topic discovery works, but no messages transmitted when running rmw-zenoh with mixed ROS2 distros

[berndpfrommer]

I encounter the following strange issue when running inside a docker container (and I'm not sure the docker container is the problem, actually):

There are 3 hosts involved on a fixed wire LAN, where Host A sends messages to Host B and Host C:

Host B <- Host A -> Host C

Host A runs a zenoh router without a docker container, with default config except for:

   endpoints: [
       "tcp/host_b:7447",
       "tcp/host_c:7447"
    ],

and analogously configured are Host B and Host C. The only host running ROS humble inside a docker container is Host C, which is an older Jetson that can not run Humble natively because the OS cannot be upgraded past Ubuntu 20.04.

Running the demo talker on Host A, and the demo listener on Host B works as expected. No problems there.

On Host C however (the Jetson running ROS2 humble inside a docker container), I can query the topics and node lists fine, however no messages are transmitted upon subscription. Host A, for some reason never even sends the messages (checked with tcpdump), although (from experiments with other nodes where I modified the source code) the talker node on Host A does see the subscription occurring, and does call publish().

By tcpdumping the request packets from Host C -> Host A I found a potential problem:

/%chatter/std_msgs::msg::dds_::String_/TypeHashNotSupported/

Whereas the request packets from Host B -> Host A have:

%chatter/std_msgs::msg::dds_::String_/RIHS01_df668c74.....

Is the problem that Humble and Rolling cannot communicate with each other due to different message types? Any other reasons why this is failing, or any suggestions how to fix this?

Quite a bummer if I cannot make this work because all my newer hosts are running on Rolling and Jazzy and even upgrading to the latest Jetson would not allow me to run Jazzy/Rolling since NVidia only support Ubuntu 22.04.

[Yadunund]

Is the problem that Humble and Rolling cannot communicate with each other due to different message types?

In ROS 2, we don't guarantee communication between different distributions. While some middleware might still support it, rmw_zenoh does not. This is particularly true between Humble and later distributions, as we introduced Message Type Hashes starting from Iron. This Type Hash is included in the zenoh keyexpression for topics from Iron onwards, but it's absent in Humble. Consequently, while nodes might be discovered via the liveliness mechanism, they won't be able to communicate due to the difference in keyexpressions, as you've observed.

We added type hashes to keyexpressions to ensure that entities send and receive messages with matching versions. This prevents crashes during deserialization especially when: 1) publisher/subscriber topic names match but message types differ (e.g., #117), or 2) topic names and types match, but message fields differ.

Jetson would not allow me to run Jazzy/Rolling since NVidia only supports Ubuntu 22.04.

Nvidia provides 24.04 ROS 2 Jazzy containers, so you might be able to upgrade depending on your hardware: https://github.com/dusty-nv/jetson-containers/blob/master/docs/build.md#2404-containers. However, if I recall correctly, Jazzy is installed from source, so you'll also need to build rmw_zenoh and its dependencies from source. It's worth trying.

Closing this out since it's a duplicate of #425.

We'd also appreciate a PR to update our README to make this point clear if you have the time!