draft-ietf-ipv6-host-load-sharing-00.txt   draft-ietf-ipv6-host-load-sharing-01.txt 
INTERNET-DRAFT R. Hinden/Nokia IPv6 Working Group R. Hinden
January 4, 2002 INTERNET-DRAFT Nokia
January 26, 2004 D. Thaler
Expires July 2004 Microsoft
IPv6 Host to Router Load Sharing IPv6 Host to Router Load Sharing
<draft-ietf-ipv6-host-load-sharing-01.txt>
<draft-ietf-ipv6-host-load-sharing-00.txt>
Status of this Memo Status of this Memo
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This internet draft expires on July 4, 2002. Copyright Notice
Abstract Copyright (C) The Internet Society (2004). All Rights Reserved.
This document defines a change to IPv6 Neighbor Discovery that IPv6 Draft IPv6 Host to Router Load Sharing January 2004
hosts can use to load share their outgoing traffic between multiple
default routers.
1. Introduction Abstract
IPv6 hosts on a LAN will usually learn about default routers by The original IPv6 conceptual sending algorithm does not require
receiving Router Advertisements sent using the IPv6 Neighbor load-sharing among equivalent IPv6 routers, and suggests schemes
Discovery protocol [ND]. If there are multiple routers the hosts which can be problematic in practice. This document updates the
will automatically learn about them and have multiple default routers conceptual sending algorithm so that traffic to different
to send off link traffic. destinations is distributed among routers in an efficient fashion.
IPv6 Neighbor Discovery protocol does not require any specific 1. Introduction
procedure for hosts to divide (i.e., load share) outgoing traffic
between these routers. This document defines procedures that IPv6
hosts can use to load share their outgoing traffic between multiple
default routers.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", In the conceptual sending algorithm in [ND] and in the optional
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this extension in [ROUTERSEL], a next hop is chosen when no destination
document are to be interpreted as described in [RFC 2119]. 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 router is found to be unreachable).
2. Background 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.
RFC2461 "Neighbor Discovery for IPv6" [ND] defines in section 6.3.6 It is desirable when there is more than one equivalent router that
an algorithm for selecting default routers. This algorithm is hosts distribute their outgoing traffic among these routers. This
invoked during next hop determination when no destination cache entry shares the load among multiple routers and provides better
exists for an off-link destination or when communication through an performance for the host's traffic.
existing router is failing. Normally a router would be selected the
first time traffic is sent to a specific destination. Subsequent
traffic to the same destination would continue to use this router
unless there was some other reason to change to a different router
(e.g., redirect message received, etc.).
ND further specifies that when there are multiple reachable default [ND] does not require any particular behavior in this respect. It
routers, an implementation may always return the same router (e.g., specifies that an implementation may always choose the same router
the first in the list) or may cycle through the list of reachable (e.g., the first in the list) or may cycle through the routers in
default routers in a round robin manner. It does not require any a round-robin manner. Both of these suggestions are problematic.
specific behavior in the case of multiple default routers.
It is desirable when there is more than one default router that the Clearly, always choosing the same router does not provide load
hosts distribute their outgoing traffic among these routers. This sharing. Some problems with naive tie-breaking techniques such as
document changes the ND behavior to require that an implementation round-robin and random are discussed in [MULTIPATH]. While the
cycle through the list of default routers in a random order. 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
3. Load Sharing Draft IPv6 Host to Router Load Sharing January 2004
The load sharing algorithm changes the currently specified default router at a time.
router selection algorithm to cycle through the list of reachable
default routers in random order. This should have the effect of
distributing outgoing traffic for new destinations among the default
routers. Random selection, versus round robin, is used to avoid
synchronization in the hosts selection of a default router.
Bullet 1) in section 6.3.6 "Default Router Selection" [ND] is In the remainder of this document, the key words "MUST", "MUST
replaced with the following: NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in [RFC2119].
1) Routers that are reachable or probably reachable (i.e., in any 2. Load Sharing
state other than INCOMPLETE) SHOULD be preferred over routers
whose reachability is unknown or suspect (i.e., in the
INCOMPLETE state, or for which no Neighbor Cache entry exists).
An implementation SHOULD pick routers from the default router
list in random order while making sure it always returns a
reachable or a probably reachable router when one is available.
4. Acknowledgments When a host chooses from multiple equivalent routers, it MUST
choose using some method which distributes load for different
destinations among the equivalent routers. That is, a host MUST
NOT always choose the same router (e.g., the first in the list).
A host SHOULD use a hash-based scheme, such as those described in
[MULTIPATH], which takes the destination IP address into account.
The author of this document would like to thank Erik Nordmark, Brian Note that traffic for a given destination address will use the
Haberman, Steve Deering, Aron Silverton, and Christian Huitema for same router as long as the Destination Cache Entry for the
their helpful suggestions. 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.
5. Security Considerations 3. Acknowledgments
This document requires an node to cycle through a the list of default The authors of this document would like to thank Erik Nordmark,
routers. There are no known security issues with this change to IPv6 Brian Haberman, Steve Deering, Aron Silverton, and Christian
Neighbor Discovery. Huitema for their helpful suggestions.
6. References 4. Security Considerations
[ADD-ARH] Hinden, R., S. Deering, "IP Version 6 Addressing As mentioned in [MULTIPATH], when next-hop selection is
Architecture", RFC2373, July 1988. 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. 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.
[ICMPv6] Conta, A., S. Deering, "Internet Control Message Protocol Draft IPv6 Host to Router Load Sharing January 2004
(ICMPv6) for the Internet Protocol Version 6 (IPv6)",
RFC2463, December 1998.
[IPv6] Deering, S., R. Hinden, "Internet Protocol, Version 6 5. Normative References
(IPv6) Specification", RFC2460, December 1998.
[ND] Narten, T., E. Nordmark, W. Simpson, "Neighbor Discovery [ND] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC2461, December 1998. for IP Version 6 (IPv6)", RFC2461, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, BCP0014, March 1997. Requirement Levels", RFC2119, BCP0014, March 1997.
7. Author's Address 6. Informative References
[MULTIPATH]
Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991, November 2000.
[ROUTERSEL]
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 Robert Hinden
Nokia Nokia
313 Fairchild Drive 313 Fairchild Drive
Mountain View, CA 94043 Mountain View, CA 94043
US
Phone: +1 650 625-2004 Phone: +1 650 625-2004
Email: hinden@iprg.nokia.com Email: bob.hinden@nokia.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
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 to Router Load Sharing January 2004
o Split load sharing back into its own document.
o Made hash-based, rather than random, the rule.
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 End of changes. 

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