Measuring minrelaytxfee across the Bitcoin network

In an exploration of Bitcoin nodes' behaviors, particularly relating to their response to fee filters, an interesting observation was made regarding nodes running different versions of the Bitcoin software. Initially, it seemed plausible that nodes operating on versions predating the 2016 update might not engage with feefilter due to technological constraints or those using versions before the introduction of the -feefilter configuration option in 2021 intentionally disabling it. This hypothesis was partly based on the premise that older versions lacked the capability or configuration to either send or process feefilter information correctly.

However, upon a detailed analysis, it was discovered that a significant proportion of nodes that failed to send a reply were actually utilizing Core 27.x versions. Data indicated that out of the nodes not returning a feefilter, a considerable number were identified across various recent versions, suggesting that these versions should technically support and return a feefilter value of 9170997. This revelation prompted further investigation into the possible reasons behind this anomaly.

A crucial insight emerged from examining the operational state of these nodes at the time of initial connection attempts. It was found that many of these nodes were in Initial Block Download (IBD) phase, a condition likely affecting their ability to process or respond to feefilter requests as expected. This finding was corroborated by reviewing logs and observing that some nodes did indeed reply with the specific feefilter value when not in the IBD phase upon subsequent connections. This nuanced understanding underscores the complexity of node behavior and the influence of operational states like IBD on protocol interactions within the Bitcoin network.