diff --git a/internal/quic/sent_packet.go b/internal/quic/sent_packet.go
new file mode 100644
index 0000000..03d5e53
--- /dev/null
+++ b/internal/quic/sent_packet.go
@@ -0,0 +1,100 @@
+// 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.
+
+package quic
+
+import (
+	"sync"
+	"time"
+)
+
+// A sentPacket tracks state related to an in-flight packet we sent,
+// to be committed when the peer acks it or resent if the packet is lost.
+type sentPacket struct {
+	num  packetNumber
+	size int       // size in bytes
+	time time.Time // time sent
+
+	ackEliciting bool // https://www.rfc-editor.org/rfc/rfc9002.html#section-2-3.4.1
+	inFlight     bool // https://www.rfc-editor.org/rfc/rfc9002.html#section-2-3.6.1
+	acked        bool // ack has been received
+	lost         bool // packet is presumed lost
+
+	// Frames sent in the packet.
+	//
+	// This is an abbreviated version of the packet payload, containing only the information
+	// we need to process an ack for or loss of this packet.
+	// For example, a CRYPTO frame is recorded as the frame type (0x06), offset, and length,
+	// but does not include the sent data.
+	b []byte
+	n int // read offset into b
+}
+
+var sentPool = sync.Pool{
+	New: func() interface{} {
+		return &sentPacket{}
+	},
+}
+
+func newSentPacket() *sentPacket {
+	sent := sentPool.Get().(*sentPacket)
+	sent.reset()
+	return sent
+}
+
+// recycle returns a sentPacket to the pool.
+func (sent *sentPacket) recycle() {
+	sentPool.Put(sent)
+}
+
+func (sent *sentPacket) reset() {
+	*sent = sentPacket{
+		b: sent.b[:0],
+	}
+}
+
+// The append* methods record information about frames in the packet.
+
+func (sent *sentPacket) appendNonAckElicitingFrame(frameType byte) {
+	sent.b = append(sent.b, frameType)
+}
+
+func (sent *sentPacket) appendAckElicitingFrame(frameType byte) {
+	sent.ackEliciting = true
+	sent.inFlight = true
+	sent.b = append(sent.b, frameType)
+}
+
+func (sent *sentPacket) appendInt(v uint64) {
+	sent.b = appendVarint(sent.b, v)
+}
+
+func (sent *sentPacket) appendOffAndSize(start int64, size int) {
+	sent.b = appendVarint(sent.b, uint64(start))
+	sent.b = appendVarint(sent.b, uint64(size))
+}
+
+// The next* methods read back information about frames in the packet.
+
+func (sent *sentPacket) next() (frameType byte) {
+	f := sent.b[sent.n]
+	sent.n++
+	return f
+}
+
+func (sent *sentPacket) nextInt() uint64 {
+	v, n := consumeVarint(sent.b[sent.n:])
+	sent.n += n
+	return v
+}
+
+func (sent *sentPacket) nextRange() (start, end int64) {
+	start = int64(sent.nextInt())
+	end = start + int64(sent.nextInt())
+	return start, end
+}
+
+func (sent *sentPacket) done() bool {
+	return sent.n == len(sent.b)
+}
diff --git a/internal/quic/sent_packet_test.go b/internal/quic/sent_packet_test.go
new file mode 100644
index 0000000..08a3d8f
--- /dev/null
+++ b/internal/quic/sent_packet_test.go
@@ -0,0 +1,51 @@
+// 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.
+
+package quic
+
+import "testing"
+
+func TestSentPacket(t *testing.T) {
+	frames := []interface{}{
+		byte(frameTypePing),
+		byte(frameTypeStreamBase),
+		uint64(1),
+		i64range{1 << 20, 1<<20 + 1024},
+	}
+	// Record sent frames.
+	sent := newSentPacket()
+	for _, f := range frames {
+		switch f := f.(type) {
+		case byte:
+			sent.appendAckElicitingFrame(f)
+		case uint64:
+			sent.appendInt(f)
+		case i64range:
+			sent.appendOffAndSize(f.start, int(f.size()))
+		}
+	}
+	// Read the record.
+	for i, want := range frames {
+		if done := sent.done(); done {
+			t.Fatalf("before consuming contents, sent.done() = true, want false")
+		}
+		switch want := want.(type) {
+		case byte:
+			if got := sent.next(); got != want {
+				t.Fatalf("%v: sent.next() = %v, want %v", i, got, want)
+			}
+		case uint64:
+			if got := sent.nextInt(); got != want {
+				t.Fatalf("%v: sent.nextInt() = %v, want %v", i, got, want)
+			}
+		case i64range:
+			if start, end := sent.nextRange(); start != want.start || end != want.end {
+				t.Fatalf("%v: sent.nextRange() = [%v,%v), want %v", i, start, end, want)
+			}
+		}
+	}
+	if done := sent.done(); !done {
+		t.Fatalf("after consuming contents, sent.done() = false, want true")
+	}
+}