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Mohamed G. Gouda					CS 356
Spring 2017						Exercise 1
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Consider an ISP tree that has
	one Tier-1 ISP: T1,
	two Regional ISPs: R1 and R2, and
	four Access Networks: A1, A2, A3, and A4

Connections in this tree are as follows. T1 is connected to both R1 and R2,
R1 is connected to both A1 and A2, and R2 is connected to both A3 and A4.

Show that if any one of the ISPs in this tree, namely T1, R1, or R2, fails 
and can no longer receive and send messages, then there is a message whose
original source is in an Access Network Ai and whose ultimate destination
is in another Access Network Aj and this message cannot be routed from its
original source to its ultimate destination.

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Solution:
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If T1 fails, then any message whose original source is in A1 and whose
ultimate destination is in A3 can't be routed.

If R1 fails, then any message whose original source is in A1 and whose
ultimate destination is in A2 can't be routed.

If R2 fails, then any message whose original source is in A3 and whose
ultimate destination is in A4 can't be routed.