One document matched: draft-williams-avtext-avbsync-00.txt
Audio/Video Transport Extensions A. Williams
Internet-Draft Audinate
Intended status: Standards Track February 2011
Expires: August 5, 2011
IEEE 1588/802.1AS Synchronisation for RTP Streams
draft-williams-avtext-avbsync-00
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 5, 2011.
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Internet-Draft AVB/RTP Synchronisation February 2011
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Timestamp formats . . . . . . . . . . . . . . . . . . . . . . . 3
3. Header Extension . . . . . . . . . . . . . . . . . . . . . . . 4
4. IEEE 1733 / RTCP . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. An Appendix . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
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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:
o IEEE 1588-2008: the second version of the "Standard for a
Precision Clock Synchronization Protocol for Networked Measurement
and Control Systems". This is a revised version of the original
IEEE1588-2002 standard and is often called IEEE1588v2 or PTPv2.
o IEEE 802.1AS: "Timing and Synchronization for Time Sensitive
Applications in Bridged Local Area Networks". This is a Layer-2
only profile of IEEE 1588-2008 for use in Audio/Video Bridged
LANs.
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:
AS_sec: 48 bits seconds since epoch
AS_nsec: 32 bits nanoseconds
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A shorter 32 bit timestamp is defined for use in streaming media
protocols in the following way:
as_timestamp = (AS_sec * 10^9 + AS_nsec) modulo 2^32
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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IEEE 1588/802.1AS Synchronisation Header Extension
The fields are defined as follows:
subtype: IEEE 1733 RTCP subtype field, see Section 4.
as_timestamp: a 32 bit IEEE 1588/802.1AS timestamp as defined in
Section 4.
reserved: as this specification evolves, additional fields are
expected to be included in this header.
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:
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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 (Section 2) 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:
urn:ietf:params:rtp-hdrext:avb-sync
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
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Author's Address
Aidan Williams
Audinate
Level 1, 458 Wattle St
Ultimo, NSW 2007
Australia
Phone: +61 2 8090 1000
Fax: +61 2 8090 1001
Email: aidan.williams@audinate.com
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