I'm a little bit confused regarding the keystream. When installing the same session key again the nonce will be reset and start again.

-Session Key + Nonce = Keystream?

-The Keystream is now the part which is used to encrypt the plain text?

-How is it possible to decrypt anything? Do I need a piece of plain text and an encrypted message from what I suggest this plain text is in there to then derive the keystream?

-Does every single packet get a new Keystream for encryption or does the keystream renwew after a complete new 4-way handshake.

Sorry but I try to understand..