In my prior article on the mempool, I laid out a simple conceptual framework to reason about the basic functionality of the mempool, and how it was used by different kinds of users of the Bitcoin network. In this piece I will be looking at the differences between relay policy and consensus rules, and why by default Bitcoin nodes do not relay some types of bitcoin transactions despite being consensus valid. 

First and foremost, regardless of the peer-to-peer network refusing to relay certain kinds of consensus valid transactions, if those transactions were to find up in a miner’s mempool and be selected for inclusion in a block, they will be received and downloaded by nodes when…

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