The link state advertisement header contains the LS type, Link State ID and Advertising Router fields. The combination of these three fields uniquely identifies the link state advertisement.
There may be several instances of an advertisement present in the Autonomous System, all at the same time. It must then be determined which instance is more recent. This determination is made by examining the LS sequence, LS checksum and LS age fields. These fields are also contained in the 20-byte link state advertisement header.
Several of the OSPF packet types list link state advertisements. When the instance is not important, an advertisement is referred to by its LS type, Link State ID and Advertising Router (see Link State Request Packets). Otherwise, the LS sequence number, LS age and LS checksum fields must also be referenced.
A detailed explanation of the fields contained in the link state advertisement header follows.