Network Working Group                                           S. HanksRequest for Comments: 1702                               NetSmiths, Ltd.Category: Informational                                            T. Li                                                            D. Farinacci                                                               P. Traina                                                           cisco Systems                                                            October 1994            Generic Routing Encapsulation over IPv4 networksStatus 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 beHanks, 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 1994Authors' 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.comReferences   RFC 1701      Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing      Encapsulation", RFC 1701, NetSmiths, Ltd., and cisco Systems,      October 1994.Hanks, Li, Farinacci & Traina                                   [Page 4]