http://msdn.microsoft.com/msdnmag/issues/1000/Winsock/default.aspx

Who Manages the Buffers?
As just mentioned, AFD.SYS handles buffer management for applications that use Winsock to talk to the transport protocol drivers. This means that when an application calls the send or WSASend function to send data, the data gets copied by AFD.SYS to its internal buffers (up to the SO_SNDBUF setting) and the send or WSASend function returns immediately. The data is then sent by AFD.SYS behind the application's back, so to speak. Of course, if the application wants to issue a send for a buffer larger than the SO_SNDBUF setting, the WSASend call blocks until all the data is sent.
Similarly, on receiving data from the remote client, AFD.SYS will copy the data to its own buffers as long as there is no outstanding data to receive from the application, and as long as the SO_RCVBUF setting is not exceeded. When the application calls recv or WSARecv, the data is copied from AFD.SYS's buffers to the application-provided buffer.

后面一段的 'outstanding data'，这是什么意思啊？