I was reading about Round Trip Time (RTT) and then I came across this formula

EstimatedRTT = 0.9 * PreviousEstimatedRTT + 0.1 * Sample RTT

What is the value of PreviousEstimatedRTT when it is calculated for the very first time??Is it assumed to be 0 or any other value??

  • Where did you find this formula? Are you talking about the estimation of the RTT in the context of TCP congestion control?
    – Gerben
    Aug 23, 2014 at 15:25
  • yes it is in context with TCP.it was on wikipedia..i had just assumed a = 0.1
    – rock321987
    Aug 23, 2014 at 15:32

1 Answer 1


According to RFC 6928, the SRTT is only calculated with the formula

SRTT <- (1 - alpha) * SRTT + alpha * R'

after two RTT measurements have been made. After the first measurement, SRTT is initilized to the value of this first measurement, ie. SRTT <- R'.

You were close, according to the RFC alpha SHOULD be 1/8 (= 0.125).

Note, in RFC's, SHOULD means "there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course." (RFC 2119)

  • can you please give a detailed example of how to calculate EstimatedRTT for 2 or 3 iterations. I am following the book by F. Kurose, Computer Networking. And what does SRTT mean in the rfc? If it is SampleRTT, then it seems to contradict with that in Kurose's book. Please throw some light into it.
    – user19152
    Sep 19, 2015 at 14:51
  • never mind, I have taken a look into the rfc, SRTT stands for SmoothedRTT. I still need to know the rest please. That is a worked out example.
    – user19152
    Sep 19, 2015 at 15:44

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