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Role of Transport Layer - Responsibilities and Protocols

The transport layer is the fourth layer in OSI Reference Model. It responds to service requests from Session layer and issues service requests to Network layer. Transport Layer is responsible for delivering messages between hosts. In the Transport layer, data travels in the form of segments. The transport layer is responsible for creating an end to end connection between source IP and the destination IP. For establishing this end to end connection, the Transport layer is using two major protocols TCP and UDP.

TCP is connection oriented protocol where UDP is connectionless protocol. So by using TCP, we can create an end to end reliable connection between the source and destination hosts.

The transport layer is called the host-to-host transport layer in the TCP/IP model.


Duties of  The Transport Layer

Major duties of the Transport layer are:

  1. Creating an end-to-end connection between hosts in different networks,

  2. Error recovery,

  3. Flow control,

  4. Ensuring complete data transfer in TCP

  5. Congestion avoidance


The stateful protocol TCP helps to establish the end to end connection in the Transport layer. It offers reliable delivery of data to the destination host.

Transport Layer is also responsible for error recovery and is responsible for the resending of data segments which had an error. TCP provides this feature.

Flow control is the process of ensuring where the amount of data sending to the destination is affordable by it. Without flow control, a faster source might flood destination with so much information. To accomplish it we are using many techniques like windowing.

As I explained above, TCP is a connection-oriented protocol. As the title suggests, it ensures the complete transfer of segments. TCP ensures the resending of segments If the destination host did not receive them.

For example, let us consider two hosts A and B. A is sending data to B, but B fails to receive a few of them. Then B will request A to resend them. When A receives the request, it will resend the missing part. So, this connection ensures the complete data transfer.

Datagram Congestion Control Protocol (DCCP) helps to prevent congestion avoidance in this layer. By removing the possibilities of bottleneck issues, DCCP assures congestion avoidance.

Major Transport Layer protocols

List of the major protocols works in the Transport later are:
  1. TCP( Transmission Control Protocol)

  2. UDP( User Datagram Protocol)

  3. DCCP (Datagram Congestion Control Protocol)

  4. SCTP (Stream Control Transmission Protocol)



Suggested Readings
  1. Physical layer

  2. Data link layer

  3. Network layer

  4. Transport layer

  5. Session layer

  6. Presentation layer

  7. Application layer

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