Max BTU Size Limit Increased for SNA Server Connections

• Microsoft SNA Server, versions 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 4.0, 4.0 SP1

SYMPTOMS

The maximum Basic Transmission Unit (BTU) length that can be specified on a connection has been increased to 65,536 bytes for channel connections and 32,768 bytes for non-channel connections. These are limits imposed by SNA XID formats, which allow 2 bytes for maximum BTU size for channel connections and 15 bits for non-channel connections.

CAUSE

The maximum BTU length was previously hardcoded for a maximum length of 16,393 bytes, which is the largest size supported by Token Ring networks.

RESOLUTION

Microsoft has confirmed this to be a problem in SNA Server versions 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 4.0, and 4.0 SP1. We are researching this problem in SNA Server version 3.0 and will post more information here in the Knowledge Base as it becomes available.

This problem was corrected in the latest SNA Server version 4.0 U.S. Service Pack. For information on obtaining this Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

MORE INFORMATION

Setting the maximum BTU length to the limit does not guarantee that the SNA Server connection will actually use this value. The individual link service or the host can negotiate it down. For information on how the actual Max BTU value is determined for 802.2 DLC connection, refer to the following Knowledge Base article:

Q148972 How SNA Server Determines Max BTU Over 802.2 DLC

Additional query words: snacfg

Keywords :
Version : WINDOWS:3.0,3.0 SP1,3.0 SP2,3.0 SP3,4.0,4.0 SP1
Platform : WINDOWS
Issue type : kbbug