Re: [netext] Chair review of I-D: draft-ietf-netext-pd-pmip (05)

[Peter McCann <[e-mail]>]

Hi, Jouni,

jouni korhonen wrote:
> 
> Pete,
> 
> On Nov 26, 2012, at 5:54 PM, Peter McCann wrote:
> 
> [budget cut scale removal of text]
> 
>>>> hop router, and traffic is always tunneled directly back to that
>>>> router. In contrast, PMIP has the MAG as the first hop router. This
>>>> is a fundamental difference that IMHO makes it more likely to see a
>>>> link flap on a MAG to MAG handover, even when the same LMA is
>>>> reachable from both.
>>> 
>>> If the LMA stays the same, the link does not flap.
>> 
>> I don't see how you can make a blanket statement like this for all
>> link layers that might be used for PMIP.  In particular, it seems
>> to require a knowledge, at the L2, of whether the current MAG can
>> see the same LMA.  This knowledge needs to be propagated across the
>> link to the client, and used to determine whether the link down/up
>> event propagates through the IP stack.  Perhaps your particular
>> implementation experience involved carrying such a signal but I
>> cannot imagine that it holds true for every L2, especially if they
>> were not designed with PMIP in mind.
> 
> I am just reflecting the nature of existing point-to-point L2
> technologies. They tend to be session oriented, ref PPP and 3GPP
> stuff. Even when a WLAN type of technology is twisted to resemble
> point-to-point (and with authentication) you can make it session
> aware.. with limitations. And if the whole user session is between
> UE and the LMA, then any leg losing the session usually torn down
> the whole pipe including your L2.

My point is that L2 is torn down/rebuilt even when the LMA is 
reachable and the PMIP session stays up.

> I am curious to learn what point-to-point L2 technologies you have
> in mind? Something already on the field or on the drawing board?

Take PPP as an example.  The PPP session would be between the 
MN and the MAG.  There would clearly need to be a new LCP/IPCP
exchange between the MN and a new MAG, even if the new MAG could
get back to the same LMA.  In this case PPP experiences link down/
up but the PMIP session is preserved.

This is distinct from the 3GPP link layer where the p2p session
is between MN and PGW.  The PGW is the first hop router.

PMIP has the MAG as the first hop router.  When you disconnect
from one first hop router and connect to another first hop
router, the link flaps unless you have taken special measures
to hide this event from the IP stack.

> [smaller cost cut scale removal of text]
> 
>>> I repeat what I said earlier. If the LMA changes the L2 session goes
>>> down, prefixes may change etc. It is a bit more than just link up/down
>>> because it also involves setting up a new PMIP session & L2 session.
>>> (on L2 technologies I am concerned with).
>> 
>> You seem to be implying that the L2 technology has a notion of a PMIP
>> session.  The layered protocol model would seem to discourage this,
>> and I can imagine that many link layers are designed without PMIP
>> in mind.
> 
> I do not have a PMIP aware L2 in mind. See above. Just a point-to-point
> L2 that has a notion of session itself i.e. one has to explicitly set it
> up and it can be disconnected at will by either end.

But in PMIP the L2 is terminated between MN and MAG.  Change of MAG means
tearing down one L2 session and establishing a new one.  Hence it seems
more natural to me to experience a link flap than to go to the extra
effort to hide this from the IP stack in some special cases (which 
involve the network topology behind the MAG, something that is usually
not visible to L2).

> [snip]
> 
>>>>> Assuming RFC6543 is not used and we do assign different link-
>>>>> locals, then it is up to the LMA decide when to use which
>>>>> link-local. Two examples: each LMA has its own or each PMIP session
>>>>> gets its own "unique" link-local. Both cases have possibility of
>>>>> address collisions but are quite unlikely. I do not really care
>>>>> about coverage areas rather the case when something happened on the
>>>>> emulated home link and for that case the two examples would be
>>>>> sufficient. Note that I am not prompting such "solution".
>>>> 
>>>> Did you mean to say "up to the MAG"?  I am not sure I like the idea
>>>> of the LMA controlling the MAG's link-local address.
>>> 
>>> No.. or lets say my wording was imprecise. Whether this feature is
>>> enabled depends on the MAG configuration. If MAG allows the LMA to
>>> generate the link-local, then LMA does it and MAG uses that address.
>>> In that case the LMA decides when to use which link-local address.
>> 
>> It would still require coordination between the old LMA and the new
>> LMA, precisely in those situations where no such coordination is
>> possible (the PMIP domains are distinct and unconnected).
> 
> It is just a matter of will. OUIs are invented and EUI-64 is big hefty
> number. Collision would be less probable than me winning in lottery.
> However, this would probably be a task to some other SDO to arrange
> than IETF.

Sure, I agree this is possible.  But, it seems even more natural to
me to allow every MAG to have its own link-local address that is
globally unique.  I guess I am calling into question the assumption
that all MAGs in a PMIP domain have the same link-local address.
Perhaps that was a PMIP design mistake.  If the IP stack is going
to see a link flap anyway, we might as well deal with a proper change
of first-hop router (although that router might still advertise the
same prefix.  It is as if one router became unreachable but another
router pops up to take its place.  MNs should be able to handle this).

> [oh-snap]
> 
>>>> If the link down/up event has been hidden from the IP stack, then
>>>> sitting there doing nothing is exactly what an RFC3315 compliant
>>>> implementation would do.  Hence my use of MUST in (2) above.
>>> 
>>> Sure. Never claimed it to be different. But as said, I cannot control
>>> the client side implementation. That is the point.
>> 
>> But you seem to be making assumptions about the L2 implementation
>> and that, in particular, it knows when a PMIP session is
> disconnected.
> 
> See my earlier notes on L2 sessions.
> 
>> I am sorry to be making a big deal about these issues.  I am raising
>> them because I think it will be important to get these details right
>> when it comes to the DMM solution.  There, I think we will have some
> 
> Figured that ;)
> 
>> set of prefixes assigned to the MN at any given time, and some
>> arbitrary subset will need to be deprecated and/or torn down
>> proactively upon each handover (change of MAG if a PMIP-style solution
>> is used to re-route the packets).  However, we can discuss more details
>> on the DMM list if you prefer (and this may get us a bit into DMM
>> solution space.  But, I am trying to think ahead).
> 
> Yep, I am familiar with this specific problem space. See e.g.
> http://tools.ietf.org/html/draft-korhonen-dmm-local-prefix-00 for one
> solution approach. This, however, assumes the MN always has at least on
> LMA that remains the same as long as the MN is within the PMIP domain.
> But that was enough DMM for my needs ;)

Ok, just read your draft.  It raises the right issues, I think.  However,
I wonder if we could make it more general by decoupling the MN-visible
events like RAs and DHCP interaction and link flaps from the PMIP
protocol used to re-route packets and transfer context.
There are many ways to re-route packets and many ways to get the
context about the old prefixes from one AR to another AR.

Also, we should deal with cases where the old LMA is not reachable
and the DHCP server changes.

> Thinking ahead is more than welcome!

Good to hear.  Feel free to snip the netext mailing list from the
CC line when you think it's appropriate.

-Pete