Compatibility requirements for hard or soft forks

[gavinandresen]

Cross-posted from the bitcoin-dev mailing list: (I'm curious to see if there is better discussion here or there)

Should it be a requirement that ANY one-megabyte transaction that is valid under the existing rules also be valid under new rules?

Pro: There could be expensive-to-validate transactions created and given a lockTime in the future stored somewhere safe. Their owners may have no other way of spending the funds (they might have thrown away the private keys), and changing validation rules to be more strict so that those transactions are invalid would be an unacceptable confiscation of funds.

Con: It is extremely unlikely there are any such large, timelocked transactions, because the Core code has had a clear policy for years that 100,000-byte transactions are "standard" and are relayed and mined, and larger transactions are not. The requirement should be relaxed so that only
valid 100,000-byte transaction under old consensus rules must be valid under new consensus rules (larger transactions may or may not be valid).


I had to wrestle with that question when I implemented BIP101/Bitcoin XT when deciding on a limit for signature hashing (and decided the right answer was to support any "non-attack"1MB transaction; see https://bitcoincore.org/~gavin/ValidationSanity.pdf for more details).

[edmundedgar]

Weighing in with my not-particularly-informed bikeshed-colour opinions since this seems like a reasonable place for them...

It seems nice to support these theoretical past transactions if it's practical (eg max 1 >0.1 MB tx per block, or is that too complex?) but I think people can get too carried away with the unacceptable confiscation type of things; Ultimately the reason you don't want your bitcoins confiscated is because they're valuable, and they'll only continue to be valuable if bitcoin is able to evolve.

[acoindr]

Maybe the larger question is where is what is valid defined? Is it the software? A written manifesto? If something isn't explicitly forbidden in the software does that equal a promise of validity? I don't think that should be the case.

[BitcoinXio]

Ugh, the link is broken because bitcoin "core" .org folks aren't even bothering to keep your files alive. I'm sure you have this stuff locally but I archived the PDF file (it doesn't look good since it's converted by Google cache from PDF to HTML), but it's here: https://archive.is/OhHqi

Also, here is the talk you did on Validation Sanity on YouTube.

https://youtu.be/j5OjMdqkiiI?t=1m27s