M_RCV_BUFSIZ
#define G_RCV_BUFSIZ(x) (((x) >> S_RCV_BUFSIZ) & M_RCV_BUFSIZ)
if (csk->rcv_win > (M_RCV_BUFSIZ << 10))
csk->rcv_wup -= csk->rcv_win - (M_RCV_BUFSIZ << 10);