3.1.5 Message Processing Events and Sequencing Rules
The MS-SNTP protocol supports modes according to [RFC1305] section 3.4.2 (Table 5 "Modes and Actions"), except that it does not support broadcast clients, NTP control message, or symmetric passive mode. If the client receives a message with an unsupported mode, the message MUST be ignored as specified in [RFC1305] section 3.4.3. The NTP Authentication Extensions are defined only for the following NTP and SNTP association modes (as specified in [RFC1305] sections 3.2.1 and 3.3): client, server, and symmetric active.
The client prepares either a Client NTP Request or Client ExtendedAuthenticator NTP Request message header in the same way it occurs in the base Network Time Protocol (NTP), which is described in [RFC1305] section 3.4.2. The message format used is dependent on the value of the ExtendedAuthenticatorSupported ADM element (see section 3.1.1).