nRF9160 (Thingy:91) MTU for UDP using Serial LTE modem app

[Michal Mühlpachr]

I want to send larger UDP packets from nRF9160 (Thingy:91) using Serial LTE modem app.

I am not able to send more than 288 bytes of binary data.

How to send larger binary data by UDP in Serial LTE modem please ?

- Thingy:91 v1.0.0

- LTE modem app from ncs v1.4.0

- Modem firmware AT+CGMR mfw_nrf9160_1.2.2

- NB IoT Vodafone CZ, +CEREG: 5,"AE38","000AC51F",9,,,"00000000","00000111"

- +CGCONTRDP: 0,,"hardwario.com","","",,,,,,,1430

According https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.4.0/nrf/applications/serial_lte_modem/doc/TCPIP_AT_commands.html#udp-send-data-xsendto there is maximum size for NET_IPV4_MTU 576 bytes. 

Where is this restriction from please ?

How it can be changed in Serial LTE modem app please ?

I am getting ERROR while using 

AT#XSENDTO=<url>,<port>,0,"<HEX>..."

with more than 576 hex characters (288 binary data bytes payload in UDP packet).

How to fix that please ?

BTW there is error in documentation https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.4.0/nrf/applications/serial_lte_modem/doc/TCPIP_AT_commands.html#udp-send-data-xsendto either <datatype> optional or missing in example.

Thank you.

[Hakon]

Hello,

 

Where is this restriction from please ?

 It looks like it is defined in this file.

 

How it can be changed in Serial LTE modem app please ?

 You can try changing the number in net_ip.h, but I can't guarantee that will work. Also, be aware that size will likely be restricted by the bsdlib buffer size BSD_IP_MAX_MESSAGE_SIZE, which is 4k bytes.