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It is a common meme in some parts of the Bitcoin sphere that RBF erodes the option of accepting zero confirmation transactions. It would be beneficial if the following questions were answered:
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What restrictions are placed on RBF in the deployed "opt-in RBF" variant?
Opt-in Replace-by-Fee defines a way to mark transactions as replaceable until they are confirmed in a block. This is done by setting the sequence number smaller than MAX-1. (MAX corresponds to standard final transactions and MAX-1 marks locktimed transactions.)
To replace the transaction before it is confirmed, the authoring user may broadcast an update of the transaction (a doublespend!) with a higher fee. This update version may also set the sequence number to a final value, i.e. MAX-1 or MAX, marking it as non-replaceable.
Bitcoin Core v0.12.0 and following versions will allow replacement under the above description, but will not create opt-in RBF transactions by default.
How does opt-in RBF interact with unconfirmed transactions?
MAX-1 or MAX are not affected. Nodes that implement opt-in RBF treat them exactly as before. Doublespends of final transactions are still treated under the first seen paradigm.What influence does this have on zero confirmation payment processing?
Users get a new type of transactions that should not be accepted without confirmation. Accepting final transactions without confirmation has exactly the same risks as before the introduction of opt-in RBF.