An interesting experiment would be a transaction "proof of publication" chain.

Each transaction would be added to that chain when it is received.  It could be merge mined with the main chain.

If the size was limited, then it doesn't even require spam protection.

Blocks could be "discouraged" if they have transactions which violate the ordering in that chain.  Miners could still decide which transactions they include, but couldn't include transactions which are double spends.

The locktime/final field could be used for transactions which want to be replaceable.

The chain could use some of the fast block proposals.  For example, it could include orphans of a block when computing the block's POW.



On Wed, Apr 23, 2014 at 9:53 PM, Gregory Maxwell <gmaxwell@gmail.com> wrote:
On Wed, Apr 23, 2014 at 1:44 PM, Adam Ritter <aritter@gmail.com> wrote:
> Isn't a faster blockchain for transactions (maybe as a sidechain) solving
> the problem? If there would be a safe way for 0-confirmation transactions,
> the Bitcoin blockchain wouldn't even be needed.

Large scale consensus can't generally provide instantly irreversible
transactions directly: Increasing the block speed can't help past the
point where the time starts getting close to the network diameter...
you simply can't tell what a consensus of a group of nodes is until
several times the light cone that includes all of them.  And if you
start getting close to the limit you dilute the power working on the
consensus and potentially make life easier for a large attacker.

Maybe other chains with different parameters could achieve a different
tradeoff which was better suited to low value retail transactions
(e.g. where you want a soft confirmation fast). A choice of tradeoffs
could be very useful, and maybe you can practically get close enough
(e.g. would knowing you lost a zero-conf double spend within 30
seconds 90% of the time be good enough?)... but I'm not aware of any
silver bullet there which gives you something identical to what a
centralized service can give you without invoking at least a little
bit of centralization.

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