CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 1 Lecture 5. WWW, HTTP and URL Objective: to review the concepts of WWW to understand how HTTP works URL World Wide Web (the Web) A collection of documents of information –distributed over the internet, and connected by links –Based on TCP/IP, using HTTP –Web server software and Web browsers –It grows to involve many existing applications, such , file transfer Tim Berners-Lee at CERN proposed the World Wide Web application HyperText Transfer Protocol (HTTP) is the core of the Web, which was defined in HTTP1.0 (RFC:1945) and currently use HTTP1.1 RFC:2616

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 2 The documents used in the Web, can contains links, called hypertext. The ideas of hypertext was also proposed and used by other researchers. HTML (Hypertext Markup Language) is a coding language create hypertext. HTTP is based on TCP/IP, and implemented in two programs: a client program (e.g., Web browser) executing on a client machine and a sever program (Web server) executing on a server machine, they talk each other by exchanging HTTP massages A Web document (or Web page) is an object such as an HTML file, a JPEG image, GIF image, Java applet, etc. A Web page is stored in a Web server machine, and has an location, and is accessible by URL

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 3 Uniform Resource Locator (URL) is the global address of documents and other resources on the World Wide Web. –The first part of the address indicates what protocol to use. –The second part specifies the IP address or the domain name of the server host. –Third part is the path of the object in the server directory. –Example: A browser is user agent for the Web, it implements client side HTTP, and displays the requested Web page and provides many navigation and configuration functions. A Web server houses Web objects, each addressable by URL, Web server implements server side HTTP.

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 4 HTTP functions like the FTP and SMTP –Like FTP, it does file transfer from server to client. But it uses only one connection, i.e., it uses the services of TCP on port 80. –Like SMTP, a client sends a request which looks like a mail. The server responds like send a mail to the client –The server is stateless, i.e., it does not keep records of previous requests. HTTP1.0 is nonpersistent, i.e., the connection is closed after a request is done HTTP/1.1 is persistent, i.e., the connection will keep alive for a short period time, another request coming within the time can use the same connection. PC running a browser TCP Open ? OK HTTP GET … Data TCP Close Server running a Server program Initiate TCP connection Request and response

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 5 A request massage –Request line: request type + URL+HTTP version Request types –HTTP/1.0 GET, POST, HEAD asks server to leave requested object out of response –HTTP/1.1 GET, POST, HEAD, PUT: uploads file in entity body to path specified in URL field DELETE: deletes file specified in the URL field URL scheme://host:port/path –scheme : http, ftp, telnet, file, mailto, news. Request line General header Request header Entity header Blank line Body

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 6 –General header cache control, connection, date, MIME- version, upgrade –Request header: accept, accept-charset, accept-encoding, accept-language, authorization, from, host, referrer, user-agent –Entity header: allow, content-encoding, content-language, content-range, content- type, expires, last-modified, location –Example GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 7 Response massage : –Status line: HTTP version+status code+status phase Status code3 100 = continues, 101=switching, 200=OK, 201=created, 202=accepted, 204=no content 301=multiple choices, 302=moved permanently 304=moved temporarily, 400=bad request, 401=unauthorized, 403=forbidden, 404=not found 405=method not allowed, 406=not acceptable 500=internal service error, 501=not implemented 503=service unavailable General header –Response header: accept-range, age, public, retry-after, server Status line General header Response header Entity header Blank line Body

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 8 –Example HTTP/ OK Date: Mon, 27 Jun :22:47 GMT Server: Apache/ (Unix) (Red-Hat/Linux) Last-modified: Wed, 26 Jun :12:29 GMT Etag: "841fb-4b-3d1a0179" Accept-ranges: bytes Content-length: 75 Connection: close Content-type: text/html data data data data data... Proxy server: a computer that keeps copies of responses to recent requests. In the presence of a proxy server, the HTTP client sends a request to the proxy server. The proxy server sends the Request to the corresponding server. Incoming responses are sent to the proxy server, and stored for future requests from the other clients.

CP476 Internet Computing Lecture 5 : HTTP, WWW and URL 9 Cookies –Stateless in server site. Cookies are used on client side to store access information to a page –Four components 1. cookie header line in the HTTP response message 2. cookie header line in HTTP request message 3. cookie file kept on user’s host and managed by user’s browser