CSD008/COMP3616, Networks & Internet Systems: Problem Set 4
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                       Solutions
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("Tanenbaum" refers to the 4th Edition)


1. Draw a diagram to show at which layers of the OSI model
 the following internetworking devices operate: repeater;
 bridge; router; gateway.

Solution: see lecture notes on "Internetworking Devices".


2. (Tanenbaum, Ch. 5, problem 3) Datagram subnets route each
 packet as a separate unit, independent of all others. Virtual
 circuit subnets do not have to do this, since each data
 packet follows a predetermined route. Does this mean that
 virtual circuit subnets do not need the capability to route
 isolated packets from an arbitrary source to an arbitrary
 destination?

Solution: No. A virtual circuit subnet still needs to be
 able to route isolated packets from an arbitrary source
 to an arbitrary destination to set up the connection.


3. (Tanenbaum, Ch. 5, problem 6) Assuming that all routers
 and hosts are working properly and that all software in
 both is free of all errors, is there any chance (however
 small) that a packet will be delivered to the wrong
 destination?

Solution: Yes. Even if all routers and hosts are working
 correctly and all software in them is error-free, a large
 enough noise burst could be undetected by the error
 protection mechanisms. Since all practical error
 protection mechanisms have undetectable errors, this means
 we can't rule out a valid packet for destination A being
 corrupted to a valid packet for destination B, which will
 result in mis-routing.

4. See lecture notes

5. See lecture notes

6. See lecture notes