6MAN S. Amante
Internet-Draft Level 3
Obsoletes: 3697 (if approved) B. Carpenter
Updates: 2205, 2460 Univ. of Auckland
(if approved) S. Jiang
Intended status: Standards Track Huawei Technologies Co., Ltd
Expires: August 4, 2011 J. Rajahalme
Nokia-Siemens Networks
January 31, 2011
IPv6 Flow Label Specification
draft-ietf-6man-flow-3697bis-00
Abstract
This document specifies the IPv6 Flow Label field and the minimum
requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding
labeled packets, and flow state establishment methods. Even when
mentioned as examples of possible uses of the flow labeling, more
detailed requirements for specific use cases are out of scope for
this document.
The usage of the Flow Label field enables efficient IPv6 flow
classification based only on IPv6 main header fields in fixed
positions.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 August 1, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. IPv6 Flow Label Specification . . . . . . . . . . . . . . . . 5
3. Flow Labeling Requirements . . . . . . . . . . . . . . . . . . 6
4. Flow State Establishment Requirements . . . . . . . . . . . . 8
5. Essential correction to RFC 2205 . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Theft and Denial of Service . . . . . . . . . . . . . . . 9
6.2. IPsec and Tunneling Interactions . . . . . . . . . . . . . 10
6.3. Security Filtering Interactions . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
A flow is a sequence of packets sent from a particular source to a
particular unicast, anycast, or multicast destination that a node
desires to label as a flow. A flow could consist of all packets in a
specific transport connection or a media stream. However, a flow is
not necessarily 1:1 mapped to a transport connection.
Traditionally, flow classifiers have been based on the 5-tuple of the
source and destination addresses, ports, and the transport protocol
type. However, some of these fields may be unavailable due to either
fragmentation or encryption, or locating them past a chain of IPv6
extension headers may be inefficient. Additionally, if classifiers
depend only on IP layer headers, later introduction of alternative
transport layer protocols will be easier.
The usage of the 3-tuple of the Flow Label and the Source and
Destination Address fields enables efficient IPv6 flow
classification, where only IPv6 main header fields in fixed positions
are used.
The minimum level of IPv6 flow support consists of labeling the
flows. A specific goal is to enable and encourage the use of the
flow label for various forms of stateless load distribution,
especially across Equal Cost Multi-Path (EMCP) and/or Link
Aggregation Group (LAG) paths. ECMP and LAG are methods to bond
together multiple physical links used to procure the required
capacity necessary to carry an offered load greater than the
bandwidth of an individual physical link. IPv6 source nodes SHOULD
be able to label known flows (e.g., TCP connections, application
streams), even if the node itself does not require any flow-specific
treatment. Node requirements for flow labeling are given in
Section 3.
The flow label can be used most simply in stateless models, but
stateful mechanisms are also possible. Specific flow state
establishment methods and the related service models are out of scope
for this specification, but the generic requirements enabling co-
existence of different methods in IPv6 nodes are set forth in
Section 4. The associated scaling characteristics (such as nodes
involved in state establishment, amount of state maintained by them,
and state growth function) will be specific to particular service
models.
This document replaces [RFC3697] and Appendix A of [RFC2460]. A
rationale for the changes made is documented in
[I-D.ietf-6man-flow-update]. The present document also includes a
correction to [RFC2205] concerning the flow label.
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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. IPv6 Flow Label Specification
The 20-bit Flow Label field in the IPv6 header [RFC2460] is used by a
node to label packets of a flow. A Flow Label of zero is used to
indicate packets not part of any flow. Packet classifiers can use
the triplet of Flow Label, Source Address, and Destination Address
fields to identify which flow a particular packet belongs to.
Packets are processed in a flow-specific manner by nodes that are
able to do so in a stateless manner, or that have been set up with
flow-specific state. The nature of the specific treatment and the
methods for flow state establishment are out of scope for this
specification.
Once set to a non-zero value, the Flow Label MUST be delivered
unchanged to the destination node(s). A forwarding node MUST NOT
change the flow label value in an arriving packet if it is non-zero.
However, there are two qualifications to this rule:
1. Implementers are advised that forwarding nodes, especially those
acting as domain border devices, might nevertheless be configured
to change the flow label value in packets (e.g., to a new pseudo-
random value). This is undetectable, unless some future version
of IPsec authentication [RFC4302] protects the flow label value.
2. To enable stateless load distribution at any point in the
Internet, a network domain MUST NOT forward packets outside the
domain whose flow label values are other than zero or pseudo-
random. Neither domain border egress routers nor intermediate
routers/devices (using a flow-label, for example, as a part of an
input-key for a load-distribution hash) can determine by
inspection that a value is not pseudo-random. Therefore, if
nodes within a domain ignore the above recommendations to set
zero or pseudo-random flow label values, and such packets are
forwarded outside the domain, this would likely result in
undesirable operational implications (e.g., congestion,
reordering) for not only the inappropriately flow-labelled
packets, but also well-behaved flow-labelled packets, during
forwarding at various intermediate devices. Thus, a domain must
protect its peers by never exporting inappropriately labelled
packets. This document does not specify the method for enforcing
this rule. The suggested way to enforce it is that nodes within
a domain MUST NOT set the flow label to a non-zero and non-
pseudo-random number if the packet will leave the domain. If
this is not known to be the case, the border router will need to
change outgoing flow labels.
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There is no way to verify whether a flow label has been modified en
route. Therefore, no Internet-wide mechanism can depend
mathematically on immutable flow labels; they have a "best effort"
quality. This leads to the following formal rules:
IPv6 nodes MUST NOT assume that the Flow Label value in a incoming
packet is identical to the value set by the source node.
Forwarding nodes such as routers and load balancers MUST NOT depend
only on Flow Label values being randomly distributed. In any usage
such as a hash key for load distribution, the Flow Label bits MUST be
combined with bits from other sources within the packet, so as to
produce a constant hash value for each flow and a suitable
distribution of hash values across flows.
Although a pseudo-random flow label is recommended, and will always
be helpful for load balancing, it is unsafe to assume its presence in
the general case, and the use case needs to work even if the flow
label value is zero.
Nodes keeping dynamic flow state MUST NOT assume packets arriving 120
seconds or more after the previous packet of a flow still belong to
the same flow, unless a flow state establishment method in use
defines a longer flow state lifetime or the flow state has been
explicitly refreshed within the lifetime duration.
The use of the Flow Label field does not necessarily signal any
requirement on packet reordering. Especially, the zero label does
not imply that significant reordering is acceptable.
An IPv6 node that does not set or make use of the flow label MUST
ignore it when receiving or forwarding a packet.
3. Flow Labeling Requirements
To enable Flow Label based classification, source nodes SHOULD assign
each unrelated transport connection and application data stream to a
new flow. It is RECOMMENDED that source hosts support the flow label
by setting the flow label field for all packets of a flow to the same
pseudo-random value. Both stateful and stateless methods of
assigning a pseudo-random value could be used, but it is outside the
scope of this specification to mandate an algorithm.
An OPTIONAL algorithm for generating such a pseudo-random value is
described in [I-D.gont-6man-flowlabel-security].
[[ QUESTION TO WG: Should we incorporate that algorithm here, or
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leave it as a separate draft? ]]
A source node which does not otherwise set the flow label MUST set
its value to zero.
A node that forwards a flow whose flow label value in arriving
packets is zero MAY set the flow label value. In that case, it is
RECOMMENDED that the forwarding node sets the flow label field for a
flow to a pseudo-random value.
o The same considerations apply as to source hosts setting the flow
label.
o This option, if implemented, would presumably be used by ingress
routers. It would place a considerable per-packet processing load
on them, even if they adopted a stateless method of flow
identification and label assignment. This is why the principal
recommendation is that the source host should set the label.
The preceding rules taken together allow a given network domain to
include routers that set flow labels on behalf of hosts that do not
do so. They also recommend that flow labels exported to the Internet
are always either zero or pseudo-random.
The node that sets the flow label MAY also take part in flow state
establishment methods that result in assigning certain packets to
specific flows.
To enable applications and transport protocols to define what packets
constitute a flow, the source node MUST provide means for the
applications and transport protocols to specify the Flow Label values
to be used with their flows. The use of the means to specify Flow
Label values is subject to appropriate privileges (see Section 6.1).
The source node SHOULD be able to select unused Flow Label values for
flows not requesting a specific value to be used.
[[ QUESTION TO WG: Should we reduce this whole paragraph to a MAY? ]]
A source node MUST ensure that it does not unintentionally reuse Flow
Label values it is currently using or has recently used when creating
new flows. Flow Label values previously used with a specific pair of
source and destination addresses MUST NOT be assigned to new flows
with the same address pair within 120 seconds of the termination of
the previous flow. The source node SHOULD provide the means for the
applications and transport protocols to specify quarantine periods
longer than the default 120 seconds for individual flows.
To avoid accidental Flow Label value reuse, the source node SHOULD
select new Flow Label values in a well-defined way and use an initial
value that avoids reuse of recently used Flow Label values each time
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the system restarts. The initial value SHOULD be derived from a
previous value stored in non-volatile memory, or in the absence of
such history, a randomly generated initial value using techniques
that produce good randomness properties SHOULD be used
[I-D.gont-6man-flowlabel-security].
4. Flow State Establishment Requirements
To enable stateful flow-specific treatment, flow state needs to be
established on all or a subset of the IPv6 nodes on the path from the
source to the destination(s). The methods for the state
establishment, as well as the models for flow-specific treatment will
be defined in separate specifications.
To enable co-existence of different methods in IPv6 nodes, the
methods MUST meet the following basic requirements:
1. The method MUST provide the means for flow state clean-up from
the IPv6 nodes providing the flow-specific treatment. Signaling
based methods where the source node is involved are free to
specify flow state lifetimes longer than the default 120 seconds.
2. Flow state establishment methods MUST be able to recover from the
case where the requested flow state cannot be supported.
5. Essential correction to RFC 2205
[RFC2460] reduced the size of the flow label field from 24 to 20
bits. The references to a 24 bit flow label field on pages 87 and 88
of [RFC2205] are updated accordingly.
6. Security Considerations
This section considers security issues raised by the use of the Flow
Label, primarily the potential for denial-of-service attacks, and the
related potential for theft of service by unauthorized traffic
(Section 6.1). Section 6.2 addresses the use of the Flow Label in
the presence of IPsec including its interaction with IPsec tunnel
mode and other tunneling protocols. We also note that inspection of
unencrypted Flow Labels may allow some forms of traffic analysis by
revealing some structure of the underlying communications. Even if
the flow label were encrypted, its presence as a constant value in a
fixed position might assist traffic analysis and cryptoanalysis.
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
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[I-D.gont-6man-flowlabel-security] for further discussion.
6.1. Theft and Denial of Service
Since the mapping of network traffic to flow-specific treatment is
triggered by the IP addresses and Flow Label value of the IPv6
header, an adversary may be able to obtain better service by
modifying the IPv6 header or by injecting packets with false
addresses and/or labels. Taken to its limits, such theft-of-service
becomes a denial-of-service attack when the modified or injected
traffic depletes the resources available to forward it and other
traffic streams. A curiosity is that if a DoS attack were undertaken
against a given Flow Label (or set of Flow Labels), then traffic
containing an affected Flow Label might well experience worse-than-
best-effort network performance.
Note that since the treatment of IP headers by nodes is typically
unverified, there is no guarantee that flow labels sent by a node are
set according to the recommendations in this document. Therefore,
any assumptions made by the network about header fields such as flow
labels should be limited to the extent that the upstream nodes are
explicitly trusted.
Since flows are identified by the 3-tuple of the Flow Label and the
Source and Destination Address, the risk of theft or denial of
service introduced by the Flow Label is closely related to the risk
of theft or denial of service by address spoofing. An adversary who
is in a position to forge an address is also likely to be able to
forge a label, and vice versa.
There are two issues with different properties: Spoofing of the Flow
Label only, and spoofing of the whole 3-tuple, including Source and
Destination Address.
The former can be done inside a node which is using or transmitting
the correct source address. The ability to spoof a Flow Label
typically implies being in a position to also forge an address, but
in many cases, spoofing an address may not be interesting to the
spoofer, especially if the spoofer's goal is theft of service, rather
than denial of service.
The latter can be done by a host which is not subject to ingress
filtering [RFC2827] or by an intermediate router. Due to its
properties, such is typically useful only for denial of service. In
the absence of ingress filtering, almost any third party could
instigate such an attack.
In the presence of ingress filtering, forging a non-zero Flow Label
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on packets that originated with a zero label, or modifying or
clearing a label, could only occur if an intermediate system such as
a router was compromised, or through some other form of man-in-the-
middle attack. However, the risk is limited to traffic receiving
better or worse quality of service than intended. For example, if
Flow Labels are altered or cleared at random, flow classification
will no longer happen as intended, and the altered packets will
receive default treatment. If a complete 3-tuple is forged, the
altered packets will be classified into the forged flow and will
receive the corresponding quality of service; this will create a
denial of service attack subtly different from one where only the
addresses are forged. Because it is limited to a single flow
definition, e.g., to a limited amount of bandwidth, such an attack
will be more specific and at a finer granularity than a normal
address-spoofing attack.
Since flows are identified by the complete 3-tuple, ingress filtering
[RFC2827] will, as noted above, mitigate part of the risk. If the
source address of a packet is validated by ingress filtering, there
can be a degree of trust that the packet has not transited a
compromised router, to the extent that ISP infrastructure may be
trusted. However, this gives no assurance that another form of man-
in-the-middle attack has not occurred.
Only applications with an appropriate privilege in a sending host
will be entitled to set a non-zero Flow Label. Mechanisms for this
are operating system dependent. Related policy and authorization
mechanisms may also be required; for example, in a multi-user host,
only some users may be entitled to set the Flow Label. Such
authorization issues are outside the scope of this specification.
6.2. IPsec and Tunneling Interactions
The IPsec protocol, as defined in [RFC4301], [RFC4302], [RFC4303]
does not include the IPv6 header's Flow Label in any of its
cryptographic calculations (in the case of tunnel mode, it is the
outer IPv6 header's Flow Label that is not included). Hence
modification of the Flow Label by a network node has no effect on
IPsec end-to-end security, because it cannot cause any IPsec
integrity check to fail. As a consequence, IPsec does not provide
any defense against an adversary's modification of the Flow Label
(i.e., a man-in-the-middle attack).
IPsec tunnel mode provides security for the encapsulated IP header's
Flow Label. A tunnel mode IPsec packet contains two IP headers: an
outer header supplied by the tunnel ingress node and an encapsulated
inner header supplied by the original source of the packet. When an
IPsec tunnel is passing through nodes performing flow classification,
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the intermediate network nodes operate on the Flow Label in the outer
header. At the tunnel egress node, IPsec processing includes
removing the outer header and forwarding the packet (if required)
using the inner header. The IPsec protocol requires that the inner
header's Flow Label not be changed by this decapsulation processing
to ensure that modifications to label cannot be used to launch theft-
or denial-of-service attacks across an IPsec tunnel endpoint. This
document makes no change to that requirement; indeed it forbids
changes to the Flow Label.
When IPsec tunnel egress decapsulation processing includes a
sufficiently strong cryptographic integrity check of the encapsulated
packet (where sufficiency is determined by local security policy),
the tunnel egress node can safely assume that the Flow Label in the
inner header has the same value as it had at the tunnel ingress node.
This analysis and its implications apply to any tunneling protocol
that performs integrity checks. Of course, any Flow Label set in an
encapsulating IPv6 header is subject to the risks described in the
previous section.
6.3. Security Filtering Interactions
The Flow Label does nothing to eliminate the need for packet
filtering based on headers past the IP header, if such filtering is
deemed necessary for security reasons on nodes such as firewalls or
filtering routers.
However, security devices that clear or rewrite non-zero flow label
values would be in violation of this specification.
7. IANA Considerations
This document requests no action by IANA.
8. Acknowledgements
Steve Deering and Alex Conta were co-authors of RFC 3697, on which
this document is based.
Valuable comments and contributions were made by Fred Baker, Steve
Blake, Remi Despres, Alan Ford, Fernando Gont, Brian Haberman, Tony
Hain, Joel Halpern, Qinwen Hu, Chris Morrow, Thomas Narten, Mark
Smith, Pascal Thubert, Iljitsch van Beijnum, and other participants
in the 6man working group.
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Contributors to the development of RFC 3697 included Ran Atkinson,
Steve Blake, Jim Bound, Francis Dupont, Robert Elz, Tony Hain, Robert
Hancock, Bob Hinden, Christian Huitema, Frank Kastenholz, Thomas
Narten, Charles Perkins, Pekka Savola, Hesham Soliman, Michael
Thomas, Margaret Wasserman, and Alex Zinin.
This document was produced using the xml2rfc tool [RFC2629].
9. Change log
draft-ietf-6man-flow-3697bis-00: original version, built from RFC3697
and draft-ietf-6man-flow-update-01, 2011-01-31
10. References
10.1. Normative References
[I-D.gont-6man-flowlabel-security]
Gont, F., "Security Assessment of the IPv6 Flow Label",
draft-gont-6man-flowlabel-security-01 (work in progress),
November 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
10.2. Informative References
[I-D.ietf-6man-flow-update]
Amante, S., Carpenter, B., and S. Jiang, "Update to the
IPv6 flow label specification",
draft-ietf-6man-flow-update-01 (work in progress),
January 2011.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
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[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
Authors' Addresses
Shane Amante
Level 3 Communications, LLC
1025 Eldorado Blvd
Broomfield, CO 80021
USA
Email: shane@level3.net
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
Huawei Building, No.3 Xinxi Rd.,
Shang-Di Information Industry Base, Hai-Dian District, Beijing
P.R. China
Email: shengjiang@huawei.com
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Jarno Rajahalme
Nokia-Siemens Networks
TBD
TBD
Finland
Email: jarno.rajahalme@nsn.com
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