diff --git a/src/mux_h2.c b/src/mux_h2.c
index d9f0b56..855bd50 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -952,6 +952,91 @@
 	return ret;
 }
 
+/* processes a WINDOW_UPDATE frame whose payload is <payload> for <plen> bytes.
+ * Returns > 0 on success or zero on missing data. It may return an error in
+ * h2c or h2s. Described in RFC7540#6.9.
+ */
+static int h2c_handle_window_update(struct h2c *h2c, struct h2s *h2s)
+{
+	int32_t inc;
+	int error;
+
+	if (h2c->dfl != 4) {
+		error = H2_ERR_FRAME_SIZE_ERROR;
+		goto conn_err;
+	}
+
+	/* process full frame only */
+	if (h2c->dbuf->i < h2c->dfl)
+		return 0;
+
+	inc = h2_get_n32(h2c->dbuf, 0);
+
+	if (h2c->dsi != 0) {
+		/* stream window update */
+		if (h2s->st == H2_SS_IDLE) {
+			error = H2_ERR_PROTOCOL_ERROR;
+			goto conn_err;
+		}
+
+		/* it's not an error to receive WU on a closed stream */
+		if (h2s->st == H2_SS_CLOSED)
+			return 1;
+
+		if (!inc) {
+			error = H2_ERR_PROTOCOL_ERROR;
+			goto strm_err;
+		}
+
+		if (h2s->mws >= 0 && h2s->mws + inc < 0) {
+			error = H2_ERR_FLOW_CONTROL_ERROR;
+			goto strm_err;
+		}
+
+		h2s->mws += inc;
+		if (h2s->mws > 0 && (h2s->flags & H2_SF_BLK_SFCTL)) {
+			h2s->flags &= ~H2_SF_BLK_SFCTL;
+			if (h2s->cs && LIST_ISEMPTY(&h2s->list) &&
+			    (h2s->cs->flags & CS_FL_DATA_WR_ENA)) {
+				/* This stream wanted to send but could not due to its
+				 * own flow control. We can put it back into the send
+				 * list now, it will be handled upon next send() call.
+				 */
+				LIST_ADDQ(&h2c->send_list, &h2s->list);
+			}
+		}
+	}
+	else {
+		/* connection window update */
+		if (!inc) {
+			error = H2_ERR_PROTOCOL_ERROR;
+			goto conn_err;
+		}
+
+		if (h2c->mws >= 0 && h2c->mws + inc < 0) {
+			error = H2_ERR_FLOW_CONTROL_ERROR;
+			goto conn_err;
+		}
+
+		h2c->mws += inc;
+	}
+
+	return 1;
+
+ conn_err:
+	h2c_error(h2c, error);
+	return 0;
+
+ strm_err:
+	if (h2s) {
+		h2s_error(h2s, error);
+		h2c->st0 = H2_CS_FRAME_A;
+	}
+	else
+		h2c_error(h2c, error);
+	return 0;
+}
+
 /* process Rx frames to be demultiplexed */
 static void h2_process_demux(struct h2c *h2c)
 {
@@ -1056,6 +1141,11 @@
 				ret = h2c_ack_ping(h2c);
 			break;
 
+		case H2_FT_WINDOW_UPDATE:
+			if (h2c->st0 == H2_CS_FRAME_P)
+				ret = h2c_handle_window_update(h2c, h2s);
+			break;
+
 			/* FIXME: implement all supported frame types here */
 		default:
 			/* drop frames that we ignore. They may be larger than