op: Operation type, for example subscribe, unsubscribe, or ping
data (optional): Parameters specific to an operation
id (optional): The request ID, an arbitrary integer passed to link requests with responses
data: The JSON payload of the response or the channel message
error (optional): The error message returned if an error occurred
id (optional): The ID of the corresponding request, if it is provided
channel (optional): The channel for the sent message, only added to the response when there is a response on the channel