diff --git a/src/mux_h2.c b/src/mux_h2.c
index 6268465d8..7cdf31c32 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -14,6 +14,7 @@
 #include <common/config.h>
 #include <common/h2.h>
 #include <common/hpack-tbl.h>
+#include <common/net_helper.h>
 #include <proto/applet.h>
 #include <proto/connection.h>
 #include <proto/stream.h>
@@ -391,6 +392,15 @@ static inline void h2s_error(struct h2s *h2s, enum h2_err err)
 	}
 }
 
+/* writes the 24-bit frame size <len> at address <frame> */
+static inline void h2_set_frame_size(void *frame, uint32_t len)
+{
+	uint8_t *out = frame;
+
+	*out = len >> 16;
+	write_n16(out + 1, len);
+}
+
 
 /*********************************************************/
 /* functions below are I/O callbacks from the connection */