It is intended primarily to create an everyday understanding for developing networking standards and protocols.
The model is pretty useful to help understand the basics of computer networking.
However, it is very worth keeping in mind that the real-world implementation of protocols differs somewhat.
There are plenty of layer-crossing protocols.
Modern networking concepts also dont necessarily fit into the OSI model well.
Still, it is helpful to understand the basics.
The transport layer is layer four.
The transport layer is responsible for providing end-to-end communication services between communicating devices.
Transport layer protocols can be connection-oriented.
They can, however, also be connectionless.
TCP, which stands for Transmission Control Protocol, is the best-known and most-used transport layer protocol.
It is connection-oriented forming bidirectional communication, and features error detection and retransmission functionality.
UDP, the most popular transport layer protocol, stands for Universal Datagram Protocol.
It is connectionless, and the sender never knows if the recipient ever received a transmission.
Together these two protocols make up most of the web link traffic on the Internet.
They are also the only two protocols implemented on all major operating systems.
TLS, or Transport Layer Security, is an encryption protocol used in HTTPS, amongst other things.
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Features of Transport Layer Protocols
Transport layer protocols can be connection-oriented.
The sequence of data is also generally important.
Segments can be numbered so that they can be reordered if shown in the incorrect order.
Error detection can be implemented using error-detecting codes such as a checksum.
A receipt message can be sent confirming successful receipt with an ACK and an error with a NACK.
Upon receipt of a NACK or a timeout, the sender may automatically repeat the transmission.
Flow control can ensure that the sender does not transmit so fast that the receiver cant keep up.
Congestion avoidance helps to ensure optimum data pipe performance even under heavy loads.
None of these features are mandatory for transport layer protocols.
TCP, for example, does support all of the above features.
UDP, however, only supports Multiplexing.
Connection-Oriented vs. Connectionless
Typically, most web connection communications are bidirectional and follow a request-response pattern.
Web traffic is an excellent example of this.
For request-response use cases, TCP is ideal.
It offers a reliable connection between the two parties.
If a segment fails to be appropriately received, this could cause a webpage not to display correctly.
However, automatic error checking and replaying help detect and correct these errors as fast as possible.
The additional transport time is less of an issue than broken content.
This isnt always the case, though.
As such, real-time and streaming content tends to use UDP.
Its lighter weight and faster because it doesnt guarantee a successful transmission, unlike TCP.
Conclusion
The transport layer is layer 4 of the OSI model.
It is responsible for providing end-to-end communication services between communicating devices.
Both options are helpful and have many use cases.
Combined, TCP and UDP make up most of the connection traffic on the Internet.
