OSI Transport Layer

The transport layer is responsible for organizing incoming data packets using their content, and assigning them a port number.

The content of the packets suddenly becomes important, since we need to read the content of the packet in order to assign it a port number corresponding to it's network application. For example, data read from a packet sent by an HTTP application can be funneled to port 80. To do this, data from the upper layers is organized as layer 4 PDUs or segments, each segment tagged with it's associated port number.

The packet is then forwarded to the Network layer for delivery. At that point, outgoing packets from many different hosts are multiplexed on a single link for speed and efficiency, using the port numbers and the destination network addresses. Other devices that help out on the transport layer include multilayer switches, load balancers, firewalls, IDSs, and more.