Audio/Video Transport Working Group H.M. Stokking Internet Draft M.O. van Deventer Intended status: Informational O.A. Niamut Expires: April 2011 F.A. Walraven R. van Brandenburg TNO Netherlands I. Vaishnavi CWI Netherlands October 11, 2010 RTCP XR Block Type for inter-destination media synchronization draft-brandenburg-avt-rtcp-for-idms-02.txt 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on April 11, 2011. Copyright Notice Copyright (c) 2010 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. Brandenburg Expires April 11, 2011 [Page 1] Internet-Draft RTCP for IDMS October 2010 Abstract This document specifies an RTCP XR Block Type and associated SDP parameter for inter-destination media synchronization (IDMS). The RTCP Block Type is used to collect media play-out information from participants in a group watching a specific RTP media stream and to distribute a summary of the collected information so that the participants can synchronize play-out. Typical applications for inter-destination media synchronization are social TV, watching apart together and shared service control (i.e. applications where two or more geographically separated users are watching a media stream together). Table of Contents 1. Introduction.................................................2 1.1. Inter-destination Media Synchronization.................2 1.2. Applicability of RTCP to IDMS...........................3 1.3. Applicability of SDP to IDMS............................3 1.4. This document and ETSI TISPAN...........................4 2. Inter-destination media synchronization use cases............4 2.1. Watching Apart Together.................................4 2.2. Shared Service Control..................................4 3. Architecture for inter-destination media synchronization.....4 3.1. Media Synchronization Application Server (MSAS).........5 3.2. Synchronization Client (SC).............................5 3.3. Mixer...................................................5 4. RTCP XR Block Type for IDMS..................................5 5. SDP parameter for IDMS.......................................8 6. Security Considerations......................................9 7. IANA Considerations..........................................9 8. Conclusions..................................................9 9. References..................................................10 9.1. Normative References...................................10 9.2. Informative References.................................10 10. Acknowledgments............................................10 1. Introduction 1.1. Inter-destination Media Synchronization Inter-destination media synchronization (IDMS) refers to the play-out of media streams at two or more geographically distributed locations in a temporally synchronized manner. It can be applied to both unicast and multicast media streams and can be applied to any type and/or combination of streaming media, such as audio, video and text Brandenburg Expires April 11, 2011 [Page 2] Internet-Draft RTCP for IDMS October 2010 (subtitles). [Boronat2009] provides an overview of technologies and algorithms for IDMS. IDMS requires the exchange of information on media receipt and playout times. It may also require signaling for the initiation and maintenance of IDMS sessions and groups. The presented RTCP-XR specification for IDMS is independent of the used synchronization algorithm, which is out-of-scope of this document. 1.2. Applicability of RTCP to IDMS RTP and RTCP [RFC3550] are protocols that are typically used in conjunction. RTP (Real-time Transport Protocol) provides end-to-end network transport functions suitable for applications requiring real- time data transport, such as audio, video or data, over multicast or unicast network services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner that is scalable to large multicast networks, and to provide minimal control and identification functionality. RTP receivers and - senders provide reception quality feedback by sending out RTCP Receiver Report (RR) and Sender Report (SR) packets [RFC3550], which may be augmented by eXtended Reports (XR) [RFC3611]. IDMS involves the collection, summarizing and distribution of RTP packet arrival and play-out times. RTCP is applicable to IDMS, as information on RTP packet arrival times and play-out times can be considered reception quality feedback information. 1.3. Applicability of SDP to IDMS RTCP XR [RFC3611] defines the Extended Report (XR) packet type for the RTP Control Protocol (RTCP), and defines how the use of XR packets can be signaled by an application using the Session Description Protocol (SDP). SDP signaling is used to set up and maintain a synchronization group between Synchronization Clients (SCs). Brandenburg Expires April 11, 2011 [Page 3] Internet-Draft RTCP for IDMS October 2010 1.4. This document and ETSI TISPAN ETSI TISPAN [TS 183 063] has specified architecture and protocol for IDMS using RTCP XR exchange and optional SDP signaling. This internet draft is an excerpt of the IDMS part of that specification. 2. Inter-destination media synchronization use cases Social TV is the combination of media content consumption by two or more users at different devices and locations and real-time communication between those users. 2.1. Watching Apart Together Watching Apart Together is an example of Social TV, where two or more users watch the same television broadcast at different devices and locations, while communicating with each other using text, audio and/or video. A skew in the media play-out of the two or more users can have adverse effects on their experience. A well-known use case here is one friend experiencing a goal in a football match well before or after other friend(s).Thus IDMS is required to provide play-out synchronization. 2.2. Shared Service Control Shared Service Control is an example of Social TV, where two or more users experience some content-on-demand together, while sharing the trick-play controls (play, pause, fast forward, rewind) of the content on demand. Similar to the previous use case, without IDMS, differences in play- out speed and the effect of transit delay of trick-play control signals would desynchronize content play-out. 3. Architecture for inter-destination media synchronization The architecture for IDMS, which is based around a sync-maestro architecture [Boronat2009], is sketched below. The SC and MSAS entities are shown as additional functionality for the RTP receiver and sender respectively. It should be noted that a master-slave type of architecture is also supported by having one of the SC devices also act as an MSAS. Brandenburg Expires April 11, 2011 [Page 4] Internet-Draft RTCP for IDMS October 2010 +-----------------------+ +-----------------------+ | | | | | Receiver | | Sender | | | SR+XR | | | +-----------------+ | <----- | +-----------------+ | | | | | | | | | | | Synchronization | | | | Media | | | | Client | | | | Synchronization | | | | (SC) | | | | Application | | | | or | | | | Server | | | | Mixer | | RR+XR | | (MSAS) | | | | | | -----> | | | | | +-----------------+ | | +-----------------+ | | | | | +-----------------------+ +-----------------------+ 3.1. Media Synchronization Application Server (MSAS) An MSAS collects RTP packet arrival times and play-out times from one or more SC(s) in a synchronization group. The MSAS summarizes and distributes this information to the SCs in the synchronization group, e.g. by determining the SC with the most lagged play-out and using its reported RTP packet arrival time and play-out time as a summary. 3.2. Synchronization Client (SC) An SC reports RTP packet arrival times and play-out times of a media stream. It can receive summaries of such information, and use that to adjust its play-out buffer. 3.3. Mixer A Mixer re-originates an RTP media stream. Therefore the SSRC and RTP time stamps of an outgoing RTP media stream of a mixer are unrelated to the SSRC and RTP time stamps of the incoming RTP stream. A Mixer can report the correlation between SSCRs and RTP time stamps of incoming and outgoing RTP media streams. 4. RTCP XR Block Type for IDMS This section specifies the RTCP XR Block Type for reporting inter- destination media synchronization information on an RTP media stream. Its definition is based on [RFC5576]. The RTCP XR is used to report information on receipt times and presentation times of RTP packets to e.g. a Sender [RFC3611], a Feedback Target [RFC3550] or a Third Party Monitor [RFC3611]. The RTCP XR is also used to indicate synchronization settings instructions to a receiver of the RTP media stream. Brandenburg Expires April 11, 2011 [Page 5] Internet-Draft RTCP for IDMS October 2010 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| Resrv | PT=XR=207 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of packet sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | BT=12 | SPST |Resrv|P| block length=7 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PT | Resrv | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Media Stream Correlation Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of media source | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Received NTP timestamp, most significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Received NTP timestamp, least significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Received RTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Presented NTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The first 64 bits form the header of the RTCP XR, as defined in [RFC3611]. The SSRC of packet sender identifies the sender of the specific RTCP packet. Block Type (BT): 8 bits. It identifies the block format. Its value shall be set to 12. Synchronization Packet Sender Type (SPST): 4 bits. This field indentifies the role of the packet sender for this specific eXtended Report. It can have the following values: SPST=0 Reserved For future use. SPST=1 The packet sender is an SC. It uses this XR to report synchronization status information. Timestamps relate to the SC input. SPST=2 The packet sender is an MSAS. It uses this XR to report synchronization settings instructions. Timestamps relate to the input of a virtual SC, which acts as reference to which the SCs belonging to this session are synchronized. Brandenburg Expires April 11, 2011 [Page 6] Internet-Draft RTCP for IDMS October 2010 SPST=3 The packet sender is a Mixer [RFC3550]. It uses this XR to report synchronization correlation information related to its incoming media stream. Timestamps relate to its input. SPST=4 The packet sender is a Mixer [RFC3550]. It uses this XR to report synchronization correlation information related to a specific outgoing media stream of SC'. Timestamps relate to its input. (see Note below) SPST=5-15 Reserved For future use. NOTE: Following the RTP/RTCP specification [RFC3611], RTP timestamps relate to the arrival time of the first octet of an RTP packet. In case of SPST=4 (Mixer output), there is not such an arrival time as the media stream is re-originated at the Mixer. In this case, the timestamp would relate to the arrival time of the equivalent octet (representing e.g. the same video pixel or audio sample) of the incoming media steam. Reserved bits (Resrv): 3 bits. These bits are reserved for future use and shall be set to 0. Packet Presented NTP timestamp flag (P): 1 bit. Bit set to 1 if the Packet Presented NTP timestamp contains a value, 0 if it is empty. If this flag is set to zero, then the Packet Presented NTP timestamp shall not be inspected. Block Length: 16 bits. This shall be set to 7, as this RTCP Block Type has a fixed length. Payload Type (PT): 7 bits. This field identifies the format of the media payload, according to [RFC3551]. The media payload is associated with an RTP timestamp clock rate. This clock rate provides the time base for the RTP timestamp counter. Reserved bits (Resrv): 25 bits. These bits are reserved for future use and shall be set to 0. Media Stream Correlation Identifier: 32 bits. This identifier is used to correlate synchronized media streams. The value 0 (all bits are set "0") indicates that this field is empty. The value 2^32-1 (all bits are set "1") is reserved for future use. If the RTCP Packet Sender is an SC or an MSAS (SPST=1 or SPST=2), then the Media Stream Correlation Identifier maps on the SyncGroupId. If the RTCP Packet Sender is an Mixer (SPST=3 or SPST=4), related incoming and outgoing media streams have the same Media Stream Correlation Identifier. Brandenburg Expires April 11, 2011 [Page 7] Internet-Draft RTCP for IDMS October 2010 SSRC: 32 bits. The SSRC of the media source shall be set to the value of the SSRC identifier carried in the RTP header [RFC3550] of the RTP packet to which the XR relates. Packet Received NTP timestamp: 64 bits. This NTP timestamp [RFC5905] is the arrival wall clock time of the first octet of the RTP packet to which the XR relates. For SPST=2 it relates to a virtual SC to which the other SCs in the synchronization group may synchronize. For SPST=4 the SC' should calculate backwards when the content (video frame, audio sample) associated with the first octet of the RTP packet arrived. SCs shall be time-synchronized using e.g. NTP. Packet Received RTP timestamp: 32 bits. This timestamp has the value of the RTP time stamp carried in the RTP header [RFC3550] of the RTP packet to which the XR relates. Packet Presented NTP timestamp: 32 bits. This timestamp reflects the NTP time when the data contained in the first octet of the associated RTP packet is presented to the user. It consists of the least significant 16 bits of the NTP seconds part and the most significant 16 bits of the NTP fractional second part. If this field is empty, then it shall be set to 0 and the Packet Presented NTP timestamp flag (P) shall be set to 0. It shall be empty for SPST=3 and SPST=4. 5. SDP parameter for IDMS The SDP parameter sync-group is used to signal the use of the RTCP XR block for inter-destination media synchronization. This SDP parameter extends rtcp-xr-attrib as follows, using Augmented Backus-Naur Form [RFC5234]. rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF ; Original definition from [RFC3611], section 5.1 xr-format =/ grp-sync ; Extending xr-format for inter-destination media synchronization grp-sync = "grp-sync" [",sync-group=" SyncGroupId] SyncGroupId = 1*DIGIT ; Numerical value from 0 till 4294967295 DIGIT = %x30-39 SyncGroupId is a 32-bit unsigned integer in network byte order and represented in decimal. SyncGroupId identifies a group of SCs for inter-destination media synchronization. It maps on the Media Stream Correlation Identifier of Annex W.1 for SPST=1 and SPST=2. The value Brandenburg Expires April 11, 2011 [Page 8] Internet-Draft RTCP for IDMS October 2010 SyncGroupId=0 represents an empty SyncGroupId. The value 4294967295 (2^32-1) is reserved for future use. The following is an example of the SDP attribute for inter- destination media synchronization. a=rtcp-xr:grp-sync,sync-group=42 6. Security Considerations The specified RTCP XR Block Type in this document is used to collect, summarize and distribute information on packet-receipt times and play-out times of streaming media. The information may be used to orchestrate the media play-out at multiple devices. Errors in the information, either accidental or malicious, may lead to undesired behavior. For example, if one device erroneously reports a two-hour delayed play-out, then another device in the same synchronization group could decide to delay its play-out by two hours as well, in order to keep its play-out synchronized. A user would likely interpret this two hour delay as a malfunctioning service. Therefore, the application logic of both Synchronization Clients and Media Synchronization Application Servers should check for inconsistent information. Differences in play-out time of more than e.g. ten seconds could be an indication of such inconsistent information. 7. IANA Considerations New block types for RTCP XR are subject to IANA registration. For general guidelines on IANA considerations for RTCP XR, refer to [RFC3611]. [TS 183 063] assigns the block type value 12 in the RTCP XR Block Type Registry to "Inter-destination Media Synchronization Block". [TS 183 063] also registers the SDP [RFC4566] parameter "grp-sync" for the "rtcp-xr" attribute in the RTCP XR SDP Parameters Registry. 8. Conclusions This document specifies the RTCP XR and the associated SDP parameter for inter-destination media synchronization. Brandenburg Expires April 11, 2011 [Page 9] Internet-Draft RTCP for IDMS October 2010 9. References 9.1. Normative References [RFC5234] Crocker, D. and Overell, P., "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, January 2008. [RFC3550] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [RFC3551] Schulzrinne, H. and Casner S., "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003 [RFC3611] Friedman, T. "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [RFC4566] Handley, M., "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5576] Lennox, J., "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, June 2009. [RFC5905] Mills, D., "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [TS 183 063] ETSI TISPAN, "IMS-based IPTV stage 3 specification", TS 183 063 v3.4.1, June 2010. 9.2. Informative References [Boronat2009] Boronat, F., et al, "Multimedia group and inter-stream synchronization techniques: A comparative study", Elsevier Information Systems 34 (2009), pp. 108-131 10. Acknowledgments The authors thank TNO and KPN for each partially funding the work behind this internet draft. Brandenburg Expires April 11, 2011 [Page 10] Internet-Draft RTCP for IDMS October 2010 Authors' Addresses Hans M. Stokking TNO Brassersplein 2, Delft, the Netherlands Phone: +31 15 28 57078 Email: hans.stokking@tno.nl M. Oskar van Deventer TNO Brassersplein 2, Delft, the Netherlands Phone: +31 15 28 57078 Email: oskar.vandeventer@tno.nl Omar A. Niamut TNO Brassersplein 2, Delft, the Netherlands Phone: +31 15 28 57078 Email: omar.niamut@tno.nl Fabian A. Walraven TNO Brassersplein 2, Delft, the Netherlands Phone: +31 15 28 57078 Email: fabian.walraven@tno.nl Ray van Brandenburg TNO Brassersplein 2, Delft, the Netherlands Phone: +31 15 28 57078 Email: ray.vanbrandenburg@tno.nl Ishan Vaishnavi CWI Science Park 123, Amsterdam, the Netherlands Phone: +31 20 592 4323 Email: i.vaishnavi@cwi.nl Brandenburg Expires April 11, 2011 [Page 11]