TCP/IP Retransmissions

TCP/IP is employed between processes physically tens of centimetres (i.e. nano-seconds) apart, as well as processes tens of thousands of kilometres (near a second) apart.

As TCP/IP uses a sliding window protocol, timeouts are employed to force re-transmissions. As there are so many destination hosts, what should be the timeout value?

To cope with the widely varying network delays, TCP maintains a dynamic estimate of the current round trip time (RTT) for each connection. Because the RTTs vary tremendously, TCP averages RTTs into a smoothed round trip time (SRTT) that minimizes the effects of unusually short or long RTTs.

where α is a smoothing factor that determines how much weight the new values are given. When α=1, the new value of RTT is ignored, when α=0 all previous values are ignored. Typically α is between 0.8 and 0.9.

The SRTT estimates the average round trip time. To also allow for queuing and transmission delays, TCP also calculates the mean deviation (MDEV) of the RTT from the measured value.

This is also smoothed: