Download - Ch.1: Introduction Computer Networking Eliezer Dor (eliezer [email protected])eliezer dor@ Teaching Assistant: Allon Wagner 1.

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Ch.1: Introduction

Computer Networking

Eliezer Dor (eliezer [email protected])

Teaching Assistant: Allon Wagner

1

Ch.1: Introduction

Course InformationLectures: Thursday 5 – 8 Dach 005Recitation: Tuesday 16 – 17, 17 – 18 Dan David 001

Web site: http://www.cs.tau.ac.il/~allonwag/comnet2011B/index.html

1. Keshav : An Engineering Approach to Computer Networking

2. Tanenbaum : Computer Networks3. Bertsekas and Gallager : Data Networks

• Kurose-Ross: A Top-down Approach to Computer Networking

Main Book:

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Additional Books:

Ch.1: Introduction

Practical Information

Homework assignment: Mandatory Both theoretical and programming

Grades:Final Exam: 60% theory exercises: 20%Programming exercises: 20%

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Ch.1: Introduction

Chapter 1

4

Introduction

Ch.1: Introduction

Motivation

1st stage society: Agriculture, handicraft 2nd stage society: Industrial, labor intensive Today’s society:

automated industry with sophisticated logistics information intensive:

business, knowledge, advertising, news, social interaction, recreation

Future society is likely to be even more information-dominated

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Ch.1: Introduction

The Purpose of the Network

serves network applications residing in hosts applications at distinct hosts need to

co-ordinate actions / co-operate thus they need to communicate information

to each other network must deliver that information

to the right host to the right application process / thread

network serves applications which serve users

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Ch.1: Introduction

Information A representation of knowledge Examples:

text, music, video, technical specifications software, instructions, reports, alarms

Can be represented in two ways analog (pictures / ideograms) digital (bits)

the Digital Revolution convert information as pictures to information as

bits networks move around bits instead of pictures

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Ch.1: Introduction

Challenges make order in the jungle of applications organize information into manageable units keep track of info units sent/ moving/

received take account of errors / misunderstandings etc.

move the bits through the network find the destination host in the network jungle

using an efficient path learn automatically the current network topology

make efficient use of link / router capacities resolve competition for use of same resource Cheaply, Securely, with Quality of Service,

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Ch.1: Introduction

Internet Physical Infrastructure

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Ch.1: Introduction

This course’s Challenge

To discuss this complexity in an organized way, so that we understand the issues / alternatives can follow/design/troubleshoot processes

Need to divide the job into functional layers

Understand the interrelation between them These problems are beyond a specific

technology

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Ch.1: Introduction

Early communications systems

telegraph, telephone first used direct point to point links when number of users grew: introduced switching points/ configurable

circuits each call had a dedicated circuit for its

duration

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Switched connection

phone linetrunk group

Ch.1: Introduction

Data Networks

set of interconnected nodes exchanging information links are common usage switching node must:

choose for each data unit a link bringing it closer to dest. schedule their transmission on the common usage links

(resolve the competition for the usage of the link)

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Ch.1: Introduction

Qwest backbone

http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf13/71

Ch.1: Introduction

Networking Tasks – phone net. sol’n

Addressing - identify the end user phone number 1-201-222-2673 = country code + region code + exchange + number

Routing – Find route from source to destination. determined from phone number by static routing tables

Forwarding – how information is moved circuit switching::a fixed circuit along path to destination

Information Units - How information is sent voice samples; no addressing attached samples sent continuously , 8000/sec network must prepare source-dest. circuit in advance

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Ch.1: Introduction

Networking Tasks – Internet Solution

Addressing - identify the end user IP addresses 132.66.48.37, = network number || host #

Routing- How to get from source to destination routers learn automatically network topology build routing tables / updated frequently

Forwarding – how information is moved packet switching: move packets 1 by 1 through routers.

Information Units - How information is sent. self-descriptive packet = data + header header contains destination address

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Ch.1: Introduction

Telephone networks support a high end-to-end quality of service, but is expensive

Internet supports no quality of service but is flexible and cheap

Future networks will have to support a wide range of service qualities at a reasonable cost

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Ch.1: Introduction

History

1961: Kleinrock shows effectiveness of packet-switching1964: Baran - packet-switching concept in military

networks1967: ARPAnet – by Advanced Research Projects Agency1972: ARPAnet demonstrated publicly (15 nodes total)

first transport. email, protocols1970: ALOHAnet satellite network in Hawaii1973: Metcalfe’s PhD thesis proposes Ethernet1974: Cerf and Kahn - architecture for internetworkinglate70’s: proprietary architectures: DECnet, SNA, XNA1982-5: TCP/IP, SMTP mail, DNS, FTP1988: TCP congestion control1991: ARPAnet commercialized: NSFnet , Internet1989-93: WWW, browser, http, html, URL

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LAN idea

Ch.1: Introduction

Cerf and Kahn’s internetworking principles:

autonomy - no internal changes required to interconnect networks

best effort service model stateless routers decentralized control

Defines today’s Internet architecture

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Ch.1: Introduction

Why do we need Protocols

Communication is between applications or other S/W entities

Its objective: enable cooperation on a common task

Need protocols to understand each other Semantics: what I report/ want of you to do Syntax/ format: how write/ read this info

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Ch.1: Introduction

Open/ Proprietary Protocols

Open protocol can be used by anyone it is published by a standards organization

or a public consortium e.g. draft standard. standard

Proprietary protocol is owned by a company may be used subject to company’s agreement

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Ch.1: Introduction

Why do we need Standards

Communication happens between entities Hosts (personal computers, servers) Routers

H/W entities produced by different vendors S/W applications/ OS entities also Need agreement to ensure correct, efficient

and meaningful communication this is called Interworking

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Ch.1: Introduction

Organizations that Issue Standards

IETF (Internet Engineering Task Force)

IEEE (Institute for Electrical and Electronic

Engineers)

ITU (International Telecommunications Union)

ISO (International Organization for Standardization)

W3C (World Wide Web Consortium)

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Ch.1: Introduction

Why Layering

Communication is a very complex task What we need is:

communication btw applications at distant hosts What is reasonably feasible in one piece is:

the ability to transfer a series of bits over a link We need to bridge between very sophisticated

applications and very primitive physical layer What is needed is to divide the task’s

functionality into well chosen parts each part should be reasonably ‘easy’ to do they should work well together

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Ch.1: Introduction

How to do Layering

Define a conceptual Layering Model means: what is the function of each layer how they cooperate / use each other’s services

Set principles for proper usage of the model Build protocols for each layer

protocol is between same layer entities @ distinct nodes

there may be several protocols in each layer providing different type service for the layer’s function

Define interfaces between layers interface (here) is between distinct layer entities at

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Ch.1: Introduction

Layering PrinciplesModularity each layer works independently of the others

information exchange only according to Interfaces defined in the Model

analogous to the Object Oriented principle in S/W eng.

this means: don’t change/peek into internal variables of other

layers modularity is bypassed very seldom

only when there is no other solution to a problem

Transparency layering should be invisible to user

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Ch.1: Introduction

Layering Benefits

Layering enables: discussion/understanding of the issues

enables clear visualizing of relationships btw. functions

it’s impossible to think about all layers @ once efficient development of protocols

each layer has a different functional focus no need to think other layers when designing it

easy replacement/maintenance of protocols as long as modularity & interfaces are adhered to

Layering is a good reference model for discussion

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Ch.1: Introduction

A mail system layering model

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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Ch.1: Introduction

How do we Communicate?

Send a mail from Alice to Bob Alice in Champaign, Bob in Hollywood

Example: US Postal Service

Bob

Champaign, Illinois

Hollywood, California

Alice

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Ch.1: Introduction

What does Alice do?

Bob’s address (his mailbox) Bob’s name – in case people share mailbox Postage – have to pay! Alice’s own name and address

in case Bob wants to return a message In case the mail has to be returned.

Bob100 Santa Monica Blvd.Hollywood, CA 90028

Alice200 Cornfield Rd.Champaign, IL 61820

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Ch.1: Introduction

What does Bob do?

Install a mailbox Receive the mail Get rid of envelope Read the message

Bob100 Santa Monica Blvd.Hollywood, CA 90028

Alice200 Cornfield Rd.Champaign, IL 61820

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Ch.1: Introduction

Layers Peer entities

each layer implements a service via its own internal-layer actions relying on services provided by layer belowQn: Find scenarios justifying adding extra layers to the mail model. Name the layers and specify their place in model

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Champaign

HollywoodUser give parcel to P.O pick up parcel at P.O

Post office (P.O) counter handling put parcel in mailbox

Ground transfer: on truck to airport from airport to dest. P.O

Airport transfer: loading on airplane take off the airplane

Airplane routing from source to destination

Ch.1: Introduction

What Layers are Necessary?

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Application:

I received your msg …I want you to do …

Application

Transmitter:Physical Layer

1011001…

RouterHost A

Host B

how to get it to B ?

NETWORK

Receiver

how to make

sense?

msg

Ch.1: Introduction

What Layers are Necessary?

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Application:

I received your msg …I want you to do …

Application

Transmitter:Physical Layer

1011001…

RouterHost A

Host B

how to get it to B ?

NETWORK

Receiver

how to make

sense?

Link Layer

Frame

1011001…

Frame Link L.

msg

Ch.1: Introduction

What Layers are Necessary?

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Application:

I received your msg …I want you to do …

Application

Transmitter:Physical Layer

1011001…

RouterHost A

Host B

NETWORK

Receiver

Link Layer

Frame

1011001…

Frame Link L.

Net Layer packet

1011001…

packetNet L.too many details !!

too many pieces!!

msg

Ch.1: Introduction

What Layers are Necessary?

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Application:

I received your msg …I want you to do …

Application

Transmitter:Physical Layer

1011001…

RouterHost A

Host B

NETWORK

Receiver

Link Layer

Frame

1011001…

Frame Link L.

Net Layer packet

1011001…

packetNet L.

Transport Transport

Net Layer

msg

THE FIVE LAYER MODEL

Ch.1: Introduction

Application Layer L5

Tasks: write messages serving needs of application

proper type of semantics (meaning, information) appropriate syntax/format, so that semantics is understood

keep track of the interaction process / state machine Focus: on needs of a specific application

type Data unit: Message Peer:the Application Layer at destination host Uses: the Transport Layer Used by: the application itself Run by: the application

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Ch.1: Introduction

Transport Layer L4

Main Tasks: prepare data for transfer

fragment data into proper size segments / reassemble at dest.

add header which enables delivery to the correct appl. process

optional: error- /flow- /congestion-control Data Unit: Segment Focus: on control of End-to-End data transfer Peer:the Transport Layer at destination host Uses: the Network Layer Used by: the Application Layer Run by: the OS of the host

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Ch.1: Introduction

Network Layer L3 Main Tasks:

learn network topology in real time prepare routing tables for fast usage in forwarding data network layer (WAN) addressing forward data from source to destination

Data Unit: Datagram / “packet” Focus: on network and data fowarding Peers: the Network Layer along the whole

path Uses: the Link Layer Used by: the Transport Layer Run by: the OS of the host, the router S/W

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Ch.1: Introduction

Link Layer L2 Main Tasks:

insert delimiters so start/end of frame can be known physical layer may transfer an endless stream of bits this is part of the task of the Link header and Link trailer

in LAN, access control/ link layer addressing Data Unit: Frame Focus: data transfer over a link Peer:Link Layer at the other end of the link Uses: the Physical Layer Used by: the Network Layer Run by: the NIC (Network interface card, כרטיס

(רשת

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Ch.1: Introduction

Physical Layer L1

Main Tasks: transmit signals that encode bits 1 and 0 receive such signals and decode bits from them synchronize the bit rate clocks of the peer nodes

Data Unit: Bit Focus: bit transfer over a link Peer:the Physical Layer @ other end of the link

Uses: the raw media: cable/ space Used by: the Link Layer Run by: transmitter/ receiver /wave

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Ch.1: Introduction

Protocols

A protocol is a set of rules and formats that govern the communication between communicating peers set of valid message formats - syntax meaning of each message -

semantics Necessary for any function that

requires cooperation between peers

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Ch.1: Introduction

A protocol provides a service For example: the post office “registered”

protocol for reliable parcel transfer service Peer entities use a protocol to provide a

service to a higher-level peer entity for example, truck drivers use a protocol to

present post offices with the abstraction of an unreliable parcel transfer service

In the layering model: each layer gives service to next higher layer

Protocols

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Ch.1: Introduction

ISO OSI reference model

Reference model formally defines what is meant by a layer, a

service etc. Service architecture

describes the services provided by each layer and the service access point

Protocol architecture set of protocols that implement the service

architecture compliant service architectures may still use

non-compliant protocol architectures

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Ch.1: Introduction

The seven/five Layers

Presentation

Application

Session

Transport

Network

Data Link

Physical

Presentation

Application

Session

Transport

Network

Data Link

Physical

Network

Data Link

Physical

End system End systemIntermediate system

There are only 5 (!!) in most architectures

Application

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Transport

Ch.1: Introduction

The seven Layers - protocol stack

Presentation

Application

Session

Transport

Network

Data Link

Physical

Presentation

Application

Session

Transport

Network

Data Link

Physical

data

DH+data+DT

bits

data

data

data

data

AH

PH

SH

TH

Network

Data Link

Physical

dataNH

Session and presentation layers are not so important, and are often ignoredSession and presentation layers are not so important, and are often ignored

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Ch.1: Introduction

Postal network

Application: people using the postal system Session and presentation: chief clerk sends

some priority mail, and some by regular mail ; translator translates letters going abroad.

Transport layer: mail clerk sends a message, retransmits if not acked

Network layer: postal system computes a route and forwards the letters

Datalink layer: letters loaded on planes, trains, trucks

Physical layer: the driver/pilot carrying letters in sack

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Ch.1: Introduction

Internet protocol stack

application: supporting network applications ftp, smtp, http

transport: host-host data transfer tcp, udp

network: routing of datagrams from source to destination ip, routing protocols

link: data transfer between neighboring network elements ppp, ethernet, WiFi, token ring

physical: bits “on the wire”

application

transport

network

link

physical

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etw

ork

acc

ess

Ch.1: Introduction 1-48

HtHn Mpacketdatagram/

source host

application

transportnetwork

linkphysical

segment Ht M

message M

destination hostapplicatio

ntransportnetwork

linkphysical

HtHn M

Ht M

M

networklink

physical

HtHn M HtHn M

router

Encapsulation

frame HtHnHl M Tl

M – message

Ht – transport header

Hn – network header

Hl – link header

Tl – link trailer

HtHnHl M TlHtHnHl’ M Tl’

HtHnHl’ M Tl’

1011………

Ch.1: Introduction 1-49

Protocols and Interfaces

Ch.1: Introduction 1-50

Service & protocol at layer k

Service received by layer k from layer k-1

to layer k+1

Ch.1: Introduction 1-51

H3datagram

T2H2 frameH4

segment

Packet structure: sending host view

L5: application layer generates a message and passes it to transport layer

L4: transport layer adds its header (H4=Ht) this generates a segment which is passed to netwk

layer(one message may be fragmented into several segments)

L3: network layer adds its header (H3=Hn) this generates a datagram, which is passed to link layer

L2: link layer adds header (H2=Hl), trailer (T2=Tl) this generates a frame which is passed to physical layer

L1: physical layer sends the frame as a sequence of bytes is on link

Message

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Ch.1: Introduction

Packet structure: router viewReceiving stage:• L1: physical layer receives frame, passes it to L2• L2: link layer checks H2+T2 and removes them

– this makes a datagram which is passed to network layer

H3datagram

L3 payload T2H2frame

Sending stage:L3: network layer decides on which link to

sendtransfers datagram to L2

L2: link layer adds new H2+T2 and makes a frame

frame is passed to L1 which sends its bits on link H3

datagram

L3 payloadH2* T2*frame

Ch.1: Introduction 1-53

T2H2 frame

Packet structure: destination view L1: physical layer receives frame from link L2: link layer recognizes frame boundaries

checks H2+T2 and removes them this makes a datagram, passed to network layer

L3: network layer checks H3 and removes it this generates a segment, passed to transport layer

L4: transport layer checks H4 and removes it this leaves the message which is saved in receive buffer

L5: application layer takes message from buffer

MessageH4

segment

H3datagram

Ch.1: Introduction

Physical layer L1 Link Types:

Point to Point (usually continuous transmission) LAN/multiple access (intermittent transmission)

What is contained in a standard: shapes/sizes/material of connectors and

cables/media coding scheme to represent a bit bit-level synchronization

Nodes: Repeater Hub (on LAN only)

Located: in transmitter/receiver of NIC ( רשת (כרטיס

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Ch.1: Introduction

Datalink layer L2 Protocol Types

PTP protocol (HDLC, PPP, LAPD) LAN protocol (Ethernet, Token Ring, WiFI)

contains a MAC sublayer) What is contained in a protocol

header & trailer format indication of start & end frame (delimitation) in cont. transmission links: filler frame/marker

format in LAN: media access (MAC) rules, addressing rules

Nodes (in LAN only) Bridge, (L2-) Switch

Located: in NIC of hosts, routers, swithces55/71

Ch.1: Introduction

Network layer L3 Network Types

Circuit switching/Packet switching (datagram or VC) Protocol Types: Routing/ Forwarding What is contained in a forwarding protocol

header format, address formats forwarding rules (how to use routing tables)

What is contained in a routing protocol rules/ messages for learning topology info rules for building routing tables

Nodes: Router Metaphor: Welds links into Host into Host (ETE)

“channel” Location: Host OS, Router S/W

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Ch.1: Introduction

Network layer (more) L3 In datagram networks

provides both routing and data forwarding In connection-oriented network

separate data plane and control plane data plane only forwards and schedules

data control plane prepares (virtual) circuits

before data is sent Internet

forwarding by IP protocol (a datagram protocol)

best effort service (no reliability tools) several routing protocols (RIP, OSFP, BGP)

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Ch.1: Introduction

At intermediate systems participates in routing protocol to create

routing tables responsible for forwarding packets schedules the transmission order of packets chooses which packets to drop

Network layer (contd.)

At end-systems

primarily hides details of datalink layer segments and reassemble detects errors

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Ch.1: Introduction

Transport layer L4 Protocol Types:

Reliable stream protocols (TCP, SCTP, SSL) Unreliable datagram protocols (UDP)

What is contained a protocol header format user-process multiplexing rules (using port)in Reliable protocols, also: error control (ack, seq. #s, retransmission) flow control (don’t overwhelm destination) congestion control (don’t overload network)

Metaphor: Gives a Process to Process ETE channel Location: Hosts only, part of OS

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Ch.1: Introduction

Application layer L5 Application Types:

User-oriented applications (Web, Mail, File xfer..) Protocols: HTTP, SMTP+POP, FTP

Infrastructure applications (DNS, NTP) Protocol Types:

Each application type has a separate protocol What is contained a protocol

header format rules for mutual interaction of peer processes

Metaphor: Talks to peer application about common job

Location: Hosts only, run by the application S/W

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Layer Model Summary Studied (basically) the Internet 5 Layer Model OSI model (defined earlier, by ISO)

Contains 2 more layers: Layer 5 (Sessiion) Layer 6 (Presentation)

Application Layer is pushed to Layer 7 Not used in the Internet

Session layer Duplex ctrl, Data priority, Special session controls

Presentation layer Data structure standardization, encoding,

encryption see Extra slides for more details

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Ch.1: Introduction

History 1961-1972: Early packet-switching principles

1961: Kleinrock - queuing theory shows effectiveness of packet-switching

1964: Baran - packet-switching in military networks1967: ARPAnet – conceived by Advanced Research

Projects Agency1969: first ARPAnet node operational

1972: ARPAnet demonstrated publicly– NCP (Netwk Control Protocol) 1st host-host protocol – first e-mail program– ARPAnet has 15 nodes

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History 1972-1980: Internetworking, new and

proprietary nets

1970: ALOHAnet satellite network in Hawaii1973: Metcalfe’s PhD thesis proposes Ethernet1974: Cerf and Kahn - architecture for interconnecting

networkslate70’s: proprietary architectures: DECnet, SNA, XNAlate 70’s: switching fixed length packets (ATM precursor)1979: ARPAnet has 200 nodes

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Cerf and Kahn’s internetworking principles:

– minimalism, autonomy - no internal changes required to interconnect networks

– best effort service model– stateless routers– decentralized control

Defines today’s Internet architecture

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Ch.1: Introduction

History 1980-1990: new protocols,

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, Minitel

100,000 hosts connected to confederation of networks

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Ch.1: Introduction

History 1990 - : commercialization and WWW

early 1990’s: ARPAnet decommissioned1991: NSF lifts restrictions on commercial use of NSFnet

(decommissioned, 1995)early 1990s: WWW

hypertext [Bush 1945, Nelson 1960’s]HTML, http: Berners-Lee1994: Mosaic, later Netscapelate 1990’s: commercialization of WWW

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Ch.1: Introduction

Demand and Supply

• Huge growth in users– The introduction of the web

• Faster home access– Better user experience.

• Infrastructure– Significant portion of telecommunication.

• New evolving industries– Although, sometimes temporary setbacks

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Ch.1: Introduction

Internet: Users

0

200

400

600

800

1000

1200

1400

Mil

lion

use

rs

1995 1997 1999 2001 2003 2005 2007 2009

year 68/71

Ch.1: Introduction

Penetration around the Globe (2009)

Africa

Asia/Pacific

Europe

Middle EastUSA+Canada

Latin America

Australia

Africa

Asia/Pacific

Europe

Middle East

USA+Canada

Latin America

Australia

0

10

20

30

40

50

60

70

80

Africa

Asia/P

acific

Europ

e

Mid

dle E

ast

USA+Can

ada

Latin

Am

erica

Austra

lia

%Population %Penetration

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Ch.1: Introduction

Users around the Globe (2002/5/9)

Africa

Asia/Pacific

Europe

Middle East

USA+Canada

Latin America

Australia

0

100

200

300

400

500

600

700

800

2009

2005

2002

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Ch.1: Introduction

Technology: Modem speed

300 12002400960014400

2880033600

56000

0

20000

40000

60000

80000

100000

1979

1980

1984

1987

1991

1993

1995

1997

2008

year

bp

s

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Ch.1: Introduction

Today’s options

• Modem: 56 K

• ISDN: 64K – 128K

• Frame Relay: 56K ++

• Today High Speed Connections– Cable, ADSL, Satellite.– All are available at

• 5Mb (2005)• 30 Mb (2009)

OBSOLETE

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Ch.1: Introduction

Coming soon (1999)

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Today (2005)

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Ch.1: Introduction

Session layer

• Not common

• Provides full-duplex service, expedited data delivery, and session synchronization

• Internet– doesn’t have a standard session layer

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Ch.1: Introduction

• Duplex– if transport layer is simplex, concatenates two transport

endpoints together

• Expedited data delivery– allows some messages to skip ahead in end-system

queues, by using a separate low-delay transport layer endpoint

• Synchronization– allows users to place marks in data stream and to roll back

to a prespecified mark

Session layer (cont.)

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Ch.1: Introduction

Presentation layer• Usually ad hoc• Touches the application data (Unlike other layers which deal with headers)

• Hides data representation differences between applications– characters (ASCII, unicode, EBCDIC.)

• Can also encrypt data• Internet

– no standard presentation layer– only defines network byte order for 2- and 4-byte

integers

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Ch.1: Introduction

עיקרון השכבות

VoIP

UDP

Network (IPv4)

Ethernet

Application

Transport

Network

Data-Link

Network

Source Destination

Email(smtp) ftp

TCP

WiFiModem

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Ch.1: Introduction

עיקרון השכבות

Network

Source Destination

app1

UDP

Network (IPv4)

Ethernet

app2 app3

TCP

WiFiModem

app1

UDP

Network (IPv4)

Ethernet

app2 app3

TCP

WiFiModem

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Ch.1: Introduction

Discussion

• Layers break a complex problem into smaller, simpler pieces.

• Why seven layers?– Need a top and a bottom 2– Need to hide physical link; so need datalink 3– Need both end-to-end and hop-by-hop actions; so

need at least the network and transport layers 5

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