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Versions: (draft-wenger-avtext-avpf-ccm-layered) 00 01 02 03 04 RFC 8082

Network Working Group                                          S. Wenger
Internet-Draft                                                 J. Lennox
Updates: 5104 (if approved)                                  Vidyo, Inc.
Intended status: Standards Track                               B. Burman
Expires: November 18, 2016                                 M. Westerlund
                                                                Ericsson
                                                            May 17, 2016


   Using Codec Control Messages in the RTP Audio-Visual Profile with
                      Feedback with Layered Codecs
                 draft-ietf-avtext-avpf-ccm-layered-01

Abstract

   This document fixes a shortcoming in the specification language of
   the Codec Control Message Full Intra Request (FIR) as defined in
   RFC5104 when using with layered codecs.  In particular, a Decoder
   Refresh Point needs to be sent by a media sender when a FIR is
   received on any layer of the layered bitstream, regardless on whether
   those layers are being sent in a single or in multiple RTP flows.
   The other payload-specific feedback messages defined in RFC 5104 and
   RFC 4585 as updated by RFC 5506 have also been analyzed, and no
   corresponding shortcomings have been found.

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."

   This Internet-Draft will expire on November 18, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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 and Problem Statement  . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Updated definition of Decoder Refresh Point . . . . . . . . .   4
   4.  Full Intra Request for Layered Codecs . . . . . . . . . . . .   4
   5.  Identifying the use of Layered Codecs (Informative) . . . . .   5
   6.  Layered Codecs and non-FIR codec control messages
       (Informative) . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Picture Loss Indication (PLI) . . . . . . . . . . . . . .   6
     6.2.  Slice Loss Indication (SLI) . . . . . . . . . . . . . . .   6
     6.3.  Reference Picture Selection Indication (RPSI) . . . . . .   6
     6.4.  Temporal-Spatial Trade-off Request and Notification
           (TSTR/TSTN) . . . . . . . . . . . . . . . . . . . . . . .   7
     6.5.  H.271 Video Back Channel Message (VBCM) . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction and Problem Statement

   RFC 4585 [RFC4585] and RFC 5104 [RFC5104] specify a number of
   payload-specific feedback messages which a media receiver can use to
   inform a media sender of certain conditions, or make certain
   requests.  The feedback messages are being sent as RTCP receiver
   reports, and RFC 4585 specifies timing rules that make the use of
   those messages practical for time-sensitive codec control.

   Since the time those RFCs were developed, layered codecs have gained
   in popularity and deployment.  Layered codecs use multiple sub-
   bitstreams called layers to represent the content in different
   fidelities.  Depending on the media codec and its RTP payload format
   in use, single layers or groups of layers may be sent in their own



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   RTP streams (in MRST or MRMT mode as defined in RFC 7656 [RFC7656]),
   or multiplexed (using media-codec specific multiplexing mechanisms)
   in a single RTP stream (SRST mode as defined in RFC 7656 [RFC7656]).
   The dependency relationship between layers forms a directed graph,
   with the base layer at the root.  Enhancement layers depend on the
   base layer and potentially on other enhancement layers, and the
   target layer and all layers it depends on have to be decoded jointly
   in order to re-create the uncompressed media signal at the fidelity
   of the target layer.

   Implementation experience has shown that the Full Intra Request
   command as defined in RFC 5104 [RFC5104] is underspecified when used
   with layered codecs and when more than one RTP stream is used to
   transport the layers of a layered bitstream at a given fidelity.  In
   particular, from the RFC 5104 [RFC5104] specification language it is
   not clear whether an FIR received for only a single RTP stream of
   multiple RTP streams covering the same layered bitstream necessarily
   triggers the sending of a Decoder Refresh Point (as defined in
   RFC 5104 [RFC5104] section 2.2) for all layers, or only for the layer
   which is transported in the RTP stream which the FIR request is
   associated with.

   This document fixes this shortcoming by:

   a.  Updating the definition of the Decoder Refresh Point (as defined
       in RFC 5104 [RFC5104] section 2.2) to cover layered codecs, in
       line with the corresponding definitions used in a popular layered
       codec format, namely H.264/SVC [H.264].  Specifically, a decoder
       refresh point, in conjunction with layered codecs, resets the
       state of the whole decoder, which implies that it includes hard
       or gradual single-layer decoder refresh for all layers;

   b.  Requiring that, when a media sender receives a Full Intra Request
       over the RTCP stream associated with any of the RTP streams over
       which a part of the layered bitstream is transported, to send a
       Decoder Refresh Point;

   c.  Require that a media receiver sends the FIR on the RTCP stream
       associated with the base layer (the option of receiving FIR on
       enhancement layer-associated RTCP stream as specified in point b)
       above is kept for backward compatibility); and

   d.  Providing guidance on how to detect that a layered codec is in
       use for which the above rules apply.

   While, clearly, the reaction to FIR for layered codecs in RFC 5104
   [RFC5104] and companion documents is underspecified, it appears that
   this is not the case for any of the other payload-specific codec



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   control messages defined in any of RFC 4585 [RFC4585], RFC 5104
   [RFC5104], or RFC 5506 [RFC5506].  A brief summary of the analysis
   that led to this conclusion is also included in this document.

2.  Requirements Language

   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].

3.  Updated definition of Decoder Refresh Point

   The text below updates the definition of Decoder Refresh Point in
   section 2.2 of RFC 5104 [RFC5104].

   Decoder Refresh Point: A bit string, packetized in one or more RTP
   packets, that completely resets the decoder to a known state.

   Examples for "hard" single layer decoder refresh points are Intra
   pictures in H.261 [H.261], H.263 [H.263], MPEG-1 [MPEG-1], MPEG-2
   [MPEG-2], and MPEG-4 [MPEG-4]; Instantaneous Decoder Refresh (IDR)
   pictures in H.264 [H.264], and H.265 [H.265]; and Keyframes in VP8
   [RFC6386] and VP9 [I-D.grange-vp9-bitstream].  "Gradual" decoder
   refresh points may also be used; see for example H.264 [H.264].
   While both "hard" and "gradual" decoder refresh points are acceptable
   in the scope of this specification, in most cases the user experience
   will benefit from using a "hard" decoder refresh point.

   A decoder refresh point also contains all header information above
   the syntactical level of the picture layer (or equivalent, depending
   on the video compression standard) that is conveyed in-band.  In
   H.264 [H.264], for example, a decoder refresh point contains
   parameter set Network Adaptation Layer (NAL) units that generate
   parameter sets necessary for the decoding of the following slice/data
   partition NAL units (and that are not conveyed out of band).

   When a layered codec is in use, the above definition (and, in
   particular, the requirement to COMPLETELY reset the decoder to a
   known state) implies that the decoder refresh point includes hard or
   gradual single layer decoder refresh points for all layers.

4.  Full Intra Request for Layered Codecs

   When a media receiver or middlebox has decided to send a FIR command
   (based on the guidance provided in Section 4.3.1 of RFC 5104
   [RFC5104], it MUST do so in the RTCP stream related to the forward
   RTP stream that carries the base layer of the layered bitstream, and
   the Feedback Control Information (FCI, and in particular the SSRC



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   field therein) MUST also refer to the forward RTP stream that carries
   the base layer.

   When a Full Intra Request Command is received by the designated media
   sender in the RTCP stream associated with any of the RTP streams in
   which any layer of a layered bitstream are sent, the designated media
   sender MUST send a Decoder Refresh Point (Section 3) as defined above
   at its earliest opportunity.  The requirements related to congestion
   control on the forward RTP streams as specified in sections 3.5.1.5
   of RFC 5104 [RFC5104] apply for the RTP streams both in isolation and
   combined.

   Note: the requirement to react to FIR commands associated with
   enhancement layers is included for robustness and backward
   compatibility reasons.

5.  Identifying the use of Layered Codecs (Informative)

   The above modifications to RFC 5104 unambiguously define how to deal
   with FIR when layered bitstreams are in use.  However, it is
   surprisingly difficult to identify this situation.  In general, it is
   expected that implementers know when layered coding (in its commonly
   understood sense: with inter-layer prediction between pyramided-
   arranged layers) is in use and when not, and can therefore implement
   the above updates to RFC 5104 correctly.  However, there are use
   cases of the use of layered codecs that may be viewed as somewhat
   exotic today but clearly are supported by the video coding syntax, in
   which the above rules would lead to suboptimal system behavior.
   Nothing would break, and there would not be an interop failure, but
   the user experience may suffer through the sending or receiving of
   Decoder Refresh Points at times or on parts of the bitstream that are
   unnecessary from a user experience viewpoint.  Therefore, this
   informative section is included that provides the current
   understanding of when a layered codec is in use and when not.

   The key observation made here is that the RTP payload format
   negotiated for the RTP streams, in isolation, is not necessarily an
   indicator for the use of layering.  Some layered codecs (including
   H.264/SVC) can form decodable bitstreams including only (one or more)
   enhancement layers, without the base layer, effectively creating
   simulcastable sub-bitstreams in a scalable bitstream that does not
   take advantage of inter-layer prediction.  In such a scenario, it is
   potentially (though not necessarily) unnecessary--or even counter-
   productive--to send a decoder refresh point on all RTP streams using
   that payload format and SSRC.

   One good indication of the likely use of layering with interlayer
   prediction is when the various RTP streams are "bound" together on



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   the signaling level.  In an SDP environment, this would be the case
   if they are marked as being dependent from each other using the
   grouping framework RFC 4588 [RFC4588] and the layer dependency
   RFC 5583 [RFC5583].

6.  Layered Codecs and non-FIR codec control messages (Informative)

   Between them, RFC 4585 [RFC4585] (as updated by RFC 5506 [RFC5506])
   and RFC 5104 [RFC5104] define a total of seven Payload-specific
   Feedback messages.  For the FIR command message, guidance has been
   provided above.  In this section, some information is provided with
   respect to the remaining six codec control messages.

6.1.  Picture Loss Indication (PLI)

   PLI is defined in RFC 4585 [RFC4585] section 6.3.1.  The prudent
   response to a PLI message received for an enhancement layer is to
   "repair" (through whatever source-coding specific means) that
   enhancement layer and all dependent enhancement layers, but not the
   reference layer(s) used by the enhancement layer for which the PLI
   was received.  The encoder can figure out by itself what constitutes
   a dependent enhancement layer and does not need help from the system
   stack in doing so.  Insofar, there is nothing that needs to be
   specified herein.

6.2.  Slice Loss Indication (SLI)

   SLI is defined in RFC 4585 [RFC4585] section 6.3.2.  The authors'
   current understanding is that the prudent response to a SLI message
   received for an enhancement layer is to "repair" (through whatever
   source-coding specific means) the affected spatial area of that
   enhancement layer and all dependent enhancement layers, but not the
   reference layers used by the enhancement layer for which the SLI was
   received.  The encoder can figure out by itself what constitutes a
   dependent enhancement layer and does not need help from the system
   stack in doing so.  Insofar, there is nothing that needs to be
   specified herein.  SLI has seen very little implementation and, as
   far as it is known, none in conjunction with layered systems.

6.3.  Reference Picture Selection Indication (RPSI)

   RPSI is defined in RFC 4585 [RFC4585] section 6.3.3.  While a
   technical equivalent of RPSI has been in use with non-layered systems
   for many years, no implementations are known in conjunction of
   layered codecs.  The authors' current understanding is that the
   reception of an RPSI message on any layer forces the encoder to
   "repair" the bitstream on that layer and all dependent layers without




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   the need of any system-provided guidance.  Insofar, RPSI should work
   without further need for specification language.

6.4.  Temporal-Spatial Trade-off Request and Notification (TSTR/TSTN)

   TSTN/TSTR are defined in RFC 5104 [RFC5104] section 4.3.2 and 4.3.3,
   respectively.  The TSTR request allows to communicate (typically
   user-interface-obtained) guidance of the preferred trade-off between
   spatial quality and frame rate.  A technical equivalent of TSTN/TSTR
   has seen deployment for many years in non-scalable systems.

   The Temporal-Spatial Trade-off request and notification messages
   include an SSRC target, which (similarly to FIR) may refer to an RTP
   stream carrying a base layer, an enhancement layer, or multiple
   layers.  Therefore, the authors' current understanding is that the
   semantics of the message applies to the layers present in the
   targeted RTP stream.

   It is noted that per-layer TSTR/TSTN is a mechanism that is, in some
   ways, counterproductive in a system using layered codecs.  Given a
   sufficiently complex layered bitstream layout, a sending system has
   flexibility in adjusting the spatio/temporal quality balance by
   adding and removing temporal, spatial, or quality enhancement layers.
   At present it is unclear whether an allowed (or even recommended)
   option to the reception of a TSTR is to adjust the bit allocation
   within the layer(s) present in the addressed RTP stream, or to adjust
   the layering structure accordingly--which can involve more than just
   the addressed RTP stream.

   Until there is a sufficient critical mass of implementation practice,
   it is probably prudent for an implementer not to assume either of the
   two options (or any middleground that may exist between the two), be
   liberal in accepting TSTR messages, perhaps responding in TSTN
   indicating "no change," not sending TSTR messages except when
   operating in SRST mode as defined in RFC 7656 [RFC7656], and
   contribute to the IETF documentation of any implementation
   requirements that make per-layer TSTR/TSTN useful.

6.5.  H.271 Video Back Channel Message (VBCM)

   VBCM is defined in RFC 5104 [RFC5104] section 4.3.4.  What was said
   above for RPSI (Section 6.3) applies here as well.

7.  Acknowledgements

   The authors want to thank Mo Zanaty for useful discussions.





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8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   The security considerations of RFC 4585 [RFC4585] (as updated by
   RFC 5506 [RFC5506]) and RFC 5104 [RFC5104] apply.  The clarified
   response to FIR does not require any updates.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <http://www.rfc-editor.org/info/rfc5104>.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
              2009, <http://www.rfc-editor.org/info/rfc5506>.

10.2.  Informative References

   [H.261]    ITU-T, "ITU-T Rec. H.261: Video codec for audiovisual
              services at p x 64 kbit/s", 1993,
              <http://handle.itu.int/11.1002/1000/1088>.

   [H.263]    ITU-T, "ITU-T Rec. H.263: Video coding for low bit rate
              communication", 2005,
              <http://handle.itu.int/11.1002/1000/7497>.

   [H.264]    ITU-T, "ITU-T Rec. H.264: Advanced video coding for
              generic audiovisual services", 2014,
              <http://handle.itu.int/11.1002/1000/12063>.



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   [H.265]    ITU-T, "ITU-T Rec. H.265: High efficiency video coding",
              2015, <http://handle.itu.int/11.1002/1000/12455>.

   [I-D.grange-vp9-bitstream]
              Grange, A. and H. Alvestrand, "A VP9 Bitstream Overview",
              draft-grange-vp9-bitstream-00 (work in progress), February
              2013.

   [I-D.ietf-mmusic-rid]
              Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B.,
              Roach, A., and B. Campen, "RTP Payload Format
              Constraints", draft-ietf-mmusic-rid-05 (work in progress),
              March 2016.

   [I-D.ietf-mmusic-sdp-simulcast]
              Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty,
              "Using Simulcast in SDP and RTP Sessions", draft-ietf-
              mmusic-sdp-simulcast-04 (work in progress), February 2016.

   [MPEG-1]   ISO/IEC, "ISO/IEC 11172-2:1993 Information technology --
              Coding of moving pictures and associated audio for digital
              storage media at up to about 1,5 Mbit/s -- Part 2: Video",
              1993.

   [MPEG-2]   ISO/IEC, "ISO/IEC 13818-2:2013 Information technology --
              Generic coding of moving pictures and associated audio
              information -- Part 2: Video", 2013.

   [MPEG-4]   ISO/IEC, "ISO/IEC 14496-2:2004 Information technology --
              Coding of audio-visual objects -- Part 2: Visual", 2004.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <http://www.rfc-editor.org/info/rfc4588>.

   [RFC5583]  Schierl, T. and S. Wenger, "Signaling Media Decoding
              Dependency in the Session Description Protocol (SDP)",
              RFC 5583, DOI 10.17487/RFC5583, July 2009,
              <http://www.rfc-editor.org/info/rfc5583>.

   [RFC6386]  Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
              Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
              Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
              <http://www.rfc-editor.org/info/rfc6386>.






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   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <http://www.rfc-editor.org/info/rfc7656>.

Appendix A.  Change Log

   NOTE TO RFC EDITOR: Please remove this section prior to publication.

   draft-wenger-avtext-avpf-ccm-layered-00-00: initial version

   draft-ietf-avtext-avpf-ccm-layered-00: resubmit as avtext WG draft
   per IETF95 and list confirmation by Rachel 4/25/2016

   draft-ietf-avtext-avpf-ccm-layered-00: In section "Identifying the
   use of Layered Codecs (Informative)", removed last sentence that
   could be misread that the explicit signaling of simulcasting in
   conjunction with payload formats supporting layered coding implies no
   layering.

Authors' Addresses

   Stephan Wenger
   Vidyo, Inc.

   Email: stewe@stewe.org


   Jonathan Lennox
   Vidyo, Inc.

   Email: jonathan@vidyo.com


   Bo Burman
   Ericsson
   Kistavagen 25
   SE - 164 80 Kista
   Sweden

   Email: bo.burman@ericsson.com









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   Magnus Westerlund
   Ericsson
   Farogatan 6
   SE- 164 80 Kista
   Sweden

   Phone: +46107148287
   Email: magnus.westerlund@ericsson.com











































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