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@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 > _______________________________________________ > Lightning-dev mailing list > Lightning-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev _______________________________________________ Lightning-dev mailing list Lightning-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
