Although anonymizing Peer-to-Peer (P2P) networks often means extra cost in terms of transfer efficiency, many systems try to mask the identities of their users for privacy consideration. By comparison and analysis of existing approaches, we investigate the properties of unstructured P2P anonymity, and summarize current attack models on these designs. Most of these approaches are path-based, which require peers to pre-construct anonymous paths before transmission, thus suffering significant overhead and poor reliability. We also discuss the open problems in this field and propose several future research directions.