RFC 2003 (rfc2003) - Page 3 of 14

IP Encapsulation within IP

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
   disadvantage of encapsulation is not as serious as it might seem at
   first.

3. IP in IP Encapsulation

   To encapsulate an IP datagram using IP in IP encapsulation, an outer
   IP header [10] is inserted before the datagram's existing IP header,
   as follows:

                                         +---------------------------+
                                         |                           |
                                         |      Outer IP Header      |
                                         |                           |
     +---------------------------+       +---------------------------+
     |                           |       |                           |
     |         IP Header         |       |         IP Header         |
     |                           |       |                           |
     +---------------------------+ ====> +---------------------------+
     |                           |       |                           |
     |                           |       |                           |
     |         IP Payload        |       |         IP Payload        |
     |                           |       |                           |
     |                           |       |                           |
     +---------------------------+       +---------------------------+

   The outer IP header Source Address and Destination Address identify
   the "endpoints" of the tunnel.  The inner IP header Source Address
   and Destination Addresses identify the original sender and recipient
   of the datagram, respectively.  The inner IP header is not changed by
   the encapsulator, except to decrement the TTL as noted below, and
   remains unchanged during its delivery to the tunnel exit point.  No
   change to IP options in the inner header occurs during delivery of
   the encapsulated datagram through the tunnel.  If need be, other
   protocol headers such as the IP Authentication header [1] may be
   inserted between the outer IP header and the inner IP header.  Note
   that the security options of the inner IP header MAY affect the
   choice of security options for the encapsulating (outer) IP header.









Perkins                     Standards Track