|
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
|