Types of HTTP and Its Message Format

hypertext transfer protocol http message format

Moving on with our discussion on Hyper Text Transfer Protocol (HTTP). If you have forget something about HTTP, then you can go to this link and revise it, and then start with this post. HTTP Revise.

In this post, we are going to discuss about the types of HTTP connections and HTTP message format.

First we will start with the HTTP connection types. There are basically two types of HTTP connections: Persistent and Non-Persistent. Lets have a look at each one of them.

When you or any host communicate with the server, there is a long series of request-response messages, that is being exchanged between you and the server. Therefore, depending on the type of application, the Application developer has to make a decision, that should all the request-response messages should be sent over a single TCP connection or they should be sent under individual TCP connection for every pair of request-response message.

i) If all the request-response message pair are sent over different TCP connection, it is known as Non-Persistent Connections and

ii) if there is only one TCP connection for a series of messages, it is known as Persistent Connection.

Hyper Text Transfer Protocol (HTTP) with   non-persistent connection :

http message format 
Let me start by giving you an example that will make you understand about HTTP non-persistent connection. The transfer of a Web Page from server to client under non-persistent connection is as follows:
 

Let us suppose that a web page consists of a base HTML file and 20 PNG images, all these 21 files reside on the same server. Say, the address or the URL of the base HTML file is http://www.fitnytech.com/technology/images.html.

 Now the process starts.
 

1. The HTTP client process initiates a TCP connection to the server http://fitnytech.com on port number 80, the default port number for HTTP. There will be a socket at the client and a socket at the server associated with the TCP connection.

2. The HTTP client sends an HTTP request message to the server via its socket. The request message includes the path name /technology/images.html.
 

3. The HTTP server process receives the request message via its socket, retrieves the object /technology/images.html from its storage, encapsulates or embed the object in an HTTP response message, and sends the response message to the client via its socket.

4. The HTTP server process tells TCP to close the TCP connection.
 

5. The HTTP client receives the response message. The TCP connection terminates. The message indicates that the encapsulated object is an HTML file. The client extracts the file from the response message, examines the HTML file and finds references to the 20 PNG objects.

6. Now to get those 20 PNG images, the first four steps are again repeated.
 

The steps above explains you the use of non-persistent connections, where each TCP connection is closed after the server sends the object—the connection does not persist or remains for other objects. Each TCP connection transports only one request message and one response message. Therefore in this case, when a user requests the Web page, 21 TCP connections must be generated.

 
 Lets now see into the depth of non-persistent connection and calculate the time taken from requesting a web page and till the entire file is received.
 

Here we will describe you the time taken by a small packet to travel from client to server and back to client. This is known as Round-Trip Time (RTT). The RTT includes the packet propagation delay, queuing delay at the router and switches and the the processing delay.

 What happens when you click on a link or a hyperlink , the client or your browser initiates a TCP connection with the server. This connection involves 3 steps that are often called as “3-way handshake“. This handshake includes, (a) The client sends a TCP segment to the server, (b) The server acknowledges and responds with a  TCP segment, and finally (c) The client acknowledges back to the server.
 

The 1st two parts (a) and (b) takes 1 RTT. After completing the 1st two steps , the client sends a request message along with the 3rd part of handshake i.e. acknowledgement to server. After receiving the request message and acknowledgement , the server respond by sending the HTML file into the TCP connection. This HTTP request-response takes 1 RTT. So approximately, we can say that, Total response time taken is 2 RTT’s plus the transmission time taken by the server. Now as usual, I will clear  it by drawing a figure for you.

 
 

round trip time file transmission

         Figure: TIME TAKEN IN TRANSMISSION OF A HTML FILE

 
 
Hyper Text Transfer Protocol (HTTP) with persistent connection :
 
 As we can clearly see that , non-persistent connections have certain disadvantages. For every requesting object, you have to set-up a new TCP connection that is a huge overhead for a server that is serving millions of requests. And also, it will take 2 RTT for every requested object.
 

To overcome theses issues, HTTP is used with persistent connections. In persistent connection, the server doesn’t close the connection after sending a response. More than one request and response can be sent between the same client and the same server over the same TCP connection. The example we have taken above of an HTML file and 20 PNG images. So these 21 objects can be sent over a single persistent connection.The request for different objects are made simultaneously without waiting for the replies to the pending requests. This phenomenon is often called as pipelining.  When the server receives back-to-back requests, it sends the objects back-to-back. The server closes the connection , when the connection remains useless or idle for a particular time period.

 
  • The default mode of HTTP uses persistent connections. However an application developer can modify it to non-persistent according to his need.
 
 
 

**************************************                                                   ********************************************       ************************************                                                   ******************************************

 

Now lets move on and have a look at the types of HTTP message formats. As you already know, there are two types of HTTP message. One is Request message and the other is the Response message.

 

HTTP REQUEST MESSAGE

GET    /technology/images.html    HTTP/1.1

Host : http://fitnytech.com

Connection : close

User-agent : Chrome/7.0

Language : Fr

Have a look at this message and try to understand it. You can see that the message is written in ASCII text, so that an ordinary human being can read it. As this request message contains 5 lines. Similarly a request message can have more lines and as few as 1 line also.

In this example, the 1st line of the message is said to be the request line and the further are called the header lines.

Request line: It has 3 fields: (a) Method field, (b) URL field and (c) HTTP version field.

There are various methods that can be there in a method field such as GET, POST, PUT, HEAD, DELETE.

GET:    The GET method is used for general web page request.

POST:  The POST method is used to ad parameters to the server.

The URL field contains the address of the requested object. As here, it is /department/images.html.

Version field: Version of the HTTP that browser implements, here it is version HTTP/1.1.

Now the header fields.

1st header field contains the host i.e. http://fitnytech.com, tells the name of the host on which the objects reside.

2nd header field Connection:close , browser tells the server that it doesn’t want to bother with persistent connection. It wants server to close the connection after sending the requested object.

3rd header field User-agent : Chrome/7.0, a chrome browser. This field is useful in the cases where the server has different copies of the files for different version of user agent.

4th header field Language: Fr. It tells the server in which language , the desired copy of the requested file is wanted. Here it is French. If the server has the requested copy, it will send that, but if it doesn’t , then it will response with the default version.

Now let me give you a  general format of HTTP request message.

HTTP request message connection

HTTP RESPONSE MESSAGE

 

HTTP/1.1    200     OK

Connection: close

Date: Fri, 06 Sept 2013 11:45:07 (GMT +05:30)

Server: Apache/2.2.3 

Last-Modified: Sun, 01 Sept 2013 15:11:03 (GMT+05:30)

Content-Length: 4951

Content-Type: text/html

(data data data data data …)

 

A response message could have more number of entries also. But we will discuss this basic message with 8 entries.

Have a look at this message and try to understand this. It basically has 3 sections. 1. An initial status line 2. Header lines 3. Entity body.

1. Status Line: It has 3 fields. (a) The protocol version. (b) A status code. (c) Status message

In the given message, the server is using HTTP/1.1 version and the code is 200 , that means everything is OK and executed properly (The server has found the object and sending it).

  • Some common Status Code that you will generally get as response from the server, while   requesting for a Web page are:

                         (i)  200 OK : It means , The Request succeed and the requested information has been sent.

                        (ii) 301 Moved permanently : It indicate that the requested URL has been permanently                  moved to a new address. This new URL will be indicated in the Location, header of the response message.

                        (iii) 400 Bad Request : When server doesn’t understand the request, it shows this code.

                         (iv) 404 Not Found : The requested information doesn’t exist on this server.


2. Now the header fields.

1st header field Connection : close, The server wants to inform the client that it is going to close the connection after sending the object.

2nd header field DATE : It indicated the time and the date when the HTTP response was sent by the server.

3rd header field Server : The name of the server used to generate the response message.

4th header field Last-Modified : Its the time and the date when the object was last created or modified.

5th header field Content-Length : It tells about the number of bytes in the response message that are being sent.

6th header field Content Type : It tells that the object in the entity body is a HTML text.

3. Entity body that is being denoted by data data data data……. is the meat of the message . It contains the requested object.

Let me give you a general format of HTTP response message.

http header fields

Thank you for reading this article. We hope you enjoyed it.

This is all from us on HTTP protocol and its message format. Do you have something to share with our readers ?

Raman Deep Singh Chawla

Raman Deep Singh Chawla

Raman is the founder of FitnyTech . He is a fitness App Developer and a Blogger. He is fond of his fitness and sports. He has great passion for Cricket , Tennis , Soccer and Table Tennis. In his free time , he loves to learn about technology , write about it , share his thoughts with others. His passion for technology can be seen at his blogs.

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