Reliable communication

Best-effort communication is a wrapper around UDP/IP. That is, the packet(s) containing a sample are sent to the addresses where the readers reside. No state is maintained on the writer. If a packet is lost, the reader ignores the samples contained in the lost packet and continue with the next one.

When using reliable communication, the writer maintains a copy of the sample in the Writer History Cache (WHC) of the DDSI writer. If a reader detects that there are lost packets, the reader can request a re-transmission of the sample. To ensure that all readers learn of new samples in the WHC, the DDSI writer periodically sends heartbeats to its readers. If all matched readers have acknowledged all samples in the WHC, the DDSI writer can suppress these periodic heartbeats.

If a reader receives a heartbeat and detects it did not receive all samples, it requests a re-transmission by sending an AckNack message to the writer.

Note

The heartbeat timing is adjustable. If the latency is longer than the heartbeat interval, this can result in multiple re-transmit requests for a single sample.

In addition to requesting the re-transmission of some samples, a reader also uses the AckNack messages to inform the writer of samples received and not received. Whenever the writer indicates that it requires a response to a heartbeat, the readers sends an AckNack message (even when no samples are missing).

Combining these behaviours allows the writer to remove old samples from its WHC when it fills up the cache, enabling readers to receive all data reliably.

The default Cyclone DDS behaviour is to never to consider readers unresponsive. The DDSI specification does not define how to handle a situation where readers do not respond to a heartbeat, or fail to receive samples from a writer after a re-transmission request. A solution to this situation is to periodically check the participant containing the reader.

Note

The DDSI specification allows for the suppressing of heartbeats, merging AckNacks, and re-transmissions, and so on. These techniques are required to allow for a performant DDSI implementation while avoiding the need for sending redundant messages.