    Network Working Group                                         C. Perkins
    Request for Comments: 2004                                           IBM
    Category: Standards Track                                   October 1996

                        Minimal Encapsulation within IP

    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

       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.

    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.

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    RFC 2004              Minimal Encapsulation for IP          October 1996

    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.  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        |
         +---------------------------+       |                           |
                                             |                           |
                                             +---------------------------+

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    RFC 2004              Minimal Encapsulation for IP          October 1996

       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.

        Note that unlike IP-in-IP encapsulation [4], the Time to Live
        (TTL) field in the IP header is not modified during encapsulation;
        if the encapsulator is forwarding the datagram, it will decrement
        the TTL as a result of doing normal IP forwarding.  Also, since
        the original TTL remains in the IP header after encapsulation,
        hops taken by the datagram within the tunnel are visible, for
        example, to "traceroute".

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    RFC 2004              Minimal Encapsulation for IP          October 1996

       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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    RFC 2004              Minimal Encapsulation for IP          October 1996

       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] Perkins, C., Editor, "IPv4 Mobility Support", Work in Progress,
           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] Mogul, J.,  and S. Deering, "Path MTU Discovery", RFC 1191,
           November 1990.

       [4] Perkins, C., "IP Encapsulation within IP", RFC 2003,
           October 1996.

       [5] Perkins, C., Editor, "IP Mobility Support", RFC 2002,
           October 1996.

       [6] Postel, J., Editor, "Internet Protocol", STD 5, 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
       EMail: 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
       EMail: solomon@comm.mot.com

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