RE: [RPRWG] Class A and B Guarantees
John,
Once the STQ at a station starts to build up, the
traffic that it sends to the downstream station will
be 100% of link rate until the STQ has completely
drained. This is regardless of A0 reservations
because the scheduling at the transit queues is
work-conserving (i.e. if there is a frame in the
transit queue, either something from the insert path
is sent depending on congestion thresholds, etc.,
or something from the transit queue is sent).
-Anoop
-----Original Message-----
From: John Lemon [mailto:JLemon@xxxxxxxxxxxx]
Sent: Tuesday, April 22, 2003 5:33 PM
To: Jon Schuringa; stds-802-17@xxxxxxxx
Subject: RE: [RPRWG] Class A and B Guarantees
Jon,
You have a few incorrect assumptions in your paper.
1) You state that "During this time, the STQ of station 14 grows with a
constant rate because it sends with a constant rate its own class A0 traffic
and receives frames at 100% line rate." But this can't be true because
Station 13 is not allowed to send at higher than 80% line rate since Station
14 has an A0 rate of 20% line rate.
2) Priority inversions that are minor priority inversions are not considered
violations of the guaranties, and are allowed. Only major priority
inversions are prohibited and prevented.
jl
-----Original Message-----
From: Jon Schuringa [mailto:jon.schuringa@xxxxxxxxxxxx]
Sent: Tuesday, April 22, 2003 8:24 AM
To: stds-802-17@xxxxxxxx
Subject: [RPRWG] Class A and B Guarantees
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 wrong
clause. Although at the wrong address, I got the answer that the
statement 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".
A detailed description and an example scenario can be found here:
http://grouper.ieee.org/groups/802/17/member/draftballots/d2_1/refs/js_issue
s_1.pdf
This document contains other issues as well.
Opinions?
Best regards,
Jon
-----------
Jon Schuringa
Institute of Communication Networks
Vienna University of Technology
Favoritenstraße 9/388
A-1040 Vienna
+43/1/58801-38814
www.ikn.tuwien.ac.at