Previously Bitcoin would send 1/4 of transactions out to all peers
instantly. This causes high overhead because it makes >80% of
INVs size 1. Doing so harms privacy, because it limits the
amount of source obscurity a transaction can receive.
These randomized broadcasts also disobeyed transaction dependencies
and required use of the orphan pool. Because the orphan pool is
so small this leads to poor propagation for dependent transactions.
When the bypass wasn't in effect, transactions were sent in the
order they were received. This avoided creating orphans but
undermines privacy fairly significantly.
This commit:
Eliminates the bypass. The bypass is replaced by halving the
average delay for outbound peers.
Sorts candidate transactions for INV by their topological
depth then by their feerate (then hash); removing the
information leakage and providing priority service to
higher fee transactions.
Limits the amount of transactions sent in a single INV to
7tx/sec (and twice that for outbound); this limits the
harm of low fee transaction floods, gives faster relay
service to higher fee transactions. The 7 sounds lower
than it really is because received advertisements need
not be sent, and because the aggregate rate is multipled
by the number of peers.
(cherry picked from commit f2d3ba73860e875972738d1da1507124d0971ae5)
Zcash: Candidate transactions for INV are not sorted by their
topological depth because we haven't backported bitcoin/bitcoin#6654.
Strictly speaking this is not a true deduplication; the test output will be
slightly different (due to inconsistent print statements) but I think this is
close enough.
Jack Grigg
Gregory Maxwell
mdr0id
Daira Hopwood
Per Grön
Jay Graber