Transport Layer is the second layer of the TCP/IP model. It is an end-to-end layer used to deliver messages to a host. It is termed as an end-to-end layer because it provides a point-to-point connection rather than hop-to- hop, between the source host and destination host to deliver the services reliably. The unit of data encapsulation in Transport Layer is a segment.
The two main Transport layer protocols are:
- Transmission Control Protocol (TCP) which provides reliable communication between two hosts.
- User Datagram Protocol (UDP) provides unreliable communication between two hosts.
Functions of Transport Layer :
Specific functions of the transport layer are as follows:
- Process to process delivery :
While Data Link Layer requires the MAC address, Transport Layer header includes service point address which is port address. - Segmentation and Reassembling :
A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission. - Connection Control – It includes 2 types:
- (i). Connectionless Transport Layer –
Each segment is considered as an independent packet and delivered to the transport layer at the destination machine. - (ii). Connection Oriented Transport Layer –
Before delivering packets, connection is made with transport layer at the destination machine.
- (i). Connectionless Transport Layer –
- Multiplexing and Demultiplexing :
Multiplexing allows simultaneous use of different applications over a network which is running on a host. The transport layer provides this mechanism which enables us to send packet streams from various applications simultaneously over a network. - Flow control :
The transport layer provides a flow control mechanism between the adjacent layers of the TCP/IP model. TCP also prevents data loss due to a fast sender and slow receiver by imposing some flow control techniques. It uses the method of sliding window protocol which is accomplished by the receiver by sending a window back to the sender informing the size of data it can receive. - Error Control :
Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission. - Congestion Control :
Congestion is a situation in which too many sources over a network attempt to send data and the router buffers start overflowing due to which loss of packets occur. TCP provides AIMD- additive increase multiplicative decrease, leaky bucket technique for congestion control. - Data integrity and Error correction :
Transport layer checks for errors in the messages coming from application layer by using error detection codes, computing checksums, it checks whether the received data is not corrupted and uses the ACK and NACK services to inform the sender if the data has arrived or not and checks for the integrity of data. - End-to-end Connection between hosts :
The transport layer is also responsible for creating the end-to-end Connection between hosts for which it mainly uses TCP and UDP.
Please write comments if you find anything incorrect. A gentle request to share this topic on your social media profile.