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IPv6 Datagram Options
(Page 1 of 2)
In IPv4, all “extra” information
required for various purposes is placed into the datagram in the form
of options
that appear in the IPv4 header. In IPv6,
the new concept of extension
headers is introduced; these headers take
the place of many of the predefined IPv4 options. However, the concept
of options is still maintained in IPv6, for a slightly different purpose.
Options allow the IPv6 datagram to be supplemented with arbitrary sets
of information that aren't defined in the regular extension headers.
They provide maximum flexibility, allowing the basic IPv6 protocol to
be extended in ways the designers never anticipated, with the goal of
reducing the chances of the protocol becoming obsolete.
IPv6 Option Extension Header Types
I said that IPv6 options supplement
extension headers; in fact, they are actually implemented as extension
headers. There are two different ones used to encode options. These
two headers only differ in terms of how the options they contain are
to be processed by devices; otherwise, they are formatted the same and
used in the same way.
The two extension header types are:
- Destination Options: Contains options
that are intended only for the ultimate destination of the datagram
(and perhaps a set of routers specified in a Routing header, if present).
- Hop-By-Hop Options: Contains options
that carry information for every device (router) between the source
and destination
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