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Versions: 00 01 02 03 04 rfc4311                         Standards Track
IPv6 Working Group                                       R. Hinden
INTERNET-DRAFT                                               Nokia
June 23, 2005                                            D. Thaler
Expires December 2005                                    Microsoft

                 IPv6 Host to Router Load Sharing

Status of this Memo

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Copyright Notice

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Draft            IPv6 Host to Router Load Sharing        June 2005

Copyright (C) The Internet Society (2005).  All Rights Reserved.


The original IPv6 conceptual sending algorithm does not do load-
sharing among equivalent IPv6 routers, and suggests schemes which
can be problematic in practice.  This document updates the
conceptual sending algorithm in RFC 2461 so that traffic to
different destinations can be distributed among routers in an
efficient fashion.

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 the router is found to be unreachable).

In addition, as described in [ADDRSEL], the choice of next hop may
also affect the choice of source address, and hence indirectly
(and to a lesser extent) may affect the router used for inbound
traffic as well.

In both the base sending 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 often 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.

On the other hand, load sharing can be undesirable in situations
where sufficient capacity is available through a single router and
the traffic patterns could be more predictable by using a single
router; in particular, this helps to diagnose connectivity
problems beyond the first-hop routers.

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[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 selection may also result in
synchronization issues among hosts, where in the worst case the
load is concentrated on one 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 SHOULD
support choosing using some method which distributes load for
different destinations among the equivalent routers rather than
always choosing the same router (e.g., the first in the list).
This memo takes no stance on whether the support for load sharing
should be turned on or off by default.  Furthermore, a host that
does attempt to distribute load among routers SHOULD use a hash-
based scheme which takes (at least) the destination IP address
into account, such as those described in [MULTIPATH], for choosing
a router to use.

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.

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3.  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
more unpredictable the hash is, the harder it becomes to conduct a
denial-of-service attack against any single router.

However, a malicious local application can bypass the algorithm
for its own traffic by using mechanisms such as raw sockets, and
remote attackers can still overload the routers directly.  Hence,
the mechanisms discussed herein have no significant incremental
impact on Internet infrastructure security.

4.  IANA Considerations

This document has no actions for IANA.

5.  Acknowledgments

The authors of this document would like to thank Erik Nordmark,
Brian Haberman, Steve Deering, Aron Silverton, Christian Huitema,
and Pekka Savola.

6.  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.

     Draves, R., "Default Address Selection for Internet Protocol
     version 6 (IPv6)", RFC 3484, February 2003.

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     Draves, R. and D. Thaler, "Default Router Preferences and
     More-Specific Routes", Work in progress, draft-ietf-
     ipv6-router-selection-07.txt, January 2005.

7.  Informative References

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

8.  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

9.  Full Copyright Statement

Copyright (C) The Internet Society (2005).  This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their

This document and the information contained herein are provided on

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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
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specification can be obtained from the IETF on-line IPR repository
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The IETF invites any interested party to bring to its attention
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to implement this standard.  Please address the information to the
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