Re: Understanding IPv6-PD over PPPoE

[Andrei Borzenkov <arvidjaar gmail com>]

On Wed, Jun 16, 2021 at 12:13 PM Beniamino Galvani <bgalvani redhat com> wrote:
> On Wed, Jun 16, 2021 at 11:43:36AM +0300, Andrei Borzenkov wrote:
>
> > It is explicitly prohibited to assign any IA_PD prefix to the same
> > interface via which this was obtained.
> >
> >                                               the requesting router MUST
> >    NOT assign any delegated prefixes or subnets from the delegated
> >    prefix(es) to the link through which it received the DHCP message
> >    from the delegating router.
>
> You are right, RFC 3633 forbids it. However, if I understand correctly
> this approach is the one mentioned in [1], which refers to an expired
> IETF draft [2] saying:
>
>    As stated in [RFC 3633], "the requesting router MUST NOT assign any
>    delegated prefixes or subnets from the delegated prefix(es) to the
>    link through which is received the DHCP message from the delegating
>    router", however the approach described in this document may still
>    be useful in other DHCPv6 scenarios or non-DHCPv6 scenarios.
>    Moreover, [RFC 3769] has no explicit requirement that avoids the
>    approach described in this document.  Furthermore, this has been
>    tested in DHCPv6-PD implementations and worked well, so we must say
>    that it may be implementation-dependent.
>
> From [1], it seems that some ISPs are using this method. It would be
> interesting to research what e.g OpenWRT or other OSes do.
>
> Beniamino
>
> [1] 
> https://www.ripe.net/publications/docs/ripe-690#4-1-4---64-prefix-out-of-the-ipv6-prefix-assigned-to-the-end-user
> [2] https://datatracker.ietf.org/doc/html/draft-palet-v6ops-point2point-01

Well, there is RFC6603. You still need to have some reasonable
fallback in case prefix exclusion is not supported by the upstream
router. And even *with* prefix exclusion the actual assignment of
prefix to link goes via RA and actual address assignment via SLAAC,
not via DHCP (at least, not mandatory). Prefix exclusion just informs
which part of delegated prefixes cannot be assigned downstream.