...As NatTrav development was stopped quite a while ago, please consider looking at more production-ready libraries like STUN (https://sourceforge.net/projects/stun/).
For educational purposes, I still strongly recommend to have a look at both library and example application (chatapp) which are both documented fairly well. To download the latest version of NatTrav, please access the SVN sources in the Code section (project sits in "/nattrav/trunk/").