| Internet-Draft | RTP Payload Format for VP9 | June 2021 |
| Uberti, et al. | Expires 6 December 2021 | [Page] |
This specification 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.¶
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This specification 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 above VP8, VP9 is also designed in a way that allows spatially-scalable video encoding.¶
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.¶
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 key frames.¶
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, and "picture" to refer to all the representations (frames) at a single instant in time. A picture thus consists of one or more frames, encoding different spatial layers.¶
Within a picture, a frame with spatial layer ID equal to SID, where SID > 0, can depend on a frame of the same picture with a lower 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 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 frame can depend only on the immediately previous spatial layer SID-1 frame, when S > 0. Additionally, if inter-picture dependency is used, a spatial layer SID frame is assumed to only depend on a previously coded spatial layer SID frame.¶
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 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 key frame with temporal layer ID equal to 0.¶
In flexible mode, each packet can contain up to 3 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 (PG). In this mode, the inter-picture dependencies (the reference indices) of the Picture Group 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", as used in this document, is not the same thing as the term "Group of Pictures" as it is traditionally used in video coding, i.e. to mean an independently-decoadable 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 modes.¶
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 are encoded as unsigned integers in network octet order.¶
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The VP9 payload descriptor will be described in Section 4.2; the VP9 payload is described in [VP9-BITSTREAM]. OPTIONAL RTP padding MUST NOT be included unless the P bit is set.¶
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].¶
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 |
| .. |
+-+-+-+-+-+-+-+-+
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 |
| .. |
+-+-+-+-+-+-+-+-+
The mandatory first octet is followed by the extension data fields that are enabled:¶
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.¶
When P and F are both set to one, indicating a non-key frame in flexible mode, then at least one reference index MUST 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.¶
The scalability structure (SS) data describes the resolution of each frame within a picture as well as the inter-picture dependencies for a picture group (PG). If the VP9 payload descriptor's "V" bit is set, the SS data is present in the position indicated in Figure 2 and Figure 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 .
+-+-+-+-+-+-+-+-+ -/ -/
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, SID or L bit equal to zero, 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).¶
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.¶
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; it can however 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.¶
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 | - |
This structure is constructed such that the "U" bit can always be set.¶
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 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.¶
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 an 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.¶
The Layer Refresh Request (LRR) [I-D.ietf-avtext-lrr] 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 shows the format of LRR's layer index fields for VP9 streams. The two "RES" fields MUST be set to 0 on transmission and ingnored 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 a LRR, those packets should contain layer indices and the reference field(s) so that the decoder or a selective forwarding middleboxes [RFC7667] can make this derivation.¶
Example:¶
LRR {1,0}, {2,1} is sent by an MCU when it is currently relaying {1,0} to a receiver and which 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}, 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.¶
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 receivers max-fs and max-fr value; in such scenarios, a sender MAY exceed the signaled values.)¶
A receiver MUST ignore any parameter unspecified in this specification.¶
| Profile | profile-id |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 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 |
The media type video/VP9 string is mapped to fields in the Session Description Protocol (SDP) [RFC8866] as follows:¶
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¶
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:¶
This registration is done using the template defined in [RFC6838] and following [RFC4855].¶
[RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.]¶
VP9 bitstream format [VP9-BITSTREAM] and RFC XXXX.¶
[RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.]¶
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 Protocol 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 lays on 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. The rest of this security consideration section discusses the security impacting properties of the payload format itself.¶
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 mis-behave. 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, 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.¶
Congestion control for RTP SHALL be used in accordance with RFC 3550 [RFC3550], and with any applicable RTP profile; e.g., RFC 3551 [RFC3551]. The congestion control mechanism can, in a real-time encoding scenario, adapt the transmission rate by instructing the encoder to encode at a certain target rate. Media aware network elements MAY use the information in the 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).¶
The IANA is requested to register the media type registration "video/vp9" as specified in Section 7. The media type is also requested to be added to the IANA registry for "RTP Payload Format MIME types" <http://www.iana.org/assignments/rtp-parameters>.¶
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 comments and feedback.¶