Called by the framework to notify the receiving socket that the sending socket has out-of-band data to send.
Out-of-band data is a logically independent channel that is associated with each pair of connected sockets of type SOCK_STREAM. The channel is generally used to send urgent data.
MFC supports out-of-band data, but users of class CAsyncSocket are discouraged from using it. The easier way is to create a second socket for passing such data. For more information about out-of-band data, see Windows Sockets: Socket Notifications.