1. 1 计算机网络 胥正川 （ XU Zhengchuan ） 管理学院 信息管理与信息系统系 办公室：思源楼 708 室，电话： 25011237 Email: zcxu@fudan.edu.cn

2. 2 4 Course evaluation Class Contribution 10% Case study 10% （ 3 人组） Presentation 10% Final Exam 70% 自编教材： 上 — 网络基础知识材料汇编 下 – 移动通信技术材料汇编

3. An Introduction to Networking Chapter 1 Updated January 2009 XU Zhengchuan Fudan University

4. Part I: Basic Networks Concepts Concepts we will see throughout the book

5. 5 Figure 1-1: Basic Networking Concepts What Is a Network? –A network is a transmission system that connects two or more applications running on different computers. Network

6. 6 Figure 1-1: Basic Networking Concepts Client/Server Applications –Most Internet applications are client/server applications –Clients receive service from servers –The client is often a browser Client Computer Server Computer Server Program Client Program Services

7. Part II: The Nine Elements of a Network Although the idea of “network” is simple, you must understand the nine elements found in most networks

8. 8 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Message (Frame) Access Line Trunk Line Server ApplicationClient Application 1. Networks connect applications on different computers. 1. Networks connect applications on different computers. Networks connect computers: 2. Clients (fixed and mobile) and 3. Servers Networks connect computers: 2. Clients (fixed and mobile) and 3. Servers

9. 9 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 3 Message (Frame) Trunk Line Server ApplicationClient Application 4. Computers (and routers) usually communicate by sending messages called frames 4. Computers (and routers) usually communicate by sending messages called frames

10. 10 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 4 Message (Frame) Trunk Line Server ApplicationClient Application Switch 2 Switch 1 Switch 3 Client Sends Frame to Sw1 Client Sends Frame to Sw1 Sw1 Sends Frame to Sw2 Sw1 Sends Frame to Sw2 Sw2 Sends Frame To Sw3 Sw2 Sends Frame To Sw3 Sw3 Sends Frame to Server Sw3 Sends Frame to Server 5. Switches Forward Frames Sequentially 5. Switches Forward Frames Sequentially

11. 11 Figure 1-5: Ethernet Switch Operation A1-44-D5-1F-AA-4CB2-CD-13-5B-E4-65 Switch D4-47-55-C4-B6-F9 C3-2D-55-3B-A9-4F Port 15 Frame to C3… A1- sends a frame to C3- Frame to C3… Switch sends frame to C3- Switching Table PortHost 10A1-44-D5-1F-AA-4C 13B2-CD-13-5B-E4-65 15C3-2D-55-3B-A9-4F 16 D4-47-55-C4-B6-F9 Switching Table PortHost 10A1-44-D5-1F-AA-4C 13B2-CD-13-5B-E4-65 15C3-2D-55-3B-A9-4F 16 D4-47-55-C4-B6-F9 15C3-2D-55-3B-A9-4F C3- is out Port 15 1 2 3

12. 12 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application 6. Wireless Access Points Connect Wireless Stations to Switches 6. Wireless Access Points Connect Wireless Stations to Switches 消息（帧）

13. 13 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application 7. Routers connect networks to the outside world; Treated just like computers in single networks 7. Routers connect networks to the outside world; Treated just like computers in single networks Yes, single networks can contain routers

14. 14 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application 8. Access Lines Connect Computers to Switches （接入线） 8. Access Lines Connect Computers to Switches （接入线） 9. Trunk Lines Connect Switches to Switches and Switches to Routers （中继线） 9. Trunk Lines Connect Switches to Switches and Switches to Routers （中继线）

15. 15 Figure 1-4: Packet Switching and Multiplexing （多路复用） Client Computer A Mobile Client Computer B Router D Server Computer C AC BD Access Line Trunk Line Multiplexed Packets Share Trunk Lines So Packet Switching Reduces the Cost of Trunk Lines Breaking Communications into Small Messages is Called Packet Switching, even if the Messages are Frames

16. 16 Network Elements: Recap Name the 9 Elements of Single networks. –Without looking back through your handout Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased

17. Part III: Transmission Speed

18. 18 Figure 1-6: Transmission Speed Measuring Transmission Speed –Measured in bits per second (bps) –In metric notation: Increasing factors of 1,000 … –Not factors of 1,024 Kilobits per second (kbps)-note the lowercase k Megabits per second (Mbps) Gigabits per second (Gbps) Terabits per second (Tbps)

19. 19 Figure 1-6: Transmission Speed Measuring Transmission Speed –What is 23,000 bps in metric notation? –What is 3,000,000,000 in metric notation? –What is 15,100,000 bps in metric notation? Occasionally measured in bytes per second If so, written as Bps Usually seen in file download speeds

20. 20 Figure 1-6: Transmission Speed Writing Transmission Speeds in Proper Form –The rule for writing speeds (and metric numbers in general) in proper form is that there should be 1 to 3 places before the decimal point –23.72 Mbps is correct (2 places before the decimal point). –2,300 Mbps has four places before the decimal point, so it should be rewritten as 2.3 Gbps (1 place). –0.5 Mbps has zero places to the left of the decimal point. It should be written as 500 kbps (3 places).

21. 21 Figure 1-6: Transmission Speed Writing Transmission Speeds in Proper Form –How to convert 1,200 Mbps to proper form Divide the number 1,200 by 1000 –Move decimal point three places to the left: 1.200 Multiply the metric suffix Mbps by 1,000 –Gbps Result: –1.2 Gbps

22. 22 Figure 1-6: Transmission Speed Writing Transmission Speeds in Proper Form –How to convert 0.036 Mbps to proper form Multiply the number 0.036 by 1000 –Move decimal point three places to the right: 36 Divide the metric suffix Mbps by 1,000 –kbps Result: –36 kbps

23. 23 Figure 1-6: Transmission Speed Writing Transmission Speeds in Proper Form –How should you write the following in proper form? 549.73 kbps 0.47 Gbps 11,200 Mbps.0021 Gbps

24. 24 Figure 1-6: Transmission Speed Rated Speed （额定速率） –The speed in bits per second that you should get (advertised or specified in the standard). Throughput （吞吐率） –The speed you actually get –Almost always lower than the rated speed On Shared Transmission Lines –Aggregate throughput—total throughput for all users –Individual throughput—what individual users get

25. Part IV: LANs and WANs

26. 26 Figure 1-8: LANs Versus WANs Characteristics Scope LANs WANs For transmission within a site. Campus, building, and SOHO (Small Office or Home Office) LANs For transmission within a site. Campus, building, and SOHO (Small Office or Home Office) LANs For transmission between sites For transmission between sites Building LAN Building LAN Home LAN Home LAN Campus LAN Campus LAN Wide Area Network

27. 27 WANsCharacteristicsLANs Cost per bit TransmittedLowHigh Figure 1-8: LANs Versus WANs Typical Speed Unshared 100 Mbps to a gigabit per second to each desktop. Even faster trunk line speeds. Shared 128 kbps to several megabits per second trunk line speeds It’s simple economics. If the cost per unit is higher, the number of units demanded will be lower. Corporations cannot afford high-speed for most of their WAN transmission

28. 28 Figure 1-8: LANs Versus WANs Characteristics Management LANs WANs On own premises, so firm builds and manages its own LAN or outsources the Work On own premises, so firm builds and manages its own LAN or outsources the Work Must use a carrier with rights of way for transmission in public Area. Carrier handles most work but Charges a high price. Must use a carrier with rights of way for transmission in public Area. Carrier handles most work but Charges a high price. Choices Unlimited Only those offered by carrier Only those offered by carrier

29. 29 Figure 1-9: Local Area Network (LAN) in a Large Building Router Core Switch Workgroup Switch 2 Workgroup Switch 1 Wall Jack To WAN Wall Jack Server Client Frames from the client to the server go through Workgroup Switch 2, through the Core Switch, through Workgroup Switch 1, and then to the server

30. 30

31. Part V: Internets

32. 32 Figure 1-11: Internets Single LANs Versus Internets –In single networks (LANs and WANs), all devices connect to one another by switches—our focus so far. –In contrast, an internet is a group of networks connected by routers so that any application on any host on any single network can communicate with any application on any other host on any other network in the internet. LAN WAN LAN Application Router

33. 33 Figure 1-11: Internets Internet Components –All computers in an internet are called hosts –Clients as well as servers Cat (Ignores Internet) Internet Client PC (Host) Cellphone (Host) VoIP Phone (Host) PDA (Host) Server (Host) Host

34. 34 Figure 1-11: Internets Hosts Have Two Addresses IP Address –This is the host’s official address on its internet –32 bits long –Expressed for people in dotted decimal notation (e.g., 128.171.17.13) Single-Network Addresses –This is the host’s address on its single network –Ethernet addresses, for instance, are 48 bits long –Expressed in hexadecimal notation (e.g., AF-23-9B- E8-67-47)

35. 35 Figure 1-11: Internets Networks are connected by devices called routers –Switches provide connections within networks, while routers provide connections between networks in an internet. Frames and Packets –In single networks, message are called frames –In internets, messages are called packets

36. 36

37. 37 Figure 1-11: Internets Packets are carried within frames –One packet is transmitted from the source host to the destination host across the internet Its IP destination address is that of the destination host Frame Packet LAN WAN LAN Router

38. 38 Figure 1-11: Internets Packets are carried within frames –In each network, the packet is carried in (encapsulated in) a frame –If there are N networks between the source and destination hosts, there will be one packet and N networks between the source and destination hosts, there will be one packet and N frames for a transmission Frame Packet LAN WAN LAN Router

39. 39 Figure 1-12: Internet with Three Networks Host B Host A Network X Network Y Network Z R1 R2 Route A-B Packet A packet goes all the way across the internet; It’s path is its route A packet goes all the way across the internet; It’s path is its route

40. 40 Figure 1-12: Internet with Three Networks Mobile Client Host Server Host Switch X2 Switch X1 Switch Router R1 D6-EE-92-5F-C1-56 Network X Route A-B A route is a packet’s path through the internet A route is a packet’s path through the internet Details in Network X Details in Network X Data link A-R1 Data link A-R1 A data Link is a frame’s path through its single network A data Link is a frame’s path through its single network In Network X, the Packet is Placed in Frame X Packet Frame X Host A 10.0.0.23 AB-23-D1-A8-34-DD

41. 41 Figure 1-12: Internet with Three Networks Router R1 Router R2 AF-3B-E7-39-12-B5 Packet Frame Y To Network X To Network Z Network Y Data Link R1-R2 Route A-B Details in Network Y Details in Network Y

42. 42 Figure 1-12: Internet with Three Networks Host B www.pukanui.com 1.3.45.111 55-6B-CC-D4-A7-56 Mobile Client Host Switch Z1 Switch Z2 Switch Packet Frame Z Network Z Router R2 Router Data Link R2-B Details in Network Z Details in Network Z Mobile Client Computer

43. 43 Figure 1-12: Internet with Three Networks In this internet with three networks, in a transmission, –There is one packet –There are three frames (one in each network) If a packet in an internet must pass through 10 networks, –How many packets will be sent? –How many frames must carry the packet?

44. 44 10000000101010110001000100001101 Figure 1-13: Converting IP Addresses into Dotted Decimal Notation Divided into 4 bytes. These are segments. 100000001010101100010001 00001101 Dotted decimal notation (4 segments separated by dots) Dotted decimal notation (4 segments separated by dots) IP Address (32 bits long) Convert each byte to decimal (result will be between 0 and 255)* 12817117 13 *The conversion process is described in the Hands On section at the end of the chapter. 128.171.17.13

45. 45 Figure 1-17: The Internet 2. User PC’s Internet Service Provider 2. Webserver’s Internet Service Provider ISP 1. User PC Host Computer 1. Webserver Host Computer 4. NAPs = Network Access Points Connect ISPs Router NAP ISP 3. Internet Backbone (Multiple ISP Carriers) Access Line Access Line

46. 46 Figure 1-18: Subnets in an Internet LAN 1 LAN 2 LAN Subnet 10.1.x.x WAN Subnet 123.x.x.x LAN Subnet 60.4.3.x LAN Subnet 10.2.x.x LAN Subnet 10.3.x.x LAN Subnet 60.4.15.x LAN Subnet 60.4.7.x Note: Subnets are single networks (collections of switches, transmission lines) Often drawn as simple lines to focus on routers for internetworking Router R1 Router R3 Router R4 Router R2 LAN Subnet 60.4.131.x

47. 47 Figure 1-19: Terminology Differences for Single- Network and Internet Professionals By Single-Network Professionals By Internet Professionals By Internet Professionals Single Networks Are Called Networks Subnets Internets Are Called Internets Networks In this book, we will usually call internets “internets” and subnets “single networks”

48. 48 Figure 1-14: The Internet, internets, Intranets, and Extranets Lower-case internet –Any internet Upper-case Internet –The global Internet Intranet –An internet restricted to users within a single company Extranet –A group of resources that can be accessed by authorized people in a group of companies

49. 49 Figure 1-20: IP Address Management Every Host Must Have a Unique IP address –Server hosts are given static IP addresses (unchanging) –Clients get dynamic (temporary) IP addresses that may be different each time they use an internet Dynamic Host Configuration Protocol (DHCP) (Figure 1-21) –Clients get these dynamic IP addresses from Dynamic Host Configuration Protocol (DHCP) servers (Figure 1-21)

50. 50 Figure 1-21: Dynamic Host Configuration Protocol (DHCP) Client PC A3-4E-CD-59-28-7F DHCP Server 1. DHCP Request Message: “My 48-bit Ethernet address is A3-4E-CD-59-28-7F”. Please give me a 32-bit IP address.” 2. Pool of IP Addresses 3. DHCP Response Message: “Computer at A3-4E-CD-59-28-7F, your 32-bit IP address is 11010000101111101010101100000010”. (Usually other configuration parameters as well.)

51. 51 Figure 1-20: IP Address Management Domain Name System (DNS) (Figure 1-22) –IP addresses are official addresses on the Internet and other internets –Hosts can also have host names (e.g., cnn.com) Not official—like nicknames –If you only know the host name of a host that you want to reach, your computer must learn its IP address DNS servers tell our computer the IP address of a target host whose name you know. (Figure 1-22)

52. 52 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table 1. Client Host wishes to reach Voyager.cba.hawaii.edu; Needs to know its IP Address 2. Sends DNS Request Message “The host name is Voyager.cba.hawaii.edu” Voyager.cba.hawaii.edu 128.171.17.13 Local DNS Host

53. 53 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table 4. DNS Response Message “The IP address is 128.171.17.13” Voyager.cba.hawaii.edu 128.171.17.13 5. Client sends packets to 128.171.17.13 3. DNS Host looks up the target host’s IP address DNS Host

54. 54 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table Client Host 1. DNS Request Message Anther DNS Host Local DNS Host 3. DNS Response Message The local DNS host sends back the response; the user is unaware that other DNS hosts were involved The local DNS host sends back the response; the user is unaware that other DNS hosts were involved If local DNS host does not have the target host’s IP address, it contacts other DNS hosts to get the IP address If local DNS host does not have the target host’s IP address, it contacts other DNS hosts to get the IP address 2. Request & Response

55. Part VI: Security

56. 56 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Legitimate Packet Border Firewall Hardened Server Allowed Legitimate Packet Hardened Client PC Internal Corporate Network Border firewall should pass legitimate packets Border firewall should pass legitimate packets The Internet Attacker Log File

57. 57 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Attack Packet Denied Attack Packet Hardened Server Hardened Client PC Internal Corporate Network Border firewall should deny (drop) and log attack packets Border firewall should deny (drop) and log attack packets The Internet Border Firewall Attacker Log File

58. 58 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Attacker Attack Packet Denied Attack Packet Internal Corporate Network The Internet Border Firewall Hardened Server Hardened Server Hardened Client PC Hardened Client PC Attack Packet Attack Packet Log File Hosts should be hardened against attack packets that get through Hosts should be hardened against attack packets that get through

59. 59 Figure 1-24: Cryptographic Protections Cryptography –The use of mathematical operations to thwart attacks on message dialogues between pairs of communicating parties (people, programs, or devices) Initial Authentication –Determine the other party’s identity to thwart impostors

60. 60 Figure 1-24: Cryptographic Protections Message-by-Message Protections –Encryption to provide confidentiality so that an eavesdropper cannot reach intercepted messages –Electronic signatures provide message-by-message authentication to prevent the insertion of messages by an impostor after initial authentication –Electronic signatures usually also provide message integrity; this tells the receiver whether anyone has changed the message en route

61. Topics Covered

62. 62 Network Elements: Recap Applications (the only element that users care about) Computers –Clients –Servers Switches and Routers Transmission Lines –Trunk lines –Access Lines Messages (Frames) Wireless Access Points Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased

63. 63 Recap: LANs and WANs LANs transmit data within corporate sites WANs transmit data between corporate sites Each LAN or WAN is a single network LAN costs are low and speeds are high WAN costs are high and speeds are lower WAN

64. 64 LAN WAN LAN Recap: Internets Most firms have multiple LANs and WANs. They must create internets –An internet is a collection of networks connected by routers so that any application on any host on any single network can communicate with any application on any other host on any other network in the internet. Application Router

65. 65 LAN WAN LAN Recap: Internets Elements of an Internet –Computers connected to the internet are called hosts Both servers and client PCs are hosts –Routers connect the networks of the internet together In contrast, switches forward frames within individual networks Router Client PC Host Server Host Router

66. 66 Recap: Internets Hosts Have Two Addresses IP Address –This is the host’s official address on its internet –32 bits long –Expressed for people in dotted decimal notation (e.g., 128, 171, 17.13) Single Network Addresses –This is the host’s address on its single network –Ethernet addresses, for instance, are 48 bits long –Expressed in hexadecimal notation, e.g., AF-23-9B- E8-67-47

67. 67 Recap: Internets Switches versus Routers –Switches move frames through a single network (LAN or WAN) –Routers move packets through internets Messages –Messages in single networks are called frames –Messages in internets are called packets –Packets are encapsulated within (carried inside) frames

68. 68 Recap: Security Security –Firewalls –Hardened Hosts –Cryptographic security for sensitive dialogues Initial authentication Encryption for confidentiality Electronic signatures for authentication and message integrity

69. 69 Quality of Service It is not enough that networks work –They must work well Quality of Service (QoS) defines quantitative measures of service quality –Speed 速度 –Delay (Latency) 延迟 –Reliability 可靠性 Security (not a QoS measure but crucial)

70. 70 Figure 1.9: Quality of Service (QoS) Speed –Bits per second (bps) –Multiples of 1,000 (not 1,024) –Kilobits per second (kbps)—Note the Lower-case “k” –Megabits （兆位） per second (Mbps) –Gigabits （千兆位） per second (Gbps) –Terabits （兆兆位） per second (Tbps) –Petabits （千兆兆位） per second (Pbps)

71. 71 Figure 1.9: Quality of Service (QoS) Congestion and Latency （拥塞和延迟） –Congestion because traffic chronically or momentarily exceeds capacity –Latency delay measured in milliseconds (ms) –Especially bad for some services such as voice communication or highly interactive applications

72. 72 Figure 1.9: Quality of Service (QoS) Reliability 可靠性 –Availability 可用性 Percent of time the network is available to users for transmission and reception Want 24x7x365 availability Telephone network: Five 9s (99.999%) –Error Rate 错误率 Percent of lost or damaged messages or bits

73. 73 Figure 1.9: Quality of Service (QoS) Service Level Agreements (SLAs) –Quantitative guarantees for various service parameters –Example: Better than 99% availability and a packet loss error rate of less 0.5% measured over each day; latency not exceeding 45 ms 99% of the time. –Network provider pays performance penalties if guarantees are not met