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COMP 561: "Computer Networks"
 

COMP 561: "Computer Networks"

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    COMP 561: "Computer Networks" COMP 561: "Computer Networks" Presentation Transcript

    • COMP 561: “Computer Networks” Qian Zhang Fall 2008 HKUST Introduction 1-1
    • Course Info Instructor: Qian Zhang www.cs.ust.hk/~qianzh Course web site http://www.cse.ust.hk/~qianzh/COMP561- Fall2008/index.html contains all notes, announcements, etc. Check it regularly! Lecture schedule Tuesday/Thursday 15:00-16:20 Rm 4503 Introduction 1-2
    • Course Info Textbook: James Kurose and Keith Ross Computer Networking: A Top Down Approach, 4th ed. Addison Wesley, 2007 http://wps.aw.com/aw_kurose_network_4/ with useful resource material The reading materials online for paper reading and reviewing Experience networking research through team projects (1- 2 students) Understand what is good research Hands-on experience in networking research Appreciate team work / collaborations Survey oriented (own idea is encouraged) Introduction 1-3
    • Course Info Grading scheme Homework (2) 30 points Project (1) 10 points Paper review 10 points Final Exam 50 points Paper review Everyone reviews 1 paper Email me ids of 3 papers that you’d like to review by Sept. 26 Submit the review for one paper of your choice before final exam (1 page) Introduction 1-4
    • Course Schedule Introduction of computer networking Application layer Transport layer Networking layer Link layer and local area networks Mobile and wireless computing Multimedia networking Introduction 1-5
    • Chapter 1 Introduction A note on the use of these ppt slides: The notes used in this course are substantially based on powerpoint slides developed and Computer Networking: copyrighted by J.F. Kurose and K.W. Ross, 2007 A Top Down Approach , 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007. Introduction 1-6
    • Chapter 1: Introduction Our goal: Overview: Get “feel” and What’s the Internet? terminology What’s a protocol? More depth, detail later in course Network edge; hosts, access Approach: net, physical media use Internet as Network core: packet/circuit example switching, Internet structure Performance: loss, delay, throughput Security Protocol layers, service models History Introduction 1-7
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-8
    • What’s the Internet: “nuts and bolts” view PC Millions of connected Mobile network server computing devices: Global ISP wireless hosts = end systems laptop Running network cellular handheld apps Home network Regional ISP Communication links access Fiber, copper, points radio, satellite Institutional network wired links Transmission rate = bandwidth Routers: forward router packets (chunks of data) Introduction 1-9
    • What’s the Internet: “nuts and bolts” view Mobile network Protocols control sending, receiving of msgs Global ISP E.g., TCP, IP, HTTP, Skype, Ethernet Home network Internet: “network of Regional ISP networks” Loosely hierarchical Public Internet versus Institutional network private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force Introduction 1-10
    • What’s the Internet: A Service View Communication infrastructure enables distributed applications: Web, VoIP, email, games, e-commerce, file sharing Communication services provided to apps: Reliable data delivery from source to destination “Best effort” (unreliable) data delivery Introduction 1-11
    • What’s a Protocol? Human protocols: Network protocols: “What’s the time?” Machines rather than “I have a question” humans Introductions All communication activity in Internet … specific msgs sent governed by protocols … specific actions taken Protocols define format, when msgs received, order of msgs sent and or other events received among network entities, and actions taken on msg transmission, receipt Introduction 1-12
    • What’s a Protocol? A human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection Got the response time? Get http://www.awl.com/kurose-ross 2:00 time <file> Q: Other human protocols? Introduction 1-13
    • Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-14
    • A Closer Look at Network Structure: Network edge: applications and hosts Access networks, physical media: wired, wireless communication links Network core: Interconnected routers Network of networks Introduction 1-15
    • The Network Edge: End systems (hosts): Run application programs E.g. Web, email At “edge of network” peer-peer Client/server model Client host requests, receives service from always-on server E.g. Web browser/server; client/server email client/server Peer-peer model: Minimal (or no) use of dedicated servers E.g. Skype, BitTorrent Introduction 1-16
    • Network Edge: Reliable Data Transfer Service Goal: data transfer TCP service [RFC 793] between end systems Reliable, in-order byte- Handshaking: setup stream data transfer (prepare for) data Loss: acknowledgements transfer ahead of time and retransmissions Hello, hello back human Flow control: protocol Sender won’t overwhelm Set up “state” in two receiver communicating hosts Congestion control: TCP - Transmission Senders “slow down Control Protocol sending rate” when Internet’s reliable data network congested transfer service Introduction 1-17
    • Network Edge: Best Effort (Unreliable) Data Transfer Service Goal: data transfer App’s using TCP: between end systems HTTP (Web), FTP (file same as before! transfer), Telnet UDP - User Datagram (remote login), SMTP Protocol [RFC 768]: (email) Connectionless Unreliable data App’s using UDP: transfer streaming media, No flow control teleconferencing, DNS, No congestion control Internet telephony Introduction 1-18
    • 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 1-19
    • Residential Access: Point to Point Access Dialup via modem Up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: can’t be “always on” DSL: digital subscriber line deployment: telephone company (typically) up to 1 Mbps upstream (today typically < 256 kbps) up to 8 Mbps downstream (today typically < 1 Mbps) dedicated physical line to telephone central office Introduction 1-20
    • Residential Access: Cable Modems HFC: hybrid fiber coax Asymmetric: up to 30Mbps downstream, 2 Mbps upstream Is shared broadcast medium Network of cable and fiber attaches homes to ISP router Homes share access to router Deployment: available via cable TV companies Introduction 1-21
    • Company Access: Local Area Networks Company/univ local area network (LAN) connects end system to edge router Ethernet: 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet Modern configuration: end systems connect into Ethernet switch LANs: chapter 5 Introduction 1-22
    • Wireless Access Networks Shared wireless access network connects end system to router router Via base station aka “access point” base Wireless LANs: station 802.11b/g (WiFi): 11 or 54 Mbps Wider-area wireless access Provided by telco operator ~1Mbps over cellular system mobile (EVDO, HSDPA) hosts Next up (?): WiMAX (10’s Mbps) over wide area Introduction 1-23
    • Wireless Technologies coverage Bluetooth WWAN (3G,4G?) UWB RFID WMAN (Wi-Max) WLAN (Wi-Fi) WPAN Introduction 1-24
    • Home Networks Typical home network components: ADSL or cable modem Router/firewall/NAT Ethernet Wireless access point wireless laptops to/from cable router/ cable modem firewall headend wireless access Ethernet point Introduction 1-25
    • Physical Media Twisted Pair (TP) Bit: propagates between Two insulated copper transmitter/rcvr pairs wires Physical link: what lies Category 3: traditional between transmitter & phone wires, 10 Mbps receiver Ethernet Category 5: Guided media: 100Mbps Ethernet Signals propagate in solid media: copper, fiber, coax Unguided media: Signals propagate freely, e.g., radio Introduction 1-26
    • Physical Media: Coax, Fiber Coaxial cable: Fiber optic cable: Two concentric copper Glass fiber carrying light conductors pulses, each pulse a bit Bidirectional High-speed operation: Baseband: High-speed point-to-point Single channel on cable transmission (e.g., 10’s- Legacy Ethernet 100’s Gps) Broadband: Low error rate: repeaters Multiple channels on spaced far apart ; immune cable to electromagnetic noise HFC Introduction 1-27
    • Physical Media: Radio Signal carried in electromagnetic spectrum No physical “wire” Bidirectional Propagation environment effects: Reflection Obstruction by objects Interference Multipath propagation Signal at Receiver Signal at Sender Introduction 1-28
    • Physical Media: Radio Radio link types: Terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., Wifi) 11Mbps, 54 Mbps Wide-area (e.g., cellular) e.g. 3G: hundreds of kbps Satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay Geosynchronous versus low altitude Introduction 1-29
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-30
    • 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 1-31
    • Network 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 1-32
    • Network Core: Circuit Switching Network resources Dividing link bandwidth (e.g., bandwidth) into “pieces” divided into “pieces” Frequency division Pieces allocated to calls Time division Resource piece idle if not used by owning call (no sharing) Introduction 1-33
    • Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time Introduction 1-34
    • Network Core: Packet Switching Each end-end data stream Resource contention: divided into packets Aggregate resource User A, B packets share demand can exceed network resources amount available Each packet uses full link Congestion: packets bandwidth queue, wait for link use Resources used as needed Store and forward: packets move one hop at a time Bandwidth division into “pieces” Node receives complete Dedicated allocation packet before forwarding Resource reservation Introduction 1-35
    • Packet Switching: Statistical Multiplexing 100 Mb/s A Ethernet statistical multiplexing C 1.5 Mb/s B queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing TDM: each host gets same slot in revolving TDM frame Introduction 1-36
    • Packet-Switching: Store-and-Forward L R R R Takes L/R seconds to Example: transmit (push out) packet L = 7.5 Mbits of L bits on to link or R R = 1.5 Mbps bps delay = 15 sec Entire packet must arrive at router before it can be transmitted on next link: store and forward Delay = 3L/R (assuming zero propagation delay) more on delay shortly … Introduction 1-37
    • Packet Switching versus Circuit Switching Is packet switching a “slam dunk winner?” 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 needed for audio/video apps Still an unsolved problem (chapter 7) Introduction 1-38
    • Internet Structure: Network of Networks Roughly hierarchical At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage Treat each other as equals Tier-1 providers Tier 1 ISP interconnect (peer) privately Tier 1 ISP Tier 1 ISP Introduction 1-39
    • Tier-1 ISP: e.g., Sprint POP: point-of-presence to/from backbone peering … … . … … … to/from customers Introduction 1-40
    • 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-2 ISPs Tier-2 ISP pays Tier-2 ISP also peer Tier-2 ISP privately with tier-1 ISP for connectivity to Tier 1 ISP each other. rest of Internet Tier-2 ISP is customer of tier-1 provider Tier 1 ISP Tier 1 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Introduction 1-41
    • Internet Structure: Network of Networks “Tier-3” ISPs and local ISPs Last hop (“access”) network (closest to end systems) local ISP Tier 3 local local local ISP ISP ISP ISP Local and tier- Tier-2 ISP Tier-2 ISP 3 ISPs are customers of Tier 1 ISP higher tier ISPs connecting them to rest Tier 1 ISP of Internet Tier 1 ISP Tier-2 ISP local Tier-2 ISP Tier-2 ISP ISP local local local ISP ISP ISP Introduction 1-42
    • Internet Structure: Network of Networks A packet passes through many networks! local ISP Tier 3 local local local ISP ISP ISP ISP Tier-2 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local Tier-2 ISP Tier-2 ISP ISP local local local ISP ISP ISP Introduction 1-43
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-44
    • 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 packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction 1-45
    • Four Sources of Packet Delay 1. Nodal processing: 2. Queueing Check bit errors Time waiting at output Determine output link link for transmission Depends on congestion level of router A B nodal processing queueing Introduction 1-46
    • Delay in Packet-Switched Networks 3. Transmission delay: 4. Propagation delay: R=link bandwidth (bps) d = length of physical link L=packet length (bits) s = propagation speed in Time to send bits into medium (~2x108 m/sec) link = L/R propagation delay = d/s Note: s and R are very different quantities! transmission A propagation B nodal processing queueing Introduction 1-47
    • Caravan Analogy 100 km 100 km ten-car toll toll caravan booth booth Cars “propagate” at Time to “push” entire 100 km/hr caravan through toll Toll booth takes 12 sec to booth onto highway = service a car 12*10 = 120 sec (transmission time) Time for last car to car~bit; caravan ~ packet propagate from 1st to 2nd toll both: Q: How long until caravan 100km/(100km/hr)= 1 hr is lined up before 2nd toll booth? A: 62 minutes Introduction 1-48
    • Caravan Analogy (more) 100 km 100 km ten-car toll toll caravan booth booth Yes! After 7 min, 1st car Cars now “propagate” at at 2nd booth and 3 cars 1000 km/hr still at 1st booth. Toll booth now takes 1 1st bit of packet can min to service a car arrive at 2nd router Q: Will cars arrive to before packet is fully 2nd booth before all transmitted at 1st router! cars serviced at 1st See Ethernet applet at AWL booth? Web site Introduction 1-49
    • Nodal Delay d nodal = d proc + d queue + d trans + d prop dproc = processing delay Typically a few microsecs or less dqueue = queuing delay Depends on congestion level in the router dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay A few microsecs to hundreds of msecs Introduction 1-50
    • Queueing Delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite! Introduction 1-51
    • “Real” Internet Delays and Routes What do “real” Internet delay & loss look like? Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: Sends three packets that will reach router i on path towards destination Router i will return packets to sender Sender times interval between transmission and reply 3 probes 3 probes 3 probes Introduction 1-52
    • Packet Loss Queue (aka buffer) preceding link in buffer has finite capacity Packet arriving to full queue dropped (aka lost) Lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost Introduction 1-53
    • Throughput Throughput: rate (bits/time unit) at which bits transferred between sender/receiver Instantaneous: rate at given point in time Average: rate over long(er) period of time link capacity server sends bits pipe that can carry server, with link capacity pipe that can carry (fluid) of F bits file into pipe Rs bits/sec fluid at rate c bits/sec Rfluid at rate to send to client Rs bits/sec) Rc bits/sec) Introduction 1-54
    • Throughput (more) Rs < Rc What is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec bottleneck link link on end-end path that constrains end-end throughput Introduction 1-55
    • Throughput: Internet Scenario Rs Per-connection Rs Rs end-end throughput: R min(Rc,Rs,R/10) In practice: Rc or Rc Rc Rs is often Rc bottleneck 10 connections (fairly) share backbone bottleneck link R bits/sec Introduction 1-56
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-57
    • Protocol “Layers” Networks are complex! Many “pieces”: Hosts Question: Routers Is there any hope of Links of various organizing structure of media network? Applications Protocols Or at least our discussion Hardware and of networks? software Introduction 1-58
    • Why Layering? Dealing with complex systems: Explicit structure allows identification, relationship of complex system’s pieces Layered reference model for discussion Modularization eases maintenance, updating of system Change of implementation of layer’s service transparent to rest of system E.g., change in gate procedure doesn’t affect rest of system Layering considered harmful? Introduction 1-59
    • Internet Protocol Stack Application: supporting network applications application FTP, SMTP, HTTP Transport: process-process data transport transfer TCP, UDP network Network: routing of datagrams from source to destination link IP, routing protocols Link: data transfer between physical neighboring network elements PPP, Ethernet Physical: bits “on the wire” Introduction 1-60
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-61
    • Network Security Attacks on Internet infrastructure: Infecting/attacking hosts: malware, spyware, worms, unauthorized access (data stealing, user accounts) Denial of service: deny access to resources (servers, link bandwidth) Internet not originally designed with (much) security in mind Original vision: “a group of mutually trusting users attached to a transparent network” ☺ Internet protocol designers playing “catch-up” Security considerations in all layers! Introduction 1-62
    • What Can Bad Guys Do: Malware? Spyware: Worm: Infection by downloading Infection by passively web page with spyware receiving object that gets Records keystrokes, web itself executed sites visited, upload info Self-replicating: propagates to collection site to other hosts, users Virus Sapphire Worm: aggregate scans/sec Infection by receiving in first 5 minutes of outbreak (CAIDA, UWisc data) object (e.g., e-mail attachment), actively executing Self-replicating: propagate itself to other hosts, users Introduction 1-63
    • Denial of Service Attacks Attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic 1. Select target 2. Break into hosts around the network (see malware) target 3. Send packets toward target from compromised hosts Introduction 1-64
    • Sniff, Modify, Delete Your Packets Packet sniffing: Broadcast media (shared Ethernet, wireless) Promiscuous network interface reads/records all packets (e.g., including passwords!) passing by A C src:B dest:A payload B Ethereal software used for end-of-chapter labs is a (free) packet-sniffer More on modification, deletion later Introduction 1-65
    • Masquerade as you IP spoofing: send packet with false source address A C src:B dest:A payload B Introduction 1-66
    • Masquerade as you IP spoofing: send packet with false source address Record-and-playback: sniff sensitive info (e.g., password), and use later Password holder is that user from system point of view C A src:B dest:A user: B; password: foo B Introduction 1-67
    • Masquerade as you IP spoofing: send packet with false source address Record-and-playback: sniff sensitive info (e.g., password), and use later Password holder is that user from system point of view later ….. C A src:B dest:A user: B; password: foo B Introduction 1-68
    • Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction 1-69
    • Internet History 1961-1972: Early packet-switching principles 1961: Kleinrock - The first link in the queueing theory shows SRI Internet backbone effectiveness of crash packet-switching UCLA 1964: Baran - packet- switching in military nets The first message: LO! 1967: ARPAnet conceived by Advanced What was the first message ever Research Projects sent on the Internet? (LOGIN) Agency 1969: first ARPAnet We sent an “L” - did you get the “L”? YEP! We sent an “O” - did you get the “O”? YEP! node operational We sent a “G” - did you get the “G”? Introduction 1-70
    • Internet History 1961-1972: Early packet-switching principles The Internet is Born! 1972: at UCLA on October ARPAnet public 29, 1969 demonstration What it looked like at NCP (Network Control the end of 1969 Protocol) first host-host protocol First e-mail program ARPAnet has 15 nodes Introduction 1-71
    • Internet History 1972-1980: Internetworking, new and proprietary nets 1970: ALOHAnet satellite Cerf and Kahn’s network in Hawaii internetworking principles: 1974: Cerf and Kahn - minimalism, autonomy - architecture for no internal changes interconnecting networks required to 1976: Ethernet at Xerox interconnect networks PARC best effort service ate70’s: proprietary model architectures: DECnet, SNA, XNA stateless routers late 70’s: switching fixed decentralized control length packets (ATM define today’s Internet precursor) architecture 1979: ARPAnet has 200 nodes Introduction 1-72
    • Internet History 1980-1990: new protocols, a proliferation of networks 1983: deployment of New national TCP/IP networks: Csnet, 1982: SMTP e-mail BITnet, NSFnet, protocol defined Minitel 1983: DNS defined 100,000 hosts for name-to-IP- connected to address translation confederation of 1985: FTP protocol networks defined 1988: TCP congestion control Introduction 1-73
    • Internet History 1990, 2000’s: commercialization, the Web, new apps Early 1990’s: ARPAnet Late 1990’s – 2000’s: decommissioned More killer apps: instant 1991: NSF lifts restrictions on messaging, P2P file sharing commercial use of NSFnet Network security to (decommissioned, 1995) forefront Early 1990s: Web Est. 50 million host, 100 Hypertext [Bush 1945, million+ users Nelson 1960’s] Backbone links running at HTML, HTTP: Berners-Lee Gbps 1994: Mosaic, later Netscape Late 1990’s: commercialization of the Web Introduction 1-74
    • Internet History 2007: ~500 million hosts Voice, Video over IP P2P applications: BitTorrent (file sharing), Skype (VoIP), PPLive (video) More applications: YouTube, gaming Wireless, mobility Introduction 1-75
    • Introduction: Summary Covered a “ton” of material! You now have: Internet overview Context, overview, What’s a protocol? “feel” of networking Network edge, core, More depth, detail to access network follow! Packet-switching versus circuit-switching Internet structure Performance: loss, delay, throughput Layering, service models Security History Introduction 1-76