Internet Engineering Task Force J. Jaeggli
Internet-Draft Zynga
Intended status: Informational L. Colitti
Expires: April 19, 2013 W. Kumari
Google
E. Vyncke
Cisco
M. Kaeo
Double Shot Security
T. Taylor, Ed.
Huawei Technologies
October 16, 2012
Why Operators Filter Fragments and What It Implies
draft-taylor-v6ops-fragdrop-00
Abstract
This memo is written to make application developers and network
operators aware of the significant probability that IPv6 packets
containing fragmentation extension headers will fail to reach their
destination. Some assumptions about the ability to use TCP or UDP
datagrams larger than a single packet may accordingly need
adjustment. This memo provides observational evidence for the
dropping of IPv6 fragments along a significant number of paths,
explores the operational impact of fragmentation and the reasons why
dropping occurs, and considers the effect of fragment dropping on
applications particularly including DNS.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 19, 2013.
Copyright Notice
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Internet-Draft Why Operators Filter Fragments October 2012
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Observations and Rationale . . . . . . . . . . . . . . . . . . 3
2.1. Possible Causes . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Impact on Applications . . . . . . . . . . . . . . . . . . 5
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Informative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Measurements of whether internet service providers and edge networks
deliver IPv6 fragments to their destination reveal that for IPv6 in
particular, fragments are being dropped along a substantial number of
paths. IPv6 datagrams with fragmentation headers are a non-issue in
the core of the internet, where fragments are routed just like any
other IPv6 datagram. However, fragmentation creates operational
issues at the edge of the network that may lead to administratively
imposed filtering or inadvertent failure to deliver the fragment to
the application.
Section 2 begins with some observations on how often IPv6 fragment
loss occurs in practice. We go on to look at the operational reasons
for filtering fragments, a key aspect of which is the threats they
pose to security policy and appliances. Section 2.2 then looks at
the impact on key applications, particularly DNS.
In the longer run, as network operators gain a better understanding
of the risks and non-risks of fragmentation and as middlebox,
customer premise equipment (CPE), and host implementations improve,
we believe that some incidences of fragment dropping will diminish.
However, some of the justifications for filtering will persist in the
longer term, and application developers must remain aware of the
implications.
This document deliberately refrains from discussing possible
responses to the problem posed by the dropping of IPv6 fragments.
Such a discussion will quickly turn up a number of possibilities,
application-specific or more general; but the amount of time needed
to specify and deploy a given resolution will be a major constraint
in choosing amongst them. In any event, that discussion is likely to
proceed in multiple directions and is therefore considered beyond the
scope of this memo.
2. Observations and Rationale
[Blackhole] is a good public reference for the incidence of IPv6
fragment filtering. It describes experiments run to determine the
incidence and location of ICMP Packet Too Big and fragment filtering.
The authors used fragmented DNS packets to determine the latter and
found for IPv6 that filtering appeared to be occurring on some 10% of
the tested paths. The filtering appeared to be located at the edge
(enterprise and customer networks) rather than in the core.
[Co-authors, more to contribute?]
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2.1. Possible Causes
Why does such filtering happen? One cause is non-conforming
implementations in CPE and low-end routers. Along with that, some
network managers filter fragments on principle, without taking
account of the specific risks involved, just as they may filter ICMP
Packet Too Big. Both implementations and management should improve
over time, reducing the problem somewhat.
Some filtering and dropping of fragments is done for hardware,
performance, or topological considerations. Stateful inspection
devices or destination hosts can experience resource exhaustion if
they are flooded with fragments not followed by the remaining
fragments of the unfragmented packet. Stateless ACLs may be
difficult to apply to fragments other than the one in which the upper
layer header is present. As [Attacks] demonstrates, inconsistencies
in reassembly logic between middleboxes or CPEs and hosts can cause
fragments to be wrongfully discarded, or can allow exploits to pass
undetected through middleboxes. Stateless Load balancing schemes may
hash fragmented datagrams from the same flow to different paths
because the 5-tuple may available on only the initial fragment.
Leaving aside these incentives towards fragment dropping, other
considerations may weigh on the operator's mind. One example cited
on the NANOG list was that of a router where fragment processing was
done by the control plane processor rather than in the forwarding
plane hardware, with a consequent hit on performance. Another
incentive toward dropping of fragments is the disproportionate number
of software errors still being encountered in fragment processing.
Since this code is exercised less frequently than the rest of the
stack, bugs remain longer in the code before they are detected. Some
of these software errors can introduce vulnerabilities subject to
exploitation. It is common practice [RFC6192] to recommend that
control-plane ACLs protecting routers and network devices be
configured to drop all fragments.
Operators weigh the risks associated with each of the considerations
just enumerated, and come up with the most suitable policy for their
circumstances. It is likely that at least some operators will find
it desirable to drop fragments in at least some cases.
The IETF can help this effort by identifying specific classes of
fragments that do not represent legitimate use cases and hence should
always be dropped. Examples of this work are given by
[draft-6man-atomic] and [I-D.ietf-6man-oversized-header-chain]. The
problem of inconsistent implementations may also be mitigated by
providing further advice on the more difficult points. However, some
cases will remain where legitimate fragments are discarded for
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legitimate reasons. The potential problems these cases pose for
applications is our next topic.
2.2. Impact on Applications
Some applications can live without fragmentation, some cannot. DNS
is one application that may be vulnerable when fragment dropping
occurs. EDNS0 extensions [RFC2761] allow for responses in UDP PDU
greater than 512 bytes. Particularly with DNSSEC, responses may be
larger than the MTU and fragmentation at the sending host in order to
respond using UDP is desirable and legal. The current choices open
to the operators of DNS servers in this situation are to defer
deployment of DNSSEC, fragment responses, or use TCP if there are
cases where the rrset would be expected to exceed the MTU. The use
of fallback to TCP will impose a major resource and performance hit
and increases vulnerability to denial of service attacks.
Other applications, such as Network File System, NFS, are also known
to fragment their large UDP packets but this is most often kept
within a single organization network and should not be impacted by
fragment dropping at the Internet core or edges.
3. Acknowledgements
TBD.
4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
[Obviously a few things to say here, or we can find a few good
references.]
6. Informative References
[Attacks] Atlasis, A., "Attacking IPv6 Implementation Using
Fragmentation", March 2012.
http://media.blackhat.com/bh-eu-12/Atlasis/
bh-eu-12-Atlasis-Attacking_IPv6-WP.pdf
[Blackhole]
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de Boer, M. and J. Bosma, "Discovering Path MTU black
holes on the Internet using RIPE Atlas", July 2012.
http://www.nlnetlabs.nl/downloads/publications/
pmtu-black-holes-msc-thesis.pdf
[I-D.ietf-6man-ipv6-atomic-fragments]
Gont, F., "Processing of IPv6 "atomic" fragments",
draft-ietf-6man-ipv6-atomic-fragments-01 (work in
progress), August 2012.
[]
Gont, F. and V. Manral, "Security and Interoperability
Implications of Oversized IPv6 Header Chains",
draft-ietf-6man-oversized-header-chain-01 (work in
progress), July 2012.
[RFC2761] Dunn, J. and C. Martin, "Terminology for ATM
Benchmarking", RFC 2761, February 2000.
[RFC6192] Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
Router Control Plane", RFC 6192, March 2011.
[draft-6man-atomic]
Gont, F., "Processing of IPv6 "atomic" fragments (Work in
progress)", August 2012.
Authors' Addresses
Joel Jaeggli
Zynga
924 Mouton Circle
East Palo Alto, CA 94303
USA
Email: jjaeggli@zynga.com
Lorenzo Colitti
Google
Phone:
Email: lorenzo@google.com
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Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Phone:
Email: warren@kumari.net
Eric Vyncke
Cisco
De Kleetlaan 6A
Diegem, 1831
Belgium
Phone:
Email: evyncke@cisco.com
Merike Kaeo
Double Shot Security
Phone:
Email: merike@doubleshotsecurity.com
Tom Taylor (editor)
Huawei Technologies
Ottawa, Ontario
Canada
Phone:
Email: tom.taylor.stds@gmail.com
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