NetBT Improvements for Connections Between Multihomed Computers

ID: Q216471

The information in this article applies to:
  • Microsoft Windows NT Server version 4.0
  • Microsoft Windows NT Workstation version 4.0

SYMPTOMS

When connecting from a multihomed computer to another multihomed computer, where both computers have two separate networks in common, the optimal route may not be chosen. Additionally, the outbound path may differ from the inbound path.


CAUSE

When a call is placed using NetBIOS over TCP/IP (NetBT), for example, "net use \\mycomputer\myshare" on a multihomed computer, the outbound calls are placed on both interfaces. Each interface should query WINS and then choose the best target IP address for the called computer. Two improvements are now available in this algorithm.


RESOLUTION

The algorithm used in determining the best target IP address has been changed to include two improvements, available in this new version of Netbt.sys.

Previously, a match was chosen in the order below. For additional information, please see the following article(s) in the Microsoft Knowledge Base:

Q161425 WinNT 4.0 SP2 Multihomed Computer Connection Enhancement
  1. Check for a matching subnet address for the calling interface.


  2. Check for a matching subnet address for any interface in the computer.


  3. Pick a target address randomly.


The new version adds one more check, as shown here:
  1. Check for a matching subnet address for the calling interface.


  2. Check for a matching network address for the calling interface.


  3. Check for a matching subnet address for any interface in the computer.


  4. Pick a target address randomly.


Also added is support for a "per-interface" NetBIOS name cache. The per-interface cache allows each adapter to choose target IP addresses for multihomed computers independently, instead of picking up an IP address from the cache that was chosen in the context of another adapter.

To illustrate:

                              ---------
           ------------------| Router1 |----------------
          | Net1SubnetA       ---------     Net1SubnetB |
          | NIC1                                   NIC1 |
    -----------                                     -----------
   | Computer1 |                                   | Computer2 |
    -----------                                     -----------
          | NIC2                                   NIC2 |
          | Net2SubnetA       ---------     Net2SubnetB |
           ------------------| Router2 |----------------
                              ---------

In the scenario shown above, if a NetBT call is placed from Computer1 to Computer2, a call will go out each interface (NIC1 and NIC2). NIC1 will query WINS, and, assuming that WINS returns an IP address for each interface on Computer2, it will choose the IP address associated with Computer2NIC1 as the best IP address to connect to, because both share the same network ID (even though the subnet ID is different). This IP address will be cached for NIC1 in the per-interface name cache. NIC2 will also query WINS, and assuming that WINS returns an IP address for each interface on Computer2, it will choose the IP address associated with NIC2, and this IP address will be cached in the per-interface name cache for NIC2.

Calls are placed on both interfaces (and the WINS server for each interface is queried separately) because the computer cannot assume that the target computer is reachable on both networks in all cases.

This enhancement can improve the path used for connections from a multihomed computer to another multihomed computer, when both have two networks in common. It does not offer any changes for other scenarios.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:
For information on obtaining the individual software update, contact Microsoft Product Support Services. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
http://www.microsoft.com/support/supportnet/overview/overview.asp


STATUS

Microsoft has confirmed this to be a problem in Windows NT 4.0. This problem was first corrected in Windows NT version 4.0 Service Pack 5.

Additional query words: 4.00 multi-homed multi homed

Keywords : kbbug4.00 kbfix4.00 nt4sp5fix
Version : winnt:4.0
Platform : winnt
Issue type : kbbug


Last Reviewed: August 27, 1999
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