range-based (negentropy): assume sorted ids {1,2,3} on client, server has {1,3,4}. client initiates with full range hash (e.g., keccak(1-4)=h1); server responds with subranges where diffs (e.g., [1,3) present, [3,4] missing). diffs resolve in 1-2 rounds: client sends id 4 (new), server skips 2 (client has). message size ~3: 50 bytes (hashes + bounds). for 1000 elements, ~4 rounds, 2-5kb total if clustered diffs. gcs filter sync: client builds gcs filter (n=3,p=20 false positives) from wanted ids {1,2,3}, serializes (~80 bytes incl metadata) and queries server. server matches its set, sends full events for positives + proofs. handles fps via client verification. for 1000, filter ~4kb serialized, plus ~10kb events if half match. logperiodic.com/rbsr.html github.com/btcd/btcd/tree/master/btcutil/gcs