Add comment describing the RDT packet header. See discussion in "[PATCH]

RDT/Realmedia patches #2" thread on the mailinglist.

Originally committed as revision 15823 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rdt.c

@@ -184,6 +184,57 @@ ff_rdt_parse_header(const uint8_t *buf, int len,
     }
     if (len < 10)
         return -1;
+    /**
+     * Layout of the header (in bits):
+     * 1:  len_included
+     *     Flag indicating whether this header includes a length field;
+     *     this can be used to concatenate multiple RDT packets in a
+     *     single UDP/TCP data frame and is used to precede RDT data
+     *     by stream status packets
+     * 1:  need_reliable
+     *     Flag indicating whether this header includes a "reliable
+     *     sequence number"; these are apparently sequence numbers of
+     *     data packets alone. For data packets, this flag is always
+     *     set, according to the Real documentation [1]
+     * 5:  set_id
+     *     ID of a set of streams of identical content, possibly with
+     *     different codecs or bitrates
+     * 1:  is_reliable
+     *     Flag set for certain streams deemed less tolerable for packet
+     *     loss
+     * 16: seq_no
+     *     Packet sequence number; if >=0xFF00, this is a non-data packet
+     *     containing stream status info, the second byte indicates the
+     *     type of status packet (see wireshark docs / source code [2])
+     * if (len_included) {
+     *     16: packet_len
+     * } else {
+     *     packet_len = remainder of UDP/TCP frame
+     * }
+     * 1:  is_back_to_back
+     *     Back-to-Back flag; used for timing, set for one in every 10
+     *     packets, according to the Real documentation [1]
+     * 1:  is_slow_data
+     *     Slow-data flag; currently unused, according to Real docs [1]
+     * 5:  stream_id
+     *     ID of the stream within this particular set of streams
+     * 1:  is_no_keyframe
+     *     Non-keyframe flag (unset if packet belongs to a keyframe)
+     * 32: timestamp (PTS)
+     * if (set_id == 0x1F) {
+     *     16: set_id (extended set-of-streams ID; see set_id)
+     * }
+     * if (need_reliable) {
+     *     16: reliable_seq_no
+     *         Reliable sequence number (see need_reliable)
+     * }
+     * if (stream_id == 0x3F) {
+     *     16: stream_id (extended stream ID; see stream_id)
+     * }
+     * [1] https://protocol.helixcommunity.org/files/2005/devdocs/RDT_Feature_Level_20.txt
+     * [2] http://www.wireshark.org/docs/dfref/r/rdt.html and
+     *     http://anonsvn.wireshark.org/viewvc/trunk/epan/dissectors/packet-rdt.c
+     */
     if (sn)  *sn  = (buf[0]>>1) & 0x1f;
     if (seq) *seq = AV_RB16(buf+1);
     if (ts)  *ts  = AV_RB32(buf+4);