Chapter overview:
  * introduce multi-hop HTLC forwarding + onion routing at a lower level

Relevant questions to answer:
  * Onion Routing:
    * What is onion routing? What security guarantees does it offer?
    * What is the Sphinx format? How does it differ from prior mixnet packet formats?
    * How does DH randomization work? (possibly too low-level)
    * Why is it important to keep the packet fixed sized at all times?
    * What routing information is contained in the payload for an node?
    * How can the routing information be extended in the future?
  * HTLC Forwarding:
    * How does a node gain fees when it forwards a payment?
    * What is the "cltv delta" why does it matter? 
    * What does a successful HTLC forwarding look like?
    * How can a node cancel back an HTLC off-chain?
    * What is a typical network switch/hub? How can a similar analogy be applied to forwarding HTLCs?
    * What is the circuit map, why does a node need to maintain this?
    * Can fails or settles be safely pipelined on the network?
    * How does a node send an error back to the sender without knowing who they are?
    * What dangers exist w.r.t time-locks and timely on-chain confirmation?