AVTCore R. van Brandenburg
Internet-Draft H. Stokking
Intended status: Standards Track O. van Deventer
Expires: December 15, 2012 TNO
F. Boronat
M. Montagud
Universitat Politecnica de
Valencia
K. Gross
AVA Networks
June 13, 2012
RTCP for inter-destination media synchronization
draft-ietf-avtcore-idms-05
Abstract
This document gives information on an RTCP Packet Type and RTCP XR
Block Type including associated SDP parameters for Inter-Destination
Media Synchronization (IDMS). The RTCP XR Block Type, registered
with IANA based on an ETSI specification, is used to collect media
play-out information from participants in a group playing-out
(watching, listening, etc.) a specific RTP media stream. The RTCP
packet type specified by this document is used to distribute a common
target play-out point to which all the distributed receivers, sharing
a media experience, can synchronize.
Typical use cases in which IDMS is usefull are social TV, shared
service control (i.e. applications where two or more geographically
separated users are watching a media stream together), distance
learning, networked video walls, networked loudspeakers, etc.
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."
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This Internet-Draft will expire on December 15, 2012.
Copyright Notice
Copyright (c) 2012 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
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Inter-Destination Media Synchronization . . . . . . . . . 4
1.2. Applicability of RTCP to IDMS . . . . . . . . . . . . . . 4
1.3. Applicability of SDP to IDMS . . . . . . . . . . . . . . . 5
1.4. This document and ETSI TISPAN . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview of IDMS operation . . . . . . . . . . . . . . . . . . 5
4. Inter-Destination Media Synchronization use cases . . . . . . 7
5. Architecture for Inter-Destination Media Synchronization . . . 8
5.1. Media Synchronization Application Server (MSAS) . . . . . 8
5.2. Synchronization Client (SC) . . . . . . . . . . . . . . . 9
5.3. Communication between MSAS and SCs . . . . . . . . . . . . 9
6. RTCP XR Block Type for IDMS . . . . . . . . . . . . . . . . . 9
7. RTCP Packet Type for IDMS (IDMS report) . . . . . . . . . . . 11
8. Timing and NTP Considerations . . . . . . . . . . . . . . . . 13
9. SDP Parameter for RTCP XR IDMS Block Type . . . . . . . . . . 14
10. SDP Parameter for RTCP IDMS Packet Type . . . . . . . . . . . 15
11. Compatibility with ETSI TISPAN . . . . . . . . . . . . . . . . 15
12. On the use of presentation timestamps . . . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . . 18
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
16.1. Normative References . . . . . . . . . . . . . . . . . . . 19
16.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
1.1. Inter-Destination Media Synchronization
Inter-Destination Media Synchronization (IDMS) refers to the play-out
of media streams at two or more geographically distributed locations
in a time synchronized manner. It can be applied to both unicast and
multicast media streams and can be applied to any type and/or
combination of streaming media, such as audio, video and text
(subtitles).[Ishibashi2006] and [Boronat2009] provide an overview of
technologies and algorithms for IDMS.
IDMS requires the exchange of information on media receipt and play-
out times among participants in an IDMS session. It may also require
signaling for the initiation and maintenance of IDMS sessions and
groups of receivers.
The presented RTCP specification for IDMS is independent of the used
synchronization algorithm, which is out-of-scope of this document.
1.2. Applicability of RTCP to IDMS
Currently, a large share of real-time applications make use of RTP
and RTCP [RFC3550]. RTP provides end-to-end network transport
functions suitable for applications requiring real-time data
transport, such as audio, video or data, over multicast or unicast
network services. The timestamps, sequence numbers, and payload
(content) type identification mechanisms provided by RTP packets are
very useful for reconstructing the original media timing, and for
reordering and detecting packet loss at the client side.
The data transport is augmented by a control protocol (RTCP) to allow
monitoring of the data delivery in a manner that is scalable to large
multicast networks, and to provide minimal control and identification
functionality. RTP receivers and senders provide reception quality
feedback by sending out RTCP Receiver Report (RR) and Sender Report
(SR) packets [RFC3550], respectively, which may be augmented by
eXtended Reports (XR) [RFC3611]. Both RTP and RTCP are intended to
be tailored through modifications in order to include profile-
specific information required by particular applications, and the
guidelines on doing so are specified in [RFC5868].
IDMS involves the collection, summarizing and distribution of RTP
packet arrival and play-out times. As information on RTP packet
arrival times and play-out times can be considered reception quality
feedback information, RTCP is well suited for carrying out IDMS,
which may facilitate the implementation and deployment in typical
multimedia applications.
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1.3. Applicability of SDP to IDMS
RTCP XR [RFC3611] defines the Extended Report (XR) packet type for
the RTP Control Protocol (RTCP), and defines how the use of XR
packets can be signaled by an application using the Session
Description Protocol (SDP) [RFC4566].
SDP signaling is used to set up and maintain a synchronization group
between Synchronization Clients (SCs). This document describes two
SDP parameters for doing this, one for the RTCP XR block type and one
for the new RTCP packet type.
1.4. This document and ETSI TISPAN
ETSI TISPAN [TS183063] has specified architecture and protocol for
IDMS using RTCP XR exchange and SDP signaling. For more information
on how this document relates to [TS183063], see Section 11.
2. Terminology
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] and
indicate requirement levels for compliant implementations.
3. Overview of IDMS operation
This section provides a brief example of how the RTCP functionality
is used for achieving IDMS. The section is tutorial in nature and
does not contain any normative statements.
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Alice's . . . . . . .tv:abc.com . . . . . . . . . Bob's
TV (Sync Client) (Sync Server) Laptop (Sync Client)
| | |
| Media Session | |
|<=====================>| |
| Invite(URL,Sync-group ID) |
|------------------------------------------------->|
| | Media Session Set-up |
| |<========================>|
| | |
| Call set-up |
|<================================================>|
| | |
| RTP Packet | RTP Packet |
|<----------------------|------------------------->|
| RR + IDMS XR | |
|---------------------->| RR + IDMS XR |
| |<-------------------------|
| RTCP IDMS packet | RTCP IDMS packet |
|<----------------------|------------------------->|
| | |
Alice is watching TV in her living room. At some point she sees that
a football game of Bob's favorite team is on. She sends him an
invite to watch the program together. Embedded in the invitation is
the link to the media server and a unique sync-group identifier.
Bob, who is also at home, receives the invite on his laptop. He
accepts Alice's invitation and the RTP client on his laptop sets up a
session to the media server. A VoIP connection to Alice's TV is also
set up, so that Alice and Bob can talk while watching the game
together.
As is common with RTP, both the RTP client in Alice's TV as well as
the one in Bob's laptop send periodic RTCP Receiver Reports (RR) to
the media server. However, in order to make sure Alice and Bob see
the events in the football game at (approximately) the same time,
their clients also periodically send an IDMS XR block to the sync
server function of the media server. Included in the XR blocks are
timestamps on when both Alice and Bob received (or played out) a
particular RTP packet.
The sync server function in the media server calculates a reference
client from the received IDMS XR blocks (e.g. by selecting whichever
client received the packet the latest as the reference client). It
then sends an RTCP IDMS packet containing the play-out information of
this reference client to the sync clients of both Alice and Bob.
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In this case Bob's connection has the longest delay and the reference
client therefore includes a delay similar to the one experienced by
Bob. Upon reception of this information, Alice's RTP client can
choose what to do with this information. In this case it decreases
its play-out rate temporarily until the play-out time matches with
the reference client play-out (and thus matches Bob's play-out).
Another option for Alice's TV would be to simply pause playback until
it catches up. The exact implementation of the synchronization
algorithm is up to the client.
Upon reception of the reference client RTCP IDMS packet, Bob's client
does not have to do anything since it is already synchronized to the
reference client (since it is based on Bob's delay). Note that other
synchronization algorithms may introduce even more delay than the one
experienced by the most delayed client, e.g. to account for delay
variations, for new clients joining an existing synchronization
group, etc.
4. Inter-Destination Media Synchronization use cases
There are a large number of use cases in which IDMS might be useful.
This section will highlight some of them. It should be noted that
this section is in no way meant to be exhaustive.
A first usage scenario for IDMS is Social TV. Social TV is the
combination of media content consumption by two or more users at
different devices and locations combined with the real-time
communication between those users. An example of Social TV is when
two or more users are watching the same television broadcast at
different devices and locations, while communicating with each other
using text, audio and/or video. A skew in their media play-out
processes can have adverse effects on their experience. A well-known
use case here is one friend experiencing a goal in a football match
well before or after other friend(s).
Another potential use case for IDMS is a networked video wall. A
video wall consists of multiple computer monitors, video projectors,
or television sets tiled together contiguously or overlapped in order
to form one large screen. Each of the screens reproduces a portion
of the larger picture. In some implementations, each screen may be
individually connected to the network and receive its portion of the
overall image from a network-connected video server or video scaler.
Screens are refreshed at 60 hertz (every 16-2/3 milliseconds) or
potentially faster. If the refresh is not synchronized, the effect
of multiple screens acting as one is broken.
A third usage scenario is that of the networked loudspeakers, in
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which two or more speakers are connected to the network individually.
Such situations can for example be found in large conference rooms,
legislative chambers, classrooms (especially those supporting
distance learning) and other large-scale environments such as
stadiums. Since humans are more susceptible to differences in audio
delay, this use case needs even more accuracy than the video wall use
case. Depending on the exact application, the need for accuracy can
then be in the range of microseconds.
5. Architecture for Inter-Destination Media Synchronization
The architecture for IDMS, which is based on a sync-maestro
architecture [Boronat2009], is sketched below. The Synchronization
Client (SC) and Media Synchronization Application Server (MSAS)
entities are shown as additional functionality for the RTP receiver
and sender respectively.
It should be noted that a master/slave type of architecture is also
supported by having one of the SC devices also act as an MSAS. In
this case the MSAS functionality is thus embedded in an RTP receiver
instead of an RTP sender.
+-----------------------+ +-----------------------+
| | SR + | |
| RTP Receiver | RTCP | RTP Sender |
| | IDMS | |
| +-----------------+ | <----- | +-----------------+ |
| | | | | | | |
| | Synchronization | | | | Media | |
| | Client | | | | Synchronization | |
| | (SC) | | | | Application | |
| | | | | | Server | |
| | | | RR+XR | | (MSAS) | |
| | | | -----> | | | |
| +-----------------+ | | +-----------------+ |
| | | |
+-----------------------+ +-----------------------+
5.1. Media Synchronization Application Server (MSAS)
An MSAS collects RTP packet arrival times and play-out times from one
or more SC(s) in a synchronization group. The MSAS summarizes and
distributes this information to the SCs in the synchronization group
as synchronization settings, e.g. by determining the SC with the most
lagged play-out and using its reported RTP packet arrival time and
play-out time as a summary.
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5.2. Synchronization Client (SC)
An SC reports on RTP packet arrival times and play-out times of a
media stream. It can receive summaries of such information, and use
that to adjust its play-out buffer.
5.3. Communication between MSAS and SCs
Two different message types are used for the communication between
MSAS and SCs. For the SC->MSAS message containing the play-out
information of a particular client, an RTCP XR Block Type is used
(see Section 6). For the MSAS->SC message containing the
synchronization settings instructions, a new RTCP Packet Type is
defined (see Section 7).
6. RTCP XR Block Type for IDMS
This section describes the RTCP XR Block Type for reporting IDMS
information on an RTP media stream. Its definition is based on
[RFC3611]. The RTCP XR is used to provide feedback information on
receipt times and presentation times of RTP packets to e.g. a Sender
[RFC3611], a Feedback Target [RFC5760] or a Third Party Monitor
[RFC3611].
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| Resrv | PT=XR=207 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| BT=12 | SPST |Resrv|P| block length=7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PT | Resrv |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Media Stream Correlation Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Presented NTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The first 64 bits form the header of the RTCP XR, as defined in
[RFC3611]. The SSRC of packet sender identifies the sender of the
specific RTCP packet.
The IDMS report block consists of 8 32-bit words, with the following
fields:
Block Type (BT): 8 bits. It identifies the block format. Its value
SHALL be set to 12.
Synchronization Packet Sender Type (SPST): 4 bits. This field
identifies the role of the packet sender for this specific eXtended
Report. It can have the following values:
SPST=0 Reserved For future use.
SPST=1 The packet sender is an SC. It uses this XR to report
synchronization status information. Timestamps relate to the SC
input.
SPST=2 This setting is reserved in order to preserve compatibility
with ETSI TISPAN [TS183063]. See Section 11 for more information.
SPST=3-15 Reserved For future use.
Reserved bits (Resrv): 3 bits. These bits are reserved for future
definition. In the absence of such a definition, the bits in this
field MUST be set to zero and MUST be ignored by the receiver.
Packet Presented NTP timestamp flag (P): 1 bit. Bit set to 1 if the
Packet Presented NTP timestamp field contains a value, 0 if it is
empty. If this flag is set to zero, then the Packet Presented NTP
timestamp SHALL be ignored.
Block Length: 16 bits. This field indicates the length of the block
in 32 bit words minus one and SHALL be set to 7, as this RTCP Block
Type has a fixed length.
Payload Type (PT): 7 bits. This field identifies the format of the
media payload, according to [RFC3551]. The media payload is
associated with an RTP timestamp clock rate. This clock rate
provides the time base for the RTP timestamp counter.
Reserved bits (Resrv): 25 bits. These bits are reserved for future
use and SHALL be set to 0.
Media Stream Correlation Identifier: 32 bits. This identifier is
used to correlate synchronized media streams. The value 0 (all bits
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are set "0") indicates that this field is empty. The value 2^32-1
(all bits are set "1") is reserved for future use. If the RTCP
Packet Sender is an SC (SPST=1) or an MSAS (SPST=2), then the Media
Stream Correlation Identifier maps on the Synchronization Group
Identifier (SyncGroupId) to which the report applies.
SSRC: 32 bits. The SSRC of the media source SHALL be set to the
value of the SSRC identifier carried in the RTP header [RFC3550] of
the RTP packet to which the XR relates.
Packet Received NTP timestamp: 64 bits. This timestamp reflects the
wall clock time at the moment of arrival of the first octet of the
RTP packet to which the XR relates. It is formatted based on the NTP
timestamp format as specified in [RFC5905]. See Section 8 for more
information on how this field is used.
Packet Received RTP timestamp: 32 bits. This timestamp has the value
of the RTP timestamp carried in the RTP header [RFC3550] of the RTP
packet to which the XR relates. Several consecutive RTP packets will
have equal timestamps if they are (logically) generated at once,
e.g., belong to the same video frame. It may well be the case that
one receiver reports on the first RTP packet having a certain RTP
timestamp and a second receiver reports on the last RTP packet having
that same RTP timestamp. This would lead to an error in the
synchronization algorithm due to the faulty interpretation of
considering both reports to be on the same RTP packet. To solve
this, an SC SHOULD report on RTP packets in which a certain RTP
timestamp shows up for the first time.
Packet Presented NTP timestamp: 32 bits. This timestamp reflects the
wall clock time at the moment the rendered frame contained in the
first byte of the associated RTP packet is presented to the user. It
is based on the time format used by NTP and consists of the least
significant 16 bits of the NTP seconds part and the most significant
16 bits of the NTP fractional second part. If this field is empty,
then it SHALL be set to 0 and the Packet Presented NTP timestamp flag
(P) SHALL be set to 0. Presented here means the moment the data is
played out to the user of the system, i.e. sound played out through
speakers, video images being displayed on some display, etc. The
accuracy resulting from the synchronization algorithm will only be as
good as the accuracy with which the receivers can determine the delay
between receiving packets and presenting them to the end-user.
7. RTCP Packet Type for IDMS (IDMS report)
This section specifies the RTCP Packet Type for indicating
synchronization settings instructions to the receivers of the RTP
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media stream. Its definition is based on [RFC3550].
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| Resrv | PT=TBD | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Media Stream Correlation Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Received RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Presented NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Presented NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first 64 bits form the header of the RTCP Packet Type, as defined
in [RFC3550]. The SSRC of packet sender identifies the sender of the
specific RTCP packet.
The RTCP IDMS packet consists of 7 32-bit words, with the following
fields:
SSRC: 32 bits. The SSRC of the media source SHALL be set to the
value of the SSRC identifier carried in the RTP header [RFC3550] of
the RTP packet to which the RTCP IDMS packet relates.
Media Stream Correlation Identifier: 32 bits. This identifier is
used to correlate synchronized media streams. The value 0 (all bits
are set "0") indicates that this field is empty. The value 2^32-1
(all bits are set "1") is reserved for future use. The Media Stream
Correlation Identifier maps on the SyncGroupId of the group to which
this packet is sent.
Packet Received NTP timestamp: 64 bits. This timestamp reflects the
wall clock time at the reference client at the moment it received the
first octet of the RTP packet to which this packet relates. It can
be used by the synchronization algorithm on the receiving SC to
adjust its play-out timing in order to achieve synchronization, e.g.
to set the required play-out delay. The timestamp is formatted based
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on the NTP timestamp format as specified in [RFC5905]. See Section 8
for more information on how this field is used.
Packet Received RTP timestamp: 32 bits. This timestamp has the value
of the RTP timestamp carried in the RTP header [RFC3550] of the RTP
packet to which the XR relates. This SHOULD relate to the first
arriving RTP packet containing this particular RTP timestamp, in case
multiple RTP packets contain the same RTP timestamp.
Packet Presented NTP timestamp: 64 bits. This timestamp reflects the
wall clock time at the reference client at the moment it presented
the rendered frame contained in the first octet of the associated RTP
packet to the user. The timestamp is formatted based on the NTP
timestamp format as specified in [RFC5905]. If this field is empty,
then it SHALL be set to 0. This field MAY be left empty if none or
only one of the receivers reported on presentation timestamps.
Presented here means the moment the data is played out to the user of
the system.
In some use cases (e.g. phased array transducers), the level of
control an MSAS might need to have over the exact moment of play-out
is so precise that a 32bit Presented Timestamp will not suffice. For
this reason, this RTCP Packet Type for IDMS includes a 64bit
Presented Timestamp field. Since an MSAS will in practice always add
some extra delay to the delay reported by the most lagged receiver
(to account for packet jitter), it suffices for the IDMS XR Block
Type with which the SCs report on their play-out to have a 32bit
Presented Timestamp field.
8. Timing and NTP Considerations
To achieve IDMS, the different receivers involved need synchronized
clocks as a common timeline for synchronization. Depending on the
synchronization accuracy required, different clock synchronization
methods can be used. For social TV, synchronization accuracy should
be achieved on the order of hundreds of milliseconds. In that case,
correct use of NTP on receivers will in most situations achieve the
required accuracy. As a guideline, to deal with clock drift of
receivers, receivers should synchronize their clocks at the beginning
of a synchronized session. In case of high required accuracy, the
synchronized clocks of different receivers should not drift beyond
the accuracy required for the synchronization mechanism. In
practice, this can mean that receivers need to synchronize their
clocks repeatedly during a synchronization session.
Because of the stringent synchronization requirements for achieving
good audio in some use cases, a high accuracy will be needed. In
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this case, use of the global NTP system may not be sufficient. For
improved accuracy, a local NTP server could be set up, or some other
more accurate clock synchronization mechanism can be used, such as
GPS time or the Precision Time Protocol [IEEE-1588].
[I-D.draft-williams-avtcore-clksrc] defines a set of SDP parameters
for signaling the clock synchronization source or sources available
to and used by the individual receivers. Using these paramenters, an
SC can indicate which synchronization source is being used at the
moment, the last time the SC synchronized with this source and the
synchronization frequency. An SC can also indicate any other
synchronization sources available to it. This allows multiple SCs in
an IDMS session to use the same or a similar clock source for their
session.
Applications performing IDMS may or may not be able to choose a
synchronization method for the system clock, because this may be a
system-wide setting which the application cannot change. How
applications deal with this is up to the implementation. The
application might control the system clock, or it might use a
separate application clock or even a separate IDMS session clock. It
might also report on the system clock and the synchronization method
used, without being able to change it.
[I-D.draft-gross-leap-second] presents some guidelines on how RTP
senders and receivers should deal with leap seconds. When relying on
NTP for clock synchronization, IDMS is particularly sensitive to leap
second induced timing discrepancies. It is therefore recommended to
take the guideline specified in [I-D.draft-gross-leap-second] into
account when implementing IDMS.
9. SDP Parameter for RTCP XR IDMS Block Type
The SDP parameter sync-group is used to signal the use of the RTCP XR
block for IDMS. It is also used to carry an identifier of the
synchronization group to which clients belong or will belong. This
SDP parameter extends rtcp-xr-attrib as follows, using Augmented
Backus-Naur Form [RFC5234].
rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF
; Original definition from [RFC3611], section 5.1
xr-format =/ grp-sync ; Extending xr-format for Inter-Destination
Media Synchronization
grp-sync = "grp-sync" [",sync-group=" SyncGroupId]
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SyncGroupId = 1*DIGIT ; Numerical value from 0 through 4294967294
DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer represented in decimal.
SyncGroupId identifies a group of SCs for IDMS. It maps on the Media
Stream Correlation Identifier as described in Section 6 and
Section 7. The value SyncGroupId=0 represents an empty SyncGroupId.
The value 4294967294 (2^32-1) is reserved for future use.
The following is an example of the SDP attribute for IDMS
a=rtcp-xr:grp-sync,sync-group=42
10. SDP Parameter for RTCP IDMS Packet Type
The SDP parameter rtcp-idms is used to signal the use of the RTCP
IDMS Packet Type for IDMS. It is also used to carry an identifier of
the synchronization group to which clients belong or will belong.
The SDP parameter is used as a media-level attribute during session
setup. This SDP parameter is defined as follows, using Augmented
Backus-Naur Form [RFC5234].
rtcp-idms = "a=" "rtcp-idms" ":" [sync-grp] CRLF
sync-grp = "sync-group=" SyncGroupId
SyncGroupId = 1*DIGIT ; Numerical value from 0 through 4294967294
DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer and represented in decimal.
SyncGroupId identifies a group of SCs for IDMS. The value
SyncGroupId=0 represents an empty SyncGroupId. The value 4294967294
(2^32-1) is reserved for future use.
The following is an example of the SDP attribute for IDMS.
a=rtcp-idms:sync-group=42
11. Compatibility with ETSI TISPAN
The SDP usage for IDMS follows the rules defined in RFC3611 in
section 5 on SDP signalling, with the exception of what is stated
here. The IDMS usage of RTCP is a (loosely coupled) collaborative
parameter, in the sense that receivers sent their status information
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and in response (asynchronously) the MSAS sents synchronization
instructions. Both the sync-group parameter (defined by ETSI TISPAN)
and the rtcp-idms parameter (defined in this document) thus indicate
the ability to sent and the ability to receive indicated RTCP
messages. This section defines how these SDP parameters should be
used, and thus also explains how compatibility with the TISPAN
solution is arranged for.
As described in Section 1.4, ETSI TISPAN has described its mechanism
for IDMS in [TS183063]. One of the main differences between the
TISPAN document and this document is the fact that the TISPAN
solution uses an RTCP XR block for both the SC->MSAS message and the
MSAS->SC message (by selecting SPST-type 2), while this document
specifies a new RTCP Packet Type for the MSAS->SC message. The
message from MSAS to SC is not in any way a report on how a receiver
sees a session, and therefore a separate RTCP packet type is more
appropriate than the XR block solution chosen in ETSI TISPAN. To
achieve compatibility, MSAS implementations SHOULD implement both the
TISPAN RTCP block and the new RTCP IDMS report for MSAS->SC messages.
SCs MAY implement support for both types of messages. For the
MSAS->SC signaling, it is recommended to use the RTCP IDMS report
defined in this document. The TISPAN RTCP XR block with SPST=2 MAY
be used for purposes of compatibility with the TISPAN solution, but
MUST NOT be used if all nodes involved support the new RTCP IDMS
report.
Most of the times, the IDMS SDP parameters will be used in the offer/
answer context. Receivers will indicate in their SDP which RTCP
messages they support.
For a unicast situation, three situations are possible in offer/
answer context:
- If a receiver indicates at least the rtcp-idms SDP parameter, the
MSAS SHOULD reply with only the rtcp-idms parameter and use only
the RTCP IDMS report for MSAS->SC communication
- If a receiver indicates only the sync-group SDP parameter, and the
MSAS also supports this, it SHOULD reply with only the sync-group
parameter and use only the RTCP XR block with SPST=2 for MSAS->SC
communication
- If a receiver indicates only the sync-group SDP parameter, and the
MSAS does not support this, the media sender MUST ignore the
parameter. This receiver will not become part of the
synchronization session
Note that it is possible that for a certain synchronization group,
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that the MSAS sends RTCP IDMS packets to one receiver and RTCP XR
IDMS blocks with SPST=2 to another receiver.
In a multicast situation using the offer/answer context, it will work
a bit differently. The negotiation is the same as in the unicast
situation. But, the MSAS will multicast all RTCP messages to all
receivers. So:
- If all receivers support the RTCP IDMS report, the MSAS SHOULD
only sent the RTCP IDMS report for MSAS->SC messages
- If not all receivers support the RTCP IDMS report, but all
receivers support the TISPAN solution, the MSAS SHOULD only sent
the RTCP XR block with SPST=2 for MSAS->SC messages.
- If some receivers support only the RTCP IDMS report and other
receivers support only the TISPAN solution, the MSAS SHOULD sent
both the RTCP IDMS report and the RTCP XR block with SPST=2 for
MSAS->SC messages. This is less efficient, since the information
sent is duplicated, but this is the only way to include all
receivers in a synchronization session in this scenario.
In certain multicast situations, there is no offer/answer context.
In that case, the MSAS SHOULD use only the RTCP IDMS packet type and
thus use only the SDP parameter rtcp-idms. Receivers that do not
support the RTCP IDMS packet will just ignore both the SDP parameter
and the RTCP IDMS packets, and will thus not join the synchronization
session. For compatability with the TISPAN solution, the MSAS MAY
choose to use the RTCP XR IDMS block type instead, using the SDP
parameter sync-group. The media sender SHOULD NOT use both
parameters at the same time in this case of no offer/answer context.
12. On the use of presentation timestamps
A receiver can report on different timing events, i.e. on packet
arrival times and on play-out times. A receiver SHALL report on
arrival times and a receiver MAY report on play-out times. RTP
packet arrival times are relatively easy to report on. Normally, the
processing and play-out of the same media stream by different
receivers will take roughly the same amount of time. Synchronizing
on packet arrival times, may lead to some accuracy loss, but it will
be adequate for many applications, such as social TV.
Also, if the receivers are in some way controlled, e.g. having the
same buffer settings and decoding times, high accuracy can be
achieved. However, if all receivers in a synchronization session
have the ability to report on, and thus synchronize on, actual play-
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out times, or packet presentation times, this may be more accurate.
It is up to applications and implementations of this RTCP extension
whether to implement and use this.
13. Security Considerations
The security considerations described in [RFC3611] apply to this
document as well.
The specified RTCP XR Block Type in this document is used to collect,
summarize and distribute information on packet reception- and play-
out-times of streaming media. The information may be used to
orchestrate the media play-out at multiple devices.
Errors in the information, either accidental or malicious, may lead
to undesired behavior. For example, if one device erroneously
reports a two-hour delayed play-out, then another device in the same
synchronization group could decide to delay its play-out by two hours
as well, in order to keep its play-out synchronized. A user would
likely interpret this two hour delay as a malfunctioning service.
Therefore, the application logic of both Synchronization Clients and
Media Synchronization Application Servers should check for
inconsistent information. Differences in play-out time exceeding
configured limits (e.g. more than ten seconds) could be an indication
of such inconsistent information.
No new mechanisms are introduced in this document to ensure
confidentiality. Encryption procedures, such as those being
suggested for a Secure RTP (SRTP) at the time that this document was
written, can be used when confidentiality is a concern to end hosts.
14. IANA Considerations
This document defines a new RTCP packet type called IDMS report in
the IANA registry of RTP parameters, based on the specification in
Section 10.
Further, this document defines a new SDP parameter "rtcp-idms" within
the existing IANA registry of SDP Parameters.
The SDP attribute "rtcp-idms" defined by this document is registered
with the IANA registry of SDP Parameters as follows:
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SDP Attribute ("att-field"):
Attribute name: rtcp-idms
Long form: RTCP report block for IDMS
Type of name: att-field
Type of attribute: media level
Subject to charset: no
Purpose: see sections 7 and 10 of this document
Reference: this document
Values: see this document
15. Contributors
The following people have participated as co-authors or provided
substantial contributions to this document: Omar Niamut, Fabian
Walraven, Ishan Vaishnavi, Rufael Mekuria and Rob Koenen.
16. References
16.1. Normative References
[I-D.draft-williams-avtcore-clksrc]
Williams, A., van Brandenburg, R., Stokking, H., and K.
Gross, "RTP Clock Source Signalling,
draft-williams-avtcore-clksrc-00", March 2012.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels, RFC 2119", March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications, RFC3550", July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video conferences with Minimal Control, RFC3551",
July 2003.
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[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR),
RFC3611", November 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol, RFC4566", July 2006.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications, RFC5234", January 2008.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback, RFC5760", February 2010.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specifications, RFC5905", February 2010.
[TS183063]
"IMS-based IPTV stage 3 specification, TS 183 063 v3.4.1",
June 2010.
16.2. Informative References
[Boronat2009]
Boronat, F., Lloret, J., and M. Garcia, "Multimedia group
and inter-stream synchronization techniques: a comparative
study, Elsevier Information Systems 34 (2009), pp. 108-
131".
[I-D.draft-gross-leap-second]
Gross, K. and R. Brandenburg, van, "RTP and Leap Seconds,
draft-gross-leap-seconds-01".
[IEEE-1588]
"1588-2008 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", 2008.
[Ishibashi2006]
Ishibashi, Y., Nagasaka, M., and N. Fujiyoshi, "Subjective
Assessment of Fairness among users in multipoint
communications, Proceedings of the 2006 ACM SIGCHI
internation conference on Advances in computer
entertainment technology, 2006".
[RFC5868] Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP), RFC5968", September 2010.
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Authors' Addresses
Ray van Brandenburg
TNO
Brassersplein 2
Delft 2612CT
the Netherlands
Phone: +31-88-866-7000
Email: ray.vanbrandenburg@tno.nl
Hans Stokking
TNO
Brassersplein 2
Delft 2612CT
the Netherlands
Phone: +31-88-866-7000
Email: hans.stokking@tno.nl
M. Oskar van Deventer
TNO
Brassersplein 2
Delft 2612CT
the Netherlands
Phone: +31-88-866-7000
Email: oskar.vandeventer@tno.nl
Fernando Boronat
Universitat Politecnica de Valencia
IGIC Institute, Universitat Politecnica de Valencia-Campus de Gandia (UPV), C/ Paraninfo, 1, Grao de Gandia, C/ Paraninfo, 1, Grao de Gandia
Valencia 46730
Spain
Phone: +34 962 849 341
Email: fboronat@dcom.upv.es
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Mario Montagud
Universitat Politecnica de Valencia
IGIC Institute, Universitat Politecnica de Valencia-Campus de Gandia (UPV), C/ Paraninfo, 1, Grao de Gandia, C/ Paraninfo, 1, Grao de Gandia
Valencia 46730
Spain
Phone: +34 962 849 341
Email: mamontor@posgrado.upv.es
Kevin Gross
AVA Networks
Phone: +1-303-447-0517
Email: Kevin.Gross@AVAnw.com
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