Characterizing Orphan Transactions in the Bitcoin Network

Orphan transactions are those whose parental income sources are missing at the time that they are processed. These transactions typically languish in a local buffer until evicted or all their parents are discovered, at which point they may be propagated further. So far, there has been little work in the literature on characterizing the nature and impact of such orphans. Yet, it is intuitive that they should affect performance of the Bitcoin network. This work thus seeks to methodically research such effects through a measurement campaign of orphan transactions on live Bitcoin nodes. Our data show that, surprisingly, orphan transactions tend to have fewer parents on average than non-orphan transactions. The salient features of their missing parents are a lower fee, a larger size, and a lower transaction fee per byte than all other received transactions. We also find out that the network overhead incurred by these orphan transactions can be significant, exceeding 17% when using the default orphan memory pool size (i.e.,100 transactions). However, this overhead can be made negligible, without significant computational or memory demands, if the pool size is simply increased to 1000 transactions.