One document matched: draft-ietf-payload-vp8-17.xml
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<rfc category="std" docName="draft-ietf-payload-vp8-17" ipr="trust200902">
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<!-- generate table of contents -->
<front>
<title abbrev="RTP Payload Format for VP8">RTP Payload Format for VP8
Video</title>
<author fullname="Patrik Westin" initials="P." surname="Westin">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>1600 Amphitheatre Parkway</street>
<city>Mountain View</city>
<region>CA</region>
<code>94043</code>
<country>USA</country>
</postal>
<email>patrik.westin@gmail.com</email>
</address>
</author>
<author fullname="Henrik F Lundin" initials="H.F." surname="Lundin">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>Kungsbron 2</street>
<city>Stockholm</city>
<region/>
<code>11122</code>
<country>Sweden</country>
</postal>
<email>hlundin@google.com</email>
</address>
</author>
<author fullname="Michael Glover" initials="M." surname="Glover">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>5 Cambridge Center</street>
<city>Cambridge</city>
<region>MA</region>
<code>02142</code>
<country>USA</country>
</postal>
</address>
</author>
<author fullname="Justin Uberti" initials="J." surname="Uberti">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>747 6th Street South</street>
<city>Kirkland</city>
<region>WA</region>
<code>98033</code>
<country>USA</country>
</postal>
</address>
</author>
<author fullname="Frank Galligan" initials="F." surname="Galligan">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>1600 Amphitheatre Parkway</street>
<city>Mountain View</city>
<region>CA</region>
<code>94043</code>
<country>USA</country>
</postal>
</address>
</author>
<date day="9" month="September" year="2015"/>
<area>General</area>
<workgroup>Payload Working Group</workgroup>
<keyword>RFC</keyword>
<keyword>Request for Comments</keyword>
<keyword>RTP</keyword>
<keyword>VP8</keyword>
<keyword>WebM</keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>This memo describes an RTP payload specification applicable to the
transmission of video streams encoded using the VP8 video codec <xref
target="RFC6386"/>. The format described in this document can be used
both in peer-to-peer and video conferencing applications.</t>
<t>VP8 is based on decomposition of frames into square
sub-blocks of pixels known as "macroblocks" (see Section 2 of
<xref target="RFC6386"/>). Prediction of such sub-blocks using
previously constructed blocks, and adjustment of such
predictions (as well as synthesis of unpredicted blocks) is done 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
<xref target="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.</t>
<t>In summary, the payload format described in this document enables a
number of features in VP8, including: <list style="symbols">
<t>Taking partition boundaries into consideration, to improve loss
robustness and facilitate efficient packet loss concealment at the
decoder.</t>
<t>Temporal scalability.</t>
<t>Advanced use of reference frames to enable efficient error
recovery.</t>
<t>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.</t>
</list></t>
</section>
<section anchor="conventions"
title="Conventions, Definitions and Acronyms">
<t>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 <xref
target="RFC2119"/>.</t>
<t>This document uses the definitions of <xref
target="RFC6386"/>. In particular, the following terms are
used.</t>
<t><list style="hanging">
<t hangText="Key frames:"> Frames that are decoded without reference to any other frame
in a sequence. (also called interframes and
I-frames)</t>
<t hangText="Interframes:"> Frames that are encoded with reference to prior frames,
specifically all prior frames up to and including the most
recent key frame. (also called prediction frames
and P-frames)</t>
<t hangText="Golden and altref frames:"> alternate
prediction frames. 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.</t>
<t hangText="Macroblock:"> a square array of pixels whose
Y (luminance) dimensions are 16x16 pixels and whose U and
V (chrominance) dimensions are 8x8 pixels.</t>
</list></t>
<t>Two definitions from <xref
target="RFC4585"/> are also used in this document.</t>
<t><list style="hanging">
<t hangText="RPSI:"> Reference picture selection index. A
feedback message to let the encoder know that the decoder
has correctly decoded a certain frame.</t>
<t hangText="SLI:"> Slice loss indication. A feedback
message to let a decoder inform an encoder that it has
detected the loss or corruption of one or several
macroblock(s)</t>
</list></t>
</section>
<section anchor="mediaFormatDescription" title="Media Format Description">
<t>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.</t>
<t>One specific use case of the three reference frame types is
temporal scalability. By setting up the reference hierarchy in
the 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.) Support for
temporal scalability is provided by the optional TL0PICIDX and
TID/Y/KEYIDX fields described in <xref target="VP8payloadDescriptor"/>
For a general description about temporal scalability for video coding,
see e.g., <xref target="Sch07"/>.</t>
<t>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" <xref target="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. Accordingly, this
document RECOMMENDS that the frame is packetized by the sender
with each data partition in a separate packet or packets. This
may be beneficial for decoder error concealment, and the payload
format described in <xref target="payloadFormat"/> provides
fields that allow the partitions to be identified even if the
first partition is not available. The sender can,
alternatively, aggregate the data partitions into a single data
stream and, optionally, split it into several packets without
consideration of the partition boundaries. The receiver can use
the length information in the first partition to identify the
partitions during decoding.</t>
<t>The format specification is described in <xref target="payloadFormat"/>.
In <xref target="RPSIandSLI"/>, a method to
acknowledge receipt of reference frames using RTCP techniques is
described.</t>
<t>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.</t>
</section>
<section anchor="payloadFormat" title="Payload Format">
<t>This section describes how the encoded VP8 bitstream is encapsulated
in RTP. To handle network losses usage of RTP/AVPF <xref
target="RFC4585"/> is RECOMMENDED. All integer fields in the
specifications are encoded as unsigned integers in network octet
order.</t>
<section anchor="RTPHeaderUsage" title="RTP Header Usage">
<figure anchor="figureRTPHeader">
<preamble>The general RTP payload format for VP8 is depicted
below.</preamble>
<artwork><![CDATA[
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 #octets) |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : VP8 payload header (3 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VP8 pyld hdr : |
+-+-+-+-+-+-+-+-+ |
: Octets 4..N of VP8 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
<postamble>The VP8 payload descriptor and VP8 payload header will be
described in <xref target="VP8payloadDescriptor"/> and
<xref target="VP8payloadHeader"/>. OPTIONAL RTP padding MUST NOT
be included unless the P bit is set. The figure specifically shows
the format for the first packet in a frame. Subsequent packets will
not contain the VP8 payload header, and will have later octets in
the frame payload.</postamble>
</figure>
<t><list style="hanging">
<t hangText="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.</t>
<t hangText="Payload type (PT):"> In line with the policy
in Section 3 of <xref target="RFC3551"/>, applications
using the VP8 RTP payload profile MUST assign a dynamic
payload type number to be used in each RTP session and
provide a mechanism to indicate the mapping. See
<xref target="SdpParameters"/> for the mechanism to be used
with the Session Description Protocol (SDP)
<xref target="RFC4566"/>.</t>
<t hangText="Timestamp:"> The RTP timestamp indicates the
time when the frame was sampled. The granularity of the
clock is 90 kHz, so a delta of 1 represents 1/90,000 of a
second.</t>
<t>The remaining RTP Fixed Header Fields (V, P, X, CC,
sequence number, SSRC and CSRC identifiers) are used as
specified in Section 5.1 of <xref target="RFC3550"/>.</t>
</list></t>
</section>
<section anchor="VP8payloadDescriptor" title="VP8 Payload Descriptor">
<figure anchor="figureVP8payloadDescriptor">
<preamble>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.</preamble>
<artwork><![CDATA[
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)
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="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. Note: this X bit is not to be confused with the
X bit in the RTP header.</t>
<t hangText="R:">Bit reserved for future use. MUST be set to zero
and MUST be ignored by the receiver.</t>
<t hangText="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. <list style="empty">
<t>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.</t>
</list></t>
<t hangText="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.</t>
<t hangText="PID:">Partition index. Denotes which VP8 partition
the first payload octet of the packet belongs to. The first VP8
partition (containing modes and motion vectors) MUST be labeled
with PID = 0. PID SHOULD be incremented by 1 for each subsequent
partition, but MAY be kept at 0 for all packets. PID cannot 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.</t>
</list></t>
<t>When the X bit is set to 1 in the first octet, the
Extended Control Bits field octet MUST be provided as the second
octet. If the X bit is 0, the Extended Control Bits field octet MUST
NOT be present, and no extensions (I, L, T, or K) are
permitted.
<list style="hanging">
<t hangText="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.</t>
<t hangText="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>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="RSV:">Bits reserved for future use. MUST be set to
zero and MUST be ignored by the receiver.</t>
</list></t>
<t>After the extension bit field follow the extension data fields that
are enabled. <list style="hanging">
<t hangText="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:
<list style="hanging">
<t hangText="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. Note: this M
bit is not to be confused with the M bit in the RTP header.</t>
<t hangText="PictureID:">7 or 15 bits (shown left and
right, respectively, in <xref
target="figureVP8payloadDescriptor"/>) not including the M
bit. This is a running index of the frames, which MAY
start at a random value, 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.</t></list></t>
<t hangText="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:
<list style="hanging">
<t hangText="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 MAY start at a
random value, and MUST wrap to 0 after reaching the
maximum number 255. Use of TL0PICIDX depends on the presence
of TID. It is therefore RECOMMENDED that the TID is used
whenever TL0PICIDX is.</t></list></t>
<t hangText="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:
<list style="hanging">
<t hangText="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.</t>
<t hangText="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. Additionally, the Y bit MUST be set to 0 if any frame
following the current frame depends on a non-base layer frame older
than the base layer 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 three codec
buffers (as defined by <xref target="RFC6386"/>) that have been
updated since the last time the KEYIDX field was changed.</t><!-- <list
style="empty">-->
<t>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.</t>
<!--</list></t>-->
<t hangText="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 at a random value, and MUST wrap to 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. </t><!--<list style="empty">-->
<t>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.</t>
<!--</list></t>--></list></t>
<t hangText="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.</t>
</list></t>
<t><vspace blankLines="100"/></t>
<!-- force a pagebreak-->
</section>
<section anchor="VP8payloadHeader" title="VP8 Payload Header">
<t>The beginning of an encoded VP8 frame is referred to as an
"uncompressed data chunk" in Section 9.1 of <xref target="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.</t>
<t>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 <xref target="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 or mode) partition. In this RTP payload
specification, the location offsets are considered to be part
of the first partition.</t>
<figure anchor="figureVP8payloadHeader">
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Size0|H| VER |P|
+-+-+-+-+-+-+-+-+
| Size1 |
+-+-+-+-+-+-+-+-+
| Size2 |
+-+-+-+-+-+-+-+-+
| Octets 4..N of|
| VP8 payload |
: :
+-+-+-+-+-+-+-+-+
| OPTIONAL RTP |
| padding |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>A packetizer needs access to the P bit. The remaining
fields are left unexplained with reference to <xref
target="RFC6386"/></t>
<t><list style="hanging">
<t hangText="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 <xref target="RFC6386"/></t>
</list></t>
</section>
<section title="Aggregated and Fragmented Payloads">
<t>An encoded VP8 frame can be divided into two or more
partitions, as described in <xref target="intro"/>. 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.</t>
<t>If the preferred usage suggested in <xref
target="mediaFormatDescription"/> is followed, with each packet
carrying data from exactly one partition, the S bit and PID
fields described in <xref target="VP8payloadDescriptor"/>
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.</t>
<t>If the packetizer does not pay attention to the partition
boundaries, one packet can contain a fragment of a partition,
a complete partition, or an aggregate of fragments and
partitions. There is no explicit signaling of partition
boundaries in the payload and the partition lengths at the end
of the first partition have to be used to identify the
boundaries. Partitions MUST be aggregated in decoding order.
Two fragments from different partitions MAY be aggregated into
the same packet along with one or more complete
partitions.</t>
<t>In all cases, the payload of a packet MUST contain data
from only one video frame. Consequently the set of packets
carrying the data from a particular frame will contain exactly
one VP8 Payload Header (see <xref target="VP8payloadHeader"/>)
carried in the first packet of the frame. The last, or only,
packet carrying data for the frame MUST have the M bit set in
the RTP header.</t>
</section>
<section title="Example algorithms">
<section title="Frame reconstruction algorithm">
<t>Example of frame reconstruction algorithm. <list style="hanging">
<t hangText="1:">Collect all packets with a given RTP
timestamp.</t>
<t hangText="2:">Go through packets in order, sorted by sequence
numbers, if packets are missing, send NACK as defined in <xref
target="RFC4585"/> or decode with missing partitions, see <xref
target="FrameReconstructionWithLoss"/> below.</t>
<t hangText="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.</t>
</list></t>
</section>
<section anchor="FrameReconstructionWithLoss"
title="Partition reconstruction algorithm">
<t>Example of partition reconstruction algorithm. The algorithm only
applies for the RECOMMENDED use case with partitions in separate
packets.
<list style="hanging">
<t hangText="1:">Scan for the start of a new partition; S=1.</t>
<t hangText="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.</t>
<t hangText="3:">Store the packets in the complete partition,
continue the scan repeating from step 1 until end of frame is
reached.</t>
<t hangText="4:">Send all complete partitions to the decoder. If
no complete partition is found discard the whole frame.</t>
</list></t>
</section>
</section>
<section title="Examples of VP8 RTP Stream">
<t>A few examples of how the VP8 RTP payload can be used are included
below.</t>
<section title="Key frame in a single RTP packet">
<figure>
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="Non-discardable VP8 interframe in a single RTP packet; no PictureID">
<figure>
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t><vspace blankLines="100"/></t>
<!-- force a pagebreak-->
</section>
<section title="VP8 partitions in separate RTP packets">
<figure>
<preamble>First RTP packet; complete first partition.</preamble>
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+
| Octets 4..L of|
| first VP8 |
| partition |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<figure>
<preamble>Second RTP packet; complete second partition.</preamble>
<artwork><![CDATA[
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 |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="VP8 frame fragmented across RTP packets">
<figure>
<preamble>First RTP packet; complete first partition.</preamble>
<artwork><![CDATA[
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 |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<figure>
<preamble>Second RTP packet; first fragment of second
partition.</preamble>
<artwork><![CDATA[
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 |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<figure>
<preamble>Third RTP packet; second fragment of second
partition.</preamble>
<artwork><![CDATA[
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 |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<figure>
<preamble>Fourth RTP packet; last fragment of second
partition.</preamble>
<artwork><![CDATA[
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 |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="VP8 frame with long PictureID">
<figure>
<preamble>PictureID = 4711 = 001001001100111 binary (first 7 bits:
0010010, last 8 bits: 01100111).</preamble>
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+
| Octets 4..N of|
| VP8 payload |
: :
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
</section>
</section>
<section anchor="RPSIandSLI" title="Using VP8 with RPSI and SLI Feedback">
<t>The VP8 payload descriptor defined in <xref
target="VP8payloadDescriptor"/> 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 <xref target="RFC4585"/>.</t>
<section anchor="RPSI" title="RPSI">
<t>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
for point to point (unicast) communication. 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 <xref target="RFC4585"/>) is simply the PictureID of the
received frame.</t>
</section>
<section anchor="SLI" title="SLI">
<t>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 <xref
target="RFC4585"/>:</t>
<figure anchor="figureSLIHeader">
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>Here, First is the macroblock address (in scan order) of
the first lost block and Number is the number of lost blocks,
as defined in <xref target="RFC4585"/>. 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 part of the frame is
corrupted. When the sender receives an SLI message, it can
make use of the knowledge from the latest received RPSI
message. Knowing that the last golden or altref frame was
successfully received, it can encode the next frame with
reference to that established reference.</t>
</section>
<section title="Example">
<t>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.</t>
<texttable anchor="table_example_timing"
title="Exemple signaling between sender and receiver">
<ttcol align="left">Event</ttcol>
<ttcol align="left">Sender</ttcol>
<ttcol align="left">Receiver</ttcol>
<ttcol align="left">Established reference</ttcol>
<c>1000</c>
<c>Send golden frame PictureID = 0</c>
<c/>
<c/>
<c/>
<c/>
<c>Receive and decode golden frame</c>
<c/>
<c>1001</c>
<c/>
<c>Send RPSI(0)</c>
<c/>
<c>1002</c>
<c>Receive RPSI(0)</c>
<c/>
<c>golden</c>
<c>...</c>
<c>(sending regular frames)</c>
<c/>
<c/>
<c>1100</c>
<c>Send altref frame PictureID = 100</c>
<c/>
<c/>
<c/>
<c/>
<c>Altref corrupted or lost</c>
<c>golden</c>
<c>1101</c>
<c/>
<c>Send SLI(100)</c>
<c>golden</c>
<c>1102</c>
<c>Receive SLI(100)</c>
<c/>
<c/>
<c>1103</c>
<c>Send frame with reference to golden</c>
<c/>
<c/>
<c/>
<c/>
<c>Receive and decode frame (decoder state restored)</c>
<c>golden</c>
<c>...</c>
<c>(sending regular frames)</c>
<c/>
<c/>
<c>1200</c>
<c>Send altref frame PictureID = 200</c>
<c/>
<c/>
<c/>
<c/>
<c>Receive and decode altref frame</c>
<c>golden</c>
<c>1201</c>
<c/>
<c>Send RPSI(200)</c>
<c/>
<c>1202</c>
<c>Receive RPSI(200)</c>
<c/>
<c>altref</c>
<c>...</c>
<c>(sending regular frames)</c>
<c/>
<c/>
<c>1300</c>
<c>Send golden frame PictureID = 300</c>
<c/>
<c/>
<c/>
<c/>
<c>Receive and decode golden frame</c>
<c>altref</c>
<c>1301</c>
<c/>
<c>Send RPSI(300)</c>
<c>altref</c>
<c>1302</c>
<c>RPSI lost</c>
<c/>
<c/>
<c>1400</c>
<c>Send golden frame PictureID = 400</c>
<c/>
<c/>
<c/>
<c/>
<c>Receive and decode golden frame</c>
<c>altref</c>
<c>1401</c>
<c/>
<c>Send RPSI(400)</c>
<c/>
<c>1402</c>
<c>Receive RPSI(400)</c>
<c/>
<c>golden</c>
</texttable>
<t>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 <xref target="RFC4585"/>, as long as the picture is
corrupted.</t>
</section>
</section>
<section anchor="payloadFormatParameters"
title="Payload Format Parameters">
<t>This payload format has two optional parameters.</t>
<section anchor="mediaTypeRegistration" title="Media Type Definition">
<t>This registration is done using the template defined in <xref
target="RFC6838"/> and following <xref target="RFC4855"/>. <list
style="hanging">
<t hangText="Type name:">video</t>
<t hangText="Subtype name:">VP8</t>
<t hangText="Required parameters:">None.</t>
<t hangText="Optional parameters:"><vspace
blankLines="0"/> 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. <list style="hanging">
<t hangText="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.</t>
<t hangText="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.</t>
<t>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).</t>
</list></t>
<t hangText="Encoding considerations:"><vspace blankLines="0"/>
This media type is framed in RTP and contains binary data; see
Section 4.8 of <xref target="RFC6838"/>.</t>
<t hangText="Security considerations:">See <xref
target="securityConsiderations"/> of RFC xxxx. <vspace
blankLines="0"/> [RFC Editor: Upon publication as an RFC, please
replace "XXXX" with the number assigned to this document and
remove this note.]</t>
<t hangText="Interoperability considerations:">None.</t>
<t hangText="Published specification:">VP8 bitstream format <xref
target="RFC6386"/> and RFC XXXX. <vspace blankLines="0"/> [RFC
Editor: Upon publication as an RFC, please replace "XXXX" with the
number assigned to this document and remove this note.] <vspace
blankLines="0"/></t>
<t hangText="Applications which use this media type:"><vspace
blankLines="0"/> For example: Video over IP, video
conferencing.</t>
<t hangText="Fragment identifier considerations:">N/A.</t>
<t hangText="Additional information:">None.</t>
<t
hangText="Person & email address to contact for further information:"><vspace
blankLines="0"/> Patrik Westin, patrik.westin@gmail.com</t>
<t hangText="Intended usage:">COMMON</t>
<t hangText="Restrictions on usage:"><vspace blankLines="0"/> This
media type depends on RTP framing, and hence is only defined for
transfer via RTP <xref target="RFC3550"/>.</t>
<t hangText="Author:">Patrik Westin, patrik.westin@gmail.com</t>
<t hangText="Change controller:"><vspace blankLines="0"/> IETF
Payload Working Group delegated from the IESG.</t>
</list></t>
</section>
<section anchor="SdpParameters" title="SDP Parameters">
<t>The receiver MUST ignore any fmtp parameter unspecified in this
memo.</t>
<section title="Mapping of Media Subtype Parameters to SDP">
<t>The media type video/VP8 string is mapped to fields in the
Session Description Protocol (SDP) <xref target="RFC4566"/> as
follows: <list style="symbols">
<t>The media name in the "m=" line of SDP MUST be video.</t>
<t>The encoding name in the "a=rtpmap" line of SDP MUST be VP8
(the media subtype).</t>
<t>The clock rate in the "a=rtpmap" line MUST be 90000.</t>
<t>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.</t>
</list></t>
<section title="Example">
<t>An example of media representation in SDP is as follows:</t>
<t>m=video 49170 RTP/AVPF 98<vspace blankLines="0"/> a=rtpmap:98
VP8/90000<vspace blankLines="0"/> a=fmtp:98 max-fr=30;
max-fs=3600;<vspace blankLines="0"/></t>
</section>
</section>
<section title="Offer/Answer Considerations">
<t>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 <xref target="mediaTypeRegistration"/>,
where the macroblock size is 16x16 pixels as defined in <xref
target="RFC6386"/>, the max-fs and max-fr parameters MUST be used to
establish these limits.</t>
</section>
</section>
</section>
<section anchor="securityConsiderations" title="Security Considerations">
<t>RTP packets using the payload format defined in this
specification are subject to the security considerations
discussed in the RTP specification <xref target="RFC3550"/> ,
and in any applicable RTP profile such as RTP/AVP <xref
target="RFC3551"/>, RTP/AVPF <xref target="RFC4585"/>, RTP/SAVP
<xref target="RFC3711"/> or RTP/SAVPF <xref target="RFC5124"/>.
However, as "Securing the RTP Protocol Framework: Why RTP Does
Not Mandate a Single Media Security Solution" <xref
target="RFC7202"/> discusses, it is not an RTP payload format's
responsibility to discuss or mandate what solutions are used to
meet the basic security goals like confidentiality, integrity
and source authenticity for RTP in general. This responsibility
lays on anyone using RTP in an application. They can find
guidance on available security mechanisms and important
considerations in Options for Securing RTP Sessions <xref
target="RFC7201"/>. Applications SHOULD use one or more
appropriate strong security mechanisms. The rest of this
security consideration section discusses the security impacting
properties of the payload format itself.</t>
<t>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.</t>
</section>
<section anchor="congestionControl" title="Congestion Control">
<t>Congestion control for RTP SHALL be used in accordance with RFC 3550
<xref target="RFC3550"/>, and with any applicable RTP profile; e.g., RFC
3551 <xref target="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 <xref target="VP8payloadDescriptor"/> 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).</t>
</section>
<section anchor="IANAConsiderations" title="IANA Considerations">
<t>The IANA is requested to register the following values:<vspace
blankLines="0"/> - Media type registration as described in <xref
target="mediaTypeRegistration"/>.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc6386;
&rfc2119;
&rfc4585;
&rfc3550;
&rfc3551;
&rfc4566;
&rfc6838;
&rfc4855;
</references>
<references title="Informative References">
&rfc3711;
&rfc5124;
&rfc7201;
&rfc7202;
<reference anchor="Sch07" target="http://dx.doi.org/10.1109/TCSVT.2007.905532">
<front>
<title>Overview of the Scalable Video Coding Extension of the H.264/AVC Standard</title>
<author initials="H." surname="Schwarz" fullname="Heiko Schwarz"><organization/></author>
<author initials="D." surname="Marpe" fullname="Detlev Marpe"><organization/></author>
<author initials="T." surname="Wiegand" fullname="Thomas Wiegand"><organization/></author>
<date year="2007" />
</front>
<seriesInfo name="Circuits and Systems for Video Technology, IEEE Transactions on," value="17(9)" />
<seriesInfo name="DOI" value="10.1109/TCSVT.2007.905532" />
</reference>
</references>
</back>
</rfc>
<!-- LocalWords: PictureID DCT Hadamard WHT SSRC CSRC pyld hdr FI VER RPSI
-->
<!-- LocalWords: stPartitionSize SLI SDP AVPF SRTP IANA PID PICIDX TID
-->
<!-- LocalWords: RTP Amphitheatre Kungsbron WebM RTCP KEYIDX NACK
-->
<!-- LocalWords: partId IP IETF IESG AVP SAVP SAVPF
-->
| PAFTECH AB 2003-2026 | 2026-04-23 19:40:05 |