Re: [iccrg] AIMD versus AIAD

[Jonathan Morton <[e-mail]>]

> On 20 Nov, 2020, at 9:59 am, Jana Iyengar <[e-mail]> wrote:
> 
> This goes all the way back to Chiu and Jain.

I see in that paper that a primitive type of AQM signalling is described - one that is only capable of sending one signal per "time slot", possibly an abstract representation of RTT.  This has a fairly obvious lineage to RFC-3168 ECN, which is also capable of handling only one congestion signal per RTT.  Under these conditions, an MD response to that singular signal is the only one that is correct.

With high-fidelity congestion signalling applied to DCTCP (or TCP Prague), that same single Congestion Experienced signal, issued once per RTT, produces a very different response.  It only subtracts a constant from the cwnd, instead of reducing it to some fraction.  That is clearly consistent with the Additive Decrease formula in the Chiu/Jain paper.

The added capability of high-fidelity congestion feedback is that there can now be *more than one* congestion signal per RTT.  But it means that O(N) signals must be issued by the network in order to reduce cwnd by an O(N) factor.  Each signal only produces an O(1) response.

Regardless of whether you accept the above argument, there is a clear qualitative difference between the conventional AIMD response and that of DCTCP, which needs accurate terminology to describe.  That is my point here.

 - Jonathan Morton