Discovery of DDSI participants and endpoints

DDSI participants discover each other using the Simple Participant Discovery Protocol (SPDP). This protocol periodically sends a message containing the specifics of the participant to a set of known addresses. By default, this address is a standardised multicast address (239.255.0.1 where the port number is derived from the domain ID) that all DDSI implementations listen to.

In the SPDP message, the unicast and multicast addresses are important, as that is where the participant can be reached. Typically, each participant has a unique unicast address, which means all participants on a node have a different UDP/IP port number in their unicast address. The actual address (including port number) in a multicast-capable network is unimportant because all participants learn them through the SPDP messages.

The SPDP protocol allows for unicast-based discovery. This requires listing the addresses of machines where participants are located, and ensuring that each participant uses one of a small set of port numbers. Because of this, some port numbers are derived from the domain ID and a participant index, which is a small non-negative integer, unique to a participant within a node. Eclipse Cyclone DDS adds an indirection and uses one participant index for a domain for each process, regardless of how many DCPS participants are created by the process.

When two participants have discovered each other and both have matched the DDSI built-in endpoints that their peer is advertising in the SPDP message, the Simple Endpoint Discovery Protocol (SEDP) takes over. The SEDP exchanges information about the DCPS Readers and writers with the two participants. For Eclipse Cyclone DDS, SEDP also exchanges information about publishers, subscribers and topics in a manner compatible with OpenSplice.

The SEDP data is handled as reliable (see Reliable communication), transient-local data (see DDSI-Specific Transient-Local Behaviour). Therefore, the SEDP Writers send Heartbeats. If the SEDP Readers detect they have not yet received all samples and send AckNacks requesting re-transmissions, the Writer responds to these and eventually receives a pure acknowledgement informing it that the reader has now received the complete set.

Note

The discovery process creates a burst of traffic each time a participant is added to the system: all existing participants respond to the SPDP message, which all start exchanging SEDP data.