Re: [ledbat] Fwd: New Version Notification for draft-ietf-ledbat-congestion-08.txt

Gorry Fairhurst <gorry@erg.abdn.ac.uk> Sat, 15 October 2011 07:40 UTC

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Date: Sat, 15 Oct 2011 08:41:11 +0100
From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
Organization: University of Aberdeen
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To: Joe Touch <touch@isi.edu>
References: <4E92338F.1030601@fandm.edu> <4E93032B.7090006@isi.edu> <201110101846.26115.mirja.kuehlewind@ikr.uni-stuttgart.de> <4E932F77.5060006@isi.edu>
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Cc: Janardhan Iyengar <jana.iyengar@gmail.com>, ledbat@ietf.org
Subject: Re: [ledbat] Fwd: New Version Notification for draft-ietf-ledbat-congestion-08.txt
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  The latest text for the LEDBAT draft says:

    INIT_CWND SHOULD be 4, and MIN_CWND SHOULD be 2.  An INIT_CWND of 4
    should help seed FILTER() at the sender when there are no samples at
    the beginning of a flow, and a MIN_CWND of 2 allows FILTER() to use
    more than a single instantaneous delay estimate while not being too
    aggressive.  Slight deviations may be warranted, for example, when
    these values of MIN_CWND and INIT_CWND interact poorly with the
    framing protocol.

I commented previously on this, and here are my thoughts on the new text.

I would personally be OK with starting with a window of 4.

I do not agree that an update to TCP should be tracked by LEDBAT in the 
general case, because as I understand, the value 4 is derived from the 
operation of LEDBAT itself - i.e. use of the filter.

  In general, I like the idea of explaining when the SHOULD may be 
violated and what the implications are. However, the present clause at 
the end of this para seems to suggest ANY other value could also be OK, 
I do not agree this is to be left to the implementer - I was hoping here 
to see this limited to a fraction of the current TCP initial window, 
e.g. "but SHOULD NOT be larger than Max (4, one half of the initial 
window specified for TCP, currently [RFC3390])." However, I suspect that 
starting small is a very good idea for a longer-lived background flow 
that intends to be less aggressive than other flows, and should have 
little impact on the overall download time, therefore it may be better 
to just leave the recommendation as should use 4 in the first RTT.

Gorry
On 10/10/2011 18:46, Joe Touch wrote:
>
>
> On 10/10/2011 9:46 AM, Mirja Kuehlewind wrote:
>> Hi,
>>
>> I wouldn't want to refer to TCP's value because of a 
>> less-than-best-effort
>> traffic there might always be a motivation to take different/lower 
>> values
>> than TCP.
>
> The values currently suggested are TCP's current values. I don't mind 
> if you want to pick smaller ones, but I'd suggest spec'ing them 
> relative to TCP's current values regardless.
>
> Joe
>
>>
>> Mirja
>>
>>
>> On Monday 10 October 2011 16:37:31 Joe Touch wrote:
>>> On 10/9/2011 4:51 PM, Janardhan Iyengar wrote:
>>>> Dear all,
>>>>
>>>> A new version of the congestion control draft is in the repository.
>>>> There are two major mods in this revision:
>>>
>>> ...
>>>
>>>> 2/ We have set the values for INIT_CWND to 4 and MIN_CWND to 2, and 
>>>> have
>>>> clarified the discussion of CURRENT_DELAYS and INIT_CWND/MIN_CWND in
>>>> Section 3.5.
>>>
>>> It might be more stable to refer to TCP's INIT_CWND and MIN_CWND 
>>> values,
>>> i.e., that ledbat should use TCP's values for these.
>>>
>>> (they've been under discussion for revision in TCPM, FWIW)
>>>
>>> Joe
>>> _______________________________________________
>>> ledbat mailing list
>>> ledbat@ietf.org
>>> https://www.ietf.org/mailman/listinfo/ledbat
>>
>>
>>
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