M_RCV_BUFSIZ
#define G_RCV_BUFSIZ(x) (((x) >> S_RCV_BUFSIZ) & M_RCV_BUFSIZ)
#define G_RCV_BUFSIZ(x) (((x) >> S_RCV_BUFSIZ) & M_RCV_BUFSIZ)
MPASS(cp->opt0_bufsize >= 0 && cp->opt0_bufsize <= M_RCV_BUFSIZ);
cp->opt0_bufsize = min(wnd >> 10, M_RCV_BUFSIZ);
cp->opt0_bufsize = min(wnd >> 10, M_RCV_BUFSIZ);