Hiya,

I like the text below as a starter. I'd suggest it also
include something to take into account the ECH issue
mentioned on the dpriv list [1]

S

[1] https://mailarchive.ietf.org/arch/msg/dns-privacy/3xL59_1P0ZHOUEYsDJ1Q22ZZVvo/

On 30/04/2021 07:46, Martin Thomson wrote:
On Fri, Apr 30, 2021, at 16:25, Valery Smyslov wrote:
The original motivation for 7525bis was to update RFC 7525 in light
of TLS 1.3 appearance. However, I believe that recommendations for
using ALPN are in scope of this document.

How about a new Section 3.7 "Application-Layer Protocol
Negotiation":

--- TLS implementations MUST support the Application-Layer Protocol
Negotiation (ALPN) extension [RFC7301].  Correct use of ALPN ensures
that clients and servers agree on a negotiated protocol.

Newly defined application protocols that use TLS MUST define an ALPN
identifier and mandate the use of ALPN for negotiating the protocol.

An existing application protocol might not have been assigned an ALPN
identifier.  For other protocols the ALPN identifier might not have
been part of the original protocol definition, or use of ALPN might
have been defined originally as being optional.  In all of these
cases, implementations cannot require the use of ALPN.  A server
implementation MUST fail a connection attempt with a fatal
"no_application_protocol" alert if it is configured to use a protocol
that has no assigned ALPN identifier and a client offers an
"application_layer_protocol_negotiation" extension. ---

This last bit might be an update to RFC 7301, but it's important for
protecting against cross-protocol attacks on clients that support
protocols with ALPN identifiers where the use of ALPN is not
guaranteed.

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