AVTCore Working Group                                          J. Uberti
Internet-Draft                                                 S. Holmer
Intended status: Standards Track                              M. Flodman
Expires: July 16, 2020 January 8, 2021                                         D. Hong
                                                               J. Lennox
                                                             8x8 / Jitsi
                                                        January 13,
                                                            July 7, 2020

                    RTP Payload Format for VP9 Video


   This memo describes an RTP payload format for the VP9 video codec.
   The payload format has wide applicability, as it supports
   applications from low bit-rate peer-to-peer usage, to high bit-rate
   video conferences.  It includes provisions for temporal and spatial

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 16, 2020. January 8, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions and Acronyms . . . . . . . . . . . .   3
   3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
   4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   5
     4.2.  VP9 Payload Descriptor  . . . . . . . . . . . . . . . . .   6
       4.2.1.  Scalability Structure (SS): . . . . . . . . . . . . .  11
     4.3.  Frame Fragmentation . . . . . . . . . . . . . . . . . . .  12  13
     4.4.  Scalable encoding considerations  . . . . . . . . . . . .  13
     4.5.  Examples of VP9 RTP Stream  . . . . . . . . . . . . . . .  13  14
       4.5.1.  Reference picture use for scalable structure  . . . .  13  14
   5.  Feedback Messages and Header Extensions . . . . . . . . . . .  14
     5.1.  Reference Picture Selection Indication (RPSI) . . . . . .  14  15
     5.2.  Full Intra Request (FIR)  . . . . . . . . . . . . . . . .  15
     5.3.  Layer Refresh Request (LRR) . . . . . . . . . . . . . . .  15
     5.4.  Frame Marking . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  16  17
     6.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  16  17
     6.2.  SDP Parameters  . . . . . . . . . . . . . . . . . . . . .  19
       6.2.1.  Mapping of Media Subtype Parameters to SDP  . . . . .  19
       6.2.2.  Offer/Answer Considerations . . . . . . . . . . . . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   This memo describes an RTP payload specification applicable to the
   transmission of video streams encoded using the VP9 video codec
   [VP9-BITSTREAM].  The format described in this document can be used
   both in peer-to-peer and video conferencing applications.

   The VP9 video codec was developed by Google, and is the successor to
   its earlier VP8 [RFC6386] codec.  Above the compression improvements
   and other general enhancements above VP8, VP9 is also designed in a
   way that allows spatially-scalable video encoding.

2.  Conventions, Definitions and Acronyms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Media Format Description

   The VP9 codec can maintain up to eight reference frames, of which up
   to three can be referenced by any new frame.

   VP9 also allows a frame to use another frame of a different
   resolution as a reference frame.  (Specifically, a frame may use any
   references whose width and height are between 1/16th that of the
   current frame and twice that of the current frame, inclusive.)  This
   allows internal resolution changes without requiring the use of key

   These features together enable an encoder to implement various forms
   of coarse-grained scalability, including temporal, spatial and
   quality scalability modes, as well as combinations of these, without
   the need for explicit scalable coding tools.

   Temporal layers define different frame rates of video; spatial and
   quality layers define different and possibly dependent
   representations of a single input frame.  Spatial layers allow a
   frame to be encoded at different resolutions, whereas quality layers
   allow a frame to be encoded at the same resolution but at different
   qualities (and thus with different amounts of coding error).  VP9
   supports quality layers as spatial layers without any resolution
   changes; hereinafter, the term "spatial layer" is used to represent
   both spatial and quality layers.

   This payload format specification defines how such temporal and
   spatial scalability layers can be described and communicated.

   Temporal and spatial scalability layers are associated with non-
   negative integer IDs.  The lowest layer of either type has an ID of
   0, and is sometimes referred to as the "base" temporal or spatial

   Layers are designed (and MUST be encoded) such that if any layer, and
   all higher layers, are removed from the bitstream along either of the
   two dimensions, the remaining bitstream is still correctly decodable.

   For terminology, this document uses the term "frame" to refer to a
   single encoded VP9 frame for a particular resolution/quality, and
   "picture" to refer to all the representations (frames) at a single
   instant in time.  A picture thus consists of one or more frames,
   encoding different spatial layers.

   Within a picture, a frame with spatial layer ID equal to SID, where
   SID > 0, can depend on a frame of the same picture with a lower
   spatial layer ID.  This "inter-layer" dependency can result in
   additional coding gain compared to the case where only traditional
   "inter-picture" dependency is used, where a frame depends on
   previously coded frame in time.  For simplicity, this payload format
   assumes that, within a picture and if inter-layer dependency is used,
   a spatial layer SID frame can depend only on the immediately previous
   spatial layer SID-1 frame, when S > 0.  Additionally, if inter-
   picture dependency is used, a spatial layer SID frame is assumed to
   only depend on a previously coded spatial layer SID frame.

   Given above simplifications for inter-layer and inter-picture
   dependencies, a flag (the D bit described below) is used to indicate
   whether a spatial layer SID frame depends on the spatial layer SID-1
   frame.  Given the D bit, a receiver only needs to additionally know
   the inter-picture dependency structure for a given spatial layer
   frame in order to determine its decodability.  Two modes of
   describing the inter-picture dependency structure are possible:
   "flexible mode" and "non-flexible mode".  An encoder can only switch
   between the two on the first packet of a key frame with temporal
   layer ID equal to 0.

   In flexible mode, each packet can contain up to 3 reference indices,
   which identify all frames referenced by the frame transmitted in the
   current packet for inter-picture prediction.  This (along with the D
   bit) enables a receiver to identify if a frame is decodable or not
   and helps it understand the temporal layer structure.  Since this is
   signaled in each packet it makes it possible to have very flexible
   temporal layer hierarchies and patterns which are changing

   In non-flexible mode, the inter-picture dependency (the reference
   indices) of a Picture Group (PG) MUST be pre-specified as part of the
   scalability structure (SS) data.  In this mode, each packet has an
   index to refer to one of the described pictures in the PG, from which
   the pictures referenced by the picture transmitted in the current
   packet for inter-picture prediction can be identified.

   (Editor's Note: A "Picture Group", as used in this document, is not
   the same thing as a the term "Group of Pictures" as it is
   traditionally used in video coding, i.e. to mean an independently-
   decoadable run of pictures beginning with a keyframe.  Suggestions
   for better terminology are welcome.)
   The SS data can also be used to specify the resolution of each
   spatial layer present in the VP9 stream for both flexible and non-
   flexible modes.

4.  Payload Format

   This section describes how the encoded VP9 bitstream is encapsulated
   in RTP.  To handle network losses usage of RTP/AVPF [RFC4585] is
   RECOMMENDED.  All integer fields in the specifications are encoded as
   unsigned integers in network octet order.

4.1.  RTP Header Usage

   The general RTP payload format for VP9 is depicted below.

      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|X|  CC   |M|     PT      |       sequence number         |
     |                           timestamp                           |
     |           synchronization source (SSRC) identifier            |
     |            contributing source (CSRC) identifiers             |
     |                             ....                              |
     |            VP9 payload descriptor (integer #octets)           |
     :                                                               :
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               : VP9 pyld hdr  |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
     |                                                               |
     +                                                               |
     :                   Bytes 2..N of VP9 payload                   :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               :    OPTIONAL RTP padding       |

   The VP9 payload descriptor will be described in Section 4.2; the VP9
   payload header is described in [VP9-BITSTREAM].  OPTIONAL RTP padding
   MUST NOT be included unless the P bit is set.  The figure
   specifically shows the format for the first packet in a frame.
   Subsequent packets will not contain the VP9 payload header, and will
   have later octets in the frame payload.

                                 Figure 1

   Marker bit (M):  MUST be set to 1 for the final packet of the highest
      spatial layer frame (the final packet of the picture), and 0
      otherwise.  Unless spatial scalability is in use for this picture,
      this will have the same value as the E bit described below.  Note
      this bit MUST be set to 1 for the target spatial layer frame if a
      stream is being rewritten to remove higher spatial layers.

   Payload Type (PT):  In line with the policy in Section 3 of
      [RFC3551], applications using the VP9 RTP payload profile MUST
      assign a dynamic payload type number to be used in each RTP
      session and provide a mechanism to indicate the mapping.  See
      Section 6.2 for the mechanism to be used with the Session
      Description Protocol (SDP) [RFC4566].

   Timestamp:  The RTP timestamp indicates the time when the input frame
      was sampled, at a clock rate of 90 kHz.  If the input picture is
      encoded with multiple layer frames, all of the frames of the
      picture MUST have the same timestamp.

      If a frame has the VP9 show_frame field set to 0 (i.e., it is
      meant only to populate a reference buffer, without being output)
      its timestamp MAY alternately be set to be the same as the
      subsequent frame with show_frame equal to 1.  (This will be
      convenient for playing out pre-encoded content packaged with VP9
      "superframes", which typically bundle show_frame==0 frames with a
      subsequent show_frame==1 frame.)  Every frame with show_frame==1,
      however, MUST have a unique timestamp modulo the 2^32 wrap of the

   The remaining RTP Fixed Header Fields (V, P, X, CC, sequence number,
   SSRC and CSRC identifiers) are used as specified in Section 5.1 of

4.2.  VP9 Payload Descriptor
   In flexible mode (with the F bit below set to 1), The first octets
   after the RTP header are the VP9 payload descriptor, with the
   following structure.

         0 1 2 3 4 5 6 7
        |I|P|L|F|B|E|V|Z| (REQUIRED)
        +-+-+-+-+-+-+-+-+                             -\
   P,F: | P_DIFF      |N| (CONDITIONALLY REQUIRED)    - up to 3 times
        +-+-+-+-+-+-+-+-+                             -/
   V:   | SS            |
        | ..            |

                                 Figure 2

   In non-flexible mode (with the F bit below set to 0), The first
   octets after the RTP header are the VP9 payload descriptor, with the
   following structure.

         0 1 2 3 4 5 6 7
        |I|P|L|F|B|E|V|Z| (REQUIRED)
   V:   | SS            |
        | ..            |

                                 Figure 3

   I: Picture ID (PID) present.  When set to one, the OPTIONAL PID MUST
      be present after the mandatory first octet and specified as below.

      Otherwise, PID MUST NOT be present.  If the SS field was present
      in the stream's most recent start of a keyframe (i.e., non-
      flexible scalability mode is in use), then the PID MUST also be
      present in every packet.

   P: Inter-picture predicted frame.  When set to zero, the frame does
      not utilize inter-picture prediction.  In this case, up-switching
      to a current spatial layer's frame is possible from directly lower
      spatial layer frame.  P SHOULD also be set to zero when encoding a
      layer synchronization frame in response to an LRR
      [I-D.ietf-avtext-lrr] message (see Section 5.3).  When P is set to
      zero, the TID field (described below) MUST also be set to 0 (if
      present).  Note that the P bit does not forbid intra-picture,
      inter-layer prediction from earlier frames of the same picture, if

   L: Layer indices present.  When set to one, the one or two octets
      following the mandatory first octet and the PID (if present) is as
      described by "Layer indices" below.  If the F bit (described
      below) is set to 1 (indicating flexible mode), then only one octet
      is present for the layer indices.  Otherwise if the F bit is set
      to 0 (indicating non-flexible mode), then two octets are present
      for the layer indices.

   F: Flexible mode.  F set to one indicates flexible mode and if the P
      bit is also set to one, then the octets following the mandatory
      first octet, the PID, and layer indices (if present) are as
      described by "Reference indices" below.  This MUST only be set to
      1 if the I bit is also set to one; if the I bit is set to zero,
      then this MUST also be set to zero and ignored by receivers.  The
      value of this F bit MUST only change on the first packet of a key
      picture.  A key picture is a picture whose base spatial layer
      frame is a key frame, and which thus completely resets the encoder
      state.  This packet will have its P bit equal to zero, SID or D
      bit (described below) equal to zero, and B bit (described below)
      equal to 1.

   B: Start of a frame.  MUST be set to 1 if the first payload octet of
      the RTP packet is the beginning of a new VP9 frame, and MUST NOT
      be 1 otherwise.  Note that this frame might not be the first frame
      of a picture.

   E: End of a frame.  MUST be set to 1 for the final RTP packet of a
      VP9 frame, and 0 otherwise.  This enables a decoder to finish
      decoding the frame, where it otherwise may need to wait for the
      next packet to explicitly know that the frame is complete.  Note
      that, if spatial scalability is in use, more frames from the same
      picture may follow; see the description of the M bit above.

   V: Scalability structure (SS) data present.  When set to one, the
      OPTIONAL SS data MUST be present in the payload descriptor.
      Otherwise, the SS data MUST NOT be present.

   -: Bit reserved

   Z: Not a reference frame for future use.  MUST be upper spatial layers.  If set to zero 1,
      indicates that frames with higher spatial layers SID+1 of the
      current and MUST be
      ignored by following pictures do not depend on the receiver. current
      spatial layer SID frame.  This enables a decoder which is
      targeting a higher spatial layer to know that it can safely
      discard this packet's frame without processing it, without having
      to wait for the "D" bit in the higher-layer frame (see below).

   The mandatory first octet is followed by the extension data fields
   that are enabled:

   M: The most significant bit of the first octet is an extension flag.
      The field MUST be present if the I bit is equal to one.  If set,
      the PID field MUST contain 15 bits; otherwise, it MUST contain 7
      bits.  See PID below.

   Picture ID (PID):  Picture ID represented in 7 or 15 bits, depending
      on the M bit.  This is a running index of the pictures.  The field
      MUST be present if the I bit is equal to one.  If M is set to
      zero, 7 bits carry the PID; else if M is set to one, 15 bits carry
      the PID in network byte order.  The sender may choose between a 7-
      or 15-bit index.  The PID SHOULD start on a random number, and
      MUST wrap after reaching the maximum ID.  The receiver MUST NOT
      assume that the number of bits in PID stay the same through the

      In the non-flexible mode (when the F bit is set to 0), this PID is
      used as an index to the picture group (PG) specified in the SS
      data below.  In this mode, the PID of the key frame corresponds to
      the first specified frame in the PG.  Then subsequent PIDs are
      mapped to subsequently specified frames in the PG (modulo N_G,
      specified in the SS data below), respectively.

      All frames of the same picture MUST have the same PID value.

      Frames (and their corresponding pictures) with the VP9 show_frame
      field equal to 0 MUST have distinct PID values from subsequent
      pictures with show_frame equal to 1.  Thus, a Picture as defined
      in this specification is different than a VP9 Superframe.

      All frames of the same picture MUST have the same value for

   Layer indices:  This information is optional but recommended whenever
      encoding with layers.  For both flexible and non-flexible modes,
      one octet is used to specify a layer frame's temporal layer ID
      (TID) and spatial layer ID (SID) as shown both in Figure 2 and
      Figure 3.  Additionally, a bit (U) is used to indicate that the
      current frame is a "switching up point" frame.  Another bit (D) is
      used to indicate whether inter-layer prediction is used for the
      current frame.

      In the non-flexible mode (when the F bit is set to 0), another
      octet is used to represent temporal layer 0 index (TL0PICIDX), as
      depicted in Figure 3.  The TL0PICIDX is present so that all
      minimally required frames - the base temporal layer frames - can
      be tracked.

      The TID and SID fields indicate the temporal and spatial layers
      and can help middleboxes and and endpoints quickly identify which
      layer a packet belongs to.

      TID:  The temporal layer ID of current frame.  In the case of non-
         flexible mode, if PID is mapped to a picture in a specified PG,
         then the value of TID MUST match the corresponding TID value of
         the mapped picture in the PG.

      U: Switching up point.  If this bit is set to 1 for the current
         picture with temporal layer ID equal to TID, then "switch up"
         to a higher frame rate is possible as subsequent higher
         temporal layer pictures will not depend on any picture before
         the current picture (in coding order) with temporal layer ID
         greater than TID.

      SID:  The spatial layer ID of current frame.  Note that frames
         with spatial layer SDI > 0 may be dependent on decoded spatial
         layer SID-1 frame within the same picture.  Different frames of
         the same picture MUST have distinct spatial layer IDs, and
         frames' spatial layers MUST appear in increasing order within
         the frame.

      D: Inter-layer dependency used.  MUST be set to one if current
         spatial layer SID frame depends on spatial layer SID-1 frame of
         the same picture.  MUST only be set to zero if current spatial
         layer SID frame does not depend on spatial layer SID-1 frame of
         the same picture.  For the base layer frame (with SID equal to
         0), this D bit MUST be set to zero.

      TL0PICIDX:  8 bits temporal layer zero index.  TL0PICIDX is only
         present in the non-flexible mode (F = 0).  This is a running
         index for the temporal base layer pictures, i.e., the pictures
         with TID set to 0.  If TID is larger than 0, TL0PICIDX
         indicates which temporal base layer picture the current picture
         depends on.  TL0PICIDX MUST be incremented when TID is equal to
         0.  The index SHOULD start on a random number, and MUST restart
         at 0 after reaching the maximum number 255.

   Reference indices:  When P and F are both set to one, indicating a
      non-key frame in flexible mode, then at least one reference index
      has to be specified as below.  Additional reference indices (total
      of up to 3 reference indices are allowed) may be specified using
      the N bit below.  When either P or F is set to zero, then no
      reference index is specified.

      P_DIFF:  The reference index (in 7 bits) specified as the relative
         PID from the current picture.  For example, when P_DIFF=3 on a
         packet containing the picture with PID 112 means that the
         picture refers back to the picture with PID 109.  This
         calculation is done modulo the size of the PID field, i.e.,
         either 7 or 15 bits.

      N: 1 if there is additional P_DIFF following the current P_DIFF.

4.2.1.  Scalability Structure (SS):

   The scalability structure (SS) data describes the resolution of each
   frame within a picture as well as the inter-picture dependencies for
   a picture group (PG).  If the VP9 payload descriptor's "V" bit is
   set, the SS data is present in the position indicated in Figure 2 and
   Figure 3.

   V:   | N_S |Y|G|-|-|-|
        +-+-+-+-+-+-+-+-+              -\
   Y:   |     WIDTH     | (OPTIONAL)    .
        +               +               .
        |               | (OPTIONAL)    .
        +-+-+-+-+-+-+-+-+               . - N_S + 1 times
        |     HEIGHT    | (OPTIONAL)    .
        +               +               .
        |               | (OPTIONAL)    .
        +-+-+-+-+-+-+-+-+              -/
   G:   |      N_G      | (OPTIONAL)
        +-+-+-+-+-+-+-+-+                            -\
   N_G: | TID |U| R |-|-| (OPTIONAL)                 .
        +-+-+-+-+-+-+-+-+              -\            . - N_G times
        |    P_DIFF     | (OPTIONAL)    . - R times  .
        +-+-+-+-+-+-+-+-+              -/            -/

                                 Figure 4

   N_S:  N_S + 1 indicates the number of spatial layers present in the
      VP9 stream.

   Y: Each spatial layer's frame resolution present.  When set to one,
      the OPTIONAL WIDTH (2 octets) and HEIGHT (2 octets) MUST be
      present for each layer frame.  Otherwise, the resolution MUST NOT
      be present.

   G: PG description present flag.

   -: Bit reserved for future use.  MUST be set to zero and MUST be
      ignored by the receiver.

   N_G:  N_G indicates the number of pictures in a Picture Group (PG).
      If N_G is greater than 0, then the SS data allows the inter-
      picture dependency structure of the VP9 stream to be pre-declared,
      rather than indicating it on the fly with every packet.  If N_G is
      greater than 0, then for N_G pictures in the PG, each picture's
      temporal layer ID (TID), switch up point (U), and the R reference
      indices (P_DIFFs) are specified.

      The first picture specified in the PG MUST have TID set to 0.

      G set to 0 or N_G set to 0 indicates that either there is only one
      temporal layer or no fixed inter-picture dependency information is
      present going forward in the bitstream.

      Note that for a given picture, all frames follow the same inter-
      picture dependency structure.  However, the frame rate of each
      spatial layer can be different from each other and this can be
      controlled with the use of the D bit described above.  The
      specified dependency structure in the SS data MUST be for the
      highest frame rate layer.

   In a scalable stream sent with a fixed pattern, the SS data SHOULD be
   included in the first packet of every key frame.  This is a packet
   with P bit equal to zero, SID or D bit equal to zero, and B bit equal
   to 1.  The SS data MUST only be changed on the picture that
   corresponds to the first picture specified in the previous SS data's
   PG (if the previous SS data's N_G was greater than 0).

4.3.  Frame Fragmentation

   VP9 frames are fragmented into packets, in RTP sequence number order,
   beginning with a packet with the B bit set, and ending with a packet
   with the E bit set.  There is no mechanism for finer-grained access
   to parts of a VP9 frame.

4.4.  Scalable encoding considerations

   In addition to the use of reference frames, VP9 has several
   additional forms of inter-frame dependencies, largely involving
   probability tables for the entropy and tree encoders.  In VP9 syntax,
   the syntax element "error_resilient_mode" resets this additional
   inter-frame data, allowing a frame's syntax to be decoded

   Due to the requirements of scalable streams, a VP9 encoder producing
   a scalable stream needs to ensure that a frame does not depend on a
   previous frame (of the same or a previous picture) that can
   legitimately be removed from the stream.  Thus, a frame that follows
   a removable frame (in full decode order) MUST be encoded with
   "error_resilient_mode" set to true.

   For spatially-scalable streams, this means that
   "error_resilient_mode" needs to be turned on for the base spatial
   layer; it can however be turned off for higher spatial layers,
   assuming they are sent with inter-layer dependency (i.e. with the "D"
   bit set).  For streams that are only temporally-scalable without
   spatial scalability, "error_resilient_mode" can additionally be
   turned off for any picture that immediately follows a temporal layer
   0 frame.

4.5.  Examples of VP9 RTP Stream

4.5.1.  Reference picture use for scalable structure

   As discussed in Section 3, the VP9 codec can maintain up to eight
   reference frames, of which up to three can be referenced or updated
   by any new frame.  This section illustrates one way that a scalable
   structure (with three spatial layers and three temporal layers) can
   be constructed using these reference frames.

               | Temporal | Spatial | References | Updates |
               |    0     |    0    |     0      |    0    |
               |          |         |            |         |
               |    0     |    1    |    0,1     |    1    |
               |          |         |            |         |
               |    0     |    2    |    1,2     |    2    |
               |          |         |            |         |
               |    2     |    0    |     0      |    6    |
               |          |         |            |         |
               |    2     |    1    |    1,6     |    7    |
               |          |         |            |         |
               |    2     |    2    |    2,7     |    -    |
               |          |         |            |         |
               |    1     |    0    |     0      |    3    |
               |          |         |            |         |
               |    1     |    1    |    1,3     |    4    |
               |          |         |            |         |
               |    1     |    2    |    2,4     |    5    |
               |          |         |            |         |
               |    2     |    0    |     3      |    6    |
               |          |         |            |         |
               |    2     |    1    |    4,6     |    7    |
               |          |         |            |         |
               |    2     |    2    |    5,7     |    -    |

                       Example scalability structure

   This structure is constructed such that the "U" bit can always be

5.  Feedback Messages and Header Extensions
5.1.  Reference Picture Selection Indication (RPSI)

   The reference picture selection index is a payload-specific feedback
   message defined within the RTCP-based feedback format.  The RPSI
   message is generated by a receiver and can be used in two ways.
   Either it can signal a preferred reference picture when a loss has
   been detected by the decoder -- preferably then a reference that the
   decoder knows is perfect -- or, it can be used as positive feedback
   information to acknowledge correct decoding of certain reference
   pictures.  The positive feedback method is useful for VP9 used for
   point to point (unicast) communication.  The use of RPSI for VP9 is
   preferably combined with a special update pattern of the codec's two
   special reference frames -- the golden frame and the altref frame --
   in which they are updated in an alternating leapfrog fashion.  When a
   receiver has received and correctly decoded a golden or altref frame,
   and that frame had a PictureID in the payload descriptor, the
   receiver can acknowledge this simply by sending an RPSI message back
   to the sender.  The message body (i.e., the "native RPSI bit string"
   in [RFC4585]) is simply the PictureID of the received frame.

   Note: because all frames of the same picture must have the same
   inter-picture reference structure, there is no need for a message to
   specify which frame is being selected.

5.2.  Full Intra Request (FIR)

   The Full Intra Request (FIR) [RFC5104] RTCP feedback message allows a
   receiver to request a full state refresh of an encoded stream.

   Upon receipt of an FIR request, a VP9 sender MUST send a picture with
   a keyframe for its spatial layer 0 layer frame, and then send frames
   without inter-picture prediction (P=0) for any higher layer frames.

5.3.  Layer Refresh Request (LRR)

   The Layer Refresh Request [I-D.ietf-avtext-lrr] allows a receiver to
   request a single layer of a spatially or temporally encoded stream to
   be refreshed, without necessarily affecting the stream's other

               |   RES   | TID | RES     | SID |

                                 Figure 5

   Figure 5 shows the format of LRR's layer index fields for VP9
   streams.  The two "RES" fields MUST be set to 0 on transmission and
   ingnored on reception.  See Section 4.2 for details on the TID and
   SID 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


   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.

5.4.  Frame Marking

   The Frame Marking RTP header extension [I-D.ietf-avtext-framemarking]
   is a mechanism to provide information about frames of video streams
   in a largely codec-independent manner.  However, for its extension
   for scalable codecs, the specific manner in which codec layers are
   identified needs to be specified specifically for each codec.  This
   section defines how frame marking is used with VP9.

       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
      |  ID=2 |  L=2  |S|E|I|D|B| TID |0|0|0|0|0| SID |    TL0PICIDX  |

                                 Figure 6

   When this header extension is used with VP9, the TID and SID fields
   MUST match the values in the packet which the header extension is
   attached to; see Section 4.2 for details on these fields.

   See [I-D.ietf-avtext-framemarking] for explanations of the other
   fields, which are generic.

6.  Payload Format Parameters

   This payload format has two optional parameters.

6.1.  Media Type Definition

   This registration is done using the template defined in [RFC6838] and
   following [RFC4855].

   Type name:  video

   Subtype name:  VP9

   Required parameters:  None.

   Optional parameters:
      These parameters are used to signal the capabilities of a receiver
      implementation.  If the implementation is willing to receive
      media, both parameters MUST be provided.  These parameters MUST
      NOT be used for any other purpose.

      max-fr:  The value of max-fr is an integer indicating the maximum
         frame rate in units of frames per second that the decoder is
         capable of decoding.

      max-fs:  The value of max-fs is an integer indicating the maximum
         frame size in units of macroblocks that the decoder is capable
         of decoding.

         The decoder is capable of decoding this frame size as long as
         the width and height of the frame in macroblocks are less than
         int(sqrt(max-fs * 8)) - for instance, a max-fs of 1200 (capable
         of supporting 640x480 resolution) will support widths and
         heights up to 1552 pixels (97 macroblocks).

      profile-id:  The value of profile-id is an integer indicating the
         default coding profile, the subset of coding tools that may
         have been used to generate the stream or that the receiver
         supports).  Table 1 lists all of the profiles defined in
         section 7.2 of [VP9-BITSTREAM] and the corresponding integer
         values to be used.

         If no profile-id is present, Profile 0 MUST be inferred.

         Informative note: See Table 2 for capabilities of coding
         profiles defined in section 7.2 of [VP9-BITSTREAM].

   Encoding considerations:

      This media type is framed in RTP and contains binary data; see
      Section 4.8 of [RFC6838].

   Security considerations:  See Section 7 of RFC xxxx.
      [RFC Editor: Upon publication as an RFC, please replace "XXXX"
      with the number assigned to this document and remove this note.]

   Interoperability considerations:  None.

   Published specification:  VP9 bitstream format [VP9-BITSTREAM] and
      RFC XXXX.
      [RFC Editor: Upon publication as an RFC, please replace "XXXX"
      with the number assigned to this document and remove this note.]

   Applications which use this media type:
      For example: Video over IP, video conferencing.

   Fragment identifier considerations:  N/A.

   Additional information:  None.

   Person & email address to contact for further information:
      Jonathan Lennox <jonathan.lennox@8x8.com>

   Intended usage:  COMMON

   Restrictions on usage:
      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [RFC3550].

   Author:  Jonathan Lennox <jonathan.lennox@8x8.com>

   Change controller:
      IETF AVTCore Working Group delegated from the IESG.

                         | Profile | profile-id |
                         |    0    |     0      |
                         |         |            |
                         |    1    |     1      |
                         |         |            |
                         |    2    |     2      |
                         |         |            |
                         |    3    |     3      |

    Table 1: Table 1.  Table of profile-id integer values representing
    the VP9 profile corresponding to the set of coding tools supported.

   | Profile | Bit Depth | SRGB Colorspace |    Chroma Subsampling    |
   |    0    |     8     |        No       |        YUV 4:2:0         |
   |         |           |                 |                          |
   |    1    |     8     |       Yes       | YUV 4:2:0,4:4:0 or 4:4:4 |
   |         |           |                 |                          |
   |    2    |  10 or 12 |        No       |        YUV 4:2:0         |
   |         |           |                 |                          |
   |    3    |  10 or 12 |       Yes       | YUV 4:2:0,4:4:0 or 4:4:4 |

             Table 2: Table 2.  Table of profile capabilities.

6.2.  SDP Parameters

   The receiver MUST ignore any fmtp parameter unspecified in this memo.

6.2.1.  Mapping of Media Subtype Parameters to SDP

   The media type video/VP9 string is mapped to fields in the Session
   Description Protocol (SDP) [RFC4566] as follows:

   o  The media name in the "m=" line of SDP MUST be video.

   o  The encoding name in the "a=rtpmap" line of SDP MUST be VP9 (the
      media subtype).

   o  The clock rate in the "a=rtpmap" line MUST be 90000.

   o  The parameters "max-fs", and "max-fr", MUST be included in the
      "a=fmtp" line of SDP if SDP is used to declare receiver
      capabilities.  These parameters are expressed as a media subtype
      string, in the form of a semicolon separated list of
      parameter=value pairs.

   o  The OPTIONAL parameter profile-id, when present, SHOULD be
      included in the "a=fmtp" line of SDP.  This parameter is expressed
      as a media subtype string, in the form of a parameter=value pair.
      When the parameter is not present, a value of 0 MUST be used for
      profile-id.  Example

   An example of media representation in SDP is as follows:

   m=video 49170 RTP/AVPF 98
   a=rtpmap:98 VP9/90000
   a=fmtp:98 max-fr=30; max-fs=3600; profile-id=0;

6.2.2.  Offer/Answer Considerations

   When VP9 is offered over RTP using SDP in an Offer/Answer model
   [RFC3264] for negotiation for unicast usage, the following
   limitations and rules apply:

   o  The parameter identifying a media format configuration for VP9 is
      profile-id.  This media format configuration parameter MUST be
      used symmetrically; that is, the answerer MUST either maintain all
      configuration parameters or remove the media format (payload type)
      completely if one or more of the parameter values are not

   o  To simplify the handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [RFC3264].  An answer MUST NOT
      contain the payload type number used in the offer unless the
      configuration is exactly the same as in the offer.

7.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550], and in any applicable RTP profile such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
   SAVPF [RFC5124].  SAVPF [RFC5124].  However, as "Securing the RTP
   Protocol Framework: Why RTP Does Not Mandate a Single Media Security
   Solution" [RFC7202] discusses, it is not an RTP payload format's
   responsibility to discuss or mandate what solutions are used to meet
   the basic security goals like confidentiality, integrity and source
   authenticity for RTP in general.  This responsibility lays on anyone
   using RTP in an application.  They can find guidance on available
   security mechanisms in Options for Securing RTP Sessions [RFC7201].
   Applications SHOULD use one or more appropriate strong security
   mechanisms.  The rest of this security consideration section
   discusses the security impacting properties of the payload format

   This RTP payload format and its media decoder do not exhibit any
   significant non-uniformity in the receiver-side computational
   complexity for packet processing, and thus are unlikely to pose a
   denial-of-service threat due to the receipt of pathological data.
   Nor does the RTP payload format contain any active content.

8.  Congestion Control

   Congestion control for RTP SHALL be used in accordance with RFC 3550
   [RFC3550], and with any applicable RTP profile; e.g., RFC 3551
   [RFC3551].  The congestion control mechanism can, in a real-time
   encoding scenario, adapt the transmission rate by instructing the
   encoder to encode at a certain target rate.  Media aware network
   elements MAY use the information in the VP9 payload descriptor in
   Section 4.2 to identify non-reference frames and discard them in
   order to reduce network congestion.  Note that discarding of non-
   reference frames cannot be done if the stream is encrypted (because
   the non-reference marker is encrypted).

9.  IANA Considerations

   The IANA is requested to register the media type registration "video/
   vp9" as specified in Section 6.1.  The media type is also requested
   to be added to the IANA registry for "RTP Payload Format MIME types"

10.  Acknowledgments

   Alex Eleftheriadis, Yuki Ito, Won Kap Jang, Sergio Garcia Murillo,
   Roi Sasson, Timothy Terriberry, Emircan Uysaler, and Thomas Volkert
   commented on the development of this document and provided helpful
   comments and feedback.

11.  References

11.1.  Normative References

              Zanaty, M., Berger, E., and S. Nandakumar, "Frame Marking
              RTP Header Extension", draft-ietf-avtext-framemarking-10
              (work in progress), November 2019.

              Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
              Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
              Message", draft-ietf-avtext-lrr-07 (work in progress),
              July 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [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,

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,

   [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, <https://www.rfc-editor.org/info/rfc5104>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

              Grange, A., de Rivaz, P., and J. Hunt, "VP9 Bitstream &
              Decoding Process Specification", Version 0.6, March 2016,

11.2.  Informative References

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

   [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,

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <https://www.rfc-editor.org/info/rfc7202>.

Authors' Addresses

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

   Email: justin@uberti.name
   Stefan Holmer
   Google, Inc.
   Kungsbron 2
   Stockholm  111 22

   Email: holmer@google.com

   Magnus Flodman
   Google, Inc.
   Kungsbron 2
   Stockholm  111 22

   Email: mflodman@google.com

   Danny Hong
   Google, Inc.
   1585 Charleston Road
   Mountain View, CA  94043

   Email: dannyhong@google.com

   Jonathan Lennox
   8x8, Inc. / Jitsi
   1350 Broadway
   New York, NY  10018

   Email: jonathan.lennox@8x8.com