discovery_server client's <transport_descriptors><listening_ports><port> automation [13610]

[yuraSomatic]

Automatic assigning available port number to new client, without need to specify different port numbers for each client, resulting in redundant config files.

Expected Behavior

I have 2 configs - discovery server and it's client and I can start ros2 foxy nodes - 1 server and 2+ clients, all clients using same config (look at client without <listening ports> config below and running on same machine. In this case client's <listening ports> not provided, so they suppose to be automaticly set to available ones.

Server

<?xml version="1.0" encoding="UTF-8" ?>
<profiles>
    <transport_descriptors>
        <transport_descriptor>
            <transport_id>LAN server tcp transport</transport_id>
            <type>TCPv4</type>
            <listening_ports>
                <port>5100</port>
            </listening_ports>
        </transport_descriptor>
    </transport_descriptors>

    <participant profile_name="LAN server" is_default_profile="true">
        <rtps>
            <prefix>
                4D.49.47.55.45.4c.5f.42.41.52.52.4f
            </prefix>
            <userTransports>
                <transport_id>LAN server tcp transport</transport_id>
            </userTransports>
            <useBuiltinTransports>false</useBuiltinTransports>
            <builtin>
                <discovery_config>
                    <discoveryProtocol>SERVER</discoveryProtocol>
                </discovery_config>
                <metatrafficUnicastLocatorList>
                    <locator>
                        <tcpv4>
                            <address>192.168.88.247</address>
                            <port>65215</port>
                            <physical_port>5100</physical_port>
                        </tcpv4>
                    </locator>
                </metatrafficUnicastLocatorList>
            </builtin>
        </rtps>
    </participant>
</profiles>

Client without <listening_ports>

<?xml version="1.0" encoding="UTF-8" ?>
<profiles>
    <transport_descriptors>
        <transport_descriptor>
            <transport_id>LAN tcp transport</transport_id>
            <type>TCPv4</type>
        </transport_descriptor>
    </transport_descriptors>

    <participant profile_name="LAN client" is_default_profile="true">
        <rtps>
            <userTransports>
                <transport_id>LAN tcp transport</transport_id>
            </userTransports>
            <useBuiltinTransports>false</useBuiltinTransports>
            <builtin>
                <discovery_config>
                    <discoveryProtocol>CLIENT</discoveryProtocol>
                    <discoveryServersList>
                        <RemoteServer prefix="4D.49.47.55.45.4c.5f.42.41.52.52.4f">
                            <metatrafficUnicastLocatorList>
                                <locator>
                                    <tcpv4>
                                        <address>192.168.88.247</address>
                                        <port>65215</port>
                                        <physical_port>5100</physical_port>
                                    </tcpv4>
                                </locator>
                            </metatrafficUnicastLocatorList>
                        </RemoteServer>
                    </discoveryServersList>
                </discovery_config>
            </builtin>
        </rtps>
    </participant>
</profiles>

Current Behavior

When I starting client, using client without <listening ports> config above - I got this error:

2021-04-29 10:17:49.890 [RTPS_PARTICIPANT Error] Participant / with GUID 1.f.3a.1a.bb.2d.0.0.1.0.0.0|0.0.1.c1 tries to use discovery server over TCP without providing a proper listening port -> Function RTPSParticipantImpl

So I must create new config for every new client with only difference - <listening ports>, when I try create 2+ cli with same port (Client with <listening_ports> config below) - got following error:

2021-04-29 10:17:13.617 [RTCP_MSG_OUT Error] TCPTransport Error binding at port: (64865) with msg: bind: Address already in use -> Function create_acceptor_socket

Client with <listening_ports>

<?xml version="1.0" encoding="UTF-8" ?>
<profiles>
    <transport_descriptors>
        <transport_descriptor>
            <transport_id>LAN tcp transport</transport_id>
            <type>TCPv4</type>
            <listening_ports>
                <port>64865</port> <!--__HERE__-->
            </listening_ports>
        </transport_descriptor>
    </transport_descriptors>

    <participant profile_name="LAN client" is_default_profile="true">
        <rtps>
            <userTransports>
                <transport_id>LAN tcp transport</transport_id>
            </userTransports>
            <useBuiltinTransports>false</useBuiltinTransports>
            <builtin>
                <discovery_config>
                    <discoveryProtocol>CLIENT</discoveryProtocol>
                    <discoveryServersList>
                        <RemoteServer prefix="4D.49.47.55.45.4c.5f.42.41.52.52.4f">
                            <metatrafficUnicastLocatorList>
                                <locator>
                                    <tcpv4>
                                        <address>192.168.88.247</address>
                                        <port>65215</port>
                                        <physical_port>5100</physical_port>
                                    </tcpv4>
                                </locator>
                            </metatrafficUnicastLocatorList>
                        </RemoteServer>
                    </discoveryServersList>
                </discovery_config>
            </builtin>
        </rtps>
    </participant>
</profiles>

Steps to Reproduce

1. Run ros2 daemon with discovery_server config
   ros2 daemon stop
export FASTRTPS_DEFAULT_PROFILES_FILE=path/to/discovery_server.xml
ros2 daemon start
2. Run some ros2 node
   export FASTRTPS_DEFAULT_PROFILES_FILE=path/to/discovery_client.xml
ros2 run my_package my_node1
3. Check server comunicate with client
   ros2 topic list

System information

• fastrtps: 2.0.2
• OS: Ubuntu 20.04
• ROS2: foxy

[JLBuenoLopez]

I am going to move this issue to the Ideas discussion forum. According to our CONTRIBUTING guidelines the feature requests and improvements are discussed there. The behavior described is not a malfunction but something that can be improved in the library to help the user configure Fast DDS.