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

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

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

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

Component: Message Transfer Agent

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.

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: