One document matched: draft-williams-avtext-avbsync-01.txt
Differences from draft-williams-avtext-avbsync-00.txt
Audio/Video Transport Extensions A. Williams
Internet-Draft Audinate
Intended status: Standards Track March 11, 2011
Expires: September 12, 2011
IEEE 1588/802.1AS Synchronisation for RTP Streams
draft-williams-avtext-avbsync-01
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 [1].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Williams Expires September 12, 2011 [Page 1]
Internet-Draft AVB/RTP Synchronisation March 2011
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Timestamp formats . . . . . . . . . . . . . . . . . . . . . . . 3
3. Header Extension . . . . . . . . . . . . . . . . . . . . . . . 4
4. IEEE 1733 / RTCP AVB Packet . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. An Appendix . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Williams Expires September 12, 2011 [Page 2]
Internet-Draft AVB/RTP Synchronisation March 2011
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 many applications
(e.g. professional, commercial and automotive AV), the NTP clock
synchronisation protocol does not meet the necessary time alignment
and synchronisation speed requirements.
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 AV
signals and can support latencies less than 1ms through a gigabit
LAN.
Three flavours of IEEE1588 are in use today:
o IEEE 1588-2002 [3]: the original "Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and Control
Systems". This is often called IEEE1588v1 or PTPv1.
o IEEE 1588-2008 [4]: 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 [5]: "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:
Williams Expires September 12, 2011 [Page 3]
Internet-Draft AVB/RTP Synchronisation March 2011
AS_sec: 48 bits seconds since epoch
AS_nsec: 32 bits nanoseconds
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
Figure 1 shows the fields of the AVB sync header extension. It uses
the standard RTP header extension mechanism defined in RFC 5285 [2].
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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IEEE 1588/802.1AS Synchronisation Header Extension
The fields are defined as follows:
subtype: RTCP AVB packet subtype field, see Section 4.
as_timestamp: a 32 bit IEEE 1588/802.1AS timestamp as defined in
Section 2.
reserved: as this specification evolves, additional fields are
expected to be included in this header.
Williams Expires September 12, 2011 [Page 4]
Internet-Draft AVB/RTP Synchronisation March 2011
The as_timestamp MUST correspond to the same instant as the RTP
timestamp in the packet's header, and MUST be derived from the same
clock used to generate the as_timestamps in the RTCP AVB packets.
Provided that it has knowledge of the SSRC to CNAME mapping, either
from prior receipt of an RTCP CNAME packet or via out of band
signalling [RFC5576], the receiver can use the information provided
as input to the synchronization algorithm, in exactly the same way as
if an additional RTCP AVB packet had been received for the flow.
4. IEEE 1733 / RTCP AVB Packet
IEEE 1733 [6] defines the "AVB RTCP packet" type reproduced in
Figure 2. RTCP AVB packets contain a mapping between RTP timestamp
and an 802.1AS timestamp as well as additional clock and QoS
information.
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|subtype=0| PT=208 | length=9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name (ASCII) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+R
| gmTimeBaseIndicator | |T
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +C
| |P
+ gmIdentity (80 bits) +
| |A
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+V
| |B
+ stream_id (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| as_timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rtp_timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: IEEE 1733/RTCP AVB packet format
A brief description of the major fields follows:
Williams Expires September 12, 2011 [Page 5]
Internet-Draft AVB/RTP Synchronisation March 2011
gmIdentity an 80 bit field uniquely identifying the current 802.1AS
grand master clock used by the source to generate as_timestamps
for this flow
stream_id a 64 bit number identifying the 802.1Qat [7] 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
Please consult the IEEE 1733 specification [6] for more details.
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
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Singer, D. and H. Desineni, "A General Mechanism for RTP Header
Extensions", RFC 5285, July 2008.
7.2. Informative References
[3] Institute of Electrical and Electronics Engineers, "IEEE
Standard for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems", IEEE Std 1588-2002,
2002,
<http://standards.ieee.org/findstds/standard/1588-2002.html>.
[4] Institute of Electrical and Electronics Engineers, "IEEE
Standard for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems", IEEE Std 1588-2008,
2008,
Williams Expires September 12, 2011 [Page 6]
Internet-Draft AVB/RTP Synchronisation March 2011
<http://standards.ieee.org/findstds/standard/1588-2008.html>.
[5] Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and Metropolitan Area Networks - IEEE Draft
Standard for Local and Metropolitan Area Networks - Timing and
Synchronization for Time-Sensitive Applications in Bridged Local
Area Networks", IEEE Std 802.1AS-2011, 2011,
<http://standards.ieee.org/findstds/standard/802.1AS-2011.html>.
[6] Institute of Electrical and Electronics Engineers, "IEEE P1733/
D7.0 Draft Standard for Layer 3 Transport Protocol for Time
Sensitive Applications in Local Area Networks", IEEE Draft Std
1733/D7.0, February 2011,
<http://grouper.ieee.org/groups/1733/>.
[7] Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and Metropolitan Area Networks---Virtual
Bridged Local Area Networks Amendment 14: Stream Reservation
Protocol (SRP)", IEEE Std 802.1Qat-2010 (Revision of IEEE Std
802.1Q-2005), 2010, <http://standards.ieee.org/about/get/>.
Appendix A. An Appendix
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
Williams Expires September 12, 2011 [Page 7]
| PAFTECH AB 2003-2026 | 2026-04-24 07:27:30 |