Hypertext Transfer Protocol

HTTP  stands  for  Hypertext  Transfer Protocol.  It  is  specically aTCP/IP  based  communication protocol  which  is  used  to  deliver  virtually  all  files  and  other  data,  jointly  known as resources, on the World Wide Web. These resources may be HTML files, image files, query results, or anything else.

A browser is works as an HTTP client because it sends requests to an HTTP server which is known as Web server. The Web Server then sends responses back to the client.Here,the standard and default port for HTTP servers to listen on is 80 but it can be changed to any other port like 8080 etc.

There are 3 main things about HTTP of which you should be aware:

• HTTP is connectionless: After a request is made, the client will disconnects from the server and waits for a answer. The server must re-establish the connection after it process the request.
• HTTP is media independent: Every type of data can be sent by HTTP as long as both the client and server know how to manage the data content. How content is managed is determined by the MIME requirement.
• HTTP is stateless: This is a direct result of HTTP's being connectionless. Now,The server and client are aware of each other only during a request. Afterwards, all forgets the other. Due to this reason neither the client nor the browser could retain information among different request across the web pages.

Like the majority of network protocols, HTTP mostly uses the client-server model: An HTTP client opens a connection and sends a request message to an HTTP server; the server then returns a response message, typically having the resource that was requested. After delivering the answer, the server closes the connection.

The standard format of the request and response or answer messages are similar and will have following structure:

• An initial line CRLF
• Zero or more header lines CRLF
• A blank line ie. a CRLF
• An optional message body such as file, query data and query output.

Initial lines and headers should end in CRLF. Though you should gracefully manages lines ending in just LF. More precisely, CR and LF here mean ASCII values 13 and 10.

Initial Line : Request

The initial line is different for the request than for the response. A request line has 3 parts, divided by spaces:

• An HTTP Method Name
• The local path of the specific requested resource.
• The version of HTTP being used.

Here is an exampple of initial line for Request Message. GET /path/to/file/index.html HTTP/1.0

• GET is the most common HTTP method. Other methods can be POST, HEAD etc.
• The path is the part of the URL after the host name. This path is also know as the request Uniform Resource Identifier (URI). A URI is like a URL, but more general.
• The HTTP version always takes the form "HTTP/x.x", uppercase.

Initial Line : Response

The initial response line, called the status line, also has 3 parts separated by spaces:

• The version of HTTP being used.

• A response status code that will gives the result of the request.
• An English reason phrase which is describing the status code.

Here is an example of initial line for Response or reply Message. HTTP/1.0 200 OK or HTTP/1.0 404 Not Found

Header Lines

Header lines mostly  provide information about the request or response, or about the object sent in the message body.

The header lines are in the usual text header format, which is: one line per headera & of the form "Header-Name: value",It must be ending ending with CRLF. It is the same format which is used for email and news postings, which is defined in RFC 822.

• A header line ought to end in CRLF, but you be supposed to handle LF correctly.
• The header name is must not be case-sensitive.
• Any number of spaces or tabs may be among the ":" and the value.
• Header lines start with space or tab are actually part of the previous header line, folded into multiple lines for easy reading.

Here is an exampple of ione header line

User-agent: Mozilla/3.0Gold or

Last-Modified: Fri, 31 Dec 1999 23:59:59 GMT

The Message Body

An HTTP message may have a body of data sent after the header lines. In a reply and this is where the requested resource is returned to the client (the most common use of the message body), or maybe explanatory text if there's an error. In a request and this is generally where user-entered data or uploaded files are sent to the server.

If an HTTP message includes a body, there are generally header lines in the message that describe the body. In particular:

• The Content-Type: header gives the MIME-type of the data in its body, like text/html or image/gif.

• The Content-Length: In this header gives the number of bytes in the body.

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