From Java I2P:
I2P's Network Database is a specialized distributed database, containing just two types of data - router contact information (Router infos) and Destination contact information (LeaseSets). Each piece of data is signed by the appropriate party and verified by anyone who uses or stores it. In addition, the data has liveliness information within it, allowing irrelevant entries to be dropped, newer entries to replace older ones, and protection against certain classes of attack.
Read Network-Database for details.