Computer Networking

Transport Layer

Transport Layer


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

Services of Transport Layer

Transport Layer use the TCP or UDP protocols to provide its various services. When the data came at Transport layer, then it adds the TCP or UDP header with this data. This header adds the some functionality. The services provided by the transport layer through TCP or UDP protocols are given below

1. End-To-End Delivery

Transport layer is responsible of Port to Port Delivery. It ensures that the receiving port must obtain the same data as it transmitted by sending port. It use 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 identifying by its unique segment number.  These segments are converted into packets at network layer.

3. Full Duplex

Instead of UDP, transport layer use TCP to achieve the functionality of 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 the considerable load on network.


5. Reliable Delivery

The transport layer provides the services of 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.
  • Transport layer use checksum method to error control which ensures the end-to-end reliability. As we know, the data link layer also provides the facility of error handling by using CRC method, but CRC method ensures only node-to-node error-free delivery.

ii. Sequence Control

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

iii. Loss Control

  • The Reliability of 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 the aspect of reliability. The transport layer also ensure 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 transmission of too much data of sender, then the receiver discards some packets and request for the retransmission of discarded packets. This phenomena cause to reducing the system performance.

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

7. Multiplexing

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

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

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