Computer Networks study guide

5.1-1. Routing versus forwarding. Which of the following statements correctly identify the differences between routing and forwarding. Select one or more statements.

Forwarding refers to moving packets from a router's input to appropriate router output, and is implemented in the data plane Routing refers to determining the route taken by packets from source to destination, and is implemented in the control plane.

5.4-5. eBGP or iBGP? Consider routers 2c and 2d in Autonomous System AS2 in the figure below. Indicate the flavor of BGP and the router from which each of 2c and 2d learns about the path to destination x. How does router 2c learn of the path AS3, X to destination network X? How does router 2d learn of the path AS3, X to destination network X?

From 3a via eBGP. From 2c via iBGP.

4.3.10. Network Address Translation (NAT). Which one of the following operations is not performed by NAT.?

Generating ACKs back to the TCP sender and then taking responsibility for reliably delivery the segment to its destination, possibly using a non-TCP reliable data transfer protocol.

4.4-6. Crafting network-wide forwarding using flow tables (more). Consider the network below. We want to specify the match+action rules at s3 so that s3 acts only as a relay for traffic between 137.220/16 and 67.56/16. In particular s3 should not accept/forward and traffic to/from 128.119/16. From the list of match+action rules below, select the rules to include in s3's flow table to implement this forwarding behavior. Assume that if a packet arrives and finds no matching rule, it is dropped.

Input port: 3; Dest: 137.220/16 Action: forward(2) Input port: 2; Dest: 67.56/16 Action: forward(3)

4.2-4. Longest prefix matching. Consider the following forwarding table below. Indicate the output to link interface to which a datagram with the destination addresses below will be forwarded under longest prefix matching. (Note: The list of addresses is ordered below. If two addresses map to the same output link interface, map the first of these two addresses to the first instance of that link interface.)

Just match the prefixes. Not hard

4.1-3. The control plane versus the data plane. For each of the actions below, select those actions below that are primarily in the network-layer data plane. The other actions that you don't select below then correspond to control-plane actions.

Moving an arriving datagram from a router's input port to output port Looking up address bits in an arriving datagram header in the forwarding table. Dropping a datagram due to a congested (full) output buffer.

Can you decrypt the hash of a message to get the original message? Explain your answer?

No. This is because a hash function is a one-way function. That is, given any hash value, the original message cannot be recovered (given h such that h=H(m), one cannot recover m from h).

5.4-4. Path advertisement and policy (Part 3). Again, suppose a provider network only wants to carry traffic to/from its customer networks (i.e., to provide no transit service), and customer networks only want to carry traffic to/from itself. Suppose C has advertised path Cy to x. To implement this policy, to which of the following networks would network x advertise the path xCy?

None of these other networks

5.5-4. Internal structure of the SDN controller (3). Which of the functions below belong in the controller layer labeled "Communication to/from controlled device"? Check all below that apply.

OpenFlow protocol

6.3-4. Characteristics of Multiple Access Protocols (a). Consider the following multiple access protocols that we've studied: (1) TDMA, and FDMA (2) CSMA (3) Aloha, and (4) polling. Which of these protocols are collision-free (e.g., collisions will never happen)?

Polling TDMA and FDMA

6.3-5. Characteristics of Multiple Access Protocols (b). Consider the following multiple access protocols that we've studied: (1) TDMA, and FDMA (2) CSMA (3) Aloha, and (4) polling. Which of these protocols requires some form of centralized control to mediate channel access?

Polling TDMA and FDMA

6.3-6. Characteristics of Multiple Access Protocols (c). Consider the following multiple access protocols that we've studied: (1) TDMA, and FDMA (2) CSMA (3) Aloha, and (4) polling. For which of these protocols is the maximum channel utilization 1 (or very close to 1)?

Polling TDMA and FDMA

6.3-7. Characteristics of Multiple Access Protocols (d). Consider the following multiple access protocols that we've studied: (1) TDMA, and FDMA (2) CSMA (3) Aloha, and (4) polling. For which of these protocols ss there a maximum amount of time that a node knows that it will have to wait until it can successfully gain access to the channel?

Polling TDMA and FDMA

5.5-3. Internal structure of the SDN controller (2). Which of the functions below belong in the controller layer labeled "Network-wide distributed, robust state management"? Check all below that apply.

Switch information Statistics Flow tables Link-state information Host information

4.5-3. The end-to-end principle. Which of the statements below are true statements regarding the "end-to-end principle"? Check all that apply.

The end-to-end argument allows that some redundant functionality might be placed both in-network and at the network edge in order to enhance performance. The end-to-end argument advocates placing functionality at the network edge because some functionality cannot be completely and correctly implemented in the network, and so needs to be placed at the edge in any case, making in-network implementation redundant.

6.3-3. Polling and token-passing protocols. Which of the following statements is true about polling and token-passing protocols?

These protocol can achieve close 100% utilization, in the case that there is only one node that always has frames to send (the fact that the utilization is close to, but not exactly, 100% is due to a small amount of medium access overhead but not due to collisions) These protocol can achieve close to 100% channel utilization, in the case that all nodes always have frames to send (the fact that the utilization is close to, but not exactly, 100% is due to a small amount of medium access overhead but not due to collisions)

6.4-3. Different types of addressing (c). We've now learned about both IPv4 addresses and MAC addresses. Consider the address properties below, and use the pulldown menu to indicate which of these properties is a property of both IPv4 addresses and MAC addresses.

This address must be unique among all hosts in a subnet.

6.4-1. Different types of addressing (a). We've now learned about both IPv4 addresses and MAC addresses. Consider the address properties below, and use the pulldown menu to indicate which of these properties is only a property of MAC addresses (and therefore is not a property of IPv4 addresses - careful!).

This is a 48-bit address. This address remains the same as a host moves from one network to another. This is a link-layer address.

4.3-9. Purpose of DHCP. What is the purpose of the Dynamic Host Configuration Protocol?

To obtain an IP address for a host attaching to an IP network.

5.4-3. Path advertisement and policy (Part 2). Again, suppose a provider network only wants to carry traffic to/from its customer networks (i.e., to provide no transit service), and customer networks only want to carry traffic to/from itself. Suppose C has advertised path Cy to A. To implement this policy, to which of the following networks would network A advertise the path ACy?

W

4.4-1. Destination-based match+action. Destination-based forwarding, which we studied in section 4.2, is a specific instance of match+action and generalized forwarding. Select the phrase below which best completes the following sentence: "In destination-based forwarding, ..."

... after matching on the destination IP address in the datagram header, the action taken is to forward the datagram to the output port associated with that destination IP address.

6.3-1. Multiple Access protocols (a). Consider the figure below, which shows the arrival of 6 messages for transmission at different multiple access wireless nodes at times t = 0.3, 1.7, 1.8, 2.5, 4.2, 4.6. Each transmission requires exactly one time unit. For the pure ALOHA protocol, indicate which packets are successfully transmitted. You can assume that if a packet experiences a collision, a node will not attempt a retransmission of that packet until sometime after t=5.

1

6.3-2. Multiple Access protocols (b). Consider the figure below, which shows the arrival of 6 messages for transmission at different multiple access wireless nodes at times t = 0.3, 1.7, 1.8, 2.5, 4.2, 4.6. Each transmission requires exactly one time unit. For the pure ALOHA protocol, indicate which packets are successfully transmitted. You can assume that if a packet experiences a collision, a node will not attempt a retransmission of that packet until sometime after t=5.

1 4

6.3-4. Multiple Access protocols (d). Consider the figure below, which shows the arrival of 6 messages for transmission at different multiple access wireless nodes at times t = 0.3, 1.7, 1.8, 2.5, 4.2, 4.6. Each transmission requires exactly one time unit. For the CSMA/CD protocol (with collision detection), indicate which packets are successfully transmitted. You should assume that it takes .2 time units for a signal to propagate from one node to each of the other nodes. You can assume that if a packet experiences a collision or senses the channel busy and that that node will not attempt a retransmission of that packet until sometime after t=5. If a node senses a collision, it stops transmitting immediately (although it will still take .2 time units for the last transmitted bit to propagate to all other nodes). Hint: consider propagation times carefully here.

1 4 5

6.3-3. Multiple Access protocols (c). Consider the figure below, which shows the arrival of 6 messages for transmission at different multiple access wireless nodes at times t = 0.3, 1.7, 1.8, 2.5, 4.2, 4.6. Each transmission requires exactly one time unit. For the CSMA protocol (without collision detection), indicate which packets are successfully transmitted. You should assume that it takes .2 time units for a signal to propagate from one node to each of the other nodes. You can assume that if a packet experiences a collision or senses the channel busy and that that node will not attempt a retransmission of that packet until sometime after t=5. Hint: consider propagation times carefully here.

1 5

4.2-7a.Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under FCFS scheduling. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

1 2 3 4 5 6 7

4.2-8a. Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one-time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under FCFS scheduling. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

1 2 3 4 5 6 7

4.2-7c.Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under round robin scheduling, where red starts a round if there are both red and green packets ready to transmit after an empty slot. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1)

1 2 3 4 5 7 6

4.2-7b.Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one-time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under priority scheduling, where red packets have higher priority. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

1 2 3 5 4 7 6

4.2-8b.Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under priority scheduling, where red packets have higher priority. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

1 2 4 3 5 6 7

4.2-8c. Packet scheduling. Consider the pattern of red and green packet arrivals to a router's output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival. Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under round robin scheduling, where red starts a round if there are both red and green packets ready to transmit after an empty slot. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

1 2 4 3 5 6 7

6.4-1 Network- and Link-layer addressing: an end-to-end-scenario (1a). Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router's interfaces. Consider an IP datagram being sent from node B to node D. Match the source/destination network- or link-layer address at the location (3) by choosing a value from the pulldown list What is the source MAC address on the frame at point (3)? What is the destination MAC address on the frame at point (3)? What is the source IP address of the datagram at point (3)? What is the destination IP address of the datagram at point (3)? What is the destination IP address of the datagram at point (3)?

68-01-BC-58-AF-24 49-FA-B0-3C-E2-7C 128.119.50.107 128.119.240.116

6.4-8 Network- and Link-layer addressing: an end-to-end-scenario (2b). Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router's interfaces. Consider an IP datagram being sent from node A to node C. Match the source/destination network- or link-layer address at the location (4) by choosing a value from the pulldown list What is the source MAC address on the frame at point (4)? What is the destination MAC address on the frame at point (4)? What is the source IP address of the datagram at point (4)? What is the destination IP address of the datagram at point (4)?

72-9E-4A-31-9C-42 4C-9D-AA-74-D6-1F 128.119.97.18 128.119.240.52

6.4-2 Network- and Link-layer addressing: an end-to-end-scenario (1b). Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router's interfaces. Consider an IP datagram being sent from node B to node D. Match the source/destination network- or link-layer address at the location (4) by choosing a value from the pulldown list What is the source MAC address on the frame at point (4)? What is the destination MAC address on the frame at point (4)? What is the source IP address of the datagram at point (4)? What is the destination IP address of the datagram at point (4)?

72-9E-4A-31-9C-42 D5-A0-EE-9A-73-D5 128.119.50.107 128.119.240.116

4.3-8. IPv4 versus IPv6. Which of the following fields occur ONLY in the IPv6 datagram header (i.e., appear in the IPv6 header but not in the IPv4 header)? Check all that apply.

128-bit source and destination IP addresses. The flow label field

4.3-5c. Subnetting. Consider the three subnets in the diagram below. Which of the following addresses can not be used by an interface in the 223.1.3/29 network? Check all that apply.

223.1.3.28 223.1.2.6 223.1.3.16

4.3-5a. Subnetting. Consider the three subnets in the diagram below. What is the maximum # of interfaces in the 223.1.2/24 network?

256

6.2-1. Two dimensional parity. Which of the following statements is true about a two-dimensional parity check (2D-parity) computed over a payload?

2D-parity can detect any case of a single bit flip in the payload. 2D-parity can detect and correct any case of a single bit flip in the payload. 2D-parity can detect any case of two bit flips in the payload.

5.2-5. Dijkstra's link-state routing algorithm (Part 2). Consider the graph shown below and the use of Dijkstra's algorithm to compute a least cost path from a to all destinations. Suppose that nodes b and d have already been added to N'. What is the path cost to the next node to be added to N' (refer to the text for an explanation of notation)

4

6.4-7 Network- and Link-layer addressing: an end-to-end-scenario (2a). Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router's interfaces. Consider an IP datagram being sent from node A to node C. Match the source/destination network- or link-layer address at the location (2) by choosing a value from the pulldown list What is the source MAC address on the frame at point (2)? What is the destination MAC address on the frame at point (2)? What is the source IP address of the datagram at point (2)? What is the destination IP address of the datagram at point (2)?

77-34-F1-EF-14-72 CC-A5-81-0B-AE-33 128.119.97.18 128.119.240.52

4.3-5b. Subnetting. Consider the three subnets in the diagram below. What is the maximum # of interfaces in the 223.1.3/29 network?

8

4.2-6. HOL blocking. What is meant by Head of the Line (HOL) blocking?

A queued datagram waiting for service at the front of a queue prevents other datagrams in queue from moving forward in the queue.

4.2-2. Where does destination address lookup happen? Where in a router is the destination IP address looked up in a forwarding table to determine the appropriate output port to which the datagram should be directed?

At the input port where a packet arrives.

4.2-3. Where does "match+action" happen? Where in a router does "match plus action" happen to determine the appropriate output port to which the arriving datagram should be directed?

At the input port where a packet arrives.

6.1-1. Link-layer services. Which of the following services may be implemented in a link-layer protocol? Select one or more statements.

Bit-level error detection and correction. Multiplexing down from / multiplexing up to a network-layer protocol. Reliable data transfer between directly connected nodes. Flow control between directly connected nodes. Coordinated access to a shared physical medium.

5.2-3. What type of routing? Match the name of a general approach to routing with characteristics of that approach.

Centralized, global routing-All routers have complete topology, and link cost information. Decentralized routing-An iterative process of computation, exchange of informatoin with neighbors. Routers may initially only know link costs to directly-attached neighbors. Static routing-Routes change slowly over time Dynamic routing-Routing changes quickly over time.

5.2-4. Dijkstra's link-state routing algorithm (Part 1). Consider the graph shown below and the use of Dijkstra's algorithm to compute a least cost path from a to all destinations. Suppose that nodes b and d have already been added to N'. What is the next node to be added to N' (refer to the text for an explanation of notation).

E

5.3-3. Open Shortest Path First (OSPF). Consider the OSPF routing protocol. Which of the following characteristics are associated with OSPF (as opposed to BGP)?+

Finds a least cost path from source to destination. Floods link state control information. Is an intra-domain routing protocol.

4.5-1. What's a "middlebox"? Which of the following network devices can be thought of as a "middlebox"? Check all that apply.

HTTP cache HTTP load balancer Network Address Translation box

5.6-1. ICMP: Internet control message protocol. Which of the statements below about ICMP are true?

ICMP messages are carried directly in IP datagrams rather than as payload in UDP or TCP segments. ICMP is used by hosts and routers to communicate network-level information The TTL-expired message type in ICMP is used by the traceroute program

4.5-2. The "thin waist" of the Internet. What protocol (or protocols) constitutes the "thin waist" of the Internet protocol stack? Check all that apply.

IP

Suppose N people want to communicate with each of N - 1 other people using symmetric key encryption. All communication between any two people, i and j, is visible to all other people in this group of N, and no other person in this group should be able to decode their communication. How many keys are required in the system as a whole? Now suppose that public key encryption is used. How many keys are required in this case?

If each user wants to communicate with N other users, then each pair of users musthave a shared symmetric key. There are N*(N-1)/2 such pairs and thus there areN*(N-1)/2 keys. With a public key system, each user has a public key which isknown to all, and a private key (which is secret and only known by the user). Thereare thus 2N keys in the public key system.

Suppose that an intruder has an encrypted message as well as the decrypted version of that message. Can the intruder mount a ciphertext-only attack, a known-plaintext attack, or a chosen-plaintext attack?

In this case, a known plaintext attack is performed. If, somehow, the messageencrypted by the sender was chosen by the attacker, then this would be a chosenplaintext attack.

6.2-1 Two dimensional parity: error detection and correction. Suppose that a packet's payload consists of 10 eight-bit values (e.g., representing ten ASCII-encoded characters) shown below. (Here, we have arranged the ten eight-bit values as five sixteen-bit values). The received data (including parity) bits are shown. Even parity is used. One received data bit has been flipped. Which one is it? (row and column numbering start at 1) Use the pulldown boxes to specify the row and column where the flipped bit appears (indicates start at 1)

In which row has the bit flip occurred? 3 In which column has the bit flip occurred? 9

5.5-2. Internal structure of the SDN controller (1). Which of the functions below belong in the controller layer labeled "Interface, abstractions for network control apps"? Check all below that apply.

Intent Network graph

5.3-1. Routing within or among networks. Match the terms "interdomain routing" and intradomain routing" with their definitions. Recall that in Internet parlance, an "AS" refers to "Autonomous System" - a network under the control of a single organization.

Interdomain routing-Routing among different ASes ("networks"). Intradomain routing-Routing among routers within same AS ("network")

4.3-6. Plug-and-play. What is meant by saying that DHCP is a "plug and play" protocol?

No manual configuration is needed for the host to join the network.

4.3-1. What is the Internet Protocol? What are the principal components of the IPv4 protocol (check all that apply)?

Packet handling conventions at routers (e.g., segmentation/reassembly) IPv4 datagram format. IPv4 addressing conventions.

5.2-1. What's a "good" path? What is the definition of a "good" path for a routing protocol? Choose the best single answer.

Routing algorithms typically work with abstract link weights that could represent any of, or combinations of, all of the other answers.

4.4-4. Match+action in Openflow 1.0. Consider the figure below that shows the generalized forwarding table in a router. Recall that a * represents a wildcard value. Now consider an arriving datagram with the IP source and destination address fields indicated below. For each source/destination IP address pair, indicate which rule is matched. Note: assume that a rule that is earlier in the table takes priority over a rule that is later in the table and that a datagram that matches none of the table entries is dropped. Source: 1.2.56.32 Destination:128.116.40.186 Source: 65.92.15.27 Destination: 3.4.65.76 Source: 10.1.2.3 Destination: 7.8.9.2 Source: 10.1.34.56 Destination: 54.72.29.90

Rule 2, with action drop Rule 1, with action forward(2) Rule 3, with action send to controller No match to any rule.

5.5-1. SDN Layers. Consider the SDN layering shown below. Match each layer name below with a layer label (a), (b) or (c) as shown in the diagram.

SDN Controller (network operating system)-B SDN-controlled switches-C. Network-control applications-A.

5.2-2. Dijkstra's link-state routing algorithm. Consider Dijkstra's link-state routing algorithm that is computing a least-cost path from node a to other nodes b, c, d, e, f. Which of the following statements is true. (Refer to Section 5.2 in the text for notation.)

Suppose nodes b, c, and d are in the set N'. These nodes will remain in N' for the rest of the algorithm, since the least-cost paths from a to b, c, and d are known. In the initialization step, the initial cost from a to each of these destinations is initialized to either the cost of a link directly connecting a to a direct neighbor, or infinity otherwise. The values computed in the vector D(v), the currently known least cost of a path from a to any node v, will never increase following an iteration.

4.1-1. The network layer - where is it? Check all of the statements below about where (in the network) the network layer is implemented that are true.

The network layer is implemented in hosts at the network's edge. The network layer is implemented in routers in the network core.

4.2-5. Packet dropping. Suppose a datagram is switched through the switching fabric and arrives to its appropriate output to find that there are no free buffers. In this case:

The packet will either be dropped or another packet will be removed (lost) from the buffer to make room for this packet, depending on policy. But the packet will definitely not be sent back to the input port.

What is the purpose of nonce in authentication protocols?

The purpose of the nonce is to defend against the replay attack.

6.3-2. Pure Aloha and CSMA. Which of the following statements is true about both Pure Aloha, and CSMA (both with and without collision detection?

There can be simultaneous transmissions resulting in collisions. Pure Aloha and CSMA can achieve 100% utilization, in the case that there is only one node that always has frames to send

6.4-2. Different types of addressing (b). We've now learned about both IPv4 addresses and MAC addresses. Consider the address properties below, and use the pulldown menu to indicate which of these properties is only a property of IPv4 addresses (and therefore is not a property of MAC addresses - careful!).

This address is allocated by DHCP. This is a 32-bit address. This is a network-layer address.

RSA is a public cryptography algorithm True/False

True

RSA is slower than DES True/False

True

6.4-6. Self Learning Switches (c). Consider the network below with six nodes, star-connected into an Ethernet switch. Suppose that A sends a frame to A', A' replies to A, then B sends a message to B' and B' replies to B, and then A sends to B and B replies to A. In this sequence of frame transmissions, how many frames have appeared at the interface at C'? Assume that the switch's table is initially empty

Two frames appear at the C' interface.

4.4-3. What fields can be matched in generalized match+action. Which of the following fields in the frame/datagram/segment/application-layer message can be matched in OpenFlow 1.0? Check all that apply.

Upper layer protocol field IP source address IP type-of-service field IP destination address Source and/or destination port number

4.3-2. The IPv4 header. Match each of the following fields in the IP header with its description, function or use.

Version field-This field contains the IP protocol version number Type-of-service field-This field contains ECN and differentiated service bits. Fragmentation offset field-This field is used for datagram fragmentation/reassembly. Time-to-live field-The value in this field is decremented at each router; when it reaches zero, the packet must be dropped. Header checksum field-This field contains the Internet checksum of this datagram's header fields. Upper layer field-This field contains the "protocol number" for the transport-layer protocol to which this datagram's payload will be demultiplexed - UDP or TCP, for example. Payload/data field-This field contains a UDP or TCP segment, for example. Datagram length field- This field indicates the total number of bytes in datagram.

4.4-5. Crafting network-wide forwarding using flow tables. Consider the network below. We want to specify the match+action rules at s3 so that only the following network-wide behavior is allowed: 1 traffic from 128.119/16 and destined to 137.220/16 is forwarded on the direct link from s3 to s1; 2. traffic from 128.119/16 and destined to 67.56/16 is forwarded on the direct link from s3 to s2; 3. incoming traffic via port 2 or 3, and destined to 128.119/16 is forwarded to 128.119/16 via local port 1. 4. No other forwarding should be allowed. In particular s3 should not forward traffic arriving from 137.220/16 and destined for 67.56/16 and vice versa. From the list of match+action rules below, select the rules to include in s3's flow table to implement this forwarding behavior. Assume that if a packet arrives and finds no matching rule, it is dropped.

input port: 2; Dest: 128.119/16 Action: forward(1) Input port:1 ; Dest: 137.220/16 Action: forward(2) Input port: 1; Dest: 67.56/16 Action: forward(3) Input port: 3; Dest: 128.119/16 Action: forward(1)

5.4-1. Routing within networks? Among the following protocols, terminology or considerations, indicate those that are associated with "routing within a single network (typically owned and operated by one organization)."

intra-AS routing intra-domain routing Driven more by performance than by routing policy OSPF

6.4-3 Network- and Link-layer addressing: an end-to-end-scenario (1c). Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router's interfaces. Consider an IP datagram being sent from node B to node D. Match the source/destination network- or link-layer address at the location (5) by choosing a value from the pulldown list. What is the source MAC address on the frame at point (5)? What is the destination MAC address on the frame at point (5)? What is the source IP address of the datagram at point (5)? What is the destination IP address of the datagram at point (5)?

72-9E-4A-31-9C-42 D5-A0-EE-9A-73-D5 128.119.50.107 128.119.240.116

6.4-5. Self Learning Switches (b). Consider the network below with six nodes, star-connected into an Ethernet switch. Suppose that A sends a frame to A', A' replies to A, then B sends a message to B' and B' replies to B. Suppose you are node C, and consider the frames arriving to node C's interface (whether they are destined to C or not). From what senders do these frames arrive?

A B C'

4.3-7. DHCP request message. Which of the following statements about a DHCP request message are true (check all that are true). Hint: check out Figure 4.24 in the 7th and 8th edition of our textbook.

A DHCP request message may contain the IP address that the client will use. The transaction ID in a DHCP request message will be used to associate this message with future DHCP messages sent from, or to, this client. A DHCP request message is sent broadcast, using the 255.255.255.255 IP destination address.

4.1-2. Forwarding versus routing. Consider the travel analogy discussed in the textbook - some actions we take on a trip correspond to forwarding and other actions we take on a trip correspond to routing. Which of the following travel actions below correspond to forwarding? The other travel actions that you don't select below then correspond to routing.

A car waits at light and then turns left at the intersection. A car takes the 3rd exit from a roundabout. A car stops at an intersection to "gas-up" and take a "bathroom break"

4.1-4. What type of control plane? We've seen that there are two approaches towards implementing the network control plane - a per-router control-plane approach and a software-defined networking (SDN) control-plane approach. Which of the following actions occur in a per-router control-plane approach? The other actions that you don't select below then correspond to actions in an SDN control plane.

A router exchanges messages with another router, indicating the cost for it (the sending router) to reach a destination host. Routers send information about their incoming and outgoing links to other routers in the network.

6.4-4. Self Learning Switches (a). Consider the network below with six nodes, star-connected into an Ethernet switch. Suppose that A sends a frame to A', A' replies to A, then B sends a message to B' and B' replies to B. Enter the values that are present in the switch's forwarding table after B'-to-B frame is sent and received. Assumed that the table is initially empty and that entries are added to the table sequentially. Answer the questions below from the pulldown list What is the first entry added to the table? What is the second entry added to the table? What is the third entry added to the table? What is the fourth entry added to the table?

A,1 A',4 B,2 B',5

4.2-1. What's inside a router? Match the names of the principal router components (A,B,C,D below) with their function and whether they are in the network-layer data plane or control plane.

A. input ports, operating primarily in the data plane. B. the switching fabric, operating primarily in the data plane. C. output ports, operating primarily in the data plane. D. the routing processor, operating primarily in the control plane.

Consider an 8-bit block cipher. How many possible input blocks does this cipher have? How many possible mappings are there? If we view each mapping as a key, then how many possible keys does this cipher have?

An 8-block cipher has 2^8possible input blocks. Each mapping is a permutation of the2^8input blocks; so there are 2^8! possible mappings; so there are 2^8! possible keys.

4.3-3. What is an IP address actually associated with? Which of the following statements is true regarding an IP address? (Zero, one or more of the following statements is true).

An IP address is associated with an interface. If a router has more than one interface, then it has more than one IP address at which it can be reached If a host has more than one interface, then it has more than one IP address at which it can be reached.

6.4-8. Learning switch state removal. Consider the simple star-connected Ethernet LAN shown below, and suppose the switch table contains entries for each of the 6 hosts. How will those entries be removed from the switch table?

An entry for a host will be removed if that host doesn't transmit any frames for a certain amount of time (that is, table entries will timeout).

5.4-2. Path advertisement and policy (Part 1). Suppose a provider network only wants to carry traffic to/from its customer networks (i.e., to provide no transit service), and customer networks only want to carry traffic to/from itself. Consider the figure below. To implement this policy, to which of the following networks would network C advertise the path Cy?

B x A

6.3-1. Channel partitioning protocols. Which of the following statements is true about channel partitioning protocols?

Channel partitioning protocols can achieve 100% channel utilization, in the case that all nodes always have frames to send There can be times when the channel is idle, when a node has a frame to send, but is prevented from doing so by the medium access protocol

6.4-4. Fields in an Ethernet frame. Use the pulldown menus below to match the name of the field with the function/purpose of a field within an Ethernet frame.

Cyclic redundancy check (CRC) field Used to detect and possibly correct bit-level errors in the frame. Source address field 48-bit MAC address of the sending node Data (payload) field The contents of this field is typically (bit not always) a network-layer IP datagram. Type field. Used to demultiplex the payload up to a higher level protocol at the receiver. Sequence number field This field does not exist in the Ethernet frame

4.4-2. Generalized match+action. Which of the following match+actions can be taken in the generalized OpenFlow 1.0 match+action paradigm that we studied in Section 4.4? Check all that apply.

after matching on the 48-bit link-layer destination MAC address, the action taken is to forward the datagram to the output port associated with that link-layer address after matching on the port number in the segment's header, the action taken is to forward the datagram to the output port associated with that destination IP address. after matching on the port number in the segment's header, the action taken is to decide whether or not to drop that datagram containing that segment. after matching on the source and destination IP address in the datagram header, the action taken is to forward the datagram to the output port associated with that source and destination IP address pair. after matching on the destination IP address in the datagram header, the action taken is to forward the datagram to the output port associated with that destination IP address. after matching on the destination IP address in the datagram header, the action taken is to decide whether or not to drop that datagram. Literally everything except this: ... after matching on the URL contained in an HTTP GET request in the TCP segment within the IP datagram, the action taken is to determine the IP address of the server associated with that URL, and to forward the datagram to the output port associated with that destination IP address.****