Basic IPv6 Concepts 6.10 Formatting Guidelines for Options This appendix gives some advice on how to lay out the fields when designing new options to be used in the Hop-by-Hop Options header or the Destination Options header, as described in Section 6.4.2. These guide- lines are based on the following assumptions: One desirable feature is that any multioctet fields within the Option Data area of an option be aligned on their natural boundaries, i.e., fields of width n octets should be placed at an integer multiple of n octets from the start of the Hop-by-Hop or Destination Options header, for n = 1, 2, 4, or 8. Another desirable feature is that the Hop-by-Hop or Destination Options header take up as little space as possible, subject to the requirement that the header be an integer multiple of 8 octets long. It may be assumed that, when either of the option-bearing headers are present, they carry a very small num- ber of options, usually only one. These assumptions suggest the following approach to laying out the fields of an option: order the fields from smallest to largest, with no interior padding, then derive the alignment requirement for the entire op- tion based on the alignment requirement of the largest field (up to a maximum alignment of 8 octets). This approach is illustrated in the following examples: Example 1 If an option X required two data fields, one of length 8 octets and one of length 4 octets, it would be laid out as follows: