Test by Bikram | Mock GATE | Test 3 | Question: 53

Question

The $TCP$ congestion window is set to $20$ $KB$ and a timeout occurs. Assume that the maximum segment size is $1$ $KB$.

Then, if the next four transmission bursts are all successful, the window size is _______  $KB$.

Comments

Why the answer is not 8 KB?

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@Lucky sunda

The threshold value what we calculated is  20 KB/2=  10 KB.

since the threshold is 10 KB, the window size can only be 10 KB. ( as it says timeout thats why consider Threshold value ) ..

see my below answer please.

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After timeout occurs

$threshold\leftarrow cwnd/2$ (10KB)

$cwnd \leftarrow 1$ (1KB)

 

cwnd would change like (until some event happens):

$1KB \rightarrow 2KB \rightarrow 4KB \rightarrow 8KB \rightarrow \color{blue}{10KB} \rightarrow 11KB \rightarrow 12KB...$

Answer 1

When a timeout occurs, three things happend.

• First, slow start will be initiated.
• Second, the congestion window would start at 1.
• Third, the threshold will be reset to  20 KB/2=  10 KB.

• If the next four transmission are all successful, then

1st transmission: 1 segment, 1KB
2nd transmission: 2 segments, 2KB
3rd transmission: 4 segments, 4KB
4th transmission: 8 segments, 8KB

After these four successful transmissions, the window size is supposed to be 16.

However, since the threshold is 10 KB, the window size can only be 10 KB. ( as it says timeout thats why consider Threshold value ) ..

Comments

The window size increases per ack received till the threshold limit in Slow Start state i.e. consider the 3rd transmission which has Ws of 4.
For each of the 4 acks the Ws increases at Slow start state.Thus the Ws increases to 8.

At 4th transmission, when the 1st ack is received, the Ws increases to 9 (since it is still at slow start and below the threshold of 10)
2nd Ack increases the Ws to 10.

Once the threshold is reached, the Ws increases by 1 for the number of acks being equal to Ws(which is now 10)
Thus the Ws is 10 here.

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can anyone plz tell me how to understand whether Q asked about nth or n+1 th MSS in most of such Question its not clearly mentioned sometime at end of kth Transmission whether i write for Kth one or next One In some test series their correct key is Kth whereas somewhere K+1 th please clearify @Deepanshu @Shaik Masthan @MiNiPanda

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Abhisek Tiwari 4 i dont know about this topic only this part  of n/w i left  for gate ...