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Suppose $A$ is connected to $B$ via an intermediate router $R$, as in the previous problem. The $A - R$ link is instantaneous, but the $R - B$ link transmits only one packet each second, one at a time (so two packets take $2$ seconds). Assume $A$ sends to $B$ using the sliding window protocol with $SWS = 4$. For Time = $0,1,2,3,4$, state what packets arrive at and are sent from A and B. How large does the queue at $R$ grow?

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At time t=0,

A has queue contain packets 1, 2, 3, 4. As A - R link is instantaneous, this window is immediately available at R. 1 starts travelling from R. Now window contains packets 2, 3, 4.

At time t=1,

1 reaches B. Ack starts from B to R. 2 starts travelling from A. Now window contains 3, 4.

At time t = 2,

2 reaches B. Ack for 1 reaches R. Which is immediately available at A. Then window slides left. Now packets 3, 4, 5 are in queue. 3 starts travelling from A. Then packets 4 & 5 are in queue.

At time t = 3,

3 reaches B. Ack for 2 reaches R. Which is immediately available at A. Then window slides left. Now packets 4, 5, 6 are in queue. 4 starts travelling from A. Then packets 5 & 6 are in queue.

At time t = 4,

4 reaches B. Ack for 3 reaches R. Which is immediately available at A. Then window slides left. Now packets 5, 6, 7 are in queue. 5 starts travelling from A. Then packets 6 & 7 are in queue.

At any time a+1, a+2 are waiting in queue.

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