Ccna routing and Switching Complete Study Guide

User Datagram Protocol (UDP)

User Datagram Protocol (UDP) is basically the scaled-down economy model of TCP, which is 

why UDP is sometimes referred to as a thin protocol. Like a thin person on a park bench, a thin 

protocol doesn’t take up a lot of room—or in this case, require much bandwidth on a network.

UDP doesn’t offer all the bells and whistles of TCP either, but it does do a fabulous job 

of transporting information that doesn’t require reliable delivery, using far less network 

resources. (UDP is covered thoroughly in Request for Comments 768.)

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Chapter 3 

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  Introduction to TCP/IP 

So clearly, there are times that it’s wise for developers to opt for UDP rather than TCP, 

one of them being when reliability is already taken care of at the Process/Application layer. 

Network File System (NFS) handles its own reliability issues, making the use of TCP both 

impractical and redundant. But ultimately, it’s up to the application developer to opt for 

using UDP or TCP, not the user who wants to transfer data faster!

UDP does not sequence the segments and does not care about the order in which the seg-

ments arrive at the destination. UDP just sends the segments off and forgets about them. 

It doesn’t follow through, check up on them, or even allow for an acknowledgment of safe 

arrival—complete abandonment. Because of this, it’s referred to as an unreliable protocol. This 

does not mean that UDP is ineffective, only that it doesn’t deal with reliability issues at all.

Furthermore, UDP doesn’t create a virtual circuit, nor does it contact the destination 

before delivering information to it. Because of this, it’s also considered a connectionless 

protocol. Since UDP assumes that the application will use its own reliability method, it 

doesn’t use any itself. This presents an application developer with a choice when running 

the Internet Protocol stack: TCP for reliability or UDP for faster transfers.

It’s important to know how this process works because if the segments arrive out of 

order, which is commonplace in IP networks, they’ll simply be passed up to the next layer 

in whatever order they were received. This can result in some seriously garbled data! On the 

other hand, TCP sequences the segments so they get put back together in exactly the right 

order, which is something UDP just can’t do.