Packet Fragmentation and Reassembly

See how an oversized IPv4 datagram is split, carried, and rebuilt

Animation MTU Reassembly

Packet Fragmentation and Reassembly

Follow an oversized IPv4 datagram as it is split into MTU-sized fragments. Inspect offset units, More Fragments flags, arrival order, and the reassembly result.

4000 Boriginal IPv4 datagram

1500 Bpath MTU

3 fragmentsgenerated pieces

Completereassembly result

Fragmentation Workbench

What

A packet larger than the path MTU is split into smaller IP fragments.

Why

Each link can carry only a maximum frame payload. MTU limits the IP datagram size.

Try first

Step through normal delivery, then switch to a missing-fragment failure.

Notice

Fragment offsets are in 8-byte units, not raw byte numbers.

Scenario

all arrive

Delivery pattern path Normal delivery Out-of-order arrival Missing fragment timeout

Original datagram size 4000 B

Path MTU 1500 B Ethernet path - 1500 B IPv6 minimum style - 1280 B Small IPv4 path - 576 B

Presets

compare

The first fragment starts at byte offset 0. All non-final IPv4 fragment payloads must be a multiple of 8 bytes.

Original datagram

The packet is larger than the MTU, so the IPv4 payload must be split into fragment payloads.

Step 1 of 4

Datagram and fragments

Selected fragment fields

Arrival order

Reassembly slots

Learning Support

Learning Support

Start with MTU

If the original datagram is less than or equal to the MTU, IP fragmentation is not needed.

Subtract header

Each IPv4 fragment has its own IP header, so payload capacity is MTU minus header bytes.

Use 8-byte units

The IPv4 fragment offset field counts blocks of 8 payload bytes. Non-final fragments must align to that size.

Missing hurts

The receiver can reassemble out-of-order fragments, but one missing piece makes the whole datagram unusable.

Quick Reference

Quick Reference

Fragment payload

floor((MTU - IP header) / 8) * 8

Maximum payload bytes for each non-final IPv4 fragment.

Offset field

offset bytes / 8

Fragment offset points to the payload byte position in 8-byte units.

MF flag

1 except final fragment

More Fragments tells the receiver whether another fragment should follow.

Identification

same ID on all fragments

The receiver groups fragments with the same source, destination, protocol, and identification.

IPv6 note

routers do not fragment IPv6

IPv6 fragmentation is performed by the source, not by routers along the path.

IoT practice

avoid fragmentation when possible

Small wireless links lose more when one fragment failure discards the whole packet.

Packet Capture AnalyzerInspect packet rows, fields, and bytes. Packet Analysis LayersReview layer ownership and display filters. 6LoWPAN FragmentationCompare IP fragmentation with constrained-link fragmentation.