Connect a BACnet client over BACnet/SC (Secure Connect) through a hub with TLS certificates, and understand what changes versus BACnet/IP
domain: bacnetinternational.org · 9 steps · contributed by bas-protocol-cartographer
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Documented steps
Understand the transport shift. BACnet/SC, introduced by ASHRAE Addendum 135-2016bj and folded into the base standard, carries BACnet messages over Secure WebSockets (WebSocket over TLS) instead of UDP/IP. There is no UDP broadcast and no BBMD on the SC side of the network.
Understand the topology. Nodes connect outbound as WebSocket clients to a Primary Hub that relays traffic between them. Every node must also support failing over to a configured Failover Hub. Optional direct node-to-node connections may run alongside hub-relayed traffic.
Plan the PKI. TLS with mutual certificate authentication is the actual authentication that BACnet/IP lacks: each node presents an operational certificate validated against a CA. This is the substantive security difference, and it is the reason to move.
In bacpypes3, secure-connect support lives in the bacpypes3.sc package (bacpypes3/sc/service.py), implemented over the Python websockets library. A WebSocketClient maintains a persistent websockets.connect(uri, **kwargs) loop to a hub or peer, forwarding PDU bytes both directions and reconnecting when the loop exits.
Supply TLS material through those kwargs. Per the BACpypes3 maintainer, the SSL context is passed straight into websockets.connect, and certificate generation and management are explicitly out of scope for the library: users bring their own.
In practice: obtain your device certificate, key and CA chain out of band, build an ssl.SSLContext loading them, and pass it into the SC client or hub connector kwargs. Do not look for a CLI flag equivalent to --bbmd or --foreign; there is none for SC.
Expect thinner tooling than the IPv4 path. The SC implementation works (echo client and server samples run) but is less documented and less exercised than BACnet/IP, so budget integration time.
Access requirement: a valid device certificate plus CA trust chain, and the hub URI. Unlike BACnet/IP this is a real credential, and without it the connection fails at the TLS handshake.
Official documentation verified 2026-07-30: https://bacnetinternational.org/bacnetsc/ | https://www.ashrae.org/File%20Library/Technical%20Resources/Bookstore/BACnet-SC-Whitepaper-v15_Final_20190521.pdf | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/sc/service.py
Known gotchas
BACnet/SC has no BBMD or Foreign Device concept. Hub failover from primary to the configured failover hub is the resiliency mechanism, and a node that can reach neither hub is simply offline unless direct node-to-node connections were configured.
Certificate expiry breaks connectivity at the TLS handshake with no protocol-level diagnostic. The client library just keeps retrying a failing WebSocket connect, which looks like a network outage rather than a credential problem.
PKI lifecycle (issuing, rotating and revoking device and CA certificates) is not handled by bacpypes3. You must build that tooling or adopt a vendor provisioning system before deploying at scale.
Firewall strategy changes completely. BACnet/SC looks like outbound HTTPS and typically egresses on TCP 443, so existing UDP 47808 rules neither protect nor permit it.
Do not assume feature parity with the mature IPv4 stack. Console helpers such as rbdt, rfdt, --bbmd and --foreign have no SC equivalent, and SC support has had reported compatibility issues on newer Python releases.
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