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IPv6 Datagram Options
(Page 2 of 2)
IPv6 Option Format
Each of these header types has a
one-byte Next Header field, and a one-byte Header Extension
Length field that indicates the headers overall length. The
rest of the header has one or more option fields. Figure 109
illustrates the overall format of these two headers. The format of each
option is similar to that of IPv4 options, and is shown in Table 73.
Figure 109: IPv6 Hop-By-Hop Options and Destination Options Header Formats
Each of these extension headers begins with two fixed fields, Next Header and Header Extension Length. The rest of the header consists of a sequence of variable-length options. Each option has the structure shown in Table 73, consisting of a type / length / value triplet.
Table 73: IPv6 Option Format
Data Length: Specifies the length of the Option Data
subfield below. Note that this is a change in semantics from IPv4, where
the length field indicated the size of the entire option; in IPv6 the
length of the Option Type and Option Data Length fields
are not included.
Option Data: The
data to be sent as part of the option, which is specific to the option
type. Also sometimes referred to as the Option Value.
Note: The Option Type subfield is a bit strange in terms of how it is interpreted. Even though it has a substructure with three sub-subfields as shown in Table 73, that structure is informal. What I mean by this is that the 8 bits of this field are taken as a single entity. Despite the special meaning of the three highest-order bits, the entire field is called the Option Type, not just the last five bits, and the whole is used as a single value from 0 to 255. In fact, the sub-subfield names aren't even specified in the standard; I made them up to help explain these fields.
Since each option has a subfield
for type, length and value (data), they are sometimes said to be TLV-encoded.
If there are multiple options, they are placed one after each other
in the header. At the end of all the options in a Hop-By-Hop Options
or Destination Options extension header, padding may be placed
to ensure that the header is a multiple of 8 bytes in length.
Key Concept: Two IPv6 extension header types, Hop-By-Hop Options and Destination Options, are used to carry arbitrary optional information in IPv6 datagrams. Each consists of a set of variable-length options that are defined using three subfields indicating the options type, length and value.
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Version 3.0 - Version Date: September 20, 2005
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