Internet Engineering Task Force B. Volz
INTERNET DRAFT Ericsson
DHC Working Group Feb 2002
Expires: August 22, 2002
Load Balancing for DHCPv6
draft-ietf-dhc-dhcpv6-loadb-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies a load balancing algorithm for use with
DHCPv6. Load balancing enables multiple cooperating DHCPv6 servers
to decide which one should service a client, without exchanging
any information beyond initial configuration. It expands on RFC
3074 "DHC Load Balancing Algorithm" to include DHCPv6.
1. Introduction
This document extends the load balancing concepts described in
RFC 3074 "DHC Load Balancing Algorithm" [3] to DHCPv6 [2].
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [1].
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3. Terminology
This document uses terminology specific to IPv6 and DHCPv6 as defined
in the "Terminology" section of the DHCP specification [2].
This document uses many of the concepts and terminology specific to
load balancing as defined in the "Load Balancing Terminology" section
of the DHC Load Balancing specification [3].
4. DHCPv6 Server Operation
DHCPv6 uses a DUID (DHCP Unique Identifier) to identify clients. The
DUID is carried in most client-generated messages in the Client
Identifier option as described in [2]. The client's DUID is defined
to be the Service Transaction ID (STID) [3].
DHCPv6 uses two types of client messages, those that are directed to
a specific server and those that are directed to all servers. The
messages directed to a specific server contain a Server Identifier
option as described in [2] and include the Request, Renew, Release,
Decline, and Information-Request messages. The messages directed to
all servers do not include a Server Identifier option and include
the Solicit, Confirm, Rebind, and Information-Request messages.
For the messages directed to a specific server, this load balancing
algorithm does not apply and a server processes that client's
request if the Server Identifier option's DUID of the request matches
it own and discards all other requests.
For the messages directed to all servers, the load balancing
algorithm MAY be used to limit the clients that a server services
if the request contains a Client Identifier option. The server uses
the hash algorithm described in [3] on the client's DUID (the STID)
and uses the resulting hash value to determine if the client is
within the server's configured hash bucket assignment (HBA) [3]. If
the hash value is assigned to the server, the server MUST process
the client request (other server policy may of course determine how
the request is processed and whether a reply is sent to the client).
If the hash value is not assigned to the server, the server SHOULD
NOT process the request. The server MAY process the request if the
elapsed time value in the Elapsed Time option of the request exceeds
a preconfigured value (the Service Delay or SD in [3]). How the SD is
configured for a server is outside the scope of this document.
For client requests (such as Information-Request messages) which do
not contain a Client Identifier option, there is no STID and thus all
servers MUST process these requests.
The hash bucket assignments for each server must be configured and
care must be taken to assign each hash bucket to at least one
server. How the hash buckets are configured in servers is outside
the scope of this document.
If a single hash bucket is assigned to multiple servers, the logic
a client uses to select a server applies (just as if there were
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multiple servers for clients without load balancing). For example,
each server can be configured with a different server preference
value [2].
5. DHCPv6 Relay Operation
This document does not specify any techniques related to load
balancing for relays. While a similar approach to that described
in [3] could be used with DHCPv6 relays, further investigation of
the benefits and complexities this may add to DHCPv6 configurations
is needed before any recommendations can be made. This is an area
of further work and discussion.
Relays MUST be configured to forward client requests to all of
the DHCPv6 servers that may be part of a load balancing group.
6. DHCPv6 Client Operation
DHCPv6 clients need not be aware that load balancing is in use by
the servers. A client operates as described in [2].
Client operation with respect to load balancing is the same as
client operation with multiple servers. If a server that was
servicing a client becomes unavailable for some reason, the client
will eventually time-out and communicate with all servers. When
this happens, if there are multiple servers assigned to handle
that client's hash bucket, one or more of these remaining servers
will respond. If there are no other servers for that hash bucket,
other servers may respond once the elapsed time value in the
Elapsed Time option exceeds their configured SD.
If there is only one server (either for all clients or for some
of the hash buckets), failure of that server will prevent clients
from obtaining or extending the lifetimes of addresses. However,
there is no difference whether load balancing is used or not.
7. Security Considerations
This proposal in and by itself provides no security, nor does it
impact existing security. See [2] for further details as to DHCPv6
security issues.
Servers using load balancing are responsible for ensuring that if
the contents of the HBA are transmitted over the network as part
of the process of configuring any server, that message be secured
against tampering, since tempering with the HBA could result in a
denial of service for some or all clients.
8. Acknowledgements
Thanks to the DHC Working Group for their time and input into the
specification starting at IETF-52. Thanks also to the following
individuals for their comments and questions (in alphabetical
order) Stefan Berg, Herold Fagerberg, Ted Lemon, Tony Lindstr÷m,
Thomas Narten, Anders Strand, and Jack Wong.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-23 (work in progress), February
2002.
[3] Volz, B., Gonczi, S., Lemon, T., Stevens, R., "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
Author's Address
Bernie Volz
Ericsson
959 Concord Street
Framingham, MA 01701
USA
Phone: +1 508 875 3162
EMail: bernie.volz@ericsson.com
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