University of Toronto Faculty of Applied Science and Engineering

Final Exam, December 2018

ECE 461: Internetworking Examiner: J. Liebeherr

Exam Type: B Calculator: Type 2

This is a 2.5 hour exam. There are a total of 7 problems for a total of 100 Marks. Note that not all problems have the same maximum marks. Note the binary-decimal and hexadecimal-decimal conversion tables on the last pages of this handout. Write your solutions into an answer book. Make sure your name and student number is on the answer book. Do not write answers in this handout. Show all steps of your solution. Make sure that your answers are legible.

Problem 1. (20 marks) Routing Tables

Consider a router with the following routing table entries (the next hop address is abbreviated by a letter):

Network Prefix Next Hop

0.0.0.0/0 A 128.0.0.0/2 B 128.0.0.0/3 C 160.0.0.0/4 D 164.0.0.0/6 E 164.96.0.0/14 A 164.100.0.0/15 A 164.100.128.0/17 B 164.102.0.0/16 A 164.103.0.0/17 A 164.103.128.0/18 A 164.103.128.0/20 F 164.103.192.0/18 A

a. (8 marks) For each of the following IP destination addresses, indicate the longest prefix match in the routing table. (You may use the row number or the interface name to indicate an entry):

al) 167.91.111.14 164.101.89.110 175.235.84.10

A) 164.103.224.110

b. (8 marks) Prefixes in a routing table can be aggregated if their prefixes create a contiguous address block and they have the same next hop address. Your task is to aggregate the routing table above to the maximum degree possible. In your answer, state which routing table entries can be aggregated and describe the resulting changes to the routing table.

c. (4 marks) Provide the broadcast address of the network prefixes for the routing table entries in rows (2), (5), (6), (12).

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Problem 2. (20 marks) IP address allocation The purpose of this problem is to assign address blocks to seven subnets with a hierarchical allocation of IP addresses. The subnets are connected as shown in the figure.

Subnet D (2000 hosts)'N if2

ifl SubnetB Rout r 3

(1000

osts (1000 hosts Subnet E

(800 hosts)

Subnet F

(400 hosts) if2

Router 3 ifi SubnetC

(1000 (1000 hosts SubnetG if3

(200 hosts)

~iUSubnetA

~Rou~teruter 2_F-K OOsts)

if 3

Rest of Internet

Your answers in this problem must satisfy the following four requirements:

Requirement 1: The requirement for the allocation of addresses is as follows: Subnet A obtains an address block from its Regional Internet Registry (RIR);

Subnets B and C obtain their address blocks by acquiring a portion of the address block assigned to Subnet A; Subnets D and E obtain their address blocks by acquiring a portion of the address block assigned to Subnet B. Subnets F and G obtain their address blocks by acquiring a portion of the address block assigned to Subnet C.

Requirement 2: Each subnet must sunnort the following number of hosts: Subnet: I A B C D E F G Number of hosts: 1 1500 1000 1000 2000 800 400 200

Requirement 3: There is an organizational requirement on the allocation of IP addresses:

For two subnets X and Y, let X< Y mean that all IP addresses in Subnet X are smaller than any IP address in Subnet Y. Then, the organizational constraint is that B <E < D < C < G <F <

A.

Requirement 4: The RIR has the address block 11.42.0.0/16 available. From this address block you can request a network prefix of any size. However, your address block must be selected from the end of address block 11.42.0.0/16. (For example, if you request a/24 prefix, then you must select 11.42.255.0/24).

(8 marks) Determine the smallest address block that Subnet A can obtain from the RIR, which satisfies all requirements. Justify your answer.

(8 marks) Use the address block selected in (a), and create IP network prefixes for each subnet, such that all requirements are satisfied.

(4 marks) Given your solution to (b), determine the maximum number of hosts that can be added to each subnet (without a need to change the assignment of IP network prefixes).

Problem 3. (15 points) Routing Consider the statically routed IP network in the figure below with four routers. The router interfaces are labeled as Ii, 12, 13, and 14.

I]. Router 12

13

12

'/L

2I1Rou:r 13 OO.2.O/24

netRo y3 Network

I1 Ii Router

14

11

Rest of ___________ ________

Router 13

The routing tables are as follows:

Router X:

Destination Next Interface

128.100.2.0/24 Router 1 Ii

Router 3:

Router 1:

Destination Next Interface

128.100.2.32/27 Router 2 12

128.100.2.0/24 Router 3 13 128.100.2.192/264 Router 14

0.0.0.0/0 Router Ii

Router 2:

Destination Next Interface

128.100.2.0/26 Router 3 12

128.100.2.224/29 Router 4 13

128.100.2.20/30 Router 3 12

0.0.0.0/0 Router 1 12

Destination Next Flop Interface

128.100.2.16/28 Router 4 14

128.100.2.0/24 On-link 13

128.100.2.192/26 Router 2 12

128.100.2.240/28 On-link 13

0.0.0.0/0 Router 2 12

Router 4:

Destination Next Hop Interface

128.100.2.24/29 Router 1 Il

128.100.2.224/28 Router 3 13

0.0.0.0/0 Router 2 12

The routing tables are misconfigured and, as a result, some IP datagrams are forwarded in a loop.

(Problem statement continued on the next page.)

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Problem 3 (continued):

(10 points) Determine all routing loops that exist in the above configuration. For each loop, provide the range of IP addresses that will loop and the routers that the looping IP datagrams will traverse.

(5 points) Your objective is to modify the routing tables such that no loops occur and that hosts on subnet 128.100.2.0/24 can send/receive traffic to and from the rest of the Internet, subject to the following restrictions:

You are not allowed to change the routing table of Router 1; You are not allowed to change the default routing table entries of Routers 2, 3, and 4.

Problem 4. (15 Points) Routing and Switching The figure (next page) shows a network of bridges (Ethernet switches), routers and hosts. The IP configuration of the hosts and routers is as follows:

Ethernet interface ethO Ethernet interface ethi Default Gateway

Hostl 10.0.3.12/24 -- 10.0.3.3

Host2 10.0.3.21/16 -- 10.0.1.2

Host3 10.0.1.11/24 -- 10.0.1.2

Host4 10.0.4.14/16 -- 10.0.4.3

Routerl 10.0.3.3/24 10.0.1.2/24 10.0.3.2

Router2 10.0.3.2/24 10.0.4.3/24 10.0.3.3

In the figure, the ports of the bridges are labeled as p1, p2, etc.

Assume that Proxy ARP is enabled on all interfaces of Router 2 and disabled on all interfaces of Router 1. Assume that ARP caches are empty on all systems at the start of each sub-problem.

a. (10 marks) Consider the list of PING commands below. For each PING command, state if it is successful. If not, state where the command fails, and explain why it fails. Determine the route taken by the ICMP Echo Request and ICMP Echo Reply messages of the PING commands. In your answers, you only need to show the sequence of devices that are traversed by each of the ICMP packets (For example: "Host3 -Routerl -Bridge2-Host 1").

The list of PING commands is:

On Host l: ping 10.0.1.11 On Host 4: ping 10.0.3.21 On Host 2: ping 10.0.1.11

On Host 2: ping 10.0.3.12 On Host 3: ping 10.0.4.14

b. (5 marks) Suppose that all bridges execute the Spanning Tree Protocol (STP). Assume that the bridge identifiers are set to "1" for Bridge 1, to "2" for Bridge 2, and "3" for Bridge 3. Show the results of the STP protocol when the protocol has stabilized, by listing the root ports and designated ports of each bridge.

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ethO OSt 10.0.3.12/24 P1

Bridge3

p3 p2 eth0 10.0.1.11/241

I

ethO I I 1

~RKouterl 10.0.4.3/24

10.0.3.2/24 ethi I 2

Router2

)

Bridgel

p3

1 Bridge2

p4 p3

ost ethO

4 10.0.4.14/16

Host ethO 10.0.3.21/16 2

Figure. Problem 4.

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Problem 5. (10 points) IPv6

(3 points) The stateless address autoconfiguration in IPv6 provides a dynamic IP configuration (including IP address, prefix length, and default gateway) without the help of a DHCP server. Describe the steps and protocols involved in stateless address autoconfiguration.

(3 points) IPv6 can create a 64-bit interface identifier (EUI-64) from the MAC address of an interface. Determine the EUI-64 identifier for the MAC address AC:DE:48:00: 11:22.

(4 points) When transitioning from IPv4 to IPv6, some protocols have to be modified or even replaced, while others can remain unchanged. Indicate what is applicable to the following protocols (the options are "modified", "replaced", or "unchanged"), and add a brief explanation:

c DHCP ARP TCP DNS

Problem 6. (10 Points) Proxy Functions.

(5 points) Describe the purpose and the function of Proxy ARP. Present an example, where Proxy ARP provides benefits.

(5 points) Describe the purpose and the function of a DHCP Relay Agent. Present an example, where a DHCP Relay Agent provides benefits.

Problem 7. (10 Marks) IP Multicast (4 marks) By default, an Ethernet switch that receives a frame containing an IP datagram with a multicast destination IP address forwards the frame on all ports (except the port where the frame arrived). Some Ethernet switches have a feature called IGMP snooping, where the switch listens to IGMP network traffic between hosts and routers on all of its ports.

Describe how a switch can use IGMP snooping to prevent that frames with IP multicast messages are flooded on all ports (except the port where the frame arrived)

(6 marks) Consider the network of multicast routers in the figure. The routers are referred to by a letter (A, B, C, etc.). The links between routers are labeled with weights, which are used for the shortest path computation of unicast routes. Note that links have a different cost in each direction.

To denote interfaces of the routers, we use the convention that AB denotes the interface of Router A on the link to Router B, BD denotes the interface of Router B on the link to Router D, and so on.

Assume that the routers have executed a unicast routing algorithm and computed the shortest paths based on the weights in the figure. (There is no need to show the steps of the unicast routing algorithm.)

Consider that Router D is the core (rendezvous point) of a shared tree for multicast routing for a multicast group 224.2.3.4. The shared tree is constructed with reverse path forwarding (RPF) by using the reverse shortest paths to the core.

b 1) (3 marks) For each router (except Router D) provide the RPF neighbor for multicast group 224.2.3.4 with respect to the core.

b2) (3 marks) For each router (including Router D), provide the multicast routing table entries for 224.2.3.4. (Hint: The columns of the multicast routing table are Source Address, Group Address, RPF interface, outgoing interfaces)

Hexadecimal-to-Decimal Conversion Table

Hex Dec Hex Dec 1 1 10 16

2 2 20 32

3 3 30 48

4 4 40 64

5 5 50 80

6 6 60 96

7 7 70 112

8 8 80 128

9 9 90 144

A 10 AU 160

B 11 BO 176

C 12 CO 192

D 13 DO 208

E 14 EO 224 F 15 FO 240

Hex Dec 100 256 200 512

300 768

400 1024

500 1280

600 1536 700 1792

800 2048

900 2304

AOO 2560

BOO 2816

COO 3072

DUO 3328

EOO 3584

FOO 3840

Hex Dec 1000 4096

2000 8192

3000 12288

4000 16384

5000 20480

6000 24576

7000 28672

8000 32768

9000 36864

A000 40960

B000 45056

C000 49152

D000 53248

E000 57344

F000 61440

Binary-Decimal Conversion

Last 4 bits-)

First 4 bits4, 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

0000 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0001 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0010 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 0011 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 0100 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 0101 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 0110 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 0111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 1000 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 1001 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 1010 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 1011 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 1100 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 1101 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 1110 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 1111 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255

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