Re: [iccrg] AIMD versus AIAD

there is also additive increase adaptive decrease, i.e. reducing cwnd based
on measured bandwidth
sm

On Fri, Nov 20, 2020 at 8:45 AM Jonathan Morton <chromatix99@gmail.com>
wrote:

> At the end of today's session, I had the opportunity to highlight the
> central feature of TCP Prague, inherited from DCTCP, which is the change in
> response to a CE mark (additive instead of multiplicative).  The response
> from the presenters (Bob & Koen) indicated that we need to clarify some
> terminology before the discussion can begin in earnest.
>
> Specifically:  what is Multiplicative Decrease, and what is Additive
> Decrease, and which one does TCP Prague use?
>
> It is well established that NewReno and CUBIC are AIMD algorithms, indeed
> Reno was the original AIMD CC.  A single packet loss or CE mark causes the
> cwnd to reduce to some fixed fraction of its previous value within one RTT
> of the congestion signal being recognised.  Or, mathematically:
>
> MD:  O(1) signals causes O(N) reduction in cwnd.
>
> Conversely, looking at DCTCP and ignoring the low-pass filters which
> disappear in the long run, each CE mark causes half a segment to be
> subtracted from the cwnd.  This is classic Additive Decrease.  There is
> still (barring implementation bugs) a Multiplicative Decrease response to
> packet loss.  So, distilling:
>
> AD:  O(1) signals causes O(1) reduction in cwnd; to get O(N) reduction
> requires O(N) distinct signals.
>
> The above is true regardless of how closely spaced the congestion signals
> are.  With precise feedback of CE marks, you can successfully register a
> mark on every data packet during a whole RTT and obtain, effectively, a
> multiplicative decrease - but that does not make the response MD, because
> the response to a *single* CE mark is O(1) segments.
>
>  - Jonathan Morton
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Saverio Mascolo,  Professore Ordinario di Automatica

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