[Lightning-dev] Base AMP

[Christian Decker]

I'm not sure this is an improvement at all over just allowing a single
merge-point, i.e., the destination. You see as long as we don't attempt
intermediate merges the routes are independent and failures of one HTLC
do not impact any other parts. Take for example the network below:

  --------
 /        \
A----B-----C-----D
\               /
 -------E-------

For simplicity let's assume unit capacities on all channels except C-D
and a total payment of 2 from A to D.

If we use C as a merge point for the two partial payments A-C-D and
A-B-C-D, then C can only forward if both partial payment succeed, i.e.,
if for example A-C fails then we'll need to tear down the HTLCs for both
paths because it'll no longer be possible to find an alternative route
to fulfill the forwarding of 2 over C-D.

If however we have two independent routes A-B-C-D and A-C-D, then A-C-D
can fail independently and we can recover by attempting A-E-D, no need
to touch A-B-C-D at all.

Overall it seems we get very little benefit (we save some HTLC setups
and teardown) for a lot of added complexity. In the above case we would
have saved on a single C-D HTLC, and the cost of doing so is many times
larger (2 HTLCs needed to be torn down because we could no longer pass
enough capacity to C in order for it to reach the forward threshold).

Let's please stick with the simple mechanism of having the recipient be
the only merge point.

Cheers,
Christian

ZmnSCPxj via Lightning-dev <lightning-dev at lists.linuxfoundation.org>
writes:
> Good morning list,
>
> I propose the below to support Base AMP.
>
> The below would allow arbitrary merges of paths, but not arbitrary splits.  I am uncertain about the safety of arbitrary splits.
>
> ### The `multipath_merge_per_hop` type (`option_base_amp`)
>
> This indicates that payment has been split by the sender using Base AMP, and that the receiver should wait for the total intended payment before forwarding or claiming the payment.
> In case the receiving node is not the last node in the path, then succeeding hops MUST be the same across all splits.
>
> 1. type: 1 (`termination_per_hop`)
> 2. data:
>   * [`8` : `short_channel_id`]
>   * [`8` : `amt_to_forward`]
>   * [`4` : `outgoing_cltv_value`]
>   * [`8` : `intended_total_payment`]
>   * [`4` : `zeros`]
>
> The contents of this hop will be the same across all paths of the Base AMP.
> The `payment_hash` of the incoming HTLCs will also be the same across all paths of the Base AMP.
>
> `intended_total_payment` is the total amount of money that this node should expect to receive in all incoming paths to the same `payment_hash`.
>
> This may be the last hop of a payment onion, in which case the `HMAC` for this hop will be `0` (the same rule as for `per_hop_type` 0).
>
> The receiver:
>
> * MUST impose a reasonable timeout for waiting to receive all component paths, and fail all incoming HTLC offers for the `payment_hash`  if they have not totalled equal to `intended_total_payment`.
> * MUST NOT forward (if an intermediate node) or claim (if the final node) unless it has received a total greater or equal to `intended_total_payment` in all incoming HTLCs for the same `payment_hash`.
>
> The sender:
>
> * MUST use the same `payment_hash` for all paths of a single multipath payment.
>
> Regards,
> ZmnSCPxj
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