System Administration Guide, Volume 3

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 they appear.

Figure 14-1 IPv6 Header Format


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 it arrives at its final destination. This facilitates a major improvement in router performance for packets containing options. In IPv4, the presence of any options requires the router to examine all options.

The other improvement is that, unlike IPv4 options, IPv6 extension headers can be of arbitrary length and the total amount of options carried in a packet is not limited to 40 bytes. This feature, plus the manner in which they are processed, permits IPv6 options to be used for functions that were not practical in IPv4. A good example of this 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 8 octets long so that alignment of subsequent headers is retained.

The following IPv6 extension headers are currently defined.