Re: ALPN

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, [email protected], Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly