The World Wide Web

Web Growth

Web Server Statistics Apache is the most popular web server today (freely available) Microsoft IIS is gaining ground

The World Wide Web: A Brief History

Vannevar Bush 1945

MEMory EXtender system Problem: –Bush was concerned about “new knowledge not reaching the people who would benefit from it”

MEMory EXtender system Store publications, correspondence, and personal work on microfilm Items retrieved rapidly using index codes Can annotate text with margin notes, comments Can construct a trail through the material and save it Acts as an external memory

MEMory EXtender system limitations Basic unit of content is an image page –No links to/from sub-text No digital content –No keyword search, only TOC/index codes No networking –No rapid info sharing

Results The MEMory EXtender system was the inspiration for the creators of hypertext and the web

Marshall McLuhan 1964 – Hypertext Coined McLuhan is known for coining the expressions the medium is the message and the global village, and for predicting the World Wide Web almost thirty years before it was invented

Tim Berners-Lee invents Global Hypertext WWW 1991 Sir Timothy John "Tim" Berners-Lee is an English computer scientist known as the inventor of the World Wide Web. He made a proposal for an information management system in March 1989, and on 25 December 1990, with the help of Robert Cailliau and a young student at CERN, he implemented the first successful communication between a Hypertext Transfer Protocol (HTTP) client and server via the Internet. By the mid 1990s, the World Wide Web had replaced Mark P. McCahill's Gopher protocol as the dominant Internet protocol. Berners-Lee is the director of the World Wide Web Consortium (W3C), which oversees the Web's continued development. He is also the founder of the World Wide Web Foundation, and is a senior researcher and holder of the Founders Chair at the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL). He is a director of the Web Science Research Initiative (WSRI),and a member of the advisory board of the MIT Center for Collective Intelligence.

World First Browser and Web page. 3

Then… 4

What has happened through the years? Appearance of JavaScript (JS) in Late Standardization of HTML in Jan Introduction of Cascading Style Sheet (CSS) in Document Object Model (DOM) was standardized in End of Begin to be working in AJAX (Widely used by Google in 2004). 5

Web Architecture 6

The World Wide Web: HTTP Protocol

Both were invented at the same time by the same person HTTP vs HTML HTML: hypertext markup language –Definitions of tags that are added to Web documents to control their appearance HTTP: hypertext transfer protocol –The rules governing the conversation between a Web client and a Web server

What is a protocol? In diplomatic circles, a protocol is the set of rules governing a conversation between people We have seen that the client and server carry on a machine-to-machine conversation A network protocol is the set of rules governing a conversation between a client and a server There are many protocols, HTTP is just one

I would like to open a connection GET Display response Close connection OK Send page or error message OK Client Server HTTP is the set of rules governing the format and content of the conversation between a Web client and server An HTTP conversation

An HTTP example The message requesting a Web page must begin with the work “GET” and be followed by a space and the location of a file on the server, like this: GET /fac/lpress/shortbio.htm The protocol spells out the exact message format, so any Web client can retrieve pages from any Web server.

Network protocols The details are only important to developers. The rules are defined by the inventor of the protocol – may be a group or a single person. The rules must be precise and complete so programmers can write programs that work with other programs. The rules are often published as an RFC along with running client and server programs.RFC The HTTP protocol used for Web applications was invented by Tim Berners Lee.Tim Berners Lee RFC = request for comments

Tim Berners-Lee Tim Berners-Lee was knighted by Queen Elizabeth for his invention of the World Wide Web. He is shown here, along with the first picture posted on the Web and a screen shot from an early version of his Web browser.

HTTP is an application layer protocol The Web client and the Web server are application programs Application layer programs do useful work like retrieving Web pages, sending and receiving or transferring files Lower layers take care of the communication details The client and server send messages and data without knowing anything about the communication network

The application layer is boss – the top layer Your boss says: Send this package to Miami -- I don't care if you use Federal Express, UPS, or any other means. Also, let me know when it arrives or if it cannot be delivered for some reason. The application program says: Send this request to the server -- I don't care how you do it or whether it goes over phone lines, radio, or anything else about the details. Just send the message, and let me know when it arrives or if it cannot be delivered for some reason. LayerFunction ApplicationDo useful work like Web browsing, , and file transfer Lower layersHandle communication between the client and server There are five TCP/IP layers, the application layer and four lower layers.

Many application layer protocols are used on the Internet, HTTP is only one ProtocolApplication HTTP: Hypertext TransferRetrieve and view Web pages FTP: File Transfer Copy files from client to server or from server to client SMTP: Simple Mail Transport Send POP: Post OfficeRead

The TCP/IP protocol layers Application Transport Internet Data link Physical Get useful work done – retrieve Web pages, copy files, send and receive , etc. Make client-server connections and optionally control transmission speed, check for errors, etc. Route packets between networks Route data packets within the local area network Specify what medium connects two nodes, how binary ones and zeros are differentiated, etc, The application program is king – it gets work done using the lower level layers for communication between the client and server.

HTTP Operations GET: retrieves URL (most widely used) HEAD: retrieves only response header POST: posts data to server PUT: puts page on server DELETE: deletes page from server

Simple HTTP Request and Reply Request: GET HTTP/1.0 Response: HTTP-Version: HTTP/ OK Content-Length: 3012 Content-Type: text/html

HTTP 1.0 Client opens a separate TCP connection for each requested object Object is served and connection is closed Advantages –maximum concurrency Limitations –TCP connection setup/tear-down overhead –TCP slow start overhead

HTTP 1.0 Client Server ACK, GET Retrieve Data From Disk Retrieve Image From Disk SYN SYN, ACK connect() close() connect() ACK, GET image.gif SYN SYN, ACK close() write()

HTTP 1.1 Client Server ACK, GET Retrieve Data From Disk Retrieve Image From Disk SYN SYN, ACK connect() GET image.gif close() write()

Server Side Close() Client Server ACK, GET Retrieve Data From Disk Retrieve Image From Disk SYN SYN, ACK connect() GET image.gif write() Timeout! close() Set timeout Reset timeout

Dynamic Content Web pages can be created as requests arrive Advantages –Personalization (e.g., my.yahoo.com), –interaction with client input –interaction with back-end applications Disadvantages –Performance penalty Generating dynamic content (CGI, ASP, PHP, ColdFusion, JavaScript, Flash, …)

CGI Scripts CGI scripts are URLs with a.cgi extension The script is a program (e.g., C, JAVA, …) When the URL is requested, server invokes the named script, passing to it client info Script outputs HTML page to standard output (redirected to server) Server sends page to client

CGI Execution Server CGI Script fork() Send page Request Response

Active Server Pages (ASPs) Active server pages are HTML documents with extensions for embedded program execution When request arrives, server fetches and parses the HTML document Server executes embedded executable code and plugs output into page Expanded page is sent to client