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Obsoleted by: 6437 PROPOSED STANDARD
Network Working Group                                       J. Rajahalme
Request for Comments: 3697                                         Nokia
Category: Standards Track                                       A. Conta
                                                            B. Carpenter
                                                              S. Deering
                                                              March 2004

                     IPv6 Flow Label Specification

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document specifies the IPv6 Flow Label field and the minimum
   requirements for IPv6 source 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

1.  Introduction

   A flow is a sequence of packets sent from a particular source to a
   particular unicast, anycast, or multicast destination that the source
   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.

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RFC 3697             IPv6 Flow Label Specification            March 2004

   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
   option 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.  IPv6 source nodes supporting the flow labeling MUST be able
   to label known flows (e.g., TCP connections, application streams),
   even if the node itself would not require any flow-specific
   treatment.  Doing this enables load spreading and receiver oriented
   resource reservations, for example.  Node requirements for flow
   labeling are given in section 3.

   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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

2.  IPv6 Flow Label Specification

   The 20-bit Flow Label field in the IPv6 header [IPv6] is used by a
   source to label packets of a flow.  A Flow Label of zero is used to
   indicate packets not part of any flow.  Packet classifiers 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 the nodes that have been set
   up with flow-specific state.  The nature of the specific treatment
   and the methods for the flow state establishment are out of scope for
   this specification.

   The Flow Label value set by the source MUST be delivered unchanged to
   the destination node(s).

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RFC 3697             IPv6 Flow Label Specification            March 2004

   IPv6 nodes MUST NOT assume any mathematical or other properties of
   the Flow Label values assigned by source nodes.  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

   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.

   If an IPv6 node is not providing flow-specific treatment, it MUST
   ignore the field 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.  The source node MAY also take part in flow state
   establishment methods that result in assigning certain packets to
   specific flows.  A source node which does not assign traffic to flows
   MUST set the Flow Label to zero.

   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 5.1).
   The source node SHOULD be able to select unused Flow Label values for
   flows not requesting a specific value to be used.

   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 sequence (e.g.,
   sequential or pseudo-random) and use an initial value that avoids

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RFC 3697             IPv6 Flow Label Specification            March 2004

   reuse of recently used Flow Label values each time 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 [RND] SHOULD be used.

4.  Flow State Establishment Requirements

   To enable 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

   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

   (2)  Flow state establishment methods MUST be able to recover from
        the case where the requested flow state cannot be supported.

5.  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 5.1).  Section 5.2 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.

5.1.  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

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RFC 3697             IPv6 Flow Label Specification            March 2004

   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 [INGR] 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
   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

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   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
   [INGR] 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.

5.2.  IPsec and Tunneling Interactions

   The IPsec protocol, as defined in [IPSec, AH, ESP], 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.

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   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.

5.3.  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.

6.  Acknowledgements

   The discussion on the topic in the IPv6 WG mailing list has been
   instrumental for the definition of this specification.  The authors
   want to thank 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
   for their contributions.

7.  References

7.1.  Normative References

   [IPv6]      Deering, S. and R. Hinden, "Internet Protocol Version 6
               Specification", RFC 2460, December 1998.

   [KEYWORDS]  Bradner, S., "Key words for use in RFCs to indicate
               requirement levels", BCP 14, RFC 2119, March 1997.

   [RND]       Eastlake, D., Crocker, S. and J. Schiller, "Randomness
               Recommendations for Security", RFC 1750, December 1994.

7.2.  Informative References

   [AH]        Kent, S. and R. Atkinson, "IP Authentication Header", RFC
               2402, November 1998.

   [ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security
               Payload (ESP)", RFC 2406, November 1998.

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RFC 3697             IPv6 Flow Label Specification            March 2004

   [INGR]      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.

   [IPSec]     Kent, S. and R. Atkinson, "Security Architecture for the
               Internet Protocol", RFC 2401, November 1998.

Authors' Addresses

   Jarno Rajahalme
   Nokia Research Center
   P.O. Box 407

   EMail: jarno.rajahalme@nokia.com

   Alex Conta
   Transwitch Corporation
   3 Enterprise Drive
   Shelton, CT 06484

   EMail: aconta@txc.com

   Brian E. Carpenter
   IBM Zurich Research Laboratory
   Saeumerstrasse 4 / Postfach
   8803 Rueschlikon

   EMail: brc@zurich.ibm.com

   Steve Deering
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706

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RFC 3697             IPv6 Flow Label Specification            March 2004

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