3.1.5.1 RegisterResponse Message
If the resolver client is not waiting for a response to a Register or Update message, it MUST ignore the RegisterResponse message.
Upon receiving the RegisterResponse message, the resolver client MUST store the RegistrationId value locally. If this message is received in response to an Update message, the RegistrationId value MUST replace the locally stored value. Additionally, the resolver client SHOULD start a client refresh timer with the time interval indicated in the RegistrationLifetime value in the RegisterResponse message.