Protocol Data Units at Each Layer
- Networking Fundamentals & OSI Model
- TCP/IP Model
- Network Topologies
- IP Addressing & Subnetting
- Routing Algorithms & Protocols
- Switching Concepts
- Transport Layer Protocols (TCP & UDP)
- Application Layer Protocols
- Network Security & Firewalls
- Wireless Networking & Wi-Fi
- Network Address Translation (NAT)
- DNS Deep Dive
- Congestion Control & Flow Control
- Error Detection & Correction
- Network Devices
- VPN & Tunneling
- Load Balancing & CDN Networking
- Socket Programming
- Network Troubleshooting Tools
- Physical Layer & Transmission Media
- Data Link Layer & MAC Addressing
- Ethernet Protocol
- ARP (Address Resolution Protocol)
- ICMP Protocol
- Unicast, Multicast, Broadcast and Anycast
- Proxy Servers
- Port Numbers Reference
- Network Performance Metrics
- Quality of Service (QoS)
- Software-Defined Networking (SDN)
- HTTP/2 and HTTP/3 Overview
- 5G and Mobile Networks
- IoT Networking Basics
- VoIP Networking Basics
What a Protocol Data Unit Is
A Protocol Data Unit (PDU) is simply the name given to a chunk of data at a specific layer, once that layer's own header has been added. The name changes as the data moves down the stack, because each layer is technically producing a different kind of unit.
PDU Names Across the TCP/IP Layers
| Layer | PDU name | Contains |
|---|---|---|
| Application | Data / Message | The raw application content (an HTTP request, an email) |
| Transport | Segment (TCP) or Datagram (UDP) | Data plus a transport header (ports, sequence numbers) |
| Internet | Packet | Segment plus an IP header (source/destination IP) |
| Network Access | Frame | Packet plus a MAC header and trailer |
Why Precise Terminology Matters
Saying "the packet got dropped" versus "the frame got dropped" points at completely different layers - and therefore completely different causes and fixes. A dropped packet suggests a routing problem (Internet layer); a dropped frame suggests a local link problem (Network Access layer). Using the correct PDU name is a fast way to communicate exactly where in the stack an issue lives.
Visualizing the PDU Chain
Message -> [ Segment (+ TCP header) ]
-> [ Packet (+ IP header) ]
-> [ Frame (+ MAC header/trailer) ]
-> raw bitsThis is the same encapsulation process covered earlier, just viewed through the lens of what each resulting unit is properly called at each stage.