Generic Routing Encapsulation over IPv4 networks
RFC 1702

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Network Working Group                                           S. Hanks
Request for Comments: 1702                               NetSmiths, Ltd.
Category: Informational                                            T. Li
                                                            D. Farinacci
                                                               P. Traina
                                                           cisco Systems
                                                            October 1994

            Generic Routing Encapsulation over IPv4 networks

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.

Introduction

   In an earlier memo [RFC 1701], we described GRE, a mechanism for
   encapsulating arbitrary packets within an arbitrary transport
   protocol.  This is a companion memo which describes the use of GRE
   with IP.  This memo addresses the case of using IP as the delivery
   protocol or the payload protocol and the special case of IP as both
   the delivery and payload.  This memo also describes using IP
   addresses and autonomous system numbers as part of a GRE source
   route.

IP as a delivery protocol

   GRE packets which are encapsulated within IP will use IP protocol
   type 47.

IP as a payload protocol

   IP packets will be encapsulated with a Protocol Type field of 0x800.

   For the Address Family value of 0x800, the Routing Information field
   will consist of a list of IP addresses and indicates an IP source
   route.  The first octet of the Routing Information field constitute a
   8 bit 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 IP address.  The SRE Length field consists of the total
   length of the IP Address List in octets.

Hanks, Li, Farinacci & Traina                                   [Page 1]
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                                   [Page 2]
RFC 1702                 GRE over IPv4 networks             October 1994

   dropped.  Otherwise, the system should use the IP address indicated
   by the Offset field to replace the destination address in the