The strength is the precedence of the issue sent by the publication. Strength and persistence allowthe receiver to arbitrate if issues are received from several matching publications. Publication relation may be best effort (as fast as possible but not faster as the minimum separation) or strict. In the case of the strict publisher–subscriber relation, the timing is ensured with a heartbeat message sent from the publisher to the subscriber (exact timing is middleware dependent) and a replied acknowledge message. The RTPS protocol is designed to run over an unreliable transport such as UDP/IP and a message is the contents (payload) of exactly one UDP/IP datagram.