Mohamed G Gouda							CS 326E
Spring 2013							Homework 3
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1. Assume that ICANN has assigned the IP prefix 128.83.129.11 / 26 to an ISP which
   decides to distribute this prefix among 7 networks, A through G, based on their
   needs of IP addresses:
   
	Network A needs at least 15 IP addresses.
	Network B needs at least 15 IP addresses.
	Network C needs at least 14 IP addresses.
	Network D needs at least 4 IP addresses.
	Network E needs at least 4 IP addresses.
	Network F needs at least 3 IP addresses.
	Network G needs at least 3 IP addresses.

  Specify the prefixes that the ISP will assign to each of these networks.

Sol. 

   The ISP assigns 16 IP addresses to each of the networks A, B, and C. 
   Therefore, the mask for each of the networks A, B, and C is 28. 
   The ISP assigns the following prefixes:

	Prefix 128.83.129.0  / 28 is assigned to network A.
	Prefix 128.83.129.16 / 28 is assigned to network B.
	Prefix 128.83.129.32 / 28 is assigned to network C.

   The ISP assigns 4 IP addresses to each of the networks D, E, F, and G. 
   Therefore, the mask for each of the networks D, E, F, and G is 30. 
   The ISP assigns the following prefixes:

	Prefix 128.83.129.48 / 30 is assigned to network D.
	Prefix 128.83.129.52 / 30 is assigned to network E.
	Prefix 128.83.129.56 / 30 is assigned to network F.
	Prefix 128.83.129.60 / 30 is assigned to network G.

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2. If a packet P is dropped by a router R before P reaches its final 
   destination D, then an ICMP packet Q is sent back to the original 
   source S of packet P in order to inform S of why P is dropped.
   However, if the ICMP packet Q is dropped by another router R'
   before Q reaches its final destination S, then R' does not send any
   ICMP packet Q' to the original source R of Q. Mention a possible 
   motivation for this design.

Sol. 

   If the ICMP packet Q reaches the original source S of packet P, then
   S can decide to slow or stop temporarily sending future packets to the
   ultimate destination D. On the other hand, if an ICMP packet Q' reaches 
   the original source R of Q, then R may decide to stop sending ICMP 
   packets to S even though it continues to drop packets sent from S to D.
   This would be a bad design.

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3. Consider an Autonomous System that uses RIP for intra-AS routing. Assume
   that the routing table for router A in this system has the entry

	(dst subnet = x,
	 # hops to reach dst = 7,
	 best neighbour to reach dst = B)

   Assume also that router A receives the routing table from router B which
   causes the above entry to change and become 

	(dst subnet = x,
	 # hops to reach dst = 8,
	 best neighbour to reach dst = B)

   Write an entry in the received routing table from router B that caused 
   the above change.

Sol.

   The received routing table from router B has an entry of the form,
 
	 (dst subnet = x,
	 # hops to reach dst = 7,
	 best neighbour to reach dst != A)

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4. Consider an Autonomous System that uses OSPF for intra-AS routing. Assume
   that this system has 5 routers (A through E) and 6 subnets (u through z).
   Assume that the local state of each router is as follows.

	A is connected to u,w, and x but x is down.
	B is connected to u and v.
	C is connected to v,w, and z but z is down.
	D is connected to x and y but x is down.
	E is connected to y and z but z is down.

   Write down the routing table that router A computes.

Sol.

   The local states of routers D and E will not be able to reach router A 
   because subnetworks x and z are down. Therefore, router A will not hear
   of subnetwork y (and will not know whether y is up or down). Thus, the 
   routing table of router A is as follows.

	dst		best neighbour to reach dst
	
	 u		A
	 v		B
	 w		A
	 x		- (not reachable)
	 z		- (not reachable)

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