[bitcoin-dev] BIP 158 Flexibility and Filter Size

Jim Posen jim.posen at gmail.com
Tue Jun 5 17:22:04 UTC 2018

> I don't understand this comment. The bandwidth gains are not from
> address reuse, they are from the observed property that false
> positives are independent between two filters. I.e. clients that
> connect once a day will probably download 2-3 filters at most, if they
> had nothing relevant in the last ~144 blocks.

Your multi-layer digest proposal (https://bc-2.jp/bfd-profile.pdf) uses a
different type of filter which seems more like a compressed Bloom filter if
I understand it correctly. Appendix A shows how the FP rate increases with
the number of elements.

With the Golomb-Coded Sets, the filter size increases linearly in the
number of elements for a fixed FP rate. So currently we are targeting an
~1/2^20 rate (actually 1/784931 now), and filter sizes are ~20 bits * N for
N elements. With a 1-layer digest covering let's say 16 blocks, you could
drop the FP rate on the digest filters and the block filters each to ~10
bits per element, I think, to get the same FP rate for a given block by
your argument of independence. But the digest is only half the size of the
16 combined filters and there's a high probability of downloading the other
half anyway. So unless there is greater duplication of elements in the
digest filters, it's not clear to me that there are great bandwidth
savings. But maybe there are. Even so, I think we should just ship the
block filters and consider multi-layer digests later.
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