TCP Latency – What is the Value of EstimatedRTT at First Time?

latencylayer4protocol-theorytcptransport-protocol

Although this question might be marked as duplicated! I'm asking again because other question's answers like this couldn't help me. So please consider the case and don't mark it as duplicate and answer me.

I'm trying to know the value of EstimatedRTT at first time for this formula:

EstimatedRTT = (1 – α) • EstimatedRTT + α • SampleRTT

Best Answer

It can be any arbitrary value, completely dependent on the scenario. For instance, you may measure an initial RTT of 100ms. Now, the RTT measured for the next received packet/ACK may be say, 80ms. For the exponential weighted moving average model, alpha is generally taken to be equal to 0.125 (of course, that may vary as per the model you desire, but it always lies in the interval (0,1)) and the newly estimated RTT would be:

New RTT = (1-0.125)*100 + 0.125*80
        = 97.5 ms

Once again, if the next measured value of RTT is say 150ms, 

New RTT = (1-0.125)*97.5 + 0.125*150
        = 104.0625 ms

Thus, the initial RTT that you measure, which becomes a seed for future values, is completely dependent on the network scenario and its parameters.

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