Port 2873: PSRT — Industrial Pub/Sub for the Real World

What This Port Does

Port 2873 is the default port for PSRT — the PubSub Realtime Telemetry Protocol — registered with IANA by Altertech in November 2021.¹ It operates on both TCP and UDP.

PSRT is a publish-subscribe messaging protocol built for industrial IoT environments where the network is not your friend: slow WAN links, intermittent connections, large payloads, and clients that need to authenticate before they can say anything. It was designed by Bohemia Automation / Altertech, a group with over 15 years in enterprise automation and industrial infrastructure — power plants, factories, and urban systems running a million-plus sensors.²

The Problem It Solves

MQTT is the standard protocol for IoT pub/sub messaging. It works well on local networks and clean connections. It was not designed for the wide-area networks that connect real industrial equipment across cities or across a factory campus where the link is unreliable by default.

PSRT fills that gap. It handles the same logical model as MQTT — topics, subscriptions, publish/subscribe patterns — but is built to survive the conditions MQTT quietly assumes away. It processes over 100,000 messages per node with latencies under one millisecond, uses B-tree algorithms for subscription handling so it doesn't slow down as subscription counts climb, and remains performant even with payloads exceeding 1MB.³

The code that runs on PSRT can switch to MQTT (and back) with only a few lines of change. They're logically compatible — PSRT just doesn't pretend the network is reliable.

Who Uses It

PSRT is the messaging layer for EVA ICS v4 — an industrial automation platform built by the same team. If you encounter port 2873 in the wild, it's most likely a PSRT server running as part of an industrial IoT or automation deployment, not consumer software.

A Note on Older References

Some port databases reference port 2873 under the name "PASPAR2 ZoomIn." This appears to be an older, informal annotation from security scanner databases (notably SANS) that predates the formal IANA registration. There is no documented protocol by that name. The current official assignment is PSRT.

What Range This Port Falls In

Port 2873 is in the registered ports range (1024–49151). These ports are not hardcoded into operating systems like well-known ports (0–1023), but they are formally registered with IANA so that applications can claim a home. Registration means Altertech has told IANA "this is where PSRT lives" — it doesn't mean your system runs PSRT by default.

Checking What's on This Port

If you see traffic on port 2873 and want to know what's listening:

# Linux/macOS — show what process is using port 2873
ss -tlnp | grep 2873
lsof -i :2873

# Windows
netstat -ano | findstr :2873

If you're not running industrial automation software and something is listening on port 2873, it's worth investigating — but there's nothing inherently suspicious about this port. It's not a common target for exploitation.