Implement the clock filter as descirbed by David Mills:

form the last 8 replied received from a peer, find the one with the lowest
delay. Use that as the peer's update taken into account for calculating
the local clock's offset.
Invalidate that reply and all ones received earlier than it so that they do
not get used again.

src/usr.sbin/ntpd/client.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: client.c,v 1.10 2004/07/06 19:06:43 henning Exp $ */
+/*	$OpenBSD: client.c,v 1.11 2004/07/06 23:26:38 henning Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -138,16 +138,19 @@ client_dispatch(struct ntp_peer *p)
 	T2 = lfp_to_d(msg.rectime);
 	T3 = lfp_to_d(msg.xmttime);
 
-	p->offset[p->shift] = ((T2 - T1) + (T3 - T4)) / 2;
-	p->delay[p->shift] = (T2 - T1) - (T3 - T4);
+	p->reply[p->shift].offset = ((T2 - T1) + (T3 - T4)) / 2;
+	p->reply[p->shift].delay = (T4 - T1) - (T2 - T3);
+	p->reply[p->shift].error = (T2 - T1) - (T3 - T4);
+	p->reply[p->shift].rcvd = time(NULL);
+	p->reply[p->shift].good = 1;
 
 	p->state = STATE_REPLY_RECEIVED;
 	p->next = time(NULL) + INTERVAL_QUERY;
 	p->deadline = 0;
 
 	log_debug("reply received from %s: offset %f delay %f",
-	    log_sockaddr((struct sockaddr *)&fsa), p->offset[p->shift],
-	    p->delay[p->shift]);
+	    log_sockaddr((struct sockaddr *)&fsa), p->reply[p->shift].offset,
+	    p->reply[p->shift].delay);
 
 	if (++p->shift >= OFFSET_ARRAY_SIZE) {
 		p->shift = 0;

src/usr.sbin/ntpd/ntp.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntp.c,v 1.11 2004/07/05 22:12:53 henning Exp $ */
+/*	$OpenBSD: ntp.c,v 1.12 2004/07/06 23:26:38 henning Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -39,6 +39,7 @@ void	ntp_sighdlr(int);
 int	ntp_dispatch_imsg(void);
 int	ntp_dispatch(int fd);
 void	ntp_adjtime(struct ntpd_conf *);
+int	get_peer_update(struct ntp_peer *, double *);
 
 void
 ntp_sighdlr(int sig)
@@ -283,16 +284,15 @@ void
 ntp_adjtime(struct ntpd_conf *conf)
 {
 	struct ntp_peer	*p;
-	double		 offset_median = 0;
+	double		 offset, offset_median = 0;
 	int		 offset_cnt = 0;
-	u_int8_t	 idx;
 
 	TAILQ_FOREACH(p, &conf->ntp_peers, entry) {
 		if (!p->valid)
 			continue;
 
-		for (idx = 0; idx < OFFSET_ARRAY_SIZE; idx++) {
-			offset_median += p->offset[idx];
+		if (get_peer_update(p, &offset) == 0) {
+			offset_median += offset;
 			offset_cnt++;
 		}
 	}
@@ -303,3 +303,41 @@ ntp_adjtime(struct ntpd_conf *conf)
 		imsg_compose(&ibuf_main, IMSG_ADJTIME, 0,
 			    &offset_median, sizeof(offset_median));
 }
+
+int
+get_peer_update(struct ntp_peer *p, double *offset)
+{
+	int	i, best = 0, good = 0;
+
+	/*
+	 * clock filter
+	 * find the offset which arrived with the lowest delay
+	 * use that as the peer update
+	 * invalidate it and all older ones
+	 */
+
+	for (i = 0; good == 0 && i < OFFSET_ARRAY_SIZE; i++)
+		if (p->reply[i].good) {
+			good++;
+			best = i;
+		}
+
+	for (; i < OFFSET_ARRAY_SIZE; i++)
+		if (p->reply[i].good) {
+			good++;
+			if (p->reply[i].delay < p->reply[best].delay)
+				best = i;
+		}
+
+	/* XXX lower trust in the peer when too few good replies received */
+
+	if (good == 0)
+		return (-1);
+
+	for (i = 0; i < OFFSET_ARRAY_SIZE; i++)
+		if (p->reply[i].rcvd <= p->reply[best].rcvd)
+			p->reply[i].good = 0;
+
+	*offset = p->reply[best].offset;
+	return (0);
+}

src/usr.sbin/ntpd/ntpd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntpd.h,v 1.12 2004/07/05 22:12:53 henning Exp $ */
+/*	$OpenBSD: ntpd.h,v 1.13 2004/07/06 23:26:38 henning Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -36,7 +36,7 @@
 #define	NTPD_OPT_VERBOSE	0x0001
 #define	NTPD_OPT_VERBOSE2	0x0002
 
-#define	INTERVAL_ADJTIME	120	/* call adjtime every n seconds */
+#define	INTERVAL_ADJTIME	240	/* call adjtime every n seconds */
 #define	INTERVAL_QUERY		30	/* sync with peers every n seconds */
 #define	QUERYTIME_MAX		15	/* single query might take n secs max */
 #define	OFFSET_ARRAY_SIZE	8
@@ -53,6 +53,14 @@ struct listen_addr {
 	int				 fd;
 };
 
+struct ntp_offset {
+	u_int8_t	good;
+	double		offset;
+	double		delay;
+	double		error;
+	time_t		rcvd;
+};
+
 struct ntp_peer {
 	TAILQ_ENTRY(ntp_peer)		 entry;
 	struct sockaddr_storage		 ss;
@@ -60,8 +68,7 @@ struct ntp_peer {
 	enum client_state		 state;
 	time_t				 next;
 	time_t				 deadline;
-	double				 offset[OFFSET_ARRAY_SIZE];
-	double				 delay[OFFSET_ARRAY_SIZE];
+	struct ntp_offset		 reply[OFFSET_ARRAY_SIZE];
 	u_int8_t			 shift;
 	u_int8_t			 valid;
 };