The ReverseHello Message has the additional fields shown in Table 72.
Table 72 – OPC UA Connection Protocol ReverseHello Message
|
Name |
Data Type |
Description |
|
ServerUri |
String |
The ApplicationUri of the Server which sent the Message. The encoded value shall be less than 4 096 bytes. Client shall return a Bad_TcpEndpointUrlInvalid error and close the connection if the length exceeds 4 096 or if it does not recognize the Server identified by the URI. |
|
EndpointUrl |
String |
The URL of the Endpoint which the Client uses when establishing the SecureChannel. This value shall be passed back to the Server in the Hello Message. The encoded value shall be less than 4 096 bytes. Clients shall return a Bad_TcpEndpointUrlInvalid error and close the connection if the length exceeds 4 096 or if it does not recognize the resource identified by the URL. This value is a unique identifier for the Server which the Client may use to look up configuration information. It should be one of the URLs returned by the GetEndpoints Service. |
For connection-based protocols, such as TCP, the ReverseHello Message allows Servers behind firewalls with no open ports to connect to a Client and request that the Client establish a SecureChannel using the socket created by the Server.
For message-based protocols the ReverseHello Message allows Servers to announce their presence to a Client. In this scenario, the EndpointUrl specifies the Server’s specific address and any tokens required to access it.