from swp

Question

in what protocol or protocols, it is possible for the sender to receive an ack for a packet that falls outside of its current window 

a. stop and wait            b. selective repeat      c.gbn       d. all of the above

Comments

Not sure, but the answers looks to be c.) Go-Back N protocol
The ACK may be delayed due to some network issues for a particular frame, but the window moved due to cumulative acknowledgment of another frame.

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why not ans is a.stop and wait

in stop and wait if ack  delayed ,then sender wil re transmit packet,after some time if actual ack came it wil move into next window

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Yes. It can happen. In that case, the answer should be option d.) since this situation can happen in every other sliding window protocol.

Answer 1

Ans is d

this is the concept of delayed ack which can occur in any of the protocol

Answer 2

Answer(b).Only in selective repeat we can able to get ACK for a packet that is out of current window.

explanation:As we know  1)in selective repeat arq does not wait for any ack and continuously send packets.

2)if a ACK is lost then ACK for higher packet can be used by receiver to make sender know that UNACKNOWLEDGED packet is received successfully by the receiver.let imagine that after sometimes that ACKNOWLEDGED packet ACK is received by the sender.

As selective repeat ARQ supports individual ACK.