CS 742 Computer Communication Networks - Homework 1 - Name:____________

    DUE: Wednesday, September 18 (in class)


    o Please show all work on a separate sheet attached to this sheet.

    (40 points - 5 points for each problem)

    1. Imagine that you have trained your St. Bernard, Bernie, to carry 
       a box of 100 data CD instead of a flask of Budweiser. These CDs 
       each contain 700 MBs. The dog can travel at a rate of 18 km/hr. 
       For what range of distances does Bernie have a higher data rate 
       than a 45 Mbps T-3 (Transmission Level 3) line?

    2. What is the principal difference between connectionless
       communication and connection-oriented communication? Give two 
       example applications for which connection-oriented service is 
       appropriate. Also, give two example applications for 
       connectionless service.

    3. The well-known port numbers are contained in the file
       /etc/services on most Unix systems. Use "grep" to determine the 
       port number and protocol used by the following servers:

           a. HTTP:         b. SSH:       c. DNS (Domain):

       Hint: execute $ grep tftp /etc/services  to find all lines in the 
       /etc/services file that contains the string "tftp".

       Why is it important for all "ftp" servers to be assigned to the 
       same port?

    4. In which layer are the following protocols located in the OSI
       7-Layer Reference Model: FTP, HTTP, TCP, UDP, and IP?
       Find a protocol for the presentation, session, transport, and
       data link layer, respectively.
 
    5. The Unix utility ping can be used to find the RTT (Round-Trip
       Time) to various Internet hosts. Read the man page for ping,
       and use it to find the RTT to www.cs.twsu.edu and www.cisco.com.
       Measure the RTT values at different times of day, and compare
       the results. What do you think accounts for the differences?

    6. Please look at Example codes on the CS742 lab webpage, look in 
       Threads, explain why in the program wait() throws an 
       IllegalMonitor Exception?  
       (Note: please don't write more than 20 lines in answer, try to 
        explain it clearly.) 
   
    7. Please look at Example codes on the CS742 lab webpage, look in 
       url.java, write down the changes required in the program if all 
       outgoing connections to some website go through some proxy server?

    8. The use of java decompiler to decode .class code back into .java 
       file has been demonstrated in the lab, this is unsafe and of course 
       not secure. Search for techniques to make java code safe enough to 
       protect java source code from being decompiled ?   
       (Hint: Look for java obfuscators in google.)