OSI Layer 4 - Transports

The Transport layer of the OSI model is responsible for differentiating network flows.

At any given time on the user's computer there may be an open Internet browser, when music is streaming, while messenger or chat app is running. Every this application sends and receives data from the Internet, and all the data arrives in the form of 1's and 0's on that computer's NIC.

Something has to be there to tell which 1 and 0 belong to messenger or browser or streaming music. That "something" is Layer 4 : 

 

 

Layer 4 accomplishes this by using an addressing scheme known as Port Number.

Specifically, there are two methods to differentiate network flows. They are known as Transmission Control Protocol (TCP), or User Datagram Protocol (UDP). Both TCP and UDP have 65,536 port numbers (each), and application flows uniquely identified by Source and Destination ports (in combination with Source and Destination IP address).

TCP and UDP use different strategies in how data flows transferred, and the differences and inner workings of the two are interesting and significant, but unfortunately they are outside the scope of this article series. They will be the topic of an upcoming article or series. 

To summarize, if Layer 2 is responsible for hop to hop delivery, and Layer 3 is responsible for end to end delivery, it could be said that Layer 4 is responsible for service-to-service delivery.