Internet Engineering Task Force B. Volz
INTERNET DRAFT Ericsson
DHC Working Group June 2002
Expires: December 18, 2002
Load Balancing for DHCPv6
draft-ietf-dhc-dhcpv6-loadb-01.txt
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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. 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
client.
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 |
+-------+-------+ +-------+-------+
| |
| |
<---------+-------------------------+-------Network--->
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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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
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.
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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.
References
[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
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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