  Orphan transactions are those whose parental income-sources are missing at
the time that they are processed. These transactions are not propagated to
other nodes until all of their missing parents are received, and they thus end
up languishing in a local buffer until evicted or their parents are found.
Although there has been little work in the literature on characterizing the
nature and impact of such orphans, it is intuitive that they may affect
throughput on 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. Moreover, the
salient features of their missing parents are a lower fee and larger size than
their non-orphan counterparts, resulting in a lower transaction fee per byte.
Finally, we note that the network overhead incurred by these orphan
transactions can be significant, exceeding 17% when using the default orphan
memory pool size (100 transactions). However, this overhead can be made
negligible, without significant computational or memory demands, if the pool
size is merely increased to 1000 transactions.
