ROS 2 rmw_zenoh Interoperability

This document describes how nros communicates with standard ROS 2 nodes using rmw_zenoh_cpp.

Status: WORKING

nros <-> ROS 2 rmw_zenoh communication is fully operational as of January 2025.

Architecture

┌─────────────────────────┐          ┌─────────────────────────┐
│   ROS 2 Node            │          │   nros Node             │
│   (rmw_zenoh_cpp)       │<-------->│   (zenoh-pico)          │
│                         │  Zenoh   │                         │
│   ros2 topic echo       │  Router  │   native-rs-talker      │
│   /chatter              │ (zenohd) │                         │
└─────────────────────────┘          └─────────────────────────┘

Both nodes connect to the same Zenoh router (zenohd) or communicate directly in peer mode.

Quick Start

# Terminal 1: Start zenoh router
zenohd --listen tcp/127.0.0.1:7447

# Terminal 2: Run nros talker
cargo run -p native-rs-talker --features zenoh -- --tcp 127.0.0.1:7447

# Terminal 3: Run ROS 2 listener
source /opt/ros/humble/setup.bash
export RMW_IMPLEMENTATION=rmw_zenoh_cpp
export ZENOH_CONFIG_OVERRIDE='mode="client";connect/endpoints=["tcp/127.0.0.1:7447"]'
ros2 topic echo /chatter std_msgs/msg/Int32 --qos-reliability best_effort

Protocol Requirements

1. Data Topic Key Expression Format

For ROS 2 Humble, the data key expression format is:

<domain_id>/<topic_name>/<type_name>/<type_hash>

Important: For Humble, use TypeHashNotSupported as the type hash (no RIHS01_ prefix).

Example:

0/chatter/std_msgs::msg::dds_::Int32_/TypeHashNotSupported

Note: Newer ROS 2 versions (Iron+) use RIHS01_<hash> format with actual type hashes.

Implementation: TopicInfo::to_key() in the transport traits module

2. Liveliness Token Format

rmw_zenoh uses Zenoh liveliness tokens for discovery. Without these, ros2 topic list won’t show nros topics.

Node Liveliness Token:

@ros2_lv/<domain>/<zid>/0/0/NN/%/%/<node_name>

Publisher Liveliness Token:

@ros2_lv/<domain>/<zid>/0/11/MP/%/%/<node_name>/<topic>/<type>/RIHS01_<hash>/<qos>

Subscriber Liveliness Token:

@ros2_lv/<domain>/<zid>/0/11/MS/%/%/<node_name>/<topic>/<type>/RIHS01_<hash>/<qos>

Key differences from data keyexprs:

• Liveliness tokens do use RIHS01_ prefix
• Topic names use % instead of / (e.g., %chatter not /chatter)
• ZenohId must be in LSB-first hex format

Implementation: Ros2Liveliness in the transport/zenoh module

3. QoS String Format

The QoS portion of liveliness tokens uses this format:

reliability:durability:history,depth:deadline_sec,deadline_nsec:lifespan_sec,lifespan_nsec:liveliness,liveliness_sec,liveliness_nsec

Values:

• Reliability: 1=RELIABLE, 2=BEST_EFFORT
• Durability: 1=TRANSIENT_LOCAL, 2=VOLATILE
• History: 1=KEEP_LAST

For BEST_EFFORT/VOLATILE with KEEP_LAST depth 1:

2:2:1,1:,:,:,,

Important: Empty values default to RELIABLE which causes QoS mismatch. Always specify explicit values.

4. CDR Message Format

Messages use CDR (Common Data Representation) little-endian encoding:

[0x00, 0x01, 0x00, 0x00]  // CDR encapsulation header (LE)
[... CDR payload ...]     // Message data

Implementation: the serdes crate

5. RMW Attachment Format

rmw_zenoh requires metadata attached to each published message. For Humble, the serialization format is:

Total: 33 bytes

The serialization uses Zenoh’s ze_serializer to ensure compatibility with zenoh-cpp’s deserializer.

Implementation: serialize_rmw_attachment() in zenoh-pico/src/serializer.rs

6. ZenohId Format

The ZenohId in liveliness tokens must be formatted in LSB-first (little-endian) hex format.

#![allow(unused)]
fn main() {
// Correct: LSB-first
fn to_hex_string(&self) -> String {
    let mut hex = String::new();
    for byte in self.bytes.iter() {  // LSB first
        write!(&mut hex, "{:02x}", byte).ok();
    }
    hex
}
}

Common Issues and Solutions

Issue: Discovery works but no messages received

Symptom: ros2 topic list shows the topic, ros2 topic info shows correct QoS, but ros2 topic echo receives nothing.

Cause: Data keyexpr format mismatch.

Solution: For Humble, don’t use RIHS01_ prefix in data keyexprs:

• Wrong: 0/chatter/std_msgs::msg::dds_::Int32_/RIHS01_TypeHashNotSupported
• Correct: 0/chatter/std_msgs::msg::dds_::Int32_/TypeHashNotSupported

Issue: Topic not visible in ros2 topic list

Symptom: nros publishes but ROS 2 doesn’t see the topic.

Cause: Liveliness token format incorrect.

Solutions:

1. Check ZenohId is LSB-first hex format
2. Ensure topic name uses % prefix (e.g., %chatter)
3. Verify QoS string has explicit values, not defaults

Issue: QoS incompatibility warnings

Symptom: ROS 2 logs QoS incompatibility or subscriber doesn’t receive.

Cause: Publisher and subscriber QoS don’t match.

Solution: Use BEST_EFFORT for both:

ros2 topic echo /chatter std_msgs/msg/Int32 --qos-reliability best_effort

Issue: rmw_zenoh not connecting to router

Symptom: rmw_zenoh uses peer mode by default, not connecting to zenohd.

Solution: Force client mode:

export ZENOH_CONFIG_OVERRIDE='mode="client";connect/endpoints=["tcp/127.0.0.1:7447"]'

Implementation Status

Version Compatibility

References

• rmw_zenoh design docs
• Zenoh serialization RFC
• zenoh-pico