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TCP Window Calculation

I was going through some TCP windowing over a high speed WAN link, and thought it might be worth to post some info regarding this.

We do face a typical problem of having slow speed file transfer even if the dedicated pipe is capable of supporting it.

There are three factors which affects this…

  1. TCP Window Size.
  2. Round trip latency of the circuit.
  3. Bandwidth of the circuit.

Maximum throughput you can get from a line with 10ms latency and a TCP window of 32KB can be calculated with…

32KB –> 32 x 1024 x 8 = 262144 Bits

262144 ÷ 0.01 = 26214400 bps = 26.2144 Mbps

Let’s say we have an OC-3 line, which is at 155 Mbps and a round trip latency of 10ms, and we need to calculate the TCP window size to maximize the throughput…

155.52 x 106 = 155520000 bps

TCP_WINDOW = 155520000 x 0.01 = 1555200 Bits = 194400 Bytes = 189.84375 KB

Hope I got the calculation right 🙂

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