Serial LTE Modem AT#XSENDTO limited to 256 bytes of payload in one datagram

[Michal Mühlpachr]

Hi,

why AT#XSENDTO in datamode is limited to 256 bytes of payload in one datagram please ?

It seams, that limitation comes from UART_RX_LEN in slm_at_host.c
https://github.com/nrfconnect/sdk-nrf/blob/9a5b3865e22e007f68d628eb732b26127ac0e041/applications/serial_lte_modem/src/slm_at_host.c#L36

Why it is not by default at least 548 ?

Default NET_IPV4_MTU 576 - 28 (IPv4 + UDP header) = 548

Why CONFIG_SLM_SOCKET_RX_MAX (developer.nordicsemi.com/.../slm_description.html) is not used in slm_at_host.c for UART_RX_LEN ?

SLM is splitting UDP payloads into multiple datagrams now. This is not correct behavior, it should report error in case maximum payload is exceeded. UDP payload is not stream like TCP and can not be reconstructed correctly at receiver side.

Regards,
Michal

[Håkon Alseth]

Hi,

 

Could you share which version of ncs that you are using, and a bit more detailed information about the issue that you're seeing?

 

Splitting packets on the uart interface shall not be the same as splitting it on a socket interface.

The maximum size of a single udp datagram is defined in the xsendto:

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.9.0/nrf/applications/serial_lte_modem/doc/SOCKET_AT_commands.html#udp-send-data-xsendto

 

Kind regards,

Håkon

[Michal Mühlpachr]

In ncs v1.7 AT#XSENDTO with binary data (without <data> parameter, with data mode) SLM splits data to 256 bytes chunks of payload send in UDP datagram.
In ncs v1.9.1 AT#XSENDTO with binary data (without <data> parameter, with data mode) SLM splits data to 512 bytes chunks of payload send in UDP datagram.

[Michal Mühlpachr]

We would like to utilize MTU as much as possible. We are sure, that datagrams with 1400 bytes travels fine through LTE/EPS with other modems.

[Håkon Alseth]

Hi,

 

Could you please provide how to reproduce this scenario?

Michal Mühlpachr said:

In ncs v1.7 AT#XSENDTO with binary data (without <data> parameter, with data mode) SLM splits data to 256 bytes chunks of payload send in UDP datagram.
In ncs v1.9.1 AT#XSENDTO with binary data (without <data> parameter, with data mode) SLM splits data to 512 bytes chunks of payload send in UDP datagram.

SLM in ncs v1.9.0, one change:

CONFIG_NRF_MODEM_LIB_TRACE_ENABLED=y

Here's my test scenario:

at%xmodemtrace=1,2
at+cfun=1
AT#XSOCKET=1,2,0

Here's the send command:

AT#XSENDTO="example.com",1234,"Test UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDP"

Which returns:

#XSENDTO: 568

 

And here's the pcap that shows the frame sent as > 512 bytes:

trace-2022-03-09T12-13-05.383Z.pcapng

 

Kind regards,

Håkon

[Håkon Alseth]

Hi,

 

Here's my test setup:

* ncs v1.9.0

* SLM with one modification: CONFIG_NRF_MODEM_LIB_TRACE_ENABLED=y

AT commands that I send:

at%xmodemtrace=1,2
at+cfun=1
AT#XSOCKET=1,2,0

AT#XSENDTO="example.com",1234
"Test UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDPTest UDP"
++++++

Here it sends > 512 byte datagram, as shown in this pcap:

trace-2022-03-09T12-21-49.077Z.pcapng

 

Kind regards,

Håkon