[Bitcoin-development] Cost savings by using replace-by-fee, 30-90%

s7r s7r at sky-ip.org
Tue May 26 21:29:28 UTC 2015


What is wrong with the man testing some ideas on his custom branch? This
is how improvements come to life. I saw in the BIPs some really
interesting ideas and nice brainstorming which came from Peter Todd.

Now, my question, if replace by fee doesn't allow me to change the
inputs or the outputs, I can only add outputs... what can I do with this
feature? If I sent a tx and want to replace it with a higher fee one,
the higher fee one can only have maybe additional change addresses or
another payment, if the inputs suffice? Do we have any real use cases?

P.S. is it planned to include this by default in bitcoin core 10.0.3 or
it will remain just on Peter's branch?

On 5/26/2015 11:30 PM, joliver at airmail.cc wrote:
> You're the Chief Scientist of __ViaCoin__ a alt with 30 second blocks 
> and you have big banks as clients. Shit like replace-by-fee and leading 
> the anti-scaling mob is for your clients, not Bitcoin. Get the fuck out.
> 
> Peter Todd - 8930511 Canada Ltd.
> 1214-1423 Mississauga Valley Blvd.
> Mississauga ON L5A 4A5
> Canada
> 
> https://www.ic.gc.ca/app/scr/cc/CorporationsCanada/fdrlCrpDtls.html?corpId=8930511
> 
> On 2015-05-26 00:10, Peter Todd wrote:
>> On Tue, May 26, 2015 at 12:03:09AM +0200, Mike Hearn wrote:
>>> CPFP also solves it just fine.
>>
>> CPFP is a significantly more expensive way of paying fees than RBF,
>> particularly for the use-case of defragmenting outputs, with cost
>> savings ranging from 30% to 90%
>>
>>
>> Case 1: CPFP vs. RBF for increasing the fee on a single tx
>> ----------------------------------------------------------
>>
>> Creating an spending a P2PKH output uses 34 bytes of txout, and 148
>> bytes of txin, 182 bytes total.
>>
>> Let's suppose I have a 1 BTC P2PKH output and I want to pay 0.1 BTC to
>> Alice. This results in a 1in/2out transaction t1 that's 226 bytes in 
>> size.
>> I forget to click on the "priority fee" option, so it goes out with the
>> minimum fee of 2.26uBTC. Whoops! I use CPFP to spend that output,
>> creating a new transaction t2 that's 192 bytes in size. I want to pay
>> 1mBTC/KB for a fast confirmation, so I'm now paying 418uBTC of
>> transaction fees.
>>
>> On the other hand, had I use RBF, my wallet would have simply
>> rebroadcast t1 with the change address decreased. The rules require you
>> to pay 2.26uBTC for the bandwidth consumed broadcasting it, plus the 
>> new
>> fee level, or 218uBTC of fees in total.
>>
>> Cost savings: 48%
>>
>>
>> Case 2: Paying multiple recipients in succession
>> ------------------------------------------------
>>
>> Suppose that after I pay Alice, I also decide to pay Bob for his hard
>> work demonstrating cryptographic protocols. I need to create a new
>> transaction t2 spending t1's change address. Normally t2 would be
>> another 226 bytes in size, resulting in 226uBTC additional fees.
>>
>> With RBF on the other hand I can simply double-spend t1 with a
>> transaction paying both Alice and Bob. This new transaction is 260 
>> bytes
>> in size. I have to pay 2.6uBTC additional fees to pay for the bandwidth
>> consumed broadcasting it, resulting in an additional 36uBTC of fees.
>>
>> Cost savings: 84%
>>
>>
>> Case 3: Paying multiple recipients from a 2-of-3 multisig wallet
>> ----------------------------------------------------------------
>>
>> The above situation gets even worse with multisig. t1 in the multisig
>> case is 367 bytes; t2 another 367 bytes, costing an additional 367uBTC
>> in fees. With RBF we rewrite t1 with an additional output, resulting in
>> a 399 byte transaction, with just 36uBTC in additional fees.
>>
>> Cost savings: 90%
>>
>>
>> Case 4: Dust defragmentation
>> ----------------------------
>>
>> My wallet has a two transaction outputs that it wants to combine into
>> one for the purpose of UTXO defragmentation. It broadcasts transaction
>> t1 with two inputs and one output, size 340 bytes, paying zero fees.
>>
>> Prior to the transaction confirming I find I need to spend those funds
>> for a priority transaction at the 1mBTC/KB fee level. This transaction,
>> t2a, has one input and two outputs, 226 bytes in size. However it needs
>> to pay fees for both transactions at once, resulting in a combined 
>> total
>> fee of 556uBTC. If this situation happens frequently, defragmenting
>> UTXOs is likely to cost more in additional fees than it saves.
>>
>> With RBF I'd simply doublespend t1 with a 2-in-2-out transaction 374
>> bytes in size, paying 374uBTC. Even better, if one of the two inputs is
>> sufficiently large to cover my costs I can doublespend t1 with a
>> 1-in-2-out tx just 226 bytes in size, paying 226uBTC.
>>
>> Cost savings: 32% to 59%, or even infinite if defragmentation w/o RBF
>>               costs you more than you save
>>
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