SNA Clients Experience Delay Connecting to Multihomed Server

• Microsoft SNA Server, versions 3.0 Service Pack 2 and 4.0

SYMPTOMS

SNA Server clients configured to use TCP/IP sockets may experience a delay during the initial connection to a multihomed SNA Server computer. The time delay before getting the application session depends upon the TCP/IP retry timers configured on the client, but should be 45 to 55 seconds assuming the retry-timer settings have not been modified from their defaults.

CAUSE

Multihomed server support was added to SNA Server and SNA Server clients in SNA Server 3.0 Service Pack 2. This new feature allows the SnaBase service to send two entries (a new entry, TCPIP2, that lists all IP addresses of the server, and the original entry, TCPIP, that lists only one of possibly many IP addresses associated with the server) in its service-table updates for the TCP/IP transport. The original entry is retained for backward compatibility with earlier-version (pre-3.0 SP2) clients and servers.

SNA Server 3.0 SP2 (and later) clients now receive all of the TCP/IP addresses configured on the SNA Server computer. However, the clients try to connect to the SNA Server computer by trying the TCP/IP addresses in the order they are listed in the service-table update received from the SNA Server computer. If the first TCP/IP address exists on a subnet that is not reachable from the client's subnet, the client is not able to try the next TCP/IP address in its list until the TCP/IP retry timers expire. This can lead to a 45 to 55 second delay between connection attempts over the different TCP/IP addresses.

Note: Once the initial connection is made, the SNA Server client will continue to use the correct TCP/IP address for the duration of the application session. The potential delay is only noticeable upon the initial application startup.

STATUS

Microsoft has confirmed this to be a problem in SNA Server versions 3.0 SP2 and 4.0. A supported fix is now available for SNA Server 4.0, 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

With this update applied, clients that try connecting to multihomed servers will search the TCP/IP address list received from the server and will choose an IP address that it is most likely to get a successful connection based on the client's own TCP/IP address. The client will attempt to connect to this TCP/IP address first, and if that fails, it will then try the other addresses in the list.

If SNA Server 3.0 SP1 clients (and earlier versions) fail to connect to a multihomed Windows NT Server 4.0 computer running SNA Server, please refer to the following Knowledge Base article:

   Article-ID: Q161778
   Title: TCP/IP Clients Can't Connect to SNA Server on Windows NT 4.0

   Article-ID: Q177579
   Title: SNA Client Loses One TCP/IP Address on a Multihomed Server