title: Iperf tool cannot set one larger socket buffer
body: <p>Recently I try Iperf3 to test our system's throughput. I want to set tcp's window size to 2M, but failed.</p>
  
  <p><em>iperf3 -c 135.242.61.60 -w 2M</em></p>
  
  <p><em>Connecting to host 135.242.61.60, port 5201</em></p>
  
  <p><em>iperf3: error - socket buffer size not set correctly</em></p>
  
  <p>I check iperf3's source code, and find out that iperf3 will setsockopt with parameter 2M and then getsockopt to verify the paramter. The value getsockopt return is less than 2M. I think maybe there are some limitation in Kernel.</p>
  
  <p>How can I enlarge the tcp window to 2M?</p>