ROS topics are named buses in which ROS nodes exchange messages. Topics can anonymously publish and subscribe, which means that the production of messages is decoupled from the consumption. The ROS nodes are not interested in knowing which node is publishing the topic or subscribing topics; they only look for the topic name and whether the message types of the publisher and subscriber are matching.
The communication using topics are unidirectional. If we want to implement a request/response, such as communication, we have to switch to ROS services.
The ROS nodes communicate with topics using TCP/IP-based transport known as TCPROS. This method is the default transport method used in ROS. Another type of communication is UDPROS
