[bitcoin-dev] Why not Child-Pays-For-Parent?

Jeff Garzik jgarzik at gmail.com
Fri Jul 10 16:31:01 UTC 2015


This is a good explanation but it does not address reachability.  TX_a, the
first tx sent out on the network, presumably has insufficient fee to get
mined - which also means it did not necessarily even reach all miners.

Simply sending out TX_b with added fee does not guarantee that nodes
suddenly have TX_a, which they may have ignored/dropped before.



On Fri, Jul 10, 2015 at 12:28 PM, Tier Nolan <tier.nolan at gmail.com> wrote:

>
>
> On Fri, Jul 10, 2015 at 5:09 PM, Richard Moore <me at ricmoo.com> wrote:
>
>> I was also wondering, with CPFP, should the transaction fee be based on
>> total transactions size, or the sum of each transaction’s required fee? For
>> example, a third transaction C whose unconfirmed utxo from transaction B
>> has an unconfirmed utxo in transaction A (all of A’s inputs are confirmed),
>> with each A, B and C being ~300bytes, should C’s transaction fee be 0.0001
>> btc for the ~1kb it is about to commit to the blockchain, or 0.0003 btc for
>> the 3 transactions it is going to commit.
>>
>
> It should be whatever gives the highest fee.  In effect, child pays for
> parent creates compound transactions.
>
> A: 250 bytes, 0 fee
> B: 300 bytes: 0.0005 fee
> C: 400 bytes: 0.0001 fee
>
> There are 3 combinations to consider
>
> A: 0 fee for 250 bytes = 0 per byte
> A&B: 0.0005 fee for 550 bytes = 0.91 uBTC per byte
> A&B&C: 0.0006 fee for 950 bytes = 0.63uBTC per byte
>
> This means that the A&B combination has the best fee per byte value.  A&B
> should be added to the memory pool (if 0.91 uBTC per byte is above the
> threshold).
>
> Once A&B are added, then C can be reconsidered on its own.
>
> C: 0.0001 for 400 bytes = 0.25 BTC per byte
>
> If that is above the threshold, then C should be added.
>
> In practice, it isn't possible to check every combination.  If there are N
> transactions, then checking all triple combinations costs around N cubed.
>
> A 2 pass system could get a reasonably efficient result.
>
> B is 0.0005 fee for 300 bytes = 1.67 uBTC per byte and is assumed to be a
> high value transaction.
>
> The algorithm would be
>
> Pass 1:
> Process all transactions in order of BTC per byte, until block is full
>     If the transaction's parents are either already in the pool or a
> previous block, add the transaction.
>
> Pass 1:
> Process all non-included transactions in order of BTC per byte, until
> block is full
>     If the transaction's parents are either already in the pool or a
> previous block, add the transaction.
>
>     Otherwise, consider the transaction plus all non-included ancestors as
> a single transaction
>         If this combined transaction has a higher BTC per byte than the
> lowest transaction(s),
>             add the combined transaction
>             drop the other transaction(s)
>
>
>
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