NscTcpSocketImpl::SendPendingData() will write to NSC using:
m_nscTcpSocket->send_data()

If the underlying NSC socket buffer becomes full, the return value of send_data() will indicate this and SendPendingData() will return false, and some data will be sitting in the staging buffer m_txBuffer.  Later, an NSCWakeup() may trigger the emptying of this buffer (since NSC may later have space), but Socket::NotifySend() is never called to unblock the application, and the TCP transfer will stall.