Each endpoint used for an interface has its own descriptor. This descriptor contains the information
required by the host to determine the bandwidth requirements of each endpoint. An endpoint descriptor isalways returned as part of the configuration information returned by a GetDescriptor(Configuration)
request. An endpoint descriptor cannot be directly accessed with a GetDescriptor() or SetDescriptor()
request. There is never an endpoint descriptor for endpoint zero. Table 9-13 shows the standard endpoint
descriptor.
用于接口的端点都有自己的描述符。该描述符包含主机为确定每个端点的带宽需求所需的信息。一个端点描述符总是作为GetDescriptor（配置）请求返回的配置信息的一部分返回。端点描述符不能通过GetDescriptor（）或SetDescriptor（）请求直接访问。端点0不能被描述。
The bmAttributes field provides information about the endpoint’s Transfer Type (bits 1..0) and
Synchronization Type (bits 3..2). In addition, the Usage Type bit (bits 5..4) indicate whether this is an
endpoint used for normal data transfers (bits 5..4=00B), whether it is used to convey explicit feedback
information for one or more data endpoints (bits 5..4=01B) or whether it is a data endpoint that also serves
as an implicit feedback endpoint for one or more data endpoints (bits 5..4=10B). Bits 5..2 are only
meaningful for isochronous endpoints and must be reset to zero for all other transfer types.
bmAttributes 数据块的bits 1..0 表示传输的类型，
If the endpoint is used as an explicit feedback endpoint (bits 5..4=01B), then the Transfer Type must be set
to isochronous (bits1..0 = 01B) and the Synchronization Type must be set to No Synchronization
(bits 3..2=00B).
如果端点被用作显示的反馈端点，那么传输类型必须为同步模式。同步的类型必须为No Synchronization (bits 3..2=00B).
A feedback endpoint (explicit or implicit) needs to be associated with one (or more) isochronous data
endpoints to which it provides feedback service. The association is based on endpoint number matching. A
feedback endpoint always has the opposite direction from the data endpoint(s) it services. If multiple data
endpoints are to be serviced by the same feedback endpoint, the data endpoints must have ascending
ordered–but not necessarily consecutive–endpoint numbers. The first data endpoint and the feedback
endpoint must have the same endpoint number (and opposite direction). This ensures that a data endpoint
can uniquely identify its feedback endpoint by searching for the first feedback endpoint that has an endpoint
number equal or less than its own endpoint number.
一个反馈的端点需要绑定一个同步的数据端点，为这个端点进行服务。绑定是基于端点号进行的。反馈的端点和数据服务的端点是一个相反的方向。多个数据端点可以使用同一个反馈端点，数据端点必须按升序列排列，但不一定连续。第一个数据端点号必须和反馈的端点号一致（相反的方向）。这确保了数据端点可以通过搜索端点号等于或小于其自己的端点号的第一个反馈端点来唯一标识其反馈端点。