We are using modem firmware 1.3.5 on nRF9160.
When packet loss is high, we sometimes see connection resets (NRF_ECONNRESET) reported by the modem that don't seem to be explainable by the packets the server has sent.
I have documented such a case in the attached files and will explain it here. On the client side, NRF_MODEM_LIB_TRACE_LEVEL_IP_ONLY was used.
The nRF91 opens a TCP connection to a server and tries to send data:
client -> SYN -> server
client <- SYN,ACK <- server
client -> ACK LOST
client -> PSH,ACK LOST
client -> PSH,ACK LOST (retransmission)
client <- SYN,ACK <- server (retransmission)
client -> ACK LOST (duplicate)
At this point, the nRF91 aborts the nrf_poll function with POLLERR. Querying the SO_ERROR using nrf_getsockopt yields 104 (NRF_ECONNRESET).
Is this expected behaviour? As far as I know, ECONNRESET should indicate that the server has sent a RST packet, which was not the case here. Not that in the logs, you can see the server sending a RST packet, but that is much later.
Server packet trace:
Client (nRF91) packet trace (note that the timestamps are wrong):
