Support async payments in BOLT 12 #1149
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Changed up the feature bits, required single-chain offers, and disallowed setting |
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Added a fixup requiring the invreq to be included in the payment onion per spec meeting discussion a week ago! |
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Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously. We use an experimental TLV type for this new onion payload field, since the spec is still not merged in the BOLTs.
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Add a new invoice request parameter to onion_utils::build_onion_payloads. As of this commit it will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.
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Add a new invoice request parameter to onion_utils::create_payment_onion. As of this commit it will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.
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Add a new invoice request parameter to outbound_payments and channelmanager send-to-route internal utils. As of this commit the invreq will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.
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When transitioning outbound payments from AwaitingInvoice to StaticInvoiceReceived, include the invreq in the new state's outbound payment storage for future inclusion in an async payment onion. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.
joostjager
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Feb 28, 2025
| - Otherwise: | ||
| - MUST follow the requirements for the reader of `payload` in [Payload Format](04-onion-routing.md#payload-format) | ||
| - if the `hold_htlc` TLV is present: | ||
| - MUST NOT forward the HTLC until a corresponding `release_held_htlc` onion |
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Is it known what value LSPs typically use for max htlcs on their channels? If the LSP is the chan initiator, they may want to keep it low to avoid a high commit tx absolute fee. This would then limit the number of outstanding async payments.
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Good point, although maybe worth noting that the HTLC slot is only taken up on the mobile sender <> LSP channel, which isn't typically being used for any other payment forwards. LSPs may indeed want to limit the number of outstanding async payments though as you say.
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For small tips it may not be a problem because those do not add to the commit tx weight and fee risk for the LSP. Although the risk then is that the LSP might lose the htlc values in a close event.
joostjager
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| * [`1366*byte`:`onion_routing_packet`] | ||
| * [`update_add_htlc_tlvs`:`tlvs`] | ||
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| 1. `tlv_stream`: `update_add_htlc_tlvs` |
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Discussed offline: maybe signal hold_htlc in the onion, so that the option remains open to park htlcs at a more remote node? Potentially as a way to unburden the sender LSP from async payments.
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Appreciate the feedback and discussion @joostjager! |
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This is an incredibly simple first start towards the protocol sketched out at [1]. It adds the ability to have a counteraprty hold an HTLC before forwarding it. Specifically, the HTLC sender sets a required TLV on the `update_add_htlc` message and sends an onion message to the final recipient. From there, the final recipient uses the included `reply_path` to notify the sender's counterparty that they're online and ready to receive the HTLC. In order to fully flesh out the protocol as sketched, we'll need to add an onion message mailbox service (which should be as simple as a new feature bit), add PTLCs, and extensions to BOLT 12 to allow signaling that the final recipient is often-offline. While we could add such signaling to BOLT 11, there's not a whole lot of reason to - if the recipient is able to provide an invoice, they're currently online! [1] https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-October/003307.html
This builds on prior commits by adding the ability to fetch an invoice from an always-online node on behalf of an often-offline recipient, e.g. a mobile node. The idea is that often-offline recipients will supply some always-online node such as their wallet vendor with a static (i.e. `payment_hash`-less) invoice to return on its behalf. The recipient will then publish an offer containing blinded paths that terminate at this always-online node, who payers can request the invoice from if the recipient is offline at the time. After receiving the static invoice, payers will commence the protocol outlined in [1] to send the HTLC asynchronously. [1]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-October/003307.html
This field may be useful for often-offline recipients who did not receive the invreq when it was originally sent, due to another node providing a static invoice on their behalf. Recipients may want to verify the invreq or be provided some other relevant data about the payment while remaining stateless until an HTLC is actually received.
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This builds on #989 by adding the ability to fetch an invoice from an
always-online node on behalf of an often-offline recipient, e.g. a mobile node.
The idea is that often-offline recipients will supply some always-online node
such as their wallet vendor with a static keysend (i.e.
payment_hash-less)invoice to return on its behalf. The recipient will then publish an offer
containing blinded paths that terminate at this always-online node, who payers
can request the invoice from if the recipient is offline at the time. After
receiving the keysend invoice, payers will commence the protocol outlined in
1 to send the HTLC asynchronously.
Some context on the top commit where we include the invoice request in the payment onion:
This definitely warrants discussion, but the idea is that this field may be useful for often-offline recipients who did not receive the invoice request when it was originally sent. Recipients may want to verify the invreq or be provided some other relevant data about the payment, while keeping the payment stateless until an HTLC is actually received. For example, future extensions have been proposed 2 that require the recipient to know a unique token for a payment, and this field would provide that to them.
Seeking conceptual feedback! I'm also working on the implementation in LDK.
Based on
#798and #989.