Introduction 1-1

Introduction To Computer Networks

-89מבוא לרשתות תקשורת 350

פרופ' אמיר הרצברגComputer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002.

Based on foils by Kurose & Ross ©, see: http://www.aw.com/kurose-ross/

My site: http://amirherzberg.com

Course site: http://HL2.BIU.AC.IL

Introduction 1-2

Introduction To Computer Networks- פרטים89-350תקשורת ומחשוב Lecturer: Prof. Amir Herzberg

Research in applied cryptography, secure communication and commerce, networking, dist. algs.,…

Reception: Monday 2:00-3:45pm, room 324 Physical mail: bin 23 E-mail: herzbea@cs.biu.ac.il

• Will normally answer within a week

Assistants: Mor Brosh, Igal Ioffe, Fridel Fainshtain

Use the book and foils – read at home before lectures!!

Introduction 1-3

Introduction To Computer Networks- ציונים89-350תקשורת ומחשוב 80% : exam 10% : theoretical assignments (few)

Will not consider the lowest-graded But submit all – assignments are (rarely) lost

10%: programming assignment Great for work interviews, experience

Mandatory to pass both assignments and exam Copying will lead to immediate disciplinary action (to

all parties) – please do not. Class attendance optional But no noise…

You can pass notes, send e-mail – but no noise! Offenders expelled from lecture – no warning

• Additional penalties possible (points, etc.)

Introduction 1-4

Books Main book: Computer Networking: A Top

Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002. In library and should (soon?) be in shop

Also use: Tanenbaum: Computer Networks Now in 4th edition Some copies in library

Introduction 1-5

Online Content Lectures, exercises and other stuff will be posted

at Hi-Learn system: http://HL2.BIU.AC.IL Also: submit (most) exercises via Hi-Learn To login:

Use your `regular` BIU user-ID Your password consists of 6 characters: XYnnnn

• X is the first letter of your first name, e.g. a for Avi• Y is the first letter of your surname, e.g. c for Cohen• nnnn is 4 digits code you received in the printed

timetable you (should have) received, used to access the BIU information kiosks, e.g. 1234

• Example password is ac1234

Problems/not registered: elmeser@mail.biu.ac.il

Introduction 1-6

Lecture K&RChapter

Topic

1,2 1 Introduction

3,4 2 Application layer protocols

5-7 3 Transport layer, reliability, congestion

7,8 4 Network (internet) layer

9,10 5 Data-Link layer

11,12 7, but use notes

Network Security

13   Overview

Schedule

Introduction 1-7

Chapter 1: Introduction(this and next lecture)Our goal: get context,

overview, “feel” of networking

more depth, detail later in course

approach: descriptive use Internet as

example

Overview: what’s the Internet what’s a protocol? network edge network core access net, physical media Internet/ISP structure performance: loss, delay protocol layers, service

models history

Introduction 1-8

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

millions of connected computing devices: hosts, end-systems PCs workstations, servers PDAs phones, toasters

running network apps communication links

fiber, copper, radio, satellite

transmission rate = bandwidth

router/switch: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router/switch workstation

servermobile

Introduction 1-9

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

technologies: TCP, UDP, IP – and others… Allowing interoperability RFC: Request for comments IETF: Internet Engineering

Task Force

An internet: “network of networks” loosely hierarchical public Internet versus

private intranet

local ISP

companynetwork

regional ISP

router/switch workstation

servermobile

Introduction 1-10

Computer Communication Network

Connection btw computers

Each pair can communicate (using `physical` network addresses)

Different implementations

Most ensure reliable communication

1 2

3

4

5

Introduction 1-11

Network Types

Network

Ring

Link Bus

Star

Satellite

Cellular

NMS

Switched

SWEach pair can communicate

(using network addresses)

Introduction 1-12

An internet: Connection of Networks (Using TCP/IP; `Internet` is the `big, public` internet.)

Each pair can communicate (using Internet Protocol addresses (IP addresses))

Introduction 1-13

What’s the Internet: a service view communication

infrastructure enables distributed applications: Web, email, games, e-

commerce, database., voting, file (MP3) sharing

Communication transport services provided to apps: connectionless connection-oriented

Use protocols to control send, receive messages e.g., TCP, IP, HTTP, FTP,

POP3, SMTP

Introduction 1-14

What’s a protocol?human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather than

humans all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and

received among network entities, and actions taken on msg transmission, receipt

Introduction 1-15

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

Q: Other human protocols?

Hi

Hi

Got thetime?

2:00

TCP connection req

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

<file>time

Q: What is this protocol?

Introduction 1-16

A closer look at network structure: network edge:

applications and hosts

network core: routers/switches network of networks

Introduction 1-17

Chapter 1: roadmap

1.1 What is the Internet?1.3 Network core1.2 Network edge1.4 Network access and physical media1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History

Introduction 1-18

The Network Core

mesh of interconnected routers and switches

Hi speed, minimize work Routing: how is data

transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

Introduction 1-19

Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required Example: phone

network

Introduction 1-20

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls

resource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces”

Division for Multiple Access: Frequency division

(FDMA) Time division

(TDMA) Code division

(CDMA) – much later

Introduction 1-21

Circuit Switching: TDMA and FDMA

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

Introduction 1-22

Network Core: Packet Switching

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time transmit over link wait turn at next

link

Bandwidth division into “pieces”Dedicated allocationResource reservation

Introduction 1-23

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern statistical multiplexing.

In TDM each host gets same slot in revolving TDM frame.

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Introduction 1-24

Packet switching versus circuit switching

1 Mbit link each user:

100 kbps when “active”

active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

Introduction 1-25

Packet switching versus circuit switching

Great for bursty data resource sharing simpler, no call setup

Excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to guarantee bandwidth and Quality Of

Service (QOS)? (easy with circuit switching!) Needed for audio/video apps Still an unsolved problem (chapter 6)

Is packet switching always better?

Introduction 1-26

Packet-switching: store-and-forward

Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps

Entire packet must arrive at router before it can be transmitted on next link: store and forward

End to end delay = (#hops)*L/R

Example: L = 7.5 Mbits R = 1.5 Mbps One link (hop):

L/R=5sec End to end delay =

3*L/R=15 sec

R R RL

Introduction 1-27

Packet Switching: Message Segmenting and Pipelining

Now break up the message into 5000 packets

Each packet 1,500 bits 1 msec to transmit packet on one link pipelining: each link works in parallel 5 sec for each link E2E Delay=5+0.001*2 Delay reduced from 15 sec to 5.002 sec

Introduction 1-28

Packet-switched networks: routing Goal: move packets through routers from source to

destination (using which path?) we’ll study several path selection (i.e. routing) algorithms

(chapter 4)

datagram network/routing: destination address in packet determines next hop routes may change during session analogy: driving, asking directions

virtual circuit network/routing: each packet carries tag (virtual circuit ID), tag determines

next hop fixed path determined at call setup time, remains fixed thru

call routers maintain per-call state

Introduction 1-29

Chapter 1: roadmap

1.1 What is the Internet?1.3 Network core1.2 Network edge1.4 Network access and physical media1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History

Introduction 1-30

The network edge: end systems (hosts):

Provide transport services To application programs

• e.g. Web, email Idea: Do most work at edge

client/server model client host requests, receives

service from always-on server e.g. Web browser/server; email

client/server

peer-peer model: Both sides `equal` E.g. `ad-hoc network` between

palmtop personal devices

Introduction 1-31

connection-oriented transport serviceGoal: reliable data

transfer between end systems

handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human

protocol set up “state” in two

communicating hosts

TCP - Transmission Control Protocol Internet’s connection-

oriented service

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements and

retransmissions flow control:

sender won’t overwhelm receiver

congestion control: senders “slow down sending

rate” when network congested Prevents increased congestion Improves performance to all (if

all use it and delay, loss are mainly due to congestion)

Introduction 1-32

connectionless transport service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data transfer

• Don’t resend (phone,…)

no flow control no congestion control

• TCP congestion control harmful if delay, loss NOT due to congestion!

Simple, small

App’s using TCP: HTTP (Web), FTP (file

transfer), Telnet (remote login), SMTP (email)

App’s using UDP: streaming media,

teleconferencing, DNS, Internet telephony

Other transport services

Introduction 1-33

Network Taxonomy

Telecommunicationnetworks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

•Internet is Packet-switched, Datagram network•Transport services (on end-systems):

•connection-oriented (TCP)•connectionless (UDP)•Recently: others (DCCP, SCP)

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

TCPConnection

Service

UDPConnection-less

Service

Introduction 1-34

Lecture 1: Main concepts Network, internet, Internet, intranet What’s a protocol? Network edge, core Network transport services

Connectionless (UDP) and connection (TCP) Client/server vs. Peer to Peer

Routing and switching Router: connects two different networks in internet Packet-switching versus circuit-switching (FDM,TDM) Packet-switching:

• Datagram vs. virtual circuit (VC)• Packet segmentation and pipelining

Use book and course site (in Hi-Learn system)

Introduction 1-35

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History

Introduction 1-36

Internet History

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

1964: Baran - packet-switching in military nets

1967: ARPAnet conceived by Advanced Research Projects Agency

1969: first ARPAnet node operational

1972: ARPAnet

demonstrated publicly NCP (Network Control

Protocol) first host-host protocol

first e-mail program ARPAnet has 15 nodes

1961-1972: Early packet-switching principles

Introduction 1-37

Internet History

1970: ALOHAnet satellite network in Hawaii

1973: Metcalfe’s PhD thesis proposes Ethernet

1974: Cerf and Kahn - architecture for interconnecting networks

late70’s: proprietary architectures: DECnet, SNA, XNA

late 70’s: switching fixed length packets (ATM precursor)

1979: ARPAnet has 200 nodes

Cerf and Kahn’s internetworking principles: minimalism, autonomy

- no internal changes required to interconnect networks

best effort service model

stateless routers decentralized control

define today’s Internet architecture

1972-1980: Internetworking, new and proprietary nets

Introduction 1-38

Internet History

1983: deployment of TCP/IP

1982: SMTP e-mail protocol defined

1983: DNS defined for name-to-IP-address translation

1985: FTP protocol defined

1988: TCP congestion control

new national networks: Csnet, BITnet, NSFnet, Minitel

100,000 hosts connected to confederation of networks

1980-1990: new protocols, a proliferation of networks

Introduction 1-39

Internet History

Early 1990’s: ARPAnet decommissioned

1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

early 1990s: Web hypertext [Bush 1945,

Nelson 1960’s] HTML, HTTP: Berners-Lee 1994: Mosaic, later

Netscape late 1990’s:

commercialization of the Web

Late 1990’s – 2000’s:

more killer apps: instant messaging, peer2peer file sharing (e.g., Naptser)

network security to forefront

est. 50 million host, 100 million+ users

backbone links running at Gbps

1990, 2000’s: commercialization, the Web, new apps