1. In the beginning the server sends you a list of fingerprints of id ranges. 2. You check if your local fingerprints for the same ranges match. 3. When they match, you say that part matches; when they don't match, you look inside the range to see what you have. 4. If you have few ids or none for that range, you send the ids you have -- then the server will send back the ids it has; if you have way too many ids inside that range, you split the range into multiple ranges and send a fingerprint of each. 5. Repeat the process for all ranges until have narrowed them enough for you to get a list of the ids you don't have from the server -- as well as the ids you have that the server doesn't. Something like that, I may have gotten details wrong.