Minor API change. The 'factor' now ranges from 0..10. See adb.h for details.

lib/dns/adb.c

@@ -3401,14 +3401,14 @@ dns_adb_adjustsrtt(dns_adb_t *adb, dns_adbaddrinfo_t *addr,
 
 	REQUIRE(DNS_ADB_VALID(adb));
 	REQUIRE(DNS_ADBADDRINFO_VALID(addr));
-
-	if (factor == 0)
-		factor = 4;
+	REQUIRE(factor <= 10);
 
 	bucket = addr->entry->lock_bucket;
 	LOCK(&adb->entrylocks[bucket]);
 
-	new_srtt = (addr->entry->srtt * (factor - 1) + rtt) / factor;
+	new_srtt = (addr->entry->srtt / 10 * factor)
+		+ (rtt / 10 * (10 - factor));
+
 	addr->entry->srtt = new_srtt;
 	addr->srtt = new_srtt;
 

lib/dns/include/dns/adb.h

@@ -445,6 +445,12 @@ dns_adb_adjustgoodness(dns_adb_t *adb, dns_adbaddrinfo_t *addr,
  *	goodness value.  This may include changes made by others.
  */
 
+/*
+ * A reasonable default for RTT adjustments
+ */
+#define DNS_ADB_RTTADJDEFAULT		7	/* default scale */
+#define DNS_ADB_RTTADJREPLACE		0	/* replace with our rtt */
+
 void
 dns_adb_adjustsrtt(dns_adb_t *adb, dns_adbaddrinfo_t *addr,
 		   unsigned int rtt, unsigned int factor);
@@ -453,7 +459,7 @@ dns_adb_adjustsrtt(dns_adb_t *adb, dns_adbaddrinfo_t *addr,
  * (where srtt is the existing rtt value, and rtt and factor are arguments to
  * this function):
  *
- *	new_srtt = (srtt * (factor - 1) + rtt) / factor
+ *	new_srtt = (old_srtt / 10 * factor) + (rtt / 10 * (10 - factor));
  *
  * Requires:
  *
@@ -461,9 +467,9 @@ dns_adb_adjustsrtt(dns_adb_t *adb, dns_adbaddrinfo_t *addr,
  *
  *	addr be valid.
  *
- * Note:
+ *	0 <= factor <= 10
  *
- *	If factor is zero, 4 will be used.
+ * Note:
  *
  *	The srtt in addr will be updated to reflect the new global
  *	srtt value.  This may include changes made by others.

lib/dns/resolver.c

@@ -269,7 +269,7 @@ fctx_cancelquery(resquery_t **queryp, dns_dispatchevent_t **deventp,
 
 	if (finish != NULL) {
 		rtt = (unsigned int)isc_time_microdiff(finish, &query->start);
-		factor = 0;
+		factor = DNS_ADB_RTTADJDEFAULT;
 	} else {
 		/*
 		 * We don't have an RTT for this query.  Maybe the packet
@@ -280,10 +280,9 @@ fctx_cancelquery(resquery_t **queryp, dns_dispatchevent_t **deventp,
 		if (rtt > 10000000)
 			rtt = 10000000;
 		/*
-		 * We set 'factor' to 1, so that we will replace the current
-		 * RTT.
+		 * Replace the current RTT with our value.
 		 */
-		factor = 1;
+		factor = DNS_ADB_RTTADJREPLACE;
 	}
 	dns_adb_adjustsrtt(fctx->res->view->adb, query->addrinfo, rtt, factor);