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Saturday, September 21, 2013

What are the services provided to upper layers by transport layer?

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. 

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