6MAN                                                           S. Amante
Internet-Draft                                                   Level 3
Obsoletes: 3697 (if approved)                               B. Carpenter
Updates: 2205, 2460 (if approved)                      Univ. of Auckland
Intended status: Standards Track                                S. Jiang
Expires: November 3, 2011                   Huawei Technologies Co., Ltd
                                                            J. Rajahalme
                                                  Nokia Siemens Networks
                                                             May 2, 2011

                     IPv6 Flow Label Specification


   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

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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 November 3, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   than English.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  IPv6 Flow Label Specification  . . . . . . . . . . . . . . . .  5
   3.  Stateless Flow Labeling Requirements . . . . . . . . . . . . .  6
   4.  Flow State Establishment Requirements  . . . . . . . . . . . .  7
   5.  Essential correction to RFC 2205 . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     6.1.  Theft and Denial of Service  . . . . . . . . . . . . . . .  8
     6.2.  IPsec and Tunneling Interactions . . . . . . . . . . . . . 10
     6.3.  Security Filtering Interactions  . . . . . . . . . . . . . 10
   7.  Differences from RFC 3697  . . . . . . . . . . . . . . . . . . 11
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 11
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     11.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   From the viewpoint of the network layer, 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.  From an upper layer viewpoint, 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

   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 flow label could be used in both stateless and stateful
   scenarios.  A stateless scenario is one where any node that processes
   the flow label in any way does not need to store any information
   about a flow before or after a packet has been processed.  A stateful
   scenario is one where a node that processes the flow label value
   needs to store information about the flow, including the flow label
   value.  A stateful scenario might also require a signaling mechanism
   to establish flow state in the network.

   The flow label can be used most simply in stateless scenarios.  This
   specification concentrates on the stateless model and how it can be
   used as a default mechanism.  Details of stateful models, signaling,
   specific flow state establishment methods and their related service
   models are out of scope for this specification.  The basic
   requirement for stateful models is set forth in Section 4.

   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

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   streams), even if the node itself does not require any flow-specific
   treatment.  Node requirements for stateless flow labeling are given
   in Section 3.

   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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 that have not been labeled.  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

   Flow label values should be chosen such that their bits exhibit a
   high degree of variability, making them suitable for use as part of
   the input to a hash function used in a load distribution scheme.  At
   the same time, third parties should be unlikely to be able to guess
   the next value that a source of flow labels will choose.

   In statistics, a discrete uniform distribution is defined as a
   probability distribution in which each value in a given range of
   equally spaced values (such as a sequence of integers) is equally
   likely to be chosen as the next value.  The values in such a
   distribution exhibit both variability and unguessability.  Thus, as
   specified below in Section 3, an approximation to a discrete uniform
   distribution is preferable as the source of flow label values.
   Intentionally, there are no precise mathematical requirements placed
   on the distribution or the method used to achieve such a

   Once set to a non-zero value, the Flow Label MUST be delivered
   unchanged to the destination node(s).  That is, a forwarding node
   MUST NOT change the flow label value in an arriving packet if it is

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   There is no way to verify whether a flow label has been modified en
   route or whether it belongs to a uniform distribution.  Therefore, no
   Internet-wide mechanism can depend mathematically on immutable and
   uniformly distributed flow labels; they have a "best effort" quality.
   This leads to the following formal rules:
   o  Implementers should be aware that the flow label is an unprotected
      field that could have been accidentally or intentionally changed
      en route (see Section 6).
   o  Forwarding nodes such as routers and load distributors MUST NOT
      depend only on Flow Label values being uniformly distributed.  In
      any usage such as a hash key for load distribution, the Flow Label
      bits MUST be combined at least 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.
      Typically the other fields used will be some or all components of
      the usual 5-tuple.

   Although uniformly distributed flow label values are recommended
   below, and will always be helpful for load distribution, it is unsafe
   to assume their presence in the general case, and the use case needs
   to work even if the flow label value is zero.

   As a general practice, packet flows should not be reordered, and the
   use of the Flow Label field does not affect this.  In particular, a
   Flow label value of zero does not imply that reordering is

3.  Stateless Flow Labeling Requirements

   This section defines the minimum requirements for stateless methods
   of setting the flow label value.

   To enable Flow Label based classification, source nodes SHOULD assign
   each unrelated transport connection and application data stream to a
   new flow.  A typical definition of a flow for this purpose is any set
   of packets carrying the same 5-tuple {dest addr, source addr,
   protocol, dest port, source port}.

   It is desirable that flow label values should be uniformly
   distributed to assist load distribution.  It is therefore RECOMMENDED
   that source hosts support the flow label by setting the flow label
   field for all packets of a given flow to the same value chosen from
   an approximation to a discrete uniform distribution.  Both stateful
   and stateless methods of assigning a value could be used, but it is
   outside the scope of this specification to mandate an algorithm.  The
   algorithm SHOULD ensure that the resulting flow label values are
   unique with high probability.  However, if two flows are by chance

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   assigned the same flow label value, and have the same source and
   destination addresses, it simply means that they will receive the
   same treatment throughout the network.  As long as this is a low
   probability event, it will not significantly affect load

   A possible stateless algorithm is to use a suitable 20 bit hash of
   values from the IP packet's 5-tuple.  An alternative is to to use a
   pseudo-random number generator to assign a flow label value for a
   given transport session; such a method will require minimal local
   state to be kept at the source node.  Viewed externally, either
   approach will produce values that are effectively uniformly
   distributed and pseudo-random.

   An implementation in which flow labels are assigned sequentially is
   NOT RECOMMENDED, as it would then be simple for third parties to
   guess the next value.

   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 change the flow label value.  In that case, it is
   RECOMMENDED that the forwarding node sets the flow label field for a
   flow to a uniformly distributed value as just described for source
   o  The same considerations apply as to source hosts setting the flow
      label; in particular, the normal case is that a flow is defined by
      the 5-tuple.
   o  This option, if implemented, would presumably be used by first-hop
      or ingress routers.  It might 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

   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 uniformly distributed.

4.  Flow State Establishment Requirements

   A node that sets the flow label MAY also take part in a flow state
   establishment method that results in assigning specific treatments to
   specific flows, possibly including signaling.  Any such method MUST
   NOT disturb nodes taking part in the stateless model just described.

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   Thus, any node that sets flow label values according to a stateful
   scheme MUST ensure that packets conform to Section 3 of the present
   specification if they are sent outside the network domain using the
   stateful scheme.  Further details are not discussed in this document.

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, even if IPsec
   authentication [RFC4302] is in use, so it can be forged by an on-path
   attacker.  On the other hand, a uniformly distributed pseudo-random
   flow label cannot be readily guessed by an off-path attacker; see
   [I-D.gont-6man-flowlabel-security] for further discussion.

   This specification defines the flow label as immutable once it has
   been set to a non-zero value.  However, 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.  This is undetectable.

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 unintended service by
   modifying the IPv6 header or by injecting packets with false
   addresses and/or labels.  Theft of service is not further discussed
   in this document, since it can only be analysed for specific stateful
   methods of using the flow label.  However, a denial of service attack
   becomes possible in the stateless model when the modified or injected

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   traffic depletes the resources available to forward it and other
   traffic streams.  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.  A man-in-the-
   middle or injected-traffic denial of service attack specifically
   directed at flow label handling would involve setting unusual flow
   labels.  For example, an attacker could set all flow labels reaching
   a given router to the same arbitrary non-zero value, or could perform
   rapid cycling of flow label values such that the packets of a given
   flow will each have a different value.  Either of these attacks would
   cause a stateless load distribution algorithm to perform badly and
   would cause a stateful classifier to behave incorrectly.  For this
   reason, stateless classifiers should not use the flow label alone to
   control load distribution, and stateful classifiers should include
   explicit methods to detect and ignore suspect flow label values.

   Since flows are identified by the 3-tuple of the Flow Label and the
   Source and Destination Address, the risk of denial of service
   introduced by the Flow Label is closely related to the risk of 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

   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, this 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
   on packets that originated with a zero label, or modifying or
   clearing a label, could only occur if an intermediate system such as

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   a router was compromised, or through some other form of man-in-the-
   middle attack.

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,
   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.

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   However, security devices that clear or rewrite non-zero flow label
   values would be in violation of this specification.

7.  Differences from RFC 3697

   The main differences between this specification and its predecessor
   are as follows:
   o  This specification encourages non-zero flow label values to be
      used, and clearly defines how to set a non-zero value.
   o  It encourages a stateless model with uniformly distributed flow
      label values.
   o  It does not specify any details of a stateful model.
   o  It retains the rule that the flow label is immutable, but allows
      routers to set the label on behalf of hosts that do not do so.

   For further details see [I-D.ietf-6man-flow-update].

8.  IANA Considerations

   This document requests no action by IANA.

9.  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.

   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].

10.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-flow-3697bis-03: update to resolve WGLC comments,

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   o  Clarified that the network layer view of flows is agnostic about
      transport sessions.
   o  Honed the definition of stateless v stateful models.
   o  Honed the text about using a pseudo-random function.
   o  Moved material about violation of immutability to Security
      section, and rephrased accordingly.
   o  Dropped material about setting the flow label at a domain exit
      router: doesn't belong here now that we have dropped almost all
      the stateful text.
   o  Removed normative reference to draft-gont-6man-flowlabel-security.
   o  Removed the statement that a node that does not set or use the
      flow label must ignore it: this statement appears to be a no-op.
   o  Added a summary of changes from RFC 3697.
   o  Miscellaneous editorial fixes.

   draft-ietf-6man-flow-3697bis-02: update to remove most text about
   stateful methods, 2011-03-13

   draft-ietf-6man-flow-3697bis-01: update after resolving 11 initial
   issues, 2011-02-26

   draft-ietf-6man-flow-3697bis-00: original version, built from RFC3697
   and draft-ietf-6man-flow-update-01, 2011-01-31

11.  References

11.1.  Normative References

   [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.

11.2.  Informative References

              Gont, F., "Security Assessment of the IPv6 Flow Label",
              draft-gont-6man-flowlabel-security-01 (work in progress),
              November 2010.

              Amante, S., Carpenter, B., and S. Jiang, "Rationale for

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              update to the IPv6 flow label specification",
              draft-ietf-6man-flow-update-04 (work in progress),
              March 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.

   [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

   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

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   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: jiangsheng@huawei.com

   Jarno Rajahalme
   Nokia Siemens Networks
   Linnoitustie 6
   02600  Espoo

   Email: jarno.rajahalme@nsn.com

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