IPv6 Working Group R. Hinden
INTERNET-DRAFT Nokia
January 26, 2004 D. Thaler
Expires July 2004 Microsoft
IPv6 Host to Router Load Sharing
<draft-ietf-ipv6-host-load-sharing-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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Abstract
The original IPv6 conceptual sending algorithm does not require
load-sharing among equivalent IPv6 routers, and suggests schemes
which can be problematic in practice. This document updates the
conceptual sending algorithm so that traffic to different
destinations is 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 a router is found to be unreachable).
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.
It is 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.
[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 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
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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 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.
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. Acknowledgments
The authors of this document would like to thank Erik Nordmark,
Brian Haberman, Steve Deering, Aron Silverton, and Christian
Huitema for their helpful suggestions.
4. 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. 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.
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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", RFC 2119, BCP0014, March 1997.
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
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
8. Revision History
(This section to be removed before publication as an RFC)
Changes from draft-ietf-ipv6-router-selection-02.txt:
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o Split load sharing back into its own document.
o Made hash-based, rather than random, the rule.
9. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
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