Mohamed G Gouda							CS 326E
Spring 2013							Midterm 3
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Open Notes							50 Minutes
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1. The DHCP protocol is invoked by a mobile client, when this client enters a 
   network, so that the client gets an IP address from the DHCP server in this 
   network. Recall that this protocol consists of 4 messages: Discover, Offer, 
   Request, and Ack. 

   Consider the case where 2 mobile clients C1 and C2 enter the same network
   around the same time. Both clients invoke DHCP around the same time to get
   IP addresses from the DHCP server of the network. Explain how DHCP prevents
   the two clients from getting the same IP address.

Sol.

   When clients C1 and C2 enter the network, they will send Discover messages 
   (around the same time) asking for IP addresses from the DHCP server. Each 
   of these two messages has a sequence numbers that is selected at random 
   by the sending client and so the sequence numbers in the two Discover 
   messages will be different (with a very high probability).

   When the DHCP server receives each Discover message, it replies with an Offer 
   message that contains a different IP address. The reply Offer message will 
   have the same sequence number as the received Discover message. Although each 
   of the two Offer messages will reach both clients, each client can examine 
   the sequence numbers in the Offer messages and can tell which Offer message 
   is a reply to its earlier Discover message.

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2. Let A be a router in an Autonomous System that uses the RIP protocol for its
   Intra-AS routing. Assume that the routing table of A has the following 4 
   entries:
 	
	dst	Smallest #hops		Best neighbour
		for reaching dst	for reaching dst

	w	16			C
	x	3			C
	y	2			B
	z	3			B

   Assume also that A does not receive any routing table from its neighbouring
   router B for a long time, say 90 Seconds. In this case, A concludes that B 
   is down, and updates its own routing table to reflect the fact that B is down.
   Show how the above 4 entries will be changed in the updated routing table of A.

Sol.

	dst	Smallest #hops		Best neighbour
		for reaching dst	for reaching dst

	w	16			C
	x	3			C
	y	16			B
	z	16			B

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3. Consider an Autonomous System that uses OSPF for its intra-AS routing. Assume
   that this system has 6 routers (A through F) and 7 subnets (a through g).
   Assume that the local state of each router is as follows.

	A is connected to b and c.
	B is connected to a and c.
	C is connected to a,b, and g but g is down.
	D is connected to e and f.
	E is connected to d and f.
	F is connected to d,e, and g but g is down.

   Write down the routing tables of routers C and F.

Sol. 
	
   Routing table of C:

	dst		best neighbour to reach dst
	
	 a		- (directly connected to C)
	 b		- (directly connected to C)
	 c		A (or B)
	 g              - (not reachable)

	
   Routing table of F:

	dst		best neighbour to reach dst
	
	 d		- (directly connected to F)
	 e		- (directly connected to F)
	 f		D (or E)
	 g		- (not reachable)

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4. Let AS1 be an autonomous system that has the prefix pf and 2 gateways
   A and B. Also, let AS2 be an autonomous system that has 2 gateways C and D
   and 2 routers E and F. The two gateways A and C are connected by a subnetwork
   u, and the two gateways B and D are connected by a subnetwork x.
   Gateway C and router F are connected by a subnetwork v, gateway D and 
   router E are connected by a subnetwork y, and the two routers F and E 
   are connected by a subnetwork z. What route advertisements of prefix pf will
   router F receive? What entry of prefix pf will F enter in its BGP routing 
   table?

Sol.
   
   Router F will receive the following two route advertisements concerning
   prefix pf:

	(pf, u) and (pf, x)

   Router F will enter the following entry in its BGP routing table:

	(dst = pf, best neighbour for reaching dst = C)
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