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TCP and UDP Overview and Role In TCP/IP
(Page 3 of 3)
Applications of TCP and UDP
Having two transport layer protocols
with such complementary strengths and weaknesses provides considerable
flexibility to the creators of networking software:
- TCP Applications: Most typical
applications need the reliability and other services provided by TCP,
and don't care about loss of a small amount of performance to overhead.
For example, most applications that transfer files or important data
between machines use TCP, because loss of any portion of the file renders
the entire thing useless. Examples include such well-known applications
as the Hypertext
Transfer Protocol (HTTP) used by the World Wide Web (WWW),
Transfer Protocol (FTP) and the Simple
Mail Transfer Protocol (SMTP). I
describe TCP applications in more detail in the TCP section.
- UDP Applications: I'm sure you're thinking:
what sort of application doesn't care if its data gets there,
and why would I want to use it? You might be surprised: UDP is
used by lots of TCP/IP protocols. UDP is a good match for applications
in two circumstances. The first is when the application doesn't really
care if some of the data gets lost; streaming video or multimedia is
a good example, since one lost byte of data won't even be noticed. The
other is when the application itself chooses to provide some other mechanism
to make up for the lack of functionality in UDP. Applications that send
very small amounts of data, for example, often use UDP under the assumption
that if a request is sent and a reply is not received, the client will
just send a new request later on. This provides enough reliability without
the overhead of a TCP connection. I
discuss some common UDP applications in the UDP section.
Key Concept: Most classical applications, especially ones that send files or messages, require that data be delivered reliably, and therefore use TCP for transport. Applications using UDP are usually those where loss of a small amount of data is not a concern, or that use their own application-specific procedures for dealing with potential delivery problems that TCP handles more generally.
In the next few sections we'll first
examine the common transport layer addressing scheme used by TCP and
UDP, and then look at each of the two protocols in detail. Following
these sections is a summary
comparison to help you see at a glance
where the differences lie between TCP and UDP. Incidentally, if you
want a good real-world illustration of why having both UDP
and TCP is valuable, consider message
transport under the Domain Name System (DNS),
which actually uses UDP for certain types of communication and TCP for
Before leaving the subject of comparing
UDP and TCP, I want to explicitly point out that even though TCP is
often described as being slower than UDP, this is a relative
measurement. TCP is a very well-written protocol that is capable of
highly efficient data transfers. It is only slow compared to UDP because
of the overhead of establishing and managing connections. The difference
can be significant, but is not enormous, so keep that in mind.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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