diff --git a/packetcache/packetcache.go b/packetcache/packetcache.go
index 309b07a..d48eb33 100644
--- a/packetcache/packetcache.go
+++ b/packetcache/packetcache.go
@@ -526,3 +526,21 @@ func (cache *Cache) GetStats(reset bool) (uint32, uint32, uint32, uint32) {
 	}
 	return expected, lost, totalLost, eseqno
 }
+
+// ToBitmap takes a non-empty sorted list of seqnos, and computes a bitmap
+// covering a prefix of the list.  It returns the part of the list that
+// couldn't be covered.
+func ToBitmap(seqnos []uint16) (first uint16, bitmap uint16, remain []uint16) {
+	first = seqnos[0]
+	bitmap = uint16(0)
+	remain = seqnos[1:]
+	for len(remain) > 0 {
+		delta := remain[0] - first - 1
+		if delta >= 16 {
+			break
+		}
+		bitmap = bitmap | (1 << delta)
+		remain = remain[1:]
+	}
+	return
+}
diff --git a/packetcache/packetcache_test.go b/packetcache/packetcache_test.go
index 7f87163..c6bd7f3 100644
--- a/packetcache/packetcache_test.go
+++ b/packetcache/packetcache_test.go
@@ -3,6 +3,7 @@ package packetcache
 import (
 	"bytes"
 	"math/rand"
+	"reflect"
 	"sync"
 	"testing"
 
@@ -457,3 +458,39 @@ func BenchmarkCachePutGetAt(b *testing.B) {
 	}
 	wg.Wait()
 }
+
+func TestToBitmap(t *testing.T) {
+	l := []uint16{18, 19, 32, 38}
+	bb := uint16(1 | 1<<(32-18-1))
+	f, b, r := ToBitmap(l)
+	if f != 18 || b != bb {
+		t.Errorf("Expected %v %v, Got %v %v", 18, bb, f, b)
+	}
+	if len(r) != 1 || r[0] != 38 {
+		t.Errorf("Expected [38], got %v", r)
+	}
+
+	f2, b2, r2 := ToBitmap(r)
+	if f2 != 38 || b2 != 0 || len(r2) != 0 {
+		t.Errorf("Expected 38 0, got %v %v %v", f2, b2, r2)
+	}
+}
+
+func TestToBitmapNack(t *testing.T) {
+	l := []uint16{18, 19, 32, 38}
+	var nacks []rtcp.NackPair
+	m := l
+	for len(m) > 0 {
+		var f, b uint16
+		f, b, m = ToBitmap(m)
+		nacks = append(nacks, rtcp.NackPair{f, rtcp.PacketBitmap(b)})
+	}
+	var n []uint16
+	for len(nacks) > 0 {
+		n = append(n, nacks[0].PacketList()...)
+		nacks = nacks[1:]
+	}
+	if !reflect.DeepEqual(l, n) {
+		t.Errorf("Expected %v, got %v", l, n)
+	}
+}