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Date: Tue, 26 May 2015 19:47:39 +0100
Message-ID: <CALqxMTFJ1SKbfv2oux2oqytVGKT34cfLNkZiZsXaMiMSUU1KrA@mail.gmail.com>
From: Adam Back <adam@cypherspace.org>
To: Danny Thorpe <danny.thorpe@gmail.com>
Content-Type: text/plain; charset=UTF-8
Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Cost savings by using replace-by-fee,
	30-90%
Precedence: list

The general idea for replace by fee is that it would be restricted so
as to make it safe, eg all the original addresses should receive no
less bitcoin (more addresses can be added).

The scorched earth game theory stuff (allowing removing recipients) is
kind of orthogonal.

Adam

On 26 May 2015 at 19:22, Danny Thorpe <danny.thorpe@gmail.com> wrote:
> What prevents RBF from being used for fraudulent payment reversals?
>
> Pay 1BTC to Alice for hard goods, then after you receive the goods broadcast
> a double spend of that transaction to pay Alice nothing? Your only cost is
> the higher network fee of the 2nd tx.
>
> Thanks,
> -Danny
>
> On Mon, May 25, 2015 at 5:10 PM, Peter Todd <pete@petertodd.org> 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
>>
>> --
>> 'peter'[:-1]@petertodd.org
>> 0000000000000000134ce6577d4122094479f548b997baf84367eaf0c190bc9f
>>
>>
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>
>
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