diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index aa49919..036164f 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -130,7 +130,9 @@
 	return sgsn->cfg.timers.T3314 + (sgsn->cfg.timers.T3312 + 4 * 60);
 }
 
-/* Send a message through the underlying layer */
+/* Send a message through the underlying layer.
+ * For param encryptable, see 3GPP TS 24.008 § 4.7.1.2 and
+ * gsm48_hdr_gmm_cipherable(). Pass false for not cipherable messages. */
 static int gsm48_gmm_sendmsg(struct msgb *msg, int command,
 			     struct sgsn_mm_ctx *mm, bool encryptable)
 {