Rationale for Update to the IPv6 Flow Label Specification
RFC 6436

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Internet Engineering Task Force (IETF)                         S. Amante
Request for Comments: 6436                                       Level 3
Category: Informational                                     B. Carpenter
ISSN: 2070-1721                                        Univ. of Auckland
                                                                S. Jiang
                                                                  Huawei
                                                           November 2011

       Rationale for Update to the IPv6 Flow Label Specification

Abstract

   Various published proposals for use of the IPv6 flow label are
   incompatible with its original specification in RFC 3697.
   Furthermore, very little practical use is made of the flow label,
   partly due to some uncertainties about the correct interpretation of
   the specification.  This document discusses and motivates changes to
   the specification in order to clarify it and to introduce some
   additional flexibility.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6436.

Amante, et al.                Informational                     [Page 1]
RFC 6436                    Flow Label Update              November 2011

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Impact of Current Specification  . . . . . . . . . . . . . . .  3
   3.  Changes to the Specification . . . . . . . . . . . . . . . . .  6
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Alternative Approaches  . . . . . . . . . . . . . . . 12

1.  Introduction

   The flow label field in the IPv6 header was reserved but left
   Experimental by [RFC2460], which mandates only that "Hosts or routers
   that do not support the functions of the Flow Label field are
   required to set the field to zero when originating a packet, pass the
   field on unchanged when forwarding a packet, and ignore the field
   when receiving a packet."

   The flow label field was normatively specified by [RFC3697].  In
   particular, we quote three rules from that RFC:

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

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

   c.  "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."

Amante, et al.                Informational                     [Page 2]
RFC 6436                    Flow Label Update              November 2011

   Additionally, RFC 3697 does not define the method a host should adopt
   by default to choose the value of the flow label, if no specific
   method is in use.  It was expected that various signaling methods
   might be defined for agreeing on values of the flow label, but no
   such methods have been standardized, except a pre-existing option in
   RSVP [RFC2205].

   The flow label is hardly used in practice in widespread IPv6
   implementations, although some operating systems do set it
   [McGann05].  To some extent, this is due to the main focus being on
   basic deployment of IPv6, but the absence of a default method of
   choosing the flow label value means that most host implementations
   simply set it to zero.  There is also anecdotal evidence that the