| Payload Working Group | J. Uberti |
| Internet-Draft | S. Holmer |
| Intended status: Standards Track | M. Flodman |
| Expires: September 14, 2017 | |
| J. Lennox | |
| D. Hong | |
| Vidyo | |
| March 13, 2017 |
RTP Payload Format for VP9 Video
draft-ietf-payload-vp9-03
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.
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This memo describes an RTP 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.
TODO: VP9 description. Please see [VP9-BITSTREAM].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
TODO: Cite terminology from [VP9-BITSTREAM].
The VP9 codec can maintain up to eight reference frames, of which up to three can be referenced or updated by any new frame.
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.
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.
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.
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.
Within a super frame, a layer frame with spatial layer ID equal to S, where S > 0, can depend on a layer frame of the same super frame 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 super frame and 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 previously coded spatial layer S 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 S frame depends on spatial layer S-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 very 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 patterns which are changing dynamically.
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 MUST have an index to refer to one of the described frames in the GOF, from which the frames referenced by the frame transmitted in the current packet for inter-picture prediction can be identified.
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 pyld hdr | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+ |
: Bytes 2..N of VP9 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The VP9 payload descriptor and VP9 payload header will be described in Section 4.2 and Section 4.3. OPTIONAL RTP padding MUST NOT be included unless the P bit is set. The figure specifically shows the format for the first packet in a frame. Subsequent packets will not contain the VP9 payload header, and will have later octets in the frame payload.
Figure 1
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|-| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (REQUIRED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: | T |U| S |D| (CONDITIONALLY RECOMMENDED)
+-+-+-+-+-+-+-+-+ -\
P,F: | P_DIFF |N| (CONDITIONALLY REQUIRED) - up to 3 times
+-+-+-+-+-+-+-+-+ -/
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
Figure 2
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|-| (REQUIRED)
+-+-+-+-+-+-+-+-+
I: |M| PICTURE ID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
M: | EXTENDED PID | (RECOMMENDED)
+-+-+-+-+-+-+-+-+
L: | T |U| S |D| (CONDITIONALLY RECOMMENDED)
+-+-+-+-+-+-+-+-+
| TL0PICIDX | (CONDITIONALLY REQUIRED)
+-+-+-+-+-+-+-+-+
V: | SS |
| .. |
+-+-+-+-+-+-+-+-+
Figure 3
The mandatory first octet is followed by the extension data fields that are enabled:
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 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: | T |U| R |-|-| (OPTIONAL) .
+-+-+-+-+-+-+-+-+ -\ . - N_G times
| P_DIFF | (OPTIONAL) . - R times .
+-+-+-+-+-+-+-+-+ -/ -/
Figure 4
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, and B bit equal to 1. The SS data MUST only be changed on the frame that corresponds to the very first frame specified in the previous SS data's GOF (if the previous SS data's N_G was greater than 0).
TODO: need to describe VP9 payload header.
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 M set. There is no mechanism for finer-grained access to parts of a VP9 frame.
TODO
TODO: Update to indicate which frame within the picture.
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 PictureID in the payload descriptor, the receiver can acknowledge this simply by sending an RPSI message back to the sender. The message body (i.e., the "native RPSI bit string" in [RFC4585]) is simply the PictureID of the received frame.
TODO: Update to indicate which frame within the picture.
The slice loss indication is another payload-specific feedback message defined within the RTCP-based feedback format. The SLI message is generated by the receiver when a loss or corruption is detected in a frame. The format of the SLI message is as follows [RFC4585]:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First | Number | PictureID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5
Here, First is the macroblock address (in scan order) of the first lost block and Number is the number of lost blocks, as defined in [RFC4585]. 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 part of the frame is corrupted. When the sender receives an SLI message, it can make use of the knowledge from the latest received RPSI message. Knowing that the last golden or altref frame was successfully received, it can encode the next frame with reference to that established reference.
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 super frame 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 [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|
+-------------+-----------------+
| T |R| S | RES |
+-------------+-----------------+
Figure 6
Figure 6 shows the format of LRR's layer index field for VP9 streams. This is designed to follow the same layout as the "L" byte of the VP9 payload header, which carries the stream's layer information. The "R" and "RES" fields MUST be set to 0 on transmission and ingnored on reception. See Section 4.2 for details on the T and S 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 upgrade frames are being encoded in response to a LRR, those packets should contain layer indices and the reference fields so that the decoder or an MCU 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 two optional parameters.
This registration is done using the template defined in [RFC6838] and following [RFC4855].
The receiver MUST ignore any fmtp parameter unspecified in this memo.
The media type video/VP9 string is mapped to fields in the Session Description Protocol (SDP) [RFC4566] 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;
TODO: Update this for VP9
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]. 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.
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 following values:
- Media type registration as described in Section 6.1.
| [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. |
| [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. |
| [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. |
| [RFC7201] | Westerlund, M. and C. Perkins, "Options for Securing RTP Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014. |
| [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. |