Internet Key Exchange Protocol Version 2 (IKEv2) Message Fragmentation
RFC 7383

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Internet Engineering Task Force (IETF)                        V. Smyslov
Request for Comments: 7383                                    ELVIS-PLUS
Category: Standards Track                                  November 2014
ISSN: 2070-1721

 Internet Key Exchange Protocol Version 2 (IKEv2) Message Fragmentation

Abstract

   This document describes a way to avoid IP fragmentation of large
   Internet Key Exchange Protocol version 2 (IKEv2) messages.  This
   allows IKEv2 messages to traverse network devices that do not allow
   IP fragments to pass through.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7383.

Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Smyslov                      Standards Track                    [Page 1]
RFC 7383                   IKEv2 Fragmentation             November 2014

Table of Contents

   1. Introduction ....................................................2
      1.1. Problem Description ........................................2
      1.2. Proposed Solution ..........................................3
      1.3. Conventions Used in This Document ..........................4
   2. Protocol Details ................................................4
      2.1. Overview ...................................................4
      2.2. Limitations ................................................4
      2.3. Negotiation ................................................5
      2.4. Using IKE Fragmentation ....................................5
      2.5. Fragmenting Message ........................................6
           2.5.1. Selecting Fragment Size .............................8
           2.5.2. PMTU Discovery ......................................9
           2.5.3. Fragmenting Messages Containing Unprotected
                  Payloads ...........................................11
      2.6. Receiving IKE Fragment Message ............................11
           2.6.1. Replay Detection and Retransmissions ...............13
   3. Interaction with Other IKE Extensions ..........................14
   4. Transport Considerations .......................................14
   5. Security Considerations ........................................15
   6. IANA Considerations ............................................16
   7. References .....................................................16
      7.1. Normative References ......................................16
      7.2. Informative References ....................................16
   Appendix A. Design Rationale ......................................19
   Appendix B. Correlation between IP Datagram Size and Encrypted
               Payload Content Size ..................................19
   Acknowledgements ..................................................20
   Author's Address ..................................................20

1.  Introduction

1.1.  Problem Description

   The Internet Key Exchange Protocol version 2 (IKEv2), specified in
   [RFC7296], uses UDP as a transport for its messages.  Most IKEv2
   messages are relatively small, usually below several hundred bytes.
   A notable exception is the IKE_AUTH exchange, which requires fairly
   large messages, up to several KB, especially when certificates are
   transferred.  When the IKE message size exceeds the path MTU, it gets
   fragmented at the IP level.  The problem is that some network
   devices, specifically some NAT boxes, do not allow IP fragments to
   pass through.  This apparently blocks IKE communication and,
   therefore, prevents peers from establishing an IPsec Security
   Association (SA).  Section 2 of [RFC7296] discusses the impact of IP
   fragmentation on IKEv2 and acknowledges this problem.

Smyslov                      Standards Track                    [Page 2]
RFC 7383                   IKEv2 Fragmentation             November 2014