Audio/Video Transport Extensions | A.M. Williams |
Internet-Draft | Audinate |
Intended status: Standards Track | February 2011 |
Expires: August 05, 2011 |
IEEE 1588/802.1AS Synchronisation for RTP Streams
draft-williams-avtext-avbsync
Specification of an RTP header extension for carrying in-band synchronization metadata provided by the IEEE1588/802.1AS Precision Time Protocols.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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Synchronisation between RTP flows and between devices rendering RTP flows is currently facilitated by means of NTP format timestamps taken with respect to a shared reference clock. In professional AV applications, the NTP clock synchronisation protocol does not meet the time alignment and synchronisation speed requirements of many systems.
Like NTP, the IEEE1588 family of clock synchronisation protocols provide a shared reference clock in an network - typically a LAN. IEEE1588 provides sub-microsecond synchronisation between devices on a LAN and typically locks within seconds at startup rather than minutes. With support from Ethernet switches, IEEE1588 protocols can achieve nanosecond timing accuracy in LANs. Network interface chips and cards supporting hardware time-stamping of timing critical protocol messages are also available.
When using IEEE1588 clock synchronisation, networked AV systems can achieve sub 1 microsecond time alignment accuracy when rendering rendering of AV signals and can support latencies less than 1ms through a gigabit LAN.
Two main flavours of IEEE1588 are in use today:
By using an IEEE 1588 derived reference clock, synchronisation of RTP streams and devices in LANs can be considerably improved.
A global IEEE 1588/802.1AS timestamp is 80 bits in total, divided into two parts:
A shorter 32 bit timestamp is defined for use in streaming media protocols in the following way:
The shorter as_timestamp field covers just over 4 seconds of time.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|1| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+R | timestamp |T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+P | synchronisation source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | 0xBE | 0xDE | length=2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+E | ID-A | L=6 | subtype | reserved |x +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+t | as_timestamp |n +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | payload data | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are defined as follows:
IEEEE 1733 defines an RTCP header extension which also includes 802.1AS timestamp information. The IEEE 1733 RTCP extension signals infromation in addition to timestamps, including:
TBD: A URN will be required to signal the presence of this header extension, such as:
[RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
[InfRef] | ", 2004. | , "