net/netcheck: ignore PreferredDERP changes that are small

If the absolute value of the difference between the current
PreferredDERP's latency and the best latency is <= 10ms, don't change
it and instead prefer the previous value.

This is in addition to the existing hysteresis that tries to remain
on the previous DERP region if the relative improvement is small, but
handles nodes that have low latency to >1 DERP region better.

Updates #8603

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I1e34c94178f8c9a68a69921c5bc0227337514c70

net/netcheck/netcheck.go

@@ -83,6 +83,7 @@ const (
 	defaultInitialRetransmitTime = 100 * time.Millisecond
 )
 
+// Report contains the result of a single netcheck.
 type Report struct {
 	UDP         bool // a UDP STUN round trip completed
 	IPv6        bool // an IPv6 STUN round trip completed
@@ -209,7 +210,7 @@ type Client struct {
 	prev     map[time.Time]*Report // some previous reports
 	last     *Report               // most recent report
 	lastFull time.Time             // time of last full (non-incremental) report
-	curState *reportState          // non-nil if we're in a call to GetReportn
+	curState *reportState          // non-nil if we're in a call to GetReport
 	resolver *dnscache.Resolver    // only set if UseDNSCache is true
 }
 
@@ -1443,6 +1444,17 @@ func (c *Client) timeNow() time.Time {
 	return time.Now()
 }
 
+const (
+	// preferredDERPAbsoluteDiff specifies the minimum absolute difference
+	// in latencies between two DERP regions that would cause a node to
+	// switch its PreferredDERP ("home DERP"). This ensures that if a node
+	// is 5ms from two different DERP regions, it doesn't flip-flop back
+	// and forth between them if one region gets slightly slower (e.g. if a
+	// node is near region 1 @ 4ms and region 2 @ 5ms, region 1 getting
+	// 5ms slower would cause a flap).
+	preferredDERPAbsoluteDiff = 10 * time.Millisecond
+)
+
 // addReportHistoryAndSetPreferredDERP adds r to the set of recent Reports
 // and mutates r.PreferredDERP to contain the best recent one.
 func (c *Client) addReportHistoryAndSetPreferredDERP(r *Report, dm tailcfg.DERPMapView) {
@@ -1514,11 +1526,27 @@ func (c *Client) addReportHistoryAndSetPreferredDERP(r *Report, dm tailcfg.DERPM
 		}
 	}
 
-	// If we're changing our preferred DERP, the old one's still
-	// accessible, and the new one's not much better, just stick
-	// with where we are.
+	// If we're changing our preferred DERP, we want to add some stickiness
+	// to the current DERP region. We avoid changing if the old region is
+	// still accessible and one of the conditions below is true.
+	keepOld := false
 	changingPreferred := prevDERP != 0 && r.PreferredDERP != prevDERP
-	if changingPreferred && oldRegionCurLatency != 0 && bestAny > oldRegionCurLatency/3*2 {
+	oldRegionIsAccessible := oldRegionCurLatency != 0
+	if changingPreferred && oldRegionIsAccessible {
+		// bestAny < any other value, so oldRegionCurLatency - bestAny >= 0
+		if oldRegionCurLatency-bestAny < preferredDERPAbsoluteDiff {
+			// The absolute value of latency difference is below
+			// our minimum threshold.
+			keepOld = true
+		}
+		if bestAny > oldRegionCurLatency/3*2 {
+			// Old region is about the same on a percentage basis
+			keepOld = true
+		}
+	}
+	if keepOld {
+		// Reset the report's PreferredDERP to be the previous value,
+		// which undoes any region change we made above.
 		r.PreferredDERP = prevDERP
 	}
 }

net/netcheck/netcheck_test.go

@@ -327,6 +327,15 @@ func TestAddReportHistoryAndSetPreferredDERP(t *testing.T) {
 			wantPrevLen: 2,
 			wantDERP:    1, // 2 didn't get fast enough
 		},
+		{
+			name: "preferred_derp_hysteresis_no_switch_absolute",
+			steps: []step{
+				{0 * time.Second, report("d1", 4*time.Millisecond, "d2", 5*time.Millisecond)},
+				{1 * time.Second, report("d1", 4*time.Millisecond, "d2", 1*time.Millisecond)},
+			},
+			wantPrevLen: 2,
+			wantDERP:    1, // 2 is 50%+ faster, but the absolute diff is <10ms
+		},
 		{
 			name: "preferred_derp_hysteresis_do_switch",
 			steps: []step{