RE: [RPRWG] Some questions about the pseudo-code in Gandalf
Donghui,
Thanks again for the reply. Sorry, I seem
to have said something that has upset you.
There was no mention of Alladin in my email.
I'm just trying to understand the Gandalf
proposal as thoroughly as I can. And I
need help from the authors of the proposal
because I wasn't able to figure it out
on my own. Again, apologies if I have said
something that offended you...it wasn't
meant to be that way.
I have some more comments, please see below.
-Anoop
> -----Original Message-----
> From: Donghui Xie [mailto:dxie@xxxxxxxxx]
> Sent: Wednesday, January 02, 2002 7:49 PM
> To: Anoop Ghanwani
> Cc: Anoop Ghanwani; stds-802-17@xxxxxxxx
> Subject: RE: [RPRWG] Some questions about the pseudo-code in Gandalf
>
>
> Anoop,
>
> I am trying to give answers that can make you happy, though I
> recognize
> that anything short of Alladin as a whole wouldn't make you
> complete happy.
>
> Happy New Year !
> Donghui
>
> At 11:25 AM 1/2/2002 -0800, Anoop Ghanwani wrote:
>
> >Donghui,
> >
> >Many thanks for response. I'm going to go ahead and
> >delete the answers that I'm happy with, but would like
> >to continue the discussion on some of the things that
> >are still unclear in my mind.
> >
> >-Anoop
> >
> >
> > > -----Original Message-----
> > > From: Donghui Xie [mailto:dxie@xxxxxxxxx]
> > > Sent: Wednesday, January 02, 2002 11:01 AM
> > > To: Anoop Ghanwani
> > > Cc: stds-802-17@xxxxxxxx
> > > Subject: RE: [RPRWG] Some questions about the pseudo-code
> in Gandalf
> > >
> > > >- lp_allow is computed but never used. Is something missing?
> > >
> > > Please see the pseudo code in the section of "THESE ARE
> > > UPDATED EVERY CLOCL
> > > CYCLE" on page 46.
> >
> >In an actual implementation, would one really have to do these
> >updates _every single_ clock tick? Or is there some way to make
> >it less frequent? If so, what frequency of update is recommended?
>
> The pseudo code simply states that token should be added to
> the dynamic
> shaper bucket continuously. There are many implementations that can
> effectively achieve the same result without adversely
> impeding intended
> traffic shaping and transmission. I would leave system designers to
> recommend a particular implementation as fit to their system design.
>
>
OK. I just wanted to make sure that I understand what's
being proposed. Is it possible for you to share with us
what was done in the simulation? I would probably update
it at each packet arrival instant, but I'm sure there are
other ways to achieve the desired behavior as pointed
out by you.
> > > > > - What's the rationale for:
> > > > > if (recd_rate != NULL_RCVD_INFO) &&
> > > > > (lp_forward_rate > (allow_rate_congestion/WEIGHT))
> > > > > in order to determine whether to send a rev_rate?
> > >
> > > This is a part of the fairness algorithm to ensure fair rate
> > > transmission
> > > on the ring. If some downstream node is congested and
> > > upstream nodes are
> > > sending more traffic than the fair rate of the current node
> > > (in a weighted
> > > fashion), current node will pass its received fair rate
> > > further upstream.
> > > This fairness logic applies when current node is not congested.
> >
> >The reason I brought this up is because only the local node's
> >weight is being used to determine whether or not to send the
> >message upstream. A better design, IMHO, would account for
> >the weights of the other nodes as well.
>
> I see much more advantage in using local weight only, which
> simplifies
> weight calculation requirements while providing elegant
> weighted fairness
> at the same time eliminates the need for global weights
> information and its
> associated much more complex calculations. That said, in recognizing
> Alladin's needs for global weights, next Gandalf version may
> optionally
> allow people use global weights in their preferred hard way.
>
Indeed using the local weight is simpler and requires less
information. But it does end up being an approximation
that works best only when all weights are equal.
>
> > >
> > > > >
> > > > > - The pseudo-code does not appear to account for virtual
> > > > > output queueing support within the MAC client.
> > >
> > > The pseudo-code presents the basic fairness algorithm with single
> > > choke-point information. A VoQ capable MAC client would run
> > > multiple copies
> > > of the basic fairness algorithm, and keep track of all the
> > > choke-points
> > > information with each running copy corresponding to one
> > > virtual destination
> > > queue. For details, please see section 6.7 for descriptions on the
> > > relationship of the basic fairness algorithm and VoQ capable
> > > MAC client.
> >
> >Section 6.7 does indeed explain the multi-choke point case.
> >In the absence of multi-choke point capability, the algorithm
> >can be shown to be shown to have severe limitations.
> >Therefore, I think it makes sense to have the pseudo-code
> >cover the multi-choke point case as well, even if it
> >just involves adding a loop. Leaving it as an exercise for
> >the interested reader can hide complexity that it
> >introduces. I would like to see how it affects data
> >traffic making its way from the MAC client to the MAC.
>
> Claim of "severe limitations" and the "absence of multi-choke point
> capability" are not substantiated. So no points are given on
> that. "hide
> complexity" seems to contradict "Section 6.7 does indeed explain the
> multi-choke point case."
> 802.17 MAC should be able to provide all the traffic
> information to MAC
> client, which may choose to utilize the info and make
> intelligent packet
> transmission so as to optimize ring utilization. Gandalf bandwidth
> management enables all the means for a highly sophisticated
> MAC client to
> transmit packets onto the ring while optimizing ring bandwidth
> utilization. If you like to see how MAC client
> implementations affect data
> traffic "making its way from the MAC client to the MAC", you
> are welcome to
> exercise the various MAC client implementations.
Maybe "severe limitations" was too hard. I'm sorry. But
I think you have to agree that there were some limitations
that made the introduction of "multi-choke" necessary.
Again, as I stated earlier, even if it is just a for loop
that needs to be added, it would be nice to see it as
part of the pseudo-code. If all of the enhancements
don't make their way to the pseudo-code, then the
pseudo-code is not complete. The draft represents a
work in progress, so what does it cost to add it to
a later version?
The decision by the MAC of whether or not to accept
a packet from the client needs to reflect the logic
needed for VDQ; there have to be checks in there
so that the MAC can selectively deny access depending
on which choke point will be affecting the packet.
Certainly, one can figure out a way to make this
work, but since it's an integral part of the MAC,
I think it would make sense to specify that behavior.
>
>
> > > > > > - First off, it seems that the pseudo-code does not handle
> > > > > > the multi-choke point feature discussed in the proposal.
> > > > > > It would be nice to see this.
> > >
> > > ditto.
> >
> >I guess you agree with my statement above :-)
>
> Certinaly not.
>
OK, well don't add it. May be I'll try and come up
with a contribution to add it at some point. (It probably
won't be for January, because I still have a lot to
learn.)
[..]
> > > > > >
> > > > > > - Can one of the authors comment on the tolerance
> > > > > > of the scheme to message loss? The fairness message is
> > > > > > being sent hop-by-hop, and it seems like if it happens
> > > > > > to get lost, those that don't see it will continue
> > > > > > to increase their rates. If that is indeed the case,
> > > > > > I think something needs to be done about robustness.
> > >
> > > Fair rate messages get transmitted in fixed interval
> > > periodically. The
> > > interval is about 100us. In addition, Gandalf
> incorporates fair rate
> > > ramp-down and ramp-up in a low-pass filtered fashion at the
> > > intervals.
> > > Overall, these schemes integrate and establish a very
> robust Gandalf
> > > fairness algorithm, which is immune to occasional fair rate
> > > message losses.
> > > However, a persistent fair rate message loss over several
> consecutive
> > > intervals will trigger Gandalf protection procedure.
> >
> >I thought that the fairness messages are sent no more often than
> >each DECAY_INTERVAL. I haven't done the math, but maybe that does
> >work out to 100 usec. The low pass filtered ramp-up is good.
> >The low pass filtered ramp-down may not necessarily be the
> >best in times of message loss because it doesn't give up
> >bandwidth as quickly as it should (or could if the congestion
> >messages are getting lost). I think simulations should consider
> >the effect of fairness message loss on performance.
> >(I'm pushing for quantifying behavior in worst-case scenarios
> >as alluded to by other folks on the simulation thread.)
>
> If you like, Gandalf doesn't stop you from sending more often
> fairness
> messages. Actually, Gandalf doesn't do low pass in ramp-down,
> so Gandalf
> may well be the best.
> As Gandalf performance is not sensitive to occasional
> congestion message
> loss, I don't see the necessity to spend much effort to study
> its effects.
> Nonetheless, you are very much welcome to study the
> situations. I would
> certainly be interested to see your results if you may kindly
> share them.
I misunderstood your earlier response. If it doesn't
do low pass ramp-down, that's great.
As far as testing performance in the presence
of loss of fairness messages - If you were
following the thread on simulation, there were a number
of people that expressed the necessity to test the
algorithms to their limit. As a technical standards body,
I think it is our moral responsibility to find out
where things break so that we can work on fixing them,
or making sure that they break safely,
before the standard is written and published.
But anyway, I don't think this discussion is going
to go anywhere till one of us does the work, so let's
just drop this one for now.
>
> > >
> > > > > >
> > > > > > - What is the significance of the parameter AGECOEFF
> > > > > > and how does one determine what is a good value to
> > > > > > use for it?
> > >
> > > This parameter is part of Gandalf fair rate calculation,
> > > which is used for
> > > all transmission rates and not supposed to change at all.
> >
> >It's nice to know that I don't have to change it :-), but
> >it doesn't help me understand why it is needed. Basically,
> >I'd like to know why one should be so sure that 4 is best
> >value.
>
>
> Through heuristics and simulations, we made sure a value of 4
> is the best.
> Well, you are definitely welcome to try out simulations to be
> certain on
> different values for it and share your discovery with .17 WG.
>
In order to do any useful simulations, I need to understand
what the purpose of each parameter is so that I can start
with a reasonable value and measure its effect, and then
decide whether the results are even intuitive or not. As
far as AGECOEFF goes, quite frankly, I just don't know what
it does. All I was hoping to get from you was an explanation
for why the parameter was introduced. For example, LP_ALLOW
is used to apply a low pass filter function to one of the
rates. Maybe I'm too dumb, but the need for AGECOEFF is
just not obvious to me...