3.1.5.3 Receiving a Reliable Notify Acknowledgement
When an Authenticated Internet Protocol peer receives a Notify message of type NOTIFY_ACK (see section 2.2.3.5 for verification details), it MUST check to see if any Notify messages are queued waiting for the notify ACK to arrive. If so, the first non-Notify message in the Request Non-notify Message pending Queue is sent at this time (section 3.1.7.5).