Subject: Re: [Tsvwg] SCTP Max.burst and CWND

From: Mark Allman <[email-masked]>
Date: Mon, 04 Mar 2002 22:32:37 -0500

> >   * Rwnd limits.  During loss recovery we'd like to continue to send
> >     segments into the network to keep the ACK clock going and to
> >     keep our estimate of the path capacity valid.  Sometimes this is
> >     not possible due to thr limited rwnd.  So, at the end of
> >     recovery we get a large cumulative ACK that gives us permission
> >     to send a bunch of segments.  At this point we have lost the ACK
> >     clock.  But, we know how to restart that -- slow start.  You'll
> >     end up basically doing that with maxburst.  So, just make the
> >     adjustments to cwnd.
> 
> So at the end of a FR (in less than one RTT) you are proposing to
> adjust the cwnd down so that you enter slow start - Instead of
> applying a maxburst?

Yes.  Maxburst is going to show slow start behavior anyway.  So, my
point is that it doesn't matter which you do.  My mental model says
that if you get in this situation you have lost the ACK clock and so
you should slow start (because that is how we restart the ACK
clock).  And, since there are no ACKs, maxburst will end up being
roughly the same as slow start (if maxburst is specified as I think
it is -- i.e., a limit on the data segments sent per ACK -- I have
not read the current stuff, so I suppose I could be off here).
Depending on the constant that is chosen for maxburst, you might get
a little better than slow start.  But, if maxburst is too big then
that defeats the point of having it in the first place.  And, if
these two schemes are roughly the same we should just use one
control and not get involved in trying to sort out the interations
between two controls.

allman

--
Mark Allman -- NASA GRC/BBN -- http://roland.grc.nasa.gov/~mallman/

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From: "Armando L. Caro Jr." <[email-masked]>
Date: Tue, 5 Mar 2002 13:43:59 -0500 (EST)

On Mon, 4 Mar 2002, Mark Allman wrote:

> Yes.  Maxburst is going to show slow start behavior anyway.

Max.Burst as specified in the implementor's guide, doesn't show slow start
behavior. All it does is limit the first send after a FR recovery. I
always assumed cwnd was enough, but now I am starting to see arguments for
imposing a limit. The cwnd is meant to control how much is in flight, but
bursting a cwnd amount of data all at once puts strain on the bottlenceck
router(s) and can degrade performance of all flows (not just the one in
question).

Max.Burst limits how many packets can be sent for the incoming SACK which
closes the gap. However, the problem is that Max.Burst only delays the
burst to the second incoming SACK after the gap is closed. Instead,
Max.Burst should be applied for _all_ SACKs. Applying Max.Burst on all
SACKs gives a slow startish behavior (Mark: maybe this is what you were
thinking of?). Each SACK clocks out Max.Burst packets until the bw-delay
product eventually smoothens out the bursts.

Also, applying Max.Burst to all SACKs reduces some of the problem with
application-limited periods as mentioned in rfc2861, but it doesn't really
address the issue correctly. For this issue, cwnd degradation is more
appropriate.

I have attached 3 ps files which graph excerpts of simulations I ran to
show three different behaviors:

	1. burst.ps shows  - no Max.Burst
	2. delayedBurst.ps - Max.Burst applied as specified in impl-guide
	3. noBurst.ps      - Max.Burst applied always

Comments?

~armando

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| Armando L. Caro Jr.     |       [email-masked] |
| University of Delaware  |  www.cis.udel.edu/~acaro |
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From: "Armando L. Caro Jr." <[email-masked]>
Date: Tue, 5 Mar 2002 13:47:22 -0500 (EST)

sorry, forgot to attach the graphs...

On Tue, 5 Mar 2002, Armando L. Caro Jr. wrote:

> 	1. burst.ps        - no Max.Burst

~armando

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From: Randall Stewart <[email-masked]>
Date: Wed, 06 Mar 2002 07:35:24 -0600

> control and not get involved in trying to sort out the interations

I want to think on this a bit.. I will buy you and Armando a beer
in MN (assuming you are both going) and we can discuss it in
a sensible manner at a bar bof :>

See you there.

R

-- 
Randall R. Stewart
[email-masked] 815-342-5222 (cell phone)