Generic Routing Encapsulation (GRE)
A general header for carrying one network-layer packet inside another.
Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, DOI 10.17487/RFC2784, March 2000, <https://www.rfc-editor.org/info/rfc2784>.
@misc{rfc2784,
series = {Request for Comments},
number = 2784,
howpublished = {RFC 2784},
publisher = {RFC Editor},
doi = {10.17487/RFC2784},
url = {https://www.rfc-editor.org/info/rfc2784},
author = {D. Farinacci and T. Li and S. Hanks and D. Meyer and P. Traina},
title = {{Generic Routing Encapsulation (GRE)}},
pagetotal = 9,
year = 2000,
month = mar,
} RFC text
Contents
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 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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.
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
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
RFC 1701 and RFC 1702. 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, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[RFC1226] Kantor, B., "Internet Protocol Encapsulation of AX.25
Frames", RFC 1226, May 1991.
[RFC1234] Provan, D., "Tunneling IPX Traffic through IP Networks",
RFC 1234, June 1991.
[RFC1241] Woodburn, R. and D. Mills, "Scheme for an Internet
Encapsulation Protocol: Version 1", RFC 1241, July 1991.
[RFC1326] Tsuchiya, P., "Mutual Encapsulation Considered Dangerous",
RFC 1326, May 1992.
[RFC1479] Steenstrup, M., "Inter-Domain Policy Routing Protocol
Specification: Version 1", RFC 1479, 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", RFC 1701, October 1994.
[RFC1702] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation over IPv4 networks", RFC 1702,
October 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March, 1997.
[RFC2408] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October, 1998.
[RFC2637] Hamzeh, K., et al., "Point-to-Point Tunneling Protocol
(PPTP)", RFC 2637, 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.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
ISO Plain Language
On this page
§1A General Packet Encapsulation
GRE is a general protocol for encapsulating one network-layer protocol inside another. The packet being carried is the payload packet; it is placed inside a GRE packet, which can then be carried by an outer delivery protocol. The RFC describes the common GRE behavior deployed by multiple vendors.
The design aims to reduce the need for a separate encapsulation specification for every protocol pairing. It does not define when a packet should be encapsulated, and it may be less suitable where a protocol-specific encapsulation already exists.
GRE adds a common wrapper around a packet so another network protocol can carry it. The choice of which packets to wrap is left to other mechanisms.
§2Layers in a GRE Packet
A GRE-encapsulated packet has an outer delivery header, a GRE header, and the payload packet. The GRE header identifies the payload protocol. The delivery protocol is responsible for carrying the resulting packet to the other endpoint.
The base GRE header contains a checksum-present bit, reserved bits, a version field, and a 2-octet Protocol Type. A checksum and a reserved field follow only when the checksum-present bit is set.
The outer protocol carries the packet, GRE identifies what is inside, and the payload is the original packet. The checksum fields are present only when the header's checksum bit is set.
- Delivery headerOuter protocol carries the GRE packet.
- GRE headerIdentifies the payload protocol.
- Payload packetThe network-layer packet being carried.
§3Version, Protocol Type, and Checksum
The GRE version field must be zero. The Protocol Type identifies the payload using ETHER TYPE values; a receiver should discard a packet with a type not listed in the referenced ETHER TYPE registries. Reserved bits 6 through 12 must be sent as zero and ignored when received. A receiver that does not implement RFC 1701 must discard packets with any of bits 1 through 5 set.
If the checksum-present bit is one, the packet includes a 2-octet checksum and a 2-octet Reserved1 field. The checksum is the IP one's-complement checksum over the GRE header and payload, computed with the checksum field set to zero. Reserved1 must be sent as zero.
The header names the inner protocol and its GRE version. When present, the checksum covers both the GRE header and payload, while reserved fields must remain zero.
§4Carrying and Forwarding IPv4
When IPv4 is the payload, the GRE Protocol Type must be 0x800. After decapsulation, the tunnel endpoint forwards the inner IPv4 packet using its destination address and decrements its TTL. If that destination is the encapsulator at the other end of the tunnel, the endpoint must discard the packet to prevent a loop.
When GRE itself is carried over IPv4, IPv4 protocol number 47 identifies it as the delivery payload.
The tunnel endpoint removes the GRE wrapper and forwards the inner IPv4 packet toward its destination. It decreases the packet's TTL and discards a packet addressed back to the tunnel endpoint.
§5Interoperation with RFC 1701
RFC 1701 used bits now called Reserved0 as flags for optional key, sequence-number, routing, and strict-source-route fields. RFC 2784 deprecates those fields and specifies that its GRE headers do not contain them.
A sender following RFC 2784 sets Reserved0 to zero, which an RFC 1701 receiver interprets as no optional fields. Conversely, a receiver that does not implement RFC 1701 discards packets with any of bits 1 through 5 set, rather than misreading an older extended header.
RFC 2784 leaves the older optional fields unused. A receiver that does not support the older format rejects packets that mark those fields as present.
§6Filtering and Known Limitations
Routing with GRE follows IPv4 routing, but packet filtering may need to inspect the encapsulated packet or run at the tunnel endpoints. The RFC notes that a tunnel may be terminated at a firewall when filtering inside GRE is a concern.
The appendix lists issues outside this specification, including Path MTU Discovery, IPv6 as a delivery or payload protocol, interaction with ICMP and differentiated services, and multiple or looping encapsulations. Existing IPv4-delivered GRE implementations described by the RFC do not implement Path MTU Discovery or set the Don't Fragment bit.
A firewall may need to inspect the inner packet or filter traffic at a tunnel endpoint. The RFC also lists packet-size behavior, IPv6 support, and other interactions as unresolved topics.