ICMPv6 Informational Message Options
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Each of the five ICMPv6 informational messages types defined and used by the Neighbor Discovery (ND) protocol has an Options field into which one or more options may be inserted. This probably isn't the best name for these sets of data, since they are only optional in certain cases. In fact, in some cases the option is actually the entire point of the message. For example, a Neighbor Advertisement message containing a link-layer address for address resolution carries it in an option, but the message wouldn't be of much use without it.
Each option has its own structure of subfields based on the classical type, length and value triplet used in many message formats. The Type subfield indicates the option type and the Length field indicates its length so the device processing the option can determine where it ends. The value is not a single field, but rather one or more that contain the actual information for which the option is being used.
Some options are used for only one kind of ICMPv6 message, while others are used for more than one variety. So, they are best thought of as modular components used in different types of messages as needed. I describe the format of each of these five options below.
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