Network Working Group G. Dommety
Request for Comments: 2890 Cisco Systems
Category: Standards Track September 2000

Key and Sequence Number Extensions to 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

GRE (Generic Routing Encapsulation) specifies a protocol for
encapsulation of an arbitrary protocol over another arbitrary network
layer protocol. This document describes extensions by which two
fields, Key and Sequence Number, can be optionally carried in the GRE
Header [1].

1. IntrodUCtion

The current specification of Generic Routing Encapsulation [1]
specifies a protocol for encapsulation of an arbitrary protocol over
another arbitrary network layer protocol. This document describes
enhancements by which two fields, Key and Sequence Number, can be
optionally carried in the GRE Header [1]. The Key field is intended
to be used for identifying an individual traffic flow within a
tunnel. The Sequence Number field is used to maintain sequence of
packets within the GRE Tunnel.

1.1. Specification Language

The keyWords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [3].

In addition, the following words are used to signify the requirements
of the specification.

Silently discard
The implementation discards the datagram without further
processing, and without indicating an error to the
sender. The implementation SHOULD provide the
capability of logging the error, including the contents
of the discarded datagram, and SHOULD record the event
in a statistics counter.

2. Extensions to GRE Header

The GRE packet header[1] has the following format:

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)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The proposed GRE header will have the following format:

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 KS Reserved0 Ver Protocol Type
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum (optional) Reserved1 (Optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key (optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number (Optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Key Present (bit 2)

If the Key Present bit is set to 1, then it indicates that the
Key field is present in the GRE header. Otherwise, the Key
field is not present in the GRE header.

Sequence Number Present (bit 3)

If the Sequence Number Present bit is set to 1, then it
indicates that the Sequence Number field is present.
Otherwise, the Sequence Number field is not present in the GRE
header.

The Key and the Sequence Present bits are chosen to be
compatible with RFC1701 [2].

2.1. Key Field (4 octets)

The Key field contains a four octet number which was inserted by the
encapsulator. The actual method by which this Key is oBTained is
beyond the scope of the document. The Key field is intended to be
used for identifying an individual traffic flow within a tunnel. For
example, packets may need to be routed based on context information
not present in the encapsulated data. The Key field provides this
context and defines a logical traffic flow between encapsulator and
decapsulator. Packets belonging to a traffic flow are encapsulated
using the same Key value and the decapsulating tunnel endpoint
identifies packets belonging to a traffic flow based on the Key Field
value.

2.2. Sequence Number (4 octets)

The Sequence Number field is a four byte field and is inserted by the
encapsulator when Sequence Number Present Bit is set. The Sequence
Number MUST be used by the receiver to establish the order in which
packets have been transmitted from the encapsulator to the receiver.
The intended use of the Sequence Field is to provide unreliable but
in-order delivery. If the Key present bit (bit 2) is set, the
sequence number is specific to the traffic flow identified by the Key
field. Note that packets without the sequence bit set can be
interleaved with packets with the sequence bit set.

The sequence number value ranges from 0 to (2**32)-1. The first
datagram is sent with a sequence number of 0. The sequence number is
thus a free running counter represented modulo 2**32. The receiver
maintains the sequence number value of the last successfully
decapsulated packet. Upon establishment of the GRE tunnel, this value
should be set to (2**32)-1.

When the decapsulator receives an out-of sequence packet it SHOULD be
silently discarded. A packet is considered an out-of-sequence packet
if the sequence number of the received packet is less than or equal
to the sequence number of last successfully decapsulated packet. The
sequence number of a received message is considered less than or
equal to the last successfully received sequence number if its value
lies in the range of the last received sequence number and the
preceding 2**31-1 values, inclusive.

If the received packet is an in-sequence packet, it is successfully
decapsulated. An in-sequence packet is one with a sequence number
exactly 1 greater than (modulo 2**32) the last successfully
decapsulated packet, or one in which the sequence number field is not
present (S bit not set).

If the received packet is neither an in-sequence nor an out-of-
sequence packet it indicates a sequence number gap. The receiver may
perform a small amount of buffering in an attempt to recover the
original sequence of transmitted packets. In this case, the packet
may be placed in a buffer sorted by sequence number. If an in-
sequence packet is received and successfully decapsulated, the
receiver should consult the head of this buffer to see if the next
in-sequence packet has already been received. If so, the receiver
should decapsulate it as well as the following in-sequence packets
that may be present in the buffer. The "last successfully
decapsulated sequence number" should then be set to the last packet
that was decapsulated from the buffer.

Under no circumstances should a packet wait more that
OUTOFORDER_TIMER milliseconds in the buffer. If a packet has been
waiting that long, the receiver MUST immediately traverse the buffer
in sorted order, decapsulating packets (and ignoring any sequence
number gaps) until there are no more packets in the buffer that have
been waiting longer than OUTOFORDER_TIMER milliseconds. The "last
successfully decapsulated sequence number" should then be set to the
last packet so decapsulated.

The receiver may place a limit on the number of packets in any per-
flow buffer (Packets with the same Key Field value belong to a flow).
If a packet arrives that would cause the receiver to place more than
MAX_PERFLOW_BUFFER packets into a given buffer, then the packet at
the head of the buffer is immediately decapsulated regardless of its
sequence number and the "last successfully decapsulated sequence
number" is set to its sequence number. The newly arrived packet may
then be placed in the buffer.

Note that the sequence number is used to detect lost packets and/or
restore the original sequence of packets (with buffering) that may
have been reordered during transport. Use of the sequence number
option should be used appropriately; in particular, it is a good idea
a avoid using when tunneling protocols that have higher layer in-
order delivery mechanisms or are tolerant to out-of-order delivery.
If only at certain instances the protocol being carried in the GRE
tunnel requires in-sequence delivery, only the corresponding packets
encapsulated in a GRE header can be sent with the sequence bit set.

Reordering of out-of sequence packets MAY be performed by the
decapsulator for improved performance and tolerance to reordering in
the network. A small amount of reordering buffer
(MAX_PERFLOW_BUFFER) may help in improving performance when the
higher layer employs stateful compression or encryption. Since the
state of the stateful compression or encryption is reset by packet
loss, it might help the performance to tolerate some small amount of

packet reordering in the network by buffering.

3. Security Considerations

This document describes extensions by which two fields, Key and
Sequence Number, can be optionally carried in the GRE (Generic
Routing Encapsulation) Header [1]. When using the Sequence number
field, it is possible to inject packets with an arbitrary Sequence
number and launch a Denial of Service attack. In order to protect
against such attacks, IP security protocols [4] MUST be used to
protect the GRE header and the tunneled payload. Either ESP
(Encapsulating Security Payload) [5] or AH (Authentication Header)[6]
MUST be used to protect the GRE header. If ESP is used it protects
the IP payload which includes the GRE header. If AH is used the
entire packet other than the mutable fields are authenticated. Note
that Key field is not involved in any sort or security (despite its
name).

4. IANA Considerations

This document does not require any allocations by the IANA and
therefore does not have any new IANA considerations.

5. Acknowledgments

This document is derived from the original ideas of the authors of
RFC1701. Kent Leung, Pete McCann, Mark Townsley, David Meyer,
Yingchun Xu, Ajoy Singh and many others provided useful discussion.
The author would like to thank all the above people.

6. References

[1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC2784, March 2000.

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

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

[4] Kent, S. and R. Atkinson, "Security Architecture for the Internet
Protocol", RFC2401, November 1998.

[5] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC2406, November 1998.

[6] Kent, S., and R. Atkinson, " IP Authentication Header", RFC2402,
November 1998.

Author's Address

Gopal Dommety
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134

EMail: gdommety@cisco.com

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