3.1.4.1 Initiating the Login Sequence

The client initiates the login sequence by sending a REGISTER request without any credentials, as suggested in [RFC3261]. If the REGISTER request is challenged by a server request for NTLM Authentication Protocol or Kerberos authentication, the client then resends the REGISTER request with credentials. This step also establishes an SA between the client and the server that is used to sign any future messages.