XCON: How to Improve Exchange Server Performance Over Poor Network Conditions

• Microsoft Exchange Server, versions 4.0, 5.0, 5.5

SUMMARY

There are certain parameters which can be adjusted to improve connectivity between Exchange Server computers within a site over a Site Connector or over an X.400 Connector. These values are found in one of two places:

• On the Messaging Defaults tab of the MTA Site Configuration Properties page. The values found in this location affect all message transfer agents (MTAs) in the site.



• On the Override tab of the X.400 Connector Properties page. These values affect only traffic over the X.400 Connector and override the values found on the Messaging Defaults tab of the MTA Site Configuration Properties page.

MORE INFORMATION

In either location, go to the RTS Values section.

The Checkpoint Size (K) value is by default set to 30. Checkpoint Size (K) sets the amount of data to be transferred before a checkpoint is inserted. A checkpoint consists of one synchronization point (session level) inside an IP packet, the length of which is 59 bytes. Inserting a checkpoint reduces transmission speed but increases the reliability of the connection. If an error occurs and the message must be transferred again, the process restarts from the most recent checkpoint.

If you specify zero, no checkpoint is set. A checkpoint size of 0 is not recommended. A checkpoint of 5 might help reliability over an unreliable network, but reduce transmission speed. A checkpoint of 1 requires every packet to be explicitly asked for before you can send the next one.

Also in the RTS section, go to the Window Size value. By default this is set to 5. Window size is the number of checkpoints that can go unacknowledged before data transfer is suspended. The greater the window size, the greater the transfer rate. The lower the window size, the more reliable the transfer. Specify window size only if the checkpoint size is greater than zero.

Now go to the Association Parameters section, Disconnect (sec) value on the same properties page. By default, this is set to 120 seconds, or two minutes. This means that the local MTA waits two minutes for an association to be re-established by either MTA before dropping the connection. Add this to the Lifetime value to determine how long the connection stays open after message transfer completes. Increasing this value should help network connectivity in poor network conditions.

For example, say you have an X.400 Connector set up to a remote location, and the physical link is known to have problems because of the hardware limitations along the link. These conditions are verified by seeing a large number of retransmissions or resets in a network trace. It is only 64 Kbps and also used for data transfer in addition to carrying Exchange Server traffic. By lowering the Checkpoint Size value to 15, the Window Size value to 2, and increasing the Disconnect Time value to 600 seconds, you should be able to slow down message transfer somewhat but make it more reliable. Experimentation with these values is the best way to determine which ones work best for your situation. These values, if changed on an X.400 Connector, should be changed at both ends of the connector. If you are using a site connector between two sites, these changes should be made on the MTA Site Configuration Properties page in each site.

For additional information, please see the following article in the Microsoft Knowledge Base:

Q185934 XCON: Exchange Server MTA Always Uses RTSE Window Size 3

http://www.microsoft.com/technet/default.asp

Additional query words: RTS bandwidth

Keywords          : exc4 exc5 exc55 
Version           : winnt:4.0,5.0,5.5
Platform          : winnt 
Issue type        : kbhowto 

Last Reviewed: July 26, 1999