diff --git a/quic/core/crypto/crypto_protocol.h b/quic/core/crypto/crypto_protocol.h
index 6873e0f..a3abb33 100644
--- a/quic/core/crypto/crypto_protocol.h
+++ b/quic/core/crypto/crypto_protocol.h
@@ -192,6 +192,7 @@
 const QuicTag kBWS2 = TAG('B', 'W', 'S', '2');  // Server bw resumption v2.
 const QuicTag kBWS3 = TAG('B', 'W', 'S', '3');  // QUIC Initial CWND - Control.
 const QuicTag kBWS4 = TAG('B', 'W', 'S', '4');  // QUIC Initial CWND - Enabled.
+const QuicTag kBWS5 = TAG('B', 'W', 'S', '5');  // QUIC Initial CWND up and down
 
 // Enable path MTU discovery experiment.
 const QuicTag kMTUH = TAG('M', 'T', 'U', 'H');  // High-target MTU discovery.