Networks Exam 2

7-1-1. How fast is that wireless technology? Use the pulldown menus below to match the approximate transmission rate with the the wireless technology that achieves that rate. Of course, sender/receiver distance, noise and other factors determine actual transmission speed, so "your mileage may vary" (YMMV).

- 802.11 ax: 14 Gbps - 5G cellular: 10 Gbps - 802.11 ac: 3.5 Gpbs - 4G LTE: hundreds of Mbps - 802.11 g: 54 Mbps - Bluetooth: 2 Mbps

7.3.2. RTS/CTS frames. What is the purpose of RTS (request to send) and CTS (clear to send) frames in WiFi (802.11) networks? Select one or more of the answers below. [Hint: check two answers below].

- A CTS that is sent allows a receiver to force other nodes (other than the intended sender who sent the RTS) to refrain from transmitting, thus allowing the sender who sent the RTS to then transmit a frame with less likelihood of a collision. - RTC/CTS frames helps nodes in a wireless network mitigate the effects of the hidden terminal problem.

6.4-6. Self-learning switches. Which of the following statements are true about a self learning switch?

- A self learning switch associates the source MAC address on an incoming frame with the port on which it arrived, and stores this matching in a table. The switch has now learned the port that leads to that MAC address. - A self-learning switch will age-out (forget) a self-learned association of a MAC address x and switch port y if it doesn't see a frame with MAC address x incoming on switch port y after some amount of time. - A self-learning switch frees a network manager from a least one configuration task that might be associated with managing a switch

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).

7.3.2. Use of ACKs in WiFi. Why are link-layer ACKs used in WiFi (802.11) networks? [Hint: check two of the boxes below].

- Because of the hidden terminal problem, a node that is transmitting and hears no collisions still doesn't know if there was a collision at the receiver. - Wireless links are noisier than wired links, and so bit level errors are more likely to occur, making link-layer error recovery more valuable that in less-noisy wired links.

Match the name of a general approach to routing with characteristics of that approach.

- Centralized, global routing => All routers have... - Decentralized routing => an iterative process of computation... - Static routing => routes change slowly over time - Dynamic routing => routing changes quickly over time

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 - Source address field: 48-bit MAC - 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

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.

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.

- 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. - 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. - 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.

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 with same AS network

Consider the OSPF routing protocol. Which of the following characteristics are associated with OSPF (as opposed to BGP)?

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

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)."

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

Check the one or more of the following statements about the OSPF protocol that are true.

- OSPF implements hierarchical routing - OSPF is an intra-domain routing protocol. - OSFP uses a Dijkstra-like algorithm to implement least cost path routing.

7.3.2. Characteristics of wireless links. WHich of the following statements about the characteristics of wireless links are true?

- Path loss refers to the decrease in the strength of a radio signal as it propagates through space. - The bit error rate (BER) of a wireless channel decreases as the signal-to-noise ratio (SNR) increases. - The "hidden terminal problem" happens when A sends to B over a wireless channel, and an observer, C (that can be even closer to A than B), does not detect/receive A's transmission because of physical obstacles in the path between A and C. - Multipath propagation occurs when portions of the electromagnetic wave reflect off objects and the ground taking paths of different lengths between the sender and a receiver, and thus arriving at the receiver at slightly different points in time.

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

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

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

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

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

Which of the following statements is true about channel partitioning protocols?

- 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. - Channel partitioning protocols can achieve 100% channel utilization, in the case that all nodes always have frames to send.

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 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) - 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)

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

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

7.3-1. Beacon Frames. What is the purpose of a beacon frame in WiFi (802.11) networks?

A beacon frame allows an access point to advertise its existence, and the frequency channel it is operating on, to devices that want to connect to an access point.

Limiting use of resources or capabilities to given users.

Access control

7.4-2. IMSI. In 4G LTE cellular systems, what is an International Mobile SubscriberIdentity (IMSI)?

Assigned by a mobile carrier network to a device, when the device attaches to the radio access network, serving a similar link-layer role as MAC addresses in a wired network.

Proving you are who you say you are

Authentication

7.3-3. Why three addresses? Why does the WiFi (802.11) link-layer frame have three addresses? [Note: WiFi actually has four MAC addresses in the frame, but we're only focusing here on the three widely used ones].

Because both the access point that will relay this frame to the intended link-layer receiving host or router interface, as well as that intended destination host or router interface need to be specified.

7.3-6. Bluetooth. Which of the following statement are true about the Bluetooth protocol?

Bluetooth uses TDM, FDM, polling, error detection and correction, and has sleep modes to conserve device power. Pretty sophisticated for a consumer technology!

7.4-3. Reliable data transfer at the link layer. Which of the following statements is true about the link-level service of reliable data transfer (using ACKs) in WiFi (802.11) networks and in 4G cellular networks?

Both WiFi and LTE provide link-level reliable data transfer.

7.1-2. Infrastructure Mode. What is meant when we say that a network of devices is operating in "infrastructure mode"?

Devices communicate with each other and to the larger outside world via a base station (also known as an access point).

Used to detect tampering/changing of message contents, and to identify the originator of a message.

Digital signature

Provides confidentiality by encoding contents

Encryption

Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections

Firewall

6.4-5. Switch forwarding and filtering. Suppose an Ethernet frame arrives to an Ethernet switch, and the Ethernet switch does not know which of its switch ports leads to the node with the given destination MAC address? In this case, what does the switch do?

Flood the frame on all ports except the port on which the frame arrived.

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

What is the definition of a "good" path for a routing protocol? Chose 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.

Which of the functions below belong in the controller layer labeled "Network-wide distributed, robust state management"? Check all below that apply.

Statistics, host information, flow tables, link-state information, switch information

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)?

TDMA, FDMA Polling

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?

TDMA, FDMA Polling

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)?

TDMA, FDMA Polling

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?

TDMA, FDMA Polling

7.3-5. The 802.11 Media Access Control protocol. Which of the following statement are true about the 802.11 (WiFi) MAC protocol?

The 802.11 MAC protocol performs collision avoidance. That is, an 802.11 sender and receiver can use approaches such as RTS/CTS, inter-frame spacing, and explicit acknowledgments to try avoid, rather than detect, colliding transmissions from another node.

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.

Which of the functions below belong in the controller layer labeled "Communication to/from controlled device"? Check all below that apply.

openflow protocol