Transport Layer

Transport layers (Layer 4) work transparently within the layers above to deliver and receive data without errors.

The send side breaks application messages into segments (packets) and passes them on to the network layer (Layer 3).

The receiving side then reassembles segments into messages and passes them to the application layer (Layer 7).

What Services Can the Transport Layer Provide?

Connection-Oriented Communication

Devices at the end-points of a network communication establish a handshake protocol such as TCP to ensure a connection is robust before data is exchanged.

The weakness of this method is that for each delivered message, there is a requirement for an acknowledgment, adding considerable network load compared to self-error-correcting packets.

The repeated requests cause significant slowdown of network speed when defective byte streams or datagrams are sent.

Same Order Delivery

Ensures that packets are always delivered in strict sequence by assigning them a number.

Although the network layer is responsible, the transport layer can fix any discrepancies in sequence caused by packet drops or device interruption by reordering them.

Data Integrity

Using checksums, the data integrity across all the delivery layers can be ensured.

These checksums guarantee that the data transmitted is the same as the data received and that is not corrupt.

Missing or corrupted data can be resent by requesting retransmission from other layers.

Flow Control

Devices at each end of a network connection often have no way of knowing each other's capabilities in terms of data throughput.

Data can end up being sent faster than the speed at which the receiving device is able to buffer or process it. When this happens, buffer overruns can cause complete communication breakdowns.

Conversely, if the receiving device is not receiving data fast enough, this causes a buffer underrun, which may well cause an unnecessary reduction in network performance.

Flow control ensures that the data is sent at a rate that is acceptable for both sides by managing data flow.

Traffic Control

Digital communications networks are subject to bandwidth and processing speed restrictions, which can mean a huge amount of potential for data congestion on the network.

This network congestion can affect almost every part of a network. The transport layer can identify the symptoms of overloaded nodes and reduced flow rates and take the proper steps to remediate these issues.

Multiplexing

The transmission of multiple packet streams from unrelated applications or other sources (multiplexing) across a network requires some very dedicated control mechanisms, which are found in the transport layer.

This multiplexing allows the use of simultaneous applications over a network such as when different internet browsers are opened on the same computer.

In the OSI model, multiplexing is handled in the service layer.

Byte orientation

Some applications prefer to receive byte streams instead of packets; the transport layer allows for the transmission of byte-oriented data streams if required.