+/*
+ * The Windows Overlay FileSystem Filter (WOF, a.k.a. wof.sys) is a filesystem
+ * filter driver, available in Windows 8.1 and later, which allows files to be
+ * "externally backed", meaning that their data is stored in another location,
+ * possibly in compressed form.
+ *
+ * WOF implements a plug-in mechanism by which a specific "provider" is
+ * responsible for actually externally backing a given file. The currently
+ * known providers are:
+ *
+ * - The WIM provider: allows a file to be externally backed by a
+ * compressed resource in a WIM archive
+ * - The file provider: allows a file to be "externally backed" by a named
+ * data stream stored with the file itself, where that named data stream
+ * has the format of a compressed WIM resource
+ *
+ * For both of these providers, externally backed files are effectively
+ * read-only. If you try to write to such a file, Windows automatically
+ * decompresses it and turns it into a regular, non-externally-backed file.
+ *
+ * WOF provides various ioctls that control its operation. For example,
+ * FSCTL_SET_EXTERNAL_BACKING sets up a file as externally backed.
+ *
+ * WOF external backings are implemented using reparse points. One consequence
+ * of this is that WOF external backings can only be set on files that do not
+ * already have a reparse point set. Another consequence of this is that it is
+ * possible to create a WOF external backing by manually creating the reparse
+ * point, although this requires dealing with undocumented data structures and
+ * it only works when the WOF driver is not currently attached to the volume.
+ *
+ * Note that only the unnamed data stream portion of a file can be externally
+ * backed. Other NTFS streams and metadata are not externally backed.
+ */
+
+
+/* Current version of the WOF driver/protocol */
+#define WOF_CURRENT_VERSION 1
+
+/* Specifies the WIM backing provider */
+#define WOF_PROVIDER_WIM 1
+
+/* Specifies the "file" backing provider (a.k.a. System Compression) */
+#define WOF_PROVIDER_FILE 2
+
+/* The current version of the WIM backing provider */
+#define WIM_PROVIDER_CURRENT_VERSION 1
+
+/* The current version of the file backing provider */
+#define FILE_PROVIDER_CURRENT_VERSION 1