What's New in Offline Files for Windows Vista

Based on customer feedback, the Offline Files feature has been redesigned for Windows Vista®, and it utilizes many of the improvement available in this version of the Windows operating system.

This document describes some of the functionalities of and changes to Offline Files in Windows Vista. It is intended for general users of Offline Files and system administrators.

What does Offline Files do?

Before the introduction of Offline Files, Windows users could access files available on a shared network resource only when connected to the network. By using Offline Files, Windows users can access files that are available on a shared network resource and continue to work with network files when the computer is not connected to the network.

Offline Files maintains a local cache of remote files and folders on your computer, so that they are available to you when you are working offline. You continue to access these files in the same way that you accessed them when you were online because the shared network resource paths and namespaces are preserved.

When your network connection is restored, any changes that you made while working offline are updated to the network by default. If you and someone else on the network have made changes to the same file, you have the option of saving your version of the file to the network, the other user's version, or both.

Who does Offline Files benefit?

Offline Files is useful for:

• All Windows Vista users who want to be able to continue to access network files if there is a network outage.
• Mobile users who need to access network files while working offline.
• Backup administrators who want to centralize data backups from client computers.
• Network administrators who want to enhance the experience of users in branch offices who access files and folders hosted by servers on the corporate network.

What new functionality is provided by Offline Files in Windows Vista?

Offline Files has been completely redesigned for Windows Vista, offering a set of new functionalities, which include:

• Better defined modes of operation
• Seamless offline to online transitions
• Optimized file synchronization
• Improved slow-link mode
• Consistent namespaces
• Cache size management
• Per-user encryption
• Scriptable API support

Better defined modes of operation

Offline Files in Windows Vista offers four modes of operation:

• Online mode
• Auto offline mode
• Manual offline mode
• Slow-link mode

In Offline Files in Windows XP, modes of operation affect an entire network server or domain-based Distributed File System (DFS) namespace. In Windows Vista, modes of operation apply to individual DFS scopes and individual Server Message Block (SMB) shared folders.

A DFS scope is defined as a folder in a domain-based DFS namespace that corresponds to a DFS link. If a network error is detected when connecting to a folder or a file in a domain-based DFS namespace, Offline Files in Windows Vista will not bring the whole domain offline (as it does in Windows XP). It will bring offline only the DFS link that includes the folder or file that had the error.

Consider the scenario of the following domain-based DFS namespace:

\\DFSdomain\folder1\folder2\folder3\folder4\file1

Assume that the namespace has the following configuration:

• Folder2 is a DFS link to: \\server1\folder2
• Folder4 is a DFS link to: \\server2\folder4

If there is a network error connecting to file1, only folder4 and file1 will be brought offline. The rest of the DFS namespace remains online. If there is a network error connecting to folder2 or folder3, these two folders are brought offline, but folder4 and file1 remain online.

Online mode

Online mode is the default mode of operation, and it provides the following:

• Normal access to network files
• Improved performance because read requests can be satisfied from the local cache if the file in the cache is synchronized with the server

In online mode, all modifications to a file or folder (write, open, or create) are applied to the network server first and then to the local cache. When you are in online mode, the details pane in Windows Explorer shows Offline status: Online.

Auto offline mode

When Offline Files determines that there has been a network error during a file operation (browse, open, create, read, or write), it automatically transitions the SMB shared folder or DFS scope where the error occurred to auto offline mode.

In auto offline mode, all open, create, read, and write requests are satisfied from the local cache. There are certain operations that you cannot perform while in this mode—for example, changes to the namespace (changing the name of or deleting a folder) or accessing the previous version of a file. However, you can create new folders, continue to browse the part of the namespace that is available to you offline, and also see that part of the namespace for which you only have placeholders. For a detailed explanation about how this works in Offline Files for Windows Vista, see "Consistent namespaces" in this document.

While in auto offline mode, Offline Files will attempt to reconnect every two minutes by default. When a network connection is available, Offline Files automatically transitions to online mode. For more details about what happens during a transition to online mode, see "Seamless offline to online transition" in this document.

When you are in auto offline mode, the details pane in Windows Explorer shows Offline status: Offline (not connected).

Manual offline mode

While working in online mode, you can force a transition to offline mode by clicking Work offline on the Windows Explorer Command Bar.

This change in operation mode affects only the SMB shared resource or DFS scope that you are currently browsing in Windows Explorer. Other SMB shared resources and DFS scopes will continue to be online. This mode of operation persists even after you restart your computer.

Access to files and folders in manual offline mode is the same as in auto offline mode. The difference between these two offline modes is that in manual offline mode, you can initiate a synchronization process for a folder or file at any time. (In auto offline mode, you do not have this option because a network connection is not available.) After the synchronization process has completed, the file or folder continues to be available offline.

If you have an open file in the SMB shared resources or DFS scope that has not yet been copied to the local cache, a warning message appears when you force that shared resource or scope into manual offline mode. You have the following options:

• Close the open file before transitioning to offline mode
• Ignore the warning
• Choose to not go offline

If you ignore the warning and continue with the transition to offline mode, any unsaved changes to the files that have not been cached may be lost.

While in manual offline mode, you can return to online mode by clicking Work online on the Windows Explorer Command Bar. For more details about what happens during a transition to online mode, see "Seamless offline to online transition" in this document.

Note

A transition from online mode to auto offline mode, or from auto offline mode to online mode, can also be forced by running a script based on the new Offline Files application programming interface (API). For more information, see "Scriptable API support" in this document.

When you are in manual offline mode, the details pane in Windows Explorer shows Offline status: Offline (working offline).

Slow-link mode

If the Configure slow-link mode Group Policy setting has been enabled by the domain administrator, an SMB shared resource or DFS scope can automatically be transitioned to slow-link mode. This happens if you are working in online mode and the performance of your connection to that SMB shared resource or DFS scope is determined to be low. For a detailed explanation about how slow-link mode works in Offline Files for Windows Vista, see "Improved slow-link mode" in this document.

When you are in slow-link mode, the details pane in Windows Explorer shows Offline status: Offline (slow connection).

Comparing all four modes of operation

The following tables show the differences between the four modes of operation for Offline Files.

Table 1   Where access requests are satisfied for each mode of operation

Mode	Open or create file	Read from file	Write to file	Browse folder
Online	Server	Local cache (if in sync with server)	Server and cache	Server
Auto offline	Local cache	Local cache	Local cache	Local cache
Manual offline	Local cache	Local cache	Local cache	Local cache
Slow-link	Local cache	Local cache	Local cache	Local cache

Table 2   Availability of automatic synchronization vs. manual synchronization for each mode of operation

Mode	Automatic synchronization	Manual synchronization
Online	Available	Available
Auto offline	Not available	Not available
Manual offline	Not available	Available
Slow-link	Not available	Available

Seamless offline to online transitions

The transition from offline mode to online mode in Windows Vista is non-disruptive for the user. Unlike previous versions, you do not need to close applications or manually start the synchronization process.

When Offline Files in Windows Vista detects that an SMB shared resource or a DFS scope is available for reconnection, that share or scope is seamlessly transitioned to online mode. The transition to online mode of an SMB shared resource or DFS scope works as follows:

1. The namespace for the SMB shared resource or DFS scope is brought online.
2. For each file that has an open handle (that is, is in use by an application), Offline Files verifies if the file was changed on the network server or in the local cache, and then does one of the following:
   • If the file was not changed, it recreates the handle for that file on the server. This ensures that all running applications transition to work with the files on the server.
   • If the file was changed, that file continues to stay offline and a handle is not recreated on the server. The details pane in Windows Explorer shows Offline status: Offline (need to sync) for that file, and you continue to work with the version of the file that is stored in your local cache. After closing the file, you can use Sync Center at any time to start the synchronization process for that file.
3. For all files that do not have open handles, the synchronization process is automatically started in the background. This process does the following:
   1. Updates (to the server) all offline changes
   2. Updates the local cache with any changes made on the server while you were working offline
      During the synchronization process, if Offline Files finds a file that was modified on the server and in the local cache while you were offline, the file stays offline and the synchronization process continues to run. You are notified about the conflict by an icon in the notification area, and the details pane in Windows Explorer shows Offline status: Offline (need to sync) for that file. You can choose to resolve the conflict immediately or at a later time by using Sync Center. Meanwhile, you can continue to work with the version of the file that is stored in your local cache.

Important

Because Offline Files will not synchronize changes for files that have open handles, files that are usually open for long periods of time and that are prone to change, will not automatically be synchronized. An example of this type of file is an Outlook PST file. To synchronize this type of file, you must close the application that uses the file and start a manual synchronization process.

Note

Offline Files will bring online and synchronize only the SMB shared resource or DFS scope that you are currently accessing. Other shared resources and scopes remain offline until you access them and there is a network connection to the server that hosts them.

Optimized file synchronization

One of the performance enhancements of Offline Files in Windows Vista is a faster synchronization process. This is achieved by using a new synchronization algorithm that does the following:

• Reduces the time and bandwidth needed to identify differences between the local cache and the server
• Determines which parts of a file in the local cache have changed, and then updates only those parts of the file on the network server

The process of updating only parts of a file on the server is called Bitmap Differential Transfer. It takes place during synchronization of the local cache to the server.

Bitmap Differential Transfer

Offline Files in Windows Vista uses Bitmap Differential Transfer. Bitmap Differential Transfer tracks which blocks of a file in the local cache are modified while you are working offline and then sends only those blocks to the server. In Windows XP, Offline Files copies the entire file from the local cache to the server, even if only a small part of the file was modified while offline.

Because Bitmap Differential Transfer can make the synchronization more efficient, Offline Files in Windows Vista supports all file types. Offline Files in Windows XP could not synchronize large files efficiently; therefore, certain types of files were specifically excluded from the local cache.

Note

Bitmap Differential Transfer is not used when synchronizing changes from the server to the local cache. If a file is modified on the server when you are offline, Offline Files in Windows Vista will copy the whole file to your local cache during synchronization. Bitmap Differential Transfer works only for pre-existing files and not for files that were created while working offline. Because of this limitation, files modified with applications that do not modify the file in place will not work with Bitmap Differential Transfer. For example, Microsoft® Word creates a temporary file that is used to replace the original file. These types of files are considered new, and they are copied in their entirety from the local cache to the server.

Improved slow-link mode

Offline Files in Windows Vista incorporates a slow-link mode of operation to improve the experience for mobile and traveling users who connect to the corporate network with low-throughput connections. This mode of operation can also improve the experience for users who connect to servers located in remote locations where network latency is high.

Performance of a network connection is automatically determined by measuring throughput and packet latency between the client computer and the server that hosts an SMB shared resource or a DFS scope. When low performance is detected, the SMB shared resource or DFS scope is automatically transitioned to slow-link mode.

Note

Automatic transition to slow-link mode does not take place immediately. A short period of time is required for Offline Files to calculate network performance. During this time, Offline Files is working in online mode.

When you are in slow-link mode, all read and write requests are satisfied from the local cache. You can manually initiate a synchronization process at any time by clicking Sync on the Windows Explorer Command Bar, or by using Sync Center. After the synchronization process has completed, you continue to work in slow-link mode.

In this mode, all read and write requests are satisfied from the local cache, so Offline Files cannot determine the performance of the network connection. The only way to return to online mode is to manually make this transition. To do this, click Work online on the Windows Explorer Command Bar. After you have transitioned to online mode, Offline Files will reassess the performance of the network connection every five minutes (default setting). If network performance continues to be low, you will automatically be transitioned back to slow-link mode.

Enabling slow-link mode

The slow-link mode of operation is not enabled by default. The domain administrator must enable the Configure slow-link mode Group Policy setting. If this policy setting is disabled or has not been configured, Offline Files will not transition an SMB shared resource or DFS scope to slow-link mode.

The performance of a network connection is determined by measuring throughput and packet latency between the client computer and the server that hosts the SMB shared resource or DFS scope. You can configure the threshold values for minimum throughput and maximum packet latency in the Configure slow-link mode Group Policy setting. For more details about this and other new Group Policy settings available for Offline Files in Windows Vista, see "Group policy settings added or changed for Offline Files in Windows Vista" in this document.

Users can configure how often Offline Files will determine network performance after a manual transition to online mode. To configure the computer, in Control Panel, click Network and Internet, and then click Offline Files. On the Network tab, specify a number of minutes. The default setting is to check network performance every five minutes.

Note

Users will not be able to configure how often Offline Files will determine network performance unless the Configure slow-link mode Group Policy setting has been enabled.

Consistent namespaces

In Windows XP, after transitioning to offline mode, only files that have been selected to be made available offline, or have been cached automatically, remain visible to the user. For example, if a folder has ten files and only three are selected to be made available offline, when you are in offline mode, the folder will appear to only contain those three files. The other seven files are not visible until you return to online mode. This can cause confusion and an inconsistent experience between online and offline modes.

To provide a consistent experience in online and offline modes, Offline Files in Windows Vista creates a placeholder for files and folders that are not available offline. The placeholder appears as a fainter image of the file or folder, and it indicates to the user that a file or folder exists in the shared folder, but it is not currently available offline.

When a file in a shared folder is selected to be made available offline, placeholders are created for the other files and folders that are contained in the parent folder but not in any subfolders. As in the previous example, if you have selected three files to be made available offline, and the folder where those files reside contains another seven files. When you transition offline, you will see all ten files. The three files that you selected to be made available offline will look like normal files, and the other seven files will have an icon overlay that identifies them as placeholder files.

The following figure shows a folder with three files that have been made available offline, and seven files and one subfolder for which placeholders have been created.

If you try to open a placeholder, an error message alerts you that the file is not currently available.

Another instance where placeholders are created is when you access a file in a shared folder that has the automatic caching option enabled. Because the file you accessed will be made automatically available on your computer offline, Offline Files creates placeholders for all files and subfolders that are in the same folder.

Caching

When you create a shared folder on a server, there are three caching options available for that folder:

• Manual caching (default). If you want a file in the shared folder to be available on your computer when you are offline, you must manually select it to be made available offline.
• Automatic caching. When this option is selected, every time you access a file in a shared folder, the file will be made temporarily available on your computer offline.
• No caching. Files in the shared folder cannot be made available offline.

Specific files can also be selected to be cached automatically by using the Administratively assigned offline files Group Policy setting.

Cache size management

In Windows XP, there was no limit to the amount of disk space used by files that were cached manually. The only limit that could be specified was the amount of disk space occupied by files that were cached automatically.

With Offline Files in Windows Vista, you can specify a limit for the total size of the local cache. This limit includes automatically cached files and manually cached files. There is also a limit (within the total local cache size limit) for the amount of disk space used by automatically cached files.

For example, you can specify that the total cache size-limit for storing offline files on your computer is 5 gigabytes (GB), and within that limit, only 2 GB can be used to store files that are cached automatically. With this limit, files that were cached automatically are removed on a least-recently used basis as the disk space used by this type of files approaches the limit you specify.

Files that were cached manually are never removed from the local cache. When the total local cache size limit is reached and all files that were cached automatically have already been removed, you can not make files available offline until you specify a new limit or delete files from the local cache by using the Offline Files control panel item

These two local cache size limits can be specified in the Disk Usage tab of the Offline Files control panel item or with a Group Policy setting.

Per-user encryption

Offline Files in Windows XP offers the ability to encrypt your local cache, but the encryption can only occur within the context of the local system. Offline Files in Windows Vista offers enhanced security by encrypting each file in the local cache by using each user's local certificate.

This enhanced encryption not only protects all files in the local cache from unauthorized access (for example, if a laptop is lost or stolen), but it prevents cached files from becoming accessible to other users of the same computer.

Because the local cache is per computer and not per user, there is only one copy of each file stored in the local cache. When the cache is encrypted, the first user who selects a file to be made available offline is the only user who will have access to that file when working offline. Other users will be able to access that file only when working online.

Note

Even if encryption is not enabled for offline files, when a user selects an encrypted file to be made available offline, the copy of that file in the local cache is automatically encrypted using the user's local certificate.

Encryption of offline files can be enabled manually by using either of the following features:

• The Offline Files control panel item
• The Encrypt the Offline Files cache Group Policy setting

When a user manually enables encryption of the local cache, only those files in the cache to which the user has access are encrypted. If there are files in the local cache that the user does not have access to, those files are only encrypted when a user who has access to those files logs on to that computer.

When encryption is enabled by using the Encrypt the Offline Files cache Group Policy setting, the encryption process begins the next time a user logs on to the computer. As with manually-enabled encryption, only those files that a user has access to are encrypted.

Note

If a file is in use, it cannot be encrypted. Offline Files will automatically attempt to encrypt that file when the user logs on again. If encryption was enabled manually, the user can choose to close the file and manually restart the encryption process by using the Offline Files control panel item. The user cannot restart the encryption process manually if encryption was enabled with Group Policy.

The encryption process of a local cache that already contains files can take some time to complete. Newly cached files are immediately encrypted, while the encryption process of the existing files keeps running in the background. You can check the status of the encryption process at any time by using the Offline Files control panel item.

Note

The same rules, processes, and exceptions that apply to the encryption process apply to the decryption process.

Scriptable API support

Offline Files in Windows Vista provides programmability support for controlling and monitoring the behavior of offline files.

Two versions of the Offline Files API are available:

• Conventional Win32 and COM implementation
• Windows Management Instrumentation (WMI) provider for scripting purposes

For more information about the Win32 and COM implementation, see "Offline Files" on the Microsoft Web site at https://go.microsoft.com/fwlink/?LinkId=89102.

For more information about WMI scripting support, see the Offline Files WMI Provider Reference on the Microsoft Web site at https://go.microsoft.com/fwlink/?LinkId=89103.

Changes in the user interface for Offline Files in Windows Vista

The following screenshots show several of the features that are available in the user interface for Offline Files in Windows Vista. To access Offline Files, in Control Panel, click Network and Internet, and then click Offline Files. To access Indexing Options, in Control Panel, click System and Maintenance, and then click Indexing Options.

Figure 2 shows Always Available Offline and Sync, the menu items available when Offline Files is enabled and you right-click a file or folder. A check mark is present on the menu when the selected file is available offline.

Figure 3 shows the file icon overlays that tell the user which files are available and unavailable offline. Auto-cached files do not show the overlays. This figure also shows the Sync and Work Offline Command Bar buttons. These buttons are available only when Offline Files is enabled or when the files in the folder being viewed have been cached.

Figure 4 shows the two optional columns for Offline Files in Windows Explorer: Offline availability and Offline status. In the Windows Explorer Details view, you can make these columns available by right-clicking the column heading area and then selecting the columns.

Figure 5 shows the new Offline Files tab, which is accessed by right-clicking a file or folder that is available offline, and then clicking Properties. From this location, you can specify that a file or folder is always available offline, and you can manually synchronize your local copy with the version on the network.

Figure 6 shows Sync Center in Control Panel. You can use this tool to manage synchronization tasks for any available offline files and folders.

Figure 7 shows the Sync Center icon that appears in the notification area when you enable Offline Files and make a file or folder available offline for the first time.

Figure 8 shows the General tab in Offline Files. You can use the options on this tab to perform general management tasks including enabling or disabling Offline Files; opening Sync Center; and viewing any SMB shared folders, mapped network drives, or other files or folders that are available offline.

Figure 9 shows the Disk Usage tab in Offline Files. You can use this tab to view the total disk spaced that is being used by Offline Files and the space that is available to cache more files or folders in the Offline Files cache. You can also set the limits for the amount of disk space that can be used by automatically cached files and all offline files. You can use this UI to delete automatically cached files from the local cache of Offline Files.

Figure 10 shows the dialog box that opens when you click the Change Limits button that is available on the Disk Usage tab in Offline Files. Use this to set the maximum limits on your computer for automatically cached files and all offline files.

Figure 11 shows the Encryption tab in Offline Files. If you choose to encrypt the offline files cache, you encrypt only the local cached versions of the files (not the network versions of the files).

Figure 12 shows the Network tab in Offline Files. You can use this tab to set how often Offline Files will check network performance after a manual transition to online mode.

Figure 13 shows the property window for the Indexing Options control panel item. Indexing of offline files is enabled by default. You can disable this ability in Indexing Options by clicking Modify. Then, under Change selected locations, clear the Offline Files check box.

Group Policy settings added or changed for Offline Files in Windows Vista

In Windows Vista, all Group Policy settings for Offline Files can be found through two paths:

• Computer Configuration\Administrative Templates\Network\Offline Files
• User Configuration\Administrative Templates\Network\Offline Files

Table 3   New or changed Group Policy settings for Offline Files in Windows Vista

Setting name	Status in Windows Vista
Action on server disconnect	Not applicable
Administratively assigned offline files	Unchanged
Allow or Disallow use of the Offline Files feature	Unchanged
At logoff, delete local copy of user's offline files	Not applicable
Configure slow link speed	Not applicable—replaced by Configure slow-link mode
Configure slow-link mode	New—replaced Configure Slow link speed
Default cache size	Not applicable—replaced by Limit disk space used by offline files
Encrypt the Offline Files cache	Unchanged
Event logging level	Not applicable
Files not cached	Not applicable
Initial reminder balloon lifetime	Not applicable
Limit disk space used by offline files	New—replaced Default cache size
Non-default server disconnect actions	Not applicable
Prevent use of Offline Files folder	Not applicable
Prohibit 'Make Available Offline' for these files and folders	Not applicable
Prohibit user configuration of Offline Files	Not applicable
Reminder balloon frequency	Not applicable
Reminder balloon lifetime	Not applicable
Remove 'Make Available Offline'	Unchanged
Subfolders always available offline	Not applicable
Synchronize all offline files before logging off	Not applicable
Synchronize all offline files when logging on	Not applicable
Synchronize offline files before suspend	Not applicable
Turn off reminder balloons	Not applicable
Turn on economical application of administrative assigned Offline Files	New

Interoperability with server operating systems

All changes to Offline Files in Windows Vista (including Bitmap Differential Transfer) are compatible with any Microsoft Windows Server operating system that fully supports the SMB protocol. This includes: Windows Server 2000, Windows Server 2003, and Windows Server 2003 R2.

Editions of Windows Vista with Offline Files

Offline Files is available with:

• Windows Vista Enterprise
• Windows Vista Business
• Windows Vista Ultimate

Note

The Offline Files feature is enabled by default in all these editions of Windows Vista.