Transport Layer

The transport layer is the 4th  layer in the OSI model and is responsible for end-to-end (port-to-port). communication over a network.

Services of Transport Layer

Transport Layer uses the TCP or UDP protocols to provide its various services. When the data comes to the Transport layer, the TCP or UDP header is added to this data. This header adds some functionality. The services provided by the transport layer through TCP or UDP protocols are given below

1. End-to-end Delivery

The transport layer is responsible for Port-to-Port Delivery. It ensures that the receiving port must obtain the same data as it is transmitted by the sending port. It uses the checksum method for error detection. End-to-end delivery is also called port-to-port or Process-to-process delivery. 

2. Segmentation

Byte streaming from upper layers is converted into segmentations.  Through segmentation, larger pieces of data are divided into smaller segments/blocks. Each segment is identified by its unique segment number.  These segments are converted into packets at the network layer.

3. Full Duplex

Instead of UDP, the transport layer uses TCP to achieve the functionality of a full duplex.

4. Connection-Oriented Communication

For end-to-end communication, a link is established through TCP protocol to ensure a connection before to data is transferred.

The limitation of this method is that for each delivered message, there is a requirement for an acknowledgment; it adds a considerable load on the network.

5. Reliable Delivery

The transport layer provides reliability by retransmitting the lost and damaged packets. The reliable delivery has four aspects:

i. Error Control

  • It used the checksum algorithm for error deduction. The basic purpose of reliability is Error Control. In this way, the packet has arrived correctly.
  • The transport layer uses the checksum method for error control, which ensures end-to-end reliability. As we know, the data link layer also provides error handling by using the CRC method, but the CRC method ensures only node-to-node error-free delivery.

ii. Sequence Control

  • Reliability also involves the factor of sequence control. It means sending and receiving orders must be the same so that various segments of a transmission can be correctly reassembled. For example, if while sending the packet, P1 is transferred initially and then P2 and P3 are transmitted, then at the receiving end, the P1 must receive first, then P2 and P3.

iii. Loss Control

  • The reliability of the transport layer ensures that all the fragments arrive at the destination successfully without losing some of them.
  • If some segment is missing, then its sequence number identifies it while reassembling.

iv. Duplication Control

  • Duplication Control is also an aspect of reliability. The transport layer also ensures that no duplicate data arrive at the destination.
  • If some segment is duplicated, then its sequence number identifies it while reassembling. In this way, a duplicate segment is discarded.

6. Flow Control

If the receiver is overloaded due to the transmission of too much data by the sender, then the receiver discards some packets and requests for the retransmission of discarded packets. These phenomena cause a reduction in the system performance.

 The transport layer uses the “sliding window protocol” to handle the flow control.

7. Multiplexing

The transport layer maintains multiplexing. The transmission of multiple packet streams from different applications at a time is multiplexed and DE multiplexed across a network.

This multiplexing allows the use of simultaneous applications over a network at a time. For example, the Opening of different internet browsers on the same computer at a time happens due to multiplexing.