diff --git a/internal/utils/varint_packetnumber.go b/internal/utils/varint_packetnumber.go
new file mode 100644
index 00000000..b05afd4b
--- /dev/null
+++ b/internal/utils/varint_packetnumber.go
@@ -0,0 +1,50 @@
+package utils
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// ReadVarIntPacketNumber reads a number in the QUIC varint packet number format
+func ReadVarIntPacketNumber(b *bytes.Reader) (protocol.PacketNumber, protocol.PacketNumberLen, error) {
+	b1, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	if b1&0x80 == 0 {
+		return protocol.PacketNumber(b1), protocol.PacketNumberLen1, nil
+	}
+	b2, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	if b1&0x40 == 0 {
+		return protocol.PacketNumber(uint64(b1&0x3f)<<8 + uint64(b2)), protocol.PacketNumberLen2, nil
+	}
+	b3, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	b4, err := b.ReadByte()
+	if err != nil {
+		return 0, 0, err
+	}
+	return protocol.PacketNumber(uint64(b1&0x3f)<<24 + uint64(b2)<<16 + uint64(b3)<<8 + uint64(b4)), protocol.PacketNumberLen4, nil
+}
+
+// WriteVarIntPacketNumber writes a packet number in the QUIC varint packet number format
+func WriteVarIntPacketNumber(b *bytes.Buffer, i protocol.PacketNumber, len protocol.PacketNumberLen) error {
+	switch len {
+	case protocol.PacketNumberLen1:
+		b.WriteByte(uint8(i & 0x7f))
+	case protocol.PacketNumberLen2:
+		b.Write([]byte{(uint8(i>>8) & 0x3f) | 0x80, uint8(i)})
+	case protocol.PacketNumberLen4:
+		b.Write([]byte{(uint8(i>>24) & 0x3f) | 0xc0, uint8(i >> 16), uint8(i >> 8), uint8(i)})
+	default:
+		return fmt.Errorf("invalid packet number length: %d", len)
+	}
+	return nil
+}
diff --git a/internal/utils/varint_packetnumber_test.go b/internal/utils/varint_packetnumber_test.go
new file mode 100644
index 00000000..d743b1ae
--- /dev/null
+++ b/internal/utils/varint_packetnumber_test.go
@@ -0,0 +1,157 @@
+package utils
+
+import (
+	"bytes"
+	"io"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Varint packet number encoding / decoding", func() {
+	Context("Decoding", func() {
+		It("reads a 1 byte number", func() {
+			b := bytes.NewReader([]byte{0x19}) // 00011001
+			p, len, err := ReadVarIntPacketNumber(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen1))
+			Expect(p).To(Equal(protocol.PacketNumber(0x19)))
+		})
+
+		It("errors when given an empty reader", func() {
+			_, _, err := ReadVarIntPacketNumber(bytes.NewReader(nil))
+			Expect(err).To(MatchError(io.EOF))
+		})
+
+		It("reads a 2 byte number", func() {
+			b := bytes.NewReader([]byte{0xb7, 0x19}) // first byte: 10110111
+			p, len, err := ReadVarIntPacketNumber(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen2))
+			Expect(p).To(Equal(protocol.PacketNumber(0x3719)))
+		})
+
+		It("errors on EOF when reading a 2 byte number", func() {
+			b := bytes.NewReader([]byte{0xb7}) // first byte: 10110111
+			_, _, err := ReadVarIntPacketNumber(b)
+			Expect(err).To(MatchError(io.EOF))
+		})
+
+		It("reads a 4 byte number", func() {
+			b := bytes.NewReader([]byte{0xe5, 0x89, 0xfa, 0x19}) // first byte: 11100101
+			p, len, err := ReadVarIntPacketNumber(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen4))
+			Expect(p).To(Equal(protocol.PacketNumber(0x2589fa19)))
+		})
+
+		It("errors on EOF after the 3rd byte when reading a 4 byte number", func() {
+			b := bytes.NewReader([]byte{0xe5, 0x89}) // first byte: 11100101
+			_, _, err := ReadVarIntPacketNumber(b)
+			Expect(err).To(MatchError(io.EOF))
+		})
+
+		It("errors on EOF after the 4th byte when reading a 4 byte number", func() {
+			b := bytes.NewReader([]byte{0xe5, 0x89, 0xfa}) // first byte: 11100101
+			_, _, err := ReadVarIntPacketNumber(b)
+			Expect(err).To(MatchError(io.EOF))
+		})
+	})
+
+	Context("Encoding", func() {
+		It("writes a 1 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x42, protocol.PacketNumberLen1)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(1))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen1))
+			Expect(p).To(Equal(protocol.PacketNumber(0x42)))
+		})
+
+		It("only uses the least significant 7 bits when writing a 1 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x1234ea, protocol.PacketNumberLen1)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(1))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen1))
+			Expect(p).To(Equal(protocol.PacketNumber(0x6a)))
+		})
+
+		It("writes a small 2 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x42, protocol.PacketNumberLen2)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(2))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen2))
+			Expect(p).To(Equal(protocol.PacketNumber(0x42)))
+		})
+
+		It("writes a 2 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x1337, protocol.PacketNumberLen2)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(2))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen2))
+			Expect(p).To(Equal(protocol.PacketNumber(0x1337)))
+		})
+
+		It("only uses the least significant 14 bits when writing a 2 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x1234ff37, protocol.PacketNumberLen2)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(2))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen2))
+			Expect(p).To(Equal(protocol.PacketNumber(0x3f37)))
+		})
+
+		It("writes a small 4 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0xbeef, protocol.PacketNumberLen4)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(4))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen4))
+			Expect(p).To(Equal(protocol.PacketNumber(0xbeef)))
+		})
+
+		It("writes a 4 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x12beef42, protocol.PacketNumberLen4)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(4))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen4))
+			Expect(p).To(Equal(protocol.PacketNumber(0x12beef42)))
+		})
+
+		It("only uses the least significant 30 bits when writing a 4 byte packet number", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x1234deadbeef, protocol.PacketNumberLen4)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(b.Len()).To(Equal(4))
+			p, len, err := ReadVarIntPacketNumber(bytes.NewReader(b.Bytes()))
+			Expect(err).ToNot(HaveOccurred())
+			Expect(len).To(Equal(protocol.PacketNumberLen4))
+			Expect(p).To(Equal(protocol.PacketNumber(0x1eadbeef)))
+		})
+
+		It("errors when encountering invalid packet number lengths", func() {
+			b := &bytes.Buffer{}
+			err := WriteVarIntPacketNumber(b, 0x1234deadbeef, 13)
+			Expect(err).To(MatchError("invalid packet number length: 13"))
+		})
+	})
+})