UAC                         Registrar                         AS/OP
---------------------------------------------------------------------
  |                               |                               |
  | [1] The UAC interacts with the AS and obtains tokens using    |
  |     some out-of-scope mechanism.                              |
  |<=============================================================>|
  |                               |                               |
  | [2] REGISTER                  |                               |
  |     Authorization: Bearer <access_token>                      |
  |------------------------------>|                               |
  |                               | [3] HTTP POST /introspect     |
  |                               |     {access_token}            |
  |                               |       (OPTIONAL)              |
  |                               |------------------------------>|
  |                               |                               |
  |                               | [4] 200 OK {metadata}         |
  |                               |       (OPTIONAL)              |
  |                               |<------------------------------|
  |                               |                               |
  | [5] 200 OK                    |                               |
  |<------------------------------|                               |
  |                               |                               |