Internet Engineering Task Force                               J. Jaeggli
Internet-Draft                                                     Zynga
Intended status: Informational                                L. Colitti
Expires: April 19, 2013                                        W. Kumari
                                                               E. Vyncke
                                                                 M. Kaeo
                                                    Double Shot Security
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                        October 16, 2012

           Why Operators Filter Fragments and What It Implies


   This memo is written to make application developers and network
   operators aware of the significant probability that IPv6 packets
   containing fragmentation extension headers will fail to reach their
   destination.  Some assumptions about the ability to use TCP or UDP
   datagrams larger than a single packet may accordingly need
   adjustment.  This memo provides observational evidence for the
   dropping of IPv6 fragments along a significant number of paths,
   explores the operational impact of fragmentation and the reasons why
   dropping occurs, and considers the effect of fragment dropping on
   applications particularly including DNS.

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
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   Drafts is at

   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 April 19, 2013.

Copyright Notice

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   Copyright (c) 2012 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Observations and Rationale  . . . . . . . . . . . . . . . . . . 3
     2.1.  Possible Causes . . . . . . . . . . . . . . . . . . . . . . 4
     2.2.  Impact on Applications  . . . . . . . . . . . . . . . . . . 5
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Informative References  . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   Measurements of whether internet service providers and edge networks
   deliver IPv6 fragments to their destination reveal that for IPv6 in
   particular, fragments are being dropped along a substantial number of
   paths.  IPv6 datagrams with fragmentation headers are a non-issue in
   the core of the internet, where fragments are routed just like any
   other IPv6 datagram.  However, fragmentation creates operational
   issues at the edge of the network that may lead to administratively
   imposed filtering or inadvertent failure to deliver the fragment to
   the application.

   Section 2 begins with some observations on how often IPv6 fragment
   loss occurs in practice.  We go on to look at the operational reasons
   for filtering fragments, a key aspect of which is the threats they
   pose to security policy and appliances.  Section 2.2 then looks at
   the impact on key applications, particularly DNS.

   In the longer run, as network operators gain a better understanding
   of the risks and non-risks of fragmentation and as middlebox,
   customer premise equipment (CPE), and host implementations improve,
   we believe that some incidences of fragment dropping will diminish.
   However, some of the justifications for filtering will persist in the
   longer term, and application developers must remain aware of the

   This document deliberately refrains from discussing possible
   responses to the problem posed by the dropping of IPv6 fragments.
   Such a discussion will quickly turn up a number of possibilities,
   application-specific or more general; but the amount of time needed
   to specify and deploy a given resolution will be a major constraint
   in choosing amongst them.  In any event, that discussion is likely to
   proceed in multiple directions and is therefore considered beyond the
   scope of this memo.

2.  Observations and Rationale

   [Blackhole] is a good public reference for the incidence of IPv6
   fragment filtering.  It describes experiments run to determine the
   incidence and location of ICMP Packet Too Big and fragment filtering.
   The authors used fragmented DNS packets to determine the latter and
   found for IPv6 that filtering appeared to be occurring on some 10% of
   the tested paths.  The filtering appeared to be located at the edge
   (enterprise and customer networks) rather than in the core.

   [Co-authors, more to contribute?]

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2.1.  Possible Causes

   Why does such filtering happen?  One cause is non-conforming
   implementations in CPE and low-end routers.  Along with that, some
   network managers filter fragments on principle, without taking
   account of the specific risks involved, just as they may filter ICMP
   Packet Too Big. Both implementations and management should improve
   over time, reducing the problem somewhat.

   Some filtering and dropping of fragments is done for hardware,
   performance, or topological considerations.  Stateful inspection
   devices or destination hosts can experience resource exhaustion if
   they are flooded with fragments not followed by the remaining
   fragments of the unfragmented packet.  Stateless ACLs may be
   difficult to apply to fragments other than the one in which the upper
   layer header is present.  As [Attacks] demonstrates, inconsistencies
   in reassembly logic between middleboxes or CPEs and hosts can cause
   fragments to be wrongfully discarded, or can allow exploits to pass
   undetected through middleboxes.  Stateless Load balancing schemes may
   hash fragmented datagrams from the same flow to different paths
   because the 5-tuple may available on only the initial fragment.

   Leaving aside these incentives towards fragment dropping, other
   considerations may weigh on the operator's mind.  One example cited
   on the NANOG list was that of a router where fragment processing was
   done by the control plane processor rather than in the forwarding
   plane hardware, with a consequent hit on performance.  Another
   incentive toward dropping of fragments is the disproportionate number
   of software errors still being encountered in fragment processing.
   Since this code is exercised less frequently than the rest of the
   stack, bugs remain longer in the code before they are detected.  Some
   of these software errors can introduce vulnerabilities subject to
   exploitation.  It is common practice [RFC6192] to recommend that
   control-plane ACLs protecting routers and network devices be
   configured to drop all fragments.

   Operators weigh the risks associated with each of the considerations
   just enumerated, and come up with the most suitable policy for their
   circumstances.  It is likely that at least some operators will find
   it desirable to drop fragments in at least some cases.

   The IETF can help this effort by identifying specific classes of
   fragments that do not represent legitimate use cases and hence should
   always be dropped.  Examples of this work are given by
   [draft-6man-atomic] and [I-D.ietf-6man-oversized-header-chain].  The
   problem of inconsistent implementations may also be mitigated by
   providing further advice on the more difficult points.  However, some
   cases will remain where legitimate fragments are discarded for

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   legitimate reasons.  The potential problems these cases pose for
   applications is our next topic.

2.2.  Impact on Applications

   Some applications can live without fragmentation, some cannot.  DNS
   is one application that may be vulnerable when fragment dropping
   occurs.  EDNS0 extensions [RFC2761] allow for responses in UDP PDU
   greater than 512 bytes.  Particularly with DNSSEC, responses may be
   larger than the MTU and fragmentation at the sending host in order to
   respond using UDP is desirable and legal.  The current choices open
   to the operators of DNS servers in this situation are to defer
   deployment of DNSSEC, fragment responses, or use TCP if there are
   cases where the rrset would be expected to exceed the MTU.  The use
   of fallback to TCP will impose a major resource and performance hit
   and increases vulnerability to denial of service attacks.

   Other applications, such as Network File System, NFS, are also known
   to fragment their large UDP packets but this is most often kept
   within a single organization network and should not be impacted by
   fragment dropping at the Internet core or edges.

3.  Acknowledgements


4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   [Obviously a few things to say here, or we can find a few good

6.  Informative References

   [Attacks]  Atlasis, A., "Attacking IPv6 Implementation Using
              Fragmentation", March 2012.



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              de Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", July 2012.


              Gont, F., "Processing of IPv6 "atomic" fragments",
              draft-ietf-6man-ipv6-atomic-fragments-01 (work in
              progress), August 2012.

              Gont, F. and V. Manral, "Security and Interoperability
              Implications of Oversized IPv6 Header Chains",
              draft-ietf-6man-oversized-header-chain-01 (work in
              progress), July 2012.

   [RFC2761]  Dunn, J. and C. Martin, "Terminology for ATM
              Benchmarking", RFC 2761, February 2000.

   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
              Router Control Plane", RFC 6192, March 2011.

              Gont, F., "Processing of IPv6 "atomic" fragments (Work in
              progress)", August 2012.

Authors' Addresses

   Joel Jaeggli
   924 Mouton Circle
   East Palo Alto, CA  94303


   Lorenzo Colitti


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   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043


   Eric Vyncke
   De Kleetlaan 6A
   Diegem,   1831


   Merike Kaeo
   Double Shot Security


   Tom Taylor (editor)
   Huawei Technologies
   Ottawa, Ontario


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