IP-PBX                                         P-CSCF
    |                                               | 
    |----(1) REGISTER------------------------------>| 
    |        Security-Client: sdes-srtp;mediasec    |
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |                                               | 
    |<---(2) 401 UNAUTHORIZED ----------------------|
    |        Security-Server: msrp-tls;mediasec     | 
    |        Security-Server: sdes-srtp;mediasec    | 
    |        Security-Server: dtls-srtp;mediasec    |
    |                                               | 
    |----(3) REGISTER (with Authorization Header)-->| 
    |        Security-Client: sdes-srtp;mediasec    |
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |        Security-Verify: msrp-tls;mediasec     | 
    |        Security-Verify: sdes-srtp;mediasec    | 
    |        Security-Verify: dtls-srtp;mediasec    |
    |                                               | 
    |<---(4) 200 OK---------------------------------|
    |                                               |