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Q171564: TCP/IP Dead Gateway Detection Algorithm Updated for Windows NT

Article: Q171564
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbnetwork kbWinNT400sp4fix
Last Modified: 09-AUG-2001

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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 
- Microsoft Windows NT Server version 4.0, Terminal Server Edition 
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SYMPTOMS
========

You may experience a loss of connectivity to computers across the router when
your primary gateway fails, even though your computer running Windows NT is
configured with multiple default gateways for redundancy.

CAUSE
=====

Windows NT 4.0 supports dead gateway detection using the Triggered Reselection
method described in RFC 816. This implementation can cause the current default
gateway to switch to the second or third configured gateway very easily;
however, it does not test the gateway being switched to prior to making the
change.

RESOLUTION
==========

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or
Windows NT Server 4.0, Terminal Server Edition. For additional information,
please see the following article in the Microsoft Knowledge Base:

  Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack


STATUS
======

Microsoft has confirmed this to be a problem in Windows NT 4.0 and Windows NT
Server 4.0, Terminal Server Edition. This problem was first corrected in Windows
NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition
Service Pack 4.



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

The description and explanation of the Windows NT 4.0 dead gateway detection
algorithm follows:

- When any TCP connection that is routed through the default gateway has
  attempted to send a TCP packet to the destination a number of times equal to
  one-half of the registry value TcpMaxDataRetransmissions, but receives no
  response, the algorithm advises IP to switch to the next gateway in the
  list.

  The new algorithm, however, is more cautious about advising IP to switch
  gateways:

- When any TCP connection that is routed through the default gateway has
  attempted to send a TCP packet to the destination a number of times equal to
  one-half of the registry value TcpMaxDataRetransmissions, but receives no
  response, the algorithm changes the Route Cache Entry (RCE) for that one
  remote IP address to use the next default gateway in the list. When 25
  percent of the TCP connections have moved to the next default gateway, the
  algorithm advises IP to change the default gateway for the whole computer to
  the one that the connections are now using.

Example of the New Algorithm
----------------------------

Assume that there are currently TCP connections to 11 different IP addresses that
are being routed through the default gateway. Now assume that the default
gateway fails, that there is a second default gateway configured, and that the
value for TcpMaxDataRetransmissions is at the default of 5.

When the first TCP connection tries to send data, it will not receive any
acknowledgments. After the third retransmission, the RCE for that remote IP
address will be switched to use the next default gateway in the list. At this
point, any TCP connections to that one remote IP address will have switched
over, but the remaining connections will still try to use the original default
gateway.

When the second TCP connection tries to send data, the same thing will happen.
Now, two of the 11 RCEs will point to the new gateway.

When the third TCP connection tries to send data, after the third retransmission,
three of 11 RCEs will have been switched to the second default gateway. Since,
at this point, over 25 percent of the RCEs have been moved, the default gateway
for the whole computer will be moved to the new one.

At this point, that default gateway remains the primary one for the computer
until it experiences problems (causing the dead gateway algorithm to try the
next one in the list again) or until the computer is restarted.

When the end of the list of default gateways is reached, the search returns to
the beginning again.

Additional query words:

======================================================================
Keywords          : kbnetwork kbWinNT400sp4fix 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbNTTermServ400 kbNTTermServSearch
Version           : winnt:4.0
Issue type        : kbbug
Solution Type     : kbfix

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