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CITS3002 Computer Networks - Tutorial 2

1. Two people, A and B, are at the ends of a very slow landline telephone connection. They are both completely cut off from the world except for their connection with each other. In particular, they have no idea what time it is in the real world (they are doing their Computer Networks project).

   There is a delay AB in the transmission of what is being said by A to B, and a delay BA in the transmission of what is being said by B to A. The delay AB does not equal the delay BA. The delays, for the purpose of this problem, are on the order of minutes in each direction. Both A and B have watches. Before they begin to talk, the watches are not set to the correct time, nor are they set to the same time.

   Can you devise a protocol such that A and B can, via their landline telephone connection only, accurately synchronize their watches (to within a few seconds)? The watches do not have to reflect time in the real world, but must match each other. (if such a protocol exists, both A and B will know the protocol before the call begins, and each will know whether they are A or B).

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

2. [from a past mid-semester test] Provide 3 distinct reasons why a stream of data is broken into frames by the Data-Link layer.
   
   

3. 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?