[Lightning-dev] AMP: Atomic Multi-Path Payments over Lightning

[ZmnSCPxj]

Good morning Laolu,

This is excellent work!

Some minor comments...

> (Atomic Multi-path Payments). It can be experimented with on Lightning
> *today* with the addition of a new feature bit to gate this new
> feature. The beauty of the scheme is that it requires no fundamental changes
> to the protocol as is now, as the negotiation is strictly *end-to-end*
> between sender and receiver.

I think, a `globalfeatures` odd bit could be used for this.  As it is end-ot-end, `localfeatures` is not appropriate.

>   - Potential fee savings for larger payments, contingent on there being a
>     super-linear component to routed fees. It's possible that with
>     modifications to the fee schedule, it's actually *cheaper* to send
>     payments over multiple flows rather than one giant flow.

I believe, currently, fees have not this super-linear component.  Indeed, the existence of per-hop fees (`fee_base_msat`) means, splitting the payment over multiple flows will be, very likely, more expensive, compared to using a single flow.  Tiny roundoffs in computing the proportional fees (`fee_proportional_millionths`) may make smaller flows give a slight fee advantage, but I think the multiplication of per-hop fees will dominate.

>   - Using smaller payments increases the set of possible paths a partial
>     payment could have taken, which reduces the effectiveness of static
>     analysis techniques involving channel capacities and the plaintext
>     values being forwarded.

Strongly agree!

> In order to include the three tuple within the per-hop payload for the final
> destination, we repurpose the _first_ byte of the un-used padding bytes in
> the payload to signal version 0x01 of the AMP protocol (note this is a PoC
> outline, we would need to standardize signalling of these 12 bytes to
> support other protocols).

I believe the `realm` byte is intended for this.  Intermediate nodes do not need to understand realm bytes that are understood by other nodes in the route, including the realm bytes understood by the final destination, as intermediate nodes cannot, indeed, read the hop data of other nodes.  Thus, you can route over nodes that are unaware of AMP, and only provide an AMP realm byte to the destination node, who, is able to reconstruct this your AMP data as per your algorithm.

Indeed, the `realm` byte controls the interpretation of the rest of the 65-byte packet.  If you define, instead, a separate `realm` that is understood by the destination node, you can redefine the entire 64 bytes of the final hop data as you wish.

If we support AMP only at final payees, we can completely redefine the 64 bytes in the final hop data for the new AMP `realm`, and not consume the next hop (which would reduce route length by 1).

(If we want to support multiple routes converging to an intermediate node, then continue routing to a different final node after routes have merged (i.e. A->B->C->D, and A->E->C->D, with the payment being merged by C, who forwards the combination to D), then we need to follow the current hop data format, but I think supporting AMP at final payees is actually enough... AMP at intermediate nodes might not be used often enough by senders for it to matter, as taking advantage of that seems more complex than just asking your routing algo to provide you multiple routes to a destination, which you are probably already doing)

----

Overall, good work I think.

Regards,
ZmnSCPxj
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