HTLC across two Hydra Heads

[noonio]

Why

We would like to demonstrate transfer of fungible assets between Hydra heads. Notably, this should be able to be done without decommiting/commiting it (i.e. without passing via L1.) This is the "tried and true" method of all payment channel networks (e.g. Bitcoin Lightning.)

What

Scenario:

1. Two two-party Hydra Heads
2. One party participates in both heads
3. I.e. Head 1 = Alice <-> Ida and, Head 2 = Ida <-> Bob
4. Alice has some ada (in Head 1), Ida has (at least) that much ada in Head 2. We want to send Ada from Alice to Bob and have Ida not put any Ada at risk.

To build:

• Hydra-node must not change to accomodate this functionality (should already work!)
• A process-compose setup (like the #demo),
• A UI for Alice that allows Alice to specify they want to send Xyz Ada to Bob (or, in theory, any recipient they have a "path" to),
• A similar UI for Bob to see that they are expecting monies (or at least to see their balance),
• An Aiken plutus script which implements the HTLC logic, (running on L2) (see picture)
• An offchain piece for Ida to observe her state
• (All the same frontend application, as much as possible)

How

• No Haskell
• No fees for now.
• Aiken to get CBOR for the contracts
• Use one of the known client libraries
• TypeScript!
  • Can connect to the hydra-node via the WebSockets

Related links

• https://github.com/AngeYobo/htlc-contract-aiken/blob/main/htlc-contract.ak
• https://interledger.org/developers/rfcs/hashed-timelock-agreements/

[v0d1ch]

What is confusing for me in this approach is that there is a HTLC script (committed to both Heads, I presume) which should validate the transfer. Confusing part is how will it be able to do that if L2 ledger's are isolated and known only to relevant Hydra Head.
If I submit a tx that locks some ada on L2 inside HeadNo1 how would you be able to claim the locked ada in another Head if the two ledger states are completely separate and don't know of each other?