Computer Networking: A Top-Down Approach Knowledge Checks 5 and 6

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 following statements are true about MAC (link-layer) addresses? Select one or more statements below.

Generally does not change, and is associated with a device when it is manufactured/created. Generally stays unchanged as a device moves from one network to another. Has 48 bits.

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?

How does router 2c learn of the path AS3, X to destination network X? From 3a via eBGP. How does router 2d learn of the path AS3, X to destination network X? From 2c via iBGP.

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

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

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

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 networks.

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 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

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 collisions-free (e.g., collisions will never happen)?

Polling TDMA and FDMA

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

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. Multiplexing down from / multiplexing up to a network-layer protocol. Bit-level error detection and correction.

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) SDN-controlled switches Network-control applications

SDN Controller (network operating system): (b) SDN-controlled switches: (c) Network-control applications: (a)

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

Switch information Link-state information Host information Statistics Flow tables

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)

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 both IPv4 addresses and MAC addresses.

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

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 move from one network to another.

Consider the simple star-connected Ethernet LAN shown below, and suppose the Ethernet switch is a learning switch, and that the switch table is initially empty. Suppose C sends an Ethernet frame address to C' and C' replies back to C. How many of these two frames are also received at B's interface?

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 path cost to the next node to be added to N' (refer to the text for an explanation of notation).

4

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 two bit flips 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 a single bit flip in the payload.

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.

Which of the following statements correctly identify the differences between routing and forwarding. Select on 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.

Which of the statements below about ICMP are true?

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

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. Intradomain routing.

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

Which of the functions below belong in the controller layer labeled "Interface, abstractions for network control apps"? Check all below that apply.

Network graph Intent

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.

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

OpenFlow protocol

Match the name of the approach towards implementing a control plane with a description of how this approach works. Question List: Per-router control plane. Software-defined networking (SDN).

Per-router control plane: Individual routing algorithm components - with a component in each and every router - interact with each other in the control plane. The individual routing algorithm component executing in a given router computes the local forwarding table for that router. Software-defined networking (SDN): A (typically) remote controller gathers information from routers, and then computes and installs the forwarding tables in routers.

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

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.

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 be, 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. 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.

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.

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 in only a property of IPv4 addresses (and therefor is not a property of MAC address - careful!)

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

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?

X B A

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

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 frees a network manager from at least one configuration task that might be associated with managing a switch. 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.

Consider the simple star-connected Ether 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).

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 information with neighbors. Routers may initially only know link costs to directly-attached neighbors. Static routing: Routers changes slowly over time. Dynamic routing: Routing changes quickly over time.

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