Hi Val, Thanks for the proposal here. About using OM for payment probing, I think there could be an alternative of the routing hops themselves announcing their liquidity balances with an extension or new set of gossip messages.
Gossips messages could commit to a liquidity balance and duration (e.g +1000 blocks from tip), rather relying on sender-side probing, which might be less and less accurate with higher rate of network liquidity congestion. Additionally, a routing hop could commit to "private" gossip sent back to HTLC senders during HTLC successful settlement. Those "private" gossip might announce "differentiated services" of channel liquidity levels. I believe this can be guaranteed transparently from the rest of the network thanks to parallel channels. Any deviation from the gossip message could be penalized by sender's scoring algorithms as the liquidity SLA parameters have been announced explicitly by the routing hops. This is just an intuition on how public channel liquidity discovery could be worked out, and I don't know which of the alternatives would be more efficient in terms of bandwidth/convergence delays. Best, Antoine Le lun. 27 févr. 2023 à 21:32, vwallace via Lightning-dev < lightning-dev@lists.linuxfoundation.org> a écrit : > Hi list! > > I wanted to bring up the idea of using onion messages for payment probing, > which was briefly touched > on at the 2022 LN summit. Tadge Dryja has also brought up a similar idea. > > I recommend reading the gist instead since it has the relevant diagrams > in-line: > https://gist.github.com/valentinewallace/ebe1741f0438c2360eda0f80f0e075c9 > but the scheme is also > posted below for convenience. > > ## Introduction > > For context, in today’s lightning, payment reliability tends to heavily > depend on having sufficient > payment volume to determine current liquidity balances of channels, which > is how most big nodes can > tell whether a given channel has enough liquidity to forward a given > amount. If a node is using HTLC > probing to achieve this payment volume, they use a regular > `update_add_htlc` message with a bogus > payment hash, where the error returned informs the sender of whether the > payment reached the final > recipient. Note that there is a tradeoff between always routing through > nodes that are known to > rebalance their channels vs leaning on probing smaller nodes and “risking” > payments through them > based on what’s learned. > > Today’s HTLC payment probing can work well, but risks channel liquidity > being locked up if a peer > along the route goes offline. On the upside, for just-in-time probes, it > works to loosely “reserve” > the channel liquidity along the route for the actual payment. > > Onion messages (OMs) present a convenient way to probe without risking > locked liquidity along the > route. > > ## Design Rationale > > A naive approach could be to send onion message probes directly to > individual nodes along the > desired route, including those to which you don’t have a direct channel. > However, this scheme is > problematic because it would enable monitoring the payment flows of > arbitrary nodes across the > network without having to have a channel path to them. This would be a > significant degradation of > privacy because, for comparison, in HTLC probing it is quite difficult to > probe the balances of > far-off nodes. And if you can’t probe a node using HTLCs, you can’t send > over it anyway, so there’s > not a lot of benefit (and significant privacy downside). > > Therefore, it is probably best to design a scheme that probes along > channel paths, like HTLC > probing. This adds more complexity to the case where an intermediate node > cannot forward the desired > amount due to the stateless nature of OMs, discussed further down. > > ## Scheme > > Let’s go through the happy path, where Alice is probing Alice > Bob > > Carol > Dave for a 100k msat > payment. > > She’ll construct an onion message for Bob, the first hop, as such: > https://imgur.com/BZg8yt8 > > Bob receives this OM, sees that he’s able to forward 110k msats to his > next-hop Carol, and forwards > Carol’s onion packet to her. Carol sees she can forward 105k msats to > Dave, and forwards his onion > packet. Finally, Dave receives his onion packet, sees he can receive 100k > msats from Carol, and uses > the provided reply path to send a simple probe success onion message back > to Alice: > > ``` > onion_message_probe_result { > data_tlv { > type: 4242, > probe_id: [u8; 32], > can_forward_desired_amt: true, > } > .. regular OM TLVs > } > ``` > > Note that Dave will use this same onion message if he can’t receive; he’ll > just set > `can_forward_desired_amt` to false. > > As an example of the sad path for an intermediate node, if Carol can’t > forward 105k msats to Dave, > she’ll fail the probe back to Bob by sending this onion message: > https://imgur.com/a/hqlzw4I > > This step justifies why we need to encode a failure onion for each > intermediate hop of a probe. If > we hadn’t done that, and Carol responded to Bob with an empty “failed” > message, Bob would have no > idea which peer to fail the probe back to, because OMs are stateless. > Instead, Bob unwraps his error > onion and sees that he needs to fail back to Alice with her provided error > onion. Alice receives the > failure onion and can see that Carol was not able to forward the desired > amount corresponding to the > probe id, thus completing the probe. > > ## Outro > > While there is nothing stopping nodes from lying about their ability to > forward, they may be > negatively scored if they do so. Further, adopting a protocol like this > could help routing nodes > attract more forwarding traffic, which is a nice incentive. > > I view this feature as a low priority enhancement, given there are a lot > more pressing issues in LN, > but open to feedback. Mostly, I thought it could be useful to spark ideas > and highlight the > flexibility of OMs. > > Cheers, > Val > _______________________________________________ > Lightning-dev mailing list > Lightning-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev >
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