I don't know why but now OTA works better; I still get quite some NACKs on the gateway but at least most of the times the OTA is successful even with signing enabled. With ACK enabled the transfer speed is at least 4/5 times slower

In the BACnet protocol, the messages are not sent directly, but pushed into an internal state machine that remembers (some) of the previously messages sent (i.e. memorize(_msg); send(_msg); ). The state machine also peeks incoming messages and can trigger a message re-send in case of a missed ACK. the difference with MySensors is that all the messages are numbered, otherwise you would not know which message of the last N sent previously was ACKed, but well.. ACKing the last similar message might be good enough. Anyway @SiLeX : if you want to try, this is not that complicated, (if you don't take into account the STREAMs) it will just costs about (1 status byte + 1 timestamp + 1 frame size) x N ; for N frames memorized.

...no .. I know .. but I must explain this .. : I know .. receiver on first upload data for TX Fifo .. and listen .. if receive valid packet then send ack with data from TX fifo . but I mean this : repeater periodicaly contact gateway and udate some easy data .. for example date time , network structure , how many node are in network , if some registered node are visible or not , etc .. and if see gateway .. this is example : my node is not directly connected to gateway .. byt by two repeater .. all repeate every 10minutes ask for gateway just for easy ping only ACK , if receive ACK then know all information what I wrote.. but will know that see gateway .. and update ACK payload .. if my sensor node send data to gateway and don't want ACK .. then received hardware ACK and will know that last 10minutes repeater seen gateway .. yes it's not for 100% .. but wery efective .. to inform all other sensor node but not network load . only sensor node that really need to know 100% if gateway received packet use ACK for gateway.. look I'm not tested .. but in fact ... if sensor send data trought repeater and repeater not see gateway .. still received hardware ACK ?? why ?? not efective contact repeater what don't see gateway.. then i have this idea.. this will be very efective network.. I thinks that serial gateway need RTC on I2C and some small ram .. and inform automaticaly network for this information.. gateway will do more important work