implement a function to parse the version number of a Long Header packet

internal/wire/header.go

@@ -40,6 +40,15 @@ func IsLongHeaderPacket(firstByte byte) bool {
 	return firstByte&0x80 > 0
 }
 
+// ParseVersion parses the QUIC version.
+// It should only be called for Long Header packets (Short Header packets don't contain a version number).
+func ParseVersion(data []byte) (protocol.VersionNumber, error) {
+	if len(data) < 5 {
+		return 0, io.EOF
+	}
+	return protocol.VersionNumber(binary.BigEndian.Uint32(data[1:5])), nil
+}
+
 // IsVersionNegotiationPacket says if this is a version negotiation packet
 func IsVersionNegotiationPacket(b []byte) bool {
 	if len(b) < 5 {

internal/wire/header_test.go

@@ -111,6 +111,24 @@ var _ = Describe("Header Parsing", func() {
 			Expect(Is0RTTPacket(append(zeroRTTHeader, []byte("foobar")...))).To(BeTrue())
 		})
 	})
+	Context("parsing the version", func() {
+		It("parses the version", func() {
+			b := []byte{0x80, 0xde, 0xad, 0xbe, 0xef}
+			v, err := ParseVersion(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(v).To(Equal(protocol.VersionNumber(0xdeadbeef)))
+		})
+
+		It("errors with EOF", func() {
+			b := []byte{0x80, 0xde, 0xad, 0xbe, 0xef}
+			_, err := ParseVersion(b)
+			Expect(err).ToNot(HaveOccurred())
+			for i := range b {
+				_, err := ParseVersion(b[:i])
+				Expect(err).To(MatchError(io.EOF))
+			}
+		})
+	})
 
 	Context("Identifying Version Negotiation Packets", func() {
 		It("identifies version negotiation packets", func() {