CPFP

[kuk]

Hi

to do a cpfp transaction where does the miners fee comes from? Is it payed from the input of the parent transaction?

Thx in advance from your answer(s)

[ranochigo]

Not directly. A CPFP transaction spends from the output of the unconfirmed transaction; this means that the extra fees can come from the UTXO that was created from the unconfirmed transaction. Alternatively, you can spend another UTXO in the CPFP transaction to supplement the fees as well.

[kuk]

oh got it. thanks for your fast answer

[Charles-Tim]

oh got it. thanks for your fast answer

In addition, know that you must include a change address UTXO in the child transaction if you are the sender. If change address UTXO is not included, you will not be able to use CPFP as the second transaction.

The miner fee still comes from part of the bitcoin you are transferring, but you must include change address UTXO (not necessarily all the bitcoin in the UTXO)

If you are the receiver, you can also spend the unconfirmed input on some wallets, like electrum.

The fee that would be paid should be a fee that can get two transactions confirmed at ones when the last fee paid has been added along.

CPFP fee= fee that can get two transactions confirmed minus the last fee paid.

[kuk]

You mean as a sender and rbf is not possible i can use cpfp also but must include a change utxo?! Does this change utxo has to be the initial change address from the parent transaction? Doesnt it? If i sent with max amount is a cpfp even possible as sender?

[hosseinimr93]

You mean as a sender and rbf is not possible i can use cpfp also but must include a change utxo?! Does this change utxo has to be the initial change address from the parent transaction? Doesnt it? If i sent with max amount is a cpfp even possible as sender?

The general rule is that, for implementing CPFP, you need to have control over one of outputs of the unconfirmed transaction.
If you you send the entire balance of inputs to another person, you can't implement CPFP.
If you don't send the entire balance, your transaction will include a change address and you will be able to implement CPFP.

Let's say you send bitcoin from address A to address B. (You have control over address A and have no control over address B)
If you haven't sent the total balance, your transaction will be as follows.

Address A -----> Address B
                         Address C

Where address C is your change address and contains the remaining balance of address A.

CPPF can be implemented by owner of address B and owner of address C (you).

[Charles-Tim]

It will be good to use a wallet that support replace-by-fee, the fee to accelerate transaction will be much lower if replace-by-fee is used. Only what will be required is to pump the fee, rather than making another (child) transaction that will require more fee to be paid.

You mean as a sender and rbf is not possible i can use cpfp also but must include a change utxo?

Yes. But, not necessary mean all the change address UTXO, you can use just some of the bitcoin from the total change address UTXO.

Does this change utxo has to be the initial change address from the parent transaction?

Yes.

If i sent with max amount is a cpfp even possible as sender?

If you send all the amount of bitcoin, that means there will not be change address. In this case, you will not be able to use child-pay-for-parent if you are the sender, because no change address UTXO is available to be used as no change address in the parent transaction. If the receiver is using a wallet that can spend unconfirmed transaction, like electrum, the receiver can use child-pay-for-parent with high fee as explained above and send the coin to another address on his/her wallet. Or, if he wants to send bitcoin to another person, he can use the opportunity to use child-pay-for-parent by sending the unconfirmed transaction, or part of the unconfirmed transaction, both (parent and child transactions) will be confirmed at ones.

[kuk]

Wow for now no more question. thank you both