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CONFIGURE THE MAX LIMIT FOR CONCURRENT TCP CONNECTIONS

March 6, 2004 by Snakefoot | 15 Comment

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 (MS KB812873) 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 KB196271
> More Info MS KB319502
> More Info MS KB319504
> More Info MS KB328476
> More Info MS KB836429

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 KB151418
> More Info MS KB224585

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 KB137984
> More Info MS KB149532
> More Info MS KB832954
> 
> 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 KB140325

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 KB296265

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 KB127144

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 www.LvlLord.de
> 
> 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 KB969710

Related No more than 10 connections to a remote computer

TAGS:

 * crippled, ephemeral, tcpip

CATEGORY:

 * Network,
 * Network,
 * Network,
 * Network


COMMENTS:

 1.  twl says:
     7 March 2009 at 1:32
     
     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.
     
     Reply
     
 2.  Talgat-_- says:
     5 January 2010 at 17:00
     
     hi guys! im from kz... my provider is megaline.kz 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
     taxa_91_virtual@inbox.ru thx
     
     Reply
     
 3.  James Watt says:
     27 February 2010 at 4:51
     
     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.
     
     Reply
     
 4.  Snakefoot says:
     2 March 2010 at 0:32
     
     > 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
     
     Reply
     
 5.  Emrah says:
     2 August 2010 at 17:52
     
     Hi,
     
     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?
     
     Reply
     
 6.  Yehuda says:
     26 May 2011 at 18:27
     
     Very nicely written
     
     ill try to apply it on my production servers
     
     Thanks
     
     i
     
     Reply
     
 7.  Tasneem says:
     30 May 2012 at 10:49
     
     Brilliant information. Thinking of making these changes on our servers for
     our clients. Thank You!
     
     Reply
     
 8.  SUNDAR says:
     13 September 2013 at 8:51
     
     Hi,
     
     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
     
     Reply
     
 9.  AndrewK says:
     7 November 2013 at 4:07
     
     Hi,
     
     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?
     
     Reply
     
 10. Duder says:
     21 January 2014 at 18:59
     
     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.
     
     Reply
     * Lame says:
       18 May 2015 at 4:36
       
       Duder you moron, you are replying to an article that was written 10 years
       ago, obviously its outdated and you're mentioning about Win7?
       
       And I'm also a moron for replying to your comment a year after.
       
       But I just had to.
     
       
     * Oly says:
       11 February 2019 at 16:37
       
       Duder obviously knows nothing about Windows or TCP. This article is still
       as relevant and useful as it ever was.
     
       
     
 11. ho909 says:
     4 December 2015 at 1:25
     
     MaxFreeTWTcbs = 1000 - ????
     
     MaxFreeTWTcbs = 65000 !
     
     Reply
     
 12. Paul says:
     28 February 2016 at 16:22
     
     Hello- I am really desperate as we built a chat application that uses
     websocket and stomp protocol, after about 1500 connections, users cannot
     connect anymore.
     I believe it has to do with some limit set to the number of concurrent open
     to connections by the OS, but I am not sure how to increase this limit or
     to determine if this really is the cause.
     We are running our application on Windows 2008 server, the app is written
     in Java and hosted in Tomcat.
     Any help would be very much appreciated.
     Thanks in advance
     
     Reply
     * Snakefoot says:
       5 March 2016 at 10:56
       
       TomCat/Apache can work in two modes. BIO - One thread per connection. NIO
       - multiple connections for each thread.
       
       Setup your connector in server.xml to use NIO. Http11Nio2Protocol (Tomcat
       8+) or Http11NioProtocol (Tomcat 6+) and configure maxConnections.
       
       For a chat-application consider to use small connector-buffers
       (socket.appWriteBufSize="1024" and socket.appReadBufSize="1024") to lower
       footprint for each chat-connection.
     
       
     


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