Connect a ROS2 node to the Discovery node and to another peer (directly) [12811]

[zemskymax]

Is it possible to connect a ROS2 node (we work with Galactic) to both a Discovery server (to discover other ROS2 nodes) and to another peer that uses a multicast discovery?

I am trying to configure the DEFAULT_FASTRTPS_PROFILES.xml file according to this link.

<?xml version="1.0" encoding="UTF-8" ?>
<profiles>
    <participant profile_name="participant_profile" is_default_profile="true">
        <rtps>
            <builtin>
                <discovery_config>
                    <discoveryProtocol>SUPER_CLIENT</discoveryProtocol>
                    <discoveryServersList>
                        <RemoteServer prefix="x.x.x.x.x.x.x.x.x.x.x.x">
                            <metatrafficUnicastLocatorList>
                                <locator>
                                    <udpv4>
                                        <address>127.0.0.1</address>
                                        <port>11810</port>
                                    </udpv4>
                                </locator>
                            </metatrafficUnicastLocatorList>
                        </RemoteServer>
                    </discoveryServersList>
                </discovery_config>
                <initialPeersList>
                    <locator>
                    	<udpv4>
                            <!-- <port>11818</port> -->
                            <address>127.0.0.1</address>
                        </udpv4>
                    </locator>
                </initialPeersList>
            </builtin>
        </rtps>
    </participant>
</profiles>

However, it doesn't seem to work. Do you have any suggestions?

I would like to provide any other required information.

[jparisu]

Hi @zemskymax

The important point here is to distinguish between discovery protocols and transports.

Fast DDS Discovery Server Protocol is a Discovery Protocol, and so you can configure a Participant as CLIENT or SERVER regarding this protocol. A Participant that is using a protocol will not be able to discover other Participant that is using a different protocol. Thus, if one of your Participants is using Fast DDS Discovery Server Protocol and the other is using Simple Discovery Protocol they will never match.
Fast DDS Discovery Server Protocol does not allow to directly connect two Clients without a middle Server. For this propose you can use Static Discovery Protocol.

In the other hand, there are Transports. Multicast is a specific transport, and it could be used by any Participant regardless of the Discovery Protocol it is using. Thus, you can configure a Discovery Server with multicast listening address, and it will discover every Participant using that multicast address as long as they are using Fast DDS Discovery Server Protocol.

Please, check the documentation for further information about discovery protocols:
https://fast-dds.docs.eprosima.com/en/latest/fastdds/discovery/discovery.html#discovery-mechanisms
and about transports:
https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/transport.html

[zemskymax]

Hello @jparisu

Thank you for your fast reply!

Here is our scenario:
On first machine, I have several ROS2 nodes running with discovery server - unicast (with rmw_fastdds).
On another machine, I have an application running with different implementation of DDS (RTI).

On one hand, I cannot use multicast, since my network is wireless. On the other hand, it is a third party application and I can't change the DDS vendor or DDS configurations.

Is there any way we can use the discovery server of eProsima, with a client of RTI?
Is there a different way to bridge between RTI and eProsima?

[jparisu]

Hi @zemskymax

Sadly, the answer is no. There is not a current way to do it. Fast DDS Discovery Server is a eProsima Fast DDS feature, is not part of the DDS specification, and thus other implementations are not prepared to use it.

However, there are 2 possibilities that we are developing right now:

1. Integration Service allows to communicate two different "middlewares". In this case, the middlewares are no different but the Discovery Protocol are, so its a similar scenario. Here is the documentation of this product: https://integration-service.docs.eprosima.com/en/latest/ and in this branch of the FastDDS-SH there is a work in progress to add a Discovery Server to one side: Add Databroker Participant type [12151] FastDDS-SH#82 .

2. We are currently working in a new product that communicate DDS via WAN. It is not ready yet, and it will take some months to be operative, but it will be able to connect networks with Discovery Server and networks with Simple Discovery (similar to the Integration Service but with a huge efficiency improvement).

We would like to know more about your use case and how we could help. If you are interested in this new features, please feel free to contact our support team danielcabezas@eprosima.com and we can arrange a quick meeting.

[zemskymax]

Hi @jparisu

Just a clarification, the RTI application can discover only one node (we can add a bridge node that will use discovery server protocol from one side and simple protocol from another). Is it possible?

[jparisu]

Hi @zemskymax

Is it possible?

It is indeed.
In the schema that you have attached, the orange node would be one of the applications I mentioned: Integration Service or Databroker.

⚠️ Be aware that both options are not currently prepared to support this scenario.

1. Integration Service has a PR already done, but it is not currently moving forward.
2. The Databroker is an application that is in the company road map for next year. It is a generic bridge for DDS implementations (different domains, different discovery protocols, WAN communication, etc.). However, this project is in a very early stage, and it would take several months to be ready. If you are interested in contributing with this project and discuss about schedules and deadlines, feel free to contact out support team danielcabezas@eprosima.com