1、Introduction,1-1,Chapter 1: Introduction,Our goal: get context, overview, “feel” of networking more depth, detail later in course approach: descriptive use Internet as example,Overview: whats the Internet whats a protocol? network edge network core access net, physical media Internet/ISP structure p
2、erformance: loss, delay protocol layers, service models history,Introduction,1-2,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, serv
3、ice models 1.8 History,Introduction,1-3,Whats the Internet: “nuts and bolts” view,millions of connected computing devices: hosts, end-systems PCs workstations, servers PDAs phones, toasters running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers
4、: forward packets (chunks of data),Introduction,1-4,“Cool” internet appliances,Worlds smallest web server http:/www-ccs.cs.umass.edu/shri/iPic.html,IP picture frame http:/ toaster+weather forecaster,Introduction,1-5,Whats the Internet: “nuts and bolts” view,protocols control sending, receiving of ms
5、gs 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,local ISP,company network,regional ISP,router,workstation,server,mobile,Introduction,1-6,Wh
6、ats the Internet: a service view,communication infrastructure enables distributed applications: Web, email, games, e-commerce, database., voting, file (MP3) sharing communication services provided to apps: connectionless connection-oriented,cyberspace Gibson: “a consensual hallucination experienced
7、daily by billions of operators, in every nation, “,Introduction,1-7,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 ac
8、tivity in Internet governed by protocols,protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt,Introduction,1-8,Whats a protocol?,a human protocol and a computer network protocol:,Q: Other human protocols?,Hi,Hi,TCP connection
9、req,Introduction,1-9,Key Elements of a Protocol,Syntax Data formats Signal levels Semantics Control information Error handling Timing Speed matching Sequencing,Introduction,1-10,A closer look at network structure:,network edge: applications and hosts network core: routers network of networks access
10、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 packet-switched networks 1.7 Protocol layers, service models 1.8 Hi
11、story,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 client/server peer-peer model:minimal (or no) use of dedicated
12、servers e.g. Gnutella, KaZaA,Introduction,1-13,Network 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
13、 Internets connection-oriented service,TCP service RFC 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,1-14,Network edge:
14、connectionless service,Goal: data transfer between end systems same as before! UDP - User Datagram Protocol RFC 768: Internets connectionless service unreliable data transfer no flow control no congestion control,Apps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)App
15、s using UDP: streaming media, teleconferencing, DNS, Internet telephony,Introduction,1-15,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 lay
16、ers, service models 1.8 History,Introduction,1-16,The Network Core,mesh 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”,Introduction,1-17,Ne
17、twork Core: Circuit Switching,End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required,Introduction,1-18,Network Core: Circuit Switching,network resources (e.g., bandwidth) divided into “pieces” pi
18、eces allocated to calls resource piece idle if not used by owning call (no sharing),dividing link bandwidth into “pieces” frequency division time division,Introduction,1-19,Circuit Switching: TDMA and TDMA,Introduction,1-20,Network Core: Packet Switching,each end-end data stream divided into packets
19、 user A, B packets share network resources each packet uses full link bandwidth resources used as needed,resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time transmit over link wait t
20、urn at next link,Introduction,1-21,Packet Switching: Statistical Multiplexing,Sequence of A & B packets does not have fixed pattern statistical multiplexing. In TDM each host gets same slot in revolving TDM frame.,A,B,C,10 Mbs Ethernet,1.5 Mbs,statistical multiplexing,queue of packets waiting for ou
21、tput link,Introduction,1-22,Packet switching versus circuit switching,1 Mbit link each user: 100 kbps 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 l
22、ink,Introduction,1-23,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: How to provide circuit-like behavior? bandwidth guarantees need
23、ed for audio/video apps still an unsolved problem (chapter 6),Is packet switching a “slam dunk winner?”,Introduction,1-24,Packet-switching: store-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
24、 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,1-25,Packet Switching: Message Segmenting,Now break up the message into 5000 packets,Each packet 1,500 bits 1 msec to transmit packet on one link pipelining: each link works in paral
25、lel Delay reduced from 15 sec to 5.002 sec,Introduction,1-26,Packet-switched networks: forwarding,Goal: move packets through routers from source to destination well study several path selection (i.e. routing)algorithms (chapter 4) datagram network: destination address in packet determines next hop r
26、outes may change during session 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,1-27,Network Taxonomy,Tele
27、communication networks,Datagram network is not either connection-oriented or connectionless.Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.,Introduction,1-28,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access
28、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,1-29,Access networks and physical media,Q: How to connection end systems to edge router? residential access nets institutional access networks
29、(school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?,Introduction,1-30,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
30、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 downstream4 kHz - 50 kHz for upstream0 kHz - 4 kHz for ordinary telephone,Introduction,1-31,Residential access: cable modems,HFC: hyb
31、rid fiber coax asymmetric: up to 10Mbps upstream, 1 Mbps downstream network of cable and fiber attaches homes to ISP router shared access to router among home issues: congestion, dimensioning deployment: available via cable companies, e.g., MediaOne,Introduction,1-32,Residential access: cable modems
32、,Diagram: http:/ Network Architecture: Overview,home,cable headend,cable distribution network (simplified),Typically 500 to 5,000 homes,Introduction,1-34,Cable Network Architecture: Overview,home,cable headend,cable distribution network (simplified),Introduction,1-35,Cable Network Architecture: Over
33、view,home,cable headend,cable distribution network,Introduction,1-36,Cable Network Architecture: Overview,home,cable headend,cable distribution network,FDM:,Introduction,1-37,Company access: local area networks,company/univ local area network (LAN) connects end system to edge router Ethernet: shared
34、 or dedicated link connects end system and router 10 Mbs, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs happening now LANs: chapter 5,Introduction,1-38,Wireless access networks,shared wireless access network connects end system to router via base station aka “access point” wireless L
35、ANs: 802.11b (WiFi): 11 Mbps wider-area wireless access provided by telco operator 3G 384 kbps Will it happen? WAP/GPRS in Europe,Introduction,1-39,Home networks,Typical home network components: ADSL or cable modem router/firewall/NAT Ethernet wireless accesspoint,wireless access point,wireless lapt
36、ops,router/ firewall,cable modem,to/from cable headend,Ethernet (switched),Introduction,1-40,Physical Media,Bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax unguided media: signa
37、ls propagate freely, e.g., radio,Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5 TP: 100Mbps Ethernet,Introduction,1-41,Physical Media: coax, fiber,Coaxial cable: two concentric copper conductors bidirectional baseband: single channel on
38、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 far apart ; immune to electromagnetic noise,Introduction,1-4
39、2,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 channels LAN (e.g., WaveLAN) 2Mbps, 11Mbps wide-area (e.g., c
40、ellular) e.g. 3G: hundreds of kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus LEOS,Introduction,1-43,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 struc
41、ture and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History,Introduction,1-44,Internet structure: network of networks,roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage treat each othe
42、r as equals,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-45,Tier-1 ISP: e.g., Sprint,Sprint US backbone network,Introduction,1-46,Internet structure: network of networks,“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs,Tier 1 ISP,Tier 1 I
43、SP,Tier 1 ISP,Introduction,1-47,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,1-48,Internet structure: network of networks,a packet passes through many networks!,Tier 1 ISP,Tier
44、1 ISP,Tier 1 ISP,Introduction,1-49,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,1-50,How d
45、o 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,1-51,Four sources of packet delay,1. nodal processing: check bit errors determine output link,2. queuing time waiting at output link for tran
46、smission depends on congestion level of router,Introduction,1-52,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 of physical link s = propagation speed in medium (2x108 m/sec) pr
47、opagation delay = d/s,Note: s and R are very different quantities!,Introduction,1-53,Packet Processing inside switches,Forwarding Decision,Forwarding Decision,Forwarding Decision,Forwarding Table,Forwarding Table,Forwarding Table,Interconnect,Output Scheduling,Introduction,1-54,Switching Fabric,Intr
48、oduction,1-55,Switching Interconnects Two basic techniques,Input Queuing,Output Queuing,Usually a non-blocking switch fabric (e.g. crossbar),Usually a fast bus,Introduction,1-56,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 caravan is lined up before 2nd toll booth?,
