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Re: [RPRWG] Class A and B Guarantees



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