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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.
+
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+Hanks, Li, Farinacci & Traina                                   [Page 1]
+
+RFC 1702                 GRE over IPv4 networks             October 1994
+
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+   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
+   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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+Hanks, Li, Farinacci & Traina                                   [Page 3]
+
+RFC 1702                 GRE over IPv4 networks             October 1994
+
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+Authors' Addresses
+
+   Stan Hanks
+   NetSmiths, Ltd.
+   2025 Lincoln Highway
+   Edison, NJ 08817
+
+   EMail: stan@netsmiths.com
+
+
+   Tony Li
+   cisco Systems, Inc.
+   1525 O'Brien Drive
+   Menlo Park, CA 94025
+
+   EMail: tli@cisco.com
+
+
+   Dino Farinacci
+   cisco Systems, Inc.
+   1525 O'Brien Drive
+   Menlo Park, CA 94025
+
+   EMail: dino@cisco.com
+
+
+   Paul Traina
+   cisco Systems, Inc.
+   1525 O'Brien Drive
+   Menlo Park, CA 94025
+
+   EMail: pst@cisco.com
+
+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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+Hanks, Li, Farinacci & Traina                                   [Page 4]
+