We are trying to design the architecture between our LNS on premise and our cumulocity SaaS. We can find some documentation about the interconnection through MQTT protocol for both uplink and downlink but on HTTP it is not clear to me. 

Do we have an uplink on HTTP from LNS to the Server for uplink or is it done by long polling from Cumulocity to the LNS ?

In other sens is the connection always initiated by the server or the LNS do need to intiate it for uplink?