[Cake] Control theory and congestion control

[Sebastian Moeller]

Hi Jonathan,

interesting post, lots of points to think about ;)


On May 9, 2015, at 21:02 , Jonathan Morton <chromatix99 at gmail.com> wrote:

> > The "right" amount of buffering is *1* packet, all the time (the goal is nearly 0 latency with 100% utilization). We are quite far from achieving that on anything...
> 
> And control theory shows, I think, that we never will unless the mechanisms available to us for signalling congestion improve. ECN is good, but it's not sufficient to achieve that ultimate goal. I'll try to explain why.

	I wonder, given the potentially hostile state of the internet, can we realistically expect more than what we have right now?

> 
> Aside from computer networking, I also dabble in computer simulated trains. Some of my bigger projects involve detailed simulations of what goes on inside them, especially the older ones which are relatively simple. These were built at a time when the idea of putting anything as delicate as a transistor inside what was effectively a megawatt-class power station was unthinkable, so the control gear tended to be electromechanical or even electropneumatic. The control laws therefore tended to be the simplest ones they could get away with.
> 
> The bulk of the generated power went into the main traction circuit, where a dedicated main generator is connected rather directly to the traction motors through a small amount of switchgear (mainly to reverse the fields on the motors at either end off the line). Control of the megawatts of power surging through this circuit was effected by varying the excitation of the main generator. Excitation is in turn provided by shunting the auxiliary voltage through an automatic rheostat known as the Load Regulator before it reaches the field winding of the generator. Without field current, the generator produces no power.
> 
> The load regulator is what I want to focus on here. Its job was to adjust the output of the generator to match the power - more precisely the torque - that the engine was capable of producing (or, in English Electric locos at least, the torque set by the driver's controls, which wasn't always the maximum). The load regulator had a little electric motor to move it up and down. A good proxy for engine torque was available in the form of the fuel rack position; the torque output of a diesel engine is closely related to the amount of fuel injected per cycle. The fuel rack, of course, was controlled by the governor which was set to maintain a particular engine speed; a straightforward PI control problem solved by a reasonably simple mechanical device.
> 
> So it looks like a simple control problem; if the torque is too low, increase the excitation, and vice versa.
> 
> Congestion control looks like a simple problem too. If there is no congestion, increase the amount of data in flight; if there is, reduce it. We even have Explicit Congestion Notification now to tell us that crucial data point, but we could always infer it from dropped packets before.

	I think we critically depend on being able to interpret lost packets as well, as a) not all network nodes use ECN signaling, and b) even those that do can go into “drop-everything” mode if overloaded.

> 
> So what does the load regulator's control system look like? It has as many as five states: fast down, slow down, hold, slow up, fast up. It turns out that trains really like changes in tractive effort to be slow and smooth, and as infrequent as possible. So while a very simple "bang bang" control scheme would be possible, it would inevitably oscillate around the set point instead of settling on it. Introducing a central hold state allows it to settle when cruising at constant speed, and the two slow states allow the sort of fine adjustments needed as a train gradually accelerates or slows, putting the generator only slightly out of balance with the engine. The fast states remain to allow for quick response to large changes - the driver moves the throttle, or the motors abruptly reconfigure for a different speed range (the electrical equivalent of changing gear).

	I think I see where you are going with this ;). Question: how would a 5 state system look at an intermediate network router? I have two [points I do not see clear at all. 1) Competiton with simple greedy non-ECN flows, if these push the router into the dropping regime how will well behaved ECN flows be able to compete? And how can the intermediate router control/check that a flow truly is well-behaved, especially with all the allergies against keeping per-flow state that router’s seem to have?

> 
> On the Internet, we're firmly stuck with bang-bang control. As big an improvement as ECN is, it still provides only one bit of information to the sender: whether or not there was congestion reported during the last RTT. Thus we can only use the "slow up" and "fast down" states of our virtual load regulator (except for slow start, which ironically uses the "fast up" state), and we are doomed to oscillate around the ideal congestion window, never actually settling on it.

	Is the steady state, potentially outside of the home, link truly likely enough that an non-oscillating congestion controller will effectively work better? In other words would the intermediate node ever signal hold sufficiently often that implementing this stage seems reasonable?

> 
> Bufferbloat is fundamentally about having insufficient information at the endpoints about conditions in the network. We've done a lot to improve that, by moving from zero information to one bit per RTT. But to achieve that holy grail, we need more information still.

	True, but how stable is a network path actually over seconds time frames?

> 
> Specifically, we need to know when we're at the correct BDP, not just when it's too high. And it'd be nice if we also knew if we were close to it. But there is currently no way to provide that information from the network to the endpoints.

	Could an intermediate router actually figure out what signal to send all flows realistically?

Best Regards
	Sebastian

> 
> - Jonathan Morton
> _______________________________________________
> Cake mailing list
> Cake at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cake