RFC 1701 (rfc1701) - Page 2 of 8


Generic Routing Encapsulation (GRE)



Alternative Format: Original Text Document



RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


   protocol the delivery protocol. The algorithms for processing this
   packet are discussed later.

Overall packet

   The entire encapsulated packet would then have the form:

                  ---------------------------------
                  |                               |
                  |       Delivery Header         |
                  |                               |
                  ---------------------------------
                  |                               |
                  |       GRE Header              |
                  |                               |
                  ---------------------------------
                  |                               |
                  |       Payload packet          |
                  |                               |
                  ---------------------------------

Packet header

   The GRE packet header 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |C|R|K|S|s|Recur|  Flags  | Ver |         Protocol Type         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Checksum (optional)      |       Offset (optional)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Key (optional)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Sequence Number (optional)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Routing (optional)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Flags and version (2 octets)

      The GRE flags are encoded in the first two octets.  Bit 0 is the
      most significant bit, bit 15 is the least significant bit.  Bits
      13 through 15 are reserved for the Version field.  Bits 5 through
      12 are reserved for future use and MUST be transmitted as zero.






Hanks, Li, Farinacci & Traina