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6man Working Group                                             R. Bonica
Internet-Draft                                          Juniper Networks
Updates: RFC 2460 (if approved)                                W. Kumari
Intended status: Standards Track                            Google, Inc.
Expires: December 23, 2013                                       R. Bush
                                               Internet Initiative Japan
                                                           June 21, 2013

                    IPv6 Fragment Header Deprecated


   This memo deprecates the IPv6 Fragment Header.  It provides reasons
   for deprecation and updates RFC 2460.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 December 23, 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

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   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.  Case For Deprecation  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Resource Conservation . . . . . . . . . . . . . . . . . .   3
     2.2.  Fragmentation Is Rare . . . . . . . . . . . . . . . . . .   3
       2.2.1.  UDP-based Applications That Rely on Fragmentation . .   4
     2.3.  Attack Vectors  . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Operator Behavior . . . . . . . . . . . . . . . . . . . .   5
   3.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Each link on the Internet is characterized by a Maximum Transmission
   Unit (MTU).  A link's MTU represents the maximum packet size that can
   be conveyed over the link, without fragmentation.  MTU is a
   unidirectional metric.  A bidirectional link may be characterized by
   one MTU in the forward direction and another MTU in the reverse
   direction.  IPv6 [RFC2460] requires that every link in the Internet
   have an MTU of 1280 octets or greater.  On any link that cannot
   convey a 1280-octet packet in one piece, link-specific fragmentation
   and reassembly must be provided at a layer below IPv6.  Therefore,
   the PMTU between any two IPv6 nodes is 1280 bytes or greater.

   Likewise, for any given source node, the path to a particular
   destination node is characterized by a path MTU (PMTU).  At a given
   source, the PMTU associated with a destination is equal to the
   minimum MTU of all of the links that contribute to the path between
   the source and the destination.

   [RFC2460] strongly recommends that IPv6 nodes implement Path MTU
   Discovery (PMTUD) [RFC1981], in order to discover and take advantage
   of PMTUs greater than 1280 octets.  However, a minimal IPv6
   implementation (e.g., in a boot ROM) may simply restrict itself to

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   sending packets no larger than 1280 octets, and omit implementation
   of PMTUD.

   In order to send a packet larger than a path's MTU, a node may use
   the IPv6 Fragment header to fragment the packet at the source and
   have it reassembled at the destination(s).  However, the use of such
   fragmentation is discouraged in any application that is able to
   adjust its packets to fit the measured path MTU (i.e., down to 1280

   In IPv6, a packet can be fragmented only by the host that originates
   it.  This constitutes a departure from the IPv4 [RFC0791]
   fragmentation strategy, in which a packet can be fragmented by its
   originator or by any router that it traverses en route to its

   This memo deprecates the IPv6 Fragment Header.  It provides reasons
   for deprecation and updates [RFC2460].

2.  Case For Deprecation

   This section presents a case for deprecating the IPv6 Fragment

2.1.  Resource Conservation

   Packets that are fragmented at their source need to be reassembled at
   their destination.  [Kent87] points out that the reassembly process
   is resource intensive.  It consumes significant compute and memory
   resources.  While the cited reference refers to IPv4 fragmentation
   and reassembly, many of its criticisms are equally applicable to

   By comparison, if a source node were to execute PMTUD procedures, and
   if applications were to avoid sending datagrams that would result in
   IP packets that exceed the PMTU, the task of reassembly could be
   avoided, altogether.

2.2.  Fragmentation Is Rare

   Today, most popular operating systems implement PMTUD or an extension
   thereof, called Packetization Layer MTU Discovery (PMTUD) [RFC4821].
   Most popular TCP [RFC0793] implementations leverage this technology
   and restrict their segment size so that IP fragmentation is not
   required.  As a result, IPv6 fragments carrying TCP payload are
   rarely observed on the Internet.

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   Likewise, many UDP-based [RFC0768] applications follow the
   recommendations of [RFC5405].  According to [RFC5405], "an
   application SHOULD NOT send UDP datagrams that result in IP packets
   that exceed the MTU of the path to the destination.  Consequently, an
   application SHOULD either use the path MTU information provided by
   the IP layer or implement path MTU discovery itself to determine
   whether the path to a destination will support its desired message
   size without fragmentation.  Applications that do not follow this
   recommendation to do PMTU discovery SHOULD still avoid sending UDP
   datagrams that would result in IP packets that exceed the path MTU.
   Because the actual path MTU is unknown, such applications SHOULD fall
   back to sending messages that are shorter than the default effective
   MTU for sending."  The effective MTU for IPv6 is 1280 bytes.

   Because many UDP-based applications follow the above-quoted
   recommendation, IPv6 fragments carrying UDP traffic are also rarely
   observed on the Internet.

2.2.1.  UDP-based Applications That Rely on Fragmentation

   The following is a list of UDP-based applications that do not follow
   the recommendation of [RFC5405] and rely in IPv6 fragmentation:

   o  DNSSEC [RFC4035].  (However, it is useful to note the DNS queries
      and responses can run over TCP.)

   The effectiveness of these protocols may currently be degraded by
   operator behavior.  SeeSection 2.4 for details.

2.3.  Attack Vectors

   Security researchers have found and continue to find attack vectors
   that rely on IP fragmentation.  For example,
   [I-D.ietf-6man-oversized-header-chain] and
   [I-D.ietf-6man-nd-extension-headers] describe variants of the tiny
   fragment attack [RFC1858].  In this attack, a packet is crafted so
   that it can evade stateless firewall filters.  The stateless firewall
   filter matches on fields drawn from the IPv6 header and an upper
   layer header.  However, the packet is fragmented so that the upper
   layer header, or a significant part of that header, does not appear
   in the first fragment.  Because a stateless firewall cannot parse
   payload beyond the first fragment, the packet evades detection by the

   Security researcher have also studied reassembly algorithms on
   popular computing platforms, with the following goals:

   o  to discover fragility in seldom exercised parts of the IP stack

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   o  to engineer flows that maximize resources consumed by the
      reassembly process

   The Dawn and Rose Attacks [Hollis] are the products of such research.

   All of the attack vectors mentioned above can be mitigated with
   firewalls and increasingly sophisticated reassembly algorithms.
   However, the continued investment required to mitigate newly
   discovered vulnerabilities detracts from the cost effectiveness of
   IPv6 as a networking solution.

2.4.  Operator Behavior

   For reasons described above, and also articulated in
   [I-D.taylor-v6ops-fragdrop], many network operators filter all IPv6
   fragments.  Also, the default behavior of many currently deployed
   firewalls is to discard IPv6 fragments.

   In one recent study [DeBoer], two researchers distributed probes to
   423 IPv6 enabled sites.  The researchers then tested connectivity
   between an experimental control center and the probes.  They found
   that during any given trial period, sixty percent of the sites that
   could be reached with unfragmented packets could also be reached with
   fragmented packets.  The remaining forty percent appeared to be
   filtering IPv6 fragments

3.  Recommendation

   This memo deprecates IPv6 fragmentation and the IPv6 fragment header.
   New application and transport layer protocols MUST NOT send datagrams
   that result in IPv6 packets exceeding the MTU of the path to the
   destination.  However, legacy applications and transport layer
   protocols will continue to do so.

   New IPv6 host implementations MAY support IPv6 fragmentation and
   reassembly, but are not required to do so.

   Network operators MAY filter IPv6 fragments.

4.  IANA Considerations

   IANA is requested to mark the Fragment Header for IPv6 (44) as
   deprecated in the Protocol Numbers registry.

5.  Security Considerations

   Deprecation of the IPv6 Fragment Header will improve network security
   by eliminating attacks that rely on fragmentation.

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6.  Acknowledgements

   The author wishes to acknowledge Bob Hinden and Ole Troan for their
   review and constructive comments.

7.  References

7.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November

7.2.  Informative References

   [DeBoer]   De Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", July 2012, <http:

   [Hollis]   Hollis, K., "The Rose Attack Explained", , <http://

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              Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", draft-ietf-6man-nd-
              extension-headers-05 (work in progress), June 2013.

              Gont, F. and V. Manral, "Security and Interoperability
              Implications of Oversized IPv6 Header Chains", draft-ietf-
              6man-oversized-header-chain-02 (work in progress),
              November 2012.

              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-00 (work in progress), March 2013.

              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-01 (work in
              progress), June 2013.

   [Kent87]   Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
              In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
              Communications Technology , August 1987.

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

Authors' Addresses

   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, Virginia  20170

   Email: rbonica@juniper.net

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   Warren Kumari
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountainview, California  94043

   Email: warren@kumari.net

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island  Washington

   Email: randy@psg.com

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