Re: [v6ops] RFC4861 question - short prefixes in PIOs

[神明達哉 <[e-mail]>]

(I'm only copying 6man, as I believe it's purely a protocol spec
question)

At Wed, 26 Jun 2019 15:56:36 +0000,
"Templin (US), Fred L" <[e-mail]> wrote:
>
> I have an RFC4861 question (several actually) on short prefixes in RA
PIOs:
>
> 1) If a PIO includes a prefix with length less than 64 (e.g.,
2001:db8::/32) and with L=1, does it
>
> mean that 2001:db8::/32 should be added to the interface prefix list?

In my interpretation (ditto for subsequent questions), yes.

> 2) If yes to 1), does it mean that packets forwarded to the interface for
any destination covered
>
> by 2001:db8::/32 will trigger Address Resolution instead of forwarding to
a default router?

Yes.

> 3) If the PIO instead has L=0, does it mean that 2001:db8::/32 is
“associated” with the link but
> not necessarily “on-link”?

I'm not sure how to interpret it (in particular I'm not sure what
"associated with the link" means), but my interpretation of L=0 is
that the RA doesn't say anything about the on-link-ness of that
prefix.  See also the description of the L flag in RFC4861:

      L              1-bit on-link flag.  [...]  When
                     not set the advertisement makes no statement about
                     on-link or off-link properties of the prefix.  In
                     other words, if the L flag is not set a host MUST
                     NOT conclude that an address derived from the
                     prefix is off-link.  That is, it MUST NOT update a
                     previous indication that the address is on-link.

> 4) If yes to 3), does it mean that 2001:db8::/32 should be added to the
IPv6 forwarding table
>
> as a “route-to-interface” with the receiving interface as the next hop?

No.  See the second MUST NOT of the RFC4861 text cited above.

> 5) Does A=1 have any meaning for prefixes with length less than 64? Or,
must prefixes with
>
> length less than 64 set A=0?

As far as RFC4861 is concerned, the A flag has no meaning, regardless
of the prefix length.  It only matters in RFC4862.  In terms of
RFC4862, whether "A=1 has any meaning for prefixes with length less
than 64" depends on the length of the IID of the link; if the prefix
length != 128-IIDLength, the validation rule 5.5.3 d) of RFC4862 makes
the prefix ignored.  If non-64 prefix length is invalid in terms of
RFC4862 in that sense, it'd be *safe* to avoid setting the A flag, but
the protocol specification doesn't say it *must* be so.

You may also want to check
https://tools.ietf.org/html/draft-jinmei-6man-prefix-clarify-00
I believe it clarifies many of the above questions.

--
JINMEI, Tatuya