Exercise 3 (2.-5.10.2001)

1. An eager internet user, student Iitu Iivari sits in the computer science department surfing the Web and opens the URL link http://www.encyclopedia.com/articles/12910.html from the document he is reading by clickin the mouse. What all happens after that!
   1. What do the different parts of the URL link mean and what are they used for?
   2. What happens in the application layer? Who are communicating and how? What messages are sent?
   3. Try out the HTTP protocol and see its messages with your own eyes. Make a telnet connection to eg. the WWW Server at the department (telnet www.cs.helsinki.fi 80) and ask for eg. this exam page: GET /u/marttine/tili/syksy01/harj/ex3.html. What happens when you ask for a page that doesn't exist or a page for which you don't have access rights?

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.
   1. How long does it take to transmit a 100 Kbits file over the LAN to another host machine.
   2. If the rate of requests in the LAN causes 20 typical fetches (= 100 Kbit file) in a second, how high load or traffic does this cause in the Internet?
   3. How much does the load caused by the LAN users decrease if there were a local cache in the LAN with hit rate of i) 40%, ii) 60% or iii) 80% meaning that this procentige of the pages is found in the local cache?

3. More about the cache.
   1. 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?
   2. How much capacity is necessary in case each LAN had its own local cache that provides pages to local hosts?
   3. Find out, from the web or otherwise, how the Internet caches are organized.

4. 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?

5. 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?

6. 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?