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Networking Fundamentals · Foundational

Protocol Data Unit (PDU)

The name for the unit of data at a given layer, such as bits, frame, packet, segment or data.

Definition

PDUs are named by layer: Layer 1 bits, Layer 2 frame, Layer 3 packet, Layer 4 segment (TCP) or datagram (UDP), and Layers 5-7 data. Each PDU consists of the upper-layer PDU as payload plus that layer's control information. The term is also used generically for any protocol message, such as an SNMP PDU.

Example

A TCP segment becomes the payload of an IP packet, which becomes the payload of an Ethernet frame.

Exam tip

Mnemonic for Layers 4 down to 1 is "Some People Fear Birthdays": segment, packet, frame, bits.

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