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Web Protocols and Practice

Dogan KesdoganAachen University of TechnologyDepartment of Computer Science IVCommunication and Distributed Systems

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Staff

Instructor: Dogan Kesdogan, kesdogan@informatik.rwth-aachen.de

2 hours lectureOffice: AachenOffice hours: no hours, please send me an email

Hiwi: Adnan Tariq, see i4 web siteAdnan.Tariq@rwth-aachen.de1 hour exercise

Note the course web pages at www-i4.informatik.rwth-aachen.de

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What Is This Course About?

What are the principles behind the Internet?What are the principles behind Web?How do these relate to many of today’s hot topics?

Web servers, HTML, HTTP, …Peer-to-peerApplication serversContent distribution networksWeb searchWeb services, Grid Computing…

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Textbooks

Two textbooks:Computer Networking: A Top Down Approach Featuring the Internet, Jim Kurose, Keith RossWeb Protocols and Practice, BalachanderKrishnamurthy, Jennifer Rexford

Course reader:Excerpts from several booksMany recent research papers

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Related courses

Multimedia Systems, Spaniol Thißen(V3/Ü1), Prerequisite the lecutre ''Data Communications''. Verteilte Systeme, Thißen, Diepolder, Spaniol, (V2/P3), Prerequisite the lecutre''Data Communications''. Verlässliche Verteilte Systeme 2, Freiling, (V3, Ü1)

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Prerequisites, Workload, etc.

No special PrerequisitesWorkload:

Homework assignments (with solutions)Final examYou can start to experiment with the Internet

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What Exactly Is the Internet?

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Chapter 1Computer Networksand the Internet

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

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:

If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)

If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2002J.F Kurose and K.W. Ross, All Rights Reserved

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Chapter 1: IntroductionOur goal:

get context, overview, “feel” of networkingmore depth, detail later in courseapproach:

descriptiveuse Internet as example

Overview:what’s the Internetwhat’s a protocol?network edgenetwork coreaccess net, physical mediaInternet/ISP structureperformance: loss, delayprotocol layers, service modelshistory

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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 ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service models1.8 History

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What’s the Internet: “nuts and bolts” viewmillions of connected computing devices: hosts, end-systems

PCs workstations, serversPDAs phones, toasters

running network appscommunication links

fiber, copper, radio, satellitetransmission rate = bandwidth

routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstationserver

mobile

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

protocols control sending, receiving of msgs

e.g., TCP, IP, HTTP, FTP, PPPInternet: “network of networks”

loosely hierarchicalpublic Internet versus private intranet

Internet standardsRFC: Request for commentsIETF: Internet Engineering Task Force

local ISP

companynetwork

regional ISP

router workstationserver

mobile

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What’s the Internet: a service viewcommunication infrastructure enables distributed applications:

Web, email, games, e-commerce, database., voting, file (MP3) sharing

communication services provided to apps:

connectionlessconnection-oriented

The Internet does not provide a service that makes promises about how long it will take to get the service!The Internet is an infrastructure in which new applications are being constantly invented

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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 humansall 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

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

Some protocols are simple and straightforward, while others are complex and intellectually deep

Hi

HiGot thetime?2:00

TCP connectionreq

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

<file>time

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A closer look at network structure:

network edge:applications and hostsnetwork core:

routersnetwork of networks

access networks, physical media:communication links

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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 ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service models1.8 History

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The network edge:end systems (hosts):

run application programse.g. Web, emailat “edge of network”

client/server modelclient host requests, receives service from always-on servere.g. Web browser/server; email client/server

peer-peer model:minimal (or no) use of

dedicated serverse.g. Gnutella, KaZaA

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Network edge: connection-oriented service

Goal: data transfer between end systemshandshaking: setup (prepare for) data transfer ahead of time

Hello, hello back human protocolset 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

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Network edge: connectionless service

Goal: data transfer between end systems

same as before!UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service

unreliable data transferno flow controlno congestion control

App’s using TCP:HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)

App’s using UDP:streaming media, teleconferencing, DNS, Internet telephony

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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 ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service models1.8 History

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The Network Core

mesh of interconnected routersthe fundamental question: how is data transferred through net?

circuit switching:dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete “chunks”

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Network Core: Circuit Switching

End-end resources reserved for “call”link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup required

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Network Core: Circuit Switchingnetwork resources

(e.g., bandwidth) divided into “pieces”pieces allocated to callsresource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces”

frequency divisiontime division

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Circuit Switching: TDMA and TDMA

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

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Network Core: Packet Switchingeach end-end data stream

divided into packetsuser A, B packets sharenetwork resourceseach packet uses full link bandwidth resources used as needed

resource contention:aggregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a time

transmit over linkwait turn at next link

Bandwidth division into “pieces”Dedicated allocationResource reservation

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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

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Packet switching versus circuit switching

1 Mbit linkeach 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 users1 Mbps link

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Packet switching versus circuit switching

Great for bursty dataresource sharingsimpler, no call setup

Excessive congestion: packet delay and lossprotocols needed for reliable data transfer, congestion control

Q: How to provide circuit-like behavior?bandwidth guarantees needed for audio/video appsstill an unsolved problem

Is packet switching a “slam dunk winner?”

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Packet-switching: store-and-forward

Takes L/R seconds to transmit (push out) packet of L bits on to link or R bpsEntire packet must arrive at router before it can be transmitted on next link: store and forwarddelay = 3L/R

Example:L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 sec

R R RL

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Packet Switching: Message Segmenting

Now break up the message into 5000 packetsEach packet 1,500 bits1 msec to transmit packet on one linkpipelining: each link works in parallelDelay reduced from 15 sec to 5.002 sec

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Packet-switched networks: forwarding

Goal: move packets through routers from source to destination

we’ll study several path selection (i.e. routing) algorithmsdatagram network:

destination address in packet determines next hoproutes may change during sessionanalogy: driving, asking directions

virtual circuit network:each packet carries tag (virtual circuit ID), tag determines next hopfixed path determined at call setup time, remains fixed thru callrouters maintain per-call state

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Network TaxonomyTelecommunication

networks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

• Datagram network is not either connection-oriented or connectionless.• Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.