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Q161425: WinNT 4.0 SP2 Multihomed Computer Connection Enhancement

Article: Q161425
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 21-FEB-2002

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The information in this article applies to:

- Microsoft Windows NT Workstation version 4.0 
- Microsoft Windows NT Server version 4.0 
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SYMPTOMS
========

When one of the network interfaces on a multihomed computer is out of service,
it is impossible to establish a NetBIOS-over-TCP/IP connection to the multihomed
computer.

CAUSE
=====

NetBIOS over TCP (NetBT) only binds to one IP address for each physical network
interface. From the NetBT viewpoint, a computer is multihomed only when it has
more than one network interface card (NIC) installed. When a name registration
packet is sent from a multihomed computer, it is flagged as a multihomed name
registration so that it will not conflict with the same name being registered by
another interface in the same computer.

If a broadcast name query is received by a multihomed computer, all
NetBT/interface bindings receiving the query will respond with their addresses,
and, by default, the client will choose the first response and connect to the
address supplied by it. This behavior can be controlled by the RandomAdapter
registry parameter.

When a directed name query is sent to a Windows Internet Name Service (WINS)
server, the WINS server responds with a list of all IP addresses that were
registered with WINS by the multihomed computer.

Choosing the best IP address to connect to on a multihomed computer is a client
function. The following algorithm is employed, in the order listed:

- If one of the IP addresses in the name query response list is on the same
  logical subnet as the calling binding of NetBT on the local computer, that
  address is selected. If more than one of the addresses meets this criteria,
  one is picked at random from those that match.

- If one of the IP addresses in the list is on the same logical subnet as any
  binding of NetBT on the local computer, that address is selected. If more
  than one of the addresses meets this criteria, one of these is picked at
  random.

- If none of the IP addresses in the list is on the same subnet as any binding
  of NetBT on the local computer, an address is selected at random from the
  list.

This algorithm provides a reasonably good way of balancing connections to a
server across multiple NICs, while still favoring direct connections when they
are available.

However, with computers running versions of Windows NT 4.0 prior to Windows NT
4.0 Service Pack 2 (SP2), if the IP address chosen from the list does not
respond, the connection attempt will fail. In some cases (specifically, the ones
listed above where an address was selected at random), a second or third attempt
to connect the resource may succeed; however, the user or application may
receive an error and the retries may need to be instigated manually.

RESOLUTION
==========

Windows NT 4.0 SP2 includes an enhancement to NetBT. NetBT still uses the
algorithm listed above to choose a best IP address to connect to on a multihomed
computer; however, now it retains the list of addresses and orders them by
preference. It then attempts to ping each of the addresses in the list in order,
until one of them responds. Two ping attempts are made for each address, with a
two-second wait for a response after each attempt. If there is a successful
response to one of the pings, then a TCP connection and NetBIOS session are
established to that address. If none of the addresses respond to the ping
attempts, NetBT will revert to the old behavior, and attempt to establish a TCP
connection to the best address in the list.

In addition, if a session is already in existence and the network interface
responsible for the session on the multihomed computer fails, the failure will
be detected and a new session will be established over one of the working
interfaces, provided that one exists. The status of open files may not be
preserved, as file handles will be invalidated when the old session is deleted.

NOTE: This feature is specific to NetBT sessions, and does not apply to TCP
connections used by other higher-level protocols or interfaces, such as Windows
Sockets.

An additional enhancement was added in a hotfix after Windows NT 4.0 SP3 was
released that provides better load balancing between NICs on a multihomed server
and fixes some cases where a failed NIC on the server could prevent the client
from connecting, even though other NICs might still be available.

As a result, NetBT now uses the following algorithm for selecting an address from
the list returned by WINS:

1. Check for addresses on local subnets or networks:

  a. If one or more of the addresses in the name query response list are on the
     same logical subnet as the calling binding of NetBT on the local computer,
     these addresses are put into a sublist of addresses.

  b. If no addresses were found in step 1a, and then if one or more of the
     addresses in the list are on the same logical subnet as any binding of
     NetBT on the local computer, those addresses are put into a sublist.

  c. If no addresses were found in steps 1a or 1b, and then if one or more
     addresses in the list are on the same logical network as any binding of
     NetBT on the local computer, those addresses are put into a sublist.

2. Randomize the lists:

  a. If a sublist was generated in step 1, that list is put in a random order
     and the remaining addresses in the name query response list are added to
     the end of that list, also in a random order.

  b. If no sublist was generated in step 1, the main list is sorted in a random
     order.

3. Picking the address:

  a. The client will take the new, randomized list and ping the first address
     on that list; if the server responds to the ping, that address will be
     used.

  b. If pinging the first address fails, the next address in the list will be
     pinged, until a good address is found.

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

  ARTICLE-ID: Q182781
  TITLE : Client Connections to Multihomed Server Not Load Balanced

STATUS
======

Windows NT 4.0 Service Pack 2 includes an enhancement to NetBT that improves the
ability to connect to multihomed computers even when one or more of the network
interfaces are disabled. There are currently no plans to port this enhancement
back to prior versions of Windows NT.


Additional query words: multi-homed multi homed

======================================================================
Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : :4.0
Issue type        : kbbug

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