OTA (over the air) updates were introduced with iOS 5. They allow a user to update the firmware on their device without the desktop iTunes program.
There are three known versions of the update file containers.
Version 1 updates came first, and were replaced with version 2 with iOS ???. They contain three main files:
archive.cpio.gz: The actual encrypted patch archive. It is a collection of BSDIFF40 and the requisite flashing tools (bbupdater, imeisv, etc.).Info.plist and Info.plist.signature: Contains information about the update along with a signature.libupdate_brain.dylib: A code library for stage 2 of the update process.Version 2 updates were introduced with iOS ???. Instead of an archive.cpio.gz file, they contain two folders: added and patches. As their names imply, added contains unencrypted files to be added to the filesystem while patches contains BSDIFF40 files of files that need to be patched.
Version 3 updates were introduced with iOS ???. Quite a number of changes were made to the structure. The most major changes are listed below:
added folder no longer exists.links.txt was added. It contains a list of symbolic links (symlinks) to be created.patches folder contents no longer use the BSDIFF40 file format, but a modified version known as BXDIFF41.payload and prepare_payload were added. They hold the filesystem contents in a proprietary format.removed.txt was added. It contains a list of files to be deleted prior to updating.This is a comprehensive list of every OTA update that has been released to the public.