Minimal Encapsulation within IP
draft-ietf-mobileip-minenc-02

Internet Engineering Task Force C. Perkins
INTERNET DRAFT IBM
 31 May 1996
 Minimal Encapsulation within IP
 draft-ietf-mobileip-minenc-02.txt
Status of This Memo
 This document is a submission by the Mobile IP Working Group of the
 Internet Engineering Task Force (IETF). Comments should be submitted
 to the mobile-ip@SmallWorks.COM mailing list.
 Distribution of this memo is unlimited.
 This document is an Internet-Draft. Internet-Drafts are working
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 and its working groups. Note that other groups may also distribute
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Abstract
 This document specifies a method by which an IP datagram may be
 encapsulated (carried as payload) within an IP datagram, with less
 overhead than "conventional" IP encapsulation that adds a second IP
 header to each encapsulated datagram. Encapsulation is suggested as
 a means to alter the normal IP routing for datagrams, by delivering
 them to an intermediate destination that would otherwise not be
 selected by the (network part of the) IP Destination Address field
 in the original IP header. Encapsulation may be serve a variety
 of purposes, such as delivery of a datagram to a mobile node using
 Mobile IP.
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1. Introduction
 This document specifies a method by which an IP datagram may be
 encapsulated (carried as payload) within an IP datagram, with
 less overhead than "conventional" IP encapsulation [4] that adds a
 second IP header to each encapsulated datagram. Encapsulation is
 suggested as a means to alter the normal IP routing for datagrams, by
 delivering them to an intermediate destination that would otherwise
 not be selected by the (network part of the) IP Destination Address
 field in the original IP header. The process of encapsulation and
 decapsulation of a datagram is frequently referred to as "tunneling"
 the datagram, and the encapsulator and decapsulator are then
 considered to be the the "endpoints" of the tunnel; the encapsulator
 node is refered to as the "entry point" of the tunnel, and the
 decapsulator node is refered to as the "exit point" of the tunnel.
2. Motivation
 The Mobile IP working group has specified the use of encapsulation
 as a way to deliver packets from a mobile node's "home network" to
 an agent that can deliver datagrams locally by conventional means
 to the mobile node at its current location away from home [5]. The
 use of encapsulation may also be indicated whenever the source (or
 an intermediate router) of an IP datagram must influence the route
 by which a datagram is to be delivered to its ultimate destination.
 Other possible applications of encapsulation include multicasting,
 preferential billing, choice of routes with selected security
 attributes, and general policy routing.
 See [4] for a discussion concerning the advantages of encapsulation
 versus use of the IP loose source routing option. Using IP headers
 to encapsulate IP datagrams requires the unnecessary duplication of
 several fields within the inner IP header; it is possible to save
 some additional space by specifying a new encapsulation mechanism
 that eliminates the duplication. The scheme outlined here comes from
 the Mobile IP Working Group (in earlier Internet Drafts), and is
 similar to that which had been defined in [2].
3. Minimal Encapsulation
 A minimal forwarding header is defined for datagrams which are not
 fragmented prior to encapsulation. Use of this encapsulating method
 is optional. Minimal encapsulation MUST NOT be used when an original
 datagram is already fragmented, since there is no room in the minimal
 forwarding header to store fragmentation information.
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 To encapsulate an IP datagram using minimal encapsulation, the
 minimal forwarding header is inserted into the datagram, as follows:
 +---------------------------+ +---------------------------+
 | | | |
 | IP Header | | Modified IP Header |
 | | | |
 +---------------------------+ ====> +---------------------------+
 | | | Minimal Forwarding Header |
 | | +---------------------------+
 | IP Payload | | |
 | | | |
 | | | IP Payload |
 +---------------------------+ | |
 | |
 +---------------------------+
 The IP header of the original datagram is modified, and the minimal
 forwarding header is inserted into the datagram after the IP header,
 followed by the unmodified IP payload of the original datagram (e.g.,
 transport header and transport data). No additional IP header is
 added to the datagram.
 In encapsulating the datagram, the original IP header [6] is modified
 as follows:
 - The Protocol field in the IP header is replaced by protocol
 number 55 for the minimal encapsulation protocol.
 - The Destination Address field in the IP header is replaced by the
 IP address of the exit point of the tunnel.
 - If the encapsulator is not the original source of the datagram,
 the Source Address field in the IP header is replaced by the IP
 address of the encapsulator.
 - The Total Length field in the IP header is incremented by the
 size of the minimal forwarding header added to the datagram.
 This incremental size is either 12 or 8 octets, depending on
 whether or not the Original Source Address Present (S) bit is set
 in the forwarding header.
 - The Header Checksum field in the IP header is recomputed [6] or
 updated to account for the changes in the IP header described
 here for encapsulation.
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 The format of the minimal forwarding header is as follows:
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Protocol |S| reserved | Header Checksum |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Original Destination Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : (if present) Original Source Address :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Protocol
 Copied from the Protocol field in the original IP header.
 Original Source Address Present (S)
 0 The Original Source Address field is not present. The
 length of the minimal tunneling header in this case is
 8 octets.
 1 The Original Source Address field is present. The
 length of the minimal tunneling header in this case is
 12 octets.
 reserved
 Sent as zero; ignored on reception.
 Header Checksum
 The 16-bit one's complement of the one's complement sum of all
 16-bit words in the minimal forwarding header. For purposes of
 computing the checksum, the value of the checksum field is 0.
 The IP header and IP payload (after the minimal forwarding
 header) are not included in this checksum computation.
 Original Destination Address
 Copied from the Destination Address field in the original IP
 header.
 Original Source Address
 Copied from the Source Address field in the original IP header.
 This field is present only if the Original Source Address
 Present (S) bit is set.
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 When decapsulating a datagram, the fields in the minimal forwarding
 header are restored to the IP header, and the forwarding header is
 removed from the datagram. In addition, the Total Length field in
 the IP header is decremented by the size of the minimal forwarding
 header removed from the datagram, and the Header Checksum field in
 the IP header is recomputed [6] or updated to account for the changes
 to the IP header described here for decapsulation.
 The encapsulator may use existing IP mechanisms appropriate for
 delivery of the encapsulated payload to the tunnel exit point. In
 particular, use of IP options are allowed, and use of fragmentation
 is allowed unless the "Don't Fragment" bit is set in the IP header.
 This restriction on fragmentation is required so that nodes employing
 Path MTU Discovery [3] can obtain the information they seek.
4. Routing Failures
 The use of any encapsulation method for routing purposes brings
 with it increased susceptibility to routing loops. To cut down the
 danger, a router should follow the same procedures outlined in [4].
5. ICMP Messages from within the Tunnel
 ICMP messages are to be handled as specified in [4], including the
 maintenance of tunnel "soft state".
6. Security Considerations
 Security considerations are not addressed in this document, but are
 generally similar to those outlined in [4].
7. Acknowledgements
 The original text for much of Section 3 was taken from the Mobile IP
 draft [1]. Thanks to David Johnson for improving consistency and
 making many other improvements to the draft.
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References
 [1] IPv4 Mobility Support. ietf-draft-mobileip-protocol-10.txt --
 outdated draft, May 1995.
 [2] David B. Johnson. Scalable and Robust Internetwork Routing
 for Mobile Hosts. In Proceedings of the 14th International
 Conference on Distributed Computing Systems, pages 2--11, June
 1994.
 [3] J. Mogul and S. Deering. Path MTU Discovery. RFC 1191, November
 1990.
 [4] Charles Perkins. IP Encapsulation within IP.
 draft-ietf-ip4inip4-01.txt (work in progress), May 1996.
 [5] C. Perkins, Editor. IPv4 Mobility Support.
 ietf-draft-mobileip-protocol-17.txt (work in progress), May 1996.
 [6] J. B. Postel, Editor. Internet Protocol. RFC 791, September
 1981.
Author's Address
 Questions about this memo can be directed to:
 Charles Perkins
 Room H3-D34
 T. J. Watson Research Center
 IBM Corporation
 30 Saw Mill River Rd.
 Hawthorne, NY 10532
 Work: +1-914-784-7350
 Fax: +1-914-784-6205
 E-mail: perk@watson.ibm.com
 The working group can be contacted via the current chair:
 Jim Solomon
 Motorola, Inc.
 1301 E. Algonquin Rd.
 Schaumburg, IL 60196
 Work: +1-847-576-2753
 E-mail: solomon@comm.mot.com
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