Problem set 2

1. Message, packet and packet size
   The size of a message is 10 Mbytes and its destination three hops ahead (there are two routers between). The transmit rate is 1 MBps and propagation delay and other delays can be ignored.

   1. How long does it take to transmit the whole message from the source to the destination.
   2. If the message is divided into ten packets, 1 Mbyte each, how long does it take to transmit the message from source to its destination?
   3. How can the results of a) and b) be useful in data communications? Could it be possible to split the message into smaller and smaller packets?
   4. If the transmission link is very error prone, is it better to use small or large packet size? If the line is almost error free what size is better then?
   5. Try out the "Message Segmentation" applet from the support page of the course book . What values do you get, when using the message size of 16 Kbits you vary the packet size with 16, 8, 4, 2 or 1 kbits? How does the propagation delay of the links affect the values?

2. Optimal message size
   X bits of the user's data are sent as consecutive packages to a destination k hops away. The size of a network package is p databits and h header bits (and x>>p+h). The speed of data transmission is b bps and the propagation delay is insignificant and can be ignored. What value for p minimizes the total transfer time? ( Use derivation to get the minimum value.)

3. Propagation delay
   

   1. Try out the "Transmission versus Propagation Delay" applet from the support page of the course book . How does the propagation delay proportion of the total time vary, when using packet size 1 kB you vary the distance (10 km or 1000 km) and transfer rate (1 Mbps or 100 Mbps)?
   3. Suppose that machines A and B are communicating using a geostationary satellite that is situated about 36 000 kms from the earth. The transmission rate of the satellite link is 1 Gbps. The propagation speed for the bits is 200 000 km/s (about 2/3 of the speed of light).
      1. How long is the propagation delay when the machine A is sending to the machine B.
      2. How long does it take to transmit a short 1500 byte message from the machine A to the satellite?
      3. How many bits can the satellite link carry at the same time when the bits are travelling serially one after another? How long then is one bit?

4. Loss of packets due to buffer overflow
   Experiment with the "Queuing and Loss Applet" situated in the course book support site.

   1. Set the average arrival rate (= Emission rate ) to 500 packets per second and the transmission rate to 350 packets per second. How many packets of the arriving hundred packets are lost due to buffer overload?
   2. If the average arriving rate is 500 packets per second, is it enough to raise the transfer rate to 500 packets per second to prevent buffer overflow and packet losses due to buffer overflow? What happens if you ran the applet for about 500 arriving packets? And why?
   3. Would it be a good idea to solve the overflow problem just by using a very large (even infinite) buffer?

5. Internet under attack
   In what different ways the security of Internet is nowadays under attack?
   (Information about Internet attacks and threats is found in the Chapter 1.6 of the 4th edition course book. There is also plenty of this type information available in the Web.)
   Answer this question as if it were an exam question worth 10 points.

6. Surfing the Web.
   

   1. An eager internet user, student Iitu Iivari sits in the computer science department surfing the Web and opens the URL link http://www.helsinki.fi/yliopisto/ from the document he is reading by clicking the mouse. What happens in the application layer? Who are communicating and how? What messages are sent?
   2. Try out the HTTP protocol and see its messages with your own eyes. Make a telnet connection to eg. the WWW server telnet www.poly.edu 80 and ask for the home page of professor Ross: GET /user/ross/index.html HTTP/1.0. (Here press enter two times.) What happens if you use HEAD instead of GET? What happens if you try to use the HTTP/1.1 version?