Managing TCP Retransmission Timeout

[vytautas.virvicius]

During our evaluation of MQTT, we observed that if we send data every five minutes, the server stops receiving it. We currently believe this is related to CG-NAT, which drops the tunnel due to inactivity, preventing the packet from being routed. This leads to the module retransmitting the TCP packet 10 times with varying delays before determining that the TCP connection is dead. The entire process takes approximately 18 minutes.

Is there a way to control the frequency of retransmissions or the total time before considering the TCP connection dead?

[Charlie]

Hi,

THanks for checking with us. Looks https://docs.nordicsemi.com/bundle/ncs-2.7.0/page/kconfig/index.html#CONFIG_NET_TCP_RETRY_COUNT is what you need, it has default 9.

The following two configuration may also help to avoide retransimisson.

MQTT Keep-Alive: CONFIG_MQTT_KEEPALIVE

Socket Timeout: You can also set a timeout on the socket to prevent indefinite waiting. For example:

err = setsockopt(client.transport.tls.sock, SOL_SOCKET , SO_SNDTIMEO, &timeout_mqtt, sizeof(timeout_mqtt));

Best regards,

Charlie

[vytautas.virvicius]

Charlie said:

THanks for checking with us. Looks https://docs.nordicsemi.com/bundle/ncs-2.7.0/page/kconfig/index.html#CONFIG_NET_TCP_RETRY_COUNT is what you need, it has default 9.

The CONFIG_NET_TCP_RETRY_COUNT is only applicable to Zephyr's native networking stack. It is not used in Nordic's offloaded sockets. While SO_SNDTIMEO doesn't seem to do anything.

[Charlie]

Hi,

nRF91 modem only proivde socket for straight forward TCP connection, and it does not have an comprehensive TCP stack like Zephyr network libary.

Cellular: Modem Shell sample has an implementiaon that demostrates run nRF91 Series DK with Zephyr native TCP/IP stack. You can refer to it or adding you own implmentaion for TCP retry count based funtion on current TCP socket simple usage.

Best regards,

Charlie

[vytautas.virvicius]

Using the native Zephyr TCP/IP stack isn't an option due to the increased size of the firmware.

Is there no alternative way to control the timeout mechanism for the modem TCP/IP stack?

[Charlie]

Hi,

The retransmission count feature is built on top of the basic TCP/IP socket and is not inherently part of the core TCP stack. Due to the limited resources on the modem running on the nRF9151, this feature is not supported.

As I mentioned, I would recommend referring to the Zephyr TCP stack. Zephyr's implementation provides enhanced capabilities, including retransmission counting. If resource constraints are a concern, you might consider porting only the specific parts of the code relevant to your needs to minimize size increases

Best regards,

Charlie