6LoWPAN Fragmentation Animation

See how IPv6 datagrams are split into 6LoWPAN fragments for 127-byte IEEE 802.15.4 frames

6LoWPAN Fragmentation Animation

Animation 6LoWPAN Fragmentation

6LoWPAN Fragmentation Animation

IEEE 802.15.4 frames are tiny compared with IPv6 packets. This workbench shows how a 6LoWPAN sender adds FRAG1/FRAGN headers, how offsets are counted in 8-byte units, and why one lost fragment can make the receiver discard the whole datagram.

Reassembling current receiver state

6 fragments frames needed on this hop

88 B typical FRAGN data capacity

60 s reassembly timeout budget

What

A single IPv6 datagram is split into small 6LoWPAN fragments that fit inside 127-byte 802.15.4 MAC frames.

Why

IPv6 expects much larger packets than low-power radio links can carry in one frame, especially after MAC and security overhead.

Try First

Press Step Fragment. Watch each fragment fill the reassembly buffer and compare the first header with later headers.

Notice

FRAGN uses an offset measured in 8-byte units. Every non-final payload must align so the next offset is an integer.

Controls

Datagram

Scenario Compressed CoAP Compressed CoAP IPv6 minimum MTU Secure mesh hop Small telemetry

Datagram size 420 B

MAC + FCS + security 31 B

Delivery model All arrive All arrive Drop fragment 3 Out of order

Scenario Presets

Fragmentation and Reassembly Workbench

A compressed CoAP datagram needs several 802.15.4 frames. Step through the fragments and watch the receiver buffer fill.

Reassembling

Learning Support

Fragmentation is per hop

6LoWPAN fragmentation happens on the low-power link. A router may reassemble before forwarding, depending on the routing approach.

Tag keeps fragments together

The datagram tag lets the receiver match fragments from the same original datagram.

Offset is not bytes

The FRAGN offset is the byte offset divided by 8. This is why non-final fragments are sized on 8-byte boundaries.

Loss is expensive

The 6LoWPAN fragmentation layer does not repair a missing fragment by itself. The buffer is discarded on timeout.

Quick Reference

802.15.4 limit

max_frame = 127 B

The workbench subtracts MAC, FCS, and security overhead before adding 6LoWPAN fragment headers.

FRAG1

dispatch 11000
header 4 B

The first fragment carries datagram_size and datagram_tag, but no offset field.

FRAGN

dispatch 11100
header 5 B

Every later fragment repeats size and tag, then adds datagram_offset.

Offset rule

datagram_offset = byte_offset / 8

The last fragment may be shorter; earlier fragments must keep the next offset aligned.

Size field

11 bits: up to 2047 B

This animation keeps the slider within IPv6 minimum MTU examples, but the field can encode more.

Tag field

16-bit datagram_tag

The tag must not be reused too quickly while old reassembly buffers might still exist.

Timeout

60 s reassembly window

If all fragments do not arrive in time, the receiver discards the incomplete datagram.

Practical warning

more fragments = more loss risk

Compression, smaller payloads, and application block transfer can reduce fragmentation pressure.

Guided Practice

Find the first FRAGN

Step once or twice. The first fragment is FRAG1; later fragments become FRAGN and show an offset.

Raise overhead

Move MAC + security overhead upward. The payload capacity falls and the fragment count rises.

Drop one fragment

Select Drop fragment 3, play to the end, then step the timer until the reassembly buffer is discarded.

Compare small telemetry

Use Small telemetry. Notice when a datagram fits in one frame, no FRAG1/FRAGN header is needed.

6LoWPAN IPHCSee how compression reduces the datagram before fragmentation pressure grows. IEEE 802.15.4Review the low-power MAC frame that sets the 127-byte ceiling. Packet FragmentationCompare with the general fragmentation and reassembly concept. CoAP Block-WiseExplore application-layer block transfer as an alternative design choice.