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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----- 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_issues_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 |