[ledbat] INIT_CWND and MIN_CWND (WAS Re: New Version Notification for draft-ietf-ledbat-congestion-08.txt)

Hi

Change of subject name. 
I have no strong opinion on this but I would like to ask how likely it is that LEDBAT is used for shortlived flows in the first place. The CC-algo does not seem to me as the obvious pick of I was looking for instance for fast HTTP request/response times. Given that it seems like minor issue what the INIT_CWND is if the flows are 10minutes in duration or perhaps more.
MIN_CWND is another matter, that should possibly be fixed to relatively small value like 2.

/Ingemar

> 
> Message: 3
> Date: Wed, 19 Oct 2011 10:14:53 -0700
> From: Joe Touch <touch@isi.edu>
> To: gorry@erg.abdn.ac.uk
> Cc: Janardhan Iyengar <jana.iyengar@gmail.com>, ledbat@ietf.org
> Subject: Re: [ledbat] New Version Notification	for
> 	draft-ietf-ledbat-congestion-08.txt
> Message-ID: <4E9F058D.4040909@isi.edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Hi, Gorry,
> 
> I agree with your logic, but it suggests INIT_CWND and 
> MIN_CWND should be 2 - those are both clearly safe, clearly 
> smaller than the current deployed base, and would not 
> increase to track TCP's increases.
> 
> Joe
> 
> On 10/19/2011 12:26 AM, Gorry Fairhurst wrote:
> > On 18/10/2011 20:27, Joe Touch wrote:
> >>
> >>
> >> On 10/18/2011 8:29 AM, David Ros wrote:
> >> ...
> >>> Hi all,
> >>>
> >>> Sorry if I'm getting this totally wrong, but: is it really
> >>> *necessary*
> >>> that LEDBAT's INIT_CWND be *smaller* than TCP's? As far as I 
> >>> understand, the point of LEDBAT is to be 
> less-than-best-effort over 
> >>> (relatively) long timescales, or at least over time scales longer 
> >>> than one (initial) RTT. And just starting up as TCP won't make it 
> >>> *more* aggressive than TCP. Is this correct??
> >>
> >> No, but it could make it a lot like TCP if the offered load is in 
> >> short bursts. If that's not the intent, then the INIT_CWND 
> needs to 
> >> be smaller than TCP's.
> >>
> >> Joe
> >>
> >>
> >
> > I think INIT_CWND should not be significantly bigger than 
> *deployed* 
> > TCP INIT_CWND. (It may of course be desirable to be smaller or the 
> > same, and that would benefit in the way Joe suggested).
> >
> > When I suggested "4" could be OK, this was only 1 larger 
> than current 
> > usage for a 1500B MTU, and equivalent for some smaller MTU. 
> That to me 
> > is not "significantly bigger". It would seem OK, because if this 
> > induced congestion LEDBAT would react within INIT_CWND 
> segments in a 
> > conservative way.
> >
> > If we care about LEDBAT being conservative compared to other TCP 
> > sessions, then I really think we should not track future 
> new proposals 
> > to raise INIT_CWND. I suggest this would have side effects:
> >
> > - It could make LEDBAT more aggressive than *deployed* TCP 
> > implementations, that I think would be bad.
> >
> > - It may require LEDBAT to implement additional algorithms 
> to ensure 
> > it is conservative when the larger INIT_CWND induces congestion. 
> > Addressing this would likely add complexity to LEDBAT and make it 
> > dependent on these TCP updates (if any).
> >
> > I think a larger INIT_CWND (e.g. by tracking any evolution of TCP's
> > INIT_CWND) is unwarranted if the goal is for LEDBAT to target bulk 
> > less-than-best-effort use. I cannot see the case yet for a LEDBAT 
> > INIT-CWND beyond 4. Raising this could indeed save an RTT 
> at the start 
> > of a LEDBAT flow, but this would be at the expense of making it
> > (slightly) more aggressive than presently deployed TCP. This seems 
> > undesirable.
> >
> > I also suggested that MIN_CWND should similarly be fixed to 
> a small number.
> >
> > Gorry
> 
> 
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> End of ledbat Digest, Vol 33, Issue 9
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