Routehints are nothing complicated. If you happen to check out the code used to generate them routehints are fairly easy. The code merely grabs the primary 20 (arbitrary restrict) or so nodes with whom it has channels that match sure parameters (wholesome, energetic, in any other case usable and public).
They’re only a technique for personal nodes (we will outline a node as non-public after they have solely non-public channels, as this immediately ends in none of their friends broadcasting their existence) to keep away from broadcasting their existence however nonetheless reliably obtain funds (as long as their friends are additionally accessible).
To rephrase, routehints bridge the hole left by the gossip layer by manually extending a payer’s graph of recognized nodes/channels by embedding information in an bill.
Radar places it properly:
Routing hints present data to seek out non-advertised, or non-public, channels. This enables routing of funds to nodes that aren’t publicly seen on the community. A routing trace gives a partial route between the receiver’s non-public node and a public node. This routing trace is then included within the bill generated by the receiver and offered to the payer.
Lastly, as a result of route hints are simply depth 0 channels/nodes from the receiving node’s perspective, it is doable for routehints to fail of their function if the receiving node is barely in a position to provide nodes that are additionally thought-about non-public, for the reason that payer is not going to have the ability to path to them and in flip the receiving node.