Oct 18, 2009 Computer Networking Yishay Mansour (mansour at cs.tau.ac.il)mansour at cs.tau.ac.il Teaching Assistant: Hillel Avni 1

Oct 18, 2009

Computer Networking

Yishay Mansour (mansour at cs.tau.ac.il)

Teaching Assistant: Hillel Avni

1

mailto:[email protected]

Lecture 1: Oct 18, 2009

Course InformationLectures: Sunday 4 – 7 Orenstein 111Exercises: Wednesday 11 –12, 12 – 1 Super Center 315 מרכזי על

Web site: http://www.cs.tau.ac.il/research/hillel.avni/courses/comnet10.html

1. An Engineering Approach to Computer Networking / Keshav

2. Computer Networks / Tanenbaum 3. Data Networks / Bertsekas and Gallager

•A Top-down Approach to Computer Networking / Kurouse-Ross Books:

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Practical Information

Homework assignment: Mandatory Both theoretical and programming

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

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Motivation

Today’s economy manufacturing, distributing, and retailing goods but also creating and disseminating

information publishing banking film making….

part of the ‘information economy’ Future economy is likely to be dominated

by information!

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Information? A representation of knowledge Examples:

books bills CDs & DVDs

Can be represented in two ways analog (atoms) digital (bits)

the Digital Revolution convert information as atoms to information as bits use networks to move bits around instead of atoms

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Lecture 1: Oct 18, 2009The Challenges

represent all types of information as bits.

move the bits In large quantities, everywhere, cheaply, Securely, with quality of service, ….

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Today’s Networks are complex!

hosts routers links of various media applications protocols hardware, software

Tomorrow’s will be even more! 7/71

Oct 18, 2009 8

Backbone ISPISP ISP

Internet Physical InfrastructureResidential access

Cable Fiber DSL Wireless

Campus access, e.g.,

Ethernet Wireless

The Internet is a network of networks

Each individually administrated network is called an Autonomous System (AS) 8/71


This course’s Challenge To discuss this complexity in an

organized way, that will make today’s computer networks (and their limitations) more comprehensive.

identification, and understanding relationship of complex system’s pieces.

Problems that are beyond a specific technology

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Lecture 1: Oct 18, 2009Early communications systems I.e. telephone point-to-point links directly connect together the users wishing to

communicate use dedicated communication circuit if distance between users increases beyond the

length of the cable, the connection is formed by a number of sections connected end-to-end in series.

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Data Networks

set of interconnected nodes exchange information sharing of the transmission circuits= "switching". many links allow more than one path between

every 2 nodes. network must select an appropriate path for each

required connection.

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Lecture 1: Oct 18, 2009Qwest backbone

http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf12/71

Oct 18, 2009

Networking Issues - Telephone

Addressing - identify the end user

phone number 1-201-222-2673 = country code + city code + exchange + number

Routing - How to get from source to destination.

Telephone circuit switching: Based on the phone number.

Information Units - How is information sent

telephone Samples @ Fixed sampling rate. not self descriptive! have to know where and when a sample came

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Networking Issues - Internet

Addressing - identify the end user

IP addresses 132.66.48.37, Refer to a host interface = network number + host number

Routing- How to get from source to destination

Packet switching: move packets (chunks) of data among routers from source to destination independently.

Information Units - How is information sent.

Self-descriptive data: packet = data + metadata (header).

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

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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Lecture 1: Oct 18, 2009History 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 (Network Control Protocol) first host-host

protocol first e-mail program ARPAnet has 15 nodes

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Lecture 1: Oct 18, 2009History 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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Lecture 1: Oct 18, 2009History 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, Minitel100,000 hosts connected to confederation of

networks

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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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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

setbacks21/71


Internet: Users

0

200

400

600

800

1000

1200

1400

Mil

lion

use

rs

1995 1997 1999 2001 2003 2005 2007 2009

year 22/71

Lecture 1: Oct 18, 2009Penetration 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

http://www.internetworldstats.com/stats.htm 23/71

Lecture 1: Oct 18, 2009Users 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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Technology: Modem speed

300 12002400960014400

2880033600

56000

0

20000

40000

60000

80000

100000

year

bp

s

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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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Coming soon (1999)

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Today (2005)

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Lecture 1: Oct 18, 2009Why do we need Standards

Networks (and other media) support communication between different entities

Need agreement to ensure correct, efficient and meaningful communication

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Lecture 1: Oct 18, 2009Various Organizations Issue Standards

IEEE (Institute for Electrical and Electronic

Engineers)

IETF (Internet Engineering Task Force)

ITU (International Telecommunications Union)

ISO (International Organization for Standardization)

W3C (World Wide Web Consortium)30/71


Protocol Layers

A way for organizing structure of network

The idea: a series of steps

… Or at least our discussion of networks

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Protocol Layering

Necessary because communication is complex

Intended primarily for protocol designers

Divides the problem into intellectually manageable pieces

Provides a conceptual framework that can help us understand protocols

Think of layering as a guideline, not a rigid specification

Understand that optimizations may violate strict layering

Should be invisible to users32/71


Mail system functionality

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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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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What does Alice do?

Bob’s address (to a 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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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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Oct 18, 2009

Layers:

Person delivery of parcel

Post office counter handling

Ground transfer: loading on trucks

Airport transfer: loading on airplane

Airplane routing from source to destination

each layer implements a service

via its own internal-layer actions

relying on services provided by layer below

Peer entities

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Advantages of Layering

explicit structure allows identification & relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance & updating of system change of implementation of layer’s

service transparent to rest of system

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Protocols

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

semantics

Necessary for any function that requires cooperation between peers

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A protocol provides a service For example: the post office 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

Protocols

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Protocol Layers

A network that provides many services needs many protocols

Some services are independent, But others depend on each other

A Protocol may use another protocol as a step in its execution for example, ground transfer is one step in the

execution of the example reliable parcel transfer protocol

This form of dependency is called layering Post office handling is layered above parcel

ground transfer protocol. 41/71

Lecture 1: Oct 18, 2009Open protocols and systems

A set of protocols is open if protocol details are publicly available changes are managed by an organization whose

membership and transactions are open to the public A system that implements open protocols is

called an open system International Organization for Standards (ISO)

prescribes a standard to connect open systems open system interconnect (OSI)

Has greatly influenced thinking on protocol stacks

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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 architectures43/71


The seven 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!!

Application

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Lecture 1: Oct 18, 2009The 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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עיקרון השכבות

Application

Transport

Network

Data-Link

Application

Transport

Network

Data-Link

Network

Identical message

Identical message

Identical message

Source Destination בשכבהXמתקבלת הודעה זהה להודעה ששכבה

Xמסרה בצד המקור

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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 carried by planes, trains, automobiles

physical layer: the letter itself

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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

physical: bits “on the wire”

application

transport

network

link

physical

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applicationtransportnetwork

Linkphysical

applicationtransportnetwork

Linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

Protocol layering and data

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Physical layer L1

Moves bits between physically connected end-systems

Standard prescribes coding scheme to represent a bit shapes and sizes of connectors bit-level synchronization

Internet technology to move bits on a wire, wireless link,

satellite channel etc.

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Datalink layer L2

(Reliable) communication over a single link. Introduces the notion of a frame

set of bits that belong together Idle markers tell us that a link is not carrying a

frame Begin and end markers delimit a frame Internet

a variety of datalink layer protocols most common is Ethernet others are FDDI, SONET, HDLC

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Datalink layer (contd.)

Datalink layer protocols are the first layer of software Very dependent on underlying physical link properties Usually bundle both physical and datalink in hardware.

Ethernet (broadcast link) end-system must receive only bits meant for itneed datalink-layer addressalso need to decide who gets to speak nextthese functions are provided by Medium ACcess sublayer (MAC)

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Network layer L3

Carries data from source to destination. Logically concatenates a set of links to form the

abstraction of an end-to-end link Allows an end-system to communicate with any other

end-system by computing a route between them Hides idiosyncrasies of datalink layer Provides unique network-wide addresses Found both in end-systems and in intermediate

systems

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Network layer types

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 (touches every byte) control plane responsible for routing, call-

establishment, call-teardown (doesn’t touch data bytes)

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Internet network layer is provided by Internet

Protocol (IP) found in all end-systems and intermediate

systems provides abstraction of end-to-end link segmentation and reassembly packet-forwarding, routing, scheduling unique IP addresses can be layered over anything, but only best-

effort service

Network layer (contd.)

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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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Transport layer L4

Reliable end-to-end communication. creates the abstraction of an error-controlled,

flow-controlled and multiplexed end-to-end link(Network layer provides only a ‘raw’ end-to-end service)

Some transport layers provide fewer services e.g. simple error detection, no flow control, and no retransmission

Internet TCP provides error control, flow control, multiplexing

UDP provides only multiplexing57/71


Transport layer (contd.)

Error control GOAL: message will reach destination despite packet loss,

corruption and duplication ACTIONS: retransmit lost packets; detect, discard, and

retransmit corrupted packets; detect and discard duplicated packets

Flow control match transmission rate to rate currently sustainable on

the path to destination, and at the destination itself Multiplexes multiple applications to the

same end-to-end connection adds an application-specific identifier (port number) so

that receiving end-system can hand in incoming packet to the correct application 58/71


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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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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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 61/71


Application layer

The set of applications that use the network Doesn’t provide services to any other layer

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VoIP

UDP

Network (IPv4)

Ethernet

Application

Transport

Network

Data-Link

Network

Source Destination

Email(smtp) ftp

TCP

WiFiModem

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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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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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1Introduction and Layering

2Data Link: Multi Access

3Hubs, Bridges and Routers

4Scheduling and Buffer Management

5Switching Fabrics

6Routing

7Reliable Data Transfer

8End to End Window Based Protocols

9Flow Control

10Multimedia and QoS

11Network Security

12Distributed Algorithms

Course outline

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Documents

Oct 18, 2009 Computer Networking Yishay Mansour (mansour at cs.tau.ac.il)mansour at cs.tau.ac.il Teaching Assistant: Hillel Avni 1