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October 20, 2020 23:18

ROS 2 Middleware Layer for RTI Connext DDS

This repository contains two novel implementations of the ROS 2 RMW layer which allow developers to deploy their ROS applications on top of RTI Connext DDS Professional and RTI Connext DDS Micro.

The repository provides two RMW packages:

  • rmw_connextdds

  • rmw_connextddsmicro

Package rmw_connextdds is meant to be a replacement for rmw_connext_cpp. This new implementation resolves several performance issues, and it improves out-of-the-box interoperability with DDS applications.

The repository is undergoing stabilization, with some features still in active development. Please consider reporting any issue that you may experience, while monitoring the repository for frequent updates.

For any questions or feedback, feel free to reach out to

Quick Start

  1. Load ROS into the shell environment (Rolling if using the master branch, see Support for different ROS 2 Releases)

    source /opt/ros/rolling/setup.bash
  2. Configure RTI Connext DDS Professional and/or RTI Connext DDS Micro on your system (see Requirements). Make the installation(s) available via environment variables, e.g. by using the provided rtisetenv_<architecture>.bash script (replace ~/rti_connext_dds-6.0.1 with the path of your Connext installation):

     source ~/rti_connext_dds-6.0.1/resource/scripts/rtisetenv_x64Linux4gcc7.3.0.bash
  3. Create an overlay directory and clone the repository:

    mkdir -p ~/ros2_connextdds/src/ros2
    cd ~/ros2_connextdds
    git clone src/ros2/rmw_connextdds
  4. Build the RMW:

    colcon build --symlink-install
  5. Load the generated environment script:

    source ~/ros2_connextdds/install/setup.bash
  6. Run ROS applications with RTI Connext DDS Professional:

    RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_cpp talker
  7. Run ROS applications with RTI Connext DDS Micro:

    RMW_IMPLEMENTATION=rmw_connextddsmicro ros2 run demo_nodes_cpp talker

Support for different ROS 2 Releases

rmw_connextdds, and rmw_connextddsmicro support multiple versions of ROS 2.

The following table summarizes which branch of the repository should be checked out in order to compile the RMW implementations for a specific ROS 2 release:

ROS 2 Release Branch Status
Rolling master Developed
Foxy foxy LTS (May 2023)
Eloquent eloquent EOL (Nov 2020)
Dashing dashing LTS (May 2021)

Branch master is actively developed and maintained. It is used to create other branches for specific ROS 2 releases (starting from Galactic).

Branches marked as LTS will receive updates for critical bug fixes and important patches only (until they reach EOL).

Branches marked as EOL will not receive any future updates.

RTI Connext DDS Requirements

Both RMW packages require the appropriate version of RTI Connext DDS to be available on the build and target systems.

rmw_connextdds requires RTI Connext DDS Professional (version 5.3.1 or later), while rmw_connextddsmicro requires RTI Connext DDS Micro (version 3 or later).

The installations must be made available via environment variables. If no valid installation is detected, the packages will be skipped and not be built.

RMW RTI Product Environment Variable(s) Required Default
rmw_connextdds RTI Connext DDS Professional 5.3.1, or 6.x CONNEXTDDS_DIR, or NDDSHOME Yes None
rmw_connextddsmicro RTI Connext DDS Micro 3.x RTIMEHOME No (if RTI Connext DDS Professional 6.x is available) Guessed from contents of RTI Connext DDS Professional installation (6.x only, 5.3.1 users must specify RTIMEHOME).

Multiple versions of RTI Connext DDS Professional

Package rti_connext_dds_cmake_module will first check variable ${CONNEXTDDS_DIR}, and then fall back to ${NDDSHOME} to determine the location of the RTI Connext DDS Professional libraries used by rmw_connextdds

This behavior allows users of the old Connext RMW (rmw_connext_cpp) who have installed RTI Connext DDS Professional 5.3.1 via the apt package rti-connext-dds-5.3.1, to have both that version, and a more recent one (e.g. 6.0.1) installed on their system, but configured via different variables.

If rmw_connext_cpp is installed via debian package ros-<version>-rmw-connext-cpp, variable ${NDDSHOME} will always be hard-coded to the install location of the apt package (/opt/

In this case, you can use ${CONNEXTDDS_DIR} to point to a Connext 6.x installation, making sure to source script rti_connext_dds-6.x.x/resource/scripts/rtisetenv_<architecture>.bash after loading your ROS installation, so that the Connext 6.x libraries and paths will be found first in the relevant environement variables (e.g. ${LD_LIBRARY_PATH}).

If you encounter any errors with selecting your desired Connext installation, consider uninstalling rmw_connext_cpp and connext_cmake_module (e.g. sudo apt remove ros-<version>-rmw-connext-cpp ros-<version>-connext-cmake-module).

Runtime Configuration

In addition to standard configuration facilities provided by the ROS 2 RMW interface, rmw_connextdds, and rmw_connextddsmicro support the additional configuration of some aspects of their runtime behavior via custom environment variables.


Enable different policies to improve interoperability with rmw_cyclonedds_cpp.

By default, ROS 2 applications using rmw_connextdds will be able to communicate with those using rmw_cyclonedds_cpp only via ROS 2 publishers and subscribers, while ROS 2 clients and services will not interoperate across vendors.

The reason for this incompatibility lies in rmw_cyclonedds_cpp's use of a custom mapping for propagating request metadata between clients and services.

When this "compatibility mode" is enabled, rmw_connextdds (and rmw_connextddsmicro) will use this non-standard profile in order to interoperate with rmw_cyclonedds_cpp, instead of using one the two standard profiles defined by the DDS-RPC specification (see RMW_CONNEXT_REQUEST_REPLY_MAPPING).


By default, rmw_connextdds will try to detect the use of "large data" types, and automatically optimize the QoS of DDS DataWriters and DataReaders using these types, to improve out of the box performance on reliable streams.

These optimizations will be applied to any endpoint whose type has a serialized size of at least 1MB (configured by a compile-time limit).

rmw_connextdds will modify a "large data" endpoint's RTPS reliability protocol parameters to more quickly recover samples, which typically improves performance in the presence of very fragmented data, but it might also end up increasing network traffic unnecessarily, particularly if data is not exchanged at a fast periodic pace.

Variable RMW_CONNEXT_DISABLE_LARGE_DATA_OPTIMIZATIONS may be used to disable these automatic optimizations, and revert to Connext's default behavior.


By default, rmw_connextdds modifies the QoS of its DomainParticipant to enable the optimizations defined by RTI Connext DDS' built-in QoS snippet Optimization.Discovery.Endpoint.Fast.

These optimizations speed up the discovery process between different applications but they also introduce an overhead in network traffic, which might be undesirable for larger systems.

Variable RMW_CONNEXT_DISABLE_FAST_ENDPOINT_DISCOVERY may be used to disable these automatic optimizations, and to leave the DomainParticipant's QoS to its defaults.


When this variable is not set or set to always, the QoS settings specified in the default profile will be used and the ros QoS profile will be applied on top of it. You can use topic filters in XML profile files to have different defaults for different topics, but you have to use the mangled topic names (see ROS topic mangling conventions).

In case this variable is set to never, the QoS settings will be loaded from the default profile as before but the ros QoS profile will be ignored. Be aware of configuring the QoS of rcl topics (rt/rosout, rt/parameter_events, etc.) and the rmw internal topic ros_discovery_info correctly.

This variable can also be set to dds_topics: <regex>, e.g.: dds_topics: rt/my_topic|rt/my_ns/another_topic. In that case, QoS settings for topics matching the provided regex will be loaded in the same way as the never policy, and the ones that don't match will be loaded in the same way as the always policy.

ROS topic mangling conventions

ROS mangles topic names in the following way:

  • Topics are prefixed with rt. e.g.: /my/fully/qualified/ros/topic is converted to rt/my/fully/qualified/ros/topic.
  • The service request topics are prefixed with rq and suffixed with Request. e.g.: /my/fully/qualified/ros/service request topic is rq/my/fully/qualified/ros/serviceRequest.
  • The service response topics are prefixed with rr and suffixed with Response. e.g.: /my/fully/qualified/ros/service response topic is rr/my/fully/qualified/ros/serviceResponse.


Variable RMW_CONNEXT_INITIAL_PEERS can be used to specify a list of comma-separated values of "address locators" that the DomainParticipant created by the RMW will use to try to make contact with remote peer applications during the DDS discovery phase.

The values will be parsed, trimmed, and stored in QoS field DDS_DomainParticipantQos::discovery::initial_peers, overwriting any value it previously contained.

While both rmw_connextdds and rmw_connextddsmicro will honor this variable, equivalent, and more advanced, functionality is already available in RTI Connext DDS, for example using variable NDDS_DISCOVERY_PEERS.

For this reason, only users of rmw_connextddsmicro should consider specifying RMW_CONNEXT_INITIAL_PEERS.

For example, rmw_connextddsmicro will use lo as its default UDP network interface (see RMW_CONNEXT_UDP_INTERFACE), which will prevent it from accessing the default discovery peer (multicast address The default peer configuration will also prevent the DomainParticipant from carrying out discovery over the built-in shared-memory transport. To enable discovery over this transport, in addition to the default multicast peer:

RMW_IMPLEMENTATION=rmw_connextddsmicro \
  ros2 run demo_nodes_cpp listener


ROS 2 applications using rmw_connextdds will not be able to interoperate with applications using the previous RMW implementation for RTI Connext DDS, rmw_connext_cpp, unless variable RMW_CONNEXT_LEGACY_RMW_COMPATIBILITY_MODE is used to enable a "compatibility" mode with these older implementation.

In particular, when this mode is enabled, rmw_connextdds will revert to adding a suffix (_) to the end of the names of the attributes of the ROS 2 data types propagated via DDS discovery.


Control how rmw_connextdds will override the default DomainParticipantQos obtained from Connext.

If this variable is unspecified, or set to all, then rmw_connextdds will modify the default DomainParticipantQos with settings derived from ROS 2 options (e.g. "localhost only", or "node enclave"), and some additional optimizations meant to improve the out of the box experiene (e.g. speed up endpoint discovery, and increase the size of type information shared via discovery).

If the variable is set to basic, then only those settings associated with ROS 2 options will be modified.

If the variable is set to never, then no settings will be modified and the DomainParticipantQos will be used as is.

Note that values basic and never will disable the same endpoint discovery optimizations controlled by RMW_CONNEXT_DISABLE_FAST_ENDPOINT_DISCOVERY.


The DDS-RPC specification defines two profiles for mapping "request/reply" interactions over DDS messages (e.g. ROS 2 clients and services):

  • the basic profile conveys information about the originator of a request as an inline payload, serialized before the actual request/reply payloads.

  • The extended profile relies on DDS' metadata to convey request/reply information out of band.

By default, rmw_connextdds uses the extended profile when sending requests from a ROS 2 client to a service, while rmw_connextddsmicro uses the basic one.

Variable RMW_CONNEXT_REQUEST_REPLY_MAPPING can be used to select the actual profile used at runtime. Either "basic" or "extended" may be specified.

At the moment, the extended profile is only available with rmw_connextdds. In this configuration, rmw_connextdds will interoperate with rmw_fastrtps_cpp, e.g.:

RMW_IMPLEMENTATION=rmw_connextdds \
  ros2 run demo_nodes_cpp add_two_ints_server

RMW_IMPLEMENTATION=rmw_fastrtps_cpp \
  ros2 run demo_nodes_cpp add_two_ints_client

When using the basic profile, rmw_connextdds will interoperate with rmw_connextddsmicro, e.g.:

RMW_IMPLEMENTATION=rmw_connextdds \
  ros2 run demo_nodes_cpp add_two_ints_server

RMW_IMPLEMENTATION=rmw_connextddsmicro \
  ros2 run demo_nodes_cpp add_two_ints_client

Use variable RMW_CONNEXT_CYCLONE_COMPATIBILITY_MODE to enable interoperability with rmw_cyclonedds_cpp using a non-standard version of the basic profile, e.g.:

RMW_IMPLEMENTATION=rmw_connextdds \
  ros2 run demo_nodes_cpp add_two_ints_server

RMW_IMPLEMENTATION=rmw_cyclonedds_cpp \
  ros2 run demo_nodes_cpp add_two_ints_client


RTI Connext DDS Micro requires applications to explicitly configure the network interface to use for UDPv4 communication.

rmw_connextddsmicro makes the arbitrary decision of using lo as the default interface.

This is undesireable if non-local communication is required, and/or if the default DDS multicast peer ( is to be used.

Variable RMW_CONNEXT_UDP_INTERFACE may be used to customize the network interface actually used by RTI Connext DDS Micro's UDPv4 transport, e.g. to use eth0:

RMW_IMPLEMENTATION=rmw_connextddsmicro \
  ros2 run demo_nodes_cpp listener

This variable is not used by rmw_connextdds.


rmw_connextdds will always set DDS_DataWriterQos::publish_mode::kind of any DataWriter it creates to DDS_ASYNCHRONOUS_PUBLISH_MODE_QOS, in order to enable out of the box support for "large data".

This behavior might not be always desirable, and it can be disabled by setting RMW_CONNEXT_USE_DEFAULT_PUBLISH_MODE to a non-empty value.

This variable is not used by rmw_connextddsmicro, since it doesn't automatically override DDS_DataWriterQos::publish_mode::kind.