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

ID: Q235849

The information in this article applies to:

Microsoft SNA Server, versions 4.0, 4.0 SP1, 4.0 SP2

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, please see the following article in the Microsoft Knowledge Base:

Q215838 How to Obtain the Latest SNA Server Version 4.0 Service Pack

STATUS

Microsoft has confirmed this to be 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.

Additional query words:

Keywords : sna4sp3fix sna4 sna4sp1 sna4sp2
Version : WINDOWS:4.0,4.0 SP1,4.0 SP2
Platform : WINDOWS
Issue type : kbbug