IP Encapsulation within IP
RFC 2003

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Network Working Group                                         C. Perkins
Request for Comment: 2003                                            IBM
Category: Standards Track                                   October 1996

                       IP 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.
   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 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.
   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 based on the (network part of the) IP
   Destination Address field in the original IP header.  Once the
   encapsulated datagram arrives at this intermediate destination node,
   it is decapsulated, yielding the original IP datagram, which is then
   delivered to the destination indicated by the original Destination
   Address field.  This use 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
   "endpoints" of the tunnel.

   In the most general tunneling case we have

      source ---> encapsulator --------> decapsulator ---> destination

   with the source, encapsulator, decapsulator, and destination being
   separate nodes.  The encapsulator node is considered the "entry

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RFC 2003                      IP-within-IP                  October 1996

   point" of the tunnel, and the decapsulator node is considered the
   "exit point" of the tunnel.  There in general may be multiple
   source-destination pairs using the same tunnel between the
   encapsulator and decapsulator.

2. Motivation

   The Mobile IP working group has specified the use of encapsulation as
   a way to deliver datagrams 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 [8].  The use of
   encapsulation may also be desirable 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.

   It is generally true that encapsulation and the IP loose source
   routing option [10] can be used in similar ways to affect the routing
   of a datagram, but there are several technical reasons to prefer
   encapsulation:

    -  There are unsolved security problems associated with the use of
       the IP source routing options.

    -  Current Internet routers exhibit performance problems when
       forwarding datagrams that contain IP options, including the IP
       source routing options.

    -  Many current Internet nodes process IP source routing options
       incorrectly.

    -  Firewalls may exclude IP source-routed datagrams.

    -  Insertion of an IP source route option may complicate the
       processing of authentication information by the source and/or
       destination of a datagram, depending on how the authentication is
       specified to be performed.

    -  It is considered impolite for intermediate routers to make
       modifications to datagrams which they did not originate.

   These technical advantages must be weighed against the disadvantages
   posed by the use of encapsulation:

    -  Encapsulated datagrams typically are larger than source routed
       datagrams.

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RFC 2003                      IP-within-IP                  October 1996

    -  Encapsulation cannot be used unless it is known in advance that
       the node at the tunnel exit point can decapsulate the datagram.

   Since the majority of Internet nodes today do not perform well when
   IP loose source route options are used, the second technical