[ros-users] ROS 2.0 Strategy review

Brian Gerkey gerkey at osrfoundation.org
Mon Sep 28 21:08:39 UTC 2015

To further emphasize the difference between DDS and things like ZeroMQ
(zmq), and to motivate our decision to go with the former (see also

While you might consider zmq a "known thing," and indeed it is widely used
in a number of distributed systems, it's insufficient to say, "use zmq."
zmq specifies only the transport part of the system (how sockets are
handled), saying nothing about discovery (how participants find each other)
or serialization (how data is encoded on the wire).  Adding those features
isn't impossible, or even necessarily that difficult.  You could, for
example, combine zmq for transport with protobuf for serialization and UDP
multicast for discovery [*].  zmq doesn't support unreliable transport, so
you'd also need to add your own solution there (e.g., managing UDP sockets
manually).  Still, it's all doable.

The problem is not in the effort required to build that system.  It's that
you then have to define, document, and defend your custom combination of
techniques and protocols.  When it comes time to convince someone to rely
on it, you have to make the argument that your bespoke system is reliable,
robust, free of nasty corner cases, and ready to be used in serious
domains, whether that's a classroom full of undergrads, a government-funded
R&D program, or a commercial product.

That argument can be made and won; after all, ROS today is a custom system
combining various protocols and techniques (TCP, UDP, XML/RPC, custom
serialization, discovery via a central master, etc.), and yet it is widely
used and there are many ROS-based products and services in the
marketplace.  But there are many, many more current and future robotics
applications where ROS will *not* be chosen, in large part because of its
bespoke nature.

At OSRF, we looked carefully at this issue, considered a wide variety of
options, and came to the conclusion that while we could build on things
like zmq, we would really be defining and building another custom
middleware.  And things would just get more custom as we want to add
features like quality-of-service (QoS).  Do you want to support
store-and-forward of messages?  Priority among messages?  Limited-duration
delivery retry?  Well, you have to invent your own rules for handling those
cases, in addition to writing (and testing and debugging) the code to
implement them.

By contrast, DDS (and its underlying wire protocol, RTPS) is an open,
end-to-end middleware specification that is relied upon for serious
applications in labs and industries around the world.  And the
specification includes extensive QoS settings that give you essentially any
kind of behavior between UDP fire-and-forget and TCP retry-forever.  That's
exactly what we need for robotics applications.  And there are multiple
well-tested implementations of the specification.

DDS is by no means perfect, but we believe that it's the best foundation
for ROS 2.


[*] We're intimately familiar with this approach, as we've done exactly
that in the ignition-transport library, which is being used for
communication within Gazebo: http://ignitionrobotics.org/libraries/transport.
It works great for its intended use case, but it has the drawbacks inherent
to any custom middleware solution, described above.

On Sat, Sep 26, 2015 at 6:02 PM, BiggsGeoffrey via ros-users <
ros-users at lists.ros.org> wrote:

> I always feel a little sad when someone paints the entire catalogue of
> OMG specifications with complaints against a technology from several
> decades ago. To anyone who has heard about how bad CORBA is, I encourage
> them to try out a recent version (in particular, the C++11 API). It’s not
> bad and it’s not slow. It’s good at what it was designed for, which is a
> distributed object system.
> Having said that, DDS is much better for our needs, because it has a
> different focus.
> Regarding the patents issue; the OMG specifications are open and freely
> available. There may be patents covering parts of them, but this is the
> same risk that any software, open or closed, faces in many parts of the
> world these days. I would be very surprised if someone couldn’t find a part
> of the zmq source that infringes some unknown, ambiguous software patent.
> As for zmg being a “known thing”, DDS is known, too. It may not be as
> well-known by those who prefer open source, but for the many, many
> companies and institutes who use it, it is known and trusted. If it wasn’t
> meeting their needs, they would be instead pouring those expensive
> licensing fees into improving zmq or developing their own in-house
> technology. They don’t keep their experience a secret, either. I have
> spoken with people from industry who have enjoyed complaining about the
> problems they’ve had with DDS – and then gone on to mention that their
> chosen implementer fixed those problems promptly because they wanted to
> keep getting paid. Sure, in the open-source world we can fix problems we
> find ourselves (I hope you don’t need safety certification), but this
> doesn’t mean everyone keeps their experience a secret.
> The OSRF made a sensible choice to chose an open, standardised protocol
> with many implementations, both commercial and open-source, available.
> Anyone who wants to implement the RTPS protocol can do so, while anyone who
> doesn’t has the choice between open-source implementations (fun fact: at
> least one of them uses CORBA internally) and commercial implementations.
> (Not every issue about this has been resolved yet; there are still concerns
> that Thibault has pointed out with different nodes using different
> implementations.)
> Good wire protocols are hard, and leaving it up to the experts gives the
> OSRF more developer time for the robotics things that go on top.
> If you still don’t like or want to use DDS, well, ROS is open source! The
> OSRF has abstracted DDS behind a messaging API. You can implement a version
> of the API that uses IPoAC if you like! I would travel to ROSCon just to
> see that talk.
> Geoff
> From: ros-users <ros-users-bounces at lists.ros.org> on behalf of Linas
> Vepstas via ros-users <ros-users at lists.ros.org>
> Reply-To: "linasvepstas at gmail.com" <linasvepstas at gmail.com>, User
> discussions <ros-users at lists.ros.org>
> Date: Sunday, September 27, 2015 at 02:08
> To: Aaron Schiffman <aarondsc at yahoo.com>, User discussions <
> ros-users at lists.ros.org>
> Subject: Re: [ros-users] ROS 2.0 Strategy review
> Hi Aaron,
> Can you clarify? Do you mean "IP of DDS", or IP of something else?  Are
> DDS algos patented?  There used to be talk of zero-mq-based ROS, but that
> seems to have disappeared from the table.
> My knee-jerk reaction is to be a bit suspicious of OMG-created
> technologies; they sound great at first, but are often over-wrought (e.g.
> corba).  I'd never even heard a whisper about DDS before yesterday; I'm
> nervous about adopting a technology that has not yet gained any acceptance
> at all in the open-source community.  So, for example, whatever one's
> opinion of zmq might be, positive or negative, its a "known thing"; many
> people have used it, there is developer experience, a track record.
> There's no such track record for DDS -- the proprietary world seems to be
> the primary consumer of the thing, and their experience with it is secret,
> and not shared. We don't actually know how well it works (although I admit
> it sounds really great, based on the wikipedia article).
> Anyway: please clarify: IP of what? And who "owns" that IP, who has rights
> to it?
> -- Linas.
> On Sat, Sep 26, 2015 at 11:03 AM, Aaron Schiffman via ros-users <
> ros-users at lists.ros.org> wrote:
>> This doesn't feel right sharing my some of my thoughts I held back since
>> Roscon 2014 about ROS 2.0, but here goes:
>> The ip ownership and patent of the underlying ROS 2.0 distributed udp
>> protocol are of concern as a third party protocol implementor. Yes, ROS.org
>> or OSRF may have explicit legal permission to use said protocol, but it is
>> not truly an open/free platform when the public is at the mercy of the IP
>> owner, unless the entire platform is contractually opened up and made free.
>> As a ROS protocol implementor Ive personally held off on implementing ROS
>> 2.0 protocols, while waiting to see how it pans out. I am still of the
>> belief that the UDPROS protocol with enhancements can do everything the new
>> protocol can do, but better. That really doesn't matter now though.
>> I appreciate that osrf took the focus from protocols and put their
>> limited resources to work on tools. In an r&d organization that would be
>> the path I would expect to be the most rewarding, except that I've grown to
>> appreciate think of ROS as a rock that the open robotics universe revolves
>> around. Like I think of Linux, as an open operating system, except that ROS
>> is more an open set of design frameworks like tcpip is a standard protocol
>> with many implementors.
>> Wish I could be there in Hamburg with you all! The birds of a feather
>> meetings, and the couple hours socializing with drinks were the most
>> influential on my development direction this past year. Watching roscon on
>> YouTube just will not be the same.
>> I am so stoked about this upcoming year in Robotics I can hardly contain
>> myself (probably a good reason for me to not be there in October:)
>> God bless Roscon 2015 in Hamburg!
>> Aaron
>> Sent from Yahoo Mail on Android
>> <https://overview.mail.yahoo.com/mobile/?.src=Android>
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