOULD IMPLEMENT Kerberos, S-Key, or a similar authentication
 procedure.
 DISCUSSION
 Allowing your properly identified network operator to twiddle with
 your routers is necessary; allowing anyone else to do so is
 foolhardy.
 A router MUST NOT have undocumented back door access and master
 passwords. A vendor MUST ensure any such access added for purposes
 of debugging or product development are deleted before the product is
 distributed to its customers.
 DISCUSSION
 A vendor has a responsibility to its customers to ensure they are
 aware of the vulnerabilities present in its code by intention -
 e.g., in-band access. Trap doors, back doors and master passwords
 intentional or unintentional can turn a relatively secure router
 into a major problem on an operational network. The supposed
 operational benefits are not matched by the potential problems.
11. REFERENCES
 Implementors should be aware that Internet protocol standards are
 occasionally updated. These references are current as of this
 writing, but a cautious implementor will always check a recent
 version of the RFC index to ensure that an RFC has not been updated
 or superseded by another, more recent RFC. Reference [INTRO:6]
 explains various ways to obtain a current RFC index.
 APPL:1.
 Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC
 951, Stanford University, Sun Microsystems, September 1985.
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 APPL:2.
 Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
 Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
 University, October 1993.
 APPL:3.
 Wimer, W., "Clarifications and Extensions for the Bootstrap
 Protocol", RFC 1542, Carnegie Mellon University, October 1993.
 ARCH:1.
 DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006 (three
 volumes), DDN Network Information Center, SRI International,
 Menlo Park, California, USA, December 1985.
 ARCH:2.
 V. Cerf and R. Kahn, "A Protocol for Packet Network
 Intercommunication", IEEE Transactions on Communication, May
 1974. Also included in [ARCH:1].
 ARCH:3.
 J. Postel, C. Sunshine, and D. Cohen, "The ARPA Internet
 Protocol", Computer Networks, volume 5, number 4, July 1981.
 Also included in [ARCH:1].
 ARCH:4.
 B. Leiner, J. Postel, R. Cole, and D. Mills, :The DARPA
 Internet Protocol Suite", Proceedings of INFOCOM '85, IEEE,
 Washington, DC, March 1985. Also in: IEEE Communications
 Magazine, March 1985. Also available from the Information
 Sciences Institute, University of Southern California as
 Technical Report ISI-RS-85-153.
 ARCH:5.
 D. Comer, "Internetworking With TCP/IP Volume 1: Principles,
 Protocols, and Architecture", Prentice Hall, Englewood Cliffs,
 NJ, 1991.
 ARCH:6.
 W. Stallings, "Handbook of Computer-Communications Standards
 Volume 3: The TCP/IP Protocol Suite", Macmillan, New York, NY,
 1990.
 ARCH:7.
 Postel, J., "Internet Official Protocol Standards", STD 1, RFC
 1780, Internet Architecture Board, March 1995.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 ARCH:8.
 Information processing systems - Open Systems Interconnection -
 Basic Reference Model, ISO 7489, International Standards
 Organization, 1984.
 ARCH:9
 R. Braden, J. Postel, Y. Rekhter, "Internet Architecture
 Extensions for Shared Media", 05/20/1994
 FORWARD:1.
 IETF CIP Working Group (C. Topolcic, Editor), "Experimental
 Internet Stream Protocol", Version 2 (ST-II), RFC 1190, October
 1990.
 FORWARD:2.
 Mankin, A., and K. Ramakrishnan, Editors, "Gateway Congestion
 Control Survey", RFC 1254, MITRE, Digital Equipment Corporation,
 August 1991.
 FORWARD:3.
 J. Nagle, "On Packet Switches with Infinite Storage", IEEE
 Transactions on Communications, volume COM-35, number 4, April
 1987.
 FORWARD:4.
 R. Jain, K. Ramakrishnan, and D. Chiu, "Congestion Avoidance
 in Computer Networks With a Connectionless Network Layer",
 Technical Report DEC-TR-506, Digital Equipment Corporation.
 FORWARD:5.
 V. Jacobson, "Congestion Avoidance and Control", Proceedings of
 SIGCOMM '88, Association for Computing Machinery, August 1988.
 FORWARD:6.
 W. Barns, "Precedence and Priority Access Implementation for
 Department of Defense Data Networks", Technical Report MTR-
 91W00029, The Mitre Corporation, McLean, Virginia, USA, July
 1991.
 FORWARD:7
 Fang, Chen, Hutchins, "Simulation Results of TCP Performance
 over ATM with and without Flow Control", presentation to the ATM
 Forum, November 15, 1993.
 FORWARD:8
 V. Paxson, S. Floyd "Wide Area Traffic: the Failure of Poisson
 Modeling", short version in SIGCOMM '94.
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 FORWARD:9
 Leland, Taqqu, Willinger and Wilson, "On the Self-Similar Nature
 of Ethernet Traffic", Proceedings of SIGCOMM '93, September,
 1993.
 FORWARD:10
 S. Keshav "A Control Theoretic Approach to Flow Control",
 SIGCOMM 91, pages 3-16
 FORWARD:11
 K.K. Ramakrishnan and R. Jain, "A Binary Feedback Scheme for
 Congestion Avoidance in Computer Networks", ACM Transactions of
 Computer Systems, volume 8, number 2, 1980.
 FORWARD:12
 H. Kanakia, P. Mishara, and A. Reibman]. "An adaptive
 congestion control scheme for real-time packet video transport",
 In Proceedings of ACM SIGCOMM 1994, pages 20-31, San Francisco,
 California, September 1993.
 FORWARD:13
 A. Demers, S. Keshav, S. Shenker, "Analysis and Simulation of
 a Fair Queuing Algorithm",
 93 pages 1-12
 FORWARD:14
 Clark, D., Shenker, S., and L. Zhang, "Supporting Real-Time
 Applications in an Integrated Services Packet Network:
 Architecture and Mechanism", 92 pages 14-26
 INTERNET:1.
 Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
 Sciences Institute, September 1981.
 INTERNET:2.
 Mogul, J., and J. Postel, "Internet Standard Subnetting
 Procedure", STD 5, RFC 950, Stanford, USC/Information Sciences
 Institute, August 1985.
 INTERNET:3.
 Mogul, J., "Broadcasting Internet Datagrams in the Presence of
 Subnets", STD 5, RFC 922, Stanford University, October 1984.
 INTERNET:4.
 Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
 1112, Stanford University, August 1989.
Baker Standards Track [Page 136]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 INTERNET:5.
 Kent, S., "U.S. Department of Defense Security Options for the
 Internet Protocol", RFC 1108, BBN Communications, November 1991.
 INTERNET:6.
 Braden, R., Borman, D., and C. Partridge, "Computing the
 Internet Checksum", RFC 1071, USC/Information Sciences
 Institute, Cray Research, BBN Communications, September 1988.
 INTERNET:7.
 Mallory T., and A. Kullberg, "Incremental Updating of the
 Internet Checksum", RFC 1141, BBN Communications, January 1990.
 INTERNET:8.
 Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
 USC/Information Sciences Institute, September 1981.
 INTERNET:9.
 A. Mankin, G. Hollingsworth, G. Reichlen, K. Thompson, R.
 Wilder, and R. Zahavi, "Evaluation of Internet Performance -
 FY89", Technical Report MTR-89W00216, MITRE Corporation,
 February, 1990.
 INTERNET:10.
 G. Finn, A "Connectionless Congestion Control Algorithm",
 Computer Communications Review, volume 19, number 5, Association
 for Computing Machinery, October 1989.
 INTERNET:11.
 Prue, W., and J. Postel, "The Source Quench Introduced Delay
 (SQuID)", RFC 1016, USC/Information Sciences Institute, August
 1987.
 INTERNET:12.
 McKenzie, A., "Some comments on SQuID", RFC 1018, BBN Labs,
 August 1987.
 INTERNET:13.
 Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
 PARC, September 1991.
 INTERNET:14.
 Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191,
 DECWRL, Stanford University, November 1990.
Baker Standards Track [Page 137]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 INTERNET:15
 Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
 Domain Routing (CIDR): an Address Assignment and Aggregation
 Strategy" RFC 1519, BARRNet, cisco, Merit, OARnet, September
 1993.
 INTERNET:16
 St. Johns, M., "Draft Revised IP Security Option", RFC 1038,
 IETF, January 1988.
 INTERNET:17
 Prue, W., and J. Postel, "Queuing Algorithm to Provide Type-
 of-service For IP Links", RFC 1046, USC/Information Sciences
 Institute, February 1988.
 INTERNET:18
 Postel, J., "Address Mappings", RFC 796, USC/Information
 Sciences Institute, September 1981.
 INTRO:1.
 Braden, R., and J. Postel, "Requirements for Internet
 Gateways", STD 4, RFC 1009, USC/Information Sciences Institute,
 June 1987.
 INTRO:2.
 Internet Engineering Task Force (R. Braden, Editor),
 "Requirements for Internet Hosts - Communication Layers", STD 3,
 RFC 1122, USC/Information Sciences Institute, October 1989.
 INTRO:3.
 Internet Engineering Task Force (R. Braden, Editor),
 "Requirements for Internet Hosts - Application and Support", STD
 3, RFC 1123, USC/Information Sciences Institute, October 1989.
 INTRO:4.
 Clark, D., "Modularity and Efficiency in Protocol
 Implementations", RFC 817, MIT Laboratory for Computer Science,
 July 1982.
 INTRO:5.
 Clark, D., "The Structuring of Systems Using Upcalls",
 Proceedings of 10th ACM SOSP, December 1985.
 INTRO:6.
 Jacobsen, O., and J. Postel, "Protocol Document Order
 Information", RFC 980, SRI, USC/Information Sciences Institute,
 March 1986.
Baker Standards Track [Page 138]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 INTRO:7.
 Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
 1700, USC/Information Sciences Institute, October 1994. This
 document is periodically updated and reissued with a new number.
 It is wise to verify occasionally that the version you have is
 still current.
 INTRO:8.
 DoD Trusted Computer System Evaluation Criteria, DoD publication
 5200.28-STD, U.S. Department of Defense, December 1985.
 INTRO:9
 Malkin, G., and T. LaQuey Parker, Editors, "Internet Users'
 Glossary", FYI 18, RFC 1392, Xylogics, Inc., UTexas, January
 1993.
 LINK:1.
 Leffler, S., and M. Karels, "Trailer Encapsulations", RFC 893,
 University of California at Berkeley, April 1984.
 LINK:2
 Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
 1661, Daydreamer July 1994.
 LINK:3
 McGregor, G., "The PPP Internet Protocol Control Protocol
 (IPCP)", RFC 1332, Merit May 1992.
 LINK:4
 Lloyd, B., and W. Simpson, "PPP Authentication Protocols", RFC
 1334, L&A, Daydreamer, May 1992.
 LINK:5
 Simpson, W., "PPP Link Quality Monitoring", RFC 1333,
 Daydreamer, May 1992.
 MGT:1.
 Rose, M., and K. McCloghrie, "Structure and Identification of
 Management Information of TCP/IP-based Internets", STD 16, RFC
 1155, Performance Systems International, Hughes LAN Systems, May
 1990.
 MGT:2.
 McCloghrie, K., and M. Rose (Editors), "Management Information
 Base of TCP/IP-Based Internets: MIB-II", STD 16, RFC 1213,
 Hughes LAN Systems, Inc., Performance Systems International,
 March 1991.
Baker Standards Track [Page 139]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 MGT:3.
 Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
 Network Management Protocol", STD 15, RFC 1157, SNMP Research,
 Performance Systems International, MIT Laboratory for Computer
 Science, May 1990.
 MGT:4.
 Rose, M., and K. McCloghrie (Editors), "Towards Concise MIB
 Definitions", STD 16, RFC 1212, Performance Systems
 International, Hughes LAN Systems, March 1991.
 MGT:5.
 Steinberg, L., "Techniques for Managing Asynchronously Generated
 Alerts", RFC 1224, IBM Corporation, May 1991.
 MGT:6.
 Kastenholz, F., "Definitions of Managed Objects for the
 Ethernet-like Interface Types", RFC 1398, FTP Software, Inc.,
 January 1993.
 MGT:7.
 McCloghrie, K., and R. Fox "IEEE 802.4 Token Bus MIB", RFC 1230,
 Hughes LAN Systems, Inc., Synoptics, Inc., May 1991.
 MGT:8.
 McCloghrie, K., Fox R., and E. Decker, "IEEE 802.5 Token Ring
 MIB", RFC 1231, Hughes LAN Systems, Inc., Synoptics, Inc., cisco
 Systems, Inc., February 1993.
 MGT:9.
 Case, J., and A. Rijsinghani, "FDDI Management Information
 Base", RFC 1512, The University of Tennesse and SNMP Research,
 Digital Equipment Corporation, September 1993.
 MGT:10.
 Stewart, B., Editor "Definitions of Managed Objects for RS-232-
 like Hardware Devices", RFC 1317, Xyplex, Inc., April 1992.
 MGT:11.
 Kastenholz, F., "Definitions of Managed Objects for the Link
 Control Protocol of the Point-to-Point Protocol", RFC 1471, FTP
 Software, Inc., June 1992.
 MGT:12.
 Kastenholz, F., "The Definitions of Managed Objects for the
 Security Protocols of the Point-to-Point Protocol", RFC 1472,
 FTP Software, Inc., June 1992.
Baker Standards Track [Page 140]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 MGT:13.
 Kastenholz, F., "The Definitions of Managed Objects for the IP
 Network Control Protocol of the Point-to-Point Protocol", RFC
 1473, FTP Software, Inc., June 1992.
 MGT:14.
 Baker, F., and R. Coltun, "OSPF Version 2 Management
 Information Base", RFC 1253, ACC, Computer Science Center,
 August 1991.
 MGT:15.
 Willis, S., and J. Burruss, "Definitions of Managed Objects for
 the Border Gateway Protocol (Version 3)", RFC 1269, Wellfleet
 Communications Inc., October 1991.
 MGT:16.
 Baker, F., and J. Watt, "Definitions of Managed Objects for the
 DS1 and E1 Interface Types", RFC 1406, Advanced Computer
 Communications, Newbridge Networks Corporation, January 1993.
 MGT:17.
 Cox, T., and K. Tesink, Editors "Definitions of Managed Objects
 for the DS3/E3 Interface Types", RFC 1407, Bell Communications
 Research, January 1993.
 MGT:18.
 McCloghrie, K., "Extensions to the Generic-Interface MIB", RFC
 1229, Hughes LAN Systems, August 1992.
 MGT:19.
 Cox, T., and K. Tesink, "Definitions of Managed Objects for the
 SIP Interface Type", RFC 1304, Bell Communications Research,
 February 1992.
 MGT:20
 Baker, F., "IP Forwarding Table MIB", RFC 1354, ACC, July 1992.
 MGT:21.
 Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
 1724, Xylogics, Inc., Cisco Systems, November 1994
 MGT:22.
 Throop, D., "SNMP MIB Extension for the X.25 Packet Layer", RFC
 1382, Data General Corporation, November 1992.
Baker Standards Track [Page 141]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 MGT:23.
 Throop, D., and F. Baker, "SNMP MIB Extension for X.25 LAPB",
 RFC 1381, Data General Corporation, ACC, November 1992.
 MGT:24.
 Throop, D., and F. Baker, "SNMP MIB Extension for MultiProtocol
 Interconnect over X.25", RFC 1461, Data General Corporation, May
 1993.
 MGT:25.
 Rose, M., "SNMP over OSI", RFC 1418, Dover Beach Consulting,
 Inc., March 1993.
 MGT:26.
 Minshall, G., and M. Ritter, "SNMP over AppleTalk", RFC 1419,
 Novell, Inc., Apple Computer, Inc., March 1993.
 MGT:27.
 Bostock, S., "SNMP over IPX", RFC 1420, Novell, Inc., March
 1993.
 MGT:28.
 Schoffstall, M., Davin, C., Fedor, M., and J. Case, "SNMP over
 Ethernet", RFC 1089, Rensselaer Polytechnic Institute, MIT
 Laboratory for Computer Science, NYSERNet, Inc., University of
 Tennessee at Knoxville, February 1989.
 MGT:29.
 Case, J., "FDDI Management Information Base", RFC 1285, SNMP
 Research, Incorporated, January 1992.
 OPER:1.
 Nagle, J., "Congestion Control in IP/TCP Internetworks", RFC
 896, FACC, January 1984.
 OPER:2.
 Sollins, K., "TFTP Protocol (revision 2)", RFC 1350, MIT, July
 1992.
 ROUTE:1.
 Moy, J., "OSPF Version 2", RFC 1583, Proteon, March 1994.
 ROUTE:2.
 Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
 Environments", RFC 1195, DEC, December 1990.
Baker Standards Track [Page 142]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 ROUTE:3.
 Hedrick, C., "Routing Information Protocol", RFC 1058, Rutgers
 University, June 1988.
 ROUTE:4.
 Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3
 (BGP-3)", RFC 1267, cisco, T.J. Watson Research Center, IBM
 Corp., October 1991.
 ROUTE:5.
 Gross, P, and Y. Rekhter, "Application of the Border Gateway
 Protocol in the Internet", RFC 1772, T.J. Watson Research
 Center, IBM Corp., MCI, March 1995.
 ROUTE:6.
 Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
 904, UDEL, April 1984.
 ROUTE:7.
 Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827, BBN,
 October 1982.
 ROUTE:8.
 Seamonson, L, and E. Rosen, "STUB" "Exterior Gateway Protocol",
 RFC 888, BBN, January 1984.
 ROUTE:9.
 Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
 Multicast Routing Protocol", RFC 1075, BBN, Stanford, November
 1988.
 ROUTE:10.
 Deering, S., Multicast Routing in Internetworks and Extended
 LANs, Proceedings of '88, Association for Computing Machinery,
 August 1988.
 ROUTE:11.
 Almquist, P., "Type of Service in the Internet Protocol Suite",
 RFC 1349, Consultant, July 1992.
 ROUTE:12.
 Rekhter, Y., "Experience with the BGP Protocol", RFC 1266, T.J.
 Watson Research Center, IBM Corp., October 1991.
 ROUTE:13.
 Rekhter, Y., "BGP Protocol Analysis", RFC 1265, T.J. Watson
 Research Center, IBM Corp., October 1991.
Baker Standards Track [Page 143]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 TRANS:1.
 Postel, J., "User Datagram Protocol", STD 6, RFC 768,
 USC/Information Sciences Institute, August 1980.
 TRANS:2.
 Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
 USC/Information Sciences Institute, September 1981.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS
 Subject to restrictions given below, a host MAY be able to act as an
 intermediate hop in a source route, forwarding a source-routed
 datagram to the next specified hop.
 However, in performing this router-like function, the host MUST obey
 all the relevant rules for a router forwarding source-routed
 datagrams [INTRO:2]. This includes the following specific
 provisions:
 (A) TTL
 The TTL field MUST be decremented and the datagram perhaps
 discarded as specified for a router in [INTRO:2].
 (B) ICMP Destination Unreachable
 A host MUST be able to generate Destination Unreachable messages
 with the following codes:
 4 (Fragmentation Required but DF Set) when a source-routed
 datagram cannot be fragmented to fit into the target network;
 5 (Source Route Failed) when a source-routed datagram cannot be
 forwarded, e.g., because of a routing problem or because the
 next hop of a strict source route is not on a connected
 network.
 (C) IP Source Address
 A source-routed datagram being forwarded MAY (and normally will)
 have a source address that is not one of the IP addresses of the
 forwarding host.
 (D) Record Route Option
 A host that is forwarding a source-routed datagram containing a
 Record Route option MUST update that option, if it has room.
 (E) Timestamp Option
 A host that is forwarding a source-routed datagram containing a
 Timestamp Option MUST add the current timestamp to that option,
 according to the rules for this option.
 To define the rules restricting host forwarding of source-routed
 datagrams, we use the term local source-routing if the next hop will
 be through the same physical interface through which the datagram
 arrived; otherwise, it is non-local source-routing.
 A host is permitted to perform local source-routing without
 restriction.
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 A host that supports non-local source-routing MUST have a
 configurable switch to disable forwarding, and this switch MUST
 default to disabled.
 The host MUST satisfy all router requirements for configurable policy
 filters [INTRO:2] restricting non-local forwarding.
 If a host receives a datagram with an incomplete source route but
 does not forward it for some reason, the host SHOULD return an ICMP
 Destination Unreachable (code 5, Source Route Failed) message, unless
 the datagram was itself an ICMP error message.
APPENDIX B. GLOSSARY
 This Appendix defines specific terms used in this memo. It also
 defines some general purpose terms that may be of interest. See also
 [INTRO:9] for a more general set of definitions.
 Autonomous System (AS)
 An Autonomous System (AS) is a connected segment of a network
 topology that consists of a collection of subnetworks (with
 hosts attached) interconnected by a set of routes. The
 subnetworks and the routers are expected to be under the control
 of a single operations and maintenance (O&M) organization.
 Within an AS routers may use one or more interior routing
 protocols, and sometimes several sets of metrics. An AS is
 expected to present to other ASs an appearence of a coherent
 interior routing plan, and a consistent picture of the
 destinations reachable through the AS. An AS is identified by
 an Autonomous System number.
 Connected Network
 A network prefix to which a router is interfaced is often known
 as a local network or the subnetwork of that router. However,
 these terms can cause confusion, and therefore we use the term
 Connected Network in this memo.
 Connected (Sub)Network
 A Connected (Sub)Network is an IP subnetwork to which a router
 is interfaced, or a connected network if the connected network
 is not subnetted. See also Connected Network.
 Datagram
 The unit transmitted between a pair of internet modules. Data,
 called datagrams, from sources to destinations. The Internet
 Protocol does not provide a reliable communication facility.
 There are no acknowledgments either end-to-end or hop-by-hop.
 There is no error no retransmissions. There is no flow control.
 See IP.
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 Default Route
 A routing table entry that is used to direct any data addressed
 to any network prefixes not explicitly listed in the routing
 table.
 Dense Mode
 In multicast forwarding, two paradigms are possible: in Dense
 Mode forwarding, a network multicast is forwarded as a data link
 layer multicast to all interfaces except that on which it was
 received, unless and until the router is instructed not to by a
 multicast routing neighbor. See Sparse Mode.
 EGP
 Exterior Gateway Protocol A protocol that distributes routing
 information to the gateways (routers) which connect autonomous
 systems. See IGP.
 EGP-2
 Exterior Gateway Protocol version 2 This is an EGP routing
 protocol developed to handle traffic between Autonomous Systems
 in the Internet.
 Forwarder
 The logical entity within a router that is responsible for
 switching packets among the router's interfaces. The Forwarder
 also makes the decisions to queue a packet for local delivery,
 to queue a packet for transmission out another interface, or
 both.
 Forwarding
 Forwarding is the process a router goes through for each packet
 received by the router. The packet may be consumed by the
 router, it may be output on one or more interfaces of the
 router, or both. Forwarding includes the process of deciding
 what to do with the packet as well as queuing it up for
 (possible) output or internal consumption.
 Forwarding Information Base (FIB)
 The table containing the information necessary to forward IP
 Datagrams, in this document, is called the Forwarding
 Information Base. At minimum, this contains the interface
 identifier and next hop information for each reachable
 destination network prefix.
 Fragment
 An IP datagram that represents a portion of a higher layer's
 packet that was too large to be sent in its entirety over the
 output network.
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 General Purpose Serial Interface
 A physical medium capable of connecting exactly two systems, and
 therefore configurable as a point to point line, but also
 configurable to support link layer networking using protocols
 such as X.25 or Frame Relay. A link layer network connects
 another system to a switch, and a higher communication layer
 multiplexes virtual circuits on the connection. See Point to
 Point Line.
 IGP
 Interior Gateway Protocol A protocol that distributes routing
 information with an Autonomous System (AS). See EGP.
 Interface IP Address
 The IP Address and network prefix length that is assigned to a
 specific interface of a router.
 Internet Address
 An assigned number that identifies a host in an internet. It
 has two parts: an IP address and a prefix length. The prefix
 length indicates how many of the most specific bits of the
 address constitute the network prefix.
 IP
 Internet Protocol The network layer protocol for the Internet.
 It is a packet switching, datagram protocol defined in RFC 791.
 IP does not provide a reliable communications facility; that is,
 there are no end-to-end of hop-by-hop acknowledgments.
 IP Datagram
 An IP Datagram is the unit of end-to-end transmission in the
 Internet Protocol. An IP Datagram consists of an IP header
 followed by all of higher-layer data (such as TCP, UDP, ICMP,
 and the like). An IP Datagram is an IP header followed by a
 message.
 An IP Datagram is a complete IP end-to-end transmission unit.
 An IP Datagram is composed of one or more IP Fragments.
 In this memo, the unqualified term Datagram should be understood
 to refer to an IP Datagram.
 IP Fragment
 An IP Fragment is a component of an IP Datagram. An IP Fragment
 consists of an IP header followed by all or part of the higher-
 layer of the original IP Datagram.
 One or more IP Fragments comprises a single IP Datagram.
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 In this memo, the unqualified term Fragment should be understood
 to refer to an IP Fragment.
 IP Packet
 An IP Datagram or an IP Fragment.
 In this memo, the unqualified term Packet should generally be
 understood to refer to an IP Packet.
 Logical [network] interface
 We define a logical [network] interface to be a logical path,
 distinguished by a unique IP address, to a connected network.
 Martian Filtering
 A packet that contains an invalid source or destination address
 is considered to be martian and discarded.
 MTU (Maximum Transmission Unit)
 The size of the largest packet that can be transmitted or
 received through a logical interface. This size includes the IP
 header but does not include the size of any Link Layer headers
 or framing.
 Multicast
 A packet that is destined for multiple hosts. See broadcast.
 Multicast Address
 A special type of address that is recognizable by multiple
 hosts.
 A Multicast Address is sometimes known as a Functional Address
 or a Group Address.
 Network Prefix
 The portion of an IP Address that signifies a set of systems.
 It is selected from the IP Address by logically ANDing a subnet
 mask with the address, or (equivalently) setting the bits of the
 address not among the most significant <prefix-length> bits of
 the address to zero.
 Originate
 Packets can be transmitted by a router for one of two reasons:
 1) the packet was received and is being forwarded or 2) the
 router itself created the packet for transmission (such as route
 advertisements). Packets that the router creates for
 transmission are said to originate at the router.
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 Packet
 A packet is the unit of data passed across the interface between
 the Internet Layer and the Link Layer. It includes an IP header
 and data. A packet may be a complete IP datagram or a fragment
 of an IP datagram.
 Path
 The sequence of routers and (sub-)networks that a packet
 traverses from a particular router to a particular destination
 host. Note that a path is uni-directional; it is not unusual to
 have different paths in the two directions between a given host
 pair.
 Physical Network
 A Physical Network is a network (or a piece of an internet)
 which is contiguous at the Link Layer. Its internal structure
 (if any) is transparent to the Internet Layer.
 In this memo, several media components that are connected using
 devices such as bridges or repeaters are considered to be a
 single Physical Network since such devices are transparent to
 the IP.
 Physical Network Interface
 This is a physical interface to a Connected Network and has a
 (possibly unique) Link-Layer address. Multiple Physical Network
 Interfaces on a single router may share the same Link-Layer
 address, but the address must be unique for different routers on
 the same Physical Network.
 Point to Point Line
 A physical medium capable of connecting exactly two systems. In
 this document, it is only used to refer to such a line when used
 to connect IP entities. See General Purpose Serial Interface.
 router
 A special-purpose dedicated computer that connects several
 networks. Routers switch packets between these networks in a
 process known as forwarding. This process may be repeated
 several times on a single packet by multiple routers until the
 packet can be delivered to the final destination - switching the
 packet from router to router to router... until the packet gets
 to its destination.
 RPF
 Reverse Path Forwarding - A method used to deduce the next hops
 for broadcast and multicast packets.
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 Silently Discard
 This memo specifies several cases where a router is to Silently
 Discard a received packet (or datagram). This means that the
 router should discard the packet without further processing, and
 that the router will not send any ICMP error message (see
 Section [4.3.2]) as a result. However, for diagnosis of
 problems, the router should provide the capability of logging
 the error (see Section [1.3.3]), including the contents of the
 silently discarded packet, and should record the event in a
 statistics counter.
 Silently Ignore
 A router is said to Silently Ignore an error or condition if it
 takes no action other than possibly generating an error report
 in an error log or through some network management protocol, and
 discarding, or ignoring, the source of the error. In
 particular, the router does NOT generate an ICMP error message.
 Sparse Mode
 In multicast forwarding, two paradigms are possible: in Sparse
 Mode forwarding, a network layer multicast datagram is forwarded
 as a data link layer multicast frame to routers and hosts that
 have asked for it. The initial forwarding state is the inverse
 of dense-mode in that it assumes no part of the network wants
 the data. See Dense Mode.
 Specific-destination address
 This is defined to be the destination address in the IP header
 unless the header contains an IP broadcast or IP multicast
 address, in which case the specific-destination is an IP address
 assigned to the physical interface on which the packet arrived.
 subnet
 A portion of a network, which may be a physically independent
 network, which shares a network address with other portions of
 the network and is distinguished by a subnet number. A subnet
 is to a network what a network is to an internet.
 subnet number
 A part of the internet address that designates a subnet. It is
 ignored for the purposes internet routing, but is used for
 intranet routing.
 TOS
 Type Of Service A field in the IP header that represents the
 degree of reliability expected from the network layer by the
 transport layer or application.
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 TTL
 Time To Live A field in the IP header that represents how long a
 packet is considered valid. It is a combination hop count and
 timer value.
APPENDIX C. FUTURE DIRECTIONS
 This appendix lists work that future revisions of this document may
 wish to address.
 In the preparation of Router Requirements, we stumbled across several
 other architectural issues. Each of these is dealt with somewhat in
 the document, but still ought to be classified as an open issue in
 the IP architecture.
 Most of the he topics presented here generally indicate areas where
 the technology is still relatively new and it is not appropriate to
 develop specific requirements since the community is still gaining
 operational experience.
 Other topics represent areas of ongoing research and indicate areas
 that the prudent developer would closely monitor.
 (1) SNMP Version 2
 (2) Additional SNMP MIBs
 (7) More detailed requirements for leaking routes between routing
 protocols
 (8) Router system security
 (9) Routing protocol security
 (10) Internetwork Protocol layer security. There has been extensive
 work refining the security of IP since the original work writing
 this document. This security work should be included in here.
 (12) Load Splitting
 (13) Sending fragments along different paths
 (15) Multiple logical (sub)nets on the same wire. Router
 Requirements does not require support for this. We made some
 attempt to identify pieces of the architecture (e.g., forwarding
 of directed broadcasts and issuing of Redirects) where the
 wording of the rules has to be done carefully to make the right
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 thing happen, and tried to clearly distinguish logical
 interfaces from physical interfaces. However, we did not study
 this issue in detail, and we are not at all confident that all
 the rules in the document are correct in the presence of
 multiple logical (sub)nets on the same wire.
 (15) Congestion control and resource management. On the advice of
 the IETF's experts (Mankin and Ramakrishnan) we deprecated
 (SHOULD NOT) Source Quench and said little else concrete
 (Section 5.3.6).
 (16) Developing a Link-Layer requirements document that would be
 common for both routers and hosts.
 (17) Developing a common PPP LQM algorithm.
 (18) Investigate of other information (above and beyond section
 [3.2]) that passes between the layers, such as physical network
 MTU, mappings of IP precedence to Link Layer priority values,
 etc.
 (19) Should the Link Layer notify IP if address resolution failed
 (just like it notifies IP when there is a Link Layer priority
 value problem)?
 (20) Should all routers be required to implement a DNS resolver?
 (21) Should a human user be able to use a host name anywhere you can
 use an IP address when configuring the router? Even in ping and
 traceroute?
 (22) Almquist's draft ruminations on the next hop and ruminations on
 route leaking need to be reviewed, brought up to date, and
 published.
 (23) Investigation is needed to determine if a redirect message for
 precedence is needed or not. If not, are the type-of-service
 redirects acceptable?
 (24) RIPv2 and RIP+CIDR and variable length network prefixes.
 (25) BGP-4 CIDR is going to be important, and everyone is betting on
 BGP-4. We can't avoid mentioning it. Probably need to describe
 the differences between BGP-3 and BGP-4, and explore upgrade
 issues...
 (26) Loose Source Route Mobile IP and some multicasting may require
 this. Perhaps it should be elevated to a SHOULD (per Fred
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 Baker's Suggestion).
APPENDIX D. Multicast Routing Protocols
 Multicasting is a relatively new technology within the Internet
 Protocol family. It is not widely deployed or commonly in use yet.
 Its importance, however, is expected to grow over the coming years.
 This Appendix describes some of the technologies being investigated
 for routing multicasts through the Internet.
 A diligent implementor will keep abreast of developments in this area
 to properly develop multicast facilities.
 This Appendix does not specify any standards or requirements.
D.1 Introduction
 Multicast routing protocols enable the forwarding of IP multicast
 datagrams throughout a TCP/IP internet. Generally these algorithms
 forward the datagram based on its source and destination addresses.
 Additionally, the datagram may need to be forwarded to several
 multicast group members, at times requiring the datagram to be
 replicated and sent out multiple interfaces.
 The state of multicast routing protocols is less developed than the
 protocols available for the forwarding of IP unicasts. Three
 experimental multicast routing protocols have been documented for
 TCP/IP. Each uses the IGMP protocol (discussed in Section [4.4]) to
 monitor multicast group membership.
D.2 Distance Vector Multicast Routing Protocol - DVMRP
 DVMRP, documented in [ROUTE:9], is based on Distance Vector or
 Bellman-Ford technology. It routes multicast datagrams only, and
 does so within a single Autonomous System. DVMRP is an
 implementation of the Truncated Reverse Path Broadcasting algorithm
 described in [ROUTE:10]. In addition, it specifies the tunneling of
 IP multicasts through non-multicast-routing-capable IP domains.
D.3 Multicast Extensions to OSPF - MOSPF
 MOSPF, currently under development, is a backward-compatible addition
 to OSPF that allows the forwarding of both IP multicasts and unicasts
 within an Autonomous System. MOSPF routers can be mixed with OSPF
 routers within a routing domain, and they will interoperate in the
 forwarding of unicasts. OSPF is a link-state or SPF-based protocol.
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 By adding link state advertisements that pinpoint group membership,
 MOSPF routers can calculate the path of a multicast datagram as a
 tree rooted at the datagram source. Those branches that do not
 contain group members can then be discarded, eliminating unnecessary
 datagram forwarding hops.
D.4 Protocol Independent Multicast - PIM
 PIM, currently under development, is a multicast routing protocol
 that runs over an existing unicast infrastructure. PIM provides for
 both dense and sparse group membership. It is different from other
 protocols, since it uses an explicit join model for sparse groups.
 Joining occurs on a shared tree and can switch to a per-source tree.
 Where bandwidth is plentiful and group membership is dense, overhead
 can be reduced by flooding data out all links and later pruning
 exception cases where there are no group members.
APPENDIX E Additional Next-Hop Selection Algorithms
 Section [5.2.4.3] specifies an algorithm that routers ought to use
 when selecting a next-hop for a packet.
 This appendix provides historical perspective for the next-hop
 selection problem. It also presents several additional pruning rules
 and next-hop selection algorithms that might be found in the
 Internet.
 This appendix presents material drawn from an earlier, unpublished,
 work by Philip Almquist; Ruminations on the Next Hop.
 This Appendix does not specify any standards or requirements.
E.1. Some Historical Perspective
 It is useful to briefly review the history of the topic, beginning
 with what is sometimes called the "classic model" of how a router
 makes routing decisions. This model predates IP. In this model, a
 router speaks some single routing protocol such as RIP. The protocol
 completely determines the contents of the router's Forwarding
 Information Base (FIB). The route lookup algorithm is trivial: the
 router looks in the FIB for a route whose destination attribute
 exactly matches the network prefix portion of the destination address
 in the packet. If one is found, it is used; if none is found, the
 destination is unreachable. Because the routing protocol keeps at
 most one route to each destination, the problem of what to do when
 there are multiple routes that match the same destination cannot
 arise.
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 Over the years, this classic model has been augmented in small ways.
 With the deployment of default routes, subnets, and host routes, it
 became possible to have more than one routing table entry which in
 some sense matched the destination. This was easily resolved by a
 consensus that there was a hierarchy of routes: host routes should be
 preferred over subnet routes, subnet routes over net routes, and net
 routes over default routes.
 With the deployment of technologies supporting variable length subnet
 masks (variable length network prefixes), the general approach
 remained the same although its description became a little more
 complicated; network prefixes were introduced as a conscious
 simplification and regularization of the architecture. We now say
 that each route to a network prefix route has a prefix length
 associated with it. This prefix length indicates the number of bits
 in the prefix. This may also be represented using the classical
 subnet mask. A route cannot be used to route a packet unless each
 significant bit in the route's network prefix matches the
 corresponding bit in the packet's destination address. Routes with
 more bits set in their masks are preferred over routes that have
 fewer bits set in their masks. This is simply a generalization of
 the hierarchy of routes described above, and will be referred to for
 the rest of this memo as choosing a route by preferring longest
 match.
 Another way the classic model has been augmented is through a small
 amount of relaxation of the notion that a routing protocol has
 complete control over the contents of the routing table. First,
 static routes were introduced. For the first time, it was possible
 to simultaneously have two routes (one dynamic and one static) to the
 same destination. When this happened, a router had to have a policy
 (in some cases configurable, and in other cases chosen by the author
 of the router's software) which determined whether the static route
 or the dynamic route was preferred. However, this policy was only
 used as a tie-breaker when longest match didn't uniquely determine
 which route to use. Thus, for example, a static default route would
 never be preferred over a dynamic net route even if the policy
 preferred static routes over dynamic routes.
 The classic model had to be further augmented when inter-domain
 routing protocols were invented. Traditional routing protocols came
 to be called "interior gateway protocols" (IGPs), and at each
 Internet site there was a strange new beast called an "exterior
 gateway", a router that spoke EGP to several "BBN Core Gateways" (the
 routers that made up the Internet backbone at the time) at the same
 time as it spoke its IGP to the other routers at its site. Both
 protocols wanted to determine the contents of the router's routing
 table. Theoretically, this could result in a router having three
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 routes (EGP, IGP, and static) to the same destination. Because of
 the Internet topology at the time, it was resolved with little debate
 that routers would be best served by a policy of preferring IGP
 routes over EGP routes. However, the sanctity of longest match
 remained unquestioned: a default route learned from the IGP would
 never be preferred over a net route from learned EGP.
 Although the Internet topology, and consequently routing in the
 Internet, have evolved considerably since then, this slightly
 augmented version of the classic model has survived intact to this
 day in the Internet (except that BGP has replaced EGP). Conceptually
 (and often in implementation) each router has a routing table and one
 or more routing protocol processes. Each of these processes can add
 any entry that it pleases, and can delete or modify any entry that it
 has created. When routing a packet, the router picks the best route
 using longest match, augmented with a policy mechanism to break ties.
 Although this augmented classic model has served us well, it has a
 number of shortcomings:
 o It ignores (although it could be augmented to consider) path
 characteristics such as quality of service and MTU.
 o It doesn't support routing protocols (such as OSPF and Integrated
 IS-IS) that require route lookup algorithms different than pure
 longest match.
 o There has not been a firm consensus on what the tie-breaking
 mechanism ought to be. Tie-breaking mechanisms have often been
 found to be difficult if not impossible to configure in such a way
 that the router will always pick what the network manger considers
 to be the "correct" route.
E.2. Additional Pruning Rules
 Section [5.2.4.3] defined several pruning rules to use to select
 routes from the FIB. There are other rules that could also be
 used.
 o OSPF Route Class
 Routing protocols that have areas or make a distinction between
 internal and external routes divide their routes into classes
 by the type of information used to calculate the route. A
 route is always chosen from the most preferred class unless
 none is available, in which case one is chosen from the second
 most preferred class, and so on. In OSPF, the classes (in
 order from most preferred to least preferred) are intra-area,
 inter-area, type 1 external (external routes with internal
 metrics), and type 2 external. As an additional wrinkle, a
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 router is configured to know what addresses ought to be
 accessible using intra-area routes, and will not use inter-
 area or external routes to reach these destinations even when
 no intra-area route is available.
 More precisely, we assume that each route has a class
 attribute, called route.class, which is assigned by the routing
 protocol. The set of candidate routes is examined to determine
 if it contains any for which route.class = intra-area. If so,
 all routes except those for which route.class = intra-area are
 discarded. Otherwise, router checks whether the packet's
 destination falls within the address ranges configured for the
 local area. If so, the entire set of candidate routes is
 deleted. Otherwise, the set of candidate routes is examined to
 determine if it contains any for which route.class = inter-
 area. If so, all routes except those for which route.class =
 inter-area are discarded. Otherwise, the set of candidate
 routes is examined to determine if it contains any for which
 route.class = type 1 external. If so, all routes except those
 for which route.class = type 1 external are discarded.
 o IS-IS Route Class
 IS-IS route classes work identically to OSPF's. However, the
 set of classes defined by Integrated IS-IS is different, such
 that there isn't a one-to-one mapping between IS-IS route
 classes and OSPF route classes. The route classes used by
 Integrated IS-IS are (in order from most preferred to least
 preferred) intra-area, inter-area, and external.
 The Integrated IS-IS internal class is equivalent to the OSPF
 internal class. Likewise, the Integrated IS-IS external class
 is equivalent to OSPF's type 2 external class. However,
 Integrated IS-IS does not make a distinction between inter-area
 routes and external routes with internal metrics - both are
 considered to be inter-area routes. Thus, OSPF prefers true
 inter-area routes over external routes with internal metrics,
 whereas Integrated IS-IS gives the two types of routes equal
 preference.
 o IDPR Policy
 A specific case of Policy. The IETF's Inter-domain Policy
 Routing Working Group is devising a routing protocol called
 Inter-Domain Policy Routing (IDPR) to support true policy-based
 routing in the Internet. Packets with certain combinations of
 header attributes (such as specific combinations of source and
 destination addresses or special IDPR source route options) are
 required to use routes provided by the IDPR protocol. Thus,
 unlike other Policy pruning rules, IDPR Policy would have to be
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 applied before any other pruning rules except Basic Match.
 Specifically, IDPR Policy examines the packet being forwarded
 to ascertain if its attributes require that it be forwarded
 using policy-based routes. If so, IDPR Policy deletes all
 routes not provided by the IDPR protocol.
E.3 Some Route Lookup Algorithms
 This section examines several route lookup algorithms that are in
 use or have been proposed. Each is described by giving the
 sequence of pruning rules it uses. The strengths and weaknesses
 of each algorithm are presented
E.3.1 The Revised Classic Algorithm
 The Revised Classic Algorithm is the form of the traditional
 algorithm that was discussed in Section [E.1]. The steps of this
 algorithm are:
 1. Basic match
 2. Longest match
 3. Best metric
 4. Policy
 Some implementations omit the Policy step, since it is needed only
 when routes may have metrics that are not comparable (because they
 were learned from different routing domains).
 The advantages of this algorithm are:
 (1) It is widely implemented.
 (2) Except for the Policy step (which an implementor can choose to
 make arbitrarily complex) the algorithm is simple both to
 understand and to implement.
 Its disadvantages are:
 (1) It does not handle IS-IS or OSPF route classes, and therefore
 cannot be used for Integrated IS-IS or OSPF.
 (2) It does not handle TOS or other path attributes.
 (3) The policy mechanisms are not standardized in any way, and are
 therefore are often implementation-specific. This causes
 extra work for implementors (who must invent appropriate
 policy mechanisms) and for users (who must learn how to use
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 the mechanisms. This lack of a standardized mechanism also
 makes it difficult to build consistent configurations for
 routers from different vendors. This presents a significant
 practical deterrent to multi-vendor interoperability.
 (4) The proprietary policy mechanisms currently provided by
 vendors are often inadequate in complex parts of the
 Internet.
 (5) The algorithm has not been written down in any generally
 available document or standard. It is, in effect, a part of
 the Internet Folklore.
E.3.2 The Variant Router Requirements Algorithm
 Some Router Requirements Working Group members have proposed a
 slight variant of the algorithm described in the Section
 [5.2.4.3]. In this variant, matching the type of service
 requested is considered to be more important, rather than less
 important, than matching as much of the destination address as
 possible. For example, this algorithm would prefer a default
 route that had the correct type of service over a network route
 that had the default type of service, whereas the algorithm in
 [5.2.4.3] would make the opposite choice.
 The steps of the algorithm are:
 1. Basic match
 2. Weak TOS
 3. Longest match
 4. Best metric
 5. Policy
 Debate between the proponents of this algorithm and the regular
 Router Requirements Algorithm suggests that each side can show
 cases where its algorithm leads to simpler, more intuitive routing
 than the other's algorithm does. This variant has the same set of
 advantages and disadvantages that the algorithm specified in
 [5.2.4.3] does, except that pruning on Weak TOS before pruning on
 Longest Match makes this algorithm less compatible with OSPF and
 Integrated IS-IS than the standard Router Requirements Algorithm.
E.3.3 The OSPF Algorithm
 OSPF uses an algorithm that is virtually identical to the Router
 Requirements Algorithm except for one crucial difference: OSPF
 considers OSPF route classes.
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 The algorithm is:
 1. Basic match
 2. OSPF route class
 3. Longest match
 4. Weak TOS
 5. Best metric
 6. Policy
 Type of service support is not always present. If it is not
 present then, of course, the fourth step would be omitted
 This algorithm has some advantages over the Revised Classic
 Algorithm:
 (1) It supports type of service routing.
 (2) Its rules are written down, rather than merely being a part of
 the Internet folklore.
 (3) It (obviously) works with OSPF.
 However, this algorithm also retains some of the disadvantages of
 the Revised Classic Algorithm:
 (1) Path properties other than type of service (e.g., MTU) are
 ignored.
 (2) As in the Revised Classic Algorithm, the details (or even the
 existence) of the Policy step are left to the discretion of
 the implementor.
 The OSPF Algorithm also has a further disadvantage (which is not
 shared by the Revised Classic Algorithm). OSPF internal (intra-
 area or inter-area) routes are always considered to be superior to
 routes learned from other routing protocols, even in cases where
 the OSPF route matches fewer bits of the destination address.
 This is a policy decision that is inappropriate in some networks.
 Finally, it is worth noting that the OSPF Algorithm's TOS support
 suffers from a deficiency in that routing protocols that support
 TOS are implicitly preferred when forwarding packets that have
 non-zero TOS values. This may not be appropriate in some cases.
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E.3.4 The Integrated IS-IS Algorithm
 Integrated IS-IS uses an algorithm that is similar to but not quite
 identical to the OSPF Algorithm. Integrated IS-IS uses a different
 set of route classes, and differs slightly in its handling of type of
 service. The algorithm is:
 1. Basic Match
 2. IS-IS Route Classes
 3. Longest Match
 4. Weak TOS
 5. Best Metric
 6. Policy
 Although Integrated IS-IS uses Weak TOS, the protocol is only capable
 of carrying routes for a small specific subset of the possible values
 for the TOS field in the IP header. Packets containing other values
 in the TOS field are routed using the default TOS.
 Type of service support is optional; if disabled, the fourth step
 would be omitted. As in OSPF, the specification does not include the
 Policy step.
 This algorithm has some advantages over the Revised Classic
 Algorithm:
 (1) It supports type of service routing.
 (2) Its rules are written down, rather than merely being a part of
 the Internet folklore.
 (3) It (obviously) works with Integrated IS-IS.
 However, this algorithm also retains some of the disadvantages of the
 Revised Classic Algorithm:
 (1) Path properties other than type of service (e.g., MTU) are
 ignored.
 (2) As in the Revised Classic Algorithm, the details (or even the
 existence) of the Policy step are left to the discretion of the
 implementor.
 (3) It doesn't work with OSPF because of the differences between IS-
 IS route classes and OSPF route classes. Also, because IS-IS
 supports only a subset of the possible TOS values, some obvious
 implementations of the Integrated IS-IS algorithm would not
 support OSPF's interpretation of TOS.
 The Integrated IS-IS Algorithm also has a further disadvantage (which
 is not shared by the Revised Classic Algorithm): IS-IS internal
 (intra-area or inter-area) routes are always considered to be
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 superior to routes learned from other routing protocols, even in
 cases where the IS-IS route matches fewer bits of the destination
 address and doesn't provide the requested type of service. This is a
 policy decision that may not be appropriate in all cases.
 Finally, it is worth noting that the Integrated IS-IS Algorithm's TOS
 support suffers from the same deficiency noted for the OSPF
 Algorithm.
Security Considerations
 Although the focus of this document is interoperability rather than
 security, there are obviously many sections of this document that
 have some ramifications on network security.
 Security means different things to different people. Security from a
 router's point of view is anything that helps to keep its own
 networks operational and in addition helps to keep the Internet as a
 whole healthy. For the purposes of this document, the security
 services we are concerned with are denial of service, integrity, and
 authentication as it applies to the first two. Privacy as a security
 service is important, but only peripherally a concern of a router -
 at least as of the date of this document.
 In several places in this document there are sections entitled ...
 Security Considerations. These sections discuss specific
 considerations that apply to the general topic under discussion.
 Rarely does this document say do this and your router/network will be
 secure. More likely, it says this is a good idea and if you do it,
 it *may* improve the security of the Internet and your local system
 in general.
 Unfortunately, this is the state-of-the-art AT THIS TIME. Few if any
 of the network protocols a router is concerned with have reasonable,
 built-in security features. Industry and the protocol designers have
 been and are continuing to struggle with these issues. There is
 progress, but only small baby steps such as the peer-to-peer
 authentication available in the BGP and OSPF routing protocols.
 In particular, this document notes the current research into
 developing and enhancing network security. Specific areas of
 research, development, and engineering that are underway as of this
 writing (December 1993) are in IP Security, SNMP Security, and common
 authentication technologies.
 Notwithstanding all the above, there are things both vendors and
 users can do to improve the security of their router. Vendors should
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 get a copy of Trusted Computer System Interpretation [INTRO:8]. Even
 if a vendor decides not to submit their device for formal
 verification under these guidelines, the publication provides
 excellent guidance on general security design and practices for
 computing devices.
APPENDIX F: HISTORICAL ROUTING PROTOCOLS
 Certain routing protocols are common in the Internet, but the authors
 of this document cannot in good conscience recommend their use. This
 is not because they do not work correctly, but because the
 characteristics of the Internet assumed in their design (simple
 routing, no policy, a single "core router" network under common
 administration, limited complexity, or limited network diameter) are
 not attributes of today's Internet. Those parts of the Internet that
 still use them are generally limited "fringe" domains with limited
 complexity.
 As a matter of good faith, collected wisdom concerning their
 implementation is recorded in this section.
F.1 EXTERIOR GATEWAY PROTOCOL - EGP
F.1.1 Introduction
 The Exterior Gateway Protocol (EGP) specifies an EGP that is used to
 exchange reachability information between routers of the same or
 differing autonomous systems. EGP is not considered a routing
 protocol since there is no standard interpretation (i.e. metric) for
 the distance fields in the EGP update message, so distances are
 comparable only among routers of the same AS. It is however designed
 to provide high-quality reachability information, both about neighbor
 routers and about routes to non-neighbor routers.
 EGP is defined by [ROUTE:6]. An implementor almost certainly wants
 to read [ROUTE:7] and [ROUTE:8] as well, for they contain useful
 explanations and background material.
 DISCUSSION
 The present EGP specification has serious limitations, most
 importantly a restriction that limits routers to advertising only
 those networks that are reachable from within the router's
 autonomous system. This restriction against propagating third
 party EGP information is to prevent long-lived routing loops.
 This effectively limits EGP to a two-level hierarchy.
 RFC-975 is not a part of the EGP specification, and should be
 ignored.
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F.1.2 Protocol Walk-through
 Indirect Neighbors: RFC-888, page 26
 An implementation of EGP MUST include indirect neighbor
 support.
 Polling Intervals: RFC-904, page 10
 The interval between Hello command retransmissions and the
 interval between Poll retransmissions SHOULD be configurable
 but there MUST be a minimum value defined.
 The interval at which an implementation will respond to Hello
 commands and Poll commands SHOULD be configurable but there
 MUST be a minimum value defined.
 Network Reachability: RFC-904, page 15
 An implementation MUST default to not providing the external list of
 routers in other autonomous systems; only the internal list of
 routers together with the nets that are reachable through those
 routers should be included in an Update Response/Indication packet.
 However, an implementation MAY elect to provide a configuration
 option enabling the external list to be provided. An implementation
 MUST NOT include in the external list routers that were learned
 through the external list provided by a router in another autonomous
 system. An implementation MUST NOT send a network back to the
 autonomous system from which it is learned, i.e. it MUST do split-
 horizon on an autonomous system level.
 If more than 255 internal or 255 external routers need to be
 specified in a Network Reachability update, the networks reachable
 from routers that can not be listed MUST be merged into the list for
 one of the listed routers. Which of the listed routers is chosen for
 this purpose SHOULD be user configurable, but SHOULD default to the
 source address of the EGP update being generated.
 An EGP update contains a series of blocks of network numbers, where
 each block contains a list of network numbers reachable at a
 particular distance through a particular router. If more than 255
 networks are reachable at a particular distance through a particular
 router, they are split into multiple blocks (all of which have the
 same distance). Similarly, if more than 255 blocks are required to
 list the networks reachable through a particular router, the router's
 address is listed as many times as necessary to include all the
 blocks in the update.
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Unsolicited Updates: RFC-904, page 16
 If a network is shared with the peer, an implementation MUST send an
 unsolicited update upon entry to the Up state if the source network
 is the shared network.
Neighbor Reachability: RFC-904, page 6, 13-15
 The table on page 6 that describes the values of j and k (the
 neighbor up and down thresholds) is incorrect. It is reproduced
 correctly here:
 Name Active Passive Description
 -----------------------------------------------
 j 3 1 neighbor-up threshold
 k 1 0 neighbor-down threshold
 The value for k in passive mode also specified incorrectly in RFC-
 904, page 14 The values in parenthesis should read:
 (j = 1, k = 0, and T3/T1 = 4)
 As an optimization, an implementation can refrain from sending a
 Hello command when a Poll is due. If an implementation does so, it
 SHOULD provide a user configurable option to disable this
 optimization.
Abort timer: RFC-904, pages 6, 12, 13
 An EGP implementation MUST include support for the abort timer (as
 documented in section 4.1.4 of RFC-904). An implementation SHOULD
 use the abort timer in the Idle state to automatically issue a Start
 event to restart the protocol machine. Recommended values are P4 for
 a critical error (Administratively prohibited, Protocol Violation and
 Parameter Problem) and P5 for all others. The abort timer SHOULD NOT
 be started when a Stop event was manually initiated (such as through
 a network management protocol).
Cease command received in Idle state: RFC-904, page 13
 When the EGP state machine is in the Idle state, it MUST reply to
 Cease commands with a Cease-ack response.
Hello Polling Mode: RFC-904, page 11
 An EGP implementation MUST include support for both active and
 passive polling modes.
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Neighbor Acquisition Messages: RFC-904, page 18
 As noted the Hello and Poll Intervals should only be present in
 Request and Confirm messages. Therefore the length of an EGP
 Neighbor Acquisition Message is 14 bytes for a Request or Confirm
 message and 10 bytes for a Refuse, Cease or Cease-ack message.
 Implementations MUST NOT send 14 bytes for Refuse, Cease or Cease-ack
 messages but MUST allow for implementations that send 14 bytes for
 these messages.
Sequence Numbers: RFC-904, page 10
 Response or indication packets received with a sequence number not
 equal to S MUST be discarded. The send sequence number S MUST be
 incremented just before the time a Poll command is sent and at no
 other times.
F.2 ROUTING INFORMATION PROTOCOL - RIP
F.2.1 Introduction
 RIP is specified in [ROUTE:3]. Although RIP is still quite important
 in the Internet, it is being replaced in sophisticated applications
 by more modern IGPs such as the ones described above. A router
 implementing RIP SHOULD implement RIP Version 2 [ROUTE:?], as it
 supports CIDR routes. If occasional access networking is in use, a
 router implementing RIP SHOULD implement Demand RIP [ROUTE:?].
 Another common use for RIP is as a router discovery protocol.
 Section [4.3.3.10] briefly touches upon this subject.
F.2.2 Protocol Walk-Through
 Dealing with changes in topology: [ROUTE:3], page 11
 An implementation of RIP MUST provide a means for timing out
 routes. Since messages are occasionally lost, implementations
 MUST NOT invalidate a route based on a single missed update.
 Implementations MUST by default wait six times the update
 interval before invalidating a route. A router MAY have
 configuration options to alter this value.
 DISCUSSION
 It is important to routing stability that all routers in a RIP
 autonomous system use similar timeout value for invalidating
 routes, and therefore it is important that an implementation
 default to the timeout value specified in the RIP specification.
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 However, that timeout value is too conservative in environments
 where packet loss is reasonably rare. In such an environment, a
 network manager may wish to be able to decrease the timeout period
 to promote faster recovery from failures.
 IMPLEMENTATION
 There is a very simple mechanism that a router may use to meet the
 requirement to invalidate routes promptly after they time out.
 Whenever the router scans the routing table to see if any routes
 have timed out, it also notes the age of the least recently
 updated route that has not yet timed out. Subtracting this age
 from the timeout period gives the amount of time until the router
 again needs to scan the table for timed out routes.
Split Horizon: [ROUTE:3], page 14-15
 An implementation of RIP MUST implement split horizon, a scheme used
 for avoiding problems caused by including routes in updates sent to
 the router from which they were learned.
 An implementation of RIP SHOULD implement Split horizon with poisoned
 reverse, a variant of split horizon that includes routes learned from
 a router sent to that router, but sets their metric to infinity.
 Because of the routing overhead that may be incurred by implementing
 split horizon with poisoned reverse, implementations MAY include an
 option to select whether poisoned reverse is in effect. An
 implementation SHOULD limit the time in which it sends reverse routes
 at an infinite metric.
 IMPLEMENTATION
 Each of the following algorithms can be used to limit the time for
 which poisoned reverse is applied to a route. The first algorithm
 is more complex but does a more thorough job of limiting poisoned
 reverse to only those cases where it is necessary.
 The goal of both algorithms is to ensure that poison reverse is
 done for any destination whose route has changed in the last Route
 Lifetime (typically 180 seconds), unless it can be sure that the
 previous route used the same output interface. The Route Lifetime
 is used because that is the amount of time RIP will keep around an
 old route before declaring it stale.
 The time intervals (and derived variables) used in the following
 algorithms are as follows:
 Tu The Update Timer; the number of seconds between RIP updates.
 This typically defaults to 30 seconds.
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 Rl The Route Lifetime, in seconds. This is the amount of time
 that a route is presumed to be good, without requiring an
 update. This typically defaults to 180 seconds.
 Ul The Update Loss; the number of consecutive updates that have to
 be lost or fail to mention a route before RIP deletes the
 route. Ul is calculated to be (Rl/Tu)+1. The +1 is to
 account for the fact that the first time the ifcounter is
 decremented will be less than Tu seconds after it is
 initialized. Typically, Ul will be 7: (180/30)+1.
 In The value to set ifcounter to when a destination is newly
 learned. This value is Ul-4, where the 4 is RIP's garbage
 collection timer/30
 The first algorithm is:
 - Associated with each destination is a counter, called the
 ifcounter below. Poison reverse is done for any route whose
 destination's ifcounter is greater than zero.
 - After a regular (not triggered or in response to a request)
 update is sent, all the non-zero ifcounters are decremented by
 one.
 - When a route to a destination is created, its ifcounter is set
 as follows:
 - If the new route is superseding a valid route, and the old
 route used a different (logical) output interface, then the
 ifcounter is set to Ul.
 - If the new route is superseding a stale route, and the old
 route used a different (logical) output interface, then the
 ifcounter is set to MAX(0, Ul - INT(seconds that the route
 has been stale/Ut).
 - If there was no previous route to the destination, the
 ifcounter is set to In.
 - Otherwise, the ifcounter is set to zero
 - RIP also maintains a timer, called the resettimer below. Poison
 reverse is done on all routes whenever resettimer has not
 expired (regardless of the ifcounter values).
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 - When RIP is started, restarted, reset, or otherwise has its
 routing table cleared, it sets the resettimer to go off in Rl
 seconds.
 The second algorithm is identical to the first except that:
 - The rules which set the ifcounter to non-zero values are changed
 to always set it to Rl/Tu, and
 - The resettimer is eliminated.
 Triggered updates: [ROUTE:3], page 15-16; page 29
 Triggered updates (also called flash updates) are a mechanism for
 immediately notifying a router's neighbors when the router adds or
 deletes routes or changes their metrics. A router MUST send a
 triggered update when routes are deleted or their metrics are
 increased. A router MAY send a triggered update when routes are
 added or their metrics decreased.
 Since triggered updates can cause excessive routing overhead,
 implementations MUST use the following mechanism to limit the
 frequency of triggered updates:
 (1) When a router sends a triggered update, it sets a timer to a
 random time between one and five seconds in the future. The
 router must not generate additional triggered updates before
 this timer expires.
 (2) If the router would generate a triggered update during this
 interval it sets a flag indicating that a triggered update is
 desired. The router also logs the desired triggered update.
 (3) When the triggered update timer expires, the router checks the
 triggered update flag. If the flag is set then the router
 sends a single triggered update which includes all the
 changes that were logged. The router then clears the flag
 and, since a triggered update was sent, restarts this
 algorithm.
 (4) The flag is also cleared whenever a regular update is sent.
 Triggered updates SHOULD include all routes that have changed
 since the most recent regular (non-triggered) update. Triggered
 updates MUST NOT include routes that have not changed since the
 most recent regular update.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 DISCUSSION
 Sending all routes, whether they have changed recently or not, is
 unacceptable in triggered updates because the tremendous size of
 many Internet routing tables could otherwise result in
 considerable bandwidth being wasted on triggered updates.
Use of UDP: [ROUTE:3], page 18-19.
 RIP packets sent to an IP broadcast address SHOULD have their initial
 TTL set to one.
 Note that to comply with Section [6.1] of this memo, a router SHOULD
 use UDP checksums in RIP packets that it originates, MUST discard RIP
 packets received with invalid UDP checksums, but MUST NOT discard
 received RIP packets simply because they do not contain UDP
 checksums.
Addressing Considerations: [ROUTE:3], page 22
 A RIP implementation SHOULD support host routes. If it does not, it
 MUST (as described on page 27 of [ROUTE:3]) ignore host routes in
 received updates. A router MAY log ignored hosts routes.
 The special address 0.0.0.0 is used to describe a default route. A
 default route is used as the route of last resort (i.e., when a route
 to the specific net does not exist in the routing table). The router
 MUST be able to create a RIP entry for the address 0.0.0.0.
Input Processing - Response: [ROUTE:3], page 26
 When processing an update, the following validity checks MUST be
 performed:
 o The response MUST be from UDP port 520.
 o The source address MUST be on a directly connected subnet (or on a
 directly connected, non-subnetted network) to be considered valid.
 o The source address MUST NOT be one of the router's addresses.
 DISCUSSION
 Some networks, media, and interfaces allow a sending node to
 receive packets that it broadcasts. A router must not accept its
 own packets as valid routing updates and process them. The last
 requirement prevents a router from accepting its own routing
 updates and processing them (on the assumption that they were sent
 by some other router on the network).
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 An implementation MUST NOT replace an existing route if the metric
 received is equal to the existing metric except in accordance with
 the following heuristic.
 An implementation MAY choose to implement the following heuristic to
 deal with the above situation. Normally, it is useless to change the
 route to a network from one router to another if both are advertised
 at the same metric. However, the route being advertised by one of
 the routers may be in the process of timing out. Instead of waiting
 for the route to timeout, the new route can be used after a specified
 amount of time has elapsed. If this heuristic is implemented, it
 MUST wait at least halfway to the expiration point before the new
 route is installed.
F.2.3 Specific Issues
RIP Shutdown
 An implementation of RIP SHOULD provide for a graceful shutdown
 using the following steps:
 (1) Input processing is terminated,
 (2) Four updates are generated at random intervals of between two
 and four seconds, These updates contain all routes that were
 previously announced, but with some metric changes. Routes
 that were being announced at a metric of infinity should
 continue to use this metric. Routes that had been announced
 with a non-infinite metric should be announced with a metric
 of 15 (infinity - 1).
 DISCUSSION
 The metric used for the above really ought to be 16 (infinity);
 setting it to 15 is a kludge to avoid breaking certain old hosts
 that wiretap the RIP protocol. Such a host will (erroneously)
 abort a TCP connection if it tries to send a datagram on the
 connection while the host has no route to the destination (even if
 the period when the host has no route lasts only a few seconds
 while RIP chooses an alternate path to the destination).
RIP Split Horizon and Static Routes
 Split horizon SHOULD be applied to static routes by default. An
 implementation SHOULD provide a way to specify, per static route,
 that split horizon should not be applied to this route.
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RFC 1812 Requirements for IP Version 4 Routers June 1995
F.3 GATEWAY TO GATEWAY PROTOCOL - GGP
 The Gateway to Gateway protocol is considered obsolete and SHOULD NOT
 be implemented.
Acknowledgments
 O that we now had here
 But one ten thousand of those men in England
 That do no work to-day!
 What's he that wishes so?
 My cousin Westmoreland? No, my fair cousin:
 If we are mark'd to die, we are enow
 To do our country loss; and if to live,
 The fewer men, the greater share of honour.
 God's will! I pray thee, wish not one man more.
 By Jove, I am not covetous for gold,
 Nor care I who doth feed upon my cost;
 It yearns me not if men my garments wear;
 Such outward things dwell not in my desires:
 But if it be a sin to covet honour,
 I am the most offending soul alive.
 No, faith, my coz, wish not a man from England:
 God's peace! I would not lose so great an honour
 As one man more, methinks, would share from me
 For the best hope I have. O, do not wish one more!
 Rather proclaim it, Westmoreland, through my host,
 That he which hath no stomach to this fight,
 Let him depart; his passport shall be made
 And crowns for convoy put into his purse:
 We would not die in that man's company
 That fears his fellowship to die with us.
 This day is called the feast of Crispian:
 He that outlives this day, and comes safe home,
 Will stand a tip-toe when the day is named,
 And rouse him at the name of Crispian.
 He that shall live this day, and see old age,
 Will yearly on the vigil feast his neighbours,
 And say 'To-morrow is Saint Crispian:'
 Then will he strip his sleeve and show his scars.
 And say 'These wounds I had on Crispin's day.'
 Old men forget: yet all shall be forgot,
 But he'll remember with advantages
 What feats he did that day: then shall our names.
 Familiar in his mouth as household words
 Harry the king, Bedford and Exeter,
 Warwick and Talbot, Salisbury and Gloucester,
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RFC 1812 Requirements for IP Version 4 Routers June 1995
 Be in their flowing cups freshly remember'd.
 This story shall the good man teach his son;
 And Crispin Crispian shall ne'er go by,
 From this day to the ending of the world,
 But we in it shall be remember'd;
 We few, we happy few, we band of brothers;
 For he to-day that sheds his blood with me
 Shall be my brother; be he ne'er so vile,
 This day shall gentle his condition:
 And gentlemen in England now a-bed
 Shall think themselves accursed they were not here,
 And hold their manhoods cheap whiles any speaks
 That fought with us upon Saint Crispin's day.
 -- William Shakespeare
 This memo is a product of the IETF's Router Requirements Working
 Group. A memo such as this one is of necessity the work of many more
 people than could be listed here. A wide variety of vendors, network
 managers, and other experts from the Internet community graciously
 contributed their time and wisdom to improve the quality of this
 memo. The editor wishes to extend sincere thanks to all of them.
 The current editor also wishes to single out and extend his heartfelt
 gratitude and appreciation to the original editor of this document;
 Philip Almquist. Without Philip's work, both as the original editor
 and as the Chair of the working group, this document would not have
 been produced. He also wishes to express deep and heartfelt
 gratitude to the previous editor, Frank Kastenholz. Frank changed
 the original document from a collection of information to a useful
 description of IP technology - in his words, a "snapshot" of the
 technology in 1991. One can only hope that this snapshot, of the
 technology in 1994, is as clear.
 Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy
 Wittbrodt each wrote major chapters of this memo. Others who made
 major contributions to the document included Bill Barns, Steve
 Deering, Kent England, Jim Forster, Martin Gross, Jeff Honig, Steve
 Knowles, Yoni Malachi, Michael Reilly, and Walt Wimer.
 Additional text came from Andy Malis, Paul Traina, Art Berggreen,
 John Cavanaugh, Ross Callon, John Lekashman, Brian Lloyd, Gary
 Malkin, Milo Medin, John Moy, Craig Partridge, Stephanie Price, Yakov
 Rekhter, Steve Senum, Richard Smith, Frank Solensky, Rich Woundy, and
 others who have been inadvertently overlooked.
 Some of the text in this memo has been (shamelessly) plagiarized from
 earlier documents, most notably RFC-1122 by Bob Braden and the Host
Baker Standards Track [Page 174]
RFC 1812 Requirements for IP Version 4 Routers June 1995
 Requirements Working Group, and RFC-1009 by Bob Braden and Jon
 Postel. The work of these earlier authors is gratefully
 acknowledged.
 Jim Forster was a co-chair of the Router Requirements Working Group
 during its early meetings, and was instrumental in getting the group
 off to a good start. Jon Postel, Bob Braden, and Walt Prue also
 contributed to the success by providing a wealth of good advice
 before the group's first meeting. Later on, Phill Gross, Vint Cerf,
 and Noel Chiappa all provided valuable advice and support.
 Mike St. Johns coordinated the Working Group's interactions with the
 security community, and Frank Kastenholz coordinated the Working
 Group's interactions with the network management area. Allison
 Mankin and K.K. Ramakrishnan provided expertise on the issues of
 congestion control and resource allocation.
 Many more people than could possibly be listed or credited here
 participated in the deliberations of the Router Requirements Working
 Group, either through electronic mail or by attending meetings.
 However, the efforts of Ross Callon and Vince Fuller in sorting out
 the difficult issues of route choice and route leaking are especially
 acknowledged.
 The editor thanks his employer, Cisco Systems, for allowing him to
 spend the time necessary to produce the 1994 snapshot.
Editor's Address
 The address of the current editor of this document is
 Fred Baker
 Cisco Systems
 519 Lado Drive
 Santa Barbara, California 93111
 USA
 Phone:+1 805-681-0115
 EMail: fred@cisco.com
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