diff --git a/quiche/quic/core/crypto/crypto_protocol.h b/quiche/quic/core/crypto/crypto_protocol.h
index 157e2d3..9efae96 100644
--- a/quiche/quic/core/crypto/crypto_protocol.h
+++ b/quiche/quic/core/crypto/crypto_protocol.h
@@ -466,6 +466,8 @@
 const QuicTag kMCS1 = TAG('M', 'C', 'S', '1');
 const QuicTag kMCS2 = TAG('M', 'C', 'S', '2');
 const QuicTag kMCS3 = TAG('M', 'C', 'S', '3');
+const QuicTag kMCS4 = TAG('M', 'C', 'S', '4');
+const QuicTag kMCS5 = TAG('M', 'C', 'S', '5');
 
 constexpr QuicTag kBSUS = TAG('B', 'S', 'U', 'S');  // Blocks server connection
                                                     // until the SETTINGS frame