quic: correct rttvar updates

RFC errata 7539 adjusts RFC 9002 Section 5.3 to update
rttvar before smoothed_rtt.

https://www.rfc-editor.org/errata/eid7539

For golang/go#58547

Change-Id: I154ed54d3d693c3b42355de39c5907c7cc906fde
Reviewed-on: https://go-review.googlesource.com/c/net/+/502215
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/quic/rtt.go

@@ -67,7 +67,7 @@ func (r *rttState) updateSample(now time.Time, handshakeConfirmed bool, spaceID
 	if adjustedRTT < r.minRTT {
 		adjustedRTT = latestRTT
 	}
-	r.smoothedRTT = ((7 * r.smoothedRTT) + adjustedRTT) / 8
 	rttvarSample := abs(r.smoothedRTT - adjustedRTT)
 	r.rttvar = (3*r.rttvar + rttvarSample) / 4
+	r.smoothedRTT = ((7 * r.smoothedRTT) + adjustedRTT) / 8
 }

internal/quic/rtt_test.go

@@ -134,15 +134,15 @@ func TestRTTInitialRTT(t *testing.T) {
 	handshakeConfirmed = true
 	rtt.updateSample(now, handshakeConfirmed, handshakeSpace, 40*time.Millisecond, ackDelay, maxAckDelay)
 	adjustedRTT = 15 * time.Millisecond // latest_rtt (40ms) - max_ack_delay (25ms)
-	smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
-	if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
-		t.Errorf("smoothed_rtt = %v, want %v", got, want)
-	}
 	rttvarSample = abs(smoothedRTT - adjustedRTT)
 	rttvar = (3*rttvar + rttvarSample) / 4
 	if got, want := rtt.rttvar, rttvar; got != want {
 		t.Errorf("rttvar = %v, want %v", got, want)
 	}
+	smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
+	if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
+		t.Errorf("smoothed_rtt = %v, want %v", got, want)
+	}
 
 	// "[...] MUST NOT subtract the acknowledgment delay from
 	// the RTT sample if the resulting value is smaller than the min_rtt."
@@ -156,13 +156,13 @@ func TestRTTInitialRTT(t *testing.T) {
 	}
 	// latest_rtt (30ms) - ack_delay (25ms) = 5ms, which is less than min_rtt (10ms)
 	adjustedRTT = 30 * time.Millisecond // latest_rtt
-	smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
-	if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
-		t.Errorf("smoothed_rtt = %v, want %v", got, want)
-	}
 	rttvarSample = abs(smoothedRTT - adjustedRTT)
 	rttvar = (3*rttvar + rttvarSample) / 4
 	if got, want := rtt.rttvar, rttvar; got != want {
 		t.Errorf("rttvar = %v, want %v", got, want)
 	}
+	smoothedRTT = (7*smoothedRTT + adjustedRTT) / 8
+	if got, want := rtt.smoothedRTT, smoothedRTT; got != want {
+		t.Errorf("smoothed_rtt = %v, want %v", got, want)
+	}
 }