Go back protocol

Question

1. Consider a scenario of GO-BACK-4 protocol with 10 bits sequence numbers. Assume that s is the sequence number of subsequent packet at time t0, Then what can be the possible range of window size at sender. Note: medium does not change the order of packets.
   1.    [s-9, s-1]
   2.    [s-4, s-9]
   3.    [s-4, s]
   4.    [s, s+3]

Comments

4 ??

---

no ,3 is the given answer

Answer 1

In , GO-BACK-4  protocol sender window size , N = 4 .

Assume that s is the sequence number of subsequent packet at time t0,

That means at time t0 , receiver expect next packet of sequence number  s .

receiver waiting for packet s means receiver already has received packet (s − 1) , and has ACKed that and all other preceding packets like  (s-1) (s-2) , (s-3) etc...

If all of these ACK's have been received by sender, then sender's window range is  [ s to s + (N − 1) ].

Suppose  that none of the ACKs  for N packets have been received at the sender. In this case, the sender's window contains     s   to  (s-N)  packets . That is maximum s and minimum (s-N) .

So the possible range of window size at sender is [ s − N to  s ]. now put N = 4 the range of  window size becomes [ (s-4) to s ] which is option 3 .

Comments

@Bikram If none of them were ACK’ed, don’t you think, the window should contain s-1 to s-N?. Otherwise, there will be N+1 sequence no, which is more than the window size.

---

I have the same doubt