I have reviewed this document as part of the security directorate's ongoing
effort to review all IETF documents being processed by the IESG.  These
comments were written primarily for the benefit of the security area directors.
Document editors and WG chairs should treat these comments just like any other
last call comments.

The document introduces an extension to the OAuth protocol that allows resource
servers to signal to a client that the authentication event associated with the 
access token of the current request does not meet its authentication requirements 
and specify how to meet them.

The document is well written and easy to understand.

The Security Considerations section looks comprehensive. However, I think that 
one potential issue is not discussed - the possibility of DoS attacks.
The protocol allows the resource server to send the client back to the authorization 
server for a "better" authentication token. In my opinion it opens a possibility
for rogue resource servers to mount a DoS attack by constantly requesting
a "better" token. In my understanding a client should respect these replies 
and each time should ask the authorization server for a "better" (e.g. fresher) token. 
Depending on the authentication mechanism involved this may be annoying for the user 
and put an additional load on both the client and the authorization server.