RFC 2784 (rfc2784) - Page 3 of 9

Generic Routing Encapsulation (GRE)

RFC 2784             Generic Routing Encapsulation            March 2000


2.2. Checksum Present (bit 0)

   If the Checksum Present bit is set to one, then the Checksum and the
   Reserved1 fields are present and the Checksum field contains valid
   information. Note that a compliant implementation MUST accept and
   process this field.

2.3. Reserved0 (bits 1-12)

   A receiver MUST discard a packet where any of bits 1-5 are non-zero,
   unless that receiver implements RFC 1701. Bits 6-12 are reserved for
   future use. These bits MUST be sent as zero and MUST be ignored on
   receipt.

2.3.1. Version Number (bits 13-15)

   The Version Number field MUST contain the value zero.

2.4. Protocol Type (2 octets)

   The Protocol Type field contains the protocol type of the payload
   packet. These Protocol Types are defined in [RFC 1700] as "ETHER
   TYPES" and in [ETYPES]. An implementation receiving a packet
   containing a Protocol Type which is not listed in [RFC 1700] or
   [ETYPES] SHOULD discard the packet.

2.5. Checksum (2 octets)

   The Checksum field contains the IP (one's complement) checksum sum of
   the all the 16 bit words in the GRE header and the payload packet.
   For purposes of computing the checksum, the value of the checksum
   field is zero. This field is present only if the Checksum Present bit
   is set to one.

2.6. Reserved1 (2 octets)

   The Reserved1 field is reserved for future use, and if present, MUST
   be transmitted as zero. The Reserved1 field is present only when the
   Checksum field is present (that is, Checksum Present bit is set to
   one).

3. IPv4 as a Payload

   When IPv4 is being carried as the GRE payload, the Protocol Type
   field MUST be set to 0x800.






Farinacci, et al.           Standards Track