RTP Streams
Audio packets are fully RTP compliant. Control and timing packets, however, do not seem to be fully compliant with the RTP standard.
The following payload types are defined:
| payload type | port | description |
|---|---|---|
| 82 | timing_port | timing request |
| 83 | timing_port | timing reply |
| 84 | control_port | time sync |
| 85 | control_port | retransmit request |
| 86 | control_port | retransmit reply |
| 96 | server_port | audio data |
Audio packets
Audio data is sent using the DynamicRTP-Type-96 payload type. The Marker
bit is set on the first packet sent after RECORD or FLUSH requests. The RTP
payload contains optionally encrypted audio data.
Example: encrypted audio packet
client → server
0000 80 e0 b1 91 f7 79 16 c2 e8 bb 6b 2c bb 5c 8e 51
0010 aa 7c d2 96 00 c3 fd 60 eb ae 6e 41 31 38 fe ae
....
03e0 cb 1c 73 bf e7 05 93 30 fa 85 7f 32 77 8d a8 97
03f0 a0 c7 c8 78 7b e5 81 a1 4f b4 3e a3 43 db 7c
Real-Time Transport Protocol
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
1... .... = Marker: True
Payload type: DynamicRTP-Type-96 (96)
Sequence number: 45457
Timestamp: 4151908034
Synchronization Source identifier: 0xe8bb6b2c (3904596780)
Payload: bb5c8e51aa7cd29600c3fd60ebae6e413138feae909b44f1...
Sync packets
Sync packets are sent once per second to the control port. They are used to
correlate the RTP timestamps currently used in the audio stream to the NTP time
used for clock synchronization. Payload type is 84, the Marker bit is always
set and the Extension bit is set on the first packet after RECORD or
FLUSH requests. The SSRC field is not included in the RTP header.
| bytes | description |
|---|---|
| 8 | RTP header without SSRC |
| 8 | current NTP time |
| 4 | RTP timestamp for the next audio packet |
Example: sync packet
client → server
0000 80 d4 00 04 c7 cd 11 a8 83 ab 1c 49 2f e4 22 e2
0010 c7 ce 3f 1f
Real-Time Transport Protocol
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
1... .... = Marker: True
Payload type: Unassigned (84)
Sequence number: 4
Timestamp: 3352105384
Synchronization Source identifier: 0x83ab1c49 (2209029193)
Payload: 2fe422e2c7ce3f1f
Retransmit packets
AirTunes supports resending audio packets which have been lost. Payload
type is 85 for retransmit queries, the Marker bit is always set and the
SSRC field is not included in the RTP header.
| bytes | description |
|---|---|
| 8 | RTP header without SSRC |
| 2 | sequence number for the first lost packet |
| 2 | number of lost packets |
Retransmit replies have payload type 86, with a full audio RTP packet after the sequence number.
Timing packets
Timing packets are used to synchronize a master clock for audio. This is useful for clock recovery and precise synchronization of several devices playing the same audio stream.
Timing packets are sent at 3 second intervals. They always have the
Marker bit set, and payload type 82 for queries and 83 for replies.
The SSRC field is not included in the RTP header, so it takes only 8
bytes, followed by three NTP timestamps:
| bytes | description |
|---|---|
| 8 | RTP header without SSRC |
| 8 | origin timestamp |
| 8 | receive timestamp |
| 8 | transmit timestamp |
Example: timing query/reply
server → client
0000 80 d2 00 07 00 00 00 00 00 00 00 00 00 00 00 00
0010 00 00 00 00 00 00 00 00 83 c1 17 cc af ba 9b 32
Real-Time Transport Protocol
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
1... .... = Marker: True
Payload type: Unassigned (82)
Sequence number: 7
Timestamp: 0
Synchronization Source identifier: 0x00000000 (0)
Payload: 00000000000000000000000083c117ccafba9b32
client → server
0000 80 d3 00 07 00 00 00 00 83 c1 17 cc af ba 9b 32
0010 83 c1 17 cc b0 12 ce b6 83 c1 17 cc b0 14 10 47
Real-Time Transport Protocol
10.. .... = Version: RFC 1889 Version (2)
..0. .... = Padding: False
...0 .... = Extension: False
.... 0000 = Contributing source identifiers count: 0
1... .... = Marker: True
Payload type: Unassigned (83)
Sequence number: 7
Timestamp: 0
Synchronization Source identifier: 0x83c117cc (2210469836)
Payload: afba9b3283c117ccb012ceb683c117ccb0141047