In the concept of topic , I learnt that nodes ready to publish/ subscribe notify the master and the master then exchanges each other's IP addresses and these nodes then communicate . However in services , is it that the publishing node will notify the master of its service and the subscribing node also notifies about the subscription to that service and then the master "ONLY " notifies the subscription node of publishing node's location and then this node sends a request to the service node ( this is when the service node becomes aware of its client , rather than the master sharing the information ) and awaits a response from the service node ? If yes , how is it advantageous as compared to the communication via topic . please give a detailed answer . Thank you.