Processing of IPv6 "Atomic" Fragments
RFC 6946

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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 6946                           Huawei Technologies
Updates: 2460, 5722                                             May 2013
Category: Standards Track
ISSN: 2070-1721

                 Processing of IPv6 "Atomic" Fragments

Abstract

   The IPv6 specification allows packets to contain a Fragment Header
   without the packet being actually fragmented into multiple pieces (we
   refer to these packets as "atomic fragments").  Such packets are
   typically sent by hosts that have received an ICMPv6 "Packet Too Big"
   error message that advertises a Next-Hop MTU smaller than 1280 bytes,
   and are currently processed by some implementations as normal
   "fragmented traffic" (i.e., they are "reassembled" with any other
   queued fragments that supposedly correspond to the same original
   packet).  Thus, an attacker can cause hosts to employ atomic
   fragments by forging ICMPv6 "Packet Too Big" error messages, and then
   launch any fragmentation-based attacks against such traffic.  This
   document discusses the generation of the aforementioned atomic
   fragments and the corresponding security implications.  Additionally,
   this document formally updates RFC 2460 and RFC 5722, such that IPv6
   atomic fragments are processed independently of any other fragments,
   thus completely eliminating the aforementioned attack vector.

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/rfc6946.

Gont                         Standards Track                    [Page 1]
RFC 6946                  IPv6 Atomic Fragments                 May 2013

Copyright Notice

   Copyright (c) 2013 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.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................4
   3. Generation of IPv6 Atomic Fragments .............................4
   4. Updating RFC 2460 and RFC 5722 ..................................5
   5. Security Considerations .........................................6
   6. Acknowledgements ................................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   Appendix A. Survey of Processing of IPv6 Atomic Fragments by
               Different Operating Systems ............................8

1.  Introduction

   [RFC2460] specifies the IPv6 fragmentation mechanism, which allows
   IPv6 packets to be fragmented into smaller pieces such that they fit
   in the Path-MTU to the intended destination(s).  [RFC2460] allows
   fragments to overlap, thus leading to ambiguity in the result of the
   reassembly process, which could be leveraged by attackers to bypass
   firewall rules and/or evade Network Intrusion Detection Systems
   (NIDS) [RFC5722].

   [RFC5722] forbids overlapping fragments, specifying that when
   overlapping fragments are detected, all the fragments corresponding
   to that packet must be silently discarded.

   As specified in Section 5 of [RFC2460], when a host receives an
   ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller
   than 1280 (the minimum IPv6 MTU), it is not required to reduce the
   assumed Path-MTU, but must simply include a Fragment Header in all
   subsequent packets sent to that destination.  The resulting packets

Gont                         Standards Track                    [Page 2]
RFC 6946                  IPv6 Atomic Fragments                 May 2013

   will thus not actually be fragmented into several pieces but will
   just include a Fragment Header with both the "Fragment Offset" and
   the "M" flag set to 0 (we refer to these packets as "atomic