# value EstimatedRTT at first time?

What is the value of EstimatedRTT at first time?

EstimatedRTT = (1-α) EstimatedRTT + α*SampleRTT

Is it the average of all sampleRTT ? For example i have following SampleRTTs:

120 ms, 140 ms, 160 ms

Which of the following calculation would be correct ?

EstimatedRTT1 = (1-α) ((120*140*160)/3) + α*120

or

EstimatedRTT1 = (1-α) 120 + α*140

I asking again because I only found this answer(but I want to be sure). What is the value of EstimatedRTT at first time?

• Have you checked RFC 6298? – Zac67 Jul 1 '18 at 7:16
• now, yes. Here it says: (2.2) When the first RTT measurement R is made, the host MUST set SRTT <- R ; RTTVAR <- R/2. This means the first DevRTT in the Table is 130/2=65 and not 130 ? – Megumin123 Jul 1 '18 at 17:44
• Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 8:58

Ok, maybe I found the answer.

So I found a table with following values

``````Segment   SampleRTT   EstimatedRTT   DevRTT    Time Out

1         130         130.00         130.00     650.00

2         138         131.00         99.25      528.00

3         122         129.88         76.41      435.50

4         124         129.14         58.59      363.50

5         131         129.37         44.35      306.77
``````

Looks like EstimateRTT, DevRTT have at first the same value as the sampleRTT(130). The rest can be calculated with following formula as b = 0.25; a = 0.125

EstimatedRTT = (1- a)EstimatedRTTlast + aSampleRTT

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

Timeout = Estimated RTT + 4*DevRTT

Update:

Ok, thanks to Zac I was looking in RFC 6298 Where it says:

(2.2) When the first RTT measurement R is made, the host MUST set

``````        SRTT <- R
RTTVAR <- R/2
RTO <- SRTT + max (G, K*RTTVAR)

where K = 4.
``````

So in first Segment EstimatedRTT=SampleRTT, DevRTT=SampleRTT/2.

So the table would look in the first segment like this, if I understood it right:

``````Segment   SampleRTT   EstimatedRTT   DevRTT    Time Out

1         130         130.00         65.00     390.00
``````

Now which source should I rather trust ? I think the RFC 6298

• When you downvote it, leave atleast comment why. You can answer/comment atleast... or maybe post the answer how to calcute the RTTs, with a example... if you know it better... – Megumin123 Jul 1 '18 at 20:48