Network Working Group
Request for Comments: 3074
Category: Standards Track
B. Volz
S. Gonczi
Network Engines, Inc.
T. Lemon
Internet Engines, Inc.
R. Stevens
Join Systems, Inc.
February 2001

DHC Load Balancing Algorithm

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2001). All Rights Reserved.


This document proposes a method of algorithmic load balancing. It enables multiple, cooperating servers to decide which one should service a client, without exchanging any information beyond initial configuration.

The server selection is based on the servers hashing client Media Access Control (MAC) addresses when multiple Dynamic Host Configuration Protocol (DHCP) servers are available to service DHCP clients. The proposed technique provides for efficient server selection when multiple DHCP servers offer services on a network without requiring any changes to existing DHCP clients. The same method is proposed to select the target server of a forwarding agent such as a Bootstrap Protocol (BOOTP) relay.

1. Introduction

This protocol was originally devised to support a specific load balancing optimization of the DHCP Failover Protocol [FAILOVR]. The authors later realized that it could be used to optimize the behavior of cooperating DHCP servers and the BOOTP relay agents that forward packets to them. The proposal makes it possible to set up each participating server to accept a preconfigured (approximate) percentage of the client load. This is done using a deterministic hashing algorithm, that could easily be applied to other protocols having similar characteristics.

2. Terminology

This section discusses both the generic requirements terminology common to many IETF protocol specifications, and also terminology introduced by this document.

2.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119].

2.2. Load Balancing Terminology

This document introduces the following terms:

Service Delay, SD

A load balancing parameter, allowing delayed service of a client by a server participating in the load-balancing scheme, instead of ignoring the client.

Hash Bucket Assignments, HBA

A configuration directive that assigns a set of hash bucket values to a server participating in the load-balancing scheme.

Server ID, SID

An identifier that can be used to designate one of the participating Servers. In the context of DHCP, the SID is the IP address or DNS name of the server.

Service Transaction, ST

A set of client-server exchanges that lead to a server providing or denying some service to a client. Example: the DISCOVER/OFFER/ REQUEST/ACK message exchange between a DHCP server and client is a service transaction.

Service Transaction ID, STID

An attribute of the individual client requests used for load- balancing.

3. Background and External Requirements

Because DHCP clients use UDP broadcasts to contact DHCP servers, a client DHCPDISCOVER message may be received by more than one server. All servers receiving such a broadcast may respond to the client, letting the client choose which server it will use.

When a BOOTP relay agent is used, it typically forwards or rebroadcasts client broadcasts to all configured servers, so a similar inefficiency is present.

The optimization described allows a server to be chosen for each such transaction by performing a "serve" / "do not serve" computation. A forwarding agent can perform the same computation to choose a forwarding destination.

In either case, the choice of server can be computed, without the participants having to negotiate who is to respond.

The approach is probabilistic in nature, because it is nearly impossible to foresee which client will request service next. For short periods of time, the actual percentage of clients served by a given server will likely deviate from the desired percentage. As the number of requests grows, the actual percentage of the load being handled by each server will approximate the configured percentage.

4. Overview

DHCP servers MUST use the Client Identifier option as the STID if it is present. If no Client Identifier option is present, the hlen field of the DHCP packet MUST be used as the length of the data to be hashed, and the contents of the chaddr MUST be the data to be hashed. At most the first sixteen bytes of the Client Identifier or chaddr are used.

The proposal maps the STID into a hash value using the function in section 6. The resulting hash value can then be used to decide who should respond to the request, or who the forwarding target should be.

The provided hash function generates hash values 0 to 255, and yields a fairly even hash bucket distribution for random STID-s, and also for STID sequences that have some pattern. Resource allocation is accomplished by assigning a set of specific hash values to each participating server.

A server will only service a request if the STID hash of the request matches one of its assigned hash values.

Any hash buckets not assigned to servers will result in some client ST-s being entirely ignored. (In some scenarios, this may be a desirable outcome.) STID-s need not be unique, but should have sufficient variety to distribute load to each server.

HBA-s MAY be transmitted as messages, encapsulated in messages of some other protocol, e.g., e-mail, or DHCP Failover Protocol option.

DHCP server implementations may optionally be configurable to handle a case where load balancing is being done but the server that is supposed to respond is not available, or is out of suitable addresses.

DHCP server implementations that provide this capability SHOULD set the DS (Delayed Service) configuration parameter to the number of seconds to wait after the client's first request has been sent before responding to a client, where the hash would not normally permit the client to be served.

A DHCP server providing this capability SHOULD use the value in the secs field of the client request if its value is not zero. Because some clients may not correctly implement the secs field, a DHCP server MAY keep track of the first instance of a client transaction to which it would not normally respond. If the server receives a request from a client that has the same transaction ID as a previously recorded request, and if the secs field in the second packet is zero, the DHCP server MAY use the elapsed time (seconds) between the first and subsequent client request, instead of the secs field.

5. Operation

5.1 Configuration

The configuration step consists of assigning hash values to available servers. This is accomplished by providing one or more Hash Bucket Assignments (HBA-s). These may come from a configuration file, the Windows NT registry, EEPROM, etc. Alternatively, the hash bucket values could be assigned using some agreed upon algorithm. E.g., "Every odd value is serviced by server A and every even value is serviced by server B".

5.2 HBA Intended for a Server

When configuring one specific server, an HBA in the form of a simple bit map of 32 octet values SHOULD be used.

The first octet in the HBA bitmap represents HBA values 0-7, the next byte values 8-15, and so on, with the thirty-second octet representing values 248-255. In each octet, the least significant bit in that octet represents the smallest HBA value in that octet.

Each bit of the HBA is associated with one possible hash value. If a bit is set in the map, it means the recipient server MUST service each client request, where the STID yields the corresponding hash value.

For example, if a server is configured with an HBA of the following 32 octets:

            FF FF FF FF FF FF 00 00 ( 0   - 63 )
            FF FF FF FF FF FF FF FF ( 64 - 127 )
            00 00 00 00 00 00 00 00 (128 - 191 )
            00 00 00 00 00 00 00 00 (192 - 255 )

then it MUST service any client requests where the STID hashes into the bucket values of 0 through 47 and 64 through 127.

5.3 Delayed Service Parameter

The Delayed Service parameter is optional.

If the parameter is not configured, the HBA sets up a strict Serve/Do not serve policy.

If the parameter is configured, the server that is not supposed to serve a specific request (based on the HBA and the STID hash), is allowed to respond, after S seconds have elapsed since the client first attempted to get service. A server MAY use the secs field in the BOOTP header for determining the time since the client has been trying to get service, or it MAY track repeated requests some other way.

5.4 HBA Intended for a Forwarder

When configuring a forwarding agent, (e.g., BOOTP relay) HBA-s consisting of pairs of Server-ID / Hash Bucket values MAY be used.

Here, the Server ID (SID) designates the server responsible for the specified Hash Bucket. The forwarding agent forwards each client request, where the STID yields the specified hash value, to the server designated by the SID.

The Server ID may be any unique server attribute, (e.g., IP address, DNS name, etc.) that is meaningful in the context of the relay agent operation.

A forwarder may be configured to forward a given packet to more than one server. For example, a BOOTP relay could be set up to split the load between 2 primary-backup server pairs, each pair running the DHCP Failover Protocol [FAILOVR]. In this case, a packet that is intended for a server pair Will have to be forwarded to both the primary, and the secondary server of the pair.

A possible configuration file for a forwarding agent (e.g., BOOTP relay) may look like this: 0..24;  25..55;  56..128; 129 130 131 200..202;

The above configuration consists of 4 HBA-s. The first HBA example reads: "Any Client request, where the STID yields a hash value 0 to 24, will be forwarded to both server and".

The 4th HBA example states: "Any Client request, where the STID yields a hash value 129,139,131,200,201 or 202, will be forwarded to server

6. Hash Function for Load Balancing

The following hash function is a C language implementation of the algorithm known as "Pearson's hash". The Pearson's hash algorithm was originally published in [PEARSON].

The hash function is computationally inexpensive, requires an array lookup and xor operation for each key byte. To make this proposal work, all interoperable implementations MUST use this hash function, with the set of mixing table values given below:

/* A "mixing table" of 256 distinct values, in pseudo-random order. */

unsigned char  loadb_mx_tbl[256] ={
251, 175, 119, 215, 81, 14, 79, 191, 103, 49, 181, 143, 186, 157,  0,
232, 31, 32, 55, 60, 152, 58, 17, 237, 174, 70, 160, 144, 220, 90, 57,
223, 59,  3, 18, 140, 111, 166, 203, 196, 134, 243, 124, 95, 222, 179,
197, 65, 180, 48, 36, 15, 107, 46, 233, 130, 165, 30, 123, 161, 209, 23,
97, 16, 40, 91, 219, 61, 100, 10, 210, 109, 250, 127, 22, 138, 29, 108,
244, 67, 207,  9, 178, 204, 74, 98, 126, 249, 167, 116, 34, 77, 193,
200, 121,  5, 20, 113, 71, 35, 128, 13, 182, 94, 25, 226, 227, 199, 75,

27, 41, 245, 230, 224, 43, 225, 177, 26, 155, 150, 212, 142, 218, 115,
241, 73, 88, 105, 39, 114, 62, 255, 192, 201, 145, 214, 168, 158, 221,
148, 154, 122, 12, 84, 82, 163, 44, 139, 228, 236, 205, 242, 217, 11,
187, 146, 159, 64, 86, 239, 195, 42, 106, 198, 118, 112, 184, 172, 87,
2, 173, 117, 176, 229, 247, 253, 137, 185, 99, 164, 102, 147, 45, 66,
231, 52, 141, 211, 194, 206, 246, 238, 56, 110, 78, 248, 63, 240, 189,
93, 92, 51, 53, 183, 19, 171, 72, 50, 33, 104, 101, 69, 8, 252, 83, 120,
76, 135, 85, 54, 202, 125, 188, 213, 96, 235, 136, 208, 162, 129, 190,
132, 156, 38, 47, 1, 7, 254, 24, 4, 216, 131, 89, 21, 28, 133, 37, 153,
149, 80, 170, 68, 6, 169, 234, 151

unsigned char loadb_p_hash(
        const unsigned char *key,       /* The key to be hashed */
        const int len )                 /* Key length in bytes  */
unsigned char hash  = len;
int i;

        for (i=len ; i > 0 ;  )
            hash = loadb_mx_tbl  [ hash ^ key[ --i ] ];

return( hash );


int accept_service_request(
        const unsigned char HBA[32],    /* The hash bucket bitmap */
        const unsigned char *key,       /* The service transaction id
        const int len  )                /* length of the above */
unsigned char hash = loadb_p_hash(key,len);
int index          = (hash >> 3) & 31;
int bitmask        = 1 << (hash & 7);

        /* return 1 if we should service this transaction */
        return((HBA[index] & bitmask) != 0);

7. Security Considerations

This proposal in and by itself provides no security, nor does it impact existing security. Servers using this algorithm 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 tampering with the HBA could result in denial of service for some or all clients.

8. References

   [FAILOVR]  Kinnear, K,, Droms, R., Rabil, G., Dooley, M., Kapur, A.,
              Gonczi, S. and B. Volz, "DHCP Failover Protocol", Work in
   [PEARSON]  The Communications of the ACM  Vol.33, No.  6 (June 1990),
              pp. 677-680.
   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels," BCP 14, RFC 2119, March 1997.

9. Acknowledgements

Special thanks to Peter K. Pearson, the author of Pearson's hash who has kindly granted his permission to use his algorithm, free of any encumbrances.

This proposal stems from the original idea of hashing MAC addresses to a single bit by Ted Lemon, during a Failover Protocol discussion held at CISCO Systems in February, 1999. Rob Stevens suggested the potential use of this algorithm for purposes beyond those of the Failover Protocol.

Many thanks to Ralph Droms, Kim Kinnear, Mark Stapp, Glenn Waters, Greg Rabil and Jack Wong for their comments during the ongoing discussions.

10. Authors' Addresses

   Bernie Volz
   959 Concord Street
   Framingham, MA  01701
   Phone: +1-617-513-9060

Steve Gonczi
Network Engines, Inc.
25 Dan Road Canton, MA 02021-2817

   Phone: 781-332-1165

Ted Lemon
950 Charter Street
Redwood City, CA 94043


Rob Stevens
Join Systems, Inc.
1032 Elwell Ct Ste 243 Palo Alto CA 94203

   Phone: (650)-968-4470

11. Full Copyright Statement

Copyright © The Internet Society (2001). All Rights Reserved.

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.



Funding for the RFC Editor function is currently provided by the Internet Society.