    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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    RFC 4311            IPv6 Host-to-Router Load Sharing       November 2005

       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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    RFC 4311            IPv6 Host-to-Router Load Sharing       November 2005

       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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    RFC 4311            IPv6 Host-to-Router Load Sharing       November 2005

    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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    RFC 4311            IPv6 Host-to-Router Load Sharing       November 2005

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       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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