Subject: FW: [Diffserv] EF question !!!

From: Shahram Davari <[email-masked]>
Date: Thu, 6 Apr 2000 11:07:31 -0700

"The figure was messed up in previous email. Please use this one:"

Hi,

RFC-2598 says: 
"EF SHOULD average at least the configured rate when measured over any time
interval equal to or longer than the time it takes to send an output link
MTU sized packet at the configured rate"

Lets assume 2 PHB queues: an EF and a BE each of rate R (both nicely
shaped). Let's assume the output link rate is 2R so that 50% of the link BW
(=R) is assigned to EF and 50% (=R) to BE. Let's also assume that packet
sizes are all MTU size. Now on the output link we will have EF and BE
packets one after another as following:

----------------------------------------------------------
|  BE   |  EF   |  BE  |  EF  | BE  |  EF  |   .....       |
----------------------------------------------------------

                <------------------->
                         3T

The duration of each packet at the output link is T= MTU/2R. Now assume the
time from the beginning of a BE packet to the end of another BE packet; this
time = 3T (notice that there is only one EF packet in this interval). Also
the time to send an MTU size packet at EF configured rate is MTU/R = 2T.

Since 3T  > 2T then according to RFC-2598 we should average the configured
rate in the 3T interval. But as you can see the average EF rate in the 3T
interval is MTU/3T = 2R/3 which is 2/3 of the EF configured rate R. 

Either I am missing something or there is a problem with the RFC-2598.

Any comments?

Shahram Davari
Systems Engineer, Product Research
PMC-Sierra Inc. 
555 Legget drive,
Suit 834, Tower B,
Ottawa, ON K2K 2X3, Canada
Phone: +1 (613) 271-4018
Fax  : +1 (613) 271-7007

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---

From: Carlo Matarasso <[email-masked]>
Date: Fri, 07 Apr 2000 09:36:08 +0200

Shahram,

when you calculate average values in steady state, the time interval you
consider has to be independent from its position on the time axis. Yours is not:
if you take the 3T interval right after yours, you' ll have 2 EF MTU and 1 BE,
that will even up the previous average.
In you example you have to consider an interval that contains an evan amount of
Ts.

If you consider int. = 6T, in this interval the EF rate is 3MTU/6T = MTU/2T = R

Carlo

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From: Anna Charny <[email-masked]>
Date: Fri, 07 Apr 2000 10:48:37 -0700

Carlo,

>If you consider int. = 6T, in this interval the EF rate is 3MTU/6T = 

It would seem to me that the requirement that statement "X must /should be 
true for any interval of length Y or longer" means that
whatever interval of length Y or longer you choose to take, statement X 
must indeed hold in this interval.  The fact that
you can find *some* intervals for which X is true does not yet mean that X 
must hold for *any* interval of length Y or longer.  Hence your
objection to Sharams's observation does not appear to be valid.  Did I 
misunderstand what you wanted to say?

Regards,
Anna

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From: Vicent Pla <[email-masked]>
Date: Fri, 07 Apr 2000 17:26:22 +0200

> packets one after another as following:

When I read RFC-2598 I assumed EF queue didn't empty during the measured time
interval.  I think the case you described above doesn't fit this requirement.
Indeed, if such constraint isn't assumed, a much simpler EF traffic arrival
pattern, namely, no EF arrivals for an "interval equal to or longer than the
time it takes to send an output link
MTU sized packet at the configured rate" will contradict the RFC-2598
requirement.

--
Vicent