computer networking 课后答案.pdf

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Computer Networking: A Top-Down Approach Featuring the Internet, 4th Edition Solutions to Review Questions and Problems Version Date: June 21, 2007 This document contains the solutions to review questions and problems for the 4th edition of Computer Networking: A Top-Down Approach Featuring the Internet by Jim Kurose and Keith Ross. These solutions are being made available to instructors ONLY. Please do NOT copy or distribute this document to others (even other instructors). Please do not post any solutions on a publicly-available Web site. We’ll be happy to provide a copy (up-to-date) of this solution manual ourselves to anyone who asks. All material © copyright 1996-2007 by J.F. Kurose and K.W. Ross. All rights reserved

Chapter 1 Review Questions 1. There is no difference. Throughout this text, the words “host” and “end system” are used interchangeably. End systems include PCs, workstations, Web servers, mail servers, Internet-connected PDAs, WebTVs, etc. 2. Suppose Alice, an ambassador of country A wants to invite Bob, an ambassador of country B, over for dinner. Alice doesn’t simply just call Bob on the phone and say, “come to our dinner table now”. Instead, she calls Bob and suggests a date and time. Bob may respond by saying he’s not available that particular date, but he is available another date. Alice and Bob continue to send “messages” back and forth until they agree on a date and time. Bob then shows up at the embassy on the agreed date, hopefully not more than 15 minutes before or after the agreed time. Diplomatic protocols also allow for either Alice or Bob to politely cancel the engagement if they have reasonable excuses. 3. A networking program usually has two programs, each running on a different host, communicating with each other. The program that initiates the communication is the client. Typically, the client program requests and receives services from the server program. 4. 1. Dial-up modem over telephone line: residential; 2. DSL over telephone line: residential or small office; 3. Cable to HFC: residential; 4. 100 Mbps switched Etherent: company; 5. Wireless LAN: mobile; 6. Cellular mobile access (for example, WAP): mobile 5. HFC bandwidth is shared among the users. On the downstream channel, all packets emanate from a single source, namely, the head end. Thus, there are no collisions in the downstream channel. 6. Current possibilities include: dial-up; DSL; cable modem; fiber-to-the-home. 7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps. For an X Mbps Ethernet (where X = 10, 100, 1,000 or 10,000), a user can continuously transmit at the rate X Mbps if that user is the only person sending data. If there are more than one active user, then each user cannot continuously transmit at X Mbps. 8. Ethernet most commonly runs over twisted-pair copper wire and “thin” coaxial cable. It also can run over fibers optic links and thick coaxial cable. 9. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ISDN: up to 128 kbps, bandwidth is dedicated; ADSL: downstream channel is .5-8 Mbps, upstream channel

is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps and upstream channel is usually less than a few Mbps, bandwidth is shared. 10. There are two most popular wireless Internet access technologies today: In a wireless LAN, wireless users transmit/receive packets to/from a base station (wireless access point) within a radius of few tens of meters. The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network. a) Wireless LAN b) Wide-area wireless access network In these systems, packets are transmitted over the same wireless infrastructure used for cellular telephony, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a radius of tens of kilometers of the base station. 11. A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth. 12. In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame. 13. At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host completes transmission and the entire packet is received at the router (no propagation delay). Because the router has the entire packet at time t1, it can begin to transmit the packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes transmission and the entire packet is received at the receiving host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2. 14. A tier-1 ISP connects to all other tier-1 ISPs; a tier-2 ISP connects to only a few of the tier-1 ISPs. Also, a tier-2 ISP is a customer of one or more tier-1. 15. a) 2 users can be supported because each user requires half of the link bandwidth. b) Since each user requires 1Mbps when transmitting, if two or fewer users transmit simultaneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing delay before the link. Whereas, if three users transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In this case, there will be queuing delay before the link. c) Probability that a given user is transmitting = 0.2

d) Probability that all three users are transmitting simultaneously = p ( 3 1 − p − ) 33 3 3 ⎞ ⎟⎟ ⎠ ⎛ ⎜⎜ ⎝ = (0.2)3 = 0.008. Since the queue grows when all the users are transmitting, the fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is 0.008. 16. The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these delays are fixed, except for the queuing delays, which are variable. 17. Java Applet 18. 10msec; d/s; no; no 19. a) 500 kbps b) 62.5 seconds c) 100kbps; 312.5 seconds 20. End system A breaks the large file into chunks. To each chunk, it adds header generating multiple packets from the file. The header in each packet includes the address of the destination: end system B. The packet switch uses the destination address to determine the outgoing link. Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet’s address. 21. Java Applet 22. Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup. Yes, these tasks can be duplicated at different layers. For example, error control is often provided at more than one layer. 23. The five layers in the Internet protocol stack are – from top to bottom – the application layer, the transport layer, the network layer, the link layer, and the physical layer. The principal responsibilities are outlined in Section 1.5.1. 24. Application-layer message: data which an application wants to send and passed onto the transport layer; transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header; network-layer datagram: encapsulates transport-layer segment with a network-layer header; link- layer frame: encapsulates network-layer datagram with a link-layer header. 25. Routers process layers 1 through 3. (This is a little bit of a white lie, as modern routers sometimes act as firewalls or caching components, and process layer four as well.) Link layer switches process layers 1 through 2. Hosts process all five layers. 26. a) Virus

Requires some form of human interaction to spread. Classic example: E-mail viruses. numbers, looking for vulnerable processes to infect. b)Worms No user replication needed. Worm in infected host scans IP addresses and port c) Trojan horse Hidden, devious part of some otherwise useful software. 27. Creation of a botnet requires an attacker to find vulnerability in some application or system (e.g. exploiting the buffer overflow vulnerability that might exist in an application). After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable. The target is basically to compromise a series of systems by exploiting that particular vulnerability. Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability. An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet. Hence, it becomes possible for the attacker to issue a command to all the nodes, that target a single node (for example, all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target, which might result in a TCP SYN flood attack at the target). 28. Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely modify the message(s) being sent from Bob to Alice. For example, she can easily change the phrase “Alice, I owe you $1000” to “Alice, I owe you $10,000”. Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice (and vise-versa), even if the packets from Bob to Alice are encrypted. Chapter 1 Problems: Problem 1. There is no single right answer to this question. Many protocols would do the trick. Here's a simple answer below: Messages from ATM machine to Server purpose Msg name ------- -------- Let server know that there is a card in the HELO ATM machine ATM card transmits user ID to Server PASSWD User enters PIN, which is sent to server BALANCE User requests balance WITHDRAWL User asks to withdraw money BYE user all done

withdrawl purpose ------- Ask user for PIN (password) last requested operation (PASSWD, WITHDRAWL) OK last requested operation (PASSWD, WITHDRAWL) in ERROR sent in response to BALANCE request user done, display welcome screen at ATM --------------> (check if valid userid) <------------- PASSWD <------------- OK (password is OK) --------------> <------------- AMOUNT <------------- OK --------------> <------------- BYE Messages from Server to ATM machine (display) Msg name -------- PASSWD OK ERR AMOUNT BYE Correct operation: client server HELO (userid) PASSWD --------------> (check password) BALANCE WITHDRAWL --------------> check if enough $ to cover ATM dispenses $ BYE In situation when there's not enough money: HELO (userid) PASSWD --------------> (check password) BALANCE WITHDRAWL --------------> check if enough $ to cover withdrawl error msg displayed no $ given out BYE Problem 2. a) A circuit-switched network would be well suited to the application described, because the application long sessions with predictable smooth bandwidth requirements. Since the transmission rate is known and not bursty, bandwidth can be reserved for each application session circuit with no significant waste. In addition, we --------------> (check if valid userid) <------------- PASSWD <------------- OK (password is OK) --------------> <------------- AMOUNT <------------- ERR (not enough funds) --------------> <------------- BYE involves

need not worry greatly about the overhead costs of setting up and tearing down a circuit connection, which are amortized over the lengthy duration of a typical application session. b) Given such generous link capacities, the network needs no congestion control mechanism. In the worst (most potentially congested) case, all the applications simultaneously transmit over one or more particular network links. However, since each link offers sufficient bandwidth to handle the sum of all of the applications' data rates, no congestion (very little queuing) will occur. Problem 3. a) We can n connections between each of the four pairs of adjacent switches. This gives a maximum of 4n connections. b) We can n connections passing through the switch in the upper-right-hand corner and another n connections passing through the switch in the lower-left-hand corner, giving a total of 2n connections. Problem 4. Tollbooths are 100 km apart, and the cars propagate at 100km/hr. A tollbooth services a car at a rate of one car every 12 seconds. a) There are ten cars. It takes 120 seconds, or two minutes, for the first tollbooth to service the 10 cars. Each of these cars has a propagation delay of 60 minutes before arriving at the second tollbooth. Thus, all the cars are lined up before the second tollbooth after 62 minutes. The whole process repeats itself for traveling between the second and third tollbooths. Thus the total delay is 124 minutes. b) Delay between tollbooths is 7*12 seconds plus 60 minutes, i.e., 61 minutes and 24 seconds. The total delay is twice this amount, i.e., 122 minutes and 48 seconds. d prop dtrans d to end −− sm / = RL /= ( = Problem 5 a) b) c) d) The bit is just leaving Host A. e) The first bit is in the link and has not reached Host B. f) The first bit has reached Host B. g) Want seconds. seconds. RLsm / seconds. + end ) / Lm = R S = 100 10 × 3 28 ( 5.2 × 10 8 ) 893 = km.

Problem 6 Consider the first bit in a packet. Before this bit can be transmitted, all of the bits in the packet must be generated. This requires 848 ⋅ 310 64 × sec=6msec. 848 ⋅ 610 1 × sec= μ384 sec. The time required to transmit the packet is Propagation delay = 2 msec. The delay until decoding is 6msec + μ384 sec + 2msec = 8.384msec A similar analysis shows that all bits experience a delay of 8.384 msec. Problem 7 a) 10 users can be supported because each user requires one tenth of the bandwidth. b) − − n p 40 ) . 1.0=p 40 ⎛ ⎜⎜ n ⎝ p n . ( 1 c) ⎞ ⎟⎟ ⎠ 9 ∑ n = 0 40 n ⎞ ⎟⎟ ⎠ ⎛ ⎜⎜ ⎝ d) 1 − n p ( 1 − p ) 40 − n . We use the central limit theorem to approximate this probability. Let random variables such that p . = ( XP ) = 1 j jX be independent (P “11 or more users” ) −= 1 P ⎛ ⎜⎜ ⎝ 40 ∑ 1j = jX ≤ 10 ⎞ ⎟⎟ ⎠ j j 1 = ≤ j P P X ⎛ ⎜⎜ ⎝ 10 40 ∑ − ⋅ ⎞ =⎟⎟ ⎠ X 4 9.01.0 ∑ 40 ⎛ ⎜ j 1 = ⎜⎜ 40 ⋅ ⎝ ⎛ ≤ ZP ⎜ ⎝ 999 .0= (P “10 or more users” ) when Z is a standard normal r.v. Thus 6 6.3 ⎞ =⎟ ⎠ )16.3 ( ZP 40 ≤ ≤ ≈ 6 9.01.0 ⋅ ⋅ ⎞ ⎟ ⎟⎟ ⎠ .0≈ 001 .

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