USB bandwidth allocation

This article provides guidance concerning the careful management of USB bandwidth. It's the responsibility of every USB client driver to minimize the USB bandwidth it uses, and return unused bandwidth to the free bandwidth pool as promptly as possible.

Why is my USB driver getting out of bandwidth errors?

Competition for bandwidth on the USB bus comes from multiple sources, both hardware and software. It's difficult to predict exactly how much bandwidth is available for a USB client driver. The USB host controller requires a certain amount of bandwidth for its operations. The amount required depends on whether the controller is high speed or not. It varies from system to system. USB hubs that operate at high speed must sometimes translate transactions between high-speed upstream ports and low-speed devices downstream, and this translation process consumes bandwidth. But whether bandwidth is required for transaction translation depends on the kind of devices that are connected and the topology of the device tree.

The most serious strain on bandwidth resource usually comes from USB client drivers that monopolize bandwidth. The system allocates bandwidth on a first-come-first-serve basis. If the first USB driver loaded requests all of the available bandwidth, a USB driver that loads at a later time isn't allowed any bandwidth for its device. The system can't configure the device and fails to enumerate it. Since it's not apparent why the enumeration failed, the user has bad experience.

Occasionally, a client driver exhausts the available bandwidth with a high-speed interrupt transfer. But the most common case, by far, is that of a client driver that allocates too much bandwidth for an isochronous transfer, then fails to release the bandwidth in a timely fashion. The system reserves allocated bandwidth until the driver that requested it closes its endpoint (by opening another endpoint), or the device for which the bandwidth was allocated is removed. The system doesn't allocate guaranteed bandwidth for bulk transfers, so bulk transfers are never the cause of enumeration failures. However, the performance of bulk transfer devices depends on how much bandwidth is allocated for devices that do periodic (isochronous and interrupt) transfers.

The USB 2.0 specification requires an isochronous device to have zero-bandwidth endpoints on its default interface setting. This ensures that no bandwidth is reserved for the device until a function driver opens a non-default interface, which helps prevent enumeration failures caused by excessive bandwidth requests during device configuration. It doesn't prevent a client driver from allocating too much bandwidth after configuring its device, thereby preventing other devices from functioning properly.

The key to proper bandwidth management is that every USB device in the system that does isochronous transfers must offer multiple alternative (Alt) settings for each interface that contains isochronous endpoints, and client drivers must make judicious use of these Alt settings. Client drivers should begin by requesting the interface setting with the highest bandwidth. If the request fails, the client driver should request interface settings with smaller and smaller bandwidths until a request succeeds.

For instance, suppose a webcam device has the following interfaces:

Interface 0 (Default interface setting: No endpoints with nonzero isochronous bandwidth in the default setting)

Isochronous Endpoint 1: maximum packet size = 0 bytes

Isochronous Endpoint 2: maximum packet size = 0 bytes

Interface 0 Alt setting 1

Isochronous Endpoint 1: maximum packet size = 256 bytes

Isochronous Endpoint 2: maximum packet size = 256 bytes

Interface 0 Alt setting 2

Isochronous Endpoint 1: maximum packet size = 512 bytes

Isochronous Endpoint 2: maximum packet size = 512 bytes

The driver for the webcam configures the webcam to use the default interface setting when it initializes. The default setting has no isochronous bandwidth, so using the default setting during initialization avoids the danger that the webcam might fail to enumerate, because of a failed request for isochronous bandwidth.

When the client driver is ready to do an isochronous transfer, it should attempt to use Alt setting 2, because Alt setting 2 has the largest packet size. If the request fails, the driver can make a second attempt, using Alt setting 1. Since Alt setting 1 requires less bandwidth, this request might succeed, even though the first request failed. Multiple Alt settings allow the driver to make several attempts, before giving up.

After the webcam becomes idle, it can return the allocated bandwidth to the free bandwidth pool by selecting the default setting once again.

Users can see how much bandwidth a USB controller has allocated by checking the controller's properties in Windows Device Manager. Select the controller's properties then look under the Advanced tab. This reading doesn't indicate how much bandwidth USB hubs have allocated for transaction translation.

The Device Manager feature that reports the bandwidth usage of a USB controller doesn't work properly in Windows XP.

USB transfer and packet sizes

This article describes USB transfer sizes allowed in various versions of the Windows operating system.

• Maximum transfer size
• Maximum packet size
• Maximum packet size restriction on read transfer buffers
• Delimiting write transfers with short packets

Maximum transfer size

The maximum transfer size specifies hard-coded limits in the USB driver stack. It's possible that transfer sizes below these limits fail because of system resource limitations. To avoid these types of failures and to ensure compatibility across all versions of Windows, avoid using large transfer sizes for USB transfers.

Note

The MaximumTransferSize member of the USBD_PIPE_INFORMATION structure is obsolete. The USB driver stack ignores the value in MaximumTransferSize for both composite and non-composite devices.

In Windows 2000, the USB driver stack initializes MaximumTransferSize to USBD_DEFAULT_MAXIMUM_TRANSFER_SIZE. A client driver can set a smaller value while configuring the device. For a composite device, the client driver for each function can only change MaximumTransferSize for pipes in the non-default interface setting.

USB transfer sizes are subject to the following limits:

Transfer pipe	Windows 8.1, Windows 8	Windows 7, Windows Vista	Windows XP, Windows Server 2003	Windows 2000
Control	64K for SuperSpeed and high speed (xHCI) 4K for full and low speed (xHCI, EHCI, UHCI, OHCI) For UHCI, 4K on the default endpoint; 64K on non-default control pipes	64K for high speed (EHCI) 4K for full and low speed (EHCI, UHCI, OHCI) For UHCI, 4K on the default endpoint; 64K on non-default control pipes (UHCI)	64K for high speed (EHCI) 4K for full and low speed (EHCI, UHCI, OHCI) For UHCI, 4K on the default endpoint; 64K on non-default control pipes (UHCI)	4K on the default endpoint; 64K on non-default control pipes (OHCI)
Interrupt	4MB for SuperSpeed, high, full, and low speed (xHCI, EHCI, UHCI, OHCI)	4MB for high, full, and low speed (EHCI, UHCI, OHCI)	Unlimited	Undetermined(OHCI)
Bulk	32MB for SuperSpeed (xHCI) 4MB for high and full speed (xHCI) 4MB for high and full speed (EHCI and UHCI) 256K full speed (OHCI)	4MB for high and full speed (EHCI, UHCI) 256K for full speed (OHCI)	3MB for high and full speed (EHCI) Undetermined (UHCI) 256K for full speed (OHCI)	Undetermined(OHCI)
Isochronous	1024 * wBytesPerInterval for SuperSpeed (xHCI) (see USB_SUPERSPEED_ENDPOINT_COMPANION_DESCRIPTOR) 1024 * MaximumPacketSize for high speed (xHCI, EHCI) 256 * MaximumPacketSize for full speed (xHCI, EHCI) 64K for full speed (UHCI, OHCI)	1024 * MaximumPacketSize for high speed (EHCI) 256 * MaximumPacketSize for full-speed (EHCI) 64K for full speed (UHCI, OHCI)	1024 * MaximumPacketSize for high-speed (EHCI) 256 * MaximumPacketSize for full speed (EHCI) 64K for full speed (UHCI, OHCI)	64K for full speed (OHCI)

Restricting the transfer size with MaximumTransferSize doesn't directly affect how much bandwidth a device consumes. The client driver must either change the interface setting or restrict the maximum packet size set in the MaximumPacketSize member of USBD_PIPE_INFORMATION.

Maximum packet size

The maximum packet size is defined by the wMaxPacketSize field of the endpoint descriptor. A client driver can regulate the USB packet size in a select-interface request to the device. Changing this value doesn't change the wMaxPacketSize on the device.

In the URB for the request is a USBD_PIPE_INFORMATION structure for the pipe. In that structure,

• Modify the MaximumPacketSize member of the USBD_PIPE_INFORMATION structure. Set it to a value less than or equal to the value of wMaxPacketSize defined in device firmware for the current interface setting.
• Set the USBD_PF_CHANGE_MAX_PACKET flag in the PipeFlags member USBD_PIPE_INFORMATION structure.

For information about selecting an interface setting, see How to Select a Configuration for a USB Device.

Maximum packet size restriction on read transfer buffers

When a client driver makes a read request, the transfer buffer must be a multiple of the maximum packet size. Even when the driver expects data less than the maximum packet size, it must still request the entire packet. When the device sends a packet less than the maximum size (a short packet), it's an indication that the transfer is complete.

On older controllers, the client driver can override the behavior. In the TransferFlags member of the data transfer URB, the client driver must set the USBD_SHORT_TRANSFER_OK flag. That flag permits the device to send packets smaller than wMaxPacketSize.

On xHCI host controllers, USBD_SHORT_TRANSFER_OK ignored for bulk and interrupt endpoints. Transfer of short packets on EHCI controllers doesn't result in an error condition.

On EHCI host controllers, USBD_SHORT_TRANSFER_OK is ignored for bulk and interrupt endpoints.

On UHCI and OHCI host controllers, if USBD_SHORT_TRANSFER_OK isn't set for a bulk or interrupt transfer, a short packet transfer halts the endpoint and an error code is returned for the transfer.

Delimiting write transfers with short packets

The USB driver stack driver doesn't impose the same restrictions on packet size, when writing to the device, that it imposes when reading from the device. Some client drivers must make frequent transmissions of small quantities of control data to manage their devices. It's impractical to restrict data transmissions to packets of uniform size in such cases. Therefore, the driver stack doesn't assign any special significance to packets of size less than the endpoint's maximum size during data writes. This allows a client driver to break a large transfer to the device into multiple URBs of any size less than or equal to the maximum.

The driver must either end the transmission with a packet of less than maximum size, or delimit the end of the transmission with a zero-length packet. The transmission isn't complete until the driver sends a packet smaller than wMaxPacketSize. If the transfer size is an exact multiple of the maximum, the driver must send a zero-length delimiting packet to explicitly terminate the transfer

The client driver is responsible for delimiting the data transmission with zero-length packets, as required by the USB specification. The USB driver stack doesn't generate these packets automatically.

Delimiting USB data transfers with packets smaller than wMaxPacketSize

Compliant USB 2.0 and USB 1.1 drivers must transmit packets of maximum size (wMaxPacketSize) and then end the transmission with a packet of less than maximum size, or delimit the end of the transmission with a zero-length packet. The transmission isn't complete until the driver sends a packet smaller than wMaxPacketSize. If the transfer size is an exact multiple of the maximum, the driver must send a zero-length delimiting packet to explicitly terminate the transfer

The device driver is responsible for delimiting the data transmission with zero-length packets, as required by the USB specification. The system USB stack doesn't generate these packets automatically.