On Linux ifconfig gets the raw data statistics from /proc/net/dev.
The values there (with ifconfig destination in brackets added) are described by an article Exploring the /proc/net/ Directory as:
- errs (Errors): The total number of transmit or receive errorsdetected by the device driver.
- drop (Dropped): The total number of packets dropped by the devicedriver.
- fifo (Overruns): The number of FIFO buffer errors.
- frame (Frame): The number of packet framing errors.
- carrier (Carrier): The number of carrier losses detected by thedevice driver.