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RTP_Payload_Format_For_AV1.html
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<!DOCTYPE html>
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<title>RTP Payload Format For AV1</title>
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W3C <!---0.134451%-->First Public Working Draft<!---0.134451%-->
<time class="dt-published" datetime="2020-08-27">27 August 2020<!---0.134451%--></time><!---0.134451%-->
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<section id="abstract" class="introductory"><h2>Abstract</h2>
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<section>
<h2 class="no_toc" id="rtp-payload-format-for-av1-v05">RTP Payload Format For AV1 (v0.5)<a class="self-link" aria-label="§" href="#rtp-payload-format-for-av1-v05"></a></h2>
<p><strong>Status:</strong> The Alliance for Open Media AV1 Real-Time Communications Subgroup Working Draft (WD)</p>
<section>
<h3 class="no_toc" id="abstract">Abstract<a class="self-link" aria-label="§" href="#abstract"></a></h3>
<p>This document describes an RTP payload format for the <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1 video codec</a>.
The payload format has wide applicability, from low bit-rate
peer-to-peer usage, to high bit-rate multi-party video conferences. It
includes provisions for temporal and spatial scalability.</p>
</section>
<section>
<h3 class="no_toc" id="status-of-this-document">Status of this Document<a class="self-link" aria-label="§" href="#status-of-this-document"></a></h3>
<p>This document is a working draft of the Real-Time Communications Subgroup.</p>
</section>
<section>
<h3 class="no_toc" id="contents">Contents<a class="self-link" aria-label="§" href="#contents"></a></h3>
<ul id="markdown-toc">
<li><a href="#introduction" id="markdown-toc-introduction">1. Introduction</a></li>
<li><a href="#conventions-definitions-and-acronyms" id="markdown-toc-conventions-definitions-and-acronyms">2. Conventions, Definitions and Acronyms</a></li>
<li><a href="#media-format-description" id="markdown-toc-media-format-description">3. Media Format Description</a></li>
<li><a href="#payload-format" id="markdown-toc-payload-format">4. Payload Format</a> <ul>
<li><a href="#rtp-header-usage" id="markdown-toc-rtp-header-usage">4.1 RTP Header Usage</a></li>
<li><a href="#rtp-header-marker-bit-m" id="markdown-toc-rtp-header-marker-bit-m">4.2 RTP Header Marker Bit (M)</a></li>
<li><a href="#dependency-descriptor-rtp-header-extension" id="markdown-toc-dependency-descriptor-rtp-header-extension">4.3 Dependency Descriptor RTP Header Extension</a></li>
<li><a href="#av1-aggregation-header" id="markdown-toc-av1-aggregation-header">4.4 AV1 Aggregation Header</a></li>
<li><a href="#payload-structure" id="markdown-toc-payload-structure">4.5 Payload Structure</a></li>
</ul>
</li>
<li><a href="#packetization-rules" id="markdown-toc-packetization-rules">5. Packetization Rules</a> <ul>
<li><a href="#examples" id="markdown-toc-examples">5.1 Examples</a></li>
</ul>
</li>
<li><a href="#mane-and-sfm-behavior" id="markdown-toc-mane-and-sfm-behavior">6. MANE and SFM Behavior</a> <ul>
<li><a href="#simulcast" id="markdown-toc-simulcast">6.1. Simulcast</a></li>
<li><a href="#example" id="markdown-toc-example">6.1.1 Example</a></li>
</ul>
</li>
<li><a href="#payload-format-parameters" id="markdown-toc-payload-format-parameters">7. Payload Format Parameters</a> <ul>
<li><a href="#media-type-definition" id="markdown-toc-media-type-definition">7.1. Media Type Definition</a></li>
<li><a href="#sdp-parameters" id="markdown-toc-sdp-parameters">7.2 SDP Parameters</a> <ul>
<li><a href="#mapping-of-media-subtype-parameters-to-sdp" id="markdown-toc-mapping-of-media-subtype-parameters-to-sdp">7.2.1 Mapping of Media Subtype Parameters to SDP</a></li>
</ul>
</li>
<li><a href="#rid-restrictions-mapping-for-av1" id="markdown-toc-rid-restrictions-mapping-for-av1">7.2.2 RID restrictions mapping for AV1</a> <ul>
<li><a href="#usage-with-the-sdp-offeranswer-model" id="markdown-toc-usage-with-the-sdp-offeranswer-model">7.2.3 Usage with the SDP Offer/Answer Model</a></li>
<li><a href="#usage-in-declarative-session-descriptions" id="markdown-toc-usage-in-declarative-session-descriptions">7.2.4 Usage in Declarative Session Descriptions</a></li>
</ul>
</li>
<li><a href="#examples-1" id="markdown-toc-examples-1">7.3 Examples</a> <ul>
<li><a href="#level-upgrading" id="markdown-toc-level-upgrading">7.3.1 Level upgrading</a></li>
<li><a href="#simulcast-with-payload-multiplexing" id="markdown-toc-simulcast-with-payload-multiplexing">7.3.2 Simulcast with payload multiplexing</a></li>
<li><a href="#simulcast-with-ssrc-multiplexing" id="markdown-toc-simulcast-with-ssrc-multiplexing">7.3.3 Simulcast with SSRC multiplexing</a></li>
<li><a href="#single-stream-simulcast" id="markdown-toc-single-stream-simulcast">7.3.4 Single stream simulcast</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#feedback-messages" id="markdown-toc-feedback-messages">8. Feedback Messages</a></li>
<li><a href="#full-intra-request-fir" id="markdown-toc-full-intra-request-fir">8.1. Full Intra Request (FIR)</a></li>
<li><a href="#layer-refresh-request-lrr" id="markdown-toc-layer-refresh-request-lrr">8.2. Layer Refresh Request (LRR)</a></li>
<li><a href="#iana-considerations" id="markdown-toc-iana-considerations">9. IANA Considerations</a></li>
<li><a href="#security-considerations" id="markdown-toc-security-considerations">10. Security Considerations</a></li>
<li><a href="#references" id="markdown-toc-references">11. References</a> <ul>
<li><a href="#normative-references" id="markdown-toc-normative-references">11.1 Normative References</a></li>
<li><a href="#informative-references" id="markdown-toc-informative-references">11.2 Informative References</a></li>
</ul>
</li>
<li><a href="#appendix" id="markdown-toc-appendix">Appendix</a> <ul>
<li><a href="#dependency-descriptor-rtp-header-extension-1" id="markdown-toc-dependency-descriptor-rtp-header-extension-1">Dependency Descriptor RTP Header Extension</a> <ul>
<li><a href="#a1-introduction" id="markdown-toc-a1-introduction">A.1 Introduction</a></li>
<li><a href="#a2-conventions-definitions-and-acronyms" id="markdown-toc-a2-conventions-definitions-and-acronyms">A.2 Conventions, Definitions and Acronyms</a></li>
<li><a href="#a3-media-stream-requirements" id="markdown-toc-a3-media-stream-requirements">A.3 Media Stream Requirements</a></li>
<li><a href="#a4-dependency-descriptor-format" id="markdown-toc-a4-dependency-descriptor-format">A.4 Dependency Descriptor Format</a> <ul>
<li><a href="#a41-syntax" id="markdown-toc-a41-syntax">A.4.1 Syntax</a></li>
<li><a href="#a42-semantics" id="markdown-toc-a42-semantics">A.4.2 Semantics</a></li>
<li><a href="#a43-deciding-decodability-using-chains" id="markdown-toc-a43-deciding-decodability-using-chains">A.4.3 Deciding Decodability using Chains</a></li>
<li><a href="#a44-switching" id="markdown-toc-a44-switching">A.4.4 Switching</a></li>
<li><a href="#a45-templates" id="markdown-toc-a45-templates">A.4.5 Templates</a></li>
</ul>
</li>
<li><a href="#a5-signaling-setup-information" id="markdown-toc-a5-signaling-setup-information">A.5 Signaling (Setup) Information</a></li>
<li><a href="#a6-examples" id="markdown-toc-a6-examples">A.6 Examples</a></li>
<li><a href="#a61-decode-targets-decode-target-indications-and-chains" id="markdown-toc-a61-decode-targets-decode-target-indications-and-chains">A.6.1 Decode targets, Decode Target Indications, and Chains</a></li>
<li><a href="#a62-scalability-structure-examples" id="markdown-toc-a62-scalability-structure-examples">A.6.2 Scalability structure examples</a> <ul>
<li><a href="#a621-l1t3-single-spatial-layer-with-3-temporal-layers" id="markdown-toc-a621-l1t3-single-spatial-layer-with-3-temporal-layers">A.6.2.1 L1T3 Single Spatial Layer with 3 Temporal Layers</a></li>
<li><a href="#a622-l2t1-full-svc-with-occasional-switch" id="markdown-toc-a622-l2t1-full-svc-with-occasional-switch">A.6.2.2 L2T1 Full SVC with Occasional Switch</a></li>
<li><a href="#a623-l3t3-full-svc" id="markdown-toc-a623-l3t3-full-svc">A.6.2.3 L3T3 Full SVC</a></li>
<li><a href="#a624-l3t3-k-svc-with-temporal-shift" id="markdown-toc-a624-l3t3-k-svc-with-temporal-shift">A.6.2.4 L3T3 K-SVC with Temporal Shift</a></li>
</ul>
</li>
<li><a href="#a7-references" id="markdown-toc-a7-references">A.7 References</a> <ul>
<li><a href="#a71-normative-references" id="markdown-toc-a71-normative-references">A.7.1 Normative References</a></li>
<li><a href="#a72-informative-references" id="markdown-toc-a72-informative-references">A.7.2 Informative References</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</section>
<section>
<h3 id="introduction">1. Introduction<a class="self-link" aria-label="§" href="#introduction"></a></h3>
<p>This document describes an RTP payload specification applicable to the transmission of video streams encoded using the <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1 video codec</a>.
In AV1, the smallest individual video encoder entity presented for
transport is the Open Bitstream Unit (OBU). This specification allows
both for fragmentation and aggregation of OBUs in the same RTP packet,
but explicitly disallows doing so across frame boundaries.</p>
<p>Appendix A of this document describes the Dependency Descriptor (DD)
RTP Header extension, which conveys information about individual video
frames and the dependencies between them. This allows forwarding of
video frames in situations where an intermediary does not wish to
examine the RTP payload or does not have access to it, such as when the
RTP payload is encrypted end-to-end. While the DD RTP Header extension
was designed for use with AV1, it may prove useful for other codecs as
well.</p>
<p>This specification also provides several mechanisms through which
scalability structures are described. AV1 uses the concept of predefined
scalability structures. These are a set of commonly used picture
prediction structures that can be referenced simply via an indicator
value (scalability_mode_idc, residing in the sequence header). For cases
that do not fall in any of the predefined cases, there is a mechanism
for describing the scalability structure. These bitstream parameters
greatly simplify the organization of the corresponding data at the RTP
payload format level.</p>
</section>
<section>
<h3 id="conventions-definitions-and-acronyms">2. Conventions, Definitions and Acronyms<a class="self-link" aria-label="§" href="#conventions-definitions-and-acronyms"></a></h3>
<p>The key words “<em class="rfc2119">MUST<!---0.134451%--></em>”, “<em class="rfc2119">MUST NOT<!---0.134451%--></em>”, “<em class="rfc2119">REQUIRED<!---0.134451%--></em>”, “<em class="rfc2119">SHALL<!---0.134451%--></em>”, “<em class="rfc2119">SHALL NOT<!---0.134451%--></em>”, “<em class="rfc2119">SHOULD<!---0.134451%--></em>”, “<em class="rfc2119">SHOULD NOT<!---0.134451%--></em>”, “<em class="rfc2119">RECOMMENDED<!---0.134451%--></em>”, “<em class="rfc2119">MAY<!---0.134451%--></em>”, and “<em class="rfc2119">OPTIONAL<!---0.134451%--></em>” in this document are to be interpreted as described in <a href="https://tools.ietf.org/html/rfc2119">RFC2119</a>.</p>
<dl>
<dt>Coded frame</dt>
<dd>The representation of one frame before the decoding process.</dd>
<dt>Frame</dt>
<dd>A frame in this document is synonymous to a Coded frame.</dd>
</dl>
<p class="alert alert-info"><strong>Note:</strong> In contrast, in AV1, Frame is defined as the representation of video signals in the spatial domain.</p>
<p class="alert alert-info"><strong>Note:</strong> Multiple frames may be present at the same instant in time.</p>
<dl>
<dt>Media-Aware Network Element (MANE)</dt>
<dd>A middlebox that relays streams among transmitting and receiving
clients by selectively forwarding packets and which may have access to
the media (<a href="https://tools.ietf.org/html/rfc6184">RFC6184</a>).</dd>
<dt>OBU element</dt>
<dd>An OBU, or a fragment of an OBU, contained in an RTP packet.</dd>
<dt>Open Bitstream Unit (OBU)</dt>
<dd>The smallest bitstream data framing unit in AV1. All AV1 bitstream structures are packetized in OBUs.</dd>
<dt>“S” Mode</dt>
<dd>A scalability mode in which multiple encodings are sent on the
same SSRC. This includes the S2T1, S2T1h, S2T2, S2T2h, S2T3, S2T3h,
S3T1, S3T1h, S3T2, S3T2h, S3T3 and S3T3h scalability modes defined in
Section 6.7.5 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.</dd>
<dt>Selective Forwarding Middlebox (SFM)</dt>
<dd>A middlebox that relays streams among transmitting and receiving
clients by selectively forwarding packets without requiring access to
the media (<a href="https://tools.ietf.org/html/rfc7667">RFC7667</a>).</dd>
<dt>Temporal unit</dt>
<dd>Defined by the AV1 specification: A temporal unit consists of all
the OBUs that are associated with a specific, distinct time instant.</dd>
</dl>
</section>
<section>
<h3 id="media-format-description">3. Media Format Description<a class="self-link" aria-label="§" href="#media-format-description"></a></h3>
<p>The AV1 codec can maintain up to eight reference frames, of which up
to seven can be referenced by any new frame. AV1 also allows a frame to
use another frame of a different spatial resolution as a reference
frame. This allows internal resolution changes without requiring the use
of key frames. These features together enable an AV1 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.</p>
<p>Spatial and quality layers define different and possibly dependent
representations of a single input frame. For a given spatial layer,
temporal layers define different frame rates of video. Spatial layers
allow a frame to be encoded at different spatial resolutions, whereas
quality layers allow a frame to be encoded at the same spatial
resolution but at different qualities (and thus with different amounts
of coding error). AV1 supports quality layers as spatial layers without
any resolution changes; hereinafter, the term “spatial layer” is used to
represent both spatial and quality layers.</p>
<p>This payload format specification provides for specific mechanisms
through which such temporal and spatial scalability layers can be
described and communicated.</p>
<p>Temporal and spatial scalability layers are associated with
non-negative integer IDs. The lowest layer of either type has an ID
equal to 0.</p>
<p class="alert alert-info"><strong>Note:</strong> Layer dependencies
are constrained by the AV1 specification such that a temporal layer with
temporal_id T and spatial layer with spatial_id S are only allowed to
reference previously coded video data having temporal_id T’ and
spatial_id S’, where T’ <= T and S’ <= S.</p>
</section>
<section>
<h3 id="payload-format">4. Payload Format<a class="self-link" aria-label="§" href="#payload-format"></a></h3>
<p>This section describes how the encoded AV1 bitstream is encapsulated
in RTP. All integer fields in this specification are encoded as unsigned
integers in network byte order.</p>
<section>
<h4 id="rtp-header-usage">4.1 RTP Header Usage<a class="self-link" aria-label="§" href="#rtp-header-usage"></a></h4>
<p>The general RTP payload format follows the RTP header format <a href="https://tools.ietf.org/html/rfc3550">RFC3550</a> and generic RTP header extensions <a href="https://tools.ietf.org/html/rfc8285">RFC8285</a>, and is shown below.</p>
<p>The Dependency Descriptor and AV1 aggregation header are described in
this document. The payload itself is a series of OBU elements, preceded
by length information as detailed later in this document. An OBU
element is either an entire OBU or an OBU fragment.</p>
<pre><code aria-busy="false" class="hljs">
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 |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 0x100 | 0x0 | extensions length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 0x1(ID) | hdr_length | |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| |
| dependency descriptor (hdr_length #bytes) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Other rtp header extensions...|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| AV1 aggr hdr | |
+-+-+-+-+-+-+-+-+ |
| |
| Bytes 2..N of AV1 payload |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</code></pre>
</section>
<section>
<h4 id="rtp-header-marker-bit-m">4.2 RTP Header Marker Bit (M)<a class="self-link" aria-label="§" href="#rtp-header-marker-bit-m"></a></h4>
<p>The RTP header Marker bit <em class="rfc2119">MUST<!---0.134451%--></em> be set equal to 0 if the packet is not the last packet of the temporal unit, it <em class="rfc2119">SHOULD<!---0.134451%--></em> be set equal to 1 otherwise.</p>
<p class="alert alert-info"><strong>Note:</strong> It is possible for a
receiver to receive the last packet of a temporal unit without the
marker bit being set equal to 1, and a receiver should be able to handle
this case. The last packet of a temporal unit is also indicated by the
next packet, in RTP sequence number order, having an incremented
timestamp.</p>
</section>
<section>
<h4 id="dependency-descriptor-rtp-header-extension">4.3 Dependency Descriptor RTP Header Extension<a class="self-link" aria-label="§" href="#dependency-descriptor-rtp-header-extension"></a></h4>
<p>To facilitate the work of selectively forwarding portions of a
scalable video bitstream, as is done by a Selective Forwarding Middlebox
(SFM), certain information needs to be provided for each packet.
Appendix A of this specification defines how this information is
communicated.</p>
</section>
<section>
<h4 id="av1-aggregation-header">4.4 AV1 Aggregation Header<a class="self-link" aria-label="§" href="#av1-aggregation-header"></a></h4>
<p>The aggregation header is carried in the first byte of the RTP
payload and is used to indicate if the first and/or last OBU element in
the payload is a fragment of an OBU. The aggregation header is not part
of the AV1 bitstream and <em class="rfc2119">MUST NOT<!---0.134451%--></em> be presented to an AV1 decoder.</p>
<p>The structure is as follows.</p>
<pre><code aria-busy="false" class="hljs">
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Z|Y| W |N|-|-|-|
+-+-+-+-+-+-+-+-+
</code></pre>
<p data-tests="webrtc/VideoRtpDepacketizerAv1Test/ParseTreatsContinuationFlagAsNotBeginningOfFrame">Z: <em class="rfc2119">MUST<!---0.134451%--></em>
be set to 1 if the first OBU element is an OBU fragment that is a
continuation of an OBU fragment from the previous packet, and <em class="rfc2119">MUST<!---0.134451%--></em> be set to 0 otherwise.<details class="respec-tests-details removeOnSave">
<summary>
tests: 1<!---0.134451%-->
</summary>
<ul>
<li>
<a href="https://test-site.com/webrtc/VideoRtpDepacketizerAv1Test/ParseTreatsContinuationFlagAsNotBeginningOfFrame">
ParseTreatsContinuationFlagAsNotBeginningOfFrame<!---0.134451%-->
</a>
<!---0.134451%-->
</li><!---0.134451%-->
</ul>
</details></p>
<p>Y: <em class="rfc2119">MUST<!---0.134451%--></em> be set to 1 if the last OBU element is an OBU fragment that will continue in the next packet, and <em class="rfc2119">MUST<!---0.134451%--></em> be set to 0 otherwise.</p>
<p>W: two bit field that describes the number of OBU elements in the packet. This field <em class="rfc2119">MUST<!---0.134451%--></em> be set equal to 0 or equal to the number of OBU elements contained in the packet. If set to 0, each OBU element <em class="rfc2119">MUST<!---0.134451%--></em> be preceded by a length field. If not set to 0 (i.e., W = 1, 2 or 3) the last OBU element <em class="rfc2119">MUST NOT<!---0.134451%--></em>
be preceded by a length field. Instead, the length of the last OBU
element contained in the packet can be calculated as follows:</p>
<pre><code aria-busy="false" class="hljs">
Length of the last OBU element =
length of the RTP payload
- length of aggregation header
- length of previous OBU elements including length fields
</code></pre>
<p>N: <em class="rfc2119">MUST<!---0.134451%--></em> be set to 1 if the packet is the first packet of a coded video sequence, and <em class="rfc2119">MUST<!---0.134451%--></em> be set to 0 otherwise.</p>
<p class="alert alert-info"><strong>Note:</strong> if N equals 1 then Z must equal 0.</p>
</section>
<section>
<h4 id="payload-structure">4.5 Payload Structure<a class="self-link" aria-label="§" href="#payload-structure"></a></h4>
<p>The smallest high-level syntax unit in AV1 is the OBU. All AV1
bitstream structures are packetized in OBUs. Each OBU has a header,
which provides identifying information for the contained data (payload).</p>
<p>The payload contains a series of one or more OBU elements. The design
allows for a combination of aggregation and fragmentation of OBUs,
i.e., a set of OBU elements in which the first and/or last element is a
fragment of an OBU.</p>
<p>The length field is encoded using leb128. Leb128 is defined in the
AV1 specification, and provides for a variable-sized, byte-oriented
encoding of non-negative integers where the first bit of each
(little-endian) byte indicates whether or not additional bytes are used
in the representation (AV1, Section 4.10.5).</p>
<p>Whether or not the first and/or last OBU element is a fragment of an
OBU is signaled in the aggregation header. Fragmentation may occur
regardless of how the W field is set.</p>
<p>The AV1 specification allows OBUs to have an optional size field
called obu_size (also leb128 encoded), signaled by the
obu_has_size_field flag in the OBU header. To minimize overhead, the
obu_has_size_field flag <em class="rfc2119">SHOULD<!---0.134451%--></em> be set to zero in all OBUs.</p>
<p>The following figure shows an example payload where the length field
is shown as taking two bytes for the first and second OBU elements and
one byte for the last (N) OBU element.</p>
<pre><code aria-busy="false" class="hljs">
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Z|Y|0 0|N|-|-|-| OBU element 1 size (leb128) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
: :
: OBU element 1 data :
: :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | OBU element 2 size (leb128) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: OBU element 2 data :
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: ... :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OBU e... N size| |
+-+-+-+-+-+-+-+-+ OBU element N data +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</code></pre>
<p>The following figure shows an example payload containing two OBU
elements where the last OBU element omits the length field (and the W
field is set to 2). The size of the last OBU element can be calculated
given the formula described in Section 4.4.</p>
<pre><code aria-busy="false" class="hljs">
OBU element example size calculation:
Total RTP payload size = 303 bytes
AV1 aggregation header = 1 byte
OBU element 1 size = 2 bytes
OBU element 1 data = 200 bytes
OBU element 2 data = 303 - 1 - (2 + 200) = 100 bytes
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Z|Y|1 0|N|-|-|-| OBU element 1 size (leb128) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: :
: OBU element 1 data :
: :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: :
: OBU element 2 data :
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</code></pre>
</section>
</section>
<section>
<h3 id="packetization-rules">5. Packetization Rules<a class="self-link" aria-label="§" href="#packetization-rules"></a></h3>
<p>Each RTP packet <em class="rfc2119">MUST NOT<!---0.134451%--></em> contain OBUs that belong to different temporal units.</p>
<p>The temporal delimiter OBU, if present, <em class="rfc2119">SHOULD<!---0.134451%--></em> be removed when transmitting, and <em class="rfc2119">MUST<!---0.134451%--></em> be ignored by receivers. Tile list OBUs are not supported. They <em class="rfc2119">SHOULD<!---0.134451%--></em> be removed when transmitted, and <em class="rfc2119">MUST<!---0.134451%--></em> be ignored by receivers.</p>
<p>If a sequence header OBU is present in an RTP packet and
operating_points_cnt_minus_1 > 0 then for any number i where 0 <= i
< operating_points_cnt_minus_1 the following <em class="rfc2119">MUST<!---0.134451%--></em> be true: (operating_point_idc[i] & operating_point_idc[i+1]) == operating_point_idc[i+1].</p>
<p>A sender <em class="rfc2119">MAY<!---0.134451%--></em> produce a
sequence header with operating_points_cnt_minus_1 = 0 and
operating_point_idc[0] = 0xFFF and seq_level_idx[0] = 0. In such case,
seq_level_idx[0] does not reflect the level of the operating point.</p>
<p class="alert alert-info"><strong>Note:</strong> The intent is to
disable OBU dropping in the AV1 decoder. To ensure a decoder’s
capabilities are not exceeded, OBU filtering should instead be
implemented at the system level (e.g., in a MANE).</p>
<p>If more than one OBU contained in an RTP packet has an OBU extension header, then the values of the temporal_id and spatial_id <em class="rfc2119">MUST<!---0.134451%--></em> be the same in all such OBUs in the RTP packet.</p>
<p>If a sequence header OBU is present in an RTP packet, then it <em class="rfc2119">SHOULD<!---0.134451%--></em>
be the first OBU in the packet. OBUs that are not associated with a
particular layer (and thus do not have an OBU extension header) <em class="rfc2119">SHOULD<!---0.134451%--></em> be in the beginning of a packet, following the sequence header OBU if present.</p>
<p>A sequence header OBU <em class="rfc2119">SHOULD<!---0.134451%--></em>
be included in the base layer when scalable encoding is used. When
simulcast encodings are transported on the same SSRC (an “S” mode), a
sequence header OBU <em class="rfc2119">SHOULD<!---0.134451%--></em> be
aggregated with each spatial layer. This ensures that if an intermediary
removes simulcast encodings from the bitstream before forwarding, the
modified bitstream will still be decodable.</p>
<section>
<h4 id="examples">5.1 Examples<a class="self-link" aria-label="§" href="#examples"></a></h4>
<p>The following are example packetizations of OBU sequences. A
two-letter notation is used to identify the OBU type: FH - frame header,
TG - tile group, FR - frame, SH - sequence header, TD - temporal
delimitered, MD - metadata. Parentheses after the type indicate the
temporal_id and spatial_id combination. For example “TG(0,1)” indicates a
tile group OBU with temporal_id equal to 0 and spatial_id equal to 1.</p>
<p>The following is an example coded video sequence:</p>
<pre><code aria-busy="false" class="hljs">
TD SH MD MD(0,0) FH(0,0) TG0(0,0) MD(0,1) FH(0,1) TG(0,1)
</code></pre>
<p>This sequence could be packetized as follows. First, the TD OBU is
dropped. Then, the following packetization grouping (indicated using
square brackets) may be used:</p>
<pre><code aria-busy="false" class="hljs json">
[ SH MD MD(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) FH(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) TG(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) ] [ MD(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) FH(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) TG(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) ]
</code></pre>
<p>It is also possible to send each OBU in its own RTP packet:</p>
<pre><code aria-busy="false" class="hljs css">
<span class="hljs-selector-attr">[ SH ]</span> <span class="hljs-selector-attr">[ MD ]</span> <span class="hljs-selector-attr">[ MD(0,0) ]</span> <span class="hljs-selector-attr">[ FH(0,0) ]</span> <span class="hljs-selector-attr">[ TG(0,0) ]</span> ...
</code></pre>
<p>The following packetization grouping would not be allowed, since it
combines data from different spatial layers in the same packet:</p>
<pre><code aria-busy="false" class="hljs json">
[ SH MD MD(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) FH(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) TG(<span class="hljs-number">0</span>,<span class="hljs-number">0</span>) MD(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) FH(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) TG(<span class="hljs-number">0</span>,<span class="hljs-number">1</span>) ]
</code></pre>
</section>
</section>
<section>
<h3 id="mane-and-sfm-behavior">6. MANE and SFM Behavior<a class="self-link" aria-label="§" href="#mane-and-sfm-behavior"></a></h3>
<p>If a packet contains an OBU with an OBU extension header then the
entire packet is considered associated with the layer identified by the
temporal_id and spatial_id combination that are indicated in the
extension header. If a packet does not contain any OBU with an OBU
extension header, then it is considered to be associated with all
operating points.</p>
<p>The general function of a MANE or SFM is to selectively forward
packets to receivers. To make forwarding decisions a MANE may inspect
the media payload, so that it may need to be able to parse the AV1
bitstream and if so, cannot function when end-to-end encryption is
enabled. An SFM does not parse the AV1 bitstream and therefore needs to
obtain the information relevant to selective forwarding by other means,
such as the Dependency Descriptor described in Appendix A. In addition
to enabling bitstream-independent forwarding and support for end-to-end
encryption, the Dependency Descriptor also enables forwarding where the
metadata OBU provided in the AV1 bitstream is not sufficient to express
the structure of the stream.</p>
<section>
<h4 id="simulcast">6.1. Simulcast<a class="self-link" aria-label="§" href="#simulcast"></a></h4>
<p>The RTP payload defined in this specification supports two distinct
modes for transport of simulcast encodings. In either mode, simulcast
transport <em class="rfc2119">MUST<!---0.134451%--></em> only be used to convey multiple encodings from the same source. Also, in either mode, a sequence header OBU <em class="rfc2119">SHOULD<!---0.134451%--></em> be aggregated with each spatial layer. Both modes <em class="rfc2119">MUST<!---0.134451%--></em> be supported by implementations of this specification.</p>
<p>When simulcast encodings are transported each on a separate RTP
stream, each simulcast encoding utilizes a distinct bitstream containing
its own distinct Sequence Header and Scalability Metadata OBUs. This
mode utilizes distinct SSRCs and Restriction Identifiers (RIDs) for each
encoding as described in <a href="https://tools.ietf.org/html/draft-ietf-avtext-rid">I-D.ietf-avtext-rid</a> and, as a result, RTCP feedback can be provided for each simulcast encoding. This mode of simulcast transport, which <em class="rfc2119">MUST<!---0.134451%--></em> be supported by SFMs, utilizes Session Description Protocol (SDP) signaling as described in <a href="https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast">I-D.ietf-mmusic-sdp-simulcast</a> and <a href="https://tools.ietf.org/html/draft-ietf-mmusic-rid">I-D.ietf-mmusic-rid</a>.</p>
<p>When simulcast encodings are transported on a single RTP stream, RIDs
are not used and the Sequence Header and Scalability Metadata OBUs
(utilizing an ‘S’ mode) convey information relating to all encodings.
This simulcast transport mode is possible since AV1 enables multiple
simulcast encodings to be provided within a single bitstream. However,
in this mode, RTCP feedback cannot be provided for each simulcast
encoding, but only for the aggregate, since only a single SSRC is used.
This mode of simulcast transport <em class="rfc2119">MAY<!---0.134451%--></em> be supported by SFMs.</p>
</section>
<section>
<h4 id="example">6.1.1 Example<a class="self-link" aria-label="§" href="#example"></a></h4>
<p>In this example, it is desired to send three simulcast encodings,
each containing three temporal layers. When sending all encodings on a
single SSRC, scalability mode ‘S3T3’ would be indicated within the
scalability metadata OBU, and the Dependency Descriptor describes the
dependency structure of all encodings.</p>
<p>When sending each simulcast encoding on a distinct SSRC, the
scalability mode ‘L1T3’ would be indicated within the scalability
metadata OBU of each bitstream, and the Dependency Descriptor in each
stream describes only the dependency structure for that individual
encoding. A distinct spatial_id (e.g. 0, 1, 2) could be used for each
stream (if a single AV1 encoder is used to produce the three simulcast
encodings), but if distinct AV1 encoders are used, the spatial_id values
may not be distinct.</p>
</section>
</section>
<section>
<h3 id="payload-format-parameters">7. Payload Format Parameters<a class="self-link" aria-label="§" href="#payload-format-parameters"></a></h3>
<p>This section specifies the parameters that <em class="rfc2119">MAY<!---0.134451%--></em> be used to select optional features of the payload format and certain features of the bitstream.</p>
<section>
<h4 id="media-type-definition">7.1. Media Type Definition<a class="self-link" aria-label="§" href="#media-type-definition"></a></h4>
<ul>
<li>Type name:
<ul>
<li><strong>video</strong></li>
</ul>
</li>
<li>Subtype name:
<ul>
<li><strong>AV1</strong></li>
</ul>
</li>
<li>Required parameters:
<ul>
<li>None.</li>
</ul>
</li>
<li>Optional parameters:
<ul>
<li>These parameters are used to signal the capabilities of a
receiver implementation. If the implementation is willing to receive
media, <strong>profile</strong> and <strong>level-idx</strong> parameters <em class="rfc2119">MUST<!---0.134451%--></em> be provided. These parameters <em class="rfc2119">MUST NOT<!---0.134451%--></em> be used for any other purpose.
<ul>
<li><strong>profile</strong>: The value of <strong>profile</strong> is an integer indicating the highest AV1 profile supported by the receiver. The range of possible values is identical to the <strong>seq_profile</strong> syntax element specified in <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a></li>
<li><strong>level-idx</strong>: The value of <strong>level-idx</strong> is an integer indicating the highest AV1 level supported by the receiver. The range of possible values is identical to the <strong>seq_level_idx</strong> syntax element specified in <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a></li>
<li><strong>tier</strong>: The value of <strong>tier</strong> is an integer indicating tier of the indicated level. The range of possible values is identical to the <strong>seq_tier</strong> syntax element specified in <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>. If parameter is not present, level’s tier is to be assumed equal to 0</li>
</ul>
</li>
</ul>
</li>
<li>Encoding considerations:
<ul>
<li>This media type is framed in RTP and contains binary data; see Section 4.8 of <a href="https://tools.ietf.org/html/rfc6838">RFC6838</a>.</li>
</ul>
</li>
<li>Security considerations:
<ul>
<li>See Section 10.</li>
</ul>
</li>
<li>Interoperability considerations:
<ul>
<li>None.</li>
</ul>
</li>
<li>Published specification:
<ul>
<li><a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1 video codec</a></li>
</ul>
</li>
<li>Applications which use this media type:
<ul>
<li>Video over IP, video conferencing.</li>
</ul>
</li>
<li>Fragment identifier considerations:
<ul>
<li>N/A.</li>
</ul>
</li>
<li>Additional information:
<ul>
<li>None.</li>
</ul>
</li>
<li>Person & email address to contact for further information:
<ul>
<li>TODO</li>
</ul>
</li>
<li>Intended usage:
<ul>
<li>COMMON</li>
</ul>
</li>
<li>Restrictions on usage:
<ul>
<li>This media type depends on RTP framing, and hence is only defined for transfer via RTP <a href="https://tools.ietf.org/html/rfc3550">RFC3550</a>.</li>
</ul>
</li>
<li>Author:
<ul>
<li>TODO</li>
</ul>
</li>
<li>Change controller:
<ul>
<li>AoMedia Codec Group, RTC sub-group</li>
</ul>
</li>
</ul>
</section>
<section>
<h4 id="sdp-parameters">7.2 SDP Parameters<a class="self-link" aria-label="§" href="#sdp-parameters"></a></h4>
<p>The receiver <em class="rfc2119">MUST<!---0.134451%--></em> ignore any fmtp parameter not specified in this document.</p>
<section>
<h5 id="mapping-of-media-subtype-parameters-to-sdp">7.2.1 Mapping of Media Subtype Parameters to SDP<a class="self-link" aria-label="§" href="#mapping-of-media-subtype-parameters-to-sdp"></a></h5>
<p>The media type video/AV1 string is mapped to fields in the Session Description Protocol (SDP) per <a href="https://tools.ietf.org/html/rfc4566">RFC4566</a> as follows:
* The media name in the “m=” line of SDP <em class="rfc2119">MUST<!---0.134451%--></em> be video.
* The encoding name in the “a=rtpmap” line of SDP <em class="rfc2119">MUST<!---0.134451%--></em> be AV1 (the media subtype).
* The clock rate in the “a=rtpmap” line <em class="rfc2119">MUST<!---0.134451%--></em> be 90000.
* The parameters “<strong>profile</strong>”, and “<strong>level-idx</strong>”, <em class="rfc2119">MUST<!---0.134451%--></em>
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.
* Parameter “<strong>tier</strong>” <em class="rfc2119">MAY<!---0.134451%--></em> be included alongside “<strong>profile</strong>” and “<strong>level-idx</strong> parameters in “a=fmtp” line if the indicated level supports a non-zero tier.</p>
</section>
</section>
<section>
<h4 id="rid-restrictions-mapping-for-av1">7.2.2 RID restrictions mapping for AV1<a class="self-link" aria-label="§" href="#rid-restrictions-mapping-for-av1"></a></h4>
<p>The RID specification declares the set of codec-agnostic restrictions
for media streams. All the restrictions are optional and are subject to
negotiation based on the SDP Offer/Answer rules described in Section 6
in <a href="https://tools.ietf.org/html/draft-ietf-mmusic-rid">I-D.ietf-mmusic-rid</a>. When these restrictions are applied to the AV1 codec, they <em class="rfc2119">MUST<!---0.134451%--></em> have the following interpretation:
* <strong>max-width</strong>, maximum width of the frame in units of samples. The meaning of the restriction is the same as variable <strong>MaxHSize</strong> of the levels table from Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.
* <strong>max-height</strong>, maximum height of the frame in units of samples. The meaning of the restriction is the same as variable <strong>MaxVSize</strong> of the levels table from Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.
* <strong>max-fps</strong>, maximum number of temporal units per second.
* <strong>max-fs</strong>, maximum size of the frame in units of samples. The meaning of the restriction is the same as variable <strong>MaxPicSize</strong> of the levels table from Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.
* <strong>max-br</strong>, maximum bit rate in units bits per second. The meaning of the restriction is the same as variable <strong>MaxBitrate</strong> defined in Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.
* <strong>max-pps</strong>, maximum decode rate in units of samples per second. The meaning of the restriction is the same as variable <strong>MaxDecodeRate</strong> of the levels table from Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a>.
* <strong>max-bpp</strong>, maximum number of bits per pixel of any given coded frame, calculated as a ratio between <strong>CompressedSize</strong> variable defined Section A.3 of <a href="https://aomediacodec.github.io/av1-spec/av1-spec.pdf">AV1</a> and expressed in bits, and number of samples in frame.</p>
<p>If during the SDP negotiation process both parties acknowledge restrictions, then the transported media stream <em class="rfc2119">MUST<!---0.134451%--></em> have at least one operating point with the negotiated restrictions.</p>
<section>
<h5 id="usage-with-the-sdp-offeranswer-model">7.2.3 Usage with the SDP Offer/Answer Model<a class="self-link" aria-label="§" href="#usage-with-the-sdp-offeranswer-model"></a></h5>
<p>When AV1 is offered over RTP using SDP in an Offer/Answer model as described in <a href="https://tools.ietf.org/html/rfc3264">RFC3264</a> for negotiation for unicast usage, the following limitations and rules apply:
* The media format configuration is identified by <strong>level-idx</strong>, <strong>profile</strong> and <strong>tier</strong>. The answerer <em class="rfc2119">SHOULD<!---0.134451%--></em> maintain all parameters. These media configuration parameters are asymmetrical and the answerer <em class="rfc2119">MAY<!---0.134451%--></em> declare its own media configuration if the answerer capabilities are different from the offerer.
* The profile to use in the offerer-to-answerer direction <em class="rfc2119">MUST<!---0.134451%--></em>
be lesser or equal to the profile the answerer supports for receiving,
and the profile to use in the answerer-to-offerer direction <em class="rfc2119">MUST<!---0.134451%--></em> be lesser or equal to the profile the offerer supports for receiving.
* The level to use in the offerer-to-answerer direction <em class="rfc2119">MUST<!---0.134451%--></em> be lesser or equal to the level the answerer supports for receiving, and the level to use in the answerer-to-offerer direction <em class="rfc2119">MUST<!---0.134451%--></em> be lesser or equal to the level the offerer supports for receiving.
* The tier to use in the offerer-to-answerer direction <em class="rfc2119">MUST<!---0.134451%--></em> be lesser or equal to the tier the answerer supports for receiving, and the tier to use in the answerer-to-offerer direction <em class="rfc2119">MUST<!---0.134451%--></em> be lesser or equal to the tier the offerer supports for receiving.</p>
</section>
<section>
<h5 id="usage-in-declarative-session-descriptions">7.2.4 Usage in Declarative Session Descriptions<a class="self-link" aria-label="§" href="#usage-in-declarative-session-descriptions"></a></h5>
<p>When AV1 over RTP is offered with SDP in a declarative style, as in Real Time Streaming Protocol (RTSP) <a href="https://tools.ietf.org/html/rfc2326">RFC2326</a> or Session Announcement Protocol (SAP) <a href="https://tools.ietf.org/html/rfc2974">RFC2974</a>,
the following considerations apply.
* All parameters capable of indicating both stream properties and
receiver capabilities are used to indicate only stream properties. In
this case, the parameters <strong>profile</strong>, <strong>level-idx</strong> and <strong>tier</strong> declare only the values used by the stream, not the capabilities for receiving streams.
* A receiver of the SDP is required to support all parameters and values of the parameters provided; otherwise, the receiver <em class="rfc2119">MUST<!---0.134451%--></em>
reject (RTSP) or not participate in (SAP) the session. It falls on the
creator of the session to use values that are expected to be supported
by the receiving application.</p>
</section>
</section>
<section>
<h4 id="examples-1">7.3 Examples<a class="self-link" aria-label="§" href="#examples-1"></a></h4>
<p>An example of media representation in SDP is as follows:</p>
<ul>
<li>m=video 49170 RTP/AVPF 98</li>
<li>a=rtpmap:98 AV1/90000</li>
<li>a=fmtp:98 profile=2; level-idx=8; tier=1;</li>
</ul>
<section>
<h5 id="level-upgrading">7.3.1 Level upgrading<a class="self-link" aria-label="§" href="#level-upgrading"></a></h5>
<p>In the following example the offer is accepted with level upgrading.
The level to use in the offerer-to-answerer direction is Level 2.0, and
the level to use in the answerer-to-offerer direction is Level 3.0/Tier
1. The answerer is allowed to send at any level up to and including
Level 2.0, and the offerer is allowed to send at any level up to and
including Level 3.0/Tier 1:</p>
<p>Offer SDP:
* m=video 49170 RTP/AVPF 98
* a=rtpmap:98 AV1/90000
* a=fmtp:98 profile=0; level-idx=0;</p>
<p>Answer SDP:
* m=video 49170 RTP/AVPF 98
* a=rtpmap:98 AV1/90000
* a=fmtp:98 profile=0; level-idx=4; tier=1;</p>
</section>
<section>
<h5 id="simulcast-with-payload-multiplexing">7.3.2 Simulcast with payload multiplexing<a class="self-link" aria-label="§" href="#simulcast-with-payload-multiplexing"></a></h5>
<p>In the following example an offer is made by a conferencing server to
receive 3 simulcast streams with payload multiplexing. The answerer
agrees to send 3 simulcast streams at different resolutions.</p>
<p>Offer SDP:
* m=video 49170 UDP/TLS/RTP/SAVPF 98
* a=mid:0
* a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
* a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
* a=extmap:3 urn:3gpp:video-orientation
* a=extmap:4 https://aomediacodec.github.io/av1-rtp-spec/#dependency-descriptor-rtp-header-extension
* a=sendrecv
* a=rtcp-mux
* a=rtcp-rsize
* a=rtpmap:98 AV1/90000
* a=rtpmap:99 AV1/90000
* a=rtpmap:100 AV1/90000
* a=fmtp:98 profile=2; level-idx=8; tier=1;
* a=fmtp:99 profile=2; level-idx=8; tier=1;
* a=fmtp:100 profile=2; level-idx=8; tier=1;
* a=rtcp-fb:98 ccm fir
* a=rtcp-fb:98 nack
* a=rtcp-fb:98 nack pli
* a=rtcp-fb:99 ccm fir
* a=rtcp-fb:99 nack
* a=rtcp-fb:99 nack pli
* a=rtcp-fb:100 ccm fir
* a=rtcp-fb:100 nack
* a=rtcp-fb:100 nack pli
* a=rid:q recv pt=98;max-width=640;max-height=480
* a=rid:h recv pt=99;max-width=1280;max-height=720
* a=rid:f recv pt=100;max-width=1920;max-height=1080
* a=simulcast:recv q;h;f</p>
<p>Answer SDP:
* m=video 48120 UDP/TLS/RTP/SAVPF 98
* a=mid:0
* a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
* a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
* a=extmap:3 urn:3gpp:video-orientation
* a=extmap:4 https://aomediacodec.github.io/av1-rtp-spec/#dependency-descriptor-rtp-header-extension
* a=sendrecv
* a=rtcp-mux
* a=rtcp-rsize
* a=rtpmap:98 AV1/90000