Network Computer Sharing

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Preliminary UI design for Windows Vista incorporating "Castle" (from a WinHEC 2006 presentation).[1]

Network Computer Sharing (codenamed Castle) was a peer-to-peer identity and item replication service included in Windows "Longhorn" and Windows Vista that primarily aimed to simplify sharing across multiple computers by replicating user profiles. Network Computer Sharing allowed an administrator of a single machine to create and manage a secure establishment—a castle—into which other machines could participate.

Network Computer Sharing was discussed during PDC 2003 as an identity and item management service comprising Microsoft's then-new synchronization efforts, many of which relied on direct and extensive use of "WinFS". During WinHEC 2004 Microsoft envisaged that it would be used to propagate driver installations across joined machines; an installation of a printer on one machine would allow that printer to be installed on all joined machines.

In 2005, after the "Longhorn" development reset, Network Computer Sharing was still slated for inclusion in Windows; Jim Allchin described it as a "domain in the home" and discussed a scenario for identity replication over the Internet.[2] Preliminary documentation in Windows Vista indicates the use of Network Computer Sharing to distribute Windows updates across joined machines, perform simultaneous backups, and to roam parental control settings with standard user accounts; several of these scenarios were conceived during pre-reset "Longhorn" development.

Development of Network Computer Sharing reportedly ceased after the "Longhorn" development reset of 2004,[3] but it is included in post-reset Build 5098 and Build 5212.


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

Consumer-oriented editions of Windows have historically allowed users to share data across multiple machines. However, users are required to create separate profiles for each machine; inexperienced users must also deal with the complexity of configuring separate credential and permission settings. A simplified sharing solution for home users predates the development of Network Computer Sharing; Windows 98 and Windows ME allowed users to share data without password protection,[4] and Windows XP—the first version of Windows NT designed for use by home users—introduced "Simple File Sharing" that offered similar functionality.[5] However, while these features made it easier to share data without complexity of credentials and permissions, they introduced potential security risks—every user on the same network could access the data—and did not resolve the problem that prevents a single user from securely accessing data from each computer without the creation of profiles on multiple machines.

Network Computer Sharing would replicate profiles (in this sense, it is similar to roaming user profiles in server versions of Windows where an administrator can store profiles on a server, allowing users to access data regardless of the computer in use[6][2]) by allowing administrators of a single PC to create and manage a secure establishment—a castle—into which other PCs could participate.

Overview and features

The Castle Discovery Service in Windows "Longhorn" Build 4074
Depiction of an installation propagating to all PCs.[7]

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.[3]

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."[2]

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

In Windows Vista

Network Computer Sharing is included in Build 5098 and Build 5212. Extensive preliminary Help and Support Center documentation for Network Computer Sharing is present in Windows Vista Build 5219.



HomeGroup is a password-based sharing solution that debuted in Windows 7 and was retired in Windows 10 RS5. 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 to be shared 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 and printers with other members in the establishment.

During development of Windows 7, bloggers and enthusiasts compared HomeGroup to Network Computer Sharing, speculating that it was a realization of the latter feature.[11][12] HomeGroup, however, while borne out of a similar desire to create a simple and secure sharing solution, offers only a very minimal subset of Network Computer Sharing functionality.[3] With the release of Windows 8 Microsoft updated HomeGroup to work with the new File History feature, which enables users to perform simultaneous backups of machines joined in the establishment; this was a scenario envisaged for Network Computer Sharing.

Features and availability Castle HomeGroup
Folder, item, and printer sharing Yes Yes
Query all joined PCs for shared folders and items Yes Yes
Access shared content across PCs by use of same credentials Yes Yes (through traditional file sharing)
Simultaneous backups of content stored across joined PCs Yes Yes (with File History in Windows 8 and later versions)
PCs in the establishment One PC is limited to one Castle at a time One PC is limited to one HomeGroup at a time
Number of establishment participants 10 Unlimited
Join both a domain and an establishment No Yes
Stream media across joined PCs No Yes
Replication of folders, items, and user profiles Yes No
Parental control monitoring and roaming Yes No
Propagation of device installations to joined PCs Yes No
Distribution of Windows updates across joined PCs Yes No
Windows Connect Now (add a PC to the establishment by USB flash drive) Yes No

Microsoft accounts

Windows 8 introduces the ability to login with Microsoft accounts, which synchronizes user data and preferences across machines over the Internet; this is an indirect realization of the Network Computer Sharing scenario discussed by Jim Allchin.[2]

Delivery Optimization

Windows 10 introduces the optional Delivery Optimization feature that can, in a peer-to-peer fashion deliver Windows updates across PCs, which is a scenario conceived during development of Network Computer Sharing.

See also