Max Concurrent Connections in WinNT/2k/XP ?

snakefoot · Mar 6, 2004

After reading some articles at several P2P networks, then there was a certain conviction that configuring the P2P for several 1000 connection were completely out of their mind.
Since I have feeling that Microsoft have intended that Windows NT should be able to handle more than a 1000 connections, then I looked around for settings which affected max concurrent connections.

Are the findings below correct ? And are there settings that I have overlooked ?
When a client tries to make a connection to a server, then an invisible/implicit bind is made to a local anonymous (ephemeral) port number. The default range for anonymous ports in Windows is 1024 to 5000, thus giving 3977 concurrent connections. It is possible to change the upper limit with this DWORD registry key:
Code:
[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxUserPort = 10000 (Default = 5000, Max = 65534)
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Even when not having 3977 concurrent connections, then it is still possible to run out of available port numbers. This can happen if quickly opening and closing connections, because after a connection is "closed" it enters the state TIME_WAIT, and will continue to occupy the port number for 4 minutes (2*Maximum Segment Live, MSL) before it is actually removed. This behavior is specified in RFC 793 to make sure the other party has receive confirmation that the socket is closed. It is possible to change how long a socket should be in in TIME_WAIT state before it is removed:
Code:
[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
TcpTimedWaitDelay = 240 (Default = 240 secs, Range = 30-300)
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For each connections a TCP Control Block (TCB - Data structure) is maintained, the TCB for all the active connections are stored in a hash table. The hash table is optimized for finding a certain active connection, but if the table is too small compared to the total amount of active connections, then extra CPU time is required to find a connection. Configure the size of the hash table with this DWORD registry key (Is allocated from pagepool memory):
Code:
[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxHashTableSize = 1024 (Default = 512, Range = 64-65536)
Note Microsoft recommends for a multiprocessor environment, that the value should not be higher than the maximum amount of concurrent connections, and if this is the case then the registry key
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The TCP Control Blocks (TCBs) are pre-allocated, to avoid spending time on allocating/deallocating the TCBs every time connections are created/closed. The reuse/caching of TCBs improves improves memory management, but also limits how many connections TCP can support simultaneously (Active + TIME_WAIT). Configure the amount of pre-allocated TCBs with this DWORD registry key (Is allocated from pagepool memory):
Code:
[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxFreeTcbs = 2000 (Default = RAM dependent but usual 1000-2000)
Note in Win2k an emergency cleanup feature was added to decrease the chance of running out of pre-allocated TCBs. If more than a 1000 connections (MaxFreeTWTcbs) are in the TIME-WAIT state, then the connections that have been in TIME-WAIT state for more than 60 secs are closed forcefully and the corresponding TCB is marked free for use.
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Max Concurrent Connections in WinNT/2k/XP ?

snakefoot Sergeant Major