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Versions: 00 01 02

Network Working Group                                          P. Westin
Internet-Draft                                                 H. Lundin
Intended status: Experimental                                  M. Glover
Expires: September 9, 2011                                     J. Uberti
                                                             F. Galligan
                                                                  Google
                                                           March 8, 2011


      Proposal for the IETF on "RTP Payload Format for VP8 Video"
                      draft-westin-payload-vp8-02

Abstract

   This memo describes an RTP payload format for the VP8 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.

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 9, 2011.

Copyright Notice

   Copyright (c) 2011 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
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

















































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1.  Introduction

   This memo describes an RTP payload specification applicable to the
   transmission of video streams encoded using the VP8 video codec [1].
   The format described in this document can be used both in peer-to-
   peer and video conferencing applications.

   The VP8 codec uses three different reference frames for interframe
   prediction: the previous frame, the golden frame, and the altref
   frame.  The payload specification in this memo has elements that
   enable advanced use of the reference frames, e.g., for improved loss
   robustness.

   Another property of the VP8 codec is that it applies data
   partitioning to the encoded data.  Thus, an encoded VP8 frame can be
   divided into two or more partitions, as described in "VP8 Data Format
   and Decoding Guide" [1].  The first partition (prediction or mode)
   contains prediction mode parameters and motion vectors for all
   macroblocks.  The remaining partitions all contain the DCT/WHT
   coefficients for the residuals.  The first partition is decodable
   without the remaining residual partitions.  The subsequent partitions
   may be useful even if some part of the frame is lost.  This memo
   allows the partitions to be sent in the same RTP packet.
   Nevertheless, it may be beneficial for decoder error-concealment to
   use separate packets for the two partition types, even though it is
   not mandatory according to this specification.

   The format specification is described in Section 4.  Section 5
   describes a method to acknowledge receipt of reference frames using
   RTCP techniques is described.  Both these examples serve as
   motivation for two of the fields included in the payload format: the
   "1st partition size" and "PictureID" fields.



















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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 RFC 2119 [2].














































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3.  Media Format Background

   VP8 is based on decomposition of frames into square subblocks of
   pixels, prediction of such subblocks using previously constructed
   blocks, and adjustment of such predictions (as well as synthesis of
   unpredicted blocks) using a discrete cosine transform (hereafter
   abbreviated as DCT).  In one special case, however, VP8 uses a
   "Walsh-Hadamard" (hereafter abbreviated as WHT) transform instead of
   a DCT.  An encoded VP8 frame is divided into two or more partitions,
   as described in [1].  The first partition (prediction or mode)
   contains prediction mode parameters and motion vectors for all
   macroblocks.  The remaining partitions all contain the DCT/WHT
   coefficients for the residuals.






































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4.  Payload Format

   The general RTP payload format for VP8 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             |
     |                             ....                              |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |            VP8 payload descriptor (integer #bytes)            |
     :                                                               :
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               : VP8 payload header (3 octets) |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | VP8 pyld hdr  :                                               |
     +-+-+-+-+-+-+-+-+                                               |
     :                   Bytes 4..N of VP8 payload                   :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The VP8 payload descriptor and VP8 payload header will be described
   in the sequel.  OPTIONAL RTP padding MUST NOT be included unless the
   P bit is set.

                                 Figure 1

      Marker bit: The marker bit indicates the last packet part of an
      frame.  This enables an decoder to finish decoding the picture,
      where it otherwise may need to wait for the next packet to
      explicitly know that.

      Timestamp: The RTP timestamp indicate the time when the frame was
      sampled at a clock rate of 90KHz.

      Sequence number: The sequence number are monotonically increasing
      and set as packets are sent.






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      The remaining RTP header fields are used as specified in RFC 3550
      [4].

4.1.  VP8 Payload Descriptor

   The first bytes after the RTP header are the VP8 payload descriptor,
   with the following structure.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | RSV |I|N|FI |B|         PictureID (integer #bytes)            |
     +-+-+-+-+-+-+-+-+                                               |
     :                                                               :
     |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               : (VP8 data or VP8 payload header; byte aligned)|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

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

      I: PictureID present.  When set to one, a PictureID is provided
      after the first byte of the payload descriptor.  When set to zero,
      the PictureID is omitted, and the one-byte payload descriptor is
      immediately followed by the VP8 payload.

      N: Non-reference frame.  When set to one, the frame can be
      discarded without affecting any other future or past frames.

      FI: Fragmentation information field.  This field contains
      information about the fragmentation of VP8 payloads carried in the
      RTP packet.  The four different values are listed below.

      *  00 The RTP packet contains no fragmented VP8 partitions.  The
         payload is one or several complete partitions.

      *  01 The RTP packet contains the first part of a fragmented
         partition.  The fragment must be placed in its own RTP packet.

      *  10 The RTP packet contains a fragment that is neither the first
         nor the last part of a fragmented partition.  The fragment must
         be placed in its own RTP packet.

      *  11 The RTP packet contains the last part of a fragmented
         partition.  The fragment must be placed in its own RTP packet.




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      B: Beginning VP8 frame.  When set to 1 this signals that a new VP8
      frame starts in this RTP packet.

      PictureID: Multiple of 8 bits.  This is a running index of the
      frames.  The field is present only if the I bit is equal to one.
      The most significant bit of each byte is an extension flag.  The 7
      following bits carry (parts of) the PictureID.  If the extension
      flag is one, the PictureID continues in the next byte.  If the
      extension flag is zero, the 7 remaining bits are the last (and
      least significant) bits in the PictureID.  The sender may choose
      any number of bytes, smaller or equal to 9 bytes as the maximum
      PictureID.  The PictureID SHALL start on a random number, and MUST
      wrap after reaching the maximum ID.  Leading zero bytes MUST be
      supressed.

4.2.  VP8 Payload Header

   The first three bytes of an encoded VP8 frame are refered to as an
   "uncompressed data chunk" in [1], and co-serve as payload header in
   this RTP format.  Note that the header is present only in packets
   which have the B bit equal to one in the payload descriptor.
   Subsequent packets for the same frame do not carry the payload
   header.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P| VER |H|         1st partition size          |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
     |                                                               |
     :                   Bytes 4..N of VP8 payload                   :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 3

      P: Inverse key frame flag.  When set to 0 the current frame is a
      key frame.  When set to 1 the current frame is an interframe.
      Defined in [1]

      VER: A version number as defined in [1].

      H: Show frame bit as defined in [1].






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      1st partition size: 19 bits.  A field containing the size of the
      first data partition in bytes, as defined in [1].

4.3.  Aggregated and Fragmented Payloads

   An encoded VP8 frame can be divided into two or more partitions, as
   described in Section 1.  The fragmentation information described in
   Section 4.1 MUST be used to signal if any fragmentation is applied.
   Aggregation of encoded partitions is done without explicit signaling.
   Partitions MUST be aggregated in decoding order.  Two fragments from
   different partitions MUST not be agregated into the same packet.  An
   aggregation MUST have exactly one payload descriptor.  Aggregated
   partitions MUST represent parts of one and the same video frame.
   Consequently, an aggregated packet will have one or no payload
   header, depending on whether the aggregate contains the first
   partition of a frame or not, respectively.  Note that the length of
   the first partition can always be obtained from the first partition
   size parameter in the VP8 payload header.  Fragments of encoded
   partitions MUST NOT be aggregated.

4.4.  Examples of VP8 RTP Stream

   A few examples of how the VP8 RTP payload can be used are included
   below.



























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4.4.1.  Key frame in a single RTP packet

   Marker bit = 1.  I = 1.  B = 1.  PictureID = 17 = 0001001 binary.  P
   = 0.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=1                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|0: VER :1:       1st partition |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | size = L      |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                                                               |
     :                Bytes 4..L of first VP8 partition              :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :                Remaining VP8 partitions                       :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

























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4.4.2.  VP8 interframe in a single RTP packet; no PictureID

   Marker bit = 1.  I = 0.  B = 1.  P = 1.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=1                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 0 0 0 0 1|1: VER :1:       1st partition size = L        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     :                Bytes 4..L of first VP8 partition              :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :                Remaining VP8 partitions                       :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.4.3.  VP8 partitions in separate RTP packets

   First RTP packet; marker bit = 0.  I = 1.  B = 1.  PictureID = 17.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=0                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|1: VER :1:       1st partition |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | size = L      |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                                                               |
     :                Bytes 4..L of first VP8 partition              :
     |                                                               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   Second RTP packet; marker bit = 1.  B = 0.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=1                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 0:0 0 0 0 1 0 0 1|                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :                Remaining VP8 partitions                       :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.4.4.  VP8 frame fragmented across RTP packets

   First RTP packet; marker bit = 0.  I = 1.  FI = 00.  B = 1.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=0                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|1: VER :1:       1st partition |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | size = L      |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                                                               |
     :                Bytes 4..L of first VP8 partition              :
     |                                                               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
















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   Second RTP packet; marker bit = 0.  FI = 01.  B = 0.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=0                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 1 0:0 0 0 0 1 0 0 1|                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :          First fragment of second VP8 partition               :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Third RTP packet; marker bit = 0.  FI = 10.  B = 0.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=0                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 1 0 0:0 0 0 0 1 0 0 1|                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :          Middle fragment of second VP8 partition              :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Last RTP packet; marker bit = 1.  FI = 11.  B = 0.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=1                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 1 1 0:0 0 0 0 1 0 0 1|                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :          Last fragment of second VP8 partition                :
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                               :    OPTIONAL RTP padding       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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4.4.5.  VP8 frame with long PictureID

   PictureID = 4711 = 01001001100111 binary (first 7 bits: 0100100, last
   7 bits: 1100111).

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       RTP Header M=1                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1:1 0 1 0 0 1 0 0 0 1 1 0 0 1 1 1|1: VER :1: 1st |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | partition size = L            |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     :                Bytes 4..N of first VP8 frame                  :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

































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5.  Using VP8 with RPSI and SLI Feedback

   The VP8 payload descriptor defined in Section 4.1 above contains an
   optional PictureID parameter.  This parameter is included mainly to
   enable use of reference picture selection index (RPSI) and slice loss
   indication (SLI), both defined in RFC 4585 [3].

5.1.  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 VP8 used as
   unicast.  The use of RPSI for VP8 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 RFC 4585 [3]) is
   simply the PictureID of the received frame.

5.2.  SLI

   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
   [3]:

     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 4

   Here, First is the macroblock address (in scan order) of the first
   lost block and Number is the number of lost blocks.  PictureID is the
   six least significant bits of the codec-specific picture identifier
   in which the loss or corruption has occurred.  For VP8, this codec-
   specific identifier is naturally the PictureID of the current frame,



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   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 parts 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.  Example

   The use of RSPI and SLI is best illustrated in an example.  In this
   example, the encoder may not update the altref frame until the last
   sent golden frame has been acknowledged with an RPSI message.  If an
   update is not received within some time, a new golden frame update is
   sent instead.  Once the new golden frame is established and
   acknowledge, the same rule applies when updating the altref frame.

     Event  Sender              Receiver                    Established
                                                            reference
     +----+--------------------+--------------------------+------------+
     1000  Send golden frame
            PictureID = 0
                                Receive and decode
                                golden frame

     1001                       Send RPSI(0)
     1002   Receive RPSI(0)                                 golden

     ...    (sending regular frames)

     1100  Send altref frame
           PictureID = 100
                                Altref corrupted or lost    golden

     1101                       Send SLI(100)               golden
     1102  Receive SLI(100)

     1103  Send frame with
           reference to golden
                                Receive and decode frame
                                (decoder state restored)    golden

     ...   (sending regular frames)

     1200  Send altref frame



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           PictureID = 200
                                Receive and decode
                                altref frame                golden

     1201                       Send RPSI(200)
     1202  Receive RPSI(200)                                altref

     ...  (sending regular frames)

     1300  Send golden frame
           PictureID = 300
                                Receive and decode
                                golden frame                altref

     1301                       Send RPSI(300)              altref
     1302  RPSI lost

     1400  Send golden frame
           PictureID = 400
                                 Receive and decode
                                 golden frame               altref

     1401                        Send RPSI(400)
     1402  Receive RPSI(400)                                golden
     +----+--------------------+--------------------------+------------+

   Note that the scheme is robust to loss of the feedback messages.  If
   the RPSI is lost, the sender will try to update the golden (or
   altref) again after a while, without releasing the established
   reference.  Also, if an SLI is lost, the receiver can keep sending
   SLI messages at any interval, as long as the picture is corrupted.




















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6.  Payload Format Parameters

   This section specifies the parameters that MAY be used to select
   optional features of the payload format and certain features of the
   bitstream.

6.1.  Restrictions on usage

   This media type depends on RTP framing, and hence is only defined for
   transfer via RTP [RFC3550] [4].Transport within other framing
   protocols is not defined at this time.

6.2.  Media Type Registration

   This registration is done using the template defined in RFC 4288 [7]
   and following RFC 4855 [8].

   Type name:  video

   Subtype name:  VP8

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:
      This media type is framed and contains binary data; see Section
      4.8 of [7].

   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:  VP8 bitstream format [1] 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.

   Additional information:  None.

   Person & email address to contact for further information:
      Patrik Westin, patrik.westin@gmail.com





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   Intended usage:  COMMON

   Restrictions on usage:
      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [4].

   Author:  Patrik Westin, patrik.westin@gmail.com

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

6.3.  SDP Parameters

   The receiver MUST ignore any parameter unspecified in this memo.

6.3.1.  Mapping of MIME Parameters to SDP

   The MIME media type video/VP8 string is mapped to fields in the
   Session Description Protocol (SDP) [6] 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 VP8 (the
      MIME subtype).

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

   o  The OPTIONAL parameter "version", if included, MUST be in the
      a=fmtp SDP field.  This parameter matches the VP8 bitstream
      version.

6.4.  Example

   An example of media representation in SDP is as follows:

   m=video 49170 RTP/AVPF 98
   a=rtpmap:98 VP8/90000
   a=fmtp:98 version=0













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7.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [4], and in any applicable RTP profile.  The main
   security considerations for the RTP packet carrying the RTP payload
   format defined within this memo are confidentiality, integrity and
   source authenticity.  Confidentiality is achieved by encryption of
   the RTP payload.  Integrity of the RTP packets through suitable
   cryptographic integrity protection mechanism.  Cryptographic system
   may also allow the authentication of the source of the payload.  A
   suitable security mechanism for this RTP payload format should
   provide confidentiality, integrity protection and at least source
   authentication capable of determining if an RTP packet is from a
   member of the RTP session or not.  Note that the appropriate
   mechanism to provide security to RTP and payloads following this memo
   may vary.  It is dependent on the application, the transport, and the
   signalling protocol employed.  Therefore a single mechanism is not
   sufficient, although if suitable the usage of SRTP [5] is
   recommended.  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.



























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8.  Congestion Control

   Congestion control for RTP SHALL be used in accordance with RFC 3550
   [4], and with any applicable RTP profile; e.g., RFC 3551 [9].  The
   congestions 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 VP8 payload descriptor in Section 4.1 to identify
   non-reference frames and discard them in order to reduce network
   congestion.









































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9.  IANA Considerations

   The IANA is requested to register the following values:
   - Media type registration as described in Section 6.2.















































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10.  References

   [1]  Google, Inc., "VP8 Data Format and Decoding Guide", July 2010,
        <http://www.webmproject.org>.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, STD 1, December 1997.

   [3]  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, STD 1, July 2006.

   [4]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications",
        RFC 3550, STD 64, July 2003.

   [5]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
        Norrman, "The Secure Real-time Transport Protocol (SRTP)",
        RFC 3711, STD 1, March 2004.

   [6]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, STD 1, April 1998.

   [7]  Freed, N. and J. Klensin, "Media Type Specifications and
        Registrations Procedures", RFC 4288, BCP 13, December 2005.

   [8]  Casner, S., "Media Type Registration of RTP Payload Formats",
        RFC 4855, February 2007.

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




















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Authors' Addresses

   Patrik Westin
   Google, Inc.
   Kungsbron 2
   Stockholm,   11122
   Sweden

   Email: patrik.westin@gmail.com


   Henrik F Lundin
   Google, Inc.
   Kungsbron 2
   Stockholm,   11122
   Sweden

   Email: hlundin@google.com


   Michael Glover
   Google, Inc.


   Justin Uberti
   Google, Inc.


   Frank Galligan
   Google, Inc.





















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