Subject: RE: [Tsvwg] taking NewReno (RFC 2582) to Proposed Standard

From: "Henderson, Thomas R" <[email-masked]>
Date: Mon, 19 May 2003 08:59:09 -0700

> > If maxburst always applies, and is set to a not-too-small value,
> > I don't see the problem.  I agree that if maxburst only 
> > applies to
> > leaving fast recovery, that having FlightSize << cwnd can cause
> > problems later on.  But I also have the same concerns as Sally about
> > cutting down cwnd as a means to limit bursts.
> 
> What if we allow slow start after doing the cut?
> 
My initial feeling is that it would cause a net reduction in the overall 
sending rate, but how significant, I'm not sure:
i) with maxburst, while you are limiting bursts, you are still building
cwnd further during that period, while with your slow start proposal, 
you spend a few RTTs just getting back to the cwnd you had,
ii) by reducing cwnd, you are more vulnerable to additional reductions if
a loss event occurs while you are trying to build back up.

Tom
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From: Mark Allman <[email-masked]>
Date: Mon, 19 May 2003 12:48:14 -0400

> My initial feeling is that it would cause a net reduction in the
> overall sending rate, but how significant, I'm not sure:
> 
> i) with maxburst, while you are limiting bursts, you are still
> building cwnd further during that period, while with your slow
> start proposal, you spend a few RTTs just getting back to the cwnd
> you had,

I am not sure I'd agree that when using maxburst and
outstanding_data < cwnd you should be building your cwnd.  That
seems to be a violation of the spirit of CWV to me (i.e., TCP is not
using (validating) what it has, why should it be allowed to build
more?).

> ii) by reducing cwnd, you are more vulnerable to additional
> reductions = if a loss event occurs while you are trying to build
> back up.

So, if I read the above right ... Assume we use maxburst,
outstanding_data=X and cwnd=X+Y (bigger because of the maxburst
limiting).  So, you're saying that if a loss event occurs we set
cwnd to (X+Y)/2?  (Whereas if we reduce cwnd we end up doing
something like: cwnd=(X+maxburst)/2.)  Do I have that right?

If I do have that right then I disagree.  I think that if you lose
something when outstanding_data = X then you should be halving from
X regardless of cwnd (i.e., cwnd = X/2).  (And, in fact, this is
what rfc2581 would call for.)  

To take an extreme example, assume X=Y and so cwnd =
(2*outstanding_data) and there is a loss.  If you just "cwnd /= 2"
then you have not cut your sending rate at all.

Or, maybe I am misreading your intention?

Thanks!

allman

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