The user and reference documents describe how writers and readers can request different values for reliability, but say little about the implications. More specifically, if a writer offers reliable and there is a mix of readers, some requesting reliable, some best-effort, how is data distributed? I'm primarily interested in DDS with the interoperability wire protocol, not proprietary transports.
Does the writer use both TCP and UDP multicast in that case? Or is reliable QoS implemented using multicast? Or will the reader that requests best-effort effectively get reliable treatment with potentially greater latency?
Although I don't have time to read the RTPS specification in detail, a quick look suggests that the answer to my question is that a RELIABLE Writer can serve BEST_EFFORT Readers without forcing them to behave in RELIABLE fashion.