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I've made a simple experiment on TCP transportation performance. The experiment is as follows:

There are two machines, A and B, each installed with Ubuntu 12.04 Server. I've installed "iperf" on either machine, and use it to test the transportation rate. A and B are connected through a 100Mbps link. The experiment is like this:

  1. I use iperf to send from A to B using TCP mode. The result is that on both side the rate output by iperf is 100Mbps, and is very stable.

  2. I use another iperf process to send from B to A, using the same settings. The result is that on both side the rate output is a little lower, 99Mbps, stably. But this is understandable.

  3. I use one more iperf process to again send from A to B, with the presence of the previous two traffic flows. Now the wired thing happens. The rate of the three traffic flows are all 50Mbps, on both sides. The rates are all very stable.

I understand the reason that flow 1 and flow 3 share the one-direction link and both have a bandwidth of 50Mbps. But what is the reason that the backward flow, flow 2, is also affected and is also 50Mbps? Shouldn't the bidirectional link be regarded as two different links that have no interference with each other?

DsRoyster
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  • The problem is that when you add this way, A->B is 50+50=100, yet B->A is only 50. The problem is that why B->A is 50... – DsRoyster Jan 24 '14 at 04:47
  • Yeah, i was reading over the question again. The description confused me a bit. For reference, though, this seems like it'd be a better fit for http://superuser.com, as it's more a hardware / networking question, not a programming question. – cHao Jan 24 '14 at 04:47
  • Oh, what is that website? I'll go and have a look. – DsRoyster Jan 24 '14 at 04:51
  • It's a site specifically for computer and home-networking questions, run by the same company that runs SO. – cHao Jan 24 '14 at 04:54
  • Oh. I was thinking if there's some site specially for networking problems? – DsRoyster Jan 24 '14 at 04:55
  • I don't know if there's one *specifically* for networking questions, but SU knows a bit about Ethernet. :) http://serverfault.com might know quite a bit more; that's for server and network admins and such. I dunno how they feel about tiny-network questions, though, which is why it wasn't my first suggestion. – cHao Jan 24 '14 at 06:45

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