Re: [ledbat] [R-C] LEDBAT vs RTCWeb

[Randell Jesup <[e-mail]>]

On 4/23/2012 8:07 AM, Rolf Winter wrote:
> Hi,
>
> the way I remember how we got to the 100ms was a little different. We started out with 25 ms of extra delay and between version 01 and 02 of the WG draft I believe it "magically" changed. I remember asking about the motivation but I cannot recall an answer. My guess is that 25ms is very few packets in common residential broadband access networks and the algorithm might not quite work that well in those settings _today_. After some debate we ended up accepting 100 ms as a maximum. The problem is that going away from 100 ms in practice is that (assuming more than one app adopts LEDBAT congestion control) the one app that yields the most will get the least throughput, making anything less than 100 ms really unattractive in a competitive environment. So while I am personally OK with the compromise we achieved in the end, I am not happy about the lack of motivation. I am mildly optimistic that it is based on experience in the field rather than making the existing implementation at the tim
>     atch the spec.

I searched the email archives and didn't see anything discussing it 
(though I might have missed it) or discussing interaction with VoIP 
(other than that one paper, which also showed the sensitivity to the 
allowed delay and the advantage of a user willing to accept longer 
delays, plus how latecomers could mis-read the buffer state.

The problem is that a 100ms-delay-scavenger algorithm like this "poisons 
the well" for any application that desires/needs lower delay.  That's 
why I believe there are only two reasonable targets: 0 (drained queues) 
and full queues (assuming AQM isn't implemented to keep queues low).

I don't think this is just a theoretical problem, I think it's a very 
serious issue, which if LEDBAT gets used a lot will cause a lot of 
problems.  Perhaps it's *better* than old-style saturating bittorrent 
flows, in that VoIP will be possible without shutting down bittorrent, 
but that doesn't mean it's good, and the perception that bittorrent 
doesn't break other apps with the new protocols will encourage people to 
leave it running - and depending on the vagaries of bittorrent, it might 
be fine when you start the call and then slam you.  I'm especially 
worried about OS's and apps using it for background update transfers 
with no user control or knowledge, etc.

-- 
Randell Jesup
[e-mail]