Computer Networks (CITS3230) - Tutorial 2

1. Discuss this statement: "There is no justification for large Australian telecommunications companies charging their users for the volume of data they consume. Charging users based on their volume-based usage is immoral. The networking infrastructure, once in place, depreciates at the same rate whether it is used or not. Connectors corrode, routers heat up, disks keep spinning. The actual use of the infrastructure should have no additional cost over and above the fees charged just to access it."

2. You suspect that someone is regularly breaking into your home computer via its dialup connection, and that they need to modify some significant configuration files to facilitate their next intrusion. Are that checksum algorithms that we have seen in this unit of any assistance to you?

3. Consider this observation: an acknowledgment frame for the Data Link Layer carries no data, or payload. Instead it simply consists of its header and a zero-length message. When the header is represented as four integer fields we use (in any order) - a FRAMETYPE, a sequence number, a length, and a checksum - for a total of 16 bytes on a Pentium computer. For the stop-and-wait protocol, the FRAMETYPE of an acknowledgment frame is always the same constant, the length is always zero, each sequence number is always either 0 or 1, and hence the checksum field will only ever take on one of two possible values.

   So do we really need 16 bytes to send just one of two possible acknowledgment frames? Can we send fewer bytes?

4. When the concept of time was introduced into our stop-and-wait protocol of Week 3, we made a rough guess at a suitable timeout period - three times the expected propagation delay. How could an adaptive stop-and-wait protocol determine a better timeout period?

5. There are three armies, Red, Green and Blue, about to enter battle. The Red and Green armies are camped on opposite sides of a valley overlooking the Blue army in the valley. If the Red and Green armies can attack the Blue army simultaneously then, together, they can win. However, if just one of them attacks they will be defeated. The only communication possible between the Red and Green armies is to send a messenger through Blue army territory but there is the possibility that the messenger will be captured and the message will not be delivered. The Red and Green armies would like to synchronize their attack, to ensure victory, but each is unwilling to attack unless they are certain the other army will simultaneously attack.

   Either, prove that no algorithm will permit the Red and Green armies to attack together, and thus win the battle, or prove that such an algorithm does exist by providing one.

   What has this problem got to do with Computer Networks anyway?

Chris McDonald

March 2008.