Add details about audio packet transmission

1 files changed, 21 insertions, 5 deletions

diff --git a/airtunes2.rst b/airtunes2.rst
index 599e0ae..29779ca 100644
--- a/airtunes2.rst
+++ b/airtunes2.rst
@@ -61,6 +61,27 @@ Connect
 #. Prepare RTP connection for audio packets
 
 
+Transmitting Audio
+~~~~~~~~~~~~~~~~~~
+
+The audio data is encapsulated in an RTP packet (see RFC3550 Section 5.1)
+
+#. Bytes 0-1 of the RTP Header are 0x80, 0x60
+#. Bytes 2-3 store the current sequence number (whose initial value was set
+   in the RECORD RTSP request)
+#. Bytes 4-7 store the current RTP timestamp (initial value was transmitted
+   to the Airtunes device in the RECORD RTSP request)
+#. Bytes 8-11 store the SSRC (random number which will be the same in all
+   audio packets, see RFC3550 Section 5.1 for more details) (TODO: is the
+   SSRC the same as the RTSP client session ID ?)
+#. Starting at byte 12 comes the audio data (see `Packetizing audio`_ for
+   its format)
+#. Send this RTP packet as UDP on the audio data port
+#. Increase sequence number by one for next audio packet
+#. Increase timestamp by number of frames in this audio packet
+
+
+
 Disconnect
 ~~~~~~~~~~
 
@@ -520,11 +541,6 @@ Packetizing audio
         the rest stays unencrypted)
      #. Copy audio header to buffer
      #. Append encrypted audio data to buffer
-     
-#. Set bytes 2-4 to sequence number in big endian
-#. Set bytes 4-8 to timestamp in big endian
-#. Increase sequence number by one for next packet
-#. Increase timestamp by number of frames in this packet
 
 
 Metadata