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

Abstract

Specification of an RTP header extension for carrying in-band synchronization metadata provided by the IEEE1588/802.1AS Precision Time Protocols.

Requirements Language

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].

Status of this Memo

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This Internet-Draft will expire on August 05, 2011.

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Table of Contents

1. Introduction

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.

2. Timestamp formats

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.

3. Header Extension

      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:

4. IEEE 1733 / RTCP

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:

subtype
an RTCP subtype
gmIdentity
an 80 bit field uniquely identifying the current 802.1AS grand master clock for the network
stream_id
a 64 bit number identifying the 802.1Qav stream associated with this RTP flow
as_timestamp
the 32 bit 802.1AS timestamp [timestamp_formats] associated with the RTP timestamp carried in this packet
rtp_timestamp
the RTP timestamp of a media packet

5. IANA Considerations

TBD: A URN will be required to signal the presence of this header extension, such as:

6. Acknowledgements

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2. Informative References

, "
[InfRef] ", 2004.

Appendix A. An Appendix

Author's Address

Aidan Williams Audinate Level 1, 458 Wattle St Ultimo, NSW 2007 Australia Phone: +61 2 8090 1000 EMail: aidan.williams@audinate.com