PDU Transformation Workbench

Name and inspect each protocol data unit as an IoT message moves down and back up the stack

Learner-ready PDU transformation workbench for tracing application data, TCP segments, UDP datagrams, IP packets, link frames, physical bits, overhead, MTU pressure, and decapsulation.

Animation PDU Names Encapsulation IoT Link Fit

PDU Transformation Workbench

Follow one IoT message as each layer gives the same bytes a new PDU name: application data, TCP segment or UDP datagram, IP packet, link frame, and physical bits. Step forward slowly to see what is added, what becomes payload, and what the receiver removes.

100 B Application payload

158 B Transmitted frame bytes

63% Useful payload ratio

Frame Current PDU view

Learning Brief

Encapsulation means each lower layer treats the entire upper-layer unit as its payload and adds control information needed for its own job.

What is a PDU? A protocol data unit is the named unit handled by a layer. The name changes as wrappers are added or removed.

What should I notice? The original data stays inside the stack, but each layer views the whole upper-layer unit as its own payload.

Why IoT cares Small radio frames make names, byte counts, compression, and fragmentation visible design concerns.

Scenario

Controls

Payload Bytes: 100

Transport TCP base header, 20 B UDP header, 8 B

Network IPv4 minimum header, 20 B IPv6 base header, 40 B IPv6 with 6LoWPAN-style compression example

Link Ethernet II, 14 B header + 4 B FCS IEEE 802.15.4 example, 9 B header + 2 B FCS

Direction

Application Data

Stage 1 of 5

The sender starts with the application message before protocol headers are added.

PDU Layer Path

Current PDU Byte Layout

Diagnosis

PDU Transformation Trace

Quick Reference

PDU Names Application data/message, TCP segment or UDP datagram, IP packet, link frame, physical bits.

Common Header Sizes UDP 8 B, TCP base 20 B, IPv4 minimum 20 B, IPv6 base 40 B, Ethernet header 14 B plus 4 B FCS.

Layer Payload Rule At each lower layer, the whole upper-layer PDU is treated as payload and wrapped with that layer's fields.

Naming Trap TCP creates segments; UDP creates datagrams. IP creates packets. Ethernet and 802.15.4 create frames.

Payload Efficiency Useful payload ratio = application bytes divided by the final transmitted frame bytes.

MTU Pressure Large headers can force fragmentation or adaptation-layer work before the packet can cross a small radio link.

Decapsulation The receiver validates and removes each lower-layer wrapper before passing the remaining payload upward.

Physical Layer The physical layer sends bits or symbols. It does not rename the link PDU into a new header-bearing packet.

IoT Design Hint CoAP/UDP, header compression, and careful payload sizing help preserve payload room on constrained links.

How to Read the Workbench

PDU Layer Path Green layers have already handled the PDU. The orange layer is the layer currently adding or removing fields.

Byte Layout Each colored block is a field group. The application data block should stay visible as wrappers accumulate around it.

Diagnosis Use the naming note, efficiency, and MTU messages to explain what the current layer is really handling.

Technical Accuracy Notes

IPv4 and TCP The workbench uses minimum IPv4 and TCP base headers. Options can increase both values.

UDP PDU Name UDP is commonly described as carrying datagrams. This page avoids calling UDP PDUs TCP-style segments.

IPv6 and Extension Headers The IPv6 base header is 40 B; extension headers are separate and are not included unless a scenario explicitly adds them.

6LoWPAN Example The compressed case is an educational example. Real compressed sizes depend on addressing mode, context, hop limit, ports, and mesh/security fields.

Link-Layer Variability IEEE 802.15.4 frame overhead changes with addressing and security fields. This page uses a small representative frame to show MTU pressure.

Ethernet Wire Detail The frame byte total includes Ethernet header and FCS. Preamble and inter-frame gap are physical-layer timing overhead and are kept out of the frame byte count.

Layer Models The display maps practical TCP/IP behavior onto OSI-style names so learners can connect both vocabularies.

Source Links

UDP Header RFC 768 - User Datagram Protocol

IPv4 Header RFC 791 - Internet Protocol

IPv6 Header RFC 8200 - IPv6 Specification

TCP Header RFC 9293 - Transmission Control Protocol

6LoWPAN Compression RFC 6282 - IPv6 Header Compression

IEEE 802.15.4 Overview IEEE 802.15.4-2020 standard page