rfc2784.txt

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Network Working Group                                    D. Farinacci
Request for Comments: 2784                                      T. Li
Category: Standards Track                            Procket Networks
                                                             S. Hanks
                                                 Enron Communications
                                                             D. Meyer
                                                        Cisco Systems
                                                            P. Traina
                                                     Juniper Networks
                                                           March 2000


                  Generic Routing Encapsulation (GRE)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document specifies a protocol for encapsulation of an arbitrary
   network layer protocol over another arbitrary network layer protocol.

1.  Introduction

   A number of different proposals [RFC1234, RFC1226] currently exist
   for the encapsulation of one protocol over another protocol. Other
   types of encapsulations [RFC1241, RFC1479] have been proposed for
   transporting IP over IP for policy purposes. This memo describes a
   protocol which is very similar to, but is more general than, the
   above proposals.  In attempting to be more general, many protocol
   specific nuances have been ignored. The result is that this proposal
   may be less suitable for a situation where a specific "X over Y"
   encapsulation has been described.  It is the attempt of this protocol
   to provide a simple, general purpose mechanism which reduces the
   problem of encapsulation from its current O(n^2) size to a more
   manageable size. This memo purposely does not address the issue of
   when a packet should be encapsulated.  This memo acknowledges, but
   does not address problems such as mutual encapsulation [RFC1326].




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   In the most general case, a system has a packet that needs to be
   encapsulated and delivered to some destination.  We will call this
   the payload packet.  The payload is first encapsulated in a GRE
   packet.  The resulting GRE packet can then be encapsulated in some
   other protocol and then forwarded.  We will call this outer protocol
   the delivery protocol. The algorithms for processing this packet are
   discussed later.

   Finally this specification describes the intersection of GRE
   currently deployed by multiple vendors.

   The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
   SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
   in RFC 2119 [RFC2119].

2. Structure of a GRE Encapsulated Packet

   A GRE encapsulated packet has the form:

    ---------------------------------
    |                               |
    |       Delivery Header         |
    |                               |
    ---------------------------------
    |                               |
    |       GRE Header              |
    |                               |
    ---------------------------------
    |                               |
    |       Payload packet          |
    |                               |
    ---------------------------------

   This specification is generally concerned with the structure of the
   GRE header, although special consideration is given to some of the
   issues surrounding IPv4 payloads.

2.1. GRE Header

   The GRE packet header has the form:

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |C|       Reserved0       | Ver |         Protocol Type         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Checksum (optional)      |       Reserved1 (Optional)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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2.2. Checksum Present (bit 0)

   If the Checksum Present bit is set to one, then the Checksum and the
   Reserved1 fields are present and the Checksum field contains valid
   information. Note that a compliant implementation MUST accept and
   process this field.

2.3. Reserved0 (bits 1-12)

   A receiver MUST discard a packet where any of bits 1-5 are non-zero,
   unless that receiver implements RFC 1701. Bits 6-12 are reserved for
   future use. These bits MUST be sent as zero and MUST be ignored on
   receipt.

2.3.1. Version Number (bits 13-15)

   The Version Number field MUST contain the value zero.

2.4. Protocol Type (2 octets)

   The Protocol Type field contains the protocol type of the payload
   packet. These Protocol Types are defined in [RFC1700] as "ETHER
   TYPES" and in [ETYPES]. An implementation receiving a packet
   containing a Protocol Type which is not listed in [RFC1700] or
   [ETYPES] SHOULD discard the packet.

2.5. Checksum (2 octets)

   The Checksum field contains the IP (one's complement) checksum sum of
   the all the 16 bit words in the GRE header and the payload packet.
   For purposes of computing the checksum, the value of the checksum
   field is zero. This field is present only if the Checksum Present bit
   is set to one.

2.6. Reserved1 (2 octets)

   The Reserved1 field is reserved for future use, and if present, MUST
   be transmitted as zero. The Reserved1 field is present only when the
   Checksum field is present (that is, Checksum Present bit is set to
   one).

3. IPv4 as a Payload

   When IPv4 is being carried as the GRE payload, the Protocol Type
   field MUST be set to 0x800.






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3.1. Forwarding Decapsulated IPv4 Payload Packets

   When a tunnel endpoint decapsulates a GRE packet which has an IPv4
   packet as the payload, the destination address in the IPv4 payload
   packet header MUST be used to forward the packet and the TTL of the
   payload packet MUST be decremented. Care should be taken when
   forwarding such a packet, since if the destination address of the
   payload packet is the encapsulator of the packet (i.e., the other end
   of the tunnel), looping can occur. In this case, the packet MUST be
   discarded.

4. IPv4 as a Delivery Protocol

   The IPv4 protocol 47 [RFC1700] is used when GRE packets are
   enapsulated in IPv4. See [RFC1122] for requirements relating to the
   delivery of packets over IPv4 networks.

5. Interoperation with RFC 1701 Compliant Implementations

   In RFC 1701, the field described here as Reserved0 contained a number
   of flag bits which this specification deprecates. In particular, the
   Routing Present, Key Present, Sequence Number Present, and Strict
   Source Route bits have been deprecated, along with the Recursion
   Control field. As a result, the GRE header will never contain the
   Key, Sequence Number or Routing fields specified in RFC 1701.

   There are, however, existing implementations of RFC 1701. The
   following sections describe correct interoperation with such
   implementations.

5.1. RFC 1701 Compliant Receiver

   An implementation complying to this specification will transmit the
   Reserved0 field set to zero. An RFC 1701 compliant receiver will
   interpret this as having the Routing Present, Key Present, Sequence
   Number Present, and Strict Source Route bits set to zero, and will
   not expect the RFC 1701 Key, Sequence Number or Routing fields to be
   present.

5.2. RFC 1701 Compliant Transmitter

   An RFC 1701 transmitter may set any of the Routing Present, Key
   Present, Sequence Number Present, and Strict Source Route bits set to
   one, and thus may transmit the RFC 1701 Key, Sequence Number or
   Routing fields in the GRE header. As stated in Section 5.3, a packet
   with non-zero bits in any of bits 1-5 MUST be discarded unless the
   receiver implements RFC 1701.




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6. Security Considerations

   Security in a network using GRE should be relatively similar to
   security in a normal IPv4 network, as routing using GRE follows the
   same routing that IPv4 uses natively. Route filtering will remain
   unchanged. However packet filtering requires either that a firewall
   look inside the GRE packet or that the filtering is done on the GRE
   tunnel endpoints. In those environments in which this is considered
   to be a security issue it may be desirable to terminate the tunnel at
   the firewall.

7. IANA Considerations

   This section considers the assignment of additional GRE Version
   Numbers and Protocol Types.

7.1.  GRE Version Numbers

   This document specifies GRE version number 0. GRE version number 1 is
   used by PPTP [RFC2637]. Additional GRE version numbers are assigned
   by IETF Consensus as defined in RFC 2434 [RFC2434].

7.2.  Protocol Types