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Question

Consider an instance of TCP’s Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of slow start phase is 2 MSS and the threshold at the start is 1^st transmission is 16 MSS. Assume TCP use over a lossy link i.e., timeout occur after transmission of 7^th packet . The congestion window size at the end of 14 RTT is _________ (in MSS).

 

My question is after timeout does it always go to window size 1 MSS,or shall it go to initial congestion window i.e 2 MSS here?Also if min congestion window mentioned as 2 MSS,how does in increase window size in congestion avoidance phase?Will it increase additively by 1 MSS always or increase additively by the initial window size?Solution given as 12 MSS for this question.

Comments

This is solution given....question is does it go to 1 MSS always after timeout or initial cwnd?

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My question is after timeout does it always go to window size 1 MSS,or shall it go to initial congestion window i.e 2 MSS here .

No when Packets are lost then only go with 1 MSS otherwise go with normal.

Answer 1

There are two possibilities of congestion

1. weak possible( Happens when you got three duplicate ack)

2.strong possibility( when timeout  occurs)

if weak congestion then {sw becomes half of present window} and apply additive increase algo.

If strong congestion then {sw =1mss and threshold becomes half of present window} and apply slow start algo.

Comments

hello..thanks for the comment...bt my question is if initial congestion window starts from suppose 4 MSS,will it always come back to 1 MSS after timeout?.I dont think so,we need to always use whatever initial cwnd size is given. I am getting contradicting answers in different places,one I solved in gateforum old paper had different process.Even there are contradictions here also,this one is gate question,

https://gateoverflow.in/2156/gate2012_45

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i got ur doubt, according to me after timeout occurs sender window size always comes to 1mss.

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how does in increase window size in congestion avoidance phase?Will it increase additively by 1 MSS always or increase additively by the initial window size?

in congetion avoidance we add 1mss always because it dont depend upon the mss size but on the round trips time.