See Jan Seedorf, "Security Challenges for Peer-to-Peer SIP", IEEE Network, September/October 2006. http://ieeexplore.ieee.org/xpls/abs_all.jsp? isnumber=36007&arnumber=1705882&count=6&index=5 http://www.jcbroadband.com/Library/jcbvoip5.pdf Henning
The quoted paper by Jan Seedorf, "Security Challenges for Peer-to-Peer SIP" http://www.jcbroadband.com/Library/jcbvoip5.pdf mentions briefly distributed trust and reward. Solid progress has been made as shown in "p2p Trust Infrastructure" by R. Nelson and Gruia Pitigoi-Aron, see http://www.cs.ucla.edu/~rlnelson/trust.pdf
