Consider a simple protocol for transferring files over a link. After some initial negotiation, A sends data packets of size 1 kB to B; B then replies with an acknowledgment. A always waits for each ACK before sending the next data packet; this is known as stop-and-wait.

Packets that are overdue are presumed lost and are retransmitted.

(a) In the absence of any packet losses or duplications, explain why it is not necessary to include any “sequence number” data in the packet headers.

(b) Suppose that the link can lose occasional packets but that packets that do always arrive in the order sent. Is a 2-bit sequence number (that is, N mod 4) enough for A and B to detect and resend any lost packets? Is a 1-bit sequence number enough?

(c) Now suppose that the link can deliver out of order and that sometimes, a packet can be delivered as much as 1 minute after subsequent packets. How does this change the sequence number requirements?