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Re: [ietf-smtp] ALPN

John R Levine <johnl@taugh.com> Wed, 28 July 2021 18:00 UTC

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Date: Wed, 28 Jul 2021 14:00:16 -0400
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From: John R Levine <johnl@taugh.com>
To: Alexey Melnikov <alexey.melnikov@isode.com>, ietf-smtp@ietf.org
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Subject: Re: [ietf-smtp] ALPN
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On Wed, 28 Jul 2021, Alexey Melnikov wrote:
>> I dunno.  It's not immediately obvious what the problem would be
>> to share the same ALPN among all four mail applications, SMTP,
>> SUBMIT, IMAP, and POP.
>
> I think having separate allocations for them would be clean design.
>
> However, ALPN seem to be intended for multiple protocols sharing the same 
> IP/port. This doesn't seem to apply to SMTP/SUBMIT/IMAP/POP. Or am I missing 
> the point of ALPN?

I took another look and as far as I can tell, ALPN does two things.  The 
original motivation was to allow http/1 and http/2 on the same port even 
though http/2 isn't backward compatible with /1.  The ongoing motivation 
is to allow every possible protocol to run on port 443 so they can get 
through firewalls.

I think an ALPN for SUBMIT would make sense since it is somewhat common to 
run a submit-ish server on a non-standard port when your mail server is 
behind a spam filtering proxy or the like.  Since SMTP only does STARTTLS, 
by the time there was an ALPN negotiation it's too late.

Regards,
John Levine, johnl@taugh.com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly

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