Internet Engineering Task Force Gopal Dommety
INTERNET DRAFT cisco Systems
Category: Standards Track June 2000
Title: draft-dommety-gre-ext-04.txt
Expires January 2001
Key and Sequence Number Extensions to GRE
draft-dommety-gre-ext-04.txt
Status of this Memo
This document is a submission by the Network Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the gre@ops.ietf.org mailing list.
Distribution of this memo is unlimited.
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Abstract
GRE 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 (Generic Routing
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Internet Draft Key and Sequence Number Extensions to GRE June, 2000
Encapsulation) 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 RFC 2119 [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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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| |K|S| 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 RFC 1701 [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)
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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.
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Internet Draft Key and Sequence Number Extensions to GRE June, 2000
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
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Internet Draft Key and Sequence Number Extensions to GRE June, 2000
(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 RFC 1701. 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 Traina, P.,
"Generic Routing Encapsulation (GRE)," RFC 2784, March 2000.
[2] Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing
Encapsulation", RFC 1701, NetSmiths, Ltd., and cisco Systems, October
1994.
[3] Bradner S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[4] Kent S, and Atkinson R, "Security Architecture for the Internet
Protocol ", RFC 2401, November 1998.
[5] Kent S, and Atkinson R, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[6] Kent S, and Atkinson R, " IP Authentication Header", RFC 2402,
November 1998.
Authors's Address
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Internet Draft Key and Sequence Number Extensions to GRE June, 2000
Gopal Dommety
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: gdommety@cisco.com
This internet draft expires in January 2001