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TCP Functions: What TCP Does
(Page 2 of 2)
Functions Not Performed By TCP
TCP does so much that sometimes it
is described as doing everything an application needs to
use an internetwork. I may even have been guilty of this myself. However,
the protocol doesn't do everything. It has limitations
and certain areas that its designers specifically did not address. Among
the notable functions TCP does not perform include:
- Specifying Application Use: TCP defines
the transport protocol. It does not describe specifically how applications
are to use TCP.
- Providing Security: TCP does not provide
any mechanism for ensuring the authenticity or privacy of data it transmits.
If these are needed they must be accomplished using some other means,
such as IPSec,
- Maintaining Message Boundaries: TCP sends
data as a continuous stream, not as discrete messages. It is up to the
application to specify where one message ends and the next begins.
- Guaranteeing Communication: Wait a minute
isn't the whole point of TCP supposed to be that it guarantees data
will get to its destination? Well, yes and no. J
TCP will detect unacknowledged transmissions and re-send them if needed.
However, in the event of some sort of problem that prevents reliable
communication, all TCP can do is keep trying. It can't make
any guarantees because there are too many things out of its control.
Similarly, it can attempt to manage the flow of data, but cannot resolve
This last point might seem a bit
pedantic, but is important to keep in mind, especially since the tendency
is to think of TCP as somewhat bulletproof. The overall
success of communication depends entirely on the underlying internetwork
and the networks that constitute it. A chain is as strong as its weakest
link, and if there is a problem at the lower layers, nothing TCP can
do will guarantee successful data transfer.
Key Concept: TCP provides reliable communication only by detecting failed transmissions and re-sending them. It cannot guarantee any particular transmission, because it relies on IP, which is unreliable. All it can do is keep trying if an initial delivery attempt fails.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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