Testbook

Question

Consider a TCP connection using the slow start congestion control scheme with an initial threshold value of 64 kB and a maximum segment size(MSS) of 2 kB. The receiver’s advertised window is initially 32 kB. The first transmission attempt is numbered 0, and all transmission attempts are successful except for the timeouts on attempt number 4. Find the size (in kB) of the sender’s congestion window attempt number 9.

Comments

https://gateoverflow.in/39167/size-of-senders-window

Answer 1

Transmission 0: W_{C =} 2KB. (1 segment)

Transmission 1: W_{C =} 4KB. (2 segments)

Transmission 2: W_{C =} 8KB. (4 segments)

Transmission 3: W_{C =} 16KB. (8 segments)

Transmission 4: W_{C =} 32KB. (16 segments)

Now timeout occurs.

Since timeout has happened, new Threshold = ¹/₂ * Current Window size = 16KB.

Algorithm enters slow start phase.

Transmission 5: W_{C =} 2KB. (1 segment)

Transmission 6: W_{C =} 4KB. (2 segments)

Transmission 7: W_{C =} 8KB. (4 segments)

Transmission 8: W_{C =} 16KB. (8 segments)

Since Threshold has been reached, so algorithm enters Congestion Avoidance phase:

Transmission 9: W_{C =} 18KB. (9 segments)

Answer - 18KB

Comments

answer will be 9 MSS ie 18KB

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I have a DOUBT :

If TO didn't occur then what would be the next congestion window size ( As I guess it could not be greater than receiver advertised window size i.e. 32 KB) ? Plz explain...

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even i've the same doubt ;

i am not very much sure but acc to common sense i guess at 4th attempt it should be 18KB only 

becoz we know that sender whindow size = min(receiver window size, congestion window size)

= min(32,32)=32KB

since sender cannot send more than 32KB and if timeout wouldn't have occured at 4th attempt then sender window would become 64kB (acc to slow start) which is not possible;

so i guess when sender window size has become 16KB , it will automatically start additive increase phase and in next attempt it would be 18KB;

again I would say I AM NOT SURE ABOUT THIS!!THIS ASSUMPTION CAN BE COMPLETELY WRONG ALSO.