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

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Question

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.

Answer

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:

ARTICLE-ID: Q149532
TITLE : Windows 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_MACHINESYSTEMCurrentControlSetServicesTcpipParametersMaxFreeTcbs

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_MACHINESYSTEMCurrentControlSetServicesTcpipParametersMaxHashTableSize

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.

Keywords:NT tcp/ip connections time

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