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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
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hanks, Li, Farinacci & Traina [Page 3]
+
+RFC 1702 GRE over IPv4 networks October 1994
+
+
+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]
+