Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels
RFC 6438

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Internet Engineering Task Force (IETF)                      B. Carpenter
Request for Comments: 6438                             Univ. of Auckland
Category: Standards Track                                      S. Amante
ISSN: 2070-1721                                                  Level 3
                                                           November 2011

                     Using the IPv6 Flow Label for
      Equal Cost Multipath Routing and Link Aggregation in Tunnels

Abstract

   The IPv6 flow label has certain restrictions on its use.  This
   document describes how those restrictions apply when using the flow
   label for load balancing by equal cost multipath routing and for link
   aggregation, particularly for IP-in-IPv6 tunneled traffic.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6438.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
     1.1.  Choice of IP Header Fields for Hash Input . . . . . . . . . 3
     1.2.  Flow Label Rules  . . . . . . . . . . . . . . . . . . . . . 4
   2.  Normative Notation  . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 8

1.  Introduction

   When several network paths between the same two nodes are known by
   the routing system to be equally good (in terms of capacity and
   latency), it may be desirable to share traffic among them.  Two such
   techniques are known as equal cost multipath (ECMP) routing and link
   aggregation (LAG) [IEEE802.1AX].  There are, of course, numerous
   possible approaches to this, but certain goals need to be met:

   o  Maintain roughly equal share of traffic on each path.
      (In some cases, the multiple paths might not all have the same
      capacity, and the goal might be appropriately weighted traffic
      shares rather than equal shares.  This would affect the load-
      sharing algorithm but would not otherwise change the argument.)

   o  Minimize or avoid out-of-order delivery for individual traffic
      flows.

   o  Minimize idle time on any path when queue is non-empty.

   There is some conflict between these goals: for example, strictly
   avoiding idle time could cause a small packet sent on an idle path to
   overtake a bigger packet from the same flow, causing out-of-order
   delivery.

   One lightweight approach to ECMP or LAG is this: if there are N
   equally good paths to choose from, then form a modulo(N) hash
   [RFC2991] from a defined set of fields in each packet header that are
   certain to have the same values throughout the duration of a flow,
   and use the resulting output hash value to select a particular path.
   If the hash function is chosen so that the output values have a
   uniform statistical distribution, this method will share traffic
   roughly equally between the N paths.  If the header fields included
   in the hash input are consistent, all packets from a given flow will
   generate the same hash output value, so out-of-order delivery will

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RFC 6438             Flow Label for Tunnel ECMP/LAG        November 2011

   not occur.  Assuming a large number of unique flows are involved, it