> everyone "needs" police protection (and it surely can't be provided via free market)
Police protection certainly can be provided privately via the free market - and it often is even today, in the sense that there are more private security guards than public police officers in the US - but we usually call people who advocate entirely private generation and enforcement of law anarcho-capitalist rather than libertarian.
Economist David Friedman is one of the best thinkers on this subject; he bases his views on history and the economics of law - this chapter of his book Machinery of Freedom is a good starting point on the theoretical case:
(I do know what anarcho-capitalism is; I'm anarcho-capitalist myself.)
Libertarianism is a broad term. Anarcho-capitalists are as libertarian as minarchists are. "Libertarian" can refer to either. It's obvious @essrinn was referring to minarchism, but I just wanted to point out that minarchists still advocate presence of a gang with weapons to preserve the "order" in the society and steal from people along the way.
Police protection certainly can be provided privately via the free market - and it often is even today, in the sense that there are more private security guards than public police officers in the US - but we usually call people who advocate entirely private generation and enforcement of law anarcho-capitalist rather than libertarian.
Economist David Friedman is one of the best thinkers on this subject; he bases his views on history and the economics of law - this chapter of his book Machinery of Freedom is a good starting point on the theoretical case:
http://www.daviddfriedman.com/Libertarian/Machinery_of_Freed...
(I also recommend the book _The Enterprise of Law: Justice Without The State_ by Bruce L. Benson: http://www.amazon.com/The-Enterprise-Law-Justice-Without/dp/... )