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Understanding Protocol Data Units (PDUs)

Networking Fundamentals: The 0 to Hero Guide

Lesson 22: Understanding Protocol Data Units (PDUs)

A Protocol Data Unit (PDU) is the term used to describe data as it exists at a specific layer of the OSI or TCP/IP model. Understanding PDUs is fundamental to networking terminology.

Naming the PDUs

As data is encapsulated, the name of the data unit changes based on the header attached to it:

  1. Data: The general term for information used by the upper layers (L7, L6, L5).
  2. Segment (TCP) / Datagram (UDP): The PDU at the Transport Layer (L4).
  3. Packet: The PDU at the Network/Internet Layer (L3). This is the primary unit routers use for forwarding.
  4. Frame: The PDU at the Data Link Layer (L2). This unit includes the MAC header and the error-checking trailer.
  5. Bit: The PDU at the Physical Layer (L1). The raw stream of 1s and 0s.

mermaid graph TD A[Data (L7)] --> B{L4: Segmentation/Header}; B --> C[Segment/Datagram]; C --> D{L3: IP Header}; D --> E[Packet]; E --> F{L2: MAC Header/Trailer}; F --> G[Frame]; G --> H{L1: Encoding}; H --> I[Bits];

The Importance of PDUs

Network devices are defined by the PDU they process:

  • Routers are 'Packet' pushers (L3).
  • Switches are 'Frame' pushers (L2).

If you are troubleshooting a connectivity issue, you must identify at which PDU stage the failure is occurring.