[bitcoin-dev] Removal of reject network messages from Bitcoin Core (BIP61)

[Dustin Dettmer]

The reject message is helpful for figuring out why a tx was rejected.

It’s not useful for determining success, yes. Particularly when doing
segwit / newer types of tx’s as there’s always one or more pesky nodes who
still don’t support it and send a reject message for perfectly good tx’s.

But after a delay where you haven’t seen your tx propagated on the network,
it’s useful to know *why* it failed.

What would be nice is actually expanding this error message. Currently with
RBF tx’s “fee too small” is sent for both original transactions as well as
replacement transactions. So a bug accidentally sending spent txos
(currently in mempool) says “fee too small” instead of something more
appropriate like “fee too small to supersede existing unconfirmed
transaction.”

On Tue, Mar 5, 2019 at 7:26 PM Marco Falke via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Bitcoin Core may send "reject" messages as response to "tx", "block" or
> "version" messages from a network peer when the message could not be
> accepted.
>
> This feature is toggled by the `-enablebip61` command line option and has
> been
> disabled by default since Bitcoin Core version 0.18.0 (not yet released as
> of
> time of writing). Nodes on the network can not generally be trusted to send
> valid ("reject") messages, so this should only ever be used when connected
> to a
> trusted node. At this time, I am not aware of any software that requires
> this
> feature, and I would like to remove if from Bitcoin Core to make the
> codebase
> slimmer, easier to understand and maintain. Let us know if your application
> relies on this feature and you can not use any of the recommended
> alternatives:
>
> * Testing or debugging of implementations of the Bitcoin P2P network
> protocol
>   should be done by inspecting the log messages that are produced by a
> recent
>   version of Bitcoin Core. Bitcoin Core logs debug messages
>   (`-debug=<category>`) to a stream (`-printtoconsole`) or to a file
>   (`-debuglogfile=<debug.log>`).
>
> * Testing the validity of a block can be achieved by specific RPCs:
>   - `submitblock`
>   - `getblocktemplate` with `'mode'` set to `'proposal'` for blocks with
>     potentially invalid POW
>
> * Testing the validity of a transaction can be achieved by specific RPCs:
>   - `sendrawtransaction`
>   - `testmempoolaccept`
>
> * Wallets should not use the absence of "reject" messages to indicate a
>   transaction has propagated the network, nor should wallets use "reject"
>   messages to set transaction fees. Wallets should rather use fee
> estimation
>   to determine transaction fees and set replace-by-fee if desired. Thus,
> they
>   could wait until the transaction has confirmed (taking into account the
> fee
>   target they set (compare the RPC `estimatesmartfee`)) or listen for the
>   transaction announcement by other network peers to check for propagation.
>
> I propose to remove "reject" messages from Bitcoin Core 0.19.0 unless
> there are
> valid concerns about its removal.
>
> Marco
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