What is SNTP?
SNTP (Simple Network Time Protocol) is a simplified version of the NTP protocol and is used to synchronise time between network devices. Unlike full NTP, SNTP offers lower precision and no hierarchical time structure, but is lightweight and easy to implement.
In OT networks, SNTP is often used for time synchronisation of PLC, HMI, SCADA, Managed Switches, and logging systems where millisecond precision is not required.
🧠 The purpose of time synchronisation in OT
- Correct timestamps on alarms, logs, historical data, and events
- Incident analysis (root cause, sequence of events)
- Coordination between different systems (such as batch processes and data logging)
- Compliance with standards such as IEC 62443, GxP, ISO 27001
- Time-based access and security rules (e.g. log retention, time-based access)
🔧 How SNTP works
| Step | Description |
|---|---|
| 1. SNTP client sends a request | To a time server (internal or external) |
| 2. Server responds | With current UTC time + delay |
| 3. Client sets the clock | Locally or at system level, depending on OS/function |
| 4. Periodic updates | The clock is regularly synchronised |
SNTP usually uses port UDP 123 and is compatible with most NTP servers.
🔁 SNTP vs. NTP vs. PTP
| Protocol | Precision | Complexity | Application in OT |
|---|---|---|---|
| SNTP | ~100–500 ms | Low | Basic logic, switches, simple devices |
| NTP | ~1–10 ms | Medium | Historian, SCADA, servers, domain controllers |
| PTP (IEEE 1588) | <1 µs | High | Motion control, precise synchronisation (e.g. drives) |
✅ When to use SNTP?
- Managed Switches or field devices without a full NTP client
- Situations requiring only time consistency (not absolute precision)
- Backup or fallback time source if PTP or NTP fails
- In systems where low load, simplicity, and stability matter more than sub-millisecond precision
🛠️ Best practices
| Measure | Description |
|---|---|
| Use an internal time server | For example on your SCADA server or Domain Controller |
| Segment SNTP traffic | Via VLAN or firewall rules to prevent misuse |
| Combine with NTP/chrony | Some devices support both protocols |
| Monitor for time drift | Alarm on deviation or synchronisation error |
| Redundant SNTP servers | Configure at least two time sources in critical environments |
📌 In summary
SNTP is a simple, reliable method for providing devices in OT networks with consistent time. It is ideal for systems where stability and compatibility matter more than precision.