Suppose node A is attached to node B with link L1, and node B is attached to node C with link L2. It takes 1ms for a packet to go from A to B and 1ms for an acknowledgement to go from B to A. It takes 99ms for a packet to go from B to C and 99ms for an acknowledgement to go from C to B.
Each packet has an error detection code that enables the receivers to check if it arrived correctly.
(a) Assume that no packet or acknowledgement are dropped. Assume that A sends a packet to C and waits for its acknowledgement to come back from C. How long does it take until A gets that acknowledgement?
(b) Assume that link L1 drops the first n copies of the packet; link L2 does not drop packets; no acknowledgement gets dropped. A notices that an acknowledgement is not coming back from C if it does not arrive at the time calculated in part (a). When that occurs, at that precise time, A sends a copy of the packet. How long does it take until the packet arrives at C?
(c) Now assume that the receiver at B sends acknowledgements to A for each correct packet it receives, as soon as it gets the packet; similarly, C sends acknowledgements to B. Node A sends a packet, waits for the acknowledgement from B. If A does not get the acknowledgement at the expected time, it sends a copy of the packet at that time. Assume that link L1 drops the first n copies of the packet; link L2 does not drop packets; no acknowledgement gets dropped. How long does it take until the packet arrives at C?