The way that Sync clients store data on a Storage Server is defined by sets of integer storage versions. Each storage version defines specific semantics for how clients are supposed to behave.
There exists a global storage version which defines global semantics. This global version typically defines the following:
Each Sync client is coded to support 1 or more global storage formats. If a client encounters a storage format it doesn’t support, it should probably stop trying to consume data. Under no normal circumstances should a client modify data on a server that is defined with an unknown, newer storage format. Even if an old client wipes all server data and uploads data in its format, newer clients may transparently upgrade to the global storage version they support.
Because changing storage formats can cause clients to not be able to use Sync because all clients may not be upgraded to support a newer storage format at the same time, new global storage versions are rarely introduced.
These were used by an old version of Sync which was deprecated in early 2011. Historical information is available here.
These versions should not be in active use and should all be upgraded to a newer storage format.
This version initially made the switch to a new crypto model based fully on AES. Because of a faulty implementation of the crypto, version 5 was created to force alpha clients created with the faulty implementation to upgrade. Version 4 and version 5 are therefore practically identical.
Version 5 replaces version 3’s cryptographic model with one based purely on AES.
A full overview is available for reference.
Historical notes are available.
In terms of cryptography, version 6 is a natural evolution of version 5. It makes minor changes to low-level cryptography details.
One driving force behind version 6 was the need to support storage of the encrypted Sync Key on the storage server. This was required in order to support BrowserID integration.
Another driving force was the transition to version 2.0 of the Storage Service.
Strictly speaking, neither of these require a new global storage version. However, they presented an enticing opportunity to fix minor issues with version 5.
Version 6 is fully documented.
The formats of unencrypted records stored on the server are also versioned. For example, records in the bookmarks collection are all defined to be of a specific type. Strictly speaking, these versions are tied to a specific global version. However, since all storage formats to date have stored the per-collection version in a special record, these versions in effect apply across all global storage versions.
These versions are fully documented.