Java Technology and Applications

Java Technology and Applications
CoE Masters Programme, PSU Semester 2, 7. HTTP Objectives to explain the Hypertext Transfer Protocol (HTTP)

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

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

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

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

Details of a Server Response
HTTP version used by server Details of a Server Response status code and text HTTP/ OK Date: Sun, 12 Oct :20:51 GMT Server: Apache/1.3.9 (Unix) Debian/GNU PHP/4.0.3pl1 X-Powered-By: PHP/4.0.3pl1 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html; charset=iso <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

Part of my Home Page

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.

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

The Image Response HTTP/ OK Date: Sun, 12 Oct :20:55 GMT Server: Apache/1.3.9 (Unix) Debian/GNU PHP/4.0.3pl1 Last-Modified: Tue, 17 Oct :40:05 GMT ETag: "1bf ec1e75" Accept-Ranges: bytes Content-Length: 4500 Keep-Alive: timeout=15, max=99 Connection: Keep-Alive Content-Type: image/jpeg; charset=iso // ... data of the JPEG file

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=" The browser then connects to at port 80, and requests the top page: continued

sent to GET / HTTP/1.1 Referer: Host: User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/ Netscape6/6.01 Accept: */* Accept-Language: en Accept-Encoding: gzip,deflate,compress,identity Keep-Alive: 300 Connection: keep-alive

Server Response This server uses HTTP 1.0
HTTP/ OK Date: Sun, 12 Oct :08:24 GMT Server: Apache/ Ben-SSL/1.41 PHP/4.0.1pl2 Last-Modified: Fri, 11 Apr :48:54 GMT ETag: "214d69-543b-3ad3c616" Accept-Ranges: bytes Content-Length: Content-Type: text/html Age: 120 X-Cache: MISS from cache3.psu.ac.th Connection: keep-alive <HTML> <HEAD> // ... rest of Web page text

The New Page

1.4. Getting a Page with Telnet
In CoE/PSU, the request needs to be 'local'. telnet fivedots.coe.psu.ac.th 80 Trying Connected to fivedots.coe.psu.ac.th. Escape character is '^]'. GET ~ad/index.html HTTP/1.0 HTTP/ OK Date: Wed, 22 Oct :07:26 GMT Server: Apache/ Ben-SSL/1.41 PHP/4.0.1pl2 Last-Modified: Wed, 11 Jun :48:54 GMT ETag: "214d69-543b-3ad3c616" Accept-Ranges: bytes // ... rest of headers and HTML text of page two newlines required response

1.5. HTTP and Web Forms

The Form HTML Code <form method="post" action= " <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>

Form Input and Output

Form Input Request The HTTP Post method
POST /cgi-bin/ad/echoer HTTP/1.1 Referer: eform.html Host: fivedots.coe.psu.ac.th User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/ Netscape6/6.01 Accept: */* Accept-Language: en Accept-Encoding: gzip,deflate,compress,identity Keep-Alive: 300 Connection: keep-alive Content-type: application/x-www-form-urlencoded Content-Length: 39 pat1=hello&pat2=&pat3=world&pat4=&pat5=

Server Response HTTP/ OK Date: Sun, 12 Oct :30:07 GMT Server: Apache/1.3.9 Debian/GNU PHP/4.0.3pl1 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html; charset=iso <html><head><title>Query Result</title></head> <body background=" align=center>Query Result</H1> // ... rest of page

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

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

Using a Proxy with Telnet
Students should be able to do this. Using a Proxy with Telnet telnet cache.psu.ac.th 8080 Trying Connected to proxy6.psu.ac.th. Escape character is '^]'. GET HTTP/1.0 HTTP/ OK Date: Thu, 21 Nov :01:31 GMT Server: Apache/ (Unix) mod_perl/1.21 Last-Modified: Wed, 20 Nov :00:21 GMT ETag: "1b66a ddb7955" : response

: Accept-Ranges: bytes Content-Length: 8756 Content-Type: text/html Age: 3263 X-Cache: HIT from cache.psu.ac.th Proxy-Connection: close <html> // ... rest of Web page text </html> Connection closed by foreign host.

2. HTTP Transactions request network response Client browser
Method URL Version General header Request header Entity header Entity body request network response Client browser Web server Version Status Reason General header Response header Entity header Entity body

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

Request Components HTTP methods: General headers
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

Request headers Entity 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

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

Server Response Example
Version Status Reason HTTP/ OK Date: Tue,... Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Transfer-Encoding: chunked Server: Apache... Content-Type: text/html;... <html> // ... rest of page General headers Response headers Entity headers Entity body

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

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

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

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

Entity Headers Allow methods allowed on URL
Content-Location useful if a doc is stored in 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

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

DELETE TRACE OPTIONS delete the specified document on the server
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

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

CGI Diagram the Web/Internet request request becomes input response
CGI script Client browser Web server output becomes response

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/1.0 Referer: ... User-Agent: : The arguments are URL-encoded. two arguments

URL Encoding This is added to the end of the URL after a ?
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

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.

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

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.

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

HEAD using Telnet response
telnet fivedots.coe.psu.ac.th 80 Connected to fivedots.coe.psu.ac.th. HEAD /~ad/index.html HTTP/1.0 HTTP/ OK Date: Sun, 12 Oct :42:48 GMT Server: Apache/ Ben-SSL/1.41 PHP/4.0.1pl2 Last-Modified: Tue, 29 Jul :11:51 GMT ETag: "1f1f6e bbf7" Accept-Ranges: bytes Content-Length: 1314 Content-Type: text/html Age: 157 Connection: close Connection closed by foreign host. response

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

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.

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).

TRACE using Telnet response
telnet cache.psu.ac.th 8080 Trying Connected to proxy6.psu.ac.th. Escape character is '^]'. TRACE HTTP/1.0 HTTP/ OK Date: Wec, 22 Oct :11:20 GMT Server: Stronghold/2.4.2 Apache/ C2NetEU/2412 (Unix) Content-Type: message/http Age: 118 X-Cache: MISS from cache.psu.ac.th Proxy-Connection: close TRACE / HTTP/1.0 : response

: Cache-Control: max-age=259200 Connection: keep-alive Host: www. cs
: Cache-Control: max-age= Connection: keep-alive Host: Via: 1.0 cache.psu.ac.th:8080 (Squid/2.5.STABLE1) X-Forwarded-For: unknown Connection closed by foreign host.

3.7. The OPTIONS Method The OPTIONS method allows a client to obtain information about what methods a server supports OPTIONS * HTTP-version Often OPTIONS is disabled. Many servers require the Host header as well.

OPTIONS using Telnet Or use HTTP /1.0 with no extras response
telnet fivedots.coe.psu.ac.th 80 Trying Connected to fivedots.coe.psu.ac.th. Escape character is '^]'. OPTIONS * HTTP/1.1 Host: fivedots.coe.psu.ac.th Connection: close HTTP/ OK Date: Sun, 12 Oct :37:44 GMT Server: Apache/1.3.9 Debian/GNU PHP/4.0.3pl1 Content-Length: 0 Allow: GET, HEAD, OPTIONS, TRACE Connection: close Connection closed by foreign host. Or use HTTP /1.0 with no extras response

4. HTTP Protocol Versions
only supported the GET method requests and responses had no extra header information a GET of a non-existent page caused the server to return nothing no media types: only text/HTML was supported

HTTP 1.0 introduced headers, media types, more methods, caching, authentication, persistent connections headers mean that "meta" information can be transferred between clients and servers media types supported with Accept (Request)and Content-Type (Entity) headers continued

caching supported with the Last-Modified (Entity) and If-Modified-Since (Request) headers
authentication supported with the Authorization (Request) and WWW-Authenticate (Response) headers persistent connections supported with the (non-standard) Connection header, with a keep-alive value

HTTP 1.1 introduced a better implementation of persistent connections, multihoming, entity tags, byte ranges, digest authentication persistent connection is the default in HTTP 1.1 only need Connection: close at the end multihoming means that a server can respond to different hostnames. HTTP 1.1. requires the Host header in all requests continued

byte ranges make it possible to retrieve only part of a document
entity tags (etags) aid caching by representing each document (entity) with a unique identifier gets round the problem of the same document at different sites etags are used in the If-match and If-none-match request headers byte ranges make it possible to retrieve only part of a document useful for downloading after an interrupt, and for streaming media supported with the Range request header continued

digest authorization allows username and password information to be transferred as a unique number (a checksum) makes it much harder for hackers to steal password details

5. Server Response Codes The server response code is the number after the HTTP version string in the server response: HTTP/ OK Date: : The text after the number ("ok") is a description of the code.

Response Code ranges Code Range Meaning Information Client request successful Client request redirected; more action needed Client request incomplete Server error

Some Common Codes Code Meaning 200 OK response contains data
301 Moved new location given in Location response header 305 Use Proxy proxy location in Location 401 Unauthorized client lacked proper authorization to get the page; details sent in the WWW-Authenticate response header continued

404 Not Found no page at the URL
407 Proxy Authentication Required the client must obtain proxy authorization; details sent in the Proxy-Authenticate response header 503 Service Unavailable further details may be given in the Retry-After response header

6. Some Advanced Features
Details on: media types client-side caching retrieving parts of a document authorization cookies

6.1. Media Types The client tells the server which media types it can handle using the Accept request header. The server tries to return information in a preferred media type, and gives the type in the Content-Type entity header.

Typical Client Accept Headers
Newer browsers: Accept: image/gif, image/jpeg, */* Older browsers: Accept: image/gif Accept: image/jpeg Accept */*

6.2. Client-side Caching Two approaches:
caching based on the document age caching based on the document's entity tag (etag) Caching can be configured using the general header Cache-Control it can be switched off or set to a certain amount of time e.g. Cache-Control: no-cache continued

Cache-Control replaces the Pragma heading of HTTP 1
Cache-Control replaces the Pragma heading of HTTP 1.0 which could only switch off caching: Pragma: no-cache

Caching using Age The request header If-Modified-Since:
If-Modified-Since: Fri, 15-Jun-01 01:00:00 GMT The server returns response code 304 if it has not been modified, and the client can use the cached version. Otherwise it returns 200 and the page. continued

There is a If-Unmodified-Since header.
The server can return an Expires header which states when the document may change.

Caching using Etags If the server is using etags, it will return an ETag header with the document. The client can check documents in its cache by using the If-Match or If-None-Match headers with etags.

6.3. Retrieving Parts of a Doc.
In HTTP 1.1, a client does not need to get all of a document at once it can retrieve it in pieces, specified using byte ranges For this to be possible, the server must send a response containing the Accept-Ranges header: Accept-Ranges: bytes continued

Then the client can request the data in pieces:
GET /largefile.html HTTP/1.1 // other headers Range: Response includes a Content-range header: HTTP/ OK // other headers Content-range: / // data continued

The client can include an If-Range header to use a cached part unless it has been updated:
GET /largefile.html HTTP/1.1 // other headers If-Range: Fri, 15-Jun-01 01:00:00 GMT Range:

6.4. Authorization 1) Ordinary request 2) Denies access
and sends WWW-Authenticate header 3. Username and password obtained 4) Send request again but with Authorization header 5) Response continued

The WWW-Authenticate header specifies the authorization method required by the server:
usually BASIC which requires a "username:password" string encode in base64 BASIC also includes a realm, which is a class of users

1) Initial Request GET /sample.html HTTP/1.1 User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/ Netscape6/6.01 Accept: */* Accept-Language: en Accept-Encoding: gzip,deflate,compress,identity Keep-Alive: 300 Connection: keep-alive

2) Access Denied HTTP/ Unauthorized Server: Squid/2.2.STABLE5 Mime-Version: 1.0 Date: Sun, 12 Oct :59:09 GMT Content-Type: text/html WWW-Authenticate: Basic realm= "Systems Administrator"

3) The Browser Dialog

4) Send Request Again GET /sample.html HTTP/1.1 User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/ Netscape6/6.01 Accept: */* Accept-Language: en Accept-Encoding: gzip,deflate,compress,identity Authorization: BASIC jhg235gjmg5jkjkgj24g42g

5) Response HTTP/ OK Server: Squid/2.2.STABLE5 Mime-Version: 1.0 Date: Sun, 12 Oct :01:13 GMT Content-Type: text/html Conetnt-length // HTML of sample.html page

6.5 Cookies Client-side cookies are used to store client-specific information on the client's machine used by the browser when it accesses the same page again Not part of the HTTP specification, but used in every browser.

Cookie Usage 1) Ordinary request 2) Response and a Set-Cookie header
3) The browser stores the cookie 4) Later send another request with Cookie header included 5) The server uses the cookie information. 6) Customised response and an updated Set-Cookie header

1) & 2) Request and Response
POST / HTTP/1.0 // client headers type=newCust&firstname=Andrew HTTP/ OK // server headers Set-Cookie: acct=

3) & 4) Storage and Later Use
The browser stores the cookie information: acct= Days/months later, another request: POST / HTTP/1.0 // client headers here Cookie: acct= type=oldCust

7. More Information The World Wide Web Consortium:
HTTP/1.1 Specification: rfc2616/rfc2616.html

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