Tech-invite3GPPspecsSIPRFCs
898887868584838281807978777675747372717069686766656463626160595857565554535251504948474645444342414039383736353433323130292827262524232221201918171615141312111009080706050403020100

in Index   Prev   Next

RFC 2890

Key and Sequence Number Extensions to GRE

Pages: 7
Proposed Standard
Updates:  2784

ToP   noToC   RFC2890 - Page 1
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 RFC 2119 [3]. In addition, the following words are used to signify the requirements of the specification.
ToP   noToC   RFC2890 - Page 2
   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| |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].
ToP   noToC   RFC2890 - Page 3

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).
ToP   noToC   RFC2890 - Page 4
   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
ToP   noToC   RFC2890 - Page 5
   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 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.
ToP   noToC   RFC2890 - Page 6

6. References

[1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000. [2] Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing Encapsulation", RFC 1701, October 1994. [3] Bradner S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [5] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [6] Kent, S., and R. Atkinson, " IP Authentication Header", RFC 2402, November 1998.

Author's Address

Gopal Dommety Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 EMail: gdommety@cisco.com
ToP   noToC   RFC2890 - Page 7
Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   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
   copyrights defined in the Internet Standards process must be
   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.