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Mohamed G. Gouda						CS 356
Spring 2018							Midterm 3
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1. (5 points)
List three main differences between the exchanged messages in the RIP 
protocol and the exchanged messages in the OSPF protocol.

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2. (5 points)
Consider a multicast network (N, E) that has k mg-nodes which belong to the
same multicast group mg. Periodically, an mg-node generates a message m 
that needs to be multicasted (using the multicast tree protocol) to every
mg-node in the network. Execution of the multicast tree protocol causes
several copies of message m to be generated and sent within network (N, E).

How many copies of message m are generated and sent within network (N, E)?

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3. (5 points)
A route advertisement sent in the external BGP protocol to an IP prefix pf
is as follows:
		(pf, AS-path, next-hop z)
Assume that the AS-path in this advertisement is (AS5, AS12, AS3, AS15).
Also assume that this advertisement is sent from a gateway B in AS15 to a
gateway A in AS12. 

Explain why gateway A ignores this route advertisement.

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4. (5 points)
Consider a Switched Ethernet that connects 5 hosts using one switch S with
5 interfaces. The hosts have unique MAC addresses M1, .., M5 and the 
interfaces are named I1, .., I5. Write the switching table of S after S
receives and forwards the following five frames:

1. S receives a frame (src=M1, dst=M2 ..) from I2 and forwards it.  
2. S receives a frame (src=M2, dst=M3 ..) from I3 and forwards it. 
3. S receives a frame (src=M3, dst=M4 ..) from I4 and forwards it. 
4. S receives a frame (src=M4, dst=M5 ..) from I5 and forwards it. 
5. S receives a frame (src=M5, dst=M1 ..) from I1 and forwards it.

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Solutions
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1.
The main differences between the exchanged msgs in RIP and those in OSPF are
as follows:

First, a msg in RIP is egenrated by a RIP router every 30 seconds, while a 
msg in OSPF is generated by an OSPF router every 30 minutes.

Second, each msg that is generated by a router R in RIP is sent to every
neighbor of R, while each msg that is generated by a router R in OSPF is
sent to every router in the AS where R resides.

Third, each msg that is generated by a router R in RIP has a copy of the 
current routing table of R, while each msg that is generated by a router 
R in OSPF has a copy of the local state of R.
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2.
The number of mg-nodes in network (N, E) is k. Therefore, execution of the
multicast tree protocol causes (k-1) copies of a broadcast msg m to be
generated and sent within network (N, E).
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3.
If gatewat A accepts this route advertisment to the IP prefix pf, then the
AS route from A to pf will contain a cycle (AS12, AS3, AS15) which is not
acceptable.
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4.
After receiving the above five frames, the switching table of S becomes as 
follows.

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| M1 | I2 | TTL |
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| M2 | I3 | TTL |
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| M3 | I4 | TTL |
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| M4 | I5 | TTL |
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| M5 | I1 | TTL |
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