[bitcoin-dev] Forget dormant UTXOs without confiscating bitcoin

Peter Todd pete at petertodd.org
Sun Dec 20 11:24:54 UTC 2015

On Sun, Dec 13, 2015 at 02:07:36AM +0000, Gregory Maxwell via bitcoin-dev wrote:
> On Sun, Dec 13, 2015 at 1:00 AM, Vincent Truong via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org> wrote:
> > have run a node/kept their utxo before they were aware of this change and
> > then realise miners have discarded their utxo. Oops?
> I believe you have misunderstood jl2012's post.  His post does not
> cause the outputs to become discarded. They are still spendable,
> but the transactions must carry a membership proof to spend them.
> They don't have to have stored the data themselves, but they must
> get it from somewhere-- including archive nodes that serve this
> purpose rather than having every full node carry all that data forever.
> Please be conservative with the send button. The list loses its
> utility if every moderately complex idea is hit with reflexive
> opposition by people who don't understand it.
> Peter Todd has proposed something fairly similar with "STXO
> commitments". The primary argument against this kind of approach that

That's incorrect terminology - what I proposed are "TXO commitments". I
proposed that a MMR of all prior transaction outputs's, spent and
unspent, be committed too in blocks along with a spentness flag, not
just spent transaction outputs.

That's why I often use the term (U)TXO commitments to refer to both
classes of proposals.

> I'm aware of is that the membership proofs get pretty big, and if too
> aggressive this trades bandwidth for storage, and storage is usually
> the cheaper resource. Though at least the membership proofs could be
> omitted when transmitting to a node which has signaled that it has
> kept the historical data anyways.

What I proprosed is that a consensus-critical maximum UTXO age be part
of the protocol; UTXO's younger than that age are expected to be cached.
For UTXO's older than that age, they can be dropped from the cache,
however to spend them you are required to provide the proof, and that
proof counts as blockchain space to account for the fact that they do
need to be broadcast on the network.

The proofs are relatively large, but not so much larger than a CTxIn as
to make paying for that data infeasible.

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