RFC 2004 (rfc2004) - Page 2 of 6

Minimal Encapsulation within IP

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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