diff --git a/replaydetector/replaydetector.go b/replaydetector/replaydetector.go
index 4358d8f..8d4909e 100644
--- a/replaydetector/replaydetector.go
+++ b/replaydetector/replaydetector.go
@@ -8,7 +8,14 @@ package replaydetector
 type ReplayDetector interface {
 	// Check returns true if given sequence number is not replayed.
 	// Call accept() to mark the packet is received properly.
-	Check(seq uint64) (accept func(), ok bool)
+	// The return value of accept() indicates whether the accepted packet is
+	// has the latest observed sequence number.
+	Check(seq uint64) (accept func() bool, ok bool)
+}
+
+// nop is a no-op func that is returned in the case that Check() fails.
+func nop() bool {
+	return false
 }
 
 type slidingWindowDetector struct {
@@ -30,30 +37,33 @@ func New(windowSize uint, maxSeq uint64) ReplayDetector {
 	}
 }
 
-func (d *slidingWindowDetector) Check(seq uint64) (accept func(), ok bool) {
+func (d *slidingWindowDetector) Check(seq uint64) (accept func() bool, ok bool) {
 	if seq > d.maxSeq {
 		// Exceeded upper limit.
-		return func() {}, false
+		return nop, false
 	}
 
 	if seq <= d.latestSeq {
 		if d.latestSeq >= uint64(d.windowSize)+seq {
-			return func() {}, false
+			return nop, false
 		}
 		if d.mask.Bit(uint(d.latestSeq-seq)) != 0 {
 			// The sequence number is duplicated.
-			return func() {}, false
+			return nop, false
 		}
 	}
 
-	return func() {
+	return func() bool {
+		latest := seq == 0
 		if seq > d.latestSeq {
 			// Update the head of the window.
 			d.mask.Lsh(uint(seq - d.latestSeq))
 			d.latestSeq = seq
+			latest = true
 		}
 		diff := (d.latestSeq - seq) % d.maxSeq
 		d.mask.SetBit(uint(diff))
+		return latest
 	}, true
 }
 
@@ -75,10 +85,10 @@ type wrappedSlidingWindowDetector struct {
 	init       bool
 }
 
-func (d *wrappedSlidingWindowDetector) Check(seq uint64) (accept func(), ok bool) {
+func (d *wrappedSlidingWindowDetector) Check(seq uint64) (accept func() bool, ok bool) {
 	if seq > d.maxSeq {
 		// Exceeded upper limit.
-		return func() {}, false
+		return nop, false
 	}
 	if !d.init {
 		if seq != 0 {
@@ -99,21 +109,24 @@ func (d *wrappedSlidingWindowDetector) Check(seq uint64) (accept func(), ok bool
 
 	if diff >= int64(d.windowSize) {
 		// Too old.
-		return func() {}, false
+		return nop, false
 	}
 	if diff >= 0 {
 		if d.mask.Bit(uint(diff)) != 0 {
 			// The sequence number is duplicated.
-			return func() {}, false
+			return nop, false
 		}
 	}
 
-	return func() {
+	return func() bool {
+		latest := false
 		if diff < 0 {
 			// Update the head of the window.
 			d.mask.Lsh(uint(-diff))
 			d.latestSeq = seq
+			latest = true
 		}
 		d.mask.SetBit(uint(d.latestSeq - seq))
+		return latest
 	}, true
 }