OSPF
"Open Shortest Path First" — Interior Gateway Protocol (IGP) for routing inside an autonomous system. Link-state protocol, runs Dijkstra on a topological graph.
RFC 2328 (v2 for IPv4), RFC 5340 (v3 for IPv6). Routers exchange LSAs (Link State Advertisements), build an identical topology database, run Dijkstra to compute the best path. Areas for scalability (Area 0 = backbone).
Competitor: IS-IS (another link-state, popular at ISPs), EIGRP (Cisco proprietary), RIP (old, distance vector). OSPF is default in many enterprises and small-to-medium ISPs.