In the field of computer networking, the purpose of
the 4th layer or the transport layer is to provide services for the
end to end communication for the various operating applications. The
services are provided within an architectural framework that consists of
protocols and the components and is layered. It also offers convenient services
such as the following:
Ø Connection
– oriented data stream support
Ø Reliability
Ø Flow
control
Ø Multiplexing
and so on.
- Both the OSI (open systems interconnection) and
TCP/ IP model include the transport layer.
- The foundation of the internet is
based up on the TCP/ IP model whereas for the general networking, the OSI model
is followed.
- However, the transport layer is defined differently in both of
these models. Here we shall discuss about the transport layer in the TCP model
since it is used for keeping the API (application programming interface)
convenient to the internet hosts.
- This is in contrast with the definition of
the transport layer in the OSI model.
- TCP (transmission control protocol) is
the most widely used transport protocol and so the internet protocol suite has
been named after it i.e., the TCP/ IP.
- It is a connection-oriented transmission
protocol and so it is quite complex.
- This is also because it incorporates
reliable data stream and transmission services in to its state-ful design.
- Not
only TCP there are other protocols in the same category such as the SCTP
(stream control transmission protocol) and DCCP (datagram congestion control
protocol).
Now let us see what all services are provided by the transport layer
to its upper layers:
ØConnection-oriented communication: It is quite easy for the application for interpreting
the connection as a data stream instead of having to cope up with the
connectionless models that underlie it. For example, internet protocol (IP)
and the UDP’s datagram protocol.
Ø Byte
orientation: Processing the data stream is quite easy when compared with using
the communication system format for processing the messages. Because of such
simplification, it becomes possible for the applications to work up on message
formats that underlie.
Ø Same
order delivery: Usually, it is not guaranteed by the transport layer that the
data packets will be received in the same order in which they were sent. But
this is one of the desired features of the transport layer. Segment numbering
is used for incorporating this feature. The data packets are thus passed on to
the receiver in order. Head of line blocking is a consequence of
implementing this.
Ø Reliability: During the transportation some data packets might be lost because of errors and
problems such as network congestion. By using error detection mechanism such as
CRC (cyclic redundancy check), the data might be checked by the transport
protocol for any corruption and for the verification whether the correct
reception of the data by either sending a NACK or an ACK signal to the sending
host. Some schemes such as the ARR (automatic repeat request) are sometimes
used for the retransmission of the corrupted or the lost data.
Ø Flow
control: The rate with which the data is transmitted between two nodes is
managed for preventing a sending host with a fast speed from the transmission
of data more than what the receiver’s data buffer can take at a time. Otherwise
it might cause a buffer overrun.
Ø Congestion
avoidance: Traffic entry in to the network can be controlled by means of
congestion control by avoiding congestive collapse. The network might be kept
in a state of congestive collapse by automatic repeat requests.
