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Internet Engineering Task Force (IETF)                           F. GontRequest for Comments: 8021                        SI6 Networks / UTN-FRHCategory: Informational                                           W. LiuISSN: 2070-1721                                      Huawei Technologies                                                             T. Anderson                                                          Redpill Linpro                                                            January 2017Generation of IPv6 Atomic Fragments Considered HarmfulAbstract   This document discusses the security implications of the generation   of IPv6 atomic fragments and a number of interoperability issues   associated with IPv6 atomic fragments.  It concludes that the   aforementioned functionality is undesirable and thus documents the   motivation for removing this functionality from an upcoming revision   of the core IPv6 protocol specification (RFC 2460).Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   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).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8021.Gont, et al.                  Informational                     [Page 1]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................2   2. Security Implications of the Generation of IPv6 Atomic      Fragments .......................................................33. Additional Considerations .......................................54. Conclusions .....................................................85. Security Considerations .........................................86. References ......................................................96.1. Normative References .......................................96.2. Informative References ....................................10   Acknowledgements ..................................................12   Authors' Addresses ................................................121.  Introduction   [RFC2460] specifies the IPv6 fragmentation mechanism, which allows   IPv6 packets to be fragmented into smaller pieces such that they can   fit in the Path MTU to the intended destination(s).   A legacy IPv4/IPv6 translator implementing the Stateless IP/ICMP   Translation Algorithm [RFC6145] may legitimately generate ICMPv6   "Packet Too Big" (PTB) error messages [RFC4443] advertising an MTU   smaller than 1280 (the minimum IPv6 MTU).Section 5 of [RFC2460]   states that, upon receiving such an ICMPv6 error message, hosts are   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; rather, they will be "atomic" fragments [RFC6946]   (i.e., they will just include a Fragment Header with both the   "Fragment Offset" and the "M" flag set to 0).  [RFC6946] requires   that these atomic fragments be essentially processed by the   destination host(s) as non-fragmented traffic (since there are notGont, et al.                  Informational                     [Page 2]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017   really any fragments to be reassembled).  The goal of these atomic   fragments is simply to convey an appropriate Identification value to   be employed by IPv6/IPv4 translators for the resulting IPv4   fragments.   While atomic fragments might seem rather benign, there are scenarios   in which the generation of IPv6 atomic fragments can be leveraged for   performing a number of attacks against the corresponding IPv6 flows.   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, for example, having   IPv6/IPv4 translators generate an IPv4 Identification value   themselves), we conclude that this functionality is undesirable.Section 2 briefly discusses the security implications of the   generation of IPv6 atomic fragments and describes a specific   Denial-of-Service (DoS) attack vector that leverages the widespread   dropping of IPv6 fragments in the public Internet.Section 3   provides additional considerations regarding the usefulness of   generating IPv6 atomic fragments.2.  Security Implications of the Generation of IPv6 Atomic Fragments   The security implications of IP fragmentation have been discussed at   length in [RFC6274] and [RFC7739].  An attacker can leverage the   generation of IPv6 atomic fragments to trigger the use of   fragmentation in an arbitrary IPv6 flow (in scenarios in which actual   fragmentation of packets is not needed) and can subsequently perform   any type of fragmentation-based attack against legacy IPv6 nodes that   do not implement [RFC6946].  That is, employing fragmentation where   not actually needed allows for fragmentation-based attack vectors to   be employed, unnecessarily.   We note that, unfortunately, even nodes that already implement   [RFC6946] can be subject to DoS attacks as a result of the generation   of IPv6 atomic fragments.  Let us assume that Host A is communicating   with Host B and that, as a result of the widespread dropping of IPv6   packets that contain extension headers (including fragmentation)   [RFC7872], some intermediate node filters fragments between Host B   and Host A.  If an attacker sends a forged ICMPv6 PTB error message   to Host B, reporting an MTU smaller than 1280, this will trigger the   generation of IPv6 atomic fragments from that moment on (as required   by [RFC2460]).  When Host B starts sending IPv6 atomic fragments (in   response to the received ICMPv6 PTB error message), these packets   will be dropped, since we previously noted that IPv6 packets with   extension headers were being dropped between Host B and Host A.   Thus, this situation will result in a DoS scenario.Gont, et al.                  Informational                     [Page 3]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017   Another possible scenario is that in which two BGP peers are   employing IPv6 transport and they implement Access Control Lists   (ACLs) to drop IPv6 fragments (to avoid control-plane attacks).  If   the aforementioned BGP peers drop IPv6 fragments but still honor   received ICMPv6 PTB error messages, an attacker could easily attack   the corresponding peering session by simply sending an ICMPv6 PTB   message with a reported MTU smaller than 1280 bytes.  Once the attack   packet has been sent, the aforementioned routers will themselves be   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 filters as perBCP 38      [BCP38] does not help as a countermeasure.   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 filters in the style ofBCP 38 on the ICMPv6      payload, the use of extension headers (by the attacker) could make      this difficult, if not impossible.   o  Many implementations fail to perform validation checks on the      received ICMPv6 error messages as recommended inSection 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 connectionless transport protocol (or some      other connectionless protocol being encapsulated in IPv6), it may      be virtually impossible to perform validation checks on the      received ICMPv6 error message.  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 PTB error      messages, the Destination Cache [RFC4861] is usually updated to      reflect that any subsequent packets to such a destination should      include a Fragment Header.  This means that a single ICMPv6 PTB      error message might affect multiple communication instances (e.g.,      TCP connections) with such a destination.   o  As noted inSection 3, SIIT (the Stateless IP/ICMP Translation      Algorithm) [RFC6145], including derivative protocols such as      Stateful NAT64 (Network Address and Protocol Translation from IPv6      Clients to IPv4 Servers) [RFC6146], was the only technology making      use of atomic fragments.  Unfortunately, an IPv6 node cannot      easily limit its exposure to the aforementioned attack vector by      only generating IPv6 atomic fragments towards IPv4 destinationsGont, et al.                  Informational                     [Page 4]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017      behind a stateless translator.  This is due to the fact thatSection 3.3 of [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.3.  Additional Considerations   Besides the security assessment provided inSection 2, 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 destination       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   4.  The ability of the translator implementation to access the       information conveyed by the Fragment Header   5.  The value extracted from the low-order 16 bits of the IPv6       fragment header Identification field resulting in an appropriate       IPv4 Identification value   Unfortunately,   1.  There exists a fair share of evidence of ICMPv6 PTB error       messages being dropped on the public Internet (for instance, that       is one of the reasons for which Packetization Layer Path MTU       Discovery (PLPMTUD) [RFC4821] was produced).  Therefore, relying       on such messages being successfully delivered will affect the       robustness of the protocol that relies on them.   2.  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.  Additionally, the       results included inSection 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 thanGont, et al.                  Informational                     [Page 5]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017       1280 bytes.  Thus, any protocol relying on IPv6 atomic fragment       generation for proper functioning will have interoperability       problems with the aforementioned IPv6 stacks.   3.  IPv6 atomic fragment generation represents a case in which       fragmented traffic is produced where otherwise it would not be       needed.  Since there is widespread dropping of IPv6 fragments in       the public Internet [RFC7872], this would mean that the       (unnecessary) use of IPv6 fragmentation might result,       unnecessarily, in a DoS situation even in legitimate cases.   4.  The packet-handling API at the node where the translator is       running may obscure fragmentation-related information.  In such       scenarios, the information conveyed by the Fragment Header may be       unavailable to the translator.  [JOOL] discusses a sample       framework (Linux Netfilter) that hinders access to the       information conveyed in IPv6 fragments.   5.  While [RFC2460] requires that the IPv6 fragment header       Identification field of a fragmented packet be different than       that of any other fragmented packet sent recently with the same       Source Address and Destination Address, there is no requirement       on the low-order 16 bits of such a value.  Thus, there is no       guarantee that IPv4 fragment Identification collisions will be       avoided or reduced by employing the low-order 16 bits of the IPv6       fragment header Identification field of a packet sent by a source       host.  Besides, collisions might occur where two distinct IPv6       Destination Addresses are translated into the same IPv4 address,       such that Identification values that might have been generated to       be unique in the context of IPv6 end up colliding when used in       the context of translated IPv4.   We note that SIIT essentially employs the Fragment Header of IPv6   atomic fragments to signal the translator how to set the Don't   Fragment (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 a Fragment Header).  Additionally,   the translator will employ the low-order 16 bits of the IPv6 fragment   header Identification field for setting the IPv4 Identification.  At   least in theory, this is expected to reduce the IPv4 Identification   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 smaller       than 1260 bytes.  An IPv4 Path MTU of 1260 corresponds to an IPv6       Path MTU of 1280, due to an optionless IPv4 header being 20 bytes       shorter than the IPv6 header.Gont, et al.                  Informational                     [Page 6]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017   3.  ECMP routing [RFC2992] with more than one translator is employed,       for example, for redundancy purposes.   In such a scenario, if each translator were to select the IPv4   Identification on its own (rather than selecting the IPv4   Identification from the low-order 16 bits of the fragment   Identification of IPv6 atomic fragments), this could possibly lead to   IPv4 Identification collisions.  However, as noted above, the value   extracted from the low-order 16 bits of the IPv6 fragment header   Identification field might not result in an appropriate IPv4   Identification: for example, a number of implementations set the IPv6   fragment header Identification field according to the output of a   Pseudorandom Number Generator (PRNG) (seeAppendix B of [RFC7739]);   hence, if the translator only employs the low-order 16 bits of such a   value, it is very unlikely that relying on the fragment   Identification of the IPv6 atomic fragment will result in a reduced   IPv4 Identification collision rate (when compared to the case where   the translator selects each IPv4 Identification on its own).   Besides, because of the limited size of the IPv4 Identification   field, it is nevertheless virtually impossible to guarantee   uniqueness of the IPv4 Identification values without artificially   limiting the data rate of fragmented traffic [RFC6864] [RFC4963].   [RFC6145] was the only "consumer" of IPv6 atomic fragments, and it   correctly and diligently noted (in itsSection 6) the possible   interoperability problems of relying on IPv6 atomic fragments,   proposing a workaround that led to more robust behavior and   simplified code.  [RFC6145] has been obsoleted by [RFC7915], such   that SIIT does not rely on IPv6 atomic fragments.Gont, et al.                  Informational                     [Page 7]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 20174.  Conclusions   Taking all of the above considerations into account, we recommend   that IPv6 atomic fragments be deprecated.   In particular:   o  IPv4/IPv6 translators should be updated to not generate ICMPv6 PTB      error messages containing an MTU value smaller than the minimum      IPv6 MTU of 1280 bytes.  This will ensure that current IPv6 nodes      will never have a legitimate need to start generating IPv6 atomic      fragments.   o  The recommendation in the previous bullet ensures that there are      no longer any valid reasons for ICMPv6 PTB error messages      reporting an MTU value smaller than the minimum IPv6 MTU      (1280 bytes).  IPv6 nodes should therefore be updated to ignore      ICMPv6 PTB error messages reporting an MTU smaller than 1280 bytes      as invalid.   We note that these recommendations have been incorporated in   [IPv6-PMTUD], [IPv6-Spec], and [RFC7915].5.  Security Considerations   This document briefly discusses the security implications of the   generation of IPv6 atomic fragments and describes one specific DoS   attack vector that leverages the widespread dropping of IPv6   fragments in the public Internet.  It concludes that the generation   of IPv6 atomic fragments is an undesirable feature and documents the   motivation for removing this functionality from [IPv6-Spec].Gont, et al.                  Informational                     [Page 8]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 20176.  References6.1.  Normative References   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, DOI 10.17487/RFC2460,              December 1998, <http://www.rfc-editor.org/info/rfc2460>.   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:              Defeating Denial of Service Attacks which employ IP Source              Address Spoofing",BCP 38,RFC 2827, May 2000,              <http://www.rfc-editor.org/info/rfc2827>.   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet              Control Message Protocol (ICMPv6) for the Internet              Protocol Version 6 (IPv6) Specification",RFC 4443,              DOI 10.17487/RFC4443, March 2006,              <http://www.rfc-editor.org/info/rfc4443>.   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU              Discovery",RFC 4821, DOI 10.17487/RFC4821, March 2007,              <http://www.rfc-editor.org/info/rfc4821>.   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,              "Neighbor Discovery for IP version 6 (IPv6)",RFC 4861,              DOI 10.17487/RFC4861, September 2007,              <http://www.rfc-editor.org/info/rfc4861>.   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation              Algorithm",RFC 6145, DOI 10.17487/RFC6145, April 2011,              <http://www.rfc-editor.org/info/rfc6145>.   [RFC7915]  Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,              "IP/ICMP Translation Algorithm",RFC 7915,              DOI 10.17487/RFC7915, June 2016,              <http://www.rfc-editor.org/info/rfc7915>.   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",RFC 6864, DOI 10.17487/RFC6864, February 2013,              <http://www.rfc-editor.org/info/rfc6864>.Gont, et al.                  Informational                     [Page 9]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 20176.2.  Informative References   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path              Algorithm",RFC 2992, DOI 10.17487/RFC2992, November 2000,              <http://www.rfc-editor.org/info/rfc2992>.   [RFC5927]  Gont, F., "ICMP Attacks against TCP",RFC 5927,              DOI 10.17487/RFC5927, July 2010,              <http://www.rfc-editor.org/info/rfc5927>.   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly              Errors at High Data Rates",RFC 4963,              DOI 10.17487/RFC4963, July 2007,              <http://www.rfc-editor.org/info/rfc4963>.   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.              Li, "IPv6 Addressing of IPv4/IPv6 Translators",RFC 6052,              DOI 10.17487/RFC6052, October 2010,              <http://www.rfc-editor.org/info/rfc6052>.   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful              NAT64: Network Address and Protocol Translation from IPv6              Clients to IPv4 Servers",RFC 6146, DOI 10.17487/RFC6146,              April 2011, <http://www.rfc-editor.org/info/rfc6146>.   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol              Version 4",RFC 6274, DOI 10.17487/RFC6274, July 2011,              <http://www.rfc-editor.org/info/rfc6274>.   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",RFC 6946, DOI 10.17487/RFC6946, May 2013,              <http://www.rfc-editor.org/info/rfc6946>.   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment              Identification Values",RFC 7739, DOI 10.17487/RFC7739,              February 2016, <http://www.rfc-editor.org/info/rfc7739>.   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,              "Observations on the Dropping of Packets with IPv6              Extension Headers in the Real World",RFC 7872,              DOI 10.17487/RFC7872, June 2016,              <http://www.rfc-editor.org/info/rfc7872>.Gont, et al.                  Informational                    [Page 10]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017   [IPv6-Spec]              Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification", Work in Progress,draft-ietf-6man-rfc2460bis-08, November 2016.   [IPv6-PMTUD]              McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,              "Path MTU Discovery for IP version 6", Work in Progress,draft-ietf-6man-rfc1981bis-03, October 2016.   [JOOL]     Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6",              April 2015, <https://github.com/NICMx/Jool/wiki/nf_defrag_ipv4-and-nf_defrag_ipv6#implementation-gotchas>.Gont, et al.                  Informational                    [Page 11]

RFC 8021        IPv6 Atomic Fragments Considered Harmful    January 2017Acknowledgements   The authors would like to thank (in alphabetical order) Congxiao Bao,   Bob Briscoe, Carlos Jesus Bernardos Cano, Brian Carpenter, Bob   Hinden, Tatuya Jinmei, Alberto Leiva Popper, Ted Lemon, Xing Li,   Jeroen Massar, Erik Nordmark, Qiong Sun, Joe Touch, Ole Troan, Tina   Tsou, and Bernie Volz for providing valuable comments on earlier   versions of this document.   Fernando Gont would like to thank Jan Zorz / Go6 Lab   <http://go6lab.si/>, and Jared Mauch / NTT America, for providing   access to systems and networks that were employed to produce some of   the tests that resulted in the publication of this document.   Additionally, he would like to thank Ivan Arce, Guillermo Gont, and   Diego Armando Maradona for their inspiration.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   China   Email: liushucheng@huawei.com   Tore Anderson   Redpill Linpro   Vitaminveien 1A   Oslo  0485   Norway   Phone: +47 959 31 212   Email: tore@redpill-linpro.com   URI:http://www.redpill-linpro.comGont, et al.                  Informational                    [Page 12]