RTT: Round-Trip Time (RTT) can also be called as round-trip delay. It is to calculate how much time required for sending a packet or signal pulse from one source to a specific destination and comes back to the same specific source. RTT is one of the several factors that affecting latency and the time between the request for data and also the complete return or display of that data. RTT can range between a few milliseconds under some ideal conditions to several seconds between points under adverse conditions.

Estimated RTT plus can be defined as “safety margin”. It is the estimated value of RTT that is based on the combination of current RTT and the past RTT.

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

Large variation in Estimated RTT means larger safety margin. To calculate the DevRTT we need to estimate how much Sample RTT deviates from Estimated RTT i.e.,

DevRTT = (1-b)*DevRTTlast +b*|SampleRTT-EstimatedRTT| (typically, b = 0.25)

Table 1

A premature retransmission timeout occurs if there is no packet or signal loss or if the lost packet or signal can be captured by fast retransmission mechanism. With contrast, over estimation of RTT will lead to late retransmission timeout, in that case, if there is a loss and which cannot be captured by the fast retransmission mechanism. Therefore, it is crucial to have a Retransmission Timeout (RTO) value for TCP performance which is an equilibrium point in balancing between both the above cases.

Note: RTO must be smaller than RTT.

Following are the few algorithms which help in setting the retransmission timeout

Ludwig and Katz propose the Eifel algorithm to eliminate the unnecessary retransmissions that can result from a spurious retransmission timeout.