IPv6 Packet

What this chapter covers

This chapter looks inside an IPv6 packet and at the rule that decides how big it may be.

It explains the fixed header, the meaning of each field, and how optional features are added through extension headers. It then explains how a sender finds the largest packet that fits along a path.

These topics matter when you read packet captures, write firewall rules, or track down a fault where only large packets fail.

How the topics fit together

The header defines what a packet looks like, and Path MTU Discovery decides how large it can be.

The first page covers the header and its fields. You learn the eight fields, which IPv4 fields were removed, and how Next Header links extension headers. One of those extension headers is the Fragment header, which leads to the second page.

The second page covers Path MTU Discovery. In IPv6 a router never fragments a packet. It drops one that is too big and sends an ICMPv6 Packet Too Big message so that the sender can shrink its packets.

The two pages meet at fragmentation. The header chapter explains where the fragment information lives, and the discovery chapter explains why the source node, and not the router, uses it.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

The header tables on the first page are worth learning by heart. Practise listing the eight fields from memory, then compare them with the IPv4 fields.

For the second page, try a lab with a tunnel or a lower MTU on one link. Use ping with different sizes and watch where large packets stop working. Seeing the symptom once makes the theory easy to recall.