为什么大量的数据包大小是从主机发送的？

你好，
我正在尝试从GPIF读取数据，并通过端点中的批量发送到主机。但是，我发现包大小的块需要是4字节的倍数，即3字节将导致零长度包，5字节将导致4字节分组。但是大量的数据包大小是从主机（PC）发送的。为什么？
谢谢，
罗孚



     以下为原文
  Hi,
     
    I'm trying to read data from GPIF and send to host through bulk in endpoint. However ,I found that the bulk in packet size needs to be multiple of 4 bytes, i.e. 3 bytes will result a zero length packet and 5 bytes will result a 4 bytes packet. But the bulk out packet size is actual as that sent from host (PC). Why?
     
    Thanks,
    Rover