diff --git a/plugins/sip/gst-rfc3551.c b/plugins/sip/gst-rfc3551.c
index 798cb37..4fee802 100644
--- a/plugins/sip/gst-rfc3551.c
+++ b/plugins/sip/gst-rfc3551.c
@@ -56,6 +56,12 @@ media_codec_available_in_gst (MediaCodecInfo *codec) {
   return available;
 }
 
+/* media_codec_by_name:
+ *
+ * @name: The name of the codec
+ *
+ * Returns: (transfer: none) A #MediaCodecInfo, if found
+ */
 MediaCodecInfo *
 media_codec_by_name (const char *name)
 {
@@ -69,6 +75,12 @@ media_codec_by_name (const char *name)
   return NULL;
 }
 
+/* media_codec_by_payload_id:
+ *
+ * @payload_id: The payload id (see RFC 3551, 3555, 4733, 4855)
+ *
+ * Returns: (transfer: none) A #MediaCodecInfo, if found
+ */
 MediaCodecInfo *
 media_codec_by_payload_id (guint payload_id)
 {
@@ -80,6 +92,13 @@ media_codec_by_payload_id (guint payload_id)
   return NULL;
 }
 
+/* media_codec_get_gst_capabilities:
+ *
+ * @codec: A #MediaCodecInfo
+ *
+ * Returns: (transfer: full) The capability string describing GstCaps.
+ * Used for the RTP source element.
+ */
 gchar *
 media_codec_get_gst_capabilities (MediaCodecInfo *codec)
 {
@@ -90,6 +109,10 @@ media_codec_get_gst_capabilities (MediaCodecInfo *codec)
                           codec->payload_id);
 }
 
+/* media_codecs_get_candidates:
+ *
+ * Returns: (transfer: none) A #GList of codec candidates of type #MediaCodecInfo
+ */
 GList *
 media_codecs_get_candidates ()
 {