diff --git a/internal/quic/sent_packet_list.go b/internal/quic/sent_packet_list.go
new file mode 100644
index 0000000..6fb712a
--- /dev/null
+++ b/internal/quic/sent_packet_list.go
@@ -0,0 +1,95 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.21
+
+package quic
+
+// A sentPacketList is a ring buffer of sentPackets.
+//
+// Processing an ack for a packet causes all older packets past a small threshold
+// to be discarded (RFC 9002, Section 6.1.1), so the list of in-flight packets is
+// not sparse and will contain at most a few acked/lost packets we no longer
+// care about.
+type sentPacketList struct {
+	nextNum packetNumber // next packet number to add to the buffer
+	off     int          // offset of first packet in the buffer
+	size    int          // number of packets
+	p       []*sentPacket
+}
+
+// start is the first packet in the list.
+func (s *sentPacketList) start() packetNumber {
+	return s.nextNum - packetNumber(s.size)
+}
+
+// end is one after the last packet in the list.
+// If the list is empty, start == end.
+func (s *sentPacketList) end() packetNumber {
+	return s.nextNum
+}
+
+// discard clears the list.
+func (s *sentPacketList) discard() {
+	*s = sentPacketList{}
+}
+
+// add appends a packet to the list.
+func (s *sentPacketList) add(sent *sentPacket) {
+	if s.nextNum != sent.num {
+		panic("inserting out-of-order packet")
+	}
+	s.nextNum++
+	if s.size >= len(s.p) {
+		s.grow()
+	}
+	i := (s.off + s.size) % len(s.p)
+	s.size++
+	s.p[i] = sent
+}
+
+// nth returns a packet by index.
+func (s *sentPacketList) nth(n int) *sentPacket {
+	index := (s.off + n) % len(s.p)
+	return s.p[index]
+}
+
+// num returns a packet by number.
+// It returns nil if the packet is not in the list.
+func (s *sentPacketList) num(num packetNumber) *sentPacket {
+	i := int(num - s.start())
+	if i < 0 || i >= s.size {
+		return nil
+	}
+	return s.nth(i)
+}
+
+// clean removes all acked or lost packets from the head of the list.
+func (s *sentPacketList) clean() {
+	for s.size > 0 {
+		sent := s.p[s.off]
+		if !sent.acked && !sent.lost {
+			return
+		}
+		sent.recycle()
+		s.p[s.off] = nil
+		s.off = (s.off + 1) % len(s.p)
+		s.size--
+	}
+	s.off = 0
+}
+
+// grow increases the buffer to hold more packaets.
+func (s *sentPacketList) grow() {
+	newSize := len(s.p) * 2
+	if newSize == 0 {
+		newSize = 64
+	}
+	p := make([]*sentPacket, newSize)
+	for i := 0; i < s.size; i++ {
+		p[i] = s.nth(i)
+	}
+	s.p = p
+	s.off = 0
+}
diff --git a/internal/quic/sent_packet_list_test.go b/internal/quic/sent_packet_list_test.go
new file mode 100644
index 0000000..2f7f4d2
--- /dev/null
+++ b/internal/quic/sent_packet_list_test.go
@@ -0,0 +1,107 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.21
+
+package quic
+
+import "testing"
+
+func TestSentPacketListSlidingWindow(t *testing.T) {
+	// Record 1000 sent packets, acking everything outside the most recent 10.
+	list := &sentPacketList{}
+	const window = 10
+	for i := packetNumber(0); i < 1000; i++ {
+		list.add(&sentPacket{num: i})
+		if i < window {
+			continue
+		}
+		prev := i - window
+		sent := list.num(prev)
+		if sent == nil {
+			t.Fatalf("packet %v not in list", prev)
+		}
+		if sent.num != prev {
+			t.Fatalf("list.num(%v) = packet %v", prev, sent.num)
+		}
+		if got := list.nth(0); got != sent {
+			t.Fatalf("list.nth(0) != list.num(%v)", prev)
+		}
+		sent.acked = true
+		list.clean()
+		if got := list.num(prev); got != nil {
+			t.Fatalf("list.num(%v) = packet %v, expected it to be discarded", prev, got.num)
+		}
+		if got, want := list.start(), prev+1; got != want {
+			t.Fatalf("list.start() = %v, want %v", got, want)
+		}
+		if got, want := list.end(), i+1; got != want {
+			t.Fatalf("list.end() = %v, want %v", got, want)
+		}
+		if got, want := list.size, window; got != want {
+			t.Fatalf("list.size = %v, want %v", got, want)
+		}
+	}
+}
+
+func TestSentPacketListGrows(t *testing.T) {
+	// Record 1000 sent packets.
+	list := &sentPacketList{}
+	const count = 1000
+	for i := packetNumber(0); i < count; i++ {
+		list.add(&sentPacket{num: i})
+	}
+	if got, want := list.start(), packetNumber(0); got != want {
+		t.Fatalf("list.start() = %v, want %v", got, want)
+	}
+	if got, want := list.end(), packetNumber(count); got != want {
+		t.Fatalf("list.end() = %v, want %v", got, want)
+	}
+	if got, want := list.size, count; got != want {
+		t.Fatalf("list.size = %v, want %v", got, want)
+	}
+	for i := packetNumber(0); i < count; i++ {
+		sent := list.num(i)
+		if sent == nil {
+			t.Fatalf("packet %v not in list", i)
+		}
+		if sent.num != i {
+			t.Fatalf("list.num(%v) = packet %v", i, sent.num)
+		}
+		if got := list.nth(int(i)); got != sent {
+			t.Fatalf("list.nth(%v) != list.num(%v)", int(i), i)
+		}
+	}
+}
+
+func TestSentPacketListCleanAll(t *testing.T) {
+	list := &sentPacketList{}
+	// Record 10 sent packets.
+	const count = 10
+	for i := packetNumber(0); i < count; i++ {
+		list.add(&sentPacket{num: i})
+	}
+	// Mark all the packets as acked.
+	for i := packetNumber(0); i < count; i++ {
+		list.num(i).acked = true
+	}
+	list.clean()
+	if got, want := list.size, 0; got != want {
+		t.Fatalf("list.size = %v, want %v", got, want)
+	}
+	list.add(&sentPacket{num: 10})
+	if got, want := list.size, 1; got != want {
+		t.Fatalf("list.size = %v, want %v", got, want)
+	}
+	sent := list.num(10)
+	if sent == nil {
+		t.Fatalf("packet %v not in list", 10)
+	}
+	if sent.num != 10 {
+		t.Fatalf("list.num(10) = %v", sent.num)
+	}
+	if got := list.nth(0); got != sent {
+		t.Fatalf("list.nth(0) != list.num(10)")
+	}
+}