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RTP Payload Format for VP8 Video
draft-ietf-payload-vp8-16

The information below is for an old version of the document.
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This is an older version of an Internet-Draft that was ultimately published as RFC 7741.
Authors Patrik Westin , Henrik Lundin , Michael Glover , Justin Uberti , Frank Galligan
Last updated 2015-07-29 (Latest revision 2015-06-05)
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Ali C. Begen
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IESG IESG state Became RFC 7741 (Proposed Standard)
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Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
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Send notices to payload-chairs@ietf.org, draft-ietf-payload-vp8@ietf.org
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draft-ietf-payload-vp8-16
Payload Working Group                                          P. Westin
Internet-Draft                                                 H. Lundin
Intended status: Standards Track                               M. Glover
Expires: December 7, 2015                                      J. Uberti
                                                             F. Galligan
                                                                  Google
                                                            June 5, 2015

                    RTP Payload Format for VP8 Video
                       draft-ietf-payload-vp8-16

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 December 7, 2015.

Copyright Notice

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

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

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  . . . . . . . . . . . . . . . . . . . .   4
     4.2.  VP8 Payload Descriptor  . . . . . . . . . . . . . . . . .   6
     4.3.  VP8 Payload Header  . . . . . . . . . . . . . . . . . . .  10
     4.4.  Aggregated and Fragmented Payloads  . . . . . . . . . . .  11
     4.5.  Frame reconstruction algorithm  . . . . . . . . . . . . .  11
       4.5.1.  Partition reconstruction algorithm  . . . . . . . . .  12
     4.6.  Examples of VP8 RTP Stream  . . . . . . . . . . . . . . .  12
       4.6.1.  Key frame in a single RTP packet  . . . . . . . . . .  12
       4.6.2.  Non-discardable VP8 interframe in a single RTP
               packet; no PictureID  . . . . . . . . . . . . . . . .  13
       4.6.3.  VP8 partitions in separate RTP packets  . . . . . . .  14
       4.6.4.  VP8 frame fragmented across RTP packets . . . . . . .  15
       4.6.5.  VP8 frame with long PictureID . . . . . . . . . . . .  16
   5.  Using VP8 with RPSI and SLI Feedback  . . . . . . . . . . . .  17
     5.1.  RPSI  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  SLI . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.3.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  20
     6.1.  Media Type Definition . . . . . . . . . . . . . . . . . .  20
     6.2.  SDP Parameters  . . . . . . . . . . . . . . . . . . . . .  22
       6.2.1.  Mapping of Media Subtype Parameters to SDP  . . . . .  22
       6.2.2.  Offer/Answer Considerations . . . . . . . . . . . . .  22
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     10.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   This memo describes an RTP payload specification applicable to the
   transmission of video streams encoded using the VP8 video codec
   [RFC6386].  The format described in this document can be used both in
   peer-to-peer and video conferencing applications.

   VP8 is based on decomposition of frames into square sub-blocks of
   pixels, prediction of such sub-blocks using previously constructed

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   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 [RFC6386].  The first partition (prediction or mode)
   contains prediction mode parameters and motion vectors for all
   macroblocks.  The remaining partitions all contain the quantized DCT/
   WHT coefficients for the residuals.  There can be 1, 2, 4, or 8 DCT/
   WHT partitions per frame, depending on encoder settings.

   In summary, the payload format described in this document enables a
   number of features in VP8, including:

   o  Taking partition boundaries into consideration, to improve loss
      robustness and facilitate efficient packet loss concealment at the
      decoder.

   o  Temporal scalability.

   o  Advanced use of reference frames to enable efficient error
      recovery.

   o  Marking of frames that have no impact on the decoding of any other
      frame, so that these non-reference frames can be discarded in a
      server or media-aware network element if needed.

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].

3.  Media Format Description

   The VP8 codec uses three different reference frames for interframe
   prediction: the previous frame, the golden frame, and the altref
   frame.  Blocks in an interframe may be predicted using blocks in the
   immediately previous frame as well as the most recent golden frame or
   altref frame.  Every key frame is automatically golden and altref,
   and any interframe may optionally replace the most recent golden or
   altref frame.  Golden frames and altref frames may also be used to
   increase the tolerance to dropped frames.  The payload specification
   in this memo has elements that enable advanced use of the reference
   frames, e.g., for improved loss robustness.

   One specific use case of the three reference frame types is temporal
   scalability.  By setting up the reference hierarchy in the

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   appropriate way, up to five temporal layers can be encoded.  (How to
   set up the reference hierarchy for temporal scalability is not within
   the scope of this memo.)

   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" [RFC6386].  The first partition (prediction or
   mode) contains prediction mode parameters and motion vectors for all
   macroblocks.  The remaining partitions all contain the transform
   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 separately or in the same RTP
   packet.  It may be beneficial for decoder error-concealment to send
   the partitions in different packets, even though it is not mandatory
   according to this specification.

   The format specification is described in Section 4.  In Section 5, a
   method to acknowledge receipt of reference frames using RTCP
   techniques is described.

   The payload partitioning and the acknowledging method both serve as
   motivation for three of the fields included in the payload format:
   the "PID", "1st partition size" and "PictureID" fields.  The ability
   to encode a temporally scalable stream motivates the "TL0PICIDX" and
   "TID" fields.

4.  Payload Format

   This section describes how the encoded VP8 bitstream is encapsulated
   in RTP.  To handle network losses usage of RTP/AVPF [RFC4585] is
   RECOMMENDED.  All integer fields in the specifications are encoded as
   unsigned integers in network octet order.

4.1.  RTP Header Usage

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   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 (M):  MUST be set for the very last packet of each encoded
      frame in line with the normal use of the M bit in video formats.
      This enables a decoder to finish decoding the picture, where it
      otherwise may need to wait for the next packet to explicitly know
      that the frame is complete.

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

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

      The remaining RTP header fields are used as specified in
      [RFC3550].

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4.2.  VP8 Payload Descriptor

   The first octets after the RTP header are the VP8 payload descriptor,
   with the following structure.  The single-octet version of the
   PictureID is illustrated to the left (M bit set to zero), while the
   dual-octet version (M bit set to one) is show to the right.

         0 1 2 3 4 5 6 7                      0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
        |X|R|N|S|R| PID | (REQUIRED)        |X|R|N|S|R| PID | (REQUIRED)
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
   X:   |I|L|T|K| RSV   | (OPTIONAL)   X:   |I|L|T|K| RSV   | (OPTIONAL)
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
   I:   |M| PictureID   | (OPTIONAL)   I:   |M| PictureID   | (OPTIONAL)
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
   L:   |   TL0PICIDX   | (OPTIONAL)        |   PictureID   |
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
   T/K: |TID|Y| KEYIDX  | (OPTIONAL)   L:   |   TL0PICIDX   | (OPTIONAL)
        +-+-+-+-+-+-+-+-+                   +-+-+-+-+-+-+-+-+
                                       T/K: |TID|Y| KEYIDX  | (OPTIONAL)
                                            +-+-+-+-+-+-+-+-+

                                 Figure 2

   X: Extended control bits present.  When set to one, the extension
      octet MUST be provided immediately after the mandatory first
      octet.  If the bit is zero, all optional fields MUST be omitted.

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

   N: Non-reference frame.  When set to one, the frame can be discarded
      without affecting any other future or past frames.  If the
      reference status of the frame is unknown, this bit SHOULD be set
      to zero to avoid discarding frames needed for reference.

         Informative note: This document does not describe how to
         determine if an encoded frame is non-reference.  The reference
         status of an encoded frame is preferably provided from the
         encoder implementation.

   S: Start of VP8 partition.  SHOULD be set to 1 when the first payload
      octet of the RTP packet is the beginning of a new VP8 partition,
      and MUST NOT be 1 otherwise.  The S bit MUST be set to 1 for the
      first packet of each encoded frame.

   PID:  Partition index.  Denotes which VP8 partition the first payload
      octet of the packet belongs to.  The first VP8 partition

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      (containing modes and motion vectors) MUST be labeled with PID =
      0.  PID SHOULD be incremented for each subsequent partition, but
      MAY be kept at 0 for all packets.  PID MUST NOT be larger than 7.
      If more than one packet in an encoded frame contains the same PID,
      the S bit MUST NOT be set for any other packet than the first
      packet with that PID.

   When the X bit is set to 1 in the first octet, the extension bit
   field octet MUST be provided as the second octet.  If the X bit is 0,
   the extension bit field octet MUST NOT be present, and no extensions
   (I, L, T, or K) are permitted.

   I: PictureID present.  When set to one, the PictureID MUST be present
      after the extension bit field and specified as below.  Otherwise,
      PictureID MUST NOT be present.

   L: TL0PICIDX present.  When set to one, the TL0PICIDX MUST be present
      and specified as below, and the T bit MUST be set to 1.
      Otherwise, TL0PICIDX MUST NOT be present.

   T: TID present.  When set to one, the TID/Y/KEYIDX octet MUST be
      present.  The TID|Y part of the octet MUST be specified as below.
      If K (below) is set to one but T is set to zero, the TID/Y/KEYIDX
      octet MUST be present, but the TID field MUST be ignored.  If
      neither T nor K is set to one, the TID/Y/KEYIDX octet MUST NOT be
      present.

   K: KEYIDX present.  When set to one, the TID/Y/KEYIDX octet MUST be
      present.  The KEYIDX part of the octet MUST be specified as below.
      If T (above) is set to one but K is set to zero, the TID/Y/KEYIDX
      octet MUST be present, but the KEYIDX field MUST be ignored.  If
      neither T nor K is set to one, the TID/Y/KEYIDX octet MUST NOT be
      present.

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

   After the extension bit field follow the extension data fields that
   are enabled.

   The PictureID extension:  If the I bit is set to one, the PictureID
      extension field MUST be present, and MUST NOT be present
      otherwise.  The field consists of two parts:

      M: The most significant bit of the first octet is an extension
         flag.  If M is set, the remainder of the PictureID field MUST
         contain 15 bits, else it MUST contain 7 bits.

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      PictureID:  7 or 15 bits (shown left and right, respectively, in
         Figure 2) not including the M bit.  This is a running index of
         the frames, which SHOULD begin as a random number, MUST
         increase by 1 for each subsequent frame, and MUST wrap to 0
         after reaching the maximum ID (all bits set).  The 7 or 15 bits
         of the PictureID go from most significant to least significant,
         beginning with the first bit after the M bit.  The sender
         chooses a 7 or 15 bit index and sets the M bit accordingly.
         The receiver MUST NOT assume that the number of bits in
         PictureID stay the same through the session.  Having sent a
         7-bit PictureID with all bits set to 1, the sender may either
         wrap the PictureID to 0, or extend to 15 bits and continue
         incrementing.

   The TL0PICIDX extension:  If the L bit is set to one, the TL0PICIDX
      extension field MUST be present, and MUST NOT be present
      otherwise.  The field consists of one part:

      TL0PICIDX:  8 bits temporal level zero index.  TL0PICIDX is a
         running index for the temporal base layer frames, i.e., the
         frames with TID set to 0.  If TID is larger than 0, TL0PICIDX
         indicates which base layer frame the current image depends on.
         TL0PICIDX MUST be incremented when TID is 0.  The index SHOULD
         start on a random number, and MUST restart at 0 after reaching
         the maximum number 255.

   The TID/Y/KEYIDX extension:  If the any of the T or K bits are set to
      one, the TID/Y/KEYIDX extension field MUST be present.  It MUST
      NOT be present if both T and K are zero.  The field consists of
      three parts:

      TID:  2 bits temporal layer index.  The TID field MUST be ignored
         by the receiver when the T bit is set equal to 0.  The TID
         field indicates which temporal layer the packet represents.
         The lowest layer, i.e., the base layer, MUST have TID set to 0.
         Higher layers SHOULD increment the TID according to their
         position in the layer hierarchy.

      Y: 1 layer sync bit.  The Y bit SHOULD be set to 1 if the current
         frame depends only on the base layer (TID = 0) frame with
         TL0PICIDX equal to that of the current frame.  The Y bit MUST
         be set to 0 if the current frame depends on any other frame
         than the base layer (TID = 0) frame with TL0PICIDX equal to
         that of the current frame.  If the Y bit is set when the T bit
         is equal to 0 the current frame MUST only depend on a past base
         layer (TID=0) key frame as signaled by a change in the KEYIDX
         field.  Additionally this frame MUST NOT depend on any of the

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         three codec buffers (as defined by [RFC6386]) that have been
         updated since the last time the KEYIDX field was changed.

         Informative note: This document does not describe how to
         determine the dependency status for a frame; this information
         is preferably provided from the encoder implementation.  In the
         case of unknown status, the Y bit can safely be set to 0.

      KEYIDX:  5 bits temporal key frame index.  The KEYIDX field MUST
         be ignored by the receiver when the K bit is set equal to 0.
         The KEYIDX field is a running index for key frames.  KEYIDX MAY
         start on a random number, and MUST restart at 0 after reaching
         the maximum number 31.  When in use, the KEYIDX SHOULD be
         present for both key frames and interframes.  The sender MUST
         increment KEYIDX for key frames which convey parameter updates
         critical to the interpretation of subsequent frames, and SHOULD
         leave the KEYIDX unchanged for key frames that do not contain
         these critical updates.  If the KEYIDX is present, a receiver
         SHOULD NOT decode an interframe if it has not received and
         decoded a key frame with the same KEYIDX after the last KEYIDX
         wrap-around.

         Informative note: This document does not describe how to
         determine if a key frame updates critical parameters; this
         information is preferably provided from the encoder
         implementation.  A sender that does not have this information
         may either omit the KEYIDX field (set K equal to 0), or
         increment the KEYIDX on every key frame.  The benefit with the
         latter is that any key frame loss will be detected by the
         receiver, which can signal for re-transmission or request a new
         key frame.

   Informative note:  Implementations doing splicing of VP8 streams will
      have to make sure the rules for incrementing TL0PICIDX and KEYIDX
      are obeyed across the splice.  This will likely require rewriting
      values of TL0PICIDX and KEYIDX after the splice.

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4.3.  VP8 Payload Header

   The beginning of an encoded VP8 frame is referred to as an
   "uncompressed data chunk" in [RFC6386], and also serves as a payload
   header in this RTP format.  The codec bitstream format specifies two
   different variants of the uncompressed data chunk: a 3 octet version
   for interframes and a 10 octet version for key frames.  The first 3
   octets are common to both variants.  In the case of a key frame the
   remaining 7 octets are considered to be part of the remaining payload
   in this RTP format.  Note that the header is present only in packets
   which have the S bit equal to one and the PID equal to zero in the
   payload descriptor.  Subsequent packets for the same frame do not
   carry the payload header.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |Size0|H| VER |P|
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | Bytes 4..N of |
     | VP8 payload   |
     :               :
     +-+-+-+-+-+-+-+-+
     | OPTIONAL RTP  |
     | padding       |
     :               :
     +-+-+-+-+-+-+-+-+

                                 Figure 3

   H: Show frame bit as defined in [RFC6386].

   VER:  A version number as defined in [RFC6386].

   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 [RFC6386]

   SizeN:  The size of the first partition in bytes is calculated from
      the 19 bits in Size0, Size1, and Size2 as 1stPartitionSize = Size0
      + 8 * Size1 + 2048 * Size2.  [RFC6386].

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4.4.  Aggregated and Fragmented Payloads

   An encoded VP8 frame can be divided into two or more partitions, as
   described in Section 1.  One packet can contain a fragment of a
   partition, a complete partition, or an aggregate of fragments and
   partitions.  In the preferred use case, the S bit and PID fields
   described in Section 4.2 should be used to indicate what the packet
   contains.  The PID field should indicate which partition the first
   octet of the payload belongs to, and the S bit indicates that the
   packet starts on a new partition.  Aggregation of encoded partitions
   is done without explicit signaling.  Partitions MUST be aggregated in
   decoding order.  Two fragments from different partitions MAY be
   aggregated 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 beginning of 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.

   The VP8 bitstream format [RFC6386] specifies that if multiple DCT/WHT
   partitions are produced, the location of each partition start is
   found at the end of the first (prediction/mode) partition.  In this
   RTP payload specification, the location offsets are considered to be
   part of the first partition.

   It is OPTIONAL for a packetizer implementing this RTP specification
   to pay attention to the partition boundaries within an encoded frame.
   If packetization of a frame is done without considering the partition
   boundaries, the PID field MAY be set to zero for all packets, and the
   S bit MUST NOT be set to one for any other packet than the first.

4.5.  Frame reconstruction algorithm

   Example of frame reconstruction algorithm.

   1: Collect all packets with a given RTP timestamp.

   2: Go through packets in order, sorted by sequence numbers, if
      packets are missing, send NACK as defined in [RFC4585] or decode
      with missing partitions, see Section 4.5.1 below.

   3: A frame is complete if the frame has no missing sequence numbers,
      the first packet in the frame contains S=1 with partId=0 and the
      last packet in the frame has the marker bit set.

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4.5.1.  Partition reconstruction algorithm

   Example of partition reconstruction algorithm.

   1: Scan for the start of a new partition; S=1.

   2: Continue scan to detect end of partition; hence a new S=1
      (previous packet was the end of the partition) is found or the
      marker bit is set.  If a loss is detected before the end of the
      partition, abandon all packets in this partition and continue the
      scan repeating from step 1.

   3: Store the packets in the complete partition, continue the scan
      repeating from step 1 until end of frame is reached.

   4: Send all complete partitions to the decoder.  If no complete
      partition is found discard the whole frame.

4.6.  Examples of VP8 RTP Stream

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

4.6.1.  Key frame in a single RTP packet

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 1        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     |Size0|1| VER |0| P = 0
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | VP8 payload   |
     +-+-+-+-+-+-+-+-+

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4.6.2.  Non-discardable VP8 interframe in a single RTP packet; no
        PictureID

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 1        |
     +-+-+-+-+-+-+-+-+
     |0|0|0|1|0|0 0 0| X = 0; S = 1; PID = 0
     +-+-+-+-+-+-+-+-+
     |Size0|1| VER |1| P = 1
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | VP8 payload   |
     +-+-+-+-+-+-+-+-+

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4.6.3.  VP8 partitions in separate RTP packets

   First RTP packet; complete first partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 0        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     |Size0|1| VER |1| P = 1
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | Bytes 4..L of |
     | first VP8     |
     | partition     |
     :               :
     +-+-+-+-+-+-+-+-+

   Second RTP packet; complete second partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 1        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 1| X = 1; S = 1; PID = 1
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     | Remaining VP8 |
     | partitions    |
     :               :
     +-+-+-+-+-+-+-+-+

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4.6.4.  VP8 frame fragmented across RTP packets

   First RTP packet; complete first partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 0        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     |Size0|1| VER |1| P = 1
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | Complete      |
     | first         |
     | partition     |
     :               :
     +-+-+-+-+-+-+-+-+

   Second RTP packet; first fragment of second partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 0        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 1| X = 1; S = 1; PID = 1
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     | First fragment|
     | of second     |
     | partition     |
     :               :
     +-+-+-+-+-+-+-+-+

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   Third RTP packet; second fragment of second partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 0        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0|0 0 1| X = 1; S = 0; PID = 1
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     | Mid fragment  |
     | of second     |
     | partition     |
     :               :
     +-+-+-+-+-+-+-+-+

   Fourth RTP packet; last fragment of second partition.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 1        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0|0 0 1| X = 1; S = 0; PID = 1
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1
     +-+-+-+-+-+-+-+-+
     |0 0 0 1 0 0 0 1| PictureID = 17
     +-+-+-+-+-+-+-+-+
     | Last fragment |
     | of second     |
     | partition     |
     :               :
     +-+-+-+-+-+-+-+-+

4.6.5.  VP8 frame with long PictureID

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   PictureID = 4711 = 001001001100111 binary (first 7 bits: 0010010,
   last 8 bits: 01100111).

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  RTP header   |
     |  M = 1        |
     +-+-+-+-+-+-+-+-+
     |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0
     +-+-+-+-+-+-+-+-+
     |1|0|0|0|0 0 0 0| I = 1;
     +-+-+-+-+-+-+-+-+
     |1 0 0 1 0 0 1 0| Long PictureID flag = 1
     |0 1 1 0 0 1 1 1| PictureID = 4711
     +-+-+-+-+-+-+-+-+
     |Size0|1| VER |1|
     +-+-+-+-+-+-+-+-+
     |     Size1     |
     +-+-+-+-+-+-+-+-+
     |     Size2     |
     +-+-+-+-+-+-+-+-+
     | Bytes 4..N of |
     | VP8 payload   |
     :               :
     +-+-+-+-+-+-+-+-+

5.  Using VP8 with RPSI and SLI Feedback

   The VP8 payload descriptor defined in Section 4.2 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 [RFC4585].

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

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   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.  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
   [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 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,
   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 RPSI 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
   acknowledged, the same rule applies when updating the altref frame.

   +-------+-------------------+-------------------------+-------------+
   | Event | Sender            | Receiver                | Established |

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   |       |                   |                         | 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     | golden      |
   |       |                   | lost                    |             |
   |       |                   |                         |             |
   | 1101  |                   | Send SLI(100)           | golden      |
   |       |                   |                         |             |
   | 1102  | Receive SLI(100)  |                         |             |
   |       |                   |                         |             |
   | 1103  | Send frame with   |                         |             |
   |       | reference to      |                         |             |
   |       | golden            |                         |             |
   |       |                   |                         |             |
   |       |                   | Receive and decode      | golden      |
   |       |                   | frame (decoder state    |             |
   |       |                   | restored)               |             |
   |       |                   |                         |             |
   | ...   | (sending regular  |                         |             |
   |       | frames)           |                         |             |
   |       |                   |                         |             |
   | 1200  | Send altref frame |                         |             |
   |       | PictureID = 200   |                         |             |
   |       |                   |                         |             |
   |       |                   | Receive and decode      | golden      |
   |       |                   | altref frame            |             |
   |       |                   |                         |             |
   | 1201  |                   | Send RPSI(200)          |             |
   |       |                   |                         |             |
   | 1202  | Receive RPSI(200) |                         | altref      |
   |       |                   |                         |             |
   | ...   | (sending regular  |                         |             |
   |       | frames)           |                         |             |

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   |       |                   |                         |             |
   | 1300  | Send golden frame |                         |             |
   |       | PictureID = 300   |                         |             |
   |       |                   |                         |             |
   |       |                   | Receive and decode      | altref      |
   |       |                   | golden frame            |             |
   |       |                   |                         |             |
   | 1301  |                   | Send RPSI(300)          | altref      |
   |       |                   |                         |             |
   | 1302  | RPSI lost         |                         |             |
   |       |                   |                         |             |
   | 1400  | Send golden frame |                         |             |
   |       | PictureID = 400   |                         |             |
   |       |                   |                         |             |
   |       |                   | Receive and decode      | altref      |
   |       |                   | golden frame            |             |
   |       |                   |                         |             |
   | 1401  |                   | Send RPSI(400)          |             |
   |       |                   |                         |             |
   | 1402  | Receive RPSI(400) |                         | golden      |
   +-------+-------------------+-------------------------+-------------+

          Table 1: Exemple signaling between sender and receiver

   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 allowed by the RTCP sending timing
   restrictions as specified in [RFC4585], as long as the picture is
   corrupted.

6.  Payload Format Parameters

   This payload format has two required parameters.

6.1.  Media Type Definition

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

   Type name:  video

   Subtype name:  VP8

   Required parameters:  None.

   Optional parameters:

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      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:  VP8 bitstream format [RFC6386] and RFC
      XXXX.
      [RFC Editor: Upon publication as an RFC, please replace "XXXX"
      with the number assigned to this document and remove this note.]

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

   Fragment identifier considerations:  N/A.

   Additional information:  None.

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

   Intended usage:  COMMON

   Restrictions on usage:

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

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

   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/VP8 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 VP8 (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 if the 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 VP8/90000
   a=fmtp:98 max-fr=30; max-fs=3600;

6.2.2.  Offer/Answer Considerations

   The VP8 codec offers a decode complexity that is roughly linear with
   the number of pixels encoded.  The parameters "max-fr" and "max-fs"
   are defined in Section 6.1, where the macroblock size is 16x16 pixels
   as defined in [RFC6386], the max-fs and max-fr parameters MUST be
   used to establish these limits.

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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 [RFC3550], 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
   signaling protocol employed.  Therefore a single mechanism is not
   sufficient, although if suitable the usage of SRTP [RFC3711] 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.

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 VP8 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.

10.  References

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10.1.  Normative References

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
              2006.

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

   [RFC6386]  Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
              Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
              Guide", RFC 6386, November 2011.

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

10.2.  Informative References

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

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

Authors' Addresses

   Patrik Westin
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   Email: patrik.westin@gmail.com

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

   Email: hlundin@google.com

   Michael Glover
   Google, Inc.
   5 Cambridge Center
   Cambridge, MA  02142
   USA

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

   Frank Galligan
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

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