support largest observed packet number missing in ACK frames

fixes #63

frames/ack_frame.go

@@ -239,12 +239,8 @@ func ParseAckFrame(r *bytes.Reader, version protocol.VersionNumber) (*AckFrame,
 			}
 			rangeLength := uint8(rangeLengthByte)
 
-			if i == 0 && missingPacketSequenceNumberDelta == 0 {
-				return nil, errors.New("ACK frame: largest observed missing not yet implemented")
-			}
-
 			// contiguous NACK range
-			if missingPacketSequenceNumberDelta == 0 {
+			if i > 0 && missingPacketSequenceNumberDelta == 0 {
 				nackRange := &frame.NackRanges[len(frame.NackRanges)-1]
 				if uint64(nackRange.FirstPacketNumber) <= uint64(rangeLength)+1 {
 					return nil, errInvalidNackRanges
@@ -291,7 +287,7 @@ func (f *AckFrame) validateNackRanges() bool {
 		if nackRange.FirstPacketNumber > nackRange.LastPacketNumber {
 			return false
 		}
-		if nackRange.LastPacketNumber >= f.LargestObserved {
+		if nackRange.LastPacketNumber > f.LargestObserved {
 			return false
 		}
 	}

frames/ack_frame_test.go

@@ -100,6 +100,16 @@ var _ = Describe("AckFrame", func() {
 			Expect(b.Len()).To(BeZero())
 		})
 
+		It("parses a frame with the largest observed missing", func() {
+			b := bytes.NewReader([]byte{0x60, 0x2, 0xf, 0xb8, 0x1, 0x1, 0x0, 0xe5, 0x58, 0x4, 0x0, 0x1, 0x0, 0x0})
+			frame, err := ParseAckFrame(b, 32)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(frame.HasNACK()).To(Equal(true))
+			Expect(frame.NackRanges).To(HaveLen(1))
+			Expect(frame.NackRanges[0]).To(Equal(NackRange{FirstPacketNumber: 15, LastPacketNumber: 15}))
+			Expect(b.Len()).To(Equal(0))
+		})
+
 		Context("contiguous NACK ranges", func() {
 			It("parses a frame with a contiguous NACK range spanning two fields", func() {
 				b := bytes.NewReader([]byte{0x64, 0x8, 0x2E, 0x01, 0x72, 0x1, 0x1, 0x0, 0xc0, 0x15, 0x0, 0x0, 0x2, 0x1, 0x2b, 0x0, 0xff})
@@ -186,6 +196,15 @@ var _ = Describe("AckFrame", func() {
 			Expect(ack.validateNackRanges()).To(BeFalse())
 		})
 
+		It("does not reject NACKs with LasterPacketNumber equal to LargestObserved", func() {
+			nackRange := NackRange{FirstPacketNumber: 7, LastPacketNumber: 7}
+			ack := AckFrame{
+				LargestObserved: 7,
+				NackRanges:      []NackRange{nackRange},
+			}
+			Expect(ack.validateNackRanges()).To(BeTrue())
+		})
+
 		It("rejects NACKs with NackRanges in the wrong order", func() {
 			nackRanges := []NackRange{
 				{FirstPacketNumber: 2, LastPacketNumber: 2},