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Versions: 00 draft-ietf-avtext-lrr

Payload Working Group                                          J. Lennox
Internet-Draft                                                   D. Hong
Intended status: Standards Track                                   Vidyo
Expires: September 10, 2015                                    J. Uberti
                                                               S. Holmer
                                                              M. Flodman
                                                                  Google
                                                           March 9, 2015


         The Layer Refresh Request (LRR) RTCP Feedback Message
                       draft-lennox-avtext-lrr-00

Abstract

   This memo describes the RTP Payload-Specific Feedback Message "Layer
   Refresh Request" (LRR), which can be used to request a state refresh
   of one or more substreams of a layered media stream.  It also defines
   its use with several scalable media formats.

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
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   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 September 10, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions and Acronyms . . . . . . . . . . . .   2
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Layer Refresh Request . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Message Format  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Usage with specific codecs  . . . . . . . . . . . . . . . . .   5
     4.1.  H264 SVC  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  VP8 . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  H265  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  VP9 . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Usage with different scalability transmission mechanisms  . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This memo describes an RTP Payload-Specific Feedback Message
   [RFC4585] "Layer Refresh Request" (LRR).  It is designed to allow a
   receiver of a layered media stream to request that one or more of its
   substreams be refreshed, such that it can then be decoded by an
   endpoint which previously was not receiving those layers, without
   requiring that the entire stream be refreshed (as it would be if the
   receiver sent a Full Intra Request (FIR) [RFC5104].

   The message is designed to be applicable both to temporally and
   spatially scaled streams, and to both single-stream and multi-stream
   scalability modes.

2.  Conventions, Definitions and Acronyms

   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 [RFC2119].

2.1.  Terminology

   A "Layer Refresh Point" is a point in a scalable stream after which a
   decoder, which previously had been able to decode only some (possibly
   none) of the available layers of stream, is able to decode a greater
   number of the layers.



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   For spatial (or quality) layers, layer refresh typically requires
   that a spatial layer be encoded in a way that references only lower-
   layer subpictures of the current picture, not any earlier pictures of
   that spatial layer.  Additionally, the encoder must promise that no
   earlier pictures of that spatial layer will be used as reference in
   the future.

   An illustration of spatial layer refresh is shown below.

        ... <--  S1  <--  S1       S1  <--  S1  <-- ...
                  |        |        |        |
                 \/       \/       \/       \/
        ... <--  S0  <--  S0  <--  S0  <--  S0  <-- ...

                  1        2        3        4

   In this illustration, frame 3 is a layer refresh point for spatial
   layer S1; a decoder which had previously only been decoding spatial
   layer S0 would be able to decode layer S1 starting at frame 3.

                                 Figure 1

   For temporal layers, layer refresh requires that the layer be
   "temporally nested", i.e. use as reference only earlier frames of a
   lower temporal layer, not any earlier frames of this temporal layer,
   and also promise that no future frames of this temporal layer will
   reference frames of this temporal layer before the refresh point.  In
   many cases, the temporal structure of the stream will mean that all
   frames are temporally nested, in which case decoders will have no
   need to send LRR messages for the stream.

   An illustration of temporal layer refresh is shown below.

           ...  <----- T1  <------ T1          T1  <------ ...
                      /           /           /
                    |_          |_          |_
        ... <--  T0  <------ T0  <------ T0  <------ T0  <--- ...

                  1     2     3     4     5     6     7

   In this illustration, frame 6 is a layer refresh point for temporal
   layer T1; a decoder which had previously only been decoding temporal
   layer T0 would be able to decode layer T1 starting at frame 6.

                                 Figure 2






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   An illustration of an inherently temporally nested stream is shown
   below.

                       T1          T1          T1
                      /           /           /
                    |_          |_          |_
        ... <--  T0  <------ T0  <------ T0  <------ T0  <--- ...

                  1     2     3     4     5     6     7

   In this illustration, the stream is temporally nested in its ordinary
   structure; a decoder receiving layer T0 can begin decoding layer T1
   at any point.

                                 Figure 3

3.  Layer Refresh Request

   A layer refresh frame can be requested by sending a Layer Refresh
   Request (LRR), which is an RTCP payload-specific feedback message
   [RFC4585] asking the encoder to encode a frame which makes it
   possible to upgrade to a higher layer.  The LRR contains one or two
   tuples, indicating the layer the decoder wants to upgrade to, and
   (optionally) the currently highest layer the decoder can decode.

   The specific format of the tuples, and the mechanism by which a
   receiver recognizes a refresh frame, is codec-dependent.  Usage for
   several codecs is discussed in Section 4.

   LRR follows the model of the Full Intra Request (FIR)
   [RFC5104](Section 3.5.1) for its retransmission, reliability, and use
   in multipoint conferences.  TODO: expand these here.

   The LRR message is identified by RTCP packet type value PT=PSFB and
   FMT=TBD.  The FCI field MUST contain one or more FIR entries.  Each
   entry applies to a different media sender, identified by its SSRC.

3.1.  Message Format

   The Feedback Control Information (FCI) for the Layer Refresh Request
   consists of one or more FCI entries, the content of which is depicted
   in Figure 4.  The length of the LRR feedback message MUST be set to
   2+3*N, where N is the number of FCI entries.








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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              SSRC                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Seq nr.       |C| Payload Type| Reserved                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Target Layer Index            | Current Layer Index (opt)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 4

   SSRC (32 bits)  The SSRC value of the media sender that is requested
      to send a layer refresh point.

   Seq nr. (8 bits)  Command sequence number.  The sequence number space
      is unique for each pairing of the SSRC of command source and the
      SSRC of the command target.  The sequence number SHALL be
      increased by 1 modulo 256 for each new command.  A repetition
      SHALL NOT increase the sequence number.  The initial value is
      arbitrary.

   C (1 bit)  A flag bit indicating whether the "Current Layer Index"
      field is present in the FCI.  If this bit is false, the sender of
      the LRR message is requesting refresh of all layers up to and
      including the target layer.

   Payload Type (7 bits)  The RTP payload type for which the LRR is
      being requested.  This gives the context in which the target layer
      index is to be interpreted.

   Reserved (16 bits)  All bits SHALL be set to 0 by the sender and
      SHALL be ignored on reception.

   Target Layer Index (16 bits)  The target layer for which the receiver
      wishes a refresh point.  Its format is dependent on the payload
      type field.

   Current Layer Index (16 bits)  If C is 1, the current layer being
      decoded by the receiver.  This message is not requesting refresh
      of layers at or below this layer.  If C is 0, this field SHALL be
      set to 0 by the sender and SHALL be ignored on reception.

4.  Usage with specific codecs







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4.1.  H264 SVC

   H.264 SVC [RFC6190] defines temporal, dependency (spatial), and
   quality scalability modes.

               +---------------+---------------+
               |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |R| DID |  QID  | TID |RES      |
               +---------------+---------------+

                                 Figure 5

   Figure 5 shows the format of the layer index field for H.264 SVC
   streams.  This is designed to follow the same layout as the third and
   fourth bytes of the H.264 SVC NAL unit extension, which carry the
   stream's layer information.  The "R" and "RES" fields MUST be set to
   0 on transmission and ignored on reception.  See [RFC6190]
   Section 1.1.3 for details on the DID, QID, and TID fields.

   TODO: identifying layer refresh frames in an H.264 bitstream.

4.2.  VP8

   The VP8 RTP payload format [I-D.ietf-payload-vp8] defines temporal
   scalability modes.  It does not support spatial scalability.

               +---------------+---------------+
               |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |TID| RES                       |
               +---------------+---------------+

                                 Figure 6

   Figure 6 shows the format of the layer index field for VP8 streams.
   The "RES" fields MUST be set to 0 on transmission and ingnored on
   reception.  See [I-D.ietf-payload-vp8] Section 4.2 for details on the
   TID field.

   TODO: identifying layer refresh frames in an VP8 bitstream.

4.3.  H265

   The initial version of the H.265 payload format
   [I-D.ietf-payload-rtp-h265] defines temporal scalability, with
   protocol elements reserved for spatial or other scalability modes




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   (which are expected to be defined in a future version of the
   specification.

               +---------------+---------------+
               |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               | RES         |  LayerId  | TID |
               +-------------+-----------------+

                                 Figure 7

   Figure 7 shows the format of the layer index field for H.265 streams.
   This is designed to follow the same layout as the first and second
   bytes of the H.265 NAL unit header, which carry the stream's layer
   information.  The "RES" field MUST be set to 0 on transmission and
   ingnored on reception.  See [I-D.ietf-payload-rtp-h265] Section 1.1.3
   for details on the LayerId and TID fields.

   TODO: identifying layer refresh frames in an H.265 bitstream.

4.4.  VP9

   The RTP payload format for VP9 [I-D.uberti-payload-vp9] defines how
   it can be used for spatial and temporal scalability.

               +---------------+---------------+
               |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
               +-------------+-----------------+
               |  T  |R|  S  | RES             |
               +-------------+-----------------+

                                 Figure 8

   Figure 8 shows the format of the layer index field for VP9 streams.
   This is designed to follow the same layout as the "L" byte of the VP9
   payload header, which carries the stream's layer information.  The
   "R" and "RES" fields MUST be set to 0 on transmission and ingnored on
   reception.  See [I-D.uberti-payload-vp9] for details on the T and S
   fields.

   Identification of a layer refresh frame can be derived from the
   reference IDs of each frame by backtracking the dependency chain
   until reaching a point where only decodable frames are being
   referenced.  Therefore it's recommended for both the flexible and the
   non-flexible mode that, when upgrade frames are being encoded in
   response to a LRR, those packets should contain layer indices and the
   reference fields so that the decoder or an MCU can make this
   derivation.



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   Example:

   LRR {1,0}, {2,1} is sent by an MCU when it is currently relaying
   {1,0} to a receiver and which wants to upgrade to {2,1}. In response
   the encoder should encode the next frames in layers {1,1} and {2,1}
   by only referring to frames in {1,0}, or {0,0}.

   In the non-flexible mode, periodic upgrade frames can be defined by
   the layer structure of the SS, thus periodic upgrade frames can be
   automatically identified by the picture ID.

5.  Usage with different scalability transmission mechanisms

   Several different mechanisms are defined for how scalable streams can
   be transmitted in RTP.  The RTP Taxonomy
   [I-D.ietf-avtext-rtp-grouping-taxonomy] Section 3.7 defines three
   mechanisms: Single RTP Stream on a Single Media Transport (SRST),
   Multiple RTP Streams on a Single Media Transport (MRST), and Multiple
   RTP Streams on Multiple Media Transports (MRMT).

   The LRR message is applicable to all these mechanisms.  For MRST and
   MRMT mechanisms, the "media source" field of the LRR FCI is set to
   the SSRC of the RTP stream containing the layer indicated by the
   Current Layer Index (if "C" is 1), or the stream containing the base
   encoded stream (if "C" is 0).  For MRMT, it is sent on the RTP
   session on which this stream is sent.  On receipt, the sender MUST
   refresh all the layers requested in the stream, simultaneously in
   decode order.

   Note: arguably, for the MRST and MRMT mechanisms, FIR feedback
   messages could instead be used to refresh specific individual layers.
   However, the usage of FIR for MRSR/MRMT is not explicitly specified
   anywhere, and if FIR is interpreted as refreshing layers, there is no
   way to request an actual full, synchronized refresh of all the layers
   of an MRST/MRMT layered source.  Thus, the authors feel that
   interpreting FIR as refreshing the entire source, and using LRR for
   the individual layers, would be more useful.

6.  Security Considerations

   All the security considerations of FIR feedback packets [RFC5104]
   apply to LRR feedback packets as well.  Additionally, media senders
   receiving LRR feedback packets MUST validate that the payload types
   and layer indices they are receiving are valid for the stream they
   are currently sending, and discard the requests if not.






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7.  IANA Considerations

   The IANA is requested to register the following values:
   - TODO: PSFB value for LRR

8.  References

   [I-D.ietf-avtext-rtp-grouping-taxonomy]
              Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
              "A Taxonomy of Grouping Semantics and Mechanisms for Real-
              Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
              rtp-grouping-taxonomy-06 (work in progress), March 2015.

   [I-D.ietf-payload-rtp-h265]
              Wang, Y., Sanchez, Y., Schierl, T., Wenger, S., and M.
              Hannuksela, "RTP Payload Format for High Efficiency Video
              Coding", draft-ietf-payload-rtp-h265-07 (work in
              progress), December 2014.

   [I-D.ietf-payload-vp8]
              Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
              Galligan, "RTP Payload Format for VP8 Video", draft-ietf-
              payload-vp8-14 (work in progress), March 2015.

   [I-D.uberti-payload-vp9]
              Uberti, J., Holmer, S., Flodman, M., and J. Lennox, "RTP
              Payload Format for VP9 Video", draft-uberti-payload-vp9-00
              (work in progress), October 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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, July
              2006.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC6190]  Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
              "RTP Payload Format for Scalable Video Coding", RFC 6190,
              May 2011.







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Authors' Addresses

   Jonathan Lennox
   Vidyo, Inc.
   433 Hackensack Avenue
   Seventh Floor
   Hackensack, NJ  07601
   US

   Email: jonathan@vidyo.com


   Danny Hong
   Vidyo, Inc.
   433 Hackensack Avenue
   Seventh Floor
   Hackensack, NJ  07601
   US

   Email: danny@vidyo.com


   Justin Uberti
   Google, Inc.
   747 6th Street South
   Kirkland, WA  98033
   USA

   Email: justin@uberti.name


   Stefan Holmer
   Google, Inc.
   Kungsbron 2
   Stockholm  111 22
   Sweden


   Magnus Flodman
   Google, Inc.
   Kungsbron 2
   Stockholm  111 22
   Sweden








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