Transport Layer

Transport layer is a 4^th layer from the top. The role of the transport layer is to provide the communication services directly to the application processes running on different hosts.

The transport layer provides a logical communication between application processes running on different hosts. Although the application processes on different hosts are not physically connected, application processes use the logical communication provided by the transport layer to send the messages to each other.

A computer network provides more than one protocol to the network applications. For example, TCP and UDP are two transport layer protocols that provide a different set of services to the network layer. The transport layer protocols are implemented in the end systems but not in the network routers.

All transport layer protocols provide multiplexing/demultiplexing service. It also provides other services such as reliable data transfer, bandwidth guarantees, and delay guarantees.

Each of the applications in the application layer has the ability to send a message by using TCP or UDP. The application communicates by using either of these two protocols. Both TCP and UDP will then communicate with the internet protocol in the internet layer. The applications can read and write to the transport layer. Therefore, we can say that communication is a two-way process.

Services provided by the Transport Layer

1. End-to-end delivery: 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.

2. Addressing : Data generated by an application on one machine must be transmitted to the correct application on another machine. In this case, addressing is provided by the transport layer.

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 protocols need to know which upper-layer protocols are communicating.

3. Reliable delivery: The transport layer provides reliability services by retransmitting the lost and damaged packets.

The reliable delivery has four aspects:

• Error control: 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 the end-to-end reliability.

The transport layer performs the checking for the errors end-to-end to ensure that the packet has arrived correctly.

• Sequence control: 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 lay.

• Loss control: Loss Control is a 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: 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.

4. Flow Control: 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 it controls the flow of data so that the receiver does not become overwhelmed. Sliding window protocol is byte oriented rather than frame oriented.

5. Multiplexing: The transport layer uses the 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 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 the 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.

Elements of Transport Protocols

The fourth layer in the OSI model is known as the transport layer from the top. This layer provides the communication services directly to the running application processes on different hosts. Even though these processes on various hosts are not linked physically, and use logical communication to transmit the messages to each other.

The protocols of this layer are implemented within the end systems, not in the routers of the network.

For instance, the two transport layer protocols like TCP &UDP provides different services to the network layer.

All the protocols in this layer provide different services like multiplexing, de-multiplexing, reliable data transfer, bandwidth & delay guarantees.

Elements of Transport Layer

The elements of this layer include the following.

Service-point Addressing

Computers frequently run several programs at a time, because of this reason, the delivery of source to destination in two computers is not possible. So this layer is used for an exact kind of address to its header which is known as a port address or service point address. Through this address, every packet arrives at the correct computer & also transport layer attains the total message to the exact method on that computer.

Segmentation & Reassembly

In a segmentation process, a message can be separated into communicable segments where each segment includes a sequence number that enables the layer to rebuild the message. Once the message arrives from the source system to the destination system then the message can be reassembled properly, packets can be identified and replaced because of the lost data in transmission.

Connection Control

These are two types namely connectionless and connection-based.

Connectionless based Transport Layer

In connectionless, the transport layer treats every packet like an individual & transmits it to the destination computer. In this kind of transmission, the receiver doesn’t transmit an acceptance to the sender regarding a packet receipt.

Connection based Transport Layer

This layer makes a connection with the layer at the destination of the computer before sending the packets toward the target. The connection can be created by using the following steps: (Three way hand shaking mechanism)

• Establishment of Connection
• Transfer of Data
• Termination Connection

So connection-based service is used for the entire data transmission, whereas connectionless service is less consistent

Multiplexing & De-multiplexing

Several packets are transmitted across a system from various applications by using control systems and these are found within the transport layer. This layer allows packets using different methods, and these packets are distinguished through their port numbers & transmit them to the layer once adding correct headers.

In the De-multiplexing process, the data can be obtained which is transmitted from different processes. It gets the data segments from this layer to deliver it to the suitable method running on the machine at the receiver end.

Flow Control

The transport layer is also accountable for the flow control between the contiguous layers of the TCP/IP model. It doesn’t execute across a single link even it executes a continuous node. By impressive flow control methods, loss of data can be stopped from the sender & slow receiver.

For example, it uses the sliding window protocol technique. In this technique, the receiver transmits a window back toward the sender to update the received data size.

Error Control

This is also achieved back-to-back similar to the data link layer. In this layer, the total message can appear at the receiving end of the layer without any fault. The improvement of error can be attained through the packet re-transmission. The integrity of data can be checked once the data has arrived or not by using the services like the ACK & NACK to update the sender.