Well, here is the case, I learned that ubuntu dist has added some modification to the source of ppp source, which is here:
https://launchpad.net/ubuntu/trusty/+source/ppp
It seems ppp_2.4.5-5.1ubuntu2.2.diff.gz is some patches for the source code, how can I patch it to the original source folder? I tried something like patch -R, but I failed. the code remains the same.