Peer storage for nodes to distribute small encrypted blobs.

lightning · Sep 18, 2023 · a766bfc · a766bfc
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diff --git a/01-messaging.md b/01-messaging.md
@@ -476,6 +476,75 @@ every message maximally).
 Finally, the usage of periodic `ping` messages serves to promote frequent key
 rotations as specified within [BOLT #8](08-transport.md).
 
+### Peer storage
+
+## The `peer_storage` and `your_peer_storage` Messages
+
+Nodes that advertise the `option_provide_storage` feature offer storing
+arbitrary data for their peers. The data stored must not exceed 65531 bytes,
+which lets it fit in lightning messages.
+
+Nodes that advertise the `option_want_storage` feature want to have their
+data stored by their peers that support `option_provide_storage`.
+
+Nodes can verify that their `option_provide_storage` peers correctly store
+their data at each reconnection, by comparing the contents of the
+retrieved data with the last one they sent. However, nodes should not expect
+their peers to always have their latest data available.
+
+Nodes ask their peers to store data using the `peer_storage` message & expect peers to return them latest data using `your_peer_storage` message:
+
+1. type: 7 (`peer_storage`)
+2. data:
+   * [`u16`: `length`]
+   * [`length*byte`:`blob`]
+
+
+1. type: 9 (`your_peer_storage`)
+2. data:
+   * [`u16`: `length`]
+   * [`length*byte`:`blob`]
+
+
+Requirements:
+
+The sender of `peer_storage`:
+  - MUST offer `option_want_storage`.
+  - MAY send `peer_storage` whenever necessary (usually, everytime their configuration changes)
+  - MUST limit its `blob` to 65531 bytes.
+  - MUST encrypt the data in a way that can be validated when received.
+
+
+The receiver of `peer_storage`:
+  - If it offered `option_provide_storage`:
+    - if it has an open channel with the sender:
+      - MUST store the message.
+    - MAY store anyway (e.g. might get paid for the service).
+
+- If it does store the message:
+  - MAY delay storage to ratelimit peer to no more than one update per second.
+  - MUST respond with `your_peer_storage` containing `blob` after it is stored.
+  - MUST replace the old `blob` with the latest received.
+  - MUST send `your_peer_storage` again after reconnection.
+- Otherwise:
+  - MUST NOT respond with `your_peer_storage`
+
+The sender of `your_peer_storage`:
+  - MUST include the last `blob` it stored for that peer.
+  - when all channels with that peer are closed:
+    - SHOULD wait at least 2016 blocks before deleting the `blob`.
+
+The receiver of `your_peer_storage`:
+  - when it receives `your_peer_storage` with an outdated or irrelevant data:
+    - MAY send a warning.
+
+Rationale:
+If a peer supports `option_provide_storage` and we have a channel with it, we can send a `peer_storage` message with any `blob` so that peer can store it locally in their node. Everytime a node receives a `peer_storage` message, It needs to update the `blob` for the sender.
+
+Upon receiving `peer_storage` nodes should immediately respond with the latest `your_peer_storage`, which could work as an ack to the peer.
+
+Nodes supporting `option_want_storage`, should send a `peer_storage` message whenever they wish to update the `blob` stored with their peers. This `blob` can be used to distribute encrypted data, which could be helpful in restoring the node.
+
 ## Appendix A: BigSize Test Vectors
 
 The following test vectors can be used to assert the correctness of a BigSize

diff --git a/09-features.md b/09-features.md
@@ -45,6 +45,8 @@ The Context column decodes as follows:
 | 24/25 | `option_route_blinding`           | Node supports blinded paths                               | IN9      | `var_onion_optin`         | [BOLT #4](bolt04-route-blinding)                                      |
 | 26/27 | `option_shutdown_anysegwit`       | Future segwit versions allowed in `shutdown`              | IN       |                           | [BOLT #2][bolt02-shutdown]                                            |
 | 38/39 | `option_onion_messages`           | Can forward onion messages                                | IN       |                           | [BOLT #7](04-onion-routing.md#onion-messages)                         |
+| 40/41 | `option_want_storage`             | Want to use other nodes to store encrypted backup data    | IN       |                           | [BOLT #1][bolt01-Peer-storage]                                        |
+| 42/43 | `option_provide_storage`          | Can store other nodes' encrypted backup data              | IN       |                           | [BOLT #1][bolt01-Peer-storage]                                        |
 | 44/45 | `option_channel_type`             | Node supports the `channel_type` field in open/accept     | IN       |                           | [BOLT #2](02-peer-protocol.md#the-open_channel-message)               |
 | 46/47 | `option_scid_alias`               | Supply channel aliases for routing                        | IN       |                           | [BOLT #2][bolt02-channel-ready]                                       |
 | 48/49 | `option_payment_metadata`         | Payment metadata in tlv record                            | 9        |                           | [BOLT #11](11-payment-encoding.md#tagged-fields)                      |