Microsoft KB Archive/235849

= SNA Server Drops Client LAN Connection, Logs Event 631 on DEC Alpha =

Article ID: 235849

Article Last Modified on 11/8/2003

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APPLIES TO


 * Microsoft SNA Server 4.0
 * Microsoft SNA Server 4.0 Service Pack 1
 * Microsoft SNA Server 4.0 Service Pack 2

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This article was previously published under Q235849





SYMPTOMS
If an SNA application is running on a DEC Alpha processor, on either SNA Server or an SNA Server Windows NT client, the client-server LAN session may unexpectedly stop responding (hang) with the following event

Event ID: 631

Source: SNA Server (or SNA Application)

Description: Invalid message received across the LAN: remote: (IP address):1477 Bad sequence number 183 (expected 184)

where the expected sequence number may be one less or one more than expected. If this occurs while an APPC application is sending or receiving data, the APPC application may hang unexpectedly, or receive the following APPC error message:

primary_rc = F003 (AP_COMM_SUBSYSTEM_ABENDED)

The Event 631 may be logged on the DEC Alpha computer where the SNA Server or SNA client software is running, or on a separate SNA Server or SNA client computer that is communicating to the DEC Alpha computer.



CAUSE
If SNA Server data is being sent and received simultaneously over a single client-server LAN connection (to or from one client), internal "send sequence number" and "receive sequence number" variables may be incremented from different threads at the same instant. If this timing condition occurs on a DEC Alpha computer, one sequence number may fail to be updated, causing SNA Server to incorrectly detect an out-of-sequence message. These sequence number variables occur on consecutive BYTE boundaries within a non-packed structure. The DEC Alpha appears to improperly lock one of the BYTE values during a increment operation on an adjoining, separate BYTE value, preventing it from being updated.



RESOLUTION
To resolve this problem, obtain the latest service pack for SNA Server version 4.0. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

215838 How to Obtain the Latest SNA Server Version 4.0 Service Pack



STATUS
Microsoft has confirmed that this is a problem in SNA Server 4.0, 4.0 SP1 and 4.0 SP2. This problem was first corrected in SNA Server version 4.0 Service Pack 3.



MORE INFORMATION
SNA Server has worked around this problem by adding 7 pad bytes between the send and receive sequence number (BYTE) values, which forces these counters to fall on separate DWORD (8 byte) boundaries within memory. With these extra pad bytes, the Event 631 problem no longer occurs.

Keywords: kbbug kbfix kbsna400sp3fix kbqfe sna4 kbsna400sp2 kbsna400sp1 KB235849

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