Protocol Data Unit

networking

Definition

Protocol Data Unit

Let a protocol stack consist of layers $1,\dots ,n$, where layer $\ell$ provides a service to layer $\ell +1$ via encapsulation.

A protocol data unit (PDU) at layer $\ell$ is the unit of data exchanged between peer layer-$\ell$ entities across the network:

$${\text{PDU}}_{\ell }={\text{header}}_{\ell }\Vert {\text{payload}}_{\ell }$$

where ${\text{header}}_{\ell }$ is the layer-$\ell$ protocol control information and ${\text{payload}}_{\ell }$ is the service data unit (SDU) received from layer $\ell +1$:

$${\text{payload}}_{\ell }={\text{SDU}}_{\ell }={\text{PDU}}_{\ell +1}$$

Names per Layer

Layer	PDU Name
Application	Message
Transport	Segment (TCP) / Datagram (UDP)
Network	Packet / Datagram
Data link	Frame
Physical	Bit / symbol