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You must visit the site for detail www.networksorcery.com/enp/protocol/udp.htm

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Posted by HamidAliKhan

Please visit:

www.networksorcery.com/enp/protocol/udp.htm

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Posted by sagitraz

When you look at all the control information that TCP needs to work, and factor in the need to do the three-way handshake before any data is sent, you begin to realize that TCP is a pretty high-overhead operation. For every unit of data being sent, a ton of control information needs to be sent along with it. For some types of communication, we don't need all that control—sometimes; just dropping a postcard in the mail is fine. That is where UDP comes in.

UDP does not use any of the control and reliability features we just discussed in TCP. In fact, if you look at Figure, which shows what the UDP header looks like, you, can see that the only elements in common are the port numbers and the checksum.

Source Port Length
Destination Port Checksum

There is no sequencing, no acknowledgments, no window size—and no three-way handshake, either. So you can see that much less control information is sent with each segment. With UDP, there is no PAR. You ask for something, and then you get it. If it doesn't work, you have to ask all over again. Most applications, such as a TFTP server for example, will handle any errors and retransmissions—which means that the application itself (up at Layer 7) is doing the reliability, not Layer 4.

The typical UDP connection goes something like this:

Host A: "Hey Server Z, what's the IP address of www.google.com?"
Server Z: "www.google.com is 66.102.7.147."


Or perhaps:

Host B: "Hey Server Z, send me that file using TFTP."
Server Z: "Here's the file."

UDP is good when reliability is not needed—for DNS lookups or TFTP transfers, for example—or when the overhead of TCP would cause more problems than it solves—for example, when doing VoIP. TCP signaling would introduce so much delay that it would degrade the voice quality—plus, by the time any missing voice segment was retransmitted, it would be too late to use it! VoIP uses UDP because it is faster than TCP, and reliability is less important than minimizing delay.

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Posted by steve10