这在RFC 2328 - OSPF 版本 2，第 10.8 节中进行了解释

   Master
     Database Description packets are sent when either a) the slave acknowledges 
     the previous Database Description packet by echoing the DD sequence number 
     or b) RxmtInterval seconds elapse without an acknowledgment, in which case 
     the previous Database Description packet is retransmitted.

   Slave
     Database Description packets are sent only in response to Database 
     Description packets received from the master. If the Database Description 
     packet received from the master is new, a new Database Description packet 
     is sent, otherwise the previous Database Description packet is resent.

简而言之，在任何给定时间，只有一个 DBD 序列号可以不被确认。Master 负责确保正确确认 DBD 序列号。厨房里的两个厨师不工作……一个必须控制序列号（主），一个必须确认（从）。