Anchor Output channels
Recap: Channel states
- Each transaction is a new channel state
- Each new channel state is an unpublished L1 TX
- L1 TXs can be broadcasted at any time, unilateraly
- ... this closes a channel
The problem
The problem
- Fees need to be set when L1 TX is crafted
- L1 TX is crafted when upgrading to channel state ...
- ... NOT when actually publishing the TX (= channel closing)
- These two events can happen at a completely different time
Conclusion
- Fees need to be estimated for an unknown time in the future
- It won't be accurate
- Overpaying is real ...
- ... underpaying as well
The good news
- Only affects unilateral closes
- Does not affect collaborative closes!
- We don't really want the user to close the channel anyways
- Bugs
- Self-Sovereignity
Solution
- Use a low fee in Commitment TX
- CPFP the Commitment TX
- CPFP = Child Pays For Parent
- That's the basic idea behind anchor channels
- Unfortunately it cannot be done than that simple
Imposed limits for CPFP
- There are SPAM-protection measures in place for spending unconfirmed UTXOs
- Max 24 descendants of unconfirmed TX
- ... and other measures
Transaction pinning
Malicious channel counterparty can:
- Create 24 descendants
- with low feerate
- such that no TX ever gets confirmed
- That means you cannot CPFP!
- Channel closing stays in limbo (potentially forever)
CPFP carve out
A new L1 policy has been deployed
- 25th descendant is allowed, if it:
- has no more than one unconfirmed parent
- is of size no greater than 1K in virtual size
Now the scheme works, right?
- You can CPFP your output from the channel
- Your counterparty can CPFP their output
- They can at most create 24 descendants ...
- ... which leaves room for your 25th descendant spending your output
There's another problem ....
- Your counterparty may have multiple outputs (because of unresolved HTLC)
- So he can add 24 descentants on one output and the 25th on another
- ... which again prevents you from CPFPing
Anchor Output channels
- All channel outputs can only be spent 1 block after TX confirmation
- Consequently, you cannot create any descendants for them, while TX is still unconfirmed
- Two new outputs are added, with the only goal of being used for CPFPing
- ... one for you, one for your counterparty
TADAAAAA!!
Anchor Output channels
- All channel outputs can only be spent 1 block after TX confirmation
- Consequently, you cannot create any descendants for them, while TX is still unconfirmed
- Two new outputs are added, with the only goal of being used for CPFPing
- ... one for you, one for your counterparty
Hunting on the Blockchain
Visualization
Advantages
- Define L1 fees in-time
Indispensable for LSPs (Breez)
Disadvantages
- You need additional funds outside the channel to CPFP
- Construct needs more block space, not less