COMP 561: “Computer Networks”qianzh/COMP561-Fall2008/notes/Chapter1_2008.pdf · End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network

Introduction 1-1

COMP 561: “Computer Networks”

Qian ZhangFall 2008HKUST

Introduction 1-2

Course Info

Instructor: Qian Zhang www.cs.ust.hk/~qianzh

Course web sitehttp://www.cse.ust.hk/~qianzh/COMP561-Fall2008/index.htmlcontains all notes, announcements, etc. Check it regularly!Lecture schedule

Tuesday/Thursday 15:00-16:20 Rm 4503

http://www.cse.ust.hk/~qianzh/COMP561-Fall2008/index.html

http://www.cse.ust.hk/~qianzh/COMP561-Fall2008/index.html

Introduction 1-3

Course Info

Textbook: James Kurose and Keith RossComputer Networking: A Top Down Approach, 4th ed. Addison Wesley, 2007http://wps.aw.com/aw_kurose_network_4/ with useful resource material

The reading materials online for paper reading and reviewingExperience networking research through team projects (1-2 students)

Understand what is good researchHands-on experience in networking researchAppreciate team work / collaborationsSurvey oriented (own idea is encouraged)

http://wps.aw.com/aw_kurose_network_4/

Introduction 1-4

Course Info

Grading schemeHomework (2) 30 pointsProject (1) 10 pointsPaper review 10 pointsFinal Exam 50 points

Paper reviewEveryone reviews 1 paperEmail me ids of 3 papers that you’d like to review by Sept. 26Submit the review for one paper of your choice before final exam (1 page)

Introduction 1-5

Course Schedule

Introduction of computer networkingApplication layerTransport layerNetworking layerLink layer and local area networksMobile and wireless computing Multimedia networking

Introduction 1-6

Chapter 1Introduction

Computer Networking: A Top Down Approach ,4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 2007

Introduction 1-7

Chapter 1: IntroductionOur goal: Overview:

Get “feel” and terminologyMore depth, detail later in courseApproach:

use Internet as example

What’s the Internet?What’s a protocol?Network edge; hosts, access net, physical mediaNetwork core: packet/circuit switching, Internet structurePerformance: loss, delay, throughputSecurityProtocol layers, service modelsHistory

Introduction 1-8

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

End systems, access networks, links1.3 Network core

Circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched

networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-9

What’s the Internet: “nuts and bolts” view

Millions of connected computing devices: hosts = end systems

Running network apps Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wirelesslaptopcellular handheld

wiredlinks

access points

Communication linksFiber, copper, radio, satelliteTransmission rate = bandwidth

Routers: forward packets (chunks of data)

Introduction 1-10

What’s the Internet: “nuts and bolts” view

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

Protocols control sending, receiving of msgs

E.g., TCP, IP, HTTP, Skype, Ethernet

Internet: “network of networks”

Loosely hierarchicalPublic Internet versus private intranet

Internet standardsRFC: Request for commentsIETF: Internet Engineering Task Force

Introduction 1-11

What’s the Internet: A Service ViewCommunication 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 1-12

What’s a Protocol?Network protocols:Human protocols:

“What’s the time?”“I have a question”Introductions

… specific msgs sent… specific actions taken

when msgs received, or other events

Machines rather than humansAll communication activity 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 1-13

What’s a Protocol?A human protocol and a computer network protocol:

Hi

HiTCP connectionrequest

TCP connectionresponse

Get http://www.awl.com/kurose-ross

Got thetime?

2:00<file>time

Q: Other human protocols?

Introduction 1-14

Chapter 1: Roadmap





Introduction 1-15

A Closer Look at Network Structure:Network edge:applications and hosts

Access networks, physical media:wired, wireless communication links

Network core:Interconnected routersNetwork of networks

Introduction 1-16

The Network Edge:End systems (hosts):

Run application programsE.g. Web, emailAt “edge of network”

client/server

peer-peer

Client/server modelClient host requests, receives service from always-on serverE.g. Web browser/server; email client/server

Peer-peer model:Minimal (or no) use of dedicated serversE.g. Skype, BitTorrent

Introduction 1-17

Network Edge: Reliable Data Transfer Service

Goal: data transfer between end systemsHandshaking: setup (prepare for) data transfer ahead of time

Hello, hello back human protocolSet up “state” in two communicating hosts

TCP - Transmission Control Protocol

Internet’s reliable data transfer service

TCP service [RFC 793]Reliable, in-order byte-stream data transfer

Loss: acknowledgements and retransmissions

Flow control:Sender won’t overwhelm receiver

Congestion control:Senders “slow down sending rate” when network congested

Introduction 1-18

Network Edge: Best Effort (Unreliable) Data Transfer Service

Goal: App’s using TCP:data transfer between end systems

same as before!UDP - User Datagram Protocol [RFC 768]:

Connectionless Unreliable data transferNo flow controlNo congestion control

HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)

App’s using UDP:streaming media, teleconferencing, DNS, Internet telephony

Introduction 1-19

Access Networks and Physical Media

Q: How to connect end systems to edge router?Residential access netsInstitutional access networks (school, company)Mobile access networks

Keep in mind: Bandwidth (bits per second) of access network?Shared or dedicated?

Introduction 1-20

Residential Access: Point to Point Access

Dialup via modemUp to 56Kbps direct access to router (often less)Can’t surf and phone at same time: can’t be “always on”

DSL: digital subscriber linedeployment: 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 1-21

Residential Access: Cable Modems

HFC: hybrid fiber coaxAsymmetric: up to 30Mbps downstream, 2 Mbps upstreamIs 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 1-22

Company Access: Local Area Networks

Company/univ local area network (LAN) connects end system to edge routerEthernet:

10 Mbs, 100Mbps, 1Gbps, 10Gbps EthernetModern configuration: end systems connect into Ethernet switch

LANs: chapter 5

Introduction 1-23

Wireless Access NetworksShared wireless access network connects end system to router

Via base station aka “access point”

Wireless LANs:802.11b/g (WiFi): 11 or 54 Mbps

Wider-area wireless accessProvided by telco operator~1Mbps over cellular system (EVDO, HSDPA)Next up (?): WiMAX (10’s Mbps) over wide area

router

basestation

mobilehosts

Introduction 1-24

Wireless Technologies

WWAN (3G,4G?)

WLAN (Wi-Fi)

WPAN

WMAN (Wi-Max)

BluetoothUWBRFID

coverage

Introduction 1-25

Home NetworksTypical home network components:

ADSL or cable modemRouter/firewall/NATEthernetWireless access point

wirelessaccess point

router/firewall

Ethernet

wirelesslaptops

to/fromcable

headend

cablemodem

Introduction 1-26

Physical MediaTwisted Pair (TP)

Bit: propagates betweentransmitter/rcvr pairsPhysical link: what lies between transmitter & receiverGuided media:

Signals propagate in solid media: copper, fiber, coax

Unguided media:Signals propagate freely, e.g., radio

Two insulated copper wires

Category 3: traditional phone wires, 10 Mbps EthernetCategory 5: 100Mbps Ethernet

Introduction 1-27

Physical Media: Coax, FiberFiber optic cable:

Glass fiber carrying light pulses, each pulse a bitHigh-speed operation:

High-speed point-to-point transmission (e.g., 10’s-100’s Gps)

Low error rate: repeaters spaced far apart ; immune

Coaxial cable:Two concentric copper conductorsBidirectionalBaseband:

Single channel on cableLegacy Ethernet

Broadband:Multiple channels on cableHFC

to electromagnetic noise

Introduction 1-28

Physical Media: Radio

Signal carried in electromagnetic spectrumNo physical “wire”BidirectionalPropagation environment effects:

Reflection Obstruction by objectsInterference

Multipath propagation

Signal at Sender

Signal at Receiver

Introduction 1-29

Physical Media: Radio

Radio link types:Terrestrial microwave

e.g. up to 45 Mbps channelsLAN (e.g., Wifi)

11Mbps, 54 MbpsWide-area (e.g., cellular)

e.g. 3G: hundreds of kbpsSatellite

Kbps to 45Mbps channel (or multiple smaller channels)270 msec end-end delayGeosynchronous versus low altitude

Introduction 1-30

Chapter 1: roadmap





Introduction 1-31

The Network CoreMesh of interconnected routers

The fundamental question: how is data transferred through net?

Circuit-switching:dedicated circuit per call: telephone netPacket-switching: data sent thru net in discrete “chunks”

Introduction 1-32

Network Core: Circuit Switching

End-end resources reserved for “call”Link bandwidth, switch capacityDedicated resources: no sharingCircuit-like (guaranteed) performanceCall setup required

Introduction 1-33

Network Core: Circuit SwitchingNetwork resources

(e.g., bandwidth) divided into “pieces”Pieces allocated to callsResource piece idle if not used by owning call (no sharing)

Dividing link bandwidth into “pieces”

Frequency divisionTime division

Introduction 1-34

Circuit Switching: FDM and TDM

FDM

frequency

timeTDM

frequency

time

4 usersExample:

Introduction 1-35

Network Core: Packet SwitchingEach end-end data stream

divided into packetsUser A, B packets sharenetwork resourcesEach packet uses full link bandwidth Resources used as needed

Resource contention:Aggregate resource demand can exceed amount availableCongestion: packets queue, wait for link useStore and forward: packets move one hop at a time

Node receives complete packet before forwarding

Bandwidth division into “pieces”Dedicated allocationResource reservation

Introduction 1-36

Packet Switching: Statistical Multiplexing100 Mb/sEthernet CA

B

statistical multiplexing

1.5 Mb/s

D

queue of packetswaiting for output

link

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 1-37

Packet-Switching: Store-and-Forward

R R RL

Example:Takes L/R seconds to transmit (push out) packet of L bits on to link or R bpsEntire packet must arrive at router before it can be transmitted on next link: store and forwardDelay = 3L/R (assuming zero propagation delay)

L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 sec

more on delay shortly …

Introduction 1-38

Packet Switching versus Circuit Switching

Is packet switching a “slam dunk winner?”

Great for bursty dataResource sharingSimpler, no call setup

Excessive congestion: packet delay and lossProtocols needed for reliable data transfer, congestion control

Q: How to provide circuit-like behavior?Bandwidth guarantees needed for audio/video appsStill an unsolved problem (chapter 7)

Introduction 1-39

Internet Structure: Network of Networks

Roughly hierarchicalAt center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage

Treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

Introduction 1-40

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

….

………

POP: point-of-presence

Introduction 1-41


“Tier-2” ISPs: smaller (often regional) ISPsConnect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet

Tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other.

Introduction 1-42


“Tier-3” ISPs and local ISPs Last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Introduction 1-43


A packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Introduction 1-44

Chapter 1: roadmap





Introduction 1-45

How do Loss and Delay Occur?Packets queue in router buffers

Packet arrival rate to link exceeds output link capacityPackets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

Introduction 1-46

Four Sources of Packet Delay

1. Nodal processing:Check bit errorsDetermine output link

2. QueueingTime waiting at output link for transmission Depends on congestion level of router

A

Bnodal

processing queueing

Introduction 1-47

Delay in Packet-Switched Networks3. 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 links = propagation speed in medium (~2x108 m/sec)propagation delay = d/s

propagation

transmission

Note: s and R are very different quantities!

A

Bnodal

processing queueing

Introduction 1-48

Caravan Analogy

Cars “propagate” at 100 km/hrToll booth takes 12 sec to service a car (transmission time)car~bit; caravan ~ packetQ: How long until caravan is lined up before 2nd toll booth?

Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 secTime for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hrA: 62 minutes

toll booth

toll booth

ten-car caravan

100 km 100 km

Introduction 1-49

Caravan Analogy (more)

Cars now “propagate” at 1000 km/hrToll booth now takes 1 min to service a carQ: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!

See Ethernet applet at AWL Web site

toll booth

toll booth

ten-car caravan

100 km 100 km

Introduction 1-50

Nodal Delay

proptransqueueprocnodal ddddd +++=

dproc = processing delayTypically a few microsecs or less

dqueue = queuing delayDepends on congestion level in the router

dtrans = transmission delay= L/R, significant for low-speed links

dprop = propagation delayA few microsecs to hundreds of msecs

Introduction 1-51

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 smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!

Introduction 1-52

“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 destinationRouter i will return packets to senderSender times interval between transmission and reply

3 probes

3 probes

3 probes

Introduction 1-53

Packet Loss

Queue (aka buffer) preceding link in buffer has finite capacityPacket arriving to full queue dropped (aka lost)Lost packet may be retransmitted by previous node, by source end system, or not at all

Apacket being transmitted

packet arriving tofull buffer is lost

buffer (waiting area)

B

Introduction 1-54

ThroughputThroughput: rate (bits/time unit) at which bits transferred between sender/receiver

Instantaneous: rate at given point in timeAverage: rate over long(er) period of time

server, withfile of F bits

to send to client

link capacityRs bits/sec

link capacityRc bits/sec

pipe that can carryfluid at rateRs bits/sec)

pipe that can carryfluid at rateRc bits/sec)

server sends bits (fluid) into pipe

Introduction 1-55

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

link on end-end path that constrains end-end throughputbottleneck link

Introduction 1-56

Throughput: Internet Scenario

Rs

RsRs

Rc

Rc

Rc

R

Per-connection end-end throughput: min(Rc,Rs,R/10)In practice: Rc or Rs is often bottleneck

10 connections (fairly) share backbone bottleneck link R bits/sec

Introduction 1-57

Chapter 1: roadmap





Introduction 1-58

Protocol “Layers”Networks are complex!

Many “pieces”:HostsRoutersLinks of various mediaApplicationsProtocolsHardware and software

Question:Is there any hope of organizing structure of

network?

Or at least our discussion of networks?

Introduction 1-59

Why Layering?Dealing with complex systems:

Explicit structure allows identification, relationship of complex system’s pieces

Layered reference model for discussionModularization eases maintenance, updating of system

Change of implementation of layer’s service transparent to rest of systemE.g., change in gate procedure doesn’t affect rest of system

Layering considered harmful?

Introduction 1-60

Internet Protocol StackApplication: supporting network applications

FTP, SMTP, HTTPTransport: process-process data transfer

TCP, UDPNetwork: routing of datagrams from source to destination

IP, routing protocolsLink: data transfer between neighboring network elements

PPP, EthernetPhysical: bits “on the wire”

application

transport

network

link

physical

Introduction 1-61

Chapter 1: roadmap





Introduction 1-62

Network SecurityAttacks 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!

What Can Bad Guys Do: Malware?

Introduction 1-63

Spyware:Infection by downloading web page with spywareRecords keystrokes, web sites visited, upload info to collection site

VirusInfection by receiving object (e.g., e-mail attachment), actively executingSelf-replicating: propagate itself to other hosts, users

Worm:Infection by passively receiving object that gets itself executedSelf-replicating: propagates to other hosts, usersSapphire Worm: aggregate scans/sec

in first 5 minutes of outbreak (CAIDA, UWisc data)

Introduction 1-64

Denial of Service AttacksAttackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic

1. Select target2. Break into hosts

around the network (see malware)

3. Send packets toward target from compromised hosts

target

Introduction 1-65

Sniff, Modify, Delete Your PacketsPacket sniffing:

Broadcast media (shared Ethernet, wireless)Promiscuous network interface reads/records all packets (e.g., including passwords!) passing by

A

B

C

src:B dest:A payload

Ethereal software used for end-of-chapter labs is a (free) packet-snifferMore on modification, deletion later

Introduction 1-66

Masquerade as youIP spoofing: send packet with false source address

A C

src:B dest:A payload

B

Introduction 1-67

Masquerade as youIP spoofing: send packet with false source addressRecord-and-playback: sniff sensitive info (e.g., password), and use later

Password holder is that user from system point of view

C

src:B dest:A user: B; password: foo

A

B

Introduction 1-68

Masquerade as youIP spoofing: send packet with false source addressRecord-and-playback: sniff sensitive info (e.g., password), and use later

Password holder is that user from system point of view

later ….. C

src:B dest:A user: B; password: foo

A

B

Introduction 1-69

Chapter 1: roadmap


end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched


Introduction 1-70

Internet History

1961: Kleinrock -queueing theory shows effectiveness of packet-switching1964: Baran - packet-switching in military nets1967: ARPAnet conceived by Advanced Research Projects Agency1969: first ARPAnet node operational

1961-1972: Early packet-switching principles

The first link in theInternet backbone

UCLA

SRI

crash

The first message: LO!

What was the first message ever sent on the Internet? (LOGIN)

We sent an “L” - did you get the “L”? YEP!We sent an “O” - did you get the “O”? YEP!We sent a “G” - did you get the “G”?

Introduction 1-71

Internet History1961-1972: Early packet-switching principles

1972:ARPAnet public demonstrationNCP (Network Control Protocol) first host-host protocol First e-mail programARPAnet has 15 nodes

The Internet is Born!at UCLA on October 29, 1969What it looked like at the end of 1969

Introduction 1-72

Internet History1972-1980: Internetworking, new and proprietary nets1970: ALOHAnet satellite network in Hawaii1974: Cerf and Kahn -architecture for interconnecting networks1976: Ethernet at Xerox PARCate70’s: proprietary architectures: DECnet, SNA, XNAlate 70’s: switching fixed length packets (ATM precursor)1979: ARPAnet has 200 nodes

Cerf and Kahn’s internetworking principles:

minimalism, autonomy -no internal changes required to interconnect networksbest effort service modelstateless routersdecentralized control

define today’s Internet architecture

Introduction 1-73

Internet History1980-1990: new protocols, a proliferation of networks

1983: deployment of TCP/IP1982: SMTP e-mail protocol defined 1983: DNS defined for name-to-IP-address translation1985: FTP protocol defined1988: TCP congestion control

New national networks: Csnet, BITnet, NSFnet, Minitel100,000 hosts connected to confederation of networks

Introduction 1-74

Internet History1990, 2000’s: commercialization, the Web, new apps

Early 1990’s: ARPAnet decommissioned1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)Early 1990s: Web

Hypertext [Bush 1945, Nelson 1960’s]HTML, HTTP: Berners-Lee1994: Mosaic, later NetscapeLate 1990’s: commercialization of the Web

Late 1990’s – 2000’s:More killer apps: instant messaging, P2P file sharingNetwork security to forefrontEst. 50 million host, 100 million+ usersBackbone links running at Gbps

Introduction 1-75

Internet History

2007:~500 million hostsVoice, Video over IPP2P applications: BitTorrent (file sharing), Skype (VoIP), PPLive (video)More applications: YouTube, gamingWireless, mobility

Introduction 1-76

Introduction: SummaryCovered a “ton” of material!

Internet overviewWhat’s a protocol?Network edge, core, access network

Packet-switching versus circuit-switchingInternet structure

Performance: loss, delay, throughputLayering, service modelsSecurityHistory

You now have:Context, overview, “feel” of networkingMore depth, detail to follow!

Documents

COMP 561: “Computer Networks”qianzh/COMP561-Fall2008/notes/Chapter1_2008.pdf · End systems, access networks, links 1.3 Network core Circuit switching, packet switching, network