[ros-users] Generic message transport infrastructure

[Cedric Pradalier]

After a long time since my first post about a generic message transport 
architecture, we finally released it in the ETHZ-ASL message transport 
stack.
Details about the ethzasl_message_transport stack can be found at 
http://www.ros.org/wiki/ethzasl_message_transport

In addition to the framework, we also implemented a shared-memory 
transport plugin and a udp multicasting transport plugin, and 
instantiated the transport plugins for images and point-clouds.

The shared memory transport will not offer the same performance as using 
nodelets, but it can be an option when very large object needs to be 
passed around while keeping different processes (there have been several 
discussions on this topic on this list).

The UDP multicasting transport is interesting when small objects needs 
to be passed very quickly to many receivers on several machines (in 
comparison, normal tcp transmission will incur a delay increasing with 
the number of subscribers). Note that the plugin does not manage object 
fragmentation, and does not intend to do it (it would defeat the 
purpose). This means that it will refuse to send object bigger than 8092 
bytes.

Other type-specific transport plugins can quickly be implemented using 
this framework. As examples, I've adapted the theora and compressed 
image transport, and implemented a decimated point cloud transport.

Ideally, the udp multicasting stuff (and shm) should go into the core of 
ROS, but I have not understood enough of the xmlrpc negociation to 
implement it myself.

HTH

-- 
Dr. Cedric Pradalier
http://www.asl.ethz.ch/people/cedricp