452 lines
15 KiB
Plaintext
452 lines
15 KiB
Plaintext
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Network Working Group S. Hanks
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Request for Comments: 1701 NetSmiths, Ltd.
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Category: Informational T. Li
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D. Farinacci
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P. Traina
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cisco Systems
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October 1994
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Generic Routing Encapsulation (GRE)
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Status of this Memo
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This memo provides information for the Internet community. This memo
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does not specify an Internet standard of any kind. Distribution of
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this memo is unlimited.
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Abstract
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This document specifies a protocol for performing encapsulation of an
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arbitrary network layer protocol over another arbitrary network layer
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protocol.
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Introduction
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A number of different proposals [RFC 1234, RFC 1226] currently exist
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for the encapsulation of one protocol over another protocol. Other
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types of encapsulations [RFC 1241, SDRP, RFC 1479] have been proposed
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for transporting IP over IP for policy purposes. This memo describes
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a protocol which is very similar to, but is more general than, the
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above proposals. In attempting to be more general, many protocol
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specific nuances have been ignored. The result is that this proposal
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is may be less suitable for a situation where a specific "X over Y"
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encapsulation has been described. It is the attempt of this protocol
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to provide a simple, general purpose mechanism which is reduces the
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problem of encapsulation from its current O(n^2) problem to a more
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manageable state. This proposal also attempts to provide a
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lightweight encapsulation for use in policy based routing. This memo
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explicitly does not address the issue of when a packet should be
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encapsulated. This memo acknowledges, but does not address problems
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with mutual encapsulation [RFC 1326].
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In the most general case, a system has a packet that needs to be
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encapsulated and routed. We will call this the payload packet. The
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payload is first encapsulated in a GRE packet, which possibly also
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includes a route. The resulting GRE packet can then be encapsulated
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in some other protocol and then forwarded. We will call this outer
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Hanks, Li, Farinacci & Traina [Page 1]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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protocol the delivery protocol. The algorithms for processing this
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packet are discussed later.
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Overall packet
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The entire encapsulated packet would then have the form:
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---------------------------------
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| Delivery Header |
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---------------------------------
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| GRE Header |
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---------------------------------
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| Payload packet |
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---------------------------------
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Packet header
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The GRE packet header has the form:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|C|R|K|S|s|Recur| Flags | Ver | Protocol Type |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Checksum (optional) | Offset (optional) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Key (optional) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Sequence Number (optional) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Routing (optional)
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Flags and version (2 octets)
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The GRE flags are encoded in the first two octets. Bit 0 is the
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most significant bit, bit 15 is the least significant bit. Bits
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13 through 15 are reserved for the Version field. Bits 5 through
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12 are reserved for future use and MUST be transmitted as zero.
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Hanks, Li, Farinacci & Traina [Page 2]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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Checksum Present (bit 0)
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If the Checksum Present bit is set to 1, then the Checksum field
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is present and contains valid information.
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If either the Checksum Present bit or the Routing Present bit are
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set, BOTH the Checksum and Offset fields are present in the GRE
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packet.
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Routing Present (bit 1)
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If the Routing Present bit is set to 1, then it indicates that the
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Offset and Routing fields are present and contain valid
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information.
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If either the Checksum Present bit or the Routing Present bit are
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set, BOTH the Checksum and Offset fields are present in the GRE
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packet.
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Key Present (bit 2)
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If the Key Present bit is set to 1, then it indicates that the Key
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field is present in the GRE header. Otherwise, the Key field is
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not present in the GRE header.
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Sequence Number Present (bit 3)
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If the Sequence Number Present bit is set to 1, then it indicates
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that the Sequence Number field is present. Otherwise, the
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Sequence Number field is not present in the GRE header.
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Strict Source Route (bit 4)
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The meaning of the Strict Source route bit is defined in other
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documents. It is recommended that this bit only be set to 1 if
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all of the the Routing Information consists of Strict Source
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Routes.
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Recursion Control (bits 5-7)
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Recursion control contains a three bit unsigned integer which
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contains the number of additional encapsulations which are
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permissible. This SHOULD default to zero.
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Version Number (bits 13-15)
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The Version Number field MUST contain the value 0. Other values
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are outside of the scope of this document.
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Hanks, Li, Farinacci & Traina [Page 3]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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Protocol Type (2 octets)
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The Protocol Type field contains the protocol type of the payload
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packet. In general, the value will be the Ethernet protocol type
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field for the packet. Currently defined protocol types are listed
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below. Additional values may be defined in other documents.
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Offset (2 octets)
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The offset field indicates the octet offset from the start of the
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Routing field to the first octet of the active Source Route Entry
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to be examined. This field is present if the Routing Present or
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the Checksum Present bit is set to 1, and contains valid
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information only if the Routing Present bit is set to 1.
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Checksum (2 octets)
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The Checksum field contains the IP (one's complement) checksum of
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the GRE header and the payload packet. This field is present if
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the Routing Present or the Checksum Present bit is set to 1, and
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contains valid information only if the Checksum Present bit is set
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to 1.
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Key (4 octets)
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The Key field contains a four octet number which was inserted by
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the encapsulator. It may be used by the receiver to authenticate
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the source of the packet. The techniques for determining
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authenticity are outside of the scope of this document. The Key
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field is only present if the Key Present field is set to 1.
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Sequence Number (4 octets)
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The Sequence Number field contains an unsigned 32 bit integer
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which is inserted by the encapsulator. It may be used by the
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receiver to establish the order in which packets have been
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transmitted from the encapsulator to the receiver. The exact
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algorithms for the generation of the Sequence Number and the
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semantics of their reception is outside of the scope of this
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document.
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Routing (variable)
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The Routing field is optional and is present only if the Routing
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Present bit is set to 1.
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Hanks, Li, Farinacci & Traina [Page 4]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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The Routing field is a list of Source Route Entries (SREs). Each
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SRE has the form:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Address Family | SRE Offset | SRE Length |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Routing Information ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The routing field is terminated with a "NULL" SRE containing an
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address family of type 0x0000 and a length of 0.
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Address Family (2 octets)
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The Address Family field contains a two octet value which indicates
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the syntax and semantics of the Routing Information field. The
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values for this field and the corresponding syntax and semantics for
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Routing Information are defined in other documents.
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SRE Offset (1 octet)
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The SRE Offset field indicates the octet offset from the start of the
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Routing Information field to the first octet of the active entry in
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Source Route Entry to be examined.
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SRE Length (1 octet)
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The SRE Length field contains the number of octets in the SRE. If
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the SRE Length is 0, this indicates this is the last SRE in the
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Routing field.
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Routing Information (variable)
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The Routing Information field contains data which may be used in
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routing this packet. The exact semantics of this field is defined in
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other documents.
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Forwarding of GRE packets
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Normally, a system which is forwarding delivery layer packets will
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not differentiate GRE packets from other packets in any way.
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However, a GRE packet may be received by a system. In this case, the
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system should use some delivery-specific means to determine that this
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is a GRE packet. Once this is determined, the Key, Sequence Number
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and Checksum fields if they contain valid information as indicated by
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the corresponding flags may be checked. If the Routing Present bit
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Hanks, Li, Farinacci & Traina [Page 5]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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is set to 1, then the Address Family field should be checked to
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determine the semantics and use of the SRE Length, SRE Offset and
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Routing Information fields. The exact semantics for processing a SRE
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for each Address Family is defined in other documents.
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Once all SREs have been processed, then the source route is complete,
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the GRE header should be removed, the payload's TTL MUST be
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decremented (if one exists) and the payload packet should be
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forwarded as a normal packet. The exact forwarding method depends on
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the Protocol Type field.
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Current List of Protocol Types
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The following are currently assigned protocol types for GRE. Future
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protocol types must be taken from DIX ethernet encoding. For
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historical reasons, a number of other values have been used for some
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protocols. The following table of values MUST be used to identify
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the following protocols:
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Protocol Family PTYPE
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--------------- -----
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Reserved 0000
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SNA 0004
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OSI network layer 00FE
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PUP 0200
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XNS 0600
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IP 0800
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Chaos 0804
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RFC 826 ARP 0806
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Frame Relay ARP 0808
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VINES 0BAD
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VINES Echo 0BAE
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VINES Loopback 0BAF
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DECnet (Phase IV) 6003
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Transparent Ethernet Bridging 6558
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Raw Frame Relay 6559
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Apollo Domain 8019
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Ethertalk (Appletalk) 809B
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Novell IPX 8137
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RFC 1144 TCP/IP compression 876B
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IP Autonomous Systems 876C
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Secure Data 876D
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Reserved FFFF
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See the IANA list of Ether Types for the complete list of these
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values.
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URL = ftp://ftp.isi.edu/in-notes/iana/assignments/ethernet-numbers.
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Hanks, Li, Farinacci & Traina [Page 6]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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References
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RFC 1479
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Steenstrup, M. "Inter-Domain Policy Routing Protocol
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Specification: Version 1", RFC1479, BBN Systems and Technologies,
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July 1993.
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RFC 1226
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Kantor, B. "Internet Protocol Encapsulation of AX.25 Frames", RFC
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1226, University of California, San Diego, May 1991.
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RFC 1234
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Provan, D. "Tunneling IPX Traffic through IP Networks", RFC 1234,
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Novell, Inc., June 1991.
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RFC 1241
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Woodburn, R., and D. Mills, "Scheme for an Internet Encapsulation
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Protocol: Version 1", RFC 1241, SAIC, University of Delaware, July
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1991.
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RFC 1326
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Tsuchiya, P., "Mutual Encapsulation Considered Dangerous", RFC
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1326, Bellcore, May 1992.
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SDRP
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Estrin, D., Li, T., and Y. Rekhter, "Source Demand Routing
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Protocol Specification (Version 1)", Work in Progress.
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RFC 1702
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Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing
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Encapsulation over IPv4 networks", RFC 1702, NetSmiths, Ltd.,
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cisco Systems, October 1994.
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Security Considerations
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Security issues are not discussed in this memo.
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Hanks, Li, Farinacci & Traina [Page 7]
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RFC 1701 Generic Routing Encapsulation (GRE) October 1994
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Acknowledgements
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The authors would like to acknowledge Yakov Rekhter (IBM) and Deborah
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Estrin (USC) for their advice, encouragement and insightful comments.
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Authors' Addresses
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Stan Hanks
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NetSmiths, Ltd.
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2025 Lincoln Highway
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Edison NJ, 08817
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EMail: stan@netsmiths.com
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Tony Li
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cisco Systems, Inc.
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1525 O'Brien Drive
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Menlo Park, CA 94025
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EMail: tli@cisco.com
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Dino Farinacci
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cisco Systems, Inc.
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1525 O'Brien Drive
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Menlo Park, CA 94025
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EMail: dino@cisco.com
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Paul Traina
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cisco Systems, Inc.
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1525 O'Brien Drive
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Menlo Park, CA 94025
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EMail: pst@cisco.com
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Hanks, Li, Farinacci & Traina [Page 8]
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