Computer Networks

Tutorial #4

1. What is your preferred electronic mail address at Bendigo? In general, what is the format of an Internet email address?

2. In his book Computer Networks 3/e, P646, Tanenbaum quotes a study by Perry and Adam (1992) which reported that "Some companies have estimated that email has improved their productivity by as much as 30 percent." Give some reasons why such a spectacular increase could occur from the use of email in a business environment.

3. What is the general format of an RFC822 electronic mail message?

4. The DATA operation in the SMTP protocol uses a "." on a line by itself to indicate the end of the mail message. What do you think happens if this occurs in the message? Exercise: try sending yourself an email containing a dot on a line by itself.

5. The SMTP protocol, in common with many other Internet application protocols, uses a 3-digit code in all communications from the server. From the example given in the lecture, can you generalise about the significance of the first digit of this code? What about the second digit?

6. What is MIME? Explain briefly how MIME works to send a file (eg, a GIF image) as an attachment to an email message.

7. The MIME Type of an email attachment provides a hint to the "user agent" software as to how to handle the data in the attachment. What do you think the user agent software should be expected to do with an attachment of type application/octet-stream?

8. What is the Post Office Protocol (POP) used for? Why is it important that POP uses authentication, whereas SMTP does not? What sorts of commands do you think POP would implement?

9. A file which has been encoded using Base64 is bigger than the unencoded data. How much bigger, on average, would you expect it to be?

10. Deep thinking question: In today's lecture, the slides which discussed RFC821 (SMTP) and RFC822 both referred to email addresses. In SMTP, the MAIL FROM: and RCPT TO: protocol messages both specify email addresses --- these are called envelope addresses. The header of the RFC822 message which is being delivered also contains From: and To: lines -- these are header addresses. The interesting question to contemplate is: "what happens if the To: header address and the RCPT TO: envelope address are different" in an SMTP delivery operation? Where does the mail get delivered?