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Chapter: Computer Networks - Application Layer

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Hypertext Transfer Protocol (HTTP)

The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the World Wide Web.

HTTP

 

The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the World Wide Web. HTTP functions as a combination of FTP and SMTP. It is similar to FTP because it transfers files and uses the services of TCP. However, it is much simpler than FTP because it uses only one TCP connection. There is no separate control connection; only data are transferred between the client and the server. HTTP is like SMTP because the data transferred between the client and the server look like SMTP messages. In addition, the format of the messages is controlled by MIME-like headers. Unlike SMTP, the HTTP messages are not destined to be read by humans; they are read and interpreted by the HTTP server and HTTP client (browser). SMTP messages are stored and forwarded, but HTTP messages are delivered immediately. The commands from the client to the server are embedded in a request message. The contents of the requested file or other information are embedded in a response message. HTTP uses the services of TCP on well-known port 80.

 

1. HTTP Transaction

 

Although HTTP uses the services of TCP, HTTP itself is a stateless protocol. The client initializes the transaction by sending a request message. The server replies by sending a response.

Messages

 

The formats of the request and response messages are similar. A request message consists of a request line, a header, and sometimes a body. A response message consists of a status line, a header, and sometimes a body. Request and Status Lines The first line in a request message is called a request line; the first line in the response message is called the status line.

 

Request type: This field is used in the request message. In version1.1of HTTP, severalrequest types are defined.

 

Version: The most current version of HTTP is 1.1.

 

Status code: This field is used in the response message. The status code field is similar tothose in the FTP and the SMTP protocols. It consists of three digits.

 

Status phrase: This field is used in the response message. It explains the status code intext form.

 

Header: The header exchanges additional information between the client and the server.For example, the client can request that the document be sent in a special format, or the server can send extra information about the document. The header can consist of one or more header lines. Each header line has a header name, a colon, a space, and a header value.

Body: The body can be present in a request or response message. Usually, it contains thedocument to be sent or received.

 

2. Persistent Versus Nonpersistent Connection

 

HTTP prior to version 1.1 specified a nonpersistent connection, while a persistent connection is the default in version 1.1.

 

Nonpersistent Connection

 

In a nonpersistent connection, one TCP connection is made for each request/response. The following lists the steps in this strategy:

 

1. The client opens a TCP connection and sends a request.

 

2. The server sends the response and closes the connection.

 

3.   The client reads the data until it encounters an end-of-file marker; it then closes the connection.

 

Persistent Connection

 

HTTP version 1.1 specifies a persistent connection by default. In a persistent connection, the server leaves the connection open for more requests after sending a response. The server can close the connection at the request of a client or if a time-out has been reached. The sender usually sends the length of the data with each response. However, there are some occasions when the sender does not know the length of the data. This is the case when a document is created dynamically or actively.

 

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