Database Description-Used to form adjacencies. The router summarises all its linkstate advertisements and passes this information, via database description packets to the router it is forming an adjacency with Link State Request After the database description packets have been exchanged with a neighbour, the router may detect link state advertisements it requires to update or complete the topological database. Link state request packets are sent to the neighbour requesting these link state advertisements.
Link State Update-Used for transmission of link state advertisements between routers. This could be in response to a link state request packet or to flood a new or more recent link state advertisement.
Link State Acknowledgment Used to make the flooding of link state advertisements reliable. Each link state advertisement received is explicitly acknowledged