+1 vote
131 views

I have a doubt here. When timeout occurs, the congestion windows size is 18MSS.and Till now total RTT before this timeout were 6.

Now, my threshold will be set to 9MSS and congestion window will start from 1 MSS.So, now when I send 1MSS window and receive it's ack and then my window size changes to 2MSS, will this RTT be counted as 7th RTT or the 8th RTT?

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threshold will be 9 because timeout occurred at congestion window size 18 and threshold is set to 1/2 of congestion window size at which timeout occurred.
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$1|2|4|8|16|17|18|19|1|2|4|8|9|10|11$

i'm getting $11$

question says timeout occurs after transmission of $7^{th}$ packet. This means that cwnd is incremented. They are contradicting their own answer because in given solution timeout occurs during 7th transmission.
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It will be 1MSS

congestion window size counted after RTT completed

here ans will be 1MSS

@Ayush how much u got?

https://gateoverflow.in/1794/gate2014-1-27

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srestha how you got 1MSS? are you assuming timout after every 7th packet?

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1. ||. 2  ||. 4. ||. 8. ||. 16. ||. 17. ||. 18. ||. (T.O.). 1. ||  2  ||  4  ||  8  ||   9  ||. 10. || 11  || 12
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After 14 RTT there is again Timeout, so window sze will be at the end of RTT  came back to initial size again

i.e. 1MSS
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srestha how you assumed there is timout after 14th RTT?

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there is only 1 TO and if question mentions that TO will happen after every 7th packet then your answer can be considered

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ok,

but when one TO occured

then we again start from 1st packet, 2nd packet like this

we donot consider them as 8 th packet, 9th packet .......

then why not again we get a 7th packet?
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srestha can you give me a link to a similar question where packets are considered rather than RTT's?

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1st point is

The congestion avoidance phase continues as long as  the acknowledgements arrive before their corresponding timeouts. But the window size, and hence the rate at which the TCP sender can send, can not increase forever.

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srestha but this point is not clearing the doubt

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yes , no TO after 10th transmission
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srestha one point i would like to make is these test series questions relating congestion control which have "TO occur after x RTTs/ packets" are creating lots of confusion because of word after but gate use the term "during"which is appropriate term.

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@srestha-My question is when the TO occurs, then will we count that TO as RTT or not?

Like see my work below

Like TO occurred at 7th transmission and when 8th transmission will happen, then will we count the time between 7th transmission and 8th transmission as one RTT? Because actually if you see, it is not RTT, it is TO timer value and it is calculated as $RTT_s+4*RTT$. So, it is not actually RTT, your TO timer value.

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@Ayush

Actually that value will be less than RTT(7-->8) because the timeout has occured Hence for next transmission it will no wait till RTT. So the time between 7Th and 8Th transmission will be somewhat less than RTT(7-->8)

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@jatin-so will it be counted as RTT for the question?
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@ Ayush yes i think so https://gateoverflow.in/1794/gate2014-1-27

Here also range 1100 to 1300 if we consider RTT before next transmission to get started with slow start phase after timeout answer come as 1200. What is your thought on this ?