GATE IT 2004 | Question: 88

Question

Suppose that the maximum transmit window size for a TCP connection is $12000$ $\text{bytes}$. Each packet consists of $2000$ $\text{bytes}$. At some point in time, the connection is in slow-start phase with a current transmit window of $4000$ $\text{bytes}$. Subsequently, the transmitter receives two acknowledgments. Assume that no packets are lost and there are no time-outs. What is the maximum possible value of the current transmit window?

• A. $4000$ $\text{bytes}$
• B. $8000$ $\text{bytes}$
• C. $10000$ $\text{bytes}$
• D. $12000$ $\text{bytes}$

Comments

Maximum window size=12000B

Threshold =12000/2=6000B

MSS= 2000B

Current window size= 4000B

In slow start phase next window size=4000*2=8000B > 6000B(threshold)

So increment should of MSS only.

After 2 ACKs, current window size= 4000+ 2000+2000= 8000B

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Jo window size smjh gya

Use congestion control smjh me pakka ayega

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@arpit_18  Yes. We increase window size exponentially in the slow-start phase. Consider initial window size as 1. You sent 1 MSS to the receiver. Once you receive the acknowledgement from the receiver, you will increase window size by 1 MSS. Now the sender window size has become 2. So the sender will send 2 MSS. Now remember one thing, the sender will increase its window size by 1 MSS FOR EVERY ACKNOWLEDGEMENT it receives. So here, it will receive 2 acknowledgements and it will increase window size by 2. It will become 4 now. 

I hope your doubt is cleared!

Answer 1

The following image explains the slow start phase and the congestion avoidance phase and clearly explains why the answer is 8000B of window size.

Answer 2

In slow start phase we double the CNWD with each acknowledgement unless there is a timeout or 3 Ack's and once these events occur the slowstart or AI phase starts respectively . So In the question , 

The it is saying slowstart phase so , we need to double the CNWD with each Ack until we reach a threshold which is 12000 in this case , so 

4000*2 for 1st Ack and 8000 * 2 for next Ack  since the maximum threshold is 12000 at the end of slow start phase it will be 12,000 (which is asked in question) and hereafter for the next acknowledgements AI phase will start.

Please let me know If I am wrong .