HTTP works in a request-response manner. Let's take an example to understand this model better.
The following example illustrates the basic request-response model of communication between a web browser and a server over HTTP. The following sequence diagram illustrates the request and response messages sent between the client and server:
Here is the detailed explanation for the sequence of actions shown in the preceding diagram.
The user enters http://www.example.com/index.html in the browser and then submits the request. The browser establishes a connection with the server and sends a request to the server in the form of a request method (URI) and a protocol version, followed by a message containing the request modifiers, client information, and possible body content. The sample request looks as follows:
GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/htmlAccept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Let's take a minute to understand the structure of the preceding message. The following code is what you see in the first lines of request in our example:
GET /index.html HTTP/1.1
The general format for the request line is an HTTP command, followed by the resource to retrieve and the HTTP version supported by the client. The client can be any application that understands HTTP, although this example refers to a web browser as the client. The request line and other header fields must end with a carriage return character followed by a line-feed character. In the preceding example, the browser instructs the server to get the index.html file through the HTTP 1.1 protocol.
The rest of the information that you may see in the request message is the HTTP header values for use by the server. The header fields are colon-separated key-value pairs in the plain-text format, terminated by a carriage return and followed by a line feed character. The header fields in the request, such as the acceptable content types, languages, and connection type, are the operating parameters for an HTTP transaction. The server can use this information while preparing the response to the request. A blank line is used at the end of the header to indicate the end of the header portion in a request.
The last part of an HTTP request is the HTTP body. Typically, the body is left blank unless the client has some data to submit to the server. In our example, the body part is empty as this is a GET request for retrieving a page from the server.
So far, we have been discussing the HTTP request sent by the client. Now, let's take a look at what happens on the server when the message is received. Once the server receives the HTTP request, it will process the message and return a response to the client. The response is made up of the reply status code from the server, followed by the HTTP header and a response content body:
HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=604800
Content-Type: text/html
Date: Wed, 03 Dec 2014 15:05:59 GMT
Content-Length: 1270
<html>
<head>
<title>An Example Page</title>
</head>
<body>
Hello World !
</body>
</html>.
The first line in the response is a status line. It contains the HTTP version that the server is using, followed by a numeric status code and its associated textual phrase. The status code indicates one of the following parameters: informational codes, success of the request, client error, server error, or redirection of the request. In our example, the status line is as follows:
HTTP/1.1 200 OK
The next item in the response is the HTTP response header. Similar to the request header, the response header follows the colon-separated name-value pair format terminated by a carriage return and line feed characters. The HTTP response header can contain useful information about the resource being fetched, the server hosting the resource, and some parameters controlling the client behavior while dealing with resource, such as content type, cache expiry, and refresh rate.
The last part of the response is the response body. Upon successful processing of the request, the server will add the requested resource in the HTTP response body. It can be HTML, binary data, image, video, text, XML, JSON, and so on. Once the response body has been sent to the requestor, the HTTP server will disconnect if the connection created during the request is not of the keep-alive type (using the Connection: keep-alive header).