Protocol data unit

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

A protocol data unit is a unit of data consisting of a protocol payload together with the control information added at a networking layer.

A protocol data unit (PDU) is data combined with control information added at a networking layer. Its name changes as encapsulation adds headers and trailers. section 4.3.2

Match each PDU with its layer and encapsulated contents:

• A segment is a Layer 4 PDU: application data plus a Layer 4 header. section 4.3.2
• A packet is a Layer 3 PDU: a segment plus a Layer 3 header. section 4.3.2
• A frame is a Layer 2 PDU: a packet plus a Layer 2 header and trailer. section 4.3.2 section 6.0
• The contents encapsulated inside a PDU are its payload; therefore, a frame carries a packet, a packet carries a segment, and a segment carries application data. section 4.3.2 section 4.4

Figure 4.7 Application data encapsulated in a Layer 4 header is a segment (L4PDU); a segment encapsulated in a Layer 3 header is a packet (L3PDU); and a packet encapsulated in a Layer 2 header/trailer is a frame (L2PDU). The encapsulated contents of each PDU are that PDU’s payload.

Use the layered diagram to trace how application data becomes a segment, packet, and frame through successive encapsulation. section 4.3.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Defines segments, packets, frames, and payloads, then traces encapsulation and de-encapsulation without examining individual header fields.

Examples

Uses application data, Layer 4 headers, Layer 3 headers, Layer 2 headers and trailers, and physical transmission over cable or radio.

What you can do after

Evaluate network messages by identifying whether they are segments, packets, or frames and naming their payloads.

How it connects

Connects protocol data units with encapsulation, de-encapsulation, payloads, the TCP/IP model, and adjacent- and same-layer interaction.

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Related concepts

• frameincludes · An Ethernet frame is a link-layer protocol data unit that carries a network-layer packet and associated framing information across an Ethernet link.
• segmentincludes · A segment is a distinct portion or bounded region of a larger structure, system, or sequence, with boundaries or properties determined by its context.
• network packetincludes · A network packet is a formatted unit of data transmitted across a computer network, typically containing control information and a payload.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.