rfc1701.txt

this is a linux pptp software

Network Working Group                                           S. Hanks
Request for Comments: 1701                               NetSmiths, Ltd.
Category: Informational                                            T. Li
                                                            D. Farinacci
                                                               P. Traina
                                                           cisco Systems
                                                            October 1994


                  Generic Routing Encapsulation (GRE)

Status of this Memo


   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document specifies a protocol for performing encapsulation of an
   arbitrary network layer protocol over another arbitrary network layer
   protocol.

Introduction

   A number of different proposals [RFC 1234, RFC 1226] currently exist
   for the encapsulation of one protocol over another protocol. Other
   types of encapsulations [RFC 1241, SDRP, RFC 1479] have been proposed
   for transporting IP over IP for policy purposes. This memo describes
   a protocol which is very similar to, but is more general than, the
   above proposals.  In attempting to be more general, many protocol
   specific nuances have been ignored.  The result is that this proposal
   is may be less suitable for a situation where a specific "X over Y"
   encapsulation has been described.  It is the attempt of this protocol
   to provide a simple, general purpose mechanism which is reduces the
   problem of encapsulation from its current O(n^2) problem to a more
   manageable state.  This proposal also attempts to provide a
   lightweight encapsulation for use in policy based routing.  This memo
   explicitly does not address the issue of when a packet should be
   encapsulated.  This memo acknowledges, but does not address problems
   with mutual encapsulation [RFC 1326].

   In the most general case, a system has a packet that needs to be
   encapsulated and routed.  We will call this the payload packet.  The
   payload is first encapsulated in a GRE packet, which possibly also
   includes a route.  The resulting GRE packet can then be encapsulated
   in some other protocol and then forwarded.  We will call this outer



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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.






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RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


      Checksum Present (bit 0)

      If the Checksum Present bit is set to 1, then the Checksum field
      is present and contains valid information.

      If either the Checksum Present bit or the Routing Present bit are
      set, BOTH the Checksum and Offset fields are present in the GRE
      packet.

      Routing Present (bit 1)

      If the Routing Present bit is set to 1, then it indicates that the
      Offset and Routing fields are present and contain valid
      information.

      If either the Checksum Present bit or the Routing Present bit are
      set, BOTH the Checksum and Offset fields are present in the GRE
      packet.

      Key Present (bit 2)

      If the Key Present bit is set to 1, then it indicates that the Key
      field is present in the GRE header.  Otherwise, the Key field is
      not present in the GRE header.

      Sequence Number Present (bit 3)

      If the Sequence Number Present bit is set to 1, then it indicates
      that the Sequence Number field is present.  Otherwise, the
      Sequence Number field is not present in the GRE header.

      Strict Source Route (bit 4)

      The meaning of the Strict Source route bit is defined in other
      documents.  It is recommended that this bit only be set to 1 if
      all of the the Routing Information consists of Strict Source
      Routes.

      Recursion Control (bits 5-7)

      Recursion control contains a three bit unsigned integer which
      contains the number of additional encapsulations which are
      permissible.  This SHOULD default to zero.

      Version Number (bits 13-15)

      The Version Number field MUST contain the value 0.  Other values
      are outside of the scope of this document.



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RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


      Protocol Type (2 octets)

      The Protocol Type field contains the protocol type of the payload
      packet.  In general, the value will be the Ethernet protocol type
      field for the packet.  Currently defined protocol types are listed
      below.  Additional values may be defined in other documents.

      Offset (2 octets)

      The offset field indicates the octet offset from the start of the
      Routing field to the first octet of the active Source Route Entry
      to be examined.  This field is present if the Routing Present or
      the Checksum Present bit is set to 1, and contains valid
      information only if the Routing Present bit is set to 1.

      Checksum (2 octets)

      The Checksum field contains the IP (one's complement) checksum of
      the GRE header and the payload packet.  This field is present if
      the Routing Present or the Checksum Present bit is set to 1, and
      contains valid information only if the Checksum Present bit is set
      to 1.

      Key (4 octets)

      The Key field contains a four octet number which was inserted by
      the encapsulator.  It may be used by the receiver to authenticate
      the source of the packet.  The techniques for determining
      authenticity are outside of the scope of this document.  The Key
      field is only present if the Key Present field is set to 1.

      Sequence Number (4 octets)

      The Sequence Number field contains an unsigned 32 bit integer
      which is inserted by the encapsulator.  It may be used by the
      receiver to establish the order in which packets have been
      transmitted from the encapsulator to the receiver.  The exact
      algorithms for the generation of the Sequence Number and the
      semantics of their reception is outside of the scope of this
      document.

      Routing (variable)

      The Routing field is optional and is present only if the Routing
      Present bit is set to 1.






Hanks, Li, Farinacci & Traina                                   [Page 4]