



Network Working Group                                          S. Wenger
Internet-Draft                                                 J. Lennox
Updates: 5104 (if approved)                                  Vidyo, Inc.
Intended status: Standards Track                               B. Burman
Expires: July 14, 2017                                     M. Westerlund
                                                                Ericsson
                                                        January 10, 2017


   Using Codec Control Messages in the RTP Audio-Visual Profile with
                      Feedback with Layered Codecs
                 draft-ietf-avtext-avpf-ccm-layered-04

Abstract

   This document updates RFC5104 by fixing a shortcoming in the
   specification language of the Codec Control Message Full Intra
   Request (FIR) as defined in RFC5104 when using it 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 July 14, 2017.

Copyright Notice

   Copyright (c) 2017 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 . . . . . . . . . . . .   5
   5.  Identifying the use of layered bitstreams (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) . . . . . .   7
     6.4.  Temporal-Spatial Trade-off Request and Notification
           (TSTR/TSTN) . . . . . . . . . . . . . . . . . . . . . . .   7
     6.5.  H.271 Video Back Channel Message (VBCM) . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction and Problem Statement

   The Extended RTP Profile for Real-time Transport Control Protocol
   (RTCP)-Based Feedback (RTP/AVPF) [RFC4585] and Codec Control Messages
   in the RTP Audio-Visual Profile with Feedback (AVPF) [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



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   fidelities.  Depending on the media codec and its RTP payload format
   in use, a number of options exist how to transport those layers in
   RTP.  With reference to A Taxonomy of Semantics and Mechanisms for
   Real-Time Transport Protocol (RTP) Sources [RFC7656]):

      single layers or groups of layers may be sent in their own RTP
      streams in Multiple RTP streams on a Single media Transport (MRST)
      or Multiple RTP streams on Multiple media Transports (MRMT) mode;

      using media-codec specific multiplexing mechanisms, multiple
      layers may be sent in a single RTP stream in Single RTP stream on
      a Single media Transport (SRST) mode.

   The dependency relationship between layers in a truly layered,
   pyramid-shaped bitstream 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.
   Such a layering structure is assumed henceforth; for more exotic
   layering structures please see Section 5.

   Implementation experience has shown that the Full Intra Request (FIR)
   command as defined in [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 [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 [RFC5104] section
   2.2) for all layers, or only for the layer which is transported in
   the RTP stream that the FIR request is associated with.

   This document fixes this shortcoming by:

   a.  Updating the definition of the Decoder Refresh Point (as defined
       in [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.  Require a media sender to send a Decoder Refresh Point after the
       media sender has received a FIR over an RTCP stream associated
       with any of the RTP streams over which a part of the layered
       bitstream is transported;




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   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 bitstream is
       in use for which the above rules apply.

   While, clearly, the reaction to FIR for layered codecs in [RFC5104]
   and companion documents is underspecified, it appears that this is
   not the case for any of the other payload-specific codec control
   messages defined in any of [RFC4585], [RFC5104].  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 remainder of this section replaces the definition of Decoder
   Refresh Point in section 2.2 of [RFC5104] in its entirety.

   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 that is conveyed in-band.
   In [H.264], for example, a decoder refresh point contains those
   parameter set Network Adaptation Layer (NAL) units that generate
   parameter sets necessary for the decoding of the following slice/data
   partition NAL units.  (That is assuming the parameter sets have not
   been conveyed out of band.)





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   When a layered codec is in use, the above definition--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

   A media receiver or middlebox may decide to send a FIR command based
   on the guidance provided in Section 4.3.1 of [RFC5104].  When sending
   the FIR command, it MUST target the RTP stream that carries the base
   layer of the layered bitstream, and this is done by setting the
   Feedback Control Information (FCI, and in particular the SSRC field
   therein) to refer to the SSRC of 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.
   and 5. of [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 bitstreams (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 the use of a layered bitstream.
   In general, it is expected that implementers know when layered
   bitstreams (in its commonly understood sense: with inter-layer
   prediction between pyramided-arranged layers) are in use and when
   not, and can therefore implement the above updates to RFC 5104
   correctly.  However, there are scenarios in which layered codecs are
   employed creating non-pyramid shaped bitstreams.  Those scenarios may
   be viewed as somewhat exotic today but clearly are supported by
   certain video coding syntaxes, such as H.264/SVC.  When blindly
   applying the above rules to those non-pyramid-arranged layering
   structures, suboptimal system behavior would result.  Nothing would
   break, and there would not be an interoperability 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




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   informative section is included that provides the current
   understanding of when a layered bitstream 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 a layered bitstream.  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 within a single scalable
   bitstream (as defined in the video coding standard), but without
   inter-layer prediction.  In such a scenario, it is potentially,
   though not necessarily, counter-productive to send a decoder refresh
   point on all RTP streams using that payload format and SSRC.  It is
   beyond the scope of this document to discuss optimized reactions to
   FIRs received on RTP streams carrying such exotic bitstreams.

   One good indication of the likely use of pyramid-shaped layering with
   interlayer prediction is when the various RTP streams are "bound"
   together on the signaling level.  In an SDP environment, this would
   be the case if they are marked as being dependent on each other using
   The Session Description Protocol (SDP) Grouping Framework [RFC5888]
   and the layer dependency RFC 5583 [RFC5583].

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

   Between them, AVPF [RFC4585] and Codec Control Messages [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 section 6.3.1 of [RFC4585].  The prudent response
   to a PLI message received for an enhancement layer is to "repair"
   that enhancement layer and all dependent enhancement layers through
   appropriate source-coding specific means.  However, the reference
   layer(s) used by the enhancement layer for which the PLI was received
   does not require repair.  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.  Thus, there is nothing that needs to
   be specified herein.

6.2.  Slice Loss Indication (SLI)

   SLI is defined in section 6.3.2 of [RFC4585].  The current
   understanding is that the prudent response to a SLI message received
   for an enhancement layer is to "repair" the affected spatial area of



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   that enhancement layer and all dependent enhancement layers through
   appropriate source-coding specific means.  As in PLI, the reference
   layers used by the enhancement layer for which the SLI was received
   do not need to be repaired.  Again, as in PLI, the encoder can
   determine by itself what constitutes a dependent enhancement layer
   and does not need help from the system stack in doing so.  Thus,
   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 section 6.3.3 of [RFC4585].  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 current understanding is that the reception of an RPSI message on
   any layer indicating a missing reference picture forces the encoder
   to appropriately handle that missing reference picture in the layer
   indicated, and all dependent layers.  Thus, 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 section 4.3.2 and 4.3.3 of [RFC5104],
   respectively.  The TSTR request communicates 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 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.





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   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.
   Instead, it is suggested that an implementation be liberal in
   accepting TSTR messages, and upon receipt responding in TSTN
   indicating "no change".  Further, it is suggested that new
   implementations do not send TSTR messages except when operating in
   SRST mode as defined in [RFC7656].  Finally implementers are
   encouraged to 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 section 4.3.4 of [RFC5104].  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.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   The security considerations of AVPF [RFC4585] (as updated by Support
   for Reduced-Size Real-Time Transport Control Protocol (RTCP):
   Opportunities and Consequences [RFC5506]) and Codec Control Messages
   [RFC5104] apply.  The clarified response to FIR does not introduce
   additional security considerations.

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>.





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   [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>.

   [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.

   [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.

   [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>.



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   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <http://www.rfc-editor.org/info/rfc5888>.

   [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>.

   [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.

   draft-ietf-avtext-avpf-ccm-layered-01: clarifications in section 5.

   draft-ietf-avtext-avpf-ccm-layered-02: addressing WGLC comments,
   mostly editorial; see reflector discussions 09/2016

   draft-ietf-avtext-avpf-ccm-layered-03: addressing AD writeup
   comments, editorial

Authors' Addresses

   Stephan Wenger
   Vidyo, Inc.

   Email: stewe@stewe.org






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   Jonathan Lennox
   Vidyo, Inc.

   Email: jonathan@vidyo.com


   Bo Burman
   Ericsson
   Kistavagen 25
   SE - 164 80 Kista
   Sweden

   Email: bo.burman@ericsson.com


   Magnus Westerlund
   Ericsson
   Farogatan 2
   SE- 164 80 Kista
   Sweden

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




























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