Subject: Re: Last Call: IPv6 Router Alert Option to Proposed Standard

From: Jack McCann <[email-masked]>
Date: Wed, 11 Aug 1999 12:00:38 -0400 (EDT)

RFC2460 section 4 says:

   Individual options may have specific alignment requirements, to
   ensure that multi-octet values within Option Data fields fall on
   natural boundaries.  The alignment requirement of an option is
   specified using the notation xn+y, meaning the Option Type must
   appear at an integer multiple of x octets from the start of the
   header, plus y octets.

The IPv6 Router Alert option looks like it should specify an alignment
requirement of 2n (i.e. x == 2, y == 0) so that the 2-octet Value
field falls on a 2-octet boundary.

- Jack

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From: Thomas Narten <[email-masked]>
Date: Thu, 19 Aug 1999 12:39:28 -0400

Jack,

> field falls on a 2-octet boundary.

This sounds right to me. Do the authors agree?

Doing a little cross checking:

RFC 2473 seems to have neglected to state the alignment requirement,
though one would assume there wouldn't be one for a 1-octet data
value. :-)

draft-ietf-mobileip-ipv6-08.txt also would appear to need similar
text.

Thomas

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From: Steve Deering <[email-masked]>
Date: Thu, 19 Aug 1999 10:51:09 -0700

>This sounds right to me. Do the authors agree?

I'm not an author, but I agree.  Any document that defines an IPv6 option
containing a multi-octet field should specify an alignment requirement
for that option so as to cause the multi-octet field to be aligned on its
natural boundary, using the notation "alignment requirement: xn+y" as
described in section 4.2 of RFC 2460.  Appendix B of RFC 2460 provides
some examples of how the alignment requirement is determined and used.

The definition of an option that does not contain any multi-octet fields
should include the notation: "alignment requirement: none", as in the
definitions of the Pad1 and PadN options in RFC 2460.

Steve