Hello,

I have a short comment about error responses from an RS in 
draft-ietf-ace-oauth-authz-21. More specifically, my question is about section 
5.8.2. In the second paragraph, it states “The response code MUST be 4.01 
(Unauthorized) in case the client has not performed the proof-of-possession, or 
if RS has no valid access token for the client.” I am assuming this means that 
if the client is trying to access a resource and sending a pop key id that is 
not known by the RS, either because the RS has never seen it or because it is 
associated to a token that has already been removed from the RS, then this is 
how the RS should reply.

If this is the case, I am a bit confused on how to implement this when using 
the DTLS profile. When using this profile, a client will first try to establish 
a DTLS session with the RS when accessing a resource. Once the session is 
established, it will actually try to access the resource over that DTLS 
connection. The pop key id to be used is sent when establishing the DTLS 
connection in the DTLS handshake messages, but if the RS does not have a 
key+token associated to that id for whatever reason, then it will cancel the 
DTLS handshake. If the DTLS handshake is never completed, then the RS can’t 
really send a reply at all, much less a 4.01 reply.

Thanks,

Sebastian Echeverria
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