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HP OpenVMS Systemsask the wizard |
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The Question is: We're trying to find information on the ability to increase the TCP Window size within UCX. Our 5000+ Point of Sale servers are running UCX for the TCP/IP stack, but the communication is over satellite. To improve the IP transport of large data volumes, we need to increase the window (amount of data able to be sent between acknowlegements) size, so we can optimize the performance. Any help you can porovide regarding parameter changes, etc would be most helpful. Will Smith Manager, Network Engineering Blockbuster The Answer is : There may not be a good answer. Increasing window size can help with faster links, but its value on slower links is unclear. The optimal TCP window size does depend to an extent on the underlying network throughput. Consider what happens when you specify a 56K window across a 9600 baud line: the sending host will dump up to 56K of data onto the LAN in very short order, leaving the router to buffer all that data. What can be sent to the router in less than a second will take nearly a minute to move across the serial line, and that's assuming there are no network errors or timeouts to cause retransmissions. The TCP implementations at each end do have some fairly sophisticated backoff and congestion avoidance provisions, so it won't be a total disaster, but they don't do selective acknowledgement. That means a little bit of data loss can cause a lot of retransmission. And what about other traffic sharing that slow serial link -- how will interactive terminal traffic be affected, for example, if you keep the router's buffers so full it's running a minute behind? Depending upon the particular network configuration and workload, this may be acceptable, or it may be a real problem. Across a LAN running at Ethernet (or better) speeds, large windows are essential. Across slower WAN links, larger window sizes create a backlog and may well even degrade throughput. The OpenVMS Wizard will assume you are aware of the SET PROTOCOL TCP /[NO]WINDOW_SCALE, and have it enabled, and the TCPIP$FTP_WNDSIZ (or UCX$FTP_WNDSIZ on releases prior to V5.0) logical name, and the SET PROTOCOL TCP/QUOTA settings. Please contact the Compaq Customer Support Center, as additional information will likely be required.
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