Monitoring network usage excluding local traffic

I am working on an app that monitors network usage. However I noticed many ways to do this does not allow exclusion of local traffic (say, Time Machine).

I am looking for a way to exclude local traffic, and only monitors usage that goes directly to/from the internet.

Update: Thank you for your replies, now I know how to find if the traffic is local, but I still don't know how I can calculate total in/out bytes (sorry if I didn't elaborate earlier). I have no way of knowing how many bytes are sent/received locally (or to the internet) in a certain period of time, or since the OS starts. This problem is further complicated by the fact processes are launched or killed when the OS is running.

The answer to the question How to get network adapter stats in linux/Mac OSX? gives an interesting way of summing up total usage but it doesn't help because the usage it sums up are interface statistics.

Update 2: I've posted my final solution to this. Please scroll down a bit to see.