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Necdet,
NU: Did you run any simulations showing the
priority inversion happening while adding classA1 (when stqFullThreshold -
stqHighThreshold > RTT * rateA1?
Not yet, but A0, A1 or class B traffic should not
make a difference in
the scenario, they all have the same guaranteed
bandwidth problem.
I don't understand the last part of your question,
but the RTT actually
does not play a role in the example scenario. The
problem is the amount
of queued traffic on the transit path since
this cannot be stopped by FCMs.
So it doesn't matter if these FCM's come quickly or
not.
NU: Did you implement shaperD and reserved
classA0 bandwidth all around the ring?
Yes, I can dynamically make reservations for
class A0, A1 and B traffic. I'm not sure what you mean by "all around the
ring".
Simulations are great, but this scenario is not so
difficult to comprehend:
If 100 stations start sending class C packets to one
station, and the STQs are all empty, then
all these station may add local
traffic until their stqLowThreshold is reached. The amount of
traffic in all
these STQs is tremendous and will produce a very long traffic stream at
100% of
the line rate. And the worst thing: it cannot be stopped by FCMs!
Stations receiving this stream, which also have guaranteed bandwidth flows,
can get in trouble,
because their STQ may overload.
Jon
----- Original Message -----
Sent: Tuesday, April 22, 2003 8:24
PM
Subject: Re: [RPRWG] Class A and B
Guarantees
Jon,
Please see my comments in line.
Thanks.
Necdet
Jon Schuringa wrote:
Dear all,
I posted a comment (#33) at the Dallas
meeting about bandwidth
guarantees: In my opinion, bandwidth agreements cannot
always
be
guaranteed. The comment
was rejected because it was addressed to the wrongclause. Although at the wrong address, I got the
answer that thestatement in my
comment is incorrect, but without any explanation. Since then I had discussions with several people,
and checked my
simulations
with another simulation tool (ns2). As before, I strongly
believe this to be a serious technical
concern, and therefore post it
here to the mailing list.
The problem in short:
STQ's can reach the stqFullThreshold in scenarios
where both class C
and
class A traffic flows. As a result, the STQ gets precedence
over
all locally sourced
traffic, so that class A (and B) traffic has to wait,
causing bandwidth and jitter
problems.
The STQ can get
that full because fairness messages cannot stop
packets that already have been transmitted by other
stations, but did
not yet
arrive at the local station. This amount of packets that is on
the
transit path can be
very large since it is the sum of all packets in the
STQs on the transit path. This is also the reason
why larger STQs
do not
solve the problem. So
basically what happens in the problem scenarios is that:
1) the local station (S) receives
class C packets at 100% of the line rate.
All these packets need to
be forwarded by station S
2) Station S transmits guaranteed class A (local) traffic at
some rate x,
so the local STQ grows (at rate
x).
3) Station S
advertises a fair rate unequal to FULL_RATE once the STQ
exceeds the
stqLowThreshold
4)
All other stations see the advertized rate and limit their "add"
traffic.
This however does not directly prevent that station S gets less
than
100% line rate, because there is still transit traffic that needs to
be
forwarded by all stations. These stations empty their STQs.
5) If the class A rate x and the number
of STQs are "large enough", the
STQ in station S will reach its stqFullThreshold
and priority inversion is the result. Note that the potential problem scenarios are
realistic hub-scenarios, not
"pathological cases".
NU: Did you run any simulations showing
the priority inversion happening while adding classA1 (when stqFullThreshold
- stqHighThreshold > RTT * rateA1?
A detailed
description and an example scenario can be found here:
http://grouper.ieee.org/groups/802/17/member/draftballots/d2_1/refs/js_issues_1.pdf
This document contains other issues as
well.
Opinions?
NU: Did you implement shaperD and reserved
classA0 bandwidth all around the ring?
Best
regards,Jon -----------Jon
SchuringaInstitute of
Communication Networks
Vienna University of Technology
Favoritenstraße 9/388
A-1040 Vienna
+43/1/58801-38814
www.ikn.tuwien.ac.at
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