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RFC 1702

            Generic Routing Encapsulation over IPv4 networks


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



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