Improve the document in detail about psp

[insanum]

The shared key generation via the KDF is documented and straight forward. I think what you might be asking is how to get that shared secret key generated on the receiver back to the sender. Likely, this is a job for TLS borrowing its handshake. This is what QUIC does and implementations hook into the handshake function callback mechanisms found in TLS libraries.

[hlrichardson]

If we ignore the details of key exchange and just assume an out-of-band channel for communication between the sender and receiver, the steps would be:

1. Sender requests a new derived key from receiver.
2. Receiver picks a new SPI value and uses it to generate a derived key from the master key.
3. Receiver sends SPI and derived key to sender, which the sender can use to produce PSP-encapsulated packets.