[SOLVED]No downlink

[rpct]

I'm working with several Lopy. Some are configured as nodes, one is configured as single channel gateway. I'm on TTN.
Firmware info is :
nodename='LoPy', release='1.14.0.b1', version='v1.8.6-849-86da809 on 2018-01-17', machine='LoPy with ESP32', lorawan='1.0.0'

I cannot join TTN : I can see in TTN console join request is received by my gateway, join accept is sent to node. But nodes does not join.
So I tried ABP activation in order to check if it was a donwlink or a join problem. In ABP uplinks packets are received but downlink packets are never received. I can see from TTN console it is sent.
Here is node code :

s = socket.socket(socket.AF_LORA, socket.SOCK_RAW)

# set the LoRaWAN data rate
s.setsockopt(socket.SOL_LORA, socket.SO_DR, config.LORA_NODE_DR)

# make the socket blocking
s.setblocking(False)

time.sleep(1.0)

i = 0
while True :
    pkt = b'PKT #' + bytes([i])
    print('Sending:', pkt)
    s.send(pkt)
    i += 1
    #time.sleep(4)
    rx, port = s.recvfrom(256)
    if rx:
        print('Received: {}, on port: {}'.format(rx, port))
    time.sleep(4)
time.sleep(6)

SOLUTION :
TTN gateway <-> TTN server communication is done using TCP with TTN server port 1700 for nanogateway using Semtech packet forwarder. On my network incoming UDP traffic is blocked. I have to add a rule to allow UDP traffic from TTN server.
More info about this issue here