Uncovering BGP Action Communities and Community Squatters in the Wild

The Border Gateway Protocol (BGP) offers several ''knobs'' to control routing decisions, but they are coarse-grained and only affect routes received from neighboring Autonomous Systems (AS). To enhance policy expressiveness, BGP was extended with the communities attribute, allowing an AS to attach metadata to routes and influence the routing decisions of a remote AS. The metadata can carry information to (e.g., where a route was received) or request an action from a remote AS (e.g., not to export a route to one of its neighbors). Unfortunately, the semantics of BGP communities are not standardized, lack universal rules, and are poorly documented. In this work, we design and evaluate algorithms to automatically uncover BGP action communities and ASes that violate standard practices by consistently using the information communities of other ASes, revealing undocumented relationships between them (e.g., siblings). Our experimental evaluation with billions of route announcements from public BGP route collectors from 2018 to 2023 uncovers previously unknown AS relationships and shows that our algorithm for identifying action communities achieves average precision and recall of 92.5% and 86.5%, respectively.