Introduction

This chapter examines the purpose and use of the IPv6 header within the IPv6 protocol. The IPv6 header can be divided into a basic header that appears in every IPv6 packet and several extension headers. Each extension header has its own function or functions and is not required in an IPv6 packet. Examples of these headers are the fragmentation header and the authentication header.

The basic IPv6 header is only 40 bytes in length. Considering that 32 bytes are used for the source and destination IPv6 addresses, this only leaves eight additional bytes in the header. This was clearly by design the creators of IPv6 wanted to keep the standard header small and fixed in size. Keeping it small reduces the overhead on each packet, and keeping it fixed in size makes it easier for all IPv6 nodes to process the header.

The IPv6 header can have optional extension headers. None of these are required, but an IPv6 packet can have one or several of them. Each header serves a general purpose, and many have option types that can serve more specific purposes. This is similar to the way that Internet Control Message Protocol (ICMP) in IPv4 is used for general control, but has many codes for specific control functions.

When examining the IPv6 header, we can treat the header as if it follows the Open System Interconnection (OSI) reference model. In the OSI model each layer (Layer 2, Layer 3, Layer 4, etc.) has its own header.