Re: [dmarc-ietf] Some Proposed Language for a New pct Tag Defintion

Alessandro Vesely <vesely@tana.it> Sun, 01 August 2021 09:13 UTC

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Date: Sun, 1 Aug 2021 11:13:12 +0200
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Subject: Re: [dmarc-ietf] Some Proposed Language for a New pct Tag Defintion
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On Sun 01/Aug/2021 01:47:12 +0200 Douglas Foster wrote:
> 
> My core objection is the partial-enforcement algorithm.   I cannot believe that 
> it would be wise for me, or any other receiver, to implement that algorithm.


Why not?  What's wrong with it?

if DMARC fail and (p=quarantine or p=reject) then
    if (random mod 100) < pct then
       apply policy


> In the face of ambiguity, the only way to get a correct disposition is to 
> collect more data.    If I had more time, I would quarantine all 
> unauthenticated mail until I could determine whether the sender should be 
> authenticated by local policy or blacklisted by local policy.


If you collect millions DMARC-fail messages every day for some years and 
calculate the exact percentage you will get the same result as the algorithm 
above applied on each message as it arrives.  See:
https://en.wikipedia.org/wiki/Monte_Carlo_method#Overview

If you collect unauthenticated message, besides the implied delay, you'll have 
the problem of selecting which ones to select until the percentage is 
fulfilled.  The first ones?  Distribute evenly in time or in size?  Select the 
ones with highest score?  Luckily we don't have to do so.


Best
Ale
--