Human-Computer Interface

Human-Computer Interface

Course 5

ISPs and Internet connection

Domain Naming Service (DNS)

DNS provides name to IP address mappingOriginally one single central huge table.Today – each domain name provides its own DNS servers.Hierarchical structure: Root DNS servers (serving .com .org .net,

etc) Domain serves – serve domain queries.

DNS – How it works ?Connect at www.yahoo.com1. Ask Root Server for the DNS server of .com

=> a.gtld-servers.net2. Ask a.gtld-servers.net who is the DNS

server for yahoo.com => ns1.yahoo.com3. Ask ns1.yahoo.com who is www.yahoo.com

=> is alias for www.yahoo.akadns.net4. Ask ns1.yahoo.com who is

www.yahoo.akadns.net =>216.109.118.68

The World Wide Web

HTML Language – to describe Web pages =>RFC1866 and RFC1942HTTP protocol – to transmit web pagesThe Uniform Resource Locator – to name Web pagesHypertext – a way of describing documents and data that reference other documents/data.

TCP/IP Peer to peer communication

193.231.20.34Port: 1563

63.78.171.45Port: 80

HTTP

IPAddress + Port:Identify communicating applications on the source and destination machines

TCP/IP – peer to peer

IP Address – identifies the computerPort a communication channel descriptor 0-65535 possible ports on one machine 0-1024 – reserved well known ports

(ex:80 – Web Servers) Servers listen on well known ports

Client/Server –architectures

Generally involves two entities Server – application designed to service

specialized requests. Clients – applications using the services

offered by the server. They are calling the server and ask him specific tasks.

Example: Calling telephone information to find someone’s phone number

URLs and URIs

URL – described in RFC1738 and RFC1808.URI – Uniform Resource Identifiers – strings that specify how to access network resources.

Ex: URL:http://www.cs.ubbcluj.rohttp://www.cs.ubbcluj.ro/math/index.html

Download RFCs: http://www.rfc-editor.org/

TCP/IP Client Servers

Server – applications listening/waiting for client requests on a well known port.Clients – connect to the server and issue requests on the communication channel.

Requests are processed by the server and responses are sent back to clients.

URL – Locating resources

http://www.FreeSoft.org/Connected/index.html

http – we are using the HTTP protocol to access the resource.www.Freesoft.org - the DNS name of the web server that has our resource./Connected/index.html – the name of the directory where the server stores our document.

HTTP Protocol

Allows exchange of HTML and Web data.Works on TCP port 80 and is human readable.

Ex: Connect to www.cs.ubbcluj.roGET / HTTP/1.0 > >

< HTTP/1.0 200 OK < Date: Wed, 18 Sep 1996 20:18:59 GMT < Server: Apache/1.0.0< Content-type: text/html < Content-length: 1579 < Last-modified: Mon, 22 Jul 1996 22:23:34 GMT< < HTML document

The File Transfer Protocol-FTP

Provides ability to transfer files between machines heterogeneous env. and Op. SystWorks on the port 21 using the TCP protocol.

Ex: ftp ftp.ubbcluj.ro

User: anonymousPassword: dadi@cs.ubbcluj.roftp> get README

Internet Mail

Mail protocols: SMTP, IMAP, POP3SMTP – Simple Mail Transfer ProtocolIMAP, POP3 – mailbox access protocols.SMTP – works on TCP – port 25. Each domain name has >=1 mail

exchange servers Human readable text protocol Best effort mail transfer protocol.

Internet Security

Internet connectivity and access to each computer – opportunity for hackers to access distant machines or intercept communications.Protection methods: Firewalls, proxy servers. Encrypted communications Digital signatures

Internet Security – Firewalls

Firewall – piece of software or hardware implemented component that blocks IP packets for non-authorized users/machines.

Proxy Server – software component that intermediates connections between web browsers and external web servers.

Encryption, signatures

SSL – Secure Sockets Layer – protocol implementing encrypted communication between two hosts with source and destination authentication.Digital signatures allow to identify if a piece of data has been tampered during its transport.

FirewallsInternet Computers