Re: [RPRWG] Complete RPR/Java model for all three proposals
Bob,
Thanks for sharing your insightful experience re pathological traffic scenarios.
In response to your suggestion: "One of the criteria we should have in choosing
an algorithm is that the
worst case pattern simulations exceed some minimum threshold."
I would like to add that the likelihood of occurrence of such traffic scenarios
is also of utmost importance. Usually such pathological traffic patterns are
rare events, i.e. far towards the tail of the distribution. Having this
likelihood would put the scenario in perspective and also determine the required
urgency of a resolution. I think it is important to attach this "weight" to the
criteria when it comes to selection of alternative algorithms.
Regards, Siamack
RDLove wrote:
> To: 802.17 Reflector, and Stein Gjessing
>
> Stein, Khaled, and all simulation experts, this email focuses on how we
> choose
> traffic simulation patterns to run. Drawing on my experience in 802.5 Token
> Ring Working Group, I believe there are two classes of simulation patterns
> that are critical. The first class, are the patterns that push the
> boundaries of important classes of expected traffic. I expect that the
> output of the performance committee focused on this case. The second class
> are
> those cases that could "break" the standard, i.e. reduce its performance
> below acceptable limits. I have not seen much focus on this second class of
> patterns, yet that is the class of patterns that could jeopardize the
> success of RPR.
>
> In the 802.5 Token Ring Working Group we discovered a "killer frame" that
> caused a buffer to overflow. For a couple of meetings, addressing and
> solving the "killer frame" problem consumed the working group. We were also
> very concerned about the press talking about token ring having a "killer
> frame" problem, and what that would do to token ring sales. The problem was
> finally solved by significantly increasing the required buffer size, so that
> no "killer frame" existed.
>
> Early coaxial Ethernet, which used "vampire connectors" had its equivalent
> of the "killer frame". It turned out that if you inserted vampire connector
> taps into the coaxial cable at a spacing that was a simple harmonic of the
> fundamental frequency, standing waves were created that caused reception
> errors. That problem was fixed by marking the coaxial cable periodically
> where you were allowed to tap into it. The markings were spaced a distance
> that was not a simple harmonic of the fundamental frequency, so that large
> standing waves would not be created.
>
> RPR must allocate bandwidth based on an algorithm that by its nature cannot
> react instantaneously to the traffic requirements. The feedback, or
> feedforward nature of any algorithm will cause a response that is analogous
> to a circuit designed with a pole. The circuit has a resonant frequency,
> "f" and a "Q" associated with it. Depending on the parameters chosen, f and
> Q could be changed. If the Q is high, then the circuit response to an
> excitation near frequency f would be dramatically different from the
> response to other excitations.
>
> By undersanding the physics of the feedback or feedforward algorithm we are
> proposing, we should be able to create the traffic patterns that excite our
> models at their point of resonance. The requirement is that the model does
> not break under that excitation. In other words, the performance does not
> drop below acceptable parameters under "worst case" traffic.
>
> If a problem is found, there are two ways of addressing it. One is to change
> the point of resonance. This is a poor change, since it only means that the
> present worst case pattern doesn't cause the problem, but a new pattern will
> uncover the problem again, as bad as it always was.
>
> The other change is to change the Q of the model such that the degradation
> in performance of the model at its point of resonance is still within
> acceptable bounds.
>
> Clearly, each model will have its own "worst case patterns". Hopefully,
> those
> groups forwarding proposals will understand their algorithms well enough
> to be able to supply the worst case simulation patterns for that model.
>
> One of the criteria we should have in choosing an algorithm is that the
> worst case pattern simulations exceed some minimum threshold.
>
> I believe that for RPR to be successful we must require acceptable
> performance levels for worst case patterns. I assume that whatever
> algorithms are incorporated into our draft in January will be thoroughly
> simulated before the March meeting to be sure there are no "killer patterns"
> for RPR. I would be pleased to hear feedback from the simulation experts
> on the need to include this criteria as one of the bases for adopting a
> solution.
>
> Best regards,
>
> Robert D. Love
> Chair, Resilient Packet Ring Alliance
> 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 -----
> From: "Stein Gjessing" <steing@xxxxxxxxx>
> To: <stds-802-17@xxxxxxxx>
> Sent: Tuesday, November 27, 2001 8:54 AM
> Subject: [RPRWG] Complete RPR/Java model for all three proposals
>
> >
> > All,
> >
> > I will try to implement all three proposals:
> >
> > Alladin,
> > DVJ,
> > Gandalf
> >
> > in my RPR/Java-simulator and get some simple traffic
> > scenarios running before the January meeting.
> >
> > I have the early November drafts of all these proposals as they are
> > posted on the web site. However, I need working drafts of more
> > details of the fairness/flow control algorithm parts of these
> > proposals. I talked to some of you during the Austin meeting
> > and you said you could send me material that would help me in
> > my Java implementation effort. So please do that now.
> >
> > Please make specifications as precise and algorithmic (step by
> > step description on what to do in different cases) as possible.
> >
> > All Java code (that is all the RPR/Java models including the simulator
> > code itself) will be available for everyone. If the material you send
> > me contains confidential information (not related to the specific
> > fairness/flow control algorithm), I can treat that material
> > confidentially.
> >
> > I also suggest you send me traffic scenario proposals
> > (e.g. two for each of the Alladin, DVJ and Gandalf proposals)
> > that you think I should run.
> >
> > I hope to be able to do this during December, but depending
> > on how difficult this will be, as well as what else I have
> > to do in my job, I can of course not promise anything.
> >
> > Stein
> >
> >
> >
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