HTTP Overview Vijayan Sugumaran School of Business Administration Oakland University

What is HTTP?  Hypertext Transfer Protocol  Language used to describe how documents are sent over the internet  Rules by which browsers make requests and servers supply responses  Includes ways to:  Ask for a document by name  Agree on data format  Determine who the user is  Decide how to handle outdated resources  Indicate the results of the request  When specifying a URL or clicking on a hyperlink, the browser translates into HTTP commands  The server finds the resource and sends it back

HTTP Request Model  HTTP is a stateless request/response protocol  Client opens a socket to the Web server’s HTTP port (80 by default)  Client sends a ASCII text request line, followed by zero or more HTTP headers, an empty line, and any data that accompanies the request  The web server parses the request and locates the specified resource  Server writes a copy of the resource to the socket, where it is read by the client  The server closes connection  Once the connection is closed, the server doesn’t remember anything about this request

Connecting to the Web Server  Web server operates by listening for requests on a particular port (default port 80)  If the server is running on any other port, it has to be part of the URL  E.g.  Client initiates an HTTP request by opening a TCP/IP socket to the Web server port, and then opening input and output stream over the socket  Parameters required to open the socket are the Web server host name, and the port number  Server host name is extracted from the URL  Output stream used to send commands to server  Input stream used to read the server response

Sending the HTTP Request  Once the socket connection is made, Web browser writes an HTTP command to request the document  A request has up to four parts:  First part is “request line”  Second part – one or more request headers  Third part – Empty line consisting of carriage return or line feed  Fourth part – User data

Part I - Request Line  Mandatory part of HTTP Request  Consists of three tokens, separated by spaces  First Token: Request method  Second Token: request URI (derived from the URL – stuff that comes after the host name)   Third Token: HTTP version the client understands (allowable values are HTTP/1.0 and HTTP/1.1)  Example of Request line:  GET /myfile.html HTTP/1.0  There are eight possible methods defined in HTTP (GET and POST are the most frequently used)

Request Methods  GET – A simple request to retrieve the resource identified in the URI  HEAD – Same as GET, except the server doesn’t return the requested document. The server only returns the status line and headers  POST – A request for the server to accept data that will be written to the client’s output stream  PUT – A request for the server to store the data in the request as the new contents of the specified URI  DELETE – A request for the server to delete the resource named in the URI  OPTIONS – A request for information about what request methods the server supports  TRACE – A request for the Web server to echo the HTTP request and its headers  CONNECT – Currently unimplemented method reserved for use with a tunneling proxy

Part II - Request Headers  Second part – one or more request headers  Request headers inform the server of client’s identity and capabilities  Key/value pairs (key and value separated by colon)  E.g: Accept: text/html, image/bmp, text/x-wap.wml  one pair per line  Typical Request Headers  User-Agent ( The vendor and the version of the client)  Accept (A list of content types that client recognizes)  Content-Length (number of bytes of data appended to the request)

Part III – Empty Line  This informs the server that no more headers follow  Even if no headers exist, this empty line has to be sent so that the server doesn’t look for any more headers

HTTP Response from Server  Response also has up to four parts:  A status line  Zero or more response headers  An empty line signaling the end of the response headers, and  The requested data

HTTP Response - Status line  The status line consists of three tokens:  HTTP version  Just as the client indicates the highest version it can understand, the server indicates its capabilities  Response code  Three digit numeric code that indicates whether the request succeeded or failed, if it failed, the reason why.  An optional response description, which is a human readable explanation of the response code  Status line example  HTTP/ OK

Client (Browser) Asks…  User types the following URL in the browser   Browser opens a socket connection to host on port 80 and then writes the following line and an empty line after that.  GET /simple.htm HTTP/1.0

Web Server Returns… HTTP/ OK Date: Tue, 12 Feb :55:43 GMT Server: Apache/ (Win32) Content-Length: 241 Content-Type: text/html Welcome to The Lyric Note, the best Internet source for sheet music musical instruments books on musical topics music software, and musical gift items

Browser Renders…  Browser parses the status line and checks the status code – request was successful  The parses response headers  Finds out that 241 bytes of html follows  Reads html data and renders the document  If the html document contains references to other resources, the browser requests the server for those resources  E.g: if the document contains an tag, then the browser requests that image.  The server doesn’t send the image automatically along with the original request

Complex Request  HTTP Request: GET /compound.htm HTTP/1.0 Then the server sends the following….

Server Sends… HTTP/ OK Date: Tue, 12 Feb :55:43 GMT Server: Apache/ (Win32) Content-Length: 380 Content-Type: text/html Welcome to The Lyric Note, the best Internet source for sheet music musical instruments books on musical topics music software, and musical gift items

Another Request  Browser encounters style sheet request  Makes a second HTTP request  GET /lyricnote.css HTTP/1.0  Web server retrieves the style sheet and sends it to the client HTTP/ OK Date: Tue, 12 Feb :55:53 GMT Server: Apache/ (Win32) Connection: Keep-alive, close Content-Length: 73 Content-Type: text/plain H3 { font-size: 20px; font-weight: bold; color: #005A9C; }

Yet Another Request  Browser interprets the style sheet and applies it to the document  Encounters image tag and makes a request for the logo  GET /images/logo.jpg HTTP/1.0  Server responds with image data stream HTTP/ OK Date: Tue, 12 Feb :55:59 GMT Server: Apache/ (Win32) Connection: Keep-alive, close Content-Length: 1280 Content-Type: text/plain (binary image data follows)

GET Versus POST  GET  Form data appended to the URI  Simplifies the work the server needs to do  Limit to the length of the URI (4KB)  Some servers won’t even accept 2KB  If you submit form data using GET and book mark the resulting page, then, you don’t have to type the data  However, the data is visible – defeats having a password  POST  Write the data to the client’s output stream  Server takes time in processing this data

More Info on HTTP  W3C – World Wide Web Consortium   Freesoft.org   HTTP IETF (Internet Engineering Task Force) 