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Princeton COS 461 - Web Content Delivery

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Web Content Delivery Reading: Section 9.1.2 and 9.4.3Outline: Web Content DistributionWeb HistoryMain ingredients of the WebMain ingredients of the Web: HTMLSlide 6Main ingredients of the Web: HTTPSlide 8HTTP Example: Request and Response MessageHTTP Request MessageHTTP Response MessageHTTP: Request Methods and Response CodesHTTP is a Stateless ProtocolSlide 14Web Systems ComponentsWeb BrowserTypical Web TransactionTypical Web Transaction (Continued)Slide 19Web ServerWeb Server: Generating a ResponseHosting: Multiple Sites Per MachineHosting: Multiple Machines Per SiteSlide 24DNS Query in Web DownloadMultiple DNS QueriesWhen are DNS Queries Unnecessary?Directing Web Clients to ReplicasClever Load Balancing SchemesSlide 30TCP Interaction: Multiple TransfersTCP Interaction: Short TransfersSlide 33TCP Interaction: Persistent ConnectionsWeb Content DeliveryScalability LimitationSlide 37Server Farms (motivated for scalability)Server FarmsSlide 40Web ProxiesWeb Proxies are IntermediariesProxy CachingGetting Requests to the ProxyOther Functions of Web ProxiesSlide 46Motivation for CDNContent Delivery NetworksCDN ArchitectureSlide 50Summary: Web Content DistributionAssignment 0: Socket programmingAssignment1: HTTP ProxyHTTP Proxy: How to get started?HTTP Proxy: tips1Web Content Delivery Reading: Section 9.1.2 and 9.4.3COS 461: Computer NetworksSpring 2008 (MW 1:30-2:50 in CS105)Yaping ZhuInstructor: Jennifer RexfordTeaching Assistants: Sunghwan Ihm and Yaping Zhuhttp://www.cs.princeton.edu/courses/archive/spring08/cos461/2Outline: Web Content Distribution•Main ingredients of the Web–URL, HTML, and HTTP–HTTP: the protocol and its stateless property•Web Systems Components–Clients–Servers–DNS (Domain Name System)•Interaction with underlying network protocol: TCP•Scalability and performance enhancement–Server farms–Web Proxy–Content Distribution Network (CDN)3Web History•Before the 1970s-1980s–Internet used mainly by researchers and academics–Log in remote machines, transfer files, exchange e-mail•Internet growth and commercialization–1988: ARPANET gradually replaced by the NSFNET–Early 1990s: NSFNET begins to allow commercial traffic•Initial proposal for the Web by Berners-Lee in 1989•Enablers for the success of the Web–1980s: Home computers with graphical user interfaces–1990s: Power of PCs increases, and cost decreases4Main ingredients of the Web•URL–Denotes the global unique location of the web resource–Formatted string e.g., http://www.princeton.edu/index.html Protocol for communicating with server (e.g., http)Name of the server (e.g., www.princeton.edu)Name of the resource (e.g., index.html)•HTML–Actual content of web resource, represented in ASCII5Main ingredients of the Web: HTML•HyperText Markup Language (HTML)–Format text, reference images, embed hyperlinks–Representation of hypertext documents in ASCII format–Interpreted by Web browsers when rendering a page•Web page–Base HTML file–referenced objects (e.g., images), Each object has its own URL •Straight-forward and easy to learn–Simplest HTML document is a plain text file–Automatically generated by authoring programs6Main ingredients of the Web•URL–Denotes the global unique location of the web resource–Formatted string e.g., http://www.princeton.edu/index.html Protocol for communicating with server (e.g., http)Name of the server (e.g., www.princeton.edu)Name of the resource (e.g., index.html)•HTML–Actual content of web resource, represented in ASCII•HTTP–Protocol for client/server communication7Main ingredients of the Web: HTTP•Client program–E.g., Web browser–Running on end host–Requests service•Server program–E.g., Web server–Provides serviceGET /index.html“Site under construction”8Outline: Web Content Distribution•Main ingredients of the Web–URL, HTML, and HTTP–HTTP: the protocol and its stateless property•Web Systems Components–Clients–Servers–DNS (Domain Name System)•Interaction with underlying network protocol: TCP•Scalability and performance enhancement–Server farms–Web Proxy–Content Distribution Network (CDN)9HTTP Example: Request and Response MessageGET /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 constructionRequestResponse10HTTP Request Message•Request message sent by a client–Request line: method, resource, and protocol version–Request headers: provide information or 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 message11HTTP 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 file12HTTP:Request Methods and Response Codes•Request methods include–GET: return current value of resource, …–HEAD: return the meta-data associated with a resource–POST: update a resource, provide input to a program, …–Etc.•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”)13HTTP is a Stateless Protocol•Stateless–Each request-response exchange treated independently–Clients and servers not required to retain state•Statelessness to improve scalability–Avoids need for the server to retain info across requests–Enables the server to handle a higher rate of requests14Outline: Web Content Distribution•Main ingredients of the Web–URL, HTML, and HTTP–HTTP: the protocol and its stateless property•Web Systems Components–Clients–Servers–DNS (Domain Name System)•Interaction with underlying


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Princeton COS 461 - Web Content Delivery

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