Port numbers

The Eclipse Cyclone DDS port numbers are configured as follows:


The first two items are defined by the DDSI specification. The third item is unique to Eclipse Cyclone DDS as a way of serving multiple participants by a single DDSI instance.

  • Two “well-known” multicast ports: B and B+1.

  • Two unicast ports at which only this instance is listening: B+PG*PI+10 and B+PG*PI+11

  • One unicast port per domain participant it serves, chosen by the kernel from the list of anonymous ports, that is, >= 32768.


The default values (taken from the DDSI specification) are in parentheses.


This shows only a sub-set of the available parameters. The other parameters in the specification have no bearing on Eclipse Cyclone DDS. However, these are configurable. For further information, refer to the DDSI-RTPS 2.1 or DDSI-RTPS 2.2 specification, section 9.6.1.

PI relates to having multiple processes in the same domain on a single node. Its configured value is either auto, none or a non-negative integer. This setting matters:

  • none (default): It ignores the “participant index” altogether and asks the kernel to pick random ports (>= 32768). This eliminates the limit on the number of standalone deployments on a single machine and works well with multicast discovery, while complying with all other parts of the specification for interoperability. However, it is incompatible with unicast discovery.

  • auto: Eclipse Cyclone DDS polls UDP port numbers on start-up, starting with PI = 0, incrementing it by one each time until it finds a pair of available port numbers, or it hits the limit. To limit the cost of unicast discovery, the maximum PI is set in: Discovery/MaxAutoParticipantIndex.

  • non-negative integer: It is the value of PI in the above calculations. If multiple processes on a single machine are required, they need unique values for PI, and therefore for standalone deployments, this alternative is of little use.

To fully control port numbers, setting (= PI) to a hard-coded value is the only possibility. Discovery/ParticipantIndex By defining PI, the port numbers needed for unicast discovery are fixed as well. This allows listing peers as IP:PORT pairs, which significantly reduces traffic.

The other non-fixed ports that are used are the per-domain participant ports, the third item in the list. These are used only because there exist some DDSI implementations that assume each domain participant advertises a unique port number as part of the discovery protocol, and hence that there is never any need for including an explicit destination participant ID when intending to address a single domain participant by using its unicast locator. Eclipse Cyclone DDS never makes this assumption, instead opting to send a few bytes extra to ensure the contents of a message are all that is needed. With other implementations, you will need to check.

If all DDSI implementations in the network include full addressing information in the messages like Eclipse Cyclone DDS does, then the per-domain participant ports serve no purpose at all. The default false setting of Compatibility/ManySocketsMode disables the creation of these ports.

This setting can have a few other side benefits, as there may be multiple DCPS participants using the same unicast locator. This improves the chances of a single unicast sufficing even when addressing multiple participants.