Interactive Transaction Construction (InteractiveTxConstructor, splicing, dual-funding)

Host: dunxen  -  PR author: jurvis


  • The interactive transaction construction protocol, outlined in lightning/bolts#851, allows parties to collaboratively construct a bitcoin transaction by exchanging inputs and outputs for that transaction. It finds its first practical use in the dual-funding protocol where both parties A and B can contribute to the value of the channel during establishment. An example interactive transaction construction session (from the spec) could look like the following:

      +-------+                         +-------+
      |       |--(1)- tx_add_input ---->|       |
      |       |<-(2)- tx_complete ------|       |
      |       |--(3)- tx_add_output --->|       |
      |       |<-(4)- tx_complete ------|       |
      |       |--(5)- tx_add_input ---->|       |
      |   A   |<-(6)- tx_add_input -----|   B   |
      |       |--(7)- tx_remove_output >|       |
      |       |<-(8)- tx_add_output ----|       |
      |       |--(9)- tx_complete ----->|       |
      |       |<-(10) tx_complete ------|       |
      +-------+                         +-------+
  • Splicing also makes use of the interactive transaction construction protocol for constructing splice-in and splice-out transactions.
  • This PR implements the interactive transaction construction protocol as a type-safe state machine.


  1. Did you review the PR? Concept ACK, approach ACK, tested ACK, or NACK?
  2. What is the SerialId of an input or output and how is it chosen in this implementation? What is the SerialId parity constraint and why is it used?
  3. Why are the number of inputs and outputs for the constructed transaction limited to 252 each?
  4. If we already have the limit for inputs and outputs above, why do we place a limit of 4096 on received TxAddInput and TxAddOutput messages?
  5. How are the state transitions defined using the define_state_transitions macro and what is the purpose of doing so in this way? How does it help holders of an InteractiveTxConstructor?
  6. What transaction fees are paid by the non-initiator of the interactive construction protocol?