Using IKE Fragmentation for Large Messages
draft-smyslov-ipsecme-ikev2-fragm-large-msg-02

Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                               A. Antony
Expires: 6 May 2027                                          S. Klassert
                                                                 secunet
                                                              T. Brunner
                                                           codelabs GmbH
                                                         2 November 2026

               Using IKE Fragmentation for Large Messages
             draft-smyslov-ipsecme-ikev2-fragm-large-msg-02

Abstract

   This document describes issues with using Internet Key Exchange
   version 2 (IKEv2) fragmentation for transmitting large messages on
   unreliable transport.  The document proposes several approaches for
   dealing with these issues: randomizing the order the fragments are
   sent, dispersing sending fragments over time and selective
   retransmission of fragments based on information from the peer about
   their receiption.

Status of This Memo

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   This Internet-Draft will expire on 6 May 2027.

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   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Issues with Using IKE Fragmentation for Large Messages  . . .   3
   4.  Proposed Techniques . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Unilateral Techniques . . . . . . . . . . . . . . . . . .   4
       4.1.1.  Sending Fragments in different Order  . . . . . . . .   4
       4.1.2.  Reducing Sending Rate . . . . . . . . . . . . . . . .   4
       4.1.3.  Reducing the Amount of Data Sent by Initiator . . . .   4
     4.2.  Bilateral Techniques  . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Selective Retransmission of Fragments . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Internet Key Exchange version 2 (IKEv2) protocol [RFC7296] is
   used for key management in IPsec architecture.  IKEv2 was originally
   defined on UDP transport, and, while later IKEv2 extensions added TCP
   as a possible transport ([RFC9329],
   [I-D.ietf-ipsecme-ikev2-reliable-transport]), UDP is still a
   preferred and mostly used transport for IKEv2.

   When UDP is used as a transport, any IKEv2 message that exceeds MTU
   size is fragmented at IP layer.  IP fragmentation is known to cause
   problems with some intermediate devices that cannot correctly
   procesess any IP fragments other than the first one.  To deal with
   this, a protocol extension was developed that allows fragmenting
   messages at IKE layer [RFC7383].  While IKEv2 fragmentation allows to
   avoid IP fragmentation of large messages, it lacks any congestion
   control mechanisms, that may cause issues when the fragmented message
   is large (with some definition of "large") and the network (or
   receiver's) capacity is limited (with some definition of "limited").
   This document defines several approaches that can be used to mitigate
   these issues.

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2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

   All multi-octet fields representing integers in this document are
   laid out in big endian order (also known as "most significant byte
   first", or "network byte order").

   It is expected that readers are familiar with the details of how
   IKEv2 fragmentation [RFC7383] works.

3.  Issues with Using IKE Fragmentation for Large Messages

   At the time the IKE fragmentation mechanism was being developed, it
   was considered that most IKEv2 messages would fit into the typical
   MTU size and only few of them could exceed it.  In particular, the
   IKE_AUTH messages containing certificates were concerned and it was
   assumed at that time that "large" IKEv2 messages would be less than
   few Kbytes in size, so that the number of IKE fragments for a message
   (with a typical MTU size) would be small.  Due to these
   considerations it was decided that no mechanism for acknowledging of
   receipt of individual fragments is needed - all fragments of a
   message are transmitted (and retransmitted) at once, as with IP
   fragmentation.

   When postquantum cryptographic mechanisms started to be incorporated
   into IKEv2, the notion of "large" for an IKEv2 message has changed
   from few Kbytes to several tens of Kbytes
   [I-D.wang-ipsecme-hybrid-kem-ikev2-frodo] and up to several hundreds
   of Kbytes [I-D.smyslov-ipsecme-ikev2-mceliece]
   [I-D.kohbrok-ipsecme-mls-gike].  When messages of this size are
   fragmented and all fragments are sent at once, it can happen that
   either intermediate network devices or the final recipient are
   uncapable of handling that much data at the rate it was sent, causing
   some fragments to be lost.  Sender will eventually retransmit all the
   message fragments, but since the disproportion between sending rate
   and network/recipient limitations remain, it is likely that some
   fragments will be lost again.  Since the restransmissions are not
   adaptive, there is no guarantee that the message will eventually be
   drlivered even after several retransmissions.

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4.  Proposed Techniques

   This document defines several techniques that can be used to improve
   reliability of transmitting large messages using IKEv2 fragmentation
   mechanism.  These techniques can be used independently of each other.
   The techniques can be classified as unilateral actionsm, which do not
   require any actiona from the peer, and bilateral actions that require
   mutual support by both sender and receiver.

4.1.  Unilateral Techniques

4.1.1.  Sending Fragments in different Order

   The simplest technique is to change the order in which fragments are
   sent with each retransmission.  This will help in situation the
   recipient have limited buffer size for incoming packets and cannot
   process the received fragments quickly enough to free the buffer.
   This means that the some number of the first sent fragments will be
   processed while the rest will be dropped.  Changing the order in
   which the fragments are sent with each retransmission ensures that
   the first sent packets will change each time, thus increasing the
   chances all fragments are delivered.

4.1.2.  Reducing Sending Rate

   Another simple technique is to add some delay between sending each
   fragment, thus reducing the rate with which data is being
   transmitted.  Since the sender does not know what rate is OK for the
   receiver, this document gives no advice what this delay should be.
   If the sender uses exponential back-off when retransmitting, a
   sensible approach would be to also increase delay between sending
   fragments with each retransmission.

4.1.3.  Reducing the Amount of Data Sent by Initiator

   In IKEv2 it is always the initiator that plays an active role in
   ensuring that both the request and the response messages of an
   exchange are delivered (Section 2.1 of [RFC7296]).  In particular,
   the initiator periodically retransmits the request until it receives
   the response.  With IKE fragmentation in case the response message is
   fragmented, a situation is possible that the initiator receives only
   some of these fragments.  In this situation the initiator must
   retransmit the request, but if the request message has been
   fragmented too (the usual case), there is no need to retransmit all
   fragments of the request message, it is enough to retransmit only the
   first one (i.e., with the Fragment Number field equal to 1).  This
   works because only this fragment triggers re-sending the response (as
   specified in Section 2.6.1 of [RFC7383], all other fragments are

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   discarded and would only waste network resources.

4.2.  Bilateral Techniques

4.2.1.  Selective Retransmission of Fragments

   Selective Retransmission of Fragments is an IKEv2 extension that
   allows, when supported by both peers, to selectively retransmit only
   those fragments that weren't delivered.

4.2.1.1.  Negotiation

   This extension is negotiated by exchanging a new notification
   FRAGMENT_ACK_SUPPORTED in the IKE_SA_INIT exchange.  The Notify
   Message Type for this notification is <TBA by IANA>, Protocol ID and
   SPI Size are both set to 0 and the notification data is empty.
   Future specifications may make use of the notification data of this
   notification.  Implementations compliant with this document MUST
   ignore the content of the notification data in the received
   FRAGMENT_ACK_SUPPORTED notification if it is not empty.

   The negotiation only takes effect if peers also negotiate the use of
   IKE fragmentation, as defined in Section 2.3 of [RFC7383].  If IKE
   fragmentation is not negotiated, the FRAGMENT_ACK_SUPPORTED
   notifications MUST be ignored.

   If the selective retransmission of fragments is negotiated, the
   maximum number of fragments that an IKEv2 sender can fragment an IKE
   message to, is limited to 65534, because the value 65535 (0xffff) in
   the Total Fragments field of the Encrypted Fragment Payload
   (Section 2.5 of [RFC7383]) is used as an indication that this is not
   a fragment, but instead a Fragment Acknowledgment message
   (Section 4.2.1.4).

      |  It is unlikely that any real IKE message sent over UDP will be
      |  needed to be fragmented into 65535 fragments and fragmenting
      |  into 65534 or less fragments will be insufficient.

4.2.1.2.  Handling Fragmented Request

   When the initiator fragments the request message, it sends the
   fragments to the responder.  If the peers negotiated this extension
   and the responder received only some of the sent fragments, then the
   responder can indicate which fragments are missing.  For this
   purpose, it sends an IKEv2 response message formatted in accordance
   with Section 4.2.1.4.

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4.2.1.3.  Handling Fragmented Response

   The initiator keeps retransmitting the request message until it
   receives at least one response message fragment.  If this extension
   is negotiated and the request message was sent fragmented and the
   responder has indicated that it has received all its fragments in the
   latest Fragment Acknowledgment message, but no response message for
   the exchange is still received by the initiator, the initiator MUST
   continue retransmitting the very first fragment of the request until
   it receives at least one response message fragment.

   When this happens and if there are still some missing response
   message fragments and the peers negotiated this extension, the
   initiator can indicate which fragments are missing.  For this
   purpose, it sends an IKEv2 request message for the same exchange
   (with the same Message ID) formatted in accordance with
   Section 4.2.1.4.

4.2.1.4.  Fragment Acknowledgment Message

   Fragment Acknowledgment is an IKEv2 message that contains only the
   Encrypted Fragment payload (see Section 2.5 of [RFC7383]) formatted
   as follows:

   *  The Next Payload field is set to 0.

   *  The Fragment Number field is set to 0xffff.

   *  The Total Fragments field is set to 0xffff.

   *  The Encrypted content field contains the information about the
      missing fragments represented in the form defined in Figure 1.

   All other fields are filled in accordance with [RFC7383] and
   [RFC7296].

   If the peers negotiated this extension, the receiver will recognize
   that this message contains the fragment acknowledgment data.  If the
   message is successfully decrypted and verified, then the receiver
   extracts the fragment acknowledgment data and obtains the information
   about the fragments that need to be retransmitted.  The receiver MUST
   ignore messages failed to pass the ICV verification.

   The content of the Encrypted Fragment payload contains the fragment
   acknowledgment data in the following format.

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                          1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Rcpt Status Packet Number   |       Total Fragments         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      First Fragment Num       |      Last Fragment Num        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~               Fragment Acknowledgment Bitmap                  ~
     |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |
     +-+-+-+-+-+-+-+-+

               Figure 1: Fragment Acknowledgment Data Format

   *  Rcpt Status Packet Number (2 octets, unsigned integer) -- the
      ordinal number of this Fragment Acknowledgment message.  Each new
      Fragment Acknowledgment message MUST have this value greater than
      previous Fragment Acknowledgment messages sent by the host in the
      context of the current IKEv2 message exchange, i.e. with the same
      IKE SPIs, Exchange Type, Message ID and a sender's role (so that
      the initiator and the responder maintain these numbers for the
      messages they send independently).  This field MUST NOT be zero,
      packets with zero value MUST be ignored on receipt.

   *  Total Fragments (2 octets, unsigned integer) -- when sending, this
      field MUST be set to the value from the Total Fragments field in
      the Encrypted Fragment payloads (see Section 2.5 in [RFC7383] that
      the sender of this packet currently uses to reconstruct the
      fragmented message.  On receipr, if the value in this field is not
      equal to the value in the Total Fragments field in the in the
      Encrypted Fragment payload most recently sent to the peer, then
      this Fragment Acknowledgment message MUST be silently ignored.

   *  First Fragment Num (2 octets, unsigned integer) -- the number of
      the fragment that corresponds to the first significant bit (the
      leftmost bit of the first octet) in the Fragment Acknowledgment
      Bitmap.  This field MUST NOT be zero and MUST NOT be greater than
      the value in the Total Fragments field.  If these conditions are
      not met in the received packet, the packet MUST be silently
      discarded.

   *  Last Fragment Num (2 octets, unsigned integer) -- the number of
      the fragment that corresponds to the last significant bit in the
      Fragment Acknowledgment Bitmap.This field MUST NOT be zero, MUST
      NOT be smaller than the value in the First Fragment Num field and
      MUST NOT be greater than the value in the Total Fragments field.
      If these conditions are not met in the received packet, the packet
      MUST be silently discarded.

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   *  Fragment Acknowledgment Bitmap (variable) -- this field contains a
      bitmap that represents receipt status for fragments with numbers
      starting from the value from the First Fragment Num field till the
      value from the Last Fragment Num field.  The bitmap is interpreted
      as starting from the most significant (the leftmost) bit of the
      first octet of the field.  The length of the bitmap in octets is
      calculated as: (((Last Fragment Num) - (First Fragment Num)) / 8)
      + 1.  The last octet of the bitmap may contain some unused bits,
      these bits are ignored.  Each bit in the bitmap represents the
      receipt status of the corresponding fragment - it is set to 1 if
      the fragment was received and successfully processed and set to 0
      if the fragment is missing.

4.2.1.5.  Implementation Details

   TBD.  Should discuss when to send messages with Fragment
   Acknowledgment data, how to handle them, how to keep these messages
   small when the number of fragments is large, how to cope with PMTU
   discovery, etc.

5.  Security Considerations

   Security of IKEv2 and the IKEv2 fragmentation extension is discussed
   in [RFC7296] and [RFC7383] respectfully.  Techniques proposed in this
   document do not affect security properties of the protocol.

6.  IANA Considerations

   This document defines a new Notify Message Type in the "IKEv2 Notify
   Message Status Types" registry under the "Internet Key Exchange
   Version 2 (IKEv2) Parameters" registry group:

   TBA       FRAGMENT_ACK_SUPPORTED

7.  References

7.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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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, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,
              <https://www.rfc-editor.org/info/rfc7383>.

7.2.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc9329>.

   [I-D.ietf-ipsecme-ikev2-reliable-transport]
              Smyslov, V. and T. Reddy.K, "Separate Transports for IKE
              and ESP", Work in Progress, Internet-Draft, draft-ietf-
              ipsecme-ikev2-reliable-transport-08, 1 October 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
              ikev2-reliable-transport-08>.

   [I-D.wang-ipsecme-hybrid-kem-ikev2-frodo]
              WANG, G., Bruckert, L., and V. Smyslov, "Post-quantum
              Hybrid Key Exchange in IKEv2 with FrodoKEM", Work in
              Progress, Internet-Draft, draft-wang-ipsecme-hybrid-kem-
              ikev2-frodo-03, 24 December 2025,
              <https://datatracker.ietf.org/doc/html/draft-wang-ipsecme-
              hybrid-kem-ikev2-frodo-03>.

   [I-D.smyslov-ipsecme-ikev2-mceliece]
              Smyslov, V. and Y. Nir, "Using Classic McEliece in the
              Internet Key Exchange Protocol Version 2 (IKEv2)", Work in
              Progress, Internet-Draft, draft-smyslov-ipsecme-ikev2-
              mceliece-02, 8 April 2026,
              <https://datatracker.ietf.org/doc/html/draft-smyslov-
              ipsecme-ikev2-mceliece-02>.

   [I-D.kohbrok-ipsecme-mls-gike]
              Kohbrok, K., Robert, R., and V. Smyslov, "MLS for IPsec
              Group Key Exchange", Work in Progress, Internet-Draft,
              draft-kohbrok-ipsecme-mls-gike-00, 6 July 2026,
              <https://datatracker.ietf.org/doc/html/draft-kohbrok-
              ipsecme-mls-gike-00>.

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   [I-D.antony-ipsecme-ikev2-fragment-acknowledgment]
              Antony, A., Klassert, S., and T. Brunner, "IKEv2 Fragment
              Acknowledgment Extension", Work in Progress, Internet-
              Draft, draft-antony-ipsecme-ikev2-fragment-acknowledgment-
              03, 21 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-antony-
              ipsecme-ikev2-fragment-acknowledgment-03>.

Acknowledgements

   This document was inspired by the draft "IKEv2 Fragment
   Acknowledgment Extension"
   [I-D.antony-ipsecme-ikev2-fragment-acknowledgment].

Authors' Addresses

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

   Antony Antony
   secunet Security Networks AG
   Email: antony.antony@secunet.com

   Steffen Klassert
   secunet Security Networks AG
   Email: steffen.klassert@secunet.com

   Tobias Brunner
   codelabs GmbH
   Email: tobias@codelabs.ch

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