1

Mecanismes d’Echange d’InformationsChapter 1Introduction

Computer Networking: A Top Down Approach Featuring the Internet,

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Introduction 1-1

Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.

following:If you use these slides (e.g., in a class) in substantially unaltered form,

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Thanks and enjoy! JFK/KWR

All material copyright 1996-2005J.F Kurose and K.W. Ross, All Rights Reserved

Chapter 1: IntroductionOur goal:

get “feel” and Overview:

what’s the Internetgterminologymore depth, detail later in courseapproach:

use Internet as example

what’s a protocol?network edgenetwork coreaccess net, physical mediaInternet/ISP structure

Introduction 1-2

exampleperformance: loss, delayprotocol layers, service modelsnetwork modeling

2

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-3

y p1.7 Protocol layers, service models

What’s the Internet: “nuts and bolts” viewmillions of connected computing devices: hosts = end systems

router workstationserver

mobile= end systemsrunning network appscommunication links

fiber, copper, radio, satellitetransmission rate = bandwidth

local ISP

regional ISP

mobile

Introduction 1-4

routers: forward packets (chunks of data)

companynetwork

3

“Cool” internet appliances

Web-enabled toaster +

IP picture framehttp://www.ceiva.com/

weather forecaster

Introduction 1-5

World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html Internet phones

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

protocols control sending, receiving of msgs

TCP IP HTTP FTP PPP

router workstationserver

mobilee.g., TCP, IP, HTTP, FTP, PPPInternet: “network of networks”

loosely hierarchicalpublic Internet versus private intranet

Internet standards

local ISP

regional ISP

mobile

Introduction 1-6

Internet standardsRFC: Request for commentsIETF: Internet Engineering Task Force company

network

4

What’s the Internet: a service viewcommunication infrastructure enables distributed applications:distributed applications

Web, email, games, e-commerce, file sharing

communication services provided to apps:

Connectionless unreliableconnection-oriented

Introduction 1-7

reliable

What’s a protocol?human protocols:

“what’s the time?”network protocols:

machines rather than h“I have a question”

introductions

… specific msgs sent… specific actions taken

when msgs received

humansall communication activity in Internet governed by protocols

protocols define format, order of msgs sent and

Introduction 1-8

when msgs received, or other events

order of msgs sent and received among network

entities, and actions taken on msg

transmission, receipt

5

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

Hi

HiGot thetime?2:00

TCP connectionrequest

TCP connectionresponseGet http://www.awl.com/kurose-ross

Introduction 1-9

Q: Other human protocols?

2:00<file>

time

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-10

y p1.7 Protocol layers, service models

6

A closer look at network structure:

network edge:applications and applications and hostsnetwork core:

routersnetwork of networks

Introduction 1-11

networksaccess networks, physical media:communication links

The network edge:end systems (hosts):

run application programse.g. Web, emailat “edge of network”

client/server modelclient host requests, receives service from always-on servere.g. Web browser/server;

il li t/

Introduction 1-12

email client/server

peer-peer model:minimal (or no) use of dedicated serverse.g. Gnutella, KaZaA, Skype

7

Network edge: connection-oriented service

Goal: data transfer between end systems

TCP service [RFC 793]reliable in order bytebetween end systems

handshaking: setup (prepare for) data transfer ahead of time

Hello, hello back human protocolset up “state” in two

reliable, in-order byte-stream data transfer

loss: acknowledgements and retransmissions

flow control:sender won’t overwhelm receiver

Introduction 1-13

set up state in two communicating hosts

TCP - Transmission Control Protocol

Internet’s connection-oriented service

receivercongestion control:

senders “slow down sending rate” when network congested

Network edge: connectionless service

Goal: data transfer between end systems

App’s using TCP:HTTP (Web) FTP (file between end systems

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

connectionless unreliable data t f

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

App’s using UDP:

Introduction 1-14

transferno flow controlno congestion control

streaming media, teleconferencing, DNS, Internet telephony

8

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-15

y p1.7 Protocol layers, service models

The Network Core

mesh of interconnected routersroutersthe fundamental question: how is data transferred through net?

circuit switching:dedicated circuit per call: telephone net

Introduction 1-16

call: telephone netpacket-switching: data sent thru net in discrete “chunks”

9

Network Core: Circuit Switching

End-end resources reserved for “call”reserved for calllink bandwidth, switch capacitydedicated resources: no sharingcircuit-like

Introduction 1-17

(guaranteed) performancecall setup required

Network Core: Packet Switchingeach end-end data stream

divided into packets B k h

resource contention:aggregate resource d d d user A, B packets share

network resourceseach packet uses full link bandwidth resources used as needed

demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop

Introduction 1-18

p pat a time

Node receives complete packet before forwarding

Bandwidth division into “pieces”Dedicated allocationResource reservation

10

Packet-switching: store-and-forward

R R RL

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

Example:L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 sec

Introduction 1-19

it can be transmitted on next link: store and forwarddelay = 3L/R (assuming zero propagation delay)

more on delay shortly …

Packet-switched networks: forwarding

Goal: move packets through routers from source to destination

’ll t d l th l ti (i ti ) l ith we’ll study several path selection (i.e. routing) algorithms (chapter 4)

datagram network:destination address in packet determines next hoproutes may change during sessionanalogy: driving, asking directions

virtual circuit network:

Introduction 1-20

virtual circuit network:each packet carries tag (virtual circuit ID), tag determines next hopfixed path determined at call setup time, remains fixed thru callrouters maintain per-call state

11

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-21

y p1.7 Protocol layers, service models

Access networks and physical media

Q: How to connect end systems to edge router?

d l residential access netsinstitutional access networks (school, company)mobile access networks

Keep in mind:

Introduction 1-22

Keep in mind: bandwidth (bits per second) of access network?shared or dedicated?

12

Residential access: point to point access

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

ADSL: asymmetric digital subscriber lineup to 1 Mbps upstream (today typically < 256 kbps)

Introduction 1-23

up to 1 Mbps upstream (today typically 256 kbps)up to 8 Mbps downstream (today typically < 1 Mbps)Internet + phone (different frequencies)

0 kHz - 4 kHz for ordinary telephone4 kHz - 50 kHz for upstream50 kHz - 1 MHz for downstream

Residential access: cable modems

HFC: hybrid fiber coaxt i t 30Mb d t 2 asymmetric: up to 30Mbps downstream, 2

Mbps upstreamnetwork of cable and fiber attaches homes to ISP router

homes share access to router deployment: available via cable TV companies

Introduction 1-24

p y p

13

Cable Network Architecture: Overview

Typically 500 to 5,000 homes

Introduction 1-25

home

cable headend

cable distributionnetwork (simplified)

Cable Network Architecture: Overview

Introduction 1-26

home

cable headend

cable distributionnetwork (simplified)

14

Cable Network Architecture: Overview

server(s)

Introduction 1-27

home

cable headend

cable distributionnetwork

Company access: local area networks

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

shared or dedicated link connects end system and router10 Mbs, 100Mbps,

Introduction 1-28

pGigabit Ethernet

LANs: chapter 5

15

Wireless access networksshared wireless access network connects end system to routerto router

via base station aka “access point”

wireless LANs:802.11b (WiFi): 11 Mbps

wider-area wireless accessprovided by telco operator

basestation

router

Introduction 1-29

provided by telco operator3G AP/GPRS in Europe

mobilehosts

Home networksTypical home network components:

ADSL or cable modemt /fi ll/NATrouter/firewall/NAT

Ethernetwireless accesspoint

wirelesslaptops

lto/from

Introduction 1-30

wirelessaccess point

laptopsrouter/firewall

cablemodem

to/fromcable

headend

Ethernet

16

Physical Media

Bit: propagates betweentransmitter/rcvr pairs

Twisted Pair (TP)two insulated copper wirestransmitter/rcvr pairs

physical link: what lies between transmitter & receiverguided media:

signals propagate in solid media: copper fiber coax

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

Introduction 1-31

media: copper, fiber, coaxunguided media:

signals propagate freely, e.g., radio

Physical Media: coax, fiber

Coaxial cable:two concentric copper

d t

Fiber optic cable:glass fiber carrying light pulses each pulse a bitconductors

bidirectionalbaseband:

single channel on cablelegacy Ethernet

broadband:multiple channels on

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

Introduction 1-32

multiple channels on cableHFC

spaced far apart ; immune to electromagnetic noise

17

Physical media: radio

signal carried in electromagnetic spectrum

Radio link types:terrestrial microwave

t 45 Mb h lspectrumno physical “wire”bidirectionalpropagation environment effects:

reflection

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

2Mbps, 11Mbps, 54 Mbpswide-area (e.g., cellular)

e.g. 3G: hundreds of kbpssatellite

Introduction 1-33

obstruction by objectsinterference

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

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-34

y p1.7 Protocol layers, service models

18

Internet structure: network of networks

a packet passes through many networks!

Tier 1 ISPNAP

Tier-2 ISPTier-2 ISP

localISPlocal

ISPlocalISP

localISP Tier 3

ISP

Introduction 1-35

Tier 1 ISP Tier 1 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISP

localISP

localISP

localISP

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-36

y p1.7 Protocol layers, service models

19

How do loss and delay occur?packets queue in router buffers

packet arrival rate to link exceeds output link capacityp p p ypackets queue, wait for turn

A

packet being transmitted (delay)

Introduction 1-37

Bpackets queueing (delay)

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

Four sources of packet delay

1. nodal processing:check bit errors

2. queueingtime waiting at output check bit errors

determine output link

A propagation

transmission

time waiting at output link for transmission depends on congestion level of router

Introduction 1-38

B

propagation

nodalprocessing queueing

20

Delay in packet-switched networks3. Transmission delay:

R=link bandwidth (bps)4. Propagation delay:

d = length of physical linkL=packet length (bits)time to send bits into link = L/R

s = propagation speed in medium (~2x108 m/sec)propagation delay = d/s

t smissi

Note: s and R are very different quantities!

Introduction 1-39

A

B

propagation

transmission

nodalprocessing queueing

Nodal delayproptransqueueprocnodal ddddd +++=

dproc = processing delaytypically a few microsecs or less

dqueue = queuing delaydepends on congestion

dtrans = transmission delay

Introduction 1-40

trans y= L/R, significant for low-speed links

dprop = propagation delaya few microsecs to hundreds of msecs

21

“Real” Internet delays and routes

What do “real” Internet delay & loss look like? Traceroute program: provides delay Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination.

Introduction 1-41

3 probes

3 probes

3 probes

Packet loss

queue (aka buffer) preceding link in buffer has finite capacityhas finite capacitywhen packet arrives to full queue, packet is dropped (aka lost)lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all

Introduction 1-42

not retransmitted at all

22

Chapter 1: roadmap

1.1 What is the Internet?1 2 N t k d1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks

Introduction 1-43

y p1.7 Protocol layers, service models

Protocol “Layers”Networks are complex!

many “pieces”:hostsrouterslinks of various mediaapplicationsprotocols

Question:Is there any hope of organizing structure of

network?

Or at least our discussion

Introduction 1-44

protocolshardware, software

Or at least our discussion of networks?

23

Organization of air travel

ticket (purchase) ticket (complain)

lbaggage (check)

gates (load)

runway takeoff

airplane routing

baggage (claim)

gates (unload)

runway landing

airplane routing

l

Introduction 1-45

a series of steps

airplane routing

ticket (purchase) ticket (complain) ticket

Layering of airline functionality

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

baggage (claim

gates (unload)

runway (land)

airplane routing

baggage

gate

takeoff/landing

airplane routing

L h l i l t i

Introduction 1-46

Layers: each layer implements a servicevia its own internal-layer actionsrelying on services provided by layer below

24

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 system

Introduction 1-47

transparent to rest of systeme.g., change in gate procedure doesn’t affect rest of system

Internet protocol stackapplication: supporting network applications application

FTP, SMTP, HTTPtransport: host-host data transfer

TCP, UDPnetwork: routing of datagrams from source to destination

IP, routing protocols

pp

transport

network

link

Introduction 1-48

link: data transfer between neighboring network elements

PPP, Ethernetphysical: bits “on the wire”

physical

25

messagesegment

datagramframe

sourceapplicationtransportnetwork

linkphysical

HtHnHl MHtHn MHt M

MEncapsulation

destination network

linkphysical

HtHn M HtHn M

HtHnHl M HtHnHl M

switch

Introduction 1-49

applicationtransportnetwork

linkphysical

HtHnHl MHtHn MHt M

M linkphysical

HtHnHl M HtHnHl M

router

Introduction: SummaryCovered a “ton” of material!

Internet overviewYou now have:

context, overview, what’s a protocol?network edge, core, access network

packet-switching versus circuit-switching

Internet/ISP structure

context, o er ew, “feel” of networkingmore depth, detail to follow!

Introduction 1-50

performance: loss, delaylayering and service models