Human-Computer Interface Course 5

ISPs and Internet connection

Domain Naming Service (DNS) DNS provides name to IP address mapping Originally 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 1. Ask Root Server for the DNS server of.com => a.gtld-servers.net 2. Ask a.gtld-servers.net who is the DNS server for yahoo.com => ns1.yahoo.com 3. Ask ns1.yahoo.com who is => is alias for 4. Ask ns1.yahoo.com who is =>

The World Wide Web HTML Language – to describe Web pages =>RFC1866 and RFC1942 HTTP protocol – to transmit web pages The Uniform Resource Locator – to name Web pages Hypertext – a way of describing documents and data that reference other documents/data.

TCP/IP Peer to peer communication Port: Port: 80 HTTP IPAddress + Port: Identify communicating applications on the source and destination machines

TCP/IP – peer to peer IP Address – identifies the computer Port a communication channel descriptor possible ports on one machine – 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.RFC1738RFC1808 URI – Uniform Resource Identifiers – strings that specify how to access network resources. Ex: URL: Download RFCs:

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 – we are using the HTTP protocol to access the resource. - 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 GET / HTTP/1.0 > > < HTTP/ OK < Date: Wed, 18 Sep :18:59 GMT < Server: Apache/1.0.0 < Content-type: text/html < Content-length: 1579 < Last-modified: Mon, 22 Jul :23:34 GMT < < HTML document

The File Transfer Protocol-FTP Provides ability to transfer files between machines heterogeneous env. and Op. Syst Works on the port 21 using the TCP protocol. Ex: ftp ftp.ubbcluj.roftp.ubbcluj.ro User: anonymous Password: ftp> get README

Internet Mail Mail protocols: SMTP, IMAP, POP3 SMTP – Simple Mail Transfer Protocol IMAP, 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.

Firewalls Internet Computers