Congestion Control

Question

Q1 Let the size of the congestion window of a TCP connection be 64KB when a timeout occurs. The RTT of the connection is 50m sec and maximum segment size is 2 KB. The time taken by the TCP to get back to 64KB congestion window is _________.

I am getting difficulty in finding the RTT's of the connection.

Comments

First answer 1000 msec is actual correct answer .. you reach window size 64 MSS ..

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at time t = 1000 msec we rich to 64 MSS, and our work is over since we have delivered 64 MSS.

and at t = 1050 msec we satisfied that our data is successfully reached to receiver because of ACK.

BUT in the question it is asked that

The time taken by the TCP to get back to 64KB congestion window.

So we are only good when we sent full window size i.e. 64 MSS then after that either we get ACK or not it doesn't matter.

right sir??

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at time t = 1000 msec we rich to 64 MSS, and our work is over since we have delivered 64 MSS.

and at t = 1050 msec we satisfied that our data is successfully reached to receiver because of ACK.

Both are correct here as what is full window that is not defined.

In GATE official key for both options marks given.

Answer 1

Size of the window = 64 KB

RTT = 50 msec

maximum segment size (MSS) = 2 KB

so threshld value is half of the Cwnd Window ( as timeout occurs mention in qstn )

Threshold = 64/2 = 32 KB

so we start with a slow start until we reach threshold level

2|4|8|16|32 ( now at this point we reach our threshold level so from now on additive increase starts , MSS is given 2 KB)

|34|36|38|40|42|44|46|48|50|52|54|56|58|60|62|64

so number of RTT = 20 and each RTT time = 50 msec

so total time it takes to get back 64 KB congestion window is (20 * 50) msec = 1000 msec

This answer range can vary from 1000 to 1200 , within this range ( inclusive ) any value is correct answer .

Comments

but correct will be n*RTT when it is given in the question that . The time taken by the TCP to send full window size of xyz KB.

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send full window size - it means just reaching 64 or after 64 ( what full window means )

..both meaning possible..it is ambiguous and also in GATE paper for both options full marks given, so you no need to worry  about.

This word " full window " it creates confusion actually..

|64 or |64|  both are correct .

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Thank you soooo much sir you have cleared my doubt,

Thanks alot @Bikram Sir,