Network Computer Sharing

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The Castle Discovery Service in Windows "Longhorn" build 4074

Network Computer Sharing (codenamed Castle) was a file sharing and identity replication service included in the Microsoft Windows "Longhorn" and Windows Vista operating systems. It was designed to simplify file sharing across multiple computers on the same network, and initially relied on WinFS and other pre-reset operating system features. The feature was reportedly shelved after the development reset of the operating system in 2004,[1] but it is included in post-reset builds of Windows Vista, particularly build 5098.winmain beta1.050628-1740 and build 5212.winmain.050726-1915.


Historically, consumer-oriented editions of the Windows operating system have allowed users to share data across multiple machines. However, in order for users to share data, they are required to create individual user profiles for each machine and connect each machine to the same local network. More inexperienced users must also deal with the complexity of configuring login credential and user permission settings. The idea of creating a simplified file sharing solution for home users of the Windows operating system predates the development of Network Computer Sharing. Earlier consumer-oriented editions of Windows, including Windows 98 and Windows ME, allowed users to share data without protecting it with a password;[2] the first consumer edition of Windows NT, Windows XP, introduced a Simple File Sharing feature which offered similar functionality.[3] While these features made it easier for more inexperienced users to share data without the complexity of traditional sharing settings, they also introduced a potential security risk in that they allowed every user on the same local network to access it. Moreover, they did not solve the problem of allowing a single user to securely access his or her data from any computer without the complexity of creating individual accounts on multiple machines. Network Computer Sharing aimed to solve this problem by securely replicating user profiles across multiple machines; in this sense, it is similar to the roaming user profile feature in server versions of Windows where an administrator can store user profiles on a centralized server, thereby allowing users to access their data regardless of which computer they are using.[4][5]

Overview and features

A preliminary UI design of the Windows Vista Network Map feature illustrating a command for the scrapped "Castle" feature on the Command Bar.
Preliminary UI design for Windows Vista incorporating "Castle"; as seen in a presentation from WinHEC 2006.[6]
An illustration from WinHEC 2004[7] depicting a printer installation on a single PC propagating to all PCs on the same subnet.

To join an existing Castle, users would be required to know the login credentials of an administrator account that was already joined to the Castle (in order to ensure that only authorized users had access). Once a relationship between the computer and existing Castle had been established, account profiles on the newly joined computer would automatically be replicated across other computers joined to the Castle and kept in sync, with each computer inheriting and respecting all policies pertaining to the shared data.[8] Although Castle was originally conceived of as a way to simplify file sharing, other potential uses were envisioned, including application and setting synchronization on the same account across multiple machines, Windows Update patch distribution, and new licensing techniques for content protected by digital rights management technology. These features would not have necessarily have been included in Windows "Longhorn" as Castle in that operating system was intended to provide the foundation for these features in future versions of Windows.[1]

According to Paul Thurrott, Jim Allchin remarked on Microsoft's plans for Castle, including an envisaged scenario where users could share their data with other users across the Internet:

"The idea is to replicate identities across the home machines (if the user wants this to happen). Think about it as a domain in the home. For the scenario across the Internet we are trying to work out a solution so that it is easy to share and publish photos (or other data) with users across the Internet. Our goal is to do this so that standard Windows security IDs can be used."[5]

In addition to replicating user data, Castle was also designed to replicate device installations across PCs connected to the same local network.[9]

Castle as it appears in Windows Vista 5212


In Windows Vista

The animation for Network Computer Sharing illustrating peer-to-peer account replication.

Castle—known by its latest name in development, Network Computer Sharing—is incorporated into the post-reset Windows Vista operating system in build 5212.winmain.050726-1915. The earliest build known to include it is build 5098.winmain beta1.050628-1740.


HomeGroup is a password-based sharing solution that debuted in the Microsoft Windows 7 operating system. HomeGroup allows users on the same home network to share individual files, folders, and printers, and to set access control permissions on shared content. A HomeGroup can be created during Windows installation or after Windows has been installed.[10] Once a HomeGroup is created, a password is generated which is required for other users to join the HomeGroup; the feature will instruct the user to save this password after it is generated. Once other users join the HomeGroup, they can share their data with other members of the HomeGroup.

During development of Windows 7, technology bloggers compared HomeGroup to the Castle feature that was shelved after the development reset of Windows "Longhorn" and speculated that it was meant to be a reinstatement of the feature.[11][12] Although HomeGroup shares some design similarities with Castle in that they were both designed to create a simple and secure file sharing solution, HomeGroup only offers a subset of the functionality that was originally intended for Castle.[1]

See also