Published by: Zaya
Published date: 22 Jun 2021
The transport layer transmits the entire message to the destination. Therefore, it ensures the end-to-end delivery of an entire message from a source to the destination.
The transport layer provides the user address which is specified as a station or port. The port variable represents a particular TS-user of a specified station known as a Transport Service access point (TSAP). Each station has only one transport entity.
The transport layer provides reliability services by retransmitting the lost and damaged packets.
Reliable delivery has four aspects:
The primary role of reliability is Error Control. In reality, no transmission will be 100 percent error-free delivery. Therefore, transport layer protocols are designed to provide error-free transmission.
The data link layer also provides the error handling mechanism, but it ensures only node-to-node error-free delivery. However, node-to-node reliability does not ensure end-to-end reliability.
The data link layer checks for the error between each network. If an error is introduced inside one of the routers, then this error will not be caught by the data link layer. It only detects those errors that have been introduced between the beginning and end of the link. Therefore, the transport layer performs the checking for the errors end-to-end to ensure that the packet has arrived correctly.
The second aspect of the reliability is sequence control which is implemented at the transport layer.
On the sending end, the transport layer is responsible for ensuring that the packets received from the upper layers can be used by the lower layers. On the receiving end, it ensures that the various segments of a transmission can be correctly reassembled.
Loss Control is the third aspect of reliability.
The transport layer ensures that all the fragments of a transmission arrive at the destination, not some of them.
On the sending end, all the fragments of transmission are given sequence numbers by a transport layer.
These sequence numbers allow the receiver's transport layer to identify the missing segment.
Duplication Control is the fourth aspect of reliability.
The transport layer guarantees that no duplicate data arrive at the destination.
Sequence numbers are used to identify the lost packets; similarly, it allows the receiver to identify and discard duplicate segments.
Flow control is used to prevent the sender from overwhelming the receiver.
If the receiver is overloaded with too much data, then the receiver discards the packets and asking for the retransmission of packets. This increases network congestion and thus, reducing the system performance.
The transport layer is responsible for flow control. It uses the sliding window protocol that makes the data transmission more efficient as well as controls the flow of data so that the receiver does not become overwhelmed. Sliding window protocol is byte-oriented rather than frame-oriented.
The transport layer uses multiplexing to improve transmission efficiency.
Multiplexing can occur in two ways:
Upward multiplexing: Upward multiplexing means multiple transport layer connections use the same network connection. To make it more cost-effective, the transport layer sends several transmissions bound for the same destination along the same path; this is achieved through upward multiplexing.
Downward multiplexing: Downward multiplexing means one transport layer connection uses multiple network connections. Downward multiplexing allows the transport layer to split a connection among several paths to improve the throughput. This type of multiplexing is used when networks have a low or slow capacity.