Networking

Introduction

Networking - Introduction

• Computers are lucky…• …but this is not trivial!

Networking - Introduction

RAM CPUEasy….

Networking - Introduction

My smartphonein Denmark

www.surf.au in Australia

Hard….

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• Networking – transmitting data between physically separated computers

• No networking – no Internet• The Internet is ”just another

computer network”• We will discuss networking within

the context of the Internet

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• What does the Internet look like…?

?My device

Networking - Introduction

• What does the Internet look like…?

?My device A Computer

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• What does the Internet look like…?

?Client Server

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• Through the Internet, my device (the client) can exchange data with a remote computer (the server)

• How is the the client ”connected” to the server?

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• What does the Internet look like…?

Client Server

C S

Routers

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• Client is connected to a router C• Server is connected to a router S• Client sends data to C• C sends data to another router, who in turn

sends data to another router, etc.• At some point, data reaches the router S• The server gets the data from S• How hard can it be…?

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• Questions…– How do C and S get to know each other?– How does data find its way from C to S?– What format should data have?– How fast will the data get there?– Can we be sure that the data will reach S?– Can we be sure that the data is not altered?– …?

Networking - Introduction

• Communication requires a protocol• This is true for both humans and computers!

HiHi

Do you have the time?

Yes, it is 4.30pm

Okay, thank you You’re

welcome

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• What can go wrong…?

Hi … Я не говорю по английски

Oh, I guess he doesn’t speak

English…

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• What can go wrong…?

Hi

Hmmm, maybe he didn’t hear me…

I’ll try again in a minute…

Networking - Introduction

• What can go wrong…?

HiI’m busy!

I’ll try again in a

minute…

Networking - Introduction

• What can go wrong…?

HiHi

Do you have the time?

Yes, it is &.3¤p?

Hmmm, I’ll have to ask him again…

Networking - Introduction

• A protocol is about agreement– When has a connection been established– What is the proper format of the transmitted data– What to do when data has been sent– What to do when data has been received– What is the proper order of data– When can data be considered lost– When can a connection be considered broken

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• What is a connection really…?• Logical aspect– Connection-oriented service– Connectionless service

• Physical aspect– Circuit switching– Packet switching

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• Connection-oriented service• A (logical) way of communication, with certain

properties– All data will be delivered– All data will be delivered in the correct order– Nobody ”floods” the counterpart with data– No part of the connection becomes congested

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• Connection-oriented service

Are you ready to receive data? Sure, go ahead

OK, here is part 1 of data OK, I got

part 1

OK, here is part 2 of data OK, I got

part 2

…and so on (handshaking)

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• Connection-oriented service

Here is part n of data

Here is part n of data OK, I got

part n

Hmmm, I have to send part n again…

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• Connection-oriented service

Here is part n of data

Here is part n+1 of data

OK, I got part n

OK, I will send data more

slowly

Whoa, slow down a bit!

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• Connection-oriented service• Many good properties,

BUT…– More control data is sent– Cannot ”skip” data– Speed of transmission is

uncertain

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• Connectionless service• A (logical) way of communication, with certain

properties– No guarantee on delivery– No handshaking– No flow or congestion control

• BUT, can potentially deliver data faster

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• Connectionless service

Are you ready to receive data? Sure, go ahead

OK, bla bla bla bla bla bla…

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• Why would we ever use a connectionless service?

• Where could it be appropriate?– Sending E-mail?– File transmission– Viewing a web page– Streaming audio– Streaming video

Networking - Introduction

• Why would we ever use a connectionless service?

• Where could it be appropriate?– Sending E-mail?– File transmission– Viewing a web page– Streaming audio– Streaming video

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• On the Internet…

Connection-oriented service

Connectionless service

TCP – Transmission Control Protocol

UDP – User Datagram Protocol

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• Circuit switching• Like in the old days…

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• Using circuit switching, a dedicated end-to-end circiut is set up

Client Server

C S

Networking - Introduction

• Using circuit switching, a dedicated end-to-end circiut is set up

Client Server

C S

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• Properties of circuit switching– Guaranteed transmission rate– Wasteful in idle periods

• Best for transmissions with low transmission rate variation (phonecall?)

• Very wasteful for ”burst” transmissions

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• Packet switching• The postal approach…

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• In packet switching, we no longer use dedicated connnections

• A recipient of data has an address• Data is divided into packets, which are

”stamped” with the destination address• The magic lies in the router…

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• Delivering a letter…

Postbox

Local post office

Regional post office

National post center

Letterbox

Local post office

Regional post office

National post center

Sender Sender

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R

1

3

2

68.122.87.224

(DATA)

Router must choose a destination router…

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• Router maintains a routing table of appropriate desitination routers for a given destination adress (or interval of addresses)

• How is the routing table created…?

• MAGIC…for now

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• On the Internet, the IP (Internet Protocol) uses packet switching

• Formally, the IP implements a datagram network – a specific variant of packet switching

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• Circuit Switching vs. Packet Switching• I have a 100 megabit/second (mbps)

connection available (bandwidth guaranteed)• I want to give all my users a 10 mbps

connection• How many users can share the connection?

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• Circuit Switching guarantees the bandwidth at all times!

• Everybody must be guaranteed a 10 mbit connection…

• …so the answer must be 100/10 = 10 users

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• In packet switching, packets are sent– With full transmission rate– Intermixed with packets from other users

• No guaranteed bandwidth• We have to know about the expected use of

bandwidth per user

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• Theoretical use per day, given a 10 mbps connection:– 10 mbps ≈ 1 megabyte/sec– 1 megabyte/sec ≈ 4 gigabyte/hour– 4 gigabyte/hour ≈ 100 gigabyte/day

• In practice, probably much less!

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• Bandwidth usage for a typical (?) private user– 10 gigabyte/month– Peak: 1 gigabyte/day– 00-08: ≈ 10 %– 08-16: ≈ 20 % – 16-24: ≈ 70 %

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• What is my required ”level of service”?• Specified bandwidth available– 50 % of the time?– 90 % of the time?– 99 % of the time?– 99,99 % of the time?

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• We wish to cover ”peak usage”, but assume that bandwidth usage during peak time is constant…– Peak usage: 1 gigabyte/day, 70 % between 16-24– 700 megabyte within 8 hours– 90 megabyte within an hour– 25 kilobytes per second… – about 250 kilobits per second (0,25 mbps)

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• A 100 mbps connection…• …and a ”peak average” of 0,25 mbps…• …gives about 400 users

• Circuit switching: 10 users• Packet switching: 400 users• ..if our assumptions hold up!

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• Now we know about two protocols– TCP – implements connection-oriented service– IP – implements datagram packet switching

• Is that enough to e.g. implement a Web browser…?

• Still too low-level, we need a protocol ”on top of” TCP

• Enter HTTP

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• HTTP – HyperText Transfer Protocol• Specifically developed for ”distributed,

hypermedia-based information systems”• Developed by Tim Berners-Lee (and team)

around 1990• HTTP + HTML + URL = WWW (almost )

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• HTTP is essentially just a set of methods that can be executed within a program

• First HTTP version – one method! (GET)

• Current version has nine methods

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• Note that HTTP is not the only protocol on this level of abstraction

• SMTP – Simple Mail Transfer Protocol• FTP – File Transfer Protocol• …and some lesser known protocols

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• Now we know about (at least) three protocols– HTTP – implements hypertext-based distributed

computer systems– TCP – implements connection-oriented service– IP – implements datagram packet switching

• How are the protocols related?• Are other protocols needed?• How do we ”organise” protocols?

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• A standard model for layering functions in a communication system exists: the OSI model

• OSI – Open Systems Interconnection• Model consists of 7 layers, each of which

contains specific functionality

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• The OSI model is a logical model – does not necessarily map one-to-one to actual model

• The model for the Internet is considered to have 4-5 layers

• Kurose & Ross: 5 layers• For simplicity, we consider four layers• The set of protocols is called the Internet

Protocol Stack (or Suite)

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Hardware…

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• We will discuss the three top levels in the Internet protocol stack in further detail– Application Layer – focus on HTTP– Transport Layer – focus on TCP– Network Layer – focus on IP

Networking - Introduction