RE: Preemption with MAM RE:

["Lai, Wai S (Waisum), ALABS" <wlai@att.com>]

Francois,
  Preemption across CTs in MAM was presented to the TEWG, at least as early as June 2002, when the 00 version of <draft-wlai-tewg-bcmodel> was submitted.  Dimitry's posting also confirmed this mode of operation.
  The MAM definition simply says that Reserved (CTb) <= BCb, i.e., the reserved bandwidth of a CT is *either less than or equal to* the bandwidth constraint for the CT.  Thus, when contention resulting in preemption across CTs occurs, the reserved bandwidth of a CT can/may be less than the bandwidth constraint for the CT, depending on the relative preemption priorities of the different CTs involved.
  We can add this clarification to the MAM spec.
Thanks, Wai Sum

-----Original Message-----
From: Francois Le Faucheur (flefauch) [mailto:flefauch@cisco.com]
Sent: Monday, March 24, 2003 5:31 PM
To: Lai, Wai S (Waisum), ALABS; Geunhyung Kim; Te-Wg
Subject: Preemption with MAM RE: 


Waisum,

>> -----Original Message-----
>> From: Lai, Wai S (Waisum), ALABS [mailto:wlai@att.com] 
>> Sent: 24 March 2003 19:11
>> To: Geunhyung Kim; Te-Wg
>> Subject: RE: 
>> 
>> 
>> Geunhyung,
>>   Thanks for your query.  If preemption is disabled, then of 
>> course there will be no preemption, whether within a given 
>> CT, or across CTs, regardless of any BC model used.  
>> However, if preemption is enabled, then there can certainly 
>> be preemption across CTs in the MAM model.
>>   Francois, the DS-TE Requirements document does not place 
>> any restrictions on preemption in MAM.

That is correct.

>>   As stated in Section 4.4 of the DS-TE Requirements 
>> document, "if LSP1 contends with LSP2 for resources, LSP1 
>> may preempt LSP2 if LSP1 has a higher set-up preemption 
>> priority (i.e. lower numerical priority value) than LSP2's 
>> holding preemption priority regardless of LSP1's OA/CT and 
>> LSP2's OA/CT."  

That statement is correct. But note that it starts with " *IF* LSP1 contends with LSP2". With MAM as it has been defined for a long time in the TEWG, LSP1 can only be contending with LSP2 if they are in the same CT. If they are in different CT, they are each subject to independent bandwidth constraints and therefore simply can not be in a situation where they contend. So while preemption across CTs is not "forbidden", it is simply not applicable.

>>Different methods can be used to meet this 
>> requirement.  The MAM model takes care of it simply by an 
>> implicit aggregate constraint.

There is no such thing as "implicit constraint". There are Bandwidth Constraints which are all pretty explicit. The reason we've written down definitions for these models is so that everybody knows what are the bandwidth constraints.

MAM definition says:
"
           o Maximum Number of Bandwidth Constraints (MaxBC)= Maximum 
               Number of Class-Types (MaxCT) = 8 
             o for each value of b in the range 0 <= b <= (MaxCT - 1): 
                       Reserved (CTb) <= BCb, 
"
MAM involves just one Bandwidth Constraint per CT, and those are independent.

It may be that another model which has independent bandwidth constraint + aggregate constraint (this would basically be a hybrid between MAM and RDM) is a reasonable idea; but this is not MAM as it is currently defined.

Cheers

Francois


>>  Other models may use an 
>> explicit constraint.
>>   Geunhyung, your example below reflects the correct 
>> operation in MAM with preemption across CTs (assuming that 
>> CT0 has the highest set-up preemption priority, and CT3 is a 
>> typo of CT2).
>> Thanks, Wai Sum
>> 
>> -----Original Message-----
>> From: Geunhyung Kim [mailto:geunkim@postech.ac.kr]
>> Sent: Wednesday, March 19, 2003 11:57 PM
>> To: Te-Wg
>> Subject: RE: 
>> 
>> 
>> Hi all,
>> 
>> At first, I would like to ask one question. 
>> Is there preemption across CTs in the MAM model ?
>> 
>> If there is preemption across CTs, there is aggregate limit 
>> implicitly in MAM with preemption.
>> 
>> In the Faucheur's example (link of 100 and BC0=50, BC1=50, 
>> BC2=50), when the link is congested, there are only CT0 and 
>> CT1 traffics.
>> 
>> in the first phase, there is only CT2 traffic with 50.
>> after that, there are requests of CT0 traffic demand with 40 
>> and CT1 traffic demand 30.
>> if these requests are accepted, the link capacity is divided 
>> into CT0(40), CT1(30), CT3(30), because there is preemption 
>> across CTs.
>> 
>> However, if there is not preemption across CTs, there is 
>> aggregate limit in MAM with preepmtion(neither explicit nor implicit)
>> 
>> Regards,
>> 
>> Geunhyung
>> 
>> None of us is as smart as all of us
>> ==========================================
>> Geunhyung Kim
>> 
>> E-mail: geunkim@postech.edu
>> 
>> Tel: +82-54-279-5655
>> Fax: +82-54-279-5699
>> 
>> Networking & Distributed Systems Lab.
>> CSE
>> POSTECH
>> ===========================================
>> 
>> -----Original Message-----
>> From: owner-te-wg@ops.ietf.org 
>> [mailto:owner-te-wg@ops.ietf.org]On Behalf Of Francois Le 
>> Faucheur (flefauch)
>> Sent: Thursday, March 20, 2003 12:32 PM
>> To: Lai, Wai S (Waisum), ALABS; LE ROUX Jean-Louis FTRD/DAC/LAN
>> Cc: te-wg@ops.ietf.org
>> Subject: RE: 
>> 
>> 
>> 
>> Waisum,
>> As Jean-Louis said, there is no aggregate limit with MAM 
>> (neither explicit nor implicit).
>> If you have a link of 100 and BC0=50, BC1=50 and BC2=50, 
>> then you may very well endup with a load of up to 150 across 
>> the three CTs.
>> Hence, with MAM:
>> 	- you may have preemption within a CT (ie an LSP of CTx 
>> may need to preempt another LSP of same CTx)
>> 	- you will not have preemption across CTs (ie an LSP of 
>> CTx will not preempt another LSP of Cty, since those don't 
>> contend for bandwidth).
>> Cheers
>> Francois
>> 
>> 
>> >> -----Original Message-----
>> >> From: Lai, Wai S (Waisum), ALABS [mailto:wlai@att.com] 
>> >> Sent: 20 March 2003 04:13
>> >> To: LE ROUX Jean-Louis FTRD/DAC/LAN
>> >> Cc: te-wg@ops.ietf.org
>> >> Subject: RE: 
>> >> 
>> >> 
>> >> JL,
>> >>    Do you mean that there is no *explicit* constraint for 
>> >> the aggregate bandwidth reserved from different classes?  As 
>> >> I described in my previous reply below, when the constraints 
>> >> sum up to link capacity, there is "total isolation" with no 
>> >> preemption among classes (which is not necessary of course). 
>> >>  When this is not the case, then the link capacity will act 
>> >> implicitly as the aggregate constraint.  This is a natural 
>> >> aggregate constraint (or an appropriately scaled aggregate 
>> >> constraint in the case of overbooking) that does not need to 
>> >> be explicitly spelled out, right?  When this aggregate 
>> >> constraint is to be exceeded, then preemption among classes 
>> >> will act in accordance with the definition, which says that 
>> >> Reserved (CTb) <= BCb, i.e., the reserved bandwidth of a 
>> >> class is *either less than or equal to* the bandwidth 
>> >> constraint for the class, depending on the relative 
>> >> preemption priorities of the different classes involved.
>> >> Thanks, Wai Sum
>> >> 
>> >> -----Original Message-----
>> >> From: LE ROUX Jean-Louis FTRD/DAC/LAN
>> >> [mailto:jeanlouis.leroux@rd.francetelecom.com]
>> >> Sent: Wednesday, March 19, 2003 8:25 PM
>> >> To: Lai, Wai S (Waisum), ALABS
>> >> Cc: te-wg@ops.ietf.org
>> >> Subject: RE: 
>> >> 
>> >> 
>> >> Wai Sum,
>> >> 
>> >> Preemption among classes can definitively not occur with 
>> >> current MAM defintion, whatever the preemtion priorities, 
>> >> because there is no constraint for the aggregate bandwidth 
>> >> reserved from different classes
>> >> 
>> >> If you define BC2= 5M, BC1= 7M, BC0= 15M, then you can 
>> >> reserve simultaneously 5M of CT2 LSPs and 7M of CT1 LSP and 
>> >> 15M of CT0 LSPs.
>> >> 
>> >> To allow preemtion between classes you need constraint on 
>> >> the cumulated bandwidth reserved from diffrerent classes, 
>> >> which does not exist in MAM. 
>> >> 
>> >> 
>> >> Regards
>> >> 
>> >> JL
>> >> 
>> >> -----Message d'origine-----
>> >> De : Lai, Wai S (Waisum), ALABS [mailto:wlai@att.com]
>> >> Envoyé : mercredi 19 mars 2003 02:29
>> >> À : LE ROUX Jean-Louis FTRD/DAC/LAN
>> >> Cc : te-wg@ops.ietf.org
>> >> Objet : RE: 
>> >> 
>> >> 
>> >> Jean-Louis,
>> >>    Associated with each class-type (or simply referred to as 
>> >> a class in my draft, as stated in the 2nd paragraph of 
>> >> Section A.3) there is a preemption priority, which together 
>> >> form a TE-class.  This enables preemption among class-types.   
>> >>    "Total isolation between classes" is provided in MAM only 
>> >> when the bandwidth constraints for different classes add up 
>> >> exactly to the link capacity.  When this is not the case 
>> >> (e.g., with overbooking), there will be interference among 
>> >> classes.  As shown in my draft, the degree of this 
>> >> interference depends on the degree of bandwidth sharing, 
>> >> whether preemption is used or not, and the relative 
>> >> preemptin priority.  This is a general property for any BC 
>> >> models: the higher the degree of sharing, the less robust 
>> >> the service isolation.
>> >>    My view of overbooking is concerned with dimensioning a 
>> >> link to carry the different classes of traffic offered while 
>> >> meeting service objectives.  I have not explicitly used a 
>> >> multiplier to scale the bandwidth of might appear to be 
>> >> available and advertised, if that's what you are referring 
>> >> to.  But I think I have done that implicitly, so as to show 
>> >> the performance impacts, and the need for a judicious choice 
>> >> of overbooking multipliers.  Thus, my example of twice the 
>> >> normal traffic (while discussed in the context of overload) 
>> >> is effectively scaling with a factor of 2. 
>> >> Thanks, Wai Sum
>> >> 
>> >> -----Original Message-----
>> >> From: LE ROUX Jean-Louis FTRD/DAC/LAN 
>> >> >> [mailto:jeanlouis.leroux@rd.francetelecom.com]
>> >> Sent: Monday, 
>> >> 
>> >> March 17, 2003 8:59 PM
>> >> To: Lai, Wai S (Waisum), ALABS
>> >> Cc: te-wg@ops.ietf.org
>> >> Subject: 
>> >> 
>> >> 
>> >> Hi Wai Sum and all 
>> >> I have a question regarding draft-wlai-tewg-bcmodel-01 
>> >> Section A.3 : 
>> >> "Preemption is enabled so that, when necessary, class 1 can 
>> >> preempt class 2...." 
>> >> How can you apply this to MAM ??  
>> >> If I refer to 3.0 definition,MAM ensures total isolation 
>> >> between classes, preemption can occurs only inside a class, 
>> >> but not between classes
>> >> 
>> >> 
>> >> "Overbooking is allowed as it is to be described below..." 
>> >> How do you define overbooking here ? 
>> >> Overbooking is definitively not allowed in your RDM 
>> example (BC0=15) 
>> >> Regards 
>> >> JL 
>> >> 
>> >> 
>> 
>>