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DDS and ROS middleware implementations
ROS 2 and different DDS/RTPS vendors
ROS 2 is built on top of DDS/RTPS as its middleware, which provides discovery, serialization and transportation. This article explains the motivation behind using DDS implementations, and/or the RTPS wire protocol of DDS, in detail, but in summary DDS is an end-to-end middleware that provides features which are relevant to ROS systems, such as distributed discovery (not centralized like in ROS 1) and control over different "Quality of Service" options for the transportation.
DDS is an industry standard which is then implemented by a range of vendors, such as RTI's implementation Connext or ADLink's implementation OpenSplice RTPS (a.k.a. DDSI-RTPS) is the wire protocol used by DDS to communicate over the network, and there are implementations of that which do not fulfill the full DDS API, but provide sufficient functionality for ROS 2, such as eProsima's implementation Fast RTPS.
ROS 2 supports multiple DDS/RTPS implementations because it is not necessarily "one size fits all" when it comes to choosing a vendor/implementation. There are many factors you might consider while choosing a middleware implementation: logistical considerations like the license, or technical considerations like platform availability, or computation footprint. Vendors may provide more than one DDS or RTPS implementation targeted at meeting different needs. For example, RTI has a few variations of their Connext implementation that vary in purpose, like one that specifically targets microcontrollers and another which targets applications requiring special safety certifications (we only support their standard desktop version at this time).
In order to use a DDS/RTPS implementation with ROS 2, a "ROS Middleware interface" (a.k.a.
rmw interface or just
rmw) package needs to be created that implements the abstract ROS middleware interface using the DDS or RTPS implementation's API and tools.
It's a lot of work to implement and maintain RMW packages for supporting DDS implementations, but supporting at least a few implementations is important for ensuring that the ROS 2 codebase is not tied to any one particular implementation, as users may wish to switch out implementations depending on their project's needs.
Supported RMW implementations
|Product name||License||RMW implementation||Status|
|eProsima Fast RTPS||Apache 2||
||Full support. Default RMW. Packaged with binary releases.|
|RTI Connext||commercial, research||
||Full support. Support included in binaries, but Connext installed separately.|
|RTI Connext (dynamic implementation)||commercial, research||
||Support paused. Full support until alpha 8.*|
|PrismTech Opensplice||LGPL (only v6.4), commercial||
||Partial support. Support included in binaries, but OpenSplice installed separately.|
|OSRF FreeRTPS||Apache 2||--||Partial support. Development paused.|
"Partial support" means that one or more of the features required by the rmw interface is not implemented.
For practical information on working with multiple RMW implementations, see the Working with multiple RMW implementations page.