重大本科四门专业课计算机网络

Computer

NetworksCollege

of

Computer

ScienceChongqing

UniversityUnderstand

how

computer

networks

worksGet

familiar

with

current

computer

networks,particularly

the

Internet

research

effortsTo

grasp

the

current

direction

of

computernetworks

research.Appreciate

what

is

good

research√Problem

selection√Solution

&

research

methodology√PresentationTo

prepare

students

to

conduct

research.Course

Objectives□Textbook√

Computer

Networks,

5/eby

A.

Tanenbaum

and

D.Wetherall√amazon□Reference

books√

Computer

Networks

and

Internets,

5/e

by

D.

Comer√

Computer

Networking:

A

Top-

Down

Approach,

5/e

by

Kurose

and

Ross√

Computer

Networks,

A

Systems

Approach

by

L.

Peterson

and

B.

Davie√计算机网络（第五版）,谢希仁,电子工业出版社,2008√

TCP/IP

Illustrated,

Volume

1:The

Protocols

by

W.

RichardTextbookGrading

and

Schedule4Homework310%Project210%Experiment420%Midsemester110%Exams150%ContentsIntroduction

61The

Data

Link

Layer8The

MAC

Sub-layer

85

The

Network

Layer

122

The

Physical

Layer

6The

Transport

Layer

66The

Application

Layer6Network

SecurityGray

units

can

be

optionally

omitted

without

causing

later

gapsChapter

1:

Introduction□History

of

Computer

Networks□Uses

of

Computer

Networks□Network

Hardware□NetworkSoftware□Reference

Models□Example

Networks□NetworkStandardization□Metric

UnitsComputer

networks

are

collections

of

autonomouscomputers,

e.g.,

the

Internet

The

difference

between

Computer

Network

andDistributed

System√Distributed

System:

A

collection

of

autonomous(independent)

computers

is

transparency

to

its

users.

Itis

a

software

system

built

on

top

of

a

network.√Thus,

the

key

distinction

between

a

network

and

adistributed

system

is

the

software

(especially

theoperating

system:

NOS

and

Distributed

OS),

rather

thanthe

hardware.History

of

Computer

NetworksNetworking

History

TimelineIn

the

1960sJ.C.R

Licklider:

Internet

pioneer

with

an

early

vision

of

a

world-wcomputer

networkLeonard

Kleinrock:

established

a

mathematical

theory(queuingtheory)

of

packet

networksPaul

Baran:

invented

packet

switching

techniquesLawrence

Roberts

&

Robert

Taylor

:

created

the

ARPANETIn

the

1970sVinton

Cerf

and

Bob

Kahn:

co-invented

the

TransmissionControl

Protocol

(TCP)

and

the

Internet

Protocol

(IP),

whare

recognized

as

‘the

fathers

of

the

Internet’.Ray

Tomlinson:

programed

the

first

e-mail

andintroduced

the

notation

"user@host"Robert

M.

Metcalfe:

invented

Ethernet,

alongwith

David

BoggsIn

the

1980s

and1990sTim

Berners

Lee:

invented

the

World

Wide

Web,regarded

as

‘the

father

of

Web’Marc

Andreessen:

invented

Mosaic

and

later

NetscapeGray

units

can

be

optionally

omitted

without

causing

later

gapsChapter

1:

Introduction□History

of

Computer

Networks□Uses

of

Computer

Networks□Network

Hardware□NetworkSoftware□Reference

Models□Example

Networks□NetworkStandardization□Metric

UnitsContrast

computer

networks

withdistributed

systems,

in

which

a

model

on

top

of

the

network

is

used

to

present

the

independent

computers

tousers

as

a

single

coherent

system,

e.g.,

the

Web.Uses

of

Computer

Networks□Business

Applications□Home

Applications□Mobile

Users□Social

IssuesResource

sharing

was

initially

about

physical

resources,

such

as

printers,

but

is

now

often

about

access

to

information,

such

as

afile

server.The

Web

is

an

example

of

client-server

computing.Business

Applications□Companies

use

networks

and

computers

forresource

sharing

with

the

client-server

model:□Other

popular

uses

are

communication,

e.g.,email,

VoIP,

and

e-commercerequestresponseP2P

contrasts

with

client-server.

Why

is

it

under

home

applications?

Because

unlike

cloud

there

is

no

need

to

haveabusiness

run

dedicatedinfrastructure

for

the

app

to

work.Home

Applications□Access

to

remote

information□Person-to-person

communication□Electronic

commerce□Interactive

entertainment□Ubiquitous

computing普适计算Inpeer-to-peer

system

there

are

no

fixed

clients

and

servers.Some

forms

of

e-commerceO2O

Online

To

Offline航空，保险Laptop

sales

outpaced

desktop

sales

in

2010,

and

there

are

many

more

mobile

phones

(but

not

smart

phones)

than

personal

computers.Mobile

Users□Tablets,

laptops,

and

smart

phones

are

populardevices;

WiFi

hotspots

and

3G

cellular

providewireless

connectivity.□Mobile

users

communicate,

e.g.,

voice

and

texts,consume

content,

e.g.,

video

and

Web,

and

usesensors,

e.g.,

GPS.□Wireless

and

mobile

are

related

but

different:In

the

US,

DMCA

(Digital

Millennium

Copyright

Act)

takedowns

are

automated

notices

sent

by

content

owners

to

parties

theybelieve

areinappropriately

putting

copyrighted

content

online.

They

instruct

the

party

to

take

down

the

content

or

face

legal

measures.Social

Issues□Network

neutrality

–

no

network

restrictions□Content

ownership,

e.g.,

DMCA

takedowns□Anonymity匿名and

censorship审查□Privacy,e.g.,Web

tracking跟踪and

profiling□Theft,e.g.,botnets

and

phishing网络仿冒不造谣不传谣不信谣斯诺登Gray

units

can

be

optionally

omitted

without

causing

later

gapsChapter

1:

Introduction□History

of

Computer

Networks□Uses

of

Computer

Networks□Network

Hardware□NetworkSoftware□Reference

Models□Example

Networks□NetworkStandardization□Metric

UnitsAn

“internetwork”

is

any

larger

network

made

up

of

smaller

component

networks.

The

“Internet”

(with

acapital

I)is

the

set

of

all

connectednetworks.Network

Hardware□

Transmission

technology√

Broadcast

links√

Point-to-point

linksScaleClassification

of

interconnected

processors

byscale.Personal

Area

NetworkConnect

devices

over

the

range

of

a

personExample

of

a

Bluetooth

(wireless)

PAN:Local

Area

Networks□Connect

devices

in

a

home

or

office

building□Called

enterprise

network

in

a

companyWireless

LANwith

802.11Wired

LAN

withswitched

EthernetThis

is

acommon

way

in

which

home

subscribers

obtain

access

to

the

Internet

in

the

US.MetropolitanArea

NetworksConnect

devices

over

a

metropolitan

areaExample

MAN

based

on

cable

TV:The

company

probably

leases

the

transmissionlines

(since

most

companies

do

not

have

their

own

lines).Wide

Area

Networks

(1)□Connect

devices

over

a

country布里斯班墨尔本WAN

that

connects

three

branch

offices

in

Australia珀斯Now

the

company/customer

uses

the

Internet

(might

be

multiple

ISPs)

for

connectivity.

The

links

are

virtual

in

the

sense

that

they

refer

to

somepath

via

the

Internet

rather

than

a

particular

transmission

line.Wide

Area

Networks

(2)□A

VPN

(Virtual

Private

Network)

is

a

WAN

builtfrom

virtual

links

that

run

on

top

of

the

InternetWAN

using

a

virtual

private

networkNow

the

company/customer

buys

service

froman

ISP

who

uses

its

own

lines

to

deliver

packets.Wide

Area

Networks

(3)□An

ISP

(Internet

Service

Provider)

network

isalso

a

WAN.WAN

using

an

ISP

networkGray

units

can

be

optionally

omitted

without

causing

later

gapsChapter

1:

Introduction□History

of

Computer

Networks□Uses

of

Computer

Networks□Network

Hardware□NetworkSoftware□Reference

Models□Example

Networks□NetworkStandardization□Metric

UnitsNetwork

Software□Protocol

hierarchies□Design

issues

for

the

layers□Connection-oriented

vs.

connectionlessservice□Service

primitives□Relationshipofservicesto

protocolsProtocol

hierarchies

(1)No

layering:

each

new

application

has

to

be

re-implemented

for

every

network

technology!TelnetFTPNFSPacketradioCoaxialcableFiberopticApplicationTransmissionMediaHTTPTo

reduce

their

design

complexity,

most

networks

areorganized

as

a

stack

of

layers

or

levels.Solution:

introduce

an

intermediate

layer

that

providesunique

abstraction

for

various

network

technologiesTelnetFTPNFSPacketradioCoaxialcableFiberopticApplicationTransmissionMediaHTTPIntermediatelayer□Advantages

of

Protocol

Hierarchies√

Independence

of

layers:

These

layers

are

relative

independence.√

If

any

layer

is

changed,

other

layers

are

not

affected,

so

far

as

notchanging

interface.√

Each

layer

can

be

realized

in

right

technology.√

It

is

easy

to

realize

and

maintain.√

Standardization.Disadvantages

of

Protocol

Hierarchies:√

Some

functions

may

be

realized

repeatedly.√

The

number

of

layer

is

not

certain.Protocol

hierarchies

(2)Protocol

layering

is

the

main

structuring

method

used

to

divide

up

networkfunctionality.√

Entity:

An

active

elements

embodying

a

set

of

capabilities

defined

for

alayer√

Peer

entities:

Entities

within

the

same

layer

on

different

machines√

Protocol:

an

agreement

between

the

communicating

parties

on

howcommunication

is

to

proceed

(a

set

of

rules

and

formats

that

govern

thecommunication

between

two

peers)SyntaxSemanticssynchronization

of

communication√

service:

A

capability

of

one

layer

and

the

layers

beneath

it

(a

set

ofprimitives

provided

by

one

layer

to

layer

above)√

Interface:

defining

which

primitive

operations

and

services

the

lower

layemakes

available

to

the

upper

oneProtocolService

PrimitivesService

PrimitivesService

UserServiceService

providerLayer

nLayer

n

+

1Service

UserSAPSAPInterfaceInterfaceSAP--Service

Access

Point√

Each

protocol

instancetalks

virtually

to

its

p

√

Each

layer

communicateonly

by

using

the

onebeloweerse√

Lower

layer

services

araccessed

by

an

interface√

At

bottom,

messages

arecarried

by

the

mediumVirtual

CommunicationsActual

Communications□Example:

the

philosopher-translator-secretaryarchitecture□Each

protocol

at

different

layers

serves

a

differenpurposeProtocol

hierarchies

(3)Virtual

&

Actual

CommunicationsImportant

to

understand

difference

between:

virtualand

actual

communications,

protocols

and

interfaces.Peer

process

idea

is

key

to

network

design.√

Peer

processes

‘think’

of

communications

as

being‘horizontal’

using

protocol.

Peers

exchangeProtocol

Data

Units

(PDUs)√

Actual

communications

is

via

interfaces

(and

thephysical

communications

medium).

layers

exchangeService

Data

Units

(SDUs)Basic

concepts:√

(N)-

service

access

point(SAP):

the

logical

interface

between

the

(N)-entities

an(N

+

1)-entities.√

(N)-

Protocol

Control

Information(PCI):

information

exchanged

between

two

(N)

-entities,

using

an

(N

-

1)

connection,

to

coordinate

their

joint

operation.√

(N)-

User

Data:

the

data

transferred

between

two

(N)

-entities

on

behalf

of

the

(N

+entities

for

whom

the

(N)

-entities

are

providing

services.√

(N)-

Protocol

Data

Unit(PDU):

a

unit

of

data

which

contains

(N)-

Protocol

ControlInformation

and

possibly

(N)

-User

Data.√

(N)-Interface

Control

Information

(ICI):

information

exchanged

between

an

(N

+

1)entity

and

an

(N)-entity

to

coordinate

their

joint

operation.√

(N)-Interface

Data:

information

transferred

from

an

(N

+

1)-entity

to

an

(N)-entit

transmission

to

a

correspondent

(N

+

1)-entity

over

an

(N)-connection,

or

converse

information

transferred

from

an

(N)-entity

to

an

(N

+

1)-entity

which

has

been

rece

over

an

(N)-connection

from

a

correspondent

(N+

1)-entity.√

(N)-Interface

Data

Unit

(IDU):

the

unit

of

information

transferred

across

the

servaccess

point

between

an

(N

+

1)-entity

and

an

(N)-entity

in

a

single

interaction.√(N)-Service

Data

Unit(SDU):

the

amount

of

(N)-interface

data

whose

identity

ispreserved

from

one

end

of

an

(N)

connection

to

the

other.Interrelationship

between

data

unitsControlDataCombined(N)-(N)Peer

Entities(N)-ProtocolControlInformation(PCI)(N)-User

Data(N)-ProtocolData

Unit(PDU)(N+1)-(N)Adjacent

Layers(N)-InterfaceControl

Information(ICI)(N)-InterfaceData(N)-InterfaceData

Unit

(IDU)Logical

relationship

between

data

units

in

adjacent

la(N+1)ICI

(N+1)PDU(N+1)IDUICI(N)SDU(N)PCI(N)PCI

(N+1)PDU(N)PDU(N)ICI(N)ICI

(N)PDU

(N)IDU(N+1)-layer(N)-layerSAPSAP:

Service

Access

PointPCI:

Protocol

Control

InformationPDU:

Protocol

Data

UnitICI:

Interface

Control

InformatioIDU:

Interface

Data

UnitSDU:

Service

Data

Unit压箱底钱包信件包裹Protocol

hierarchies

(4)

Each

lower

layer

adds

its

own

header

(with

controlinformation)

to

the

message

to

transmit

and

removes

it

onreceiveLayers

may

also

split

and

join

messages,

etc.The

point

is

that

there

are

some

issues

that

are

not

wholly

the

responsibility

of

any

one

layer,

and

they

crop

up

again

and

again

in

the

text.

Forexample,

reliability

is

often

considered

a

keyfunction

of

the

transport

layer

(i.e.,

making

transport

reliable)

yet

reliability

mechanisms

also

appearin

other

layers

(error

codes

in

the

link

layer,

routing

around

failures

in

the

network

layer,

and

replication

at

the

application

layer).Design

Issues

for

the

LayersEach

layer

solves

a

particular

problem

but

mustinclude

mechanisms

to

address

a

set

ofrecurring

design

issuesIssueExample

mechanisms

at

different

layersReliability

despitefailuresCodes

for

error

detection/correction

(§3.2,

3.Routing

around

failures

(§5.2)Network

growthand

evolutionAddressing

(§5.6)

and

naming

(§7.1)Protocol

layering

(§1.3)Allocation

of

resourceslike

bandwidthMultiple

access

(§4.2)Congestion

control

(§5.3,

6.3)Security

againstvarious

threatsConfidentiality

of

messages

(§8.2,

8.6)Authentication

of

communicating

parties

(§8.7)TCP

provides

a

reliable

bytestream

service

at

the

Transport

layer,

IP

provides

unreliable

datagramservice

at

the

Network

layer.More

examples:

RTP

(used

to

carry

VoIP

data)

provides

unreliable

connection

service;

802.11

(WiFi)

provides

acknowledged

datagramservice;

Ethernet

provides

unreliable

datagramservice.Connection-Oriented

vs.ConnectionlessService

provided

by

a

layer

may

be

kinds

of

either:√Connection-oriented,

must

be

set

up

for

ongoing

use(and

torn

down

after

use),

e.g.,

phone

call√Connectionless,

messages

are

handled

separately,

e.gpostal

deliveryService

Primitives

(1)A

service

is

provided

to

the

layer

above

as

primitives

Hypothetical

example

of

service

primitives

that

may

providereliable

byte

stream

(connection-oriented)

service:The

primitives

are

called

at

the

client

and

server

by

the

higher

layer

using

the

service.

The

layer

implements

the

primitives

bysendingmessagesusing

the

services

of

the

lower

layer;

these

messages

are

assumed

to

be

reliable

for

simplicity

and

the

lower

layer

service

is

not

otherwisedescribed.This

is

similar

to

the

way

that

simple

Web

browsers

and

Web

servers

work

today.Service

Primitives

(2)□Hypothetical

example

of

how

these

primitivesmay

be

used

for

a

client-server

interactionClientServerLISTEN

(0)ACCEPTRECEIVESEND

(4)CONNECT

(1)SENDRECEIVEDISCONNECT

(5)Connect

requestAccept

responseRequest

for

dataReplyDisconnectDISCONNECT

(6)Disconnect(2)(3)Relationship

of

Services

to

ProtocoRecap:√

Service

–

says

what

a

layer

does√

Interface

–

says

how

to

access

the

service√

Protocol

–

says

how

is

the

service

implemented.

：A

set

of

rulesand

formats

that

govern

the

communication

between

two

peers√

A

layer

provides

a

service

to

the

one

above√

A

layer

talks

to

its

peer

using

a

protocol[vertical][horizontal]Gray

units

can

be

optionally

omitted

without

causing

later

gapsChapter

1:

Introduction□History

of

Computer

Networks□Uses

of

Computer

Networks□Network

Hardware□NetworkSoftware□Reference

Models□Example

Networks□NetworkStandardization□Metric

UnitsReference

ModelsReference

models

describe

the

layers

in

anetwork

architecture□OSI

reference

model□TCP/IP

reference

model□Model

used

for

this

text□Critique

of

OSI

&

TCP/IP

reference

modelOSI

Reference

Model(1)OSI

Reference

Model(2)□A

principled,

international

standard,

seven

layemodel

to

connect

different

systemsProvides

functions

needed

by

usersConverts

different

representationsManages

task

dialogsProvides

end-to-end

deliverySends

packets

over

multiple

linksSends

frames

of

informationSends

bits

as

signalsOSI

:

7

layersThis

partitioninto

‘lower’

and

‘upper’

layers

is

a

widely

used

way

to

distinguish

between

the

communications-oriented

layers

and

theapplications-

oriented

layers.

In

fact

layers

5and

6are

often

ignored

in

practical

applications

(the

Internet

protocol

hierarchy

has

no

equivalentsto

layers

5

and

6).Lower/Upper

LayersLayers

1-4

often

referred

to

as

lower

layers.Layers

5-7

are

the

upper

layers.Lower

layers

relate

more

closely

to

thecommunications

technology.Layers

1

–

3

manage

the

communications

subnet.the

entire

set

of

communications

nodes

requiredto

manage

comms.

between

a

pair

of

machines.Layers

4

–

7

are

true

‘end-to-end’

protocols.Upper

layers

relate

to

application.The

Physical

Layer

is

concerned

withthe

details

of

bit

transmissionovera

physical

channel.Designissues

for

this

layer

include:the

definition

of

0

and1,

e.g.

how

many

volts

represents

a

1,

and

howlong

a

bit

lasts,whether

the

channel

is

simplex

or

duplex,howmany

pins

a

connector

has,

and

what

the

function

of

each

pin

is.More

generally,

designissues

here

deal

with

Mechanical,

Electrical

and

Procedural

matters.Layer

1: Physical

Layer√Concerned

with

bit

transmission

over

physichannel.√Issues

include:definition

of

0/1,whether

channel

simplex/duplex,connector

design.√Mechanical,

electrical,

procedural

mattersThis

layer

takes

the

‘raw’

transmissionfacilityprovided

by

the

Physical

Layer

(Layer

1)

and

uses

it

to

provide

a

reliable,

error-free

transmissionservice.It

does

this

by

breaking

the

data

streamup

into

frames,

typically

of

thousands

of

bytes

in

length.

Where

necessary,

special

acknowledgementframes

can

be

sent

back

by

the

receivingData

Link

entity

to

indicate

successful

receipt

of

each

frame.The

Physical

Layer

transmits

a

continuous

sequence

of

bits,

and

so

Layer

2

must

create

and

recognise

frame

boundaries.

This

is

typically

doneby

the

insertion

of

special

bit

patterns.Channel

errors

can

completely

destroy

a

frame,

and

hence

retransmission

may

be

necessary.

This,

in

turn,

leads

to

the

possibility

of

duplicateframes

being

received.

Data

Link

protocols

must

deal

with

these

problems.A

Data

Link

protocolmay

offer

several

different

‘service

classes’

to

the

Network

Layer,

eachwitha

different

quality

and

cost.The

Data

Link

Layer

must

also

regulate

traffic

flow

to

prevent

‘swamping’

of

aslowreceiver.In

a

duplexchannel

there

may

be

competition

between

data

and

acknowledgment

frames;

one

solutionis

known

as

piggybacking,

whereacknowledgement

information

is

attached

to

data

frames.Layer

2:

Data

Link

Layer√Provides

reliable,

error-free

service

on

toraw

Layer

1

service.√Breaks

data

into

frames.

Requires

creationof

frame

boundaries.√Frames

used

to

manage

errors

viaacknowledgements

and

selective

frameretransmission.The

Network

Layer

controls

the

subnet.The

key

design

issue

is

routing

of

data

within

the

subnet.

Routing

can

be:based

on

static

tables

(rarely

changed),determined

at

the

startof

each

session,

orhighly

dynamic:

individually

determined

for

each

packet,

reflecting

the

current

network

load.Too

many

packets

in

the

subnet

simultaneously

can

formbottlenecks;

this

is

dealt

withby

congestion

control

procedures

in

the

Network

Layer.Accounting

for

subnet

use

is

also

typicallythe

responsibility

of

the

Network

Layer.When

a

packet

travels

fromone

network

to

another,

address

conversion

may

be

necessary;

this

is

also

performed

by

the

Network

Layer.

In

Broadcast

networks

(e.g.

many

LANs),

the

routing

problemis

very

straightforward,

and

hence

Layer3

will

be

very

simple,

or

even

non-existent.Layer

3:

Network

Layer√

Key

responsibilityDeliver

a

packet

to

specified

destinationPerform

segmentation/reassemble(fragmentation/defragmentation)

is

control

ofrouting

in

the

subnet.√

Routing

can

be

based

on:static

tables,determined

at

start

of

session,highly

dynamic

(varying

for

each

packetdepending

on

network

load).√

Also

responsible

for

congestion

control

and

usagemonitoring.The

basic

function

of

the

Transport

Layer

is

to

take

data

from

the

Session

Layer

(Layer

5),

split

it

up

into

smaller

units

(as

necessary),

and

passthese

units

on

to

the

Network

Layer.

It

is

then

also

responsible

for

ensuring

that

all

the

pieces

are

received

correctly

and

reassembled

in

thecorrect

order.Typically

the

Transport

Layer

will

create

a

distinct

Network

Layer

connectionfor

each

Transport

connection

requested

by

the

SessionLayer.

However,

depending

on

the

data

load

and

the

capacity

of

a

single

Session

channel:multiple

Network

connections

might

be

used

to

supporta

single

high-bandwidthSession

connection,

orone

high-bandwidth

Network

connectionmight

be

used

to

support

several

Session

connections.The

Transport

Layer

also

determines

what

Typeof

Service

to

provide

to

the

Session

Layer

and,

ultimately,

to

the

network

users.

For

example:an

error-free,

point-to-point

channel,

guaranteeingdata

is

delivered

in

the

correct

order

(the

most

common

type

of

service),transportof

isolated

messages

withno

guarantee

of

correct

ordering,

ormessage

broadcast

to

multiple

destinationsTransport

is

the

first

true

end-to-end

layer,

i.e.

the

Transport

protocol

communicates

between

end

parties

and

not

to

any

of

the

intermediaries.Layer

4:

Transport

Layer√

Basic

function

is

to

take

data

from

Session

Layer,split

it

up

into

smaller

units,

and

ensure

that

theunits

arrive

correctly.√

Concerned

with

efficient

provision

of

service.√

The

Transport

Layer

also

determines

the

‘typeof

service’

to

provide

to

the

Session

Layer.Service:

Provide

an

error-free

and

flow-controlled

end-to-end

connection

Multiplex

multiple

transport

connections

toone

network

connection

Split

one

transport

connection

in

multiplenetwork

connectionsThe

Session

Layer

allows

the

establishmentof

sessions

between

machines,

e.g.

to

allow

remote

logins

to

amulti-user

system,

or

to

perform

filetransfer

betweenmachines.One

of

the

Sessi