[Ace] How to specify DTLS MTI in COAP-EST

Michael Richardson <mcr+ietf@sandelman.ca> Wed, 06 June 2018 23:32 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
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Date: Wed, 06 Jun 2018 19:32:13 -0400
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Subject: [Ace] How to specify DTLS MTI in COAP-EST
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In draft-ietf-ace-coap-est, we would like to specify some mandatory to
implement algorithms for DTLS.

We write:
   The mandatory cipher suite for DTLS in EST-coaps is
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 defined in [RFC7251] which is the
   mandatory-to-implement cipher suite in CoAP.

   Additionally, the curve secp256r1 MUST be supported [RFC4492]; this curve
   is equivalent to the NIST P-256 curve.

And this is fine for now, but we'd like to signal that Curve25519 should be
considered as an alternative, but we don't want to make it a MUST *today*,
and we don't want to force implementations 15 years down the road that have
it to include secp256r1.

IPsec(ME) has published things like: https://datatracker.ietf.org/doc/rfc8247/
which include language like:

   SHOULD+   This term means the same as SHOULD.  However, it is likely
             that an algorithm marked as SHOULD+ will be promoted at
             some future time to be a MUST.

   SHOULD-   This term means the same as SHOULD.  However, an algorithm
             marked as SHOULD- may be deprecated to a MAY in a future
             version of this document.

   MUST-     This term means the same as MUST.  However, it is expected
             at some point that this algorithm will no longer be a MUST
             in a future document.  Although its status will be
             determined at a later time, it is reasonable to expect that
             if a future revision of a document alters the status of a
             MUST- algorithm, it will remain at least a SHOULD or a
             SHOULD- level.

I don't think TLS has done this... maybe TLS plans to.
We think that we'd like to use SHOULD+ for Curve25519 and MUST- for
secp256r1, but we aren't sure that the WG will like us to use so many
words as IPsec to say so.

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