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I came across following problem:

Node A sends packets to B through router R. The link A-R is instantaneous , but the R->B link transmits only 1 packet each second , one at a time . Assume A sends to B using sliding window protocol with window size =4 , For time t=2 , state what packets arrive at R and what are the packets present in the queue at R ?

The solution given was:

Since link A to B is instantaneous, at time 0, all frames 0,1,2,3,4... will be at router R. The window is of size 4. So, lets represent it as [0,1,2,3],4,…. Link R to B can transmit only 1 packet each second. So, at time 1, R will send frame 0 to B. At time 1, B will send ACK for packet 0 to R. So window at R will slide making it: 0,[1,2,3,4],…

The doubt

Go Back N uses cumulative acknowledgements, so what if at both time 0 and 1, router sends packet 0 and 1 to receiver? The router window at time 2, will still contain frames [0,1,2,3] as frame zero is not yet acknowledged. Is this wrong? Or I am wrong and sliding window protocol means Selective Repeat and it sends ACK for each frame?

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The solution is wrong (at least with the details you've given us).

At t=0, packets 0-3 (one send window worth) have been sent to R. Queue is 0-3.

At t=1, packet 0 has been transmitted R->B. Queue at R is 1-3.

At t=2 with an assumed full-duplex link R-B, B has acknowledged packet 0 (B->R->A), A has instantaneously advanced the sliding window to packet 4, instantaneously sent it to R, where it's queued. Meanwhile, R has transmitted packet 1, queue is 2-4.

  • ohhh this give rise to more doubts. (1) Dont we mean window by "queue at R"? This question has to do with next question. (2) Isnt all nodes on path apply sliding window to manage frames transmission between each adjacent hops or these techniques are employed at source and destination only, being transport layer techniques. (3) About fill duplex, the question explicitly states " R->B link transmits only 1 packet each second ". Does that mean given quoted solution is correct? – anir Jan 12 at 13:07
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    (1) No. The sliding window is between the end nodes - it is used with the transport-layer protocol, most prominently TCP segments. (2) The other nodes on the path are routers that queue packets (network layer), no sliding window. (3) Literally taken, that makes the link half-duplex, so it's unclear whether packet 0 has been acknowledged by B or if R has transmitted packet 1. – Zac67 Jan 12 at 14:01

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