Hi,

I'm proposing to stop the processing of unrequested transactions in Bitcoin
Core 22.0+ at TX message reception. An unrequested transaction is one
defined by which a "getdata" message for its specific identifier (either
txid or wtxid) has not been previously issued by the node [0].

This change is motivated by reducing the CPU DoS surface of Bitcoin Core
around mempool acceptance. Currently, an attacker can open multiple inbound
connections to a node and send expensive to validate, junk transactions.
Once the canonical INV/GETDATA sequence is enforced on the network, a
further protection would be to deprioritize bandwidth and validation
resources allocation, or even to wither connections with such DoSy peers. A
permissioned peer (PF_RELAY) will still be able to bypass such restrictions.

Raw TX message processing has always been tolerated by Core and as such
some Bitcoin clients aren't bothering with an INV/GETDATA sequence. Such
change will break their tx-relay capabilities on the p2p network and
require adaptation from them. Given deployment time of any release, I hope
it provides a window time wide enough before the old tx-processing behavior
becomes the minority.

Eager to gather feedback on this proposal, especially if such change is
deemed as too much constraining or fast on any Bitcoin software.

Cheers,
Antoine

[0] See https://github.com/bitcoin/bitcoin/pull/20277