XCON: Exchange MTA TCP/IP and TP4 connection Limits

The information in this article applies to:

• Microsoft Exchange Server, version 4.0
• Microsoft Exchange Server, version 5.0

SYMPTOMS

The Microsoft Exchange message transfer agent (MTA) is capable of 64 concurrent connections using TCP/IP and TP4 protocols. When a single MTA reaches this limit, all inbound (and outbound) connections are refused until a control on TCP/IP or TP4 becomes available. The control blocks allocation between TCP/IP and TP4 can be adjusted so that one protocol is given more control blocks. However, the maximum combined control blocks cannot exceed 64.

CAUSE

In Microsoft Exchange Server 5.0 Service Pack 1, the maximum number of concurrent connections for the MTA is increased from 64 to 1024 total.

RESOLUTION

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall Windows NT. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

Connections control blocks can be changed in the Windows NT registry by using Windows NT Registry Editor. To modify the MTA connection limits, do the following:

1. Start Windows NT Registry Editor. Use Regedt32.exe if you are running Windows NT 3.51 and either Regedit.exe or Regedt32.exe for Windows NT 4.0.

2. Modify the following registry values:

   HKEY_Local_Machine\System\CurrentControlSet\Services\ MSExchangeMTA\parameters

   Value Name: TCP/IP control blocks Data Type: REG_DWORD

      Value:      2 to 1024
   
   
      Value Name: TP4 control blocks
      Data Type:  REG_DWORD
      Value:      2 to 1024
   
      NOTE: If you choose to use both TCP/IP control blocks and TP4 control
      blocks, the maximum value for both protocols combined is 1024. The
      Microsoft Exchange MTA fails to start if the maximum value is exceeded.
      For example, you can configure TCP/IP for 1022 connections and TP4 for
      2 connections or any combination of values that does not exceed 1024.
      A minimum of 2 control blocks is required per protocol. The Microsoft
      Exchange MTA fails to start if either of the protocols has fewer than
      2 connections configured, regardless of whether the particular
      protocol is being used.
   
   

3. Exit Registry Editor. Stop and restart the MTA for the change to take effect.

STATUS

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 4.0 and 5.0.

We are researching this problem in Microsoft Exchange Server version 4.0 and will post new information here in the Microsoft Knowledge Base as it becomes available.

This problem was corrected in the latest Microsoft Exchange Server 5.0 U.S. Service Pack. 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