Alphabet Soup MAC (ethernet address e.g. 00-00-6c-4f-3d-1a) - - PDF document

1

CL1 2007/08

Lecture 15: The Internet and the Web

CL1 2007/08

Alphabet Soup

– MAC (ethernet address e.g. 00-00-6c-4f-3d-1a) » Differentiates between machines on one ethernet segment) – UDP (datagram) – TCP (transport – gives you a reliable connection) – IP (end-to-end addressing e.g. 129.215.200.7) – HTTP (Web) – SMTP (mail) – SSL (secure socket layer) » Supports creation of Virtual Private Networks (VPNs) – secure tunnels through Internet – And many others

CL1 2007/08

OSI 7 layer networking model

• 7: Application layer e.g. terminal emulation, file transfer
• 6: Presentation layer (encryption, compression etc. )
• 5: Session layer manages connections between applications
• 4: Transport layer is responsible for ensuring correct end-to-end

delivery

• 3: Network layer finds routes for packets and transmits them to

next node

• 2: Link layer breaks packets into frames and sends them;

handles point-to-point issues e.g. error recovery, flow control

• 1: Physical layer sends bits
• TCP/IP model is rather different
• Both cases, each layer makes assumptions about layer below

CL1 2007/08

Put it all together - home

• Internet connection

– Dial-up – Cable – DSL (ADSL) over phone network

• Connection point

– Dial-up, cable or DSL modem

• Attach to PC or switch/router

CL1 2007/08

Switches and hubs

PC PC PC PC PC Wired ethernet – lots of cable, everyone sees all traffic Hub – electronics replace cable But otherwise same as wire Switch – redundant traffic is filtered out

CL1 2007/08

Switch / Router / WAP / hub

• Hub – ethernet ‘in a box’
• Router – routes packets between broadband

and local network.

• Switch, often combined with router filters
• ut redundant traffic – better use of capacity

and more secure

• Switch/router often combined with Wireless

Access Point for home use

2

CL1 2007/08

Small network euipment

Print server Wireless router Wireless media streamers Cable modem Office router Domestic fileserver Wireless Access point router

CL1 2007/08

Simple home networks

PC PC PC WAP PC Printer Simplest – PC to broadband via cable or dialup modem WiFi router Network Network modem modem

Bluetooth

Phone etc. More complex – multiple machines to broadband via router

CL1 2007/08

DHCP

• Many hosts have a fixed IP address

– 129.215.200.7 = University DNS server

• Alternative is to request an address from a

DHCP server

• Leased for a period then reused.
• More efficient use of addresses
• Host can be moved from wire to wire

CL1 2007/08

NAT addressing

• (Network Address Translation)

PC PC router Network modem This side of router presents a unique Internet address to the world (for now; may be a DHCP address leased by Internet Service Provider) This side of router is private. Can use any numbering scheme you like (192.168.x.x usual) These PCs will lease 192.168.x.x addresses from your home router/DNS server Router translates between your non-unique NAT addresses and a ‘real’ IP address. Main limitation is that your PCs are invisible to the Network so can’t work as servers (without help).

CL1 2007/08

Introduction

• Networks

– Internet – Organisation of networks – Peer-2-peer networks

• Applications

– The web – File-sharing

CL1 2007/08

Put it all together - 1

• Internet

– Wide-area network backbone

• Backbone of up to 10 Gbit/sec
• Irregular network of point-to-point links
• End-to-end protocols over the top

– Routed out to local networks – Gateways to Intranets

3

CL1 2007/08

ARPANET - revision

• Aim

– Military communications network – Robust to damage

• Result

– Packet-switching – All computers equally able to communicate – The internet!

CL1 2007/08

ARPANET

CL1 2007/08

University network

• University LAN

– EdLAN

• Edinburgh & Stirling MAN

– EaStMAN

• UK education WAN

– JANET – SuperJANET5

CL1 2007/08

Computers as connectors

• Different networks can use different packet-

switching methods

• Bridge connects networks of the same type
• Gateway connects networks of different type
• Routers and Switches decide path of packets
• Repeaters boost, resend packets
• Hubs replace wire, connect within a network
• Firewalls and proxies limit access

CL1 2007/08 CL1 2007/08

4

CL1 2007/08 CL1 2007/08

Intranet and Extranet

• Intranet

– Network with access restricted to members

• f an organisation
• Extranet

– Network outside intranet – Access restricted to authorised users – E.g. business clients

CL1 2007/08

Client-server model

• Server machine

– Stores data – Or has access to data

• Client machine

– Runs application – Application requests data from server

• Server finds data and sends it to the

client

CL1 2007/08

Client-server model: the Web

• The Web: method of communication

using internet

• Client is the Web browser
• Server stores Web pages
• Communication requires

– Protocol – Addresses – Shared language

CL1 2007/08

The Web

• 3 standards

– HTTP: data exchange protocol – URL: address – HTML: shared language

• Browser

– Sends out URL through HTTP – Displays HTML data

• Server

– Receives URLs through HTTP – Finds pages and sends back

CL1 2007/08

URL: Web page address

• Protocol method

– Usually http, also https (secure http)

• Server name

– www.inf.ed.ac.uk

• Web page file path

– /teaching/courses/cl1

5

CL1 2007/08

Web Cache

• Copies of visited Web pages put in a cache
• Advantages

– Faster – Frees Internet bandwidth

• Implementation

– HTTP: methods for expressing whether a page can be copied – Cache size limit on your computer

CL1 2007/08

Client-side

• Client-side Web cache

– On client computer – Or ISP server

• Example client-side Web application

– Java applets

CL1 2007/08

Server-side

• Cache on Web server

– Recently visited pages put in cache – Reduces time to retrieve frequently visited pages

• Speed of Web services not just

connection speed!

• Example server-side Web application

– CGI scripts

CL1 2007/08

Web proxy

• Connections to a page are usually direct
• Web proxy: Mediating connection to the

page

• Used to manage access

– From a network to the Internet – From outside the network to the intranet

• Can be a cache: faster access

CL1 2007/08

Cookies

• Web page stores information on your

computer

• Used to

– Remember login details – Shopping baskets – Observe your use of web site

• Security problem?

CL1 2007/08

Peer-to-peer networks

• Network where each network node is a client

and a server

– No dedicated servers – P2P overlaid on Internet structure

• Cost

– Use internet infrastructure

• Reliability

– Not affected by server downtime Bandwidth

6

CL1 2007/08

P2P Scalability

• As the number of users and files grow

– Routing becomes time consuming

• But

– Total bandwidth increases – Not limited by access to servers – Kazaa P2P network “rewards” high bandwidth users: supernodes

CL1 2007/08

P2P Routing

• Napster

– Centralised file lists – Client server searching – P2P file distribution

• Limewire, Gnutella, FastTrack

– Distributed file lists – P2P searching + file distribution

CL1 2007/08

P2P Security

• Have to be careful what files P2P

software can access

• P2P software could observe your Web

movements (“spyware”)

• Freenet, GNUnet, AntsP2P…

– Protect user and file identity – Slower – Safe? For who?

CL1 2007/08

P2P applications

• Payment schemes (PayPal)

www.paypal.com/html/gartner-020102.html

• VoIP, e.g. Skype www.skype.com
• Spam detection razor.sourceforge.net/
• Distributed computing: GIMPS prime number

search www.mersenne.org/prime

• Instant Messenger
• etc

CL1 2007/08

Key Points

• Network organisation

– Bridges, Gateways, Routers, Repeaters – Client-server model – Peer-2-peer networks

• Applications

– How organisation affects implementation – Advantages and disadvantages: scalability, speed, security