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Versions: 00 01 draft-ietf-6man-deprecate-atomfrag-generation

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 2460, 6145 (if approved)                                 W. Liu
Intended status: Standards Track                     Huawei Technologies
Expires: February 28, 2015                                   T. Anderson
                                                          Redpill Linpro
                                                         August 27, 2014


          Deprecating the Generation of IPv6 Atomic Fragments
            draft-gont-6man-deprecate-atomfrag-generation-01

Abstract

   The core IPv6 specification requires that when a host receives an
   ICMPv6 "Packet Too Big" message reporting a "Next-Hop MTU" smaller
   than 1280, the host includes a Fragment Header in all subsequent
   packets sent to that destination, without reducing the assumed Path-
   MTU.  The simplicity with which ICMPv6 "Packet Too Big" messages can
   be forged, coupled with the widespread filtering of IPv6 fragments,
   results in an attack vector that can be leveraged for Denial of
   Service purposes.  This document briefly discusses the aforementioned
   attack vector, and formally updates RFC2460 such that generation of
   IPv6 atomic fragments is deprecated, thus eliminating the
   aforementioned attack vector.  Additionally, it formally updates
   RFC6145 such that the Stateless IP/ICMP Translation Algorithm (SIIT)
   does not rely on the generation of IPv6 atomic fragments, thus
   improving the robustness of the protocol.

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 February 28, 2015.






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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Denial of Service (DoS) attack vector . . . . . . . . . . . .   3
   4.  Additional Considerations . . . . . . . . . . . . . . . . . .   5
   5.  Updating RFC2460  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Updating RFC6145  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Small Survey of OSes that Fail to Produce IPv6
                Atomic Fragments . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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).

   Section 5 of [RFC2460] states that, when a host receives an ICMPv6
   "Packet Too Big" message [RFC4443] advertising a "Next-Hop MTU"
   smaller than 1280 (the minimum IPv6 MTU), the host 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 will thus *not* be actually fragmented into several
   pieces, but rather just include a Fragment Header with both the
   "Fragment Offset" and the "M" flag set to 0 (we refer to these
   packets as "atomic fragments").  As required by [RFC6946], these



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   atomic fragments are essentially processed by the destination host as
   non-fragment traffic (since there are not really any fragments to be
   reassembled).  IPv6/IPv4 translators will typically employ the
   Fragment Identification information found in the Fragment Header to
   select an appropriate Fragment Identification value for the resulting
   IPv4 fragments.

   While atomic fragments might seem rather benign, there are scenarios
   in which the generation of IPv6 atomic fragments can introduce an
   attack vector that can be exploited for denial of service purposes.
   Since there are concrete security implications arising from the
   generation of IPv6 atomic fragments, and there is no real gain in
   generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4
   translators generate a Fragment Identification value themselves),
   this document formally updates [RFC2460], forbidding the generation
   of IPv6 atomic fragments, such that the aforementioned attack vector
   is eliminated.  Additionally, it formally updates [RFC6145] such that
   the Stateless IP/ICMP Translation Algorithm (SIIT) does not rely on
   the generation of IPv6 atomic fragments.

   Section 3 describes some possible attack scenarios.  Section 4
   provides additional considerations regarding the usefulness of
   generating IPv6 atomic fragments.  Section 5 formally updates RFC2460
   such that this attack vector is eliminated.  Section 6 formally
   updates RFC6145 such that it does not relies on the generation of
   IPv6 atomic fragments.

2.  Terminology

   IPv6 atomic fragments
      IPv6 packets that contain a Fragment Header with the Fragment
      Offset set to 0 and the M flag set to 0 (as defined by [RFC6946]).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Denial of Service (DoS) attack vector

   Let us assume that Host A is communicating with Server B, and that,
   as a result of the widespread filtering of IPv6 packets with
   extension headers (including fragmentation)
   [I-D.gont-v6ops-ipv6-ehs-in-real-world], some intermediate node
   filters fragments between Host A and Server B.  If an attacker sends
   a forged ICMPv6 "Packet Too Big" (PTB) error message to server B,
   reporting a Next-Hop MTU smaller than 1280, this will trigger the
   generation of IPv6 atomic fragments from that moment on (as required
   by [RFC2460]).  When server B starts sending IPv6 atomic fragments



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   (in response to the received ICMPv6 PTB), these packets will be
   dropped, since we previously noted that packets with IPv6 EHs were
   being dropped between Host A and Server B.  Thus, this situation will
   result in a Denial of Service (DoS) scenario.

   Another possible scenario is that in which two BGP peers are
   employing IPv6 transport, and they implement ACLs to drop IPv6
   fragments (to avoid control-plane attacks).  If the aforementioned
   BGP peers drop IPv6 fragments but still honor received ICMPv6 Packet
   Too Big error messages, an attacker could easily attack the peering
   session by simply sending an ICMPv6 PTB message with a reported MTU
   smaller than 1280 bytes.  Once the attack packet has been fired, it
   will be the aforementioned routers themselves the ones dropping their
   own traffic.

   The aforementioned attack vector is exacerbated by the following
   factors:

   o  The attacker does not need to forge the IPv6 Source Address of his
      attack packets.  Hence, deployment of simple BCP38 filters will
      not help as a counter-measure.

   o  Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6
      payload need to be forged.  While one could envision filtering
      devices enforcing BCP38-style filters on the ICMPv6 payload, the
      use of extension (by the attacker) could make this difficult, if
      at all possible.

   o  Many implementations fail to perform validation checks on the
      received ICMPv6 error messages, as recommended in Section 5.2 of
      [RFC4443] and documented in [RFC5927].  It should be noted that in
      some cases, such as when an ICMPv6 error message has (supposedly)
      been elicited by a connection-less transport protocol (or some
      other connection-less protocol being encapsulated in IPv6), it may
      be virtually impossible to perform validation checks on the
      received ICMPv6 error messages.  And, because of IPv6 extension
      headers, the ICMPv6 payload might not even contain any useful
      information on which to perform validation checks.

   o  Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
      error messages, the Destination Cache [RFC4861] is usually updated
      to reflect that any subsequent packets to such destination should
      include a Fragment Header.  This means that a single ICMPv6
      "Packet Too Big" error message might affect multiple communication
      instances (e.g., TCP connections) with such destination.

   o  As noted in Section 4, SIIT [RFC6145] is the only technology which
      currently makes use of atomic fragments.  Unfortunately, an IPv6



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      node cannot easily limit its exposure to the aforementioned attack
      vector by only generating IPv6 atomic fragments towards IPv4
      destinations behind a stateless translator.  This is due to the
      fact that Section 3.3 of RFC6052 [RFC6052] encourages operators to
      use a Network-Specific Prefix (NSP) that maps the IPv4 address
      space into IPv6.  When an NSP is being used, IPv6 addresses
      representing IPv4 nodes (reached through a stateless translator)
      are indistinguishable from native IPv6 addresses.

4.  Additional Considerations

   Besides the security assessment provided in Section 3, it is
   interesting to evaluate the pros and cons of having an IPv6-to-IPv4
   translating router rely on the generation of IPv6 atomic fragments.

   Relying on the generation of IPv6 atomic fragments implies a reliance
   on:

   1.  ICMPv6 packets arriving from the translator to the IPv6 node

   2.  The ability of the nodes receiving ICMPv6 PTB messages reporting
       an MTU smaller than 1280 bytes to actually produce atomic
       fragments

   3.  Support for IPv6 fragmentation on the IPv6 side of the translator

   Unfortunately,

   o  There exists a fair share of evidence of ICMPv6 Packet Too Big
      messages being dropped on the public Internet (for instance, that
      is one of the reasons for which PLPMTUD [RFC4821] was produced).
      Therefore, relying on such messages being successfully delivered
      will affect the robustness of the protocol that relies on them.

   o  A number of IPv6 implementations have been known to fail to
      generate IPv6 atomic fragments in response to ICMPv6 PTB messages
      reporting an MTU smaller than 1280 bytes (see Appendix A for a
      small survey).  Additionally, results included in Section 6 of
      [RFC6145] note that 57% of the tested web servers failed to
      produce IPv6 atomic fragments in response to ICMPv6 PTB messages
      reporting an MTU smaller than 1280 bytes.  Thus, any protocol
      relying on IPv6 atomic fragment generation for proper functioning
      will have interoperability problems with the aforementioned IPv6
      stacks.

   o  IPv6 atomic fragment generation represents a case in which
      fragmented traffic is produced where otherwise it would not be
      needed.  Since there is widespread filtering of IPv6 fragments in



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      the public Internet [I-D.gont-v6ops-ipv6-ehs-in-real-world], this
      would mean that the (unnecessary) use of IPv6 fragmentation might
      result, unnecessarily, in a Denial of Service situation even in
      legitimate cases.

   Finally, we note that SIIT essentially employs the Fragment Header of
   IPv6 atomic fragments to signal the translator how to set the DF bit
   of IPv4 datagrams (the DF bit is cleared when the IPv6 packet
   contains a Fragment Header, and is otherwise set to 1 when the IPv6
   packet does not contain an IPv6 Fragment Header).  Additionally, the
   translator will employ the low-order 16-bits of the IPv6 Fragment
   Identification for setting the IPv4 Fragment Identification.  At
   least in theory, this is expected to reduce the Fragment ID collision
   rate in the following specific scenario:

   1.  An IPv6 node communicates with an IPv4 node (through SIIT)

   2.  The IPv4 node is located behind an IPv4 link with an MTU < 1260

   3.  ECMP routing [RFC2992] with more than one translator are employed
       for e.g., redundancy purposes

   In such a scenario, if each translator were to select the IPv4
   Fragment Identification on its own (rather than selecting the IPv4
   Fragment ID from the low-order 16-bits of the Fragment Identification
   of atomic fragments), this could possibly lead to IPv4 Fragment ID
   collisions.  However, since a number of implementations set IPv6
   Fragment ID according to the output of a Pseudo-Random Number
   Generator (PRNG) (see Appendix B of
   [I-D.ietf-6man-predictable-fragment-id]) and the translator only
   employs the low-order 16-bits of such value, it is very unlikely that
   relying on the Fragment ID of the IPv6 atomic fragment will result in
   a reduced Fragment ID collision rate (when compared to the case where
   the translator selects each IPv4 Fragment ID on its own).

   Finally, we note that [RFC6145] is currently the only "consumer" of
   IPv6 atomic fragments, and it correctly and diligently notes (in
   Section 6) the possible interoperability problems of relying on IPv6
   atomic fragments, proposing as a workaround something very similar to
   what we propose in Section 6.  We believe that, by making the more
   robust behavior the default behavior of the "IP/ICMP Translation
   Algorithm", robustness is improved, and the corresponding code is
   simplified.








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5.  Updating RFC2460

   The following text from Section 5 of [RFC2460]:

      "In response to an IPv6 packet that is sent to an IPv4 destination
      (i.e., a packet that undergoes translation from IPv6 to IPv4), the
      originating IPv6 node may receive an ICMP Packet Too Big message
      reporting a Next-Hop MTU less than 1280.  In that case, the IPv6
      node is not required to reduce the size of subsequent packets to
      less than 1280, but must include a Fragment header in those
      packets so that the IPv6-to-IPv4 translating router can obtain a
      suitable Identification value to use in resulting IPv4 fragments.
      Note that this means the payload may have to be reduced to 1232
      octets (1280 minus 40 for the IPv6 header and 8 for the Fragment
      header), and smaller still if additional extension headers are
      used."

   is formally replaced with:

      "An IPv6 node that receives an ICMPv6 Packet Too Big error message
      that reports a Next-Hop MTU smaller than 1280 bytes (the minimum
      IPv6 MTU) MUST NOT include a Fragment header in subsequent packets
      sent to the corresponding destination.  That is, IPv6 nodes MUST
      NOT generate IPv6 atomic fragments."

6.  Updating RFC6145

   The following text from Section 4 (Translating from IPv4 to IPv6) of
   [RFC6145]:

   ---------------- cut here -------------- cut here ----------------
   When the IPv4 sender does not set the DF bit, the translator SHOULD
   always include an IPv6 Fragment Header to indicate that the sender
   allows fragmentation.  The translator MAY provide a configuration
   function that allows the translator not to include the Fragment
   Header for the non-fragmented IPv6 packets.

   The rules in Section 4.1 ensure that when packets are fragmented,
   either by the sender or by IPv4 routers, the low-order 16 bits of the
   fragment identification are carried end-to-end, ensuring that packets
   are correctly reassembled.  In addition, the rules in Section 4.1 use
   the presence of an IPv6 Fragment Header to indicate that the sender
   might not be using path MTU discovery (i.e., the packet should not
   have the DF flag set should it later be translated back to IPv4).
   ---------------- cut here -------------- cut here ----------------

   is formally replaced with:




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   ---------------- cut here -------------- cut here ----------------
   The rules in Section 4.1 ensure that when packets are fragmented,
   either by the sender or by IPv4 routers, the low-order 16 bits of the
   fragment identification are carried end-to-end, ensuring that packets
   are correctly reassembled.
   ---------------- cut here -------------- cut here ----------------

   The following text from Section 4.1 ("Translating IPv4 Headers into
   IPv6 Headers") of [RFC6145]:

   ---------------- cut here -------------- cut here ----------------
   If there is a need to add a Fragment Header (the DF bit is not set or
   the packet is a fragment), the header fields are set as above with
   the following exceptions:
   ---------------- cut here -------------- cut here ----------------

   is formally replaced with:

      ---------------- cut here -------------- cut here ----------------
      If there is a need to add a Fragment Header (the packet is a
      fragment), the header fields are set as above with the following
      exceptions:
      ---------------- cut here -------------- cut here ----------------

   The following text from Section 4.2 ("Translating ICMPv4 Headers into
   ICMPv6 Headers") of [RFC6145]:

    ---------------- cut here -------------- cut here ----------------
             Code 4 (Fragmentation Needed and DF was Set):  Translate to
                an ICMPv6 Packet Too Big message (Type 2) with Code set
                to 0.  The MTU field MUST be adjusted for the difference
                between the IPv4 and IPv6 header sizes, i.e.,
                minimum(advertised MTU+20, MTU_of_IPv6_nexthop,
                (MTU_of_IPv4_nexthop)+20).  Note that if the IPv4 router
                set the MTU field to zero, i.e., the router does not
                implement [RFC1191], then the translator MUST use the
                plateau values specified in [RFC1191] to determine a
                likely path MTU and include that path MTU in the ICMPv6
                packet.  (Use the greatest plateau value that is less
                than the returned Total Length field.)
    ---------------- cut here -------------- cut here ----------------

   is formally replaced with:








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   ---------------- cut here -------------- cut here ----------------
            Code 4 (Fragmentation Needed and DF was Set):  Translate to
               an ICMPv6 Packet Too Big message (Type 2) with Code set
               to 0.  The MTU field MUST be adjusted for the difference
               between the IPv4 and IPv6 header sizes, but MUST NOT be
               set to a value smaller than the minimum IPv6 MTU
               (1280 bytes). That is, it should be set to maximum(1280,
               minimum(advertised MTU+20, MTU_of_IPv6_nexthop,
               (MTU_of_IPv4_nexthop)+20)).  Note that if the IPv4 router
               set the MTU field to zero, i.e., the router does not
               implement [RFC1191], then the translator MUST use the
               plateau values specified in [RFC1191] to determine a
               likely path MTU and include that path MTU in the ICMPv6
               packet.  (Use the greatest plateau value that is less
               than the returned Total Length field, but that is larger
               than or equal to 1280.)
   ---------------- cut here -------------- cut here ----------------

   The following text from Section 5 ("Translating from IPv6 to IPv4")
   of [RFC6145]:































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   ---------------- cut here -------------- cut here ----------------
   There are some differences between IPv6 and IPv4 (in the areas of
   fragmentation and the minimum link MTU) that affect the translation.
   An IPv6 link has to have an MTU of 1280 bytes or greater.  The
   corresponding limit for IPv4 is 68 bytes.  Path MTU discovery across
   a translator relies on ICMP Packet Too Big messages being received
   and processed by IPv6 hosts, including an ICMP Packet Too Big that
   indicates the MTU is less than the IPv6 minimum MTU.  This
   requirement is described in Section 5 of [RFC2460] (for IPv6's
   1280-octet minimum MTU) and Section 5 of [RFC1883] (for IPv6's
   previous 576-octet minimum MTU).

   In an environment where an ICMPv4 Packet Too Big message is
   translated to an ICMPv6 Packet Too Big message, and the ICMPv6 Packet
   Too Big message is successfully delivered to and correctly processed
   by the IPv6 hosts (e.g., a network owned/operated by the same entity
   that owns/operates the translator), the translator can rely on IPv6
   hosts sending subsequent packets to the same IPv6 destination with
   IPv6 Fragment Headers.  In such an environment, when the translator
   receives an IPv6 packet with a Fragment Header, the translator SHOULD
   generate the IPv4 packet with a cleared Don't Fragment bit, and with
   its identification value from the IPv6 Fragment Header, for all of
   the IPv6 fragments (MF=0 or MF=1).

   In an environment where an ICMPv4 Packet Too Big message is filtered
   (by a network firewall or by the host itself) or not correctly
   processed by the IPv6 hosts, the IPv6 host will never generate an
   IPv6 packet with the IPv6 Fragment Header.  In such an environment,
   the translator SHOULD set the IPv4 Don't Fragment bit.  While setting
   the Don't Fragment bit may create PMTUD black holes [RFC2923] if
   there are IPv4 links smaller than 1260 octets, this is considered
   safer than causing IPv4 reassembly errors [RFC4963].
   ---------------- cut here -------------- cut here ----------------

   is formally replaced with:
















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    ---------------- cut here -------------- cut here ----------------
    There are some differences between IPv6 and IPv4 (in the areas of
    fragmentation and the minimum link MTU) that affect the translation.
    An IPv6 link has to have an MTU of 1280 bytes or greater.  The
    corresponding limit for IPv4 is 68 bytes.  Path MTU discovery across
    a translator relies on ICMP Packet Too Big messages being received
    and processed by IPv6 hosts.

    The difference in the minimum MTUs of IPv4 and IPv6 is accommodated
    as follows:

      o  When translating an ICMPv4 "Fragmentation Needed" packet, the
         indicated MTU in the resulting ICMPv6 "Packet Too Big" will
         never be set to a value lower than 1280. This ensures that the
         IPv6 nodes will never have to encounter or handle Path MTU
         values lower than the minimum IPv6 link MTU of 1280. See
         Section 4.2.

      o  When the resulting IPv4 packet is smaller than or equal to 1260
         bytes, the translator MUST send the packet with a cleared Don't
         Fragment bit. Otherwise, the packet MUST be sent with the Don't
         Fragment bit set. See Section 5.1.

    This approach allows Path MTU Discovery to operate end-to-end for
    paths whose MTU are not smaller than minimum IPv6 MTU of 1280 (which
    corresponds to MTU of 1260 in the IPv4 domain). On paths that have
    IPv4 links with MTU < 1260, the IPv4 router(s) connected to those
    links will fragment the packets in accordance with Section 2.3 of
    [RFC0791].
    ---------------- cut here -------------- cut here ----------------

   The following text from Section 5.1 ("Translating IPv6 Headers into
   IPv4 Headers") of [RFC6145]:


















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   ---------------- cut here -------------- cut here ----------------
   Identification:  All zero.  In order to avoid black holes caused by
      ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
      workaround is discussed in Section 6), the translator MAY provide
      a function to generate the identification value if the packet size
      is greater than 88 bytes and less than or equal to 1280 bytes.
      The translator SHOULD provide a method for operators to enable or
      disable this function.

   Flags:  The More Fragments flag is set to zero.  The Don't Fragment
      (DF) flag is set to one.  In order to avoid black holes caused by
      ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
      workaround is discussed in Section 6), the translator MAY provide
      a function as follows.  If the packet size is greater than 88
      bytes and less than or equal to 1280 bytes, it sets the DF flag to
      zero; otherwise, it sets the DF flag to one.  The translator
      SHOULD provide a method for operators to enable or disable this
      function.
   ---------------- cut here -------------- cut here ----------------

   is formally replaced with:

     ---------------- cut here -------------- cut here ----------------
     Identification:  Set according to a Fragment Identification
        generator at the translator.

     Flags:  The More Fragments flag is set to zero.  The Don't Fragment
        (DF) flag is set as follows: If the packet size is less than or
        equal to 1260 bytes, it is set to zero; otherwise, it is set to
        one.
     ---------------- cut here -------------- cut here ----------------

   The following text from Section 5.1.1 ("IPv6 Fragment Processing") of
   [RFC6145]:

    ---------------- cut here -------------- cut here ----------------
    If a translated packet with DF set to 1 will be larger than the MTU
    of the next-hop interface, then the translator MUST drop the packet
    and send the ICMPv6 Packet Too Big (Type 2, Code 0) error message to
    the IPv6 host with an adjusted MTU in the ICMPv6 message.
    ---------------- cut here -------------- cut here ----------------

   is formally replaced with:








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   ---------------- cut here -------------- cut here ----------------
   If an IPv6 packet that is smaller than or equal to 1280 bytes results
   (after translation) in an IPv4 packet that is larger than the MTU of
   the next-hop interface, then the translator MUST perform IPv4
   fragmentation on that packet such that it can be transferred over the
   constricting link.
   ---------------- cut here -------------- cut here ----------------

   Finally, the following text from 6 ("Special Considerations for
   ICMPv6 Packet Too Big") of [RFC6145]:

   ---------------- cut here -------------- cut here ----------------
   Two recent studies analyzed the behavior of IPv6-capable web servers
   on the Internet and found that approximately 95% responded as
   expected to an IPv6 Packet Too Big that indicated MTU = 1280, but
   only 43% responded as expected to an IPv6 Packet Too Big that
   indicated an MTU < 1280.  It is believed that firewalls violating
   Section 4.3.1 of [RFC4890] are at fault.  Both failures (the 5% wrong
   response when MTU = 1280 and the 57% wrong response when MTU < 1280)
   will cause PMTUD black holes [RFC2923].  Unfortunately, the
   translator cannot improve the failure rate of the first case (MTU =
   1280), but the translator can improve the failure rate of the second
   case (MTU < 1280).  There are two approaches to resolving the problem
   with sending ICMPv6 messages indicating an MTU < 1280.  It SHOULD be
   possible to configure a translator for either of the two approaches.

   The first approach is to constrain the deployment of the IPv6/IPv4
   translator by observing that four of the scenarios intended for
   stateless IPv6/IPv4 translators do not have IPv6 hosts on the
   Internet (Scenarios 1, 2, 5, and 6 described in [RFC6144], which
   refer to "An IPv6 network").  In these scenarios, IPv6 hosts, IPv6-
   host-based firewalls, and IPv6 network firewalls can be administered
   in compliance with Section 4.3.1 of [RFC4890] and therefore avoid the
   problem witnessed with IPv6 hosts on the Internet.

   The second approach is necessary if the translator has IPv6 hosts,
   IPv6-host-based firewalls, or IPv6 network firewalls that do not (or
   cannot) comply with Section 5 of [RFC2460] -- such as IPv6 hosts on
   the Internet.  This approach requires the translator to do the
   following:

   1.  In the IPv4-to-IPv6 direction: if the MTU value of ICMPv4 Packet
       Too Big (PTB) messages is less than 1280, change it to 1280.
       This is intended to cause the IPv6 host and IPv6 firewall to
       process the ICMP PTB message and generate subsequent packets to
       this destination with an IPv6 Fragment Header.

       Note: Based on recent studies, this is effective for 95% of IPv6



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       hosts on the Internet.

   2.  In the IPv6-to-IPv4 direction:

       A.  If there is a Fragment Header in the IPv6 packet, the last 16
           bits of its value MUST be used for the IPv4 identification
           value.

       B.  If there is no Fragment Header in the IPv6 packet:

           a.  If the packet is less than or equal to 1280 bytes:

               -  The translator SHOULD set DF to 0 and generate an IPv4
                  identification value.

               -  To avoid the problems described in [RFC4963], it is
                  RECOMMENDED that the translator maintain 3-tuple state
                  for generating the IPv4 identification value.

           b.  If the packet is greater than 1280 bytes, the translator
               SHOULD set the IPv4 DF bit to 1.
   ---------------- cut here -------------- cut here ----------------

   is formally replaced with:

   ---------------- cut here -------------- cut here ----------------
   A number of studies (see e.g. ) indicate that it not unusual for networks
   to drop ICMPv6 Packet Too Big error messages. Such packet drops will
   result in PMTUD blackholes [RFC2923], which can only be overcome with
   PLPMTUD [RFC4821].
   ---------------- cut here -------------- cut here ----------------

7.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.

8.  Security Considerations

   This document describes a Denial of Service (DoS) attack vector that
   leverages the widespread filtering of IPv6 fragments in the public
   Internet by means of ICMPv6 PTB error messages.  Additionally, it
   formally updates [RFC2460] such that this attack vector is
   eliminated, and also formally updated [RFC6145] such that it does not
   rely on IPv6 atomic fragments.





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9.  Acknowledgements

   The authors would like to thank (in alphabetical order) Bob Briscoe,
   Brian Carpenter, Tatuya Jinmei, Jeroen Massar, and Erik Nordmark, for
   providing valuable comments on earlier versions of this document.

   Fernando Gont would like to thank Jan Zorz and Go6 Lab
   <http://go6lab.si/> for providing access to systems and networks that
   were employed to produce some of tests that resulted in the
   publication of this document.  Additionally, he would like to thank
   SixXS <https://www.sixxs.net> for providing IPv6 connectivity.

10.  References

10.1.  Normative References

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

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

   [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.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

10.2.  Informative References

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
              2923, September 2000.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, November 2000.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.






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   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
              6946, May 2013.

   [I-D.ietf-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-01 (work in progress), April 2014.

   [I-D.gont-v6ops-ipv6-ehs-in-real-world]
              Gont, F., Linkova, J., Chown, T., and W. Will, "IPv6
              Extension Headers in the Real World", draft-gont-v6ops-
              ipv6-ehs-in-real-world-00 (work in progress), August 2014.

   [Morbitzer]
              Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis.
              Thesis number: 670. Department of Computing Science,
              Radboud University Nijmegen. August 2013,
              <https://www.ru.nl/publish/pages/578936/
              m_morbitzer_masterthesis.pdf>.

Appendix A.  Small Survey of OSes that Fail to Produce IPv6 Atomic
             Fragments

   [This section will probably be removed from this document before it
   is published as an RFC].

   This section includes a non-exhaustive list of operating systems that
   *fail* to produce IPv6 atomic fragments.  It is based on the results
   published in [RFC6946] and [Morbitzer].

   The following Operating Systems fail to generate IPv6 atomic
   fragments in response to ICMPv6 PTB messages that report an MTU
   smaller than 1280 bytes:

   o  FreeBSD 8.0

   o  Linux kernel 2.6.32

   o  Linux kernel 3.2

   o  Mac OS X 10.6.7

   o  NetBSD 5.1




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Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com


   Will(Shucheng) Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com


   Tore Anderson
   Redpill Linpro
   Vitaminveien 1A
   NO-0485 Oslo
   NORWAY

   Phone: +47 959 31 212
   Email: tore@redpill-linpro.com





















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