don't read timestamps in truncated acks

fixes #59

frames/ack_frame.go

@@ -156,34 +156,37 @@ func ParseAckFrame(r *bytes.Reader) (*AckFrame, error) {
 	}
 	frame.DelayTime = time.Duration(delay) * time.Microsecond
 
-	numTimestampByte, err := r.ReadByte()
-	if err != nil {
-		return nil, err
-	}
-	numTimestamp := uint8(numTimestampByte)
+	if !frame.Truncated {
+		var err error
+		numTimestampByte, err := r.ReadByte()
+		if err != nil {
+			return nil, err
+		}
+		numTimestamp := uint8(numTimestampByte)
 
-	// Delta Largest observed
-	_, err = r.ReadByte()
-	if err != nil {
-		return nil, err
-	}
-	// First Timestamp
-	_, err = utils.ReadUint32(r)
-	if err != nil {
-		return nil, err
-	}
-
-	for i := 0; i < int(numTimestamp)-1; i++ {
 		// Delta Largest observed
 		_, err = r.ReadByte()
 		if err != nil {
 			return nil, err
 		}
-		// Time Since Previous Timestamp
-		_, err = utils.ReadUint16(r)
+		// First Timestamp
+		_, err = utils.ReadUint32(r)
 		if err != nil {
 			return nil, err
 		}
+
+		for i := 0; i < int(numTimestamp)-1; i++ {
+			// Delta Largest observed
+			_, err = r.ReadByte()
+			if err != nil {
+				return nil, err
+			}
+			// Time Since Previous Timestamp
+			_, err = utils.ReadUint16(r)
+			if err != nil {
+				return nil, err
+			}
+		}
 	}
 
 	// Invalid NACK Handling:

frames/ack_frame_test.go

@@ -82,10 +82,11 @@ var _ = Describe("AckFrame", func() {
 		})
 
 		It("accepts truncated acks", func() {
-			b := bytes.NewReader([]byte{0x50, 0xA4, 0x03, 0x23, 0x45, 0x01, 0x02, 0xFF, 0xEE, 0xDD, 0xCC})
+			b := bytes.NewReader([]byte{0x50, 0xA4, 0x03, 0x23, 0x45})
 			frame, err := ParseAckFrame(b)
 			Expect(err).ToNot(HaveOccurred())
 			Expect(frame.Truncated).To(BeTrue())
+			Expect(b.Len()).To(BeZero())
 		})
 
 		Context("contiguous NACK ranges", func() {