OSI reference model uses the term Protocol Data Unit (PDU) for a specific block of information from a specific layer, which is transferred over the network from sender to receiver. Protocol Data Unit (PDU) is often used in reference to the OSI reference model. A Protocol Data Unit (PDU) includes the headers, trailers and actual data.
Following table explains about different PDUs at important layers of OSI model. Presentation and session layers are removed, because those layers are not much relevant these days.
|Layer Number||Layer Name||Description|
|Layer 7||Application Layer||Original data generated from network application.|
|Layer 4||Transport Layer||Original data with TCP/UDP headers containing sender’s and receiver’s TCP/UDP port numbers.|
|Layer 3||Network Layer||Data with TCP/UDP headers, with Network Layer header containing sender’s and receiver’s IP addresses.|
|Layer 2||Datalink Layer||Original data with TCP/UDP header, network layer header, and Datalink Layer header containing sender’s and receiver’s MAC addresses. Datalink layer adds a trailer also.|
|Layer 1||Physical Layer||Bits for network transmission.|