Goals of Today s Lecture Main ingredients of the Web EE 122 The World Wide Web Ion Stoica TAs Junda Liu DK Moon David Zats http inst eecs berkeley edu ee122 Request response stateless and resource meta data Performance of HTTP Materials with thanks to Vern Paxson Jennifer Rexford and colleagues at UC Berkeley URIs HTML HTTP Key properties of HTTP Parallel connections persistent connections pipelining Web components Clients proxies and servers Caching vs replication 1 The Web History I Vannevar Bush 1890 1974 2 The Web History II 1945 Vannevar Bush Memex a device in which an individual stores all his books records and communications and which is mechanized so that it may be consulted with exceeding speed and flexibility 1967 Ted Nelson Xanadu Ted Nelson A world wide publishing network that would allow information to be stored not as separate files but as connected literature Owners of documents would be automatically paid via electronic means for the virtual copying of their documents Coined the term Hypertext See http www iath virginia edu elab hfl0051 html 3 4 Memex The Web History III World Wide Web WWW a distributed database of pages linked through Hypertext Transport Protocol HTTP Simple GET command for the Web Send requests Receive responses Servers Tim Berners Lee at CERN Objects Clients Receive requests Send responses Store or generate the responses Proxies Placed between clients and servers Provide extra functions HTTP 1 0 1992 HTTP 0 9 1991 Content First HTTP implementation 1990 Tim Berners Lee Web Components Client Server information simple caching HTTP 1 1 1996 5 Act as a server for the client and a client to the server Caching anonymization logging transcoding filtering access Explicit or transparent interception 6 1 HTML Base HTML file Referenced objects e g images Uniform Resource Locator URL Provides a means to get the resource http www ietf org rfc rfc3986 txt HyperText Markup Language HTML Representation of hypertext documents in ASCII format Web browsers interpret HTML when rendering a page Several functions Content How Uniform Resource Identifier URI A Web page has several components URI Content How Format text reference images embed hyperlinks HREF Straight forward to learn Syntax easy to understand Authoring programs can auto generate HTML Source almost always available Uniform Resource Name URN Names a resource independent of how to get it urn ietf rfc 3986 is a standard URN for RFC 3986 7 URL Syntax Content How 8 HTTP Client Server How protocol hostname port directorypath resource e g http inst eecs berkeley edu ee122 fa08 index html protocol http ftp https smtp rtsp etc hostname Fully Qualified Domain Name FQDN IP address Defaults to protocol s standard port port HyperText Transfer Protocol HTTP Client server protocol for transferring resources Important properties e g http 80 tcp https 443 tcp directory path Hierarchical often reflecting file system resource Identifies the desired resource Can also extend to program executions Request response protocol Resource metadata Stateless ASCII format telnet www cs berkeley edu 80 http us f413 mail yahoo com ym ShowLetter box 40B 40Bulk MsgId 2604 1744106 29699 1123 1261 0 28917 9 3552 1289957100 Search Nhead f YY 31454 order do wn sort date pos 0 view a head b GET istoica HTTP 1 0 blank line i e CRLF Client to Server Communication HTTP Big Picture Server Client Request image 10 1 age 1 Transfer im Request image 2 HTTP Request Message 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 request line age 2 Transfer im Request text Transfer text Finish display page GET somedir page html HTTP 1 1 Host www someschool edu header User agent Mozilla 4 0 lines Connection close Accept language fr blank line 11 carriage return line feed indicates end of message 12 2 Client to Server Communication HTTP Request Message Server to Client Communication 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 Request methods include HTTP Response Message Status line protocol version status code status phrase Response headers provide information Body optional data status line protocol status code status phrase GET Return current value of resource run program HEAD Return the meta data associated with a resource POST Update resource provide input to a program header lines Headers include blank line Useful info for the server e g desired language 13 Server to Client Communication data Status line protocol version status code status phrase Response headers provide information Body optional data Similar to other ASCII app protocols like SMTP Example 1xx Informational 100 Continue 2xx Success 200 OK 3xx Redirection 304 Not Modified 4xx Client error 404 Not Found 5xx Server error 503 Service Unavailable Info about a resource A separate entity Size of a resource Last modification time Type of the content Each request response exchange treated independently Servers not required to retain state This is good e g Content Type text html Usage example Conditional GET Request Stateless protocol Enables browser to automatically launch an appropriate viewer Borrowed from e mail s Multipurpose Internet Mail Extensions MIME Client requests object If modified since If object hasn t changed server returns HTTP 1 1 304 Not Modified No body in the server s response only a header Improves scalability on the server side Don t have to retain info across requests Can handle higher rate of requests Order of requests doesn t matter This is bad 17 Client Server How Data format classification 16 Examples Return meta data with no body HTTP is Stateless Meta data URL triggers a program on the server Server runs program and sends output to client 15 HTTP Resource Meta Data URL matches a file e g www index html Server returns file as the response Server generates appropriate response header Generate response dynamically Class 14 Return a file Response code classes Code data data data data data e g requested HTML file Web Server Generating a Response HTTP Response Message HTTP 1 1 200 OK Connection close Date Thu 06 Aug 2006 12 00 15 GMT Server Apache 1 3 0 Unix Last Modified Mon 22 Jun 2006 Content Length 6821 Content Type text html Some applications need persistent state Need to uniquely identify user or store temporary info e g Shopping cart user preferences and
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