Subject: Re: Proposal for the inclusion of peak rate

From: [email-masked] (Fred Baker)
Date: Mon, 28 Aug 1995 14:25:16 -0700

At 1:59 AM 8/26/95, Vinod Peris wrote:
>The problem with modifying the TSpec is that it can result in an
>incorrect computation of the end-to-end delay.  By lowering the peak
>rate because of a lower speed link on the path, the receiver will be
>unable to determine the true peak rate of the flow.

The peak rate as received by the receiver beyond the router might exceed
the rate at which the intermediate router can transmit the data? NOT! If
the router doesn't reduce the advertised peak rate to the peak at which it
can forward the data, I guarantee that the receiver will incorrectly
calculate the flow's attributes as received by itself.

>This in turn,
>will prevent the receiver from asking for the right rate R that is
>necessary to meet its end-to-end delay guarantees.  What this means,
>is that the router should not modify the TSpec, even if it includes a
>peak rate which is higher than any of its link speeds.
>
> FB> The only value in advertising peak rate is if
> FB> the router can in fact verify that it will be able to maintain its delay
> FB> commitments AND admit the new flow. What I have asked for from others who
> FB> proposed doing so was an equation that I can put in the router that will
> FB> give me that. I'm going to have to think about whether your math enables m
>e
> FB> to do that.
>
>Exactly.  These are just the kind of things (buffer requirements, number
>of accepted calls) that adding the peak rate will help you improve on.

That remains to be proven. Unfortunately, your paper doesn't describe the
way the formulae were derived, only presents some pretty general formulae.
I'm not convinced at this point.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
So Bill Gates has paid Mick and the boys 12 million dollars to use "Start Me
Up" to sell Windows 95.

For those who don't know the song, it's the one that goes "You make a grown
man cry...".

Food for thought

---

From: Vinod Peris <[email-masked]>
Date: Mon, 28 Aug 1995 18:28:16 -0400

>>>>> "FB" == Fred Baker's note dated Mon, 28 Aug 1995 14:25:16 -0700

 FB> The peak rate as received by the receiver beyond the router might exceed
 FB> the rate at which the intermediate router can transmit the data? NOT! If
 FB> the router doesn't reduce the advertised peak rate to the peak at which it
 FB> can forward the data, I guarantee that the receiver will incorrectly
 FB> calculate the flow's attributes as received by itself.

First, let me clear up a possible misunderstanding. The peak rate
field that we propose in the TSpec is not an indication of the peak
rate at which the receiver will receive data. Rather, it is the peak
rate of the source. In light of this, the intermediate routers should
not change the peak rate field in the TSpec, even though the speed of
their links are smaller than the peak rate of the source.

The better end-to-end delay bounds that are in the proposal to include
a peak rate, are a result of accounting for the peak rate at the
source. Thus, it is important that the peak rate field in the TSpec
reach the receiver without modification.

Hope this helps,

--Vinod

---

From: John Wroclawski <[email-masked]>
Date: Mon, 28 Aug 1995 19:09:41 -0400

From: [email-masked] (Fred Baker)

If I have a (bursty) source with a peak rate of 10mb/s, which is then
reduced to a peak rate of 1mb/s by a router in the middle of the net,
the bursts will be smoothed somewhat. If a later router in the path
has 155mb/s outgoing links, the traffic at that point might end up
very bursty, with a higher peak rate than 10mb/s.

Routers lowering the peak rate isn't the real issue. Routers that
raise it seem more problematic.

It seems to me that two possibly correct answers when considering
a single source are:

 1) routers do not modify the Tspec when the peak rate is lowered, but
may modify the peak rate -upward- if required, or

 2) routers do not modify the Tspec at all, but are required to
reshape traffic to prevent it exceeding the TSpec peak rate.

Of these, the second seems to impose more requirements on a router
than the first.

			-john