





Network Working Group                                          R. Hinden
Request for Comments: 4311                                         Nokia
Updates: 2461                                                  D. Thaler
Category: Standards Track                                      Microsoft
                                                           November 2005

                    IPv6 Host-to-Router Load Sharing

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 (2005).

Abstract

   The original IPv6 conceptual sending algorithm does not do load
   sharing among equivalent IPv6 routers, and suggests schemes that 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.






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   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.

   [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 NOT",
   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "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 that 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 that takes (at least) the destination IP address into account,
   such as those described in [MULTIPATH], for choosing a router to use.



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   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.  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.  Acknowledgements

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




















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5.  Normative References

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

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [ROUTERSEL]  Draves, R. and D. Thaler, "Default Router Preferences
                and More-Specific Routes", RFC 4191, November 2005.

6.  Informative References

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

Authors' Addresses

   Robert Hinden
   Nokia
   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












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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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