Subject: Selection of Tspec parameters

From: [email-masked] (Konstantinos Dovrolis)
Date: Mon, 9 Jun 1997 15:19:03 -0500

Hello,

My question relates to the token bucket parameters of the Tspec.
The values of the bucket depth b and of the token bucket rate r  
are not unique of course. One can find "infinite" (b,r) pairs
that give valid envelopes for the sender's traffic.

How do the senders decide on these parameters?

At some small experiments that we did here we found out that the 
reserved rate R for a guaranteed service flow has a strong
dependence on b and r for a certain end2end delay bound D (in these
experiments we knew the traffic a priori; so for a given b,
r is uniquely determined).

On the other hand, the source sends the Tspec at the Path message,
before it knows the delay bound D that the receiver will request
at the Rspec. So we cannot assume that the sender calculates in 
some way the "best" (b,r) given a certain delay bound that the receiver
has requested. Is this a problem regarding how RSVP works at the
conceptual level or am I missing something?

Thanks a lot in advance for any reply on the above.

                      -- Constantinos Dovrolis 
--------
Graduate student		              [email-masked]
Dept. of Electrical & Computer Engr.          Tel: (608) 262-5130
University of Wisconsin-Madison               Fax: (608) 265-4623
1415 Engineering Drive                        http://www.cae.wisc.edu/~dovrolis
Madison, WI 53706.

---

From: Craig Partridge <[email-masked]>
Date: Mon, 09 Jun 97 16:10:44 -0700

Constantinos:

Hi -- that's interesting work you've done.  I don't have a complete
answer, in part because I'm not sure I fully understand your result.

You said 

Certainly given (r,b,p) and desired bound D, R is pretty tightly determined.
I assume what you're also saying here is that modest changes in (r,b,p)
yield big changes in R (yes?) and so the question is where on the (r,b,p)
set of choices, should a sender sit?

Clearly the range of choices the sender has it not infinite.  The
sender can estimate the range of D values available and pick (r,b,p)
accordingly.  For instance, suppose the application knows that the
max acceptable value of D is 300ms.  It knows approximately the
Internet diameter (say 30) and can approximate the per-hop values of
C and D.

Then you can find the values (r,b,p) along your r,b,p curve such that

    [(b-M)/R*(p-R)/(p-r)]+(M+[30 * C-approx])/R+[30 * D-approx] < 300 ms

for a range of reasonable R values.

Presumably there are also host system constraints.  For instance, r and p
are clearly limited by the bandwidth of the interface, and presumably the
system reserves at least some bandwidth for non-guaranteed traffic.  b is
limited by buffer space and the difference between r and p.

Once you've imposed these restrictions, how does the problem look?

Thanks!

Craig