OSPF packets
OSPF messages are carried directly over IP. The protocol number is 89.
OSPF uses the following packet types:
Hello—Periodically sent to find and maintain neighbors, containing timer values, information about the DR, BDR, and known neighbors.
Database description (DD)—Describes the digest of each LSA in the LSDB, exchanged between two routers for data synchronization.
Link state request (LSR)—Requests needed LSAs from a neighbor. After exchanging the DD packets, the two routers know which LSAs of the neighbor are missing from their LSDBs. They then exchange LSR packets requesting the missing LSAs. LSR packets contain 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).