One document matched: draft-uberti-payload-vp9-01.xml
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]>
<rfc category="std" docName="draft-uberti-payload-vp9-01" ipr="trust200902">
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes" ?>
<!-- alphabetize the references -->
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<?rfc toc="yes" ?>
<!-- generate table of contents -->
<front>
<title abbrev="RTP Payload Format for VP9">RTP Payload Format for VP9
Video</title>
<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>
<email>justin@uberti.name</email>
</address>
</author>
<author fullname="Stefan Holmer" initials="S." surname="Holmer">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>Kungsbron 2</street>
<code>111 22</code>
<city>Stockholm</city>
<country>Sweden</country>
</postal>
</address>
</author>
<author fullname="Magnus Flodman" initials="M." surname="Flodman">
<organization abbrev="Google">Google, Inc.</organization>
<address>
<postal>
<street>Kungsbron 2</street>
<code>111 22</code>
<city>Stockholm</city>
<country>Sweden</country>
</postal>
</address>
</author>
<author fullname="Jonathan Lennox" initials="J." surname="Lennox">
<organization abbrev="Vidyo">Vidyo, Inc.</organization>
<address>
<postal>
<street>433 Hackensack Avenue</street>
<street>Seventh Floor</street>
<city>Hackensack</city>
<region>NJ</region>
<code>07601</code>
<country>US</country>
</postal>
<email>jonathan@vidyo.com</email>
</address>
</author>
<author fullname="Danny Hong" initials="D." surname="Hong">
<organization abbrev="Vidyo">Vidyo, Inc.</organization>
<address>
<postal>
<street>433 Hackensack Avenue</street>
<street>Seventh Floor</street>
<city>Hackensack</city>
<region>NJ</region>
<code>07601</code>
<country>US</country>
</postal>
<email>danny@vidyo.com</email>
</address>
</author>
<date/>
<area>RAI</area>
<workgroup>Payload Working Group</workgroup>
<keyword>RFC</keyword>
<keyword>Request for Comments</keyword>
<keyword>RTP</keyword>
<keyword>VP9</keyword>
<keyword>WebM</keyword>
<abstract>
<t>This memo describes an RTP payload format for the VP9 video codec.
The payload format has wide applicability, as it supports applications
from low bit-rate peer-to-peer usage, to high bit-rate video
conferences. It includes provisions for temporal and spatial scalability.</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 VP9 video codec <xref
target="I-D.grange-vp9-bitstream"/>. The format described in this document can be used
both in peer-to-peer and video conferencing applications.</t>
<t>TODO: VP9 description. Please see <xref
target="I-D.grange-vp9-bitstream"/>.</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>
</section>
<section anchor="mediaFormatDescription" title="Media Format Description">
<t>The VP9 codec can maintain up to eight reference frames, of
which up to three can be referenced or updated by any new frame.</t>
<t>VP9 also allows a reference frame to be resampled and used as a
reference for another frame of a different resolution. This
allows internal resolution changes without requiring the use of
key frames.</t>
<t>These features together enable an encoder to
implement various forms of coarse-grained scalability,
including temporal, spatial and quality scalability modes, as
well as combinations of these, without the need for explicit
scalable coding tools.</t>
<t>Temporal layers define different frame rates of video;
spatial and quality layers define different and possibly dependent
representations of a single input frame. Spatial layers allow
a frame to be encoded at different resolutions, whereas
quality layers allow a frame to be encoded at the same
resolution but at different qualities (and thus with different
amounts of coding error). VP9 supports quality layers as
spatial layers without any resolution changes; hereinafter,
the term "spatial layer" is used to represent both spatial and
quality layers.</t>
<t>This payload format specification defines how such
temporal and spatial scalability layers can be described and communicated.</t>
<t>Layers are designed (and MUST be encoded) such that if
any layer, and all higher layers, are removed from the bitstream
along any of the two dimensions, the remaining bitstream is
still correctly decodable.</t>
<t>For terminology, this document uses the term "layer frame" to refer
to a single encoded VP9 frame for a particular resolution/quality, and
"super frame" to refer to all the representations (layer frames) at a single
instant in time. A super frame thus consists of one or more layer frames,
encoding different spatial layers.</t>
<t>Within a super frame, a layer frame with spatial layer ID equal to S,
where S > 0, can depend on a frame with a lower spatial layer ID. This
"inter-layer" dependency results in additional coding gain to the
traditional "inter-picture" dependency, where a frame depends on previously
coded frame in time. For simplicity, this payload format assumes that,
within a super frame if inter-layer dependency is used, a spatial layer S frame
can only depend on spatial layer S-1 frame when S > 0. Additionally, if
inter-picture dependency is used, spatial layer S frame is assumed to only
depend on prevously coded spatial layer S frame.</t>
<t>TODO: Describe how simulcast can be supported?</t>
<t>Given above simplifications for inter-layer and inter-picture
dependencies, a flag (the D bit described below) is used to indicate whether a
spatial layer S frame depends on spatial layer S-1 frame. Then a receiver
only needs to know the inter-picture dependency structure for a given
spatial layer frame in order to determine its decodability. Two modes
of describing the inter-picture dependency structure are possible:
"flexible mode" and "non-flexible mode". An encoder can only switch
between the two on the very first packet of a key frame with temporal
layer ID equal to 0. </t>
<t>In flexible mode, each packet can contain up to 3 reference
indices, which identifies all frames referenced by the frame
transmitted in the current packet for inter-picture prediction.
This (along with the D bit) enables a receiver to identify if a frame
is decodable or not and helps it understand the temporal layer structure
so that it can drop packets as it sees fit. Since this is signaled in
each packet it makes it possible to have very flexible temporal layer
hierarchies and patterns which are changing dynamically.</t>
<t>In non-flexible mode, the inter-picture dependency (the reference
indices) of a group of frames (GOF) MUST be pre-specified as part of the
scalability structure (SS) data. In this mode, each packet will have
an index to refer to one of the described frames, from which the
frames referenced by the frame transmitted in the current packet
for inter-picture prediction can be identified.</t>
<t>The SS data can also be used to specify the resolution of each
spatial layer present in the VP9 stream.</t>
</section>
<section anchor="payloadFormat" title="Payload Format">
<t>This section describes how the encoded VP9 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 VP9 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 |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| VP9 payload descriptor (integer #bytes) |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : VP9 pyld hdr | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+ |
: Bytes 2..N of VP9 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
<postamble>The VP9 payload descriptor and VP9 payload header will be
described in the next section. OPTIONAL RTP padding MUST NOT be included
unless the P bit is set.</postamble>
</figure>
<t><list style="hanging">
<t hangText="Marker bit (M):">MUST be set to 1 for the final packet
of the highest spatial layer frame (the final packet of the super frame),
and 0 otherwise. Unless spatial scalability is in use for this super frame,
this will have the same value as the E bit described below. Note that a
MANE MUST set this value to 1 for the target spatial layer frame
when shaping out higher spatial layers.</t>
<t hangText="Timestamp:">The RTP timestamp indicates the time when
the input frame was sampled, at a clock rate of 90 kHz. If the
input frame is encoded with multiple layer frames, all of the
layer frames of the super frame MUST have the same timestamp.</t>
<t hangText="Sequence number:">The sequence numbers are
monotonically increasing in order of the encoded bitstream.</t>
</list></t>
<t>The remaining RTP header fields are used as specified in <xref
target="RFC3550"/>.</t>
</section>
<section anchor="VP9payloadDescriptor" title="VP9 Payload Description">
<figure anchor="figureVP9payloadDescriptor">
<preamble>In flexible mode (with the F bit below set to 1), The first octets
after the RTP header are the VP9 payload descriptor, with the following
structure.</preamble>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|P|L|F|B|E|V|-| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: | T |U| S |D| (CONDITIONALLY RECOMMENDED)
+-+-+-+-+-+-+-+-+ -\
P,F: | P_DIFF |X|N| (CONDITIONALLY RECOMMENDED) .
+-+-+-+-+-+-+-+-+ . - up to 3 times
X: |EXTENDED P_DIFF| (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -/
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<figure anchor="figureVP9payloadDescriptorNonFlexible">
<preamble>In non-flexible mode (with the F bit below set to 0), The first octets
after the RTP header are the VP9 payload descriptor, with the following
structure.</preamble>
<artwork><![CDATA[
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|P|L|F|B|E|V|-| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: |GOF_IDX| S |D| (CONDITIONALLY RECOMMENDED)
+-+-+-+-+-+-+-+-+
| TL0PICIDX | (CONDITIONALLY REQUIRED)
+-+-+-+-+-+-+-+-+
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="I:">Picture ID (PID) present. When set to one, the
OPTIONAL PID MUST be present after the mandatory first octet and
specified as below. Otherwise, PID MUST NOT be present.</t>
<t hangText="P:">Inter-picture predicted layer frame. When set to zero, the
layer frame does not utilize inter-picture prediction. In this case,
up-switching to current spatial layer's frame is possible from directly
lower spatial layer frame. P SHOULD also be set to zero when
encoding a layer synchronization frame in response to an <xref target='I-D.lennox-avtext-lrr'>LRR</xref>.</t>
<t hangText="L:">Layer indices present. When set to one,
the one or two octets following the mandatory first octet and the PID
(if present) is as described by "Layer indices" below. If the F bit (described below)
is set to 1 (indicating flexible mode), then only one octet is present for the
layer indices. Otherwise if the F bit is set to 0 (indicating non-flexible mode),
then two octets are present for the layer indices.</t>
<t hangText="F:">Flexible mode. F set to one indicates
flexible mode and if the P bit is also set to one, then the octets following
the mandatory first octet, the PID, and layer indices (if present) are
as described by "Reference indices" below. This MUST only be set to one if the I
bit is also set to one; if the I bit is set to zero, then this MUST also be
set to zero and ignored by receivers. The value of this F bit CAN ONLY CHANGE
on the very first packet of a key picture. This is a packet with the P bit
equal to zero, S or D bit (described below) equal to zero, B bit (described below)
equal to 1, and temporal layer ID equal to 0.</t>
<t hangText="B:">Start of a layer frame. MUST be set to 1 if
the first payload octet of the RTP packet is the beginning of a
new VP9 layer frame, and MUST NOT be 1 otherwise. Note that this
layer frame might not be the very first layer frame of a super frame.</t>
<t hangText="E:">End of a layer frame. MUST be set to 1 for the final
RTP packet of a VP9 layer frame, and 0 otherwise. This enables a
decoder to finish decoding the layer frame, where it otherwise may need to
wait for the next packet to explicitly know that the layer frame is complete.
Note that, if spatial scalability is in use, more layer frames from the
same super frame may follow; see the description of the M bit above.</t>
<t hangText="V:">Scalability structure (SS) data present. When set
to one, the OPTIONAL SS data MUST be present in the payload descriptor.
Otherwise, the SS data MUST NOT be present.</t>
<t hangText="-:">Bit reserved for future use. MUST be set to
zero and MUST be ignored by the receiver.</t>
</list></t>
<t>The mandatory first octet is followed by the extension data fields that
are enabled:<list style="hanging">
<t hangText="M:">The most significant bit of the first octet is an
extension flag. The field MUST be present if the I bit is equal to
one. If set, the PID field MUST contain 15 bits; otherwise, it MUST
contain 7 bits. See PID below.</t>
<t hangText="Picture ID (PID):">Picture ID represented in 7 or 15 bits,
depending on the M bit. This is a running index of the pictures. The
field MUST be present if the I bit is equal to one. If M is set to zero,
7 bits carry the PID; else if M is set to one, 15 bits carry the PID.
The sender may choose between 7 or 15 bits index. The PID SHOULD start on a
random number, and MUST wrap after reaching the maximum ID. The receiver
MUST NOT assume that the number of bits in PID stay the same through the
session.</t>
<t hangText="Layer indices:">This information is optional but recommended
whenever encoding with layers. In the flexible mode (when the F bit is set to 1),
one octet is used to specify a layer frame's temporal layer ID (T) and spatial layer ID (S)
as shown in <xref target="figureVP9payloadDescriptor"/>.
Additionally, a bit (U) is used to indcate that the current frame is a
"switching up point" frame. Another bit (D) is used to indicate whether inter-layer
prediction is used for the current layer frame.</t>
<t>In the non-flexible mode (when the F bit is set to 0), two octets are used
as depicted in <xref target="figureVP9payloadDescriptorNonFlexible"/>.
Like the flexible mode, the first byte contains the spatial layer ID and the
D bit. Unlike the flexible mode, instead of the T and U fields, a
group of frames index (GOF_IDX) is specified, which can be used to obtain the
values of T and U fields from the scalable structure (SS) data described below.
An additional octet to represent the temporal layer 0 index, TL0PICIDX, is present
so that all minimally required frames can be tracked.</t>
<t>The T and S fields, whether obtained directly or indirectly from the SS data,
indicate the temporal and spatial layers and can help MCUs measure bitrates
per layer and can help them make a quick decision on whether to relay a packet
or not. They can also help receivers determine what layers they are currently
decoding.
<list style="hanging">
<t hangText="T:">The temporal layer ID of current frame. This field is only present
in the flexible mode (F = 1).</t>
<t hangText="U:">Switching up point. This bit is only present in the flexible mode (F = 1).
If this bit is set to 1 for the current frame with temporal
layer ID equal to T, then "switch up" to a higher frame rate is possible as subsequent higher temporal
layer frames will not depend on any frame before the current frame (in coding time) with temporal layer
ID greater than T.</t>
<t hangText="S:">The spatial layer ID of current frame. Note that frames with spatial layer S > 0
may be dependent on decoded spatial layer S-1 frame within the same super frame.</t>
<t hangText="D:">Inter-layer dependency used. MUST be set to one if current spatial layer S frame
depends on spatial layer S-1 frame of the same super frame. MUST only be set to zero if current spatial
layer S frame does not depend on spatial layer S-1 frame of the same super frame. For the base layer frame
with S equal to 0, this D bit MUST be set to zero.</t>
<t hangText="GOF_IDX:">An index to a frame in the group of frames (GOF) described by the
SS data. This field is only present in the non-flexible mode (F = 0).
In this mode, the SS data SHOULD have been received and the temporal characteristics of
each frame must have been speficied as group of frames in the SS data
(see the description of "Scalability structure" below). Here, the values of the T and
the U fields are derived from the SS data. Additionally, the frame's inter-picture dependecy can also be
obtained from the SS data. In the case no SS data has been received or the received SS data
does not specify GOF (N_G is set to 0), then GOF_IDX MUST be ignored and the stream is assumed to
have no temporal hierarchy with both T and U equal to 0.</t>
<t hangText="TL0PICIDX:">8 bits temporal layer zero index. TL0PICIDX is only present
in the non-flexible mode (F = 0). This is a running index for the temporal
base layer frames, i.e., the frames with temporal layer ID (TID) set to 0. If TID is larger than 0,
TL0PICIDX indicates which temporal base layer frame the current frame 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.</t>
</list></t>
<t hangText="Reference indices:">These bytes are optional, but recommended when encoding with
temporal layers in the flexible mode. When P and F are both set to one, then at least
one reference index has to be specified as below. Additional reference indices (total of up to
3 reference indices are allowed) may be specified using the N bit below. When either P or F is
set to zero, then no reference index is specified.
<list style="hanging">
<t hangText="P_DIFF:">The reference index specified as the
relative PID from the current frame. For example, when P_DIFF=3
on a packet containing the frame with PID 112 means
that the frame refers back to the frame with PID
109. This calculation is done modulo the size of the PID field,
i.e., either 7 or 15 bits. For most layer
structures a 6-bit relative PID will be enough;
however, the X bit can be used to refer to older frames.</t>
<t hangText="X:">1 if this layer index has an extended P_DIFF.</t>
<t hangText="N:">1 if there is additional P_DIFF following the current P_DIFF.</t>
</list></t>
</list></t>
<section anchor="VP9payloadDescriptorSS" title="Scalability Structure (SS):">
<t>The scalability structure (SS) data describes the resolution of
each layer frame within a super frame as well as the inter-picture dependencies
for a group of frames (GOF). If the VP9 payload descriptor's "V"
bit is set, the SS data is present in the position indicated in
<xref target="figureVP9payloadDescriptor"/> and <xref target="figureVP9payloadDescriptorNonFlexible"/>.</t>
<figure anchor="figureVP9ScalabilityStructure">
<artwork><![CDATA[
+-+-+-+-+-+-+-+-+
V: | N_S |Y| N_G |
+-+-+-+-+-+-+-+-+ -\
Y: | WIDTH | (OPTIONAL) .
+ + .
| | (OPTIONAL) .
+-+-+-+-+-+-+-+-+ . - N_S + 1 times
| HEIGHT | (OPTIONAL) .
+ + .
| | (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -/ -\
N_G: | T |U| R |-|-| (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -\ . - N_G + 1 times
| P_DIFF | (OPTIONAL) . - R times .
+-+-+-+-+-+-+-+-+ -/ -/
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="N_S:">N_S + 1 indicates the number of spatial
layers present in the VP9 stream.</t>
<t hangText="Y:">Each spatial layer's frame resolution present.
When set to one, the OPTIONAL WIDTH (2 octets) and HEIGHT
(2 octets) MUST be present for each layer frame. Otherwise, the
resolution MUST NOT be present.</t>
<t hangText="N_G:">N_G + 1 indicates the number of frames
in a GOF. If N_G is greater than 0, then the SS data allows
the inter-picture dependency structure of the VP9 stream to
be pre-declared, rather than indicating it on the fly with
every packet. If N_G is greater than 0, then for N_G + 1
pictures in the GOF, each frame's temporal layer ID (T), switch up point (U),
and the R reference indices (P_DIFFs) are specified.</t>
<t>N_G=0 indicates that either there is only one temporal
layer or no fixed inter-picture dependency information is present
going forward in the bitstream.</t>
<t>Note that for a given super frame, all layer frames follow the
same inter-picture dependency structure. However, the frame rate
of each spatial layer can be different from each other and this can
be controlled with the use of the D bit described above. The
specified dependency structure in the SS data MUST be for the highest
frame rate layer.</t>
</list></t>
<t>In a scalable stream sent with a fixed pattern, the SS data
SHOULD be included in the first packet of every key frame. This is a packet
with P bit equal to zero, S or D bit equal to zero, B bit equal to 1, and
temporal layer ID (TID) equal to 0. The SS data SHOULD also be included in the
first packet of the first frame in which the SS changes.
If the SS data is included in a frame with TID not equal to 0,
it MUST also be repeated in the first packet of the first
frame with a lower TID, until TID equals to 0.</t>
</section>
</section>
<section anchor="VP9payloadHeader" title="VP9 Payload Header">
<t>TODO: need to describe VP9 payload header.</t>
</section>
<section title="Frame Fragmentation">
<t>VP9 frames are fragmented into packets, in RTP sequence
number order, beginning with a
packet with the B bit set, and ending with a packet with the
RTP marker bit set. There is no mechanism for finer-grained
access to parts of a VP9 frame.</t>
</section>
<section title="Examples of VP9 RTP Stream">
<t>TODO</t>
</section>
</section>
<section anchor="RPSIandSLI" title="Using VP9 with RPSI and SLI Feedback">
<t>The VP9 payload descriptor defined in <xref
target="VP9payloadDescriptor"/> above contains an optional PictureID
parameter. One use of this parameter is included 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>TODO: Update to indicate which frame within the picture.</t>
<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 VP9 used as
unicast. The use of RPSI for VP9 is preferably combined with a special
update pattern of the codec's two special reference frames -- the
golden frame and the altref frame -- in which they are updated in an
alternating leapfrog fashion. When a receiver has received and
correctly decoded a golden or altref frame, 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>TODO: Update to indicate which frame within the picture.</t>
<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. PictureID is the
six least significant bits of the codec-specific picture identifier in
which the loss or corruption has occurred. For VP9, this
codec-specific identifier is naturally the PictureID of the current
frame, as read from the payload descriptor. If the payload descriptor
of the current frame does not have a PictureID, the receiver MAY send
the last received PictureID+1 in the SLI message. The receiver MAY set
the First parameter to 0, and the Number parameter to the total number
of macroblocks per frame, even though only 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.</t>
</section>
<section title="Example">
<t>TODO: this example is copied from the VP8 payload format specification,
and has not been updated for VP9. It may be incorrect.</t>
<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="Example 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 required 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:">VP9</t>
<t hangText="Required parameters:"><vspace blankLines="0"/> These
parameters MUST be used to signal the capabilities of a receiver
implementation. 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="Optional parameters:">none</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:">VP9 bitstream format <xref
target="I-D.grange-vp9-bitstream"/> 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="Additional information:">None.</t>
<t
hangText="Person & email address to contact for further information:"><vspace
blankLines="0"/> TODO [Pick a contact]</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:">TODO [Pick a contact]</t>
<t hangText="Change controller:"><vspace blankLines="0"/> IETF
Payload Working Group delegated from the IESG.</t>
</list></t>
</section>
<section 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/VP9 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 VP9
(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 of SDP. 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
VP9/90000<vspace blankLines="0"/> a=fmtp:98 max-fr=30;
max-fs=3600;<vspace blankLines="0"/></t>
</section>
</section>
<section title="Offer/Answer Considerations">
<t>TODO: Update this for VP9</t>
<!--
<t>The VP9 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>
<t>NOTE IN DRAFT: If closer control of width and height is desired,
the mechanism described in
draft-nandakumar-payload-sdp-max-video-resolution is a possible
candidate for signalling, but since that document appears to be far
from finalization, this document does not make a reference to that
document. This note is only intended for facilitating WG discussion,
and should be deleted before publication of this document as an
RFC.</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. 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 <xref target="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.</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 VP9 payload descriptor
in <xref target="VP9payloadDescriptor"/> 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>
&vp9;
&rfc2119;
&rfc4585;
&rfc3550;
&rfc3711;
&rfc4566;
&rfc6838;
&rfc4855;
&rfc3551;
<!-- Replace with proper ref once it's in the I-D repo -->
<reference anchor="I-D.lennox-avtext-lrr">
<front>
<title>The Layer Refresh Request (LRR) RTCP Feedback MessageVideo</title>
<author initials="J" surname="Lennox" fullname="Jonathan Lennox"><organization/></author>
<author initials="J" surname="Uberti" fullname="Justin Uberti"><organization/></author>
<author initials="S" surname="Holmer" fullname="Stefan Holmer"><organization/></author>
<author initials="M" surname="Flodman" fullname="Magnus Flodman"><organization/></author>
<date month="March" day="9" year="2015"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-lennox-avtext-lrr-00"/>
<format type="TXT" target="http://www.ietf.org/internet-drafts/draft-lennox-avtext-lrr-00.txt"/>
</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
-->
| PAFTECH AB 2003-2026 | 2026-04-23 14:17:46 |