An Argument for Increasing TCP’s Initial Congestion Window

WC 97 / RT 1min

TCP congestion window

TCP flows start with initial congestion window of 4 segments (4KB of data).

Window if critical for how quickly flows can finish.

Increase in 15KB congestion window improves average HTTP latency by 10%, mostly benefits RTT and BDP.

Slow start increases congestion window by the number of data segments acknowledged for each received ACK.

TCP latency is dominated by the number of round-trip times in slow-start phase.

Increasing init_cwnd enables transfers to finish in fewer RTT.


BDP - bandwidth delay product.

RTT - round trip delay time.

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