MadeEasy Test Series 2019: Computer Networks - Congestion Control

Question

Consider a TCP connection using the multiplicative additive congestion control algorithm where the window size is 1 MSS and the threshold is 32 MSS. At the $8^{th}$ transmission timeout occurs and enters in the congestion detection phase. The value of the window size (in MSS) at the end of $12^{th}$ transmission is ______.

Ans given 8 but I got 16.

Though there are numerous questions of this sort being posted here on GO. But since my answer didn’t match with them, I wanted to clarify.

It says at the end of $12^{th}$ transmission. So we have to take the window size after the 12 RTTs right and not at 12th RTT?

Comments

1 || 2 || 4 || 8 || 16 || 32 || 33 || 34(TO) || 1 || 2 || 4 || 8...

it should be 8... at the end of here means when it is about to end...

like the end of the story(last stage) moment before the end... means when 12th round is about to end here...

.

//as far as i am getting this.

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@arvin

Why did congestion avoidance phase start after 16MSS? We can go upto the threshold value i.e. 32MSS ..right?

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mistake!!!

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@arvin

Its okay. But "about to end" means the 12th RTT has just finished i.e. all the ACKs for 8 MSS have been received by the sender and now the window size should get doubled i.e. 16 MSS.. na? :(

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at the end of , i think there is some grammar thing ... to be noticed i think it means just before the end..

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what if we don't need 13th transmission ? there is no need to increase or decrease the window size. so it should be 8 only.

i also had doubt in this .

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At first  I'm also face the same prob

But in everywhere i observe  they follow the same rule

Ath the end  12 transmission  means just before end

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@Magma sure ?

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Yeah i solved similar type question around 3 times

Ist time I'm did the same thing which @MiNiPanda did 

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But here

https://gateoverflow.in/2156/gate2012-45

https://gateoverflow.in/190389/tcp-congestion-control?show=190440#c190440

they have done the other way.. @Magma

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gate 2012  was ambiguous and marks were given to all.

https://gateoverflow.in/272917/me-topicwise-computer-networks-2-gate-2019-q12

this is also made easy question and in every made easy question they have not incremented the value.

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In gate I think they would specifically mention what does end means, and if they don't then end means end we should increment, or else maybe they say tending to end.

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@MiNiPanda

yes I checked ..but then why made easy follow different rule :/

and yes acc to me also 16 is correct  

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https://gateoverflow.in/295101/made-easy-gate-mock-1-question-56

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i guess 16 is correct

at the end of 12th transmission means beg of 13th transmission

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1| 2| 4| 8| 16| 32| 33| 34 timeout

threshold = 16

1| 2| 4| 8

so answer should be 16 i think.

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Window size during 12th transmission is 8KB and at the end of 12th transmission is 16KB.

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https://gateoverflow.in/295101/made-easy-gate-mock-1-question-56

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16 MSS.

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https://gateoverflow.in/2156/gate2012-45

According to this answer 16  

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https://gateoverflow.in/295101/made-easy-gate-mock-1-question-56

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This is my point of view I want to share.

In TCP AIMD, window size doubles or increases after successful acknowledgement. i.e; it is very much possible that 1 RTT (i.e; my minimum time we take to receive an acknowledgement) is greater than the transmission time of whole window. I meant to say that, it is possible, and most of the time happens, that once sender sends the whole window, it has to wait for sometime to receive an acknowledgement and increase its window size (whether doubles or single increment, depending on the phase going on).

Now coming to the question, when it says that at the end of 12th transmission, then we are talking only about transmission... i.e; the time when the whole window of the 12th transmission is sent. And as discussed earlier, there might be some time gap between the whole window transmission and 1Rtt, i.e; receiving and ACK. And until we receive an acknowledgement, how can the window size be adjusted?

So according to me 8 is the correct answer.

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@aambazinga I got it know..thanks a lot!! :)

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MiNiPanda

So then answer will be 8?

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@Somoshree Datta 5 

yes the logic of aambazinga seems right. Isn't it?

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supporting to aambazinga,

there may be chance of TO or 3 duplicates, then sender window changes

∴ we have to return the window size at 12th transmission if it ask what is the window size at end of 12 th transmission.

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Ok..got it..thanks! :) @MiNiPanda @Shaik Masthan

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there may be chance of TO or 3 duplicates, then sender window changes

or may be not.

so both answers should be correct.(8 and 16).

gate question will either mention it clearly or they will provide a range in the answer.