Changes

Now Bob says "I think you have AaBbCcDd; I want you to add 'Ff' after the d." Alice then says "I think you have AaBbCcDdFf; I want you to take the part up to b, then add 'Ee', then the part starting with D". Now they are synchronized.

Now Bob says "I think you have AaBbCcDd; I want you to add 'Ff' after the d." Alice then says "I think you have AaBbCcDdFf; I want you to take the part up to b, then add 'Ee', then the part starting with D". Now they are synchronized.

Say that, instead, Bob misses Alice's reply. As far as Bob knows, their last common version is "AaBbCcDd", yet Alice thinks it's "AaBbEeDdFf". Alice takes the opportunity to forget the "AaBbCcDd" state, (even though her versioning file system happens to still have kept "AaBbCc", that's irrelevant). Bob makes another change - from AaBbCcDdFf to AaBbCGcDdFf. Now he says "I think you have AaBbCcDd; I want you to add 'G' between C and c, and 'Ff' after the d.". Alice (without figuring out the contents of chunks C and c) replies "Up to b, then 'Ee', then G, then starting from D", and if Bob can hear that they are synchronized.

Say that, instead, Bob misses Alice's reply. As far as Bob knows, their last common version is "AaBbCcDd", yet Alice thinks it's "AaBbEeDdFf". Alice takes the opportunity to forget the "AaBbCcDd" state, (even though her versioning file system happens to still have kept "AaBbCc", that's irrelevant). Bob makes another change - from AaBbCcDdFf to AaBbCGcDdFf. Now he says "I think you have AaBbCcDd; I want you to add 'G' between C and c, and 'Ff' after the d.". (Alice "what are the contents of C and c" and Bob replies. This is not actually helpful, but I can't think of an easy algorithm to avoid it.) Then she replies "Up to b, then 'Ee', then G, then starting from D", and if Bob can hear that they are synchronized.

Note that this protocol makes use of old versions; it is possible that it could include an undo stack and/or versioning storage for free.

Note that this "protocol" makes use of old versions; it is possible that it could include an undo stack and/or versioning storage for free. It is also remarkably non-chatty and so should scale well as long as communication reliability is good.