Just for future reference, LoRaWAN Network Server Demonstration: Gateway to Server Interface Definition:
5.2.3 PULL_DATA message
The PULL_DATA messages are periodically transmitted to the LoRa network server in order to inform the server of the UDP port number to which the network server should send any PULL_RESP message.
The PULL_DATA message also keeps open any firewall that protects the LoRa gateway.