OSI Layer 4 – Transport

Layer 4 of the OSI model is the Transport layer.

aka end-to-end or host-to-host layer
content of the packets becomes significant
- a host on a network will be communicating with many other hosts using many different types of networking data
transport layer identifies each type of network application by assigning it a port number
on the sending host:
- data from upper layers is packaged as a series of layer 4 PDUs, called segments
- each segment is tagged with the application’s port number
- segment is passed to the Network layer for delivery
many hosts can transmit multiple packets at the same time
- these are multiplexed using the port numbers along with the source and destination network address onto the same link

Network and Data Link layers ignore port numbers
- becomes part of the payload and is invisible to the routers and switches at these layers
at the receiving host:
- each segment is decapsulated, identified by its port number, and passed to the relevant handler at the Application layer
- aka, traffic stream is de-multiplexed
can implement reliable data delivery mechanisms
- any lost or damaged packets are resent

Devices Operating at the Transport Layer