Question

How does TCP estimate round-trip-times (RTT) despite it being impossible to measure
one way latency on the Internet? Expand on the idea of EWMA and it’s application to
RTT estimation.

Answer 1

For every packet TCP sender starts a timer and stops it when it receives Acknowledge of that packet, It then calculate time elapsed ( i.e time between sending packet and receiving ACK) and save it as SampleRTT. Since network condition changes with time, SampleRTT value also changes with time.

EWMA ( Exponential Weighted Moving Average) also called EstimatedRTT, is calculated using previous RTT ( Previous EstimatedRTT) and current RTT ( SampleRTT) using below equation:

EstimatedRTT = (1 - a) * EstimatedRTT + a * SampleRTT.

(normally a = 0.125)

EWMA means more recent RTT values are given more importance than previous RTT values.

With the help current EstimatedRTT, deviation in RTT is calculated using below equation:

DevRTT = (1-b) * DevRTT + b * | SampleRTT - EstimatedRTT |

here | sampleRTT - EstimatedRTT | = modulus of difference between past RTT and current RTT

also normally b = 0.25

Given these two averages ( i.e EstimatedRTT and DevRTT ) one can calculate Timoutout value for next timer with equation

Timeout = EstimatedRTT + 4 * DevRTT

i.e Next timeout value should be atleast current EstimatedRTT plus deviation. If deviation is less timeoit will be nearly costant.