I do not see any transaction in the mempool having differing values for the fee and modified fee fields. Does nobody modify or accelerate their transactions? How do transactions with modified fees show up in the mempool?

  • What software are you using to inspect transactions? – Murch Aug 22 '20 at 1:00
  • I have access to the transactions through JSON file receiver which will be called every 1 minute. – Motiv Aug 24 '20 at 15:14
  • I meant, what is the source of the transaction data? Is it Bitcoin Core or a different Bitcoin implementation? – Murch Aug 24 '20 at 20:04
  • official bitcoin core implementation – Motiv Aug 25 '20 at 16:28
  • Hi Motiv, I've edited your question to make it clearer what was being asked. I was struggling with the last sentence of your original post. Did I get the meaning of that right? Please feel free to edit it further, otherwise. Thank you. – Murch Jan 31 at 0:49

As Pieter wrote in his answer, modified fee indicates that the node operator instructed his node to treat a transaction as if it had a higher fee. However, I think that you might be asking due to a misconception of how RBF affects transactions.

Most transactions are immutable: once they have been defined, the signatures on the inputs commit to the transaction's exact content and composition. Updating the fee of a transaction would require at least changing some output amounts. However, changing even one byte of the transaction would invalidate the signatures. (The exception is transaction malleability, which seems irrelevant in the context, so I'm going to ignore that here.) Therefore, transactions cannot be updated.

However, BIP-125 Opt-in Full Replace-by-Fee Signaling specifies how to mark transactions as replaceable which causes nodes to allow conflicting transactions with higher fees to supersede them. While the old transaction remains unchanged and valid in that case, the newer transaction is more lucrative to miners and therefore expected to be prioritized.

  • This is all correct, but I don't think it has anything to do with the "modified fee" reported by Bitcoin Core mempool RPCs. – Pieter Wuille Jan 30 at 22:40
  • Hello, Thank you. That was exactly the thing I was looking for. Special thanks. – Motiv Jan 31 at 0:26
  • @PieterWuille: That's right, but it looks like you answered the question, and I answered the underlying question why they were asking in the first place. :) TBH, I didn't quite understand what they meant with "modified fee", but the "Why does nobody accelerate their transaction?" seemed like a good lead. I only understood the question fully now that I saw your answer, and their reaction to our answers, though. – Murch Jan 31 at 0:37
  • I still don't understand what that has to do with modifying transactions, but ok! – Pieter Wuille Jan 31 at 1:00
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    My read is that Motiv thought RBF would leave transactions with the same txid, but a modified fee. – Murch Jan 31 at 1:01

I can not see any transaction resting in Mempool having different values for Fee and Modified Fee fields?

That's probably because you haven't modified any.

The prioritisetransaction RPC exists for miners to signal that particular transactions are paid for out of band, or are the miner's own transactions that should prioritized for block construction over what is implied by their feerate.

If you're not a miner, there is no need to ever use this RPC, and the fee and modified fee will be identical for all transactions.

  • Hello, I am looking at the integrated pool of transactions waiting in Mempool and I can not see any modified transaction there. I expected to see some at least in a month or in 6 months. Thank you – Motiv Jan 31 at 0:29
  • Have you ever invoked the prioritisetransaction RPC on your own node? If not, the modified fee will always be identical to the normal fee, because you haven't modified any. The "modified fee" is not a global network-wide property; it's local to your own node, and unless you're a miner, you never need it. – Pieter Wuille Jan 31 at 0:54
  • Reading the other comments I can maybe clarify better: the "modified fee" field in the RPC output has nothing to do with RBF. It's a mechanism for miners to accelerate their own transactions. – Pieter Wuille Jan 31 at 1:11

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