rfc1701.txt

this is a linux pptp software


RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


      The Routing field is a list of Source Route Entries (SREs).  Each
      SRE 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Routing Information ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The routing field is terminated with a "NULL" SRE containing an
   address family of type 0x0000 and a length of 0.

   Address Family (2 octets)

   The Address Family field contains a two octet value which indicates
   the syntax and semantics of the Routing Information field.  The
   values for this field and the corresponding syntax and semantics for
   Routing Information are defined in other documents.

   SRE Offset (1 octet)

   The SRE Offset field indicates the octet offset from the start of the
   Routing Information field to the first octet of the active entry in
   Source Route Entry to be examined.

   SRE Length (1 octet)

   The SRE Length field contains the number of octets in the SRE.  If
   the SRE Length is 0, this indicates this is the last SRE in the
   Routing field.

   Routing Information (variable)

   The Routing Information field contains data which may be used in
   routing this packet.  The exact semantics of this field is defined in
   other documents.

Forwarding of GRE packets

   Normally, a system which is forwarding delivery layer packets will
   not differentiate GRE packets from other packets in any way.
   However, a GRE packet may be received by a system.  In this case, the
   system should use some delivery-specific means to determine that this
   is a GRE packet.  Once this is determined, the Key, Sequence Number
   and Checksum fields if they contain valid information as indicated by
   the corresponding flags may be checked.  If the Routing Present bit



Hanks, Li, Farinacci & Traina                                   [Page 5]

RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


   is set to 1, then the Address Family field should be checked to
   determine the semantics and use of the SRE Length, SRE Offset and
   Routing Information fields.  The exact semantics for processing a SRE
   for each Address Family is defined in other documents.

   Once all SREs have been processed, then the source route is complete,
   the GRE header should be removed, the payload's TTL MUST be
   decremented (if one exists) and the payload packet should be
   forwarded as a normal packet.  The exact forwarding method depends on
   the Protocol Type field.

Current List of Protocol Types

   The following are currently assigned protocol types for GRE.  Future
   protocol types must be taken from DIX ethernet encoding.  For
   historical reasons, a number of other values have been used for some
   protocols.  The following table of values MUST be used to identify
   the following protocols:

       Protocol Family                     PTYPE
       ---------------                     -----
       Reserved                            0000
       SNA                                 0004
       OSI network layer                   00FE
       PUP                                 0200
       XNS                                 0600
       IP                                  0800
       Chaos                               0804
       RFC 826 ARP                         0806
       Frame Relay ARP                     0808
       VINES                               0BAD
       VINES Echo                          0BAE
       VINES Loopback                      0BAF
       DECnet (Phase IV)                   6003
       Transparent Ethernet Bridging       6558
       Raw Frame Relay                     6559
       Apollo Domain                       8019
       Ethertalk (Appletalk)               809B
       Novell IPX                          8137
       RFC 1144 TCP/IP compression         876B
       IP Autonomous Systems               876C
       Secure Data                         876D
       Reserved                            FFFF

   See the IANA list of Ether Types for the complete list of these
   values.

   URL = ftp://ftp.isi.edu/in-notes/iana/assignments/ethernet-numbers.



Hanks, Li, Farinacci & Traina                                   [Page 6]

RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


References

   RFC 1479
      Steenstrup, M. "Inter-Domain Policy Routing Protocol
      Specification: Version 1", RFC1479, BBN Systems and Technologies,
      July 1993.

   RFC 1226
      Kantor, B. "Internet Protocol Encapsulation of AX.25 Frames", RFC
      1226, University of California, San Diego, May 1991.

   RFC 1234
      Provan, D. "Tunneling IPX Traffic through IP Networks", RFC 1234,
      Novell, Inc., June 1991.

   RFC 1241
      Woodburn, R., and D. Mills, "Scheme for an Internet Encapsulation
      Protocol: Version 1", RFC 1241, SAIC, University of Delaware, July
      1991.

   RFC 1326
      Tsuchiya, P., "Mutual Encapsulation Considered Dangerous", RFC
      1326, Bellcore, May 1992.

   SDRP
      Estrin, D., Li, T., and Y. Rekhter, "Source Demand Routing
      Protocol Specification (Version 1)", Work in Progress.

   RFC 1702
      Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing
      Encapsulation over IPv4 networks", RFC 1702, NetSmiths, Ltd.,
      cisco Systems, October 1994.

Security Considerations

   Security issues are not discussed in this memo.















Hanks, Li, Farinacci & Traina                                   [Page 7]

RFC 1701          Generic Routing Encapsulation (GRE)       October 1994


Acknowledgements

   The authors would like to acknowledge Yakov Rekhter (IBM) and Deborah
   Estrin (USC) for their advice, encouragement and insightful comments.

Authors'  Addresses

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

   EMail: [e-mail]


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

   EMail: [e-mail]


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

   EMail: [e-mail]


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

   EMail: [e-mail]














Hanks, Li, Farinacci & Traina                                   [Page 8]