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  1. Java Technology and Applications 240-527 CoE Masters Programme, PSUSemester 2, 2003-2004 • Objectives • to explain the Hypertext Transfer Protocol (HTTP) 7. HTTP

  2. Overview 1. How a Browser Works 2. HTTP Transactions 3. Client Request Methods 4. HTTP Protocol Versions 5. Server Response Codes 6. Some Advanced Features 7. More Information

  3. 1. How a Browser Works • Browsers use the HTTP protocol to communicate with Web servers • HTTP is a request/response protocol request network response Client browser Web server

  4. 1.1. Details of a Client Request • From a browser, I request: http://fivedots.coe.psu.ac.th/~ad/ • The browser connects to the site fivedots.coe.psu.ac.th at port 80, and sends the request: continued

  5. HTTP method/ command URL HTTP version used by client GET /~ad/ HTTP/1.1Host: fivedots.coe.psu.ac.thUser-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20010131 Netscape6/6.01Accept: */*Accept-Language: enAccept-Encoding: gzip,deflate,compress,identityKeep-Alive: 300Connection: keep-alive various header information; one per line

  6. HTTP version used by server Details of a Server Response status code and text HTTP/1.1 200 OKDate: Sun, 12 Oct 2003 04:20:51 GMTServer: Apache/1.3.9 (Unix) Debian/GNU PHP/4.0.3pl1X-Powered-By: PHP/4.0.3pl1Keep-Alive: timeout=15, max=100Connection: Keep-AliveTransfer-Encoding: chunkedContent-Type: text/html; charset=iso-8859-1<html><head><title>Andrew Davison's Home Page at PSU</title></head><body bgcolor=#ffffff test=#000000> : // rest of HTML text for page HTML for Page

  7. Part of my Home Page

  8. 1.2. Web Page Images • My home page contains several images. • The browser sees them in the text of the Web page: • e.g. <img src="me.jpg" align="right" alt="[PIC of Andrew]"> • The browser automatically requests each one.

  9. An Image Request the page where the link to the image is located GET /~ad/me.jpg HTTP/1.1Referer: http://fivedots.coe.psu.ac.th/~ad/Host: fivedots.coe.psu.ac.thUser-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20010131 Netscape6/6.01Accept: */*Accept-Language: enAccept-Encoding: gzip,deflate,compress,identityKeep-Alive: 300Connection: keep-alive

  10. The Image Response HTTP/1.1 200 OKDate: Sun, 12 Oct 2003 04:20:55 GMTServer: Apache/1.3.9 (Unix) Debian/GNU PHP/4.0.3pl1Last-Modified: Tue, 17 Oct 2000 09:40:05 GMTETag: "1bf29-1194-39ec1e75"Accept-Ranges: bytesContent-Length: 4500Keep-Alive: timeout=15, max=99Connection: Keep-AliveContent-Type: image/jpeg; charset=iso-8859-1// ... data of the JPEG file

  11. 1.3. Clicking on a Link • In the browser, if I click on the link labelled 'AIT', then the browser examines the associated HTML: • <a href="http://www.cs.ait.ac.th/">AIT</a> • The browser then connects to www.cs.ait.ac.th at port 80, and requests the top page: continued

  12. sent to www.cs.ait.ac.th GET / HTTP/1.1Referer: http://fivedots.coe.psu.ac.th/~ad/Host: www.cs.ait.ac.thUser-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20010131 Netscape6/6.01Accept: */*Accept-Language: enAccept-Encoding: gzip,deflate,compress,identityKeep-Alive: 300Connection: keep-alive

  13. Server Response This server uses HTTP 1.0 HTTP/1.0 200 OKDate: Sun, 12 Oct 2003 06:08:24 GMTServer: Apache/1.3.12 Ben-SSL/1.41 PHP/4.0.1pl2Last-Modified: Fri, 11 Apr 2003 02:48:54 GMTETag: "214d69-543b-3ad3c616"Accept-Ranges: bytesContent-Length: 21563Content-Type: text/htmlAge: 120X-Cache: MISS from cache3.psu.ac.thConnection: keep-alive<HTML><HEAD> // ... rest of Web page text

  14. The New Page

  15. 1.4. Getting a Page with Telnet In CoE/PSU, the request needs to be 'local'. ad@calvin$ telnet fivedots.coe.psu.ac.th 80Trying 172.30.0.5...Connected to fivedots.coe.psu.ac.th.Escape character is '^]'.GET ~ad/index.html HTTP/1.0HTTP/1.0 200 OKDate: Wed, 22 Oct 2003 05:07:26 GMTServer: Apache/1.3.12 Ben-SSL/1.41 PHP/4.0.1pl2Last-Modified: Wed, 11 Jun 2003 02:48:54 GMTETag: "214d69-543b-3ad3c616"Accept-Ranges: bytes // ... rest of headers and HTML text of page two newlines required response

  16. 1.5. HTTP and Web Forms

  17. The Form HTML Code • <form method="post" action= "http://fivedots.coe.psu.ac.th/cgi-bin/ad/echoer"> <input TYPE="text" NAME="pat1" SIZE="15" MAXLENGTH="15" VALUE=""> <input TYPE="text" NAME="pat2" SIZE="15" MAXLENGTH="15" VALUE=""> <input TYPE="text" NAME="pat3" SIZE="15" MAXLENGTH="15" VALUE=""> <input TYPE="text" NAME="pat4" SIZE="15" MAXLENGTH="15" VALUE=""> <input TYPE="text" NAME="pat5" SIZE="15" MAXLENGTH="15" VALUE=""></p> <br> <p><input TYPE="submit" VALUE="Submit"> <input TYPE="reset" VALUE="Clear"> </form>

  18. Form Input and Output

  19. Form Input Request The HTTP Post method POST /cgi-bin/ad/echoer HTTP/1.1Referer: http://fivedots.coe.psu.ac.th/~ad/echoer/ eform.htmlHost: fivedots.coe.psu.ac.thUser-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20010131 Netscape6/6.01Accept: */*Accept-Language: enAccept-Encoding: gzip,deflate,compress,identityKeep-Alive: 300Connection: keep-aliveContent-type: application/x-www-form-urlencodedContent-Length: 39pat1=hello&pat2=&pat3=world&pat4=&pat5=

  20. Server Response HTTP/1.1 200 OKDate: Sun, 12 Oct 2003 08:30:07 GMTServer: Apache/1.3.9 Debian/GNU PHP/4.0.3pl1Keep-Alive: timeout=15, max=100Connection: Keep-AliveTransfer-Encoding: chunkedContent-Type: text/html; charset=iso-8859-1<html><head><title>Query Result</title></head><body background="http://fivedots.coe.psu.ac.th/~ad/chalk.jpg"><H1 align=center>Query Result</H1> // ... rest of page

  21. 1.6 Proxies • Most clients and servers do not communicate directly • the client must send its request via a proxy • the proxy acts as a firewall and/or cache • At PSU, most Web requests must go through the cache.psu.ac.th proxy • this is set up in the browser's preferences continued

  22. In other applications, it may be necessary to explicitly communicate with the proxy • this is done by connecting to the proxy, and sending it the full URL of the page required

  23. Students should be able to do this. Using a Proxy with Telnet ad@fivedots$ telnet cache.psu.ac.th 8080Trying 192.168.98.6... Connected to proxy6.psu.ac.th.Escape character is '^]'.GET http://www.student.math.uwaterloo.ca/~cs488/ HTTP/1.0HTTP/1.0 200 OKDate: Thu, 21 Nov 2002 06:01:31 GMTServer: Apache/1.3.27 (Unix) mod_perl/1.21Last-Modified: Wed, 20 Nov 2002 12:00:21 GMTETag: "1b66a-2234-3ddb7955" : response

  24. :Accept-Ranges: bytesContent-Length: 8756Content-Type: text/htmlAge: 3263X-Cache: HIT from cache.psu.ac.thProxy-Connection: close<html> // ... rest of Web page text</html>Connection closed by foreign host.ad@fivedots$

  25. 2. HTTP Transactions Method URL VersionGeneral headerRequest headerEntity headerEntity body request network response Client browser Web server Version Status ReasonGeneral headerResponse headerEntity headerEntity body

  26. Client Request Example Method URL Version POST /cgi-bin/ad/echoer HTTP/1.1Referer: http://fivedots...User-Agent: Mozilla/5.0 ...Accept: */*Accept-Language: enAccept-Encoding: gzip,...Keep-Alive: 300Connection: keep-aliveContent-type: application/x-www-form-urlencodedContent-Length: 39pat1=hello&pat2=&pat3=world&pat4=&pat5= Request headers General headers Entity headers Entity body

  27. Request Components • HTTP methods: • GET, POST, HEAD, PUT, DELETE • OPTIONS and TRACE (HTTP 1.1.) • other non-standardized methods • General headers • optional general information such as the current date/time, or network characteristics continued

  28. Request headers • information about the client, used by the server • e.g. browser info., document formats that the client can understand • Entity headers • used when an entity (a Web document) is about to be sent • e.g. encoding scheme, length, type, origin continued

  29. Headers may be sent in any order. • Header names are case-insensitive • e.g. Content-Type == Content-type

  30. Server Response Example Version Status Reason HTTP/1.1 200 OKDate: Tue,...Keep-Alive: timeout=15, max=100Connection: Keep-AliveTransfer-Encoding: chunkedServer: Apache...Content-Type: text/html;...<html> // ... rest of page General headers Response headers Entity headers Entity body

  31. Server Components • The general and entity headers are the same as those used in a client request. • Response header • gives the client information about the server configuration • e.g. what HTTP methods are supported, request authorization details, or server time-out report

  32. Some Other headers • General Headers • Cache-Control caching behaviour • Connection should connection close after this transaction • MIME-Version message encoding • Pragma directives for proxies • Via info about processing by gateways and proxies between the client and server continued

  33. Request Headers • Authorization to request restricted docs. • Cookie send name=value info • Host required address & port info • If-Modified-Since get doc. if newer • If-Match get doc. if matches etags • If-Range get part of a doc. if changed • Max-Forwards limits no. of proxies/gateways • Proxy-Authorization for proxy • Range only get part of a doc continued

  34. Response Headers • Accept-Ranges will accept range requests • Age age of doc in seconds • Proxy-Authenticate gives auth. scheme • Public supported methods • Retry-After try again after given time • Set-Cookie sends a name=value pair • Warning info used for caching • WWW-Authentication gives auth scheme for access to Web pages continued

  35. Entity Headers • Allow methods allowed on URL • Content-Location useful if a doc is storedin several locations • Content-Range range of partial doc sent • ETag entity tag for the doc • Expires when content may change • Last-Modified when doc last changed

  36. 3. Client Request Methods • GET • retrieve the specified document • POST • for sending (form) information • HEAD • get information about the document, but not the actual document • PUT • store the specified document on the server continued

  37. DELETE • delete the specified document on the server • TRACE • asks that proxies/gateways add information to the headers of the request, which is sent back in the response • OPTIONS • ask the server to send info about the HTTP methods it supports

  38. 3.1. The GET Method • The main purpose of GET is to request a document from a server • see earlier examples in section 1 • But the response can be generated in various ways: • a file on the Web server • the output of a CGI script • the script may examine server-side hardware, files, or do some special calculations

  39. CGI Diagram the Web/Internet request becomesinput request response CGI script Client browser Web server output becomes response

  40. A CGI Request • Data for a CGI script is passed as extra name=value arguments added to the URL: GET /cgi-bin/create.pl?user=util-tester& pass=1234 HTTP/1.0Referer: ...User-Agent: ... : • The arguments are URL-encoded. two arguments

  41. URL Encoding • name=value pairs are combined into a single string separated by &'s. • This is added to the end of the URL after a ? • Certain special characters are converted to hexadecimal preceded by a %. • e.g. '#' becomes %23, '/' becomes %2F

  42. 3.2. The POST Method • The main purpose of the POST method is to send form information to a server • see the example in section 1.5 • Most servers use CGI programs to process form requests. • The text in the form name=value data is URL encoded.

  43. Forms can use GET • The <form> tag in HTML can also be used to send data in the GET format: <form method="get" action="http://fivedots.coe.psu.ac.th/ cgi-bin/create.pl"> <input name="user"> <input name="pass" type="password"> <input type="submit" value="Submit"></form>

  44. Which Method to Use? • The GET method adds form input to the end of the URL, and there is often a maximum length limit • e.g. the URL string must be 255 chars or less • For large input, the POST method is better since there is no limit on the size of the entity body in the request.

  45. 3.3. The HEAD Method • The HEAD method returns information about a document: • this includes its modification time, its size, its type, and details about its server • this information is useful in guiding/speeding up search engines and browsers

  46. HEAD using Telnet ad@calvin$ telnet fivedots.coe.psu.ac.th 80Connected to fivedots.coe.psu.ac.th.HEAD /~ad/index.html HTTP/1.0HTTP/1.0 200 OKDate: Sun, 12 Oct 2003 06:42:48 GMTServer: Apache/1.3.12 Ben-SSL/1.41 PHP/4.0.1pl2Last-Modified: Tue, 29 Jul 2003 11:11:51 GMTETag: "1f1f6e-522-3982bbf7"Accept-Ranges: bytesContent-Length: 1314Content-Type: text/htmlAge: 157Connection: closeConnection closed by foreign host.ad@calvin$ response

  47. 3.4. The PUT Method • The PUT method is used for uploading files to a server • PUT URL HTTP-version • used in HTML editors such as FrontPage • Usually involves an authorization phase when the server asks for a user name and password before accepting the PUT • this is processed by FrontPage using details entered by the user

  48. 3.5. The DELETE Method • The DELETE method deletes the specified file: • DELETE URL HTTP-version • The server will usually ask for authorization information before carrying out the request.

  49. 3.6. The TRACE Method • The TRACE method allows a programmer to see how the client's request is passed through proxies/gateways to the server • TRACE URL HTTP-version • The server echoes the request back together with a Via header (and other optional headers).

  50. TRACE using Telnet • ad@calvin$ telnet cache.psu.ac.th 8080Trying 192.16898.6...Connected to proxy6.psu.ac.th.Escape character is '^]'.TRACE http://www.cs.ait.ac.th HTTP/1.0HTTP/1.0 200 OKDate: Wec, 22 Oct 2003 07:11:20 GMTServer: Stronghold/2.4.2 Apache/1.3.6 C2NetEU/2412 (Unix)Content-Type: message/httpAge: 118X-Cache: MISS from cache.psu.ac.thProxy-Connection: closeTRACE / HTTP/1.0 : response