Extension header are used to support parameters that are not required int every IP packet, such as fragmentation and IPSec.
The list of extension headers and the sequence in which the extension headers are
processed are as follows.
IPv6 header
Hop-by-Hop Options header
Destination Options header
Routing header
Fragment header
Authentication header
Encapsulating Security Payload header
Destination Options header
Upper-layer header (as in IPv6 tunneling)
Each extension header should occur at most once, except for the Destination
Options header which should occur at most twice (once before a Routing header
and once before the upper-layer header). Another key aspect of the extension
headers is the introduction of IPSec protocols in the form of AH and ESP to
enhance the overall security of IPv6.
