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Explain Operating System Interface OSI Layer 4 – Transport Layer

About OSI – Transport Layer

1. The OSI layer 4 is the transport layer.
2. As the name of this layer suggests, it facilitates the transfer of the data between different networks.
3. It acts as a reliable data transfer service for the layers above it.
4. It is this layer that controls the reliability of the given data links through three processes namely:
– Flow control
– Segmentation and desegmentation
– Error control

5. There exist some protocols that are both connection and state oriented.
6. This makes it possible for the transport layer to keep a track of the segments while at the same time re-transmitting those that failed due to some reason.
7. The acknowledgement regarding whether the success or failure of the data transmission is provided by this layer.
8. If the data transmission was successful, the layer sends the next data.
9. Five classes are defined by the OSI model concerning the connection – mode transport protocols. 10. These classes range from class 0 to class 4.
11. The class 0 is more commonly known as TP0 and has got the least features.
12. Class 4 is contrary to class 4.
13. Its common name is TP4 and has been designed for the networks that are less reliable such as internet.
14. There is no error recovery feature in class 0 and is therefore meant for the network layers providing connections free of any errors.
15. The class closest to TCP is the class 4.
16. All of the classes are meant for the connection oriented networks.
17. Out of the 5 classes, only the TP4 class can be applied for the connection-less network.
18. Concatenation and separation is available in all classes except the class 0.
19. Separation and reassembling feature is available in all the classes.
20. Only class 1 and class 3 have the capability to re-initiate a connection.
21. Classes 2, 3 and 4 offer multiplexing as well as de-multiplexing over a virtual circuit and provide an explicit flow control.
22. Re-transmitting the data when timeout occurs can only be done by class 4.
23. Out of all these, class 1, 3 and 4 only offer the reliable transport service.
24. It is the tunneling protocols that operate on the transport layer.
25. These include the non – IP protocols (like novell’s IPX or IBM’s SNA) which run with the IPsec. 26. Two protocols namely the UDP (user data gram protocol) and the TCP (transmission control protocol) as considered as the layer 4 protocols in the OSI model.
27. End – to – end communication services are provided by this layer only.
28. However, these services are only meant for the applications that have network protocols and components having a layered architecture.
29. Some of the services provided by the transport layer are:
– Connection – oriented data stream support
– Reliability
– Flow control
– Multiplexing

30. Transport layer is found in both the models i.e., the OSI (open systems interconnection model) model and the TCP/ IP model.
31. OSI model is not so convenient for the application programming interface when compared to the TCP/ IP model.
32. TCP (transmission control protocol) is the most popular transport protocol.
33. All the connection – oriented transmissions are based up on this protocol whereas all the connection-less transmissions are dependent on the user data-gram protocol (UDP) and is also used for messaging transmissions that are simpler.
34. But TCP is quite complex.
35. This is because of its state ful design that incorporates reliable data stream and transmission services.
36. Some other prominent protocols in this layer are the DCCP (datagram congestion control protocol) and the SCTP (stream control transmission protocol).

Below mentioned are some services offered by the transport layer:
1. Byte orientation: Application finds it easy to operate up on the data supplied as a sequence of bytes. Such byte orientation lets the application deal with a number of underlying message formats.

2. Congestion avoidance: Traffic entry in to any telecommunication network is controlled by the congestion control. Congestion control is important to keep the congestion collapse at bay. Congestive collapse occurs because of the over subscription of the link capabilities of the networks.

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