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ECE438 Homework 4 Solution
• This assignment has a total of 100 points.
• Please write your answer in the white space to the right of the corresponding problem.
1 T/F (no need for justification) - 2 x 5 points
1. Packets flowing through virtual circuit carry destination host address.
2. In Virtual circuits, every router on source-dest path maintains ”state” for each passingconnection.
3. Consider the network layer. Routers contain state about end-to-end connections.
5. Suppose that a packet is flowing from your laptop to a server. When the packet flowsthrough routers, say R4 to R5, the packet header contains the IP address of R5.
2 Forwarding table - 2 × 4 points
Suppose that IP addresses have 4 bits only, from 0000 to 1111. Of these 16 addresses, the first 5 should be forwarded to interface 1; the next 4 ones be forwarded to interface 2; the 2 addresses after then should be forwarded to interface 3, and the last 5 ones to interface 4. Create the most optimal (7 row) forwarding table with 2column forwarding table (column 1 = Prefix, and column 2 = interface number) that the router should use. Choose all apply to answer the following questions.
1. What will the entry/entries in the prefix column be for Interface 1?
(a) 00
(b) 01
(c) 000
(d) 001
(e) 010
(f) 0100
2. What will the entry/entries in the prefix column be for Interface 2?
(a) 01
(b) 011
(c) 100
(d) 001
(e) 0101 (f) 1000
3. What will the entry/entries in the prefix column be for Interface 3?
(a) 1
(b) 10
(c) 100
(d) 101
(e) 1001 (f) 1010
4. What will the entry/entries in the prefix column be for Interface 4?
(a) 10
(b) 11
(c) 101
(d) 110
(e) 111
(f) 1011
3 IP addressing - 3 x 3 points
For every IP in the sub-questions, please select the correct forwarding rule according to the routing table as shown in the choices:
1. 192.168.1.2
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
2. 192.168.32.2
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
3. 192.168.255.5
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
4 Subnet - 6 points
Consider a router that interconnects three subnets: Subnet1, Subnet2, Subnet3. Suppose all of the interfaces in each of these three subnets are required to have the prefix 203.1.17/24. Also suppose that Subnet 1 required to support at least 60 interfaces, Subnet 2 is to support at least 90 interfaces, Subnet3 is to support at least 12 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.
5 Dijkstra’s Algorithm - 5 + 3 points
Consider the network topology as shown in figure
2. Sort the following protocols by the amount of state each node maintains, and give clearexplanation.
• Link State
• Distance Vector
6 Distance Vector Routing - 3 + 6 + 5 points
Poison reverse. The idea is simple. Suppose three routers x, y, z. If z routes through y to get to destination x, then z will advertise to y that its distance to x is infinity; that is, z will advertise to y that Dz(x)= infinity (even though z knows Dz(x) in truth). z will continue telling this little white lie to y as long as it routes to x via y. Since y believes that z has no path to x, y will never attempt to route to x via z, as long as z continues to route to x via y (and lies about doing so). (Notice for a more detailed explanation, see the textbook in Distance-Vector Algorithm)
Assume four routers x, y, z, and w are connected as follows, and the cost of each link is given in the picture. Suppose that poison reverse is used in the distance vector routing algorithm.
1. Why is poison reverse needed?
2. When distance vector routing has stabilized (by starting with the initial costs specifiedabove), routers w,y, and z communicate to each other their distance vectors to router x (i.e., Dw(x),Dy(x),Dz(x) ). What are the values of these distance vectors? Fill in the following questions. ”Da(b) to c” denotes the value of router a’s distance vector to router b, which is sent to router c. If the answer is infinity, please write “inf”.
(a) Dy(x) to z
(b) Dy(x) to w
(c) Dz(x) to y
(d) Dz(x) to w
(e) Dw(x) to y
(f) Dw(x) to z
3. Now suppose that the link cost between x and y increase to 60. Will there be a count-to-infinity problem even if poisoned reverse is used? Why or why not? If there is a count-to-infinity problem, then how many iterations are needed for the distance-vector routing to reach a stable state again? Justify your answer.
7 AS Routing - 2 x 2 + 3 x 2 points
Consider the network shown above. Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Suppose there is no physical link between AS2 and AS4.
1. Router 3c learns about prefix x from which routing protocol?
(a) OSPF
(b) RIP
(c) eBGP
(d) iBGP
2. Router 3a learns about x from which routing protocol?
(a) OSPF
(b) RIP
(c) eBGP
(d) iBGP
3. Once router 1d learns about x it will put an entry (x, I) in its forwarding table.Now suppose there is a physical link between AS2 and AS4, shown by the dotted line. Suppose router 1d learns that x is accessible via AS2 as well as via AS3. Will I be set to I1 or I2? Explain why.
4. Now suppose there is another AS, called AS5, which lies on the path between AS2 andAS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as AS3 AS4. Will I be set to I1 or I2? Explain why.
8 Switch - 5+5+2+2 points
A Slotted ALOHA network of N = 32 nodes gets separated into 4 smaller networks using a switch. Each smaller network now contains N/4 nodes.
1. Before the switch was installed, calculate the probability of collisions in the network.Assume that each node attempts transmission in a given slot with a probability p = 0.3. Your answer should be correct up to 4 decimal places. Please also write the equation in terms of N and p.
2. After the switch was installed, assume that sender-receiver pairs are always within asmaller network (i.e., traffic does not cross the switch). Calculate the probability of collisions in the whole network, i.e, probability that collision occurs in any of the three smaller networks. Your answer should be correct up to 4 decimal places. Please also write the equation in terms of N and p.
3. Explain the advantages of the switch in terms of collision probability and overall networkthroughput.
4. In this scenario, would it make any difference if the switch was replaced by a hub?
9 Wireless - 3 + 3 + 6 points
In the diagram below, each wireless node is shown along with its transmission radius. E.g., A’s transmission radius is the circle with the dashed line.
1.List all the hidden terminals in the above wireless network?
2. The network uses CSMA/CA. When B wants to transmit to A it sends an RTS and Areplies with a CTS to reserve the channel. Is this guaranteed to avoid collisions, explain why or why not?
3. Suppose the nodes F, A, B, C, D are equally spaced by a distance of d. Assume all nodes are identical and transmit at same power level on the same frequency. Also assume the signal attenuates based on free space pathloss model.
A is transmitting to B while C is transmitting to D. Compute the SINR of C’s signal at D in the following cases?
(a) The noise power at D is zero.
(b) The noise power at D is not zero and in the absence of any interference, the SNR ofC’s signal at D is 20.
10 Short Answer Questions - 3 x 3
1. Alice and Bob wanted to share files with each other by setting up a socket connection.Both of them typed ifconfig on their Linux machine to obtain their IP addresses. Alice’s IP address is 130.126.255.2. Bob’s IP address is 192.168.34.102. Assume they both have access to the Internet. Can they set up the socket connection without requiring other external servers? Explain your answer
2. When they both log on to https://www.iplocation.net/ to check their IP address, willthey observe same IP address as running ifconfig?
3. Now Carol came in. She typed ifconfig and saw her IP address is 192.168.34.103. Canshe communicate directly with Bob without an external server?
• Please write your answer in the white space to the right of the corresponding problem.
1 T/F (no need for justification) - 2 x 5 points
1. Packets flowing through virtual circuit carry destination host address.
2. In Virtual circuits, every router on source-dest path maintains ”state” for each passingconnection.
3. Consider the network layer. Routers contain state about end-to-end connections.
5. Suppose that a packet is flowing from your laptop to a server. When the packet flowsthrough routers, say R4 to R5, the packet header contains the IP address of R5.
2 Forwarding table - 2 × 4 points
Suppose that IP addresses have 4 bits only, from 0000 to 1111. Of these 16 addresses, the first 5 should be forwarded to interface 1; the next 4 ones be forwarded to interface 2; the 2 addresses after then should be forwarded to interface 3, and the last 5 ones to interface 4. Create the most optimal (7 row) forwarding table with 2column forwarding table (column 1 = Prefix, and column 2 = interface number) that the router should use. Choose all apply to answer the following questions.
1. What will the entry/entries in the prefix column be for Interface 1?
(a) 00
(b) 01
(c) 000
(d) 001
(e) 010
(f) 0100
2. What will the entry/entries in the prefix column be for Interface 2?
(a) 01
(b) 011
(c) 100
(d) 001
(e) 0101 (f) 1000
3. What will the entry/entries in the prefix column be for Interface 3?
(a) 1
(b) 10
(c) 100
(d) 101
(e) 1001 (f) 1010
4. What will the entry/entries in the prefix column be for Interface 4?
(a) 10
(b) 11
(c) 101
(d) 110
(e) 111
(f) 1011
3 IP addressing - 3 x 3 points
For every IP in the sub-questions, please select the correct forwarding rule according to the routing table as shown in the choices:
1. 192.168.1.2
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
2. 192.168.32.2
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
3. 192.168.255.5
(a) 192.168.0.0/17 to port A
(b) 192.168.0.0/23 to port B
(c) 192.168.2.0/24 to port C
(d) 10.0.0.0/0 to port D
4 Subnet - 6 points
Consider a router that interconnects three subnets: Subnet1, Subnet2, Subnet3. Suppose all of the interfaces in each of these three subnets are required to have the prefix 203.1.17/24. Also suppose that Subnet 1 required to support at least 60 interfaces, Subnet 2 is to support at least 90 interfaces, Subnet3 is to support at least 12 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.
5 Dijkstra’s Algorithm - 5 + 3 points
Consider the network topology as shown in figure
2. Sort the following protocols by the amount of state each node maintains, and give clearexplanation.
• Link State
• Distance Vector
6 Distance Vector Routing - 3 + 6 + 5 points
Poison reverse. The idea is simple. Suppose three routers x, y, z. If z routes through y to get to destination x, then z will advertise to y that its distance to x is infinity; that is, z will advertise to y that Dz(x)= infinity (even though z knows Dz(x) in truth). z will continue telling this little white lie to y as long as it routes to x via y. Since y believes that z has no path to x, y will never attempt to route to x via z, as long as z continues to route to x via y (and lies about doing so). (Notice for a more detailed explanation, see the textbook in Distance-Vector Algorithm)
Assume four routers x, y, z, and w are connected as follows, and the cost of each link is given in the picture. Suppose that poison reverse is used in the distance vector routing algorithm.
1. Why is poison reverse needed?
2. When distance vector routing has stabilized (by starting with the initial costs specifiedabove), routers w,y, and z communicate to each other their distance vectors to router x (i.e., Dw(x),Dy(x),Dz(x) ). What are the values of these distance vectors? Fill in the following questions. ”Da(b) to c” denotes the value of router a’s distance vector to router b, which is sent to router c. If the answer is infinity, please write “inf”.
(a) Dy(x) to z
(b) Dy(x) to w
(c) Dz(x) to y
(d) Dz(x) to w
(e) Dw(x) to y
(f) Dw(x) to z
3. Now suppose that the link cost between x and y increase to 60. Will there be a count-to-infinity problem even if poisoned reverse is used? Why or why not? If there is a count-to-infinity problem, then how many iterations are needed for the distance-vector routing to reach a stable state again? Justify your answer.
7 AS Routing - 2 x 2 + 3 x 2 points
Consider the network shown above. Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Suppose there is no physical link between AS2 and AS4.
1. Router 3c learns about prefix x from which routing protocol?
(a) OSPF
(b) RIP
(c) eBGP
(d) iBGP
2. Router 3a learns about x from which routing protocol?
(a) OSPF
(b) RIP
(c) eBGP
(d) iBGP
3. Once router 1d learns about x it will put an entry (x, I) in its forwarding table.Now suppose there is a physical link between AS2 and AS4, shown by the dotted line. Suppose router 1d learns that x is accessible via AS2 as well as via AS3. Will I be set to I1 or I2? Explain why.
4. Now suppose there is another AS, called AS5, which lies on the path between AS2 andAS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as AS3 AS4. Will I be set to I1 or I2? Explain why.
8 Switch - 5+5+2+2 points
A Slotted ALOHA network of N = 32 nodes gets separated into 4 smaller networks using a switch. Each smaller network now contains N/4 nodes.
1. Before the switch was installed, calculate the probability of collisions in the network.Assume that each node attempts transmission in a given slot with a probability p = 0.3. Your answer should be correct up to 4 decimal places. Please also write the equation in terms of N and p.
2. After the switch was installed, assume that sender-receiver pairs are always within asmaller network (i.e., traffic does not cross the switch). Calculate the probability of collisions in the whole network, i.e, probability that collision occurs in any of the three smaller networks. Your answer should be correct up to 4 decimal places. Please also write the equation in terms of N and p.
3. Explain the advantages of the switch in terms of collision probability and overall networkthroughput.
4. In this scenario, would it make any difference if the switch was replaced by a hub?
9 Wireless - 3 + 3 + 6 points
In the diagram below, each wireless node is shown along with its transmission radius. E.g., A’s transmission radius is the circle with the dashed line.
1.List all the hidden terminals in the above wireless network?
2. The network uses CSMA/CA. When B wants to transmit to A it sends an RTS and Areplies with a CTS to reserve the channel. Is this guaranteed to avoid collisions, explain why or why not?
3. Suppose the nodes F, A, B, C, D are equally spaced by a distance of d. Assume all nodes are identical and transmit at same power level on the same frequency. Also assume the signal attenuates based on free space pathloss model.
A is transmitting to B while C is transmitting to D. Compute the SINR of C’s signal at D in the following cases?
(a) The noise power at D is zero.
(b) The noise power at D is not zero and in the absence of any interference, the SNR ofC’s signal at D is 20.
10 Short Answer Questions - 3 x 3
1. Alice and Bob wanted to share files with each other by setting up a socket connection.Both of them typed ifconfig on their Linux machine to obtain their IP addresses. Alice’s IP address is 130.126.255.2. Bob’s IP address is 192.168.34.102. Assume they both have access to the Internet. Can they set up the socket connection without requiring other external servers? Explain your answer
2. When they both log on to https://www.iplocation.net/ to check their IP address, willthey observe same IP address as running ifconfig?
3. Now Carol came in. She typed ifconfig and saw her IP address is 192.168.34.103. Canshe communicate directly with Bob without an external server?
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