计算机网络.doc

1、#QuestionAnswers (choose only one)1What are the two types of services that the Internet provides to its applications?A.Text transfer and file transferB.Reliable data delivery and “best effort” data deliveryC.FDM and TDMD.Network core and network edge2What advantage does a circuit-switched network ha

2、ve over a packet-switched network?A.No call setup is requiredB.Packets of different users always share network resourcesC.Resources used as neededD.Guaranteed transmission rate during the communication3What advantage does TDM have over FDM in a circuit-switched network?A.TDM may dynamically adjust t

3、he bandwidth of different usersB.TDM always transmits the data immediately without any delayC.FDM always requires analog-to-digital and digital-to-analog converters while TDM does notD.It is not possible to use FDM in radio physical media4What is the key distinguishing difference between a tier-lISP

4、 and a tier-2ISP?A.Tier-1 ISPs need to lease some tier-2 ISPs linksB.End-users are always directly connected to tier-1 ISPsC.Tier-1 ISPs treat each other as equals while tier-2 ISPs are the customers of tier-1 ISPsD.Tier-1 ISPs are local providers and tier-2 ISPs are global providers5The time needed

5、 for a packet to travel by a physical link from one host to another is calledA.Processing delayB.Queuing delayC.Transmission delayD.Propagation delay6How is distributed denial of service (DDoS) attack done?A.The target host is sent many requests from a network of compromised nodesB.A malicious host

6、sends to end-users invalid packets that have target host address as the source addressC.The target host is hacked to distribute some malware instead of the original hosts contentD.A malicious hosts modifies the routing tables of the routers so that the target host becomes unreachable7What is the mai

7、n reason of packet loss in packet-switched networks?A.The noise that corrupts some bits in packetsB.The overflow of the routers output bufferC.Invalid routing tables that make packets to go wrong wayD.Different link layer protocols that are incompatible to each other8Why are packet-switched networks

8、 sometimes said to implement statistical multiplexing?A.Link is shared on a packet-by-packet basis only among those users who have packets to be transmittedB.Routers use statistical information in order to implement packet switchingC.We can only measure the probability the packet is sent to one or a

9、nother directionD.This is because TDM and FDM are chosen in random order9What is end-to-end throughput?A.This is the transmission speed measured on the sending end of a linkB.This is the total delay to transfer a packet from source to destinationC.This is the maximum of the source link bandwidth and

10、 the destination link bandwidthD.This is the rate at which the recipient receives the data10Which layers in the Internet protocol stack does a router process?A.Transport and network layersB.Physical, link, and transport layersC.Physical, link, and network layersD.Link, transport, and network layers#

11、QuestionsAnswers (choose only one)1Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A and Host B each send a UDP segment to Host C with destination port number 6789 How will the process at Host C know that these two segments originated from two different hosts?A.

12、By source IP addressesB.By source port numberC.The segments will be directed to different socketsD.The segments will have different sequence numbers2Which state of TCP congestion control performs exponential bandwidth growth?A.Fast retransmitB.Congestion avoidanceC.Slow startD.Timeout3TCP socket is

13、identified byA.Destination IP addressB.Destination port numberC.Destination IP address and destination port numberD.Source IP address, source port number, destination IP address, and destination port number4Consider Go-back-N (GBN) and Selective Repeat (SR) protocols. Choose the only right statement

14、.A.GBN acknowledges individual packets, SR uses cumulative ACKsB.GBN uses cumulative ACKs, SR acknowledges individual packetsC.Both GBN and SR use cumulative ACKsD.Both GBN and SR acknowledge individual packets5What are transport-layer multiplexing and demultiplexing?A.Gathering data at source host

15、from different sockets and passing the segments to the network layer and vice versaB.FDM and TDM techniquesC.Using different application-layer protocols over the same transport-layer connectionD.A form of flow control6Does TCP connection allow bidirectional simultaneous data exchange between two pro

16、cesses?A.No, TCP connection is unidirectionalB.No, TCP connection is half-duplexC.Yes, TCP connection is full-duplexD.Yes, but only one process has congestion control7Is there any difference between flow control (FC) and congestion control (CC)?A.These are the same thingsB.FC is implemented by netwo

17、rk core; CC is implemented by network edgeC.FC throttles the sender according to the network core bandwidth; CC limits the senders transmission speed to the receivers processing speedD.FC limits the senders transmission speed to the receivers processing speed; CC throttles the sender according to th

18、e network core bandwidth8In routers, head-of-line (HOL) blocking may occurA.In input portsB.In output portsC.In switching fabricD.In forwarding table9UDP socket is identified byA.Destination IP addressB.Destination port numberC.Source IP address, source port number, destination IP address, and desti

19、nation port numberD.Destination IP address and destination port number10Do routers have IP addresses? If so, how many?A.No, they do not have IP addressesB.Yes, exactly oneC.Yes, the number may vary due to congestion controlD.Yes, the number of IP addresses is equal to the number of interfaces11Suppo

20、se Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host B receives the datagram, how does the network layer in Host B know it should pass the segment to TCP rather than to UDP or to something else?A.This is determined by the destination port numberB.This is determined by a spe

21、cial field in an IP datagramC.Network layer is unaware of TCP and UDP; this is determined by transport layer protocolD.Network layer is unaware of TCP and UDP; this is determined by application layer protocol12What is DHCP used for?A.It allows host to resolve other hosts IP address from its network

22、nameB.It translates IP address to its hosts MAC addressC.It is only used by routers to configure their forwarding tablesD.It allows host to obtain its IP address from network server when it joins network13What technique can be used to allow LAN clients with local network addresses to connect to WAN

23、servers with global Internet addresses?A.ARPB.NATC.DNSD.ICMP14Flood and prune technique is used inA.Multicast routingB.Broadcast routingC.Unicast routingD.Anycast routing15For the controlled flooding approach for broadcast communication, are the following statements true or false?(1)A node may recei

24、ve multiple copies of the same packet.(2)A node may forward multiple copies of a packet over the same outgoing link.A.(1)FALSE, (2)FALSEB.(1)FALSE, (2)TRUEC.(1)TRUE, (2)FALSED.(1)TRUE, (2)TRUE16What information does BGP provide to autonomous systems?A.Subnet reachabilityB.Adjacent link costsC.Distan

25、ce vectorsD.NIC MAC addresses17For the IPv6 datagram, are the following statements true or false?(1)Header has fixed length(2)Fragmentation is allowed(3)Header checksum is calculatedA.(1)FALSE, (2)TRUE, (3)FALSEB.(1)TRUE, (2)FALSE, (3)FALSEC.(1)FALSE, (2)FALSE, (3)TRUED.(1)TRUE, (2)TRUE, (3)TRUE18Fo

26、r the virtual circuit network, are the following statements true or false?(1)Call setup is needed(2)Routers do not keep information about end-to-end connections(3)Packets between same source-destination pair may take different pathsA.(1)FALSE, (2)FALSE, (3)TRUEB.(1)TRUE, (2)TRUE, (3)FALSEC.(1)FALSE,

27、 (2)TRUE, (3)TRUED.(1)TRUE, (2)FALSE, (3)FALSE19For the datagram network, are the statements of the question #18 true or false?A.(1)FALSE, (2)FALSE, (3)TRUEB.(1)TRUE, (2)TRUE, (3)FALSEC.(1)FALSE, (2)TRUE, (3)TRUED.(1)TRUE, (2)FALSE, (3)FALSE20Which routing protocol provides two-level hierarchy (loca

28、l area and backbone)?A.OSPFB.RIPC.BGPD.DVMRP#QuestionsAnswers (choose only one)1What is the key difference between switches and routers?ASwitches work on physical layer and link layer and process frames only; routers support network layer and process datagramsBSwitches have IP addresses and routers

29、dontCSwitches can only run local routing protocols while routers implement global routing protocolsDRouters divide network into different collision domains while switches dont2How big is the MAC/IPv4/IPv6 address space (in bits)?A32/64/128B64/48/64C16/16/32D48/32/1283As a mobile node gets farther an

30、d farther away from a base station, what are two actions that a base station could take to ensure that the loss probability of a transmitted frame does not increase?AIncrease signal level and decrease transmission rateBDecrease signal level and decrease transmission rateCIncrease signal level and in

31、crease transmission rateDDecrease signal level and increase transmission rate4What is MPLS designed for?AMPLS is used to determine nodes MAC addresses from its IP addressBMPLS improves the forwarding speed in IP routersCMPLS is used to provide reliable data transfer between routersDMPLS is only used

32、 for sending multimedia content5Before an 802.11 station transmits a data frame, must it first send an RTS frame and receive a corresponding CTS frame?AYes, it mustBNo, 802.11 protocol does not support RTS/CTS framesCIt depends on base stations settingsDIt may, but it is not required to6Which protoc

33、ols support link layer acknowledgments?ABoth Ethernet and 802.11BEthernetCNeither Ethernet nor 802.11D802.117Is there any difference between Ethernet and 802.11 frame structure?ANo, they are identicalBThey are alike, but differentC802.11 does not use MAC addressesDFrame structure is the same, but 80

34、2.11 supports longer frames8What are the reasons of using Manchester encoding in Ethernet?AManchester encoding allows all NICs to synchronize with the sender and allows receivers to detect signal transitions instead of measuring signal levelsBManchester encoding is the most bandwidth effective encod

35、ingCManchester encoding allows receiver to detect and correct some bit errorsDThere are no special reasons9How CSMA/CD struggles against collisions?AEvery node is assigned its own time slot to send the dataBOne node is chosen as a master and controls other nodesCEvery node verifies the channel is id

36、le before transmission and listens to the channel during transmission to detect collisionsDNo special action is taken; nodes start transmission as soon as they have data to transmit10Suppose a wireless station moves from one BSS to another within the same subnet. When the APs are interconnected with

37、 a switch, an AP may need to send a frame with a mobile nodes source MAC address. What for?ATo obtain a new IP address via DHCPBTo update switch forwarding tableCTo allow node to traverse NATDTo inform nodes correspondents of its new position#QuestionsAnswers (choose only one)1What security aspect d

38、oes certification authority provide?AMessage integrityBConfidentialityCEnd-point authenticationDOperational security2Consider a variation of the message authentication code (MAC) algorithm where the sender sends (m,H(m)+s), where H(m)+ s is the concatenation of H(m) and s (secret). Is this variation

39、 good?AThis algorithm is as strong as the original MAC algorithmBThis is not a MAC algorithm; the secret is trivially extracted from the codeCIt depends on the secretDThis algorithm is stronger than the original MAC algorithm3How does cipher block chaining (CBC) work?AIt encrypts hash for every plai

40、ntext blockBIt encrypts blocks twice using the same secret keyCIt uses a sequence of secret keys to encrypt different plaintext blocksDIt combines current plaintext block with the previous cipher block before encryption4What is the difference between end-to-end delay and packet jitter?AThese words m

41、ean the same thingBEnd-to-end delay is the time needed for a packet to travel from source to destination; packet jitter is the measure of network packet lossCEnd-to-end delay is the time needed for a packet to travel from source to destination; packet jitter is the fluctuation of end-to-end delaysDE

42、nd-to-end delay is the playout delay; packet jitter is the fluctuation of end-to-end delays5What is the role of the DNS in a CDN?ADNS is only used by the content provider to push the content to a CDN nodeBWhen a client searches for the content DNS server responses to the clients search requestsCTher

43、e is no special role; DNS is used the same way as with other applications and servicesDDNS authoritative server returns the IP address of the CDN server that is likely the best for the requesting browser6Which protocol can be used to control the playback of continuous media by pausing, fast-forwardi

44、ng, rewinding, and so on?ARTPBRTSPCRSVPDRTCP7In what way does a hash provide a better message integrity check than a checksum (such as the Internet checksum)?AHash is calculated faster than a checksumBFor a given message, it is much more difficult to find another one having the same hash rather than

45、 the same checksumCHash uses a symmetric-key cryptography systemDHash allows recovering the data from some bit errors8What is the role of RSVP in multimedia applications?ATo reserve resources and to admit/to block callsBTo resolve other sides IP address and port number from its symbolic nameCTo enca

46、psulate multimedia framesDTo send statistical information from receivers to senders9Which link-scheduling discipline provides equal minimum bandwidth to all network flows?AWeighted Fair QueuingBPriority QueuingCRound RobinDFirst-In-First-Out10What layer does Secure Sockets Layer (SSL) work at?ANetworkBLinkCApplicationDTransport11What security aspects does digital sig