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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                R. van Brandenburg
Request for Comments: 7272                                   H. Stokking
Category: Standards Track                                O. van Deventer
ISSN: 2070-1721                                                      TNO
                                                              F. Boronat
                                                             M. Montagud
                                                                     UPV
                                                                K. Gross
                                                            AVA Networks
                                                               June 2014


             Inter-Destination Media Synchronization (IDMS)
                 Using the RTP Control Protocol (RTCP)

Abstract

   This document defines a new RTP Control Protocol (RTCP) Packet Type
   and an RTCP Extended Report (XR) Block Type to be used for achieving
   Inter-Destination Media Synchronization (IDMS).  IDMS is the process
   of synchronizing playout across multiple media receivers.  Using the
   RTCP XR IDMS Report Block defined in this document, media playout
   information from participants in a synchronization group can be
   collected.  Based on the collected information, an RTCP IDMS Settings
   Packet can then be sent to distribute a common target playout point
   to which all the distributed receivers, sharing a media experience,
   can synchronize.

   Typical use cases in which IDMS is useful 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7272.





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Copyright Notice

   Copyright (c) 2014 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Applicability of RTCP to IDMS . . . . . . . . . . . . . .   3
     2.2.  IDMS and ETSI . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Inter-Destination Media Synchronization (IDMS) Use Cases  . .   4
   4.  Overview of IDMS Operation  . . . . . . . . . . . . . . . . .   5
   5.  Architecture for Inter-Destination Media Synchronization  . .   7
     5.1.  Media Synchronization Application Server (MSAS) . . . . .   7
     5.2.  Synchronization Client (SC) . . . . . . . . . . . . . . .   8
     5.3.  Communication between MSAS and SCs  . . . . . . . . . . .   8
   6.  RTCP XR IDMS Report Block . . . . . . . . . . . . . . . . . .   8
   7.  RTCP Packet Type for IDMS (IDMS Settings Packet)  . . . . . .  11
   8.  Timing and NTP Considerations . . . . . . . . . . . . . . . .  13
   9.  On the Use of Presentation Timestamps . . . . . . . . . . . .  14
   10. SDP Signaling for RTCP IDMS Settings Packet . . . . . . . . .  15
   11. SDP Rules . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Offer/Answer Rules . . . . . . . . . . . . . . . . . . .  16
     11.2.  Declarative Cases  . . . . . . . . . . . . . . . . . . .  17
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     13.1.  RTCP IDMS Packet Type  . . . . . . . . . . . . . . . . .  18
     13.2.  RTCP XR IDMS Report Block  . . . . . . . . . . . . . . .  19
     13.3.  RTCP-IDMS SDP Attribute  . . . . . . . . . . . . . . . .  19
     13.4.  IDMS XR Block SPST Registry  . . . . . . . . . . . . . .  19
     13.5.  Contact Information for Registrations  . . . . . . . . .  20
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     15.2.  Informative References . . . . . . . . . . . . . . . . .  21




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1.  Introduction

   IDMS refers to the playout 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.

   Inter-Destination Media Synchronization (IDMS) requires the exchange
   of information on media arrival and presentation 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
   synchronization algorithm employed, which is out of scope of this
   document.

1.1.  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].

2.  Rationale

2.1.  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 the
   original order of packets and for detecting packet loss at the
   receiver.

   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
   groups 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 report (XR) blocks [RFC3611].




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   IDMS involves the collection, summarization, and distribution of RTP
   packet arrival and presentation times.  As information on RTP packet
   arrival times and presentation times can be considered reception
   quality feedback information, RTCP is well suited for carrying out
   IDMS.

2.2.  IDMS and ETSI

   A first version of IDMS for use with RTP/RTCP was standardized by
   ETSI Telecommunications and Internet converged Services and Protocols
   for Advanced Networking (TISPAN) in [TS183063], resulting in an IANA
   registration for an RTCP XR Block Type.  This work was then brought
   as input to the IETF AVTCORE WG for further standardization,
   leveraging the RTP/RTCP expertise present in the AVTCORE WG.  This
   document is the result of that effort.

   Although the IDMS protocol described in this document has evolved
   significantly from the version that was originally specified by ETSI
   TISPAN, it is still backwards compatible with the ETSI version.  As
   such, it had been decided in ETSI to update the TS 183 063 document
   to reference this document as the normative specification of IDMS.
   This update can be found in newer versions of TS 183 063 (i.e.,
   versions higher than 3.5.2).  In accordance, this document proposes
   to update the IANA registration for the RTCP XR IDMS Report Block to
   point to this document.  Finally, this document proposes an IANA
   registry for Synchronization Packet Sender Type (SPST) values,
   allowing the registration of extensions to this document.

3.  Inter-Destination Media Synchronization (IDMS) Use Cases

   There is 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 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 playout 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 another 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



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   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, with users noticing
   tearing effects and no longer perceiving a single image.

   A third usage scenario is that of networked loudspeakers in which two
   or more speakers are connected to the network individually.  Such
   situations can, for example, be found in large conference rooms,
   legislative chambers, classrooms (especially those supporting
   distance learning), and other large-scale environments such as
   stadiums.  Since humans are more sensitive to differences in audio
   delay compared to video 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.

4.  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.

             Alice's  . . . . . . .tv:abc.com . . . . . . . Bob's
        TV (Sync Client)         (Sync Server)      Laptop (Sync Client)
               |                       |                          |
               |      Media Session    |                          |
               |<=====================>|                          |
               |            Invite(URL, SyncGroupId)              |
               |------------------------------------------------->|
               |                       |   Media Session Setup    |
               |                       |<========================>|
               |                       |                          |
               |                 Call Setup                       |
               |<================================================>|
               |                       |                          |
               |       RTP Packets     |        RTP Packets       |
               |<----------------------|------------------------->|
               |  RR + XR IDMS Report  |                          |
               |---------------------->|    RR + XR IDMS Report   |
               |                       |<-------------------------|
               |   RTCP IDMS Settings  |    RTCP IDMS Settings    |
               |<----------------------|------------------------->|
               |                       |                          |

                  Figure 1: Example of a Typical IDMS Session



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   Alice is watching TV in her living room.  At some point, she sees
   that Bob's favorite team is playing football.  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 with the media server.  A Voice over IP (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 RRs to the media server.
   However, in order to make sure Alice and Bob see the events in the
   football game at the same time, their clients also periodically send
   an RTCP XR IDMS Report Block to the Sync Server function of the media
   server.  Included in the RTCP XR IDMS Report Blocks are timestamps on
   when both Alice and Bob received (and, optionally, when they played
   out) a particular RTP packet.

   The Sync Server function in the media server calculates a reference
   client from the received RTCP XR IDMS Report Blocks (e.g., by
   selecting the most lagged client as the reference for IDMS).  It then
   sends an RTCP IDMS Settings Packet containing the playout information
   of this reference client to the sync clients of both Alice and Bob.

   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 playout rate temporarily until the playout time matches
   with the reference client playout (and, thus, matches Bob's playout).
   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 RTCP IDMS Settings 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.

   For this functionality to work correctly, it is necessary that the
   wallclocks of the receivers are synchronized with each other.  While
   Alice and Bob both report when they receive, and optionally when they




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   playout, certain RTP packets, in order to correlate their reports to
   each other, it is necessary that their wallclocks are synchronized.

5.  Architecture for Inter-Destination Media Synchronization

   The architecture for IDMS, which is based on a sync-maestro
   architecture [Boronat2009], is diagrammed below.  In this particular
   case, the Synchronization Client (SC) and Media Synchronization
   Application Server (MSAS) entities are shown as additional
   functionality for the RTP receiver and sender, respectively.

      +-----------------------+        +-----------------------+
      |                       |  SR +  |                       |
      |      RTP Receiver     |  RTCP  |      RTP Sender       |
      |                       |  IDMS  |                       |
      |  +-----------------+  | <----- |  +-----------------+  |
      |  |                 |  |        |  |                 |  |
      |  | Synchronization |  |        |  |      Media      |  |
      |  |     Client      |  |        |  | Synchronization |  |
      |  |      (SC)       |  |        |  |   Application   |  |
      |  |                 |  |        |  |      Server     |  |
      |  |                 |  | RR+XR  |  |      (MSAS)     |  |
      |  |                 |  | -----> |  |                 |  |
      |  +-----------------+  |        |  +-----------------+  |
      |                       |        |                       |
      +-----------------------+        +-----------------------+

                  Figure 2: IDMS Architecture Diagram

5.1.  Media Synchronization Application Server (MSAS)

   An MSAS collects RTP packet arrival times and presentation times from
   one or more SCs in a synchronization group by receiving RTCP XR IDMS
   reports.  The MSAS summarizes and distributes this information to the
   SCs in the synchronization group as synchronization settings via the
   RTCP IDMS Settings Packet messages, e.g., by determining the SC with
   the most lagged playout and using its reported RTP packet arrival
   time and presentation time as a summary.

   It should be noted that while the diagram above shows the MSAS as
   part of the RTP sender, this is not necessary.  For example, an MSAS
   might also be implemented as an independent function in the network
   or in a master/slave type of architecture where one of the SC devices
   also acts as an MSAS.  Wherever the MSAS is implemented, it is
   important that the MSAS has access to the RTP stream to which the XR
   reports apply, so that it is able to correlate the RTCP XR IDMS
   reports coming from different SCs.




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5.2.  Synchronization Client (SC)

   An SC reports on RTP packet arrival times and presentation times of a
   media stream.  It can receive IDMS Settings Packets containing
   summaries of such information and use that to adjust its playout
   buffer.  The SC sends RTCP XR IDMS reports to the MSAS.

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 arrival and
   playout information of a particular client, an RTCP XR IDMS Report
   Block is used (see Section 6).  For the MSAS->SC message containing
   the synchronization settings instructions, a new RTCP IDMS Settings
   Packet is defined (see Section 7).

6.  RTCP XR IDMS Report Block

   This section specifies a new RTCP XR Block Type, the RTCP XR IDMS
   Report Block, for reporting IDMS information to an MSAS.  In
   particular, it is used to provide feedback information on arrival
   times and presentation times of RTP packets.  Its definition is based
   on [RFC3550] and [RFC3611].

   In most cases, a single RTP receiver will only be part of a single
   IDMS session, i.e., it will report on arrival and presentation times
   of RTP packets from a single RTP stream in a certain synchronization
   group.  In some cases, however, an RTP receiver may be a member of
   multiple synchronization groups for the same RTP stream, e.g.,
   watching a single television program simultaneously with different
   groups.  In even further cases, a receiver may wish to synchronize
   different RTP streams at the same time, either as part of the same
   synchronization group or as part of multiple synchronization groups.
   These are all valid scenarios for IDMS and will require multiple
   reports by an SC.

   This document does not define new rules for when to send RTCP
   reports, but uses the existing rules specified in [RFC3550] for
   sending RTCP reports.  When the RTCP reporting timer allows an SC to
   send an IDMS report, the SC SHOULD report on an RTP packet received
   during the period since the last RTCP XR IDMS Report Block was sent.
   Because of RTP timestamp rollover, there is ambiguity in mapping RTP
   timestamps to NTP timestamps.  The recommendation to report on recent
   RTP packets serves to manage this ambiguity.  For more details on
   which packet to report on, see below under "Packet Received RTP
   timestamp".





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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     BT=12     | SPST  |Resrv|P|         block length=7        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     PT      |               Resrv                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Media Stream Correlation Identifier              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SSRC of media source                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Packet Received NTP timestamp, most significant word     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Packet Received NTP timestamp, least significant word    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Packet Received RTP timestamp                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Packet Presented NTP timestamp                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RTCP XR 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
   is 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, as enumerated in a
   registry maintained by IANA (see Section 13.4):

      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=2-4 Values defined by ETSI TISPAN (see [TS183063]).

      SPST=5-15 Unassigned.

   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 at transmission and MUST be ignored by the
   receiver.






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   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 0, then the Packet Presented NTP
   timestamp SHALL be ignored by the receiver.

   Block Length: 16 bits.  This field indicates the length of the block
   in 32-bit words minus one and is set to 7, as this RTCP XR IDMS Block
   Report has a fixed length.

   Payload Type (PT): 7 bits.  This field identifies the format of the
   media payload, according to [RFC3551].  This is the payload type of
   the RTP packet reported upon.  The PT field is needed in the case
   where the MSAS is neither the media server nor a receiver of the
   media stream, i.e., it is implemented as a third-party entity.  In
   such cases, the MSAS needs the PT to determine the rate of
   advancement of the timestamps of the RTP media stream to be able to
   relate reports from different SCs on different RTP timestamp values.

   Reserved bits (Resrv): 25 bits.  These bits are reserved for future
   use and SHALL be set to 0 at transmission and MUST be ignored by the
   receiver.

   Media Stream Correlation Identifier: 32 bits.  This identifier is
   used to correlate synchronized media streams.  The value 0 (all bits
   are set "0") indicates that this field is empty.  The value 2^32-1
   (all bits are set "1") is reserved for future use.  If the RTCP
   Packet Sender is an SC (SPST=1), then the Media Stream Correlation
   Identifier field contains the Synchronization Group Identifier
   (SyncGroupId) to which the report applies.

   Synchronization Source (SSRC): 32 bits.  The SSRC of the media source
   is set to the value of the SSRC identifier carried in the RTP header
   [RFC3550] of the RTP packet to which the XR IDMS relates.

   Packet Received NTP timestamp: 64 bits.  This timestamp reflects the
   wallclock time at the moment of arrival of the first octet of the RTP
   packet to which the XR IDMS 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 IDMS 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 that has a certain RTP
   timestamp, and a second receiver reports on the last RTP packet that
   has that same RTP timestamp.  This would lead to an error in the



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   synchronization algorithm due to the faulty interpretation of
   considering both reports to be on the same RTP packet.  When
   reporting on an RTP packet, which is one of several consecutive RTP
   packets having equal timestamps, an SC SHOULD report on the RTP
   packet it received with the lowest sequence number.  Note that
   'lowest sequence number' here is meant to be the first in the
   sequence of RTP packets just received, not from an earlier time
   before the last wrap around of RTP timestamps (unless this wrap
   around occurs during the sequence with equal RTP timestamps).

   Packet Presented NTP timestamp: 32 bits.  This timestamp reflects the
   wallclock time at the moment the rendered media unit (e.g., video
   frame or audio sample) 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 no Packet Presented NTP timestamp is available, this
   field SHALL be set to 0 and be considered empty, and the Packet
   Presented NTP timestamp flag (P) SHALL be set to 0.  With regards to
   NTP epoch and rollover, the value of the Packet Presented NTP
   timestamp is considered to always be greater than the Packet Received
   NTP timestamp and to be within 2^16 seconds of it.  Presented in this
   context 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 SCs can determine the delay between receiving packets and
   presenting them to the end user.  If no presentation timestamps are
   reported by SCs, the ability to accurately synchronize playout may be
   limited.

7.  RTCP Packet Type for IDMS (IDMS Settings Packet)

   This section specifies the RTCP packet type for indicating
   synchronization settings instructions to the receivers of the RTP
   media stream.  Its definition is based on [RFC3550].  Synchronization
   settings take the form of a report referencing a real or hypothetical
   RTP packet selected or contrived by the MSAS.













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    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=211    |             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 the packet sender identifies the sender of
   the specific RTCP packet.

   The RTCP IDMS Settings Packet consists of 7 32-bit words, with the
   following fields:

   PT: 211, as registered by IANA.

   SSRC: 32 bits.  The SSRC of the media source is set to the value of
   the SSRC identifier of the media source carried in the RTP header
   [RFC3550] of the RTP packet to which the RTCP IDMS Settings 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 contains the SyncGroupId of the group to which
   this packet is sent.

   Packet Received NTP timestamp: 64 bits.  This timestamp reflects the
   wallclock 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 playout timing in order to achieve synchronization, e.g.,



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   to set the required playout delay.  The timestamp is formatted based
   on the NTP timestamp format as specified in [RFC5905].  See Section 8
   for more information on how this field is used.  Because RTP
   timestamps do wrap around, the sender of this packet MUST use recent
   values, i.e., choose NTP timestamps that reflect current time and not
   too far in the future or in the past so as to create ambiguity with
   regards to RTP timestamp wrap around.

   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 IDMS relates.  This SHOULD relate to the first
   arriving RTP packet containing this particular RTP timestamp, in case
   multiple consecutive RTP packets contain the same RTP timestamp.

   Packet Presented NTP timestamp: 64 bits.  This timestamp reflects the
   wallclock time at the reference client at the moment it presented the
   rendered media unit (e.g., video frame or audio sample) 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 no Packet Presented NTP timestamp is available,
   this field SHALL be set to 0 and be considered empty.  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 playout
   is so precise that a 32-bit Presented timestamp will not suffice.
   For this reason, this RTCP packet type for IDMS includes a 64-bit
   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 RTCP XR IDMS
   Report Block with which the SCs report on their playout to have a
   32-bit Presented Timestamp field.

8.  Timing and NTP Considerations

   To achieve IDMS, the different receivers involved need synchronized
   wallclocks as a common timeline for synchronization.  This
   synchronized clock is used for reporting the Packet Received NTP
   timestamp and the Packet Presented NTP timestamp, and for
   interpretation of these fields in received IDMS Settings Packets.
   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



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   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 quality in some use cases, a high accuracy will be needed.
   In 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 [IEEE1588-2008].

   [RFC7273] defines a set of Session Description Protocol (SDP)
   parameters for signaling the clock synchronization source or sources
   available to and used by the individual receivers.  SCs MAY use
   [RFC7273] to indicate their clock synchronization source or sources
   in use and available.  Using these parameters, an SC can indicate
   which synchronization source is being used at the moment.  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 that 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.

   [RFC7164] 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 RECOMMENDED to take
   the guidelines specified in [RFC7164] into account when implementing
   IDMS.

9.  On the Use of Presentation Timestamps

   A receiver can report on different timing events, i.e., on packet
   arrival times and on playout or presentation times.  A receiver SHALL
   report on arrival times and a receiver MAY report on playout times.
   RTP packet arrival times are relatively easy to report on.  Normally,
   the processing and playout of the same media stream by different
   receivers will take roughly the same amount of time.  Synchronizing




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   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 and rendering 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 presentation times, this will be more accurate.  It is up to
   the applications and implementations of this RTCP extension whether
   to implement and use presentation timestamps.

10.  SDP Signaling for RTCP IDMS Settings Packet

   The SDP attribute rtcp-idms is used to signal the use of the RTCP
   IDMS Settings Packet and the associated RTCP XR IDMS Report Block.
   It is also used to carry an identifier of the synchronization group
   to which clients belong or will belong.  The SDP attribute is used as
   a media-level attribute during session setup.  This means that in
   case of multiple related streams, IDMS is performed on one of them.
   The other streams will be synchronized to this reference or master
   stream using existing inter-stream synchronization (such as lip-sync)
   solutions, i.e., using sender reports based on a common clock source.
   Basic guidelines for choosing the media stream for IDMS is to choose
   audio above video, as humans are most sensitive to degradation in
   audio synchronization.  When using multi-description or multi-view
   codecs, the IDMS control should be performed on the base layer.

   This SDP attribute 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*10DIGIT ; Decimal 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 4294967295
   (2^32-1) is reserved for future use.  For a description on the value
   of SyncGroupId to include, see Section 11.

   The following is an example of the SDP attribute for IDMS.

   a=rtcp-idms:sync-group=42




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11.  SDP Rules

11.1.  Offer/Answer Rules

   The SDP usage for IDMS follows the rules defined in [RFC4566] and
   Section 5 of [RFC3611] on SDP signaling with the exception of what is
   stated here.  The IDMS usage of RTCP is a loosely coupled
   collaborative attribute, in the sense that receivers send their
   status information and, in response, the MSAS asynchronously sends
   synchronization setting instructions.  The rtcp-idms attribute, thus,
   indicates the ability to send and receive indicated RTCP messages.
   This section defines how this SDP attribute should be used with
   regard to an offer/answer context.

   It is expected that, in most cases, the rtcp-idms attribute will be
   used in an offer/answer context where receivers will have
   predetermined, through some means outside the scope of this document,
   a SyncGroupId before the media session is set up.  However, A sender
   that assigns a SyncGroupId is also supported for cases, for example,
   where the MSAS contains group management functionality and is
   co-located with or otherwise communicates with the sender.  Thus,
   both senders and receivers can insert the attribute and the
   SyncGroupId.  Furthermore, the attribute is allowed to be inserted
   for more than one media stream, allowing an SC to become part of
   multiple synchronization groups simultaneously.  This effectively
   couples two (or more) synchronization groups to each other.  If the
   rtcp-idms attribute is inserted more than once for a particular media
   session, each SyncGroupId SHALL only be inserted once.

   In order to join an IDMS session, the receiver (the SC) inserts the
   rtcp-idms attribute as a media-level attribute in the SDP offer.
   This SDP offer can be an initial offer if the media session is
   starting as a synchronized session.  The SDP offer can also be an
   update to an existing media session, converting the session to an
   IDMS session.  If the receiver has a predetermined SyncGroupId value,
   it SHOULD use this value for setting the SyncGroupId parameter in the
   rtcp-idms attribute.  If the receiver does not know the SyncGroupId
   to be used, it MAY leave the SyncGroupId parameter empty by setting
   its value to 0.

   The sender SHALL include the rtcp-idms attribute in its answer.  If
   the value of the SyncGroupId parameter in the offer is not empty (not
   equal to 0), the sender SHOULD NOT change the SyncGroupId in its
   answer.  If the SyncGroupId is empty, the sender SHALL include the
   proper SyncGroupId in its answer.  If the sender receives an offer
   with the value of the SyncGroupId parameter set to 0, and cannot
   determine the proper SyncGroupId, it SHALL remove the attribute from
   its answer.



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   A sender receiving an SDP offer without the rtcp-idms attribute can
   also decide that IDMS is applicable to that media session.  In such a
   case, the sender MAY insert the rtcp-idms attribute, including a non-
   empty SyncGroupId, as part of its answer.

   A receiver receiving an rtcp-idms attribute as part of the SDP answer
   from a sender SHALL start sending RTCP XR IDMS reports (following all
   the normal RTCP rules for sending RTCP XR IDMS Report Blocks) and
   SHALL be ready to start receiving IDMS Settings.  As usual, if a
   receiver does not support the attribute (e.g., in case of an MSAS-
   inserted IDMS attribute), it SHALL ignore the attribute.

   Different updates are applicable to such an IDMS session.  Updates
   can be sent omitting the rtcp-idms attribute, thereby ending
   involvement in the synchronization session.  Updates can also be sent
   including the rtcp-idms attribute, but with a different SyncGroupId.
   This indicates a switch in the synchronization group.

11.2.  Declarative Cases

   In certain situations, there is no offer/answer context, but only a
   declarative modus.  In this case, the MSAS just inserts the rtcp-idms
   attribute and a valid SyncGroupId.  Any receiver receiving the rtcp-
   idms attribute in such a declarative case SHALL start sending RTCP XR
   IDMS Report Blocks and SHALL be ready to start receiving RTCP IDMS
   Settings Packets.

12.  Security Considerations

   The security considerations described in [RFC3611] apply to this
   document as well.

   The RTCP XR IDMS Report Block defined in this document is used to
   collect, summarize, and distribute information on packet reception
   and playout times of streaming media.  The information may be used to
   orchestrate the media playout at multiple devices.

   Errors in the information, either accidental or malicious, may lead
   to undesired behavior.  For example, if one device erroneously or
   maliciously reports a two-hour delayed playout, then another device
   in the same synchronization group could decide to delay its playout
   by two hours as well, in order to keep its playout synchronized.  A
   user would likely interpret this two-hour delay as a malfunctioning
   service.







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   Therefore, the application logic of both SCs and MSASs should check
   for out-of-bound information.  Differences in playout time exceeding
   configured limits (e.g., more than ten seconds) could be an
   indication of such out-of-bound information.

   Apart from checking for out-of-bound information in the endpoints, an
   IDMS implementation can reduce its vulnerability to attacks by
   including source authentication and message integrity measures,
   reducing the potential for man-in-the-middle attacks.  [RFC7201]
   provides an overview of the security options in RTP environments and
   includes a set of recommendations for message integrity and source
   authentication that are applicable to IDMS.  In addition to
   preventing man-in-the-middle attacks from inserting erroneous IDMS
   reports, the message confidentiality mechanisms outlined in [RFC7201]
   also prevent third parties from determining that two or more end
   hosts are receiving the same stream by looking at the Media Stream
   Correlation Identifier.

   Apart from attacking an IDMS session directly by sending incorrect
   IDMS reports, and with it introducing delays for all devices in a
   synchronization group, another potential vulnerability comes from the
   clock synchronization method used.  Should an attacker succeed in
   adjusting an SC's wallclock, that SC will report incorrect IDMS
   reports.  In order to prevent such clock synchronization attacks, it
   is recommended to use a secure time synchronization service.

13.  IANA Considerations

   This document defines a new RTCP packet type, the RTCP IDMS Packet
   (IDMS Settings), within the existing Internet Assigned Numbers
   Authority (IANA) registry of RTCP Control Packet Types.  This
   document also defines a new RTCP XR Block Type, the RTCP XR IDMS
   Report Block, within the existing IANA registry of RTCP Extended
   Reports (RTCP XR) Block Types.

   Further, this document defines a new SDP attribute "rtcp-idms" within
   the existing IANA registry of SDP Parameters, which is part of the
   "att-field (media level only)".  Finally, this document defines a new
   IANA registry subordinate to the IANA RTCP Extended Reports (RTCP XR)
   Block Type Registry: the IDMS XR Block SPST Registry.

13.1.  RTCP IDMS Packet Type

   This document assigns the packet type value 211 in the IANA 'RTCP
   Control Packet types (PT)' registry to the RTCP IDMS Packet (IDMS
   Settings).





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13.2.  RTCP XR IDMS Report Block

   This document updates the assignment of value 12 from the RTCP XR
   Block Type for reporting IDMS information as per [TS183063] to the
   RTCP XR IDMS Report Block defined in this document.

   The RTCP XR IDMS Report Block contains an extensible SPST value
   field; therefore, a new registry for this field is required.  This
   new registry is defined in Section 13.4.

13.3.  RTCP-IDMS SDP Attribute

   The SDP attribute "rtcp-idms" defined by this document is registered
   with the IANA registry of SDP Parameters as follows:

      SDP Attribute ("att-field"):

         Attribute name: rtcp-idms

         Long form: RTCP IDMS Parameters

         Type of name: att-field

         Type of attribute: media level

         Subject to charset: no

         Purpose: see Section 10 of this document

         Reference: this document

         Values: see this document

13.4.  IDMS XR Block SPST Registry

   This document defines a new IANA registry subordinate to the IANA
   RTCP Extended Reports (RTCP XR) Block Type Registry: the IDMS XR
   Block SPST Registry.

   Initial values for the IDMS XR Block SPST Registry are given below;
   future assignments are to be made through the Specification Required
   policy [RFC5226].  The registry is limited to 16 entries (numbered
   0-15), with 0 being Reserved.  Values 5-15 are available for
   assignment.

   In accordance with [RFC5226], a Designated Expert will review any
   applications made to IANA for the registry.  Primary criteria for the
   Designated Expert to use when reviewing new applications are clarity



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   of the specification and, due to the relatively small value range of
   SPST values available, potential overlap in functionality with
   existing SPST registrations.

   Value           Name                         Reference
   -----           ----                         ---------
   1               Synchronization Client       This document, Section 7
   2               MSAS                         [TS183063]
   3               SC Prime Input               [TS183063]
   4               SC Prime Output              [TS183063]

13.5.  Contact Information for Registrations

   The contact information for the registrations is:

   Ray van Brandenburg (ray.vanbrandenburg@tno.nl)
   Brassersplein 2
   2612CT, Delft, The Netherlands

14.  Contributors

   The following people have provided substantial contributions to this
   document: Omar Niamut, Fabian Walraven, Ishan Vaishnavi, and Rufael
   Mekuria.  In addition, the authors would like to thank Aidan
   Williams, Colin Perkins, Magnus Westerlund, Roni Even, Peter
   Musgrave, Ali Begen, Qin Wu, and Rob Koenen for their review comments
   and contributions to the text.
























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15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
              Protocol Extended Reports (RTCP XR)", RFC 3611, November
              2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC7273]  Williams, A., Gross, K., van Brandenburg, R., and H.
              Stokking, "RTP Clock Source Signalling", RFC 7273, June
              2014.

15.2.  Informative References

   [Boronat2009]
              Boronat, F., Lloret, J., and M. Garcia, "Multimedia group
              and inter-stream synchronization techniques: a comparative
              study", Elsevier Information Systems 34, Pages 108-131,
              March 2009,
              <http://www.sciencedirect.com/science/article/pii/
              S0306437908000525>.





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   [IEEE1588-2008]
              IEEE, "1588-2008 - IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", July 2008,
              <http://standards.ieee.org/findstds/
              standard/1588-2008.html>.

   [Ishibashi2006]
              Ishibashi, Y., Nagasaka, M., and N. Fujiyoshi, "Subjective
              assessment of fairness among users in multipoint
              communications", Proceedings of the 2006 ACM SIGCHI
              international conference on Advances in computer
              entertainment technology, Article No. 69, June 2006,
              <http://dl.acm.org/citation.cfm?id=1178905>.

   [RFC7164]  Gross, K. and R. Brandenburg, "RTP and Leap Seconds", RFC
              7164, March 2014.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, April 2014.

   [TS183063] ETSI, "Telecommunications and Internet converged Services
              and Protocols for Advanced Networking (TISPAN); IMS-based
              IPTV stage 3 specification", TS 183 063 v3.5.2, March
              2011.


























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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 (UPV)
   Valencia  46730
   Spain
   Phone: +34 962 849 341
   EMail: fboronat@dcom.upv.es

   Mario Montagud
   Universitat Politecnica de Valencia (UPV)
   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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