[R-C] [ledbat] LEDBAT vs RTCWeb

[Randell Jesup]

On 4/27/2012 7:48 AM, Stefan Holmer wrote:
>
>
> On Thu, Apr 26, 2012 at 7:12 PM, Randell Jesup <randell-ietf at jesup.org
> <mailto:randell-ietf at jesup.org>> wrote:
>
>     On 4/26/2012 11:49 AM, Nicholas Weaver wrote:
>>     On Apr 26, 2012, at 8:05 AM, Rolf Winter wrote:
>>>>     http://www.infres.enst.fr/~drossi/DATA/PRJ-Ledbat+AQM.pdf
>>>>
>>>>     It seems like AQM causes LEDBAT to promote to behavior similar to TCP,
>>>>     but with low delay.  Since it still seems fair, this is ok, but it's no
>>>>     longer a background or scavenger flow, and applications using it need
>>>>     to be aware they can impact"foreground"  flows if AQM is in play.
>>>>     Perhaps applications need to be given awareness when LEDBAT detects
>>>>     active AQM (if it can), and there's no"scavenger"  CC algorithm for AQM
>>>>     that I know of.
>>>     And the document makes that clear and I am not sure LEDBAT actually can detect AQM.
>>     One thought:  Active Queue management will be either ECN or early drop.  Which is a signal separate to the delay signal used which is"friendlier than TCP".
>>
>>     In either case, the scavenger flow property might be, well, not fully maintained but at least encouraged by backing off more than conventional TCP would to the same signal.
>
>     Correct, and that's a reasonable way to proceed - though it might
>     complicate slightly (and certainly change) the LEDBAT competing with
>     LEDBAT case.
>
>     In my delay-sensitive CC algorithm, I detected drops, and in
>     particular "fishy" drops where the end-to-end delay in the following
>     packet was lower by roughly the normal arrival interval, and would
>     take those as an indication that we should drop more substantially
>     than 'normal'.  This was targeted at tail-drop detection, especially
>     congestion spikes, but for a scavenger protocol the same idea could
>     apply to make it more responsive to AQM.
>
>
> Interesting idea indeed. Drops in delay will of course also happen when,
> say, one flow stops, so it's not only an indicator of tail-drop (even
> though the probability of tail-drop may be higher if the drop in delay
> is "fishy").

A drop in delay alone isn't a trigger; it's a loss combined with a drop 
in delay.  I.e. from a packet train point of view for 30ms packets:

<- X - 31ms - X+1 - 31ms - X+2 - 31ms - X+4(!) - 31ms - X+5

That would be a fairly typical "signal" from a tail-drop router of 
buffer overflow - timing compressed by the queue. Also note the slope 
(increasing delay).  If it were:

<- X - 31ms - X+1 - 31ms - X+2 - 60ms - X+4(!) - 31ms - X+5

Then it would be much less "fishy".  Note this is most effective in a 
constrained-channel case, perhaps somewhat less so in a 
contention-for-channel case - but constrained channel is the "normal" 
unloaded access-link or WiFi bottleneck.  And in contention, it's still 
useful - you need to tune the amount a packet arrives "early", and I 
also added a rule to require multiple 'fishy' losses within a period 
(~2s) before it kicked in extra reduction, but that's a tuning/testing 
issue.

For LEDBAT, as a scavenger protocol, it may make sense for it to respond 
to all drops by reducing more than TCP would.  (Drops say either you 
have very short max queues (<~100ms), AQM, or other traffic has forced 
the queuing delay up to the limit - in either case you want to back off 
and yield.  So long as all the scavengers drop roughly similar amounts, 
they should be fair among themselves in the super-short-queue (or AQM) 
case.  In all other cases, LEDBAT would just get out of the way of 
foreground better/faster.  It might reduce efficiency slightly in some 
cases, but I suspect not much, and the only case we really care about 
(for LEDBAT) is a non-fully-loaded channel.

-- 
Randell Jesup
randell-ietf at jesup.org