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Versions: (draft-uberti-payload-vp9) 00 01 02

Payload Working Group                                          J. Uberti
Internet-Draft                                                 S. Holmer
Intended status: Standards Track                              M. Flodman
Expires: September 19, 2016                                       Google
                                                               J. Lennox
                                                                 D. Hong
                                                                   Vidyo
                                                          March 18, 2016


                    RTP Payload Format for VP9 Video
                       draft-ietf-payload-vp9-02

Abstract

   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
   scalability.

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 September 19, 2016.

Copyright Notice

   Copyright (c) 2016 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions and Acronyms . . . . . . . . . . . .   3
   3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
   4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   5
     4.2.  VP9 Payload Description . . . . . . . . . . . . . . . . .   6
       4.2.1.  Scalability Structure (SS): . . . . . . . . . . . . .  10
     4.3.  VP9 Payload Header  . . . . . . . . . . . . . . . . . . .  12
     4.4.  Frame Fragmentation . . . . . . . . . . . . . . . . . . .  12
     4.5.  Examples of VP9 RTP Stream  . . . . . . . . . . . . . . .  12
   5.  Feedback Messages . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Reference Picture Selection Indication (RPSI) . . . . . .  12
     5.2.  Slice Loss Indication (SLI) . . . . . . . . . . . . . . .  13
     5.3.  Full Intra Request (FIR)  . . . . . . . . . . . . . . . .  13
     5.4.  Layer Refresh Request (LRR) . . . . . . . . . . . . . . .  14
   6.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  14
     6.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  15
     6.2.  SDP Parameters  . . . . . . . . . . . . . . . . . . . . .  16
       6.2.1.  Mapping of Media Subtype Parameters to SDP  . . . . .  16
       6.2.2.  Offer/Answer Considerations . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This memo describes an RTP payload specification applicable to the
   transmission of video streams encoded using the VP9 video codec
   [I-D.grange-vp9-bitstream].  The format described in this document
   can be used both in peer-to-peer and video conferencing applications.

   TODO: VP9 description.  Please see [I-D.grange-vp9-bitstream].








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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].

   TODO: Cite terminology from [I-D.grange-vp9-bitstream].

3.  Media Format Description

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

   VP9 also allows a reference frame to be resampled and used as a
   reference for another frame of a different resolution.  This allows
   internal resolution changes without requiring the use of key frames.

   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.

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

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



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   Within a super frame, a layer frame with spatial layer ID equal to S,
   where S > 0, can depend on a layer frame of the same super frame 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 super frame and if inter-layer dependency is
   used, a spatial layer S frame can only depend on spatial layer S-1
   frame when S > 0.  Additionally, if inter-picture dependency is used,
   spatial layer S frame is assumed to only depend on previously coded
   spatial layer S 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 S frame depends on spatial layer S-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 very 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
   dynamically.

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

   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




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   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 and VP9 payload header will be described
   in Section 4.2 and Section 4.3.  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 super frame), and 0
      otherwise.  Unless spatial scalability is in use for this super
      frame, 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.




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   Payload Type (TP):  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 frame is
      encoded with multiple layer frames, all of the layer frames of the
      super frame MUST have the same timestamp.

   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
   [RFC3550].

4.2.  VP9 Payload Description

   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|-| (REQUIRED)
        +-+-+-+-+-+-+-+-+
   I:   |M| PICTURE ID  | (REQUIRED)
        +-+-+-+-+-+-+-+-+
   M:   | EXTENDED PID  | (RECOMMENDED)
        +-+-+-+-+-+-+-+-+
   L:   |  T  |U|  S  |D| (CONDITIONALLY RECOMMENDED)
        +-+-+-+-+-+-+-+-+                             -\
   P,F: | P_DIFF      |N| (CONDITIONALLY REQUIRED)    - up to 3 times
        +-+-+-+-+-+-+-+-+                             -/
   V:   | SS            |
        | ..            |
        +-+-+-+-+-+-+-+-+


                                 Figure 2











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   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|-| (REQUIRED)
        +-+-+-+-+-+-+-+-+
   I:   |M| PICTURE ID  | (RECOMMENDED)
        +-+-+-+-+-+-+-+-+
   M:   | EXTENDED PID  | (RECOMMENDED)
        +-+-+-+-+-+-+-+-+
   L:   |  T  |U|  S  |D| (CONDITIONALLY RECOMMENDED)
        +-+-+-+-+-+-+-+-+
        |   TL0PICIDX   | (CONDITIONALLY 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.

   P: Inter-picture predicted layer frame.  When set to zero, the layer
      frame does not utilize inter-picture prediction.  In this case,
      up-switching to 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 5.4).  When P is set to
      zero, the T bit (described below) MUST also be set to 0 (if
      present).

   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



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      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 CAN ONLY CHANGE on the very first packet of a
      key picture.  This is a packet with the P bit equal to zero, S or
      D bit (described below) equal to zero, and B bit (described below)
      equal to 1.

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

   E: End of a layer frame.  MUST be set to 1 for the final RTP packet
      of a VP9 layer frame, and 0 otherwise.  This enables a decoder to
      finish decoding the layer frame, where it otherwise may need to
      wait for the next packet to explicitly know that the layer frame
      is complete.  Note that, if spatial scalability is in use, more
      layer frames from the same super frame 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 for future use.  MUST be set to zero and MUST be
      ignored by the receiver.

   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
      session.

      In the non-flexible mode (when the F bit is set to 0), this PID is
      used as an index to the GOF specified in the SS data bleow.  In
      this mode, the PID of the key frame corresponds to the very first



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      specified frame in the GOF.  Then subsequent PIDs are mapped to
      subsequently specified frames in the GOF (modulo N_G, specified in
      the SS data below), respectively.

   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 (T)
      and spatial layer ID (S) 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
      layer 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 T and S fields indicate the temporal and spatial layers and
      can help middleboxes and and endpoints quickly identify which
      layer a packet belongs to.

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

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

      S: The spatial layer ID of current frame.  Note that frames with
         spatial layer S > 0 may be dependent on decoded spatial layer
         S-1 frame within the same super frame.

      D: Inter-layer dependency used.  MUST be set to one if current
         spatial layer S frame depends on spatial layer S-1 frame of the
         same super frame.  MUST only be set to zero if current spatial
         layer S frame does not depend on spatial layer S-1 frame of the
         same super frame.  For the base layer frame with S 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 frames, i.e., the frames with



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         T set to 0.  If T is larger than 0, TL0PICIDX indicates which
         temporal base layer frame the current frame depends on.
         TL0PICIDX MUST be incremented when T 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 frame.  For example, when P_DIFF=3 on a
         packet containing the frame with PID 112 means that the frame
         refers back to the frame 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
   layer frame within a super frame as well as the inter-picture
   dependencies for a group of frames (GOF).  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.






















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        +-+-+-+-+-+-+-+-+
   V:   | N_S |Y|G|-|-|-|
        +-+-+-+-+-+-+-+-+              -\
   Y:   |     WIDTH     | (OPTIONAL)    .
        +               +               .
        |               | (OPTIONAL)    .
        +-+-+-+-+-+-+-+-+               . - N_S + 1 times
        |     HEIGHT    | (OPTIONAL)    .
        +               +               .
        |               | (OPTIONAL)    .
        +-+-+-+-+-+-+-+-+              -/            -\
   G:   |      N_G      | (OPTIONAL)
        +-+-+-+-+-+-+-+-+                            -\
   N_G: |  T  |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: GOF 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 frames in a GOF.  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 GOF, each frame's temporal
      layer ID (T), switch up point (U), and the R reference indices
      (P_DIFFs) are specified.

      The very first frame specified in the GOF MUST have T 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.





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      Note that for a given super frame, all layer 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, S or D bit equal to zero, and B bit equal
   to 1.  The SS data MUST only be changed on the frame that corresponds
   to the very first frame specified in the previous SS data's GOF (if
   the previous SS data's N_G was greater than 0).

4.3.  VP9 Payload Header

   TODO: need to describe VP9 payload header.

4.4.  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 RTP marker bit M set.  There is no mechanism for finer-
   grained access to parts of a VP9 frame.

4.5.  Examples of VP9 RTP Stream

   TODO

5.  Feedback Messages

5.1.  Reference Picture Selection Indication (RPSI)

   TODO: Update to indicate which frame within the picture.

   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,



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   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.

5.2.  Slice Loss Indication (SLI)

   TODO: Update to indicate which frame within the picture.

   The slice loss indication is another payload-specific feedback
   message defined within the RTCP-based feedback format.  The SLI
   message is generated by the receiver when a loss or corruption is
   detected in a frame.  The format of the SLI message is as follows
   [RFC4585]:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         First           |        Number           | PictureID |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 5

   Here, First is the macroblock address (in scan order) of the first
   lost block and Number is the number of lost blocks, as defined in
   [RFC4585].  PictureID is the six least significant bits of the codec-
   specific picture identifier in which the loss or corruption has
   occurred.  For VP9, this codec-specific identifier is naturally the
   PictureID of the current frame, as read from the payload descriptor.
   If the payload descriptor of the current frame does not have a
   PictureID, the receiver MAY send the last received PictureID+1 in the
   SLI message.  The receiver MAY set the First parameter to 0, and the
   Number parameter to the total number of macroblocks per frame, even
   though only part of the frame is corrupted.  When the sender receives
   an SLI message, it can make use of the knowledge from the latest
   received RPSI message.  Knowing that the last golden or altref frame
   was successfully received, it can encode the next frame with
   reference to that established reference.

5.3.  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 super frame
   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.



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5.4.  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
   layers.

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

                                 Figure 6

   Figure 6 shows the format of LRR's 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 Section 4.2 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.

   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.

6.  Payload Format Parameters

   This payload format has two optional parameters.







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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).

   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
      [I-D.grange-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.



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   Fragment identifier considerations:  N/A.

   Additional information:  None.

   Person & email address to contact for further information:
      TODO [Pick a contact]

   Intended usage:  COMMON

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

   Author:  TODO [Pick a contact]

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

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.

6.2.1.1.  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;




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6.2.2.  Offer/Answer Considerations

   TODO: Update this for VP9

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
   itself.

   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 following values:
   - Media type registration as described in Section 6.1.





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10.  References

10.1.  Normative References

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

   [I-D.ietf-avtext-lrr]
              Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
              Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
              Message", draft-ietf-avtext-lrr-01 (work in progress),
              October 2015.

   [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>.

   [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, <http://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, <http://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,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <http://www.rfc-editor.org/info/rfc4855>.

   [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>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.



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10.2.  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,
              <http://www.rfc-editor.org/info/rfc3551>.

   [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,
              <http://www.rfc-editor.org/info/rfc3711>.

   [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, <http://www.rfc-editor.org/info/rfc5124>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

   [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, <http://www.rfc-editor.org/info/rfc7202>.

Authors' Addresses

   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

   Email: holmer@google.com







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

   Email: mflodman@google.com


   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
























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