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Versions: 00 01 02 draft-ietf-payload-vp8

Network Working Group                                          P. Westin
Internet-Draft                                                 H. Lundin
Intended status: Experimental                                  M. Glover
Expires: August 12, 2011                                       J. Uberti
                                                             F. Galligan
                                                                  Google
                                                        February 8, 2011


      Proposal for the IETF on "RTP Payload Format for VP8 Video"
                      draft-westin-payload-vp8-00

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 August 12, 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

   An encoded VP8 frame can be 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.  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.  The format specification is
   described in Section 2.  Section 3 illustrates how VP8 can be
   combined with uneven level FEC protection.  Section 4 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.  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
   marker bit is set.

                                 Figure 1

















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2.1.  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 equal 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.

2.2.  VP8 Payload Header

   The first three bytes of an encoded VP8 frame are uncompressed, 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].

2.3.  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 2.1 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.  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.

2.4.  Examples of VP8 RTP Stream

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




























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2.4.1.  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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2.4.2.  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       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.4.3.  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       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.4.4.  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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2.4.5.  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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3.  Using VP8 with Uneven Level FEC Protection

   RFC 5109 [2] specifies a payload format for generic forward error
   correction (FEC) for RTP packets.  (An errata with critical changes
   was also published.)  One salient feature of RFC 5109 is that it
   provides "uneven level protection", ULP, which enables FEC protection
   of parts of an RTP packet.  Specifically, the first part of an RTP
   packet can be given a stronger protection than the remaining part.
   The special case is where only the first part of the RTP packet is
   protected.  The length of the protected part (actually the length of
   each protection level) is chosen and changed dynamically during a
   session.  The concept of ULP FEC fits well with the VP8 video format.
   The first partition of an encoded VP8 frame consists of context
   variables and prediction parameters (mode and vectors), while the
   subsequent partitions contain encoded residual information.  For a
   decoder, the residual information is not useful without the first
   partition.  However, a decoder could successfully use the information
   in the first partition to provide good packet loss concealment, even
   if the subsequent partitions are lost.  The conclusion is that the
   first partition deserves a higher protection factor than the
   remaining data.  By including the "first partition size" parameter in
   the VP8 payload header(Section 2.2), the application, or even a media
   aware network element, can apply the ULP FEC to the VP8 payloads,
   since it can readily identify and obtain the length of the first
   partition to which (a stronger) protection should be granted.  RFC
   5109 suggests two methods for multiplexing the media data and the FEC
   data: using the RED payload type (RFC 2198, [3]) and using separate
   sessions.  The example below is based on the RED method, although the
   above RTP format for VP8 does not preclude any method.  Since we are
   only interested in protecting the first partition of the VP8 payload,
   it is in some cases possible to obtain increased robustness for that
   part even without FEC signaling.  If the first partition is isolated
   in a separate RTP packet, this packet can simply be sent twice (exact
   replica of RTP header and payload).

3.1.  Example: Using VP8 with Uneven Level FEC Protection

   The following terminology is used in the examples below:

   o  RTP: RTP header, 12 octets.
      (Special rules apply if the header is extended; see [2].)

   o  RED: Redundancy payload header, 1 or 4 octets.

   o  VP8pd: VP8 payload descriptor, 1 or more octets (see Section 2.1).

   o  VP8ph: VP8 payload header, 3 octets (see Section 2.2)).




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   o  VP8first: First partition of a VP8 frame, length given in VP8ph.

   o  VP8second: Second (or later) partition of a VP8 frame.

   o  FEC: FEC header for FEC packets, 10 octets (see [2]).

   o  ULP: FEC level header for FEC packets, 2 or 4 octets (see [2]).

3.1.1.  VP8 first partition isolated

   The first partition of the VP8 frame is sent in its own RTP packet,
   which can easily be duplicated for increased robustness.  This case
   provides a 50% protection factor (ratio of number of FEC packet to
   the sum of FEC and protected packets; sending two duplicates provides
   a 67% protection, and so on).

                   +-----+-------+------------------+
     Packet A:     | RTP | VP8pd | VP8ph : VP8first |
                   +-----+-------+------------------+

                   +-----+-------+------------------------+
     Packet B:     | RTP | VP8pd | VP8second              |
                   +-----+-------+------------------------+

                   +-----+-------+------------------+
     Duplicate A:  | RTP | VP8pd | VP8ph : VP8first |
                   +-----+-------+------------------+
























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3.1.2.  VP8 first partition split

   The first partition of the VP8 frame can be split across two (or
   more) RTP packets.  It must be done if the first partition is larger
   than the MTU, but can also be done to facilitate protection factors
   lower than 50%.  The FEC header is calculated from the RTP headers of
   packets A and B, while the FEC payload FEC(A,B) is constructed from
   the VP8 payload descriptor, payload header and first partition, as
   indicated in the figure below.

           +-----+-----+-------+------------------+
     A:    | RTP | RED | VP8pd | VP8ph : VP8first |
           +-----+-----+-------+------------------+
           [*****]     [**************************]
           +-----+-----+-------+------------------+
     B:    | RTP | RED | VP8pd | VP8first cont'd  |
           +-----+-----+-------+------------------+
           [*****]     [**************************]
              \                      \
               ----------             ---------
                         \                     \
                       [*****]     [**************************]
           +-----+-----+-----+-----+--------------------------+
     FEC1: | RTP | RED | FEC | ULP | FEC(A,B)                 |
           +-----+-----+-----+-----+--------------------------+

           +-----+-----+-------+---------------------------+
     C:    | RTP | RED | VP8pd | VP8 second                |
           +-----+-----+-------+---------------------------+

   The split of the first partition is preferably done such that the
   payloads used to calculate FEC(A,B) are close to equal size.



















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3.1.3.  VP8 partitions aggregated

   In the case when the first partition is sent in the same packet as
   one or more subsequent partitions, the level protection can be
   applied to facilitate a bit- conservative protection for only the
   first partition.

           +-----+-----+-------+------------------+--------------------+
     A:    | RTP | RED | VP8pd | VP8ph : VP8first | VP8second          |
           +-----+-----+-------+------------------+--------------------+
           [*****]     [**************************]
             \                      \
              ----------             ---------
                        \                     \
                       [*****]     [**************************]
           +-----+-----+-----+-----+--------------------------+
     FEC1: | RTP | RED | FEC | ULP | Duplicate                |
           +-----+-----+-----+-----+--------------------------+

           +-----+-----+-------+---------------------------+
     C:    | RTP | RED | VP8pd | VP8third                  |
           +-----+-----+-------+---------------------------+





























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4.  Using VP8 with RPSI and SLI Feedback

   The VP8 payload descriptor defined in Section 2.1 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 [4].

4.1.  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 [4]) is
   simply the PictureID of the received frame.

4.2.  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
   [4]:

     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.

4.3.  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   Receive RPSI(0)
                                Send RPSI(0)                golden

     ...    (sending regular frames)

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

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

     1102  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  Receive RPSI(200)
                                Send RPSI(200)              altref

     ...  (sending regular frames)

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

     1301  RPSI lost
                                Send RPSI(300)              altref

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

     1401  Receive RPSI(400)
                                 Send 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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5.  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.

5.1.  MIME Registration

   The receiver MUST ignore any unspecified parameter.

   Media Type name: video

   Media subtype name: VP8

   Required parameters: none

   Security considerations:
   - See Section 6 of RFC xxxx.

5.2.  SDP Parameters

   The receiver MUST ignore any parameter unspecified in this memo.

5.2.1.  Mapping of MIME Parameters to SDP

   The MIME media type video/VP8 string is mapped to fields in the
   Session Description Protocol (SDP) [7] 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.

5.3.  Example

   An example of media representation in SDP is as follows:

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





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6.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [5].  This implies that confidentiality of the media
   streams is achieved by encryption; one example of this is SRTP [6].













































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7.  IANA Considerations

   The IANA is requested to register the following values:
   - MIME registration as described in Section 5.1.















































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8.  References

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

   [2]  Perkins, C., Kouvelas, U., Hodson, O., Hardman, V., Handley, M.,
        Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload
        Format for Generic Forward Error Correction", RFC 5109, STD 1,
        December 2007.

   [3]  Li, A., "RTP Payload for Redundant Audio Data", RFC 2198, STD 1,
        September 1997.

   [4]  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.

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

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

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
























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Authors' Addresses

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

   Email: patrik.westin@gmail.com


   Henrik Lundin
   Google, Inc.


   Michael Glover
   Google, Inc.


   Justin Uberti
   Google, Inc.


   Frank Galligan
   Google, Inc.


























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