use RMS for jitter. we're linking with enough libraries that libm is tiny.

ok deraadt

src/usr.sbin/ntpd/Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.15 2015/10/05 17:26:22 deraadt Exp $
+#	$OpenBSD: Makefile,v 1.16 2015/11/20 18:53:42 tedu Exp $
 
 PROG=	ntpd
 SRCS=	ntpd.c log.c ntp.c ntp_msg.c parse.y config.c \
@@ -11,7 +11,7 @@ CFLAGS+= -Wmissing-declarations
 CFLAGS+= -Wshadow -Wpointer-arith -Wcast-qual
 CFLAGS+= -Wsign-compare
 YFLAGS=
-LDADD+=	-lutil -ltls -lssl -lcrypto
+LDADD+=	-lm -lutil -ltls -lssl -lcrypto
 DPADD+= ${LIBUTIL} ${LIBCRYPTO} ${LIBSSL} ${LIBTLS}
 LINKS=	${BINDIR}/ntpd ${BINDIR}/ntpctl
 MAN=	ntpd.8 ntpd.conf.5 ntpctl.8

src/usr.sbin/ntpd/control.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: control.c,v 1.7 2015/10/23 14:52:20 phessler Exp $ */
+/*	$OpenBSD: control.c,v 1.8 2015/11/20 18:53:42 tedu Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -22,6 +22,7 @@
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <errno.h>
+#include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -33,6 +34,8 @@
 
 #define	CONTROL_BACKLOG	5
 
+#define square(x) ((x) * (x))
+
 int
 control_init(char *path)
 {
@@ -354,21 +357,18 @@ build_show_peer(struct ctl_show_peer *cp, struct ntp_peer *p)
 		cp->delay /= validdelaycnt;
 	}
 
-	/*
-	 *  use simple average for jitter calculation, as the
-	 *  RFC5905-recommended RMS average needs the math library
-	 */
 	jittercnt = 0;
 	cp->jitter = 0.0;
 	for (shift = 0; shift < OFFSET_ARRAY_SIZE; shift++) {
 		if (p->reply[shift].delay > 0.0 && shift != best) {
-			cp->jitter += p->reply[shift].delay -
-			    p->reply[best].delay;
+			cp->jitter += square(p->reply[shift].delay -
+			    p->reply[best].delay);
 			jittercnt++;
 		}
 	}
 	if (jittercnt > 1)
 		cp->jitter /= jittercnt;
+	cp->jitter = sqrt(cp->jitter);
 
 	if (p->shift == 0)
 		shift = OFFSET_ARRAY_SIZE - 1;