《计算机网络自顶向下方法与特色》幻灯片

Computer

Networks

andCommunication计算机网络与通信（双语）Professor

Chunsheng

Li主讲：李春生教授Department

of

Computer

Engineering12Course

OverviewWhat

is

Computer

networks?（什么是计算机网络）

What

is

this

course

about?(本课程是干什么的）A

top-down

approach（一种从上到下的方法）Contents

of

this

course（本课程的内容）30.1

What

is

Computer

Networks?Computer

network

is

a

communication

system

toconnect

at

least

two

computers

together

for

sharinghardware,

software,

and

information

resources.(计算机网络：用于共享硬件资源、软件资源和信息资源的连接两台或更多台计算机进行通信的系统。)Categories

of

Computer

Networksr

According

to

the

communicating

distance：WAN(Wide

Area

Networks广域网),MAN(Municipal

AreaNetworks城域网),LAN(Local

Area

Networks局域网)r

According

to

the

topology：Star(星型),Tree(树型),Ring(环型),Bus(总线型结构)。40.2

What

is

this

course

about?Introductory

(first)

course

in

computer

networking❏

learn

principles

of

computer

networking❏

learn

practice

of

computer

networking❏

Internet

architecture/protocols

as

case

studyGoals:❏

learn

a

lot

(not

just

factoids,

but

principles

andpractice)❏

have

fun

(well,

it

should

be

interesting,

at

least)prerequisites:·

algorithms,

Operating

Systems,

programming

skills0.3

A

top-down

approach:This

course

will

covernetworking

top-down❏

end-system

applications,end-end

transport❏

network

core:

routing,hooking

nets

together❏

link-level

protocols,

e.g.,Ethernet❏

other

stuff:

security,mobility,

management560.4

Contents

of

this

Course:Part

1:

Introduction

(3

classes,

text:

Chapter

1)❏

what

is

the

Internet,

What

is

a

protocol?❏

network

edge,

network

core❏

network

access,

physical

media❏

Internet

backbones,

NAPs

and

ISPs❏

delay,

loss

in

packet-switched

networks❏

protocol

layers,

service

models

(Application,Transport,

Network,

Link,

Physical

Layers)❏

brief

history

of

networking,

Internet7Contents

of

this

Course

(Cont.):Part

2:

Application

Layer

(3

classes,

text:

Ch.

2)❏

principles

of

application-layer

protocols❏

World

Wide

Web:

HTTP❏

file

transfer:

FTP❏

electronic

mail

in

the

Internet❏

the

Internet"s

directory

service:

DNS❏

socket

programming

with

TCP8Contents

of

this

Course

(Cont.):Part

3:

Transport

Layer

(6

classes,

text

Ch.

3)❏

Transport-layer

services

and

principles❏

Multiplexing

and

demultiplexing

applications❏

Connectionless

transport:

UDP❏

Principles

of

reliable

of

data

transfer❏

Connection-Oriented

Transport:

TCP❏

Principles

of

congestion

control❏

TCP

congestion

control9Contents

of

this

Course

(Cont.):Part

4:

Network

Layer

(6

classes,

text:

Ch.

4)❏

introduction

and

network

service

model❏

what’s

inside

a

router?❏

routing

principles

(algorithms)❏

hierarchical

routing❏

IP:

the

Internet

Protocol❏

Internet

routing:

RIP,

OSPF,

BGP10Contents

of

this

Course

(Cont.):Part

5:

Link

Layer,

LANs

(3

classes,

text:

Ch.

5)❏

introduction,

services❏

error

detection,

correction❏

multiple

access

protocols,

LANs❏

LAN

addresses,

ARP❏

Ethernet❏

Interconnection:

Hubs

and

Switches11Chapter

1:

IntroductionOur

goal:r

get

“feel”

andterminologyr

more

depth,

detaillater

in

courser

approach:m

use

Internet

asexampleOverview:r

what’s

the

Internetr

what’s

a

protocol?r

network

edger

network

corer

access

net,

physical

mediar

Internet/ISP

structurer

performance:

loss,

delayr

protocol

layers,

service

mode12Chapter

1:

roadmapWhat

is

the

Internet?Network

edgeNetwork

coreNetwork

access

and

physical

mediaInternet

structure

and

ISPsDelay

&

loss

in

packet-switched

networksProtocol

layers,

service

modelsHistory1.1

What’s

the

Internetr

Two

ways

to

describe

the

Internet:13r

Describe

the

Nuts-and-Bolts

of

theInternet,

that

is,

the

basic

hardware

andsoftware

components

that

make

up

theInternet.r

Describe

the

Internet

in

terms

of

anetworking

infrastructure

that

provideservices

to

distributed

applications.1.1

What’s

the

Internet:

1.

“nuts

and

bolts”viewr

millions

of

connected

computingdevices:

hosts

=

end

systems(definition

p2)r

running

network

appsr

communication

linksm

fiber,

copper,

radio,

satellitem

transmission

rate

=

bandwidthr

Packet

Switches:

Routers

andLink-layer

Switches----

forwardpackets

(chunks

of

data)

towards

their

ultimatedestinationsr

Route

or

pathr

Internet

Service

Providers(ISPs)----lower-tier

ISP

&Uper-tier

ISPlocal

ISPcompanynetworkregional

ISProuter14workstationservermobile“Cool”

internet

appliancesIP

picture

frame/Web-enabled

toaster

+weather

forecasterInternet

phonesZebraNet

(Princeton)Where

is

your

zebra?15What’s

the

Internet:

“nuts

and

bolts”viewr

protocols

control

sending,receiving

of

msgsm

e.g.,

TCP,

IP,

HTTP,

FTP,PPPr

Internet:

“network

ofnetworks”m

loosely

hierarchicalm

public

Internet

versusprivate

intranetr

Internet

standardsm

IETF:

Internet

EngineeringTask

Forcem

RFC:

Request

for

commentslocal

ISPcompanynetworkregional

ISProuter16workstationservermobile1.1

What’s

the

Internet:

2.

a

service

viewr

communication

infrastructureenables

distributedapplications:m

Web,

email,

games,

e-commerce,file

sharingr

communication

servicesprovided

to

applications:m

Connectionless

unreliablem

connection-oriented

reliabler

The

internet

does

notprovide

a

service

thatpromises

about

how

long

itwill

take

to

deliver

thedata

from

sender

toreceiver173.

What’s

a

protocol?human

protocols:r

“what’s

thetime?”r

“I

have

aquestion”r

introductionsThere

are

specificmsgs

we

send,

andspecific

actions

wetake

in

response

tothe

received

replymsgs

or

othereventsnetwork

protocols:r

machines

rather

thanhumansr

all

communicationactivity

in

Internetgoverned

by

protocolsprotocols

define

format,

order

ofmsgs

sent

and

received

amongnetwork

entities,

and

actions

takon

msg

transmission,

receipt18What’s

a

protocol?a

human

protocol

and

a

computer

network

protocol:HiHiGot

thetime?2:00TCP

connectionrequestTCP

connectionresponseGet

/index.htm<file>time1920Chapter

1:

roadmapWhat

is

the

Internet?Network

edgeNetwork

coreNetwork

access

and

physical

mediaInternet

structure

and

ISPsDelay

&

loss

in

packet-switched

networksProtocol

layers,

service

modelsHistoryA

closer

look

at

network

structure:r

network

edge:applications

andhostsr

network

core:m

routersm

network

ofnetworksr

access

networks,physical

media:communicationlinks211.2

The

network

edge:

1.

end

systems,

Clients,

serversr

end

systems

(hosts):m

run

application

programsm

e.g.

Web,

emailm

at

“edge

of

network”(Definitions

in

P9

)r

client/server

modelm

client

host

requests,

receivesservice

from

always-on

serverm

e.g.

Web

browser/server;

emailclient/server(Two

kinds

of

definitions

in

P9

)r

peer-peer

model:m

minimal

(or

no)

use

of

dedicatedserversm

e.g.

Skype,

BitTorrent,

KaZaA221.2

Network

edge:

2.

connection-orientedservice

(two

types

of

services

of

Internet)Goal:

data

transferbetween

end

systemsr

handshaking:

setup(prepare

for)

datatransfer

ahead

oftimem

Hello,

hello

back

humanprotocolm

set

up

“state”

in

twocommunicating

hostsr

TCP

-

TransmissionControl

Protocolm

Internet’sconnection-orientedserviceTCP

services

[RFC

793]r

reliable,

in-order

byte-stream

data

transferm

loss:

acknowledgementsand

retransmissionsr

flow

control:m

sender

won’t

overwhelmreceiverr

congestion

control:m

senders

“slow

downsending

rate”

whennetwork

congested23241.2

Network

edge:

3.

connectionless

serviceGoal:

data

transferbetween

end

systemsm

same

as

before!r

UDP

-

User

Datagram

Protocol

[RFC

768]:

m

connectionlessm

unreliable

datatransferm

no

flow

controlm

no

congestion

controlApp’s

using

TCP:r

HTTP

(Web),

FTP

(filetransfer),Telnet(remote

login),

SMTP(email)App’s

using

UDP:r

streaming

media,teleconferencing,DNS,

Internettelephony25Chapter

1:

roadmapWhat

is

the

Internet?Network

edgeNetwork

coreNetwork

access

and

physical

mediaInternet

structure

and

ISPsDelay

&

loss

in

packet-switched

networksProtocol

layers,

service

modelsHistory1.3

The

Network

Corer

Meaning

of

the

networkcore:

mesh

of

routersthat

interconnect

theInternet’s

endsystems.r

There

are

two

ways

toconstruct

the

networkcore:m

circuit

switching:dedicated

circuit

percall:

telephone

netm

packet-switching:data

sent

thru

net

indiscrete

“chunks”261.3

Network

Core:

1.

Circuit

SwitchingThe

resources

needed

for

a

path

to

provide

forcommunication

betweenthe

end

systems

arereserved

for

theduration

of

thecommunication

session.r

link

bandwidth,

switchcapacityr

dedicated

resources:

nosharingr

circuit-like

(guaranteed)performancer

call

setup

required27281.3

Network

Core:

1.

Circuit

Switchingnetwork

resources(e.g.,

bandwidth)divided

into“pieces”r

pieces

allocated

to

callsr

resource

piece

idle

ifnot

used

by

owning

call(no

sharing)

dividing

link

bandwidthinto

“pieces”–

frequency

division:

FDM----

Frequency-DivisionMultiplexing–

time

division:

TDM

----Time-DivisionMultiplexingTwo

simple

multiple

access

control

techniques.Each

mobile’s

share

of

the

bandwidth

is

divided

into

portions

for

the

uplink

and

the

downlink.

Also,

possibly,

out

of

band

signaling.As

we

will

see,

used

in

AMPS,

GSM,

IS-54/1361.

Circuit

Switching:

1)

FDM

and

TDMFDMfrequencytimeTDMfrequencytime4

usersExample:FrameSlot29302)

Numerical

exampler

How

long

does

it

take

to

send

a

file

of640,000

bits

from

host

A

to

host

B

over

acircuit-switched

network?m

All

links

are

1.536

Mbpsm

Each

link

uses

TDM

with

24

slotsm

500

msec

to

establish

end-to-end

circuitLet’s

work

it

out!Each

circuit

has

transmission

rate

of

1.536mbps/24=64kbpsIt

takes

640000bits/64kbps=10seconds

to

transmit

the

fileTotal

time

=

10+0.5

=

10.5

seconds1.3

Network

Core:

2.

Packet

Switchingeach

end-end

data

stream(message

)

divided

intopacketsr

user

A,

B

packets

sharenetwork

resourcesr

each

packet

uses

full

linkbandwidthr

resources

used

as

neededresource

contention:

aggregate

resourcedemand

can

exceedamount

available(queue

grows)congestion:

packetsqueue,

wait

for

link

us

store

and

forward:packets

move

one

hopat

a

time–

Node

receives

complete

packebefore

forwardingBandwidth

division

into

“pieces”Dedicated

allocation

Resource

reservation312.

Packet

Switching:

1)

Statistical

MultiplexingSequence

of

A

&

B

packets

does

not

have

fixed

pattern,shared

on

demand

On-demand

sharing

of

resourcesis

called

statistical

multiplexing.TDM:

each

host

gets

same

slot

in

revolving

TDM

frame.AB10

Mb/sEthernet1.5

Mb/sDEstatistical

multiplexing

Cqueue

of

packetswaiting

for

outputlink322.

Packet-switching:

2)

store-and-forwardr

Takes

L/R

seconds

totransmit

(push

out)

packetof

L

bits

on

to

link

or

R

bpsr

Entire

packet

must

arriveat

router

before

it

can

betransmitted

on

next

link:store

and

forwardr

delay

=

3L/R

(assumingzero

propagation

delay)Example:r

L

=

7.5

Mbitsr

R

=

1.5

Mbpsr

delay

=

15

secRRRL333.

Packet

switching

versus

circuit

switchingr

circuit-switching:m

10

usersr

packet

switching:m

with

35

users,probability

>

10

activeless

than

.0004<=10

activemore

than

0.9996Packet

switching

allows

more

users

to

use

network!r

1

Mb/s

linkr

each

user:m

100

kb/s

when“active”m

active

10%

of

timeN

users1

Mbps

linkQ