ROS 2 Nodes and DDS Middleware: default QoS and discovery interoperability

Goal is to clarify expected interoperability behavior at the ROS 2 node to DDS middleware boundary for Quality of Service and discovery.

ROS 2 uses DDS as the middleware layer, so node-to-node communication is mediated by the DDS vendor implementation of discovery, QoS and security profiles. QoS settings for reliability, durability, deadline and history are documented ROS 2 features, but their effective semantics depend on the underlying DDS implementation.

Uncertainty remains about default QoS compatibility and discovery timing when ROS 2 nodes using different DDS vendors interact. This creates an unresolved decision point around non-deterministic topic visibility and message delivery at the integration boundary.

What conditions define consistent mapping of default ROS 2 QoS profiles across DDS vendors? Which documented behavior governs topic visibility when QoS compatibility is not explicitly declared between nodes and middleware?