SNA Server May Use Max BTU Larger Than Connection Can Support

• Microsoft SNA Server, versions 3.0, 3.0 SP1, 3.0 SP2, and 4.0

SYMPTOMS

SNA Server may use a Maximum BTU (that is, maximum frame size) length greater than the configured host connection can support if the remote device (that is, mainframe or AS/400) specifies a maximum frame size greater than 32,000 bytes during its XID exchange with SNA Server.

This problem was observed while testing SNA Server using a channel connection to a mainframe. The channel link service reported back to the SNA Server service that it could support a Maximum BTU length of 16,393 bytes. The mainframe indicated that it could support a maximum frame size of 38,400 bytes during the XID exchange with SNA Server. Instead of using the smaller value of 16,393, SNA Server used 38,400 as the Maximum BTU length.

One of the possible consequences of SNA Server and the remote device using a Maximum BTU length greater than the underlying connection (that is, channel, 802.2/DLC, and so on) can support is an application exception in the SNA Server link service.

NOTE: SNA Server service internal and data link control message traces during connection activation are needed to determine the Maximum BTU length that will be used for the configured connections.

CAUSE

The SNA Server service does not correctly handle the Maximum BTU length limits if the maximum length is greater than 32,000 bytes, because the parameters that handle these limits are incorrectly defined as integers instead of unsigned short integers.

STATUS

Microsoft has confirmed this to be a problem in SNA Server versions 3.0, 3.0 SP1, and 3.0 SP2. Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

Microsoft has confirmed this to be a problem in SNA Server version 4.0. A supported fix is now available but has not been fully regression-tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.

MORE INFORMATION

For more information about how SNA Server determines the Maximum BTU length for 802.2/DLC connections, please refer to the following Knowledge Base article:

   Article-ID: Q148972
   Title     : How SNA Server Determines Max BTU Over 802.2 DLC

Keywords          : prodsna snachannel snadlc snaserverservice
Version           : WINDOWS:3.0,3.0SP1,3.0SP2,4.0
Platform          : WINDOWS
Issue type        : kbbug
Solution Type     : kbfix