RFC 1622 (rfc1622) - Page 3 of 16

Pip Header Processing

RFC 1622                 Pip Header Processing                  May 1994


2.  Pip Specification

   The Pip header is partitioned into three parts, the Initial Part, the
   Transit Part, and the Options Part.


           +===========================+
           |       Initial Part        |
           +===========================+
           |       Transit Part        |
           +===========================+
           |       Options Part        |
           +===========================+
           |                           |
           |         Payload           |
           |                           |


   Each part falls on a 32-bit boundary (as indicated by the double
   lines shown), and the Transit Part falls on a 64 bit boundary.

   The concept of tunneling in an integral part of Pip.  Pip achieves
   tunneling by encapsulating the Transit Part of the Pip header in
   another Transit Part.  Therefore, when tunneling, there is one
   Transit Part for each (nested) tunnel:


           +===========================+
           |       Initial Part        |
           +===========================+
           |       Transit Part        |
           +===========================+
           |       Transit Part        |
           +===========================+
                       .
                       .
                       .
           +===========================+
           |       Transit Part        |
           +===========================+
           |       Options Part        |
           +===========================+


   Because each Transit Part has only what is necessary for router
   forwarding and handling, this method of tunneling is reasonably
   efficient in terms of packet size.




Francis