AVTCore R. van Brandenburg
Internet Draft H. Stokking
Intended status: Standards Track M. van Deventer
Expires: May 3, 2012 TNO Netherlands
F. Boronat
M. Montagud
Universidad Politecnica de Valencia
Kevin Gross
AVA Networks
October 31, 2011
RTCP for inter-destination media synchronization
draft-ietf-avtcore-idms-02.txt
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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 summary of the
collected information so that the participants can synchronize play-
out.
Typical applications for IDMS 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 speakers, etc.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Inter-destination Media Synchronization . . . . . . . . . . 3
1.2. Applicability of RTCP to IDMS . . . . . . . . . . . . . . . 3
1.3. Applicability of SDP to IDMS . . . . . . . . . . . . . . . 4
1.4. This document and ETSI TISPAN . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of IDMS operation . . . . . . . . . . . . . . . . . . 4
4. Inter-destination media synchronization use cases . . . . . . . 6
5. Architecture for inter-destination media synchronization . . . 7
5.1. Media Synchronization Application Server (MSAS) . . . . . . 7
5.2. Synchronization Client (SC) . . . . . . . . . . . . . . . . 8
5.3. Communication between MSAS and SCs . . . . . . . . . . . . 8
6. RTCP XR Block Type for IDMS . . . . . . . . . . . . . . . . . . 9
7. RTCP Packet Type for IDMS (IDMS report) . . . . . . . . . . . . 11
8. Timing and NTP Considerations . . . . . . . . . . . . . . . . . 13
8.1. Leap Seconds . . . . . . . . . . . . . . . . . . . . . . . 14
9. SDP Parameter for RTCP XR IDMS Block Type . . . . . . . . . . . 15
10. SDP Parameter for RTCP IDMS Packet Type . . . . . . . . . . . 15
11. SDP parameter for clock source . . . . . . . . . . . . . . . . 16
12. Compatibility with ETSI TISPAN . . . . . . . . . . . . . . . . 18
13. On the use of presentation timestamps . . . . . . . . . . . . 19
14. Security Considerations . . . . . . . . . . . . . . . . . . . 19
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
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16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
17. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 21
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
18.1. Normative References . . . . . . . . . . . . . . . . . . . 22
18.2. Informative References . . . . . . . . . . . . . . . . . . 22
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 temporally 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
playout times. It may also require signaling for the initiation and
maintenance of IDMS sessions and groups.
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, most multimedia applications make use of RTP and RTCP
[RFC3550]. RTP (Real-time Transport Protocol) 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 and sequence number
mechanisms provided by RTP are very useful to reconstruct the
original media timing, reorder and detect some packet loss at the
receiver 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]. Thus, the feedback reporting features provided by
RTCP make QoS monitoring possible and can be used for troubleshooting
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and fault tolerance management in multicast distribution services
such as IPTV.
These protocols are intended to be tailored through modification
and/or additions in order to include profile-specific information
required by particular applications, and the guidelines on doing so
are specified in [RFC5968].
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 becomes a promising candidate for carrying
out IDMS, which may facilitate implementation in typical multimedia
applications.
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.
This document also allows for a receiver to indicate a used clock
source for synchronizing the receiver clock used in the IDMS session.
This is also done using an SDP parameter, which is described in this
document.
1.4. This document and ETSI TISPAN
ETSI TISPAN [TS 183 063] has specified architecture and protocol for
IDMS using RTCP XR exchange and SDP signaling.
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 IDMS RTCP
functionality is used. The section is tutorial in nature and does not
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contain any normative statements.
Alice's . . . . . . .tv:abc.com . . . . . . . . . Bob's
TV (Sync Client) (Sync Server) Laptop (SC)
| | |
| 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 program.
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 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 have 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
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this reference client to both Alice and Bob.
In this case Bob has the slowest connection 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 it 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 imaginable 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 and 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 the media play-out of the two or more users
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). Thus IDMS is required to provide
play-out synchronization.
Another potential use case for IDMS is the 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.
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A third usage scenario is that of the networked loudspeakers, in
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 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 in Section 7.
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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 [RFC5576] 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 7 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:
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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 [TS 183 063]. See section 12. 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 not be inspected.
Block Length: 16 bits. This field indicates the length of the block
in 32 bit words 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 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), then the Media Stream Correlation
Identifier maps on the SyncGroupId to which the SC belongs.
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 set.
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Packet Received RTP timestamp: 32 bits. This timestamp has the value
of the RTP time stamp 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 data contained in the first octet
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 presented 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 a receiver of the RTP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 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 set the
required playout delay. The timestamp is formatted based on the NTP
timestamp format as specified in [RFC5905]. See section 8 for more
information on how this field is set.
Packet Received RTP timestamp: 32 bits. This timestamp has the value
of the RTP time stamp carried in the RTP header [RFC3550] of the RTP
packet to which the XR relates. This SHOULD relate to the first 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 data 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 presented to the user of the system.
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In some use cases (e.g. phased array transducers), the level of
control a MSAS might need to have over the exact moment of playout is
so precise that a 32bit Presentation Timestamp might not suffice. For
this reason, this RTCP Packet Type for IDMS includes a 64bit
Presentation 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 playout to have a 32bit
Presentation 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 in 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, a high accuracy will be needed. In this case, NTP usage
may not be sufficient. Either a local NTP server could be setup, or
some other more accurate clock synchronization mechanism could be
used, such as using GPS time or the Precision Time Protocol [IEEE-
1588].
In this document, a new SDP parameter is introduced to signal the
clock synchronization source or sources used or able to be used (see
section 10). An SC can indicate which synchronization source is being
used at the moment and the last time the SC synchronized with this
source. 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 synchronization source for their session.
Applications performing IDMS may or may not be able to choose a
synchronization method for the system clock. 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.
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8.1. Leap Seconds
IDMS implementation is simplified by using a clock reference with a
timescale which does not include leap seconds. IEEE 1588, GPS and
other TAI (Inernational Atomic Time) references do not include leap
seconds. NTP time, operating system clocks and other UTC (Coordinated
Universal Time) references include leap seconds (though the ITU is
studying a proposal which could eventually eliminate leap seconds
from UTC).
Leap seconds are potentially scheduled at the end of the last day of
December and June each year. NTP inserts a leap second at the
beginning of the last second of the day. This results in the clock
freezing for one second immediately prior to the last second of the
affected day. Most system clocks insert the leap second at the end of
the last second. This results in repetition of the last second of the
day. Generating or using timestamps during the entire last second of
a day on which a leap second has been scheduled should therefore be
avoided. Note that the period to be avoided has a real-time duration
of two seconds.
It is also important that all participants correctly implement leap
seconds and have a working communications channel to receive
notification of leap second scheduling. Without prior knowledge of
leap second schedule, NTP servers and clients may be offset by
exactly one second with respect to their UTC reference. This
potential discrepancy begins when a leap second occurs and ends when
all participants receive a time update from a server or peer (which,
depending on the operating system and/or implementation, could be
anywhere from a few minutes to a week). Such a long-lived discrepancy
can be particularly disruptive to RTP and IDMS operation.
Apart from the long-lived discrepancy due to dependence on both
timing (e.g. NTP) updates and leap seconds scheduling updates, there
is also the potential for a short-lived timing discontinuity having
an effect on RTP and IDMS playout (even though leap seconds are quite
rare).
If a timescale with leap seconds is used for IDMS:
- SCs must take care not to generate any IDMS XR reports in the
immediate vicinity of the leap second. An MSAS must ignore any such
reports that may be inadvertently generated.
- RTP Senders using a leap-second-bearing reference must not generate
sender reports (SR) containing an originating NTP timestamp in the
vicinity of a leap second. Receivers should ignore timestamps in any
such reports inadvertently generated.
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- Receivers working to a leap-second-bearing reference must be
careful to take leap seconds into account if a leap second occurs
between the time a RTP packet is originated and when it is to be
presented.
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 inter-destination media synchronization. It is also used to
carry an identifier for 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]
SyncGroupId = 1*DIGIT ; Numerical value from 0 till 4294967295
DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer in network byte order and
represented in decimal. SyncGroupId identifies a group of SCs for
IDMS. It maps on the Media Stream Correlation Identifier as described
in sections 6 and 7. The value SyncGroupId=0 represents an empty
SyncGroupId. The value 4294967295 (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 for
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
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SyncGroupId = 1*DIGIT ; Numerical value from 0 till 4294967295
DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer in network byte order and
represented in decimal. SyncGroupId identifies a group of SCs for
IDMS. The value SyncGroupId=0 represents an empty SyncGroupId. The
value 4294967295 (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. SDP parameter for clock source
The SDP parameter clocksource is used to signal the source for clock
synchronization. This SDP parameter is specified as follows, using
Augmented Backus-Naur Form [RFC5234].
clocksource = "a=" "clocksource" ":" source SP [last-synced] CRLF
source = local / ntp / gps / gal / ptp
local = "local"
ntp = "ntp" ["=" ntp-server]
ntp-server = host [ ":" port ]
host = hostname / IPv4address / IPv6reference
hostname = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum
/ alphanum *( alphanum / "-" ) alphanum
toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
IPv6reference = "[" IPv6address "]"
IPv6address = hexpart [ ":" IPv4address ]
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
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hex4 = 1*4HEXDIG
port = 1*DIGIT
gps = "gps"
gal = "gal"
ptp = "ptp" SP ptp-version [ ":" ptp-id]
ptp-version = "IEEE 1588-2002" / "IEEE 1588-2008" / "IEEE 802.1AS-
2011"
ptp-id = 1*alphanum
last-synced = date SP time UTCoffset
date = 2DIGIT "-" 2DIGIT "-" 4DIGIT
; day month year (e.g., 02-06-1982)
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT "." 3DIGIT
; 00:00:00.000 - 23:59:59.999
UTCoffset = plusoffset / minusoffset
plusoffset = + 2DIGIT ":" 2DIGIT
; +01:00 (+HH:MM)
minusoffset = - 2DIGIT ":" 2DIGIT
; -00:00 (-HH:MM)
alphanum = ALPHA / DIGIT
EXAMPLE
a=clocksource:ntp=139.63.192.5:123 19-02-2011 21:03:20.345+01:00
A client MAY include this attribute multiple times. If multiple time
synchronization sources were used in the past, the client MUST only
report the 'last synced' parameter on the latest synchronization
performed. If a client supports a specific synchronization method,
but does not know any sources to use for synchronization, it SHOULD
indicate the method without specifying the source. A client MAY
indicate itself as source if it is a clock synchronization source,
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but it SHOULD do so using a publicly reachable address.
The parameter can be used as both a session or media level attribute.
It will normally be a session level parameter, since it is not
directly media-related. In case of IDMS however, it can be used in
conjunction with the rtcp-idms SDP parameter, and then it SHOULD be
used as a media-level parameter as well.
The meaning of 'local' is that no clock synchronization is performed.
Allthough not re-used, the meaning of 'gps' (Global Positioning
System), 'gal' (Galileo Positioning System) and ptp (Precision Time
Protocol) follows the use of reference identifiers in NTP [RFC5905].
When using ptp, the ptp-id MUST contain the proper identifier from
the used IEEE specification.
The 'last synced' parameter is used as an indication for the receiver
of the parameter on the accuracy of the clock. If the indicated last
synchronization time is very recent, this is an indication that the
clock can be trusted to be accurate, given the method of clock
synchronization used. If the indicated last synchronization time is
longer ago or in the future, either the clock synchronization has
been performed long ago, or the clock is synchronized to an incorrect
synchronization source. Either way, this shows that the clock used
can not be trusted to be accurate.
12. Compatibility with ETSI TISPAN
As described in section 1.4, ETSI TISPAN has also described a
mechanism for IDMS in [TS 183 063]. One of the main differences
between the TISPAN document and this document is the fact that the
TISPAN solution uses an RTPC XR block for both the SC->MSAS message
as well as for the MSAS->SC message (by selecting different SPST-
types), 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 then the XR block solution
chosen in ETSI TISPAN.
In order to maintain backward-compatibility, the RTCP XR block used
for SC->MSAS signaling specified in this document is fully compatible
with the TISPAN defined XR block.
For the MSAS->SC signaling, it is recommended to use the RTCP IDMS
Packet Type defined in this document. The TISPAN 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
Packet Type.
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The above means that the IANA registry contains two SDP parameters
for the MSAS->SC signaling; one for the ETSI TISPAN solution and one
for the IETF solution. This also means that if all elements in the
SDP negotiation support the IETF solution they SHOULD use the new
RTCP IDMS Packet Type.
13. On the use of presentation timestamps
A receiver can report on different timing events, i.e. on packet
arrival times and on playout times. A receiver SHALL report on
arrival times and a receiver MAY report on playout 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. By synchronizing
on packet arrival times, you may loose some accuracy, 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 playout 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.
14. Security Considerations
The specified RTCP XR Block Type in this document is used to collect,
summarize and distribute information on packet reception- and playout
-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.
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15. IANA Considerations
New RTCP Packet Types and RTCP XR Block Types are subject to IANA
registration. For general guidelines on IANA considerations for RTCP
XR, refer to [RFC3611].
[TS 183 063] assigns the block type value 12 in the RTCP XR Block
Type Registry to "Inter-destination Media Synchronization Block". [TS
183 063] also registers the SDP [RFC4566] parameter "grp-sync" for
the "rtcp-xr" attribute in the RTCP XR SDP Parameters Registry.
Further, this document defines a new RTCP packet type called IDMS
report. This new packet type is registered with the IANA registry of
RTP parameters, based on the specification in section 7.
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:
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
Further, this document defines a new SDP attribute, "clocksource",
within the existing IANA registry of SDP Parameters.
The SDP attribute "clocksource" defined by this document is
registered with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"):
Attribute name: clocksource
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Long form: clock synchronization source
Type of name: att-field
Type of attribute: session level
Subject to charset: no
Purpose: see sections 8 and 11 of this document
Reference: this document
Values: see this document and registrations below
The attribute has an extensible parameter field and therefore a
registry for these parameters is required. This document creates an
IANA registry called the Clocksource Source Parameters Registry. It
contains the five parameters defined in Section 11: "local", "ntp",
"gps", "gal" and "ptp".
16. Contributors
The following people have participated as co-authors or provided
substantial contributions to this document: Omar Niamut, Fabian
Walraven, Ishan Vaishnavi, Rufael Mekuria
17. Conclusions
This document describes the RTCP XR block type for IDMS, the RTCP
IDMS report and the associated SDP parameters for inter-destination
media synchronization. It also describes an SDP parameter for
indicating which source is used for synchronizing a (systems) (wall)
clock.
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18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, November 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[TS 183 063] ETSI TISPAN, "IMS-based IPTV stage 3 specification", TS
183 063 v3.4.1, June 2010.
18.2. Informative References
[RFC5968] Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)", RFC 5968, September 2010.
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[Boronat2009] Boronat, F., et al, "Multimedia group and inter-stream
synchronization techniques: A comparative study", Elsevier
Information Systems 34 (2009), pp. 108-131
[Ishibashi2006] Ishibashi Y. et al, "Subjective Assesment of Fairness
among users in multipoint communications". Proceedings of
the 2006 ACM SIGCHI international conference on Advances
in computer entertainment technology, 2006.
[IEEE-1588] IEEE Standards Association, "1588-2008 - IEEE Standard
for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems", 2008
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Authors' Addresses
Ray van Brandenburg
TNO
Brassersplein 2, Delft, the Netherlands
Phone: +31 88 86 63609
Email: ray.vanbrandenburg@tno.nl
Hans M. Stokking
TNO
Brassersplein 2, Delft, the Netherlands
Phone: +31 88 86 67278
Email: hans.stokking@tno.nl
M. Oskar van Deventer
TNO
Brassersplein 2, Delft, the Netherlands
Phone: +31 88 86 67078
Email: oskar.vandeventer@tno.nl
Fernando Boronat
IGIC Institute, Universidad Politecnica de Valencia-Campus de Gandia
C/ Paraninfo, 1, Grao de Gandia, 46730, Valencia, Spain
Phone: +34 962 849 341
Email: fboronat@dcom.upv.es
Mario Montagud
IGIC Institute, Universidad Politecnica de Valencia-Campus de Gandia
C/ Paraninfo, 1, Grao de Gandia, 46730, Valencia, Spain
Phone: +34 962 849 341
Email: mamontor@posgrado.upv.es
Kevin Gross
AVA Networks
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