Exercise 3 (5th February 2002)

NOTE! The English Exercise Group in on Tuesdays 12-14 in C476. Your instructor will be Krishnan Narayanan.

1. Answer shortly the following questions. The answers are usually almost directly found from the course book or from the lectures.
   1. What is File Transfer Protocol and how does it differ from HTTP?
   2. What do SMTP and HTTP protocols have in common? Hoe do they differ?
   3. Where and why is MIME needed?
   4. What other services do DNSs provide except domain name to IP address?

2. Assuming it takes about 2 seconds in average to get a page across the Internet ie. to fetch the page from a server in the Internet and the size of the file fetched is in average 100 Kbits. The transmission capacity of the local LAN is 100 Mbps. There is no congestion in the LAN so there is no extra waiting delays. Neither is it necessary to take the propagation delay into consideration as the distances in the LAN are rathert small. It is necessary to take into consideration only the time spent in data transfer.
   1. How much does a cache in the local network speed up the information fetches, if 70% of the information requestsare found in the local cache?
   2. If the rate of requests in the LAN causes 20 typical fetches (= 100 Kbit file) in a second to the servers in the Internet, how much less traffic is caused if 70% of the requests are found in the local cache?
   3. Suppose a very popular news server is used by 100 LANs that are much like the LAN in the former problems but without local caches. How much capacity must the server have to be able to serve the requests? How much capacity is necessary in case each LAN had its own local cache that provides pages to local hosts?

3. Student T. Terävä from the University of Helsinki sends email to her friend M. Smart to the University of Berkeley in California. She starts a mail program in her PC, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it. What happens to the message after that?
   1. What does the mail program of the sender do to the message?
   2. How does the mail server know where to send the message?
   3. How does the mail server forward the messge to the mail system of the receiver?
   4. What changes in the mail system would be needed if T.Terävä would like to include into the messge a picture of herself?

4. Supposing the mail systems use SMTP protocol. What SMTP messages are changed between the mail servers in order to transmit the message to the receiving system? What is the content of the messages?

5. Reading email.
   1. M. Smart reads his email using a mail access program running in his own PC. This program uses POP3 protocol. What messages are changed in this protocol?
   2. What are the advantages if the mail access protocol would use IMAP protocol instead?

6. UDP and TCP use one's complement in their checksums.
   1. Calculate the UDP checksum for the following three 8 bit words: 0101 0101, 0111 0000, 1100 1100.
   2. How does the receiver detect that an error has occured in the transfer?
   3. Is it possible that the receiver does not detect one bit error (ie, the change of one bit from on to zero or vice verse)? What about two or more bit errors?