R14. Telnet into a Web server and send a multiline request message, Include in the request message the If-modified-since: header line to force a response message with the 304 Not Modified status code. R15. List several popular messaging apps. Do they use the same protocols as SMS? R16. Suppose Alice, with a Web-based e-mail account (such as Hotmail or Gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts. R17. Print out the header of an e-mail message you have recently received. How many Recelved: header lines are there? Analyze each of the header lines in the message. R18. From a user's perspective, what is the difference between the download-and-delete mode and the download-and-keep mode in POP3