[core] ALPN "coap" for DTLS

Christian Amsüss <christian@amsuess.com> Sun, 26 March 2023 10:24 UTC

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Date: Sun, 26 Mar 2023 12:24:29 +0200
From: Christian Amsüss <christian@amsuess.com>
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Subject: [core] ALPN "coap" for DTLS
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Hi DTLS experts in CoRE,

processing reviews for DNS-over-CoAP[1] showed that while we do have an
ALPN and usage guidance for CoAP-over-TLS (ALPN "coap"), there is no
referencable text on whether and how CoAP-over-DTLS can be disambiguated
in an ALPN. While the general specification situation is a bit dire on
mixing DTLS with ALPNs[^2], it appears to be readily used[3].

Having an ALPN sounds like a prerequisite to announcing DNS-over-CoAP
through the DNR mechanism[4]. Can we Simply™ use the ALPN defined for
CoAP-over-TCP over there, or would that need a dedicated document that
says something along the lines of "The 'coap' ALPN can be used both with
TLS and DTLS (there is no confusion as they don't mix); unlike on TLS
where it has been a requirement from the start, its use stays optional
with DTLS"?


[1]: https://datatracker.ietf.org/doc/draft-ietf-core-dns-over-coap/
[^2]: The only update to the DTLS-unaware ALPN RFC 7031 is 8447, which
  mentions DTLS, but not in the context of ALPNs.
[3]: https://www.rfc-editor.org/rfc/rfc8833.html
[4]: https://datatracker.ietf.org/doc/draft-ietf-add-dnr/

PS. This is exactly the footnote 2 of
-- starting the work on OSCORE/EDHOC in SVCBs is yet another task.

This may seem a bit weird, but that's okay, because it is weird.
  -- perldata(1) about perl variables