HTTP is the protocol that allows web servers and browsers to send and receive data over the Internet. It is a request and response protocol. The client requests a file and the server responds to the request. HTTP uses reliable TCP connections—by default on TCP port 80. The first version of HTTP was HTTP/0.9, which was then overridden by HTTP/1.0. Replacing HTTP/1.0 is the current version of HTTP/1.1, which is defined in Request for Comments (RFC) 2616 and downloadable from http://www.w3.org/Protocols/HTTP/1.1/rfc2616.pdf.
Note: This section covers HTTP 1.1 only briefly and is intended to help you understand the messages sent by web server applications. If you are interested in more details, read RFC 2616.
In HTTP, it is always the client who initiates a transaction by establishing a connection and sending an HTTP request. The web server is in no position to contact a client or make a callback connection to the client. Either the client or the server can prematurely terminate a connection. For example, when using a web browser you can click the Stop button on your browser to stop the download process of a file, effectively closing the HTTP connection with the web server.
An HTTP request consists of three components:
An example of an HTTP request is the following:
POST /examples/default.jsp HTTP/1.1
Accept: text/plain; text/html Accept-Language: en-gb Connection: Keep-Alive
Host: localhost
User-Agent: Mozilla/4.0 (compatible; MSIE 4.01; Windows 98) Content-Length: 33
Content-Type: application/x-www-form-urlencoded Accept-Encoding: gzip, deflate
lastName=Franks&firstName=Michael
The method—URI—protocol version appears as the first line of the request.
POST /examples/default.jsp HTTP/1.1
where POST is the request method, /examples/default.jsp represents the URI and HTTP/1.1 the Protocol/Version section.
Each HTTP request can use one of the many request methods as specified in the HTTP standards. The HTTP 1.1 supports seven types of request: GET, POST, HEAD, OPTIONS, PUT, DELETE, and TRACE. GET and POST are the most commonly used in Internet applications.
The URI specifies an Internet resource completely. A URI is usually interpreted as being relative to the server's root directory. Thus, it should always begin with a forward slash /. A Uniform Resource Locator (URL) is actually a type of URI (see http://www.ietf.org/rfc/rfc2396.txt). The protocol version represents the version of the HTTP protocol being used.
The request header contains useful information about the client environment and the entity body of the request. For example, it could contain the language the browser is set for, the length of the entity body, and so on. Each header is separated by a carriage return/linefeed (CRLF) sequence.
Between the headers and the entity body, there is a blank line (CRLF) that is important to the HTTP request format. The CRLF tells the HTTP server where the entity body begins. In some Internet programming books, this CRLF is considered the fourth component of an HTTP request.
Between the headers and the entity body, there is a blank line (CRLF) that is important to the HTTP request format. The CRLF tells the HTTP server where the entity body begins. In some Internet programming books, this CRLF is considered the fourth component of an HTTP request.
In the previous HTTP request, the entity body is simply the following line:
lastName=Franks&firstName=Michael
The entity body can easily become much longer in a typical HTTP request.
Similar to an HTTP request, an HTTP response also consists of three parts:
The following is an example of an HTTP response:
HTTP/1.1 200 OK
Server: Microsoft-IIS/4.0
Date: Mon, 5 Jan 2004 13:13:33 GMT Content-Type: text/html
Last-Modified: Mon, 5 Jan 2004 13:13:12 GMT Content-Length: 112
<html>
<head>
<title>HTTP Response Example</title> </head>
<body>
Welcome to Brainy Software
</body>
</html>
The first line of the response header is similar to the first line of the request header. The first line tells you that the protocol used is HTTP version 1.1, the request succeeded (200 = success), and that everything went okay.
The response headers contain useful information similar to the headers in the request. The entity body of the response is the HTML content of the response itself. The headers and the entity body are separated by a sequence of CRLFs.