Microsoft will use its customers' upload bandwidth to deliver Windows 10's updates and apps with a peer-to-peer technology resembling BitTorrent, a fact that has caught some by surprise.

Baked into Windows 10 is a new technology Microsoft dubbed "Windows Update Delivery Optimization" (WUDO) that is turned on by default for all editions of Windows 10. However, only some SKUs (stock-keeping units) -- notably Windows 10 Home and Windows 10 Pro -- are set to provide updates and apps to other devices when connected to the public Internet.

Windows 10 Enterprise and Windows 10 Education, volume-licensed SKUs for large companies and organizations, also have WUDO enabled, but default to sharing updates and apps only within a local network.

WUDO resembles BitTorrent in its basics, and like that file-sharing technology, uses a peer-to-peer delivery system to spread the load to PCs worldwide rather than relying on a centralized-servers model. WUDO is not a substitute for Microsoft's standard delivery service, Windows Update, but in addition to it.

If WUDO is enabled, Microsoft can point others to locally-cached copies of updates and apps on users' Windows 10 devices that are connected to the Internet. When that happens, a user's Windows 10 PC acts as a substitute server for others, and any customer whose device is tapped for WUDO delivery has given Microsoft access to their upload bandwidth.

Download bandwidth should not be impacted by WUDO.

On Monday, Microsoft said that it was not using WUDO to spread the distribution burden of the Windows 10 upgrade among those who have already migrated to the new OS. "No. Downloads of the Windows 10 upgrade are happening through Windows Update," a Microsoft spokeswoman said in an email reply to questions.

WUDO was no secret: The company first mentioned peer-to-peer update delivery in May, when it touted it as a new feature for Windows Update for Business (WUB), the service targeting organizations running Windows 10 Pro, Windows 10 Enterprise and Windows 10 Education.

The service was also enabled by default in Windows Insider builds prior to the July 29 launch to mainstream customers.

But with the wider distribution of Windows 10 last week, many were caught off-guard when they heard that their upload bandwidth could be automatically used by Microsoft. A thread on Reddit that kicked off Friday, for example, was largely negative about WUDO.

"P2P [peer-to-peer] isn't bad. But it should not be turned on by default!" wrote someone identified as RMJ1984 on Reddit. "It's totally unacceptable just using peoples' bandwidth."

Microsoft has yet to publish technical documentation about WUDO, leaving customers to do their own research. "A quick check on my system here shows that when DoSvc starts up, it creates a TCP listener on port 7680 and a UDP receiver on port 3544. I wouldn't class this as definitive information, but until there's documentation on how Delivery Optimization works, experimentation is all you've got," Alun Jones, a Seattle-based application and product security manager who is also a Microsoft Most Valuable Professional (MVP), wrote on a Microsoft forum Friday.

DoSvc is the name given to the WUDO mechanism built into Windows 10.

Users can disable WUDO entirely or limit its reach by changing settings in Windows 10.

From "Settings," users can click the "Update & security" icon, then the "Advanced options" link under "Windows Update." In the ensuing window, click on "Choose how updates are delivered" to turn off WUDO by moving the slider to Off. To leave it on, but restrict sending locally cached updates and apps only to devices on the same local network, select "PCs on my local network" near the bottom.