diff --git a/src/shrpx_http.h b/src/shrpx_http.h
index e4c20958..fbff3380 100644
--- a/src/shrpx_http.h
+++ b/src/shrpx_http.h
@@ -39,6 +39,9 @@ std::string create_error_html(unsigned int status_code);
 
 std::string create_via_header_value(int major, int minor);
 
+// Returns generated RFC 7239 Forwarded header field value.  The
+// |params| is bitwise-OR of zero or more of shrpx_forwarded_param
+// defined in shrpx_config.h.
 std::string create_forwarded(int params, const std::string &node_by,
                              const std::string &node_for,
                              const std::string &host, const std::string &proto);