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Versions: 00 01 02                                                      
Internet Engineering Task Force                                  B. Volz
INTERNET DRAFT                                                  Ericsson
DHC Working Group                                               Feb 2002
Expires: August 22, 2002

                      Load Balancing for DHCPv6

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 22, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   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

   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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   [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

   [3]  Volz, B., Gonczi, S., Lemon, T., Stevens, R., "DHC Load
        Balancing Algorithm", RFC 3074, February 2001.

Author's Address

   Bernie Volz
   959 Concord Street
   Framingham, MA  01701

   Phone: +1 508 875 3162
   EMail: bernie.volz@ericsson.com

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