Subject: RE: [mobile-ip] Mobile Prefix Advertisements

From: "Powell, Ken" <[email-masked]>
Date: Tue, 26 Jun 2001 11:43:40 -0400

I think we have a serious issue here.

> > > >  >9.8.1. Building Aggregate List of Home Network Prefixes
> > > >  >
> > > >  > To support this, the home agent monitors prefixes 
> > > >  > advertised on
> > > >  > the home link by itself and other home agents, and 
> > > >  > passes the ...
> > > >                            =====================
> > > >
> > > >    Why limit this to prefixes from other home agents 
> > > >    only? Why not use prefixes from any router?
> > >
> > > I don't think that was the intention.
> >
> > Actually it was my intention, so please correct me here 
> > because this is important. In the general case, I do not
> > see how a MN can use a prefix
> > being advertised on its home link, unless a home agent is willing to
> > "serve" that prefix by accepting registrations for addresses on the
> > prefix, and thus forwarding packets to the MN while it is roaming.
> >
> > So, are you suggesting that a home agent would want to serve mobile
> > nodes that do not use a home address on a prefix that the HA is
> > advertising?
> 
> No, my wording was bad.  I was trying to say that the intention was
> _not_ to use addresses from routers that are not home agents, 
> but since I was in a hurry I didn't put enough text around it to
> be clear.

I disagree. When a mobile node is home, it will autoconfigure
to all prefixes advertised by any router. When the mobile node
moves off-link, the home agent will need to intercept all
traffic for the mobile node, otherwise you will get dropped
connections. The reason for creating the aggregate list
was to handle this problem.

While playing in the lab, I discovered I could set up a home
agent that doesn't set the A or L bits on any prefixes. The
mobile node autoconfigures to another router on the home net.
I realized this could be a very handy feature, particularly
in cases where the network administrator doesn't want the
home agent to effect addresses configured on the non-mobile
nodes.

Ken

---

From: "T.J. Kniveton" <[email-masked]>
Date: Tue, 26 Jun 2001 09:18:45 -0700

> home agent to effect addresses configured on the non-mobile

Well first of all, if the home agent is willing to serve home addresses
that are on prefixes it doesn't advertise (which it probably would be),
then there is no issue with a mobile node leaving the home network -
that will work fine regardless of what's in the MPA.

As far as a mobile renewing an address, or configuring a new one, based
on the MPA: while I'm not sure I agree 100%, I certainly see your point
and it is valid: it would be very nice for a network administrator to be
able to drop in an HA without having to reconfigure anything else.

It doesn't seem great to require every HA to advertise all of the
prefixes on its link to all MNs (and my intention was to pare the
aggregate list by removing unnecessary prefixes). Perhaps we could reach
a compromise of a router rule under "should": add received prefixes that
have the A bit set to the aggregate list. I would also suggest that the
rule should specify that the prefixes should be added only if they do
*not* have the L bit set. To me, it seems problematic for a mobile node
to be autoconfiguring home-network link-local addresses while roaming.
How do you feel about that?

As far as the rule that the mobile should be able to autoconfigure at
least all of the prefixes (which are served by HAs) that it would on the
home network, I think that still holds: the prefix still needs to be
served by the HA for it to be relevant while roaming, even if the prefix
is not explicitly advertised by the HA on the home link.

-- 
        T.J. Kniveton 
  Communications Systems Lab
     Nokia Research Center