Database Description Packet (DDP) – This contains the summary of all the LSAs, it only contains the headers and not the complete LSA. This allows router to see if its own link state database is updated. This is type 2 OSPF packet.
I(nitial)-bit – If set means this is the first DDP sent.
M(ore)-bit – If set means this is not the last DDP sent.
M(aster)S(lave)-bit – If set means DDP sent by master.
Link State Request(LSR) – This is type 3 OSPF packet, sent when router requests for current LSA state.
Link State Update (LSU) – this is type 4 OSPF packet, all update must be individually acknowledged. Link state retransmission list is built for unacknowledged LSU, acknowledged LSU will be removed from link state retransmission list.
Link State Acknowledge – This is type 5 OSPF packet. Explicit acknowledgement means LSA header is received. Implicit acknowledgement LSA of the same instance is received.