draft-ietf-avtcore-idms-04.txt   draft-ietf-avtcore-idms-05.txt 
AVTCore R. van Brandenburg AVTCore R. van Brandenburg
Internet-Draft H. Stokking Internet-Draft H. Stokking
Intended status: Standards Track O. van Deventer Intended status: Standards Track O. van Deventer
Expires: November 23, 2012 TNO Expires: December 15, 2012 TNO
F. Boronat F. Boronat
M. Montagud M. Montagud
Universitat Politecnica de Universitat Politecnica de
Valencia Valencia
K. Gross K. Gross
AVA Networks AVA Networks
May 22, 2012 June 13, 2012
RTCP for inter-destination media synchronization RTCP for inter-destination media synchronization
draft-ietf-avtcore-idms-04 draft-ietf-avtcore-idms-05
Abstract Abstract
This document gives information on an RTCP Packet Type and RTCP XR This document gives information on an RTCP Packet Type and RTCP XR
Block Type including associated SDP parameters for Inter-Destination Block Type including associated SDP parameters for Inter-Destination
Media Synchronization (IDMS). The RTCP XR Block Type, registered Media Synchronization (IDMS). The RTCP XR Block Type, registered
with IANA based on an ETSI specification, is used to collect media with IANA based on an ETSI specification, is used to collect media
playout information from participants in a group playing-out play-out information from participants in a group playing-out
(watching, listening, etc.) a specific RTP media stream. The RTCP (watching, listening, etc.) a specific RTP media stream. The RTCP
packet type specified by this document is used to distribute a common packet type specified by this document is used to distribute a common
target playout point to which all the distributed receivers, sharing target play-out point to which all the distributed receivers, sharing
a media experience, can synchronize. a media experience, can synchronize.
Typical use cases in which IDMS is usefull are social TV, shared Typical use cases in which IDMS is usefull are social TV, shared
service control (i.e. applications where two or more geographically service control (i.e. applications where two or more geographically
separated users are watching a media stream together), distance separated users are watching a media stream together), distance
learning, networked video walls, networked loudspeakers, etc. learning, networked video walls, networked loudspeakers, etc.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 23, 2012. This Internet-Draft will expire on December 15, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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5. Architecture for Inter-Destination Media Synchronization . . . 8 5. Architecture for Inter-Destination Media Synchronization . . . 8
5.1. Media Synchronization Application Server (MSAS) . . . . . 8 5.1. Media Synchronization Application Server (MSAS) . . . . . 8
5.2. Synchronization Client (SC) . . . . . . . . . . . . . . . 9 5.2. Synchronization Client (SC) . . . . . . . . . . . . . . . 9
5.3. Communication between MSAS and SCs . . . . . . . . . . . . 9 5.3. Communication between MSAS and SCs . . . . . . . . . . . . 9
6. RTCP XR Block Type for IDMS . . . . . . . . . . . . . . . . . 9 6. RTCP XR Block Type for IDMS . . . . . . . . . . . . . . . . . 9
7. RTCP Packet Type for IDMS (IDMS report) . . . . . . . . . . . 11 7. RTCP Packet Type for IDMS (IDMS report) . . . . . . . . . . . 11
8. Timing and NTP Considerations . . . . . . . . . . . . . . . . 13 8. Timing and NTP Considerations . . . . . . . . . . . . . . . . 13
9. SDP Parameter for RTCP XR IDMS Block Type . . . . . . . . . . 14 9. SDP Parameter for RTCP XR IDMS Block Type . . . . . . . . . . 14
10. SDP Parameter for RTCP IDMS Packet Type . . . . . . . . . . . 15 10. SDP Parameter for RTCP IDMS Packet Type . . . . . . . . . . . 15
11. Compatibility with ETSI TISPAN . . . . . . . . . . . . . . . . 15 11. Compatibility with ETSI TISPAN . . . . . . . . . . . . . . . . 15
12. On the use of presentation timestamps . . . . . . . . . . . . 16 12. On the use of presentation timestamps . . . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . . 16 13. Security Considerations . . . . . . . . . . . . . . . . . . . 18
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 18 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
16. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 18 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 16.1. Normative References . . . . . . . . . . . . . . . . . . . 19
17.1. Normative References . . . . . . . . . . . . . . . . . . . 18 16.2. Informative References . . . . . . . . . . . . . . . . . . 20
17.2. Informative References . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
1.1. Inter-Destination Media Synchronization 1.1. Inter-Destination Media Synchronization
Inter-Destination Media Synchronization (IDMS) refers to the playout Inter-Destination Media Synchronization (IDMS) refers to the play-out
of media streams at two or more geographically distributed locations of media streams at two or more geographically distributed locations
in a time synchronized manner. It can be applied to both unicast and 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 multicast media streams and can be applied to any type and/or
combination of streaming media, such as audio, video and text combination of streaming media, such as audio, video and text
(subtitles).[Ishibashi2006] and [Boronat2009] provide an overview of (subtitles).[Ishibashi2006] and [Boronat2009] provide an overview of
technologies and algorithms for IDMS. technologies and algorithms for IDMS.
IDMS requires the exchange of information on media receipt and IDMS requires the exchange of information on media receipt and play-
playout times among participants in an IDMS session. It may also out times among participants in an IDMS session. It may also require
require signaling for the initiation and maintenance of IDMS sessions signaling for the initiation and maintenance of IDMS sessions and
and groups of receivers. groups of receivers.
The presented RTCP specification for IDMS is independent of the used The presented RTCP specification for IDMS is independent of the used
synchronization algorithm, which is out-of-scope of this document. synchronization algorithm, which is out-of-scope of this document.
1.2. Applicability of RTCP to IDMS 1.2. Applicability of RTCP to IDMS
Currently, a large share of real-time applications make use of RTP Currently, a large share of real-time applications make use of RTP
and RTCP [RFC3550]. RTP provides end-to-end network transport and RTCP [RFC3550]. RTP provides end-to-end network transport
functions suitable for applications requiring real-time data functions suitable for applications requiring real-time data
transport, such as audio, video or data, over multicast or unicast transport, such as audio, video or data, over multicast or unicast
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multicast networks, and to provide minimal control and identification multicast networks, and to provide minimal control and identification
functionality. RTP receivers and senders provide reception quality functionality. RTP receivers and senders provide reception quality
feedback by sending out RTCP Receiver Report (RR) and Sender Report feedback by sending out RTCP Receiver Report (RR) and Sender Report
(SR) packets [RFC3550], respectively, which may be augmented by (SR) packets [RFC3550], respectively, which may be augmented by
eXtended Reports (XR) [RFC3611]. Both RTP and RTCP are intended to eXtended Reports (XR) [RFC3611]. Both RTP and RTCP are intended to
be tailored through modifications in order to include profile- be tailored through modifications in order to include profile-
specific information required by particular applications, and the specific information required by particular applications, and the
guidelines on doing so are specified in [RFC5868]. guidelines on doing so are specified in [RFC5868].
IDMS involves the collection, summarizing and distribution of RTP IDMS involves the collection, summarizing and distribution of RTP
packet arrival and playout times. As information on RTP packet packet arrival and play-out times. As information on RTP packet
arrival times and playout times can be considered reception quality arrival times and play-out times can be considered reception quality
feedback information, RTCP is well suited for carrying out IDMS, feedback information, RTCP is well suited for carrying out IDMS,
which may facilitate the implementation and deployment in typical which may facilitate the implementation and deployment in typical
multimedia applications. multimedia applications.
1.3. Applicability of SDP to IDMS 1.3. Applicability of SDP to IDMS
RTCP XR [RFC3611] defines the Extended Report (XR) packet type for RTCP XR [RFC3611] defines the Extended Report (XR) packet type for
the RTP Control Protocol (RTCP), and defines how the use of XR the RTP Control Protocol (RTCP), and defines how the use of XR
packets can be signaled by an application using the Session packets can be signaled by an application using the Session
Description Protocol (SDP) [RFC4566]. Description Protocol (SDP) [RFC4566].
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the media server. However, in order to make sure Alice and Bob see the media server. However, in order to make sure Alice and Bob see
the events in the football game at (approximately) the same time, the events in the football game at (approximately) the same time,
their clients also periodically send an IDMS XR block to the sync their clients also periodically send an IDMS XR block to the sync
server function of the media server. Included in the XR blocks are server function of the media server. Included in the XR blocks are
timestamps on when both Alice and Bob received (or played out) a timestamps on when both Alice and Bob received (or played out) a
particular RTP packet. particular RTP packet.
The sync server function in the media server calculates a reference The sync server function in the media server calculates a reference
client from the received IDMS XR blocks (e.g. by selecting whichever client from the received IDMS XR blocks (e.g. by selecting whichever
client received the packet the latest as the reference client). It client received the packet the latest as the reference client). It
then sends an RTCP IDMS packet containing the playout information of then sends an RTCP IDMS packet containing the play-out information of
this reference client to the sync clients of both Alice and Bob. this reference client to the sync clients of both Alice and Bob.
In this case Bob has the slowest connection and the reference client In this case Bob's connection has the longest delay and the reference
therefore includes a delay similar to the one experienced by Bob. client therefore includes a delay similar to the one experienced by
Upon reception of this information, Alice's RTP client can choose Bob. Upon reception of this information, Alice's RTP client can
what to do with this information. In this case it decreases its choose what to do with this information. In this case it decreases
playout rate temporarily until it matches with the reference client its play-out rate temporarily until the play-out time matches with
playout (and thus matches Bob's playout). Another option for Alice's the reference client play-out (and thus matches Bob's play-out).
TV would be to simply pause playback until it catches up. The exact Another option for Alice's TV would be to simply pause playback until
implementation of the synchronization algorithm is up to the client. 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 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 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 reference client (since it is based on Bob's delay). Note that other
synchronization algorithms may introduce even more delay than the one synchronization algorithms may introduce even more delay than the one
experienced by the most delayed client, e.g. to account for delay experienced by the most delayed client, e.g. to account for delay
variations, for new clients joining an existing synchronization variations, for new clients joining an existing synchronization
group, etc. group, etc.
4. Inter-Destination Media Synchronization use cases 4. Inter-Destination Media Synchronization use cases
There are a large number of use cases imaginable in which IDMS might There are a large number of use cases in which IDMS might be useful.
be useful. This section will highlight some of them. It should be This section will highlight some of them. It should be noted that
noted that this section is in no way meant to be exhaustive. this section is in no way meant to be exhaustive.
A first usage scenario for IDMS is Social TV. Social TV is the A first usage scenario for IDMS is Social TV. Social TV is the
combination of media content consumption by two or more users at combination of media content consumption by two or more users at
different devices and locations and real-time communication between different devices and locations combined with the real-time
those users. An example of Social TV, is when two or more users are communication between those users. An example of Social TV is when
watching the same television broadcast at different devices and two or more users are watching the same television broadcast at
locations, while communicating with each other using text, audio different devices and locations, while communicating with each other
and/or video. A skew in their media playout processes can have using text, audio and/or video. A skew in their media play-out
adverse effects on their experience. A well-known use case here is processes can have adverse effects on their experience. A well-known
one friend experiencing a goal in a football match well before or use case here is one friend experiencing a goal in a football match
after other friend(s). well before or after other friend(s).
Another potential use case for IDMS is a networked video wall. A Another potential use case for IDMS is a networked video wall. A
video wall consists of multiple computer monitors, video projectors, video wall consists of multiple computer monitors, video projectors,
or television sets tiled together contiguously or overlapped in order or television sets tiled together contiguously or overlapped in order
to form one large screen. Each of the screens reproduces a portion to form one large screen. Each of the screens reproduces a portion
of the larger picture. In some implementations, each screen may be of the larger picture. In some implementations, each screen may be
individually connected to the network and receive its portion of the individually connected to the network and receive its portion of the
overall image from a network-connected video server or video scaler. overall image from a network-connected video server or video scaler.
Screens are refreshed at 60 hertz (every 16-2/3 milliseconds) or Screens are refreshed at 60 hertz (every 16-2/3 milliseconds) or
potentially faster. If the refresh is not synchronized, the effect potentially faster. If the refresh is not synchronized, the effect
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| | (SC) | | | | Application | | | | (SC) | | | | Application | |
| | | | | | Server | | | | | | | | Server | |
| | | | RR+XR | | (MSAS) | | | | | | RR+XR | | (MSAS) | |
| | | | -----> | | | | | | | | -----> | | | |
| +-----------------+ | | +-----------------+ | | +-----------------+ | | +-----------------+ |
| | | | | | | |
+-----------------------+ +-----------------------+ +-----------------------+ +-----------------------+
5.1. Media Synchronization Application Server (MSAS) 5.1. Media Synchronization Application Server (MSAS)
An MSAS collects RTP packet arrival times and playout times from one 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 or more SC(s) in a synchronization group. The MSAS summarizes and
distributes this information to the SCs in the synchronization group distributes this information to the SCs in the synchronization group
as synchronization settings, e.g. by determining the SC with the most as synchronization settings, e.g. by determining the SC with the most
lagged playout and using its reported RTP packet arrival time and lagged play-out and using its reported RTP packet arrival time and
playout time as a summary. play-out time as a summary.
5.2. Synchronization Client (SC) 5.2. Synchronization Client (SC)
An SC reports on RTP packet arrival times and playout times of a 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 media stream. It can receive summaries of such information, and use
that to adjust its playout buffer. that to adjust its play-out buffer.
5.3. Communication between MSAS and SCs 5.3. Communication between MSAS and SCs
Two different message types are used for the communication between Two different message types are used for the communication between
MSAS and SCs. For the SC->MSAS message containing the play-out MSAS and SCs. For the SC->MSAS message containing the play-out
information of a particular client, an RTCP XR Block Type is used information of a particular client, an RTCP XR Block Type is used
(see Section 6). For the MSAS->SC message containing the (see Section 6). For the MSAS->SC message containing the
synchronization settings instructions, a new RTCP Packet Type is synchronization settings instructions, a new RTCP Packet Type is
defined (see Section 7). defined (see Section 7).
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e.g., belong to the same video frame. It may well be the case that 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 one receiver reports on the first RTP packet having a certain RTP
timestamp and a second receiver reports on the last RTP packet having timestamp and a second receiver reports on the last RTP packet having
that same RTP timestamp. This would lead to an error in the that same RTP timestamp. This would lead to an error in the
synchronization algorithm due to the faulty interpretation of synchronization algorithm due to the faulty interpretation of
considering both reports to be on the same RTP packet. To solve 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 this, an SC SHOULD report on RTP packets in which a certain RTP
timestamp shows up for the first time. timestamp shows up for the first time.
Packet Presented NTP timestamp: 32 bits. This timestamp reflects the Packet Presented NTP timestamp: 32 bits. This timestamp reflects the
wall clock time at the moment the data contained in the first octet wall clock time at the moment the rendered frame contained in the
of the associated RTP packet is presented to the user. It is based first byte of the associated RTP packet is presented to the user. It
on the time format used by NTP and consists of the least significant is based on the time format used by NTP and consists of the least
16 bits of the NTP seconds part and the most significant 16 bits of significant 16 bits of the NTP seconds part and the most significant
the NTP fractional second part. If this field is empty, then it 16 bits of the NTP fractional second part. If this field is empty,
SHALL be set to 0 and the Packet Presented NTP timestamp flag (P) then it SHALL be set to 0 and the Packet Presented NTP timestamp flag
SHALL be set to 0. Presented here means the moment the data is (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 played out to the user of the system, i.e. sound played out through
speakers, video images being displayed on some display, etc. The speakers, video images being displayed on some display, etc. The
accuracy resulting from the synchronization algorithm will only be as accuracy resulting from the synchronization algorithm will only be as
good as the accuracy with which the receivers can determine the delay good as the accuracy with which the receivers can determine the delay
between receiving packets and presenting them to the end-user. between receiving packets and presenting them to the end-user.
7. RTCP Packet Type for IDMS (IDMS report) 7. RTCP Packet Type for IDMS (IDMS report)
This section specifies the RTCP Packet Type for indicating This section specifies the RTCP Packet Type for indicating
synchronization settings instructions to the receivers of the RTP synchronization settings instructions to the receivers of the RTP
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used to correlate synchronized media streams. The value 0 (all bits 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 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 (all bits are set "1") is reserved for future use. The Media Stream
Correlation Identifier maps on the SyncGroupId of the group to which Correlation Identifier maps on the SyncGroupId of the group to which
this packet is sent. this packet is sent.
Packet Received NTP timestamp: 64 bits. This timestamp reflects the Packet Received NTP timestamp: 64 bits. This timestamp reflects the
wall clock time at the reference client at the moment it received 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 first octet of the RTP packet to which this packet relates. It can
be used by the synchronization algorithm on the receiving SC to be used by the synchronization algorithm on the receiving SC to
adjust its playout timing in order to achieve synchronization, e.g. adjust its play-out timing in order to achieve synchronization, e.g.
to set the required playout delay. The timestamp is formatted based to set the required play-out delay. The timestamp is formatted based
on the NTP timestamp format as specified in [RFC5905]. See Section 8 on the NTP timestamp format as specified in [RFC5905]. See Section 8
for more information on how this field is used. for more information on how this field is used.
Packet Received RTP timestamp: 32 bits. This timestamp has the value Packet Received RTP timestamp: 32 bits. This timestamp has the value
of the RTP timestamp carried in the RTP header [RFC3550] of the RTP 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 packet to which the XR relates. This SHOULD relate to the first
arriving RTP packet containing this particular RTP timestamp, in case arriving RTP packet containing this particular RTP timestamp, in case
multiple RTP packets contain the same RTP timestamp. multiple RTP packets contain the same RTP timestamp.
Packet Presented NTP timestamp: 64 bits. This timestamp reflects the Packet Presented NTP timestamp: 64 bits. This timestamp reflects the
wall clock time at the reference client at the moment it presented 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 rendered frame contained in the first octet of the associated RTP
the user. The timestamp is formatted based on the NTP timestamp packet to the user. The timestamp is formatted based on the NTP
format as specified in [RFC5905]. If this field is empty, then it timestamp format as specified in [RFC5905]. If this field is empty,
SHALL be set to 0. This field MAY be left empty if none or only one then it SHALL be set to 0. This field MAY be left empty if none or
of the receivers reported on presentation timestamps. Presented here only one of the receivers reported on presentation timestamps.
means the moment the data is played out to the user of the system. 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 In some use cases (e.g. phased array transducers), the level of
control an MSAS might need to have over the exact moment of playout 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 is so precise that a 32bit Presented Timestamp will not suffice. For
this reason, this RTCP Packet Type for IDMS includes a 64bit this reason, this RTCP Packet Type for IDMS includes a 64bit
Presented Timestamp field. Since an MSAS will in practice always add Presented Timestamp field. Since an MSAS will in practice always add
some extra delay to the delay reported by the most lagged receiver some extra delay to the delay reported by the most lagged receiver
(to account for packet jitter), it suffices for the IDMS XR Block (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 Type with which the SCs report on their play-out to have a 32bit
Presented Timestamp field. Presented Timestamp field.
8. Timing and NTP Considerations 8. Timing and NTP Considerations
To achieve IDMS, the different receivers involved need synchronized To achieve IDMS, the different receivers involved need synchronized
clocks as a common timeline for synchronization. Depending on the clocks as a common timeline for synchronization. Depending on the
synchronization accuracy required, different clock synchronization synchronization accuracy required, different clock synchronization
methods can be used. For social TV, synchronization accuracy should methods can be used. For social TV, synchronization accuracy should
be achieved on the order of hundreds of milliseconds. In that case, be achieved on the order of hundreds of milliseconds. In that case,
correct use of NTP on receivers will in most situations achieve the correct use of NTP on receivers will in most situations achieve the
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SDP parameter extends rtcp-xr-attrib as follows, using Augmented SDP parameter extends rtcp-xr-attrib as follows, using Augmented
Backus-Naur Form [RFC5234]. Backus-Naur Form [RFC5234].
rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF
; Original definition from [RFC3611], section 5.1 ; Original definition from [RFC3611], section 5.1
xr-format =/ grp-sync ; Extending xr-format for Inter-Destination xr-format =/ grp-sync ; Extending xr-format for Inter-Destination
Media Synchronization Media Synchronization
grp-sync = "grp-sync" [",sync-group=" SyncGroupId] grp-sync = "grp-sync" [",sync-group=" SyncGroupId]
SyncGroupId = 1*DIGIT ; Numerical value from 0 through 4294967294
SyncGroupId = 1*DIGIT ; Numerical value from 0 till 4294967295
DIGIT = %x30-39 DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer represented in decimal. SyncGroupId is a 32-bit unsigned integer represented in decimal.
SyncGroupId identifies a group of SCs for IDMS. It maps on the Media SyncGroupId identifies a group of SCs for IDMS. It maps on the Media
Stream Correlation Identifier as described in Section 6 and Stream Correlation Identifier as described in Section 6 and
Section 7. The value SyncGroupId=0 represents an empty SyncGroupId. Section 7. The value SyncGroupId=0 represents an empty SyncGroupId.
The value 4294967295 (2^32-1) is reserved for future use. The value 4294967294 (2^32-1) is reserved for future use.
The following is an example of the SDP attribute for IDMS The following is an example of the SDP attribute for IDMS
a=rtcp-xr:grp-sync,sync-group=42 a=rtcp-xr:grp-sync,sync-group=42
10. SDP Parameter for RTCP IDMS Packet Type 10. SDP Parameter for RTCP IDMS Packet Type
The SDP parameter rtcp-idms is used to signal the use of the RTCP 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 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 synchronization group to which clients belong or will belong.
The SDP parameter is used as a media-level attribute during session The SDP parameter is used as a media-level attribute during session
setup. This SDP parameter is defined as follows, using Augmented setup. This SDP parameter is defined as follows, using Augmented
Backus-Naur Form [RFC5234]. Backus-Naur Form [RFC5234].
rtcp-idms = "a=" "rtcp-idms" ":" [sync-grp] CRLF rtcp-idms = "a=" "rtcp-idms" ":" [sync-grp] CRLF
sync-grp = "sync-group=" SyncGroupId sync-grp = "sync-group=" SyncGroupId
SyncGroupId = 1*DIGIT ; Numerical value from 0 till 4294967295 SyncGroupId = 1*DIGIT ; Numerical value from 0 through 4294967294
DIGIT = %x30-39 DIGIT = %x30-39
SyncGroupId is a 32-bit unsigned integer and represented in decimal. SyncGroupId is a 32-bit unsigned integer and represented in decimal.
SyncGroupId identifies a group of SCs for IDMS. The value SyncGroupId identifies a group of SCs for IDMS. The value
SyncGroupId=0 represents an empty SyncGroupId. The value 4294967295 SyncGroupId=0 represents an empty SyncGroupId. The value 4294967294
(2^32-1) is reserved for future use. (2^32-1) is reserved for future use.
The following is an example of the SDP attribute for IDMS. The following is an example of the SDP attribute for IDMS.
a=rtcp-idms:sync-group=42 a=rtcp-idms:sync-group=42
11. Compatibility with ETSI TISPAN 11. Compatibility with ETSI TISPAN
As described in Section 1.4, ETSI TISPAN has also described a The SDP usage for IDMS follows the rules defined in RFC3611 in
mechanism for IDMS in [TS183063]. One of the main differences section 5 on SDP signalling, with the exception of what is stated
between the TISPAN document and this document is the fact that the here. The IDMS usage of RTCP is a (loosely coupled) collaborative
TISPAN solution uses an RTPC XR block for both the SC->MSAS message parameter, in the sense that receivers sent their status information
and the MSAS->SC message (by selecting different SPST-types), while and in response (asynchronously) the MSAS sents synchronization
this document specifies a new RTCP Packet Type for the MSAS->SC instructions. Both the sync-group parameter (defined by ETSI TISPAN)
message. The message from MSAS to SC is not in any way a report on and the rtcp-idms parameter (defined in this document) thus indicate
how a receiver sees a session, and therefore a separate RTCP packet the ability to sent and the ability to receive indicated RTCP
type is more appropriate than the XR block solution chosen in ETSI messages. This section defines how these SDP parameters should be
TISPAN. used, and thus also explains how compatibility with the TISPAN
solution is arranged for.
In order to maintain backward-compatibility, the RTCP XR block used As described in Section 1.4, ETSI TISPAN has described its mechanism
for SC->MSAS signaling specified in this document is fully compatible for IDMS in [TS183063]. One of the main differences between the
with the TISPAN defined XR block. 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.
For the MSAS->SC signaling, it is recommended to use the RTCP IDMS Most of the times, the IDMS SDP parameters will be used in the offer/
Packet Type defined in this document. The TISPAN XR block with answer context. Receivers will indicate in their SDP which RTCP
SPST=2 MAY be used for purposes of compatibility with the TISPAN messages they support.
solution, but MUST NOT be used if all nodes involved support the new
RTCP IDMS Packet Type.
The above means that the IANA registry contains two SDP parameters For a unicast situation, three situations are possible in offer/
for the MSAS->SC signaling; one for the ETSI TISPAN solution and one answer context:
for the IETF solution. This also means that if all elements in the
SDP negotiation support the IETF solution they SHOULD use the new - If a receiver indicates at least the rtcp-idms SDP parameter, the
RTCP IDMS Packet Type. 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,
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 12. On the use of presentation timestamps
A receiver can report on different timing events, i.e. on packet 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 on play-out times. A receiver SHALL report on
arrival times and a receiver MAY report on playout times. RTP packet arrival times and a receiver MAY report on play-out times. RTP
arrival times are relatively easy to report on. Normally, the packet arrival times are relatively easy to report on. Normally, the
processing and playout of the same media stream by different processing and play-out of the same media stream by different
receivers will take roughly the same amount of time. It can suffice receivers will take roughly the same amount of time. Synchronizing
for many applications, such as social TV, to synchronize on packet on packet arrival times, may lead to some accuracy loss, but it will
arrival times. Also, if the receivers are in some way controlled, be adequate for many applications, such as social TV.
e.g. having the same buffer settings and decoding times, high
accuracy can be achieved. However, if all receivers in a Also, if the receivers are in some way controlled, e.g. having the
synchronization session have the ability to report on, and thus same buffer settings and decoding times, high accuracy can be
synchronize on packet presented times, this may be more accurate. It achieved. However, if all receivers in a synchronization session
is up to applications and implementations of this RTCP extension have the ability to report on, and thus synchronize on, actual play-
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. whether to implement and use this.
13. Security Considerations 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, The specified RTCP XR Block Type in this document is used to collect,
summarize and distribute information on packet reception- and summarize and distribute information on packet reception- and play-
playout-times of streaming media. The information may be used to out-times of streaming media. The information may be used to
orchestrate the media playout at multiple devices. orchestrate the media play-out at multiple devices.
Errors in the information, either accidental or malicious, may lead Errors in the information, either accidental or malicious, may lead
to undesired behavior. For example, if one device erroneously to undesired behavior. For example, if one device erroneously
reports a two-hour delayed playout, then another device in the same reports a two-hour delayed play-out, then another device in the same
synchronization group could decide to delay its playout by two hours synchronization group could decide to delay its play-out by two hours
as well, in order to keep its playout synchronized. A user would as well, in order to keep its play-out synchronized. A user would
likely interpret this two hour delay as a malfunctioning service. likely interpret this two hour delay as a malfunctioning service.
Therefore, the application logic of both Synchronization Clients and Therefore, the application logic of both Synchronization Clients and
Media Synchronization Application Servers should check for Media Synchronization Application Servers should check for
inconsistent information. Differences in playout time exceeding inconsistent information. Differences in play-out time exceeding
configured limits (e.g. more than ten seconds) could be an indication configured limits (e.g. more than ten seconds) could be an indication
of such inconsistent information. of such inconsistent information.
No new mechanisms are introduced in this document to ensure No new mechanisms are introduced in this document to ensure
confidentiality. Encryption procedures, such as those being confidentiality. Encryption procedures, such as those being
suggested for a Secure RTP (SRTP) at the time that this document was 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. written, can be used when confidentiality is a concern to end hosts.
14. IANA Considerations 14. IANA Considerations
New RTCP Packet Types and RTCP XR Block Types are subject to IANA This document defines a new RTCP packet type called IDMS report in
registration. For general guidelines on IANA considerations for RTCP the IANA registry of RTP parameters, based on the specification in
XR, refer to [RFC3611]. Section 10.
[TS 183 063] assigns the block type value 12 in the RTCP XR Block
Type Registry to "Inter-Destination Media Synchronization Block".
[TS183063] 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 10.
Further, this document defines a new SDP parameter "rtcp-idms" within Further, this document defines a new SDP parameter "rtcp-idms" within
the existing IANA registry of SDP Parameters. the existing IANA registry of SDP Parameters.
The SDP attribute "rtcp-idms" defined by this document is registered The SDP attribute "rtcp-idms" defined by this document is registered
with the IANA registry of SDP Parameters as follows: with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"): SDP Attribute ("att-field"):
Attribute name: rtcp-idms Attribute name: rtcp-idms
skipping to change at page 18, line 22 skipping to change at page 19, line 27
Purpose: see sections 7 and 10 of this document Purpose: see sections 7 and 10 of this document
Reference: this document Reference: this document
Values: see this document Values: see this document
15. Contributors 15. Contributors
The following people have participated as co-authors or provided The following people have participated as co-authors or provided
substantial contributions to this document: Omar Niamut, Fabian substantial contributions to this document: Omar Niamut, Fabian
Walraven, Ishan Vaishnavi, Rufael Mekuria. Walraven, Ishan Vaishnavi, Rufael Mekuria and Rob Koenen.
16. 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.
17. References 16. References
17.1. Normative References 16.1. Normative References
[I-D.draft-williams-avtcore-clksrc] [I-D.draft-williams-avtcore-clksrc]
Williams, A., van Brandenburg, R., Stokking, H., and K. Williams, A., van Brandenburg, R., Stokking, H., and K.
Gross, "RTP Clock Source Signalling, Gross, "RTP Clock Source Signalling,
draft-williams-avtcore-clksrc-00", March 2012. draft-williams-avtcore-clksrc-00", March 2012.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels, RFC 2119", March 1997. Requirement Levels, RFC 2119", March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
skipping to change at page 19, line 27 skipping to change at page 20, line 27
Sessions with Unicast Feedback, RFC5760", February 2010. Sessions with Unicast Feedback, RFC5760", February 2010.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specifications, RFC5905", February 2010. Specifications, RFC5905", February 2010.
[TS183063] [TS183063]
"IMS-based IPTV stage 3 specification, TS 183 063 v3.4.1", "IMS-based IPTV stage 3 specification, TS 183 063 v3.4.1",
June 2010. June 2010.
17.2. Informative References 16.2. Informative References
[Boronat2009] [Boronat2009]
Boronat, F., Lloret, J., and M. Garcia, "Multimedia group Boronat, F., Lloret, J., and M. Garcia, "Multimedia group
and inter-stream synchronization techniques: a comparative and inter-stream synchronization techniques: a comparative
study, Elsevier Information Systems 34 (2009), pp. 108- study, Elsevier Information Systems 34 (2009), pp. 108-
131". 131".
[I-D.draft-gross-leap-second] [I-D.draft-gross-leap-second]
Gross, K. and R. Brandenburg, van, "RTP and Leap Seconds, Gross, K. and R. Brandenburg, van, "RTP and Leap Seconds,
draft-gross-leap-seconds-01". draft-gross-leap-seconds-01".
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