Renaming Extended Sequence Number (ESN) Transform Type in the Internet Key Exchange Protocol Version 2 (IKEv2)
draft-ietf-ipsecme-ikev2-rename-esn-00

Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Updates: 7296 (if approved)                              5 December 2024
Intended status: Informational                                          
Expires: 8 June 2025

 Renaming Extended Sequence Number (ESN) Transform Type in the Internet
                Key Exchange Protocol Version 2 (IKEv2)
                 draft-ietf-ipsecme-ikev2-rename-esn-00

Abstract

   This documents clarifies and extends the meaning of transform type 5
   in IKEv2.  It updates RFC 7296 by renaming the transform type 5 from
   "Extended Sequence Numbers (ESN)" to "Sequence Numbers Properties
   (SNP)".  It also renames two currently defined values for this
   transform type: value 0 from "No Extended Sequence Numbers" to
   "32-bit Sequential Numbers" and value 1 from "Extended Sequence
   Numbers" to "Partially Transmitted 64-bit Sequential Numbers".

Status of This Memo

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   This Internet-Draft will expire on 8 June 2025.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Description . . . . . . . . . . . . . . . . . . . . .   3
   3.  Extending the Semantics of Transform Type 5 . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   IP Security (IPsec) Architecture [RFC4301] defines a set of security
   services provided by IPsec protocols Authentication Header (AH)
   [RFC4302] and Encapsulating Security Payload (ESP) [RFC4303].  One of
   these services is anti-replay protection.  In IPsec the anti-replay
   protection service is optional, each receiver of AH and ESP packets
   individually selects whether to enable it.  The anti-replay
   protection in AH and ESP is achieved by means of a monotonically
   increasing counter that never wraps around, which is sent in each AH
   or ESP packet in the Sequence Number field.  The receiver maintains
   sliding window that allows to detect duplicate packets.

   Both AH and ESP allow to use either a 32-bit counter or a 64-bit
   counter.  The latter case is referred to as Extended Sequence Numbers
   (ESN) in AH and ESP specifications.  Since the Sequence Number field
   in both AH and ESP headers is only 32 bits in size, in case of ESN
   the high-order 32 bits of the counter are not transmitted and are
   instead deducted on the receiver based on previously received
   packets.

   Since the decision whether to enable anti-replay protection is
   ultimately taken by the receiver, the sender in accordance with AH
   ([RFC4302] Section 3.3.2) and ESP ([RFC4303] Section 3.3.3)
   specifications should always assume that the replay protection is
   enabled on receiving side.  Thus the sender should always send the
   increasing counter values and should take care that the counter never
   wraps around.  AH and ESP specifications also discuss situations when
   anti-replay protection is not possible to achieve even if senders do
   all as prescribed -- like in multicast Security Associations (SAs)
   with multiple unsynchronized senders.  Both AH and ESP specifications

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   allow the sender to relax their duties of maintaining the counter if
   there is a way to notify the sender that the anti-replay protection
   is disabled by the receiver or is not possible to achieve.

   AH and ESP rely on the Internet Key Exchange protocol version 2
   (IKEv2) [RFC7296] for establishing Security Associations.  The
   process of SA establishment includes calculation of a shared key and
   negotiation of various SA parameters, such as cryptographic
   algorithms.  This negotiation in IKEv2 is performed via so called
   transforms (see Section 3.3.2 of [RFC7296]).  The type of transform
   determines what parameter is being negotiated.  Each transform type
   has an associated list of possible values (called Transform IDs),
   that determine the possible options for negotiation.  See
   [IKEV2-IANA] for the list of transform types and associated transform
   IDs.

   Transform type 5 "Extended Sequence Numbers (ESN)" is used in IKEv2
   to negotiate the way sequence numbers for anti-replay protection are
   generated, transmitted and processed in the context of an SA.  For
   this transform type two values are defined -- "No Extended Sequence
   Numbers" and "Extended Sequence Numbers".

2.  Problem Description

   IKEv2 currently has no means to negotiate the case when both peers
   agree that anti-replay protection is not needed.  Even when both
   peers locally disable anti-replay service as receivers, they still
   need to maintain increasing sequence numbers as senders, taking care
   that they never wrap around (see
   [I-D.pan-ipsecme-anti-replay-notification]).

   There is also no way to inform receivers that anti-replay protection
   is not possible for a particular SA (for example in case of a
   multicast SA with several unsynchronized senders).

   Future IPsec security protocols may provide more options for the
   handling of anti-replay protection counters, like sending full 64-bit
   sequence numbers or completely omitting them in packets (see
   [I-D.klassert-ipsecme-eesp]).  These options will require means to be
   negotiated in IKEv2.

   Transform type 5 looks like an appropriate candidate for addressing
   these issues: it is already used for negotiation of how sequence
   numbers are handled in AH and ESP and it is possible to define
   additional transform IDs that could be used in the corresponding
   situations.  However, the current definition of transform type 5 is
   too narrow -- its name implies that this transform can only be used
   for negotiation of using ESN.

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3.  Extending the Semantics of Transform Type 5

   This document extends the semantics of transform type 5 in IKEv2 to
   the following definition.

   Transform type 5 defines the set of properties of sequence numbers of
   IPsec packets of a given SA when these packets enter the network.

   This definition requires some clarifications.

   *  By "sequence numbers" here we assume logical entities (like
      counters) that can be used for anti-replay protection on receiving
      sides.  In particular, these entities are not necessary the
      content of the Sequence Number field in the IPsec packets, but may
      be constructed using some information, that is not necessary
      transmitted.

   *  The properties are interpreted as a characteristic of IPsec SA
      packets, and not as a result of a sender actions.  For example, in
      multicast SA with multiple unsynchronized senders, even if each
      sender ensures the uniqueness of sequence numbers it generates,
      the uniqueness of sequence numbers for all IPsec packets is not
      guaranteed.

   *  The properties are defined for the packets just entering the
      network and not for the packets that receivers get.  This is
      because network behavior may break some of these properties (e.g.,
      break sequence numbers uniqueness by packet duplication).

   *  The properties of sequence numbers are interpreted in a broad
      sense, that includes the case when sequence numbers are absent.

   Given this definition, transform type 5 in the IANA registries for
   IKEv2 [IKEV2-IANA] is renamed from "Extended Sequence Numbers (ESN)"
   to "Sequence Numbers Properties (SNP)".

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   It is expected that new transform IDs will be defined for this
   transform type in future (like in G-IKEv2 [I-D.ietf-ipsecme-g-ikev2]
   for the case of multicast SAs).  Documents defining new transform IDs
   should include description of the properties the sequence numbers
   would have if the new transform ID is selected.  In particular, this
   description should include discussion whether these properties allow
   to achieve anti-replay protection.  Some existing protocols (like
   Implicit IV in ESP [RFC8750] or Aggregation and Fragmentation for ESP
   [RFC9347]) rely on properties that are guaranteed for the currently
   defined transform IDs, but this might not be true for other transform
   IDs.  The description of the sequence numbers properties for a new
   transform ID should also include discussion whether these protocols
   can be used if this transform ID is selected.

   The two currently defined transform IDs for this transform type have
   the following sequence numbers properties.

   *  Transform ID 0 defines sequence numbers as monotonically
      increasing 32-bit counters that are transmitted in the Sequence
      Number field of AH and ESP packets.  They never wrap around and
      are guaranteed to be unique, thus they are suitable for anti-
      replay protection.  They can also be used with protocols that rely
      on sequence numbers uniqueness (like [RFC8750]) or their monotonic
      increase (like [RFC9347]).  The sender and the receiver actions
      are defined in Sections 3.3.2 and 3.4.3 of [RFC4302] for AH and in
      Sections 3.3.3 and 3.4.3 of [RFC4303] for ESP.

   *  Transform ID 1 defines sequence numbers as monotonically
      increasing 64-bit counters.  The low-order 32 bits are transmitted
      in the Sequence Number field of AH and ESP packets and the high-
      order 32 bits are implicitly determined on receivers based on
      previously received packets.  The sequence numbers never wrap
      around and are guaranteed to be unique, thus they are suitable for
      anti-replay protection.  They can also be used with protocols that
      rely on sequence numbers uniqueness (like [RFC8750]) or their
      monotonic increase (like [RFC9347]).  To be able to correctly
      process the incoming packets on receivers the packets must be
      authenticated (even when the anti-replay protection is not used).
      The sender and the receiver actions are defined in Sections 3.3.2
      and 3.4.3 of [RFC4302] for AH and in Sections 3.3.3 and 3.4.3 of
      [RFC4303] for ESP.

   Given the descriptions above and the new definition of transform type
   5, the two currently defined transform IDs are renamed to better
   reflect the properties of sequence numbers they assume.

   *  Transform ID 0 is renamed from "No Extended Sequence Numbers" to
      "32-bit Sequential Numbers".

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   *  Transform ID 1 is renamed from "Extended Sequence Numbers" to
      "Partially Transmitted 64-bit Sequential Numbers".

   Note, that the above descriptions do not change the existing
   semantics of these transform IDs, they only provide clarification.
   Note also, that ESP and AH packet processing for these transform IDs
   is not affected, and bits on the wire do not change.

4.  Security Considerations

   This document clarifies and extends the meaning of Transform Type 5
   in the IKEv2 IANA registries and also renames it to reflect the new
   definition.  It also renames two currently allocated values for this
   transform type to better reflect their existing semantics, which this
   document preserves.  These actions do not affect security of the AH,
   ESP and IKEv2 protocols.

5.  IANA Considerations

   This document makes the following changes in the "Internet Key
   Exchange Version 2 (IKEv2) Parameters" IANA registries [IKEV2-IANA].
   It is assumed that RFCXXXX refers to this specification.

   *  The existing transform type 5 "Extended Sequence Numbers (ESN)" in
      the "Transform Type Values" registry is renamed to "Sequence
      Numbers Properties (SNP)".

   *  Appended [RFCXXXX] to the Reference column of Transform Type 5 in
      the "Transform Type Values" registry.

   *  Added this note to the "Transform Type Values" registry:

      "Sequence Numbers Properties (SNP)" transform type was originally
      named "Extended Sequence Numbers (ESN)" and was referenced by that
      name in a number of RFCs published prior to [RFCXXXX], which gave
      it the current title.

   *  The "Transform Type 5 - Extended Sequence Numbers Transform IDs"
      registry is renamed to "Transform Type 5 - Sequence Numbers
      Properties Transform IDs".

   *  The existing Transform ID 0 "No Extended Sequence Numbers" in what
      was the "Transform Type 5 - Extended Sequence Numbers Transform
      IDs" registry is renamed to "32-bit Sequential Numbers".

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   *  The existing Transform ID 1 "Extended Sequence Numbers" in what
      was the "Transform Type 5 - Extended Sequence Numbers Transform
      IDs" registry is renamed to "Partially Transmitted 64-bit
      Sequential Numbers".

   *  Appended [RFCXXXX] to the Reference column of Transform ID 0 and
      Transform ID 1 in what was the "Transform Type 5 - Extended
      Sequence Numbers Transform IDs" registry.

   *  Added these notes to what was the "Transform Type 5 - Extended
      Sequence Numbers Transform IDs" registry:

      This registry was originally named "Transform Type 5 - Extended
      Sequence Numbers Transform IDs" and was referenced using that name
      in a number of RFCs published prior to [RFCXXXX], which gave it
      the current title.

      "32-bit Sequential Numbers" transform ID was originally named "No
      Extended Sequence Numbers" and was referenced by that name in a
      number of RFCs published prior to [RFCXXXX], which gave it the
      current title.

      "Partially Transmitted 64-bit Sequential Numbers" transform ID was
      originally named "Extended Sequence Numbers" and was referenced by
      that name in a number of RFCs published prior to [RFCXXXX], which
      gave it the current title.

6.  Acknowledgements

   This document was created as a result of discussions with Russ
   Housley, Tero Kivinen, Paul Wouters and Antony Antony about the best
   way to extend the meaning of the Extended Sequence Numbers transform
   in IKEv2.

7.  References

7.1.  Normative References

   [IKEV2-IANA]
              IANA, "Internet Key Exchange Version 2 (IKEv2)
              Parameters", <http://www.iana.org/assignments/ikev2-
              parameters/ikev2-parameters.xhtml>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

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   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

7.2.  Informative References

   [I-D.ietf-ipsecme-g-ikev2]
              Smyslov, V. and B. Weis, "Group Key Management using
              IKEv2", Work in Progress, Internet-Draft, draft-ietf-
              ipsecme-g-ikev2-17, 19 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
              g-ikev2-17>.

   [I-D.klassert-ipsecme-eesp]
              Klassert, S., Antony, A., and C. Hopps, "Enhanced
              Encapsulating Security Payload (EESP)", Work in Progress,
              Internet-Draft, draft-klassert-ipsecme-eesp-01, 14 October
              2024, <https://datatracker.ietf.org/doc/html/draft-
              klassert-ipsecme-eesp-01>.

   [I-D.pan-ipsecme-anti-replay-notification]
              Pan, W., He, Q., and P. Wouters, "IKEv2 Support for Anti-
              Replay Status Notification", Work in Progress, Internet-
              Draft, draft-pan-ipsecme-anti-replay-notification-01, 21
              October 2024, <https://datatracker.ietf.org/doc/html/
              draft-pan-ipsecme-anti-replay-notification-01>.

   [RFC8750]  Migault, D., Guggemos, T., and Y. Nir, "Implicit
              Initialization Vector (IV) for Counter-Based Ciphers in
              Encapsulating Security Payload (ESP)", RFC 8750,
              DOI 10.17487/RFC8750, March 2020,
              <https://www.rfc-editor.org/info/rfc8750>.

   [RFC9347]  Hopps, C., "Aggregation and Fragmentation Mode for
              Encapsulating Security Payload (ESP) and Its Use for IP
              Traffic Flow Security (IP-TFS)", RFC 9347,
              DOI 10.17487/RFC9347, January 2023,
              <https://www.rfc-editor.org/info/rfc9347>.

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Author's Address

   Valery Smyslov
   ELVIS-PLUS
   Russian Federation
   Email: svan@elvis.ru

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