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Re: [ietf-smtp] ALPN

2021-07-28 13:00:39
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,

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.

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