cwnd - TCP Congestion Window Size Too Large? -


i try emulate network comprised of 2 host , 1 switch using mininet. 1 host sender, sending packets continuously other host (receiver) using iperf tool.

h1----------------------------switch--------------------------h2

-------100mbps|0.125ms-----------100mbps|0.125ms------

the link between host , switch has bandwidth of 100mbps , delay of 0.125ms.

each packets sent has size of 1.5kb , switch has buffer of 400 packets.

delay of each link 0.125ms rtt between h1, h2 4*0.125=0.5ms

cwnd (congestion window) number of packets sender send in 1 rtt, throughput computed as: throughput = cwnd/rtt.

because max(througput) < bandwidth cwnd < rtt*bandwidth=0.5*10^(-3)*100*10^6=50000b~6kb = 4packets

but when monitor cwnd using tcp_probe tool, surprisingly display cwnd bigger 200kb (~120packets), bigger expected.

tcp cwnd plot

even buffer 400 packets, cannot has cwnd large that.

please explain me, i'm stuck @ problem. thank you!

i don't think can calculate cwnd , rtt way do, because argue time packet stays in switch , in network stacks of h1 , h2 zero.

the congestion window (cwnd) amount of data can transferred without packet loss, e.g. increased long gets acked , decreased on packet loss.

according data cwnd gets downgraded @ 600, packet loss starts @ 400 packets, buffer size of switch. in moment there not 4 packets in transit between h1 , h2, 400 , rtt larger 0.5ms.


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