HTTP (HYPERTEXT TRANSFER PROTOCOL)

•   The HyperText Transfer Protocol (HTTP) is used to define how the clientserver programs can be written to retrieve web pages from the Web.
•   It a protocal to access the World Wide Web (WWW).
•   The HTTP protocol's also used to transfer the data in plain text, hypertext, audio, video, and et..
•   HTTP is a stateless request or response protocol that governs client/server communication.
•    The HTTP  sends a request and the  HTTP server returns a response of it.
Contents
•   The server uses the port number 80; the client uses a temporary port number. HTTP uses the services of TCP , a connection-oriented and reliable protocol.
•   HTTP is a text-oriented protocol. It  contains embedded URL known as links.
•   When hypertext is clicked, browser opens a new connection, retrieves file from the server and displays the file.
•   Each HTTP message has the general form

START_LINE

MESSAGE_HEADER

MESSAGE_BODY

           where stands for carriage-return-line-feed. 

Features of HTTP 

Connectionless protocol:

HTTP is a connectionless protocol. HTTP request a signal and it will response by the server in some second's. When the server receives the request, the server processes the request and sends back the response to the HTTP client after which the client disconnects the connection. The connection between client and server exist only during the current request and response time only.

o   Media independent:

HTTP protocol is a media independent as data can be sent as long as both the client and server know how to handle the data content. It was required to the client and server to specify the content type in MIME-type of  header.

o   Stateless:

HTTP is a stateless protocol as both the client and server know each other only during the current request. Due to this nature of the protocol, both the client and server do not retain the information between various requests of the web pages.

HTTP REQUEST AND RESPONSE MESSAGES

•   The HTTP protocol defines the format of the request and response messages

•   Request Message: The request message is sent by the client that consists of a request line, headers, and sometimes a body.

HTTP REQUEST MESSAGE

•   The first line in a request message is called a request line.
•   After the request line, we can have zero or more request header lines.
•   The body is an optional one. It contains the comment to be sent or the file to be published on the website when the method is PUT or POST.

Request Line

•   There are three fields in this request line - Method, URL and Version.
•   The Method field defines the request types.
•   The URL field defines the address and name of the corresponding web page. 
•   The Version field  gives the version of the protocol; the most current version of     HTTP is 1.1.
•   Some of the Method types are 

Request Header

•   Each request header line sends additional information from the client to the server.
•   Each header line has a header name, a colon, a space, and a header value.
•   The value field defines the values associated with each header name. Headers defined for request message include

Body

•   The body can be present in a request message. It is optional.
•   Usually, it contains the comment to be sent or the file to be published on the website when the method is PUT or POST.

Conditional Request

•   A client can add a condition in its request.
•   In this case, the server will send the requested web page if the condition is met or inform the client otherwise.
•   One of the most common conditions imposed by the client is the time and date the web page is modified. 
•   The client can send the header line If-Modified-Since with the request to tell the server that it needs the page only if it is modified after a certain point in time.

HTTP RESPONSE MESSAGE

•   The first line in a request message is called a status line.
•   After the request line, we can have zero or more response header lines.
•   The body is an optional one. The body is present unless the response is an error message

Status Line

•   The Status line contains three fields - HTTP version , Status code,  Status phrase
•   The first field defines the version of HTTP protocol, currently 1.1.
•   The status code field defines the status of the request. It classifies the HTTP result. It consists of three digits.

                 1xx–Informational, 2xx– Success, 3xx–Redirection,

               4xx–Client error,   5xx–Server error

•   The Status phrase field gives brief description about status code in text form.
•   Some of the Status codes are 

Response Header

•   Each header provides additional information to the client.
•   Each header line has a header name, a colon, a space, and a header value.
•   Some of the response headers  are:

Body

•   The body contains the document to be sent from the server to the client.
•   The body is present unless the response is an error message.