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Network Working Group                                          R. HindenRequest for Comments: 4311                                         NokiaUpdates:2461                                                  D. ThalerCategory: Standards Track                                      Microsoft                                                           November 2005IPv6 Host-to-Router Load SharingStatus 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 inRFC 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.Hinden & Thaler             Standards Track                     [Page 1]

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.Hinden & Thaler             Standards Track                     [Page 2]

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.Hinden & Thaler             Standards Track                     [Page 3]

RFC 4311            IPv6 Host-to-Router Load Sharing       November 20055.  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.comHinden & Thaler             Standards Track                     [Page 4]

RFC 4311            IPv6 Host-to-Router Load Sharing       November 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hinden & Thaler             Standards Track                     [Page 5]