Data Communications I, Spring 2004

Exercise 5 (25th February 2004)

Answer shortly the following questions. The answers are usually almost directly found from the course book.
1. What differences there are between distance vector routing and link state routing?
2. How does a router function? How can it forward packets to their right direction? Can a router loose packets? In which situations?
3. Why, in your opinion, does IP protocol count the checksum over the header only and not include the data part?
4. What is the count-to-infinity problem and what solution has been proposed to solve it?
5. The header field for IP address is 32 bits long and with that one can have over 4 miljard IP addresses. In spite of that there has been worries about IP addresses running out. Why?

Dijkstra's shortest path algorithm in routing

Find using Dijkstra's algorithm the shortest (lowest cost) routes from node A to all other nodes in the network below.

                            3
                    B . . . . . . .F
                  .  .            .  . 
               .      .          .    .
          3  .       1 .      4 .      . 2
           .            .      .        .
          .     3        .   .     3     .
network  A . . . . . . .  D . . . . . . . .E
           .             .  .             .
            .           .    .           .
             .         . 2    . 2       .
           2  .       .        .       .  1
               .     .          .     .
                .   .            .  .
                 C . . . . . . .  G
 
                          2

Give, based on the routes found, the routing table for node A. In the routing table there is, for each node the outlink to be used and the cost of the route. The outlinks are numbered so that the link from A to be is number 1, from A to D number 2 and from A to C number 3.

For routing the distance vector algorithm is used. Supposing each node in the network below, first knows only the distances to its neighbors. What is the content of routing table of the node E after the node E has changed routing information with its neighbors?

              1
     A --------------- B
     |               / |
     |              /  |
     |           5 /   |
     |            /    | 
     |           /     |
     |2         /      |15
     |        E        |
     |      /   \      |
     |     /     \     |
     |    /       \    |
     |   / 2    10 \   |
     |  /           \  |
     | /             \ |
     C --------------- D
             1

With a program called traceroute you can find out what routes packets really travel in the Internet. Links to many traceroute services are to be found from the net address http://www.traceroute.org.
For example the traceroute service at the Arizona University ( http://netops.telcom.arizona.edu/cgi-bin/trace) seems to be working pretty fine. Find out how packets are routed from threre to the Computer Science Department at the University of Helsinki (cs.helsinki.fi) or some other domain or IP address chosen by you. You can also try out other traceroute service, e.g. the traceroute service of the Monash University in Australia ( http://www-personal.monash.edu.au/~steve/traceroute.html) or the traceroute service provided by Wiskit in Oragano, USA (http://wiskit.com/cgi-bin/tracecon).
What information do you get about the routes? How is the traceroute program really working to get that information?
The network layer of host A gets from its transport layer 3000 bytes of data to be sent to the host B. A and B are situated in different networks that are connected by one router. A's LAN can carry at most 1500 bytes of data in one packet and the maximum size of data in B's LAN is only 1000 bytes. Suppose that the next sequence number for A's IP packet is 100.
```
        A - XXXXXXXXX  -  router  -  XXXXXXXXXXXX - B

             LAN  a                     LAN b 
      max. 1500 bytes data        max. 1000 bytes  data
```
What kind of IP packets does A send? What does the router do to these packets? What kind of IP packets does B receive? Give the contents of the IP header fields involved in fragmentation in the sent and received IP packets.
In the SMTP connection the first message sent is the HELO message.
1. What kind of TCP segment carries the HELO message in the TCP connection? What information is included in the header fields of this TCP segment? What TCP segments have already been sent before this TCP segment carrying the HELO message?
2. How are these TCP segments transferred in IP packets? What information do the header fields of these IP packets contain?