计算机网络

# Question Answers (choose only one)1 What are the two types ofservices that the Internetprovides to its applications?A. Text transfer and file transferB. Reliable data delivery and “best effort” data deliveryC. FDM and TDMD. Network core and network edge2 What advantage does acircuit-switched networkhave over a packet-switchednetwork?A. No call setup is requiredB. Packets of different users always share network resourcesC. Resources used as neededD. Guaranteed transmission rate during the communication3 What advantage does TDMhave over FDM in acircuit-switched network?A.TDM may dynamically adjust the bandwidth of differentusersB.TDM always transmits the data immediately without anydelayC.FDM always requires analog-to-digital and digital-to-analogconverters while TDM does notD. It is not possible to use FDM in radio physical media4 What is the keydistinguishing differencebetween a tier-l ISP and atier-2 ISP?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 arethe customers of tier-1 ISPsD.Tier-1 ISPs are local providers and tier-2 ISPs are globalproviders5 The time needed for a packetto travel by a physical linkA. Processing delayfrom one host to another iscalled… B. Queuing delayC. Transmission delayD. Propagation delay6 How is distributed denial ofservice (DDoS) attack done?A.The target host is sent many requests from a network ofcompromised nodesB.A malicious host sends to end-users invalid packets thathave target host address as the source addressC.The target host is hacked to distribute some malware insteadof the original host’s contentD.A malicious hosts modifies the routing tables of the routersso that the target host becomes unreachable7 What is the main reason ofpacket loss inpacket-switched networks?A. The noise that corrupts some bits in packetsB. The overflo w of the routers’ output bufferC. Invalid routing tables that make packets to go wrong wayD.Different link layer protocols that are incompatible to eachother8 Why are packet-switchednetworks sometimes said toimplement statisticalmultiplexing?A.Link is shared on a packet-by-packet basis only amongthose users who have packets to be transmittedB.Routers use statistical information in order to implementpacket switchingC.We can only measure the probability the packet is sent toone or another directionD. This is because TDM and FDM are chosen in random order9 What is end-to-endthroughput?A.This is the transmission speed measured on the sending endof a linkB.This is the total delay to transfer a packet from source todestinationC.This is the maximum of the source link bandwidth and thedestination link bandwidthD. This is the rate at which the recipient receives the data1 0 Which layers in the Internetprotocol stack does a routerprocess?A. Transport and network layersB. Physical, link, and transport layersC. Physical, link, and network layersD. Link, transport, and network layers# Questions Answers (choose only one)1 Suppose a process in Host Chas a UDP socket with portnumber 6789. Suppose bothHost A and Host B eachsend a UDP segment to HostC with destination portnumber 6789 How will theprocess at Host C know thatthese two segmentsoriginated from twodifferent hosts?A. By source IP addressesB. By source port numberC. The segments will be directed to different socketsD. The segments will have different sequence numbers2 Which state of TCPcongestion control performsexponential bandwidthgrowth?A. Fast retransmitB. Congestion avoidanceC. Slow startD. Timeout3 TCP socket is identifiedby…A. Destination IP addressB. Destination port numberC. Destination IP address and destination port numberD.Source IP address, source port number, destination IPaddress, and destination port number4 Consider Go-back-N (GBN)and Selective Repeat (SR)protocols. Choose the onlyright statement.A.GBN acknowledges individual packets, SR uses cumulativeACKsB.GBN uses cumulative ACKs, SR acknowledges individualpacketsC. Both GBN and SR use cumulative ACKsD. Both GBN and SR acknowledge individual packets5 What are transport-layermultiplexing anddemultiplexing?A.Gathering data at source host from different sockets andpassing the segments to the network layer and vice versaB. FDM and TDM techniquesC.Using different application-layer protocols over the sametransport-layer connectionD. A form of flow control6 Does TCP connection allowbidirectional simultaneousdata exchange between twoprocesses?A. No, TCP connection is unidirectionalB. No, TCP connection is half-duplexC. Y es, TCP connection is full-duplexD. Y es, but only one process has congestion control7 Is there any differencebetween flow control (FC)and congestion control(CC)?A. These are the same thingsB.FC is implemented by network core; CC is implemented bynetwork edgeC.FC throttles the sender according to the network corebandwidth; CC limits the sender’s transmission speed to thereceiver’s processing speedD.FC limits the sender’s transmission speed to the receiver’sprocessing speed; CC throttles the sender according to thenetwork core bandwidth8 In routers, head-of-line(HOL) blocking mayoccur…A. In input portsB. In output portsC. In switching fabricD. In forwarding table9 UDP socket is identifiedby…A. Destination IP addressB. Destination port numberC.Source IP address, source port number, destination IPaddress, and destination port numberD. Destination IP address and destination port number10 Do routers have IPaddresses? If so, how many?A. No, they do not have IP addressesB. Y es, exactly oneC. Y es, the number may vary due to congestion controlY es, the number of IP addresses is equal to the number of D.interfaces1 1 Suppose Host A sends Host B aTCP segment encapsulated in anIP datagram. When Host Breceives the datagram, how doesthe network layer in Host Bknow it should pass the segmentto TCP rather than to UDP or tosomething else?A. This is determined by the destination port numberB. This is determined by a special field in an IP datagramC.Network layer is unaware of TCP and UDP; this isdetermined by transport layer protocolD.Network layer is unaware of TCP and UDP; this isdetermined by application layer protocol12 What is DHCP used for?A.It allows host to resolve other host’s IP address from itsnetwork nameB. It translates IP address to its host’s MAC addressC.It is only used by routers to configure their forwardingtablesD.It allows host to obtain its IP address from networkserver when it joins network1 3 What technique can be used toallow LAN clients with localnetwork addresses to connect toW AN servers with globalInternet addresses?A. ARPB. NA TC. DNSD. ICMP1 4 Flood and prune technique isused in…A. Multicast routingB. Broadcast routingC. Unicast routingD. Anycast routing1 5 For the controlled floodingapproach for broadcastcommunication, are thefollowing statements true orfalse?(1) A node may receive multiplecopies of the same packet.(2) A node may forwardmultiple copies of a packet overthe same outgoing link.A. (1) FALSE, (2) FALSEB. (1) FALSE, (2) TRUEC. (1) TRUE, (2) FALSED. (1) TRUE, (2) TRUE1What information does BGP provide to autonomous systems? A. Subnet reachabilityB. Adjacent link costs6 C. Distance vectorsD. NIC MAC addresses1 7 For the IPv6 datagram, are thefollowing statements true orfalse?(1) Header has fixed length(2) Fragmentation is allowed(3) Header checksum iscalculatedA. (1) FALSE, (2) TRUE, (3) FALSEB. (1) TRUE, (2) FALSE, (3) FALSEC. (1) FALSE, (2) FALSE, (3) TRUED. (1) TRUE, (2) TRUE, (3) TRUE1 8 For the virtual circuit network,are the following statements trueor false?(1) Call setup is needed(2) Routers do not keepinformation about end-to-endconnections(3) Packets between samesource-destination pair may takedifferent pathsA. (1) FALSE, (2) FALSE, (3) TRUEB. (1) TRUE, (2) TRUE, (3) FALSEC. (1) FALSE, (2) TRUE, (3) TRUED. (1) TRUE, (2) FALSE, (3) FALSE1 9 For the datagram network, arethe 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) FALSE2 0 Which routing protocol providestwo-level hierarchy (local areaand backbone)?A. OSPFB. RIPC. BGPD. DVMRP# Questions Answers (choose only one)1 What is the key differencebetween switches and routers?ASwitches work on physical layer and link layer and process frames only; routerssupport network layer and process datagramsB Switches have IP addresses and routers don'tCSwitches can only run local routing protocols while routers implement global routingprotocolsD Routers divide network into different collision domains while switches don't2 How big is theMAC/IPv4/IPv6 address space(in bits)?A32/64/128B64/48/64C16/16/32D48/32/1283 As a mobile node gets fartherand farther away from a basestation, what are two actionsthat a base station could take toensure that the loss probabilityof a transmitted frame does notincrease?A Increase signal level and decrease transmission rateB Decrease signal level and decrease transmission rateC Increase signal level and increase transmission rateD Decrease signal level and increase transmission rate4 What is MPLS designed for? A MPLS is used to determine node's MAC addresses from its IP addressB MPLS improves the forwarding speed in IP routersC MPLS is used to provide reliable data transfer between routersD MPLS is only used for sending multimedia content5 Before an 802.11 stationtransmits a data frame, must itfirst send an R TS frame andreceive a corresponding CTSframe?A Y es, it mustB No, 802.11 protocol does not support R TS/CTS framesC It depends on base station's settingsD It may, but it is not required to6 Which protocols support linklayer acknowledgments?A Both Ethernet and 802.11B EthernetC Neither Ethernet nor 802.11D802.117 Is there any difference betweenEthernet and 802.11 framestructure?A No, they are identicalB They are alike, but differentC802.11 does not use MAC addressesD Frame structure is the same, but 802.11 supports longer frames8 What are the reasons of usingManchester encoding inEthernet?AManchester encoding allows all NICs to synchronize with the sender and allowsreceivers to detect signal transitions instead of measuring signal levelsB Manchester encoding is the most bandwidth effective encodingC Manchester encoding allows receiver to detect and correct some bit errorsD There are no special reasons9 How CSMA/CD strugglesagainst collisions?A Every node is assigned its own time slot to send the dataB One node is chosen as a master and controls other nodesCEvery node verifies the channel is idle before transmission and listens to the channelduring transmission to detect collisionsDNo special action is taken; nodes start transmission as soon as they have data totransmit1 0 Suppose a wireless stationmoves from one BSS toA To obtain a new IP address via DHCPB To update switch forwarding tableanother within the same subnet. When the APs are interconnected with a switch, an AP may need to send a frame with a mobile node's source MAC address. What for? C To allow node to traverse NATD To inform node's correspondents of its new position# Questions Answers (choose only one)1 What security aspect doescertification authority provide?A Message integrityB ConfidentialityC End-point authenticationD Operational security2 Consider a variation of themessage authentication code(MAC) algorithm where thesender sends (m, H(m) + s),where H(m) + s is theconcatenation of H(m) and s(secret). Is this variation good?A This algorithm is as strong as the original MAC algorithmB This is not a MAC algorithm; the secret is trivially extracted from the codeC It depends on the secretD This algorithm is stronger than the original MAC algorithm3 How does cipher block chaining(CBC) work?A It encrypts hash for every plaintext blockB It encrypts blocks twice using the same secret keyC It uses a sequence of secret keys to encrypt different plaintext blocksDIt combines current plaintext block with the previous cipher block beforeencryption4 What is the difference betweenend-to-end delay and packetjitter?A These words mean the same thingBEnd-to-end delay is the time needed for a packet to travel from source todestination; packet jitter is the measure of network packet lossCEnd-to-end delay is the time needed for a packet to travel from source todestination; packet jitter is the fluctuation of end-to-end delaysDEnd-to-end delay is the playout delay; packet jitter is the fluctuation ofend-to-end delays5 What is the role of the DNS in aCDN?A DNS 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 client'ssearch requestsCThere is no special role; DNS is used the same way as with other applicationsand servicesDDNS authoritative server returns the IP address of the CDN server that islikely the best for the requesting browser6 Which protocol can be used tocontrol the playback of continuous media by pausing,fast-forwarding, rewinding, andso on?A RTPB RTSPC RSVPD RTCP7 In what way does a hash providea better message integrity checkthan a checksum (such as theInternet checksum)?A Hash is calculated faster than a checksumBFor a given message, it is much more difficult to find another one having thesame hash rather than the same checksumC Hash uses a symmetric-key cryptography systemD Hash allows recovering the data from some bit errors8 What is the role of RSVP inmultimedia applications?A To reserve resources and to admit/to block callsB To resolve other side's IP address and port number from its symbolic nameC To encapsulate multimedia framesD To send statistical information from receivers to senders9 Which link-schedulingdiscipline provides equalminimum bandwidth to allnetwork flows?A Weighted Fair QueuingB Priority QueuingC Round RobinD First-In-First-Out10 What layer does Secure SocketsLayer (SSL) work at?A NetworkB LinkC ApplicationD Transport1 1 What security aspectsdoes digital signatureprovide?A Confidentiality and end-point authenticationB Operational security and confidentialityC Message integrity and operational securityD End-point authentication and message integrity1 2 Which firewall type cantreat packets differentlyaccording to usercredentials?A Stateful packet filtersB Application gatewayC Intrusion detection systemsD Stateless packet filters1 3 What is the differencebetween messageauthentication code(MAC) and digitalsignature (DS)?A MAC requires a shared secret; DS uses public-key cryptographyB MAC uses public-key cryptography; DS uses symmetric-key cryptographyC These are different names of the same techniqueD DS requires a shared secret; MAC uses public-key cryptography1How is the role of an SIP A SIP is used in virtual circuit networks; HA is used in datagram networks4 registrar different fromthat of a home agent (HA) in Mobile IP? BHA resolves client's name to its IP address and the port number; SIP registrar forwards all packets to the client's visited networkCSIP registrar resolves client's name to its IP address and the port number; HA forwards all packets to the client's visited networkD They act just the same1 5 What class of multimediais there in IP telephony?A Download-and-play multimediaB Streaming stored multimediaC Real-time interactive multimediaD Streaming live multimedia1 6 What is the differencebetween SSL and IPSec?AIPSec provides connection security; SSL provides host to host (network tonetwork) securityB IPSec uses public-key cryptography; SSL uses symmetric-key cryptographyC SSL uses public-key cryptography; IPSec uses symmetric-key cryptographyDSSL provides connection security; IPSec provides host to host (network tonetwork) security1 7 What is an importantdifference between asymmetric-key system(SKS) and a public-keysystem (PKS)?A PKS has better performance than SKSB SKS needs a secure channel to share the secret; PKS does notC SKS requires a certification authorityD PKS needs a secure channel to share the secret; SKS does not1 8 Consider recovery frompacket loss. Whichscheme does NOTincrease the transmissionrate?A Forward Error CorrectionB Piggyback lower quality streamC Duplicate packet transmissionD Interleaving1 9 What is the purpose of anonce in an end-pointauthentication protocol?A It defends the communication against record-and-playback attacksB It is used to synchronize the TCP server and the TCP clientC It is used as the key of a symmetric-key systemD It is used instead of public-key cryptography2 0 Can you "decrypt" a hashof a message to get theoriginal message?A Only if the original message is longB Yes, providing you know a secret keyC Yes, if it uses a public-key encryption systemD No此次考试总共四种类型题目：选择题（20/40）、填空题（5/20）、简答题（5/28）、计算题（1/12）一、选择题内容涉及所学各个章节，包括应用层（例如：web和HTTTP）、运输层（例如：TCP/UDP）、网络层（例如：广播和多播）、链路层和局域网（例如：MAC）、无线网络和移动网络（例如：802.11）、多媒体网络（例如：SIP）、计算机网络安全（例如：完整性）。