Why might the number of messages exchanged in a protocol be more significant to performance than the total amount of data sent? Design a variant of the RRA protocol in which the acknowledgement is piggy-backed on, – that is, transmitted in the same message as – the next request where appropriate, and otherwise sent as a separate message. (Hint: use an extra timer in the client.)

What will be an ideal response?

The time for the exchange of a message = A + B* length, where A is the fixed processing overhead and B is the rate of transmission. A is large because it represents significant processing at both sender and receiver; the sending of data involves a system call; and the arrival of a message is announced by an interrupt which must be handled and the receiving process is scheduled. Protocols that involve several rounds of messages tend to be expensive because of paying the A cost for every message.



The new version of RRA has:



The client always sends an acknowledgement, but it is piggy-backed on the next request if one arises in the next T seconds. It sends a separate acknowledgement if no request arises. Each time the server receives a request or an acknowledgement message from a client, it discards any reply message saved for that client.