Configure the max limit for concurrent TCP connections

6 March 2004 by Snakefoot | Comment » | Trackback Off
To keep the TCP/IP stack from taking all resources on the computer, there are different parameters that control how many connections it can handle. If running applications that are constantly opening and closing connections (P2P), or are providing a service which many tries to connect to at the same time (Web-server like IIS), then one can improve the performance of these applications by changing the restriction limits.

There is a parameter that limits the maximum number of connections that TCP may have open simultaneously.

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
TcpNumConnections = 0x00fffffe (Default = 16,777,214)

Note a 16 Million connection limit sounds very promising, but there are other parameters (See below), which keeps us from ever reaching this limit.

When a client makes a connect() call to make a connection to a server, then the client invisible/implicit bind the socket to a local dynamic (anonymous, ephemeral, short-lived) port number. The default range for dynamic ports in Windows is 1024 to 5000, thus giving 3977 outbound concurrent connections for each IP Address. It is possible to change the upper limit with this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxUserPort = 5000 (Default = 5000, Max = 65534)

Note it is possible to reserve port numbers so they aren't used as dynamic ports in case one have a certain application that needs them. This is done by using the ReservedPorts (Q812873) setting.

Note Vista changes the default range from 1024-5000 to 49152-65535, which can be controlled with the dynamicport setting using netsh. More Info MS KB929851.

More Info The Cable Guy - Ephemeral, Reserved, and Blocked Port Behavior
More Info MS KB Q196271
More Info MS KB Q319502
More Info MS KB Q319504
More Info MS KB Q328476
More Info MS KB Q836429

For each connection a TCP Control Block (TCB - Data structure using 0.5 KB pagepool and 0.5 KB non-pagepool) is maintained. The TCBs are pre-allocated and stored in a table, to avoid spending time on allocating/deallocating the TCBs every time connections are created/closed. The TCB Table enables reuse/caching of TCBs and improves memory management, but the static size limits how many connections TCP can support simultaneously (Active + TIME_WAIT). Configure the size of the TCB Table with this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxFreeTcbs = 2000 (Default = RAM dependent, but usual Pro = 1000, Srv=2000)

To make lookups in the TCB table faster a hash table has been made, which is optimized for finding a certain active connection. If the hash 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):

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
MaxHashTableSize = 512 (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 (MaxFreeTcbs), also if multiprocessor then it might be interesting to look at the registry-key NumTcbTablePartitions (Recommended value CPU-count multiplied by 4).

More Info MS KB Q151418
More Info MS KB Q224585

If having allocated a 1000 TCBs then it doesn't mean that one will be able to have a 1000 active connections. Especially if the application is 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, and prevents attempts to reconnect to the same party, before the old socket is recognized as closed at both sides. It is possible to change how long a socket should be in TIME_WAIT state before it can be re-used freely:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
TcpTimedWaitDelay = 120 (Default = 240 secs, Range = 30-300)

More Info MS KB Q137984
More Info MS KB Q149532
More Info MS KB Q832954

Note with Win2k the reuse of sockets have been changed, so when reaching the limit of more than 1000 connections in TIME-WAIT state, then it starts to mark sockets that have been in TIME_WAIT state for more than 60 secs as free. It is possible to configure this limit:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
MaxFreeTWTcbs = 1000 (Default = 1000 sockets)

Note with Win2k3 SP1 the reuse of sockets have been changed, so when it has to re-use sockets in TIME_WAIT state, then it checks whether the other party is different from the old socket. Eliminating the need to fiddle with (TcpTimedWaitDelay) and (MaxFreeTWTcbs) any more.

If using an application protocol that doesn't implement timeout checking, but relies on the TCPIP timeout checking without specifying how often it should be done, then it is possible to get connections that "never" closes, if the remote host disconnects without closing the connection properly. The TCPIP timeout checking is by default done every 2 hour, by sending a keep alive packet. It is possible to change how often TCPIP should check the connections (Affects all TCPIP connections):

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \services \Tcpip \Parameters]
KeepAliveTime = 1800000 (Default = 7,200,000 milisecs)

More Info MS KB Q140325

When data is sent/received the data is copied back and forth to non-paged pool memory for buffering. If there are many connections receiving/sending data, then it is possible to exhaust the non-paged pool memory. The max size of the non-paged pool buffer allocated for each connection is controlled by MaxBufferredReceiveBytes or TCPIP Receive Window depending on which is smallest. More Info MS KB Q296265

Note if using the Professional/Home edition of Windows then it is very likely that it is crippled (By Microsoft) not to handle many concurrent TCP connections. Ex. Microsoft have officially stated that the backlog limit is 5 (200 when Server), so the Professional edition is not able to accept() more than 5 new connections concurrently. More Info MS KB Q127144

Note even if having optimized Windows to handle many concurrent connections, then connections might still be refused when reaching a certain limit, in case a NAT-Router/Firewall is placed infront of it, which is unable to handle so many concurrent connections.

Note if having activated SYN-Attack-Protection (Enabled by default in Win2k3 SP1) or installed WinXP SP2, a limit is introduced on how many connection attempts (half-open) one can make simultaneously (XP SP2 & Vista = 10; Vista SP2 = no limit). This will keep worms like blaster and sasser from spreading too fast, but it will also limit other applications that creates many new connections simultaneously (Like P2P).

EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts

More Info

Windows Vista SP2 removes the limit again, but it can be enabled with the following DWORD registry setting:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters]
EnableConnectionRateLimiting = 1

More Info MS KB 969710

Related No more than 10 connections to a remote computer

Updated: 22 May 2009


Comment by twl - 7 March 2009 @ 1:32 Reply

Quite some article there. Hell you lost me couple times on definitions, but considering I did understand the point of article and use of it this is very god information to have.

As it seems ever since tcpip limitations the boxes has been suffering huge lags in connecting/ons while now it seems that these methods compined and bottom level (auto detections) disabled windows actually could work as some sort of decent server.

Comment by Talgat-_- - 5 January 2010 @ 17:00 Reply

hi guys! im from kz… my provider is i have problems with my connection speed also high ping in online games! I tried to call to operator but in my town cant help me! speed is very low until morning to evening. but sometimes in the night speed goes up.. i think that they connect to one internet port many peoples because of this speed is low :( how can i protect my connection or my port? plz help me! sorry for my english… imho i can explain my situation write to my mailbox thx

Comment by James Watt - 27 February 2010 @ 4:51 Reply

I know this information is a bit dated, but I have to give you credit for such an in depth analysis of the Windows TCP/IP stack. Any changes with Windows 7? Also, do 64bit Windows operating systems have different values? Thanks.

Comment by Snakefoot - 2 March 2010 @ 0:32 Reply

James Watt wrote:
Any changes with Windows 7? Also, do 64bit Windows operating systems have different values? Thanks.

Vista/Win7 introduces the Next Generation TCP/IP stack, that requires less fine tuning. Registry settings like TcpWindowSize, MaxFreeTcbs, MaxHashTableSize, NumTcbTablePartitions are now ignored. But for outbound connections there are still “only” 65534 port numbers available.

More Info Performance Tuning Guidelines for Windows Server 2008
More Info TCP/IP Registry Values for Vista and Win2k8

Comment by Emrah - 2 August 2010 @ 17:52 Reply


My company hasgot a 50 terminal server licence but only 5 clients log on at the same time. I check the terminal server configuration->RDP-TCP->properties->network adapter->Maximum connections is 5 I can’t change it. Could you please help me?

Comment by Yehuda - 26 May 2011 @ 18:27 Reply

Very nicely written

ill try to apply it on my production servers



Comment by Tasneem - 30 May 2012 @ 10:49 Reply

Brilliant information. Thinking of making these changes on our servers for our clients. Thank You!

Comment by SUNDAR - 13 September 2013 @ 8:51 Reply


My company hasgot a 50 terminal server licence but only 10 clients log on at the same time. I check the terminal server configuration->RDP-TCP->properties->network adapter->Maximum connections is 10 I can’t change it. Could you please help me

Comment by AndrewK - 7 November 2013 @ 4:07 Reply


Very nicely written article! Recently, all our computers were upgraded to Windows 7. One particular application that runs makes multiple tcp connections to a another application when the connection is made! Then application has to be closed and opened again and sometimes it still makes more than one connection. Any suggestion?

Comment by Duder - 21 January 2014 @ 18:59 Reply

This article is totally useless and doesn’t give any good info on what most people are trying to figure out: the max limit of connections from other computers. All these numbers in the thousands are irrelevant. Windows XP limits it to 10, and Win7 to 20, and it seems some people don’t like to admit they don’t know how to do anything about it, or they confuse it with the half-open connection garbage.

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