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.







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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



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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
  delivery header and forward the packet.

Autonomous system source routes

  When a system is processing a SRE with an Address Family indicating
  an AS source route, it MUST use the SRE Offset field to determine the
  next autonomous system.  If the next autonomous system is the local
  autonomous system, the SRE Offset field should be increased by two
  (the size of an autonomous system number).  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 autonomous system number is not
  equal to the local autonomous system number 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 autonomous system is NOT an adjacent autonomous system, the
  packet should be dropped.  Otherwise, the system should use the
  autonomous system number indicated by the SRE Offset field to replace
  the destination address in the delivery header and forward the
  packet.  The exact mechanism for determining the next delivery
  destination address given the AS number is outside of the scope of
  this document.

Security Considerations

  Security issues are not discussed in this memo.





















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Authors' Addresses

  Stan Hanks
  NetSmiths, Ltd.
  2025 Lincoln Highway
  Edison, NJ 08817

  EMail: [email protected]


  Tony Li
  cisco Systems, Inc.
  1525 O'Brien Drive
  Menlo Park, CA 94025

  EMail: [email protected]


  Dino Farinacci
  cisco Systems, Inc.
  1525 O'Brien Drive
  Menlo Park, CA 94025

  EMail: [email protected]


  Paul Traina
  cisco Systems, Inc.
  1525 O'Brien Drive
  Menlo Park, CA 94025

  EMail: [email protected]

References

  RFC 1701
     Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing
     Encapsulation", RFC 1701, NetSmiths, Ltd., and cisco Systems,
     October 1994.












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