The USB Mass Storage Class - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 54

Table 3-4: A bulk-only mass storage device must have two endpoint descriptors.
Byte

Field Description

0 bLength Descriptor size in bytes (07h).

1 bDescriptorType The constant Endpoint (05h).

2 bEndpointAddress Endpoint number and direction.

3 bmAttributes Transfer type supported. Bulk = 02h.

4 wMaxPacketSize Maximum packet size supported.

6 bInterval Maximum NAK rate for high-speed bulk OUT endpoints.

Otherwise ignored for bulk endpoints.

value indicates the endpoint's maximum NAK rate. This value is relevant
when the device has received data and has returned ACK, and the host has
more data to send. By returning ACK, the device is saying that it expects to
be able to accept the next transaction's data. (Otherwise the device would
return NYET.) If the next data packet arrives and for some reason the device
can't accept the packet, the endpoint returns NAK. The bInterval value says
that the endpoint expects to return NAK no more than once in each period
specified by bInterval. The value can range from 0 to 255 microframes. A
value of zero means the endpoint doesn't ever expect to return NAK imme-
diately after an ACK.

String Descriptors

A string descriptor contains descriptive text. Support for most string
descriptors is optional, but every mass-storage device that uses the bulk-only
transport protocol must have a string descriptor that contains a serial num-
ber. The serial number must have at least 12 characters and must contain
only characters in the range 0-9 (0030h-0039h) and A-F (0041h-0046h).
Note that lower-case text, hyphens, and many other characters are not
allowed. The last 12 characters must be different from the last 12 characters
of the serial number of any device with the same values in the idVendor and
idProduct fields in the device descriptor. The serial number enables a host to
retain properties such as the drive letter and access policies after a user moves
a device to another port or attaches multiple devices with the same Vendor
ID and Product ID.