Both CPFP and RBF are relay/mining policy and cannot be made consensus rules because you cannot know which transactions have been received by a givrn peer and which have not (or at what time). Consensus rules can only validate information that's in the blockchain.
(I haven't been following this development recently so apologies in advance if I've made assumptions about RBF)
If you made CPFP consensus critical for all Full-RBF transactions, RBF should be safer to use. I see RBF as a necessity for users to fix mistakes (and not for transaction prioritisation), but we can't know for sure if miners are playing with this policy fairly or not. It is hard to spot a legitimate RBF and a malicious one, but if the recipient signs off on the one they know about using CPFP, there should be no problems. This might depend on the CPFP implementation, because you'll need a way for the transaction to mark which output is a change address and which is a payment to prevent the sender from signing off his own txns. (This might be bad for privacy, but IMO a lot safer than allowing RBF double spending sprees... If you value privacy then don't use RBF?) Or maybe let them sign it off but make all outputs sign off somehow.
Copy/Paste from my reddit post:
https://www.reddit.com/r/Bitcoin/comments/3ul1kb/slug/cxgegkj
Going to chime in my opinion: opt-in RBF eliminates the trust required with miners. You don't know if they're secretly running RBF right now anyway. Whether Peter Todd invented this is irrelevant, it was going to happen either way either with good intentions or with malice, so better to develop this with good intentions.
Perhaps the solution to this problem is simple. Allow Full-RBF up to the point where a recipient creates a CPFP transaction. Any transaction with full RBF that hasn't been signed off with a CPFP cannot go into a block, and this can become a consensus rule rather than local policy thanks to the opt-in flags that's inside transactions.
> P.S. (When I wrote this, I'm actually not sure how the flag looks like and am just guessing it can be used this way. I'm not familiar with the implementation.)
CPFP is needed so that merchants can bear the burden of fees (double bandwidth costs aside, and frankly if RBF is allowed bandwidth is going to increase regardless anyway). That's always the way I've being seeing its purpose. And this makes RBF much safer to use by combining the two.
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