draft-ietf-6man-deprecate-atomfrag-generation-07.txt		draft-ietf-6man-deprecate-atomfrag-generation-08.txt
	IPv6 maintenance Working Group (6man) F. Gont		IPv6 maintenance Working Group (6man) F. Gont
	Internet-Draft SI6 Networks / UTN-FRH		Internet-Draft SI6 Networks / UTN-FRH
	Intended status: Informational W. Liu		Intended status: Informational W. Liu
	Expires: January 18, 2017 Huawei Technologies		Expires: March 16, 2017 Huawei Technologies
	T. Anderson		T. Anderson
	Redpill Linpro		Redpill Linpro
	July 17, 2016		September 12, 2016
	Generation of IPv6 Atomic Fragments Considered Harmful		Generation of IPv6 Atomic Fragments Considered Harmful
	draft-ietf-6man-deprecate-atomfrag-generation-07		draft-ietf-6man-deprecate-atomfrag-generation-08
	Abstract		Abstract
	This document discusses the security implications of the generation		This document discusses the security implications of the generation
	of IPv6 atomic fragments and a number of interoperability issues		of IPv6 atomic fragments and a number of interoperability issues
	associated with IPv6 atomic fragments, and concludes that the		associated with IPv6 atomic fragments, and concludes that the
	aforementioned functionality is undesirable, thus documenting the		aforementioned functionality is undesirable, thus documenting the
	motivation for removing this functionality in the revision of the		motivation for removing this functionality in the revision of the
	core IPv6 protocol specification.		core IPv6 protocol specification.
	skipping to change at page 1, line 38 ¶		skipping to change at page 1, line 38 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 18, 2017.		This Internet-Draft will expire on March 16, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2016 IETF Trust and the persons identified as the		Copyright (c) 2016 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 17 ¶		skipping to change at page 2, line 17 ¶
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Security Implications of the Generation of IPv6 Atomic		2. Security Implications of the Generation of IPv6 Atomic
	Fragments . . . . . . . . . . . . . . . . . . . . . . . . . . 3		Fragments . . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. Additional Considerations . . . . . . . . . . . . . . . . . . 5		3. Additional Considerations . . . . . . . . . . . . . . . . . . 5
	4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7		4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 7		6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8		7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
	8.1. Normative References . . . . . . . . . . . . . . . . . . 8		8.1. Normative References . . . . . . . . . . . . . . . . . . 8
	8.2. Informative References . . . . . . . . . . . . . . . . . 9		8.2. Informative References . . . . . . . . . . . . . . . . . 9
	Appendix A. Small Survey of OSes that Fail to Produce IPv6		Appendix A. Small Survey of OSes that Fail to Produce IPv6
	Atomic Fragments . . . . . . . . . . . . . . . . . . 10		Atomic Fragments . . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Introduction		1. Introduction
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	of Service (DoS) attack vector that leverages the widespread		of Service (DoS) attack vector that leverages the widespread
	filtering of IPv6 fragments in the public Internet. Section 3		filtering of IPv6 fragments in the public Internet. Section 3
	provides additional considerations regarding the usefulness of		provides additional considerations regarding the usefulness of
	generating IPv6 atomic fragments.		generating IPv6 atomic fragments.
	2. Security Implications of the Generation of IPv6 Atomic Fragments		2. Security Implications of the Generation of IPv6 Atomic Fragments
	The security implications of IP fragmentation have been discussed at		The security implications of IP fragmentation have been discussed at
	length in [RFC6274] and [RFC7739]. An attacker can leverage the		length in [RFC6274] and [RFC7739]. An attacker can leverage the
	generation of IPv6 atomic fragments to trigger the use of		generation of IPv6 atomic fragments to trigger the use of
	fragmentation in an arbitrary IPv6 flow and subsequently perform any		fragmentation in an arbitrary IPv6 flow (in scenarios in which actual
			fragmentation of packets is not needed), and subsequently perform any
	fragmentation-based attack against legacy IPv6 nodes that do not		fragmentation-based attack against legacy IPv6 nodes that do not
	implement [RFC6946].		implement [RFC6946]. That is, employing fragmentation where not
			actually needed allows for fragmentation-based attack vectors to be
			employed, unnecessarily.
	Unfortunately, even nodes that already implement [RFC6946] can be		We note that, Unfortunately, even nodes that already implement
	subject to DoS attacks as a result of the generation of IPv6 atomic		[RFC6946] can be subject to DoS attacks as a result of the generation
	fragments. Let us assume that Host A is communicating with Server B,		of IPv6 atomic fragments. Let us assume that Host A is communicating
	and that, as a result of the widespread dropping of IPv6 packets that		with Server B, and that, as a result of the widespread dropping of
	contain extension headers (including fragmentation) [RFC7872], some		IPv6 packets that contain extension headers (including fragmentation)
	intermediate node filters fragments between Host A and Server B. If		[RFC7872], some intermediate node filters fragments between Server B
	an attacker sends a forged ICMPv6 "Packet Too Big" (PTB) error		and Host A. If an attacker sends a forged ICMPv6 "Packet Too Big"
	message to server B, reporting an MTU smaller than 1280, this will		(PTB) error message to server B, reporting an MTU smaller than 1280,
	trigger the generation of IPv6 atomic fragments from that moment on		this will trigger the generation of IPv6 atomic fragments from that
	(as required by [RFC2460]). When server B starts sending IPv6 atomic		moment on (as required by [RFC2460]). When server B starts sending
	fragments (in response to the received ICMPv6 PTB), these packets		IPv6 atomic fragments (in response to the received ICMPv6 PTB), these
	will be dropped, since we previously noted that IPv6 packets with		packets will be dropped, since we previously noted that IPv6 packets
	extension headers were being dropped between Host A and Server B.		with extension headers were being dropped between Server B and Host
	Thus, this situation will result in a Denial of Service (DoS)		A. Thus, this situation will result in a Denial of Service (DoS)
	scenario.		scenario.
	Another possible scenario is that in which two BGP peers are		Another possible scenario is that in which two BGP peers are
	employing IPv6 transport, and they implement Access Control Lists		employing IPv6 transport, and they implement Access Control Lists
	(ACLs) to drop IPv6 fragments (to avoid control-plane attacks). If		(ACLs) to drop IPv6 fragments (to avoid control-plane attacks). If
	the aforementioned BGP peers drop IPv6 fragments but still honor		the aforementioned BGP peers drop IPv6 fragments but still honor
	received ICMPv6 Packet Too Big error messages, an attacker could		received ICMPv6 Packet Too Big error messages, an attacker could
	easily attack the peering session by simply sending an ICMPv6 PTB		easily attack the peering session by simply sending an ICMPv6 PTB
	message with a reported MTU smaller than 1280 bytes. Once the attack		message with a reported MTU smaller than 1280 bytes. Once the attack
	packet has been sent, the aforementioned routers will themselves be		packet has been sent, the aforementioned routers will themselves be
	skipping to change at page 5, line 25 ¶		skipping to change at page 5, line 25 ¶
	2. The ability of the nodes receiving ICMPv6 PTB messages reporting		2. The ability of the nodes receiving ICMPv6 PTB messages reporting
	an MTU smaller than 1280 bytes to actually produce atomic		an MTU smaller than 1280 bytes to actually produce atomic
	fragments		fragments
	3. Support for IPv6 fragmentation on the IPv6 side of the translator		3. Support for IPv6 fragmentation on the IPv6 side of the translator
	4. The ability of the translator implementation to access the		4. The ability of the translator implementation to access the
	information conveyed by the IPv6 Fragment Header		information conveyed by the IPv6 Fragment Header
			5. The value extracted from the low-order 16-bits of the IPv6
			fragment Identification resulting in an appropriate IPv4
			Identification value
	Unfortunately,		Unfortunately,
	1. There exists a fair share of evidence of ICMPv6 Packet Too Big		1. There exists a fair share of evidence of ICMPv6 Packet Too Big
	messages being dropped on the public Internet (for instance, that		messages being dropped on the public Internet (for instance, that
	is one of the reasons for which PLPMTUD [RFC4821] was produced).		is one of the reasons for which PLPMTUD [RFC4821] was produced).
	Therefore, relying on such messages being successfully delivered		Therefore, relying on such messages being successfully delivered
	will affect the robustness of the protocol that relies on them.		will affect the robustness of the protocol that relies on them.
	2. A number of IPv6 implementations have been known to fail to		2. A number of IPv6 implementations have been known to fail to
	generate IPv6 atomic fragments in response to ICMPv6 PTB messages		generate IPv6 atomic fragments in response to ICMPv6 PTB messages
	skipping to change at page 6, line 12 ¶		skipping to change at page 6, line 14 ¶
	unnecessarily, in a Denial of Service situation even in		unnecessarily, in a Denial of Service situation even in
	legitimate cases.		legitimate cases.
	4. The packet-handling API at the node where the translator is		4. The packet-handling API at the node where the translator is
	running may obscure fragmentation-related information. In such		running may obscure fragmentation-related information. In such
	scenarios, the information conveyed by the Fragment Header may be		scenarios, the information conveyed by the Fragment Header may be
	unavailable to the translator. [JOOL] discusses a sample		unavailable to the translator. [JOOL] discusses a sample
	framework (Linux Netfilter) that hinders access to the		framework (Linux Netfilter) that hinders access to the
	information conveyed in IPv6 atomic fragments.		information conveyed in IPv6 atomic fragments.
			5. While [RFC2460] requires that the IPv6 fragment Identification of
			a fragmented packet be different 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
			value. Thus, there is no guarantee that, by employing the low-
			order 16-bits of the IPv6 fragment Identification of a packet
			sent by a source host, IPv4 fragment identification collisions
			will be avoided or reduced. 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 IPv6 context end up
			colliding when used in the translated IPv4 context.
	We note that SIIT essentially employs the Fragment Header of IPv6		We note that SIIT essentially employs the Fragment Header of IPv6
	atomic fragments to signal the translator how to set the DF bit of		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		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		Fragment Header, and is otherwise set to 1 when the IPv6 packet does
	not contain an IPv6 Fragment Header). Additionally, the translator		not contain an IPv6 Fragment Header). Additionally, the translator
	will employ the low-order 16-bits of the IPv6 Fragment Identification		will employ the low-order 16-bits of the IPv6 Fragment Identification
	for setting the IPv4 Fragment Identification. At least in theory,		for setting the IPv4 Fragment Identification. At least in theory,
	this is expected to reduce the IPv4 Identification collision rate in		this is expected to reduce the IPv4 Identification collision rate in
	the following specific scenario:		the following specific scenario:
	skipping to change at page 6, line 36 ¶		skipping to change at page 6, line 51 ¶
	Path MTU of 1280, due to an option-less IPv4 header being 20		Path MTU of 1280, due to an option-less IPv4 header being 20
	bytes shorter than the IPv6 header.		bytes shorter than the IPv6 header.
	3. ECMP routing [RFC2992] with more than one translator is employed		3. ECMP routing [RFC2992] with more than one translator is employed
	for e.g., redundancy purposes.		for e.g., redundancy purposes.
	In such a scenario, if each translator were to select the IPv4		In such a scenario, if each translator were to select the IPv4
	Identification on its own (rather than selecting the IPv4		Identification on its own (rather than selecting the IPv4
	Identification from the low-order 16-bits of the Fragment		Identification from the low-order 16-bits of the Fragment
	Identification of IPv6 atomic fragments), this could possibly lead to		Identification of IPv6 atomic fragments), this could possibly lead to
	IPv4 Identification collisions. However, since a number of		IPv4 Identification collisions. However, as noted above, the value
	implementations set the IPv6 Fragment Identification according to the		extracted from the low-order 16-bits of the IPv6 fragment
	output of a Pseudo-Random Number Generator (PRNG) (see Appendix B of		Identification might not result in an appropriate IPv4
	[RFC7739]) and the translator only employs the low-order 16-bits of		identification: for example, a number of implementations set the IPv6
	such value, it is very unlikely that relying on the Fragment		Fragment Identification according to the output of a Pseudo-Random
	Identification of the IPv6 atomic fragment will result in a reduced		Number Generator (PRNG) (see Appendix B of [RFC7739]); hence,if the
	IPv4 Identification collision rate (when compared to the case where		translator only employs the low-order 16-bits of such value, it is
	the translator selects each IPv4 Identification on its own).		very unlikely that relying on the Fragment Identification of the IPv6
	Besides, because of the limited sized of the IPv4 identification		atomic fragment will result in a reduced IPv4 Identification
	field, it is nevertheless virtually impossible to guarantee		collision rate (when compared to the case where the translator
	uniqueness of the IPv4 identification values without artificially		selects each IPv4 Identification on its own). Besides, because of
	limiting the data rate of fragmented traffic [RFC6864] [RFC4963].		the limited sized 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		[RFC6145] was the only "consumer" of IPv6 atomic fragments, and it
	correctly and diligently noted (in Section 6) the possible		correctly and diligently noted (in Section 6) the possible
	interoperability problems of relying on IPv6 atomic fragments,		interoperability problems of relying on IPv6 atomic fragments,
	proposing a workaround that led to more robust behavior and		proposing a workaround that led to more robust behavior and
	simplified code. [RFC6145] has been obsoleted by [RFC7915], such		simplified code. [RFC6145] has been obsoleted by [RFC7915], such
	that SIIT does not rely on IPv6 atomic fragments.		that SIIT does not rely on IPv6 atomic fragments.
	Finally, we believe that IPv4 links with an MTU smaller than 1260
	bytes are very uncommonly found in the modern Internet. At the same
	time, we note that the sole purpose of IPv6 atomic fragments is to
	make such links compatible with IPv4/IPv6 translation. We surmise,
	therefore, that IPv6 atomic fragments are useful in only a minuscule
	number of "real world" situations.
	4. Conclusions		4. Conclusions
	Taking all of the above considerations into account, we recommend		Taking all of the above considerations into account, we recommend
	that IPv6 atomic fragments be deprecated.		that IPv6 atomic fragments be deprecated.
	In particular:		In particular:
	o IPv4/IPv6 translators should be updated to not generate ICMPv6		o IPv4/IPv6 translators should be updated to not generate ICMPv6
	Packet Too Big errors containing a Path MTU value smaller than the		Packet Too Big errors containing a Path MTU value smaller than the
	minimum IPv6 MTU of 1280 bytes. This will ensure that current		minimum IPv6 MTU of 1280 bytes. This will ensure that current
	skipping to change at page 7, line 42 ¶		skipping to change at page 8, line 8 ¶
	We note that these recommendations have been incorporated in		We note that these recommendations have been incorporated in
	[I-D.ietf-6man-rfc1981bis], [I-D.ietf-6man-rfc2460bis] and [RFC7915].		[I-D.ietf-6man-rfc1981bis], [I-D.ietf-6man-rfc2460bis] and [RFC7915].
	5. IANA Considerations		5. IANA Considerations
	There are no IANA registries within this document.		There are no IANA registries within this document.
	6. Security Considerations		6. Security Considerations
	This document briefly discusses the security implications of the		This document briefly discusses the security implications of the
	generation of IPv6 atomic fragments, and describes a specific Denial		generation of IPv6 atomic fragments, and describes one specific
	of Service (DoS) attack vector that leverages the widespread		Denial of Service (DoS) attack vector that leverages the widespread
	filtering of IPv6 fragments in the public Internet. It concludes		filtering of IPv6 fragments in the public Internet. It concludes
	that the generation of IPv6 atomic fragments is an undesirable		that the generation of IPv6 atomic fragments is an undesirable
	feature, and documents the motivation for removing this functionality		feature, and documents the motivation for removing this functionality
	from [I-D.ietf-6man-rfc2460bis].		from [I-D.ietf-6man-rfc2460bis].
	7. Acknowledgements		7. Acknowledgements
	The authors would like to thank (in alphabetical order) Congxiao Bao,		The authors would like to thank (in alphabetical order) Congxiao Bao,
	Carlos Jesus Bernardos Cano, Bob Briscoe, Brian Carpenter, Tatuya		Carlos Jesus Bernardos Cano, Bob Briscoe, Brian Carpenter, Tatuya
	Jinmei, Bob Hinden, Alberto Leiva, Ted Lemon, Xing Li, Jeroen Massar,		Jinmei, Bob Hinden, Alberto Leiva, Ted Lemon, Xing Li, Jeroen Massar,
	Erik Nordmark, Qiong Sun, Ole Troan, Tina Tsou, and Bernie Volz, for		Erik Nordmark, Joe Touch, Qiong Sun, Ole Troan, Tina Tsou, and Bernie
	providing valuable comments on earlier versions of this document.		Volz, for providing valuable comments on earlier versions of this
			document.
	Fernando Gont would like to thank Jan Zorz / Go6 Lab		Fernando Gont would like to thank Jan Zorz / Go6 Lab
	<http://go6lab.si/>, and Jared Mauch / NTT America, for providing		<http://go6lab.si/>, and Jared Mauch / NTT America, for providing
	access to systems and networks that were employed to produce some of		access to systems and networks that were employed to produce some of
	the tests that resulted in the publication of this document.		the tests that resulted in the publication of this document.
	Additionally, he would like to thank SixXS <https://www.sixxs.net>		Additionally, he would like to thank SixXS <https://www.sixxs.net>
	for providing IPv6 connectivity.		for providing IPv6 connectivity.
	8. References		8. References
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