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4 votes
4 votes
Suppose TCP’s additive increase and multiplicative decrease algorithm is used as a congestion control algorithm. The slow start phase begins with $4$ MSS and threshold at start of first transmission is $16$ MSS. Assume that a time-out occurs during $6$th transmission, what will be the congestion window at $10$th transmission.

3 Answers

6 votes
6 votes

The slow start phase begins with 4 MSS .It is not saying that a maximum segment size(MSS) of 4kb.

AIIMD works with MSS ( 1mss 2mss 4mss 8mss exponentialy increase in SLOW START PHASE then linearly increse in CONGESTION AVOIDANCE PHASE then constant)

Since SLOW START is used, window size is increased by the number of segments successfuly sent. This happens until either threshold value is reached or time out occurs.
In both of the above situations AIIMD is used to avoid congestion. If threshold is reached, window size will be increased linearly. If there is timeout, window size will be reduced to half.

Window size for 1st transmission = 4 MSS
Window size for 2nd transmission = 8 MSS
Window size for 3rd transmission = 16 MSS
threshold reached, increase linearly (according to AIMD)
Window size for 4th transmission = 17 MSS
Window size for 5th transmission = 18 MSS

Window size for 6th transmission = 19 MSS

time out occurs,

Calculate new thresold= current window size/2 => 19/2=>9

resend 19th with window size starts with as slow start.
Window size for 7th transmission = 4 MSS

Window size for 8th transmission = 8 MSS

Window size for 9th transmission = 9 MSS
threshold reached, now increase linearly (according to AIMD)
Window size for 10th transmission = 10 MSS

Congestion window at 10th transmission =10

refrence:http://www.geeksforgeeks.org/computer-networks-set-2/

edited by
2 votes
2 votes
since it starts with 4 MSS so in slow start phase: it will increase like

4MSS 8MSS 12MSS 16MSS(here thresold is reached) so it will enter in congestion avoidance phase and will go linearly with 4 MSS

now:

4MSS 8MSS 16MSS(slow start) 20MSS 24MSS 28MSS(6th transmisstion so Time out) 4MSS 8MSS 14MSS 18MSS(10th transmission)

                                                                          Th = 28/2 = 14MSS

 

hence at 10th transmission window size will be 18MSS
1 votes
1 votes
BEGIN WITH 4MSS

TRESHOLD=16 MSS SO WSIZE= 32 MSS

TIME OUTR = 6TH TRANSMISSION ,

4MSS

8MSS

16MSS --------TRESHOLD

17MSS

18MSS

19MSS--------TIME OUT

NOW TH= 16/2=8 MSS, CONTINUE

4MSS

8MSS--------TH

9 MSS

10 MSS------------CONGESTION WINDOW AT THE 10TH TRANSMISSION. ANS

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