What is the transport layer in the OSI model?

The OSI model transport layer takes data from the session layer and moves it to the network layer. The transport layer breaks down packets, manages transfer speed and ensures secure and transparent communication between two systems.

The transport layer is the fourth layer in the OSI model and facilitates communication between two systems via a continuous, secure and transparent end-to-end data transfer. The transport layer is also responsible for avoiding congestion.

The OSI model is a standard used to create a connection between two end devices within a network, even if the participants are using different software or hardware. This model was first developed in the 1970s and was published for the first time in 1983. It consists of seven different levels, which are built one on top of the other. Each layer has its own unique set of tasks. The seven layers are as follows:

1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Presentation layer
7. Application layer

The main task of the transport layer is to prepare a functional and secure end-to-end data transfer within a network. To do this the transfer layer takes the data from the session layer (layer 5) and then sends it to the network layer (layer 3). The transport layer can, if needed, separate the data into smaller units or pull the data together into larger packets to send them on more easily.

Transfers can occur in either a connection-oriented or connectionless mode. The transport layer can use a network connection, a connection for multiple connections or spread a transport connection across different network connections. This is always done in a transparent way.

There are many services offered by the transport layer to the higher levels. These are relevant for different aspects of the data transfer. Among the most important services include the following:

• Connection-oriented transfers: the transport layer enables connection-oriented transfers such as TCP (Transmission Control Protocol). To do this, it assigns port numbers between 0 and 65.535 and relies on the confirmation process described above.
• Connectionless protocols: unlike connection-oriented protocols, connectionless transfers do not need a confirmation. Although this security feature is not used, this method can still be very practical, especially with real-time transfers such as video conferencing. Protocols such as UDP (User Datagram Protocol) also use ports between 0 and 65.535.
• Same order delivery: this service ensures that data packets are sent and received in a set order. To this end, the individual packets are numbered and can be ordered correctly.
• Reliability: when sending data between two systems, data may become damaged, get lost or be received in the wrong order. The transport layer uses troubleshooting codes to ensure that the data arrives as intended. To this end, the transport layer sends a confirmation message to the sender.
• Flow control: flow control regulates and optimises data flow. This allows for adjustments to be made to the data exchange by increasing or throttling the transmission speed. This prevents the receiver from being overloaded.
• Congestion avoidance: if there is still some congestion on the nodes and connections, the transport layer can implement measures to avoid long-term congestion. This includes, for example, a reduction in the transfer rate.
• Multiplexing: packets which are transferred from one system to another can come from many different sources. With multiplexing, the transport layer allows users to open applications and services from different sources within a network.

There are many protocols which use the OSI model transport layer. These include among others:

• DCCP (datagram congestion control protocol): a network protocol to transfer media in IP networks in real time without sending an obligatory confirmation
• FCP (fibre channel protocol): a SCSI interface protocol for a standard interface within a memory network
• IL Protocol: a simpler form of TCP
• MPTCP (multipath TCP): a recommended standard which pulls together different paths
• NORM (NACK-oriented reliable multicast): a protocol used for reliable transport in multicast groups within a network
• RDP (reliable data protocol): a transport protocol for transferring images and data
• RUDP (reliable user datagram protocol): A protocol for the Plan 9 operating system
• SCTP (stream control transmission protocol): a network protocol that is placed on a potentially unreliable packet service
• TCP (transmission control protocol): a widespread network protocol that determines the way data is transferred between network components
• UDP (user datagram protocol): a minimalistic network protocol that allows for datagrams to be sent on IP networks