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David,
Let me first say that I also agree with Necdet; for a given STQ size,
one
can find out the maximum rateA1. Anoop said something similar before;
RPR can only provide guarantees if there are sufficient resources in the
MAC and the parameters are configured correctly.
So we need to find out these parameters and that is what I am trying to
do. The concern I have is that the values for these parameters can not be
freely chosen; there are trade-off and cost aspects. For example, a low
stqLowThreshold will allow an higher rateA1 but might result in bad link
utilization, a larger STQ will also help the maximum rateA1, but is more
expensive and might produce very large class B/C delays, etc.
Anyhow, my feeling is that even with reasonable STQ sizes and
thresholds settings, the maximum amount of classA1 traffic that can be
transmitted is very low for larger rings (we need more simulations /
calculations on this). I am not sure whether class A0 reservations will
bring something here, since the problem scenario in my paper also used
classA0 traffic. The inconsistency that John Lemon found in my paper, is in
my view not an inconsistency (yes, I studied the draft a bit more
since then). The unreserved rate was 80% of the line rate, but that doesn't
mean that the total class C rate on a link cannot exceed this value (see
e.g. Anoop's mail: http://grouper.ieee.org/groups/802/17/email/msg01901.html)
About your second point, the mechanism for "immediately" stopping
fairness eligible traffic, is by advertising a localFairRate of 0. Since
heavily
congested stations cannot add FE-traffic, localFairRate will be 0 or
very small.
Regards,
Jon
----- Original Message -----
Sent: Sunday, April 27, 2003 1:06
AM
Subject: RE: [RPRWG] Class A and B
Guarantees
Jon,
I
believe there are two possible concerns on guaranteed classA
traffic.
1)
After sending a "helpMe" indication upstream, and assuming
the
upstream station immediately stops
further classB/classC transmissions,
can a station ensure that classA
traffic conflicts will not fill up its STQ?
2)
After sending a "helpMe" indication upstream, can we guarantee
that
the upstream station immediately stops
further classB/classC transmissions?
I am
not concerned with (1), since one can simply limit the level of
subclassA1
transmissions to the amount that can be sustained
during a worst cast round-trip
time. For longer distances, this level decreases, and
more of the classA traffic
requires the use of less efficient (but still
functionally correct) subclassA0.
I
agree with Necdet on this point.
I
am, however concerned with (2). The mechanism for stopping
"immediately"
is
not clear to me, perhaps simply because of my difficulty in
reading/understanding
clause 9. However, that could be solved by (in the
worst case) providing additional
control warningLevel
indications.
The
meaning of such a warningLevel would be something
like:
don't send either classB or classC,
until your classB excess credits are comparable to mine
So,
I am not greatly concerned about the viability of our protocols, in
general.
I
do, however, have a specific concern that we do not currently have
such
a
warningLevel indication. However, since there are reserved fields in
the frairness
frame, concern (if validated through review) can be
resolved within the context
of
these reserved fields.
DVJ
David V. James
3180
South Ct
Palo Alto, CA 94306
Home:
+1.650.494.0926
+1.650.856.9801
Cell:
+1.650.954.6906
Fax: +1.360.242.5508
Base: dvj@xxxxxxxxxxxx
Bob,
I am sure that we don't have any issues with latency and jitter of classA
traffic. For a given STQ size, one can find out the maximum amount of
classA1 the station can transmit, if the station needs to transmit more
classA than classA1 calculated, then the balance of them has to be provided
by classA0 through reservations all around the ring.
If there are issues, I am for correcting them.
Thanks.
Necdet
"Robert D. Love" wrote:
Necdet, thank
you for supplying values that would be of general interest.I have one
further question for you. You have indicated in your note
"This would give
us how much of a buffering needed ..." I thought that Jon indicated
that additional buffering may not help, it will just change the latency /
traffic load on the ring - conditions for which the problem occurs.
If Jon's simulations with your recommended values confirm this hypothesis,
would you say that we have a problem that needs
correction?I'm trying to pin
down what simulations with what results would indicate a problem because
Jon's hypothesis is that we have a problem. Until we can get
agreement on what simulations would suggest we have a condition that needs
fixing, we are likely to just end up arguing about data, rather than about
what problems we may have and what it will take to fix them.
Thank you again Necdet for your assistance
in carefully defining the conditions to be simulated, and the results that
would indicate changes are required. Best regards, Robert D.
Love
President, LAN Connect Consultants
7105 Leveret
Circle Raleigh, NC 27615
Phone: 919
848-6773 Mobile: 919 810-7816
email: rdlove@xxxxxxxx
Fax: 208 978-1187
----- Original Message -----
Sent: Tuesday, April 22, 2003 8:08
PM
Subject: Re: [RPRWG] Class A and B
Guarantees Bob,
I would like to see simulations for the scenario that Jon described
with the following conditions:
stqLowThreshold is fixed to a value
(say 100kB)
stqHighThreshold is fixed to a value (say 200kB)
stqFullThreshold is set to infinity (or to a very large value, say
100MB)
head node is adding classA1 traffic at 10% of line rate.
It would be nice to see the maximum stq buffer occupancy in the head
node with respect to number of nodes on the ring. This would give us how
much of a buffering needed in order not to hit the stqFullThreshold.
This result can also be used as a check mechanism for the formulas
that Annex G editor(s) provided.
Thanks.
Necdet
"Robert D. Love" wrote:
Necdet,
with regards to my question: RDL: Are there any conditions that would lead you to conclude
there is a potential problem with our present algorithm?
...and your answer
NU: No. But, I want to make sure that things are clear to all
of us.
Let me first apologize for not
making my question clear enough, and now let me try again.What I would
like to know, and what I believe would be most helpful to Jon as he
runs his simulations is the following: What initial conditions must Jon use in his
simulations so that you will agree that a valid simulation with these
conditions produces a meaningful result. And then, what result
with those initial conditions, would have you in agreement that we
have a problem (assuming the simulation was done correctly).Necdet, I
want to avoid having a running argument where whatever is simulated is
challenged. If we are to make good progress on this issue, we
need your input as to what initial conditions need to be set, so that
you will not be challenging those conditions. If Jon or others
believe that other initial conditions should be used, then let's have
a dialog about which conditions need simulation, rather than focusing
arguments on challenging results because we disagree on the initial
conditions. I am hoping
that you, Jon, and other simulation experts can work as a team in
establishing those runs we need to evaluate, and in agreeing, in
advance, what types of results would indicate the algorithms we are
using have a problem. - Of course, if the algorithms have
no problem, then those agreed to simulations should not produce any
alarming results.Thank you Necdet. Best regards, Robert
D. Love
President, LAN Connect Consultants
7105 Leveret
Circle Raleigh, NC 27615
Phone: 919
848-6773 Mobile: 919 810-7816
email: rdlove@xxxxxxxx
Fax: 208 978-1187
----- Original Message -----
Sent: Tuesday, April 22, 2003
4:39 PM
Subject: Re: [RPRWG] Class A
and B Guarantees Bob,
"Robert D. Love" wrote:
Necdet, please fill us in by adding a bit more
verbiage. i.e. Why are you asking these particular
questions?
NU: I am trying to understand
the scenarious that he mentioned so that we can speak the same
language.
What are the implications of a
yes response, of a no response?
NU: Yes, for example not having
a shaperD would not provide any guarantees for classA0
traffic.
Are there particular conditions
that Jon should be simulating?
NU: If he thinks that there are
scenarious that we have not looked at, we need to look at them.
However, so far I have not seen anything in his e-mail that we
have not looked at.
Are there any conditions that
would lead you to conclude there is a potential problem with our
present algorithm?
NU: No. But, I want to make sure
that things are clear to all of us.
Thank you
Necdet.
Best regards,
Robert D. Love
President, LAN Connect Consultants
7105
Leveret Circle Raleigh, NC 27615
Phone: 919 848-6773
Mobile: 919 810-7816
email: rdlove@xxxxxxxx
Fax: 208 978-1187
----- Original Message -----
Sent: Tuesday, April 22,
2003 2:24 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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