Question

Suppose that TCP's current estimated values for the round-trip time (estimatedRTT) and deviation in the RTT (DevRTT) are 300 msec and 35 msec, respectively. Suppose that the next three measured values of the RTT are 250 msec, 200 msec, and 330 msec respectively.

Compute TCP's new value of DevRTT, estimatedRTT, and the TCP timeout value after each of these three measured RTT values is obtained. Use the values of α = 0.125, and β = 0.25.

Answer 1

EstimateRTT = (1-alpha) * estimatedRTT-prev + alpha * SampleRTT

DevRTT = (1-beta) * DevRTT-prev + beta * | SampleRTT - EstimatedRTT |

timeout = EstimatedRTT + 4*DevRTT

EstimatedRTT -> for current iteration

EstimatedRTT-prev -> for previous iteration

DevRTT -> for current iteration

DevRTT-prev -> for previous iteration

SampleRTT -> this is measured RTT values on each iteration

timeout -> this is timeout value based on RTT (round-trip-time)

at the beginning we have estimatedRTT = 300 ms, DevRTT = 35 ms , alpha = 0.125, beta = 0.25

1) for first measurement (iteration - 1) SampleRTT = 250 ms

EstimateRTT = (1-0.125) * 300 + 0.125 * 250 = 293.75 ms

DevRTT = (1-0.25) * 35 + 0.25 * | 250 - 293.75 | = 37.1875 ms

timeout = 293.75 + 4 * 37.1875 = 442.5 ms

2) for second measurement (iteration - 2) SampleRTT = 200 ms

EstimateRTT = (1-0.125) * 293.75 + 0.125 * 200 = 282.03125 ms

DevRTT = (1-0.25) * 37.1875 + 0.25 * | 200 - 282.03125 | = 48.3984375 ms

timeout =282.03125 + 4 *  48.3984375 = 475.625 ms

3) for third measurement (iteration - 3) SampleRTT = 330 ms

EstimateRTT = (1-0.125) * 282.03125 + 0.125 * 330 = 288.02734375 ms

DevRTT = (1-0.25) * 48.3984375 + 0.25 * | 330 - 288.02734375 | = 46.791992187 ms

timeout = 288.02734375 + 4 *  46.791992187 = 475.1953125 ms