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I have been measuring the throughput of file transfers from remote hosts and analysing the results, I understand about payload to overhead ratios, retransmission and other causational factors of throughput speeds.

I have downloaded a large amount of data from a remote server for testing purposes and also measured the RTT of ping packets to the server during transferrence of data.

However, the ping RTT times don't seem to be affected whilst I'm downloading. I'm not sure what this means, surely the ping RTTs would increase whilst downloading from their server? Can anyone offer some insight as to why the ping requests have consistently low RTTs regardless of whether a file is being transferred or not.

Many thanks in advance!

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There are a couple reasons it might not have affected the ping packets:

  • You weren't using all the available bandwidth - by default, ping uses very small packets, so they don't take up much bandwidth. Even if you were using 95% of your available connection, they wouldn't be affected much.
  • Some routers optimize either specifically ICMP traffic, or small packets in general.

You will usually see a high RTT when one of the network connections is completely maxed out and packets have to start queuing to be sent. This effect is especially noticable on connections that do a lot of extra queuing, such as most DSL internet connections.

If you try again using larger packets, you may notice the increased RTT - on windows, it's ping -l [size, up to 65500 bytes]. On linux, the -s flag specifies the size.

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Fantastic, thank you, I did presume that due to the small packet size of pings that would be a factor. This is a great answer, I shall try with larger packet sizes and see what happens! –  Ben Duttywood Maxfield Mar 22 '13 at 13:44
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