add support for receiving truncated acks

fixes #57

frames/ack_frame.go

@@ -17,6 +17,7 @@ type AckFrame struct {
 	Entropy         byte
 	DelayTime       time.Duration
 	NackRanges      []NackRange // has to be ordered. The NACK range with the highest FirstPacketNumber goes first, the NACK range with the lowest FirstPacketNumber goes last
+	Truncated       bool
 }
 
 // Write writes an ACK frame.
@@ -126,9 +127,7 @@ func ParseAckFrame(r *bytes.Reader) (*AckFrame, error) {
 	if typeByte&0x20 == 0x20 {
 		hasNACK = true
 	}
-	if typeByte&0x10 == 0x10 {
-		panic("truncated ACKs not yet implemented.")
-	}
+	frame.Truncated = typeByte&0x10 > 0
 
 	largestObservedLen := 2 * ((typeByte & 0x0C) >> 2)
 	if largestObservedLen == 0 {

frames/ack_frame_test.go

@@ -81,6 +81,13 @@ var _ = Describe("AckFrame", func() {
 			Expect(err).To(Equal(errInvalidNackRanges))
 		})
 
+		It("accepts truncated acks", func() {
+			b := bytes.NewReader([]byte{0x50, 0xA4, 0x03, 0x23, 0x45, 0x01, 0x02, 0xFF, 0xEE, 0xDD, 0xCC})
+			frame, err := ParseAckFrame(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(frame.Truncated).To(BeTrue())
+		})
+
 		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})