Subject: RE: [Diffserv] Re: Mapping IP QoS Info to ATM QoS

From: Shahram Davari <[email-masked]>
Date: Fri, 16 Jun 2000 12:29:28 -0700

Hi,

I think Grenville has a point. There are 3 cases in mapping PHB <--> ATM
services:

1) Mapping ATM services to PHBs. This case is useful when the e2e service is
an ATM service and it is just transiting a Diffserv network. In that case
for each ATM service a PDB should be defined that is equivalent to that ATM
service. The PDBs use predefined (or new PHBs) together with specific
traffic conditioning, etc. This kind of mapping is doable but requires new
PDBs to be defined by IETF, and therefore the ATM forum probably needs to
wait. You could think of this scenario as service interworking.

2)Mapping PHBs to ATM services in traditional ATM networks. This case is
useful for building a Diffserv network that partially consists of an ATM
network. In this scenario you could think of the ATM network as a single
Diffserv node. It is possible to use or perhaps define new ATM services
across an ATM network that can behave as a single hop PHB. This is out of
the scope of IETF and ATM Forum could start working on it without needing to
wait for IETF.

3)Mapping PHBs to ATM services in a Diffserv enabled ATM network (i.e., an
ATM network in which each ATM switch could actually implement PHBs). This
case is useful for building a Diffserv network that partially consists of a
Diffserv enabled ATM network. In this scenario you could think of each of
the ATM switches as a single Diffserv node. It is possible to enhance an ATM
switch (through H/W and S/W) so that it could behave as a single Diffserv
node. However due to ATM restrictions (no DSCP field), a kind of signaling
is required, so that ATM nodes could identify the Diffserv PHB (very similar
to L-LSP). This is the UBR+ work at ATM Forum.  Again, this is out of the
scope of IETF and ATM Forum could work on it without needing IETF. 

I believe Grenville was addressing the 2nd case. while John was talking of
the first case and Scott mentioned the 3rd case.

Regards,
-Shahram     

>-----Original Message-----
>From: Grenville Armitage [mailto:[email-masked]]
>Sent: Friday, June 16, 2000 2:37 PM
>To: John Schnizlein
>Cc: Dan Grossman; [email-masked]
>Subject: Re: [Diffserv] Re: Mapping IP QoS Info to ATM QoS
>
>
>
>
>John Schnizlein wrote:
>	[..]
>> Since ATM services are end-to-end, and PHBs are not, it is necessary
>> to form an end-to-end (or at least edge-to-edge or cross-domain)
>> service level for comparison to the ATM service level.
>
>But why? Perhaps I missed the question (which I thought was
>triggerd by someone saying the ATM Forum is waiting for DS
>services to be defined before it could figure out mappings
>from PHBs to VC service classes).  If I've gotten the question
>right, I dont see the above as a 'necessity'.
>
>	[..]
>> Dan's "by whatever name" refers to the linguistic debate spawned
>> by the attempt to move a litte closer to service specification
>> named "behavior aggregate" and renamed "per domain behavior" (PDB).
>
>That part I understand, but I still dont see why DiffServ needs
>to define services before anyone can figure out what ATM
>level services can usefully support particular DS PHBs.
>
>> While we are out here in the weeds, note that the tidy lawn view of
>> this is as Brian said - ATM virtual circuits connect router hops
>> through a virtual link. And the link behavior is part of the per-hop
>> behavior.
>
>Well, that's pretty much what I said in different words. IP PHB
>is DS PHB + link service class. So what's the question that cannot
>be answered by this approach?
>
>cheers,
>gja
>_______________________________________________________________
>_________
>Grenville Armitage                    
http://members.home.net/garmitage/
Bell Labs Research Silicon Valley

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---

From: Grenville Armitage <[email-masked]>
Date: Fri, 16 Jun 2000 15:11:48 -0700

your three scenarios are a useful clarification of what different
people might be thinking. i certainly hadn't considered #1 in
this thread.

cheers,
gja

---

From: Dan Grossman <[email-masked]>
Date: Tue, 20 Jun 2000 09:31:08 -0400

> an ATM service and it is just transiting a Diffserv network. 
I think that this case has so many problems as to not be worth pursuing.  ATM 
provides tight service definitions that probably would be too difficult to 
support with Diffserv, at least in a semi-efficient fashion.  ATM over IP in 
general has problems, starting with the fact that ATM is by definition order 
preserving and IP by definition tolerates misordering.  Besides, why would 
anyone want to do a layering inversion like that?
 
> scope of IETF and ATM Forum could work on it without needing IETF. 
I don't understand the difference between 2 and 3.  In both cases, the ATM 
network is an LIS that supports Diffserv in some fashion. The only difference 
is the degree of accomodation that the ATM service specifications make to 
Diffserv.
 
> the first case and Scott mentioned the 3rd case.
The more interesting question is whether the ATM service supports a PHB (ATM 
network plus the two edge routers behaves like a single router) or whether is 
supports a PDB (ATM network plus the two edge routers behaves like a DS 
domain).  The answer to this gets into the dark shadows of Diffserv.  PHBs 
deal with forwarding in DS core routers.  The issues surrounding resource 
management in DS core routers have never been addressed in Diffserv, except 
for occasional handwaving about a mythical bandwidth broker.  The question is 
whether one can specify a way to use a complete network to behave as if it was 
a single router. I'm no longer sure I know the answer to that question.

Dan

---

From: Brandon Ross <[email-masked]>
Date: Tue, 20 Jun 2000 21:32:05 -0400 (EDT)

On Tue, 20 Jun 2000, Dan Grossman wrote:

> ATM over IP in general has problems, starting with the fact that ATM is
> by definition order preserving and IP by definition tolerates
> misordering.  Besides, why would anyone want to do a layering inversion
> like that? 

Legacy and migration.  It's not so much IP itself that someone would want
to run ATM over, but MPLS.  When migrating a legacy ATM network to an MPLS
backbone, it's quite useful to be able to run ATM switch-to-switch trunks
over MPLS.  Then there's always legacy ATM customers that will simply
refuse to ever change technologies that have to continue to be supported
even after the ATM backbone is gone.  There's most certainly a large
demand for such things.  Rumor has it that a couple of major backbones are
already using a certain vendor's proprietary solution.

Brandon Ross                                                 404-522-5400
VP Engineering, NetRail                            http://www.netrail.net
AIM:  BrandonNR                                             ICQ:  2269442
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