The WWW Willem Visser RW334

Internet A global network of networks connecting computers using the internet protocol suite (TCP/IP) The WWW is built on top of the internet using hyperlinked web pages to display information

OSI vs. TCP/IP Stack Layering: FTP Example Network Link Transport Application Presentation Session Transport Network Link Physical The 7-layer Open System Interconnection Model The 4-layer Internet model Application FTP ASCII/Binary IP TCP Ethernet

TCP/IP Transmission Control Protocol/Internet Protocol IP is at the Network Layer TCP is at the Transport Layer UDP – Forgotten stepchild in all of this – User Datagram Protocol – Essentially Exposes IP functionality at the Transport Layer IP TCPUDP Network Transport

Internet Protocol Routing packets Connectionless No guarantees of delivery – Best Effort delivery Out of order delivery Only packet header is guaranteed to be error- free due to checksum usage

Transmission Control Protocol TCP is built on top of IP Two-way connections – Write here, see there and vice versa Reliable – Destination assembles packets in order – Resend if packet doesn’t make it Continuous byte-stream – The order is preserved from sender to receiver Flow Control – If network gets congested the ACK packets are spaced out Use sliding window to improve performance

TCP multiplexing: multiple programs using the same IP address – port: a number given to each program or service – port 80: web browser – port 25: – port 22: ssh – port 5190: AOL Instant Messenger – ort_numbers ort_numbers

TCP vs IP IP is like sending a long letter as a lot of short postcards, each one taking a potentially different route to its destination. IP is communication between computers TCP is communication between applications TCP/IP – TCP takes care of the communication between your application software (i.e. your browser) and your network software. – IP takes care of the communication with other computers. – TCP is responsible for breaking data down into IP packets before they are sent, and for assembling the packets when they arrive. – IP is responsible for sending the packets to the correct destination.

IP Addresses Encodes both the Network and the Host IPv4 – 32 bits, i.e. 4 bytes and thus values 0…255 – Running out of addresses IPv6 (will soon take over) – 128 bits IP addresses are assigned statically or dynamically (using Dynamic Host Configuration Protocol) Network Host

Domain Names is a domain name Much easier to remember than – Which is exactly the same thing! Domain Name System (DNS) servers converts domain names into IP addresses – Often just Domain Name Servers or DNS

Standards Organizations World Wide Web Consortium (W3C)W3C – web standards Internet Engineering Task Force (IETF):IETF – internet protocol standards Internet Corporation for Assigned Names and Numbers (ICANN):ICANN – domain names

The Players Web Browsers – Software Application – Displays web pages – IE, Firefox, Chrome, Safari, Opera, etc. Web Servers – Machine running web server software that listens on TCP port 80 for web page requests (from browsers) – Apache, Microsoft Internet Information Server, etc.

Hypertext Transfer Protocol HTTP is the language of the WWW – Commands sent from a web browser, the request – Understood by a web server, which sends a response – You hardly ever see these, since the browser handles them internally Application layer – Above TCP (Transport) which is above IP (Network) Tim Berners-Lee first proposed the "WorldWideWeb" project in 1989 – With Robert Cailliau wrote the formal version in 1990 – Credited with inventing the original HTTP protocol along with HTML and the associated technology for a web server and a text-based web browser – First version of HTTP standard only had GET command Commands – GET filename : download – POST filename : send a web form response – PUT filename : upload

Hypertext Transfer Protocol Sample Commands – GET filename : download – POST filename : send a web form response – PUT filename : upload Don’t need a browser in fact, one can just telnet into port 80

HTTP Return Codes HTTP returns a code in its response, possibly followed by HTML content Some of the common codes: – 200 OK – 404 Page Not Found – 500 Internal Server Error Code classes: – 1XX Informational – 2XX Success – 3XX Redirection – 4XX Client Error – 5XX Server Error Complete list is here:

Uniform Resource Locator Subclass of URI (Uniform Resource Identifier) – But few care about this and just stick with URL – URI also contains Uniform Resource Name (URN) URN is analogous to a person’s name URL is analogous to a person’s street address URL: – Protocol: http – Host: – Path: /rw334/Marks.xls upon entering this URL into the browser: – it would ask the DNS server for the IP address of – connect to that IP address at port 80 – ask the server to GET /rw334/Marks.xls – display the resulting page on the screen

More URLs anchor: jumps to a given section of a web page – – the above URL fetches index.html and then jumps downward to a part of the page labeled downloads port: for web servers on ports other than the default 80 – query string: a set of parameters passed to a web program – – the above URL asks the server at to run the program named search and pass it two parameters: q (set to "miserable+failure") start (set to 10)

Web Technologies Hypertext Markup Language (HTML)HTML – used for writing web pages Cascading Style Sheets (CSS)CSS – supplies stylistic info to web pages Javascript: Javascript – allows interactive and programmable web pages Asynchronous Javascript and XML (AJAX)AJAX – allows fetching of web documents in the background for enhanced web interaction PHP Hypertext Processor (PHP)PHP – allows the web server to create pages dynamically

HTML It is not WYSIWYG – What you write is not what you see in the browser Next generation HTML: – XHTML (started 2000) XML +HTML – HTML5 is the new future (combining HTML and XHTML) New features to handle multimedia Very far from a recommended standard from W3C, but the reality is that many browsers support it already

Cascading Style Sheets (CSS) describe the appearance, layout, and presentation of information on a web page – as opposed to HTML, which describes the content of the page describe how information is to be displayed – not what is being displayed can be embedded in HTML document or placed into separate.css file – advantage of.css file one style sheet can be shared across many HTML documents

Javascript No relation to JAVA – Although you’d be more and more surprised to what lengths SUN and Netscape seemed to have gone to make this non-relationship even more confusing! Client-side scripting language – Functional, borrowing syntax from C Browsers offer support for its execution (with a VM) Nowadays there is also server-side Javascript Example: – – Best practice to embed the JS within a file

Javascript Example function displayDate() { document.getElementById("demo").innerHTML=Date(); } My First Web Page This is a paragraph. Display Date

Asynchronous Javascript and XML AJAX Client-side interactive webpages – Old days pages got loaded all at once Retrieve data from the server asynchronously in the background without interfering with the display and behaviour of the existing page Avoids full browser reloads

PHP Hypertext Processor General purpose scripting language Widely used for generating dynamic webpages Embedded in the HTML and executed on the server-side to generate the page to be displayed – PHP processor runs on the web-server

Roadmap Server-side architectures Google App Engine Google Web Toolkit Security Web APIs AJAX using jQuery Scalability Testing