What Router-Busy-To-Network Is
Router-Busy-To-Network is a network-layer message in BACnet, defined in Clause 6.4.5 of ANSI/ASHRAE 135-2024. A router broadcasts it to tell other devices to stop sending it traffic for specific destination networks, or for all networks, while it is congested. It is carried in an NPDU (Network Protocol Data Unit, the network-layer packet) rather than as an application service, so it is unacknowledged. It is one half of the network layer’s congestion control, paired with Router-Available-To-Network, which lifts the restriction.
Like the other network-layer messages, it operates below the object and property layer. It manages the flow of routed packets, not object data.
How It Works
The message is always broadcast, and optionally carries a list of 2-octet destination network numbers. When it lists networks, senders should stop routing traffic to those networks through this router; when the list is absent, the router is asking senders to hold traffic for all networks it serves. The condition is temporary: the router later broadcasts Router-Available-To-Network to re-enable traffic. It is a request to back off, letting a router recover from a burst without dropping messages wholesale.
Where You’ll See It
- Congestion relief. A router under a heavy traffic burst broadcasts it to slow inbound messages for the affected networks until it catches up.
- Slow downstream segments. When a destination network is a slow MS/TP (Master-Slave/Token-Passing, BACnet’s RS-485 serial transport) segment, the router uses it to keep its queue for that segment from overflowing.
- Diagnostics. Seeing it repeatedly for a network points to a chronic bottleneck at that router or on the downstream segment.
Router-Busy-To-Network Error Codes
Router-Busy-To-Network is an unacknowledged network-layer message, so it has no application-layer error response. It is a flow-control signal, not an error. What to keep in mind:
- It is temporary: traffic for the named networks should resume only after the router broadcasts Router-Available-To-Network.
- A sender that ignores it and keeps sending may see its messages dropped or rejected while the router is busy.
- Frequent busy messages indicate a capacity problem to fix at the router or segment, not a transient to ignore.
Profiles That Require It
The network-layer messages are the router’s job. Router-Busy-To-Network is part of the router configuration capability NM-RC-B, which is required of the router profile B-RTR. Non-routing devices react to the message by holding traffic but do not originate it.
What Chipkin Provides
The CAS BACnet Stack supports the router congestion-control messages, including Router-Busy-To-Network and its Router-Available-To-Network counterpart. Talk to us about adding BACnet client or server support to your device.