6man Working Group R. Bonica
Internet-Draft Juniper Networks
Updates: RFC 2460 (if approved) W. Kumari
Intended status: Standards Track Google, Inc.
Expires: December 22, 2013 June 20, 2013
IPv6 Fragment Header Deprecated
draft-bonica-6man-frag-deprecate-00
Abstract
This memo deprecates the IPv6 Fragment Header. It provides reasons
for deprecation and updates RFC 2460.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 December 22, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Case For Deprecation . . . . . . . . . . . . . . . . . . . . 3
2.1. Resource Conservation . . . . . . . . . . . . . . . . . . 3
2.2. Fragmentation Is Rare . . . . . . . . . . . . . . . . . . 3
2.2.1. UDP-based Applications That Rely on Fragmentation . . 4
2.3. Attack Vectors . . . . . . . . . . . . . . . . . . . . . 4
2.4. Operator Behavior . . . . . . . . . . . . . . . . . . . . 5
3. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Each link on the Internet is characterized by a Maximum Transmission
Unit (MTU). A link's MTU represents the maximum packet size that can
be conveyed over the link, without fragmentation. MTU is a
unidirectional metric. A bidirectional link may be characterized by
one MTU in the forward direction and another MTU in the reverse
direction. IPv6 [RFC2460] requires that every link in the Internet
have an MTU of 1280 octets or greater. On any link that cannot
convey a 1280-octet packet in one piece, link-specific fragmentation
and reassembly must be provided at a layer below IPv6. Therefore,
the PMTU between any two IPv6 nodes is 1280 bytes or greater.
Likewise, for any given source node, the path to a particular
destination node is characterized by a path MTU (PMTU). At a given
source, the PMTU associated with a destination is equal to the
minimum MTU of all of the links that contribute to the path between
the source and the destination.
[RFC2460] strongly recommends that IPv6 nodes implement Path MTU
Discovery (PMTUD) [RFC1981], in order to discover and take advantage
of PMTUs greater than 1280 octets. However, a minimal IPv6
implementation (e.g., in a boot ROM) may simply restrict itself to
sending packets no larger than 1280 octets, and omit implementation
of PMTUD.
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In order to send a packet larger than a path's MTU, a node may use
the IPv6 Fragment header to fragment the packet at the source and
have it reassembled at the destination(s). However, the use of such
fragmentation is discouraged in any application that is able to
adjust its packets to fit the measured path MTU (i.e., down to 1280
octets).
In IPv6, a packet can be fragmented only by the host that originates
it. This constitutes a departure from the IPv4 [RFC0791]
fragmentation strategy, in which a packet can be fragmented by its
originator or by any router that it traverses en route to its
destination.
This memo deprecates the IPv6 Fragment Header. It provides reasons
for deprecation and updates [RFC2460].
2. Case For Deprecation
This section presents a case for deprecating the IPv6 Fragment
Header.
2.1. Resource Conservation
Packets that are fragmented at their source need to be reassembled at
their destination. [Kent87] points out that the reassembly process
is resource intensive. It consumes significant compute and memory
resources. While the cited reference refers to IPv4 fragmentation
and reassembly, many of its criticisms are equally applicable to
IPv6.
By comparison, if a source node were to execute PMTUD procedures, and
if applications were to avoid sending datagrams that would result in
IP packets that exceed the PMTU, the task of reassembly could be
avoided, altogether.
2.2. Fragmentation Is Rare
Today, most popular operating systems implement PMTUD or an extension
thereof, called Packetization Layer MTU Discovery (PMTUD) [RFC4821].
Most popular TCP [RFC0793] implementations leverage this technology
and restrict their segment size so that IP fragmentation is not
required. As a result, IPv6 fragments carrying TCP payload are
rarely observed on the Internet.
Likewise, many UDP-based [RFC0768] applications follow the
recommendations of [RFC5405]. According to [RFC5405], "an
application SHOULD NOT send UDP datagrams that result in IP packets
that exceed the MTU of the path to the destination. Consequently, an
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application SHOULD either use the path MTU information provided by
the IP layer or implement path MTU discovery itself to determine
whether the path to a destination will support its desired message
size without fragmentation. Applications that do not follow this
recommendation to do PMTU discovery SHOULD still avoid sending UDP
datagrams that would result in IP packets that exceed the path MTU.
Because the actual path MTU is unknown, such applications SHOULD fall
back to sending messages that are shorter than the default effective
MTU for sending." The effective MTU for IPv6 is 1280 bytes.
Because many UDP-based applications follow the above-quoted
recommendation, IPv6 fragments carrying UDP traffic are also rarely
observed on the Internet.
2.2.1. UDP-based Applications That Rely on Fragmentation
The following is a list of UDP-based applications that do not follow
the recommendation of [RFC5405] and rely in IPv6 fragmentation:
o DNSSEC [RFC4035]
The effectiveness of these protocols may currently be degraded by
operator behavior. SeeSection 2.4 for details.
2.3. Attack Vectors
Security researchers have found and continue to find attack vectors
that rely on IP fragmentation. For example,
[I-D.ietf-6man-oversized-header-chain] and
[I-D.ietf-6man-nd-extension-headers] describe variants of the tiny
fragment attack [RFC1858]. In this attack, a packet is crafted so
that it can evade stateless firewall filters. The stateless firewall
filter matches on fields drawn from the IPv6 header and an upper
layer header. However, the packet is fragmented so that the upper
layer header, or a significant part of that header, does not appear
in the first fragment. Because a stateless firewall cannot parse
payload beyond the first fragment, the packet evades detection by the
firewall.
Security researcher have also studied reassembly algorithms on
popular computing platforms, with the following goals:
o to discover fragility in seldom exercised parts of the IP stack
o to engineer flows that maximize resources consumed by the
reassembly process
The Dawn and Rose Attacks [Hollis] are the products of such research.
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All of the attack vectors mentioned above can be mitigated with
firewalls and increasingly sophisticated reassembly algorithms.
However, the continued investment required to mitigate newly
discovered vulnerabilities detracts from the cost effectiveness of
IPv6 as a networking solution.
2.4. Operator Behavior
For reasons described above, and also articulated in
[I-D.taylor-v6ops-fragdrop], many network operators filter all IPv6
fragments. Also, the default behavior of many currently deployed
firewalls is to discard IPv6 fragments.
In one recent study [DeBoer], two researchers distributed probes to
423 IPv6 enabled sites. The researchers then tested connectivity
between an experimental control center and the probes. They found
that during any given trial period, sixty percent of the sites that
could be reached with unfragmented packets could also be reached with
fragmented packets. The remaining forty percent appeared to be
filtering IPv6 fragments
3. Recommendation
This memo deprecates IPv6 fragmentation and the IPv6 fragment header.
New application and transport layer protocols MUST NOT send datagrams
that result in IPv6 packets exceeding the MTU of the path to the
destination. However, legacy applications and transport layer
protocols will continue to do so.
New IPv6 host implementations MAY support IPv6 fragmentation and
reassembly, but are not required to do so.
Network operators MAY filter IPv6 fragments.
4. IANA Considerations
IANA is requested to mark the Fragment Header for IPv6 (44) as
deprecated in the Protocol Numbers registry.
5. Security Considerations
Deprecation of the IPv6 Fragment Header will improve network security
by eliminating attacks that rely on fragmentation.
6. Acknowledgements
The author wishes to acknowledge Bob Hinden and Ole Troan for their
review and constructive comments.
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7. References
7.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, November
2008.
7.2. Informative References
[DeBoer] 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>.
[Hollis] Hollis, K., "The Rose Attack Explained", , <http://
digital.net/~gandalf/Rose_Frag_Attack_Explained.htm>.
[]
Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery", draft-ietf-6man-nd-
extension-headers-05 (work in progress), June 2013.
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[]
Gont, F. and V. Manral, "Security and Interoperability
Implications of Oversized IPv6 Header Chains", draft-ietf-
6man-oversized-header-chain-02 (work in progress),
November 2012.
[I-D.ietf-6man-predictable-fragment-id]
Gont, F., "Security Implications of Predictable Fragment
Identification Values", draft-ietf-6man-predictable-
fragment-id-00 (work in progress), March 2013.
[I-D.taylor-v6ops-fragdrop]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-01 (work in
progress), June 2013.
[Kent87] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology , August 1987.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
Authors' Addresses
Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, Virginia 20170
USA
Email: rbonica@juniper.net
Warren
Google, Inc.
1600 Amphitheatre Parkway
Mountainview, California 94043
USA
Email: warren@kumari.net
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