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Re: [ietf-smtp] ALPN: implementation / behaviour

2021-07-10 03:23:45
On Thu, Jul 08, 2021, Jeremy Harris wrote:

In client MTA mode I'd expect the coding to make an ALPN request to
be similarly simple. Administrative controls for 
would probably be more work than just the library interface.

It seems there are the following possible cases to handle:
1. the other side did not offer/specify ALPN.
2. the other side did offer/specify ALPN and there is
  2.1  a match
  2.2  no match

but there is no way to determine case 1 for the client.  At least
that's what I found out when I implemented it in my MTA.

The ALPACA authors have this recommendation:
  ALPN for Clients
  We also recommend that the client aborts the handshake if the server
  acknowledges the ALPN extension, but does not select a protocol
  from the client list.

AFAICT it's no possible to implement this with ALPN. I suggested a
possible solution on the ietf-tls list but there was no interest
in it.

Hopefully someone who understand this topic better can explain
what needs to be done?

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