Mikael,
The shallow marking theshold is not the most significant feature of
the AQM in DCTCP. More importantly, it does not delay congestion
signals by averaging them over a period equivalent to a worst-case
RTT (about 100ms), like all other AQMs do (RED, PIE, CoDel etc).
The shallow marking threshold certainly keeps standing queuing delay
low. However, that's only under long-running constant conditions.
During dynamics, not waiting a few hundred msec to respond to a
change in the queue is what keeps the queuing delay predictably low.
Dynamics are the norm, not constant conditions.
For instance, the AQM in DCTCP signals to a flow in slow-start as
soon as it crosses the threshold. So a flow with 20ms RTT will get
the signal in 20ms, and a flow with 3ms RTT will get the signal in
3ms (e.g. to or from a CDN cache). Whereas all other AQMs will delay
all signals for of the order of 100ms, even if the flow has a much
shorter RTT (because all these other AQMs don't know the RTT of each
flow, so they use a nominal worst-case RTT).
The source can use the signal as soon as it arrives (which it does at
the end of slow-start), or it can smooth the signal itself (which it
does once it's in congestion avoidance phase). But it can smooth the
signal over its own RTT, rather than guessing a worst-case RTT.
CoDel taught us that the best line-rate auto-tuning an AQM can do is
to use service time. DCTCP teaches us that the best RTT auto-tuning
an AQM can do is /not/ to try to guess the RTT in the first place.
Instead it is best to defer anything to do with RTT to the end-system.
We should learn from both lessons.
Bob
At 08:04 28/09/2013, Eggert, Lars wrote:
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On Sep 28, 2013, at 9:01, Mikael Abrahamsson <swm...@swm.pp.se>
wrote:
> So in datacenter one wants to start marking ECN on packets very
soon into buffer depth, hoping sender will get feedback and
throttle back the speed way before one gets taildrops?
Yep. There have been a bunch of papers on datacenter TCP variants
recently (look through the last 2-3 years of SIGCOMM papers, all online.)
Lars
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