Network Working Group W. Simpson, Editor
Request for Comments: 1661 Daydreamer
STD: 51 July 1994
Obsoletes: 1548
Category: Standards Track
 The Point-to-Point Protocol (PPP)
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.
Abstract
 The Point-to-Point Protocol (PPP) provides a standard method for
 transporting multi-protocol datagrams over point-to-point links. PPP
 is comprised of three main components:
 1. A method for encapsulating multi-protocol datagrams.
 2. A Link Control Protocol (LCP) for establishing, configuring,
 and testing the data-link connection.
 3. A family of Network Control Protocols (NCPs) for establishing
 and configuring different network-layer protocols.
 This document defines the PPP organization and methodology, and the
 PPP encapsulation, together with an extensible option negotiation
 mechanism which is able to negotiate a rich assortment of
 configuration parameters and provides additional management
 functions. The PPP Link Control Protocol (LCP) is described in terms
 of this mechanism.
Table of Contents
 1. Introduction .......................................... 1
 1.1 Specification of Requirements ................... 2
 1.2 Terminology ..................................... 3
 2. PPP Encapsulation ..................................... 4
 3. PPP Link Operation .................................... 6
 3.1 Overview ........................................ 6
 3.2 Phase Diagram ................................... 6
 3.3 Link Dead (physical-layer not ready) ............ 7
 3.4 Link Establishment Phase ........................ 7
 3.5 Authentication Phase ............................ 8
 3.6 Network-Layer Protocol Phase .................... 8
 3.7 Link Termination Phase .......................... 9
 4. The Option Negotiation Automaton ...................... 11
 4.1 State Transition Table .......................... 12
 4.2 States .......................................... 14
 4.3 Events .......................................... 16
 4.4 Actions ......................................... 21
 4.5 Loop Avoidance .................................. 23
 4.6 Counters and Timers ............................. 24
 5. LCP Packet Formats .................................... 26
 5.1 Configure-Request ............................... 28
 5.2 Configure-Ack ................................... 29
 5.3 Configure-Nak ................................... 30
 5.4 Configure-Reject ................................ 31
 5.5 Terminate-Request and Terminate-Ack ............. 33
 5.6 Code-Reject ..................................... 34
 5.7 Protocol-Reject ................................. 35
 5.8 Echo-Request and Echo-Reply ..................... 36
 5.9 Discard-Request ................................. 37
 6. LCP Configuration Options ............................. 39
 6.1 Maximum-Receive-Unit (MRU) ...................... 41
 6.2 Authentication-Protocol ......................... 42
 6.3 Quality-Protocol ................................ 43
 6.4 Magic-Number .................................... 45
 6.5 Protocol-Field-Compression (PFC) ................ 48
 6.6 Address-and-Control-Field-Compression (ACFC)
 SECURITY CONSIDERATIONS ...................................... 51
 REFERENCES ................................................... 51
 ACKNOWLEDGEMENTS ............................................. 51
 CHAIR'S ADDRESS .............................................. 52
 EDITOR'S ADDRESS ............................................. 52
1. Introduction
 The Point-to-Point Protocol is designed for simple links which
 transport packets between two peers. These links provide full-duplex
 simultaneous bi-directional operation, and are assumed to deliver
 packets in order. It is intended that PPP provide a common solution
 for easy connection of a wide variety of hosts, bridges and routers
 [1].
 Encapsulation
 The PPP encapsulation provides for multiplexing of different
 network-layer protocols simultaneously over the same link. The
 PPP encapsulation has been carefully designed to retain
 compatibility with most commonly used supporting hardware.
 Only 8 additional octets are necessary to form the encapsulation
 when used within the default HDLC-like framing. In environments
 where bandwidth is at a premium, the encapsulation and framing may
 be shortened to 2 or 4 octets.
 To support high speed implementations, the default encapsulation
 uses only simple fields, only one of which needs to be examined
 for demultiplexing. The default header and information fields
 fall on 32-bit boundaries, and the trailer may be padded to an
 arbitrary boundary.
 Link Control Protocol
 In order to be sufficiently versatile to be portable to a wide
 variety of environments, PPP provides a Link Control Protocol
 (LCP). The LCP is used to automatically agree upon the
 encapsulation format options, handle varying limits on sizes of
 packets, detect a looped-back link and other common
 misconfiguration errors, and terminate the link. Other optional
 facilities provided are authentication of the identity of its peer
 on the link, and determination when a link is functioning properly
 and when it is failing.
 Network Control Protocols
 Point-to-Point links tend to exacerbate many problems with the
 current family of network protocols. For instance, assignment and
 management of IP addresses, which is a problem even in LAN
 environments, is especially difficult over circuit-switched
 point-to-point links (such as dial-up modem servers). These
 problems are handled by a family of Network Control Protocols
 (NCPs), which each manage the specific needs required by their
 respective network-layer protocols. These NCPs are defined in
 companion documents.
 Configuration
 It is intended that PPP links be easy to configure. By design,
 the standard defaults handle all common configurations. The
 implementor can specify improvements to the default configuration,
 which are automatically communicated to the peer without operator
 intervention. Finally, the operator may explicitly configure
 options for the link which enable the link to operate in
 environments where it would otherwise be impossible.
 This self-configuration is implemented through an extensible
 option negotiation mechanism, wherein each end of the link
 describes to the other its capabilities and requirements.
 Although the option negotiation mechanism described in this
 document is specified in terms of the Link Control Protocol (LCP),
 the same facilities are designed to be used by other control
 protocols, especially the family of NCPs.
1.1. Specification of Requirements
 In this document, several words are used to signify the requirements
 of the specification. These words are often capitalized.
 MUST This word, or the adjective "required", means that the
 definition is an absolute requirement of the specification.
 MUST NOT This phrase means that the definition is an absolute
 prohibition of the specification.
 SHOULD This word, or the adjective "recommended", means that there
 may exist valid reasons in particular circumstances to
 ignore this item, but the full implications must be
 understood and carefully weighed before choosing a
 different course.
 MAY This word, or the adjective "optional", means that this
 item is one of an allowed set of alternatives. An
 implementation which does not include this option MUST be
 prepared to interoperate with another implementation which
 does include the option.
1.2. Terminology
 This document frequently uses the following terms:
 datagram The unit of transmission in the network layer (such as IP).
 A datagram may be encapsulated in one or more packets
 passed to the data link layer.
 frame The unit of transmission at the data link layer. A frame
 may include a header and/or a trailer, along with some
 number of units of data.
 packet The basic unit of encapsulation, which is passed across the
 interface between the network layer and the data link
 layer. A packet is usually mapped to a frame; the
 exceptions are when data link layer fragmentation is being
 performed, or when multiple packets are incorporated into a
 single frame.
 peer The other end of the point-to-point link.
 silently discard
 The implementation discards the packet without further
 processing. The implementation SHOULD provide the
 capability of logging the error, including the contents of
 the silently discarded packet, and SHOULD record the event
 in a statistics counter.
2. PPP Encapsulation
 The PPP encapsulation is used to disambiguate multiprotocol
 datagrams. This encapsulation requires framing to indicate the
 beginning and end of the encapsulation. Methods of providing framing
 are specified in companion documents.
 A summary of the PPP encapsulation is shown below. The fields are
 transmitted from left to right.
 +----------+-------------+---------+
 | Protocol | Information | Padding |
 | 8/16 bits| * | * |
 +----------+-------------+---------+
 Protocol Field
 The Protocol field is one or two octets, and its value identifies
 the datagram encapsulated in the Information field of the packet.
 The field is transmitted and received most significant octet
 first.
 The structure of this field is consistent with the ISO 3309
 extension mechanism for address fields. All Protocols MUST be
 odd; the least significant bit of the least significant octet MUST
 equal "1". Also, all Protocols MUST be assigned such that the
 least significant bit of the most significant octet equals "0".
 Frames received which don't comply with these rules MUST be
 treated as having an unrecognized Protocol.
 Protocol field values in the "0***" to "3***" range identify the
 network-layer protocol of specific packets, and values in the
 "8***" to "b***" range identify packets belonging to the
 associated Network Control Protocols (NCPs), if any.
 Protocol field values in the "4***" to "7***" range are used for
 protocols with low volume traffic which have no associated NCP.
 Protocol field values in the "c***" to "f***" range identify
 packets as link-layer Control Protocols (such as LCP).
 Up-to-date values of the Protocol field are specified in the most
 recent "Assigned Numbers" RFC [2]. This specification reserves
 the following values:
 Value (in hex) Protocol Name
 0001 Padding Protocol
 0003 to 001f reserved (transparency inefficient)
 007d reserved (Control Escape)
 00cf reserved (PPP NLPID)
 00ff reserved (compression inefficient)
 8001 to 801f unused
 807d unused
 80cf unused
 80ff unused
 c021 Link Control Protocol
 c023 Password Authentication Protocol
 c025 Link Quality Report
 c223 Challenge Handshake Authentication Protocol
 Developers of new protocols MUST obtain a number from the Internet
 Assigned Numbers Authority (IANA), at IANA@isi.edu.
 Information Field
 The Information field is zero or more octets. The Information
 field contains the datagram for the protocol specified in the
 Protocol field.
 The maximum length for the Information field, including Padding,
 but not including the Protocol field, is termed the Maximum
 Receive Unit (MRU), which defaults to 1500 octets. By
 negotiation, consenting PPP implementations may use other values
 for the MRU.
 Padding
 On transmission, the Information field MAY be padded with an
 arbitrary number of octets up to the MRU. It is the
 responsibility of each protocol to distinguish padding octets from
 real information.
3. PPP Link Operation
3.1. Overview
 In order to establish communications over a point-to-point link, each
 end of the PPP link MUST first send LCP packets to configure and test
 the data link. After the link has been established, the peer MAY be
 authenticated.
 Then, PPP MUST send NCP packets to choose and configure one or more
 network-layer protocols. Once each of the chosen network-layer
 protocols has been configured, datagrams from each network-layer
 protocol can be sent over the link.
 The link will remain configured for communications until explicit LCP
 or NCP packets close the link down, or until some external event
 occurs (an inactivity timer expires or network administrator
 intervention).
3.2. Phase Diagram
 In the process of configuring, maintaining and terminating the
 point-to-point link, the PPP link goes through several distinct
 phases which are specified in the following simplified state diagram:
 +------+ +-----------+ +--------------+
 | | UP | | OPENED | | SUCCESS/NONE
 | Dead |------->| Establish |---------->| Authenticate |--+
 | | | | | | |
 +------+ +-----------+ +--------------+ |
 ^ | | |
 | FAIL | FAIL | |
 +<--------------+ +----------+ |
 | | |
 | +-----------+ | +---------+ |
 | DOWN | | | CLOSING | | |
 +------------| Terminate |<---+<----------| Network |<-+
 | | | |
 +-----------+ +---------+
 Not all transitions are specified in this diagram. The following
 semantics MUST be followed.
3.3. Link Dead (physical-layer not ready)
 The link necessarily begins and ends with this phase. When an
 external event (such as carrier detection or network administrator
 configuration) indicates that the physical-layer is ready to be used,
 PPP will proceed to the Link Establishment phase.
 During this phase, the LCP automaton (described later) will be in the
 Initial or Starting states. The transition to the Link Establishment
 phase will signal an Up event to the LCP automaton.
 Implementation Note:
 Typically, a link will return to this phase automatically after
 the disconnection of a modem. In the case of a hard-wired link,
 this phase may be extremely short -- merely long enough to detect
 the presence of the device.
3.4. Link Establishment Phase
 The Link Control Protocol (LCP) is used to establish the connection
 through an exchange of Configure packets. This exchange is complete,
 and the LCP Opened state entered, once a Configure-Ack packet
 (described later) has been both sent and received.
 All Configuration Options are assumed to be at default values unless
 altered by the configuration exchange. See the chapter on LCP
 Configuration Options for further discussion.
 It is important to note that only Configuration Options which are
 independent of particular network-layer protocols are configured by
 LCP. Configuration of individual network-layer protocols is handled
 by separate Network Control Protocols (NCPs) during the Network-Layer
 Protocol phase.
 Any non-LCP packets received during this phase MUST be silently
 discarded.
 The receipt of the LCP Configure-Request causes a return to the Link
 Establishment phase from the Network-Layer Protocol phase or
 Authentication phase.
3.5. Authentication Phase
 On some links it may be desirable to require a peer to authenticate
 itself before allowing network-layer protocol packets to be
 exchanged.
 By default, authentication is not mandatory. If an implementation
 desires that the peer authenticate with some specific authentication
 protocol, then it MUST request the use of that authentication
 protocol during Link Establishment phase.
 Authentication SHOULD take place as soon as possible after link
 establishment. However, link quality determination MAY occur
 concurrently. An implementation MUST NOT allow the exchange of link
 quality determination packets to delay authentication indefinitely.
 Advancement from the Authentication phase to the Network-Layer
 Protocol phase MUST NOT occur until authentication has completed. If
 authentication fails, the authenticator SHOULD proceed instead to the
 Link Termination phase.
 Only Link Control Protocol, authentication protocol, and link quality
 monitoring packets are allowed during this phase. All other packets
 received during this phase MUST be silently discarded.
 Implementation Notes:
 An implementation SHOULD NOT fail authentication simply due to
 timeout or lack of response. The authentication SHOULD allow some
 method of retransmission, and proceed to the Link Termination
 phase only after a number of authentication attempts has been
 exceeded.
 The implementation responsible for commencing Link Termination
 phase is the implementation which has refused authentication to
 its peer.
3.6. Network-Layer Protocol Phase
 Once PPP has finished the previous phases, each network-layer
 protocol (such as IP, IPX, or AppleTalk) MUST be separately
 configured by the appropriate Network Control Protocol (NCP).
 Each NCP MAY be Opened and Closed at any time.
 Implementation Note:
 Because an implementation may initially use a significant amount
 of time for link quality determination, implementations SHOULD
 avoid fixed timeouts when waiting for their peers to configure a
 NCP.
 After a NCP has reached the Opened state, PPP will carry the
 corresponding network-layer protocol packets. Any supported
 network-layer protocol packets received when the corresponding NCP is
 not in the Opened state MUST be silently discarded.
 Implementation Note:
 While LCP is in the Opened state, any protocol packet which is
 unsupported by the implementation MUST be returned in a Protocol-
 Reject (described later). Only protocols which are supported are
 silently discarded.
 During this phase, link traffic consists of any possible combination
 of LCP, NCP, and network-layer protocol packets.
3.7. Link Termination Phase
 PPP can terminate the link at any time. This might happen because of
 the loss of carrier, authentication failure, link quality failure,
 the expiration of an idle-period timer, or the administrative closing
 of the link.
 LCP is used to close the link through an exchange of Terminate
 packets. When the link is closing, PPP informs the network-layer
 protocols so that they may take appropriate action.
 After the exchange of Terminate packets, the implementation SHOULD
 signal the physical-layer to disconnect in order to enforce the
 termination of the link, particularly in the case of an
 authentication failure. The sender of the Terminate-Request SHOULD
 disconnect after receiving a Terminate-Ack, or after the Restart
 counter expires. The receiver of a Terminate-Request SHOULD wait for
 the peer to disconnect, and MUST NOT disconnect until at least one
 Restart time has passed after sending a Terminate-Ack. PPP SHOULD
 proceed to the Link Dead phase.
 Any non-LCP packets received during this phase MUST be silently
 discarded.
 Implementation Note:
 The closing of the link by LCP is sufficient. There is no need
 for each NCP to send a flurry of Terminate packets. Conversely,
 the fact that one NCP has Closed is not sufficient reason to cause
 the termination of the PPP link, even if that NCP was the only NCP
 currently in the Opened state.
4. The Option Negotiation Automaton
 The finite-state automaton is defined by events, actions and state
 transitions. Events include reception of external commands such as
 Open and Close, expiration of the Restart timer, and reception of
 packets from a peer. Actions include the starting of the Restart
 timer and transmission of packets to the peer.
 Some types of packets -- Configure-Naks and Configure-Rejects, or
 Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
 Discard-Requests -- are not differentiated in the automaton
 descriptions. As will be described later, these packets do indeed
 serve different functions. However, they always cause the same
 transitions.
 Events Actions
 Up = lower layer is Up tlu = This-Layer-Up
 Down = lower layer is Down tld = This-Layer-Down
 Open = administrative Open tls = This-Layer-Started
 Close= administrative Close tlf = This-Layer-Finished
 TO+ = Timeout with counter > 0 irc = Initialize-Restart-Count
 TO- = Timeout with counter expired zrc = Zero-Restart-Count
 RCR+ = Receive-Configure-Request (Good) scr = Send-Configure-Request
 RCR- = Receive-Configure-Request (Bad)
 RCA = Receive-Configure-Ack sca = Send-Configure-Ack
 RCN = Receive-Configure-Nak/Rej scn = Send-Configure-Nak/Rej
 RTR = Receive-Terminate-Request str = Send-Terminate-Request
 RTA = Receive-Terminate-Ack sta = Send-Terminate-Ack
 RUC = Receive-Unknown-Code scj = Send-Code-Reject
 RXJ+ = Receive-Code-Reject (permitted)
 or Receive-Protocol-Reject
 RXJ- = Receive-Code-Reject (catastrophic)
 or Receive-Protocol-Reject
 RXR = Receive-Echo-Request ser = Send-Echo-Reply
 or Receive-Echo-Reply
 or Receive-Discard-Request
4.1. State Transition Table
 The complete state transition table follows. States are indicated
 horizontally, and events are read vertically. State transitions and
 actions are represented in the form action/new-state. Multiple
 actions are separated by commas, and may continue on succeeding lines
 as space requires; multiple actions may be implemented in any
 convenient order. The state may be followed by a letter, which
 indicates an explanatory footnote. The dash ('-') indicates an
 illegal transition.
 | State
 | 0 1 2 3 4 5
Events| Initial Starting Closed Stopped Closing Stopping
------+-----------------------------------------------------------
 Up | 2 irc,scr/6 - - - -
 Down | - - 0 tls/1 0 1
 Open | tls/1 1 irc,scr/6 3r 5r 5r
 Close| 0 tlf/0 2 2 4 4
 |
 TO+ | - - - - str/4 str/5
 TO- | - - - - tlf/2 tlf/3
 |
 RCR+ | - - sta/2 irc,scr,sca/8 4 5
 RCR- | - - sta/2 irc,scr,scn/6 4 5
 RCA | - - sta/2 sta/3 4 5
 RCN | - - sta/2 sta/3 4 5
 |
 RTR | - - sta/2 sta/3 sta/4 sta/5
 RTA | - - 2 3 tlf/2 tlf/3
 |
 RUC | - - scj/2 scj/3 scj/4 scj/5
 RXJ+ | - - 2 3 4 5
 RXJ- | - - tlf/2 tlf/3 tlf/2 tlf/3
 |
 RXR | - - 2 3 4 5
 | State
 | 6 7 8 9
Events| Req-Sent Ack-Rcvd Ack-Sent Opened
------+-----------------------------------------
 Up | - - - -
 Down | 1 1 1 tld/1
 Open | 6 7 8 9r
 Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
 |
 TO+ | scr/6 scr/6 scr/8 -
 TO- | tlf/3p tlf/3p tlf/3p -
 |
 RCR+ | sca/8 sca,tlu/9 sca/8 tld,scr,sca/8
 RCR- | scn/6 scn/7 scn/6 tld,scr,scn/6
 RCA | irc/7 scr/6x irc,tlu/9 tld,scr/6x
 RCN |irc,scr/6 scr/6x irc,scr/8 tld,scr/6x
 |
 RTR | sta/6 sta/6 sta/6 tld,zrc,sta/5
 RTA | 6 6 8 tld,scr/6
 |
 RUC | scj/6 scj/7 scj/8 scj/9
 RXJ+ | 6 6 8 9
 RXJ- | tlf/3 tlf/3 tlf/3 tld,irc,str/5
 |
 RXR | 6 7 8 ser/9
 The states in which the Restart timer is running are identifiable by
 the presence of TO events. Only the Send-Configure-Request, Send-
 Terminate-Request and Zero-Restart-Count actions start or re-start
 the Restart timer. The Restart timer is stopped when transitioning
 from any state where the timer is running to a state where the timer
 is not running.
 The events and actions are defined according to a message passing
 architecture, rather than a signalling architecture. If an action is
 desired to control specific signals (such as DTR), additional actions
 are likely to be required.
 [p] Passive option; see Stopped state discussion.
 [r] Restart option; see Open event discussion.
 [x] Crossed connection; see RCA event discussion.
4.2. States
 Following is a more detailed description of each automaton state.
 Initial
 In the Initial state, the lower layer is unavailable (Down), and
 no Open has occurred. The Restart timer is not running in the
 Initial state.
 Starting
 The Starting state is the Open counterpart to the Initial state.
 An administrative Open has been initiated, but the lower layer is
 still unavailable (Down). The Restart timer is not running in the
 Starting state.
 When the lower layer becomes available (Up), a Configure-Request
 is sent.
 Closed
 In the Closed state, the link is available (Up), but no Open has
 occurred. The Restart timer is not running in the Closed state.
 Upon reception of Configure-Request packets, a Terminate-Ack is
 sent. Terminate-Acks are silently discarded to avoid creating a
 loop.
 Stopped
 The Stopped state is the Open counterpart to the Closed state. It
 is entered when the automaton is waiting for a Down event after
 the This-Layer-Finished action, or after sending a Terminate-Ack.
 The Restart timer is not running in the Stopped state.
 Upon reception of Configure-Request packets, an appropriate
 response is sent. Upon reception of other packets, a Terminate-
 Ack is sent. Terminate-Acks are silently discarded to avoid
 creating a loop.
 Rationale:
 The Stopped state is a junction state for link termination,
 link configuration failure, and other automaton failure modes.
 These potentially separate states have been combined.
 There is a race condition between the Down event response (from
 the This-Layer-Finished action) and the Receive-Configure-
 Request event. When a Configure-Request arrives before the
 Down event, the Down event will supercede by returning the
 automaton to the Starting state. This prevents attack by
 repetition.
 Implementation Option:
 After the peer fails to respond to Configure-Requests, an
 implementation MAY wait passively for the peer to send
 Configure-Requests. In this case, the This-Layer-Finished
 action is not used for the TO- event in states Req-Sent, Ack-
 Rcvd and Ack-Sent.
 This option is useful for dedicated circuits, or circuits which
 have no status signals available, but SHOULD NOT be used for
 switched circuits.
 Closing
 In the Closing state, an attempt is made to terminate the
 connection. A Terminate-Request has been sent and the Restart
 timer is running, but a Terminate-Ack has not yet been received.
 Upon reception of a Terminate-Ack, the Closed state is entered.
 Upon the expiration of the Restart timer, a new Terminate-Request
 is transmitted, and the Restart timer is restarted. After the
 Restart timer has expired Max-Terminate times, the Closed state is
 entered.
 Stopping
 The Stopping state is the Open counterpart to the Closing state.
 A Terminate-Request has been sent and the Restart timer is
 running, but a Terminate-Ack has not yet been received.
 Rationale:
 The Stopping state provides a well defined opportunity to
 terminate a link before allowing new traffic. After the link
 has terminated, a new configuration may occur via the Stopped
 or Starting states.
 Request-Sent
 In the Request-Sent state an attempt is made to configure the
 connection. A Configure-Request has been sent and the Restart
 timer is running, but a Configure-Ack has not yet been received
 nor has one been sent.
 Ack-Received
 In the Ack-Received state, a Configure-Request has been sent and a
 Configure-Ack has been received. The Restart timer is still
 running, since a Configure-Ack has not yet been sent.
 Ack-Sent
 In the Ack-Sent state, a Configure-Request and a Configure-Ack
 have both been sent, but a Configure-Ack has not yet been
 received. The Restart timer is running, since a Configure-Ack has
 not yet been received.
 Opened
 In the Opened state, a Configure-Ack has been both sent and
 received. The Restart timer is not running.
 When entering the Opened state, the implementation SHOULD signal
 the upper layers that it is now Up. Conversely, when leaving the
 Opened state, the implementation SHOULD signal the upper layers
 that it is now Down.
4.3. Events
 Transitions and actions in the automaton are caused by events.
 Up
 This event occurs when a lower layer indicates that it is ready to
 carry packets.
 Typically, this event is used by a modem handling or calling
 process, or by some other coupling of the PPP link to the physical
 media, to signal LCP that the link is entering Link Establishment
 phase.
 It also can be used by LCP to signal each NCP that the link is
 entering Network-Layer Protocol phase. That is, the This-Layer-Up
 action from LCP triggers the Up event in the NCP.
 Down
 This event occurs when a lower layer indicates that it is no
 longer ready to carry packets.
 Typically, this event is used by a modem handling or calling
 process, or by some other coupling of the PPP link to the physical
 media, to signal LCP that the link is entering Link Dead phase.
 It also can be used by LCP to signal each NCP that the link is
 leaving Network-Layer Protocol phase. That is, the This-Layer-
 Down action from LCP triggers the Down event in the NCP.
 Open
 This event indicates that the link is administratively available
 for traffic; that is, the network administrator (human or program)
 has indicated that the link is allowed to be Opened. When this
 event occurs, and the link is not in the Opened state, the
 automaton attempts to send configuration packets to the peer.
 If the automaton is not able to begin configuration (the lower
 layer is Down, or a previous Close event has not completed), the
 establishment of the link is automatically delayed.
 When a Terminate-Request is received, or other events occur which
 cause the link to become unavailable, the automaton will progress
 to a state where the link is ready to re-open. No additional
 administrative intervention is necessary.
 Implementation Option:
 Experience has shown that users will execute an additional Open
 command when they want to renegotiate the link. This might
 indicate that new values are to be negotiated.
 Since this is not the meaning of the Open event, it is
 suggested that when an Open user command is executed in the
 Opened, Closing, Stopping, or Stopped states, the
 implementation issue a Down event, immediately followed by an
 Up event. Care must be taken that an intervening Down event
 cannot occur from another source.
 The Down followed by an Up will cause an orderly renegotiation
 of the link, by progressing through the Starting to the
 Request-Sent state. This will cause the renegotiation of the
 link, without any harmful side effects.
 Close
 This event indicates that the link is not available for traffic;
 that is, the network administrator (human or program) has
 indicated that the link is not allowed to be Opened. When this
 event occurs, and the link is not in the Closed state, the
 automaton attempts to terminate the connection. Futher attempts
 to re-configure the link are denied until a new Open event occurs.
 Implementation Note:
 When authentication fails, the link SHOULD be terminated, to
 prevent attack by repetition and denial of service to other
 users. Since the link is administratively available (by
 definition), this can be accomplished by simulating a Close
 event to the LCP, immediately followed by an Open event. Care
 must be taken that an intervening Close event cannot occur from
 another source.
 The Close followed by an Open will cause an orderly termination
 of the link, by progressing through the Closing to the Stopping
 state, and the This-Layer-Finished action can disconnect the
 link. The automaton waits in the Stopped or Starting states
 for the next connection attempt.
 Timeout (TO+,TO-)
 This event indicates the expiration of the Restart timer. The
 Restart timer is used to time responses to Configure-Request and
 Terminate-Request packets.
 The TO+ event indicates that the Restart counter continues to be
 greater than zero, which triggers the corresponding Configure-
 Request or Terminate-Request packet to be retransmitted.
 The TO- event indicates that the Restart counter is not greater
 than zero, and no more packets need to be retransmitted.
 Receive-Configure-Request (RCR+,RCR-)
 This event occurs when a Configure-Request packet is received from
 the peer. The Configure-Request packet indicates the desire to
 open a connection and may specify Configuration Options. The
 Configure-Request packet is more fully described in a later
 section.
 The RCR+ event indicates that the Configure-Request was
 acceptable, and triggers the transmission of a corresponding
 Configure-Ack.
 The RCR- event indicates that the Configure-Request was
 unacceptable, and triggers the transmission of a corresponding
 Configure-Nak or Configure-Reject.
 Implementation Note:
 These events may occur on a connection which is already in the
 Opened state. The implementation MUST be prepared to
 immediately renegotiate the Configuration Options.
 Receive-Configure-Ack (RCA)
 This event occurs when a valid Configure-Ack packet is received
 from the peer. The Configure-Ack packet is a positive response to
 a Configure-Request packet. An out of sequence or otherwise
 invalid packet is silently discarded.
 Implementation Note:
 Since the correct packet has already been received before
 reaching the Ack-Rcvd or Opened states, it is extremely
 unlikely that another such packet will arrive. As specified,
 all invalid Ack/Nak/Rej packets are silently discarded, and do
 not affect the transitions of the automaton.
 However, it is not impossible that a correctly formed packet
 will arrive through a coincidentally-timed cross-connection.
 It is more likely to be the result of an implementation error.
 At the very least, this occurance SHOULD be logged.
 Receive-Configure-Nak/Rej (RCN)
 This event occurs when a valid Configure-Nak or Configure-Reject
 packet is received from the peer. The Configure-Nak and
 Configure-Reject packets are negative responses to a Configure-
 Request packet. An out of sequence or otherwise invalid packet is
 silently discarded.
 Implementation Note:
 Although the Configure-Nak and Configure-Reject cause the same
 state transition in the automaton, these packets have
 significantly different effects on the Configuration Options
 sent in the resulting Configure-Request packet.
 Receive-Terminate-Request (RTR)
 This event occurs when a Terminate-Request packet is received.
 The Terminate-Request packet indicates the desire of the peer to
 close the connection.
 Implementation Note:
 This event is not identical to the Close event (see above), and
 does not override the Open commands of the local network
 administrator. The implementation MUST be prepared to receive
 a new Configure-Request without network administrator
 intervention.
 Receive-Terminate-Ack (RTA)
 This event occurs when a Terminate-Ack packet is received from the
 peer. The Terminate-Ack packet is usually a response to a
 Terminate-Request packet. The Terminate-Ack packet may also
 indicate that the peer is in Closed or Stopped states, and serves
 to re-synchronize the link configuration.
 Receive-Unknown-Code (RUC)
 This event occurs when an un-interpretable packet is received from
 the peer. A Code-Reject packet is sent in response.
 Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
 This event occurs when a Code-Reject or a Protocol-Reject packet
 is received from the peer.
 The RXJ+ event arises when the rejected value is acceptable, such
 as a Code-Reject of an extended code, or a Protocol-Reject of a
 NCP. These are within the scope of normal operation. The
 implementation MUST stop sending the offending packet type.
 The RXJ- event arises when the rejected value is catastrophic,
 such as a Code-Reject of Configure-Request, or a Protocol-Reject
 of LCP! This event communicates an unrecoverable error that
 terminates the connection.
 Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
 (RXR)
 This event occurs when an Echo-Request, Echo-Reply or Discard-
 Request packet is received from the peer. The Echo-Reply packet
 is a response to an Echo-Request packet. There is no reply to an
 Echo-Reply or Discard-Request packet.
4.4. Actions
 Actions in the automaton are caused by events and typically indicate
 the transmission of packets and/or the starting or stopping of the
 Restart timer.
 Illegal-Event (-)
 This indicates an event that cannot occur in a properly
 implemented automaton. The implementation has an internal error,
 which should be reported and logged. No transition is taken, and
 the implementation SHOULD NOT reset or freeze.
 This-Layer-Up (tlu)
 This action indicates to the upper layers that the automaton is
 entering the Opened state.
 Typically, this action is used by the LCP to signal the Up event
 to a NCP, Authentication Protocol, or Link Quality Protocol, or
 MAY be used by a NCP to indicate that the link is available for
 its network layer traffic.
 This-Layer-Down (tld)
 This action indicates to the upper layers that the automaton is
 leaving the Opened state.
 Typically, this action is used by the LCP to signal the Down event
 to a NCP, Authentication Protocol, or Link Quality Protocol, or
 MAY be used by a NCP to indicate that the link is no longer
 available for its network layer traffic.
 This-Layer-Started (tls)
 This action indicates to the lower layers that the automaton is
 entering the Starting state, and the lower layer is needed for the
 link. The lower layer SHOULD respond with an Up event when the
 lower layer is available.
 This results of this action are highly implementation dependent.
 This-Layer-Finished (tlf)
 This action indicates to the lower layers that the automaton is
 entering the Initial, Closed or Stopped states, and the lower
 layer is no longer needed for the link. The lower layer SHOULD
 respond with a Down event when the lower layer has terminated.
 Typically, this action MAY be used by the LCP to advance to the
 Link Dead phase, or MAY be used by a NCP to indicate to the LCP
 that the link may terminate when there are no other NCPs open.
 This results of this action are highly implementation dependent.
 Initialize-Restart-Count (irc)
 This action sets the Restart counter to the appropriate value
 (Max-Terminate or Max-Configure). The counter is decremented for
 each transmission, including the first.
 Implementation Note:
 In addition to setting the Restart counter, the implementation
 MUST set the timeout period to the initial value when Restart
 timer backoff is used.
 Zero-Restart-Count (zrc)
 This action sets the Restart counter to zero.
 Implementation Note:
 This action enables the FSA to pause before proceeding to the
 desired final state, allowing traffic to be processed by the
 peer. In addition to zeroing the Restart counter, the
 implementation MUST set the timeout period to an appropriate
 value.
 Send-Configure-Request (scr)
 A Configure-Request packet is transmitted. This indicates the
 desire to open a connection with a specified set of Configuration
 Options. The Restart timer is started when the Configure-Request
 packet is transmitted, to guard against packet loss. The Restart
 counter is decremented each time a Configure-Request is sent.
 Send-Configure-Ack (sca)
 A Configure-Ack packet is transmitted. This acknowledges the
 reception of a Configure-Request packet with an acceptable set of
 Configuration Options.
 Send-Configure-Nak (scn)
 A Configure-Nak or Configure-Reject packet is transmitted, as
 appropriate. This negative response reports the reception of a
 Configure-Request packet with an unacceptable set of Configuration
 Options.
 Configure-Nak packets are used to refuse a Configuration Option
 value, and to suggest a new, acceptable value. Configure-Reject
 packets are used to refuse all negotiation about a Configuration
 Option, typically because it is not recognized or implemented.
 The use of Configure-Nak versus Configure-Reject is more fully
 described in the chapter on LCP Packet Formats.
 Send-Terminate-Request (str)
 A Terminate-Request packet is transmitted. This indicates the
 desire to close a connection. The Restart timer is started when
 the Terminate-Request packet is transmitted, to guard against
 packet loss. The Restart counter is decremented each time a
 Terminate-Request is sent.
 Send-Terminate-Ack (sta)
 A Terminate-Ack packet is transmitted. This acknowledges the
 reception of a Terminate-Request packet or otherwise serves to
 synchronize the automatons.
 Send-Code-Reject (scj)
 A Code-Reject packet is transmitted. This indicates the reception
 of an unknown type of packet.
 Send-Echo-Reply (ser)
 An Echo-Reply packet is transmitted. This acknowledges the
 reception of an Echo-Request packet.
4.5. Loop Avoidance
 The protocol makes a reasonable attempt at avoiding Configuration
 Option negotiation loops. However, the protocol does NOT guarantee
 that loops will not happen. As with any negotiation, it is possible
 to configure two PPP implementations with conflicting policies that
 will never converge. It is also possible to configure policies which
 do converge, but which take significant time to do so. Implementors
 should keep this in mind and SHOULD implement loop detection
 mechanisms or higher level timeouts.
4.6. Counters and Timers
 Restart Timer
 There is one special timer used by the automaton. The Restart
 timer is used to time transmissions of Configure-Request and
 Terminate-Request packets. Expiration of the Restart timer causes
 a Timeout event, and retransmission of the corresponding
 Configure-Request or Terminate-Request packet. The Restart timer
 MUST be configurable, but SHOULD default to three (3) seconds.
 Implementation Note:
 The Restart timer SHOULD be based on the speed of the link.
 The default value is designed for low speed (2,400 to 9,600
 bps), high switching latency links (typical telephone lines).
 Higher speed links, or links with low switching latency, SHOULD
 have correspondingly faster retransmission times.
 Instead of a constant value, the Restart timer MAY begin at an
 initial small value and increase to the configured final value.
 Each successive value less than the final value SHOULD be at
 least twice the previous value. The initial value SHOULD be
 large enough to account for the size of the packets, twice the
 round trip time for transmission at the link speed, and at
 least an additional 100 milliseconds to allow the peer to
 process the packets before responding. Some circuits add
 another 200 milliseconds of satellite delay. Round trip times
 for modems operating at 14,400 bps have been measured in the
 range of 160 to more than 600 milliseconds.
 Max-Terminate
 There is one required restart counter for Terminate-Requests.
 Max-Terminate indicates the number of Terminate-Request packets
 sent without receiving a Terminate-Ack before assuming that the
 peer is unable to respond. Max-Terminate MUST be configurable,
 but SHOULD default to two (2) transmissions.
 Max-Configure
 A similar counter is recommended for Configure-Requests. Max-
 Configure indicates the number of Configure-Request packets sent
 without receiving a valid Configure-Ack, Configure-Nak or
 Configure-Reject before assuming that the peer is unable to
 respond. Max-Configure MUST be configurable, but SHOULD default
 to ten (10) transmissions.
 Max-Failure
 A related counter is recommended for Configure-Nak. Max-Failure
 indicates the number of Configure-Nak packets sent without sending
 a Configure-Ack before assuming that configuration is not
 converging. Any further Configure-Nak packets for peer requested
 options are converted to Configure-Reject packets, and locally
 desired options are no longer appended. Max-Failure MUST be
 configurable, but SHOULD default to five (5) transmissions.
5. LCP Packet Formats
 There are three classes of LCP packets:
 1. Link Configuration packets used to establish and configure a
 link (Configure-Request, Configure-Ack, Configure-Nak and
 Configure-Reject).
 2. Link Termination packets used to terminate a link (Terminate-
 Request and Terminate-Ack).
 3. Link Maintenance packets used to manage and debug a link
 (Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
 Discard-Request).
 In the interest of simplicity, there is no version field in the LCP
 packet. A correctly functioning LCP implementation will always
 respond to unknown Protocols and Codes with an easily recognizable
 LCP packet, thus providing a deterministic fallback mechanism for
 implementations of other versions.
 Regardless of which Configuration Options are enabled, all LCP Link
 Configuration, Link Termination, and Code-Reject packets (codes 1
 through 7) are always sent as if no Configuration Options were
 negotiated. In particular, each Configuration Option specifies a
 default value. This ensures that such LCP packets are always
 recognizable, even when one end of the link mistakenly believes the
 link to be open.
 Exactly one LCP packet is encapsulated in the PPP Information field,
 where the PPP Protocol field indicates type hex c021 (Link Control
 Protocol).
 A summary of the Link Control Protocol packet format is shown below.
 The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Code
 The Code field is one octet, and identifies the kind of LCP
 packet. When a packet is received with an unknown Code field, a
 Code-Reject packet is transmitted.
 Up-to-date values of the LCP Code field are specified in the most
 recent "Assigned Numbers" RFC [2]. This document concerns the
 following values:
 1 Configure-Request
 2 Configure-Ack
 3 Configure-Nak
 4 Configure-Reject
 5 Terminate-Request
 6 Terminate-Ack
 7 Code-Reject
 8 Protocol-Reject
 9 Echo-Request
 10 Echo-Reply
 11 Discard-Request
 Identifier
 The Identifier field is one octet, and aids in matching requests
 and replies. When a packet is received with an invalid Identifier
 field, the packet is silently discarded without affecting the
 automaton.
 Length
 The Length field is two octets, and indicates the length of the
 LCP packet, including the Code, Identifier, Length and Data
 fields. The Length MUST NOT exceed the MRU of the link.
 Octets outside the range of the Length field are treated as
 padding and are ignored on reception. When a packet is received
 with an invalid Length field, the packet is silently discarded
 without affecting the automaton.
 Data
 The Data field is zero or more octets, as indicated by the Length
 field. The format of the Data field is determined by the Code
 field.
5.1. Configure-Request
 Description
 An implementation wishing to open a connection MUST transmit a
 Configure-Request. The Options field is filled with any desired
 changes to the link defaults. Configuration Options SHOULD NOT be
 included with default values.
 Upon reception of a Configure-Request, an appropriate reply MUST
 be transmitted.
 A summary of the Configure-Request packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
 1 for Configure-Request.
 Identifier
 The Identifier field MUST be changed whenever the contents of the
 Options field changes, and whenever a valid reply has been
 received for a previous request. For retransmissions, the
 Identifier MAY remain unchanged.
 Options
 The options field is variable in length, and contains the list of
 zero or more Configuration Options that the sender desires to
 negotiate. All Configuration Options are always negotiated
 simultaneously. The format of Configuration Options is further
 described in a later chapter.
5.2. Configure-Ack
 Description
 If every Configuration Option received in a Configure-Request is
 recognizable and all values are acceptable, then the
 implementation MUST transmit a Configure-Ack. The acknowledged
 Configuration Options MUST NOT be reordered or modified in any
 way.
 On reception of a Configure-Ack, the Identifier field MUST match
 that of the last transmitted Configure-Request. Additionally, the
 Configuration Options in a Configure-Ack MUST exactly match those
 of the last transmitted Configure-Request. Invalid packets are
 silently discarded.
 A summary of the Configure-Ack packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
 2 for Configure-Ack.
 Identifier
 The Identifier field is a copy of the Identifier field of the
 Configure-Request which caused this Configure-Ack.
 Options
 The Options field is variable in length, and contains the list of
 zero or more Configuration Options that the sender is
 acknowledging. All Configuration Options are always acknowledged
 simultaneously.
5.3. Configure-Nak
 Description
 If every instance of the received Configuration Options is
 recognizable, but some values are not acceptable, then the
 implementation MUST transmit a Configure-Nak. The Options field
 is filled with only the unacceptable Configuration Options from
 the Configure-Request. All acceptable Configuration Options are
 filtered out of the Configure-Nak, but otherwise the Configuration
 Options from the Configure-Request MUST NOT be reordered.
 Options which have no value fields (boolean options) MUST use the
 Configure-Reject reply instead.
 Each Configuration Option which is allowed only a single instance
 MUST be modified to a value acceptable to the Configure-Nak
 sender. The default value MAY be used, when this differs from the
 requested value.
 When a particular type of Configuration Option can be listed more
 than once with different values, the Configure-Nak MUST include a
 list of all values for that option which are acceptable to the
 Configure-Nak sender. This includes acceptable values that were
 present in the Configure-Request.
 Finally, an implementation may be configured to request the
 negotiation of a specific Configuration Option. If that option is
 not listed, then that option MAY be appended to the list of Nak'd
 Configuration Options, in order to prompt the peer to include that
 option in its next Configure-Request packet. Any value fields for
 the option MUST indicate values acceptable to the Configure-Nak
 sender.
 On reception of a Configure-Nak, the Identifier field MUST match
 that of the last transmitted Configure-Request. Invalid packets
 are silently discarded.
 Reception of a valid Configure-Nak indicates that when a new
 Configure-Request is sent, the Configuration Options MAY be
 modified as specified in the Configure-Nak. When multiple
 instances of a Configuration Option are present, the peer SHOULD
 select a single value to include in its next Configure-Request
 packet.
 Some Configuration Options have a variable length. Since the
 Nak'd Option has been modified by the peer, the implementation
 MUST be able to handle an Option length which is different from
 the original Configure-Request.
 A summary of the Configure-Nak packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
 3 for Configure-Nak.
 Identifier
 The Identifier field is a copy of the Identifier field of the
 Configure-Request which caused this Configure-Nak.
 Options
 The Options field is variable in length, and contains the list of
 zero or more Configuration Options that the sender is Nak'ing.
 All Configuration Options are always Nak'd simultaneously.
5.4. Configure-Reject
 Description
 If some Configuration Options received in a Configure-Request are
 not recognizable or are not acceptable for negotiation (as
 configured by a network administrator), then the implementation
 MUST transmit a Configure-Reject. The Options field is filled
 with only the unacceptable Configuration Options from the
 Configure-Request. All recognizable and negotiable Configuration
 Options are filtered out of the Configure-Reject, but otherwise
 the Configuration Options MUST NOT be reordered or modified in any
 way.
 On reception of a Configure-Reject, the Identifier field MUST
 match that of the last transmitted Configure-Request.
 Additionally, the Configuration Options in a Configure-Reject MUST
 be a proper subset of those in the last transmitted Configure-
 Request. Invalid packets are silently discarded.
 Reception of a valid Configure-Reject indicates that when a new
 Configure-Request is sent, it MUST NOT include any of the
 Configuration Options listed in the Configure-Reject.
 A summary of the Configure-Reject packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Options ...
 +-+-+-+-+
 Code
 4 for Configure-Reject.
 Identifier
 The Identifier field is a copy of the Identifier field of the
 Configure-Request which caused this Configure-Reject.
 Options
 The Options field is variable in length, and contains the list of
 zero or more Configuration Options that the sender is rejecting.
 All Configuration Options are always rejected simultaneously.
5.5. Terminate-Request and Terminate-Ack
 Description
 LCP includes Terminate-Request and Terminate-Ack Codes in order to
 provide a mechanism for closing a connection.
 An implementation wishing to close a connection SHOULD transmit a
 Terminate-Request. Terminate-Request packets SHOULD continue to
 be sent until Terminate-Ack is received, the lower layer indicates
 that it has gone down, or a sufficiently large number have been
 transmitted such that the peer is down with reasonable certainty.
 Upon reception of a Terminate-Request, a Terminate-Ack MUST be
 transmitted.
 Reception of an unelicited Terminate-Ack indicates that the peer
 is in the Closed or Stopped states, or is otherwise in need of
 re-negotiation.
 A summary of the Terminate-Request and Terminate-Ack packet formats
 is shown below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Code
 5 for Terminate-Request;
 6 for Terminate-Ack.
 Identifier
 On transmission, the Identifier field MUST be changed whenever the
 content of the Data field changes, and whenever a valid reply has
 been received for a previous request. For retransmissions, the
 Identifier MAY remain unchanged.
 On reception, the Identifier field of the Terminate-Request is
 copied into the Identifier field of the Terminate-Ack packet.
 Data
 The Data field is zero or more octets, and contains uninterpreted
 data for use by the sender. The data may consist of any binary
 value. The end of the field is indicated by the Length.
5.6. Code-Reject
 Description
 Reception of a LCP packet with an unknown Code indicates that the
 peer is operating with a different version. This MUST be reported
 back to the sender of the unknown Code by transmitting a Code-
 Reject.
 Upon reception of the Code-Reject of a code which is fundamental
 to this version of the protocol, the implementation SHOULD report
 the problem and drop the connection, since it is unlikely that the
 situation can be rectified automatically.
 A summary of the Code-Reject packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rejected-Packet ...
 +-+-+-+-+-+-+-+-+
 Code
 7 for Code-Reject.
 Identifier
 The Identifier field MUST be changed for each Code-Reject sent.
 Rejected-Packet
 The Rejected-Packet field contains a copy of the LCP packet which
 is being rejected. It begins with the Information field, and does
 not include any Data Link Layer headers nor an FCS. The
 Rejected-Packet MUST be truncated to comply with the peer's
 established MRU.
5.7. Protocol-Reject
 Description
 Reception of a PPP packet with an unknown Protocol field indicates
 that the peer is attempting to use a protocol which is
 unsupported. This usually occurs when the peer attempts to
 configure a new protocol. If the LCP automaton is in the Opened
 state, then this MUST be reported back to the peer by transmitting
 a Protocol-Reject.
 Upon reception of a Protocol-Reject, the implementation MUST stop
 sending packets of the indicated protocol at the earliest
 opportunity.
 Protocol-Reject packets can only be sent in the LCP Opened state.
 Protocol-Reject packets received in any state other than the LCP
 Opened state SHOULD be silently discarded.
 A summary of the Protocol-Reject packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rejected-Protocol | Rejected-Information ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Code
 8 for Protocol-Reject.
 Identifier
 The Identifier field MUST be changed for each Protocol-Reject
 sent.
 Rejected-Protocol
 The Rejected-Protocol field is two octets, and contains the PPP
 Protocol field of the packet which is being rejected.
 Rejected-Information
 The Rejected-Information field contains a copy of the packet which
 is being rejected. It begins with the Information field, and does
 not include any Data Link Layer headers nor an FCS. The
 Rejected-Information MUST be truncated to comply with the peer's
 established MRU.
5.8. Echo-Request and Echo-Reply
 Description
 LCP includes Echo-Request and Echo-Reply Codes in order to provide
 a Data Link Layer loopback mechanism for use in exercising both
 directions of the link. This is useful as an aid in debugging,
 link quality determination, performance testing, and for numerous
 other functions.
 Upon reception of an Echo-Request in the LCP Opened state, an
 Echo-Reply MUST be transmitted.
 Echo-Request and Echo-Reply packets MUST only be sent in the LCP
 Opened state. Echo-Request and Echo-Reply packets received in any
 state other than the LCP Opened state SHOULD be silently
 discarded.
 A summary of the Echo-Request and Echo-Reply packet formats is shown
 below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Magic-Number |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Code
 9 for Echo-Request;
 10 for Echo-Reply.
 Identifier
 On transmission, the Identifier field MUST be changed whenever the
 content of the Data field changes, and whenever a valid reply has
 been received for a previous request. For retransmissions, the
 Identifier MAY remain unchanged.
 On reception, the Identifier field of the Echo-Request is copied
 into the Identifier field of the Echo-Reply packet.
 Magic-Number
 The Magic-Number field is four octets, and aids in detecting links
 which are in the looped-back condition. Until the Magic-Number
 Configuration Option has been successfully negotiated, the Magic-
 Number MUST be transmitted as zero. See the Magic-Number
 Configuration Option for further explanation.
 Data
 The Data field is zero or more octets, and contains uninterpreted
 data for use by the sender. The data may consist of any binary
 value. The end of the field is indicated by the Length.
5.9. Discard-Request
 Description
 LCP includes a Discard-Request Code in order to provide a Data
 Link Layer sink mechanism for use in exercising the local to
 remote direction of the link. This is useful as an aid in
 debugging, performance testing, and for numerous other functions.
 Discard-Request packets MUST only be sent in the LCP Opened state.
 On reception, the receiver MUST silently discard any Discard-
 Request that it receives.
 A summary of the Discard-Request packet format is shown below. The
 fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Code | Identifier | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Magic-Number |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Code
 11 for Discard-Request.
 Identifier
 The Identifier field MUST be changed for each Discard-Request
 sent.
 Magic-Number
 The Magic-Number field is four octets, and aids in detecting links
 which are in the looped-back condition. Until the Magic-Number
 Configuration Option has been successfully negotiated, the Magic-
 Number MUST be transmitted as zero. See the Magic-Number
 Configuration Option for further explanation.
 Data
 The Data field is zero or more octets, and contains uninterpreted
 data for use by the sender. The data may consist of any binary
 value. The end of the field is indicated by the Length.
6. LCP Configuration Options
 LCP Configuration Options allow negotiation of modifications to the
 default characteristics of a point-to-point link. If a Configuration
 Option is not included in a Configure-Request packet, the default
 value for that Configuration Option is assumed.
 Some Configuration Options MAY be listed more than once. The effect
 of this is Configuration Option specific, and is specified by each
 such Configuration Option description. (None of the Configuration
 Options in this specification can be listed more than once.)
 The end of the list of Configuration Options is indicated by the
 Length field of the LCP packet.
 Unless otherwise specified, all Configuration Options apply in a
 half-duplex fashion; typically, in the receive direction of the link
 from the point of view of the Configure-Request sender.
 Design Philosophy
 The options indicate additional capabilities or requirements of
 the implementation that is requesting the option. An
 implementation which does not understand any option SHOULD
 interoperate with one which implements every option.
 A default is specified for each option which allows the link to
 correctly function without negotiation of the option, although
 perhaps with less than optimal performance.
 Except where explicitly specified, acknowledgement of an option
 does not require the peer to take any additional action other than
 the default.
 It is not necessary to send the default values for the options in
 a Configure-Request.
 A summary of the Configuration Option format is shown below. The
 fields are transmitted from left to right.
 0 1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Data ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 The Type field is one octet, and indicates the type of
 Configuration Option. Up-to-date values of the LCP Option Type
 field are specified in the most recent "Assigned Numbers" RFC [2].
 This document concerns the following values:
 0 RESERVED
 1 Maximum-Receive-Unit
 3 Authentication-Protocol
 4 Quality-Protocol
 5 Magic-Number
 7 Protocol-Field-Compression
 8 Address-and-Control-Field-Compression
 Length
 The Length field is one octet, and indicates the length of this
 Configuration Option including the Type, Length and Data fields.
 If a negotiable Configuration Option is received in a Configure-
 Request, but with an invalid or unrecognized Length, a Configure-
 Nak SHOULD be transmitted which includes the desired Configuration
 Option with an appropriate Length and Data.
 Data
 The Data field is zero or more octets, and contains information
 specific to the Configuration Option. The format and length of
 the Data field is determined by the Type and Length fields.
 When the Data field is indicated by the Length to extend beyond
 the end of the Information field, the entire packet is silently
 discarded without affecting the automaton.
6.1. Maximum-Receive-Unit (MRU)
 Description
 This Configuration Option may be sent to inform the peer that the
 implementation can receive larger packets, or to request that the
 peer send smaller packets.
 The default value is 1500 octets. If smaller packets are
 requested, an implementation MUST still be able to receive the
 full 1500 octet information field in case link synchronization is
 lost.
 Implementation Note:
 This option is used to indicate an implementation capability.
 The peer is not required to maximize the use of the capacity.
 For example, when a MRU is indicated which is 2048 octets, the
 peer is not required to send any packet with 2048 octets. The
 peer need not Configure-Nak to indicate that it will only send
 smaller packets, since the implementation will always require
 support for at least 1500 octets.
 A summary of the Maximum-Receive-Unit Configuration Option format is
 shown below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Maximum-Receive-Unit |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 1
 Length
 4
 Maximum-Receive-Unit
 The Maximum-Receive-Unit field is two octets, and specifies the
 maximum number of octets in the Information and Padding fields.
 It does not include the framing, Protocol field, FCS, nor any
 transparency bits or bytes.
6.2. Authentication-Protocol
 Description
 On some links it may be desirable to require a peer to
 authenticate itself before allowing network-layer protocol packets
 to be exchanged.
 This Configuration Option provides a method to negotiate the use
 of a specific protocol for authentication. By default,
 authentication is not required.
 An implementation MUST NOT include multiple Authentication-
 Protocol Configuration Options in its Configure-Request packets.
 Instead, it SHOULD attempt to configure the most desirable
 protocol first. If that protocol is Configure-Nak'd, then the
 implementation SHOULD attempt the next most desirable protocol in
 the next Configure-Request.
 The implementation sending the Configure-Request is indicating
 that it expects authentication from its peer. If an
 implementation sends a Configure-Ack, then it is agreeing to
 authenticate with the specified protocol. An implementation
 receiving a Configure-Ack SHOULD expect the peer to authenticate
 with the acknowledged protocol.
 There is no requirement that authentication be full-duplex or that
 the same protocol be used in both directions. It is perfectly
 acceptable for different protocols to be used in each direction.
 This will, of course, depend on the specific protocols negotiated.
 A summary of the Authentication-Protocol Configuration Option format
 is shown below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Authentication-Protocol |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Type
 3
 Length
 >= 4
 Authentication-Protocol
 The Authentication-Protocol field is two octets, and indicates the
 authentication protocol desired. Values for this field are always
 the same as the PPP Protocol field values for that same
 authentication protocol.
 Up-to-date values of the Authentication-Protocol field are
 specified in the most recent "Assigned Numbers" RFC [2]. Current
 values are assigned as follows:
 Value (in hex) Protocol
 c023 Password Authentication Protocol
 c223 Challenge Handshake Authentication Protocol
 Data
 The Data field is zero or more octets, and contains additional
 data as determined by the particular protocol.
6.3. Quality-Protocol
 Description
 On some links it may be desirable to determine when, and how
 often, the link is dropping data. This process is called link
 quality monitoring.
 This Configuration Option provides a method to negotiate the use
 of a specific protocol for link quality monitoring. By default,
 link quality monitoring is disabled.
 The implementation sending the Configure-Request is indicating
 that it expects to receive monitoring information from its peer.
 If an implementation sends a Configure-Ack, then it is agreeing to
 send the specified protocol. An implementation receiving a
 Configure-Ack SHOULD expect the peer to send the acknowledged
 protocol.
 There is no requirement that quality monitoring be full-duplex or
 that the same protocol be used in both directions. It is
 perfectly acceptable for different protocols to be used in each
 direction. This will, of course, depend on the specific protocols
 negotiated.
 A summary of the Quality-Protocol Configuration Option format is
 shown below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Quality-Protocol |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+-+-+
 Type
 4
 Length
 >= 4
 Quality-Protocol
 The Quality-Protocol field is two octets, and indicates the link
 quality monitoring protocol desired. Values for this field are
 always the same as the PPP Protocol field values for that same
 monitoring protocol.
 Up-to-date values of the Quality-Protocol field are specified in
 the most recent "Assigned Numbers" RFC [2]. Current values are
 assigned as follows:
 Value (in hex) Protocol
 c025 Link Quality Report
 Data
 The Data field is zero or more octets, and contains additional
 data as determined by the particular protocol.
6.4. Magic-Number
 Description
 This Configuration Option provides a method to detect looped-back
 links and other Data Link Layer anomalies. This Configuration
 Option MAY be required by some other Configuration Options such as
 the Quality-Protocol Configuration Option. By default, the
 Magic-Number is not negotiated, and zero is inserted where a
 Magic-Number might otherwise be used.
 Before this Configuration Option is requested, an implementation
 MUST choose its Magic-Number. It is recommended that the Magic-
 Number be chosen in the most random manner possible in order to
 guarantee with very high probability that an implementation will
 arrive at a unique number. A good way to choose a unique random
 number is to start with a unique seed. Suggested sources of
 uniqueness include machine serial numbers, other network hardware
 addresses, time-of-day clocks, etc. Particularly good random
 number seeds are precise measurements of the inter-arrival time of
 physical events such as packet reception on other connected
 networks, server response time, or the typing rate of a human
 user. It is also suggested that as many sources as possible be
 used simultaneously.
 When a Configure-Request is received with a Magic-Number
 Configuration Option, the received Magic-Number is compared with
 the Magic-Number of the last Configure-Request sent to the peer.
 If the two Magic-Numbers are different, then the link is not
 looped-back, and the Magic-Number SHOULD be acknowledged. If the
 two Magic-Numbers are equal, then it is possible, but not certain,
 that the link is looped-back and that this Configure-Request is
 actually the one last sent. To determine this, a Configure-Nak
 MUST be sent specifying a different Magic-Number value. A new
 Configure-Request SHOULD NOT be sent to the peer until normal
 processing would cause it to be sent (that is, until a Configure-
 Nak is received or the Restart timer runs out).
 Reception of a Configure-Nak with a Magic-Number different from
 that of the last Configure-Nak sent to the peer proves that a link
 is not looped-back, and indicates a unique Magic-Number. If the
 Magic-Number is equal to the one sent in the last Configure-Nak,
 the possibility of a looped-back link is increased, and a new
 Magic-Number MUST be chosen. In either case, a new Configure-
 Request SHOULD be sent with the new Magic-Number.
 If the link is indeed looped-back, this sequence (transmit
 Configure-Request, receive Configure-Request, transmit Configure-
 Nak, receive Configure-Nak) will repeat over and over again. If
 the link is not looped-back, this sequence might occur a few
 times, but it is extremely unlikely to occur repeatedly. More
 likely, the Magic-Numbers chosen at either end will quickly
 diverge, terminating the sequence. The following table shows the
 probability of collisions assuming that both ends of the link
 select Magic-Numbers with a perfectly uniform distribution:
 Number of Collisions Probability
 -------------------- ---------------------
 1 1/2**32 = 2.3 E-10
 2 1/2**32**2 = 5.4 E-20
 3 1/2**32**3 = 1.3 E-29
 Good sources of uniqueness or randomness are required for this
 divergence to occur. If a good source of uniqueness cannot be
 found, it is recommended that this Configuration Option not be
 enabled; Configure-Requests with the option SHOULD NOT be
 transmitted and any Magic-Number Configuration Options which the
 peer sends SHOULD be either acknowledged or rejected. In this
 case, looped-back links cannot be reliably detected by the
 implementation, although they may still be detectable by the peer.
 If an implementation does transmit a Configure-Request with a
 Magic-Number Configuration Option, then it MUST NOT respond with a
 Configure-Reject when it receives a Configure-Request with a
 Magic-Number Configuration Option. That is, if an implementation
 desires to use Magic Numbers, then it MUST also allow its peer to
 do so. If an implementation does receive a Configure-Reject in
 response to a Configure-Request, it can only mean that the link is
 not looped-back, and that its peer will not be using Magic-
 Numbers. In this case, an implementation SHOULD act as if the
 negotiation had been successful (as if it had instead received a
 Configure-Ack).
 The Magic-Number also may be used to detect looped-back links
 during normal operation, as well as during Configuration Option
 negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
 Request packets have a Magic-Number field. If Magic-Number has
 been successfully negotiated, an implementation MUST transmit
 these packets with the Magic-Number field set to its negotiated
 Magic-Number.
 The Magic-Number field of these packets SHOULD be inspected on
 reception. All received Magic-Number fields MUST be equal to
 either zero or the peer's unique Magic-Number, depending on
 whether or not the peer negotiated a Magic-Number.
 Reception of a Magic-Number field equal to the negotiated local
 Magic-Number indicates a looped-back link. Reception of a Magic-
 Number other than the negotiated local Magic-Number, the peer's
 negotiated Magic-Number, or zero if the peer didn't negotiate one,
 indicates a link which has been (mis)configured for communications
 with a different peer.
 Procedures for recovery from either case are unspecified, and may
 vary from implementation to implementation. A somewhat
 pessimistic procedure is to assume a LCP Down event. A further
 Open event will begin the process of re-establishing the link,
 which can't complete until the looped-back condition is
 terminated, and Magic-Numbers are successfully negotiated. A more
 optimistic procedure (in the case of a looped-back link) is to
 begin transmitting LCP Echo-Request packets until an appropriate
 Echo-Reply is received, indicating a termination of the looped-
 back condition.
 A summary of the Magic-Number Configuration Option format is shown
 below. The fields are transmitted from left to right.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Magic-Number
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Magic-Number (cont) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 5
 Length
 6
 Magic-Number
 The Magic-Number field is four octets, and indicates a number
 which is very likely to be unique to one end of the link. A
 Magic-Number of zero is illegal and MUST always be Nak'd, if it is
 not Rejected outright.
6.5. Protocol-Field-Compression (PFC)
 Description
 This Configuration Option provides a method to negotiate the
 compression of the PPP Protocol field. By default, all
 implementations MUST transmit packets with two octet PPP Protocol
 fields.
 PPP Protocol field numbers are chosen such that some values may be
 compressed into a single octet form which is clearly
 distinguishable from the two octet form. This Configuration
 Option is sent to inform the peer that the implementation can
 receive such single octet Protocol fields.
 As previously mentioned, the Protocol field uses an extension
 mechanism consistent with the ISO 3309 extension mechanism for the
 Address field; the Least Significant Bit (LSB) of each octet is
 used to indicate extension of the Protocol field. A binary "0" as
 the LSB indicates that the Protocol field continues with the
 following octet. The presence of a binary "1" as the LSB marks
 the last octet of the Protocol field. Notice that any number of
 "0" octets may be prepended to the field, and will still indicate
 the same value (consider the two binary representations for 3,
 00000011 and 00000000 00000011).
 When using low speed links, it is desirable to conserve bandwidth
 by sending as little redundant data as possible. The Protocol-
 Field-Compression Configuration Option allows a trade-off between
 implementation simplicity and bandwidth efficiency. If
 successfully negotiated, the ISO 3309 extension mechanism may be
 used to compress the Protocol field to one octet instead of two.
 The large majority of packets are compressible since data
 protocols are typically assigned with Protocol field values less
 than 256.
 Compressed Protocol fields MUST NOT be transmitted unless this
 Configuration Option has been negotiated. When negotiated, PPP
 implementations MUST accept PPP packets with either double-octet
 or single-octet Protocol fields, and MUST NOT distinguish between
 them.
 The Protocol field is never compressed when sending any LCP
 packet. This rule guarantees unambiguous recognition of LCP
 packets.
 When a Protocol field is compressed, the Data Link Layer FCS field
 is calculated on the compressed frame, not the original
 uncompressed frame.
 A summary of the Protocol-Field-Compression Configuration Option
 format is shown below. The fields are transmitted from left to
 right.
 0 1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 7
 Length
 2
6.6. Address-and-Control-Field-Compression (ACFC)
 Description
 This Configuration Option provides a method to negotiate the
 compression of the Data Link Layer Address and Control fields. By
 default, all implementations MUST transmit frames with Address and
 Control fields appropriate to the link framing.
 Since these fields usually have constant values for point-to-point
 links, they are easily compressed. This Configuration Option is
 sent to inform the peer that the implementation can receive
 compressed Address and Control fields.
 If a compressed frame is received when Address-and-Control-Field-
 Compression has not been negotiated, the implementation MAY
 silently discard the frame.
 The Address and Control fields MUST NOT be compressed when sending
 any LCP packet. This rule guarantees unambiguous recognition of
 LCP packets.
 When the Address and Control fields are compressed, the Data Link
 Layer FCS field is calculated on the compressed frame, not the
 original uncompressed frame.
 A summary of the Address-and-Control-Field-Compression configuration
 option format is shown below. The fields are transmitted from left
 to right.
 0 1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 8
 Length
 2
Security Considerations
 Security issues are briefly discussed in sections concerning the
 Authentication Phase, the Close event, and the Authentication-
 Protocol Configuration Option.
References
 [1] Perkins, D., "Requirements for an Internet Standard Point-to-
 Point Protocol", RFC 1547, Carnegie Mellon University,
 December 1993.
 [2] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
 1340, USC/Information Sciences Institute, July 1992.
Acknowledgements
 This document is the product of the Point-to-Point Protocol Working
 Group of the Internet Engineering Task Force (IETF). Comments should
 be submitted to the ietf-ppp@merit.edu mailing list.
 Much of the text in this document is taken from the working group
 requirements [1]; and RFCs 1171 & 1172, by Drew Perkins while at
 Carnegie Mellon University, and by Russ Hobby of the University of
 California at Davis.
 William Simpson was principally responsible for introducing
 consistent terminology and philosophy, and the re-design of the phase
 and negotiation state machines.
 Many people spent significant time helping to develop the Point-to-
 Point Protocol. The complete list of people is too numerous to list,
 but the following people deserve special thanks: Rick Adams, Ken
 Adelman, Fred Baker, Mike Ballard, Craig Fox, Karl Fox, Phill Gross,
 Kory Hamzeh, former WG chair Russ Hobby, David Kaufman, former WG
 chair Steve Knowles, Mark Lewis, former WG chair Brian Lloyd, John
 LoVerso, Bill Melohn, Mike Patton, former WG chair Drew Perkins, Greg
 Satz, John Shriver, Vernon Schryver, and Asher Waldfogel.
 Special thanks to Morning Star Technologies for providing computing
 resources and network access support for writing this specification.
Chair's Address
 The working group can be contacted via the current chair:
 Fred Baker
 Advanced Computer Communications
 315 Bollay Drive
 Santa Barbara, California 93117
 fbaker@acc.com
Editor's Address
 Questions about this memo can also be directed to:
 William Allen Simpson
 Daydreamer
 Computer Systems Consulting Services
 1384 Fontaine
 Madison Heights, Michigan 48071
 Bill.Simpson@um.cc.umich.edu
 bsimpson@MorningStar.com
Simpson [Page 52]