Role of Transport Layer - Responsibilities and Protocols

The transport layer is the fourth layer in the OSI Reference Model. It lies between the Network and Session layers (in the OSI Model).

It is one of the critical layers with many duties and responsibilities. This tutorial explains the working, responsibilities, and protocols in the Transport layer.

The Working Of The Transport Layer

The primary responsibility of the Transport layer is to establish an end-to-end connection between the source IP and the destination IP. TCP and UDP are the two Transport layer protocols that establish an end-to-end connection.

TCP and UDP work in the Transport Layer.

It responds to service requests from the Session layer and issues service requests to the Network layer.

In the Transport layer, data travels in the form of segments.

After creating an end-to-end connection, this layer delivers messages between the devices. Transport Layer is responsible for delivering messages between hosts.

TCP is a connection-oriented protocol, but UDP is a 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 also present in the TCP/IP model. It is called the host-to-host transport layer in the TCP/IP model. Now, let us check the duties of the Transport layer in a network.

Duties of The Transport Layer

  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 resending the data segments that had an error. The TCP protocol handles this duty.

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

How Does the Transport Layer Ensure Complete Data Delivery?

As I explained above, TCP is a connection-oriented protocol. As the title suggests, it ensures the complete transfer of segments. TCP verifies if the destination host receives them. If not, it will resend the segments.

Assurance of a Complete Data Delivery is possible with a TCP connection only.

For example, let us consider two hosts, A and B. A sends data to B, but B fails to receive a few. 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.

This way, the Transport layer assures complete data delivery.

Datagram Congestion Control Protocol = DCCP

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

Transport Layer protocols

A 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