Consider the following simple UDP protocol (based loosely on TFTP, Request for Comments 1350) for downloading files:

■ Client sends a file request.

■ Server replies with first data packet.

■ Client sends ACK, and the two proceed using stop-and-wait.

Suppose client and server possess keys K_C and K_S, respectively, and that these keys are known to each other.

(a) Extend the file downloading protocol, using these keys and MD5, to provide sender authentication and message integrity.

Your protocol should also be resistant to replay attacks.

(b) How does the extra information in your revised protocol protect against arrival of late packets from prior connection incarnations and sequence number wraparound?