Subject: A question about C term in GS

From: Zhenhai Duan <[email-masked]>
Date: Thu, 30 Jul 1998 16:30:06 -0500 (CDT)

I have a question about C term in GS. In the specification
of GS, it is said it is rate-related, and implementation-dependent, and
give a short sentence for the case of WFQ, in which it is set to M, the
maximum packet size. Then I am wondering why C_tot is needed in the
ADSPEC of PATH message of RSVP, since the hop number is recorded in the
ADSPEC, so the receiver can get C_tot from:

	C_tot = n * M, where n is the hop number.

or even more general, C_tot=n * f(M), where f is a linear function.
Of course, I am assuming WFQ is the service model along the path. 

If the scheduling principle is not WFQ, then how C term is set?

Any answer is appreciated.

--Zhenhai
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Zhenhai Duan		      PhD student
[email-masked]		      Department of Comp. Sci. and Eng.   
Tel. 612-6267526(Office)      University of Minnesota          
http://www.cs.umn.edu/~duan
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From: Roch Guerin <[email-masked]>
Date: Mon, 03 Aug 98 11:46:54 -0400

Zhenhai,

Let me try to give you some feedback on your question.

> Of course, I am assuming WFQ is the service model along the path. 

That's an assumption that the spec did not want to make, although it
certainly assumes the general rate-based model for end-to-end delay
guarantees, even if other type of schedulers can be used.  

> If the scheduling principle is not WFQ, then how C term is set?

Don't think I can give an exhaustive answer as things are different for
each individual scheduler, and you need to look at the specific bound for
each to determine what the appropriate C term should be.

For example, a self-clocked or start time fair queueing scheduler would
have a similar term as WFQ, but would differ in their D term.
Similarly, if one were to use a scheduler that can decouple rate and delay
guarantees, e.g., rate controlled EDF, you could choose any value X as it
is eventually translated into a local deadline, i.e., X/R, which is not 
constrained to a particular value by the scheduler's operation.
But again, in general, the appropriate value will be scheduler specific.

Hope this helps.

Roch