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Q151418: WinNT TCP/IP May Reuse TIME-WAIT Connections Prior to 2MSL

Article: Q151418
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 22-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
========

Under sufficiently high TCP connection rates the Windows NT TCP/IP stack may
reuse TCP Control Blocks that are in the TIME-WAIT (also called the 2MSL state)
table. This can happen on a heavily loaded Web or FTP server when using graceful
TCP closes. Reuse of the Control Blocks may terminate a TCP connection in its
TIME-WAIT state prior to the 2MSL time period expiring.

There have been no reports of problems related to this symptom; however the
behavior is not RFC-compliant.

CAUSE
=====

In Windows NT the system starts to reuse TCP Control Blocks in TIME-WAIT when
the number of active connections goes over 1000. Windows NT 4.0 Service Pack 3
increased this threshold to 2000.

For more information on configuring the TIME-WAIT period for Windows NT, please
see the following article(s) in the Microsoft Knowledge Base:

Q149532Windows NT Clients Run Out of Ports

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0. A
supported fix is now available, but has not been fully regression-tested and
should be applied only to systems experiencing this specific problem. Unless you
are severely impacted by this specific problem, Microsoft recommends that you
wait for the next Service Pack that contains this fix. Contact Microsoft
Technical Support for more information.



This hotfix allows sites with heavy TCP connection rates to configure the
threshold of active TCP connections necessary before TCP control blocks are
reused.

There are two registry keys that control the use of the TIME-WAIT table:

- MaxFreeTcbs is the threshold number of active TCP connections required before
  TCP Control Blocks in the TIME-WAIT state are reused. Changing MaxFreeTcbs
  allows the system to avoid reusing TCP Control Blocks.

     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip
        \Parameters\MaxFreeTcbs

     Value Type: REG_DWORD
     Valid Range: 0-0xFFFFFFFF
     Default: 1000 (Windows NT 4.0), 2000 (Windows NT 4.0 Service Pack 3)
     Description: Controls the number of active TCP connections required
                  before TCP control blocks in the TIME-WAIT state are
                  reused.

- MaxHashTableSize controls the size of an internal system data structure that
  controls how fast the system can find a TCP Control Block. The hash table
  allows the system to access TCP Control Blocks faster and this table should
  grow if MaxFreeTcbs is configured higher than the default value.

     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip
        \Parameters\MaxHashTableSize

     Value Type: REG_DWORD
     Valid Range: 64-65536 (0x40-0x10000)
     Default: 512
     Description: This value should be set to a power of 2 (e.g. 512,
                  1024, 2048, and so on.) If this value is not a power of
                  2, the system will configure the hash table to the next
                  power of 2 value (for example, a setting of 513 is
                  "rounded up" to 1024.) This value controls how fast the
                  system can find a TCP control block and should be
                  increased if MaxFreeTcbs is increased from the default.

Additional query words: 4.00 prodnt

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : :4.0

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