diff --git a/src/connection.c b/src/connection.c
index 8826706..5d7f788 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -411,6 +411,17 @@
 }
 
 /*
+ * default cs send() : this one is used when mux->snd_buf == NULL. It puts up to
+ * <count> bytes from <buf> into cs->txbuf. The number of bytes transferred is
+ * returned. Here we don't care if cs->txbuf is allocated or not. If not, it
+ * will be swapped with <buf>.
+ */
+size_t __cs_send(struct conn_stream *cs, struct buffer *buf, size_t count, int flags)
+{
+	return b_xfer(&cs->txbuf, buf, count);
+}
+
+/*
  * Get data length from tlv
  */
 static int get_tlv_length(const struct tlv *src)