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].

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 RFC2119 [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)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 RFC1701. 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.

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 RFC1701 Compliant Implementations

In RFC1701, 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 RFC1701.

There are, however, existing implementations of RFC1701. The
following sections describe correct interoperation with such
implementations.

5.1. RFC1701 Compliant Receiver

An implementation complying to this specification will transmit the
Reserved0 field set to zero. An RFC1701 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 RFC1701 Key, Sequence Number or Routing fields to be
present.

5.2. RFC1701 Compliant Transmitter

An RFC1701 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 RFC1701 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 RFC1701.

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 RFC2434 [RFC2434].

7.2. Protocol Types

GRE uses an ETHER Type for the Protocol Type. New ETHER TYPES are
assigned by Xerox Systems Institute [RFC1700].

8. Acknowledgments

This document is derived from the original ideas of the authors of
RFC1701 and RFC1702. Hitoshi Asaeda, Scott Bradner, Randy Bush,
Brian Carpenter, Bill Fenner, Andy Malis, Thomas Narten, Dave Thaler,
Tim Gleeson and others provided many constructive and insightful
comments.

9. Appendix -- Known Issues

This document specifies the behavior of currently deployed GRE
implementations. As such, it does not attempt to address the
following known issues:

o Interaction Path MTU Discovery (PMTU) [RFC1191]

Existing implementations of GRE, when using IPv4 as the Delivery
Header, do not implement Path MTU discovery and do not set the
Don't Fragment bit in the Delivery Header. This can cause large
packets to become fragmented within the tunnel and reassembled at
the tunnel exit (independent of whether the payload packet is using
PMTU). If a tunnel entry point were to use Path MTU discovery,
however, that tunnel entry point would also need to relay ICMP
unreachable error messages (in particular the "fragmentation needed
and DF set" code) back to the originator of the packet, which is
not a requirement in this specification. Failure to properly relay
Path MTU information to an originator can result in the following
behavior: the originator sets the don't fragment bit, the packet
gets dropped within the tunnel, but since the originator doesn't
receive proper feedback, it retransmits with the same PMTU, causing
subsequently transmitted packets to be dropped.

o IPv6 as Delivery and/or Payload Protocol

This specification describes the intersection of GRE currently
deployed by multiple vendors. IPv6 as delivery and/or payload
protocol is not included in the currently deployed versions of GRE.

o Interaction with ICMP

o Interaction with the Differentiated Services Architecture

o Multiple and Looping Encapsulations

10. REFERENCES

[ETYPES] FTP://ftp.isi.edu/in-notes/iana/assignments/ethernet-
numbers

[RFC1122] Braden, R., "Requirements for Internet hosts -
communication layers", STD 3, RFC1122, October 1989.

[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC1191,
November 1990.

[RFC1226] Kantor, B., "Internet Protocol Encapsulation of AX.25
Frames", RFC1226, May 1991.

[RFC1234] Provan, D., "Tunneling IPX Traffic through IP Networks",
RFC1234, June 1991.

[RFC1241] Woodburn, R. and D. Mills, "Scheme for an Internet
Encapsulation Protocol: Version 1", RFC1241, July 1991.

[RFC1326] Tsuchiya, P., "Mutual Encapsulation Considered Dangerous",
RFC1326, May 1992.

[RFC1479] Steenstrup, M., "Inter-Domain Policy Routing Protocol
Specification: Version 1", RFC1479, July 1993.

[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.

[RFC1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation", RFC1701, October 1994.

[RFC1702] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation over IPv4 networks", RFC1702,
October 1994.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March, 1997.

[RFC2408] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC2408, November 1998.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC2434,
October, 1998.

[RFC2637] Hamzeh, K., et al., "Point-to-Point Tunneling Protocol
(PPTP)", RFC2637, July, 1999.

11. Authors' Addresses

Dino Farinacci
Procket Networks
3850 No. First St., Ste. C
San Jose, CA 95134

EMail: dino@procket.com

Tony Li
Procket Networks
3850 No. First St., Ste. C
San Jose, CA 95134

Phone: +1 408 954 7903
Fax: +1 408 987 6166
EMail: tony1@home.net

Stan Hanks
Enron Communications

EMail: stan_hanks@enron.net

David Meyer
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134

EMail: dmm@cisco.com

Paul Traina
Juniper Networks
EMail: pst@juniper.net

12. Full Copyright Statement

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

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