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.

Other features of the transport layer include the creation, mon­i­tor­ing and ter­mi­na­tion of a con­nec­tion. If the transfer occurs in the con­nec­tion-oriented mode, the suc­cess­ful data transfer will be secured with a con­fir­ma­tion. This means that the computer sending the data knows that all units have been trans­ferred as intended. If there is no con­fir­ma­tion, it will try to carry out the transfer again. When doing this, the transport layer doesn’t pay attention to the media used in the first three layers.

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 optimizes 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 (fiber 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