Internet Engineering Task Force (IETF) J. Uberti
Request for Comments: 9628 S. Holmer
Category: Standards Track M. Flodman
ISSN: 2070-1721 D. Hong
Google
J. Lennox
8x8 / Jitsi
August
October 2024
RTP Payload Format for VP9 Video
Abstract
This specification describes an RTP payload format for the VP9 video
codec. The payload format has wide applicability as it supports
applications from low bitrate peer-to-peer usage to high bitrate
video conferences. It includes provisions for temporal and spatial
scalability.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9628.
Copyright Notice
Copyright (c) 2024 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
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions
3. Media Format Description
4. Payload Format
4.1. RTP Header Usage
4.2. VP9 Payload Descriptor
4.2.1. Scalability Structure (SS)
4.3. Frame Fragmentation
4.4. Scalable Encoding Considerations
4.5. Examples of VP9 RTP Stream
4.5.1. Reference Picture Use for Scalable Structure
5. Feedback Messages and Header Extensions
5.1. Reference Picture Selection Indication (RPSI)
5.2. Full Intra Request (FIR)
5.3. Layer Refresh Request (LRR)
6. Payload Format Parameters
6.1. SDP Parameters
6.1.1. Mapping of Media Subtype Parameters to SDP
6.1.2. Offer/Answer Considerations
7. Media Type Definition
8. Security Considerations
9. Congestion Control
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
This document describes an RTP [RFC3550] payload specification
applicable to the transmission of video streams encoded using the VP9
video codec [VP9-BITSTREAM]. The format described in this document
can be used both in peer-to-peer and video conferencing applications.
The VP9 video codec was developed by Google and is the successor to
its earlier VP8 [RFC6386] codec. Above the compression improvements
and other general enhancements to VP8, VP9 is also designed in a way
that allows spatially scalable video encoding.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Media Format Description
The VP9 codec can maintain up to eight reference frames, of which up
to three can be referenced by any new frame.
VP9 also allows a frame to use another frame of a different
resolution as a reference frame. (Specifically, a frame may use any
references whose width and height are between 1/16th that of the
current frame and twice that of the current frame, inclusive.) This
allows internal resolution changes without requiring the use of
keyframes.
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.
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.
This payload format specification defines how such temporal and
spatial scalability layers can be described and communicated.
Temporal and spatial scalability layers are associated with non-
negative integer IDs. The lowest layer of either type has an ID of 0
and is sometimes referred to as the "base" temporal or spatial layer.
Layers are designed, and MUST be encoded, such that if any layer, and
all higher layers, are removed from the bitstream along either the
spatial or temporal dimension, the remaining bitstream is still
correctly decodable.
For terminology, this document uses the term "frame" to refer to a
single encoded VP9 frame for a particular resolution/quality, resolution and/or quality,
and "picture" to refer to all the representations (frames) at a
single instant in time. Thus, a picture consists of one or more
frames, encoding different spatial layers.
Within a picture, a frame with spatial-layer ID equal to SID, S, where
SID S >
0, can depend on a frame of the same picture with a lower
spatial-layer spatial-
layer ID. This "inter-layer" dependency can result in additional
coding gain compared to the case where only traditional "inter-picture"
dependency is used, where a frame depends on a previously coded frame
in time. For simplicity, this payload format assumes that, within a
picture and if inter-layer dependency is used, a spatial-layer SID S
frame can depend only on the immediately previous spatial-layer SID-1 S-1
frame, when S > 0. Additionally, if inter-
picture inter-picture dependency is
used, a spatial-layer SID S frame is assumed to only depend on a
previously coded spatial-layer SID S frame.
Given the above simplifications for inter-layer and inter-picture
dependencies, a flag (the D bit described below) is used to indicate
whether a spatial-layer SID frame depends on the spatial-layer SID-1
frame. Given the D bit, a receiver only needs to additionally 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 first packet of a keyframe with a temporal-
layer ID equal to 0.
In flexible mode, each packet can contain up to three reference
indices, which identify 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. Since this is signaled in each packet, it makes it
possible to have very flexible temporal-layer hierarchies and
scalability structures, which are changing dynamically.
In non-flexible mode, frames are encoded using a fixed, recurring
pattern of dependencies; the set of pictures that recur in this
pattern is known as a "Picture Group" (or "PG"). In this mode, the
inter-picture dependencies (the reference indices) of the PG MUST be
pre-specified as part of the Scalability Structure (SS) data. Each
packet has an index to refer to one of the described pictures in the
PG from which the pictures referenced by the picture transmitted in
the current packet for inter-picture prediction can be identified.
| Note: A "Picture Group" or "PG", as used in this document, is
| not the same thing as the term "Group of Pictures" as it is traditionally
| commonly used in video coding, i.e., to mean an independently
| decodable run of pictures beginning with a keyframe.
|
| The SS data can also be used to specify the resolution of each
| spatial layer present in the VP9 stream for both flexible and non-
flexible
| non-flexible modes.
4. Payload Format
This section describes how the encoded VP9 bitstream is encapsulated
in RTP. To handle network losses, usage of RTP/AVPF [RFC4585] is
RECOMMENDED. All integer fields in the specifications this specification are encoded as
unsigned integers in network octet order.
4.1. RTP Header Usage
The general RTP payload format for VP9 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 |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| VP9 payload descriptor (integer #octets) |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+ |
: VP9 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: General RTP Payload Format for VP
See Section 4.2 for more information on the VP9 payload descriptor;
the VP9 payload is described in [VP9-BITSTREAM]. OPTIONAL RTP
padding MUST NOT be included unless the P bit is set.
Marker bit (M): This bit MUST be set to 1 for the final packet of
the highest spatial-layer frame (the final packet of the picture),
and 0 otherwise. picture);
otherwise, it is 0. Unless spatial scalability is in use for this
picture, this bit will have the same value as the E bit described
in Section 4.2. Note this bit MUST be set to 1 for the target
spatial-layer frame if a stream is being rewritten to remove
higher spatial layers.
Payload Type (PT): In line with the policy in Section 3 of
[RFC3551], applications using the VP9 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
Section 6.1 for the mechanism to be used with the Session
Description Protocol (SDP) [RFC8866].
Timestamp: The RTP timestamp [RFC3550] indicates the time when the
input frame was sampled, at a clock rate of 90 kHz. If the input
picture is encoded with multiple-layer frames, all of the frames
of the picture MUST have the same timestamp.
If a frame has the VP9 show_frame field set to 0 (i.e., it is
meant only to populate a reference buffer without being output),
its timestamp MAY alternatively be set to be the same as the
subsequent frame with show_frame equal to 1. (This will be
convenient for playing out pre-encoded content packaged with VP9
"superframes", which typically bundle show_frame==0 frames with a
subsequent show_frame==1 frame.) Every frame with show_frame==1,
however, MUST have a unique timestamp modulo the 2^32 wrap of the
field.
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
[RFC3550].
4.2. VP9 Payload Descriptor
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.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|P|L|F|B|E|V|Z| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (REQUIRED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: | TID |U| SID |D| (Conditionally RECOMMENDED)
+-+-+-+-+-+-+-+-+ -\
P,F: | P_DIFF |N| (Conditionally REQUIRED) - up to 3 times
+-+-+-+-+-+-+-+-+ -/
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
Figure 2: Flexible Mode Format for VP9 Payload Descriptor
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.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|P|L|F|B|E|V|Z| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: | TID |U| SID |D| (Conditionally RECOMMENDED)
+-+-+-+-+-+-+-+-+
| TL0PICIDX | (Conditionally REQUIRED)
+-+-+-+-+-+-+-+-+
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
Figure 3: Non-flexible Mode Format for VP9 Payload Descriptor
I: Picture ID (PID) present. When set to 1, the OPTIONAL PID MUST
be present after the mandatory first octet and specified as below.
Otherwise, PID MUST NOT be present. If the V bit was set in the
stream's most recent start of a keyframe (i.e., the SS field was
present) and the F bit is set to 0 (i.e., non-flexible scalability
mode is in use), then this bit MUST be set on every packet.
P: Inter-picture predicted frame. When set to 0, the frame does not
utilize inter-picture prediction. In this case, up-switching to a
current spatial layer's frame is possible from a directly lower
spatial-layer frame. P SHOULD also be set to 0 when encoding a
layer synchronization frame in response to a Layer Refresh Request
(LRR) [RFC9627] message (see Section 5.3). When P is set to 0,
the TID field (described below) MUST also be set to 0 (if
present). Note that the P bit does not forbid intra-picture,
inter-layer prediction from earlier frames of the same picture, if
any.
L: Layer indices present. When set to 1, 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.
F: Flexible mode. When set to 1, this indicates flexible mode; if
the P bit is also set to 1, then the octets following the
mandatory first octet, the PID, and layer indices (if present) are
as described by "Reference "reference indices" below. This bit MUST only be
set to 1 if the I bit is also set to 1; if the I bit is set to 0,
then this bit MUST also be set to 0 and ignored by receivers.
(Flexible mode's Reference reference indices are defined as offsets from the
Picture ID field, so they would have no meaning if I were not
set.) The value of the F bit MUST only change on the first packet
of a key picture. A "key picture" is a picture whose base
spatial-layer frame is a keyframe, and thus one which completely
resets the encoder state. This packet will have its P bit equal
to 0, SID or L bit (described below) equal to 0, and B bit
(described below) equal to 1.
B: Start of a frame. This bit MUST be set to 1 if the first payload
octet of the RTP packet is the beginning of a new VP9 frame;
otherwise, it MUST NOT be 1. Note that this frame might not be
the first frame of a picture.
E: End of a frame. This bit MUST be set to 1 for the final RTP
packet of a VP9 frame, and 0 otherwise. frame; otherwise, it is 0. This enables a decoder
to finish decoding the frame, where it otherwise may need to wait
for the next packet to explicitly know that the frame is complete.
Note that, if spatial scalability is in use, more frames from the
same picture may follow; see the description of the B bit above.
V: Scalability Structure (SS) data present. When set to 1, the
OPTIONAL SS data MUST be present in the payload descriptor.
Otherwise, the SS data MUST NOT be present.
Z: Not a reference frame for upper spatial layers. If set to 1,
indicates that frames with higher spatial layers SID+1 and greater
of the current and following pictures do not depend on the current
spatial-layer SID frame. This enables a decoder that is targeting
a higher spatial layer to know that it can safely discard this
packet's frame without processing it, without having to wait for
the D bit in the higher-layer frame (see below).
The mandatory first octet is followed by the extension data fields
that are enabled:
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 M is
set, the PID field MUST contain 15 bits; otherwise, it MUST
contain 7 bits. See PID below.
Picture ID (PID): Picture ID represented in 7 or 15 bits, depending
on the M bit. This is a running index of the pictures, where the
sender increments the value by 1 for each picture it sends.
(Note, however, that because a middlebox can discard pictures
where permitted by the SS, Picture IDs as received by a receiver
might not be contiguous.) This field MUST be present if the I bit
is equal to one. If M is set to 0, 7 bits carry the PID; else, if
M is set to 1, 15 bits carry the PID in network byte order. The
sender may choose between a 7- or 15-bit index. The PID SHOULD
start on a random number and MUST wrap after reaching the maximum
ID (0x7f or 0x7fff depending on the index size chosen). The
receiver MUST NOT assume that the number of bits in the PID stays
the same through the session. If this field transitions from 7
bits to 15 bits, the value is zero-extended (i.e., the value after
0x6e is 0x006f); if the field transitions from 15 bits to 7 bits,
it is truncated (i.e., the value after 0x1bbe is 0xbf).
In the non-flexible mode (when the F bit is set to 0), this PID is
used as an index to the PG specified in the SS data below. In
this mode, the PID of the keyframe corresponds to the first
specified frame in the PG. Then subsequent PIDs are mapped to
subsequently specified frames in the PG (modulo N_G, specified in
the SS data below), respectively.
All frames of the same picture MUST have the same PID value.
Frames (and their corresponding pictures) with the VP9 show_frame
field equal to 0 MUST have distinct PID values from subsequent
pictures with show_frame equal to 1. Thus, a picture (as defined
in this specification) is different than a VP9 superframe.
All frames of the same picture MUST have the same value for
show_frame.
Layer indices: This information field is optional but RECOMMENDED whenever
encoding with layers. For both flexible and non-flexible modes,
one octet is used to specify a layer frame's temporal-layer ID
(TID) and spatial-layer ID (SID) as shown both in Figure 2 and
Figure 3. Additionally, a bit (U) is used to indicate 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 frame.
In the non-flexible mode (when the F bit is set to 0), another
octet is used to represent temporal-layer Temporal Layer 0 index Picture Index (8 bits)
(TL0PICIDX), as depicted in Figure 3. The TL0PICIDX is present so
that all minimally required frames (the base temporal-layer
frames) can be tracked.
The TID and SID fields indicate the temporal and spatial layers
and can help middleboxes and endpoints quickly identify which
layer a packet belongs to.
TID: The temporal-layer ID of the current frame. In the case of
non-flexible mode, if a PID is mapped to a picture in a
specified PG, then the value of the TID MUST match the
corresponding TID value of the mapped picture in the PG.
U: Switching up point. If this bit is set to 1 for the current
picture with a temporal-layer ID equal to TID, value T, then "switch
"switching up" to a higher frame rate is possible as subsequent
higher temporal-layer pictures will not depend on any picture
before the current picture (in coding order) with temporal-layer a temporal-
layer ID value greater than TID. T.
SID: The spatial-layer ID of the current frame. Note that frames
with spatial-layer SID > 0 may be dependent on decoded spatial-
layer SID-1 frame within the same picture. Different frames of
the same picture MUST have distinct spatial-layer IDs, and
frames' spatial layers MUST appear in increasing order within
the frame.
D: Inter-layer dependency is used. D MUST be set to 1 if and
only if the current spatial-layer SID frame depends on spatial-
layer SID-1 frame of the same picture; otherwise, it MUST be
set to 0. For the base-layer frame (with SID equal to 0), the
D bit MUST be set to 0.
TL0PICIDX: 8 bits temporal-layer zero index. Temporal Layer 0 Picture Index (8 bits). TL0PICIDX is
only present in the non-flexible mode (F = 0). This is a
running index for the temporal base-layer pictures, i.e., the
pictures with a TID set to 0. If the TID is larger than 0,
TL0PICIDX indicates which temporal base-layer picture the
current picture depends on. TL0PICIDX MUST be incremented by 1
when the TID is equal to 0. The index SHOULD start on a random
number and MUST restart at 0 after reaching the maximum number
255.
Reference indices: When P and F are both set to 1, indicating a non-
keyframe in flexible mode, then at least one reference index MUST
be specified as below. Additional reference indices (a total of
up to three reference indices are allowed) may be specified using
the N bit below. When either P or F is set to 0, then no
reference index is specified.
P_DIFF: The reference index (in 7 bits) specified as the relative
PID from the current picture. For example, when P_DIFF=3 on a
packet containing the picture with PID 112 means that the
picture refers back to the picture with PID 109. This
calculation is done modulo the size of the PID field, i.e.,
either 7 or 15 bits. A P_DIFF value of 0 is invalid.
N: 1 if there is additional P_DIFF following the current P_DIFF.
4.2.1. Scalability Structure (SS)
The SS data describes the resolution of each frame within a picture
as well as the inter-picture dependencies for a PG. If the VP9
payload descriptor's V bit is set, the SS data is present in the
position indicated in Figures 2 and 3.
+-+-+-+-+-+-+-+-+
V: | N_S |Y|G|-|-|-|
+-+-+-+-+-+-+-+-+ -\
Y: | WIDTH | (OPTIONAL) .
+ + .
| | (OPTIONAL) .
+-+-+-+-+-+-+-+-+ . - N_S + 1 times
| HEIGHT | (OPTIONAL) .
+ + .
| | (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -/
G: | N_G | (OPTIONAL)
+-+-+-+-+-+-+-+-+ -\
N_G: | TID |U| R |-|-| (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -\ . - N_G times
| P_DIFF | (OPTIONAL) . - R times .
+-+-+-+-+-+-+-+-+ -/ -/
Figure 4: VP9 Scalability Structure
N_S: Number of Spatial Layers Minus 1. N_S + 1 indicates the number
of spatial layers present in the VP9 stream.
Y: Each spatial layer's frame resolution is present. When set to 1,
the OPTIONAL WIDTH (2 octets) and HEIGHT (2 octets) MUST be
present for each layer frame. Otherwise, the resolution MUST NOT
be present.
G: The PG description present flag.
-: A bit reserved for future use. It MUST be set to 0 and MUST be
ignored by the receiver.
N_G: N_G indicates the number of pictures in a PG. 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 pictures in the PG, each picture's
temporal-layer ID (TID), switch up point (U), and Reference reference
indices (P_DIFFs) are specified.
The first picture specified in the PG MUST have a TID set to 0.
G set to 0 or N_G set to 0 indicates that either there is only one
temporal layer (for non-flexible mode) or no fixed inter-picture
dependency information is present (for flexible mode) going
forward in the bitstream.
Note that for a given picture, all frames follow the same inter-
picture dependency structure. However, the frame rate of each
spatial layer can be different from each other; this can be
described 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.
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 the P bit equal to 0, SID or L bit equal to 0, and B bit equal
to 1. The SS data MUST only be changed on the picture that
corresponds to the first picture specified in the previous SS data's
PG (if the previous SS data's N_G was greater than 0).
4.3. Frame Fragmentation
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 E bit set. There is no mechanism for finer-grained access
to parts of a VP9 frame.
4.4. Scalable Encoding Considerations
In addition to the use of reference frames, VP9 has several
additional forms of inter-frame dependencies, largely involving
probability tables for the entropy and tree encoders. In VP9 syntax,
the syntax element "error_resilient_mode" resets this additional
inter-frame data, allowing a frame's syntax to be decoded
independently.
Due to the requirements of scalable streams, a VP9 encoder producing
a scalable stream needs to ensure that a frame does not depend on a
previous frame (of the same or a previous picture) that can
legitimately be removed from the stream. Thus, a frame that follows
a frame that might be removed (in full decode order) MUST be encoded
with "error_resilient_mode" set to true.
For spatially scalable streams, this means that
"error_resilient_mode" needs to be turned on for the base spatial
layer; however, it can be turned off for higher spatial layers,
assuming they are sent with inter-layer dependency (i.e., with the D
bit set). For streams that are only temporally scalable without
spatial scalability, "error_resilient_mode" can additionally be
turned off for any picture that immediately follows a temporal-layer
0 frame.
4.5. Examples of VP9 RTP Stream
4.5.1. Reference Picture Use for Scalable Structure
As discussed in Section 3, the VP9 codec can maintain up to eight
reference frames, of which up to three can be referenced or updated
by any new frame. This section illustrates one way that a scalable
structure (with three spatial layers and three temporal layers) can
be constructed using these reference frames.
+==========+=========+============+=========+
| Temporal | Spatial | References | Updates |
+==========+=========+============+=========+
| 0 | 0 | 0 | 0 |
+----------+---------+------------+---------+
| 0 | 1 | 0,1 | 1 |
+----------+---------+------------+---------+
| 0 | 2 | 1,2 | 2 |
+----------+---------+------------+---------+
| 2 | 0 | 0 | 6 |
+----------+---------+------------+---------+
| 2 | 1 | 1,6 | 7 |
+----------+---------+------------+---------+
| 2 | 2 | 2,7 | - |
+----------+---------+------------+---------+
| 1 | 0 | 0 | 3 |
+----------+---------+------------+---------+
| 1 | 1 | 1,3 | 4 |
+----------+---------+------------+---------+
| 1 | 2 | 2,4 | 5 |
+----------+---------+------------+---------+
| 2 | 0 | 3 | 6 |
+----------+---------+------------+---------+
| 2 | 1 | 4,6 | 7 |
+----------+---------+------------+---------+
| 2 | 2 | 5,7 | - |
+----------+---------+------------+---------+
Table 1: Example Scalability Structure
This structure is constructed such that the U bit can always be set.
5. Feedback Messages and Header Extensions
5.1. Reference Picture Selection Indication (RPSI)
The reference picture selection index RPSI 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 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 for point-to-point (unicast)
communication. 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
Picture ID 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 (7- or 15-bit) Picture ID of the received frame.
| Note: because all frames of the same picture must have the same
| inter-picture reference structure, there is no need for a
| message to specify which frame is being selected.
5.2. Full Intra Request (FIR)
The Full Intra Request (FIR) [RFC5104] RTCP feedback message allows a
receiver to request a full state refresh of an encoded stream.
Upon receipt of a FIR request, a VP9 sender MUST send a picture with
a keyframe for its spatial-layer 0 layer frame and then send frames
without inter-picture prediction (P=0) for any higher-layer frames.
5.3. Layer Refresh Request (LRR)
The Layer Refresh Request (LRR) [RFC9627] allows a receiver to
request a single layer of a spatially or temporally encoded stream to
be refreshed without necessarily affecting the stream's other layers.
+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+---------------+---------+-----+
| RES | TID | RES | SID |
+---------------+---------+-----+
Figure 5: LRR Index Format
Figure 5 shows the format of an LRR's layer index fields for VP9
streams. The two "RES" fields MUST be set to 0 on transmission and
ignored on reception. See Section 4.2 for details on the TID and SID
fields.
Identification of a layer refresh frame can be derived from the
reference IDs of each frame by backtracking the dependency chain
until reaching a point where only decodable frames are being
referenced. Therefore, it's recommended for both the flexible and
the non-flexible mode that, when switching up points are being
encoded in response to an LRR, those packets contain layer indices
and the reference field or fields so that the decoder or selective
forwarding middleboxes [RFC7667] can make this derivation.
Example:
LRR {1,0}, {2,1} is sent by a Multipoint Control Unit (MCU) when it
is currently relaying {1,0} to a receiver and which that wants to upgrade to
{2,1}. In response, the encoder should encode the next frames in
layers {1,1} and {2,1} by only referring to frames in {1,0}, {1,0} or {0,0}.
In the non-flexible mode, periodic upgrade frames can be defined by
the layer structure of the SS; thus, periodic upgrade frames can be
automatically identified by the Picture ID.
6. Payload Format Parameters
This payload format has three optional parameters: max-fr, max-fs,
and profile-id.
The max-fr and max-fs 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. A media sender SHOULD NOT
send media with a frame rate or frame size exceeding the max-fr and
max-fs values signaled. (There may be scenarios, such as pre-encoded
media or selective forwarding middleboxes [RFC7667], where a media
sender does not have media available that fits within a receiver's
max-fs and max-fr value; values; in such scenarios, a sender MAY exceed the
signaled values.)
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 each 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).
profile-id: The value of profile-id is an integer indicating the
default coding profile (the subset of coding tools that may have
been used to generate the stream or that the receiver supports).
Table 2 lists all of the profiles defined in Section 7.2 of
[VP9-BITSTREAM] and the corresponding integer values to be used.
If no profile-id is present, Profile 0 MUST be inferred. (The
profile-id parameter was added relatively late in the development
of this specification, so some existing implementations may not
send it.)
Informative note: See Table 3 for capabilities of coding profiles
defined in Section 7.2 of [VP9-BITSTREAM].
A receiver MUST ignore any parameter unspecified in this
specification.
+=========+============+
| Profile | profile-id |
+=========+============+
| 0 | 0 |
+---------+------------+
| 1 | 1 |
+---------+------------+
| 2 | 2 |
+---------+------------+
| 3 | 3 |
+---------+------------+
Table 2: Comparison of
Correspondence between
profile-id to VP9
Profile Integer
+=========+===========+=================+==========================+
| Profile | Bit Depth | SRGB Colorspace | Chroma Subsampling |
+=========+===========+=================+==========================+
| 0 | 8 | No | YUV 4:2:0 |
+---------+-----------+-----------------+--------------------------+
| 1 | 8 | Yes | YUV 4:2:2,4:4:0 or 4:4:4 |
+---------+-----------+-----------------+--------------------------+
| 2 | 10 or 12 | No | YUV 4:2:0 |
+---------+-----------+-----------------+--------------------------+
| 3 | 10 or 12 | Yes | YUV 4:2:2,4:4:0 or 4:4:4 |
+---------+-----------+-----------------+--------------------------+
Table 3: Profile Capabilities
| Note: SRGB (often sRGB) = Standard Red-Green-Blue
6.1. SDP Parameters
6.1.1. Mapping of Media Subtype Parameters to SDP
The media type video/vp9 string is mapped to fields in the Session
Description Protocol (SDP) [RFC8866] as follows:
* The media name in the "m=" line of SDP MUST be video.
* The encoding name in the "a=rtpmap" line of SDP MUST be VP9 (the
media subtype).
* The clock rate in the "a=rtpmap" line MUST be 90000.
* The parameters max-fr and max-fs MUST be included in the "a=fmtp"
line of SDP if the receiver wishes to declare its receiver
capabilities. These parameters are expressed as a media subtype
string in the form of a semicolon-separated list of
parameter=value pairs.
* The OPTIONAL parameter profile-id, when present, SHOULD be
included in the "a=fmtp" line of SDP. This parameter is expressed
as a media subtype string in the form of a parameter=value pair.
When the parameter is not present, a value of 0 MUST be inferred
for profile-id.
6.1.1.1. Example
An example of media representation in SDP is as follows:
m=video 49170 RTP/AVPF 98
a=rtpmap:98 VP9/90000
a=fmtp:98 max-fr=30;max-fs=3600;profile-id=0
6.1.2. Offer/Answer Considerations
When VP9 is offered over RTP using SDP in an Offer/Answer model
[RFC3264] for negotiation for unicast usage, the following
limitations and rules apply:
* The parameter identifying a media format configuration for VP9 is
profile-id. This media format configuration parameter MUST be
used symmetrically; that is, the answerer MUST either maintain
this configuration parameter or remove the media format (payload
type) completely if it is not supported.
* The max-fr and max-fs parameters are used declaratively to
describe receiver capabilities, even in the Offer/Answer model.
The values in an answer are used to describe the answerer's
capabilities; thus, their values are set independently of the
values in the offer.
* To simplify the handling and matching of these configurations, the
same RTP payload type number used in the offer SHOULD also be used
in the answer and in a subsequent offer, as specified in
[RFC3264]. An answer or subsequent offer MUST NOT contain the
payload type number used in the offer unless the profile-id value
is exactly the same as in the original offer. However, max-fr and
max-fs parameters MAY be changed in subsequent offers and answers,
with the same payload type number, if an endpoint wishes to change
its declared receiver capabilities.
7. Media Type Definition
This registration uses the template defined in [RFC6838] and
following [RFC4855].
Type name: video
Subtype name: VP9
Required parameters: N/A
Optional parameters: There are three optional parameters: max-fr,
max-fs, and profile-id. See Section 6 for their definition.
Encoding considerations: This media type is framed in RTP and
contains binary data; see Section 4.8 of [RFC6838].
Security considerations: See Section 8 of RFC 9628.
Interoperability considerations: None
Published specification: VP9 bitstream format [VP9-BITSTREAM] and
RFC 9628.
Applications that 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: Jonathan
Lennox <jonathan.lennox@8x8.com>
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing;
hence, it is only defined for transfer via RTP [RFC3550].
Author: Jonathan Lennox <jonathan.lennox@8x8.com>
Change controller: IETF AVTCore Working Group delegated from the
IESG.
8. 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 such as
RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
SAVPF [RFC5124]. However, as "Securing the RTP Framework: Why RTP
Does Not Mandate a Single Media Security Solution" [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 lies with anyone using RTP in an
application. They can find guidance on available security mechanisms
in "Options for Securing RTP Sessions [RFC7201]. Applications SHOULD
use one or more appropriate strong security mechanisms.
Implementations of this RTP payload format need to take appropriate
security considerations into account. It is extremely important for
the decoder to be robust against malicious or malformed payloads and
ensure that they do not cause the decoder to overrun its allocated
memory or otherwise misbehave. An overrun in allocated memory could
lead to arbitrary code execution by an attacker. The same applies to
the encoder, even though problems in encoders are (typically) rarer.
This RTP payload format and its media decoder do not exhibit any
significant non-uniformity in the receiver-side computational
complexity for packet processing; thus, they 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.
9. Congestion Control
Congestion control for RTP SHALL be used in accordance with
[RFC3550], and with any applicable RTP profile, e.g., [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 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).
10. IANA Considerations
IANA has registered the media type registration "video/vp9" as
specified in Section 7. The media type has also been added to the
"RTP Payload Format Media Types" <https://www.iana.org/assignments/
rtp-parameters> subregistry of the "Real-Time Transport Protocol
(RTP) Paramaeters" registry. registry as follows.
Media Type: video
Subtype: VP9
Clock Rate (Hz): 90000
Reference: RFC 9628
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[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,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/info/rfc5104>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
[RFC9627] Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", RFC 9627, DOI 10.17487/RFC9627, August 2024,
<https://www.rfc-editor.org/info/rfc9627>.
[VP9-BITSTREAM]
Grange, A., de Rivaz, P., and J. Hunt, "VP9 Bitstream &
Decoding Process Specification", Version 0.6, 31 March
2016,
<https://storage.googleapis.com/downloads.webmproject.org/
docs/vp9/vp9-bitstream-specification-
v0.6-20160331-draft.pdf>.
11.2. Informative References
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, <https://www.rfc-editor.org/info/rfc5124>.
[RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
<https://www.rfc-editor.org/info/rfc6386>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/info/rfc7201>.
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
2014, <https://www.rfc-editor.org/info/rfc7202>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/rfc7667>.
Acknowledgments
Alex Eleftheriadis, Yuki Ito, Won Kap Jang, Sergio Garcia Murillo,
Roi Sasson, Timothy Terriberry, Emircan Uysaler, and Thomas Volkert
commented on the development of this document and provided helpful
feedback.
Authors' Addresses
Justin Uberti
Google, Inc.
747 6th Street South
Kirkland, WA 98033
United States of America
Email: justin@uberti.name
Stefan Holmer
Google, Inc.
Kungsbron 2
SE-111 22 Stockholm
Sweden
Email: holmer@google.com
Magnus Flodman
Google, Inc.
Kungsbron 2
SE-111 22 Stockholm
Sweden
Email: mflodman@google.com
Danny Hong
Google, Inc.
1585 Charleston Road
Mountain View, CA 94043
United States of America
Email: dannyhong@google.com
Jonathan Lennox
8x8, Inc. / Jitsi
Jersey City, NJ 07302
United States of America
Email: jonathan.lennox@8x8.com