# Initial estimated round trip time in the RTT formula

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? Aug 23, 2014 at 15:25
• yes it is in context with `TCP`.it was on wikipedia..i had just assumed `a = 0.1` Aug 23, 2014 at 15:32

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