Network Working group   DHC load balancing algorithm    June 2000

Internet Draft                                  Bernie Volz
                                                IPWorks, Inc.

                                                Steve Gonczi
                                                Network Engines, Inc.

                                                Ted Lemon
                                                Internet Engines, Inc.

                                                Rob Stevens
                                                Join Systems, Inc.

June 2000                                       Expires Dec 2000

                DHC load balancing algorithm

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
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.


This draft 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 MAC
addresses, when multiple DHCP servers are available to service DHCP
clients. The benefits are similar to those enumerated in [SSO-03], but
this draft does not require modifications to existing DHCP clients. The
same method is proposed to select the target server of a forwarding
agent such as a BOOTP relay.

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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",
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.

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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

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.

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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

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 serber 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.

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For example, if a server receives a HBA with 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.

The format of the option MUST be as follows when used with the DHCP
Failover [FAILOVR] Protocol:

        Code        Len         Hash Buckets
       |  0  |  11 |  0  |  32 | b1 |  b2 | ... | b32 |

The option code and length are 2 byte NBO values. The option number is
assigned in the option number space of [FAILOVR].

5.3 Delayed Service parameter

The Delayed Service parameter is optional. If it is not sent, the HBA
sets up a strict Serve/Do not serve policy.

If the parameter is used, it MUST be sent immediately before the HBA.
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. If the parameter is used, its format MUST be as

        Code        Len         Secs
       |  0  |  10 |  0  |  1  | S  |

The option code and length are 2 byte NBO values. The option number is
assigned in the option number space of [FAILOVR]. S is a one byte value,
1..255. It represents the number of seconds to delay service. The server
MAY serve a client after S seconds elapsed from the client's first
request, even if the HBA and STID hash indicate the server should not

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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

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:

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/* 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(unsigned char *key,/* The key to be hashed */
                           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 );

An example of how to check if we should service a transaction:

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);

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7.  Security

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., Volz, B., "DHCP Failover  Protocol", Internet
             Draft <draft-ietf-dhc-failover-07.txt>, June 2000.

  [PEARSON]  The Communications of the ACM  Vol.33, No.  6 (June 1990),
             pp. 677-680.

  [RFC2131]  R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
             March 1997.

  [RFC2219]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels," RFC-2219, March 1997.

  [SSO-03]   Stump, G., Gupta, P., Droms, R. Sommerfeld, R.
             "The Server Selection Option for DHCP"

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

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.

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10.  Full Copyright Statement

Copyright (C) The Internet Society (2000). 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
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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. This document and the information contained
herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE

11.  Author's information

Bernie Volz
IPWorks, Inc.
959 Concord Street Framingham, MA 01701
Phone: (508)-879-4785

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

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