Port 355 carries DATEX-ASN (Data Exchange - Abstract Syntax Notation), the protocol that connects traffic management centers across regions and countries. When a highway accident happens in one jurisdiction and the neighboring traffic center needs to know about it, when travel times need to be coordinated across state lines, when road condition data flows between systems—that communication flows through port 355.
This isn't a port most people will ever interact with directly. It's infrastructure talking to infrastructure.
What DATEX-ASN Does
DATEX-ASN defines how traffic management centers package and exchange information. The protocol specifies the format for messages about:
- Traffic incidents and accidents
- Road conditions and closures
- Travel times and delays
- Variable message sign content
- Weather-related road information
- Planned roadwork and events
The protocol uses ASN.1 (Abstract Syntax Notation One) to encode messages and exchanges them over TCP/UDP on port 355.1 It's part of the ISO 14827 standard for "Data interfaces between centres for transport information and control systems."2
The History: Why Traffic Centers Needed to Talk
In the 1990s, European traffic management centers faced a problem: they each had their own systems, their own data formats, their own ways of describing what was happening on the roads. When an incident near a border affected traffic in multiple jurisdictions, there was no standardized way to share that information.
A European task force was established to solve this. In October 1997, they formalized the Data Exchange Memorandum of Understanding (DATEX MoU), creating a framework for international exchange of traffic data.3
The original DATEX specifications had issues. They couldn't achieve true "plug and play" interoperability between different manufacturers' systems. So in late 2003, work began on DATEX II, a complete redesign that addressed the interoperability problems and updated the technology.4 The European Standard was issued by CEN in 2018.
ISO 14827-2, which defines DATEX-ASN for port 355, was first published in 2005 and updated in 2022 as AP-DATEX (Application Profile-Data Exchange).5
How It Works
DATEX-ASN operates at the application layer. Traffic management centers establish connections on port 355 (both TCP for reliable delivery and UDP for faster, connectionless transmission) to exchange formatted messages.
The protocol defines:
- How messages are structured using ASN.1 encoding
- Rules for requesting and receiving traffic data ("subscriptions" and "publications")
- Procedures for maintaining data exchange sessions between centers
A traffic center might subscribe to incident reports from neighboring regions, then publish that data to traveler information systems, navigation services, or variable message signs on highways.
Security Considerations
Port 355 is a System Port (0-1023), which means it requires elevated privileges to bind to on Unix-like systems. Traffic management systems using DATEX-ASN should:
- Restrict access to port 355 to authorized traffic management systems only
- Use firewalls to prevent unauthorized connections
- Implement authentication between centers exchanging data
- Encrypt sensitive traffic information when transmitted across public networks
Historical malware note: Port 355 has been flagged in security databases as having been used by trojans or viruses in the past.6 This doesn't mean the protocol itself is vulnerable—it means malware has occasionally used this port for communication. Any unusual traffic on port 355 from non-traffic-management sources should be investigated.
Checking What's Listening
To see if anything is listening on port 355 on your system:
Linux/Mac:
Windows:
Unless you're running traffic management center software, you shouldn't see anything listening on this port. If you do see unexpected activity, investigate—it could be malware masquerading on this port.
Related Ports
- Port 5355: Link-Local Multicast Name Resolution (LLMNR), completely different protocol despite similar number
- Ports 1024-49151: Registered ports where many other transportation and logistics protocols operate
Why This Port Matters
Port 355 represents a category of infrastructure most people never think about: the coordination layer that makes intelligent transportation systems work. Traffic doesn't just flow—it's managed, monitored, and coordinated across jurisdictions.
When you see a highway sign warning you about an accident five miles ahead, when your GPS reroutes you around unexpected congestion, when traffic lights adjust timing based on incident data—somewhere in that chain, traffic centers exchanged data. Often through port 355.
The protocol isn't glamorous. It doesn't have millions of users. But it's part of the invisible nervous system that keeps traffic moving, that coordinates emergency responses, that prevents pile-ups by getting information where it needs to go.
Frequently Asked Questions
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