IPv6 Flow Label Specification
RFC 3697

Document Type RFC - Proposed Standard (March 2004; No errata)
Obsoleted by RFC 6437
Last updated 2015-10-14
Stream IETF
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IESG IESG state RFC 3697 (Proposed Standard)
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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.

Rajahalme, et al.           Standards Track                     [Page 1]
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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