1、Introduction,1-1,Computer Networks,Zhang Jinglei Based on the slides by Jim Kurose and Keith Ross,Introduction,1-2,About Instructor,Lecturer from 1999 Anti professor from 2008 MS from 1996-1999 Tianjin University PhD CandidateResearch Interests: Image Processing Computer Vision Pattern Recognition I
2、ndustrial Networks (Fieldbus) Contact Information Office 14-312 Office Phone 60214105 Email: ,Introduction,1-3,About this course,Textbook Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Grading: Homework 20% Lab 10% Fina
3、l exam 70%,Introduction,1-4,What we should know,List some cool ideas or killer applications for networksList some influential companies in area of networksList some influential technologies in networksList some good journals in area of networksList some good conferences in networks,Introduction,1-5,
4、Organization of Class,Introduction: Computer Networks and Internet Applications of Internet: http, ftp, P2P, Transportation Layer: TCP, UDP, The Network Layer: IP, routings Link Layer: Errors, MAC, Ethernet, Wireless and Mobile Networks: cellular, ad hoc, sensor, Industrial NetworksInfranet Intercon
5、nection between Industrial Networks and Internet,Intranet.,Introduction,1-6,Chapter 1 Introduction,Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004.,The majority of the slides are based on the draft byJ.F. Kurose and K.W.
6、Ross.,Introduction,1-7,Chapter 1: Introduction,Our goal: get “feel” and terminology more depth, detail later in course approach: use Internet as example,Overview: whats the Internet whats a protocol? network edge network core access net, physical media Internet/ISP structure performance: loss, delay
7、 protocol layers, service models network modeling,Introduction,1-8,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8
8、 History,Introduction,1-9,Whats the Internet: “nuts and bolts” view,millions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data),Introduction,1-10,Whats th
9、e Internet: “nuts and bolts” view,protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force http:/www.ietf.
10、org/,local ISP,company network,regional ISP,router,workstation,server,mobile,Introduction,1-11,Whats the Internet: a service view,communication infrastructure enables distributed applications: Web, email, games, e-commerce, file sharing communication services provided to apps: Connectionless unrelia
11、ble connection-oriented reliable,Introduction,1-12,Whats a protocol?,human protocols: “whats the time?” “I have a question” introductions specific msgs sent specific actions taken when msgs received, or other events,network protocols: machines rather than humans all communication activity in Interne
12、t governed by protocols,protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt,Introduction,1-13,Whats a protocol?,a human protocol and a computer network protocol:,Q: Other human protocols?,Hi,Hi,TCP connectionreq,Introduction
13、,1-14,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History,Introduction,1-15,A closer look at network structure
14、:,network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links,Introduction,1-16,The network edge:,end systems (hosts): run application programs e.g. Web, email at “edge of network” client/server model client host requests, recei
15、ves service from always-on server e.g. Web browser/server; email client/server peer-peer model:minimal (or no) use of dedicated servers e.g. Gnutella, KaZaA,Introduction,1-17,Network edge: connection-oriented service,Goal: data transfer between end systems handshaking: setup (prepare for) data trans
16、fer ahead of time Hello, hello back human protocol set up “state” in two communicating hosts TCP - Transmission Control Protocol Internets connection-oriented service,TCP service RFC 793 reliable, in-order byte-stream data transfer loss: acknowledgements and retransmissions flow control: sender wont
17、 overwhelm receiver congestion control: senders “slow down sending rate” when network congested,Introduction,1-18,Network edge: connectionless service,Goal: data transfer between end systems same as before! UDP - User Datagram Protocol RFC 768: connectionless unreliable data transfer no flow control
18、 no congestion control,Apps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (simple mail transfer protocol-email)Apps using UDP: streaming media, teleconferencing, DNS, Internet telephony,Introduction,1-19,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Netw
19、ork core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History,Introduction,1-20,The Network Core,mesh of interconnected routers the fundamental question: how is data transferred through net?
20、 circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks”,Introduction,1-21,Network Core: Circuit Switching,End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing among apps. circuit-like: (g
21、uaranteed) performance call setup required Admission control, resource allocation,Introduction,1-22,Network Core: Circuit Switching,network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing),dividing link bandwidth
22、into “pieces” frequency division multiplexing (FDM) time division (TDM),Introduction,1-23,Circuit Switching: FDM and TDM,Introduction,1-24,Numerical example,How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links are 1.536 Mbps Each link
23、uses TDM with 24 slots 500 msec to establish end-to-end circuitWork it out!,Rate 1.536 Mbps /24 = 64,000 bps Total time 500ms+ 640/64s=10500 ms,Introduction,1-25,Symbols,Mbps megabits per second 106 bits per second MBps megabytes per second Gbps, Kbps, bandwidth the total information flow over a giv
24、en time,Introduction,1-26,Network Core: Packet Switching,each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed Chaos?,resource contention: aggregate resource demand can exceed amount available congestion:
25、 packets queue, wait for link use store and forward: packets move one hop at a time Node receives complete packet before forwarding,Introduction,1-27,Packet Switching: Statistical Multiplexing,Sequence of A & B packets does not have fixed pattern statistical multiplexing. In TDM each host gets same
26、slot in revolving TDM frame.,A,B,C,10 Mb/s Ethernet,1.5 Mb/s,statistical multiplexing,queue of packets waiting for output link,Introduction,1-28,Packet switching versus circuit switching,1 Mb/s link each user: 100 kb/s when “active” active p=10% of timecircuit-switching: 10 users packet switching: w
27、ith 35 users, probability 10 active less than .0004 With n users, probability 10 active is,Packet switching allows more users to use network!,N users,1 Mbps link,Introduction,1-29,Packet switching versus circuit switching,Great for bursty data resource sharing simpler, no call setup Excessive conges
28、tion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 6),Is packet switching a “slam dunk winner?”,Introduction,1-30,Packet-switching: st
29、ore-and-forward,Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward delay = 3L/R 3 is the number of hops,Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec,R,R,R,L,Assume the pr
30、opagation delay is ignored,Introduction,1-31,Packet-switched networks: forwarding,Goal: move packets through routers from source to destination(s) well study several path selection (i.e. routing) algorithms (chapter 4) datagram network: destination address in packet determines next hop routes may ch
31、ange during session (depends on traffic situation) analogy: driving, asking directions virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next hop fixed path determined at call setup time, remains fixed thru call routers maintain per-call state,Introduction,1-32,Pa
32、cket Switching Advantages,Line efficiency greater Node-to-node link dynamically shared by many packets Data-rate conversion Each station connects to its node at its proper data rate Nodes act as buffers Packets accepted, even under heavy traffic, but delivery delay increases Circuit switching networ
33、ks would block new connections Priorities can be used among different packets,Introduction,1-33,Packet Switching Disadvantages,Delay Transmission delay equal to length of packet divided by incoming channel rate Variable delay due to processing and queuing Packets may vary in length May take differen
34、t routes May be subject to varying delays Overall packet delay can vary substantially (jitter) Not good for real-time applications like voice and real-time video Overheads including address of destination, sequencing information added to packet Reduces capacity available for user data More processin
35、g required at node,Introduction,1-34,Switching Techniques,Datagram Each individual packets routed independently Example: Internet Virtual Circuit A path is built first (virtual circuit) All packets follow the same path (VC ID) Examples: ATM (asynchronous transfer model), frame relay, X.25,Introducti
36、on,1-35,Switching Technique Datagram,Datagram: each packet treated independently No reference to packets that have gone before Each node chooses next node on path Packets with same destination address do not follow same route May arrive out of sequence Exit node or destination restores packets to or
37、iginal order Packet may be destroyed in transit Either exit node or destination detects loss and recovers,Introduction,1-36,Switching Technique Datagram,Call setup avoided For an exchange of a few packets, datagram quicker More flexible. E.g. Routing away from the congestion Delivery is inherently m
38、ore reliable If a node fails, subsequent packets may be re-routed,Introduction,1-37,Packet Switching: Datagram Approach,Introduction,1-38,Switching Technique Virtual Circuit,Preplanned route established before packets sent All packets follow same route Similar to circuit in circuit-switching network
39、 Hence virtual circuit (but not resource reservation) Each packet has virtual circuit identifier Nodes on route know where to direct packets No routing decisions,Introduction,1-39,Switching Technique Virtual Circuit,Not dedicated path, as in circuit switching Packet still buffered at node and queued
40、 for output Routing decision made once for that virtual circuit Network may provide services related to virtual circuit Sequencing and error control Packets should transit more rapidly If node fails, all virtual circuits through node lost,Introduction,1-40,Packet Switching: Virtual-Circuit Approach,
41、Introduction,1-41,Effect of Packet Size on Transmission Time,Introduction,1-42,Segment size,http:/ Taxonomy,Telecommunication networks,Datagram network is not either connection-oriented or connectionless.Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps. (usi
42、ng datagram),Introduction,1-44,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History,Introduction,1-45,Access ne
43、tworks and physical media,Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?,Introduction,1-46,Residential access: point to poin
44、t access,Dialup via modem up to 56Kbps direct access to router (often less) Cant surf and phone at same time: cant be “always on”,ADSL: asymmetric digital subscriber line up to 1 Mbps upstream (today typically 256 kbps) up to 8 Mbps downstream (today typically 1 Mbps) FDM: 50 kHz - 1 MHz for downstr
45、eam4 kHz - 50 kHz for upstream0 kHz - 4 kHz for ordinary telephone,Introduction,1-47,Residential access: cable modems,HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream, 2 Mbps upstream network of cable and fiber attaches homes to ISP router homes share access to router deployment: available
46、 via cable TV companies,Introduction,1-48,Residential access: cable modems,Diagram: http:/ Network Architecture: Overview,home,cable headend,cable distribution network (simplified),Typically 500 to 5,000 homes,Introduction,1-50,Cable Network Architecture: Overview,home,cable headend,cable distributi
47、on network (simplified),Introduction,1-51,Cable Network Architecture: Overview,home,cable headend,cable distribution network,Introduction,1-52,Cable Network Architecture: Overview,home,cable headend,cable distribution network,FDM:,Introduction,1-53,Company access: local area networks,company/univ lo
48、cal area network (LAN) connects end system to edge router Ethernet: shared or dedicated link connects end system and router 10 Mbs, 100Mbps, Gigabit Ethernet LANs: chapter 5,Introduction,1-54,Wireless access networks,shared wireless access network connects end system to router via base station aka “access point” AP wireless LANs: (802.11) 802.11b (WiFi): 11 Mbps 802.11, 802.11a, 802.11g wider-area wireless access provided by telco operator 3G 384 kbps Will it happen? WAP/GPRS in Europe,
