Streaming BACnet¶
Rationale¶
This addenda is a specification for sending and receiving PDU’s between networking entities when they use a connection-oriented protocol.
This specification is not restricted to a specific protocol such as TCP, nor does it preclude the use of protocols that also provide security services such as SSL.
Note
I have been using BACnet/TCP to refer to Streaming BACnet over TCP connections to differentiate it from BACnet/IP which is UDP. I expect servers to be listening on port 47808.
This design follows the model of the BACnet Virtual Link Layer established in Annex J. Connection oriented protocols provide a continuous stream of content between endpoints and there is no proscribed boundary between packets, so a BACnet Streaming Link Layer (BSLL) header is used to divide the stream into PDU’s (this is considered a presentation layer function in the OSI model).
BSLL Protocol Data Units¶
The BSLL header has a one octet type field, a one octet function field, and two octet length field. The maximum data length to be 65531 octets. To distinguish these BSLL packets from those in Annex J, the type field contains X’82’.
| Field | Size | Description |
|---|---|---|
| BSLV Type | 1-octet | Streaming BACnet (value = X‘82’) |
| BSLV Function | 1-octet | Function |
| BSLV Length | 2-octets | Length L, in octets, of the BSLL message and its contents |
| Payload | variable | Dependent on the function |
Network Entities¶
There are a three network entities in BACnet; devices, routers, and BBMDs. Similar to the way a physical BACnet devices can be any combination of the three types, for example a device can also be a router, the new network entities in this specification can be combined together.
Packets from any of the services may be used in a connection, for example, routers typically have a device object and are therefore considered to be a device as well as a router. If a device is connected to a router that has a device object, it is not necessary to establish a separate connection for device-to-device traffic to separate it from device-to-router traffic. It is, however, required that the client request the connection be used for the service.