Subject: [netlmm] Re: [Int-area] IPv6 addressing model, per-MN subnet prefix, and broadcast domain

From: Julien Laganier <[email-masked]>
Date: Tue, 8 Aug 2006 13:35:49 +0200

Hi folks,

[ Removing INT area from the recipient list ]

Thanks for your comments, please see mine below,

On Tuesday 08 August 2006 08:51, Narayanan, Vidya wrote:
>
> > El 08/08/2006, a las 0:34, Narayanan, Vidya escribió:

> > >>> How do the nodes configure their global addresses? I mean
> > >>> are they using stateless autconf?
> > >>
> > >> Not sure about what's becoming mandatory and what not.
> > >> There's a draft on MN-AR interface talking about _both_
> > >> stateless and stateful, haven't checked recently though.

The draft does not make any method (stateless and/or statefull) 
mandatory, it just describes the two approaches. FWIW the IPv6 node 
requirements RFC says stateless is a MUST and statefull a MAY.

> > >>> If they do, how do you prevent the nodes from configuring
> > >>> one address per prefix the get in the RADV?
> > >>
> > >> By putting M bit to 1 in RA thus instructing MN not to derive
> > >> an address from the prefix in RA ('M' stands for managed, 
> > >> instructing to use DHCP instead).

Yes, that's one way to do it. The other way is to send unicast RAs 
only, each one containing the prefix information corresponding to the 
MN to which it is sent.

> > >>> wouldn't some of the nodes end up with multiple addresses
> > >>> from different prefixes?
> > >>
> > >> Yes, provided stateless autoconf is used.

Not in when unicast RAs are used.

> > > No. The goal here is strangely different, actually. The
> > > goal is not to advertise all the prefixes in all the RAs - the
> > > RA itself will be tailored per mobile - so, each mobile will
> > > only see the prefix that is being "assigned" to it.

Yes, that's the idea I describe above. Note that the use of the term 
"assigned" above does not necessarily implies the use of DHCP PD. As 
explained one can use unicast RAs, or even DHCP with all leased 
addresses under a per-MN subnet prefix (the "entire" subnet prefix 
does not need to be delegated with PD, although that's a 
possibility )

> > but how do you achieve that? isn't all a single broadcast
> > link (with multiple subnets? if this is the case all the
> > nodes see all the RADV, hence all the nodes configure
> > addresses from all prefixes, right?
>
> I believe those will be unicast RAs sent to the link local address
> - there will be a huge mess without that, as you observe (I believe
> there is a mess in the addressing model even with that, but it will
> be worse with broadcast RAs).

Yes it will be based on unicast RAs

> > > The other strange thing here is that the mobile may not
> > > know that it is being "assigned" anything - so, it may continue
> > > to use stateless autoconfig, assuming it is a shared prefix.

I don't see a problem with the mobile continuing to use stateless 
autoconfig. DAD will only occurs at first attachment to the domain so 
there's no real overhead. And FWIW a subnet prefix with only one node 
is just a degenerated shared prefix (shared only by one.)

> > i am lost now.... are you using something different than
> > stateless autocnf and than dhcp? a new address conf protocol
> > for specific for netlmm?

The MN uses regular stateless autoconf as per [RFC2462]. The 
difference is on the router side: it never sends multicast RAs, and 
when queried by a MN with a RS, it replies with a unicast RA 
containing the prefix that were assigned to that MN. This confirms to 
the NetLMM charter that precludes modification of the MN's IP stack, 
but allows to modify the access router.

> I'm waiting to see some proposed text myself to see how this whole
> thing is going to work on shared media :) Based on the emails I've
> seen, I think the proposal is the following (people who are
> actually proposing this may correct anything I say incorrectly
> here):
>
> When a mobile shows up, the AR obtains a prefix for the mobile from
> the LMA using the NETLMM protocol. That prefix is sent in a unicast
> RA to the mobile by the AR. The MN doesn't really know it is a
> unique prefix - it performs SLAAC and comes up with an address and
> performs DAD.

Yes that's the idea.

> At the moment, I'm very skeptical about slapping a point-to-point
> model on shared media - I'll wait for some text to see if I can
> convince myself :)

I'll write some text if we get an agreement on the INT area mailing 
list that such an approach doesn't break the IPv6 addressing model. 

From what you wrote above I got the impression that you understood how 
this per-MN prefix would work on shared media, but at the same time 
you said you're very skeptical about it. Could you explain why?

Thanks. 

--julien

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---

From: Alexandru Petrescu <[email-masked]>
Date: Tue, 08 Aug 2006 14:48:26 +0200

Salut Julien,

Julien Laganier wrote:
> Hi folks, [ Removing INT area from the recipient list ]
> 

The draft is informational IIRC, and doesn't include SeND either :-)

> only, each one containing the prefix information corresponding to the

So there'll never be two MNs having same prefix, I prefer to see this as
point-to-point.

So there may exist two RAs containing same prefix but a different
default router address.

RAs contain also the address of the default router, not only the prefix.
  There must be one in each such RA.  It will be different for each
router.  So the host will have a default router list containing several
entries, same prefix in each entry, but different default router
address.  How does the host choose the right default router?  There's
only one possible.

Entries in the default router list are expirable (based on lifetime),
not deletable by some RA.

I'm not sure how default router list can work, but it may be known.

>>>> No. The goal here is strangely different, actually. The goal is
>>>>  not to advertise all the prefixes in all the RAs - the RA 
>>>> itself will be tailored per mobile - so, each mobile will only 
>>>> see the prefix that is being "assigned" to it.
> 
> "assigned" above does not necessarily implies the use of DHCP PD.

Ok, trying to understand what else than DHCPv6 PD...

> As explained one can use unicast RAs,

Yes, one way.

> or even DHCP with all leased addresses under a per-MN subnet prefix 
> (the "entire" subnet prefix does not need to be delegated with PD, 
> although that's a possibility )

Yes, I agree one possibility is for DHCP to distribute the address only,
not the prefix.  The address distributed to MN straight (not to AR).

>>> but how do you achieve that? isn't all a single broadcast link 
>>> (with multiple subnets? if this is the case all the nodes see all
>>>  the RADV, hence all the nodes configure addresses from all 
>>> prefixes, right?
>>  - there will be a huge mess without that, as you observe (I
>> believe there is a mess in the addressing model even with that, but
>> it will be worse with broadcast RAs).
> 

I'm not sure whether you've checked RA lifetimes coherency between ARs.
  RAs contain lifetimes.  These lifetimes are decided by the router.  For
example, a router decides a lifetime 5 seconds in RA, the host receives
it and knows that after 5 seconds it must send an RS, or it knows it
can't use its address for more than 5 seconds.

If the same prefix is managed by several routers, it may mean that all
these routers must maintain together the 5s in a coherent manner.  I.e.
one wouldn't want the oAR to have 5s for that lifetime and the nAR to
have 3s for that lifetime, or viceversa.

>>>> The other strange thing here is that the mobile may not know 
>>>> that it is being "assigned" anything - so, it may continue to 
>>>> use stateless autoconfig, assuming it is a shared prefix.
> 
>  there's no real overhead. And FWIW a subnet prefix with only one 
> node is just a degenerated shared prefix (shared only by one.)
> 
>>> stateless autocnf and than dhcp? a new address conf protocol for 
>>> specific for netlmm?
> 
>>  the LMA using the NETLMM protocol.

Not sure NETLMM protocol assigning prefixes is a good idea, there's
other ways to do it, dhcp.

>> That prefix is sent in a unicast RA to the mobile by the AR. The MN
>>  doesn't really know it is a unique prefix - it performs SLAAC and 
>> comes up with an address and performs DAD.
> 
> Yes that's the idea.

I too understand this idea.

> list that such an approach doesn't break the IPv6 addressing model.

Not sure about the IPv6 addressing model being broken.  But based on the
rfc2461 router lifetimes and default router list behaviour I'm afraid
unicast RAs to a moving host attaching to different ARs sending it same
prefix may need some host enhancements (compared to 2461) to deal with
growing default router lists and need some new inter-AR protocol to
exchange lifetimes.  Just a thought, it may be not so.

Alex