From Immad Naseer present "Paxos from the ground up"
Consensus protocols are at the heart of most distributed systems and Paxos is one of the more widely used of such protocols. Instead of
introducing Paxos with all its subtle details from the get-go, we'll instead derive the protocol step by step, starting with the simplest algorithm which comes to mind and then repeatedly refining it together till we finally reach the actual protocol. This will allow you to better appreciate its simplicity and beauty and use this important
building block with more confidence and understanding in your next distributed systems project. slides 
Key concepts:
Paxos assumes all node are non malicious (not a byzantine generals faulty network).
- Promise message - Proposer. - Ordering - Acknowledgement
Message passing ...
Promise message and ack message.
- P2’s promise msg reaches both acceptors causing them to change their mind.
- If accept and promise msg interfere with each other we have a problem.
- P2 needs to listen to all ack messages before it accepts
All msgs are in flight.
- Acceptors are trying to accept
Promise message and ack message.
- P2’s promise msg reaches both acceptors causing them to change their mind.
- If accept and promise msg interfere with each other we have a problem.
- P2 needs to listen to all ack messages before it accepts
- All msgs are in flight.
- Acceptors are trying to accept
Useful open source
Azure/RSL page
Harry's neat graphic paxos