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Versions: (draft-brandenburg-avtcore-rtcp-for-idms) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 7272

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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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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