### Protocol Updates for Atomic Multi-Path Payments

Atomic multi-path payments is one of the next generation technologies coming to Lightning, it allows a sender to send payments from multiple channels to one receiver, aggregating the payments through the route or the final node.

ZmnSCPxj started a discussion on the protocol update to handle such payments. The receiver provides a payment hash that the sender re-uses across all streams and then waits until the sum matches the invoice.

There is currently two proposals on how to do this, one requires every node in the path to accept AMPs to make every point on the paths a possible merging point, these nodes must only forward (if intermediate node) or claim (if final node) when they receive a total payment greater or equal to intended_total_payment. Receiving nodes then merge every payments to get its final payment while intermediate nodes merge payments and send it along the path, this proposal comes with a con that if any of the intermediate nodes fail in merging the payments the final node will not get its “intended total payment” and so it causes failure for every other Atomic payment.

Another proposal is that only the sender and the receiver should accept AMPs, every other node on the multiple paths just routes the payment as usual and only the receiver node is able to merge payments, this proposal is simpler and does not require the whole network to comply to this update, but might need more connections on the network.

#### Exceprt

### 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).

* 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.