IPv6 Working Group                                       R. Hinden
INTERNET-DRAFT                                               Nokia
January 26,
May 4, 2004                                              D. Thaler
Expires July November 2004                                    Microsoft

                 IPv6 Host to Router Load Sharing

Status of this Memo

This document is an Internet-Draft and is in full conformance with
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-

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

Copyright Notice

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

Draft            IPv6 Host to Router Load Sharing     January         May 2004


The original IPv6 conceptual sending algorithm does not require
load-sharing do load-
sharing among equivalent IPv6 routers, and suggests schemes which
can be problematic in practice.  This document updates the
conceptual sending algorithm so that traffic to different
destinations is can be distributed among routers in an efficient

1.  Introduction

In the conceptual sending algorithm in [ND] and in the optional
extension in [ROUTERSEL], a next hop is chosen when no destination
cache entry exists for an off-link destination or when
communication through an existing router is failing.  Normally a
router is selected the first time traffic is sent to a specific
destination IP address.  Subsequent traffic to the same
destination address continues to use the same router unless there
is some reason to change to a different router (e.g., a redirect
message is received, or a the router is found to be unreachable).

In both the base algorithm and in the optional extension,
sometimes a host has a choice of multiple equivalent routers for a
destination.  That is, all other factors are equal and a host must
break a tie via some implementation-specific means.

It is typically desirable when there is more than one equivalent
router that hosts distribute their outgoing traffic among these
routers.  This shares the load among multiple routers and provides
better performance for the host's traffic.

[ND] does not require any particular behavior in this respect.  It
specifies that an implementation may always choose the same router
(e.g., the first in the list) or may cycle through the routers in
a round-robin manner.  Both of these suggestions are problematic.

Clearly, always choosing the same router does not provide load
sharing.  Some problems with load sharing using naive tie-breaking
techniques such as round-robin and random are discussed in
[MULTIPATH].  While the destination cache provides some stability
since the determination is not per-packet, cache evictions or
timeouts can still result in unstable or unpredictable paths over
time, lowering the performance and making it harder to diagnose
problems.  Round-
robin  Round-robin selection may also result in

Draft            IPv6 Host to Router Load Sharing         May 2004

synchronization issues among hosts, where in the worst case the
load is concentrated on one

Draft            IPv6 Host to Router Load Sharing     January 2004 router at a time.

In the remainder of this document, the key words "MUST", "MUST
"RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in [RFC2119].

2.  Load Sharing

When a host chooses from multiple equivalent routers, it MUST
choose SHOULD
support choosing using some method which distributes load for
different destinations among the equivalent routers.  That is, a host MUST
NOT routers rather than
always choose choosing the same router (e.g., the first in the list).
Furthermore, a host that does attempt to distribute load among
routers SHOULD use a hash-based scheme, such as those described in
[MULTIPATH], which takes the destination IP address into account.

Note that traffic for a given destination address will use the
same router as long as the Destination Cache Entry for the
destination address is not deleted.  With a hash-based scheme,
traffic for a given destination address will use the same router
over time even if the Destination Cache Entry is deleted, as long
as the list of equivalent routers remains the same.

3.  Acknowledgments

The authors of this document would like to thank Erik Nordmark,
Brian Haberman, Steve Deering, Aron Silverton, and Christian
Huitema for their helpful suggestions.

4.  Security Considerations

As mentioned in [MULTIPATH], when next-hop selection is
predictable, an application can synthesize traffic that will all
hash the same, making it possible to launch a denial-of-service
attack against the load sharing algorithm, and overload a
particular router.  This can even be done by a remote application
that can cause a host to respond to a given destination address.
A special case of this is when the same (single) next-hop is
always selected, such as in the algorithm allowed by [ND].
Introducing hashing can make such an attack more difficult; the

Draft            IPv6 Host to Router Load Sharing         May 2004

more unpredictable the hash is, the harder it becomes to conduct a
denial-of-service attack against any single router.

Draft            IPv6 Host to Router Load Sharing     January 2004

5.  Normative References

[ND] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
     for IP Version 6 (IPv6)", RFC 2461, December 1998.

     Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, BCP0014, March 1997.

6.  Informative References

     Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
     Multicast Next-Hop Selection", RFC 2991, November 2000.

     Draves, R. and D. Thaler, "Default Router Preferences and
     More-Specific Routes", Work in progress, draft-ietf-
     ipv6-router-selection-03.txt, December 2003.

7.  Authors' Addresses

     Robert Hinden
     313 Fairchild Drive
     Mountain View, CA  94043
     Phone: +1 650 625-2004
     Email: bob.hinden@nokia.com

     Dave Thaler
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA  98052
     Phone: +1 425 703 8835
     EMail: dthaler@microsoft.com

Draft            IPv6 Host to Router Load Sharing         May 2004

8.  Revision History

(This section to be removed before publication as an RFC)

Changes from draft-ietf-ipv6-router-selection-02.txt:

Draft            IPv6 Host draft-ietf-ipv6-load-sharing-01.txt:

o    Changed load sharing from a MUST to Router Load Sharing     January 2004 a SHOULD.

o    Added standard IETF intellectual property notice.

Changes from draft-ietf-ipv6-router-selection-02.txt:

o    Split load sharing back into its own document.

o    Made hash-based, rather than random, the rule.

9.  Full Copyright Statement

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

This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implmentation may be prepared, copied,
published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
works.  However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards
process must be followed, or as required to translate it into
languages other than English.

The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on

Draft            IPv6 Host to Router Load Sharing         May 2004

10.  Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights.  Information on the procedures with respect to rights
in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard.  Please address the information to the
IETF at ietf-ipr@ietf.org.