What NM-SCDC-B Is
NM-SCDC-B (Network Management-Secure Connect Direct Connect-B) is the BACnet Interoperability Building Block (BIBB) for the optional node-to-node path in a BACnet Secure Connect network. Normally every BACnet SC message travels through the hub (NM-SCH-B). Direct connect lets two nodes that can reach each other open a WebSocket between themselves and exchange traffic without the hub in the middle.
The connection is still mutually authenticated with certificates and still encrypted with TLS. Direct connect changes the path, not the security model.
Finding the peer is what the Address-Resolution and Address-Resolution-ACK BVLC messages are for: a node asks for the WebSocket URI behind a given VMAC (virtual MAC address), and the answer is what it dials. A node advertises whether it will accept direct connections, initiate them, or both, through the SC_Direct_Connect_Accept_Enable and SC_Direct_Connect_Initiate_Enable properties of its Network Port object. The two are independent.
When Direct Connect Helps
| Scenario | Why the Hub Path Falls Short | What Direct Connect Buys |
|---|---|---|
| High-rate traffic between two specific devices | Every frame transits the hub twice (in and out) | One hop between the peers, lower latency and less hub load |
| Hub hosted off-site or in the cloud | Local device-to-device traffic hairpins through the WAN | Local traffic stays local |
| Bandwidth-sensitive links | Hub fan-out multiplies traffic on constrained paths | Point-to-point exchange only between the two peers |
Direct connect is optional. Many BACnet SC deployments never need it, and a design should treat it as an optimization rather than a requirement.
[!NOTE] Both peers need direct IP reachability for direct connect to work. Nodes separated by NAT or firewall policy that only allows outbound connections to the hub cannot use it, and fall back to the hub path.
What Chipkin Provides
The CAS BACnet Stack implements the direct-connect datalink today, alongside the hub and node roles. What is not yet in place is the evidence: per-function BTL conformance for NM-SCDC-B is unverified. The CAS BACnet Stack 6.x release exercises the BACnet SC BIBBs against the BACnet Testing Laboratory’s published test plans.
Related Pages
- BACnet Secure Connect (BACnet SC)
- NM-SCH-B: Secure Connect Hub
- NM-SCCM-A: Secure Connect Certificate Management
- BACnet
- BACnet SC in the CAS BACnet Stack: what ships today and what 6.x adds