IPv6 Working Group R. Hinden
INTERNET-DRAFT Nokia
June 23, 2005 D. Thaler
Expires December 2005 Microsoft
IPv6 Host to Router Load Sharing
<draft-ietf-ipv6-host-load-sharing-04.txt>
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Copyright Notice
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Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
The original IPv6 conceptual sending algorithm does not do load-
sharing among equivalent IPv6 routers, and suggests schemes which
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.
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.
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[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 which 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 which takes (at least) the destination IP address
into account, such as those described in [MULTIPATH], for choosing
a router to use.
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.
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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. IANA Considerations
This document has no actions for IANA.
5. Acknowledgments
The authors of this document would like to thank Erik Nordmark,
Brian Haberman, Steve Deering, Aron Silverton, Christian Huitema,
and Pekka Savola.
6. 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.
[ADDRSEL]
Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
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[ROUTERSEL]
Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", Work in progress, draft-ietf-
ipv6-router-selection-07.txt, January 2005.
7. Informative References
[MULTIPATH]
Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991, November 2000.
8. 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
9. Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
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THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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