Background Intelligent Transfer Service

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Background Intelligent Transfer Service (BITS) is a component of modern Microsoft Windows operating systems that facilitates the transfer of files between machines. It is most commonly used by recent versions of Windows Update, Windows Server Update Services, and Systems Management Server to deliver software updates to clients, and is also used by Microsoft's instant messaging products to transfer files.

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[edit] Technology

BITS uses unused bandwidth to transfer data. Normally, BITS transfers data in the background, i.e., BITS will only transfer data whenever there is bandwidth which is not being used by other applications, for example, when applications use 80% of the available bandwidth, BITS will use only the remaining 20%. BITS constantly monitors network traffic for any increase or decrease in network traffic and accordingly throttles its own transfers to ensure that other foreground applications (such as a web browser) get the bandwidth they need. BITS also supports resuming transfers in case of disruptions.

BITS version 1.0 supports only downloads. From version 1.5, BITS supports both downloads and uploads. Uploads require IIS server, with BITS server extension.

[edit] Transfers

BITS transfers files, on behalf of requesting applications, asynchronously, i.e., once an application requests the BITS service for a transfer, it will be free to do any other job, or even terminate. The transfer will continue in the background, as long as the network connection is there. Even when a user is not logged on, BITS continues the data transfer.

BITS suspends any ongoing transfer when the network connection is lost, or the operating system is shut down. It resumes the transfer, from when it left off, when the computer is turned on later and the network connection is restored. BITS supports transfers over both HTTP and HTTPS.

[edit] Jobs

A BITS session has to be started from an application by creating a Job. A job is a container, which has one or more files to transfer. A newly created job doesn't have any file, files must be added to it specifying both the source and destination URIs. While a download job can have any number of files, upload jobs can have only one. Properties can be set for individual files. Jobs inherit the security context of the application which created it.

BITS provides API access to control a job. A job can be programmatically started, stopped, paused, resumed, and queried for status. Before starting a job, a priority has to be set for it, to specify when the job is processed, relative to other jobs in the transfer queue. By default, all jobs are of Normal priority, which can be optionally set to High or Low. A High priority job runs in foreground, and competes for network resources with other applications.

[edit] Scheduling

BITS schedules each job to receive only a finite time slice, for which only that job is allowed to transfer, before it is temporarily paused to give another job a chance to transfer. Higher priority jobs get a higher chunk of time slice. Round-robin scheduling to process jobs in the same priority, and prevent a large transfer job from blocking smaller jobs.

When a job is newly created, it is automatically suspended (or paused). It has to be explicitly resumed to be activated. Resuming moves the job to queued state. When it is its turn to transfer data, it first connects to the remote server and then starts transferring. After the time slice expires, the transfer is temporarily paused and the job is moved to queued state. When it gets another time slice, it has to connect again before it can transfer. When the job is complete, BITS transfers ownership of the job to the application that created it.

BITS includes built-in mechanism for error handling and attempt recovery. Errors can be either fatal or transient, either moves a job to its respective state. A transient error is a temporary error that resolves itself after some time. For a transient error, BITS waits for sometime and then retries. For fatal errors, transfers the control of the job to its creating application, with as much information regarding the error as it can provide.

[edit] Version history

  • Version 1.0 (October 2001)
  • Version 1.2 (July 2002)
    • Included with Windows XP Service Pack 1 and Windows 2000 Service Pack 3. BITS' inclusion with Windows 2000 brought Automatic Updates capabilities into the core of that operating system.
    • No other external changes were made.
  • Version 1.5 (September 2003)
    • Included with Windows Server 2003 and made available as a separate download for Windows 2000 and Windows XP.
    • Added upload and upload-reply capability, command-line execution for events, explicit credentials, and support for Windows 2000.
  • Version 2.0 (August 2004)
    • Included with Windows XP Service Pack 2 and Windows Server 2003 Service Pack 1, and made available as a separate download for Windows 2000 Service Packs 3 and 4, and prior releases of Windows XP and Server 2003.
    • Added support for performing concurrent foreground downloads, using server message block paths for remote names, downloading portions of a file, changing the prefix or complete name of a remote name, and limiting client bandwidth usage.
    • BITS 2.0 is a minimum requirement for Windows Software Update Services.
  • Version 2.5
    • Adds support for certificate-based client authentication for secure HTTP transports and custom HTTP headers.
    • Microsoft's BITS documentation suggests that this version will be available for download for Windows 2000, XP, and Server 2003.
  • Version 3.0 (November 2006)
    • Adds the ability to cache downloaded files and to serve them to peers, receive notification when a file is downloaded, access the temporary file while the download is in progress, and control HTTP redirects. BITS 3.0 also uses Internet Gateway Device counters to more accurately calculate available bandwidth.
    • Included with Windows Vista, and is scheduled for inclusion with Windows Server "Longhorn".

[edit] List of non-Microsoft applications that use BITS

[edit] See also

[edit] External links

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