@sp193 I did some more investigation after your message…
You are right with “continuous transmitting”.
For Class-C, RX2 downlink - SF12 is used with 125kHz bandwidth.
For a 50 bytes payload it has ~2.8 sec time on air according to calculator.
It is also using only a single downlink channel where 10% duty cycle is allowed (360 out of 3600 seconds / hour).
Im not sure yet about Kerlink duty cycle restrictions (if its there it should start limitting ~120 messages / hour)
When it comes to "Chirpstack doesn’t automatically retry sending messages"
I can’t confirm that …
See the logs below from gateway packet forwarder.
These are logs after enqueuing 10 unconfirmed messages for 10 different devices on chirpstack concurrently (1 message per device).
between 15:26:05
and 15:26:07
chirpstack sent eu868/gateway/EUI/command/down
(causing Sending PULL_RESP
log) 10 times while getting 1 TX_ACK without an error.
Starting from 15:26:11.383
we are getting more and more PULL_RESP
(chirpstack retries because there was a TX_ACK only for one message).
As a result we are getting TX_ACK every ~3 seconds because this is how fast the radio can transmit and much more PULL_RESP
because Chirpstack really wants the message to be sent.
From what I observe every PULL_RESP
is queued and sent over the radio causing more and more delays and the snowball effect which I described earlier.
My general idea is to use RX1 instead of RX2 but its also a bit painful as I can’t enqeue the message earier, tell Chirpstack to wait for the Uplink and use RX1. What I actually have to do is wait for the uplink and fit into restrictive get_downlink_data_delay="100ms"
with my message.
I can see at least 2 possible improvements in chirpstack:
1. Not to flood gateway with messages as its not possible to send them in that rate using RX2
2. Prepare separate queues for RX1 and RX2 messages when class C is used.
It would be nice if someone from Chirpstack developers could confirm my observations and thoughts 
2025-03-14T15:26:05.236: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:05.236: Received downlink command, downlink_id: 1082741182, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:05.236: Sending PULL_RESP, random_token: 20926, remote: 127.0.0.1:59059
2025-03-14T15:26:06.226: TX_ACK received, random_token: 20926, remote: 127.0.0.1:59059, error: NONE
2025-03-14T15:26:06.226: Sending ack event, downlink_id: 1082741182, topic: eu868/gateway/7076ff00560908fb/event/ack
2025-03-14T15:26:06.338: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.338: Received downlink command, downlink_id: 526071507, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.338: Sending PULL_RESP, random_token: 14035, remote: 127.0.0.1:59059
2025-03-14T15:26:06.339: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.339: Received downlink command, downlink_id: 623895658, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.339: Sending PULL_RESP, random_token: 58474, remote: 127.0.0.1:59059
2025-03-14T15:26:06.339: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.339: Received downlink command, downlink_id: 1675975495, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.340: Sending PULL_RESP, random_token: 23367, remote: 127.0.0.1:59059
2025-03-14T15:26:06.340: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.340: Received downlink command, downlink_id: 3607647443, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.345: Sending PULL_RESP, random_token: 21715, remote: 127.0.0.1:59059
2025-03-14T15:26:06.345: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.348: Received downlink command, downlink_id: 45049201, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.349: Sending PULL_RESP, random_token: 25969, remote: 127.0.0.1:59059
2025-03-14T15:26:06.352: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:06.353: Received downlink command, downlink_id: 1178251610, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:06.354: Sending PULL_RESP, random_token: 45402, remote: 127.0.0.1:59059
2025-03-14T15:26:07.319: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:07.319: Received downlink command, downlink_id: 3870859697, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:07.320: Sending PULL_RESP, random_token: 41393, remote: 127.0.0.1:59059
2025-03-14T15:26:07.320: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:07.320: Received downlink command, downlink_id: 87061580, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:07.320: Sending PULL_RESP, random_token: 29772, remote: 127.0.0.1:59059
2025-03-14T15:26:07.320: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:07.321: Received downlink command, downlink_id: 555077538, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:07.321: Sending PULL_RESP, random_token: 53154, remote: 127.0.0.1:59059
2025-03-14T15:26:07.327: TX_ACK received, random_token: 14035, remote: 127.0.0.1:59059, error: UNKNOWN
// 10x PULL_RESP done and only 2 TX_ACK (one with error: UNKNOWN)
2025-03-14T15:26:07.327: Sending ack event, downlink_id: 526071507, topic: eu868/gateway/7076ff00560908fb/event/ack
2025-03-14T15:26:09.102: PUSH_DATA received, random_token: 998, remote: 127.0.0.1:55712
2025-03-14T15:26:09.102: Sending PUSH_ACK, random_token: 998 remote: 127.0.0.1:55712
2025-03-14T15:26:09.103: Sending uplink event, uplink_id: 1933197396, topic: eu868/gateway/7076ff00560908fb/event/up
2025-03-14T15:26:10.021: TX_ACK received, random_token: 58474, remote: 127.0.0.1:59059, error: NONE
2025-03-14T15:26:10.021: Sending ack event, downlink_id: 623895658, topic: eu868/gateway/7076ff00560908fb/event/ack
2025-03-14T15:26:11.383: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:11.383: Received downlink command, downlink_id: 501095965, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:11.383: Sending PULL_RESP, random_token: 7709, remote: 127.0.0.1:59059
2025-03-14T15:26:11.383: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:11.383: Received downlink command, downlink_id: 2320264485, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:11.383: Sending PULL_RESP, random_token: 27941, remote: 127.0.0.1:59059
2025-03-14T15:26:11.384: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:11.384: Received downlink command, downlink_id: 90034669, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:11.384: Sending PULL_RESP, random_token: 53741, remote: 127.0.0.1:59059
2025-03-14T15:26:11.384: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:11.384: Received downlink command, downlink_id: 3182963888, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:11.388: Sending PULL_RESP, random_token: 11440, remote: 127.0.0.1:59059
2025-03-14T15:26:11.389: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:11.390: Received downlink command, downlink_id: 1920175262, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:11.390: Sending PULL_RESP, random_token: 35998, remote: 127.0.0.1:59059
2025-03-14T15:26:11.532: PULL_DATA received, random_token: 3232, remote: 127.0.0.1:59059
2025-03-14T15:26:11.532: Sending PULL_ACK, random_token: 3232, remote: 127.0.0.1:59059
2025-03-14T15:26:12.484: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:12.484: Received downlink command, downlink_id: 1856120015, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:12.484: Sending PULL_RESP, random_token: 9423, remote: 127.0.0.1:59059
2025-03-14T15:26:12.485: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:12.485: Received downlink command, downlink_id: 1641539137, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:12.485: Sending PULL_RESP, random_token: 58945, remote: 127.0.0.1:59059
2025-03-14T15:26:12.485: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:12.485: Received downlink command, downlink_id: 3173803910, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:12.485: Sending PULL_RESP, random_token: 26502, remote: 127.0.0.1:59059
2025-03-14T15:26:12.709: TX_ACK received, random_token: 23367, remote: 127.0.0.1:59059, error: NONE
2025-03-14T15:26:12.709: Sending ack event, downlink_id: 1675975495, topic: eu868/gateway/7076ff00560908fb/event/ack
2025-03-14T15:26:15.406: TX_ACK received, random_token: 21715, remote: 127.0.0.1:59059, error: NONE
2025-03-14T15:26:15.406: Sending ack event, downlink_id: 3607647443, topic: eu868/gateway/7076ff00560908fb/event/ack
2025-03-14T15:26:16.457: Received message, topic: eu868/gateway/7076ff00560908fb/command/down, qos: 0
2025-03-14T15:26:16.457: Received downlink command, downlink_id: 1107056361, topic: eu868/gateway/7076ff00560908fb/command/down
2025-03-14T15:26:16.457: Sending PULL_RESP, random_token: 22249, remote: 127.0.0.1:59059
...
// much more PULL_RESP and TX_ACK here before all 10 messages disappear from chirpstack queues