[R-C] LEDBAT vs RTCWeb

[Harald Alvestrand]

On 04/10/2012 09:14 PM, Jim Gettys wrote:
> On 04/10/2012 02:58 PM, Randell Jesup wrote:
>> 100ms is just bad, bad, bad for VoIP on the same links.  The only case
>> where I'd say it's ok is where it knows it's competing with
>> significant TCP flows.  If it reverted to 0 queuing delay or close
>> when the channel is not saturated by TCP, then we might be ok (not
>> sure).  But I don't think it does that.
>>
> You aren't going to see delay under saturating load under 100ms unless
> the bottleneck link is running a working AQM; that's the property of
> tail drop, and the "rule of thumb" for sizing buffers has been of order
> 100ms.  This is to ensure maximum bandwidth over continental paths of a
> single TCP flow.
>
> Unfortunately, the bloat in the broadband edge is often/usually much,
> much higher than this, being best measured in seconds :-(.
> http://gettys.files.wordpress.com/2010/12/uplink_buffer_all.png
> http://gettys.files.wordpress.com/2010/12/downlink_buffer_all.png
> (thanks to the Netalyzr folks).
the encouraging thing in those (depressing) charts is that the fiber 
stuff (green subcloud) seems to be less broken than the DSL. So the 
future may actually be less depressing than the past.
> Worse yet, the broadband edge is typically a single queue today (even in
> technologies that may support multiple classifications.  So your VOIP
> and other traffic is likely stuck behind other traffic.  ISP's telephony
> services are typically bypassing these queues.
>
> If there is AQM, then you'll get packet marking going on (drop or ECN),
> and decent latencies.
>
> There is hope here for AQM algorithms that are self tuning: I now know
> of two of such beasts, though they are a long way from "running code"
> state at the moment.
>
> So the direction I'm going to to get AQM that works..... (along with
> classification...).  But the high order bit is AQM, to keep the end
> point's TCP's behaving, which you can't do solely by classification.
AQM within a class, and DSCP to separate classes. Sounds like a 
necessary one-two punch.

(The authorization of DSCP is of course ANOTHER still-unsolved problem; 
Mike O'Dell once referred to the EF marking as "DDOS on steroids"....)

                 Harald