Question

Answer true or false se to the following questions and briefly justify your answer:
a. With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window.
b. With GBN, it is possible for the sender to receive an ACK for a packet that falls outside of its current window.
c. The alternating-bit protocol is the same as the SR protocol with a sender and receiver window size of 1.
d. The alternating-bit protocol is the same as the GBN protocol with a sender and receiver window size of 1.

Answer 1

a) True. Suppose the sender has a window size of 3 and sends packets 1, 2, 3 at  t0 . At t1 (t1 > t0) the receiver ACKS 1, 2, 3. At t2 (t2 > t1) the sender times out and resends 1, 2, 3. At t3 the receiver receives the duplicates and re-acknowledges 1,  2, 3.  At t4 the sender receives the ACKs that the receiver sent at t1 and advances  its window to 4, 5, 6. At t5 the sender receives the ACKs 1, 2, 3 the receiver sent at t2 . These ACKs are outside its window.

 

b) True. By essentially the same scenario as in (a).

 

c) True.

 

d) True. Note that with a window size of 1, SR, GBN, and the alternating bit protocol are functionally equivalent. The window size of 1 precludes the possibility of out-of-order packets (within the window). A cumulative ACK is just an ordinary ACK in this situation, since it can only refer to the single packet within the window.