CCNA Cisco Certified Network Associate Study Guide - FTP Server

110 Chapter 3 � Internet Protocol
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No TCP Options
TCP Data Area:
vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35
11 2b 19 35 +. 11 2b 19
Frame Check Sequence: 0x0d00000f
Notice that everything I talked about above is in the segment. As you can
see from the number of fields in the header, TCP has a lot of overhead. Application
developers might not want to use as much reliability as TCP operates
with to save overhead, so User Datagram Protocol was also defined at the
Transport layer.
User Datagram Protocol (UDP)
Application developers can use the User Datagram Protocol (UDP) in place
of TCP. UDP is the scaled-down economy model and is considered 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, much bandwidth on a network.
UDP also doesn’t offer all the bells and whistles of TCP, but it does do a
fabulous job of transporting information that doesn’t require reliable delivery—and
it does so using far fewer network resources. (Please note that UDP
is covered thoroughly in RFC 768.)
There are some situations where it would definitely be wise for application
developers to opt for UDP rather than TCP. Remember the watchdog
SNMP up there at the Process/Application layer? SNMP monitors the network,
sending intermittent messages and a fairly steady flow of status
updates and alerts, especially when running on a large network. The cost in
overhead to establish, maintain, and close a TCP connection for each one of
those little messages would reduce what would be an otherwise healthy, efficient
network to a dammed-up bog in no time.
Another circumstance calling for UDP over TCP is when the matter of
reliability is already accomplished at the Process/Application layer. Network
File System (NFS) handles its own reliability issues, making the use of TCP
both impractical and redundant. However, the application developer decides
whether to use UDP or TCP, not the user who wants to transfer data faster.