Microsoft KB Archive/229643

= XCON: 9156 Errors Occur After Applying Exchange 5.5 Service Pack 2 =

Article ID: 229643

Article Last Modified on 10/26/2006

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APPLIES TO


 * Microsoft Exchange Server 5.5 Standard Edition

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This article was previously published under Q229643



SYMPTOMS
After you apply Service Pack 2 for Exchange Server 5.5, the following error messages may be found in the Event Viewer application log:

Event ID: 9156

Source: MSExchangeMTA

Type: Warning

Category: Resource

Description: 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)

After you receive this error message, the message transfer agent (MTA) may stop processing messages and require a restart of the service to get mail flowing again.



CAUSE
The MTA leaks control blocks. Under stress, or slow link conditions, this problem becomes apparent.



RESOLUTION
To resolve this problem, obtain the latest service pack for Exchange Server version 5.5. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

191014 XGEN: How to Obtain the Latest Exchange Server 5.5 Service Pack

The English version of this fix should have the following file attributes or later:

Component: Message Transfer Agent



STATUS
Microsoft has confirmed that this is a problem in Microsoft Exchange Server version 5.5. This problem was first corrected in Exchange Server 5.5 Service Pack 3.



MORE INFORMATION
The MTA uses one control block (CB) for each association (whether remotely or locally initiated). It also uses one CB to listen for incoming associations.

The problem occurs when an association is active and waiting for a response to a Turn-Please Request, and the remote side sends an association recovery inbound. Because the association is still active, recovery handling does not work properly, and the upper layer CBs for the association are released (and eventually reused by other associations) while the lower layers remain unaware of this fact.

Later, the lower layers get closed down and POP-4 sends a Close Request up to the Platform Kernel CB, which is now used by some other association. The Close Request is ignored so no Close Response is sent back and so the POP-4 CB is not closed down properly (or leaked).

Much later on, an Open Request comes down from Platform from a Kernel CB, which matches the old partner of the leaked POP-4 CB. This causes the Open Request to be ignored by POP-4 as a Duplicate Open. The Open Request then gets passed on to the Winsock layer by the Base Open handling and is logged as a 9156 error. For more information, please see the following articles in the Microsoft Knowledge Base:

193894 XCON: MTA Stops Processing Messages and Generates 9156 Events

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

Additional query words: 9156 restart mta exchange bridgehead slow link

Keywords: kbbug kbfix kbqfe KB229643

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