It uses the following two libraries to finish a iOS app to demonstrate the process of P2P hole punching. After establishing P2P connections, people can send messages to the others.
https://github.com/lukeredpath/libPusher. This websocket library is used to implement the signaling part.
License: MIT. Copyright (c) 2010 Luke Redpath
www.pjsip.org. This is used to implement hole punching STUN part.
License: Either GPL 2.0/Later OR Commercial license, reference: http://www.pjsip.org/licensing.htm
p2pMessenger is released under the GPL v3.0 license. This is due to using PJSIP.