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Q131736: TCP/IP: Load Balancing vs. Distributed Network Sessions

Article: Q131736
Product(s): Microsoft Windows NT
Version(s): 3.5,3.51,4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 19-MAR-2002

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The information in this article applies to:

- Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0 
- Microsoft Windows NT Server versions 3.5, 3.51, 4.0 
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SUMMARY
=======

If you have a multihomed computer, the Windows NT implementation of TCP/IP will
help to distribute the number of connections, or sessions among the adapters
according to a randomizing algorithm. However, truly balancing the network
throughput among multiple network interface cards (NICs) has no current
practical implementation on PC networks.

MORE INFORMATION
================

Perfect load balancing, meaning even network bandwidth on all NICs, is not
possible. It would necessitate connection oriented network traffic to be
dynamically switched between NICs.

However, Windows NT is able to distribute connections from clients across
multiple NICs. This helps to spread out the network traffic.

Examples Without WINS
---------------------

Case 1:

You have multiple NICs in a computer which are connected to separate segments.
These segments are not connected by any routers or bridges. No load balancing or
distribution of connections takes place at all.

Case 2:

You have a multihomed server, and all of the clients have equal access through
the network to any of the NICs in the server. Configure each NIC in the server
for its own subnet and distribute the clients among those same subnets. Again,
the load balancing achieved here is by virtue of physically distributing the
clients across multiple subnets since IP traffic from the clients will be sent
to the NIC which has the same subnet address as the client.

Case 3:

In this case, all clients have equal network access to any of the NICs on the
multihomed computer. Let all of the client's and server's NICs be on the same
subnet. A client sends out a broadcast name query. This request will be received
by all of the server's adapters. All of the server's adapters will respond to
the client's query and each server adapter's response will contain only one
returned address: its own. The client connects with the first adapter to respond
and the client drops response packets from the server's other adapters.

Case 4:

Assume that you have the same setup as case 3. Clients have equal access to all
of the server's NICs, and all clients and server NICs are on the same subnet.
Here you can help to distribute the client connections between the server NICs
by enabling the RandomAdapter in the Registry. In this way, each NIC will still
respond to the name query, but each NIC will choose the IP address randomly from
all the NICs on the server. This helps to distribute network sessions among the
NICs, but does not necessarily balance the load since network traffic may vary
greatly between the sessions.

WARNING: Using Registry Editor incorrectly can cause serious, system-wide
problems that may require you to reinstall Windows NT to correct them. Microsoft
cannot guarantee that any problems resulting from the use of Registry Editor can
be solved. Use this tool at your own risk.

To do this, you will need to add the parameter RandomAdapter under the following
subkey:

 HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netbt\Parameters


For additional information, please see the following article in the Microsoft
Knowledge Base:

  Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows
  NT

Examples with WINS Enabled
--------------------------

Case 1:

All your clients are running Windows NT, Windows for Workgroups with TCP/IP- 32,
or Windows 95.

Assume all clients and all NICs for the multihomed computer are on the same
subnet, and let all clients have equal access to any NIC on the server. Assume a
WINS server exists on the network and the WINS server is not the multihomed
server.

In this case the multihomed computer has registered each of its NIC addresses
under the same NetBIOS name with the WINS server. That is, it registered itself
as a multihomed computer. When a WINS client sends a name query, it goes to the
WINS server, and the WINS server returns a list of all IP addresses registered
for the multihomed computer. The WINS client chooses from this list of multiple
IP addresses according to the following algorithm:

1. Match the NIC's subnet address with the client's address.

2. Or, if step 1 fails, match the network address of the NIC with the client's
  network address.

3. If both steps 1 and 2 fail, choose randomly from the server's list of all
  adapter IP addresses. If both steps 1 and 2 result in multiple matches,
  choose randomly from the matching list.

NOTE: The RandomAdapter parameter has no effect for these WINS clients.

Case 2:

You have clients on the WINS network which are not WINS aware or which do not
have the WINS feature enabled. These clients respond to the RandomAdapter
parameter set on the multihomed computer in the same way as Case 4 above. In
fact, the RandomAdapter flag only applies for broadcast name queries from
non-WINS aware or non-WINS enabled clients (for example, a Windows NT computer
with WINS turned off).

Additional Notes
----------------

Employing multiple NICs may not actually improve network throughput in some
cases. Suppose, for example, that multiple NICs are configured to contend for
traffic on the same segment or wire. The improved efficiency of today's NICs are
such that slow downs may occur because of lack of bandwidth on the wire before a
single NIC loses efficiency in handling the load. Today's standard ethernet wire
transfer data at 10 MB/sec, but bandwidth for current NIC technology far exceeds
this rate. A more effective way to improve throughput may be to connect a single
ethernet NIC to a 100 MB/sec ethernet link.


Additional query words: prodnt TCP

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT351search kbWinNT350search kbWinNT400search kbWinNTW350 kbWinNTW350search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTS351 kbWinNTS350 kbWinNTS351search kbWinNTS350search
Version           : :3.5,3.51,4.0

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