RFC 2003 (rfc2003) - Page 2 of 14

IP Encapsulation within IP

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.



Perkins                     Standards Track