I am trying to troubleshoot poor telnet performance over our WAN and have come up against a figure for retransmits that I consider to be too high.
Looking at the device, the retransmits are currently 7.5% of all segments sent. I have looked at another device to try and get a feel for an acceptable level, and see figures of less than 1%.
My question is: Is 7.5% too high and very likely to be the cause of my poor performance, or have I just happened to stumble on a really good connection running at less 1%?
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I would say that 7.5% is too high. There are several things that can cause this. Are your error stats high as well? If so, you need to determine the causes of the errors. Faulty cabling can be one, buffer size to small on your router can be another and faulty electronics can also be a factor. I would also suggest that you look at the utilization of the two devices. Be sure that you are evaluating the utilization at times of use, not all day unless they are in fact in use all day. You should only examine utilization at times when packets are being transmitted. Then look at the size of your circuit. Too little bandwidth will cause retransmissions as the pipe gets full and the buffers have to discard packets. You may also want to check out "Common Network Errors and Causes" available on this site. If your error stats will show you what type of error, it is easier to determine the root cause.
This was first published in September 2005