On Mon, Jan 22, 2018 at 12:40:31PM -0500, Rhavar via bitcoin-dev wrote: > So my half-baked idea is very simple: > > Allow users to merge multiple unconfirmed transactions, stripping extraneous > inputs and change as they go. > > This is currently not possible because of the bip125 rule: > "The replacement transaction pays an absolute fee of at least the sum paid by > the original transactions." > > Because the size of the merged transaction is smaller than the original > transactions, unless there is a considerable feerate bump, this rule isn't > possible to observe. > > I my question is: is it possible or reasonable to relax this rule? If this > rule was removed in its entirety, does it introduce any DoS vectors? Or can > it be changed to allow my use-case?
It would definitely introduce DoS vectors by making it much cheaper to use relay bandwidth. You'd also be able to push others' txs out of the mempool. > --- > Full backstory: I have been trying to use bip125 (Opt-in Full Replace-by-Fee) > to do "transaction merging" on the fly. Let's say that I owe John 1 bitcoin, > and have promised to pay him immediately: Instead of creating a whole new > transaction if I have an in-flight (unconfirmed) transaction, I can follow > the rules of bip125 to create a replacement that accomplishes this goal. > > From a "coin selection" point of view, this was significantly easier than > I had anticipated. I was able to encode the rules in my linear model and > feed in all my unspent and in-flight transactions and it can solve it without > difficulty. > > However, the real problem is tracking the mess. Consider this sequence of > events: > 1) I have unconfirmed transaction A > 2) I replace it with B, which pays John 1 BTC > 3) Transaction A gets confirmed > > So now I still owe John 1 BTC, however it's not immediately clear if > it's safe to send to him without waiting $n transactions. However even > for a small $n, this breaks my promise to pay him immediately. > > One possible solution is to only consider a transaction "replaceable" if it > has change, so if the original transaction confirms -- payments can > immediately be made that source the change, and provide safety in a reorg. > > However, this will only work <50% of the time for me (most transactions > don't have change) and opens a pandora's box of complexity. Most transactions don't have change?! Under what circumstance? For most use-cases the reverse is true: almost all all transactions have change, because it's rare for the inputs to exactly math the requested payment. -- https://petertodd.org 'peter'[:-1]@petertodd.org
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