Microsoft introduced in Windows XP a new file-transfer technology called the Binary Intelligent Transfer Service, or BITS. Although similar in concept to a standard file-transfer protocol, such as FTP, BITS differs in several important ways:

BITS uses HTTP. BITS downloads files using the standard Hypertext Transfer Protocol (HTTP) and requires Internet Information Server (IIS) to be running on the server that serves the files. HTTP packets pass more easily through firewalls, which makes BITS a good choice for getting files from the Internet.

Checkpoint mechanism. BITS uses a "checkpoint" mechanism that allows an interrupted file transfer to resume where it left off, even if the user becomes disconnected from the network. Users don't need to worry about interrupting background file transfers (e.g., by turning off their computers), and when network connectivity is restored, the transfer process does not waste time and bandwidth by starting over.

Automatic network throttling. BITS is able to adjust its network use based on other network adapter activity. When a user makes an interactive network call, such as opening a file on a file server or sending a job to a network printer, BITS slows its transfer rate so that the user’s response time is minimally affected.

Both the standard Windows XP Automatic Update (AU) service and the Software Update Service (SUS) update AU service use BITS to fetch update packages from Windows Update or SUS servers. Although BITS is a proprietary service, Microsoft has published a BITS API that other developers can use to perform background file transfers for their applications.

Back to associated article: Software Update Service to Ease Patch Distribution