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IPv6 Datagram Extension Headers
(Page 6 of 6)
IPv6 Extension Header Order
Each extension header appears only
once in any datagram (with one exception; see below). Also, extension
headers are only examined by the final recipients of the datagram, not
intermediate devices (again with one exception, which we will get to
momentarily). RFC 2460 specifies that when multiple headers appear,
they should be in the following order after the main header and before
the higher-layer encapsulated header in the IPv6 datagram payload:
- Hop-By-Hop Options
- Destination Options (for options
to be processed by the destination as well as devices specified in a
Routing header)
- Routing
- Fragmentation
- Authentication Header
- Encapsulating Security Payload
- Destination Options (for options
processed only by the final destination)
Now let's look at those exceptions.
The only header that can appear twice is Destination Options.
Normally, it appears as the last header. However, a Destination Options
header may exist that contain options that must be examined by a list
of devices specified in a source route, in addition to the destination.
In this case, the Destination Options header for these options
is placed before the Routing header. A second such header containing
options only for the final destination may also appear.
Key Concept: Each extension header may appear only once in an IPv6 datagram, and they must appear in a fixed order. The exception is the Destination Options header, which may appear twice; near the start of the datagram for options to be processed by devices en route to the destination, and at the end of the extension headers for options intended only for the final destination. |
The only header normally examined
by all intermediate devices is the Hop-By-Hop Options extension
header. It is used specifically to convey management information to
all routers in a route. The Hop-By-Hop Options extension header
must appear as the first extension header if present. Since it is the
only one that must be read by every router (which represents a performance
drain on routers) it is given top billing to make it easier
and faster to find and process.
Finally, note that all extension
headers must be a multiple of 8 bytes in length for alignment purposes.
Also, remember that the Next Header value for a particular extension
header appears in the Next Header field of the preceding
header, not the header itself.
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Version 3.0 - Version Date: September 20, 2005
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