In peer-to-peer environments, peers are often located behind routers which complicates the connection set-up to them. NatTrav provides a common platform for traversal techniques which are used to establish connectivity in these cases.
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/").
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