diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 3794dad..f5af5c7 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -317,6 +317,11 @@
 
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
     defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
+/*
+ * The TLS 1.3 supported groups extension was defined to be a compatible
+ * generalization of the TLS 1.2 supported elliptic curves extension. They both
+ * share the same extension identifier.
+ */
 static int ssl_parse_supported_groups_ext( mbedtls_ssl_context *ssl,
                                            const unsigned char *buf,
                                            size_t len )