Always we have adjacency between point-to-point networks or point-to-multipoint networks if they match in their Hello packets. But in other network types same as Broadcast...

In this part start with Type 1: Hello Packets, because it is the first packet that sends by the OSPF process on their interfaces to establish and maintain neighbor relationships...

In the last two sections, Hello packet and DBD packet we learned that R1 and R2 discovered each other via Hello packet and then they sent their DBD packets to each other...

OSPF Type 4 is the response to a link-state request sent by R1 to R2 to asking for more information about each LSA. These packets contain the number of LSAs and full information for each LSA...