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7Network Working Group                                           S. Hanks
8Request for Comments: 1702                               NetSmiths, Ltd.
9Category: Informational                                            T. Li
10                                                            D. Farinacci
11                                                               P. Traina
12                                                           cisco Systems
13                                                            October 1994
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16            Generic Routing Encapsulation over IPv4 networks
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18Status of this Memo
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20   This memo provides information for the Internet community.  This memo
21   does not specify an Internet standard of any kind.  Distribution of
22   this memo is unlimited.
23
24Introduction
25
26   In an earlier memo [RFC 1701], we described GRE, a mechanism for
27   encapsulating arbitrary packets within an arbitrary transport
28   protocol.  This is a companion memo which describes the use of GRE
29   with IP.  This memo addresses the case of using IP as the delivery
30   protocol or the payload protocol and the special case of IP as both
31   the delivery and payload.  This memo also describes using IP
32   addresses and autonomous system numbers as part of a GRE source
33   route.
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35IP as a delivery protocol
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37   GRE packets which are encapsulated within IP will use IP protocol
38   type 47.
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40IP as a payload protocol
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42   IP packets will be encapsulated with a Protocol Type field of 0x800.
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44   For the Address Family value of 0x800, the Routing Information field
45   will consist of a list of IP addresses and indicates an IP source
46   route.  The first octet of the Routing Information field constitute a
47   8 bit integer offset from the start of the Source Route Entry (SRE),
48   called the SRE Offset.  The SRE Offset indicates the first octet of
49   the next IP address.  The SRE Length field consists of the total
50   length of the IP Address List in octets.
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58Hanks, Li, Farinacci & Traina                                   [Page 1]
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60RFC 1702                 GRE over IPv4 networks             October 1994
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62
63   This has the form:
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65       0                   1                   2                   3
66       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
67      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
68      |        Address Family         |  SRE Offset   |  SRE Length   |
69      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70      |                        IP Address List ...
71      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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73   For the Address Family value of 0xfffe, the Routing Information field
74   will consist of a list of Autonomous System numbers and indicates an
75   AS source route.  The third octet of the Routing Information field
76   contains an 8 bit unsigned integer offset from the start of the
77   Source Route Entry (SRE), called the SRE Offset.  The SRE Offset
78   indicates the first octet of the next AS number.  THe SRE Length
79   field consists of the total length of the AS Number list in octets.
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81       0                   1                   2                   3
82       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
83      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
84      |        Address Family         |  SRE Offset   |  SRE Length   |
85      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
86      |                         AS Number List ...
87      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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89IP as both delivery and payload protocol
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91   When IP is encapsulated in IP, the TTL, TOS, and IP security options
92   MAY be copied from the payload packet into the same fields in the
93   delivery packet.  The payload packet's TTL MUST be decremented when
94   the packet is decapsulated to insure that no packet lives forever.
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96IP source routes
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98   When a system is processing a SRE with an Address Family indicating
99   an IP source route, it MUST use the SRE Offset to determine the next
100   destination IP address.  If the next IP destination is this system,
101   the SRE Offset field should be increased by four (the size of an IP
102   address).  If the SRE Offset is equal to the SRE Length in this SRE,
103   then the Offset field in the GRE header should be adjusted to point
104   to the next SRE (if any).  This should be repeated until the next IP
105   destination is not this system or until the entire SRE has been
106   processed.
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108   If the source route is incomplete, then the Strict Source Route bit
109   is checked.  If the source route is a strict source route and the
110   next IP destination is NOT an adjacent system, the packet MUST be
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114Hanks, Li, Farinacci & Traina                                   [Page 2]
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116RFC 1702                 GRE over IPv4 networks             October 1994
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119   dropped.  Otherwise, the system should use the IP address indicated
120   by the Offset field to replace the destination address in the
121   delivery header and forward the packet.
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123Autonomous system source routes
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125   When a system is processing a SRE with an Address Family indicating
126   an AS source route, it MUST use the SRE Offset field to determine the
127   next autonomous system.  If the next autonomous system is the local
128   autonomous system, the SRE Offset field should be increased by two
129   (the size of an autonomous system number).  If the SRE Offset is
130   equal to the SRE Length in this SRE, then the Offset field in the GRE
131   header should be adjusted to point to the next SRE (if any).  This
132   should be repeated until the next autonomous system number is not
133   equal to the local autonomous system number or until the entire SRE
134   has been processed.
135
136   If the source route is incomplete, then the Strict Source Route bit
137   is checked.  If the source route is a strict source route and the
138   next autonomous system is NOT an adjacent autonomous system, the
139   packet should be dropped.  Otherwise, the system should use the
140   autonomous system number indicated by the SRE Offset field to replace
141   the destination address in the delivery header and forward the
142   packet.  The exact mechanism for determining the next delivery
143   destination address given the AS number is outside of the scope of
144   this document.
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146Security Considerations
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148   Security issues are not discussed in this memo.
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170Hanks, Li, Farinacci & Traina                                   [Page 3]
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172RFC 1702                 GRE over IPv4 networks             October 1994
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175Authors' Addresses
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177   Stan Hanks
178   NetSmiths, Ltd.
179   2025 Lincoln Highway
180   Edison, NJ 08817
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182   EMail: stan@netsmiths.com
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185   Tony Li
186   cisco Systems, Inc.
187   1525 O'Brien Drive
188   Menlo Park, CA 94025
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190   EMail: tli@cisco.com
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193   Dino Farinacci
194   cisco Systems, Inc.
195   1525 O'Brien Drive
196   Menlo Park, CA 94025
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198   EMail: dino@cisco.com
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201   Paul Traina
202   cisco Systems, Inc.
203   1525 O'Brien Drive
204   Menlo Park, CA 94025
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206   EMail: pst@cisco.com
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208References
209
210   RFC 1701
211      Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing
212      Encapsulation", RFC 1701, NetSmiths, Ltd., and cisco Systems,
213      October 1994.
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226Hanks, Li, Farinacci & Traina                                   [Page 4]
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