1、Introduction,1-1,Chapter 1 Introduction,Computer Networking: A Top Down Approach Featuring the Internet, 4rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2008.,Introduction,1-2,Chapter 1: Introduction,Our goal: get “feel” and terminology more depth, detail later in course approach: use Inter
2、net as example,Overview: whats the Internet whats a protocol? network edge network core access net, physical media Internet/ISP structure performance: loss, delay protocol layers, service models network modeling,Introduction,1-3,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Netwo
3、rk 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,1-4,Whats the Internet: “nuts and bolts” view,millions of connected computing devices: hosts = end systems running n
4、etwork apps communication links fiber, copper, radio, satellite transmission rate(bit/s) = bandwidth routers: forward packets (chunks of data),Introduction,1-5,Whats the Internet: “nuts and bolts” view,Internet: “network of networks” loosely hierarchical public Internet versus private intranet proto
5、cols control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP Internet standards IETF: Internet Engineering Task Force RFC: Request for comments(5000) IEEE 802,local ISP,company network,regional ISP,router,workstation,server,mobile,Introduction,1-6,Whats a protocol?,human protocols: “whats t
6、he 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 Internet governed by protocols,protocols define format, order of msgs sent and received among network
7、 entities, and actions taken on msg transmission, receipt,Introduction,1-7,Whats a protocol?,a human protocol and a computer network protocol:,Q: Other human protocols?,Hi,Hi,TCP connectionreq,Introduction,1-8,Whats a protocol?,Protocol is the most important content!,Introduction,1-9,Whats the Inter
8、net: a service view,communication infrastructure enables distributed applications: Web, email, games, e-commerce, file sharing communication services provided to apps: Connectionless unreliable connection-oriented reliable,Introduction,1-10,A closer look at network structure:,network edge: applicati
9、ons and hosts network core: routers network of networks access networks, physical media: communication links,Introduction,1-11,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 pack
10、et-switched networks 1.7 Protocol layers, service models 1.8 History,Introduction,1-12,The network edge:,end systems (hosts): run application programs e.g. Web, email at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; email
11、client/server peer-peer model:minimal (or no) use of dedicated servers e.g. Gnutella, KaZaA,Introduction,1-13,“Cool” internet appliances,Worlds smallest web server http:/www-ccs.cs.umass.edu/shri/iPic.html,IP picture frame http:/ toaster + weather forecaster,Internet phones,Introduction,1-14,Network
12、 edge: connection-oriented service,Goal: data transfer between end systems handshaking: setup (prepare for) data transfer 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
13、 793 reliable, in-order byte-stream data transfer loss: acknowledgements and retransmissions flow control: sender wont overwhelm receiver congestion control: senders “slow down sending rate” when network congested,Introduction,1-15,Network edge: connectionless service,Goal: data transfer between end
14、 systems same as before! UDP - User Datagram Protocol RFC 768: connectionless unreliable data transfer no flow control no congestion control,Apps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)Apps using UDP: streaming media, teleconferencing, DNS, Internet telephony,
15、Introduction,1-16,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,1-17,The Network Core,mesh
16、of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” example,Introduction,1-18,Network Core: Circuit Switching,End-end resources reserved for “ca
17、ll” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required,Introduction,1-19,Network Core: Circuit Switching,network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by ownin
18、g call (no sharing),dividing link bandwidth into “pieces” frequency division time division,Introduction,1-20,Circuit Switching: FDM and TDM,Introduction,1-21,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.
19、536 Mbps Each link uses TDM with 24 slots 500 msec to establish end-to-end circuitWork it out!,Introduction,1-22,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,resour
20、ce contention: store and forward: packets move one hop at a time Node receives complete packet before forwarding aggregate resource demand can exceed amount available congestion: packets queue, wait for link use,Introduction,1-23,Packet-switching: store-and-forward,Takes L/R seconds to transmit (pus
21、h 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,Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec,R,R,R,L,Introduction,1-24,Packet Switching: Statistical Multiplexing,Sequence of A & B packets
22、does not have fixed pattern statistical multiplexing. In TDM each host gets same 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,1-25,Packet switching versus circuit switching,1 Mb/s link each user: 100 kb/s
23、when “active” active 10% of timecircuit-switching: 10 users packet switching: with 35 users, probability 10 active less than .0004,Packet switching allows more users to use network!,N users,1 Mbps link,Introduction,1-26,Packet switching versus circuit switching,1 Mb/s link some user: 1Mb/s when “act
24、ive” Other user: Silentcircuit-switching: 10s packet switching: 1s,Introduction,1-27,Packet switching versus circuit switching,Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q:
25、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,1-28,Packet-switched networks: forwarding,Goal: move packets through routers from source to destination well study sever
26、al path selection (i.e. routing) algorithms (chapter 4) 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 datagram network: destination address in packet det
27、ermines next hop routes may change during session analogy: driving, asking directions routers dont maintain per-call state,Introduction,1-29,Network Taxonomy,Telecommunication networks,Datagram network is not either connection-oriented or connectionless.Internet provides both connection-oriented (TC
28、P) and connectionless services (UDP) to apps.,Introduction,1-30,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 Hi
29、story,Introduction,1-31,Access networks 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,1-32,
30、Residential access: point to point 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
31、) FDM: 50 kHz - 1 MHz for downstream4 kHz - 50 kHz for upstream0 kHz - 4 kHz for ordinary telephone,Introduction,1-33,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 acces
32、s to router deployment: available via cable TV companies,Introduction,1-34,Residential access: cable modems,Diagram: http:/ Network Architecture: Overview,home,cable headend,cable distribution network (simplified),Typically 500 to 5,000 homes,Introduction,1-36,Cable Network Architecture: Overview,ho
33、me,cable headend,cable distribution network (simplified),Introduction,1-37,Cable Network Architecture: Overview,home,cable headend,cable distribution network,Introduction,1-38,Cable Network Architecture: Overview,home,cable headend,cable distribution network,FDM:,Introduction,1-39,Company access: lo
34、cal area networks,company/univ local 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,1-40,Wireless access networks,shared wireless access network connects end system
35、 to router via base station aka “access point” wireless LANs: 802.11b (WiFi): 11 Mbps wider-area wireless access provided by telco operator 3G 384 kbps(first in Japan) Will it happen? WAP/GPRS in Europe,Introduction,1-41,Home networks,Typical home network components: ADSL or cable modem router/firew
36、all/NAT Ethernet wireless accesspoint,wireless access point,wireless laptops,router/ firewall,cable modem,to/from cable headend,Ethernet,Introduction,1-42,Physical Media,Bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signals propa
37、gate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g., radio,Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet,Introduction,1-43,Physical Media: coax, fiber,Coaxial cable: two concentric c
38、opper conductors bidirectional baseband: single channel on cable legacy Ethernet broadband:multiple channel on cableHFC,Fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 5 Gps) low error rate: repeaters spaced f
39、ar apart ; immune to electromagnetic noise,Introduction,1-44,Physical media: radio,signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference,Radio link types: terrestrial microwave e.g. up to 45 Mbps ch
40、annels LAN (e.g., Wifi) 2Mbps, 11Mbps wide-area (e.g., cellular) e.g. 3G: hundreds of kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude,Introduction,1-45,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Netwo
41、rk 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,1-46,Internet structure: network of networks,roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity
42、, Sprint, AT&T), national/international coverage treat each other as equals,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-47,Tier-1 ISP: e.g., Sprint,Sprint US backbone network,Introduction,1-48,Internet structure: network of networks,“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or
43、more tier-1 ISPs, possibly other tier-2 ISPs,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-49,Internet structure: network of networks,“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems),Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-50,Internet structure: network of
44、networks,a packet passes through many networks!,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-51,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 Pro
45、tocol layers, service models 1.8 History,Introduction,1-52,How do loss and delay occur?,packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn,A,B,Introduction,1-53,Four sources of packet delay,1. nodal processing: check bit errors deter
46、mine output link,2. queueing time waiting at output link for transmission depends on congestion level of router,Introduction,1-54,Delay in packet-switched networks,3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R,4. Propagation delay: d = length
47、of physical link s = propagation speed in medium (2x108 m/sec) propagation delay = d/s,Note: s and R are very different quantities!,Introduction,1-55,Caravan analogy,Cars “propagate” at 100 km/hr Toll booth takes 12 sec to service a car (transmission time) carbit; caravan packet Q: How long until ca
48、ravan is lined up before 2nd toll booth?,Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes,ten-car caravan,100 km,100 km,Introduction,1-56,Caravan analogy (more),Cars now “propagate” at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?,
