diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index b5560db..0eface3 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -1,4 +1,5 @@
 /* GPRS SGSN integration with libgtp of OpenGGSN */
+/* libgtp implements the GPRS Tunelling Protocol GTP per TS 09.60 / 29.060 */
 
 /* (C) 2010 by Harald Welte <laforge@gnumonks.org>
  * (C) 2010 by On Waves