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.
Manager, Network Engineering
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.