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Transport Layer (Layer 4)
(Page 2 of 3)
Transport Layer Functions
Lets look at the specific functions
often performed at the transport layer in more detail:
- Process-Level Addressing: Addressing at
layer two deals with hardware devices on a local network, and layer
three addressing identifies devices on a logical internetwork. Addressing
is also performed at the transport layer, where it is used to differentiate
between software programs. This is part of what enables many different
software programs to use a network layer protocol simultaneously, as
mentioned above. The best example of transport-layer process-level addressing
is the TCP
and UDP port mechanism used in TCP/IP,
which allows applications to be individually referenced on any TCP/IP
- Multiplexing and Demultiplexing: Using
the addresses I just mentioned, transport layer protocols on a sending
device multiplex the data received from many application programs
for transport, combining them into a single stream of data to be sent.
The same protocols receive data and then demultiplex it from
the incoming stream of datagrams, and direct each package of data to
the appropriate recipient application processes.
- Segmentation, Packaging and Reassembly:
The transport layer segments the large amounts of data it sends over
the network into smaller pieces on the source machine, and then reassemble
them on the destination machine. This function is similar conceptually
to the fragmentation function of the network layer; just as the network
layer fragments messages to fit the limits of the data link layer, the
transport layer segments messages to suit the requirements of the underlying
- Connection Establishment, Management and Termination:
Transport layer connection-oriented protocols are responsible for the
series of communications required to establish a connection, maintain
it as data is sent over it, and then terminate the connection when it
is no longer required.
- Acknowledgments and Retransmissions: As
mentioned above, the transport layer is where many protocols are implemented
that guarantee reliable delivery of data. This is done using a variety
of techniques, most commonly the combination of acknowledgments
and retransmission timers. Each time data is sent a timer is
started; if it is received, the recipient sends back an acknowledgment
to the transmitter to indicate successful transmission. If no acknowledgment
comes back before the timer expires, the data is retransmitted. Other
algorithms and techniques are usually required to support this basic
- Flow Control: Transport layer protocols
that offer reliable delivery also often implement flow control
features. These features allow one device in a communication to specify
to another that it must "throttle back" the rate at which it is sending
data, to avoid bogging down the receiver with data. These allow mismatches
in speed between sender and receiver to be detected and dealt with.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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