Network Working Group B. Carpenter
Internet-Draft Univ. of Auckland
Updates: 3697 (if approved) S. Jiang
Intended status: Experimental Huawei Technologies Co., Ltd
Expires: August 22, 2010 February 18, 2010
Update to the IPv6 flow label specification
draft-carpenter-6man-flow-update-00
Abstract
Various uses proposed for the IPv6 flow label are incompatible with
its existing specification. This document describes changes to the
specification that permit additional use cases as well as allowing
continued use of the previous specification.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Normative Notation . . . . . . . . . . . . . . . . . . . . . . 4
3. Changes to specification . . . . . . . . . . . . . . . . . . . 4
4. Alternative Approach . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The flow label field in the IPv6 header is reserved but left
experimental by [RFC2460] and is specified by [RFC3697]. We quote
three rules from that RFC:
1. "The Flow Label value set by the source MUST be delivered
unchanged to the destination node(s)."
2. "IPv6 nodes MUST NOT assume any mathematical or other properties
of the Flow Label values assigned by source nodes."
3. "Router performance SHOULD NOT be dependent on the distribution
of the Flow Label values. Especially, the Flow Label bits alone
make poor material for a hash key."
The second two rules essentially forbid a usage in which the bits of
the flow label are encoded with a specific semantic meaning, or are
assumed to have any particular property such as randomness. However,
both before and after these rules were laid down, a considerable
number of proposals for use of the flow label have been published
that seem incompatible with them. Examples are
[I-D.conta-ipv6-flow-label], [I-D.conta-diffserv-ipv6-fl-classifier],
[I-D.chakravorty-6lsa], [I-D.banerjee-flowlabel-ipv6-qos],
[I-D.metzler-ipv6-flowlabel], [LeeKim], [LinTseng], and [Prakash].
These authors propose use cases in which some combination of the
following options apply:
o The flow label may be changed by intermediate systems.
o It doesn't matter if the flow label is changed, because the
receiver doesn't use it.
o Some or all bits of the flow label are coded: they have specific
meanings understood by routers and switches along the path.
o The coding is related to the required quality of service, as well
as identifying a flow.
o The label is used to control forwarding or switching in some way.
These proposals all require either some form of encoding of semantics
in the bits of the flow label, or the ability for routers to modify
the flow label, or both. Thus they infringe the rules from RFC 3697
quoted above.
Although [I-D.roberts-inband-qos-ipv6] does not explicitly consider
the flow label, it requests hop-by-hop functionality in IPv6 packets
very similar to what is needed by the above proposals.
We can conclude that a considerable number of researchers and
designers are stymied by RFC 3697. On the other hand, proposals such
as [I-D.martinbeckman-ietf-ipv6-fls-ipv6flowswitching],
[I-D.martinbeckman-ietf-ipv6-amp-ipv6hcamp],
[I-D.blake-ipv6-flow-label-nonce], and [I-D.carpenter-flow-ecmp]
appear to be compatible with RFC 3697. The latter two are based on
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the originator of a packet choosing a pseudo-random flow label for
each flow. Thus, we can also conclude that there is a useful role
for this approach too. The proposal below is intended to resolve
this dilemma by allowing both approaches to co-exist.
2. Normative Notation
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].
3. Changes to specification
We note that although RFC 3697 requires the flow label to be
delivered unchanged, it is not included in any transport layer
pseudo-header checksums nor in IPsec authentication [RFC4302]. We
also note that at the time of writing, the flow label is observed to
be set to zero in an overwhelming proportion of IPv6 packets; neither
operating systems nor applications currently set it, and routers do
not rely on it. Thus there is no reason to expect operational
difficulties if a careful change is made to the rules of RFC 3697.
The purpose of the proposed change is that some flow label values
should be available for domain-specific use, with locally defined
semantics, and that other flow label values should be available for
uses essentially compatible with RFC 3697. There should be no impact
on specifications other than RFC 3697 and no impact on currently
operational software and hardware.
The proposal is as follows:
o If the most significant bit (MSB) of the flow label is 0, then the
remaining 19 bits MUST obey the rules of [RFC3697]. (Note that
this does not change the meaning of an all-zero flow label or the
requirement to deliver it unchanged.)
o If the MSB of the flow label is 1, the remaining 19 bits MAY obey
a locally defined set of rules and those bits MAY be changed en
route.
The locally defined set of rules will apply within a given Flow Label
Domain, analagous to a Differentiated Services Domain [RFC2474]. A
"boundary router" is defined as any router at the boundary between a
Flow Label Domain and other parts of the Internet. The following
rules define the consequences for compatibility:
o Sending hosts that are not updated will in practice continue to
send zero labels, which MUST be delivered unchanged.
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o Sending hosts wishing to rely on RFC 3697 behaviour MUST choose
labels with MSB = 0.
o Sending hosts wishing to use locally defined behaviour MUST choose
labels with MSB = 1 and whatever other rules apply locally.
o Receiving hosts that are not updated will continue to ignore
labels.
o Receiving hosts wishing to rely on RFC 3697 behaviour MUST verify
that MSB = 0.
o Receiving hosts wishing to use locally defined behaviour MUST
verify that MSB = 1.
o Routers wishing to implement or rely on locally defined behaviour
MUST verify that MSB = 1; if MSB = 0 they MUST NOT change the flow
label.
o Considering packets outbound from the Flow Label Domain, if MSB =
0, a boundary router MUST NOT change the flow label. If MSB = 1,
it MUST set all 20 bits of the flow label to zero, so that the
locally defined behaviour is not exported from the domain.
o Considering packets inbound to the Flow Label Domain, if MSB = 0,
a boundary router MUST NOT change the flow label. If an inbound
packet has MSB = 1, it has originated from a source not following
the current specification. This is considered to be an extremely
unlikely case, and the boundary router MUST set all 20 bits of the
flow label to zero, as the choice least likely to cause unwanted
behaviour. (Note that this means the rules for inbound and
outbound packets at the boundary router are identical.)
With the ability to define local semantics for 19 bits of the flow
label, and the above provisions for compatibility, we add a further
recommendation. Its intention is to encourage load balancing
solutions based on the flow label, or to enable the behaviour defined
in [I-D.blake-ipv6-flow-label-nonce].
o Sending hosts that do not use a locally defined flow label
behaviour SHOULD choose flow labels with MSB = 0 followed by a
pseudo-random 19 bit number between 1 and 0x7FFFF.
4. Alternative Approach
Note that an alternative approach would be possible, using a specific
differentiated services code point (DSCP)[RFC2474] in the Traffic
Class octet instead of the MSB of the flow label itself, to flag a
locally defined behaviour. In this model, the above rules would be
modified by replacing the condition "MSB = 1" by the condition "DSCP
= xxxxxx" (for a specific value xxxxxx) and other fairly
straightforward changes. A more elaborate version of this was
proposed in [I-D.martinbeckman-ietf-ipv6-fls-ipv6flowswitching].
However, there are two issues with this approach. One is that DSCP
values are themselves only locally significant, whereas the
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specification above makes the MSB a globally signficant flag,
consistent with the end-to-end nature of the original flow label
definition. Secondly, it seems unwise to meld the semantics of
differentiated services, which are currently deployed to some extent,
with the unknown future semantics of flow label usage.
5. Security Considerations
The flow label is not protected in any way and can be forged by an
on-path attacker. On the other hand, a pseudo-random flow label
cannot be readily guessed by an off-path attacker. See RFC 3697 for
further discussion.
6. IANA Considerations
This document requests no action by IANA.
7. Acknowledgements
The authors are grateful to Qinwen Hu for general discussion about
the flow label and for his work in searching the literature.
Valuable comments and contributions were made by ..., and others.
This document was produced using the xml2rfc tool [RFC2629].
8. Change log
draft-carpenter-6man-flow-update-00: original version, 2010-02-18
9. References
9.1. 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.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
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9.2. Informative References
[I-D.banerjee-flowlabel-ipv6-qos]
Banerjee, R., "A Modified Specification for use of the
IPv6 Flow Label for providing An efficient Quality of
Service using hybrid approach",
draft-banerjee-flowlabel-ipv6-qos-03 (work in progress),
April 2002.
[I-D.blake-ipv6-flow-label-nonce]
Blake, S., "Use of the IPv6 Flow Label as a Transport-
Layer Nonce to Defend Against Off-Path Spoofing Attacks",
draft-blake-ipv6-flow-label-nonce-02 (work in progress),
October 2009.
[I-D.carpenter-flow-ecmp]
Carpenter, B., "Using the IPv6 flow label for equal cost
multipath routing in tunnels",
draft-carpenter-flow-ecmp-01 (work in progress),
February 2010.
[I-D.chakravorty-6lsa]
Chakravorty, S., Bush, J., and J. Bound, "IPv6 Label
Switching Architecture", draft-chakravorty-6lsa-03 (work
in progress), July 2008.
[I-D.conta-diffserv-ipv6-fl-classifier]
Conta, A. and J. Rajahalme, "Amodel for Diffserv use of
the IPv6 Flow Label Specification",
draft-conta-diffserv-ipv6-fl-classifier-01 (work in
progress), November 2001.
[I-D.conta-ipv6-flow-label]
Conta, A. and B. Carpenter, "A proposal for the IPv6 Flow
Label Specification", draft-conta-ipv6-flow-label-02 (work
in progress), July 2001.
[I-D.martinbeckman-ietf-ipv6-amp-ipv6hcamp]
Beckman, M., "IPv6 Header Compression via Addressing
Mitigation Protocol (IPv6 AMP)",
draft-martinbeckman-ietf-ipv6-amp-ipv6hcamp-01 (work in
progress), March 2007.
[I-D.martinbeckman-ietf-ipv6-fls-ipv6flowswitching]
Beckman, M., "IPv6 Dynamic Flow Label Switching (FLS)",
draft-martinbeckman-ietf-ipv6-fls-ipv6flowswitching-03
(work in progress), March 2007.
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[I-D.metzler-ipv6-flowlabel]
Metzler, J. and S. Hauth, "An end-to-end usage of the IPv6
flow label", draft-metzler-ipv6-flowlabel-00 (work in
progress), November 2000.
[I-D.roberts-inband-qos-ipv6]
Roberts, L. and J. Harford, "In-Band QoS Signaling for
IPv6", draft-roberts-inband-qos-ipv6-00 (work in
progress), July 2005.
[LeeKim] Lee, I. and S. Kim, "A QoS Improvement Scheme for Real-
Time Traffic Using IPv6 Flow Labels", Lecture Notes in
Computer Science Vol. 3043, 2004.
[LinTseng]
Lin, C., Tseng, P., and W. Hwang, "End-to-End QoS
Provisioning by Flow Label in IPv6", JCIS , 2006.
[Prakash] Prakash, B., "Using the 20 bit flow label field in the
IPv6 header to indicate desirable quality of service on
the internet", University of Colorado (M.Sc. Thesis),
2004.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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Sheng Jiang
Huawei Technologies Co., Ltd
KuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base, Hai-Dian District, Beijing
P.R. China
Email: shengjiang@huawei.com
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