|Title||Extended Sequence Number (ESN) Addendum to IPsec Domain of
Interpretation (DOI) for Internet Security Association and Key
Management Protocol (ISAKMP)
Network Working Group S. Kent
Request for Comments: 4304 BBN Technologies
Category: Standards Track December 2005
Extended Sequence Number (ESN) Addendum to
IPsec Domain of Interpretation (DOI)
for Internet Security Association
and Key Management Protocol (ISAKMP)
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 (C) The Internet Society (2005).
The IP Security Authentication Header (AH) and Encapsulating Security
Payload (ESP) protocols use a sequence number to detect replay. This
document describes extensions to the Internet IP Security Domain of
Interpretation (DOI) for the Internet Security Association and Key
Management Protocol (ISAKMP). These extensions support negotiation
of the use of traditional 32-bit sequence numbers or extended (64-
bit) sequence numbers (ESNs) for a particular AH or ESP security
The specifications for the IP Authentication Header (AH) [AH] and the
IP Encapsulating Security Payload (ESP) [ESP] describe an option for
use of extended (64-bit) sequence numbers. This option permits
transmission of very large volumes of data at high speeds over an
IPsec Security Association, without rekeying to avoid sequence number
space exhaustion. This document describes the additions to the IPsec
DOI for ISAKMP [DOI] that are needed to support negotiation of the
extended sequence number (ESN) option.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [Bra97].
2. IPsec Security Association Attribute
The following SA attribute definition is used in Phase II of an
Internet Key Exchange Protocol (IKE) negotiation. The attribute type
is Basic (B). Encoding of this attribute is defined in the base
ISAKMP specification [ISAKMP]. Attributes described as basic MUST
NOT be encoded as variable. See [IKE] for further information on
attribute encoding in the IPsec DOI. All restrictions listed in
[IKE] also apply to the IPsec DOI and to this addendum.
class value type
Extended (64-bit) Sequence Number 11 B
This class specifies that the Security Association will be using
64-bit sequence numbers. (See [AH] and [ESP] for a description
of extended (64-bit) sequence numbers.)
64-bit Sequence Number 1
3. Attribute Negotiation
If an implementation receives a defined IPsec DOI attribute (or
attribute value) that it does not support, an ATTRIBUTES-NOT-SUPPORT
SHOULD be sent and the security association setup MUST be aborted.
If an implementation receives any attribute value but the value for
64-bit sequence numbers, the security association setup MUST be
4. Security Considerations
This memo pertains to the Internet Key Exchange protocol [IKE], which
combines ISAKMP [ISAKMP] and Oakley [OAKLEY] to provide for the
derivation of cryptographic keying material in a secure and
authenticated manner. Specific discussion of the various security
protocols and transforms identified in this document can be found in
the associated base documents and in the cipher references.
The addition of the ESN attribute does not change the underlying
security characteristics of IKE. In using ESNs with ESP, it is
important to employ an encryption mode that is secure when very large
volumes of data are encrypted under a single key. Thus, for example,
Data Encryption Standard (DES) in Cipher Block Chaining (CBC) mode
would NOT be suitable for use with the ESN, because no more than 2^32
blocks should be encrypted under a single DES key in that mode.
Similarly, the integrity algorithm used with ESP or AH should be
secure relative to the number of packets being protected. To avoid
potential security problems imposed by algorithm limitations, the SA
lifetime may be set to limit the volume of data protected with a
single key, prior to reaching the 2^64 packet limit imposed by the
5. IANA Considerations
This document contains a "magic" number to be maintained by the IANA.
No additional class values will be assigned for this attribute. The
IANA has allocated an IPsec Security Attribute value for "Attribute
Type". This value is listed under the heading "value" in the table
in Section 2.
The author would like to thank the members of the IPsec working
group. The author would also like to acknowledge the contributions
of Karen Seo for her help in the editing of this specification.
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[AH] Kent, S., "IP Authentication Header", RFC 4302, December
[DOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[ISAKMP] Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
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