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I was reading a textbook which describes how TCP managing the retransmission timeout interval as:

TimeoutInterval = EstimatedRTT + 4 *DevRTT

An initial TimeoutInterval value of 1 second is recommended. Also, when a timeout occurs, the value of TimeoutInterval is doubled to avoid a premature timeout occurring for a subsequent segment that will soon be acknowledged. However, as soon as a segment is received and EstimatedRTT is updated, the TimeoutInterval is again computed using the formula above.

I don't understand why TimeoutInterval needs to be doubled, because each packet has its own timer?

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TCP generally uses multiplicative back-off to slow transmission (assuming there is network congestion) and additive speed-up to increase transmission rate (assuming there is other traffic to contend with.)

Surprisingly, this isn't one of the more controversial features of TCP congestion control algorithms. Wikipedia contains a good high-level comparison of various TCP congestion control implementations. This is a good starting point to learn about them. You'll notice the authors of most alternative algorithms, in their own source code and academic papers, don't put a lot of weight on the function/multiplier used for back-off.

One reason for this is TCP gets other signals -- duplicate ACKs and SACKS -- to indicate that it should re-transmit segments. RTO definitely happens in the practical world, but not as often as you might guess.

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