cannot ping from a usb-serial device

[pietro canevarolo]

Hi all,
I am developing a portable device that I must connect to Internet via usb.
I connect to my linux box with pppd on ttyUSB0 and the link layer is 
working ok. I can ping my device from all the machines connected, but 
when the device pings some address known reacheable my linux box sees 
the packets ("ifconfig ppp0" returns the correct number of packets 
exchanged and "netstat -i" reports the correct number of bytes on the 
interface), but no echo replies come back to my device.
I understand that my question may be silly for this forum, but I am 
really out of resources.
Can someone out there point me to some info to understand why this is 
happening?

many tnx

pietro canevarolo
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