only add an ECN control message if ECN is supported

internal/protocol/protocol.go

@@ -37,14 +37,48 @@ func (t PacketType) String() string {
 type ECN uint8
 
 const (
-	ECNNon ECN = iota // 00
-	ECT1              // 01
-	ECT0              // 10
-	ECNCE             // 11
+	ECNUnsupported ECN = iota
+	ECNNon             // 00
+	ECT1               // 01
+	ECT0               // 10
+	ECNCE              // 11
 )
 
+func ParseECNHeaderBits(bits byte) ECN {
+	switch bits {
+	case 0:
+		return ECNNon
+	case 0b00000010:
+		return ECT0
+	case 0b00000001:
+		return ECT1
+	case 0b00000011:
+		return ECNCE
+	default:
+		panic("invalid ECN bits")
+	}
+}
+
+func (e ECN) ToHeaderBits() byte {
+	//nolint:exhaustive // There are only 4 values.
+	switch e {
+	case ECNNon:
+		return 0
+	case ECT0:
+		return 0b00000010
+	case ECT1:
+		return 0b00000001
+	case ECNCE:
+		return 0b00000011
+	default:
+		panic("ECN unsupported")
+	}
+}
+
 func (e ECN) String() string {
 	switch e {
+	case ECNUnsupported:
+		return "ECN unsupported"
 	case ECNNon:
 		return "Not-ECT"
 	case ECT1:

internal/protocol/protocol_test.go

@@ -17,13 +17,22 @@ var _ = Describe("Protocol", func() {
 	})
 
 	It("converts ECN bits from the IP header wire to the correct types", func() {
-		Expect(ECN(0)).To(Equal(ECNNon))
-		Expect(ECN(0b00000010)).To(Equal(ECT0))
-		Expect(ECN(0b00000001)).To(Equal(ECT1))
-		Expect(ECN(0b00000011)).To(Equal(ECNCE))
+		Expect(ParseECNHeaderBits(0)).To(Equal(ECNNon))
+		Expect(ParseECNHeaderBits(0b00000010)).To(Equal(ECT0))
+		Expect(ParseECNHeaderBits(0b00000001)).To(Equal(ECT1))
+		Expect(ParseECNHeaderBits(0b00000011)).To(Equal(ECNCE))
+		Expect(func() { ParseECNHeaderBits(0b1010101) }).To(Panic())
+	})
+
+	It("converts to IP header bits", func() {
+		for _, v := range [...]ECN{ECNNon, ECT0, ECT1, ECNCE} {
+			Expect(ParseECNHeaderBits(v.ToHeaderBits())).To(Equal(v))
+		}
+		Expect(func() { ECN(42).ToHeaderBits() }).To(Panic())
 	})
 
 	It("has a string representation for ECN", func() {
+		Expect(ECNUnsupported.String()).To(Equal("ECN unsupported"))
 		Expect(ECNNon.String()).To(Equal("Not-ECT"))
 		Expect(ECT0.String()).To(Equal("ECT(0)"))
 		Expect(ECT1.String()).To(Equal("ECT(1)"))