Internet Engineering Task Force                                  B. Volz
INTERNET DRAFT                                                  Ericsson
DHC Working Group                                            August 2002
Expires: February 1, 2003

                      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.

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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. Motivation for Load Balancing

   DHCP [2] provides for multiple servers to advertise service to the
   clients on links. A client is generally offered configuration service
   from each of the servers and there is no guarantee of consistency for
   the client (a different server may be selected each time).

   Load balancing provides a quick and easy way for a server to
   determine whether it should service a particular client. Only the
   selected server or servers respond to the client instead of all of
   the servers. Load balancing provides a means to efficiently and
   consistently distribute the processing load for clients across
   multiple servers rather than having each server respond to every

   In addition, rather than having multiple servers service the same
   clients, load balancing allows each server to service a different set
   of clients. If a server is down, the other servers may take over the
   clients that the downed server was to handle by monitoring the
   elapsed time option in client requests.

   The load balancing technique described here and in RFC 3074 [3] work
   well for request/reply transaction protocols where a consistent
   client identifier is available.

   For example, a high performance (non-redundant) configuration of DHCP
   servers might be as follows:

       +---------------+         +---------------+
       | DHCP Server 1 |         | DHCP Server 2 |
       |   HBA 0-127   |         |  HBA 128-255  |
       +-------+-------+         +-------+-------+
               |                         |
               |                         |

   In this example, rather than both servers servicing all clients, each
   services appropriate half the clients and each services the same set
   of clients consistently. A redundant set of servers could be added
   (each configured with appropriate HBAs).

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5. 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]. The messages directed to all servers do
   not include a Server Identifier option.

   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 its 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 configured
   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 which do not contain a Client Identifier option,
   there is no STID and thus all servers process these requests.

   A load balancing server would have the following processing flow for
   received client messages:

     1. If the Server Identifier option is present in the message,
        process the message as per [2].

     2. If no Client Identifier option is present in the message,
        process the message as per [2].

     3. If the Client Identifier option's DUID is within the server's
        hash bucket assignment, process as per [2].

     4. If the Elapsed Time option is present in the message and its
        value exceeds the configured threshold, process as per [2].

     5. Otherwise, do not process the message because load balancing
        dictates that another server should be processing the message.

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   Note: For CONFIRM messages (which do not include a Server Identifier
   option), a server MAY forgo the load balancing algorithm and respond
   to all clients.

   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
   multiple servers for clients without load balancing). For example,
   each server can be configured with a different server preference
   value [2].

6. DHCPv6 Relay Agent Operation

   Relay agents MAY be configured to relay client requests using load
   balancing. A load balancing relay agent must be configured with
   additional information as to the hash buckets assigned to each
   server, in a manner similar to that presented in [3]. Care must be
   taken to assure consistent information if both relay agents and
   servers are configured with load balancing information.

   A relay agent would have the following processing flow for received
   client messages:

     1. If no Client Identifier option is present in the client's
        message, relay the message to all configured servers
        regardless of hash bucket assignments.

     2. Otherwise, use the hash algorithm described in [3] on the DUID
        in the Client Identifier option and relay the message to the
        server or servers assigned that hash bucket.

   Relay agents MUST be configured to forward client requests to all of
   the DHCPv6 servers that may be part of a load balancing group.

   Note: If relay agents are configured to do load balancing, the
   Elapsed Time option will be ineffective in allowing any server (not
   just the servers in the load balancing group) to respond to a
   client's request.

7. 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

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   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.

8. 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.

9. 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 Lindstrom,
   Thomas Narten, Anders Strand, and Jack Wong.


   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Droms (ed.), R., Bound, J., Volz, Bernie, Lemon, Ted, Perkins,
        C., Carney, M., "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", draft-ietf-dhc-dhcpv6-26 (work in progress), June

   [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

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