draft-ietf-6man-deprecate-atomfrag-generation-08.txt		rfc8021.txt
	IPv6 maintenance Working Group (6man) F. Gont		Internet Engineering Task Force (IETF) F. Gont
	Internet-Draft SI6 Networks / UTN-FRH		Request for Comments: 8021 SI6 Networks / UTN-FRH
	Intended status: Informational W. Liu		Category: Informational W. Liu
	Expires: March 16, 2017 Huawei Technologies		ISSN: 2070-1721 Huawei Technologies
	T. Anderson		T. Anderson
	Redpill Linpro		Redpill Linpro
	September 12, 2016		January 2017
	Generation of IPv6 Atomic Fragments Considered Harmful		Generation of IPv6 Atomic Fragments Considered Harmful
	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. It concludes that the
	aforementioned functionality is undesirable, thus documenting the		aforementioned functionality is undesirable and thus documents the
	motivation for removing this functionality in the revision of the		motivation for removing this functionality from an upcoming revision
	core IPv6 protocol specification.		of the core IPv6 protocol specification (RFC 2460).
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This document is not an Internet Standards Track specification; it is
	provisions of BCP 78 and BCP 79.		published for informational purposes.
	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		This document is a product of the Internet Engineering Task Force
	and may be updated, replaced, or obsoleted by other documents at any		(IETF). It represents the consensus of the IETF community. It has
	time. It is inappropriate to use Internet-Drafts as reference		received public review and has been approved for publication by the
	material or to cite them other than as "work in progress."		Internet Engineering Steering Group (IESG). Not all documents
			approved by the IESG are a candidate for any level of Internet
			Standard; see Section 2 of RFC 7841.
	This Internet-Draft will expire on March 16, 2017.		Information about the current status of this document, any errata,
			and how to provide feedback on it may be obtained at
			http://www.rfc-editor.org/info/rfc8021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2016 IETF Trust and the persons identified as the		Copyright (c) 2017 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
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	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 .....................................................8
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		5. Security Considerations .........................................8
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 8		6. References ......................................................9
	7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8		6.1. Normative References .......................................9
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8		6.2. Informative References ....................................10
	8.1. Normative References . . . . . . . . . . . . . . . . . . 8		Acknowledgements ..................................................12
	8.2. Informative References . . . . . . . . . . . . . . . . . 9		Authors' Addresses ................................................12
	Appendix A. Small Survey of OSes that Fail to Produce IPv6
	Atomic Fragments . . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Introduction		1. Introduction
	[RFC2460] specifies the IPv6 fragmentation mechanism, which allows		[RFC2460] specifies the IPv6 fragmentation mechanism, which allows
	IPv6 packets to be fragmented into smaller pieces such that they can		IPv6 packets to be fragmented into smaller pieces such that they can
	fit in the Path-MTU to the intended destination(s).		fit in the Path MTU to the intended destination(s).
	A legacy IPv4/IPv6 translator implementing the Stateless IP/ICMP		A legacy IPv4/IPv6 translator implementing the Stateless IP/ICMP
	Translation algorithm [RFC6145] may legitimately generate ICMPv6		Translation Algorithm [RFC6145] may legitimately generate ICMPv6
	"Packet Too Big" messages [RFC4443] advertising a "Next-Hop MTU"		"Packet Too Big" (PTB) error messages [RFC4443] advertising an MTU
	smaller than 1280 (the minimum IPv6 MTU). Section 5 of [RFC2460]		smaller than 1280 (the minimum IPv6 MTU). Section 5 of [RFC2460]
	states that, upon receiving such an ICMPv6 error message, hosts are		states that, upon receiving such an ICMPv6 error message, hosts are
	not required to reduce the assumed Path-MTU, but must simply include		not required to reduce the assumed Path MTU but must simply include a
	a Fragment Header in all subsequent packets sent to that destination.		Fragment Header in all subsequent packets sent to that destination.
	The resulting packets will thus *not* be actually fragmented into		The resulting packets will thus *not* be actually fragmented into
	several pieces, but rather be "atomic fragments" [RFC6946] (i.e.,		several pieces; rather, they will be "atomic" fragments [RFC6946]
	just include a Fragment Header with both the "Fragment Offset" and		(i.e., they will just include a Fragment Header with both the
	the "M" flag set to 0). [RFC6946] requires that these atomic		"Fragment Offset" and the "M" flag set to 0). [RFC6946] requires
	fragments be essentially processed by the destination host as non-		that these atomic fragments be essentially processed by the
	fragmented traffic (since there are not really any fragments to be		destination host(s) as non-fragmented traffic (since there are not
	reassembled). The goal of these atomic fragments is simply to convey		really any fragments to be reassembled). The goal of these atomic
	an appropriate Identification value to be employed by IPv6/IPv4		fragments is simply to convey an appropriate Identification value to
	translators for the resulting IPv4 fragments.		be employed by IPv6/IPv4 translators for the resulting IPv4
			fragments.
	While atomic fragments might seem rather benign, there are scenarios		While atomic fragments might seem rather benign, there are scenarios
	in which the generation of IPv6 atomic fragments can be leveraged for		in which the generation of IPv6 atomic fragments can be leveraged for
	performing a number of attacks against the corresponding IPv6 flows.		performing a number of attacks against the corresponding IPv6 flows.
	Since there are concrete security implications arising from the		Since there are concrete security implications arising from the
	generation of IPv6 atomic fragments, and there is no real gain in		generation of IPv6 atomic fragments and there is no real gain in
	generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4		generating IPv6 atomic fragments (as opposed to, for example, having
	translators generate a Fragment Identification value themselves), we		IPv6/IPv4 translators generate an IPv4 Identification value
	conclude that this functionality is undesirable.		themselves), we conclude that this functionality is undesirable.
	Section 2 briefly discusses the security implications of the		Section 2 briefly discusses the security implications of the
	generation of IPv6 atomic fragments, and describes a specific Denial		generation of IPv6 atomic fragments and describes a 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. Section 3		dropping 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 (in scenarios in which actual		fragmentation in an arbitrary IPv6 flow (in scenarios in which actual
	fragmentation of packets is not needed), and subsequently perform any		fragmentation of packets is not needed) and can subsequently perform
	fragmentation-based attack against legacy IPv6 nodes that do not		any type of fragmentation-based attack against legacy IPv6 nodes that
	implement [RFC6946]. That is, employing fragmentation where not		do not implement [RFC6946]. That is, employing fragmentation where
	actually needed allows for fragmentation-based attack vectors to be		not actually needed allows for fragmentation-based attack vectors to
	employed, unnecessarily.		be employed, unnecessarily.
	We note that, Unfortunately, even nodes that already implement		We note that, unfortunately, even nodes that already implement
	[RFC6946] can be subject to DoS attacks as a result of the generation		[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		of IPv6 atomic fragments. Let us assume that Host A is communicating
	with Server B, and that, as a result of the widespread dropping of		with Host B and that, as a result of the widespread dropping of IPv6
	IPv6 packets that contain extension headers (including fragmentation)		packets that contain extension headers (including fragmentation)
	[RFC7872], some intermediate node filters fragments between Server B		[RFC7872], some intermediate node filters fragments between Host B
	and Host A. If an attacker sends a forged ICMPv6 "Packet Too Big"		and Host A. If an attacker sends a forged ICMPv6 PTB error message
	(PTB) error message to server B, reporting an MTU smaller than 1280,		to Host B, reporting an MTU smaller than 1280, this will trigger the
	this will trigger the generation of IPv6 atomic fragments from that		generation of IPv6 atomic fragments from that moment on (as required
	moment on (as required by [RFC2460]). When server B starts sending		by [RFC2460]). When Host B starts sending IPv6 atomic fragments (in
	IPv6 atomic fragments (in response to the received ICMPv6 PTB), these		response to the received ICMPv6 PTB error message), these packets
	packets will be dropped, since we previously noted that IPv6 packets		will be dropped, since we previously noted that IPv6 packets with
	with extension headers were being dropped between Server B and Host		extension headers were being dropped between Host B and Host A.
	A. Thus, this situation will result in a Denial of Service (DoS)		Thus, this situation will result in a 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 PTB error messages, an attacker could easily attack
	easily attack the peering session by simply sending an ICMPv6 PTB		the corresponding 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
	the ones dropping their own traffic.		the ones dropping their own traffic.
	The aforementioned attack vector is exacerbated by the following		The aforementioned attack vector is exacerbated by the following
	factors:		factors:
	o The attacker does not need to forge the IPv6 Source Address of his		o The attacker does not need to forge the IPv6 Source Address of his
	attack packets. Hence, deployment of simple BCP38 filters will		attack packets. Hence, deployment of simple filters as per BCP 38
	not help as a counter-measure.		[BCP38] does not help as a countermeasure.
	o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6		o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6
	payload needs to be forged. While one could envision filtering		payload need to be forged. While one could envision filtering
	devices enforcing BCP38-style filters on the ICMPv6 payload, the		devices enforcing filters in the style of BCP 38 on the ICMPv6
	use of extension headers (by the attacker) could make this		payload, the use of extension headers (by the attacker) could make
	difficult, if at all possible.		this difficult, if not impossible.
	o Many implementations fail to perform validation checks on the		o Many implementations fail to perform validation checks on the
	received ICMPv6 error messages, as recommended in Section 5.2 of		received ICMPv6 error messages as recommended in Section 5.2 of
	[RFC4443] and documented in [RFC5927]. It should be noted that in		[RFC4443] and documented in [RFC5927]. It should be noted that in
	some cases, such as when an ICMPv6 error message has (supposedly)		some cases, such as when an ICMPv6 error message has (supposedly)
	been elicited by a connection-less transport protocol (or some		been elicited by a connectionless transport protocol (or some
	other connection-less protocol being encapsulated in IPv6), it may		other connectionless protocol being encapsulated in IPv6), it may
	be virtually impossible to perform validation checks on the		be virtually impossible to perform validation checks on the
	received ICMPv6 error message. And, because of IPv6 extension		received ICMPv6 error message. And, because of IPv6 extension
	headers, the ICMPv6 payload might not even contain any useful		headers, the ICMPv6 payload might not even contain any useful
	information on which to perform validation checks.		information on which to perform validation checks.
	o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"		o Upon receipt of one of the aforementioned ICMPv6 PTB error
	error messages, the Destination Cache [RFC4861] is usually updated		messages, the Destination Cache [RFC4861] is usually updated to
	to reflect that any subsequent packets to such destination should		reflect that any subsequent packets to such a destination should
	include a Fragment Header. This means that a single ICMPv6		include a Fragment Header. This means that a single ICMPv6 PTB
	"Packet Too Big" error message might affect multiple communication		error message might affect multiple communication instances (e.g.,
	instances (e.g., TCP connections) with such destination.		TCP connections) with such a destination.
	o As noted in Section 3, SIIT (Stateless IP/ICMP Translation		o As noted in Section 3, SIIT (the Stateless IP/ICMP Translation
	Algorithm) [RFC6145], including derivative protocols such as		Algorithm) [RFC6145], including derivative protocols such as
	Stateful NAT64 [RFC6146], was the only technology making use of		Stateful NAT64 (Network Address and Protocol Translation from IPv6
	atomic fragments. Unfortunately, an IPv6 node cannot easily limit		Clients to IPv4 Servers) [RFC6146], was the only technology making
	its exposure to the aforementioned attack vector by only		use of atomic fragments. Unfortunately, an IPv6 node cannot
	generating IPv6 atomic fragments towards IPv4 destinations behind		easily limit its exposure to the aforementioned attack vector by
	a stateless translator. This is due to the fact that Section 3.3		only generating IPv6 atomic fragments towards IPv4 destinations
	of [RFC6052] encourages operators to use a Network-Specific Prefix		behind a stateless translator. This is due to the fact that
	(NSP) that maps the IPv4 address space into IPv6. When an NSP is		Section 3.3 of [RFC6052] encourages operators to use a
	being used, IPv6 addresses representing IPv4 nodes (reached		Network-Specific Prefix (NSP) that maps the IPv4 address space
	through a stateless translator) are indistinguishable from native		into IPv6. When an NSP is being used, IPv6 addresses representing
	IPv6 addresses.		IPv4 nodes (reached through a stateless translator) are
			indistinguishable from native IPv6 addresses.
	3. Additional Considerations		3. Additional Considerations
	Besides the security assessment provided in Section 2, it is		Besides the security assessment provided in Section 2, it is
	interesting to evaluate the pros and cons of having an IPv6-to-IPv4		interesting to evaluate the pros and cons of having an IPv6-to-IPv4
	translating router rely on the generation of IPv6 atomic fragments.		translating router rely on the generation of IPv6 atomic fragments.
	Relying on the generation of IPv6 atomic fragments implies a reliance		Relying on the generation of IPv6 atomic fragments implies a
	on:		reliance on:
	1. ICMPv6 packets arriving from the translator to the IPv6 node		1. ICMPv6 packets arriving from the translator to the destination
			IPv6 node
	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 Fragment Header
	5. The value extracted from the low-order 16-bits of the IPv6		5. The value extracted from the low-order 16 bits of the IPv6
	fragment Identification resulting in an appropriate IPv4		fragment header Identification field resulting in an appropriate
	Identification value		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 PTB error
	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 Packetization Layer Path MTU
	Therefore, relying on such messages being successfully delivered		Discovery (PLPMTUD) [RFC4821] was produced). Therefore, relying
	will affect the robustness of the protocol that relies on them.		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		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
	reporting an MTU smaller than 1280 bytes (see Appendix A for a		reporting an MTU smaller than 1280 bytes. Additionally, the
	small survey). Additionally, the results included in Section 6		results included in Section 6 of [RFC6145] note that 57% of the
	of [RFC6145] note that 57% of the tested web servers failed to		tested web servers failed to produce IPv6 atomic fragments in
	produce IPv6 atomic fragments in response to ICMPv6 PTB messages		response to ICMPv6 PTB messages reporting an MTU smaller than
	reporting an MTU smaller than 1280 bytes. Thus, any protocol		1280 bytes. Thus, any protocol relying on IPv6 atomic fragment
	relying on IPv6 atomic fragment generation for proper functioning		generation for proper functioning will have interoperability
	will have interoperability problems with the aforementioned IPv6		problems with the aforementioned IPv6 stacks.
	stacks.
	3. IPv6 atomic fragment generation represents a case in which		3. IPv6 atomic fragment generation represents a case in which
	fragmented traffic is produced where otherwise it would not be		fragmented traffic is produced where otherwise it would not be
	needed. Since there is widespread filtering of IPv6 fragments in		needed. Since there is widespread dropping of IPv6 fragments in
	the public Internet [RFC7872], this would mean that the		the public Internet [RFC7872], this would mean that the
	(unnecessary) use of IPv6 fragmentation might result,		(unnecessary) use of IPv6 fragmentation might result,
	unnecessarily, in a Denial of Service situation even in		unnecessarily, in a DoS 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 fragments.
	5. While [RFC2460] requires that the IPv6 fragment Identification of		5. While [RFC2460] requires that the IPv6 fragment header
	a fragmented packet be different that of any other fragmented		Identification field of a fragmented packet be different than
	packet sent recently with the same Source Address and Destination		that of any other fragmented packet sent recently with the same
	Address, there is no requirement on the low-order 16-bits of such		Source Address and Destination Address, there is no requirement
	value. Thus, there is no guarantee that, by employing the low-		on the low-order 16 bits of such a value. Thus, there is no
	order 16-bits of the IPv6 fragment Identification of a packet		guarantee that IPv4 fragment Identification collisions will be
	sent by a source host, IPv4 fragment identification collisions		avoided or reduced by employing the low-order 16 bits of the IPv6
	will be avoided or reduced. Besides, collisions might occur		fragment header Identification field of a packet sent by a source
	where two distinct IPv6 Destination Addresses are translated into		host. Besides, collisions might occur where two distinct IPv6
	the same IPv4 address, such that Identification values that might		Destination Addresses are translated into the same IPv4 address,
	have been generated to be unique in the IPv6 context end up		such that Identification values that might have been generated to
	colliding when used in the translated IPv4 context.		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		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 Don't
	IPv4 datagrams (the DF bit is cleared when the IPv6 packet contains a		Fragment (DF) bit of IPv4 datagrams (the DF bit is cleared when the
	Fragment Header, and is otherwise set to 1 when the IPv6 packet does		IPv6 packet contains a Fragment Header and is otherwise set to 1 when
	not contain an IPv6 Fragment Header). Additionally, the translator		the IPv6 packet does not contain a Fragment Header). Additionally,
	will employ the low-order 16-bits of the IPv6 Fragment Identification		the translator will employ the low-order 16 bits of the IPv6 fragment
	for setting the IPv4 Fragment Identification. At least in theory,		header Identification field for setting the IPv4 Identification. At
	this is expected to reduce the IPv4 Identification collision rate in		least in theory, this is expected to reduce the IPv4 Identification
	the following specific scenario:		collision rate in the following specific scenario:
	1. An IPv6 node communicates with an IPv4 node (through SIIT).		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		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		than 1260 bytes. An IPv4 Path MTU of 1260 corresponds to an IPv6
	Path MTU of 1280, due to an option-less IPv4 header being 20		Path MTU of 1280, due to an optionless IPv4 header being 20 bytes
	bytes shorter than the IPv6 header.		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 example, for 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, as noted above, the value		IPv4 Identification collisions. However, as noted above, the value
	extracted from the low-order 16-bits of the IPv6 fragment		extracted from the low-order 16 bits of the IPv6 fragment header
	Identification might not result in an appropriate IPv4		Identification field might not result in an appropriate IPv4
	identification: for example, a number of implementations set the IPv6		Identification: for example, a number of implementations set the IPv6
	Fragment Identification according to the output of a Pseudo-Random		fragment header Identification field according to the output of a
	Number Generator (PRNG) (see Appendix B of [RFC7739]); hence,if the		Pseudorandom Number Generator (PRNG) (see Appendix B of [RFC7739]);
	translator only employs the low-order 16-bits of such value, it is		hence, if the translator only employs the low-order 16 bits of such a
	very unlikely that relying on the Fragment Identification of the IPv6		value, it is very unlikely that relying on the fragment
	atomic fragment will result in a reduced IPv4 Identification		Identification of the IPv6 atomic fragment will result in a reduced
	collision rate (when compared to the case where the translator		IPv4 Identification collision rate (when compared to the case where
	selects each IPv4 Identification on its own). Besides, because of		the translator selects each IPv4 Identification on its own).
	the limited sized of the IPv4 identification field, it is		Besides, because of the limited size of the IPv4 Identification
	nevertheless virtually impossible to guarantee uniqueness of the IPv4		field, it is nevertheless virtually impossible to guarantee
	identification values without artificially limiting the data rate of		uniqueness of the IPv4 Identification values without artificially
	fragmented traffic [RFC6864] [RFC4963].		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 its 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.
	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 PTB
	Packet Too Big errors containing a Path MTU value smaller than the		error messages containing an MTU value smaller than the minimum
	minimum IPv6 MTU of 1280 bytes. This will ensure that current		IPv6 MTU of 1280 bytes. This will ensure that current IPv6 nodes
	IPv6 nodes will never have a legitimate need to start generating		will never have a legitimate need to start generating IPv6 atomic
	IPv6 atomic fragments.		fragments.
	o The recommendation in the previous bullet ensures there no longer		o The recommendation in the previous bullet ensures that there are
	are any valid reasons for ICMPv6 Packet Too Big errors containing		no longer any valid reasons for ICMPv6 PTB error messages
	a Path MTU value smaller than the minimum IPv6 MTU to exist. IPv6		reporting an MTU value smaller than the minimum IPv6 MTU
	nodes should therefore be updated to ignore them as invalid.		(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		We note that these recommendations have been incorporated in
	[I-D.ietf-6man-rfc1981bis], [I-D.ietf-6man-rfc2460bis] and [RFC7915].		[IPv6-PMTUD], [IPv6-Spec], and [RFC7915].
	5. IANA Considerations
	There are no IANA registries within this document.
	6. Security Considerations		5. 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 one specific		generation of IPv6 atomic fragments and describes one specific DoS
	Denial of Service (DoS) attack vector that leverages the widespread		attack vector that leverages the widespread dropping of IPv6
	filtering of IPv6 fragments in the public Internet. It concludes		fragments in the public Internet. It concludes that the generation
	that the generation of IPv6 atomic fragments is an undesirable		of IPv6 atomic fragments is an undesirable feature and documents the
	feature, and documents the motivation for removing this functionality		motivation for removing this functionality from [IPv6-Spec].
	from [I-D.ietf-6man-rfc2460bis].
	7. Acknowledgements
	The authors would like to thank (in alphabetical order) Congxiao Bao,
	Carlos Jesus Bernardos Cano, Bob Briscoe, Brian Carpenter, Tatuya
	Jinmei, Bob Hinden, Alberto Leiva, Ted Lemon, Xing Li, Jeroen Massar,
	Erik Nordmark, Joe Touch, Qiong Sun, 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 SixXS <https://www.sixxs.net>
	for providing IPv6 connectivity.
	8. References		6. References
	8.1. Normative References		6.1. Normative References
	[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6		[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,		(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
	December 1998, <http://www.rfc-editor.org/info/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		[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
	Control Message Protocol (ICMPv6) for the Internet		Control Message Protocol (ICMPv6) for the Internet
	Protocol Version 6 (IPv6) Specification", RFC 4443,		Protocol Version 6 (IPv6) Specification", RFC 4443,
	DOI 10.17487/RFC4443, March 2006,		DOI 10.17487/RFC4443, March 2006,
	<http://www.rfc-editor.org/info/rfc4443>.		<http://www.rfc-editor.org/info/rfc4443>.
	[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU		[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
	Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,		Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
	<http://www.rfc-editor.org/info/rfc4821>.		<http://www.rfc-editor.org/info/rfc4821>.
	skipping to change at page 9, line 18 ¶		skipping to change at page 10, line 5 ¶
	[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,		[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
	"IP/ICMP Translation Algorithm", RFC 7915,		"IP/ICMP Translation Algorithm", RFC 7915,
	DOI 10.17487/RFC7915, June 2016,		DOI 10.17487/RFC7915, June 2016,
	<http://www.rfc-editor.org/info/rfc7915>.		<http://www.rfc-editor.org/info/rfc7915>.
	[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",		[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
	RFC 6864, DOI 10.17487/RFC6864, February 2013,		RFC 6864, DOI 10.17487/RFC6864, February 2013,
	<http://www.rfc-editor.org/info/rfc6864>.		<http://www.rfc-editor.org/info/rfc6864>.
	8.2. Informative References		6.2. Informative References
	[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path		[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
	Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,		Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
	<http://www.rfc-editor.org/info/rfc2992>.		<http://www.rfc-editor.org/info/rfc2992>.
	[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,		[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
	DOI 10.17487/RFC5927, July 2010,		DOI 10.17487/RFC5927, July 2010,
	<http://www.rfc-editor.org/info/rfc5927>.		<http://www.rfc-editor.org/info/rfc5927>.
	[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly		[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
	skipping to change at page 10, line 15 ¶		skipping to change at page 11, line 5 ¶
	[RFC7739] Gont, F., "Security Implications of Predictable Fragment		[RFC7739] Gont, F., "Security Implications of Predictable Fragment
	Identification Values", RFC 7739, DOI 10.17487/RFC7739,		Identification Values", RFC 7739, DOI 10.17487/RFC7739,
	February 2016, <http://www.rfc-editor.org/info/rfc7739>.		February 2016, <http://www.rfc-editor.org/info/rfc7739>.
	[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,		[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
	"Observations on the Dropping of Packets with IPv6		"Observations on the Dropping of Packets with IPv6
	Extension Headers in the Real World", RFC 7872,		Extension Headers in the Real World", RFC 7872,
	DOI 10.17487/RFC7872, June 2016,		DOI 10.17487/RFC7872, June 2016,
	<http://www.rfc-editor.org/info/rfc7872>.		<http://www.rfc-editor.org/info/rfc7872>.
	[I-D.ietf-6man-rfc2460bis]		[IPv6-Spec]
	Deering, D. and R. Hinden, "Internet Protocol, Version 6		Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", draft-ietf-6man-rfc2460bis-05 (work		(IPv6) Specification", Work in Progress,
	in progress), June 2016.		draft-ietf-6man-rfc2460bis-08, November 2016.
	[I-D.ietf-6man-rfc1981bis]
	<>, J., <>, S., <>, J., and R. Hinden, "Path MTU Discovery
	for IP version 6", draft-ietf-6man-rfc1981bis-02 (work in
	progress), April 2016.
	[Morbitzer]		[IPv6-PMTUD]
	Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis.		McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
	Thesis number: 670. Department of Computing Science,		"Path MTU Discovery for IP version 6", Work in Progress,
	Radboud University Nijmegen. August 2013,		draft-ietf-6man-rfc1981bis-03, October 2016.
	<http://www.ru.nl/publish/pages/769526/
	m_morbitzer_masterthesis.pdf>.
	[JOOL] Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6",		[JOOL] Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6",
	April 2015, <https://github.com/NICMx/Jool/wiki/		April 2015, <https://github.com/NICMx/Jool/wiki/
	nf_defrag_ipv4-and-nf_defrag_ipv6#implementation-gotchas>.		nf_defrag_ipv4-and-nf_defrag_ipv6#implementation-gotchas>.
	Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic		Acknowledgements
	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]. It is simply meant as a
	datapoint regarding the extent to which IPv6 implementations can be
	relied upon to generate IPv6 atomic fragments.
	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		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.
	o NetBSD 5.1		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		Authors' Addresses
	Fernando Gont		Fernando Gont
	SI6 Networks / UTN-FRH		SI6 Networks / UTN-FRH
	Evaristo Carriego 2644		Evaristo Carriego 2644
	Haedo, Provincia de Buenos Aires 1706		Haedo, Provincia de Buenos Aires 1706
	Argentina		Argentina
	Phone: +54 11 4650 8472		Phone: +54 11 4650 8472
	Email: fgont@si6networks.com		Email: fgont@si6networks.com
	URI: http://www.si6networks.com		URI: http://www.si6networks.com
	Will(Shucheng) Liu		Will (Shucheng) Liu
	Huawei Technologies		Huawei Technologies
	Bantian, Longgang District		Bantian, Longgang District
	Shenzhen 518129		Shenzhen 518129
	P.R. China		China
	Email: liushucheng@huawei.com		Email: liushucheng@huawei.com
	Tore Anderson		Tore Anderson
	Redpill Linpro		Redpill Linpro
	Vitaminveien 1A		Vitaminveien 1A
	Oslo 0485		Oslo 0485
	Norway		Norway
	Phone: +47 959 31 212		Phone: +47 959 31 212
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