Internet DRAFT - draft-taylor-v6ops-fragdrop
draft-taylor-v6ops-fragdrop
Internet Engineering Task Force J. Jaeggli
Internet-Draft Zynga
Intended status: Informational L. Colitti
Expires: June 6, 2014 W. Kumari
Google
E. Vyncke
Cisco
M. Kaeo
Double Shot Security
T. Taylor, Ed.
Huawei Technologies
December 3, 2013
Why Operators Filter Fragments and What It Implies
draft-taylor-v6ops-fragdrop-02
Abstract
This memo was written to make application developers and network
operators aware of the significant possibility that IPv6 packets
containing fragmentation extension headers may fail to reach their
destination. Some protocol or application assumptions about the
ability to use messages larger than a single packet may accordingly
not be supportable in all networks or circumstances.
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 and scenarios
where drops occur, and considers the effect of fragment drops on
applications where fragmentation is known to occur, particularly
including DNS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Observations and Rationale . . . . . . . . . . . . . . . . . 3
2.1. Possible Causes . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Stateful inspection . . . . . . . . . . . . . . . . . 4
2.1.2. Stateless ACLs . . . . . . . . . . . . . . . . . . . 4
2.1.3. Performance considerations . . . . . . . . . . . . . 4
2.1.4. Other considerations . . . . . . . . . . . . . . . . 4
2.1.5. Conclusions . . . . . . . . . . . . . . . . . . . . . 5
2.2. Impact on Applications . . . . . . . . . . . . . . . . . 5
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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. The filtering of IPv6 datagrams with fragmentation headers is
presumed to be a non-issue in the core of the Internet, where
fragments are routed just like any other IPv6 datagram. However,
fragmentation can creates operational issues at the edges of the
Internet that may lead to administratively imposed filtering or
inadvertent failure to deliver the fragment to the end-system or
application.
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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 limitations
they expose in the application of security policy, at resource
bottlenecks and in forwarding decisions. 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 incidence of fragment dropping currently
required will diminish. Some of the justifications for filtering
will persist in the long-term, and application developers and network
operators 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, occur in different areas and is
therefore considered beyond the scope of this memo.
2. Observations and Rationale
[Blackhole] is a good public reference for some empirical data on
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, setting the servers to an IPv6 minimum of 1280 bytes to avoid
any PMTU issues. The tests 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.
2.1. Possible Causes
Why does such filtering happen? One cause is non-conforming
implementations in CPE and low-end routers. Some network managers
filter fragments on principle, thinking this is an easier way to
deter realizable attacks utilizing IPv6 fragments without thinking of
other network impacts, similar to the practice of filtering ICMP
Packet Too Big. Both implementations and management should improve
over time, reducing the problem somewhat.
Some filtering and dropping of fragments is known to be done for
hardware, performance, or topological considerations.
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2.1.1. Stateful inspection
Stateful inspection devices or destination hosts can readily
experience resource exhaustion if they are flooded with fragments
that are not followed in a timely manner by the remaining fragments
of the original datagram. Holding fragments for reassembly even on
end-system firewalls can readily result in an effective denial of
service by memory and CPU exhaustion even if techniques, such as
virtual re-assambly exist.
2.1.2. Stateless ACLs
Stateless ACLs at layer 4 and up may be difficult to apply to
fragments other than the first one in which enough of 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 be available on only the initial fragment.
While rehashing has the possibility of reordering packets in ISP
cores it is not disastrous. However, in front of a stateful
inspection device, load balancer tier, or anycast service instance,
where headers other than the L3 header -- for example, the L4 header,
interface index (for traffic already rehashed onto different paths),
DS fields -- are considered as part of the hash, rehashing may result
in the fragments being delivered to different end-systems
2.1.3. Performance considerations
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.
2.1.4. Other considerations
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.
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2.1.5. Conclusions
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 and operators 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
[RFC6946] 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 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. UDP
DNS is one application that has the potential to be impacted when
fragment dropping occurs. EDNS0 extensions [RFC2671] allow for
responses in UDP PDUs that are greater than 512 bytes. Particularly
with DNSSEC [RFC4033], responses may be larger than the link MTU and
fragmentation would therefore occur at the sending host in order to
respond using UDP. 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 the Network File System, NFS, are also
known to fragment large UDP packets for datagrams larger than the
MTU. NFS is most often restricted to the internal networks of
organizations. In general, managing NFS connectivity should not be
impacted by decisions mananging fragment drops at network borders or
end-systems.
3. Acknowledgements
The authors of this document would like to thank the RIPE Atlas
project and NLNetlabs whose conclusions ignited this document.
4. IANA Considerations
This memo includes no request to IANA.
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5. Security Considerations
The potential for denial of service attacks, as well as limitations
inherent in upper-layer filtering when dealing with non-initial
fragments are significant issues under consideration by operators and
end-users filtering fragments. This document does not offer
alternative solutions to that problem, it does describe the impact of
those filtering practices.
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]
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-oversized-header-chain]
Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", draft-ietf-6man-oversized-
header-chain-08 (work in progress), October 2013.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC6192] Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
Router Control Plane", RFC 6192, March 2011.
[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
6946, May 2013.
Authors' Addresses
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Joel Jaeggli
Zynga
630 taylor ct #10
Mountain View, CA 94043
USA
Email: jjaeggli@zynga.com
Lorenzo Colitti
Google
Email: lorenzo@google.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: warren@kumari.net
Eric Vyncke
Cisco
De Kleetlaan 6A
Diegem 1831
Belgium
Email: evyncke@cisco.com
Merike Kaeo
Double Shot Security
Email: merike@doubleshotsecurity.com
Tom Taylor (editor)
Huawei Technologies
Ottawa, Ontario
Canada
Email: tom.taylor.stds@gmail.com
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