I am trying to test throughput in the network. But when i start iperf3 client for sending UDP packets it sends TCP SYN packets. Why does it behave like this? (it should not send udp tcp syn while connectiing to UDP server)


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That's how iperf3.x works.

The "client" wants to establish a control channel to the "server" before any UDP transfer is started. That's why you can observe some TCP before the UDP transfer starts. I haven't checked, but I believe that there is also a separate control channel connection when running TCP tests.

In contrast, iPerf 1.7 and 2.0.x do not require a TCP control channel connection to be established in UDP mode, they just "blast away".

That makes iPerf 2.0.x a tool to use with great caution in environments where (upstream) bandwidth is scarce: It is all too easy to inadvertedly start a 50Mbit/s UDP stream from a client that will completely oversubscribe the 10Mbit/s DSL upstream. On the other hand, it makes iPerf a great tool to test and validate QoS configurations across a WAN.

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