Payload Working Group J. Uberti
Internet-Draft S. Holmer
Intended status: Standards Track M. Flodman
Expires: September 14, 2017 Google
J. Lennox
D. Hong
Vidyo
March 13, 2017
RTP Payload Format for VP9 Video
draft-ietf-payload-vp9-03
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on September 14, 2017.
Copyright Notice
Copyright (c) 2017 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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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions, Definitions and Acronyms . . . . . . . . . . . . 3
3. Media Format Description . . . . . . . . . . . . . . . . . . 3
4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 5
4.2. VP9 Payload Description . . . . . . . . . . . . . . . . . 6
4.2.1. Scalability Structure (SS): . . . . . . . . . . . . . 10
4.3. VP9 Payload Header . . . . . . . . . . . . . . . . . . . 12
4.4. Frame Fragmentation . . . . . . . . . . . . . . . . . . . 12
4.5. Examples of VP9 RTP Stream . . . . . . . . . . . . . . . 12
5. Feedback Messages . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Reference Picture Selection Indication (RPSI) . . . . . . 12
5.2. Slice Loss Indication (SLI) . . . . . . . . . . . . . . . 13
5.3. Full Intra Request (FIR) . . . . . . . . . . . . . . . . 13
5.4. Layer Refresh Request (LRR) . . . . . . . . . . . . . . . 14
6. Payload Format Parameters . . . . . . . . . . . . . . . . . . 14
6.1. Media Type Definition . . . . . . . . . . . . . . . . . . 15
6.2. SDP Parameters . . . . . . . . . . . . . . . . . . . . . 16
6.2.1. Mapping of Media Subtype Parameters to SDP . . . . . 16
6.2.2. Offer/Answer Considerations . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
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].
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2. Conventions, Definitions and Acronyms
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].
3. Media Format Description
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.
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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.
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
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RECOMMENDED. All integer fields in the specifications 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 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
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 this bit MUST be set to 1 for the target spatial layer frame
if a stream is being rewritten to remove higher spatial layers.
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Payload Type (TP): 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 6.2 for the mechanism to be used with the Session
Description Protocol (SDP) [RFC4566].
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.
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 Description
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
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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
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.
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 LRR
[I-D.ietf-avtext-lrr] message (see Section 5.4). When P is set to
zero, the T bit (described below) MUST also be set to 0 (if
present).
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.
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
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1 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, and B bit (described below)
equal to 1.
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.
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.
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.
-: Bit reserved for future use. MUST be set to zero and MUST be
ignored by the receiver.
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 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. 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 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. The receiver MUST NOT
assume that the number of bits in PID stay the same through the
session.
In the non-flexible mode (when the F bit is set to 0), this PID is
used as an index to the GOF specified in the SS data bleow. In
this mode, the PID of the key frame corresponds to the very first
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specified frame in the GOF. Then subsequent PIDs are mapped to
subsequently specified frames in the GOF (modulo N_G, specified in
the SS data below), respectively.
Layer indices: This information 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 (T)
and spatial layer ID (S) 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
layer frame.
In the non-flexible mode (when the F bit is set to 0), another
octet is used to represent temporal layer 0 index (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 T and S fields indicate the temporal and spatial layers and
can help middleboxes and and endpoints quickly identify which
layer a packet belongs to.
T: The temporal layer ID of current frame. In the case of non-
flexible mode, if PID is mapped to a frame in a specified GOF,
then the value of T MUST match the corresponding T value of the
mapped frame in the GOF.
U: Switching up point. 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.
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.
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.
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
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T set to 0. If T is larger than 0, TL0PICIDX indicates which
temporal base layer frame the current frame depends on.
TL0PICIDX MUST be incremented when T 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 one, indicating a
non-key frame in flexible mode, 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.
P_DIFF: The reference index (in 7 bits) 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.
N: 1 if there is additional P_DIFF following the current P_DIFF.
4.2.1. Scalability Structure (SS):
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.
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+-+-+-+-+-+-+-+-+
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
N_S: N_S + 1 indicates the number of spatial layers present in the
VP9 stream.
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.
G: GOF description present flag.
-: Bit reserved for future use. MUST be set to zero and MUST be
ignored by the receiver.
N_G: N_G 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 pictures in the GOF, each frame's temporal
layer ID (T), switch up point (U), and the R reference indices
(P_DIFFs) are specified.
The very first frame specified in the GOF MUST have T set to 0.
G set to 0 or N_G set to 0 indicates that either there is only one
temporal layer or no fixed inter-picture dependency information is
present going forward in the bitstream.
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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.
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).
4.3. VP9 Payload Header
TODO: need to describe VP9 payload header.
4.4. 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 RTP marker bit M set. There is no mechanism for finer-
grained access to parts of a VP9 frame.
4.5. Examples of VP9 RTP Stream
TODO
5. Feedback Messages
5.1. Reference Picture Selection Indication (RPSI)
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,
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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.
5.2. Slice Loss Indication (SLI)
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.
5.3. 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 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.
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5.4. Layer Refresh Request (LRR)
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.
6. Payload Format Parameters
This payload format has two optional parameters.
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6.1. Media Type Definition
This registration is done using the template defined in [RFC6838] and
following [RFC4855].
Type name: video
Subtype name: VP9
Required parameters: None.
Optional parameters:
These parameters are used to signal the capabilities of a receiver
implementation. If the implementation is willing to receive
media, both parameters MUST be provided. These parameters MUST
NOT be used for any other purpose.
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 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).
Encoding considerations:
This media type is framed in RTP and contains binary data; see
Section 4.8 of [RFC6838].
Security considerations: See Section 7 of RFC xxxx.
[RFC Editor: Upon publication as an RFC, please replace "XXXX"
with the number assigned to this document and remove this note.]
Interoperability considerations: None.
Published specification: 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.]
Applications which use this media type:
For example: Video over IP, video conferencing.
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Fragment identifier considerations: N/A.
Additional information: None.
Person & email address to contact for further information:
TODO [Pick a contact]
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only defined
for transfer via RTP [RFC3550].
Author: TODO [Pick a contact]
Change controller:
IETF Payload Working Group delegated from the IESG.
6.2. SDP Parameters
The receiver MUST ignore any fmtp parameter unspecified in this memo.
6.2.1. Mapping of Media Subtype Parameters to SDP
The media type video/VP9 string is mapped to fields in the Session
Description Protocol (SDP) [RFC4566] as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be VP9 (the
media subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The parameters "max-fs", and "max-fr", MUST be included in the
"a=fmtp" line of SDP if SDP is used to declare receiver
capabilities. These parameters are expressed as a media subtype
string, in the form of a semicolon separated list of
parameter=value pairs.
6.2.1.1. 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;
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6.2.2. Offer/Answer Considerations
TODO: Update this for VP9
7. 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]. 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.
8. Congestion Control
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).
9. IANA Considerations
The IANA is requested to register the following values:
- Media type registration as described in Section 6.1.
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10. References
10.1. Normative References
[I-D.ietf-avtext-lrr]
Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", draft-ietf-avtext-lrr-03 (work in progress),
July 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, <http://www.rfc-editor.org/info/rfc3550>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[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,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<http://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, <http://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,
<http://www.rfc-editor.org/info/rfc6838>.
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[VP9-BITSTREAM]
Grange, A., de Rivaz, P., and J. Hunt, "VP9 Bitstream &
Decoding Process Specification", Version 0.6, March 2016,
<https://storage.googleapis.com/downloads.webmproject.org/
docs/vp9/vp9-bitstream-specification-
v0.6-20160331-draft.pdf>.
10.2. 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,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc5124>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://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, <http://www.rfc-editor.org/info/rfc7202>.
Authors' Addresses
Justin Uberti
Google, Inc.
747 6th Street South
Kirkland, WA 98033
USA
Email: justin@uberti.name
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Stefan Holmer
Google, Inc.
Kungsbron 2
Stockholm 111 22
Sweden
Email: holmer@google.com
Magnus Flodman
Google, Inc.
Kungsbron 2
Stockholm 111 22
Sweden
Email: mflodman@google.com
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
Danny Hong
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: danny@vidyo.com
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