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

Hi,

INIT_CWND = MIN_CWND = 2 sounds reasonable, and it seems to take care of the points raised -- including apps that generate nothing but short flows using LEDBAT (seems like a bad choice of CC for such apps, IMHO) -- except one.

The only thing remaining would be the filtering of delay values (the draft says INIT_CWND = 4 to initialize the filter in the first RTT), but I'd venture that this is not necessary? i.e. taking a bit more time to seed the filter doesn't look as if it'll break LEDBAT's CC?

Thanks,

David.

Le 20 oct. 2011 à 07:21, Ingemar Johansson S a écrit :

> 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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David ROS
http://www.rennes.enst-bretagne.fr/~dros/

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