3.1.5 Processing Events and Sequencing Rules
The message processing events and sequencing rules SHOULD<18> be as specified in [RFC5246], [RFC5077], and [RFC7301]. It MAY<19> be as specified in [NPN]. If a client receives an extension type in ServerHello that it did not request in the associated ClientHello, it MAY<20> abort the handshake. There can be more than one extension of the same type.