IPv6 Administration Guide

IPv6 Header and Extensions

The IPv6 protocol defines a set of headers, including the basic IPv6 header and the IPv6 extension headers.

Header Format

The following figure shows the elements that appear in the IPv6 header and the order in which the elements appear.

Figure 1–1 IPv6 Header Format

Diagram shows that the 128 bit IPv6 header consist of eight fields, including the source and destination addresses.

The following list describes the function of each header field.

Extension Headers

IPv6 includes an improved option mechanism over IPv4. IPv6 options are placed in separate extension headers that are located between the IPv6 header and the transport-layer header in a packet. Most IPv6 extension headers are not examined or processed by any router along a packet's delivery path until the packet arrives at its final destination. This feature is a major improvement in router performance for packets that contain options. In IPv4, the presence of any options requires the router to examine all options.

Unlike IPv4 options, IPv6 extension headers can be of arbitrary length. Also, the number of options that a packet carries are not limited to 40 bytes. This feature, plus the manner in which IPv6 options are processed, permits IPv6 options to be used for functions that are not practical in IPv4. A good example of IPv6 options is the IPv6 authentication and security encapsulation options.

To improve performance when handling subsequent option headers, and the transport protocol that follows, IPv6 options are always an integer multiple of eight octets long. The integer multiple of eight octets retains the alignment of subsequent headers.

The following IPv6 extension headers are currently defined.