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
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.
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
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