Adjust Parameters to Connect to LAN Man Over Slow Link

SUMMARY

Data transfer between LAN Manager 2.2 workstations and servers (using NetBEUI protocol) can sometimes fail due to slow or congested links, or intermediate systems (bridges, routers) with inadequate buffering. You can increase chances for successful data transfer by increasing T1 (and possibly dlcretries) on the workstation AND the server. You may also need to increase sesstimeout on the workstation and the related serverheuristic 15 on the server.

The symptoms of a timeout-related data transfer failure vary depending on the type of process running on the workstation. For example, if you are copying files to/from the server at the MS-DOS command line, one of the following errors may occur:

   NET805: Network device no longer exists reading drive H
   Abort, Retry, Fail?

   NET804: Network busy
   Abort, Retry, Fail?

MORE INFORMATION

On a slow link, you need to increase the "session timeout" and the "frame acknowledgment timeout" parameters to eliminate data transfer timeout failures.

Session Timeout

When the redirector and server set up a session (NET USE), they can each specify to the protocol driver a "session timeout" parameter. Later, whenever the redirector or server submits an Server Message Block (SMB) message to the underlying protocol driver for transmission on the network, the driver must manage to transmit the entire message within the "session timeout" period or it returns a timeout error.

This session timeout is exposed differently in the redirector and the server. You need to remember that if you have a slow link between redirector and server you must increase this "session timeout" on both the redirector (workstation, client) side and the server side. On the redirector side, the session timeout is exposed by means of the sesstimeout parameter. (Note: sesstimeout also determines how long the redirector is willing to wait to receive a response SMB message from the server). Contrary to early (pre- LAN Manager 2.2) documentation, as of LAN Manager 2.1, sesstimeout applies to MS-DOS REDIR as well as OS/2 REDIR.

Configure SESSTIMEOUT as follows (default = 45):

    2-127        Actual value in seconds
    128-65534    127 seconds
    65535        Don't timeout (wait forever)

    0       34 seconds
    1       68 seconds
   2-8      127 seconds
    9       Don't timeout (wait forever)

Frame Acknowledgment Timeout

Because the redirector and server can submit an SMB message up to 64K long, but the adapter can transmit only small data units (Ethernet can transmit only about 1500 bytes), NetBEUI breaks the SMB message (that it received from the redirector or server) into "frames." It labels them as belonging to the start-of-message, middle-of-message or end-of-message, sequences them and hands them off to the adapter for transmission on the network. Most data transfer between the redirector and server is "reliable connection oriented," which implies, among other things, that after NetBEUI transmits frame X it must wait for the remote station to send a frame acknowledging receipt of frame X. (For simplicity, this explanation ignores protocol optimizations such as windowing.)

The question is: once frame X has been sent, how long will NetBEUI wait to receive the frame X acknowledgment? The answer is: it will wait for T1 (default 0.5 seconds). If the acknowledgment does not arrive within that time, NetBEUI tries again as many times as are specified by the dclretries parameter (default 5), each time doubling the wait period (T1*2, T1*4, T1*8, topping out at T1*16).

Over a slow link, it takes longer for frame X to get to the remote station, and longer for the frame X acknowledgment to get back. So when you are communicating with servers across slow links and you increase the "session timeout" on both sides as described earlier, you usually also need to increase the "frame acknowledgment timeout" (T1) on both sides. And it is usually a good idea to increase dlcretries a bit also. Tuning T1 (the timeout) to accommodate the average round trip time (and increasing the retries at times when the slow link is especially busy), along with the automatic opening up of the timeout period, enables the frame- acknowledgment transaction to eventually succeed.

Note 1

Increasing netbiostimeout (and possibly netbiosretries) helps ensure that the initial connection (NET USE) succeeds, but does not affect data transfer.

Note 2

• sesstimeout is in the LANMAN.INI file [workstation] section.
• serverhuristic 15 is in the LANMAN.INI file [server] section.
• T1 and dlcretries are in the PROTOCOL.INI file [NetBEUI] section.
• netbiostimeout and netbiosretries are in the PROTOCOL.INI file [NetBEUI] section.