Howdy!
Freeman (Xiaoguang) Wang je dne 08/07/14 04:59 napisal-a:
We were test iperf UDP and found with old releases like Fedora 14, iperf TX side does not know the limitation of the link and keeps send at a data rage specified by the command line, so at the TX side we observe lots of packet drops. However, with Fedora 18 and above, it seems iperf UDP traffic is self-regulated and automatically find out the best data rate, and the RX side hardly shows any packet drop.Anybody know the reason? It looks we actually ran the same version of iperf 2.0.5 on both systems.
In general, there's no end to end control over congestions in IP networks. If there's congestion in any part of network, packets get delayed and/or dropped. If using TCP, the IP stack takes care of flow control based on end to end performance (delay, jitter, packet drop rate, etc ...). When using UDP, no such mechanism exists in IP stack, application needs to take care of it.
There's a notable exception to the above: link legs might be "buffer-less" and if the bottleneck is the first leg, this means there's no buffer between the application (iperf) and the bottleneck. In this case TX party will see "realistic" throughput as IP stack will not buffer excess data and application will get blocked when trying to send additional data until congestion gets relieved.
It might be that IP stack behaviour and/or NIC behaviour changed between F14 and F18 ...
One notable case when this behaviour happens (even using different OS): if using USB data card. Some datacards behave as if they were serial modems or simple NICs. Those data cards show real TX throughput to iperf when using UDP. Some data cards, however, behave more like routers. Those will buffer data and, if necessary, discard packets. When using such data cars, iperf will not see real throughput when TX UDP.
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