XCON: MTA Only Allows 64 TCP/IP or TP4 Connections (166602)

SYMPTOMS

The Microsoft Exchange Server message transfer agent (MTA) has a limit on the number of allowed socket control blocks for TCP and TP4. This limit is currently a shared maximum of 64 (0x40). The minimum number that must be allocated to either TCP or TP4 is 2, which means that the maximum any one transport stack can have allocated is 62 (0x3E). The default is set to 20 (0x14) for each.

When the limit is reached, the following error message is logged in the Windows NT event log:

   MSExchangeMTA   Warning   Resource   9156
   A resource limit has been reached while attempting to open an
   association. There are no free control blocks available for network type
   1. The configured count is 40. [BASE IL MAIN BASE 1 282] (10)

STATUS

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 4.0. This problem has been corrected in the latest U.S. Service Pack for Microsoft Exchange Server version 4.0. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.0. This problem has been corrected in the latest U.S. Service Pack for Microsoft Exchange Server version 5.0. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

MORE INFORMATION

The MTA uses one control block for each connection (whether remotely or locally initiated). It also uses one to listen for incoming connections.

If slow links (slower than 128Kbps) and/or large messages are involved and message backlogging is therefore likely, you can allow 10 control blocks per X.400-Link. This enables MTA traffic to be handled more efficiently by virtue of multiple connections to each remote link, without suffering from a resource shortage.

NOTE: There is no requirement to change the TP4 Thread or TCP/IP Thread values; the code will self-adjust based on the count of control blocks.

The following registry values hold the TCP/IP and TP4 control block settings:

   HKEY_LOCAL_MACHINE\System\CurrentControlSet\ 
   Services\MSExchangeMTA\Parameters\TCP/IP Control blocks

   HKEY_LOCAL_MACHINE\System\CurrentControlSet\ 
   Services\MSExchangeMTA\Parameters\TP4 Control blocks

With the fix implemented, a maximum total of 2,000 control blocks is allowed.

NOTE: If X.25 is also being used as a messaging protocol stack, the total of X.25 + TCP/IP control blocks can be no greater than 1,250.

If these values are increased beyond the acceptable ranges, the MTA will fail to start. For more information, please see the following article in the Microsoft Knowledge Base:

168815 XCON: MTA Doesn't Start, Event IDs 9228 and 4300