We are using BG95M1 and EG91NAXD on AT&T. AT&T uses different MTU depending on whether we use FirstNet or the regular ioT network:
https://iotdevices.att.com/att-iot/FirstNetMTU.aspx
We use lwip TCP stack with a PPP interface to the BG95/EG91 - however neither the BG95 nor the EG91 sends a PPP LCP MRU option during link setup.
https://datatracker.ietf.org/doc/html/rfc1661#page-41
How should our application discover the MTU for the AT&T service ?
when we send one attach reqest to network ， it include one MTU request ，by default ， MTU=1500 ， when the net work reply this request . it will send one network value approved by network , as you said , maybe it is 1430 , the device will change to 1430 , if not , the device will use the default value (1500)
you can check this value with QXDM log
Stephen, thank you for the reply.
However it does not answer my question - how would a host CPU discover the MTU assigned by the network ?
If the host CPU is using ATD and PPP and running its own TCP/IP stack (and not using the sockets AT commands) there does not seem to be a way for the TCP/IP stack on the host CPU to discover if the MTU is reduced below 1500.
I would expect the modem PPP LCP sublayer to include a MRU option but I don’t see an MRU option from either BG95M1 or EG91NAXD.
