# How the layers of Lightning Fit Together

## Reading

| Content                                                                                                                                                           | Time (min) |
| ----------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- |
| [A walk through the layers of Lightning](https://btctranscripts.com/scalingbitcoin/tel-aviv-2019/edgedevplusplus/lightning-network-layer-by-layer/)               | 40         |
| [Creating a channel](https://ellemouton.com/posts/creating-a-channel/)                                                                                            | 20         |
| [Updating State](https://ellemouton.com/posts/updating-state/)                                                                                                    | 20         |
| [Revocation in more detail](https://ellemouton.com/posts/revocation/)                                                                                             | 20         |
| [The Update Layer](https://btctranscripts.com/chaincode-residency/2019-06-26-rene-pickhardt-update-layer/)                                                        | 70         |
| [The Transfer Layer](https://btctranscripts.com/chaincode-residency/2019-06-24-fabrice-drouin-the-transfer-layer/)                                                | 50         |
| [The Multihop Layer](https://btctranscripts.com/chaincode-residency/2019-06-24-rene-pickhardt-multihop-in-lightning/)                                             | 80         |
| [The other layers (base and transport)](https://btctranscripts.com/chaincode-residency/2019-06-24-fabrice-drouin-base-and-transport-layers-of-lightning-network/) | 25         |
| [Message format](https://github.com/lightning/bolts/blob/master/01-messaging.md#lightning-message-format)                                                         | 10         |
| [Ok to be odd](https://github.com/lightning/bolts/blob/caae842bfcadd144ab3ec7ce74c317dca07ac78c/00-introduction.md#its-ok-to-be-odd)                              | 5          |
| [TLV streams](https://github.com/lightning/bolts/blob/master/01-messaging.md#type-length-value-format)                                                            | 15         |
| [Feature bits](https://github.com/lightning/bolts/blob/master/09-features.md)                                                                                     | 15         |
| [Diffie–Hellman key exchange](https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange)                                                                  | 15         |
| [How and when do I use HMAC?](https://security.stackexchange.com/questions/20129/how-and-when-do-i-use-hmac)                                                      | 15         |
| [Lightning transactions: from Zero to Hero (optional)](https://github.com/t-bast/lightning-docs/blob/master/lightning-txs.md)                                     | 60+        |

## Discussion Questions

1. How would an offline vending machine work?

### A walk through the layers of Lightning

1. How does the LN-Penalty mechanism currently leak out of the update layer?

### Creating a channel

1. What factors should be considered before opening a Lightning channel with a peer?

### The Update Layer

1. How would switching to PTLCs affect the update layer?
2. If LN were to upgrade the update layer, would the network need to end-to-end upgrade?
3. Why are the second stage HTLC transactions not part of the commitment transaction?

### The Transfer Layer

1. Explain how the timeout mechanism works for in-flight HTLCs?
2. How is the payment hash sent to the payer?
3. Can both HTLCs and PTLCs coexist in a channel?
4. What's are the required parts of invoices?

### The Multihop Layer

1. How can nodes that are not always online prevent loss of funds?
2. What are the risks of forwarding below-dust payments?
3. What is the relationship of the revocation keys for multiple subsequent commitment transactions? What would change if revocation keys were independent?
4. How can you send a payment to a node without getting an invoice? What are the tradeoffs of this payment method?
5. Why is a grace period added to the HTLC timeout?
6. What are routing hints and why are they important for private channels? Do they leak privacy?

### Diffie–Hellman key exchange

1. What is a Diffie-Hellman key exchange used for?

### How and when do I use HMAC?

1. What's the difference between a MAC and an HMAC?
2. Where are HMAC and MAC implemented in lightning and why?