diff --git a/src/mux_h2.c b/src/mux_h2.c
index ea4f90f7a..370494d5b 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -361,6 +361,18 @@ static inline int h2s_id(const struct h2s *h2s)
 	return h2s ? h2s->id : 0;
 }
 
+/* returns true of the mux is currently busy as seen from stream <h2s> */
+static inline int h2c_mux_busy(const struct h2c *h2c, const struct h2s *h2s)
+{
+	if (h2c->msi < 0)
+		return 0;
+
+	if (h2c->msi == h2s_id(h2s))
+		return 0;
+
+	return 1;
+}
+
 
 /*********************************************************/
 /* functions below are I/O callbacks from the connection */