CertPoll
                              +-----<----+
                              |          |
                              |          | CertRep(PENDING)
                              |          |
[CERT-NONEXISTENT] ------> [CERT-REQ-PENDING] --------> [CERT-ISSUED]
      ^            PKCSReq    |           CertRep(SUCCESS)
      |          RenewalReq   |
      |                       |
      +-----------------------+
      CertRep(FAILURE) or
      Max-time/max-polls exceeded