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Use of Reliable Transport in the Internet Key Exchange Protocol Version 2 (IKEv2)

Document Type Active Internet-Draft (individual)
Author Valery Smyslov
Last updated 2023-12-28
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Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                        28 December 2023
Expires: 30 June 2024

Use of Reliable Transport in the Internet Key Exchange Protocol Version
                               2 (IKEv2)


   The Internet Key Exchange protocol version 2 (IKE2) can operate
   either over unreliable (UDP) transport or over reliable (TCP)
   transport.  If TCP is used, then IPsec tunnels created by IKEv2 also
   use TCP.  This document specifies how to decouple IKEv2 and IPsec
   transports, so that IKEv2 can operate over TCP, while IPsec tunnels
   use unreliable transport.  This feature allows IKEv2 to effectively
   exchange large blobs of data (e.g. when post-quantum algorithms are
   employed) while avoiding performance problems which arise when TCP is
   used for IPsec.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 30 June 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Internet Key Exchange protocol version 2 (IKEv2) [RFC7296]
   originally used unreliable transport (UDP) for its messages.  Later
   it was extended to use TCP [RFC9329] where UDP is blocked.  UDP is
   still considered as a preferred transport for IKEv2, and TCP is only
   used if UDP datagrams cannot get through.

   Originally IKEv2 peers exchanged relatively small amount of data, so
   that simple retransmission mechanism on top of UDP with no congestion
   control sufficed.  The situation has changed when post-quantum
   cryptographic algorithms started to be incorporated into IKEv2
   [RFC9370].  Most of post-quantum algorithms require IKE peers to
   exchange much more data, than classical algorithms, up to tens (or
   even hundreds) Kbytes.  Few proposals exist that allow to overcome
   the 64 Kbytes limitation on the size of an IKE payload

   When IKE messages grow up to tens (or even hundreds) Kbytes, using
   UDP as a transport will become challenging.  Use of IKE fragmentation
   [RFC7383] avoids IP fragmentation problems and also allows each IKE
   message fragment to fit into UDP datagram, even if the original
   message doesn't.  However, all IKE fragments are always being sent
   (and retransmitted) at once, so that with the increased number of
   fragments and the lack of congestion control the simple
   retransmission mechanism of IKEv2 will perform poorly, perhaps even
   making more truble to the network.

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   Using reliable transport (like TCP) for IKEv2 would be a solution to
   the problem.  However, the current use of TCP as defined in [RFC9329]
   implies that ESP SAs are also encapsulated in TCP, which has negative
   impact on IPsec performance (see Section 9 of [RFC9329].

   This specification allows to decouple IKE and IPsec transports, so
   that it makes possible to negotiate and use reliable transport for
   IKEv2 while maintaining using unreliable transport for IPsec.

   The idea to decouple IKE and IPsec transports was originally
   presented in [I-D.tjhai-ikev2-beyond-64k-limit].

2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Details

   If the initiator supports this extension and is configured to use and
   it and also anticipates that large amount of data may be exchanged in
   this SA (e.g. it proposes Key Exchange transforms with large public
   keys), then the initiator starts the IKE_SA_INIT exchange using UDP
   port 4500 and includes a new status type notification
   RELIABLE_IKE_TRANSPORT (<TBA by IANA>) into the request message.  The
   RELIABLE_IKE_TRANSPORT notification has protocol 0, SPI size 0 and
   contains no data.  Using UDP port 4500 for the IKE_SA_INIT messages
   is explicitly allowed by [RFC7296], and ensures that IPsec packets
   can get through if they are UDP encapsulated.

   If the responder supports this extension and is configured to use it
   and the IKE_SA_INIT request contains the RELIABLE_IKE_TRANSPORT
   notification, then the responder sends back this notification in the

   Initiator (UDP:4500)               Responder (UDP:4500)
   HDR , SAi1, KEi1, Ni,
                                      HDR, SAr1, KEr1, Nr,
                                <---  N(RELIABLE_IKE_TRANSPORT)

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   In this case the initiator MUST switch to TCP using destination port
   4500 in the next exchange (IKE_INTERMEDIATE or IKE_AUTH) and the
   responder MUST be prepared to receive the next exchange request
   message on TCP port 4500.

    Initiator (TCP)                    Responder (TCP:4500)
    HDR, SK{...}  --->
                                 <---  HDR, SK{...}

   All subsequent IKE exchanges MUST continue to use TCP transport.  In
   particular, peers MUST NOT try to swich IKE transport to UDP as
   defined in Sections 7.1 and 7.3 of [RFC9329].  All recommendations of
   [RFC9329] regarding maintaning TCP connection apply accordingly.

   With this IKE extension child SAs are created as defined in [RFC7296]
   - they either use direct transport over IP or are UDP encapsulated if
   NAT is detected.  Note, that in the latter case peers are responsible
   for maintaining NAT mapping by sending NAT keepalives (see
   Section 2.23 of [RFC7296]).

4.  Security Considerations

   Section 10 of [RFC9329] discusses security implications of using TCP
   as IKE transport.

5.  IANA Considerations

   This document defines a new Notify Message Type in the "Notify
   Message Types - Status Types" registry:


6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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   [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, <>.

   [RFC9329]  Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
              Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
              DOI 10.17487/RFC9329, November 2022,

6.2.  Informative References

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,

   [RFC9370]  Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in the Internet Key Exchange Protocol
              Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
              2023, <>.

              Tjhai, C., Heider, T., and V. Smyslov, "Beyond 64KB Limit
              of IKEv2 Payloads", Work in Progress, Internet-Draft,
              draft-tjhai-ikev2-beyond-64k-limit-03, 28 July 2022,

              Nir, Y., "A Larger Internet Key Exchange version 2 (IKEv2)
              Payload", Work in Progress, Internet-Draft, draft-nir-
              ipsecme-big-payload-02, 23 July 2023,

              Smyslov, V., "Extended IKEv2 Payload Format", Work in
              Progress, Internet-Draft, draft-smyslov-ipsecme-ikev2-
              extended-pld-01, 6 March 2023,

Author's Address

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   Valery Smyslov
   PO Box 81
   Moscow (Zelenograd)
   Russian Federation
   Phone: +7 495 276 0211

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