BACnet.Protocol.BroadcastDistributionTableEntry (bacstack v0.1.0-dev.1)

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A Broadcast Distribution Table Entry describes one peer BBMD (BACnet Broadcast Management Device) or peer network that participates in the distribution of BACnet broadcasts across an IP network. The table as a whole defines the mesh of BBMDs that collectively provide broadcast emulation for an entire site and includes entries to local networks the BBMD should broadcast to.

Each entry contains the IP address and UDP port of the peer together with a subnet mask. The Broadcast Distribution Table is configured either locally or via the Write Broadcast Distribution Table service.

BACnet Specification References

  • Annex J.4.3.2 defines the exact format and the meaning of the broadcast distribution mask (the 4-octet field after the 6-octet B/IP address).
  • J.4.3 and J.4.5 describe how a BBMD uses the BDT to forward broadcasts (two-hop unicast vs. one-hop directed broadcast) and the rule that every BBMD must have an entry for itself.
  • J.7.8 covers NAT: when a NAT router sits between BBMDs the BDT entries must contain the global IP address of the far-end NAT router.

The Broadcast Distribution Mask (J.4.3.2)

The mask controls how the sending BBMD will reach the remote subnet:

  • All-1s mask (255.255.255.255) -> two-hop distribution: the message is sent as a unicast UDP datagram directly to the peer BBMD's B/IP address. The peer BBMD is then responsible for the final local broadcast on its subnet. This method always works (no special router configuration required).
  • Subnet mask of the destination (e.g. 255.255.255.0 for a /24) -> one-hop distribution: the sending BBMD emits a directed broadcast (e.g. 192.168.1.255) that the IP router is expected to forward. Many routers block directed broadcasts by default for security reasons.

All BDT entries that refer to the same remote IP subnet must carry identical masks (normative requirement). The library does not enforce this at runtime (see the TODO comment in the source).

Wire Format

Each BDT entry on the wire is exactly 10 octets: 6-octet B/IP address (IP + port) followed by the 4-octet mask. This is the format both for the Write-Broadcast-Distribution-Table payload and for the Read-BDT-Ack response.

Construction & Examples

iex> alias BACnet.Protocol.BroadcastDistributionTableEntry
iex> e = %BroadcastDistributionTableEntry{
...>   ip: {192, 168, 0, 1},
...>   port: 0xBAC0,
...>   mask: {255, 255, 255, 0}
...> }
iex> BroadcastDistributionTableEntry.encode(e)
{:ok, <<192, 168, 0, 1, 186, 192, 255, 255, 255, 0>>}

The port must be a valid 16-bit integer; the IP and mask fields must be 4-tuples. Invalid data produces {:error, :invalid_data} from encode.

See Also

Summary

Types

t()

An entry in the Broadcast Distribution Table (BDT) used by BBMDs in BACnet/IP.

Functions

decode(data)

Decodes a Broadcast Distribution Table Entry from binary data.

encode(entry)

Encodes the Broadcast Distribution Table Entry to binary data.

from_app_encoding(tags)

Parse a Broadcast Distribution Entry from application tags.

to_app_encoding(entry)

Enocde a Broadcast Distribution Table Entry to application tags.

Types

t()

@type t() :: %BACnet.Protocol.BroadcastDistributionTableEntry{
  ip: :inet.ip4_address(),
  mask: :inet.ip4_address(),
  port: :inet.port_number()
}

An entry in the Broadcast Distribution Table (BDT) used by BBMDs in BACnet/IP.

The mask field determines one-hop (directed broadcast) vs. two-hop (unicast to peer BBMD) distribution per J.4.3.2. See the moduledoc for the exact rules and the requirement that masks for the same subnet must be identical across all BBMDs.

Functions

decode(data)

@spec decode(binary()) :: {:ok, {t(), rest :: binary()}} | {:error, term()}

Decodes a Broadcast Distribution Table Entry from binary data.

encode(entry)

@spec encode(t()) :: {:ok, binary()} | {:error, term()}

Encodes the Broadcast Distribution Table Entry to binary data.

from_app_encoding(tags)

@spec from_app_encoding(BACnet.Protocol.ApplicationTags.encoding_list()) ::
  {:ok, {t(), rest :: BACnet.Protocol.ApplicationTags.encoding_list()}}
  | {:error, term()}

Parse a Broadcast Distribution Entry from application tags.

to_app_encoding(entry)

@spec to_app_encoding(t()) ::
  {:ok, BACnet.Protocol.ApplicationTags.encoding_list()} | {:error, term()}

Enocde a Broadcast Distribution Table Entry to application tags.