INT21CN Computer Networks

Tutorial #4

1. What is your preferred electronic mail address? 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 SMTP protocol, in common with many other Internet application protocols, uses a 3-digit code in all communications from the server to the client. 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?

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

6. A MIME-encoded email message is still a perfectly valid RFC822 message. What does this mean, how is it achieved, and why was it it regarded as important?

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 attachments of the following types
   1. text/plain
   2. text/html
   3. audio/basic
   4. 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? Approximate answers OK.

10. Philosophical question: in the last slide of today's lecture, some security/privacy considerations of Web-based email systems were mentioned. Do you believe these should be of concern to an average user? Is it possible that privacy may actually be enhanced by using a Web-based email system? How? Can you think of any other advantages (and disadvantages of Web-based email services?

11. 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?