Send extension indicating the TLS 1.3 draft version in Go.

Change-Id: I92425d7c72111623ddfbe8391f2d2fa88f101ef3
Reviewed-on: https://boringssl-review.googlesource.com/8818
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/test/runner/common.go

@@ -26,6 +26,10 @@ const (
 	VersionTLS13 = 0x0304
 )
 
+// The draft version of TLS 1.3 that is implemented here and sent in the draft
+// indicator extension.
+const tls13DraftVersion = 13
+
 const (
 	maxPlaintext        = 16384        // maximum plaintext payload length
 	maxCiphertext       = 16384 + 2048 // maximum ciphertext payload length
@@ -92,6 +96,7 @@ const (
 	extensionCustom                     uint16 = 1234  // not IANA assigned
 	extensionNextProtoNeg               uint16 = 13172 // not IANA assigned
 	extensionRenegotiationInfo          uint16 = 0xff01
+	extensionTLS13Draft                 uint16 = 0xff02
 	extensionChannelID                  uint16 = 30032 // not IANA assigned
 )
 

ssl/test/runner/handshake_messages.go

@@ -379,6 +379,11 @@ func (m *clientHelloMsg) marshal() []byte {
 		customExt := extensions.addU16LengthPrefixed()
 		customExt.addBytes([]byte(m.customExtension))
 	}
+	if m.vers == VersionTLS13 {
+		extensions.addU16(extensionTLS13Draft)
+		extValue := extensions.addU16LengthPrefixed()
+		extValue.addU16(tls13DraftVersion)
+	}
 
 	if extensions.len() == 0 {
 		hello.discardChild()