Hello: Periodically sent to find and maintain neighbors, containing the values of some timers, information about the DR, BDR, and known neighbors.
Database description (DBD): Describes the digest of each LSA in the LSDB, exchanged between two routers for data synchronization.
Link state request (LSR): Requests needed LSAs from the neighbor. After exchanging the DBD packets, the two routers know which LSAs of the neighbor are missing from their LSDBs. They then send an LSR packet to each other, requesting the missing LSAs. The LSA packet contains the digest of the missing LSAs.
Link state update (LSU): Transmits the requested LSAs to the neighbor.
Link state acknowledgment (LSAck): Acknowledges received LSU packets. It contains the headers of received LSAs (an LSAck packet can acknowledge multiple LSAs).