This specification describes simple node discovery, channel discovery and channel update mechanisms which do not rely on a third-party to disseminate the information.
Node and channel discovery serve two different purposes:
- Channel discovery allows the creation and maintenance of a local view of the network's topology such that the node can discover routes to the desired destination.
- Node discovery allows nodes to broadcast their ID, host and port, such that other nodes can open connections and establish payment channels.
Peers in the network exchange `channel_announcement` messages that contain information about new channels between two nodes. They can also exchange `node_announcement` messages which supply additional information about nodes, and `channel_update` messages which update information about a channel.
There can only be one valid `channel_announcement` for any channel,
but multiple `node_announcement` messages are possible (to update node
information), and at least two `channel_update` messages are expected.
This is a direct message between two endpoints of a channel and serves as an opt-in mechanism to allow the announcement of the channel to the rest of the network.
It contains the necessary signatures by the sender to construct the `channel_announcement` message.
The willingness of the endpoints to announce the channel is signaled during the connection setup by setting a `channel_public` bit in the `localfeatures` field.
Should both endpoints have signaled that they'd like to publish the channel then the `announcement_signatures` message MUST directly sent following the `funding_locked` message that established the corresponding channel.
The `announcement_signatures` message is created by constructing a `channel_announcement` message corresponding to the newly established channel, and sign it with the secrets matching their `node-id` and `bitcoin-key`, and send them using an `announcement_signatures`.
The recipient MAY fail the channel if the `node-signature` or `bitcoin-signature` is incorrect.
The recipient SHOULD queue the `channel_announcement` message for its peers if it has sent and received a valid `announcement_signatures` message.
The creating node MUST compute the double-SHA256 hash `h` of the message, starting at offset 256, up to the end of the message.
Thus the hash skips the 4 signatures, but hashes the rest of the message, including any future fields appended to the end.
`node-signature-1` and `node-signature-2` MUST be valid signatures of the hash `h` using the secret associated with `node-id-1` and `node-id-2` respectively.
`bitcoin-signature-1` and `bitcoin-signature-2` MUST be valid signatures of the hash `h` using the secret associated with `bitcoin-key-1` and `bitcoin-key-2` respectively.
key given by `node-id`. It MAY set `alias` and `rgb-color` to customize their node's appearance in maps and graphs, where the first byte of `rgb` is the red value, the second byte is the green value and the last byte is the blue value. It MUST set `alias` to a valid UTF-8 string, with any `alias` bytes following equal to zero.
match those in the already-sent `channel_announcement` message, and MUST set the least-significant bit of `flags` to 0 if the creating node is `node-id-1` in that message, otherwise 1. It MUST set other bits of `flags` to zero.
It MUST set `cltv-expiry-delta` to the number of blocks it will subtract from an incoming HTLC's `cltv-expiry`. It MUST set `htlc-minimum-msat` to the minimum HTLC value it will accept, in millisatoshi. It MUST set `fee-base-msat` to the base fee it will charge for any HTLC, in millisatoshi, and `fee-proportional-millionths` to the amount it will charge per millionth of a satoshi.
Upon establishing a connection, the two endpoints negotiate whether to perform an initial sync by setting the `initial_routing_sync` flags in the `init` message.
The endpoint SHOULD set the `initial_routing_sync` flag if it requires a full copy of the other endpoint's routing state.
Upon receiving an `init` message with the `initial_routing_sync` flag set the node sends `channel_announcement`s, `channel_update`s and `node_announcement`s for all known channels and nodes as if they were just received.
If the `initial_routing_sync` flag is not set, or initial sync was completed, then the node resumes normal operation, see the _Rebroadcasting_ section for details.
Once the announcement has been processed it is added to a list of outgoing announcements (perhaps replacing older updates) to the processing node's peers, which will be flushed at regular intervals.
This store and delayed forward broadcast is called a _staggered broadcast_
If, after applying the changes from the announcement, there are no channels associated with the announcing node, then the receiving node MAY purge the announcing node from the set of known nodes.
Otherwise the receiving node updates the metadata and stores the signature associated with the announcement.
This will later allow the receiving node to rebuild the announcement for its peers.
After processing the announcement the receiving node adds the announcement to a list of outgoing announcements.
### Requirements
Each node SHOULD flush outgoing announcements once every 60 seconds, independently of the arrival times of announcements, resulting in a staggered announcement and deduplication of announcements.
Nodes MAY re-announce their channels regularly, however this is discouraged in order to keep the resource requirements low.
Nodes SHOULD send all `channel_announcement` messages followed by the
latest `node_announcement` and `channel_update` messages upon
connection establishment.
### Rationale
Batching announcements form a natural ratelimit with low overhead.
The sending of all announcements on reconnection is naive, but simple,
and allows bootstrap for new nodes as well as updating for nodes which
