Hi everyone, At last I found it "/proc/net/dev" is what I need. This file consists of some counters for each interface, each one counts the number of packets (and bytes). ifconfig uses this file too, but after parsing it.The parser can be found here:
Also There are some software that do the network traffic monitoring (for JUST the purpose I have) such as ifstat and bwm-ng.

The other tool that can be used is, sysctl. It can be used on every Free-BSD based systems.