ASPA is short for Autonomous System Provider Authorization. The idea is that an AS holder publishes a list of who their upstream providers are into the global RPKI, and then ISPs around the world use that information to securely determine the plausibility of AS_PATHs and automatically mitigate route leaks. In other words: when you create an ASPA you explicitly authorize your upstreams to provide transit service for your AS, and in turn the world’s BGP routers can run an algorithm to check whether a BGP route involving your AS aligns with published ASPA objects. This all may sound simple enough, but there are a few gotchas!