A sender and receiver implement pipelined reliability with cumulative ACKs.

When a retransmission is triggered (e.g., due to RTO), the sender uses the information in the ACKs received so far to retransmit any sequence numbers that it thinks that the receiver has not successfully received.

With cumulative ACKs, the sender assumes that only the cumulatively ACK'ed sequence number has been dropped and is now requested by the receiver.

Note that a sender only sees the information in an ACK a full round-trip time (RTT) after the corresponding data has been transmitted. Assume that the RTO is sufficiently larger than the RTT such that all the ACKs of the prior window, if transmission were successful, would be received before the RTO triggers on any of the data in the window. Suppose in a window of size 12 Mbit, three packets are dropped---one after receiving 3 Mbit, 6 Mbit, and 9 Mbit of data each within the window. Assume there are no other losses in this window and in any window thereafter.

(a) How much data is wastefully retransmitted, i.e., data that was already received by the receiver?

(b) How many RTOs occur before the cumulative ACK number catches up with the largest sequence number that was received?

(c) Suppose the sender and receiver used selective ACKs instead. How many RTOs occur before the selective ACK number catches up with the largest sequence number that was received?