Cross Token Network Payments

[ulope]

Abstract

We want to support single payments that cross token network boundaries.

Motivation

TBD

Specification

• Implementation in Raiden itself
  External makes little sense b/c of difficulties with synchronizing, restartability etc.

• Raiden Node:

  • Swap will happen in mediator state machine (Implement token swaps in state machine #6515)
  • The API needs to gain an endpoint that supports payments across TNs (Add API endpoint for cross token payments #6516)
  • Cross network payment messages will contain the per hop token network route information
    • Ideally existing single token payments will remain backwards compatible
    • The new format should be enabled using capabilities
  • Nodes will opt in to providing swaps and inform the PFS via a new message (e.g. PFSSwapUpdate) (Create PFSSwapUpdate message #6511)
    • CLI option to enable token pairs and set rate premium (Add CLI option to enable token swaps #6514)
    • The message will contain a rate premium the node requires over uniswap base. (Create PFSSwapUpdate message #6511)
    • The PFS will update the base rate from uniswap every block (Update base rate from uniswap each block raiden-services#853)
    • The swap update need to have a validity period and be re-published before it expires.
      If not the PFS will remove the swap ability. (Create PFSSwapUpdate message #6511)

• PFS:

  • PFS will gain ability to find paths between multiple token networks (Handle cross token network routing raiden-services#851)
  • The route returned by the PFS will contain the applicable per-hop token network per hop. (Add API for requesting cross token payment routes raiden-services#852, Implement (internal) cross token route format #6512)

• WebUI:

  • An interface is needed

• MVP constraints:

  • Only uniswap as rate source for now
  • Only one rate premium per token pair for both directions
  • Only payments across TNs of the same TNR will be supported
  • PFS will use a fixed amount when querying the rates (amount TBD), i.e. we don't account for slippage for now

• Possible expansions:

  • Per path request rate calculation
  • Nodes can constrain the max slippage
  • Cross TNR payments
  • Payments with oneself
  • Cross blockchain payments

Backwards Compatibility

Ideally payments within a single TN continue to use the same message format and therefore remain backwards compatible.
Should be decided based on implementation effort.