6man B. Carpenter
Internet-Draft Univ. of Auckland
Updates: 2460, 2780 (if approved) S. Jiang
Intended status: Standards Track Huawei Technologies Co., Ltd
Expires: August 26, 2013 February 22, 2013
Transmission of IPv6 Extension Headers
draft-carpenter-6man-ext-transmit-02
Abstract
Various IPv6 extension headers have been defined since the IPv6
standard was first published. This document updates RFC 2460 to
clarify how intermediate nodes should deal with such extension
headers and with any that are defined in future. It also specifies
how extension headers should be registered by IANA, with a
corresponding minor update to RFC 2780.
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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This Internet-Draft will expire on August 26, 2013.
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction and Problem Statement . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirement to Transmit Extension Headers . . . . . . . . . . . 5
2.1. All Extension Headers . . . . . . . . . . . . . . . . . . . 5
2.2. Hop-by-Hop Options . . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction and Problem Statement
In IPv6, an extension header is any header that follows the initial
40 bytes of the packet and precedes the upper layer header (which
might be a transport header, an ICMPv6 header, or a notional "No Next
Header").
An initial set of IPv6 extension headers was defined by [RFC2460],
which also described how they should be handled by intermediate
nodes, with the exception of the hop-by-hop options header:
"...extension headers are not examined or processed
by any node along a packet's delivery path, until the packet reaches
the node (or each of the set of nodes, in the case of multicast)
identified in the Destination Address field of the IPv6 header."
This provision allowed for the addition of new extension headers,
since it means that forwarding nodes should be completely transparent
to them. Thus, new extension headers could be introduced
progressively, used only by hosts that have been updated to create
and interpret them. The extension header mechanism is an important
part of the IPv6 architecture, and several new extension headers have
been defined since RFC 2460.
Unfortunately, experience has showed that the network is not
transparent to these headers. The main reason for this is that by
design, some firewalls attempt to inspect the transport header or
payload. This means that they need to traverse the chain of
extension headers, if present, until they find the transport header
(or an encrypted payload). Unfortunately, because not all IPv6
extension headers follow a uniform TLV format, this process is clumsy
and requires knowledge of each extension header's format.
The process is potentially slow as well as clumsy, possibly
precluding its use in nodes attempting to process packets at line
speed. The present document does not intend to solve this problem,
which is caused by the fundamental architecture of IPv6 extension
headers. This document focuses on clarifying how the header chain
should be traversed in the current IPv6 architecture.
If they encounter an unrecognised extension header type, some
firewalls treat the packet as suspect and drop it. Unfortunately, it
is an established fact that several widely used firewalls do not
recognise some or all of the extension headers defined since RFC
2460. It has also been observed that certain firewalls do not even
handle all the extension headers in RFC 2460, including the fragment
header [I-D.taylor-v6ops-fragdrop], causing fundamental problems of
connectivity. This applies in particular to firewalls that attempt
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to inspect packets statelessly at very high speed, since they cannot
take the time to reassemble fragmented packets, especially when under
a denial of service attack.
Other types of middlebox, such as load balancers or packet
classifiers, might also fail in the presence of extension headers
that they do not recognise.
A contributory factor to this problem is that, because extension
headers are numbered out of the existing IP Protocol Number space,
there is no collected list of them. For this reason, it is hard for
an implementor to quickly identify the full set of defined extension
headers. An implementor who consults only RFC 2460 will miss all
extension headers defined subsequently.
This combination of circumstances creates a "Catch-22" situation
[Heller] for the deployment of any newly designed extension header.
It cannot be widely deployed, because existing firewalls will render
large parts of the Internet opaque to it. However, most firewalls
will not be updated to allow the new header to pass until it has been
proved safe and useful on the open Internet, which is impossible
until the firewalls have been updated.
The uniform TLV format now defined for extension headers [RFC6564]
will improve the situation, but only for future extensions. Some
tricky and potentially malicious cases will be avoided by forbidding
very long chains of extension headers that need to be fragmented
[I-D.ietf-6man-oversized-header-chain]. This will alleviate concerns
that stateless firewalls cannot handle a complete header chain as
required by the present document.
However, these changes are insufficient to correct the underlying
problem. The present document clarifies that the above requirement
from RFC 2460 applies to all types of node that forward IPv6 packets
and to all extension headers defined now and in the future. It also
requests IANA to create a subsidiary registry that clearly identifies
extension header types, and updates RFC 2780 accordingly.
Fundamental changes to the IPv6 extension header architecture are out
of scope for this document.
Also, Hop-by-Hop options are not handled by many high speed routers,
or are processed only on a slow path. This document also updates the
requirements for processing the Hop-by-Hop options header to make
them more realistic.
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1.1. Terminology
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 [RFC2119].
2. Requirement to Transmit Extension Headers
2.1. All Extension Headers
Any node along an IPv6 packet's path, which forwards it for any
reason, SHOULD do so regardless of any extension headers that are
present, as described in RFC 2460. Exceptionally, if this node is
designed to examine extension headers for any reason, such as
firewalling, it MUST recognise and deal appropriately with all
defined IPv6 extension header types. The list of currently defined
extension header types is maintained by IANA (see Section 4) and
implementors are advised to check this list regularly for updates.
RFC 2460 requires destination hosts to discard packets containing
unrecognised extension headers. However, intermediate forwarding
nodes MUST NOT do this by default, since that might cause them to
inadvertently discard traffic using a recently defined extension
header, not yet recognised by the intermediate node.
As mentioned above, firewalls that discard packets containing
extension headers are known to cause connectivity failures and
deployment problems. Therefore, it is important that firewalls can
parse all defined IPv6 extension headers and are able to behave
according to the above requirements. If a firewall discards a packet
containing a defined IPv6 extension header, it MUST be the result of
a configurable firewall policy, and not just the result of a failure
to recognise such a header. This means that the discard policy for
each defined type of extension header MUST be individually
configurable. The default configuration SHOULD allow all defined
extension headers. Firewalls MUST be configurable to allow packets
containing unrecognised extension headers, but such packets MUST be
dropped by default.
The IPv6 Routing Header Types 0 and 1 have been deprecated and SHOULD
NOT be used. However, as specified in [RFC5095], this does not mean
that the IPv6 Routing Header can be unconditionally dropped by
forwarding nodes. Packets containing undeprecated Routing Headers
SHOULD be forwarded by default. At the time of writing, these include
Type 2 [RFC6275], Type 3 [RFC6554], and Types 253 and 254 [RFC4727].
Others may be defined in future.
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2.2. Hop-by-Hop Options
The IPv6 Hop-by-Hop Options header SHOULD be processed by
intermediate nodes as described in [RFC2460]. However, it is to be
expected that high performance routers will either ignore it, or
assign packets containing it to a slow processing path. Designers
planning to use a Hop-by-Hop option need to be aware of this likely
behaviour.
As a reminder, in RFC 2460, it is stated that the Hop-by-Hop Options
header, if present, must be first.
3. Security Considerations
Firewall devices MUST conform to the requirements in the previous
section in order to respect the IPv6 extension header architecture.
In particular, packets containing specific extension headers are only
to be discarded as a result of a configurable policy.
When new extension headers are defined in the future, those
implementing and configuring firewalls will need to take account of
them. It is to be expected that this process will be slow. Until it
is complete, the new extension will fail in some parts of the
Internet. This aspect needs to be considered when deciding to
standardise a new extension.
4. IANA Considerations
IANA is requested to clearly mark in the Assigned Internet Protocol
Numbers registry those values which are also IPv6 Extension Header
types, for example by adding an extra column to indicate this. This
will also apply to any IPv6 Extension Header types defined in the
future.
Additionally, IANA is requested to replace the existing empty IPv6
Next Header Types registry by an IPv6 Extension Header Types
registry. It will contain only those protocol numbers which are also
marked as IPv6 Extension Header types in the Assigned Internet
Protocol Numbers registry. The initial list will be as follows:
o 0, Hop-by-Hop Options, [RFC2460]
o 43, Routing, [RFC2460], [RFC5095]
o 44, Fragment, [RFC2460]
o 50, Encapsulating Security Payload, [RFC4303]
o 51, Authentication, [RFC4302]
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o 60, Destination Options, [RFC2460]
o 135, MIPv6, [RFC6275]
o 139, HIP, [RFC5201]
o 140, shim6, [RFC5533]
The references to the IPv6 Next Header field in [RFC2780] are to be
interpreted as also applying to the IPv6 Extension Header field.
5. Acknowledgements
This document was triggered by mailing list discussions including
John Leslie, Stefan Marksteiner and others. Valuable comments and
contributions were made by Dominique Barthel, Lorenzo Colitti,
Fernando Gont, Bob Hinden, Ray Hunter, Suresh Krishnan, Marc Lampo,
Michael Richardson, Dave Thaler, Joe Touch, and others.
Brian Carpenter was a visitor at the Computer Laboratory, Cambridge
University during part of this work.
This document was produced using the xml2rfc tool [RFC2629].
6. Change log [RFC Editor: Please remove]
draft-carpenter-6man-ext-transmission-02: clarifications following WG
comments, recalibrated firewall requirements, 2013-02-22.
draft-carpenter-6man-ext-transmission-01: feedback at IETF85: clarify
scope and impact on firewalls, discuss line-speed processing and lack
of uniform TLV format, added references, restructured IANA
considerations, 2012-11-13.
draft-carpenter-6man-ext-transmission-00: original version,
2012-08-14.
7. References
7.1. Normative References
[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.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
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Values In the Internet Protocol and Related Headers",
BCP 37, RFC 2780, March 2000.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
December 2007.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.
[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.
7.2. Informative References
[Heller] Heller, J., "Catch-22", Simon and Schuster , 1961.
[]
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.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-00 (work in
progress), October 2012.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
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[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
March 2012.
Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
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