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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

AVTCore                                               R. van Brandenburg
Internet-Draft                                               H. Stokking
Intended status: Standards Track                         O. van Deventer
Expires: March 02, 2014                                              TNO
                                                              F. Boronat
                                                             M. Montagud
                                     Universitat Politecnica de Valencia
                                                                K. Gross
                                                            AVA Networks
                                                         August 29, 2013


 Inter-destination Media Synchronization using the RTP Control Protocol
                                 (RTCP)
                       draft-ietf-avtcore-idms-13

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 geographically distributed
   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 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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Copyright Notice

   Copyright (c) 2013 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
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Applicability of RTCP to IDMS . . . . . . . . . . . . . .   3
     2.2.  IDMS and ETSI . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Inter-Destination Media Synchronization (IDMS) use cases  . .   4
   5.  Overview of IDMS operation  . . . . . . . . . . . . . . . . .   5
   6.  Architecture for Inter-Destination Media Synchronization  . .   7
     6.1.  Media Synchronization Application Server (MSAS) . . . . .   7
     6.2.  Synchronization Client (SC) . . . . . . . . . . . . . . .   8
     6.3.  Communication between MSAS and SCs  . . . . . . . . . . .   8
   7.  RTCP XR Block for IDMS (IDMS Report Block)  . . . . . . . . .   8
   8.  RTCP Packet Type for IDMS (IDMS Settings) . . . . . . . . . .  11
   9.  Timing and NTP Considerations . . . . . . . . . . . . . . . .  13
   10. On the use of presentation timestamps . . . . . . . . . . . .  14
   11. SDP Signalling for RTCP IDMS Packet Type  . . . . . . . . . .  15
   12. SDP rules . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Offer/Answer rules . . . . . . . . . . . . . . . . . . .  16
     12.2.  Declarative cases  . . . . . . . . . . . . . . . . . . .  17
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     14.1.  RTCP IDMS Packet Type  . . . . . . . . . . . . . . . . .  18
     14.2.  RTCP XR IDMS Report Block  . . . . . . . . . . . . . . .  19
     14.3.  RTCP-IDMS SDP Attribute  . . . . . . . . . . . . . . . .  19
     14.4.  IDMS XR Block SPST Registry  . . . . . . . . . . . . . .  19
     14.5.  Contact Information for Registrations  . . . . . . . . .  20
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  20



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     16.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Inter-Destination Media Synchronization (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.

   IDMS requires the exchange of information on media receipt and
   playout 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.

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, the
   original order of packets and for 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
   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 Reports (XR) [RFC3611].  Both RTP and RTCP are intended to
   be tailored to 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 playout times.  As information on RTP packet



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   arrival times and playout 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.

2.2.  IDMS and ETSI

   A first version of IDMS for use with RTP/RTCP was standardized by
   ETSI TISPAN in [TS183063], resulting in an IANA registration for an
   RTCP IDMS XR Block.  At some point in time, this work was 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 (>3.5.2).
   In accordance, this document proposes to update the IANA registration
   for the RTCP IDMS XR Block to point to this document.  Finally, this
   document proposes an IANA registry for SPST values, allowing the
   registration of extensions to this document.

3.  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].

4.  Inter-Destination Media Synchronization (IDMS) 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 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 other friend(s).




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

   A third usage scenario is that of the networked loudspeakers, in
   which two or more speakers are connected to the network individually.
   Such situations can for example be found in large conference rooms,
   legislative chambers, classrooms (especially those supporting
   distance learning) and other large-scale environments such as
   stadiums.  Since humans are more susceptible to differences in audio
   delay, this use case needs even more accuracy than the video wall use
   case.  Depending on the exact application, the need for accuracy can
   then be in the range of microseconds.

5.  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 Set-up   |
               |                       |<========================>|
               |                       |                          |
               |                 Call set-up                      |
               |<================================================>|
               |                       |                          |
               |       RTP Packets     |        RTP Packets       |
               |<----------------------|------------------------->|
               |  RR + XR IDMS Report  |                          |
               |---------------------->|    RR + XR IDMS Report   |
               |                       |<-------------------------|
               |   RTCP IDMS Settings  |    RTCP IDMS Settings    |
               |<----------------------|------------------------->|



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               |                       |                          |

                  Figure 1: Example of a typical IDMS session


   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 RTCP XR IDMS Report Block to
   the Sync Server function of the media server.  Included in the XR
   IDMS 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 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.



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   For this functionality to work correctly, it is necessary that the
   wall clocks of the receivers are synchronized with each other.  Alice
   and Bob both report when they receive, and optionally when they play
   out, certain RTP packets.  In order to correlate their reports to
   each other, it is necessary that their wallclocks are synchronized.

6.  Architecture for Inter-Destination Media Synchronization

   The architecture for IDMS, which is based on a sync-maestro
   architecture [Boronat2009], is sketched 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)     |  |
   |  |                 |  | -----> |  |                 |  |
   |  +-----------------+  |        |  +-----------------+  |
   |                       |        |                       |
   +-----------------------+        +-----------------------+

                         IDMS Architecture Diagram

6.1.  Media Synchronization Application Server (MSAS)

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

   It should be noted that while the figure 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



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   apply, so that it is able to correlate the IDMS XR Reports coming
   from different SCs.

6.2.  Synchronization Client (SC)

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

6.3.  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 playout
   information of a particular client, an RTCP XR IDMS Report Block is
   used (see Section 7).  For the MSAS->SC message containing the
   synchronization settings instructions, a new RTCP IDMS Settings
   Packet Type is defined (see Section 8).

7.  RTCP XR Block for IDMS (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 receipt
   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 receipt 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 on 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 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 maintained by IANA (see
   Section 14.4):

      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 zero, 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 Block Type
   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-part 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.

   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
   wall clock 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 9
   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 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



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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 with
   'lowest sequence number' here is meant 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
   wall clock 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.

8.  RTCP Packet Type for IDMS (IDMS Settings)

   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].

       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     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      |      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:

   PT: To be determined upon registration with IANA, inserted by the RFC
   Editor upon registration with 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 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
   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 playout timing in order to achieve synchronization, e.g.
   to set the required playout delay.  The timestamp is formatted based
   on the NTP timestamp format as specified in [RFC5905].  See Section 9
   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.








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   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 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 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 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 playout to have a 32bit
   Presented Timestamp field.

9.  Timing and NTP Considerations

   To achieve IDMS, the different receivers involved need synchronized
   (wall) clocks as a common timeline for synchronization.  This
   synchronized clock is used for reporting the Packet Received NTP
   Timestamps and the Packet Presented NTP Timestamp, and for
   interpretation of these fields in received IDMS reports.  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 quality in some use cases, a high accuracy will be needed.



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   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 [IEEE-1588].

   [I-D.draft-ietf-avtcore-clksrc] defines a set of SDP parameters for
   signaling the clock synchronization source or sources available to
   and used by the individual receivers.  SCs MAY use this draft 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, 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-ietf-leap-seconds] 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 [I-D.draft-ietf-leap-seconds] into account
   when implementing IDMS.

10.  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
   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, 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
   playout times, or packet presentation times, this may be more



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   accurate.  It is up to applications and implementations of this RTCP
   extension whether to implement and use this.

11.  SDP Signalling for RTCP IDMS Packet Type

   The SDP attribute rtcp-idms is used to signal the use of the RTCP
   IDMS Packet Type for IDMS and the associated RTCP XR Block for IDMS.
   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 (i.e. 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 more sensitive to degradation in
   audio quality than in video quality.  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 12.

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

   a=rtcp-idms:sync-group=42

12.  SDP rules









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12.1.  Offer/Answer rules

   The SDP usage for IDMS follows the rules defined in [RFC4566] and
   section 5 of [RFC3611] on SDP signalling, 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 pre-
   determined, through some means outside the scope of this document, a
   SyncGroupId before the media session is setup.  However, it is also
   supported that the MSAS assigns such a SyncGroupId, for example if
   the MSAS contains group management functionality.  Thus, both the
   MSAS and the SCs can insert the attribute and the SyncGroupId.
   Furthermore, it is allowed to insert the attribute 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 pre-determined 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 MSAS SHALL include the rtcp-idms attribute in its answer.  If the
   value of the SyncGroupId parameter in the offer was not empty (not
   equal to 0), the MSAS SHOULD NOT change the SyncGroupId in its
   answer.  If the SyncGroupId was empty, the MSAS SHALL include the
   proper SyncGroupId in its answer.  If the MSAS 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.

   An MSAS receiving an SDP offer without the rtcp-idms attribute can
   also decide that IDMS is applicable to that media session.  In such a



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   case, the MSAS 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 an MSAS, SHALL start sending IDMS XR reports (following all the
   normal RTCP rules for sending RTCP XR 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 ommitting the rtcp-idms attribute, thereby ending the
   (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 synchronization group.
   Updates can also be sent including another rtcp-idms attribute,
   indicating a membership of another synchronization group, effectively
   merging the current group(s) with the new one.

12.2.  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 IDMS XR
   Report Blocks and SHALL be ready to start receiving RTCP IDMS
   Settings packets.

13.  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.

   Therefore, the application logic of both SCs and MSASs should check
   for out-of-bound information.  Differences in playout time exceeding



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   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 end-points,
   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.
   [I-D.draft-ietf-avtcore-rtp-security-options] provides an overview of
   the security options in RTP environments, and includes a set of
   recommendations for message integrity and source authentication which
   are applicable to IDMS.  In addition to preventing man-in-the-middle
   attacks from inserting erroneous IDMS Reports, the message
   confidentiality mechanisms outlined in
   [I-D.draft-ietf-avtcore-rtp-security-options] 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
   used clock synchronization method.  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.

14.  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 IDMS XR 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, 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.

14.1.  RTCP IDMS Packet Type

   This document assigns the packet type value TBD in the IANA 'RTCP
   Control Packet types (PT) Registry' to the RTCP IDMS Packet Type.

   [Note to RFC Editor: please replace TBD with the IANA-provided RTCP
   Packet Type value for this packet type, both in this section as well
   as in the figure and text in Section 8]



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14.2.  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.

   [Note to RFC Editor: this block type value is currently assigned to
   [TS183063].  This document replaces [TS183063] as the normative
   specification of the RTCP XR IDMS Report Block.  Upon publication of
   this document as RFC, [TS183063] will be changed to reflect this.

   The RTCP XR IDMS Report Block contains an extensible SPST value field
   and therefore a new registry for this field is required.  This new
   registry is defined in Section 14.4.

14.3.  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 11 of this document

            Reference: this document

            Values: see this document

14.4.  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



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

   Value           Name                         Reference
   -----           ----                         ---------
   1               Synchronization Client       section 7
   2               MSAS                         [TS183063]
   3               SC Prime Input               [TS183063]
   4               SC Prime Output              [TS183063]




14.5.  Contact Information for Registrations

   The contact information for the registrations is:

   Ray van Brandenburg (ray.vanbrandenburg@tno.nl)

   Brassersplein 2

   2612CT, Delft, The Netherlands

15.  Contributors

   The following people have participated as co-authors or 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.

16.  References

16.1.  Normative References

   [I-D.draft-ietf-avtcore-clksrc]
              Williams, A., Gross, K., van Brandenburg, R., and H.
              Stokking, "RTP Clock Source Signalling, draft-ietf-
              avtcore-clksrc-05", May 2013.



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   [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.

   [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.

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

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications, RFC5234", January 2008.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specifications, RFC5905", February 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", 2009.

   [I-D.draft-ietf-avtcore-rtp-security-options]
              Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", July 2013.

   [I-D.draft-ietf-leap-seconds]
              Gross, K. and R. Brandenburg, van, "RTP and Leap Seconds,
              draft-ietf-avtcore-leap-second-02", October 2012.

   [IEEE-1588]
              , "1588-2008 - IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", 2008.



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   [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", .

   [RFC3711]  Baughner, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Normann, "The Secure Real-time Transport Protocol (SRTP)",
              March 2004.

   [RFC5868]  Ott, J. and C. Perkins, "Guidelines for Extending the RTP
              Control Protocol (RTCP), RFC5968", September 2010.

   [TS183063]
              , "IMS-based IPTV stage 3 specification, TS 183 063
              v3.5.2", March 2011.

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



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   Fernando Boronat
   Universitat Politecnica de Valencia
   Universitat Politecnica de Valencia (UPV)
   Valencia  46730
   Spain

   Phone: +34 962 849 341
   Email: fboronat@dcom.upv.es


   Mario Montagud
   Universitat Politecnica de Valencia
   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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