RFC 1702 (rfc1702) - Page 2 of 4

Generic Routing Encapsulation over IPv4 networks

RFC 1702                 GRE over IPv4 networks             October 1994


   This has the form:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Address Family         |  SRE Offset   |  SRE Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        IP Address List ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For the Address Family value of 0xfffe, the Routing Information field
   will consist of a list of Autonomous System numbers and indicates an
   AS source route.  The third octet of the Routing Information field
   contains an 8 bit unsigned integer offset from the start of the
   Source Route Entry (SRE), called the SRE Offset.  The SRE Offset
   indicates the first octet of the next AS number.  THe SRE Length
   field consists of the total length of the AS Number list in octets.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Address Family         |  SRE Offset   |  SRE Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         AS Number List ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP as both delivery and payload protocol

   When IP is encapsulated in IP, the TTL, TOS, and IP security options
   MAY be copied from the payload packet into the same fields in the
   delivery packet.  The payload packet's TTL MUST be decremented when
   the packet is decapsulated to insure that no packet lives forever.

IP source routes

   When a system is processing a SRE with an Address Family indicating
   an IP source route, it MUST use the SRE Offset to determine the next
   destination IP address.  If the next IP destination is this system,
   the SRE Offset field should be increased by four (the size of an IP
   address).  If the SRE Offset is equal to the SRE Length in this SRE,
   then the Offset field in the GRE header should be adjusted to point
   to the next SRE (if any).  This should be repeated until the next IP
   destination is not this system or until the entire SRE has been
   processed.

   If the source route is incomplete, then the Strict Source Route bit
   is checked.  If the source route is a strict source route and the
   next IP destination is NOT an adjacent system, the packet MUST be



Hanks, Li, Farinacci & Traina