Re: [DNA] [Issue 15] [Issue 16] DAD and MLD Interaction

[James Kempf <kempf@docomolabs-usa.com>]

> > Draft-pentland in Section 5.2.3 says this:
> >
> >    Hosts MUST include a tentative source link layer address option
> >    (TSLLAO) in the RS message [13].  The router solicitation message is
> >    sent to the All_Routers_Multicast address and the source address MUST
> >    be the link local address of the host.
> >
> > The router uses the TSLLAO to route back to the host, and the definition
of
> > the TSLLAO means that it won't pollute the neighbor cache if there's a
> > duplicate. So there should be no effect on the router neighbor cache
even if
> > there is a duplicate, and the router shouldn't send the RS to any other
node
> > even if there is a duplicate because the TSLLAO will allow link routing
to
> > work correctly.
>
> This handles the RS not overwriting a previous entry in the routers
> neighbor cache, but doesn't prevent other hosts on the link (which send
> a NS for instance) from being polluted.
>

Right, it would mean suspending NS responses for the duration of the
response to the RS. Of course, that's what Optimistic does.

> > I considered that, but the problem is that if the addresses are switched
to
> > "Optimistic" . Section 3.3 of draft-ietf-ipv6-optimistic-dad says this
about
> > the "Optimistic" state:
> >
> >    * (modifies 5.4)  As soon as the initial Neighbor Solicitation is
> >         sent, the Optimistic Address is configured on the interface and
> >         available for use immediately.  The address MUST be flagged as
> >         'Optimistic'.
> >
> > so what this means to me is that the node would have to send NSes on the
> > link prior to switching its addresses to Optimistic, even if it is on
the
> > same link, because it doesn't find out that it is on the same link until
> > after the RA returns. I'm not sure whether the extra traffic overhead on
> > each move is really necessary or beneficial.
>
> There is no technical reason for optimistic DAD to require that the NS
> be sent before or even after state change AFAIK. Do you see a technical
> reason?
>

No, but unfortunately the oDAD document doesn't say anything about this.
Most implementors of oDAD will probably send it out when the state change to
Optimistic happens, because the draft says that, but when combined with DNA,
I suspect there may be some confusion unless we make it clear.

How about: the node SHOULD avoid sending the NS until after the RS is
returned?

Note that even if we do change the state to Optimistic, there is still the
issue I raised about the node originating packets with a topologically
incorrect source address if it has moved to a new IP link. The only way to
prevent that from happening is to make the addresses Tentative (and suppress
the DAD NS until after the RA returns) until the node actually finds out in
the RA whether it actually has moved to a new IP link or not.

> AFAIK The state change says "don't pollute" and the NS is there to find
> out whether there is a duplicate.
>

That is the spirit.

            jak