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

From: "Armando L. Caro Jr." <[email-masked]>
Date: Tue, 5 Mar 2002 15:44:17 -0500 (EST)

On Tue, 5 Mar 2002, Mark Allman wrote:

> proposals I have seen for maxburst all involve *implicit* limiting
> of cwnd.  My opinion is that we should get rid of this maxburst
> notion and make the limit to cwnd *explicit*.  Having two controls
> is adding complexity and possibly (subtle) interactions that do not
> need to be introduced to get the desired effect.

Yes, I agree. However, if the sender were to go back to slow start, I am
sure that many would be concerned that SCTP would act too conservatively
in comparison to TCP. So I suggest a compromise... The sender goes back to
slow start with an intial cwnd of 4*MTU. In other words, when a SACK
arrives which acknowledges a fast rtx'd tsn, the following should be done
(instead of applying Max.Burst):

	cwnd = 4 * MTU
	ssthresh stays the same

I have attached 3 graphs for comparison:

	1. noBurst.ps     - applies Max.Burst=4 _all_ the time
	2. noBurst-ss.ps  - goes back to slow start w/ cwnd = 2 * MTU
	3. noburst-ss4.ps - goes back to slow start w/ cwnd = 4 * MTU

~armando

0--                                                --0
| Armando L. Caro Jr.     |       [email-masked] |
| University of Delaware  |  www.cis.udel.edu/~acaro |
0--                                                --0

---

From: Mark Allman <[email-masked]>
Date: Tue, 05 Mar 2002 14:03:45 -0500

Armando-

> 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).

Sure.  I completely agree.  There is a difference in the notion of
the total amount we can put into the network path (cwnd) and how
fast we can send that into the network (maxburst).  But, all the
proposals I have seen for maxburst all involve *implicit* limiting
of cwnd.  My opinion is that we should get rid of this maxburst
notion and make the limit to cwnd *explicit*.  Having two controls
is adding complexity and possibly (subtle) interactions that do not
need to be introduced to get the desired effect.

> 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.

Yep.

> 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?). 

Yes.  This is what I was thinking of.  And, yes, it has a
slow-start-ish behavior.

allman

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

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