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The transport layer enables a user's
device to segment several upper-layer applications for placement on
the same Layer 4 data stream, and enables a receiving device to
reassemble the upper-layer application segments. The Layer 4 data
stream is a logical connection between the endpoints of a network, and
provides transport services from a host to a destination. This service
is sometimes referred to as end-to-end service.
The transport layer also provides two protocols:
- TCP -- a
connection-oriented, reliable protocol; provides flow
control by providing sliding windows, and reliability by
providing sequence numbers and acknowledgments. TCP re-sends
anything that is not received and supplies a virtual circuit
between end-user applications. The advantage of TCP is that
it provides guaranteed delivery of the segments.
- UDP -- connectionless and
unreliable; although responsible for transmitting messages,
no software checking for segment delivery is provided at
this layer. The advantage that UDP provides is speed. Since
UDP provides no acknowledgments, less traffic is sent across
the network, making the transfer faster.
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