What Router-Available-To-Network Is
Router-Available-To-Network is a network-layer message in BACnet, defined in Clause 6.4.6 of ANSI/ASHRAE 135-2024. A router broadcasts it to tell other devices they may resume sending it traffic for specific destination networks, or for all networks. 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 the counterpart to Router-Busy-To-Network: busy asks senders to hold, available lifts the hold.
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 may resume routing traffic to those networks through this router; when the list is absent, the router is re-enabling traffic for all networks it serves. A router sends it after a prior Router-Busy-To-Network once its congestion has cleared, so normal routing can continue.
Where You’ll See It
- Recovery after congestion. A router that curtailed traffic with a busy message broadcasts this once its queue drains, so senders resume.
- Selective re-enable. A router that slowed traffic for one network re-enables just that network by listing it, leaving any others still curtailed.
- Diagnostics. Pairing each busy message with the available message that follows shows how long a router was shedding traffic and for which networks.
Router-Available-To-Network Error Codes
Router-Available-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 clears a prior Router-Busy-To-Network; a sender that never saw the busy message simply treats the network as available all along.
- If a router restarts, senders may resume traffic without ever receiving an explicit available message, since routing defaults to enabled.
- A missed available message can leave a sender holding traffic longer than needed; the router re-broadcasts as required.
Profiles That Require It
The network-layer messages are the router’s job. Router-Available-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 it by resuming traffic but do not originate it.
What Chipkin Provides
The CAS BACnet Stack supports the router congestion-control messages, including Router-Available-To-Network and its Router-Busy-To-Network counterpart. Talk to us about adding BACnet client or server support to your device.