why UDP is used?

UDP is faster than TCP is correct.  It not only adds less data, but less logic to the underlying network protocol drivers.  TCP works with a sliding window algorithm that basically rejects packets that come out of sequence and request them to be reset.  If one wanted to optimize, they could use UDP.  Thus, if packets come out of order, the driver could hold onto them and order them.  It provides more flexibility and thus you could re-implement TCP, but more light weight especially if you can do it on a per-protocol basis.

Another reason for UDP is streaming audio or video.  If you need things to occur in real time, TCP makes sure everything is delievered.  UDP is faster and in a real time video if you miss a frame, who cares.  So, streaming packets and not caring if they get to the client or not is acceptable.  Especially when you are not saving the data, but playing it real time (Which is what I am referring to).  If your network has limitations, the video will probably play too slow anyway.  If you skip a frame or two, it's not a big deal.

In addition, there are types of traffic that simply do not warrant TCP.  DNS or WINS requests are a perfect example.  There's no need to use TCP when simply requesting name resolution.  However, WINS database changes or DNS zone transfers do need to be error checked otherwise the transmitted data might be incomplete and lead to database or zone corruption, hence error checking is required for these types of operation.

Hi ms_shubha,

I always like to explain it this way:

In UDP/IP you do not have to make connection to send/receive data.

If TCP is like handover something from your hand to my hand after we do handshake, UDP is like you just throwing something to me assuming that I will catch it.

Depends on the need, an application can be designed to use TCP if need reliability by do the handshaking first (imagine you meet your customer and handover your namecard) -or- need to be quick by just throwing the thing, no need handshaking (imagine the news paperboy throwing paper at your doorstep).

Hope this helps.
Judhi

Here's more.

1.
TCP can only be used for one-to-one data transfer, due to its connection-oriented nature. It cannot be used for one-to-many, such as broadcast or multicast. IP Broadcast and IP Multicast traffic therefore cannot use TCP, and always uses UDP.

2.
In timing-sensitive applications, sometimes the programmers prefer to retain control of the congestion control, sequencing, and other attributes of the connection. When they do this, they do not need the features of TCP, and do not want the overhead. They do what they want with the packets in the application, and send the data over UDP.