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
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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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