Re: [dmarc-ietf] Thoughts on choosing N

[Alessandro Vesely <[e-mail]>]

On Mon 15/Apr/2024 13:16:50 +0200 Douglas Foster wrote:
> Our original choice of N was based on the PSL.    The PSL could not detect 
> organizational boundaries could not boundaries below level 5, because it had no 
> entries longer than 5 labels, and we determined that the 5-label entries were 
> not used for mail.    Therefore, any increase in N is new capability.   That 
> new capability is probably desirable, but need not be limitless.  Using an N of 
> 8 introduces a lot of new capability.


8 is not needed and not justified.  A mail site using 8 labels would have 
troubles with the RFC 7489 version, which uses the PSL.  They'd have to ask for 
PSL upgrades, right?

Now, we can relax our ambition to be PSL-free and state N=max number of labels 
of public suffixes used by mail.  Or we could put N in an IANA registry that 
can be updated by expert review.  Such methods allow to have N low enough, yet 
upgradable and equal for all (compliant) implementations.

Otherwise we can drop the requirement that N be equal for all implementations, 
and just make it configurable.  After all, it is an anti-abuse measure, akin to 
SPF lookup limit.  We can also keep it fixed at 5 and be sure that 
implementations will differ anyway.


Best
Ale
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