Proxy Service¶
The proxy functionality provides a mechanism for a device to participate in a BACnet intranet that cannot use connectionless protocols in peer-to-peer or broadcast exchanges of PDU’s, either because it is impossible, due to firewall restrictions, or impractical, perhaps due to the complexity of configuring all of the intermediate networking equipment.
A proxy server is conceptually like a “port extender.” It has a BACnet address on an interface on a BACnet network, and similar to a router, knows the network number of its interface. As a BSLL server it provides a listener for connected clients. For example, it may have UDP port 47808 open and additionally be listening for clients on TCP port 47808.
After authentication, if the server does not provide the Proxy Service then it will send a Service Request Result with the error code NO_PROXY_SERVICE.
After a proxy client initiates a session with a server, the server will forward all PDU’s from the client to network and vice versa. BACnet functionality is provided by client. A proxy server does not provide any BACnet services without a connected proxy client.
There are two types of proxy servers; those that are limited to supporting a single client, and those that support multiple clients.
Single Client Proxy Service¶
A proxy server that is limited to supporting a single client forwards all BACnet traffic that it receives to the client and vice versa. It may additionally cache network topology information, such as the addresses routers and their connected networks to facilitate path discovery on behalf of clients.
Multiple Client Proxy Service¶
In addition to caching network topology information like a single client proxy server, a proxy server that supports multiple clients may provide additional functionality for processing application layer confirmed services on behalf of connected clients. For example, it may listen for unicast confirmed service requests from clients and forward the related unicast responses to the requesting client without also forwarding it to other clients.
All broadcast messages that are received by the server are forwarded to all connected clients.
Note that unlike Local Area Network Emulation Service the connected clients to a proxy service cannot exchange packets between themselves, nor are they made aware of the presence or absence of other connected clients. Only one connected client provides BACnet server functionality to the BACnet intranet, responding to Who-Is requests for example.
Protocol Data Units¶
Packets from the proxy server to the connected clients will have the SNET/SLEN/SADR fields present. The DNET/DLEN/DADR fields may also be present if the packet was broadcast, so the DLEN will always be zero and the DADR empty.
Packets from a proxy client to its connected server will have the DNET/DLEN/DADR fields present.
Proxy Client to Server Unicast Request¶
This packet is sent by a client to the server to request that the server send the request to a BACnet address on behalf of the client. The request payload is restricted to unicast messages.
The DNET/DLEN/DADR fields of the NPCI will be present, even if the destination is on the same BACnet network as the proxy server.
| Field | Size | Description |
|---|---|---|
| BSLV Type | 1-octet | Streaming BACnet (value = X‘82’) |
| BSLV Function | 1-octet | Proxy Client to Server Unicast Request (value = X‘06’) |
| BSLV Length | 2-octets | Length |
| Payload | variable | Application PDU |
Proxy Client to Server Broadcast Request¶
This packet is sent by a client to the server to request that the server broadcast the request on the local network in the case of local broadcast or global broadcast messages, or in the case of a remote network broadcast, send the request as a unicast message to the router responsible for the destination network. The request payload is restricted to broadcast messages.
The DNET/DLEN/DADR fields of the NPCI will be present. If the destination is to be considered a local broadcast by the proxy server, then the DNET will be the network of the proxy server. In all cases the DLEN will be zero and the DADR will be the empty octet string.
| Field | Size | Description |
|---|---|---|
| BSLV Type | 1-octet | Streaming BACnet (value = X‘82’) |
| BSLV Function | 1-octet | Proxy Client to Server Broadcast Request (value = X‘07’) |
| BSLV Length | 2-octets | Length |
| Payload | variable | Application PDU |
Proxy Server to Client Unicast Request¶
This packet is a forwarded copy of a unicast message received by the server to the client. The request payload is restricted to unicast messages.
The SNET/SLEN/SADR fields of the NPCI will be present, even if the destination is on the same BACnet network as the proxy server. The DNET/DLEN/DADR fields of the NPCI will be absent.
| Field | Size | Description |
|---|---|---|
| BSLV Type | 1-octet | Streaming BACnet (value = X‘82’) |
| BSLV Function | 1-octet | Proxy Server to Client Unicast Request (value = X‘08’) |
| BSLV Length | 2-octets | Length |
| Payload | variable | Application PDU |
Proxy Server to Client Broadcast Request¶
This packet is a forwarded copy of a broadcast message received by the server to the client. The request payload is restricted to broadcast messages.
The SNET/SLEN/SADR fields of the NPCI will be present, even if the destination is on the same BACnet network as the proxy server.
The DNET/DLEN/DADR fields of the NPCI will be present. If the destination was a local broadcast when it was received by the proxy server, then the DNET will be the network of the proxy server. In all cases the DLEN will be zero and the DADR will be the empty octet string.
| Field | Size | Description |
|---|---|---|
| BSLV Type | 1-octet | Streaming BACnet (value = X‘82’) |
| BSLV Function | 1-octet | Proxy Server to Client Broadcast Request (value = X‘09’) |
| BSLV Length | 2-octets | Length |
| Payload | variable | Application PDU |