Using Codec Control Messages in the RTP Audio-Visual Profile with Feedback with Layered Codecs

Internet Engineering Task Force (IETF)                         S. Wenger
Request for Comments: 8082                                     J. Lennox
Updates: 5104                                                Vidyo, Inc.
Category: Standards Track                                      B. Burman
ISSN: 2070-1721                                            M. Westerlund
                                                                Ericsson
                                                              March 2017


   Using Codec Control Messages in the RTP Audio-Visual Profile with
                      Feedback with Layered Codecs

Abstract

   This document updates RFC 5104 by fixing a shortcoming in the
   specification language of the Codec Control Message Full Intra
   Request (FIR) description 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 of 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 (which was updated by RFC 5506) have
   also been analyzed, and no corresponding shortcomings have been
   found.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8082.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction and Problem Statement  . . . . . . . . . . . . .   3
   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) . . .   6
   6.  Layered Codecs and Non-FIR Codec Control Messages
       (Informative) . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Picture Loss Indication (PLI) . . . . . . . . . . . . . .   7
     6.2.  Slice Loss Indication (SLI) . . . . . . . . . . . . . . .   7
     6.3.  Reference Picture Selection Indication (RPSI) . . . . . .   7
     6.4.  Temporal-Spatial Trade-Off Request and Notification
           (TSTR/TSTN) . . . . . . . . . . . . . . . . . . . . . . .   8
     6.5.  H.271 Video Back Channel Message (VBCM) . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 that
   a media receiver can use to inform a media sender of certain
   conditions or to 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, a number of options exist on how to transport those layers in
   RTP.  Summarizing "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 recreate
   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 a 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 that 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 (Scalable Video Coding) [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 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;

   c.  Requiring that a media receiver send the FIR on the RTCP stream
       associated with the base layer.  The option of receiving FIR on
       the 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 the companion documents is underspecified, it appears that this
   is not the case for any of the other payload-specific codec control
   messages defined in [RFC4585] and [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 [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.)

   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
   synchronization source (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 commands 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 pyramid-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
   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, counterproductive to send a decoder refresh
   point on all layers for that payload format and media source.  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
   inter-layer 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 layer dependency [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 or layers used by the enhancement layer for which the PLI was
   received do 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 an SLI message received
   for an enhancement layer is to "repair" the affected spatial area of
   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 in all dependent layers.  Thus, RPSI should work
   without further need for specification language.

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

   TSTR/TSTN are defined in Sections 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 TSTR/TSTN has seen deployment for many years
   in non-scalable systems.

   TSTR and TSTN 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.

   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 middle ground 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.  IANA Considerations

   This memo includes no request to IANA.

8.  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.

9.  References

9.1.  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>.

9.2.  Informative References

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

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

   [H.264]    ITU-T, "Advanced video coding for generic audiovisual
              services", ITU-T Recommendation H.264, Version 11, October
              2016, <http://handle.itu.int/11.1002/1000/12904>.

   [H.265]    ITU-T, "High efficiency video coding", ITU-T
              Recommendation H.265, Version 4, December 2016,
              <http://handle.itu.int/11.1002/1000/12905>.

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

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

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

   [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>.

   [VP9-BITSTREAM]
              Grange, A., de Rivaz, P., and J. Hunt, "VP9 Bitstream &
              Decoding Process Specification", Version 0.6, March 2016,
              <https://storage.googleapis.com/downloads.webmproject.org/
              docs/vp9/vp9-bitstream-specification-
              v0.6-20160331-draft.pdf>.

Acknowledgements

   The authors want to thank Mo Zanaty for useful discussions.

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


   Magnus Westerlund
   Ericsson
   Farogatan 2
   SE - 164 80 Kista
   Sweden

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