diff --git a/ggsn/pco.c b/ggsn/pco.c
index e2181e1..c71f07d 100644
--- a/ggsn/pco.c
+++ b/ggsn/pco.c
@@ -110,6 +110,7 @@ ret_broken:
 		osmo_hexdump_nospc((const uint8_t *)pco_in, pco_in->length));
 }
 
+/* Handle IP Control Protocol, RFC 1332, extensions in RFC 1877 */
 static void process_pco_element_ipcp(const struct pco_element *pco_elem, struct msgb *resp,
 				     const struct apn_ctx *apn, struct pdp_t *pdp)
 {
diff --git a/ggsn/pco.h b/ggsn/pco.h
index 4d999ef..2f95d09 100644
--- a/ggsn/pco.h
+++ b/ggsn/pco.h
@@ -42,12 +42,11 @@ struct pco_element {
 	uint8_t data[0];
 } __attribute__((packed));
 
-
-/* RFC 1332 */
+/* RFC 1332 IP Control Protocol options, extensions in RFC 1877 */
 enum ipcp_options {
-	IPCP_OPT_IPADDR = 3,
-	IPCP_OPT_PRIMARY_DNS = 129,
-	IPCP_OPT_SECONDARY_DNS = 131,
+	IPCP_OPT_IPADDR = 3, /* RFC 1332 3.3 */
+	IPCP_OPT_PRIMARY_DNS = 129, /* RFC 1877 1.1 */
+	IPCP_OPT_SECONDARY_DNS = 131,  /* RFC 1877 1.2 */
 };
 
 struct ipcp_option_hdr {