27.4 IPv6 Extension
Headers
We do not show any options with the IPv6 header
in Figure A.2 (it is
always 40 bytes in length), but an IPv6 header can be followed by
the following optional extension
headers:
-
Hop-by-hop
options must immediately follow the 40-byte IPv6 header. There are
no hop-by-hop options currently defined that are usable by an
application.
-
No destination
options are currently defined that are usable by an
application.
-
The routing
header is a source routing option, similar in concept to what we
described for IPv4 in Section
27.3.
-
The
fragmentation header is automatically generated by a host that
fragments an IPv6 datagram and then processed by the final
destination when it reassembles the fragments.
-
The use of the
authentication header (AH) is documented in RFC 2402 [Kent and
Atkinson 1998b].
-
The use of the
encapsulating security payload (ESP) header is documented in RFC
2406 [Kent and Atkinson 1998c].
We said the fragmentation header is handled
entirely by the kernel, and the AH and ESP headers are
automatically handled by the kernel based on the SADB and SPDB,
which are maintained using PF_KEY sockets (Chapter 19). This
leaves the first three options, which we will discuss in the next
two sections. The API to specify these options is defined by RFC
3542 [Stevens et al. 2003].
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