15 213 The course that gives CMU its Zip Web Services April 15 2008 Topics class23 ppt HTTP Serving static content Serving dynamic content Web History Consider a future device for individual use which is a sort of mechanized private file and library It needs a name and to coin one at random memex will do A memex is 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 It is an enlarged intimate supplement to his memory 1945 Vannevar Bush As we may think Atlantic Monthly July 1945 Describes the idea of a distributed hypertext system A memex that mimics the web of trails in our minds 2 15 213 S 08 Web History 1989 Tim Berners Lee CERN writes internal proposal to develop a distributed hypertext system Connects a web of notes with links Intended to help CERN physicists in large projects share and manage information 1990 3 Tim BL writes a graphical browser for Next machines 15 213 S 08 Web History cont 1992 NCSA server released 26 WWW servers worldwide 1993 Marc Andreessen releases first version of NCSA Mosaic browser Mosaic version released for Windows Mac Unix Web port 80 traffic at 1 of NSFNET backbone traffic Over 200 WWW servers worldwide 1994 4 Andreessen and colleagues leave NCSA to form Mosaic Communications Corp predecessor to Netscape 15 213 S 08 Internet Hosts 5 How many of the 232 IP addresses have registered names 15 213 S 08 Web Servers Clients and servers communicate using the HyperText Transfer Protocol HTTP Client and server establish TCP connection Client requests content Server responds with requested content Client and server close connection usually Web client browser HTTP request Web server HTTP response content Current version is HTTP 1 1 RFC 2616 June 1999 http www w3 org Protocols rfc2616 rfc2616 html 6 15 213 S 08 Web Content Web servers return content to clients content a sequence of bytes with an associated MIME Multipurpose Internet Mail Extensions type Example MIME types text html HTML document text plain Unformatted text application postscript Postcript document image gif Binary image encoded in GIF format image jpeg Binary image encoded in JPEG format 7 15 213 S 08 Static and Dynamic Content The content returned in HTTP responses can be either static or dynamic Static content content stored in files and retrieved in response to an HTTP request Examples HTML files images audio clips Dynamic content content produced on the fly in response to an HTTP request Example content produced by a program executed by the server on behalf of the client Bottom line All Web content is associated with a file that is managed by the server 8 15 213 S 08 URLs Each file managed by a server has a unique name called a URL Universal Resource Locator URLs for static content http www cs cmu edu 80 index html http www cs cmu edu index html http www cs cmu edu Identifies a file called index html managed by a Web server at www cs cmu edu that is listening on port 80 URLs for dynamic content http www cs cmu edu 8000 cgi bin adder 15000 213 Identifies an executable file called adder managed by a Web server at www cs cmu edu that is listening on port 8000 that should be called with two argument strings 15000 and 213 9 15 213 S 08 How Clients and Servers Use URLs Example URL http www aol com 80 index html Clients use prefix http www aol com 80 to infer What kind of server to contact Web server Where the server is www aol com What port it is listening on 80 Servers use suffix index html to Determine if request is for static or dynamic content No hard and fast rules for this Convention executables reside in cgi bin directory Find file on file system Initial in suffix denotes home directory for requested 10 content Minimal suffix is which all servers expand to some default home page e g index html 15 213 S 08 Anatomy of an HTTP Transaction unix telnet www aol com 80 Trying 205 188 146 23 Connected to aol com Escape character is GET HTTP 1 1 host www aol com Client open connection to server Telnet prints 3 lines to the terminal Client request line Client required HTTP 1 1 HOST header Client empty line terminates headers HTTP 1 0 200 OK Server response line MIME Version 1 0 Server followed by five response headers Date Mon 08 Jan 2001 04 59 42 GMT Server NaviServer 2 0 AOLserver 2 3 3 Content Type text html Server expect HTML in the response body Content Length 42092 Server expect 42 092 bytes in the resp body Server empty line r n terminates hdrs html Server first HTML line in response body Server 766 lines of HTML not shown html Server last HTML line in response body Connection closed by foreign host Server closes connection unix Client closes connection and terminates 11 15 213 S 08 HTTP Requests HTTP request is a request line followed by zero or more request headers Request line method uri version version is HTTP version of request HTTP 1 0 or HTTP 1 1 uri is typically URL for proxies URL suffix for servers A URL is a type of URI Uniform Resource Identifier See http www ietf org rfc rfc2396 txt 12 method is either GET POST OPTIONS HEAD PUT DELETE or TRACE 15 213 S 08 HTTP Requests cont HTTP methods GET Retrieve static or dynamic content Arguments for dynamic content are in URI Workhorse method 99 of requests POST Retrieve dynamic content Arguments for dynamic content are in the request body OPTIONS Get server or file attributes HEAD Like GET but no data in response body PUT Write a file to the server DELETE Delete a file on the server TRACE Echo request in response body Useful for debugging 13 15 213 S 08 HTTP Requests cont Request headers header name header data Provide additional information to the server Major differences between HTTP 1 1 and HTTP 1 0 HTTP 1 0 uses a new connection for each transaction HTTP 1 1 also supports persistent connections multiple transactions over the same connection Connection Keep Alive HTTP 1 1 requires HOST header Host kittyhawk cmcl cs cmu edu HTTP 1 1 supports chunked encoding described later Transfer Encoding chunked 14 HTTP 1 1 adds additional support for caching 15 213 S 08 HTTP Responses HTTP response is a response line followed by zero or more response headers Response line version status code status msg version is HTTP version of the response status code is numeric status status msg is corresponding English text 200 403 404 OK Request was handled without error Forbidden Server lacks permission to access file Not found Server
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