Subject: Preemption / Priority

From: "Srividhya J " <[email-masked]>
Date: Tue, 18 Sep 2001 04:03:22 +0530

Hi,

I have a doubt in the preemption / priority model described in the IETF Internet Draft draft-lefaucheur-diff-te-proto-00.txt.
The title of the draft is "Protocol Extensions for support of Diff-Serv_aware MPLS Traffic Engineering" dated July 2001 and expiring January 2002 .

Clarification is required for the preemption model in section 3.2 and the calculation shown in Appendix A of the draft.

The question follows: 
Consider there are 4 class types, CT0, CT1, CT2 and CT3, with CT3 being the highest priority CT (i-e the LSPs for CT0 have the highest priority). 
Computation:
Considering the example above, assume the configuration below:
MRB for CT3 = 35
MRB for CT3+CT3 = 75 
MRB for CT3+CT2+CT1 = 100
MRB for CT0+CT1+CT2+CT3 = 155

As per the draft:
1.	At a particular instance, suppose the bandwidth used up by the class types are as    
below.

CT	BW	
3	25	
2	40	
1	20	
0	0	

If CT2 requests for bandwidth, it is limited by 75.
The unreserved bandwidth for CT2 is 10. This is nothing but Min (70, 15, 10). as per the Appendix A of the draft.
This is fine. Now consider the following case.

2.	Suppose the bandwidth used up by the class types are as below

CT	BW	
3	10	
2	30	
1	60	
0	35	

If CT2 requests for bandwidth, it is limited by 155.
The unreserved bandwidth for CT2 is 0, i-e Min(20, 0, 35) as per the calculation given in the Appendix A of the draft.
It is also given in the draft that Maximum Reservable Bandwidth = Unreserved Bandwidth
But this seems to invalidate the point given in section 5.2 of the same draft. 

Actually, the Maximum Reservable bandwidth for CT2 should be 35 and not 0. This is because of the fact that the CT2 has higher priority over CT1 & CT0 and should be entitled to use up the bandwidth left by CT3 (25 in the example) and the bandwidth left for its own (10 in the example). But due to the calculation shown in the draft, it comes up only to 0.

Is this Correct? 
If so, does it imply that the "Unreserved Bandwidth" AKA "Maximum Reservable Bandwidth" does not consider Preemption?
Or Is "Maximum Reservable Bandwidth" is different from "Unreserved Bandwidth" ? 

Kindly respond at the earliest.

Thanks in advance,
Srividhya.

One mighty deed can change a course of actions.

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