How do two adjacent routers know that they have a two-way OSPF communication ?

Think of the answer before revealing the hidden one !
You may say "they see each other's Hello packets" - but I challenge you: if I'm a router and I see your OSPF Hello, how do I know that you also see my OSPF Hello's ?

The fact that a router sees a Hello packet it only shows an INIT state but it does not ensure a two-way communication. Routers include the list of neighbors in their Hello packets, so a 2-way state is reached when a router sees its own ID in the Hellos received from that particular peer.
The Hello packets also contains the information about DR and BDR - or use if they have not been selected yet.

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