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Mohamed G. Gouda						CS 356
Spring 2017							Homework 3
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1.
Consider a network that has four routers A, B, C, D and five subnetwoks u, v,
w, x, y. Router A is attached to subnetworks u, w, y. Router B is attaached to
subnetworks u, w, x. Router C is attached to subnetworks v, w, x. Router D is
attached to subnetworks v, w, y. Assume that the routers in this network use
the RIP protocol to maintain their routing tables. Also assume that from the
start router B is "dead". Compute the initial routing tables for routers A,
C, D in this network.
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2. 
Consider the network in Problem 1 above. Compute the final routing tables for
routers A, C, D in this network. (Hint: The routing tables for routers in a
network are final iff when any router R in this network receives the routing 
table T of a neighboring router and uses the received routing table T to 
update its own routing table then the routing table of R remains unchanged.) 
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3.
The main scenario of the DHCP protocol between a client C and a server S is as
follows:

	C --> S : Discover message	{C needs an IP address}
	C <-- S : Offer message		{S offers address 1.2.3.4}
	C --> S : Request message	{C requests address 1.2.3.4}
	C <-- S : Ack message		{S approves the request}

Consider an alternative scenario of this protocol where message loss causes S
to receive the Discover message, then send the Offer message, but never receive 
the Request message. What can S and C do to tolerate the effects of message loss
in this case.
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4.
The main scenario of the DHCP protocol between a client C and a server S is as
follows:

	C --> S : Discover message	{C needs an IP address}
	C <-- S : Offer message		{S offers address 1.2.3.4}
	C --> S : Request message	{C requests address 1.2.3.4}
	C <-- S : Ack message		{S approves the request}

Consider an alternative scenario of this protocol where message loss causes C
to send the Discover message, then receive the Offer message, and send the Request
message, but never receive the Ack message. What can S and C do to tolerate the 
effects of message loss in this case.
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Solutions:
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1.
	Initial Table A		Initial Table C		Initial Table D 
	---------------		---------------		---------------
	| u | 1 | - |		| v | 1 | - |		| v | 1 | - |
	-------------		-------------		-------------
	| w | 1 | - |		| w | 1 | - |		| w | 1 | - |
	-------------		-------------		-------------
	| y | 1 | - |		| x | 1 | - |		| y | 1 | - |
	-------------		-------------		-------------
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2.
	Final Table A		Final Table C		Final Table D 
	-------------		-------------		-------------
	| u | 1 | - |		| v | 1 | - |		| v | 1 | - |
	-------------		-------------		-------------
	| w | 1 | - |		| w | 1 | - |		| w | 1 | - |
	-------------		-------------		-------------
	| y | 1 | - |		| x | 1 | - |		| y | 1 | - |
	-------------		-------------		------------- 
	| v | 2 |C/D|		| u | 2 | A |		| u | 2 | A |
	-------------		-------------		-------------
	| x | 2 | C |		| y | 2 |A/D|		| x | 2 | C |
	-------------		-------------		-------------
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3.
There are two possible message losses that cause S never to receive a Request
message. First an Offer message is lost, and in this case C waits for an Offer
message that will never arrive. Second a Request message is lost and in this
case C waits for an Ack message that will never arrive. In these two cases, S
waits for a Request message that will never arrive. To tolerate the effect of
message loss, both C and S time-out after a long wait and return to the initial
state of the protocol, where C is still in need for an IP address from S, and 
where S withdraws its Offer of the IP address (1.2.3.4) to C.
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4.
There are two possible message losses that cause C never to receive an Ack
message. First a Request message is lost, and in this case S waits for a 
Request message that will never arrive and C waits for an Ack message that will
never arrive. Then both C and S time-out after a long wait and return to the 
initial state of the protocol, where C is still in need for an IP address from 
S, and where S withdraws its Offer of the IP address (1.2.3.4) to C. Second 
an Ack message is lost and in this case C waits for an Ack message that will 
never arrive, then C times out after a long wait and returns to its initial 
state where it is still in need for an IP adress from S. Also in this case, S 
finishes its execution of the protocol and assumes wrongly that C has gotten 
the IP address 1.2.3.4. Then after an hour, S assumes correctly that it has 
gotten back the IP address 1.2.3.4.

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