IPv6 Working Group                                          J. Rajahalme
INTERNET-DRAFT                                                     Nokia
<draft-ietf-ipv6-flow-label-02.txt>                             A. Conta
                                                            B. Carpenter
                                                              S. Deering
Expires: December 2002                                         June 2002

                     IPv6 Flow Label Specification

Status of this memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document specifies the usage of the IPv6 Flow Label field, the
   requirements for IPv6 source nodes labeling flows, and the
   requirements for flow state establishment methods.

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1.  Terminology and Definitions

   Classifier             An IP layer entity that selects packets based
                          on the content of packet headers according to
                          defined rules.

   Flow                   A sequence of related packets sent from a
                          source to a unicast, anycast, or multicast
                          destination(s). 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

                          This definition should not be confused with
                          the more restrictive definitions for "flow"
                          and "microflow" in [RSVP] and [DiffServ],
                          respectively. This definition includes, but is
                          not limited to them.

   Flow state             The information stored in an IP node driving
                          the flow classification and the flow-specific
                          treatment. The required information is
                          specified by the method defining the flow-
                          specific treatment.

   Flow state             A control mechanism used to set up the flow
   establishment method   state. A flow state establishment method can
                          be either
                          - Dynamic, under source node control (e.g.
                          - Quasi-dynamic, under network management
                          control, or
                          - Static, through manual configuration.
                          - Algorithmic (e.g. load-spreading)

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

2.  Introduction

   A flow is a sequence of related packets sent from a source to a
   unicast, anycast, or multicast destination(s). To enable specific
   processing for the flow, flow state needs to be established on the
   nodes providing the flow-specific treatment. The flow state defines
   what kind of treatment should be provided, and how to classify the
   packets to the flow.

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   Traditionally, flow classifiers have been based on the 5-tuple of the
   source and destination addresses, ports and the transport protocol
   type (e.g. the "microflow" definition in [DiffServ]). However, 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, dependence on higher layer headers by the
   IP layer represents a layer violation, possibly hindering the
   introduction of new transport protocols.

   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 specification of
   the IPv6 Flow Label field is given in section 3 below.

   The minimum level of IPv6 flow support consists of labeling the
   flows. IPv6 source nodes can label known flows (e.g. TCP connections,
   RTP streams), even if the node itself would not require any flow-
   specific treatment. Doing this enables receiver oriented resource
   reservations, e.g. [RSVP]. Requirements for flow labeling are given
   in section 4.

   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 5.

3.  IPv6 Flow Label Specification

   The 20-bit Flow Label field in the IPv6 header SHOULD be used by a
   source to label sequences of related packets sent to a specific
   unicast, anycast, or multicast destination(s). A non-zero Flow Label
   indicates that the IPv6 packet is labeled. IPv6 nodes receiving a
   labeled IPv6 packet can use the Flow Label, and Source and
   Destination Address fields to classify the packet to a certain flow.
   The packet MAY be given some flow-specific treatment based on the
   flow state established on a set of IPv6 nodes. 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(s).

   IPv6 nodes MUST NOT assume mathematical or other non-standardized
   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 key.

   If an IPv6 node is not providing flow-specific treatment, it MUST
   ignore the field when receiving or forwarding a packet.

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4.  Flow Labeling Requirements

   To enable Flow Label based classification, the source MUST label all
   packets belonging to a flow with the Flow Label value assigned to the

   The assignment of a packet to a flow takes various forms, presented

   (1)  The source MAY take part in a signaling protocol that results in
        assigning certain transport connection(s) or application data
        stream(s) to specific flow(s).

   (2)  The source MAY be configured to assign certain transport
        connection(s) or application data stream(s) to specific flow(s).

   (3)  The source SHOULD assign each new application data stream (e.g.
        RTP streams) to a new flow.

   (4)  The source SHOULD assign each new transport connection (e.g.
        TCP, SCTP) to a new flow.

   It is necessary that flow classifiers downstream from the source can
   classify packets unambiguously, i.e. that all packets which the
   source has chosen to label as a single flow can be efficiently
   identified as such.

   To enable this, the source node MUST keep track of the Flow Label
   values it is currently using or has recently used. When a new flow is
   instantiated, a unique Flow Label MUST be assigned to it. A Flow
   Label value is considered unique if it is not currently in use with
   the same Source and Destination addresses. In the case of flows with
   multiple addresses (e.g., SCTP flows) this requirement for uniqueness
   extends to all possible (Source, Destination) address pairs.

   The IPv6 source node MUST provide a facility for verifying and
   assigning new Flow Label values, and for storing the Flow Label, and
   the associated Source and Destination Addresses currently in use. The
   facility MUST be used whenever a label needs to be assigned for a new
   flow. The facility SHOULD provide a programming interface with at
   least the following functionality:

   (1)  to assign any Flow Label value for a new flow

   (2)  to assign a specific Flow Label for a new flow, and

   (3)  to delete a flow, i.e. to free a Flow Label no longer in use.

   The interface definition for the facility is beyond the scope of this

   When a dynamically instantiated flow terminates, its Flow Label value
   MUST NOT be reused until it is certain that all associated state has

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   been deleted from all nodes on the path. With some flow state
   establishment methods signaling new state may be sufficient. A
   mechanism with a sufficiently long timeout period before reusing the
   Flow Label values can also be used.

   With [RSVP] or [SDP] either the source or the destination of the flow
   could have a preference for the Flow Label value to be used. For
   example, a destination with multiple sources sending packets to it
   could require all the sources to use the same Flow Label value in
   order to collapse the classifier state to a single flow state entry,
   instead of having separate classifier state for each source (ref. the
   Wildcard-Filter reservation style in [RSVP]). Therefore the source
   SHOULD honor the destination's request to mark the packets with the
   Flow Label value specified.

   To enable the peer(s) to know the assigned or requested Flow Label
   value, the value SHOULD be included along with the Source and
   Destination addresses as part of any signaling dealing with the flow,
   e.g. transport layer connection set up, RSVP for resource
   reservation, or SDP for media session parameters.

5.  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 are defined in separate

   To enable co-existence of different methods in IPv6 nodes, the
   methods MUST meet the following basic requirements:

   (1)  A packet is classified unambiguously to a flow on the basis of
        the Flow Label, and the Source and Destination Address fields.
        Depending on the method semantics, multiple such triplets MAY
        identify the same flow state (e.g. SCTP flows with multiple
        addresses at either end-points, or a diffserv classifier with an
        address range. See also the RSVP Wildcard-Filter example in
        section 4 above). The flow state establishment method MUST
        convey this classifying information to the IPv6 nodes that need
        to perform the classification. Usage of any additional header
        fields for flow classification is beyond the scope of this

   (2)  The IPv6 node facility keeping track of the Flow Label, and the
        associated Source and Destination Addresses MUST be utilized
        when assigning Flow Label values to new flows (see section 4

   (3)  The Flow Label value 0 is reserved for non-labeled packets.

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   (4)  The method MUST provide the means for flow state clean-up from
        the IPv6 nodes providing the flow-specific treatment. Both soft-
        and hard-state methods are possible.

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

   (6)  Flow state establishment methods SHOULD include the Mobile IP
        Home Addresses of the source and the destination in the state
        establishment process in addition to the Care-of Addresses, if
        available. This enables avoiding state duplication on fixed
        portions of the path when either end changes its Care-of

Security Considerations

   Anything that facilitates flow classification also increases the
   vulnerability to traffic analysis.

   The use of the Flow Label field in general enables flow
   classification also in the presence of ESP encryption of IPv6
   payloads. This allows the transport header values to remain
   confidential, which may lessen the possibilities for some forms of
   traffic analysis.

IANA Considerations

   This specification does not define any well-known values.


   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 Steve Blake, Jim Bound, Francis Dupont, Robert Elz,
   Tony Hain, Bob Hinden, Christian Huitema, Frank Kastenholz, Charles
   Perkins, Hesham Soliman, Michael Thomas, and Margaret Wasserman for
   their contributions.

Normative References

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

Informative References

   [DiffServ]  S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W.
               Weiss, "An Architecture for Differentiated Service", RFC
               2475, December 1998.

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INTERNET-DRAFT     draft-ietf-ipv6-flow-label-02.txt           June 2002

   [Rajahalme] J. Rajahalme, A. Conta, "An IPv6 Flow Label Specification
               Proposal", Internet Draft <draft-rajahalme-ipv6-flow-
               label-00.txt>, November 2001, expires May 2002, Work in

   [RFC1809]   C. Partridge, "Using the Flow Label Field in IPv6", RFC
               1809, June 1995.

   [RSVP]      R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin,
               "Resource Reservation Protocol (RSVP) Version 1
               Functional Specification", RFC 2205, September 1997.

   [SDP]       M. Handley, V. Jacobson, "SDP: Session Description
               Protocol", RFC 2327, April 1998.

Authors' Addresses

   Jarno Rajahalme
   Nokia Research Center
   P.O. Box 407
   E-mail: 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: brian@hursley.ibm.com

   Steve Deering
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706
   Email: deering@cisco.com

Expiration Date

   This memo is filed as <draft-ietf-ipv6-flow-label-02.txt> and expires
   in December 2002.

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