vExtraTxnForCompact Considered Useful

In the discussion about Bitcoin Core's default relay policy and its implications on transaction processing, a significant point of contention is the role and utility of the extrapool mechanism. The extrapool was initially developed as a means to facilitate replacements in transaction handling. Its effectiveness, however, is questioned in the context of heterogeneous policy application, particularly given its limited applicability to Bitcoin Core's defaults. This limitation stems from the premise that the extrapool benefits nodes that engage in more extensive filtering of transactions than their peers. Specifically, it aids when a node has already observed, downloaded, and processed a transaction. In contrast, Bitcoin Core's default settings do not inherently support such selective transaction visibility, thereby rendering the extrapool less useful under these conditions.

The peer-to-peer (p2p) protocol underlying Bitcoin operates on the assumption that the behavior of honest nodes is largely homogeneous and aligns with mining practices. Deviations from this assumed homogeneity could potentially justify an increase in the extrapool size to accommodate a broader range of transactions. However, expanding the extrapool is not without its drawbacks. Such an expansion would escalate the worst-case memory requirements and could decelerate block processing due to the necessity of additional conversion and matching operations. Moreover, enlarging the extrapool contradicts one of the primary rationales behind transaction filtering—that is, to prevent unmined transactions from unnecessarily consuming memory resources.

Despite these challenges, there is a belief in the potential for improvements within the system. It is acknowledged that enhancing the functionality and efficiency of mechanisms like the extrapool could be beneficial. This perspective is grounded in the understanding that while modifications to the extrapool could enhance its utility, they are unlikely to influence the core policies of Bitcoin Core, especially considering the limitations imposed by the default relay policy. The essence of the argument is that if a transaction has never been seen by a node due to the constraints of the default settings, then the capacity or efficiency of the extrapool in handling such a transaction is moot.