World Wide WebGoals of Today’s LectureWeb HistoryEnablers for Success of the WebMain Components: URLMain Components: HTMLMain Components: HTTPExample: HyperText Transfer ProtocolHTTP: Request-Response ProtocolHTTP Request MessageExample: Conditional GET RequestHTTP Response MessageRequest Methods and Response CodesHTTP Resource Meta-DataStateless ProtocolCookiesCookies ExamplesWeb ComponentsWeb BrowserTypical Web TransactionTypical Web Transaction (Continued)Web ServerWeb Server: Generating a ResponseHosting: Multiple Sites Per MachineHosting: Multiple Machines Per SiteSlide 26Slide 27Caching vs. ReplicationCaching vs. Replication (Continued)TCP Interaction: Multiple TransfersTCP Interaction: Short TransfersSlide 32TCP Interaction: Persistent ConnectionsConclusions1World Wide WebCOS 461: Computer NetworksSpring 2006 (MW 1:30-2:50 in Friend 109)Jennifer RexfordTeaching Assistant: Mike Wawrzoniak http://www.cs.princeton.edu/courses/archive/spring06/cos461/2Goals of Today’s Lecture•Main ingredients of the Web–URL, HTML, and HTTP•Key properties of HTTP–Request-response, stateless, and resource meta-data•Web components–Clients, proxies, and servers–Caching vs. replication•Interaction with underlying network protocols–DNS and TCP–TCP performance for short transfers–Parallel connections, persistent connections, pipelining3Web History•Before the 1970s-1980s–Internet used mainly by researchers and academics–Log in remote machines, transfer files, exchange e-mail•Late 1980s and early 1990s–Initial proposal for the Web by Berners-Lee in 1989–Competing systems for searching/accessing documentsGopher, Archie, WAIS (Wide Area Information Servers), …All eventually subsumed by the World Wide Web•Growth of the Web in the 1990s–1991: first Web browser and server–1993: first version of Mosaic browser4Enablers for Success of the Web•Internet growth and commercialization–1988: ARPANET gradually replaced by the NSFNET–Early 1990s: NSFNET begins to allow commercial traffic•Personal computer–1980s: Home computers with graphical user interfaces–1990s: Power of PCs increases, and cost decreases•Hypertext–1945: Vannevar Bush’s “As We May Think”–1960s: Hypertext proposed, and the mouse invented–1980s: Proposals for global hypertext publishing systems5Main Components: URL•Uniform Resource Identifier (URI)–Denotes a resource independent of its location or value–A pointer to a “black box” that accepts request methods•Formatted string–Protocol for communicating with server (e.g., http)–Name of the server (e.g., www.foo.com)–Name of the resource (e.g., coolpic.gif)•Name (URN), Locator (URL), and Identifier (URI)–URN: globally unique name, like an ISBN # for a book–URI: identifier representing the contents of the book–URL: location of the book6Main Components: HTML•HyperText Markup Language (HTML)–Representation of hyptertext documents in ASCII format–Format text, reference images, embed hyperlinks–Interpreted by Web browsers when rendering a page•Straight-forward and easy to learn–Simplest HTML document is a plain text fileEasy to add formatting, references, bullets, etc.–Automatically generated by authoring programsTools to aid users in creating HTML files•Web page–Base HTML file referenced objects (e.g., images)–Each object has its own URL7Main Components: HTTP•HyperText Transfer Protocol (HTTP)–Client-server protocol for transferring resources–Client sends request and server sends response•Important properties of HTTP–Request-response protocol–Reliance on a global URI–Resource metadata–Statelessness–ASCII formattelnet www.cs.princeton.edu 80GET /~jrex/ HTTP/1.1Host: www.cs.princeton.edu8Example: HyperText Transfer ProtocolGET /courses/archive/spring06/cos461/ HTTP/1.1Host: www.cs.princeton.eduUser-Agent: Mozilla/4.03<CRLF>HTTP/1.1 200 OKDate: Mon, 6 Feb 2006 13:09:03 GMTServer: Netscape-Enterprise/3.5.1Last-Modified: Mon, 6 Feb 2006 11:12:23 GMTContent-Length: 21<CRLF>Site under constructionRequestResponse9HTTP: Request-Response Protocol•Client program–Running on end host–Requests service–E.g., Web browser•Server program–Running on end host–Provides service–E.g., Web serverGET /index.html“Site under construction”10HTTP Request Message•Request message sent by a client–Request line: method, resource, and protocol version–Request headers: provide information or modify request–Body: optional data (e.g., to “POST” data to the server)GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr (extra carriage return, line feed) request line(GET, POST, HEAD commands)header linesCarriage return, line feed indicates end of message11Example: Conditional GET Request•Fetch resource only if it has changed at the server•Server avoids wasting resources to send again–Server inspects the “last modified” time of the resource–… and compares to the “if-modified-since” time–Returns “304 Not Modified” if resource has not changed–…. or a “200 OK” with the latest version otherwiseGET /courses/archive/spring06/cos461/ HTTP/1.1Host: www.cs.princeton.eduUser-Agent: Mozilla/4.03If-Modified-Since: Mon, 6 Feb 2006 11:12:23 GMT<CRLF>12HTTP Response Message•Response message sent by a server–Status line: protocol version, status code, status phrase–Response headers: provide information–Body: optional dataHTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... status line(protocolstatus codestatus phrase)header linesdata, e.g., requestedHTML file13Request Methods and Response Codes•Request methods include–GET: return current value of resource, run program, …–HEAD: return the meta-data associated with a resource–POST: update a resource, provide input to a program, …•Response code classes–1xx: informational (e.g., “100 Continue”)–2xx: success (e.g., “200 OK”)–3xx: redirection (e.g., “304 Not Modified”)–4xx: client error (e.g., “404 Not Found”)–5xx: server error (e.g., “503 Service Unavailable”)•Note similarities to File Transfer Protocol (FTP)14HTTP Resource Meta-Data•Meta-data–Information relating to a resource–… but not part of the resource itself•Example
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