Audio/Video Transport WG S. Wenger
Independent
Internet Draft Y.-K. Wang
Intended status: Standards track Huawei Technologies
Expires: July 2011 T. Schierl
Fraunhofer HHI
A. Eleftheriadis
Vidyo
January 31, 2011
RTP Payload Format for Scalable Video Coding
draft-ietf-avt-rtp-svc-26.txt
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Abstract
This memo describes an RTP payload format for Scalable Video Coding
(SVC) as defined in Annex G of ITU-T Recommendation H.264, which is
technically identical to Amendment 3 of ISO/IEC International
Standard 14496-10. The RTP payload format allows for packetization
of one or more Network Abstraction Layer (NAL) units in each RTP
packet payload, as well as fragmentation of a NAL unit in multiple
RTP packets. Furthermore, it supports transmission of an SVC stream
over a single as well as multiple RTP sessions. The payload format
defines a new media subtype name "H264-SVC", but is still backwards
compatible to [I-D.ietf-avt-rtp-rfc3984bis] since the base layer,
when encapsulated in its own RTP stream, must use the H.264 media
subtype name ("H264") and the packetization method specified in [I-
D.ietf-avt-rtp-rfc3984bis]. The payload format has wide
applicability in videoconferencing, Internet video streaming, and
high bit-rate entertainment-quality video, among others.
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Table of Contents
Status of this Memo...............................................1
Abstract..........................................................2
Table of Contents.................................................3
1 . Introduction..................................................5
1.1 . The SVC Codec............................................6
1.1.1 . Overview............................................6
1.1.2 . Parameter Sets......................................8
1.1.3 . NAL Unit Header.....................................9
1.2 . Overview of the Payload Format..........................12
1.2.1 Design Principles....................................12
1.2.2 Transmission Modes and Packetization Modes...........13
1.2.3 New Payload Structures...............................15
2 . Conventions..................................................16
3 . Definitions and Abbreviations................................16
3.1 Definitions...............................................16
3.1.1 Definitions from the SVC Specification...............17
3.1.2 Definitions Specific to This Memo....................19
3.2 Abbreviations.............................................23
4 . RTP Payload Format...........................................23
4.1 RTP Header Usage..........................................23
4.2 NAL Unit Extension and Header Usage.......................24
4.2.1 NAL Unit Extension...................................24
4.2.2 NAL Unit Header Usage................................24
4.3 Payload Structures........................................26
4.4 Transmission Modes........................................28
4.5 Packetization Modes.......................................29
4.5.1 Packetization Modes for Single-Session Transmission..29
4.5.2 Packetization Modes for Multi-Session Transmission...30
4.6 Single NAL Unit Packets...................................33
4.7 Aggregation Packets.......................................33
4.7.1 Non-Interleaved Multi-Time Aggregation Packets (NI-
MTAPs).....................................................34
4.8 Fragmentation Units (FUs).................................36
4.9 Payload Content Scalability Information (PACSI) NAL Unit..36
4.10 Empty NAL unit...........................................44
4.11 Decoding Order Number (DON)..............................45
4.11.1 Cross-Session DON (CS-DON) for Multi-Session
Transmission...............................................45
5 . Packetization Rules..........................................47
5.1 Packetization Rules for Single-Session Transmission.......47
5.2 Packetization Rules for Multi-Session Transmission........48
5.2.1 NI-T/NI-TC Packetization Rules.......................48
5.2.2 NI-C/NI-TC Packetization Rules.......................51
5.2.3 I-C Packetization Rules..............................52
5.2.4 Packetization Rules for Non-VCL NAL Units............52
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5.2.5 Packetization Rules for Prefix NAL Units.............53
6 . De-Packetization Process.....................................53
6.1 De-Packetization Process for Single-Session Transmission..53
6.2 De-Packetization Process for Multi-Session Transmission...53
6.2.1 Decoding Order Recovery for the NI-T and NI-TC Modes.54
6.2.1.1 Informative Algorithm for NI-T Decoding Order
Recovery within an Access Unit..........................57
6.2.2 Decoding Order Recovery for the NI-C, NI-TC and I-C
Modes......................................................60
7 . Payload Format Parameters....................................62
7.1 Media Type Registration...................................62
7.2 SDP Parameters............................................78
7.2.1 Mapping of Payload Type Parameters to SDP............78
7.2.2 Usage with the SDP Offer/Answer Model................79
7.2.3 Dependency Signaling in Multi-Session Transmission...88
7.2.4 Usage in Declarative Session Descriptions............89
7.3 Examples..................................................90
7.3.1 Example for Offering a Single SVC Session............90
7.3.2 Example for Offering a Single SVC Session using
scalable-layer-id..........................................91
7.3.3 Example for Offering Multiple Sessions in MST........91
7.3.4 Example for Offering Multiple Sessions in MST including
operation with Answerer using scalable-layer-id............93
7.3.5 Example for Negotiating an SVC Stream with a Constrained
Base Layer in SST..........................................94
7.4 Parameter Set Considerations..............................95
8 . Security Considerations......................................95
9 . Congestion Control...........................................95
10 . IANA Consideration..........................................97
11 . Informative Appendix: Application Examples..................97
11.1 Introduction.............................................97
11.2 Layered Multicast........................................97
11.3 Streaming................................................98
11.4 Videoconferencing (Unicast to MANE, Unicast to Endpoints)99
11.5 Mobile TV (Multicast to MANE, Unicast to Endpoint)......100
12 . Acknowledgements...........................................101
13 . References.................................................102
13.1 Normative References....................................102
13.2 Informative References..................................103
14 . Authors' Addresses.........................................104
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1. Introduction
This memo specifies an RTP [RFC3550] payload format for the Scalable
Video Coding (SVC) extension of the H.264/AVC video coding standard.
SVC is specified in Amendment 3 to ISO/IEC 14496 Part 10
[ISO/IEC 14496-10], and equivalently in Annex G of ITU-T Rec. H.264
[H.264]. In this memo, unless explicitly stated otherwise,
"H.264/AVC" refers to the specification of [H.264] excluding Annex G.
SVC covers the entire application range of H.264/AVC, from low
bitrate mobile applications, to High-Definition Television (HDTV)
broadcasting, and even Digital Cinema that requires nearly lossless
coding and hundreds of mega bits per second. The scalability
features that SVC adds to H.264/AVC enable several system-level
functionalities related to the ability of a system to adapt the
signal to different system conditions with no or minimal processing.
The adaptation relates both to the capabilities of potentially
heterogeneous receivers (differing in screen resolution, processing
speed, etc.), as well as differing or time-varying network
conditions. The adaptation can be performed at the source, the
destination, or in intermediate media-aware network elements (MANEs).
The payload format specified in this memo exposes these system-level
functionalities so that system designers can take direct advantage
of these features.
Informative note: Since SVC streams contain, by design, a sub-
stream that is compliant with H.264/AVC, it is trivial for a
MANE to filter the stream so that all SVC-specific information
is removed. This memo, in fact, defines a media type parameter
("sprop-avc-ready", Section 7.2) that indicates whether or not
the stream can be converted to one compliant to [I-D.ietf-avt-
rtp-rfc3984bis] by eliminating RTP packets, and rewriting RTCP
to match the changes to the RTP packet stream as specified in
Section 7 of [RFC3550].
This memo defines two basic modes for transmission of SVC data,
single session transmission (SST) and multi-session transmission
(MST). In SST, a single RTP session is used for the transmission of
all scalability layers comprising an SVC bitstream, whereas in MST
the scalability layers are transported on different RTP sessions.
In SST, packetization is a straightforward extension of [I-D.ietf-
avt-rtp-rfc3984bis]. For MST four different modes are defined in
this memo. They differ on whether or not they allow interleaving,
i.e., transmitting Network Abstraction Layer (NAL) units in an order
different than the decoding order, and by the technique used to
effect inter-session NAL unit decoding order recovery. Decoding
order recovery is performed using either inter-session timestamp
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alignment [RFC3550] or Cross-Session Decoding Order Numbers (CS-DON).
One of the MST modes supports both decoding order recovery
techniques, so that receivers can select their preferred technique.
More details can be found in Section 1.2.2.
This memo further defines three new NAL unit types. The first type
is the Payload Content Scalability Information (PACSI) NAL unit,
which is used to provide an informative summary of the scalability
information of the data contained in an RTP packet, as well as
ancillary data (e.g., CS-DON values). The second and third new NAL
unit types are the Empty NAL unit and the Non-Interleaved Multi-time
Aggregation Packet (NI-MTAP) NAL unit. The Empty NAL unit is used to
ensure inter-session timestamp alignment required for decoding order
recovery in MST. The NI-MTAP is used as a new payload structure
allowing the grouping of NAL units of different time instances in
decoding order. More details about the new packet structures can be
found in Section 1.2.3.
This memo also defines the signaling support for SVC transport over
RTP, including a new media subtype name (H264-SVC).
A non-normative overview of the SVC codec and the payload is given
in the remainder of this section.
1.1. The SVC Codec
1.1.1. Overview
SVC defines a coded video representation in which a given bitstream
offers representations of the source material at different levels of
fidelity (hence the term "scalable"). Scalable video coding
bitstreams, or scalable bitstreams, are constructed in a pyramidal
fashion: the coding process creates bitstream components that
improve the fidelity of hierarchically lower components.
The fidelity dimensions offered by SVC are spatial (picture size),
quality (or Signal-to-Noise Ratio - SNR), as well as temporal
(pictures per second). Bitstream components associated with a given
level of spatial, quality, and temporal fidelity are identified
using corresponding parameters in the bitstream: dependency_id,
quality_id, and temporal_id (see also Section 1.1.3). The fidelity
identifiers have integer values, where higher values designate
components that are higher in the hierarchy. It is noted that SVC
offers significant flexibility in terms of how an encoder may choose
to structure the dependencies between the various components.
Decoding of a particular component requires the availability of all
the components it depends upon, either directly, or indirectly. An
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operation point of an SVC bitstream consists of the bitstream
components required to be able to decode a particular dependency_id,
quality_id, and temporal_id combination.
The term "layer" is used in various contexts in this memo. For
example, in the terms "Video Coding Layer" and "Network Abstraction
Layer" it refers to conceptual organization levels. When referring
to bitstream syntax elements such as block layer or macroblock layer,
it refers to hierarchical bitstream structure levels. When used in
the context of bitstream scalability, e.g., "AVC base layer", it
refers to a level of representation fidelity of the source signal
with a specific set of NAL units included. The correct
interpretation is supported by providing the appropriate context.
SVC maintains the bitstream organization introduced in H.264/AVC.
Specifically, all bitstream components are encapsulated in Network
Abstraction Layer (NAL) units which are organized as Access Units
(AU). An AU is associated with a single sampling instance in time.
A subset of the NAL unit types correspond to the Video Coding Layer
(VCL), and contain the coded picture data associated with the source
content. Non-VCL NAL units carry ancillary data that may be
necessary for decoding (e.g., parameter sets as explained below), or
that facilitate certain system operations but are not needed by the
decoding process itself. Coded picture data at the various fidelity
dimensions are organized in slices. Within one AU, a coded picture
of an operation point consists of all the coded slices required for
decoding up to the particular combination of dependency_id and
quality_id values at the time instance corresponding to the AU.
It is noted that the concept of temporal scalability is already
present in H.264/AVC, as profiles defined in Annex A of [H.264]
already support it. Specifically, in H.264/AVC the concept of sub-
sequences has been introduced to allow optional use of temporal
layers through Supplemental Enhancement Information (SEI) messages.
SVC extends this approach by exposing the temporal scalability
information using the temporal_id parameter, alongside (and unified
with) the dependency_id and quality_id values that are used for
spatial and quality scalability, respectively. For coded picture
data defined in Annex G of [H.264] this is accomplished by using a
new type of NAL unit, namely coded slice in scalable extension NAL
unit (type 20), where the fidelity parameters are part of its header.
For coded picture data that follow H.264/AVC, and to ensure
compatibility with existing H.264/AVC decoders, another new type of
NAL unit, namely prefix NAL unit (type 14), has been defined to
carry this header information. SVC additionally specifies a third
new type of NAL unit, namely subset sequence parameter set NAL unit
(type 15), to contain sequence parameter set information for quality
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and spatial enhancement layers. All these three newly specified NAL
unit types (14, 15 and 20) are among those reserved in H.264/AVC,
and are to be ignored by decoders conforming to one or more of the
profiles specified in Annex A of [H.264].
Within an AU, the VCL NAL units associated with a given
dependency_id and quality_id are referred to as a "layer
representation". The layer representation corresponding to the
lowest values of dependency_id and quality_id (i.e., zero for both)
is compliant by design to H.264/AVC. The set of VCL and associated
non-VCL NAL units across all AUs in a bitstream associated with a
particular combination of values of dependency_id and quality_id,
and regardless of the value of temporal_id, is conceptually a
scalable layer. For backwards compatibility with H.264/AVC, it is
important to differentiate, however, whether or not SVC-specific NAL
units are present in a given bitstream or not. This is particularly
important for the lowest fidelity values in terms of dependency_id
and quality_id (zero for both), as the corresponding VCL data are
compliant to H.264/AVC, and may or may not be accompanied by
associated prefix NAL units. This memo therefore uses the term "AVC
base layer" to designate the layer that does not contain SVC-
specific NAL units, and "SVC base layer" to designate the same layer
but with the addition of the associated SVC prefix NAL units. Note
that the SVC specification uses the term "base layer" for what in
this memo will be referred to as "AVC base layer". Similarly, it is
also important to be able to differentiate, within a layer, the
temporal fidelity components it contains. This memo uses the term
"T0" to indicate, within a particular layer, the subset that
contains the NAL units associated with temporal_id equal to 0.
SNR scalability in SVC is offered in two different ways. In what is
called Coarse-Grained Scalability (CGS), scalability is provided by
including or excluding a complete layer when decoding a particular
bitstream. In contrast, in Medium-Grained Scalability (MGS),
scalability is provided by selectively omitting the decoding of
specific NAL units belonging to MGS layers. The selection of the
NAL units to omit can be based on fixed length fields present in the
NAL unit header (see also Sections 1.1.3 and 4.2).
1.1.2. Parameter Sets
SVC maintains the parameter sets concept in H.264/AVC and introduces
a new type of sequence parameter set, referred to as subset sequence
parameter set [H.264]. Subset sequence parameter sets have NAL unit
type equal to 15, which is different from the NAL unit type value (7)
of sequence parameter sets. VCL NAL units of NAL unit type 1 to 5
must only (indirectly) refer to sequence parameter sets, while VCL
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NAL units of NAL unit type 20 must only (indirectly) refer to subset
sequence parameter sets. The references are indirect because VCL
NAL units refer to picture parameter sets (in their slice header),
which in turn refer to regular or subset sequence parameter sets.
Subset sequence parameter sets use a separate identifier value space
than sequence parameter sets.
In SVC, coded picture data from different layers may use the same or
different sequence and picture parameter sets. Let the variable
DQId be equal to dependency_id * 16 + quality_id. At any time
instant during the decoding process there is one active sequence
parameter set for the layer representation with the highest value of
DQId and one or more active layer SVC sequence parameter set(s) for
layer representations with lower values of DQId. The active
sequence parameter set or an active layer SVC sequence parameter set
remains unchanged throughout a coded video sequence in the scalable
layer in which the active sequence parameter set or active layer SVC
sequence parameter set is referred to. This means that the referred
sequence parameter set or subset sequence parameter set can only
change at IDR access units for any layer. At any time instant
during the decoding process there may be one active picture
parameter set (for the layer representation with the highest value
of DQId) and one or more active layer picture parameter set(s) (for
layer representations with lower values of DQId). The active
picture parameter set or an active layer picture parameter set
remains unchanged throughout a layer representation in which the
active picture parameter set or active layer picture parameter set
is referred to, but may change from one AU to the next.
1.1.3. NAL Unit Header
SVC extends the one-byte H.264/AVC NAL unit header by three
additional octets for NAL units of type 14 and 20. The header
indicates the type of the NAL unit, the (potential) presence of bit
errors or syntax violations in the NAL unit payload, information
regarding the relative importance of the NAL unit for the decoding
process, the layer identification information, and other fields as
discussed below.
The syntax and semantics of the NAL unit header are specified in
[H.264], but the essential properties of the NAL unit header are
summarized below for convenience.
The first byte of the NAL unit header has the following format (the
bit fields are the same as defined for the one-byte H.264/AVC NAL
unit header, while the semantics of some fields have changed
slightly, in a backwards compatible way):
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+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The semantics of the components of the NAL unit type octet, as
specified in [H.264], are described briefly below. In addition to
the name and size of each field, the corresponding syntax element
name in [H.264] is also provided.
F: 1 bit
forbidden_zero_bit. H.264/AVC declares a value of 1 as a syntax
violation.
NRI: 2 bits
nal_ref_idc. A value of "00" (in binary form) indicates that the
content of the NAL unit is not used to reconstruct reference
pictures for future prediction. Such NAL units can be discarded
without risking the integrity of the reference pictures in the
same layer. A value greater than "00" indicates that the
decoding of the NAL unit is required to maintain the integrity of
reference pictures in the same layer, or that the NAL unit
contains parameter sets.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit type as
defined in Table 7-1 of [H.264], and later within this memo. For
a reference of all currently defined NAL unit types and their
semantics, please refer to Section 7.4.1 in [H.264].
In H.264/AVC, NAL unit types 14, 15 and 20 are reserved for
future extensions. SVC uses these three NAL unit types as
follows: NAL unit type 14 is used for prefix NAL unit, NAL unit
type 15 is used for subset sequence parameter set, and NAL unit
type 20 is used for coded slice in scalable extension (see
Section 7.4.1 in [H.264]). NAL unit types 14 and 20 indicate the
presence of three additional octets in the NAL unit header, as
shown below.
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+---------------+---------------+---------------+
|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|I| PRID |N| DID | QID | TID |U|D|O| RR|
+---------------+---------------+---------------+
R: 1 bit
reserved_one_bit. Reserved bit for future extension. R must be
equal to 1. The value of R must be ignored by decoders.
I: 1 bit
idr_flag. This component specifies whether the layer
representation is an instantaneous decoding refresh (IDR) layer
representation (when equal to 1) or not (when equal to 0).
PRID: 6 bits
priority_id. This flag specifies a priority identifier for the
NAL unit. A lower value of PRID indicates a higher priority.
N: 1 bit
no_inter_layer_pred_flag. This flag specifies, when present in a
coded slice NAL unit, whether inter-layer prediction may be used
for decoding the coded slice (when equal to 1) or not (when equal
to 0).
DID: 3 bits
dependency_id. This component indicates the inter-layer coding
dependency level of a layer representation. At any access unit,
a layer representation with a given dependency_id may be used for
inter-layer prediction for coding of a layer representation with
a higher dependency_id, while a layer representation with a given
dependency_id shall not be used for inter-layer prediction for
coding of a layer representation with a lower dependency_id.
QID: 4 bits
quality_id. This component indicates the quality level of an MGS
layer representation. At any access unit and for identical
dependency_id values, a layer representation with quality_id
equal to ql uses a layer representation with quality_id equal to
ql-1 for inter-layer prediction.
TID: 3 bits
temporal_id. This component indicates the temporal level of a
layer representation. The temporal_id is associated with the
frame rate, with lower values of _temporal_id corresponding to
lower frame rates. A layer representation at a given temporal_id
typically depends on layer representations with lower temporal_id
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values, but it never depends on layer representations with higher
temporal_id values.
U: 1 bit
use_ref_base_pic_flag. A value of 1 indicates that only
reference base pictures are used during the inter prediction
process. A value of 0 indicates that the reference base pictures
are not used during the inter prediction process.
D: 1 bit
discardable_flag. A value of 1 indicates that the current NAL
unit is not used for decoding NAL units with values of
dependency_id higher than the one of the current NAL unit, in the
current and all subsequent access units. Such NAL units can be
discarded without risking the integrity of layers with higher
dependency_id values. discardable_flag equal to 0 indicates that
the decoding of the NAL unit is required to maintain the
integrity of layers with higher dependency_id.
O: 1 bit
output_flag: Affects the decoded picture output process as
defined in Annex C of [H.264].
RR: 2 bits
reserved_three_2bits. Reserved bits for future extension. RR
MUST be equal to "11" (in binary form). The value of RR must be
ignored by decoders.
This memo extends the semantics of F, NRI, I, PRID, DID, QID, TID, U,
and D per Annex G of [H.264] as described in Section 4.2.
1.2. Overview of the Payload Format
Similar to [I-D.ietf-avt-rtp-rfc3984bis], this payload format can
only be used to carry the raw NAL unit stream over RTP and not the
byte stream format specified in Annex B of [H.264].
The design principles, transmission modes, packetization modes as
well as new payload structures are summarized in this section. It
is assumed that the reader is familiar with the terminology and
concepts defined in [I-D.ietf-avt-rtp-rfc3984bis].
1.2.1 Design Principles
The following design principles have been observed for this payload
format:
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o Backward compatibility with [I-D.ietf-avt-rtp-rfc3984bis]
wherever possible.
o The SVC base layer or any H.264/AVC compatible subset of the SVC
base layer, when transmitted in its own RTP stream, must be
encapsulated using [I-D.ietf-avt-rtp-rfc3984bis]. This ensures
that such an RTP stream can be understood by [I-D.ietf-avt-rtp-
rfc3984bis] receivers.
o Media-Aware Network Elements (MANEs) as defined in [I-D.ietf-avt-
rtp-rfc3984bis] are signaling-aware, rely on signaling
information, and have state.
o MANEs can aggregate multiple RTP streams, possibly from multiple
RTP sessions.
o MANEs can perform media-aware stream thinning (selective
elimination of packets or portions thereof). By using the
payload header information identifying layers within an RTP
session, MANEs are able to remove packets or portions thereof
from the incoming RTP packet stream. This implies rewriting the
RTP headers of the outgoing packet stream, and rewriting of RTCP
packets as specified in Section 7 of [RFC3550].
1.2.2 Transmission Modes and Packetization Modes
This memo allows the packetization of SVC data for both single-
session transmission (SST) and multi-session transmission (MST). In
the case of SST all SVC data are carried in a single RTP session.
In the case of MST two or more RTP sessions are used to carry the
SVC data, in accordance with the MST-specific packetization modes
defined in this memo, which are based on the packetization modes
defined in [I-D.ietf-avt-rtp-rfc3984bis]. In MST, each RTP session
is associated with one RTP stream, which may carry one or more
layers.
The base layer is, by design, compatible to H.264/AVC. During
transmission, the associated prefix NAL units, which are introduced
by SVC and, when present, are ignored by H.264/AVC decoders, may be
encapsulated within the same RTP packet stream as the H.264/AVC VCL
NAL units, or in a different RTP packet stream (when MST is used).
For convenience, the term "AVC base layer" is used to refer to the
base layer without prefix NAL units, while the term "SVC base layer"
is used to refer to the base layer with prefix NAL units.
Furthermore, the base layer may have multiple temporal components
(i.e., supporting different frame rates). As a result, the lowest
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temporal component ("T0") of the AVC or SVC base layer is used as
the starting point of the SVC bitstream hierarchy.
This memo allows encapsulating in a given RTP stream any of the
following three alternatives of layer combinations:
1. the T0 AVC base layer or the T0 SVC base layer only;
2. one or more enhancement layers only;
3. the T0 SVC base layer, and one or more enhancement layers.
SST should be used in point-to-point unicast applications and, in
general, whenever the potential benefit of using multiple RTP
sessions does not justify the added complexity. When SST is used the
layer combination cases 1 and 3 above can be used. When an
H.264/AVC compatible subset of the SVC base layer is transmitted
using SST, the packetization of [I-D.ietf-avt-rtp-rfc3984bis] must
be used, thus ensuring compatibility with [I-D.ietf-avt-rtp-
rfc3984bis] receivers. When, however, one or more SVC quality or
spatial enhancement layers are transmitted using SST, the
packetization defined in this memo must be used. In SST, any of the
three [I-D.ietf-avt-rtp-rfc3984bis] packetization modes, namely
Single NAL Unit Mode, Non-Interleaved Mode, and Interleaved Mode,
can be used.
MST should be used in a multicast session when different receivers
may request different layers of the scalable bitstream. An
operation point for an SVC bit stream, as defined in this memo,
corresponds to a set of layers that together conform to one of the
profiles defined in Annex A or G of [H.264] and, when decoded, offer
a representation of the original video at a certain fidelity. The
number of streams used in MST should be at least equal to the number
of operation points that may be requested by the receivers.
Depending on the application, this may result in each layer being
carried in its own RTP session, or in having multiple layers
encapsulated within one RTP session.
Informative note: Layered multicast is a term commonly used to
describe the application where multicast is used to transmit
layered or scalable data that has been encapsulated into more
than one RTP session. This application allows different
receivers in the multicast session to receive different
operation points of the scalable bitstream. Layered multicast,
among other application examples, is discussed in more detail
in Section 11.2.
When MST is used, any of the three layer combinations above can be
used for each of the sessions. When an H.264/AVC compatible subset
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of the SVC base layer is transmitted in its own session in MST, the
packetization of [I-D.ietf-avt-rtp-rfc3984bis] must be used, such
that [I-D.ietf-avt-rtp-rfc3984bis] receivers can be part of the MST
and receive only this session. For MST, this memo defines four
different MST specific packetization modes, namely Non-Interleaved
Timestamp based Mode (NI-T), Non-Interleaved Cross-Layer Decoding
Order Number (CS-DON) based Mode (NI-C), Non-Interleaved Combined
Timestamp and CS-DON Mode (NI-TC), and Interleaved CS-DON based Mode
(I-C) (detailed in Section 4.5.2). The modes differ depending on
whether the SVC data are allowed to be interleaved, i.e., to be
transmitted in an order different than the intended decoding order,
and they also differ in the mechanisms provided in order to recover
the correct decoding order of the NAL units across the multiple RTP
sessions. These four MST modes re-use the packetization modes
introduced in [I-D.ietf-avt-rtp-rfc3984bis] for the packetization of
NAL units in each of their individual RTP sessions.
As the names of the MST packetization modes imply, the NI-T, NI-C
and NI-TC modes do not allow interleaved transmission, while the I-C
mode allows interleaved transmission. With any of the three non-
interleaved MST packetization modes, legacy [I-D.ietf-avt-rtp-
rfc3984bis] receivers with implementation of the Non-Interleaved
Mode specified in [I-D.ietf-avt-rtp-rfc3984bis] can join a multi-
session transmission of SVC, to receive the base RTP session
encapsulated according to [I-D.ietf-avt-rtp-rfc3984bis].
1.2.3 New Payload Structures
[I-D.ietf-avt-rtp-rfc3984bis] specifies three basic payload
structures, namely Single NAL Unit Packet, Aggregation Packet, and
Fragmentation Unit. Depending on the basic payload structure, an
RTP packet may contain a NAL unit not aggregating other NAL units,
one or more NAL units aggregated in another NAL unit, or a fragment
of a NAL unit not aggregating other NAL units. Each NAL unit of a
type specified in [H.264] (i.e., 1 to 23, inclusive) may be carried
in its entirety in a single NAL unit packet, may be aggregated in an
aggregation packet, or may be fragmented and carried in a number of
fragmentation unit packets. To enable aggregation or fragmentation
of NAL units while still ensuring that the RTP packet payload is
only comprised of NAL units, [I-D.ietf-avt-rtp-rfc3984bis]
introduced six new NAL unit types (24-29) to be used as payload
structures, selected from the NAL unit types left unspecified in
[H.264].
This memo reuses all the payload structures used in [I-D.ietf-avt-
rtp-rfc3984bis]. Furthermore, three new types of NAL units are
defined: namely Payload Content Scalability Information (PACSI) NAL
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unit, Empty NAL unit, and Non-Interleaved Multi-Time Aggregation
Packet (NI-MTAP) (specified in Sections 4.9, 4.10, and 4.7.1,
respectively).
PACSI NAL units may be used for the following purposes:
o To enable MANEs to decide whether to forward, process or discard
aggregation packets, by checking in PACSI NAL units the
scalability information and other characteristics of the
aggregated NAL units, rather than looking into the aggregated NAL
units themselves, which are defined by the video coding
specification.
o To enable correct decoding order recovery in MST using the NI-C
or NI-TC mode, with the help of the CS-DON information included in
PACSI NAL units.
o To improve resilience to packet losses, e.g. by utilizing the
following data or information included in PACSI NAL units:
repeated Supplemental Enhancement Information (SEI) messages,
information regarding the start and end of layer representations,
and the indices to layer representations of the lowest temporal
subset.
Empty NAL units may be used to enable correct decoding order
recovery in MST using the NI-T or NI-TC mode. NI-MTAP NAL units may
be used to aggregate NAL units from multiple access units but
without interleaving.
2. Conventions
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 BCP 14, RFC 2119
[RFC2119].
This specification uses the notion of setting and clearing a bit
when bit fields are handled. Setting a bit is the same as assigning
that bit the value of 1 (On). Clearing a bit is the same as
assigning that bit the value of 0 (Off).
3. Definitions and Abbreviations
3.1 Definitions
This document uses the terms and definitions of [H.264]. Section
3.1.1 lists relevant definitions copied from [H.264] for convenience.
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When there is discrepancy, the definitions in [H.264] take
precedence. Section 3.1.2 gives definitions specific to this memo.
Some of the definitions in Section 3.1.2 are also present in [I-
D.ietf-avt-rtp-rfc3984bis] and copied here with slight adaptations
as needed.
3.1.1 Definitions from the SVC Specification
access unit: A set of NAL units always containing exactly one
primary coded picture. In addition to the primary coded picture,
an access unit may also contain one or more redundant coded
pictures, one auxiliary coded picture, or other NAL units not
containing slices or slice data partitions of a coded picture.
The decoding of an access unit always results in a decoded
picture.
base layer: A bitstream subset that contains all the NAL units
with the nal_unit_type syntax element equal to 1 or 5 of the
bitstream and does not contain any NAL unit with the
nal_unit_type syntax element equal to 14, 15, or 20 and conforms
to one or more of the profiles specified in Annex A of [H.264].
base quality layer representation: The layer representation of
the target dependency representation of an access unit that is
associated with the quality_id syntax element equal to 0.
coded video sequence: A sequence of access units that consists,
in decoding order, of an IDR access unit followed by zero or more
non-IDR access units including all subsequent access units up to
but not including any subsequent IDR access unit.
dependency representation: A subset of Video Coding Layer (VCL)
NAL units within an access unit that are associated with the same
value of the dependency_id syntax element, which is provided as
part of the NAL unit header or by an associated prefix NAL unit.
A dependency representation consists of one or more layer
representations.
IDR access unit: An access unit in which the primary coded
picture is an IDR picture.
IDR picture: Instantaneous Decoding Refresh picture. A coded
picture in which all slices of the target dependency
representation within the access unit are I or EI slices that
causes the decoding process to mark all reference pictures as
"unused for reference" immediately after decoding the IDR picture.
After the decoding of an IDR picture all following coded pictures
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in decoding order can be decoded without inter prediction from
any picture decoded prior to the IDR picture. The first picture
of each coded video sequence is an IDR picture.
layer representation: A subset of VCL NAL units within an access
unit that are associated with the same values of the
dependency_id and quality_id syntax elements, which are provided
as part of the VCL NAL unit header or by an associated prefix NAL
unit. One or more layer representations represent a dependency
representation.
prefix NAL unit: A NAL unit with nal_unit_type equal to 14 that
immediately precedes in decoding order a NAL unit with
nal_unit_type equal to 1, 5, or 12. The NAL unit that
immediately succeeds in decoding order the prefix NAL unit is
referred to as the associated NAL unit. The prefix NAL unit
contains data associated with the associated NAL unit, which are
considered to be part of the associated NAL unit.
reference base picture: A reference picture that is obtained by
decoding a base quality layer representation with the nal_ref_idc
syntax element not equal to 0 and the store_ref_base_pic_flag
syntax element equal to 1 of an access unit and all layer
representations of the access unit that are referred to by inter-
layer prediction of the base quality layer representation. A
reference base picture is not an output of the decoding process,
but the samples of a reference base picture may be used for inter
prediction in the decoding process of subsequent pictures in
decoding order. Reference base picture is a collective term for
a reference base field or a reference base frame.
scalable bitstream: A bitstream with the property that one or
more bitstream subsets that are not identical to the scalable
bitstream form another bitstream that conforms to the SVC
specification[H.264].
target dependency representation: The dependency representation
of an access unit that is associated with the largest value of
the dependency_id syntax element for all dependency
representations of the access unit.
target layer representation: The layer representation of the
target dependency representation of an access unit that is
associated with the largest value of the quality_id syntax
element for all layer representations of the target dependency
representation of the access unit.
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3.1.2 Definitions Specific to This Memo
anchor layer representation: An anchor layer representation is
such a layer representation that, if decoding of the operation
point corresponding to the layer starts from the access unit
containing this layer representation, all the following layer
representations of the layer, in output order, can be correctly
decoded. The output order is defined in [H.264] as the order in
which decoded pictures are output from the decoded picture buffer
of the decoder. As H.264 does not specify the picture display
process, this more general term is used instead of display order.
An anchor layer representation is a random access point to the
layer the anchor layer representation belongs to. However, some
layer representations, succeeding an anchor layer representation
in decoding order but preceding the anchor layer representation
in output order, may refer to earlier layer representations for
inter prediction, and hence the decoding may be incorrect if
random access is performed at the anchor layer representation.
AVC base layer: The subset of the SVC base layer in which all
prefix NAL units (type 14) are removed. Note that this is
equivalent to the term "base layer" as defined in Annex G of
[H.264].
base RTP session: When multi-session transmission is used, the
RTP session that carries the RTP stream containing the T0 AVC
base layer or the T0 SVC base layer, and zero or more enhancement
layers. This RTP session does not depend on any other RTP
session as indicated by mechanisms defined in Section 7.2.3. The
base RTP session may carry NAL units of NAL unit type equal to 14
and 15.
decoding order number (DON): A field in the payload structure or
a derived variable indicating NAL unit decoding order. Values of
DON are in the range of 0 to 65535, inclusive. After reaching
the maximum value, the value of DON wraps around to 0. Note that
this definition also exists in [I-D.ietf-avt-rtp-rfc3984bis] in
exactly the same form.
Empty NAL unit: A NAL unit with NAL unit type equal to 31 and
sub-type equal to 1. An Empty NAL unit consists of only the two-
byte NAL unit header with an empty payload.
enhancement RTP session: When multi-session transmission is used,
an RTP session that is not the base RTP session. An enhancement
RTP session typically contains an RTP stream that depends on at
least one other RTP session as indicated by mechanisms defined in
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Section 7.2.3. A lower RTP session to an enhancement RTP session
is an RTP session which the enhancement RTP session depends on.
The lowest RTP session for a receiver is the RTP session that
does not depend on any other RTP session received by the receiver.
The highest RTP session for a receiver is the RTP session which
no other RTP session received by the receiver depends on.
cross-session decoding order number (CS-DON): A derived variable
indicating NAL unit decoding order number over all NAL units
within all the session-multiplexed RTP sessions that carry the
same SVC bitstream.
default level: The level indicated by the profile-level-id
parameter. In SDP Offer/Answer, the level is downgradable, i.e.,
the answer may either use the default level or a lower level.
Note that this definition also exists in [I-D.ietf-avt-rtp-
rfc3984bis] in a slightly different form.
default sub-profile: The subset of coding tools, which may be all
coding tools of one profile or the common subset of coding tools
of more than one profile, indicated by the profile-level-id
parameter. In SDP Offer/Answer, the default sub-profile must be
used in a symmetric manner, i.e. the answer must either use the
same sub-profile as the offer or reject the offer. Note that
this definition also exists in [I-D.ietf-avt-rtp-rfc3984bis] in a
slightly different form.
enhancement layer: A layer in which at least one of the values of
dependency_id or quality_id is higher than 0, or a layer in which
none of the NAL units is associated with the value of temporal_id
equal to 0. An operation point constructed using the maximum
temporal_id, dependency_id, and quality_id values associated with
an enhancement layer may or may not conform to one or more of the
profiles specified in Annex A of [H.264].
H.264/AVC compatible: The property of a bitstream subset of
conforming to one or more of the profiles specified in Annex A of
[H.264].
intra layer representation: A layer representation that contains
only slices that use intra prediction, and hence do not refer to
any earlier layer representation in decoding order in the same
layer. Note that in SVC intra prediction includes intra-layer
intra prediction as well as inter-layer intra prediction.
layer: A bitstream subset in which all NAL units of type 1, 5, 12,
14, or 20 have the same values of dependency_id and quality_id,
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either directly through their NAL unit header (for NAL units of
type 14 or 20) or through association to a prefix (type 14) NAL
unit (for NAL unit types 1, 5, or 12). A layer may contain NAL
units associated with more than one values of temporal_id.
media aware network element (MANE): A network element, such as a
middlebox or application layer gateway that is capable of parsing
certain aspects of the RTP payload headers or the RTP payload and
reacting to their contents. Note that this definition also
exists in [I-D.ietf-avt-rtp-rfc3984bis] in exactly the same form.
Informative note: The concept of a MANE goes beyond normal
routers or gateways in that a MANE has to be aware of the
signaling (e.g., to learn about the payload type mappings of
the media streams), and in that it has to be trusted when
working with SRTP. The advantage of using MANEs is that they
allow packets to be dropped according to the needs of the
media coding. For example, if a MANE has to drop packets due
to congestion on a certain link, it can identify and remove
those packets whose elimination produces the least adverse
effect on the user experience. After dropping packets, MANEs
must rewrite RTCP packets to match the changes to the RTP
packet stream as specified in Section 7 of [RFC3550].
multi-session transmission: The transmission mode in which the
SVC stream is transmitted over multiple RTP sessions. Dependency
between RTP sessions MUST be signaled according to Section 7.2.3
of this memo.
NAL unit decoding order: A NAL unit order that conforms to the
constraints on NAL unit order given in Section G.7.4.1.2 in
[H.264]. Note that this definition also exists in [I-D.ietf-avt-
rtp-rfc3984bis] in a slightly different form.
NALU-time: The value that the RTP timestamp would have if the NAL
unit would be transported in its own RTP packet. Note that this
definition also exists in [I-D.ietf-avt-rtp-rfc3984bis] in
exactly the same form.
operation point: An operation point is identified by a set of
values of temporal_id, dependency_id, and quality_id. A
bitstream corresponding to an operation point can be constructed
by removing all NAL units associated with a higher value of
dependency_id, and all NAL units associated with the same value
of dependency_id but higher values of quality_id or temporal_id.
An operation point bitstream conforms to at least one of the
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profiles defined in Annex A or Annex G of [H.264], and offers a
representation of the original video signal at a certain fidelity.
Informative Note: Additional NAL units may be removed (with
lower dependency_id or same dependency_id but lower
quality_id) if they are not required for decoding the
bitstream at the particular operation point. The resulting
bitstream, however, may no longer conform to any of the
profiles defined in Annex A or G of [H.264].
operation point representation: The set of all NAL units of an
operation point within the same access unit.
RTP packet stream: A sequence of RTP packets with increasing
sequence numbers (except for wrap-around), identical PT and
identical SSRC (Synchronization Source), carried in one RTP
session. Within the scope of this memo, one RTP packet stream is
utilized to transport one or more layers.
single-session transmission: The transmission mode in which the
SVC bitstream is transmitted over a single RTP session.
SVC base layer: The layer that includes all NAL units associated
with dependency_id and quality_id values both equal to 0,
including prefix NAL units (NAL unit type 14).
SVC enhancement layer: A layer in which at least one of the
values of dependency_id or quality_id is higher than 0. An
operation point constructed using the maximum dependency_id and
quality_id values and any temporal_id value associated with an
SVC enhancement layer does not conform to any of the profiles
specified in Annex A of [H.264].
SVC NAL unit: A NAL unit of NAL unit type 14, 15, or 20 as
specified in Annex G of [H.264].
SVC NAL unit header: A four-byte header resulting from the
addition of a three-byte SVC-specific header extension added in
NAL unit types 14 and 20.
SVC RTP session: Either the base RTP session or an enhancement
RTP session.
T0 AVC base layer: A subset of the AVC base layer constructed by
removing all VCL NAL units associated with temporal_id values
higher than 0 and non-VCL NAL units and SEI messages associated
only with the VCL NAL units being removed.
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T0 SVC base layer: A subset of the SVC base layer constructed by
removing all VCL NAL units associated with temporal_id values
higher than 0 as well as prefix NAL units, non-VCL NAL units, and
SEI messages associated only with the VCL NAL units being removed.
transmission order: The order of packets in ascending RTP
sequence number order (in modulo arithmetic). Within an
aggregation packet, the NAL unit transmission order is the same
as the order of appearance of NAL units in the packet. Note that
this definition also exists in [I-D.ietf-avt-rtp-rfc3984bis] in
exactly the same form.
3.2 Abbreviations
In addition to the abbreviations defined in [I-D.ietf-avt-rtp-
rfc3984bis], the following abbreviations are used in this memo.
CGS: Coarse-Grain Scalability
CS-DON: Cross-Session Decoding Order Number
MGS: Medium-Grain Scalability
MST: Multi-Session Transmission
PACSI: Payload Content Scalability Information
SST: Single Session Transmission
SNR: Signal-to-Noise Ratio
SVC: Scalable Video Coding
4. RTP Payload Format
4.1 RTP Header Usage
In addition to Section 5.1 of [I-D.ietf-avt-rtp-rfc3984bis] the
following rules apply.
o Setting of the M bit
The M bit of an RTP packet for which the packet payload is an NI-
MTAP MUST be equal to 1 if the last NAL unit, in decoding order, of
the access unit associated with the RTP timestamp is contained in
the packet.
o Setting of the RTP timestamp:
For an RTP packet for which the packet payload is an Empty NAL unit,
the RTP timestamp must be set according to Section 4.10.
For an RTP packet for which the packet payload is a PACSI NAL unit,
the RTP timestamp MUST be equal to the NALU-time of the next non-
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PACSI NAL unit in transmission order. Recall that the NALU-time of a
NAL unit in an MTAP is defined in [I-D.ietf-avt-rtp-rfc3984bis] as
the value that the RTP timestamp would have if that NAL unit would
be transported in its own RTP packet.
o Setting of the SSRC:
For both SST and MST, the SSRC values MUST be set according to [RFC
3550].
4.2 NAL Unit Extension and Header Usage
4.2.1 NAL Unit Extension
This memo specifies a NAL unit extension mechanism to allow for
introduction of new types of NAL units, beyond the three NAL unit
types left undefined in [I-D.ietf-avt-rtp-rfc3984bis] (i.e., 0, 30
and 31). The extension mechanism utilizes the NAL unit type value
31 and is specified as follows. When the NAL unit type value is
equal to 31, the one-byte NAL unit header consisting of the F, NRI
and Type fields as specified in Section 1.1.3 is extended by one
additional octet, which consists of a 5-bit field named Subtype and
three 1-bit fields named J, K, and L, respectively. The additional
octet is shown in the following figure.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
| Subtype |J|K|L|
+---------------+
The Subtype value determines the (extended) NAL unit type of this
NAL unit. The interpretation of the fields J, K, and L depends on
the Subtype. The semantics of the fields are as follows.
When Subtype is equal to 1, the NAL unit is an Empty NAL unit as
specified in Section 4.10. When Subtype is equal to 2, the NAL unit
is an NI-MTAP NAL unit as specified in Section 4.7.1. All other
values of Subtype (0, 3-31) are reserved for future extensions, and
receivers MUST ignore the entire NAL unit when Subtype is equal to
any of these reserved values.
4.2.2 NAL Unit Header Usage
The structure and semantics of the NAL unit header according to the
H.264 specification [H.264] were introduced in Section 1.1.3. This
section specifies the extended semantics of the NAL unit header
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fields F, NRI, I, PRID, DID, QID, TID, U, and D, according to this
memo. When the Type field is equal to 31, the semantics of the
fields in the extension NAL unit header were specified in Section
4.2.1.
The semantics of F specified in Section 5.3 of [I-D.ietf-avt-rtp-
rfc3984bis] also apply in this memo. That is, a value of 0 for F
indicates that the NAL unit type octet and payload should not
contain bit errors or other syntax violations, whereas a value of 1
for F indicates that the NAL unit type octet and payload may contain
bit errors or other syntax violations. MANEs SHOULD set the F bit to
indicate bit errors in the NAL unit.
For NRI, for a bitstream conforming to one of the profiles defined
in Annex A of [H.264] and transported using [I-D.ietf-avt-rtp-
rfc3984bis], the semantics specified in Section 5.3 of [I-D.ietf-
avt-rtp-rfc3984bis] apply, i.e., NRI also indicates the relative
importance of NAL units. For a bitstream conforming to one of the
profiles defined in Annex G of [H.264] and transported using this
memo, in addition to the semantics specified in Annex G of [H.264],
NRI also indicates the relative importance of NAL units within a
layer.
For I, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to protect
NAL units with I equal to 1 better than NAL units with I equal to 0.
MANEs MAY also utilize information of NAL units with I equal to 1 to
decide when to forward more packets for an RTP packet stream. For
example, when it is detected that spatial layer switching has
happened such that the operation point has changed to a higher value
of DID, MANEs MAY start to forward NAL units with the higher value
of DID only after forwarding a NAL unit with I equal to 1 with the
higher value of DID.
Note that, in the context of this section, "protecting a NAL unit"
means any RTP or network transport mechanism that could improve the
probability of successful delivery of the packet conveying the NAL
unit, including applying a QoS-enabled network, Forward Error
Correction (FEC), retransmissions, and advanced scheduling behavior,
whenever possible.
For PRID, the semantics specified in Annex G of [H.264] apply. Note
that MANEs implementing unequal error protection MAY use this
information to protect NAL units with smaller PRID values better
than those with larger PRID values, for example by including only
the more important NAL units in an FEC protection mechanism. The
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importance for the decoding process decreases as the PRID value
increases.
For DID, QID, TID, in addition to the semantics specified in Annex G
of [H.264], according to this memo, values of DID, QID, or TID
indicate the relative importance in their respective dimension. A
lower value of DID, QID, or TID indicates a higher importance if the
other two components are identical. MANEs MAY use this information
to protect more important NAL units better than less important NAL
units.
For U, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to protect
NAL units with U equal to 1 better than NAL units with U equal to 0.
For D, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to determine
whether a given NAL unit is required for successfully decoding a
certain Operation Point of the SVC bitstream, hence to decide
whether to forward the NAL unit.
4.3 Payload Structures
The NAL unit structure is central to H.264/AVC, [I-D.ietf-avt-rtp-
rfc3984bis], as well as SVC and this memo. In H.264/AVC and SVC,
all coded bits for representing a video signal are encapsulated in
NAL units. In [I-D.ietf-avt-rtp-rfc3984bis], each RTP packet
payload is structured as a NAL unit, which contains one or a part of
one NAL unit specified in H.264/AVC, or aggregates one or more NAL
units specified in H.264/AVC.
[I-D.ietf-avt-rtp-rfc3984bis] specifies three basic payload
structures (in Section 5.2 of [I-D.ietf-avt-rtp-rfc3984bis]): Single
NAL Unit Packet, Aggregation Packet, and Fragmentation Unit, and six
new types (24 to 29) of NAL units. The value of the Type field of
the RTP packet payload header (i.e., the first byte of the payload)
may be equal to any value from 1 to 23 for a Single NAL Unit Packet,
any value from 24 to 27 for an Aggregation Packet, and 28 or 29 for
a Fragmentation Unit.
In addition to the NAL unit types defined originally for H.264/AVC,
SVC defines three new NAL unit types specifically for SVC: coded
slice in scalable extension NAL units (type 20), prefix NAL units
(type 14), and subset sequence parameter set NAL units (type 15), as
described in Section 1.1.
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This memo further introduces three new types of NAL units, PACSI NAL
unit (NAL unit type 30) as specified in Section 4.9, Empty NAL unit
(type 31, subtype 1) as specified in Section 4.10, and NI-MTAP NAL
unit (type 31, subtype 2) as specified in Section 4.7.1.
The RTP packet payload structure in [I-D.ietf-avt-rtp-rfc3984bis] is
maintained with slight extensions in this memo, as follows. Each
RTP packet payload is still structured as a NAL unit, which contains
one or a part of one NAL unit specified in H.264/AVC and SVC, or
contains one PACSI NAL unit or one Empty NAL unit, or aggregates
zero or more NAL units specified in H.264/AVC and SVC, zero or one
PACSI NAL unit, and zero or more Empty NAL units.
In this memo, one of the three basic payload structures,
Fragmentation Unit, remains the same as in [I-D.ietf-avt-rtp-
rfc3984bis], and the other two, Single NAL Unit Packet and
Aggregation Packet, are extended as follows. The value of the Type
field of the payload header may be equal to any value from 1 to 23,
inclusive, and 30 to 31, inclusive, for a Single NAL Unit Packet,
and any value from 24 to 27, inclusive, and 31, for an Aggregation
Packet. When the Type field of the payload header is equal to 31
and the Subtype field of the payload header is equal to 2, the
packet is an Aggregation Packet (containing a NI-MTAP NAL unit).
When the Type field of the payload header is equal to 31 and the
Subtype field of the payload header is equal to 1, the packet is a
Single NAL Unit Packet (containing an Empty NAL unit).
Note that, in this memo, the length of the payload header varies
depending on the value of the Type field in the first byte of the
RTP packet payload. If the value is equal to 14, 20, or 30, the
first four bytes of the packet payload form the payload header;
otherwise if the value is equal to 31, the first two bytes of the
payload form the payload header; otherwise, the payload header is
the first byte of the packet payload.
Table 1 lists the NAL unit types introduced in SVC and this memo and
where they are described in this memo. Table 2 summarizes the basic
payload structure types for all NAL unit types when they are
directly used as RTP packet payloads according to this memo. Table
3 summarizes the NAL unit types allowed to be aggregated (i.e., used
as aggregation units in aggregation packets) or fragmented (i.e.,
carried in fragmentation units) according to this memo.
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Table 1. NAL unit types introduced in SVC and this memo
Type Subtype NAL Unit Name Section Numbers
-----------------------------------------------------------
14 - Prefix NAL unit 1.1
15 - Subset sequence parameter set 1.1
20 - Coded slice in scalable extension 1.1
30 - PACSI NAL unit 4.9
31 0 reserved 4.2.1
31 1 Empty NAL unit 4.10
31 2 NI-MTAP 4.7.1
31 3-31 reserved 4.2.1
Table 2. Basic payload structure types for all NAL unit
types when they are directly used as RTP packet payloads
Type Subtype Basic Payload Structure
------------------------------------------
0 - reserved
1-23 - Single NAL Unit Packet
24-27 - Aggregation Packet
28-29 - Fragmentation Unit
30 - Single NAL Unit Packet
31 0 reserved
31 1 Single NAL Unit Packet
31 2 Aggregation Packet
31 3-31 reserved
Table 3. Summary of the NAL unit types allowed to be
aggregated or fragmented (yes = allowed, no = disallowed,
- = not applicable/not specified)
Type Subtype STAP-A STAP-B MTAP16 MTAP24 FU-A FU-B NI-MTAP
-------------------------------------------------------------
0 - - - - - - - -
1-23 - yes yes yes yes yes yes yes
24-29 - no no no no no no no
30 - yes yes yes yes no no yes
31 0 - - - - - - -
31 1 yes no no no no no yes
31 2 no no no no no no no
31 3-31 - - - - - - -
4.4 Transmission Modes
This memo enables transmission of an SVC bitstream over one or more
RTP sessions. If only one RTP session is used for transmission of
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the SVC bitstream, the transmission mode is referred to as Single-
Session Transmission (SST); otherwise (more than one RTP session is
used for transmission of the SVC bitstream), the transmission mode
is referred to as Multi-Session Transmission (MST).
SST SHOULD be used for point-to-point unicast scenarios, while MST
SHOULD be used for point-to-multipoint multicast scenarios where
different receivers requires different operation points of the same
SVC bitstream, to improve bandwidth utilizing efficiency.
If the OPTIONAL mst-mode media type parameter (see Section 7.1) is
not present, SST MUST be used; otherwise (mst-mode is present), MST
MUST be used.
4.5 Packetization Modes
4.5.1 Packetization Modes for Single-Session Transmission
When SST is in use, Section 5.4 of [I-D.ietf-avt-rtp-rfc3984bis]
applies with the following extensions.
The packetization modes specified in Section 5.4 of [I-D.ietf-avt-
rtp-rfc3984bis], namely Single NAL Unit Mode, Non-Interleaved Mode
and Interleaved Mode, are also referred to as session packetization
modes. Table 4 summarizes the allowed session packetization modes
for SST.
Table 4. Summary of allowed session packetization modes
(denoted as "Session Mode" for simplicity) for SST (yes =
allowed, no = disallowed)
Session Mode Allowed
-------------------------------------
Single NAL Unit Mode yes
Non-Interleaved Mode yes
Interleaved Mode yes
For NAL unit types in the range of 0 to 29, inclusive, the NAL unit
types allowed to be directly used as packet payloads for each
session packetization mode are the same as specified in Section 5.4
of [I-D.ietf-avt-rtp-rfc3984bis]. For other NAL unit types, which
are newly introduced in this memo, the NAL unit types allowed to be
directly used as packet payloads for each session packetization mode
are summarized in Table 5.
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Table 5. New NAL unit types allowed to be directly used
as packet payloads for each session packetization mode
(yes = allowed, no = disallowed, - = not applicable/not
specified)
Type Subtype Single NAL Non-Interleaved Interleaved
Unit Mode Mode Mode
-------------------------------------------------------------
30 - yes no no
31 0 - - -
31 1 yes yes no
31 2 no yes no
31 3-31 - - -
4.5.2 Packetization Modes for Multi-Session Transmission
For MST, this memo specifies four MST packetization modes:
o Non-interleaved timestamp based mode (NI-T);
o Non-interleaved cross-session decoding order number (CS-DON)
based mode (NI-C);
o Non-interleaved combined timestamp and CS-DON mode (NI-TC); and
o Interleaved CS-DON (I-C) mode.
These four modes differ in two ways. First, they differ in terms of
whether NAL units are required to be transmitted within each RTP
session in decoding order (i.e., non-interleaved), or they are
allowed to be transmitted in a different order (i.e., interleaved).
Second, they differ in the mechanisms they provide in order to
recover the correct decoding order of the NAL units across all RTP
sessions involved.
The NI-T, NI-C, and NI-TC modes do not allow interleaving, and are
thus targeted for systems that require relatively low end-to-end
latency, e.g., conversational systems. The I-C mode allows
interleaving and is thus targeted for systems that do not require
very low end-to-end latency. The benefits of interleaving are the
same as that of the Interleaved Mode specified in [I-D.ietf-avt-rtp-
rfc3984bis].
The NI-T mode uses timestamps to recover the decoding order of NAL
units, whereas the NI-C and I-C modes both use the CS-DON mechanism
(explained later on) to do so. The NI-TC mode provides both
timestamps and the CS-DON method; receivers in this case may choose
to use either method for performing decoding order recovery
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The MST packetization mode in use MUST be signaled by the value of
the OPTIONAL mst-mode media type parameter. The used MST
packetization mode governs which session packetization modes are
allowed in the associated RTP sessions, which in turn govern which
NAL unit types are allowed to be directly used as RTP packet
payloads.
Table 6 summarizes the allowed session packetization modes for NI-T,
NI-C and NI-TC. Table 7 summarizes the allowed session
packetization modes for I-C.
Table 6. Summary of allowed session packetization modes
(denoted as "Session Mode" for simplicity) for NI-T, NI-C
and NI-TC (yes = allowed, no = disallowed)
Session Mode Base Session Enhancement Session
-----------------------------------------------------------
Single NAL Unit Mode yes no
Non-Interleaved Mode yes yes
Interleaved Mode no no
Table 7. Summary of allowed session packetization modes
(denoted as "Session Mode" for simplicity) for I-C (yes =
allowed, no = disallowed)
Session Mode Base Session Enhancement Session
-----------------------------------------------------------
Single NAL Unit Mode no no
Non-Interleaved Mode no no
Interleaved Mode yes yes
For NAL unit types in the range of 0 to 29, inclusive, the NAL unit
types allowed to be directly used as packet payloads for each
session packetization mode are the same as specified in Section 5.4
of [I-D.ietf-avt-rtp-rfc3984bis]. For other NAL unit types, which
are newly introduced in this memo, the NAL unit types allowed to be
directly used as packet payloads for each allowed session
packetization mode for NI-T, NI-C, NI-TC, and I-C are summarized in
Tables 8, 9, 10, and 11, respectively.
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Table 8. New NAL unit types allowed to be directly used
as packet payloads for each allowed session packetization
mode when NI-T is in use (yes = allowed, no = disallowed,
- = not applicable/not specified)
Type Subtype Single NAL Non-Interleaved
Unit Mode Mode
---------------------------------------------------
30 - yes no
31 0 - -
31 1 yes yes
31 2 no yes
31 3-31 - -
Table 9. New NAL unit types allowed to be directly used
as packet payloads for each allowed session packetization
mode when NI-C is in use (yes = allowed, no = disallowed,
- = not applicable/not specified)
Type Subtype Single NAL Non-Interleaved
Unit Mode Mode
---------------------------------------------------
30 - yes yes
31 0 - -
31 1 no no
31 2 no yes
31 3-31 - -
Table 10. New NAL unit types allowed to be directly used
as packet payloads for each allowed session packetization
mode when NI-TC is in use (yes = allowed, no = disallowed,
- = not applicable/not specified)
Type Subtype Single NAL Non-Interleaved
Unit Mode Mode
---------------------------------------------------
30 - yes yes
31 0 - -
31 1 yes yes
31 2 no yes
31 3-31 - -
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Table 11. New NAL unit types allowed to be directly used
as packet payloads for the allowed session packetization
mode when I-C is in use (yes = allowed, no = disallowed, -
= not applicable/not specified)
Type Subtype Interleaved Mode
------------------------------------
30 - no
31 0 -
31 1 no
31 2 no
31 3-31 -
When MST is in use and the MST packetization mode in use is NI-C,
Empty NAL units (type 31, subtype 1) MUST NOT be used, i.e., no RTP
packet is allowed to contain one or more Empty NAL units.
When MST is in use and the MST packetization mode in use is I-C,
both Empty NAL units (type 31, subtype 1) and NI-MTAP NAL units
(type 31, subtype 2) MUST NOT be used, i.e., no RTP packet is
allowed to contain one or more Empty NAL units or an NI-MTAP NAL
unit.
4.6 Single NAL Unit Packets
Section 5.6 of [I-D.ietf-avt-rtp-rfc3984bis] applies with the
following extensions.
The payload of a Single NAL Unit Packet MAY be a PACSI NAL unit
(Type 30) or an Empty NAL unit (Type 31 and Subtype 1), in addition
to a NAL unit with NAL unit type equal to any value from 1 to 23,
inclusive.
If the Type field of the first byte of the payload is not equal to
31, the payload header is the first byte of the payload. Otherwise
(the Type field of the first byte of the payload is equal to 31),
the payload header is the first two bytes of the payload.
4.7 Aggregation Packets
In addition to Section 5.7 of [I-D.ietf-avt-rtp-rfc3984bis], the
following applies in this memo.
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4.7.1 Non-Interleaved Multi-Time Aggregation Packets (NI-MTAPs)
One new NAL unit type introduced in this memo is the Non-Interleaved
Multi-Time Aggregation packet (NI-MTAP). An NI-MTAP consists of one
or more non-interleaved multi-time aggregation units.
The NAL units contained in NI-MTAPs MUST be aggregated in decoding
order.
A non-interleaved multi-time aggregation unit for the NI-MTAP
consists of 16 bits of unsigned size information of the following
NAL unit (in network byte order), and 16 bits (in network byte order)
of timestamp offset (TS offset) for the NAL unit. The structure is
presented in Figure 1. The starting or ending position of an
aggregation unit within a packet may or may not be on a 32-bit word
boundary. The NAL units in the NI-MTAP are ordered in NAL unit
decoding order.
The Type field of the NI-MTAP MUST be set equal to "31".
The F bit MUST be set to 0 if all the F bits of the aggregated NAL
units are zero; otherwise, it MUST be set to 1.
The value of NRI MUST be the maximum value of NRI across all NAL
units carried in the NI-MTAP packet.
The field Subtype MUST be equal to 2.
If the field J is equal to 1 the optional DON field MUST be present
for each of the non-interleaved multi-time aggregation units. For
SST the J field MUST be equal to 0. For MST, in the NI-T mode the J
field MUST be equal to 0, whereas in the NI-C or NI-TC mode the J
field MUST be equal to 1. When the NI-C or NI-TC mode is in use,
the DON field, when present, MUST represent the CS-DON value for the
particular NAL unit as defined in Section 6.2.2.
The fields K and L MUST be both equal to 0.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: NAL unit size | TS offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DON (optional) | |
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 Non-interleaved multi-time aggregation unit for
NI-MTAP
Let TS be the RTP timestamp of the packet carrying the NAL unit.
Recall that the NALU-time of a NAL unit in an MTAP is defined in [I-
D.ietf-avt-rtp-rfc3984bis] as the value that the RTP timestamp would
have if that NAL unit would be transported in its own RTP packet.
The timestamp offset field MUST be set to a value equal to the value
of the following formula:
if NALU-time >= TS, TS offset = NALU-time - TS
else, TS offset = NALU-time + (2^32 - TS)
For the "earliest" multi-time aggregation unit in an NI-MTAP the
timestamp offset MUST be zero. Hence, the RTP timestamp of the NI-
MTAP itself is identical to the earliest NALU-time.
Informative note: The "earliest" multi-time aggregation unit is
the one that would have the smallest extended RTP timestamp among
all the aggregation units of an NI-MTAP if the aggregation units
were encapsulated in single NAL unit packets. An extended
timestamp is a timestamp that has more than 32 bits and is
capable of counting the wraparound of the timestamp field, thus
enabling one to determine the smallest value if the timestamp
wraps. Such an "earliest" aggregation unit may or may not be the
first one in the order in which the aggregation units are
encapsulated in an NI-MTAP. The "earliest" NAL unit need not be
the same as the first NAL unit in the NAL unit decoding order
either.
Figure 2 presents an example of an RTP packet that contains an NI-
MTAP that contains two non-interleaved multi-time aggregation units,
labeled as 1 and 2 in the figure.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| Type | Subtype |J|K|L| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Non-interleaved Multi-time aggregation unit #1 |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Non-interleaved Multi-time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| aggregation unit #2 |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 An RTP packet including an NI-MTAP containing two
non-interleaved multi-time aggregation units
4.8 Fragmentation Units (FUs)
Section 5.8 of [I-D.ietf-avt-rtp-rfc3984bis] applies.
Informative note: In case a NAL unit with the four-byte SVC NAL
unit header is fragmented, the three-byte SVC-specific header
extension is considered as part of the NAL unit payload. That is,
the three-byte SVC-specific header extension is only available in
the first fragment of the fragmented NAL unit.
4.9 Payload Content Scalability Information (PACSI) NAL Unit
Another new type of NAL unit specified in this memo is the Payload
Content Scalability Information (PACSI) NAL unit. The Type field of
PACSI NAL units MUST be equal to 30 (a NAL unit type value left
unspecified in [H.264] and [I-D.ietf-avt-rtp-rfc3984bis]). A PACSI
NAL unit MAY be carried in a single NAL unit packet or an
aggregation packet, and MUST NOT be fragmented.
PACSI NAL units may be used for the following purposes:
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o To enable MANEs to decide whether to forward, process or discard
aggregation packets, by checking in PACSI NAL units the
scalability information and other characteristics of the
aggregated NAL units, rather than looking into the aggregated NAL
units themselves, which are defined by the video coding
specification;
o To enable correct decoding order recovery in MST using the NI-C
or NI-TC mode, with the help of the CS-DON information included in
PACSI NAL units; and
o To improve resilience to packet losses, e.g. by utilizing the
following data or information included in PACSI NAL units:
repeated Supplemental Enhancement Information (SEI) messages,
information regarding the start and end of layer representations,
and the indices to layer representations of the lowest temporal
subset.
PACSI NAL units MAY be ignored in the NI-T mode without affecting
the decoding order recovery process.
When a PACSI NAL unit is present in an aggregation packet, the
following applies.
o The PACSI NAL unit MUST be the first aggregated NAL unit in the
aggregation packet.
o There MUST be at least one additional aggregated NAL unit in the
aggregation packet.
o The RTP header fields and the payload header fields of the
aggregation packet are set as if the PACSI NAL unit was not
included in the aggregation packet.
o If the aggregation packet is an MTAP16, MTAP24, or NI-MTAP with
the J field equal to 1, the decoding order number (DON) for the
PACSI NAL unit MUST be set to indicate that the PACSI NAL unit
has an identical DON to the first NAL unit in decoding order
among the remaining NAL units in the aggregation packet.
When a PACSI NAL unit is included in a single NAL unit packet, it is
associated with the next non-PACSI NAL unit in transmission order,
and the RTP header fields of the packet are set as if the next non-
PACSI NAL unit in transmission order was included in a single NAL
unit packet.
The PACSI NAL unit structure is as follows. The first four octets
are exactly the same as the four-byte SVC NAL unit header discussed
in Section 1.1.3. They are followed by one octet containing several
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flags, then five optional octets, and finally zero or more SEI NAL
units. Each SEI NAL unit is preceded by a 16-bit unsigned size
field (in network byte order) that indicates the size of the
following NAL unit in bytes (excluding these two octets, but
including the NAL unit header octet of the SEI NAL unit). Figure 3
illustrates the PACSI NAL unit structure and an example of a PACSI
NAL unit containing two SEI NAL units.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| Type |R|I| PRID |N| DID | QID | TID |U|D|O| RR|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X|Y|T|A|P|C|S|E| TL0PICIDX (o) | IDRPICID (o) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DONC (o) | NAL unit size 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SEI NAL unit 1 |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NAL unit size 2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| SEI NAL unit 2 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 PACSI NAL unit structure. Fields suffixed by
"(o)" are OPTIONAL.
The bits A, P, and C are specified only if the bit X is equal to 1.
The bits S and E are specified, and the fields TL0PICIDX and
IDRPICID are present, only if the bit Y is equal to 1. The field
DONC is present only if the bit T is equal to 1. The field T MUST
be equal to 0 if the PACSI NAL unit is contained in an STAP-B,
MTAP16, MTAP24, or NI-MTAP with the J field equal to 1.
The values of the fields in PACSI NAL unit MUST be set as follows.
o The F bit MUST be set to 1 if the F bit in at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the next non-PACSI NAL unit in transmission order has the F bit
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the F bit MUST be set to 0.
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o The NRI field MUST be set to the highest value of NRI field among
all the remaining NAL units in the aggregation packet (when the
PACSI NAL unit is included in an aggregation packet) or the value
of the NRI field of the next non-PACSI NAL unit in transmission
order (when the PACSI NAL unit is included in a single NAL unit
packet).
o The Type field MUST be set to 30.
o The R bit MUST be set to 1. Receivers MUST ignore the value of R.
o The I bit MUST be set to 1 if the I bit of at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the I bit of the next non-PACSI NAL unit in transmission order is
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the I bit MUST be set to 0.
o The PRID field MUST be set to the lowest value of the PRID values
of the remaining NAL units in the aggregation packet (when the
PACSI NAL unit is included in an aggregation packet) or the PRID
value of the next non-PACSI NAL unit in transmission order (when
the PACSI NAL unit is included in a single NAL unit packet).
o The N bit MUST be set to 1 if the N bit of all the remaining NAL
units in the aggregation packet is equal to 1 (when the PACSI NAL
unit is included in an aggregation packet) or if the N bit of the
next non-PACSI NAL unit in transmission order is equal to 1 (when
the PACSI NAL unit is included in a single NAL unit packet).
Otherwise, the N bit MUST be set to 0.
o The DID field MUST be set to the lowest value of the DID values
of the remaining NAL units in the aggregation packet (when the
PACSI NAL unit is included in an aggregation packet) or the DID
value of the next non-PACSI NAL unit in transmission order (when
the PACSI NAL unit is included in a single NAL unit packet).
o The QID field MUST be set to the lowest value of the QID values
of the remaining NAL units with the lowest value of DID in the
aggregation packet (when the PACSI NAL unit is included in an
aggregation packet) or the QID value of the next non-PACSI NAL
unit in transmission order (when the PACSI NAL unit is included
in a single NAL unit packet).
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o The TID field MUST be set to the lowest value of the TID values
of the remaining NAL units with the lowest value of DID in the
aggregation packet (when the PACSI NAL unit is included in an
aggregation packet) or the TID value of the next non-PACSI NAL
unit in transmission order (when the PACSI NAL unit is included
in a single NAL unit packet).
o The U bit MUST be set to 1 if the U bit of at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the U bit of the next non-PACSI NAL unit in transmission order is
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the U bit MUST be set to 0.
o The D bit MUST be set to 1 if the D value of all the remaining
NAL unit in the aggregation packet is equal to 1 (when the PACSI
NAL unit is included in an aggregation packet) or if the D bit of
the next non-PACSI NAL unit in transmission order is equal to 1
(when the PACSI NAL unit is included in a single NAL unit packet).
Otherwise, the D bit MUST be set to 0.
o The O bit MUST be set to 1 if the O bit of at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the O bit of the next non-PACSI NAL unit in transmission order is
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the O bit MUST be set to 0.
o The RR field MUST be set to "11" (in binary form). Receivers
MUST ignore the value of RR.
o If the X bit is equal to 1, the bits A, P, and C are specified as
below. Otherwise, the bits A, P, and C are unspecified, and
receivers MUST ignore the values of these bits. The X bit SHOULD
be identical for all the PACSI NAL units in all the RTP sessions
carrying the same SVC bitstream.
o If the Y bit is equal to 1, the OPTIONAL fields TL0PICIDX and
IDRPICID MUST be present and specified as below, and the bits S
and E are also specified as below. Otherwise, the fields
TL0PICIDX and IDRPICID MUST NOT be present, while the S and E
bits are unspecified and receivers MUST ignore the values of
these bits. The Y bit MUST be identical for all the PACSI NAL
units in all the RTP sessions carrying the same SVC bitstream.
The Y bit MUST be equal to 0 when the parameter packetization-
mode is equal to 2.
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o If the T bit is equal to 1, the OPTIONAL field DONC MUST be
present and specified as below. Otherwise, the field DONC MUST
NOT be present. The field T MUST be equal to 0 if the PACSI NAL
unit is contained in an STAP-B, MTAP16, MTAP24, or NI-MTAP.
o The A bit MUST be set to 1 if at least one of the remaining NAL
units in the aggregation packet belongs to an anchor layer
representation (when the PACSI NAL unit is included in an
aggregation packet) or if the next non-PACSI NAL unit in
transmission order belongs to an anchor layer representation
(when the PACSI NAL unit is included in a single NAL unit packet).
Otherwise, the A bit MUST be set to 0.
Informative note: The A bit indicates whether CGS or spatial
layer switching at a non-IDR layer representation (a layer
representation with nal_unit_type not equal to 5 and idr_flag not
equal to 1) can be performed. With some picture coding
structures a non-IDR intra layer representation can be used for
random access. Compared to using only IDR layer representations,
higher coding efficiency can be achieved. The H.264/AVC or SVC
solution to indicate the random accessibility of a non-IDR intra
layer representation is using a recovery point SEI message. The
A bit offers direct access to this information, without having to
parse the recovery point SEI message, which may be buried deeply
in an SEI NAL unit. Furthermore, the SEI message may or may not
be present in the bitstream.
o The P bit MUST be set to 1 if all the remaining NAL units in the
aggregation packet have redundant_pic_cnt greater than 0 (when
the PACSI NAL unit is included in an aggregation packet) or the
next non-PACSI NAL unit in transmission order has
redundant_pic_cnt greater than 0 (when the PACSI NAL unit is
included in a single NAL unit packet). Otherwise, the P bit MUST
be set to 0.
Informative note: The P bit indicates whether a packet can be
discarded because it contains only redundant slice NAL units.
Without this bit, the corresponding information can be obtained
from the syntax element redundant_pic_cnt, which is contained in
the variable-length coded slice header.
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o The C bit MUST be set to 1 if at least one of the remaining NAL
units in the aggregation packet belongs to an intra layer
representation (when the PACSI NAL unit is included in an
aggregation packet) or if the next non-PACSI NAL unit in
transmission order belongs to an intra layer representation (when
the PACSI NAL unit is included in a single NAL unit packet).
Otherwise, the C bit MUST be set to 0.
Informative note: The C bit indicates whether a packet contains
intra slices, which may be the only packets to be forwarded, e.g.,
when the network conditions are particularly adverse.
o The S bit MUST be set to 1, if the first NAL unit following the
PACSI NAL unit in an aggregation packet is the first VCL NAL unit,
in decoding order, of a layer representation (when the PACSI NAL
unit is included in an aggregation packet) or if the next non-
PACSI NAL unit in transmission order is the first VCL NAL unit,
in decoding order, of a layer representation(when the PACSI NAL
unit is included in a single NAL unit packet). Otherwise, the S
bit MUST be set to 0.
o The E bit MUST be set to 1, if the last NAL unit following the
PACSI NAL unit in an aggregation packet is the last VCL NAL unit,
in decoding order, of a layer representation (when the PACSI NAL
unit is included in an aggregation packet) or if the next non-
PACSI NAL unit in transmission order is the last VCL NAL unit, in
decoding order, of a layer representation (when the PACSI NAL
unit is included in a single NAL unit packet). Otherwise, the E
field MUST be set to 0.
Informative note: In an aggregation packet it is always possible
to detect the beginning or end of a layer representation by
detecting changes in the values of dependency_id, quality_id, and
temporal_id in NAL unit headers, except from the first and last
NAL units of a packet. The S or E bits are used to provide this
information, for both single NAL unit and aggregation packets, so
that previous or following packets do not have to be examined.
This enables MANEs to detect slice loss and take proper action
such as requesting a retransmission as soon as possible, as well
as to allow efficient playout buffer handling similarly to the M
bit present in the RTP header. The M bit in the RTP header still
indicates the end of an access unit, not the end of a layer
representation.
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o When present, the TL0PICIDX field MUST be set to equal to
tl0_dep_rep_idx as specified in Annex G of [H.264] for the layer
representation containing the first NAL unit following the PACSI
NAL unit in the aggregation packet (when the PACSI NAL unit is
included in an aggregation packet) or containing the next non-
PACSI NAL unit in transmission order (when the PACSI NAL unit is
included in a single NAL unit packet).
o When present, the IDRPICID field MUST be set to equal to
effective_idr_pic_id as specified in Annex G of [H.264] for the
layer representation containing the first NAL unit following the
PACSI NAL unit in the aggregation packet (when the PACSI NAL unit
is included in an aggregation packet) or containing the next non-
PACSI NAL unit in transmission order (when the PACSI NAL unit is
included in a single NAL unit packet).
Informative note: The TL0PICIDX and IDRPICID fields enable the
detection of the loss of layer representations in the most
important temporal layer (with temporal_id equal to 0) by
receivers as well as MANEs. SVC provides a solution that uses
SEI messages, which are harder to parse and may or may not be
present in the bitstream. When the PACSI NAL unit is part of an
NI-MTAP packet, it is possible to infer the correct values of
tl0_dep_rep_idx and idr_pic_id for all layer representations
contained in the NI-MTAP by following the rules that specify how
these parameters are set as given in Annex G of [H.264] and by
detecting the different layer representations contained in the
NI-MTAP packet by detecting changes in the values of
dependency_id_, quality_id, and temporal_id in the NAL unit
headers as well as using the S and E flags. The only exception
is if NAL units of an IDR picture are present in the NI-MTAP in a
position other than the first NAL unit following the PACSI NAL
unit, in which case the value of idr_pic_id cannot be inferred.
In this case the NAL unit has to be partially parsed to obtain
the idr_pic_id. Note that, due to the large size of IDR pictures,
their inclusion in an NI-MTAP, and especially in a position other
than the first NAL unit following the PACSI NAL unit may be
neither practical nor useful.
o When present, the field DONC indicates the Cross-Session Decoding
Order Number (CS-DON) for the first of the remaining NAL units in
the aggregation packet (when the PACSI NAL unit is included in an
aggregation packet) or the CS-DON of the next non-PACSI NAL unit
in transmission order (when the PACSI NAL unit is included in a
single NAL unit packet). CS-DON is further discussed in Section
4.11.
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The PACSI NAL unit MAY include a subset of the SEI NAL units
associated with the access unit to which the first non-PACSI NAL
unit in the aggregation packet belongs, and MUST NOT contain SEI NAL
units associated with any other access unit.
Informative note: In H.264/AVC and SVC, within each access unit,
SEI NAL units must appear before any VCL NAL unit in decoding
order. Therefore, without using PACSI NAL units, SEI messages
are typically only conveyed in the first of the packets carrying
an access unit. Senders may repeat SEI NAL units in PACSI NAL
units, so that they are repeated in more than one packet and thus
increase robustness against packet losses. Receivers may use the
repeated SEI messages in place of missing SEI messages.
For a PACSI NAL unit included in an aggregation packet, an SEI
message SHOULD NOT be included in the PACSI NAL unit and also
included in one of the remaining NAL units contained in the same
aggregation packet.
4.10 Empty NAL unit
An Empty NAL unit MAY be included in a single NAL unit packet, an
STAP-A or an NI-MTAP packet. Empty NAL units MUST have an RTP
timestamp (when transported in a single NAL unit packet) or NALU-
time (when transported in an aggregation packet) that is associated
with an access unit for which there exists at least one NAL unit of
type 1, 5, or 20. When MST is used, the type 1, 5, or 20 NAL unit
may be in a different RTP session. Empty NAL units may be used in
the decoding order recovery process of the NI-T mode as described in
Section 5.2.1.
The packet structure is shown in the following figure.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| type | Subtype |J|K|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Empty NAL unit structure.
The fields MUST be set as follows:
- F MUST be equal to 0
NRI MUST be equal to 3
Type MUST be equal to 31
Subtype MUST be equal to 1
J MUST be equal to 0
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K MUST be equal to 0
L MUST be equal to 0
4.11 Decoding Order Number (DON)
The DON concept is introduced in [I-D.ietf-avt-rtp-rfc3984bis] and
is used to recover the decoding order when interleaving is used
within a single session. Section 5.5 of [I-D.ietf-avt-rtp-
rfc3984bis] applies when using SST.
When using MST, it is necessary to recover the decoding order across
the various RTP sessions regardless if interleaving is used or not.
In addition to the timestamp mechanism described later on, the CS-
DON mechanism is an extension of the DON facility that can be used
for this purpose, and is defined in the following section.
4.11.1 Cross-Session DON (CS-DON) for Multi-Session Transmission
The Cross-Session Decoding Order Number (CS-DON) is a number that
indicates the decoding order of NAL units across all RTP sessions
involved in MST. It is similar to the DON concept in [I-D.ietf-avt-
rtp-rfc3984bis], but contrary to [I-D.ietf-avt-rtp-rfc3984bis] where
the DON was used only for interleaved packetization, in this memo it
is used not only in the interleaved MST mode (I-C) but also in two
of the non-interleaved MST modes as well (NI-C and NI-TC).
When the NI-C or NI-TC MST modes are in use, the packetization of
each session MUST be as specified in Section 5.2.2. In PACSI NAL
units the CS-DON value is explicitly coded in the field DONC. For
non-PACSI NAL units the CS-DON value is derived as follows. Let SN
indicate the RTP sequence number of a packet.
o For each non-PACSI NAL unit carried in a session using the single
NAL unit session packetization mode, the CS-DON value of the NAL
unit is equal to (DONC_prev_PACSI + SN_diff - 1) % 65536, wherein
"%" is the modulo operation, DONC_prev_PACSI is the DONC value of
the previous PACSI NAL unit with the same NALU-time as the
current NAL unit, and SN_diff is calculated as follows:
if SN1 > SN2, SN_diff = SN1 - SN2
else SN_diff = SN2 + 65536 - SN1
where SN1 and SN2 are the SNs of the current NAL unit and the
previous PACSI NAL unit with the same NALU-time, respectively.
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o For non-PACSI NAL units carried in a session using the non-
interleaved session packetization mode, the CS-DON value of each
non-PACSI NAL unit is derived as follows.
For a non-PACSI NAL unit in a single NAL unit packet, the
following applies.
If the previous PACSI NAL unit is contained in a single
NAL unit packet, the CS-DON value of the NAL unit is
calculated as above;
otherwise (the previous PACSI NAL unit is contained in
an STAP-A packet), the CS-DON value of the NAL unit is
calculated as above, with DONC_prev_PACSI being replaced
by the CS-DON value of the previous non-PACSI NAL unit
in decoding order (i.e., the CS-DON value of the last
NAL unit of the STAP-A packet).
For a non-PACSI NAL unit in an STAP-A packet, the following
applies.
If the non-PACSI NAL unit is the first non-PACSI NAL
unit in the STAP-A packet, the CS-DON value of the NAL
unit is equal to DONC of the PACSI NAL unit in the STAP-
A packet;
otherwise (the non-PACSI NAL unit is not the first non-
PACSI NAL unit in the STAP-A packet), the CS-DON value
of the NAL unit is equal to: (the CS-DON value of the
previous non-PACSI NAL unit in decoding order + 1) %
65536, wherein "%" is the modulo operation.
For a non-PACSI NAL unit in a number of FU-A packets, the CS-
DON value of the NAL unit is calculated the same way as when
the single NAL unit session packetization mode is in use, with
SN1 being the SN value of the first FU-A packet.
For a non-PACSI NAL unit in an NI-MTAP packet, the CS-DON
value is equal to the value of the DON field of the non-
interleaved multi-time aggregation unit.
When the I-C MST packetization mode is in use, the DON values
derived according to [I-D.ietf-avt-rtp-rfc3984bis] for all the NAL
units in each of the RTP sessions MUST indicate CS-DON values.
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5. Packetization Rules
Section 6 of [I-D.ietf-avt-rtp-rfc3984bis] applies in this memo,
with the following additions.
5.1 Packetization Rules for Single-Session Transmission
All receivers MUST support the single NAL unit packetization mode to
provide backward compatibility to endpoints supporting only the
single NAL unit mode of [I-D.ietf-avt-rtp-rfc3984bis]. However, the
use of single NAL unit packetization mode (packetization-mode equal
to 0) SHOULD be avoided whenever possible, because encapsulating NAL
units of small sizes in their own packets (e.g., small NAL units
containing parameter sets, prefix NAL units, or SEI messages) is
less efficient due to the packet header overhead.
All receivers MUST support the non-interleaved mode.
Informative note: The non-interleaved mode of [I-D.ietf-avt-rtp-
rfc3984bis] does allow an application to encapsulate a single NAL
unit in a single RTP packet. Historically, the single NAL unit
mode has been included into [I-D.ietf-avt-rtp-rfc3984bis] only
for compatibility with ITU-T Rec. H.241 Annex A [H.241]. There
is no point in carrying this historic ballast towards a new
application space such as the one provided with SVC. The
implementation complexity increase for supporting the additional
mechanisms of the non-interleaved mode (namely STAP-A and FU-A)
is minor, whereas the benefits are significant. As a result, the
support of STAP-A and FU-A is required. Additionally, support
for two of the three NAL unit types defined in this memo, namely
Empty NAL units and NI-MTAP is needed, as specified in Section
4.5.1.
A NAL unit of small size SHOULD be encapsulated in an aggregation
packet together with one or more other NAL units. For example, non-
VCL NAL units such as access unit delimiters, parameter sets, or SEI
NAL units are typically small.
A prefix NAL unit and the NAL unit with which it is associated, and
which follows the prefix NAL unit in decoding order, SHOULD be
included in the same aggregation packet whenever an aggregation
packet is used for the associated NAL unit, unless this would
violate session MTU constraints or if fragmentation units are used
for the associated NAL unit.
Informative note: Although the prefix NAL unit is ignored by an
H.264/AVC decoder, it is necessary in the SVC decoding process.
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Given the small size of the prefix NAL unit, it is best if it is
transported in the same RTP packet as its associated NAL unit.
When only an H.264/AVC compatible subset of the SVC base layer is
transmitted in an RTP session, the subset MUST be encapsulated
according to [I-D.ietf-avt-rtp-rfc3984bis]. This way, an [I-D.ietf-
avt-rtp-rfc3984bis] receiver will be able to receive the H.264/AVC
compatible bitstream subset.
When a set of layers including one or more SVC enhancement layers is
transmitted in an RTP session, the set SHOULD be carried in one RTP
stream that SHOULD be encapsulated according to this memo.
5.2 Packetization Rules for Multi-Session Transmission
When MST is used, the packetization rules specified in Section 5.1
still apply. In addition, the following packetization rules MUST be
followed, to ensure that decoding order of NAL units carried in the
sessions can be correctly recovered for each of the MST
packetization modes using the de-packetization process specified in
Section 6.2.
The NI-T and NI-TC modes both use timestamps to recover the decoding
order. In order to be able to do so, it is necessary for the RTP
packet stream to contain data for all sampling instances of a given
RTP session in all enhancement RTP sessions that depend on the given
RTP session. The NI-C and I-C modes do not have this limitation,
and use the CS-DON values as a means to explicitly indicate decoding
order, either directly coded in PACSI NAL units, or inferred from
them using the packetization rules. It is noted that the NI-TC mode
offers both alternatives and it is up to the receiver to select
which one to use.
5.2.1 NI-T/NI-TC Packetization Rules
When using the NI-T mode and a PACSI NAL unit is present, the T bit
MUST be equal to 0, i.e., the DONC field MUST NOT be present.
When using the NI-T mode, the optional parameters sprop-mst-remux-
buf-size, sprop-remux-buf-req, remux-buf-cap, sprop-remux-init-buf-
time, sprop-mst-max-don-diff MUST NOT be present.
When the NI-T or NI-TC MST mode is in use, the following applies.
If one or more NAL units of an access unit of sampling time instance
t is present in RTP session A, then one or more NAL units of the
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same access unit MUST be present in any enhancement RTP session
which depends on RTP session A.
Informative note 1: The mapping between RTP and NTP format
timestamps is conveyed in RTCP SR packets. In addition, the
mechanisms for faster media timestamp synchronization discussed
in [RFC6051] may be used to speed up the acquisition of the RTP-
to-wall-clock mapping.
Informative note 2: The rule above may require the insertion of
NAL units, typically when temporal scalability is used, i.e., an
enhancement RTP session does not contain any NAL units for an
access unit with a particular NTP timestamp (media timestamp),
which however is present in a lower enhancement RTP session or
the base RTP session. There are two ways to insert additional NAL
units in order to satisfy this rule:
- One option for adding additional NAL units is to use Empty NAL
units (defined in Section 4.10), which can be used by the process
described in Section 6.2.1 for the access unit re-ordering
process.
- Additional NAL units may also be added by the encoder itself,
for example by transmitting coded data that simply instruct the
decoder to repeat the previous picture. This option, however,
may be difficult to use with pre-encoded content.
If a packet must be inserted in order to satisfy the above rule,
e.g., in case of a MANE generating multiple RTP streams out of a
single RTP stream, the inserted packet must have an RTP timestamp
that maps to the same wall-clock time (in NTP format) as the one of
the RTP timestamp of any packet of the access unit present in any
lower enhancement RTP session or the base RTP session. This is easy
to accomplish if the NAL unit or the packet can be inserted at the
time of the RTP stream generation, since the media timestamp (NTP
timestamp) must be the same for the inserted packet and the packet
of the corresponding access unit. If there is no knowledge of the
media time at RTP stream generation or if the RTP streams are not
generated at the same instance, this can be also applied later in
the transmission process. In this case the NTP timestamp of the
inserted packet can be calculated as follows.
Assume that a packet A2 of an access unit with RTP timestamp TS_A2
is present in base RTP session A, and that no packet of that access
unit is present in enhancement RTP session B, as shown in Figure 5.
Thus a packet B2 must be inserted into session B following the rule
above. The most recent RTCP sender report in session A carries NTP
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timestamp NTP_A and the RTP timestamp TS_A. The sender report in
session B with a lower NTP timestamp than NTP_A is NTP_B, and
carries the RTP timestamp TS_B.
RTP session B:..B0........B1........(B2)......................
RTCP session B:.....SR(NTP_B,TS_B).............................
RTP session A:..A0........A1........A2........................
RTCP session A:..................SR(NTP_A,TS_A)................
-----------------|--x------|-----x---|------------------------>
NTP time
--------------------+<---------->+<->+------------------------>
t1 t2 RTP TS(B) time
Figure 5 Example calculation of RTP timestamp for packet
insertion in an enhancement layer RTP session
The vertical bars ("|")in the NTP timeline in the figure above
indicate that access unit data is present in at least one of the
sessions. The "x" marks indicate the times of the sender reports.
The RTP timestamp time line for session B, shown right below the NTP
time line, indicates two time segments, t1 and t2. t1 is the time
difference between the sender reports between the two sessions,
expressed in RTP timestamp clock ticks, and t2 is the time
difference from the session A sender report to the A2 packet, again
expressed in RTP timestamp clock ticks. The sum of these differences
is added to the RTP timestamp of the session report from session B
in order to derive the correct RTP timestamp for the inserted packet
B2. In other words:
TS_B2 = TS_B + t1 + t2
Let toRTP() be a function that calculates the RTP time difference
(in clock ticks of the used clock) given an NTP timestamp difference,
and effRTPdiff() be a function that calculates the effective
difference between two timestamps, including wraparounds:
effRTPdiff( ts1, ts2 ):
if( ts1 <= ts2 ) then
effRTPdiff := ts1-ts2
else
effRTPDiff := (4294967296 + ts2) - ts1
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We have:
t1 = toRTP(NTP_A - NTP_B) and t2 = effRTPdiff(TS_A2, TS_A)
Hence in order to generate the RTP timestamp TS_B2 for the inserted
packet B2, the RTP timestamp for packet B2 TS_B2 can be calculated
as follows.
TS_B2 = TS_B + toRTP(NTP_A - NTP_B) + effRTPdiff(TS_A2, TS_A)
5.2.2 NI-C/NI-TC Packetization Rules
When the NI-C or NI-TC MST mode is in use, the following applies for
each of the RTP sessions.
o For each single NAL unit packet containing a non-PACSI NAL unit,
the previous packet, if present, MUST have the same RTP timestamp
as the single NAL unit packet, and the following applies.
o If the NALU-time of the non-PACSI NAL unit is not equal to
the NALU-time of the previous non-PACSI NAL unit in decoding
order, the previous packet MUST contain a PACSI NAL unit
containing the DONC field.
o In an STAP-A packet the first NAL unit in the STAP-A packet MUST
be a PACSI NAL unit containing the DONC field.
o For an FU-A packet the previous packet MUST have the same RTP
timestamp as the FU-A packet, and the following applies.
o If the FU-A packet is the start of the fragmented NAL unit,
the following applies.
o If the NALU-time of the fragmented NAL unit is not
equal to the NALU-time of the previous non-PACSI NAL
unit in decoding order, the previous packet MUST
contain a PACSI NAL unit containing the DONC field;
o Otherwise (the NALU-time of the fragmented NAL unit is
equal to the NALU-time of the previous non-PACSI NAL
unit in decoding order), the previous packet MAY
contain a PACSI NAL unit containing the DONC field.
o Otherwise if the FU-A packet is the end of the fragmented
NAL unit, the following applies.
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o If the next non-PACSI NAL unit in decoding order has
NALU-time equal to the NALU-time of the fragmented NAL
unit, and is carried in a number of FU-A packets or a
single NAL unit packet, the next packet MUST be a
single NAL unit packet containing a PACSI NAL unit
containing the DONC field.
o Otherwise (the FU-A packet is neither the start nor the
end of the fragmented NAL unit), the previous packet
MUST be a FU-A packet.
o For each single NAL unit packet containing a PACSI NAL unit, if
present, the PACSI NAL unit MUST contain the DONC field.
o When the optional media type parameter sprop-mst-csdon-always-
present is equal to 1, the session packetization mode in use MUST
be the Non-Interleaved Mode, and only STAP-A and NI-MTAP packets
can be used.
5.2.3 I-C Packetization Rules
When the I-C MST packetization mode is in use, the following applies.
o When a PACSI NAL unit is present, the T bit MUST be equal to 0,
i.e., the DONC field is not present, and the Y bit MUST be equal
to 0, i.e., the TL0PICIDX and IDRPICID are not present.
5.2.4 Packetization Rules for Non-VCL NAL Units
NAL units which do not directly encode video slices are known in
H.264 as non-VCL NAL units. Non-VCL units that are only used by, or
only relevant to, enhancement RTP sessions SHOULD be sent in the
lowest session to which they are relevant.
Some senders, however, such as those sending pre-encoded data, may
be unable to easily determine which non-VCL units are relevant to
which session. Thus, non-VCL NAL units MAY, instead, be sent in a
session that the session using these non-VCL NAL units depends on
(e.g., the base RTP session).
If a non-VCL unit is relevant to more than one RTP session, neither
of which depends on the other(s), the NAL unit MAY be sent in
another session which all these sessions depend on.
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5.2.5 Packetization Rules for Prefix NAL Units
Section 5.1 of this memo applies, with the following addition. If
the base layer is sent in a base RTP session using [I-D.ietf-avt-
rtp-rfc3984bis], prefix NAL units MAY be sent in the lowest
enhancement RTP session rather than in the base RTP session.
6. De-Packetization Process
6.1 De-Packetization Process for Single-Session Transmission
For single-session transmission, where a single RTP session is used,
the de-packetization process specified in Section 7 of [I-D.ietf-
avt-rtp-rfc3984bis] applies.
6.2 De-Packetization Process for Multi-Session Transmission
For multi-session transmission, where more than one RTP session is
used to receive data from the same SVC bitstream, the de-
packetization process is specified as follows.
As for a single RTP session, the general concept behind the de-
packetization process is to reorder NAL units from transmission
order to the NAL unit decoding order.
The sessions to be received MUST be identified by mechanisms
specified in Section 7.2.3. An enhancement RTP session typically
contains an RTP stream that depends on at least one other RTP
session, as indicated by mechanisms defined in Section 7.2.3. A
lower RTP session to an enhancement RTP session is an RTP session
which the enhancement RTP session depends on. The lowest RTP
session for a receiver is the base RTP session, which does not
depend on any other RTP session received by the receiver. The
highest RTP session for a receiver is the RTP session which no other
RTP session received by the receiver depends on.
For each of the RTP sessions, the RTP reception process as specified
in RFC 3550 is applied. Then the received packets are passed into
the payload de-packetization process as defined in this memo.
The decoding order of the NAL units carried in all the associated
RTP sessions is then recovered by applying one of the following
subsections, depending on which of the MST packetization modes is in
use.
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6.2.1 Decoding Order Recovery for the NI-T and NI-TC Modes
The following process MUST be applied when the NI-T packetization
mode is in use. The following process MAY be applied when the NI-TC
packetization mode is in use.
The process is based on RTP session dependency signaling, RTP
sequence numbers, and timestamps.
The decoding order of NAL units within an RTP packet stream in RTP
session is given by the ordering of sequence numbers SN of the RTP
packets that contain the NAL units, and the order of appearance of
NAL units within a packet.
Timing information according to the media timestamp TS, i.e. the NTP
timestamp as derived from the RTP timestamp of an RTP packet, is
associated with all NAL units contained in the same RTP packet
received in an RTP session.
For NI-MTAP packets the NALU-time is derived for each contained NAL
unit by using the "TS offset" value in the NI-MTAP packet as defined
in Section 4.10, and is used instead of the RTP packet timestamp to
derive the media timestamp, e.g., using the NTP wall clock as
provided via RTCP sender reports. NAL units contained in
fragmentation packets are handled as defragmented, entire NAL units
with their own media timestamps. All NAL units associated with the
same value of media timestamp TS are part of the same access unit
AU(TS). Any Empty NAL units SHOULD be kept as, effectively, access
unit indicators in the re-ordering process. Empty NAL units and
PACSI NAL units SHOULD be removed before passing access unit data to
the decoder.
Informative note: These Empty NAL units are used to associate
NAL units present in other RTP sessions with RTP sessions not
containing any data for an access unit of a particular time
instance. They act as access unit indicators in sessions that
would otherwise contain no data for the particular access unit.
The presence of these NAL units is ensured by the
packetization rules in Section 5.2.1.
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It is assumed that the receiver has established an operation point
(DID, QID, and TID values), and has identified the highest
enhancement RTP session for this operation point. The decoding
order of NAL units from multiple RTP streams in multiple RTP
sessions MUST be recovered into a single sequence of NAL units,
grouped into access units, by performing any process equivalent to
the following steps. The general process is described in Section 4.2
of [RFC6051]. For convenience the instructions of [RFC6051] are
repeated and applied to NAL units rather than to full RTP packets.
Additionally SVC specific extensions to the procedure in Section 4.2.
of [RFC6051] are presented in the following list:
o The process should be started with the NAL units received in
the highest RTP session with the first media timestamp TS (in
NTP format) available in the session's (de-jittering) buffer.
It is assumed, that packets in the de-jittering buffer are
already stored in RTP sequence number order.
o Collect all NAL units associated with the same value of media
timestamp TS, starting from the highest RTP session, from all
the (de-jittering) buffers of the received RTP sessions. The
collected NAL units will be those associated with the access
unit AU(TS).
o Place the collected NAL units in the order of session
dependency as derived by the dependency indication as
specified in Section 7.2.3, starting from the lowest RTP
session.
o Place the session ordered NAL units in decoding order within
the particular access unit by satisfying the NAL unit
ordering rules for SVC access units, as described in the
informative algorithm provided in Section 6.2.1.1.
o Remove NI-MTAP and any PACSI NAL units from the access unit
AU(TS).
o The access units can then be transferred to the decoder.
Access units AU(TS) are transferred to the decoder in the
order of appearance (given by the order of RTP sequence
numbers) of media timestamp values TS in the highest RTP
session associated with access unit AU(TS).
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Informative Note: Due to packet loss it is possible that
not all sessions may have NAL units present for the media
timestamp value TS present in the highest RTP session. In
such a case an algorithm may:
a) proceed to the next complete access unit with NAL units
present in all the received RTP sessions; or
b) consider a new highest RTP session, the highest RTP
session for which the access unit is complete, and apply
the process above. The algorithm may return to the
original highest RTP session when a complete and error-free
access unit that contains NAL units in all the sessions is
received.
The following gives an informative example.
The example shown in Figure 6 refers to three RTP sessions A, B and
C containing an SVC bitstream transmitted as 3 sources. In the
example, the dependency signaling (described in Section 7.2.3)
indicates that session A is the base RTP session, B is the first
enhancement RTP session and depends on A, and C is the second
enhancement RTP session and depends on A and B. A hierarchical
picture coding prediction structure is used, in which Session A has
the lowest frame rate and Session B and C have the same but higher
frame rate.
The figure shows NAL units contained in RTP packets which are stored
in the de-jittering buffer at the receiver for session de-
packetization. The NAL units are already re-ordered according to
their RTP sequence number order and, if within an aggregation packet,
according to the order of their appearance within the aggregation
packet. The figure indicates for the received NAL units the
decoding order within the sessions, as well as the associated media
(NTP) timestamps ("TS[..]"). NAL units of the same access unit
within a session are grouped by "(.,.)" and share the same media
timestamp TS, which is shown at the bottom of the figure. Note that
the timestamps are not in increasing order since, in this example,
the decoding order is different from the output/display order.
The process first proceeds to the NAL units associated with the
first media timestamp TS[1] present in the highest session C and
removes/ignores all preceding (in decoding order) NAL units to NAL
units with TS[1] in each of the de-jittering buffers of RTP sessions
A, B, and C. Then, starting from session C, the first media
timestamp available in decoding order (TS [1]) is selected and NAL
units starting from RTP session A, and sessions B and C are placed
in order of the RTP session dependency as required by Section 7.2.3
of this memo (in the example for TS[1]: first session B and then
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session C) into the access unit AU(TS[1]) associated with media
timestamp TS[1]. Then the next media timestamp TS[3] in order of
appearance in the highest RTP session C is processed and the process
described above is repeated. Note that there may be access units
with no NAL units present, e.g., in the lowest RTP session A (see,
e.g., TS[1]). With TS[8], the first access unit with NAL units
present in all the RTP sessions appears in the buffers.
C: ------------(1,2)-(3,4)--(5)---(6)---(7,8)(9,10)-(11)--(12)----
| | | | | | | | | |
B: -(1,2)-(3,4)-(5)---(6)--(7,8)-(9,10)-(11)-(12)--(13,14)(15,15)-
| | | | | |
A: -------(1)---------------(2)---(3)---------------(4)----(5)----
---------------------------------------------------decoding order-->
TS: [4] [2] [1] [3] [8] [6] [5] [7] [12] [10]
Key:
A, B, C - RTP sessions
Integer values in "()" - NAL unit decoding order within RTP session
"( )" - groups the NAL units of an access unit
in an RTP session
"|" - indicates corresponding NAL units of the
same access unit AU(TS[..]) in the RTP
sessions
Integer values in "[]" - media timestamp TS, sampling time
as derived, e.g., from NTP timestamp
associated with the access unit AU(TS[..]),
consisting of NAL units in the sessions
above each TS value.
Figure 6 Example of decoding order recovery in multi-source
transmission.
6.2.1.1 Informative Algorithm for NI-T Decoding Order Recovery within
an Access Unit
Within an access unit, the [H.264] specification (Sections 7.4.1.2.3
and G.7.4.1.2.3) constrains the valid decoding order of NAL units.
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These constraints make it possible to reconstruct a valid decoding
order for the NAL units of an access unit based only on the order of
NAL units in each session, the NAL unit headers, and Supplemental
Enhancement Information message headers.
This section specifies an informative algorithm to reconstruct a
valid decoding order for NAL units within an access unit. Other NAL
unit orderings may also be valid; however, any compliant NAL unit
ordering will describe the same video stream and ancillary data as
the one produced by this algorithm.
An actual implementation, of course, needs only to behave "as if"
this reordering is done. In particular, NAL units which are
discarded by an implementation's decoding process do not need to be
reordered.
In this algorithm, NAL units within an access unit are first ordered
by NAL unit type, in the order specified in Table 12 below, except
from NAL unit type 14 which is handled specially as described in the
table. NAL units of the same type are then ordered as specified for
the type, if necessary.
For the purposes of this algorithm, "session order" is the order of
NAL units implied by their transmission order within an RTP session.
For the Non-Interleaved and Single NAL unit modes, this is the RTP
sequence number order coupled with the order of NAL units within an
aggregation unit.
Table 12. Ordering of NAL unit types within in Access Unit
Type Description / Comments
-----------------------------------------------------------
9 Access unit delimiter
7 Sequence parameter set
13 Sequence parameter set extension
15 Subset sequence parameter set
8 Picture parameter set
16-18 Reserved
6 Supplemental enhancement information (SEI)
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If an SEI message with a first payload of 0 (Buffering
Period) is present, it must be the first SEI message.
If SEI messages with a Scalable Nesting (30) payload and
a nested payload of 0 (Buffering Period) are present,
these then follow the first SEI message. Such an SEI
message with the all_layer_representations_in_au_flag
equal to 1 is placed first, followed by any others,
sorted in increasing order of DQId.
All other SEI messages follow in any order.
14 Prefix NAL unit in scalable extension
1 Coded slice of a non-IDR picture
5 Coded slice of an IDR picture
NAL units of type 1 or 5 will be sent within only a
single session for any given access unit. They are
placed in session order. (Note: Any given access unit
will contain only NAL units of type 1 or type 5, not
both.)
If NAL units of type 14 are present, every NAL unit of
type 1 or 5 is prefixed by a NAL unit of type 14. (Note:
Within an access unit, every NAL unit of type 14 is
identical, so correlation of type 14 NAL units with the
other NAL units is not necessary.)
12 Filler data
The only restriction of filler data NAL units within an
access unit is that they shall not precede the first VCL
NAL unit with the same access unit.
19 Coded slice of an auxiliary coded picture without
partitioning
These NAL units will be sent within only a single
session for any given access unit, and are placed in
session order.
20 Coded slice in scalable extension
21-23 Reserved
Type 20 NAL units are placed in increasing order of DQId.
Within each DQId value, they are placed in session order.
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(Note: SVC slices with a given DQId value will be sent
within only a single session for any given access unit.)
Type 21-23 NAL units are placed immediately following
the non-reserved-type VCL NAL unit they follow in
session order.
10 End of sequence
11 End of stream
6.2.2 Decoding Order Recovery for the NI-C, NI-TC and I-C Modes
The following process MUST be used when either the NI-C or I-C MST
packetization mode is in use. The following process MAY be applied
when the NI-TC MST packetization mode is in use.
The RTP packets output from the RTP-level reception processing for
each session are placed into a re-multiplexing buffer.
It is RECOMMENDED to set the size of the re-multiplexing buffer (in
bytes) equal to or greater than the value of the sprop-remux-buf-req
media type parameter of the highest RTP session the receiver
receives.
The CS-DON value is calculated and stored for each NAL unit.
Informative note: The CS-DON value of a NAL unit may rely on
information carried in another packet than the packet
containing the NAL unit. This happens, e.g., when the CS-DON
values need to be derived for non-PACSI NAL units contained in
single NAL unit packets, as the single NAL unit packets
themselves do not contain CS-DON information. In this case,
when no packet containing required CS-DON information is
received for a NAL unit, this NAL unit has to be discarded by
the receiver as it cannot be fed to the decoder in the correct
order. When the optional media type parameter sprop-mst-csdon-
always-present is equal to 1, no such dependency exists, i.e.,
the CS-DON value of any particular NAL unit can be derived
solely according to information in the packet containing the
NAL unit, and therefore, the receiver does not need to discard
any received NAL units.
The receiver operation is described below with the help of the
following functions and constants:
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o Function AbsDON is specified in Section 8.1 of [I-D.ietf-avt-rtp-
rfc3984bis].
o Function don_diff is specified in Section 5.5 of [I-D.ietf-avt-
rtp-rfc3984bis].
o Constant N is the value of the OPTIONAL sprop-mst-remux-buf-size
media type parameter of the highest RTP session incremented by 1.
Initial buffering lasts until one of the following conditions is
fulfilled:
o There are N or more VCL NAL units in the re-multiplexing buffer.
o If sprop-mst-max-don-diff of the highest RTP session is present,
don_diff(m,n) is greater than the value of sprop-mst-max-don-diff
of the highest RTP session, where n corresponds to the NAL unit
having the greatest value of AbsDON among the received NAL units
and m corresponds to the NAL unit having the smallest value of
AbsDON among the received NAL units.
o Initial buffering has lasted for the duration equal to or greater
than the value of the OPTIONAL sprop-remux-init-buf-time media
type parameter of the highest RTP session.
The NAL units to be removed from the re-multiplexing buffer are
determined as follows:
o If the re-multiplexing buffer contains at least N VCL NAL units,
NAL units are removed from the re-multiplexing buffer and passed
to the decoder in the order specified below until the buffer
contains N-1 VCL NAL units.
o If sprop-mst-max-don-diff of the highest RTP session is present,
all NAL units m for which don_diff(m,n) is greater than sprop-
max-don-diff of the highest RTP session are removed from the re-
multiplexing buffer and passed to the decoder in the order
specified below. Herein, n corresponds to the NAL unit having
the greatest value of AbsDON among the NAL units in the re-
multiplexing buffer.
The order in which NAL units are passed to the decoder is specified
as follows:
o Let PDON be a variable that is initialized to 0 at the beginning
of the RTP sessions.
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o For each NAL unit associated with a value of CS-DON, a CS-DON
distance is calculated as follows. If the value of CS-DON of the
NAL unit is larger than the value of PDON, the CS-DON distance is
equal to CS-DON - PDON. Otherwise, the CS-DON distance is equal
to 65535 - PDON + CS-DON + 1.
o NAL units are delivered to the decoder in increasing order of CS-
DON distance. If several NAL units share the same value of CS-
DON distance, they can be passed to the decoder in any order.
o When a desired number of NAL units have been passed to the
decoder, the value of PDON is set to the value of CS-DON for the
last NAL unit passed to the decoder.
7. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features of the
bitstream. The parameters are specified here as part of the media
type registration for the SVC codec. A mapping of the parameters
into the Session Description Protocol (SDP) [RFC4566] is also
provided for applications that use SDP. Equivalent parameters could
be defined elsewhere for use with control protocols that do not use
SDP.
Some parameters provide a receiver with the properties of the stream
that will be sent. The names of all these parameters start with
"sprop" for stream properties. Some of these "sprop" parameters are
limited by other payload or codec configuration parameters. For
example, the sprop-parameter-sets parameter is constrained by the
profile-level-id parameter. The media sender selects all "sprop"
parameters rather than the receiver. This uncommon characteristic
of the "sprop" parameters may be incompatible with some signaling
protocol concepts, in which case the use of these parameters SHOULD
be avoided.
7.1 Media Type Registration
The media subtype for the SVC codec is allocated from the IETF tree.
The receiver MUST ignore any unspecified parameter.
Informative note: Requiring that the receiver ignores unspecified
parameters allows for backward compatibility of future extensions.
For example, if a future specification that is backward
compatible to this specification specifies some new parameters,
then a receiver according to this specification is capable of
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receiving data per the new payload but ignoring those parameters
newly specified in the new payload specification. This provision
is also present in [I-D.ietf-avt-rtp-rfc3984bis].
Media Type name: video
Media subtype name: H264-SVC
Required parameters: none
OPTIONAL parameters:
In the following definitions of parameters, "the stream" or "the
NAL unit stream" refers to all NAL units conveyed in the current
RTP session in SST, and all NAL units conveyed in the current RTP
session and all NAL units conveyed in other RTP sessions that the
current RTP session depends on in MST.
profile-level-id:
A base16 [RFC4648] (hexadecimal) representation of the
following three bytes in the sequence parameter set or subset
sequence parameter set NAL unit specified in [H.264]: 1)
profile_idc, 2) a byte herein referred to as profile-iop,
composed of the values of constraint_set0_flag,
constraint_set1_flag, constraint_set2_flag,
constraint_set3_flag, and reserved_zero_4bits positioned
starting from the most significant bit towards the least
significant bit (bit positions 7 through 4), and 3) level_idc.
Note that reserved_zero_4bits is required to be equal to 0 in
[H.264], but other values for it may be specified in the
future by ITU-T or ISO/IEC.
The profile-level-id parameter indicates the default sub-
profile, i.e., the subset of coding tools that may have been
used to generate the stream or that the receiver supports, and
the default level of the stream or the one that the receiver
supports.
The default sub-profile is indicated collectively by the
profile_idc byte and some fields in the profile-iop byte.
Depending on the values of the fields in the profile-iop byte,
the default sub-profile may be the same set of coding tools
supported by one profile, or a common subset of coding tools
of multiple profiles, as specified in subsection G.7.4.2.1.1
of [H.264]. The default level is indicated by the level_idc
byte, and, when profile_idc is equal to 66, 77 or 88 (the
Baseline, Main, or Extended profile) and level_idc is equal to
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11, additionally by bit 4 (constraint_set3_flag) of the
profile-iop byte. When profile_idc is equal to 66, 77 or 88
(the Baseline, Main, or Extended profile) and level_idc is
equal to 11, and bit 4 (constraint_set3_flag) of the profile-
iop byte is equal to 1, the default level is level 1b.
Table 13 lists all profiles defined in Annex A and Annex G of
[H.264] and, for each of the profiles, the possible
combinations of profile_idc and profile-iop that represent the
same sub-profile.
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Table 13. Combinations of profile_idc and profile-iop
representing the same sub-profile corresponding to the full
set of coding tools supported by one profile. In the
following, x may be either 0 or 1, while the profile names
are indicated as follows. CB: Constrained Baseline profile,
B: Baseline profile, M: Main profile, E: Extended profile,
H: High profile, H10: High 10 profile, H42: High 4:2:2
profile, H44: High 4:4:4 Predictive profile, H10I: High 10
Intra profile, H42I: High 4:2:2 Intra profile, H44I: High
4:4:4 Intra profile, C44I: CAVLC 4:4:4 Intra profile, SB:
Scalable Baseline profile, SH: Scalable High profile, and
SHI: Scalable High Intra profile.
Profile profile_idc profile-iop
(hexadecimal) (binary)
CB 42 (B) x1xx0000
same as: 4D (M) 1xxx0000
same as: 58 (E) 11xx0000
same as: 64 (H), 6E (H10), 1xx00000
7A (H42), or F4 (H44)
B 42 (B) x0xx0000
same as: 58 (E) 10xx0000
M 4D (M) 0x0x0000
same as: 64 (H), 6E (H10), 01000000
7A (H42) or F4 (H44)
E 58 00xx0000
H 64 00000000
H10 6E 00000000
H42 7A 00000000
H44 F4 00000000
H10I 64 00010000
H42I 7A 00010000
H44I F4 00010000
C44I 2C 00010000
SB 53 x0000000
SH 56 0x000000
SHI 56 0x010000
For example, in the table above, profile_idc equal to 58
(Extended) with profile-iop equal to 11xx0000 indicates the
same sub-profile corresponding to profile_idc equal to 42
(Baseline) with profile-iop equal to x1xx0000. Note that
other combinations of profile_idc and profile-iop (not listed
in Table 13) may represent a sub-profile equivalent to the
common subset of coding tools for more than one profile. Note
also that a decoder conforming to a certain profile may be
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able to decode bitstreams conforming to other profiles. For
example, a decoder conforming to the High 4:4:4 profile at
certain level must be able to decode bitstreams confirming to
the Constrained Baseline, Main, High, High 10 or High 4:2:2
profile at the same or a lower level.
If profile-level-id is used to indicate stream properties, it
indicates that, to decode the stream, the minimum subset of
coding tools a decoder has to support is the default sub-
profile, and the lowest level the decoder has to support is
the default level.
If the profile-level-id parameter is used for capability
exchange or session setup, it indicates the subset of coding
tools, which is equal to the default sub-profile, that the
codec supports for both receiving and sending. If max-recv-
level is not present, the default level from profile-level-id
indicates the highest level the codec wishes to support. If
max-recv-level is present it indicates the highest level the
codec supports for receiving. For either receiving or sending,
all levels that are lower than the highest level supported
MUST also be supported.
Informative note: Capability exchange and session setup
procedures should provide means to list the capabilities
for each supported sub-profile separately. For example,
the one-of-N codec selection procedure of the SDP
Offer/Answer model can be used (Section 10.2 of [RFC3264]).
The one-of-N codec selection procedure may also be used to
provide different combinations of profile_idc and profile-
iop that represent the same sub-profile. When there are
many different combinations of profile_idc and profile-iop
that represent the same sub-profile, using the one-of-N
codec selection procedure may result into a fairly large
SDP message. Therefore, a receiver should understand the
different equivalent combinations of profile_idc and
profile-iop that represent the same sub-profile, and be
ready to accept an offer using any of the equivalent
combinations.
If no profile-level-id is present, the Baseline Profile
without additional constraints at Level 1 MUST be implied.
max-recv-level:
This parameter MAY be used to indicate the highest level a
receiver supports when the highest level is higher than the
default level (the level indicated by profile-level-id). The
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value of max-recv-level is a base16 (hexadecimal)
representation of the two bytes after the syntax element
profile_idc in the sequence parameter set NAL unit specified
in [H.264]: profile-iop (as defined above) and level_idc. If
(the level_idc byte of max-recv-level is equal to 11 and bit 4
of the profile-iop byte of max-recv-level is equal to 1) or
(the level_idc byte of max-recv-level is equal to 9 and bit 4
of the profile-iop byte of max-recv-level is equal to 0), the
highest level the receiver supports is level 1b. Otherwise,
the highest level the receiver supports is equal to the
level_idc byte of max-recv-level divided by 10.
max-recv-level MUST NOT be present if the highest level the
receiver supports is not higher than the default level.
max-recv-base-level:
This parameter MAY be used to indicate the highest level a
receiver supports for the base layer when negotiating an SVC
stream. The value of max-recv-base-level is a base16
(hexadecimal) representation of the two bytes after the syntax
element profile_idc in the sequence parameter set NAL unit
specified in [H.264]: profile-iop (as defined above) and
level_idc. If (the level_idc byte of max-recv-level is equal
to 11 and bit 4 of the profile-iop byte of max-recv-level is
equal to 1) or (the level_idc byte of max-recv-level is equal
to 9 and bit 4 of the profile-iop byte of max-recv-level is
equal to 0), the highest level the receiver supports for the
base layer is level 1b. Otherwise, the highest level the
receiver supports for the base layer is equal to the level_idc
byte of max-recv-level divided by 10.
max-mbps, max-fs, max-cpb, max-dpb, and max-br:
The common properties of these parameters are specified in [I-
D.ietf-avt-rtp-rfc3984bis].
max-mbps: This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
max-fs: This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
max-cpb: The value of max-cpb is an integer indicating the
maximum coded picture buffer size in units of 1000 bits for
the VCL HRD parameters (see A.3.1 item i or G.10.2.2 item g of
[H.264]) and in units of 1200 bits for the NAL HRD parameters
(see A.3.1 item j or G.10.2.2 item h of [H.264]). The max-cpb
parameter signals that the receiver has more memory than the
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minimum amount of coded picture buffer memory required by the
signaled highest level conveyed in the value of the profile-
level-id parameter or the max-recv-level parameter. When max-
cpb is signaled, the receiver MUST be able to decode NAL unit
streams that conform to the signaled highest level, with the
exception that the MaxCPB value in Table A-1 of [H.264] for
the signaled highest level is replaced with the value of max-
cpb. The value of max-cpb MUST be greater than or equal to
the value of MaxCPB given in Table A-1 of [H.264] for the
highest level. Senders MAY use this knowledge to construct
coded video streams with greater variation of bit rate than
can be achieved with the MaxCPB value in Table A-1 of [H.264].
Informative note: The coded picture buffer is used in the
Hypothetical Reference Decoder (HRD, Annex C) of [H.264].
The use of the HRD is recommended in SVC encoders to verify
that the produced bitstream conforms to the standard and to
control the output bit rate. Thus, the coded picture
buffer is conceptually independent of any other potential
buffers in the receiver, including de-interleaving, re-
multiplexing and de-jitter buffers. The coded picture
buffer need not be implemented in decoders as specified in
Annex C of [H.264]; standard-compliant decoders can have
any buffering arrangements provided that they can decode
standard-compliant bitstreams. Thus, in practice, the
input buffer for video decoder can be integrated with the
de-interleaving, re-multiplexing and de-jitter buffers of
the receiver.
max-dpb: This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
max-br: The value of max-br is an integer indicating the maximum
video bit rate in units of 1000 bits per second for the VCL
HRD parameters (see A.3.1 item i or G.10.2.2 item g of [H.264])
and in units of 1200 bits per second for the NAL HRD
parameters (see A.3.1 item j or G.10.2.2 item h of [H.264]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bit rate
than is required by the signaled highest level conveyed in the
value of the profile-level-id parameter or the max-recv-level
parameter.
When max-br is signaled, the video codec of the receiver MUST
be able to decode NAL unit streams that conform to the
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signaled highest level, with the following exceptions in the
limits specified by the highest level:
o The value of max-br replaces the MaxBR value in Table A-1
of [H.264] for the highest level.
o When the max-cpb parameter is not present, the result of
the following formula replaces the value of MaxCPB in Table
A-1 of [H.264]: (MaxCPB of the signaled level) * max-br /
(MaxBR of the signaled highest level).
For example, if a receiver signals capability for Level 1.2
with max-br equal to 1550, this indicates a maximum video
bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum
video bitrate of 1860 kbits/sec for NAL HRD parameters, and a
CPB size of 4036458 bits (1550000 / 384000 * 1000 * 1000).
The value of max-br MUST be greater than or equal to the value
MaxBR given in Table A-1 of [H.264] for the signaled highest
level.
Senders MAY use this knowledge to send higher bitrate video as
allowed in the level definition of SVC, to achieve improved
video quality.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. No
assumption can be made from the value of this parameter
that the network is capable of handling such bit rates at
any given time. In particular, no conclusion can be drawn
that the signaled bit rate is possible under congestion
control constraints.
redundant-pic-cap:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
sprop-parameter-sets:
This parameter MAY be used to convey any sequence parameter
set, subset sequence parameter set and picture parameter set
NAL units (herein referred to as the initial parameter set NAL
units) that can be placed in the NAL unit stream to precede
any other NAL units in decoding order and that are associated
with the default level of profile-level-id. The parameter
MUST NOT be used to indicate codec capability in any
capability exchange procedure. The value of the parameter is
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a comma (',') separated list of base64 [RFC4648]
representations of the parameter set NAL units as specified in
Sections 7.3.2.1, 7.3.2.2 and G.7.3.2.1 of [H.264]. Note that
the number of bytes in a parameter set NAL unit is typically
less than 10, but a picture parameter set NAL unit can contain
several hundreds of bytes.
Informative note: When several payload types are offered in
the SDP Offer/Answer model, each with its own sprop-
parameter-sets parameter, then the receiver cannot assume
that those parameter sets do not use conflicting storage
locations (i.e., identical values of parameter set
identifiers). Therefore, a receiver should buffer all
sprop-parameter-sets and make them available to the decoder
instance that decodes a certain payload type.
sprop-level-parameter-sets:
This parameter MAY be used to convey any sequence, subset
sequence and picture parameter set NAL units (herein referred
to as the initial parameter set NAL units) that can be placed
in the NAL unit stream to precede any other NAL units in
decoding order and that are associated with one or more levels
different than the default level of profile-level-id. The
parameter MUST NOT be used to indicate codec capability in any
capability exchange procedure.
The sprop-level-parameter-sets parameter contains parameter
sets for one or more levels which are different than the
default level. All parameter sets targeted for use when one
level of the default sub-profile is accepted by a receiver are
clustered and prefixed with a three-byte field which has the
same syntax as profile-level-id. This enables the receiver to
install the parameter sets for the accepted level and discard
the rest. The three-byte field is named PLId, and all
parameter sets associated with one level are named PSL, which
has the same syntax as sprop-parameter-sets. Parameter sets
for each level are represented in the form of PLId:PSL, i.e.,
PLId followed by a colon (':') and the base64 [RFC4648]
representation of the initial parameter set NAL units for the
level. Each pair of PLId:PSL is also separated by a colon.
Note that a PSL can contain multiple parameter sets for that
level, separated with commas (',').
The subset of coding tools indicated by each PLId field MUST
be equal to the default sub-profile, and the level indicated
by each PLId field MUST be different than the default level.
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Informative note: This parameter allows for efficient level
downgrade or upgrade in SDP Offer/Answer and out-of-band
transport of parameter sets, simultaneously.
in-band-parameter-sets:
This parameter MAY be used to indicate a receiver capability.
The value MAY be equal to either 0 or 1. The value 1
indicates that the receiver discards out-of-band parameter
sets in sprop-parameter-sets and sprop-level-parameter-sets,
therefore the sender MUST transmit all parameter sets in-band.
The value 0 indicates that the receiver utilizes out-of-band
parameter sets included in sprop-parameter-sets and/or sprop-
level-parameter-sets. However, in this case, the sender MAY
still choose to send parameter sets in-band. When the
parameter is not present, this receiver capability is not
specified, and therefore the sender MAY send out-of-band
parameter sets only, or it MAY send in-band-parameter-sets
only, or it MAY send both.
packetization-mode:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis]. When the mst-mode parameter is present, the
value of this parameter is additionally constrained as follows.
If mst-mode is equal to "NI-T", "NI-C" or "NI-TC",
packetization-mode MUST NOT be equal to 2. Otherwise (mst-
mode is equal to "I-C"), packetization-mode MUST be equal to 2.
sprop-interleaving-depth:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
sprop-deint-buf-req:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
deint-buf-cap:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
sprop-init-buf-time:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
sprop-max-don-diff:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
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max-rcmd-nalu-size:
This parameter is as specified in [I-D.ietf-avt-rtp-
rfc3984bis].
mst-mode:
This parameter MAY be used to signal the properties of a NAL
unit stream or the capabilities of a receiver implementation.
If this parameter is present, multi-session transmission MUST
be used. Otherwise (this parameter is not present), single-
session transmission MUST be used. When this parameter is
present, the following applies. When the value of mst-mode is
equal to "NI-T", the NI-T mode MUST be used. When the value
of mst-mode is equal to "NI-C", the NI-C mode MUST be used.
When the value of mst-mode is equal to "NI-TC", the NI-TC mode
MUST be used. When the value of mst-mode is equal to "I-C",
the I-C mode MUST be used. The value of mst-mode MUST have
one of the following tokens: "NI-T", "NI-C", "NI-TC", or "I-C".
All RTP sessions in an MST MUST have the same value of mst-
mode.
sprop-mst-csdon-always-present:
This parameter MUST NOT be present when mst-mode is not
present or the value of mst-mode is equal to "NI-T" or "I-C".
This parameter signals the properties of the NAL unit stream.
When sprop-mst-csdon-always-present is present and the value
is equal to 1, packetization-mode MUST be equal to 1, and all
the RTP packets carrying the NAL unit stream MUST be STAP-A
packets containing a PACSI NAL unit that further contains the
DONC field or NI-MTAP packets with the J field equal to 1.
When sprop-mst-csdon-always-present is present and the value
is equal to 1, the CS-DON value of any particular NAL unit can
be derived solely according to information in the packet
containing the NAL unit.
When sprop-mst-csdon-always-present is present in the current
RTP session, it MUST be present also in all the RTP sessions
the current RTP session depends on and the value of sprop-mst-
csdon-always-present is identical for the current RTP session
and all the RTP sessions the current RTP session depends on.
sprop-mst-remux-buf-size:
This parameter MUST NOT be present when mst-mode is not
present or the value of mst-mode is equal to "NI-T". This
parameter MUST be present when mst-mode is present and the
value of mst-mode is equal to "NI-C", "NI-TC", or "I-C".
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This parameter signals the properties of the NAL unit stream.
It MUST be set to a value one less than the minimum re-
multiplexing buffer size (in NAL units), so that it is
guaranteed that receivers can reconstruct NAL unit decoding
order as specified in Subsection 6.2.2.
The value of sprop-mst-remux-buf-size MUST be an integer in
the range of 0 to 32767, inclusive.
sprop-remux-buf-req:
This parameter MUST NOT be present when mst-mode is not
present or the value of mst-mode is equal to "NI-T". It MUST
be present when mst-mode is present and the value of mst-mode
is equal to "NI-C", "NI-TC", or "I-C".
sprop-remux-buf-req signals the required size of the re-
multiplexing buffer for the NAL unit stream. It is guaranteed
that receivers can recover the decoding order of the received
NAL units from the current RTP session and the RTP sessions
the current RTP session depends on as specified in Section
6.2.2, when the re-multiplexing buffer size is of at least the
value of sprop-remux-buf-req in units of bytes.
The value of sprop-remux-buf-req MUST be an integer in the
range of 0 to 4294967295, inclusive.
remux-buf-cap:
This parameter MUST NOT be present when mst-mode is not
present or the value of mst-mode is equal to "NI-T". This
parameter MAY be used to signal the capabilities of a receiver
implementation and indicates the amount of re-multiplexing
buffer space in units of bytes that the receiver has available
for recovering the NAL unit decoding order as specified in
Section 6.2.2. A receiver is able to handle any NAL unit
stream for which the value of the sprop-remux-buf-req
parameter is smaller than or equal to this parameter.
If the parameter is not present, then a value of 0 MUST be
used for remux-buf-cap. The value of remux-buf-cap MUST be an
integer in the range of 0 to 4294967295, inclusive.
sprop-remux-init-buf-time:
This parameter MAY be used to signal the properties of the NAL
unit stream. The parameter MUST NOT be present if mst-mode is
not present or the value of mst-mode is equal to "NI-T".
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The parameter signals the initial buffering time that a
receiver MUST wait before starting to recover the NAL unit
decoding order as specified in Section 6.2.2 of this memo.
The parameter is coded as a non-negative base10 integer
representation in clock ticks of a 90-kHz clock. If the
parameter is not present, then no initial buffering time value
is defined. Otherwise the value of sprop-remux-init-buf-time
MUST be an integer in the range of 0 to 4294967295, inclusive.
sprop-mst-max-don-diff:
This parameter MAY be used to signal the properties of the NAL
unit stream. It MUST NOT be used to signal transmitter or
receiver or codec capabilities. The parameter MUST NOT be
present if mst-mode is not present or the value of mst-mode is
equal to "NI-T". sprop-mst-max-don-diff is an integer in the
range of 0 to 32767, inclusive. If sprop-mst-max-don-diff is
not present, the value of the parameter is unspecified.
sprop-mst-max-don-diff is calculated same as sprop-max-don-
diff as specified in [I-D.ietf-avt-rtp-rfc3984bis], with
decoding order number being replaced by cross-session decoding
order number.
sprop-scalability-info:
This parameter MAY be used to convey the NAL unit containing
the scalability information SEI message as specified in Annex
G of [H.264]. This parameter MAY be used to signal the
contained layers of an SVC bitstream. The parameter MUST NOT
be used to indicate codec capability in any capability
exchange procedure. The value of the parameter is the base64
[RFC4648] representation of the NAL unit containing the
scalability information SEI message. If present, the NAL unit
MUST contain only one SEI message which is a scalability
information SEI message.
This parameter MAY be used in an offering or declarative SDP
message to indicate what layers (operation points) can be
provided. A receiver MAY indicate its choice of one layer
using the optional media type parameter scalable-layer-id.
scalable-layer-id:
This parameter MAY be used to signal a receiver's choice of
the offers or declared operation points or layers using sprop-
scalability-info or sprop-operation-point-info. The value of
scalable-layer-id is a base16 representation of the
layer_id[ i ] syntax element in the scalability information
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SEI message as specified in Annex G of [H.264] or layer-ID
contained in sprop-operation-point-info.
sprop-operation-point-info:
This parameter MAY be used to describe the operation points of
an RTP session. The value of this parameter consists of a
comma-separated list of operation-point-description vectors.
The values given by the operation-point-description vectors
are the same as, or are derived from, the values that would be
given for a scalable layer in the scalability information SEI
message as specified in Annex G of [H.264], where the term
scalable layer in the scalability information SEI message
refers to all NAL units associated with the same values of
temporal_id, dependency_id and quality_id. In this memo such
a set of NAL units is called an operation point.
Each operation-point-description vector has ten elements,
provided as a comma-separated list of values as defined below.
The first value of the operation-point-description vector is
preceded by a '<' and the last value of the operation-point-
description vector is followed by a '>'. If the sprop-
operation-point-info is followed by exactly one operation-
point-description vector, this describes the highest operation
point contained in the RTP session. If there are two or more
operation-point-description vectors, the first describes the
lowest and the last describes the highest operation point
contained in the RTP session.
The values given by the operation-point-description vector are
as follows, in the order listed:
- layer-ID: This value specifies the layer identifier of the
operation point, which is identical to the layer_id that would
be indicated (for the same values of dependency_id, quality_id,
and temporal_id) in the scalability information SEI message.
This field MAY be empty, indicating that the value is
unspecified. When there are multiple operation-point-
description vectors with layer-ID, the values of layer-ID do
not need to be consecutive.
- temporal-ID: This value specifies the temporal_id of the
operation point. This field MUST NOT be empty.
- dependency-ID: This values specifies the dependency_id of
the operation point. This field MUST NOT be empty.
- quality-ID: This values specifies the quality_id of the
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operation point. This field MUST NOT be empty.
- profile-level-ID: This value specifies the profile-level-idc
of the operation point in the base16 format. The default sub-
profile or default level indicated by the parameter profile-
level-ID in the sprop-operation-point-info vector SHALL be
equal to or lower than the default sub-profile or default
level indicated by profile-level-id, which may be either
present or the default value is taken. This field MAY be
empty, indicating that the value is unspecified.
- avg-framerate: This value specifies the average frame rate
of the operation point. This value is given as an integer in
frames per 256 seconds. The field MAY be empty, indicating
that the value is unspecified.
- width: This value specifies the width dimension in pixels of
decoded frames for the operation point. This parameter is not
directly given in the scalability information SEI message.
This field MAY be empty, indicating that the value is
unspecified.
- height: This value gives the height dimension in pixels of
decoded frames for the operation point. This parameter is not
directly given in the scalability information SEI. This field
MAY be empty, indicating that the value is unspecified.
- avg-bitrate: This value specifies the average bit rate of
the operation point. This parameter is given as an integer in
kbits per second over the entire stream. Note that this
parameter is provided in the scalability information SEI
message in bits per second and calculated over a variable time
window. This field MAY be empty, indicating that the value is
unspecified.
- max-bitrate: This value specifies the maximum bit rate of
the operation point. This parameter is given as an integer in
kbits per second and describes the maximum bitrate per each
one second window. Note that this parameter is provided in
the scalability information SEI message in bits per second and
is calculated over a variable time window. This field MAY be
empty, indicating that the value is unspecified.
Similarly to sprop-scalability-info, this parameter MAY be
used in an offering or declarative SDP message to indicate
what layers (operation points) can be provided. A receiver
MAY indicate its choice of the highest layer it wants to send
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and/or receive using the optional media type parameter
scalable-layer-id.
sprop-no-NAL-reordering-required:
This parameter MAY be used to signal the properties of the NAL
unit stream. This parameter MUST NOT be present when mst-mode
is not present or the value of mst-mode is not equal to "NI-T".
The presence of this parameters indicates that no reordering
of non-VCL or VCL NAL units is required for the decoding order
recovery process.
sprop-avc-ready:
This parameter MAY be used to indicate the properties of the
NAL unit stream. The presence of this parameter indicates
that the RTP session, if used in SST, or used in MST combined
with other RTP sessions also with this parameter present, can
be processed by a [I-D.ietf-avt-rtp-rfc3984bis] receiver.
This parameter MAY be used with RTP sessions with media
subtype H264-SVC.
Encoding considerations:
This media type is framed and binary; see Section 4.8 of RFC
4288 [RFC4288].
Security considerations:
See Section 8 of RFC XXXX.
Published specification:
Please refer to Section 13 of RFC XXXX.
Additional information:
None
File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Ye-Kui Wang, yekuiwang@huawei.com
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
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defined for transfer via RTP [RFC3550]. Transport within
other framing protocols is not defined at this time.
Interoperability considerations:
The media subtype name contains "SVC" to avoid potential
conflict with RFC 3984 and its potential future replacement
RTP payload format for H.264 non-SVC profiles.
Applications that use this media type:
Real-time video applications like video streaming, video
telephony, and video conferencing.
Author:
Ye-Kui Wang, yekuiwang@huawei.com
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
7.2 SDP Parameters
7.2.1 Mapping of Payload Type Parameters to SDP
The media type video/H264-SVC string is mapped to fields in the
Session Description Protocol (SDP) 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 H264-SVC
(the media subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The OPTIONAL parameters "profile-level-id", "max-recv-level",
"max-recv-base-level", "max-mbps", "max-fs", "max-cpb", "max-dpb",
"max-br", "redundant-pic-cap", "in-band-parameter-sets",
"packetization-mode", "sprop-interleaving-depth", "deint-buf-cap",
"sprop-deint-buf-req", "sprop-init-buf-time", "sprop-max-don-
diff", "max-rcmd-nalu-size", "mst-mode", "sprop-mst-csdon-always-
present", "sprop-mst-remux-buf-size", "sprop-remux-buf-req",
"remux-buf-cap", "sprop-remux-init-buf-time", "sprop-mst-max-don-
diff", and "scalable-layer-id", when present, MUST be included
in the "a=fmtp" line of SDP. These parameters are expressed as a
media type string, in the form of a semicolon separated list of
parameter=value pairs.
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o The OPTIONAL parameters "sprop-parameter-sets", "sprop-level-
parameter-sets", "sprop-scalability-info", "sprop-operation-
point-info", "sprop-no-NAL-reordering-required", and "sprop-avc-
ready", when present, MUST be included in the "a=fmtp" line of
SDP or conveyed using the "fmtp" source attribute as specified in
Section 6.3 of [RFC5576]. For a particular media format (i.e.,
RTP payload type), a "sprop-parameter-sets" or "sprop-level-
parameter-sets" MUST NOT be both included in the "a=fmtp" line of
SDP and conveyed using the "fmtp" source attribute. When
included in the "a=fmtp" line of SDP, these parameters are
expressed as a media type string, in the form of a semicolon
separated list of parameter=value pairs. When conveyed using the
"fmtp" source attribute, these parameters are only associated
with the given source and payload type as parts of the "fmtp"
source attribute.
Informative note: Conveyance of "sprop-parameter-sets" and
"sprop-level-parameter-sets" using the "fmtp" source attribute
allows for out-of-band transport of parameter sets in
topologies like Topo-Video-switch-MCU [RFC5117].
7.2.2 Usage with the SDP Offer/Answer Model
When an SVC stream (with media subtype H264-SVC) is offered over RTP
using SDP in an Offer/Answer model [RFC3264] for negotiation for
unicast usage, the following limitations and rules apply:
o The parameters identifying a media format configuration for SVC
are "profile-level-id", "packetization-mode", and "mst-mode".
These media configuration parameters (except for the level part
of "profile-level-id") MUST be used symmetrically when the
answerer does not include "scalable-layer-id" in the answer; i.e.,
the answerer MUST either maintain all configuration parameters or
remove the media format (payload type) completely, if one or more
of the parameter values are not supported. Note that the level
part of "profile-level-id" includes level_idc, and, for
indication of level 1b when profile_idc is equal to 66, 77 or 88,
bit 4 (constraint_set3_flag) of profile-iop. The level part of
"profile-level-id" is changeable.
Informative note: The requirement for symmetric use does not
apply for the level part of "profile-level-id", and does not
apply for the other stream properties and capability
parameters.
Informative note: In [H.264], all the levels except for level
1b are equal to the value of level_idc divided by 10. Level
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1b is a level higher than level 1.0 but lower than level 1.1,
and is signaled in an ad-hoc manner. For the Baseline, Main
and Extended profiles (with profile_idc equal to 66, 77 and 88,
respectively), level 1b is indicated by level_idc equal to 11
(i.e. same as level 1.1) and constraint_set3_flag equal to 1.
For other profiles, level 1b is indicated by level_idc equal
to 9 (but note that level 1b for these profiles are still
higher than level 1, which has level_idc equal to 10, and
lower than level 1.1). In SDP Offer/Answer, an answer may
indicate a level equal to or lower than the level indicated in
the offer. Due to the ad-hoc indication of level 1b, offerers
and answerers must check the value of bit 4
(constraint_set3_flag) of the middle octet of the parameter
"profile-level-id", when profile_idc is equal to 66, 77 or 88
and level_idc is equal to 11.
To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer should also be
used in the answer, as specified in [RFC3264]. The same RTP
payload type number used in the offer MUST also be used in the
answer when the answer includes "scalable-layer-id". When the
answer does not include "scalable-layer-id", the answer MUST NOT
contain a payload type number used in the offer unless the
configuration is exactly the same as in the offer or the
configuration in the answer only differs from that in the offer
with a level lower than the default level offered.
Informative note: When an offerer receives an answer that does
not include "scalable-layer-id" it has to compare payload
types not declared in the offer based on the media type (i.e.,
video/H264-SVC) and the above media configuration parameters
with any payload types it has already declared. This will
enable it to determine whether the configuration in question
is new or if it is equivalent to configuration already offered,
since a different payload type number may be used in the
answer.
Since an SVC stream may contain multiple operation points, a
facility is provided so that an answerer can select a different
operation point than the entire SVC stream. Specifically,
different operation points MAY be described using the "sprop-
scalability-info" or "sprop-operation-point-info" parameters.
The first one carries the entire scalability information SEI
message defined in Annex G of [H.264], whereas the second one may
be derived, e.g. as a subset of this SEI message that only
contains key information about an operation point. Operation
points, in both cases, are associated with a layer identifier.
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If such information ("sprop-operation-point-info" or "sprop-
scalability-info") is provided in an offer, an answerer MAY
select from the various operation points offered in the "sprop-
scalability-information" or "sprop-operation-point-info"
parameters by including "scalable-layer-id" in the answer. By
this, the answerer indicates its selection of a particular
operation point in the received and/or in the sent stream. When
such operation point selection takes place, i.e., the answerer
includes "scalable-layer-id" in the answer, the media
configuration parameters MUST NOT be present in the answer.
Rather, the media configuration that the answerer will use for
receiving and/or sending is the one used for the selected
operation point as indicated in the offer.
Informative note: The ability to perform operation point
selection enables a receiver to utilize the scalable nature of
an SVC stream.
o The parameter "max-recv-level", when present, declares the
highest level supported for receiving. In case "max-recv-level"
is not present, the highest level supported for receiving is
equal to the default level indicated by the level part of
"profile-level-id". "max-recv-level", when present, MUST be
higher than the default level.
o The parameter "max-recv-base-level", when present, declares the
highest level of the base layer supported for receiving. When
"max-recv-base-level" is not present, the highest level supported
for the base layer is not constrained separately from the SVC
stream containing the base layer. The endpoint at the other side
MUST NOT send a scalable stream for which the base layer is of a
level higher than max-recv-base-level. Parameters declaring
receiver capabilities above the default level (max-mbps, max-
smbps, max-fs, max-cpb, max-dpb, max-br, and max-recv-level) do
not apply to the base layer when max-recv-base-level is present.
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o The parameters "sprop-deint-buf-req", "sprop-interleaving-depth",
"sprop-max-don-diff", "sprop-init-buf-time", "sprop-mst-csdon-
always-present", "sprop-remux-buf-req", "sprop-mst-remux-buf-
size", "sprop-remux-init-buf-time", "sprop-mst-max-don-diff",
"sprop-scalability-information", "sprop-operation-point-info",
"sprop-no-NAL-reordering-required", and "sprop-avc-ready"
describe the properties of the NAL unit stream that the offerer
or answerer is sending for the media format configuration. This
differs from the normal usage of the Offer/Answer parameters:
normally such parameters declare the properties of the stream
that the offerer or the answerer is able to receive. When
dealing with SVC, the offerer assumes that the answerer will be
able to receive media encoded using the configuration being
offered.
Informative note: The above parameters apply for any stream
sent by the declaring entity with the same configuration; i.e.,
they are dependent on their source. Rather than being bound
to the payload type, the values may have to be applied to
another payload type when being sent, as they apply for the
configuration.
o The capability parameters "max-mbps", "max-fs", "max-cpb", "max-
dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size" MAY be
used to declare further capabilities of the offerer or answerer
for receiving. These parameters MUST NOT be present when the
direction attribute is sendonly, and the parameters describe the
limitations of what the offerer or answerer accepts for receiving
streams.
o When "mst-mode" is not present and "packetization-mode" is equal
to 2, the following applies.
o An offerer has to include the size of the de-interleaving
buffer, "sprop-deint-buf-req", in the offer. To enable the
offerer and answerer to inform each other about their
capabilities for de-interleaving buffering, both parties are
RECOMMENDED to include "deint-buf-cap". It is also
RECOMMENDED to consider offering multiple payload types with
different buffering requirements when the capabilities of the
receiver are unknown.
o When "mst-mode" is present and equal to "NI-C", "NI-TC" or "I-C",
the following applies.
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o An offerer has to include "sprop-remux-buf-req" in the offer.
To enable the offerer and answerer to inform each other about
their capabilities for re-multiplexing buffering, both
parties are RECOMMENDED to include "remux-buf-cap". It is
also RECOMMENDED to consider offering multiple payload types
with different buffering requirements when the capabilities
of the receiver are unknown.
o The "sprop-parameter-sets" or "sprop-level-parameter-sets"
parameter, when present (included in the "a=fmtp" line of SDP or
conveyed using the "fmtp" source attribute as specified in
Section 6.3 of [RFC5576]), is used for out-of-band transport of
parameter sets. However, when out-of-band transport of parameter
sets is used, parameter sets MAY still be additionally
transported in-band.
The answerer MAY use either out-of-band or in-band transport of
parameter sets for the stream it is sending, regardless of
whether out-of-band parameter sets transport has been used in the
offerer-to-answerer direction. Parameter sets included in an
answer are independent of those parameter sets included in the
offer, as they are used for decoding two different video streams,
one from the answerer to the offerer, and the other in the
opposite direction.
The following rules apply to transport of parameter sets in the
offerer-to-answerer direction.
o An offer MAY include either or both of "sprop-parameter-
sets" and "sprop-level-parameter-sets". If neither "sprop-
parameter-sets" nor "sprop-level-parameter-sets" is present
in the offer, then only in-band transport of parameter sets
is used.
o If the answer includes "in-band-parameter-sets" equal to 1,
then the offerer MUST transmit parameter sets in-band.
Otherwise, the following applies.
o If the level to use in the offerer-to-answerer
direction is equal to the default level in the offer,
the following applies.
The answerer MUST be prepared to use the parameter
sets included in "sprop-parameter-sets", when
present, for decoding the incoming NAL unit stream,
and ignore "sprop-level-parameter-sets", when
present.
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When "sprop-parameter-sets" is not present in the
offer, in-band transport of parameter sets MUST be
used.
o Otherwise (the level to use in the offerer-to-answerer
direction is not equal to the default level in the
offer), the following applies.
The answerer MUST be prepared to use the parameter
sets that are included in "sprop-level-parameter-
sets" for the accepted level (i.e., the default
level in the answer, which is also the level to
use in the offerer-to-answerer direction), when
present, for decoding the incoming NAL unit stream,
and ignore all other parameter sets included in
"sprop-level-parameter-sets" and "sprop-parameter-
sets", when present.
When no parameter sets for the accepted level are
present in the "sprop-level-parameter-sets", in-
band transport of parameter sets MUST be used.
The following rules apply to transport of parameter sets in the
answerer-to-offerer direction.
o An answer MAY include either "sprop-parameter-sets" or
"sprop-level-parameter-sets", but MUST NOT include both of
the two. If neither "sprop-parameter-sets" nor "sprop-
level-parameter-sets" is present in the answer, then only
in-band transport of parameter sets is used.
o If the offer includes "in-band-parameter-sets" equal to 1,
then the answerer MUST NOT include "sprop-parameter-sets" or
"sprop-level-parameter-sets" in the answer and MUST transmit
parameter sets in-band. Otherwise, the following applies.
o If the level to use in the answerer-to-offerer
direction is equal to the default level in the answer,
the following applies.
The offerer MUST be prepared to use the parameter
sets included in "sprop-parameter-sets", when
present, for decoding the incoming NAL unit stream,
and ignore "sprop-level-parameter-sets", when
present.
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When "sprop-parameter-sets" is not present in the
answer, the answerer MUST transmit parameter sets
in-band.
o Otherwise (the level to use in the answerer-to-offerer
direction is not equal to the default level in the
answer), the following applies.
The offerer MUST be prepared to use the parameter
sets that are included in "sprop-level-parameter-
sets" for the level to use in the answerer-to-
offerer direction, when present in the answer, for
decoding the incoming NAL unit stream, and ignore
all other parameter sets included in "sprop-level-
parameter-sets" and "sprop-parameter-sets", when
present in the answer.
When no parameter sets for the level to use in the
answerer-to-offerer direction are present in
"sprop-level-parameter-sets" in the answer, the
answerer MUST transmit parameter sets in-band.
When "sprop-parameter-sets" or "sprop-level-parameter-sets" is
conveyed using the "fmtp" source attribute as specified in
Section 6.3 of [RFC5576], the receiver of the parameters MUST
store the parameter sets included in the "sprop-parameter-sets"
or "sprop-level-parameter-sets" for the accepted level and
associate them to the source given as a part of the "fmtp" source
attribute. Parameter sets associated with one source MUST only
be used to decode NAL units conveyed in RTP packets from the same
source. When this mechanism is in use, SSRC collision detection
and resolution MUST be performed as specified in [RFC5576].
Informative note: Conveyance of "sprop-parameter-sets" and
"sprop-level-parameter-sets" using the "fmtp" source attribute
may be used in topologies like Topo-Video-switch-MCU [RFC5117]
to enable out-of-band transport of parameter sets.
For streams being delivered over multicast, the following rules
apply:
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o The media format configuration is identified by "profile-level-
id", including the level part, "packetization-mode", and "mst-
mode". These media format configuration parameters (including
the level part of "profile-level-id") MUST be used symmetrically;
i.e., the answerer MUST either maintain all configuration
parameters or remove the media format (payload type) completely.
Note that this implies that the level part of "profile-level-id"
for Offer/Answer in multicast is not changeable.
To simplify handling and matching of these configurations, the
same RTP payload type number used in the offer should also be
used in the answer, as specified in [RFC3264]. An answer MUST
NOT contain a payload type number used in the offer unless the
configuration is the same as in the offer.
o Parameter sets received MUST be associated with the originating
source, and MUST be only used in decoding the incoming NAL unit
stream from the same source.
o The rules for other parameters are the same as above for unicast
as long as the above rules are obeyed.
Table 14 lists the interpretation of all the parameters that MUST be
used for the various combinations of offer, answer, and direction
attributes. Note that the two columns wherein the "scalable-layer-
id" parameter is used only apply to answers, whereas the other
columns apply to both offers and answers.
Table 14. Interpretation of parameters for various combinations of
offers, answers, direction attributes, with and without scalable-
layer-id. Columns that do not indicate offer or answer apply to
both.
sendonly --+
answer: recvonly,scalable-layer-id --+ |
recvonly w/o scalable-layer-id --+ | |
answer: sendrecv, scalable-layer-id --+ | | |
sendrecv w/o scalable-layer-id --+ | | | |
| | | | |
profile-level-id C X C X P
max-recv-level R R R R -
max-recv-base-level R R R R -
packetization-mode C X C X P
mst-mode C X C X P
sprop-avc-ready P P - - P
sprop-deint-buf-req P P - - P
sprop-init-buf-time P P - - P
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sprop-interleaving-depth P P - - P
sprop-max-don-diff P P - - P
sprop-mst-csdon-always-present P P - - P
sprop-mst-max-don-diff P P - - P
sprop-mst-remux-buf-size P P - - P
sprop-no-NAL-reordering-required P P - - P
sprop-operation-point-info P P - - P
sprop-remux-buf-req P P - - P
sprop-remux-init-buf-time P P - - P
sprop-scalability-info P P - - P
deint-buf-cap R R R R -
max-br R R R R -
max-cpb R R R R -
max-dpb R R R R -
max-fs R R R R -
max-mbps R R R R -
max-rcmd-nalu-size R R R R -
redundant-pic-cap R R R R -
remux-buf-cap R R R R -
in-band-parameter-sets R R R R -
sprop-parameter-sets S S - - S
sprop-level-parameter-sets S S - - S
scalable-layer-id X O X O -
Legend:
C: configuration for sending and receiving streams
P: properties of the stream to be sent
R: receiver capabilities
S: out-of-band parameter sets
O: operation point selection
X: MUST NOT be present
-: not usable, when present SHOULD be ignored
Parameters used for declaring receiver capabilities are in general
downgradable; i.e., they express the upper limit for a sender's
possible behavior. Thus a sender MAY select to set its encoder
using only lower/lesser or equal values of these parameters.
Parameters declaring a configuration point are not changeable, with
the exception of the level part of the "profile-level-id" parameter
for unicast usage. This expresses values a receiver expects to be
used and must be used verbatim on the sender side. If level
downgrading (for profile-level-id) is used, an answerer MUST NOT
include the scalable-layer-id parameter.
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When a sender's capabilities are declared, and non-downgradable
parameters are used in this declaration, then these parameters
express a configuration that is acceptable for the sender to receive
streams. In order to achieve high interoperability levels, it is
often advisable to offer multiple alternative configurations; e.g.,
for the packetization mode. It is impossible to offer multiple
configurations in a single payload type. Thus, when multiple
configuration offers are made, each offer requires its own RTP
payload type associated with the offer.
A receiver SHOULD understand all media type parameters, even if it
only supports a subset of the payload format's functionality. This
ensures that a receiver is capable of understanding when an offer to
receive media can be downgraded to what is supported by the receiver
of the offer.
An answerer MAY extend the offer with additional media format
configurations. However, to enable their usage, in most cases a
second offer is required from the offerer to provide the stream
property parameters that the media sender will use. This also has
the effect that the offerer has to be able to receive this media
format configuration, not only to send it.
If an offerer wishes to have non-symmetric capabilities between
sending and receiving, the offerer can allow asymmetric levels via
"level-asymmetry-allowed" equal to 1. Alternatively, the offerer
can offer different RTP sessions; i.e., different media lines
declared as "recvonly" and "sendonly", respectively. This may have
further implications on the system, and may require additional
external semantics to associate the two media lines.
7.2.3 Dependency Signaling in Multi-Session Transmission
If MST is used, the rules on signaling media decoding dependency in
SDP as defined in [RFC5583] apply. The rules on "hierarchical or
layered encoding" with multicast in Section 5.7 of [RFC4566] do not
apply, i.e., the notation for Connection Data "c=" SHALL NOT be used
with more than one address. Additionally, the order of dependencies
of the RTP sessions indicated by the "a=depend" attribute as defined
in [RFC5583] MUST represent the decoding order of the VC) NAL units
in an access unit, i.e., the order of session dependency is given
from the base or the lowest enhancement RTP session (the most
important) to the highest enhancement RTP session (the least
important).
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7.2.4 Usage in Declarative Session Descriptions
When SVC over RTP is offered with SDP in a declarative style, as in
RTSP [RFC2326] or SAP [RFC2974], the following considerations are
necessary.
o All parameters capable of indicating both stream properties and
receiver capabilities are used to indicate only stream properties.
For example, in this case, the parameter "profile-level-id"
declares the values used by the stream, not the capabilities for
receiving streams. This results in that the following
interpretation of the parameters MUST be used:
Declaring actual configuration or stream properties:
- profile-level-id
- packetization-mode
- mst-mode
- sprop-deint-buf-req
- sprop-interleaving-depth
- sprop-max-don-diff
- sprop-init-buf-time
- sprop-mst-csdon-always-present
- sprop-mst-remux-buf-size
- sprop-remux-buf-req
- sprop-remux-init-buf-time
- sprop-mst-max-don-diff
- sprop-scalability-info
- sprop-operation-point-info
- sprop-no-NAL-reordering-required
- sprop-avc-ready
Out-of-band transporting of parameter sets:
- sprop-parameter-sets
- sprop-level-parameter-sets
Not usable (when present, they SHOULD be ignored):
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- max-recv-level
- max-recv-base-level
- redundant-pic-cap
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- max-rcmd-nalu-size
- deint-buf-cap
- remux-buf-cap
- scalable-layer-id
o A receiver of the SDP is required to support all parameters and
values of the parameters provided; otherwise, the receiver MUST
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.
7.3 Examples
In the following examples, "{data}" is used to indicate a data
string encoded as base64.
7.3.1 Example for Offering a Single SVC Session
Example 1: The offerer offers one video media description including
two RTP payload types. The first payload type offers H264 and the
second offers H264-SVC. Both payload types have different fmtp
parameters as profile-level-id, packetization-mode, and sprop-
parameter-sets.
Offerer -> Answerer SDP message:
m=video 20000 RTP/AVP 97 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
sprop-parameter-sets={sps0},{pps0};
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
sprop-parameter-sets={sps0},{pps0},{sps1},{pps1};
If the answerer does not support media subtype H264-SVC, it can
issue an answer accepting only the base layer offer (payload type
96). In the following example the receiver supports H264-SVC, so it
lists payload type 97 first as the preferred option.
Answerer -> Offerer SDP message:
m=video 40000 RTP/AVP 97 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
sprop-parameter-sets={sps2},{pps2};
a=rtpmap:97 H264-SVC/90000
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a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
sprop-parameter-sets={sps2},{pps2},{sps3},{pps3};
7.3.2 Example for Offering a Single SVC Session using scalable-layer-id
Example 2: Offerer offers the same media configurations as shown in
the example above for receiving and sending the stream, but using a
single RTP payload type and including sprop-operation-point-info.
Offerer -> Answerer SDP message:
m=video 20000 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
sprop-parameter-sets={sps0},{sps1},{pps0},{pps1};
sprop-operation-point-info=<1,0,0,0,4d400a,3200,176,144,128,
256>,<2,1,1,0,53000c,6400,352,288,256,512>;
In this example the receiver supports H264-SVC and chooses the lower
operation point offered in the RTP payload type for sending and
receiving the stream.
Answerer -> Offerer SDP message:
m=video 40000 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 sprop-parameter-sets={sps2},{sps3},{pps2},{pps3};
scalable-layer-id=1;
In an equivalent example showing the use of sprop-scalabilty-info
instead using the sprop-operation-point-info, the sprop-operation-
point-info would be exchanged by the sprop-scalability-info followed
by the binary (base16) representation of the Scalability Information
SEI message.
7.3.3 Example for Offering Multiple Sessions in MST
Example 3: In this example the offerer offers a multi-session
transmission with up to three sessions. The base session media
description includes payload types which are backward compatible
with [I-D.ietf-avt-rtp-rfc3984bis], and three different payload
types are offered. The other two media are using payload types with
media subtype H264-SVC. In each media description different values
of profile-level-id, packetization-mode, mst-mode, and sprop-
parameter-sets are offered.
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Offerer -> Answerer SDP message:
a=group:DDP L1 L2 L3
m=video 20000 RTP/AVP 96 97 98
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
mst-mode=NI-T; sprop-parameter-sets={sps0},{pps0};
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=4d400a; packetization-mode=1;
mst-mode=NI-TC; sprop-parameter-sets={sps0},{pps0};
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=4d400a; packetization-mode=2;
mst-mode=I-C; init-buf-time=156320;
sprop-parameter-sets={sps0},{pps0};
a=mid:L1
m=video 20002 RTP/AVP 99 100
a=rtpmap:99 H264-SVC/90000
a=fmtp:99 profile-level-id=53000c; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps1},{pps1};
a=rtpmap:100 H264-SVC/90000
a=fmtp:100 profile-level-id=53000c; packetization-mode=2;
mst-mode=I-C; sprop-parameter-sets={sps1},{pps1};
a=mid:L2
a=depend:99 lay L1:96,97; 100 lay L1:98
m=video 20004 RTP/AVP 101
a=rtpmap:101 H264-SVC/90000
a=fmtp:101 profile-level-id=53001F; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps2},{pps2};
a=mid:L3
a=depend:101 lay L1:96,97 L2:99
It is assumed that in this example the answerer only supports the
NI-T mode for multi-session transmission. For this reason, it
chooses the corresponding payload type (96) for the base RTP session.
For the two enhancement RTP sessions the answerer also chooses the
payload types that us the NI-T mode (99 and 101).
Answerer -> Offerer SDP message:
a=group:DDP L1 L2 L3
m=video 40000 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
mst-mode=NI-T; sprop-parameter-sets={sps3},{pps3};
a=mid:L1
m=video 40002 RTP/AVP 99
a=rtpmap:99 H264-SVC/90000
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a=fmtp:99 profile-level-id=53000c; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps4},{pps4};
a=mid:L2
a=depend:99 lay L1:96
m=video 40004 RTP/AVP 101
a=rtpmap:101 H264-SVC/90000
a=fmtp:101 profile-level-id=53001F; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps5},{pps5};
a=mid:L3
a=depend:101 lay L1:96 L2:99
7.3.4 Example for Offering Multiple Sessions in MST including operation
with Answerer using scalable-layer-id
Example 4: In this example the offerer offers a multi-session
transmission of three layers with up to two sessions. The base
session media description has a payload type which is backward
compatible with [I-D.ietf-avt-rtp-rfc3984bis]. Note that no
parameter sets are provided, in which case in-band transport must be
used. The other media description contains two enhancement layers
and uses the media subtype H264-SVC. It includes two operation
point definitions.
Offerer -> Answerer SDP message:
a=group:DDP L1 L2
m=video 20000 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
mst-mode=NI-T;
a=mid:L1
m=video 20002 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53001F; packetization-mode=1;
mst-mode=NI-TC; sprop-operation-point-info=<2,0,1,0,53000c,
3200,352,288,384,512>,<3,1,2,0,53001F,6400,704,576,768,1024>;
a=mid:L2
a=depend:97 lay L1:96
It is assumed that the answerer wants to send and receive the base
layer (payload type 96), but it only wants to send and receive the
lower enhancement layer, i.e., the one with layer id equal to 2.
For this reason, the response will include the selection of the
desired layer by setting scalable-layer-id equal to 2. Note that
the answer only includes the scalable-layer-id information. The
answer could include sprop-parameter-sets in the response.
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Answerer -> Offerer SDP message:
a=group:DDP L1 L2
m=video 40000 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
mst-mode=NI-T;
a=mid:L1
m=video 40002 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 scalable-layer-id=2;
a=mid:L2
a=depend:97 lay L1:96
7.3.5 Example for Negotiating an SVC Stream with a Constrained Base
Layer in SST
Example 5: The offerer (Alice) offers one video description
including two RTP payload types with differing levels and
packetization modes.
Offerer -> Answerer SDP message:
m=video 20000 RTP/AVP 97 96
a=rtpmap:96 H264-SVC/90000
a=fmtp:96 profile-level-id=53001e; packetization-mode=0;
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53001f; packetization-mode=1;
The answerer (Bridge) chooses packetization mode 1, and indicates
that it would receive an SVC stream with the base layer being
constrained.
Answerer -> Offerer SDP message:
m=video 40000 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53001f; packetization-mode=1;
max-recv-base-level=000d
The answering endpoint must send an SVC stream at level 3.1. Since
the offering endpoint did not declare max-recv-base-level, the base
layer of the SVC stream the answering endpoint must send is not
specifically constrained. The offering endpoint (Alice) must send
an SVC stream at level 3.1, for which the base layer must be of a
level not higher than level 1.3.
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7.4 Parameter Set Considerations
Section 8.4 of [I-D.ietf-avt-rtp-rfc3984bis] applies in this memo,
with the following applies additionally for multi-session
transmission (MST).
In MST, regardless of out-of-band or in-band transport of parameter
sets is in use, parameter sets required for decoding NAL units
carried in one particular RTP session SHOULD be carried in the same
session, MAY be carried in a session that the particular RTP session
depends on, and MUST NOT be carried in a session that the particular
RTP session does not depend on.
8. Security Considerations
The security considerations of the RTP Payload Format for H.264
Video specification [I-D.ietf-avt-rtp-rfc3984bis] applies.
Additionally, the following applies.
Decoders MUST exercise caution with respect to the handling of
reserved NAL unit types and reserved SEI messages, particularly if
they contain active elements, and MUST restrict their domain of
applicability to the presentation containing the stream. The safest
way is to simply discard these NAL units and SEI messages.
When integrity protection is applied to a stream, care MUST be taken
that the stream being transported may be scalable; hence a receiver
may be able to access only part of the entire stream.
End-to-end security with either authentication, integrity or
confidentiality protection will prevent a MANE from performing
media-aware operations other than discarding complete packets. And
in the case of confidentiality protection it will even be prevented
from performing discarding of packets in a media aware way. To
allow any MANE to perform its operations, it will be required to be
a trusted entity which is included in the security context
establishment. This applies both for the media path and for the
RTCP path, if RTCP packets need to be rewritten.
9. Congestion Control
Within any given RTP session carrying payload according to this
specification, the provisions of Section 10 of [I-D.ietf-avt-rtp-
rfc3984bis] apply. Reducing the session bitrate is possible by one
or more of the following means:
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a) Within the highest layer identified by the DID field remove any
NAL units with QID higher than a certain value.
b) Remove all NAL units with TID higher than a certain value.
c) Remove all NAL units associated with a DID higher than a certain
value.
Informative note: Removal of all coded slice NAL units associated
with DIDs higher than a certain value in the entire stream is
required in order to preserve conformance of the resulting SVC
stream.
d) Utilize the PRID field to indicate the relative importance of NAL
units, and remove all NAL units associated with a PRID higher than
a certain value. Note that the use of the PRID is application-
specific.
e) Remove NAL units or entire packets according to application-
specific rules. The result will depend on the particular coding
structure used as well as any additional application-specific
functionality (e.g., concealment performed at the receiving
decoder). In general, this will result in the reception of a non-
conforming bitstream and hence the decoder behavior is not
specified by [H.264]. Significant artifacts may therefore appear
in the decoded output if the particular decoder implementation
does not take appropriate action in response to congestion control.
Informative note: The discussion above is centered on NAL units
rather than packets, primarily because that is the level where
senders can meaningfully manipulate the scalable bitstream. The
mapping of NAL units to RTP packets is fairly flexible when using
aggregation packets. Depending on the nature of the congestion
control algorithm, the "dimension" of congestion measurement
(packet count or bitrate) and reaction to it (reducing packet
count or bitrate or both) can be adjusted accordingly.
All aforementioned means are available to the RTP sender, regardless
whether that sender is located in the sending endpoint or in a
mixer-based MANE.
When a translator-based MANE is employed, then the MANE MAY
manipulate the session only on the MANE's outgoing path, so that the
sensed end-to-end congestion falls within the permissible envelope.
As all translators, in this case the MANE needs to rewrite RTCP RRs
to reflect the manipulations it has performed on the session.
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Informative note: Applications MAY also implement, in addition or
separately, other congestion control mechanisms, e.g., as
described in [RFC5775] and [Yan].
10. IANA Consideration
A new media type, as specified in Section 7.1 of this memo, should
be registered with IANA.
11. Informative Appendix: Application Examples
11.1 Introduction
Scalable video coding is a concept that has been around since at
least MPEG-2 [MPEG2], which goes back as early as 1993.
Nevertheless, it has never gained wide acceptance; perhaps partly
because applications didn't materialize in the form envisioned
during standardization.
ISO/IEC MPEG and ITU-T VCEG, respectively, performed a requirement
analysis for the SVC project. The MPEG and VCEG requirement
documents are available in [JVT-N026] and [JVT-N027], respectively.
The following introduces four main application scenarios that the
authors consider relevant and that are implementable with this
specification.
11.2 Layered Multicast
This well-understood form of the use of layered coding [McCanne]
implies that all layers are individually conveyed in their own RTP
packet streams, each carried in its own RTP session using the IP
(multicast) address and port number as the single demultiplexing
point. Receivers "tune" into the layers by subscribing to the IP
multicast, normally by using IGMP [IGMP]. Depending on the
application scenario, it is also possible to convey a number of
layers in one RTP session, when finer operation points within the
subset of layers are not needed.
Layered multicast has the great advantage of simplicity and easy
implementation. However, it has also the great disadvantage of
utilizing many different transport addresses. While the authors
consider this not to be a major problem for a professionally
maintained content server, receiving client endpoints need to open
many ports to IP multicast addresses in their firewalls. This is a
practical problem from a firewall and network address translation
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(NAT) viewpoint. Furthermore, even today IP multicast is not as
widely deployed as many wish.
The authors consider layered multicast an important application
scenario for the following reasons. First, it is well understood
and the implementation constraints are well known. Second, there
may well be large scale IP networks outside the immediate Internet
context that may wish to employ layered multicast in the future.
One possible example could be a combination of content creation and
core-network distribution for the various mobile TV services, e.g.,
those being developed by 3GPP (MBMS) [MBMS] and DVB (DVB-H) [DVB-H].
11.3 Streaming
In this scenario, a streaming server has a repository of stored SVC
coded layers for a given content. At the time of streaming, and
according to the capabilities, connectivity, and congestion
situation of the client(s), the streaming server generates and
serves a scalable stream. Both unicast and multicast serving is
possible. At the same time, the streaming server may use the same
repository of stored layers to compose different streams (with a
different set of layers) intended for other audiences.
As every endpoint receives only a single SVC RTP session, the number
of firewall pinholes can be optimized to one.
The main difference between this scenario and straightforward
simulcasting lies in the architecture and the requirements of the
streaming server, and is therefore out of the scope of IETF
standardization. However, compelling arguments can be made why such
a streaming server design makes sense. One possible argument is
related to storage space and channel bandwidth. Another is
bandwidth adaptability without transcoding -- a considerable
advantage in a congestion controlled network. When the streaming
server learns about congestion, it can reduce the sending bit rate
by choosing fewer layers when composing the layered stream; see
Section 9. SVC is designed to gracefully support both bandwidth
ramp-down and bandwidth ramp-up with a considerable dynamic range.
This payload format is designed to allow for bandwidth flexibility
in the mentioned sense. While, in theory, a transcoding step could
achieve a similar dynamic range, the computational demands are
impractically high and video quality is typically lowered --
therefore, few (if any) streaming servers implement full transcoding.
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11.4 Videoconferencing (Unicast to MANE, Unicast to Endpoints)
Videoconferencing has traditionally relied on Multipoint Control
Units (MCUs). These units connect endpoints in a star configuration
and operate as follows. Coded video is transmitted from each
endpoint to the MCU, where it is decoded, scaled, and composited to
construct output frames, which are then re-encoded and transmitted
to the endpoins(s). In systems supporting personalized layout (each
user is allowed to select the layout of his/her screen), the
compositing and encoding process is performed for each of the
receiving endpoints. Even without personalized layout, rate
matching still requires that the encoding process at the MCU is
performed separately for each endpoint. As a result, MCUs have
considerable complexity and introduce significant delay. The
cascaded encodings also reduce the video quality. Particularly for
multipoint connections, interactive communication is cumbersome as
the end-to-end delay is very high [G.114]. A simpler architecture
is the switching MCU, in which one of the incoming video streams is
redirected to the receiving endpoints. Obviously, only one user at
a time can be seen and rate matching cannot be performed, thus
forcing all transmitting endpoints to transmit at the lowest bit
rate available in the MCU-to-endpoint connections.
With scalable video coding the MCU can be replaced with an
application-level router (ALR): this unit simply selects which
incoming packets should be transmitted to which of the receiving
endpoints [Eleft]. In such a system, each endpoint performs its own
composition of the incoming video streams. Assuming, for example, a
system that uses spatial scalability with two layers, personalized
layout is equivalent to instructing the ALR to only send the
required packets for the corresponding resolution to the particular
endpoint. Similarly, rate matching at the ALR for a particular
endpoint can be performed by selecting an appropriate subset of the
incoming video packets to transmit to the particular endpoint.
Personalized layout and rate matching thus become routing decisions,
and require no signal processing. Note that scalability also allows
participants to enjoy the best video quality afforded by their links,
i.e., users no longer have to be forced to operate at the quality
supported by the weakest endpoint. Most importantly, the ALR has an
insignificant contribution to the end-to-end delay, typically an
order of magnitude less than an MCU. This makes it possible to have
fully interactive multipoint conferences with even a very large
number of participants. There are significant advantages as well in
terms of error resilience and, in fact, error tolerance can be
increased by nearly an order of magnitude here as well (e.g., using
unequal error protection). Finally, the very low delay of an ALR
allows these systems to be cascaded, with significant benefits in
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terms of system design and deployment. Cascading of traditional
MCUs is impossible due to the very high delay that even a single MCU
introduces.
Scalable video coding enables a very significant paradigm shift in
videoconferencing systems, bringing the complexity of video
communication systems (particularly the servers residing within the
network) in line with other types of network applications.
11.5 Mobile TV (Multicast to MANE, Unicast to Endpoint)
This scenario is a bit more complex, and designed to optimize the
network traffic in a core network, while still requiring only a
single pinhole in the endpoint's firewall. One of its key
applications is the mobile TV market.
Consider a large private IP network, e.g., the core network of 3GPP.
Streaming servers within this core network can be assumed to be
professionally maintained. It is assumed that these servers can
have many ports open to the network and that layered multicast is a
real option. Therefore, the streaming server multicasts SVC
scalable layers, instead of simulcasting different representations
of the same content at different bit rates.
Also consider many endpoints of different classes. Some of these
endpoints may lack the processing power or the display size to
meaningfully decode all layers; others may have these capabilities.
Users of some endpoints may wish not to pay for high quality and are
happy with a base service, which may be cheaper or even free. Other
users are willing to pay for high quality. Finally, some connected
users may have a bandwidth problem in that they can't receive the
bandwidth they would want to receive -- be it through congestion,
connectivity, change of service quality, or for whatever other
reasons. However, all these users have in common that they don't
want to be exposed too much, and therefore the number of firewall
pinholes needs to be small.
This situation can be handled best by introducing middleboxes close
to the edge of the core network, which receive the layered multicast
streams and compose the single SVC scalable bit stream according to
the needs of the endpoint connected. These middleboxes are called
MANEs throughout this specification. In practice, the authors
envision the MANE to be part of (or at least physically and
topologically close to) the base station of a mobile network, where
all the signaling and media traffic necessarily are multiplexed on
the same physical link.
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MANEs necessarily need to be fairly complex devices. They certainly
need to understand the signaling, so, for example, to associate the
PT octet in the RTP header with the SVC payload type.
A MANE may aggregate multiple RTP streams, possibly from multiple
RTP sessions, thus to reduce the number of firewall pinholes
required at the endpoints, or may optimize the outgoing RTP stream
to the MTU size of the outgoing path by utilizing the aggregation
and fragmentation mechanisms of this memo. This type of MANEs is
conceptually easy to implement and can offer powerful features,
primarily because it necessarily can "see" the payload (including
the RTP payload headers), utilize the wealth of layering information
available therein, and manipulate it.
A MANE can also perform stream thinning, in order to adhere to
congestion control principles as discussed in Section 9. While the
implementation of the forward (media) channel of such a MANE appears
to be comparatively simple, the need to rewrite RTCP RRs makes even
such a MANE a complex device.
While the implementation complexity of either case of a MANE, as
discussed above, is fairly high, the computational demands are
comparatively low.
12. Acknowledgements
Miska Hannuksela contributed significantly to the designs of the
PACSI NAL unit and the NI-C mode for decoding order recovery. Roni
Even organized and coordinated the design team for the development
of this memo, and provided valuable comments. Jonathan Lennox
contributed to the NAL unit reordering algorithm for MST and
provided input on several parts of this memo. Peter Amon, Sam
Ganesan, Mike Nilsson, Colin Perkins, and Thomas Wiegand were
members of the design team and provided valuable contributions.
Magnus Westerlund has also made valuable comments. Charles Eckel
and Stuart Taylor provided valuable comments after the first WGLC
for this document.
The work of Thomas Schierl has been supported by the European
Commission under contract number FP7-ICT-248036, project COAST.
This document was prepared using 2-Word-v2.0.template.dot.
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13. References
13.1 Normative References
[H.264] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services", 3rd Edition, November 2007.
[I-D.ietf-avt-rtp-rfc3984bis]
Wang, Y.-K., Even, R., Kristensen, T., and Jesup, R., "RTP
Payload Format for H.264 Video", draft-ietf-avt-rtp-
rfc3984bis-12.txt (work in progress), Oct. 2010.
[ISO/IEC 14496-10]
ISO/IEC International Standard 14496-10:2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
With Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and Jacobson,
V., "RTP: A Transport Protocol for Real-Time Applications",
STD 64, RFC 3550, July 2003.
[RFC4288] Freed, N. and Klensin, J., "Media Type Specification and
Registration Procedures ", RFC 4288, December 2005.
[RFC4566] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5576] Lennox, J., Ott, J., and Schierl, T., "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5583] Schierl, T. and Wenger, S., "Signaling media decoding
dependency in the Session Description Protocol (SDP)", RFC
5583, July 2009.
[RFC6051] Perkins, C. and Schierl, T., "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010
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13.2 Informative References
[DVB-H] DVB - Digital Video Broadcasting (DVB); DVB-H
Implementation Guidelines, ETSI TR 102 377, 2005.
[Eleft] Eleftheriadis, A., R. Civanlar, and O. Shapiro,
"Multipoint Videoconferencing with Scalable Video Coding",
Journal of Zhejiang University SCIENCE A, Vol. 7, Nr. 5,
April 2006, pp. 696-705. (Proceedings of the Packet Video
2006 Workshop.)
[G.114] ITU-T Rec. G.114, "One-way transmission time", May 2003.
[H.241] ITU-T Rec. H.241, "Extended video procedures and control
signals for H.300-series terminals", May 2006.
[IGMP] Cain, B., Deering S., Kovenlas, I., Fenner, B., and
Thyagarajan, A., "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[JVT-N026] Ohm J.-R., Koenen, R., and Chiariglione, L. (ed.), "SVC
requirements specified by MPEG (ISO/IEC JTC1 SC29 WG11)",
JVT-N026, available from http://ftp3.itu.ch/av-arch/jvt-
site/2005_01_HongKongGeneva/JVT-N026.doc, Hong Kong, China,
January 2005.
[JVT-N027] Sullivan, G. and Wiegand, T. (ed.), "SVC requirements
specified by VCEG (ITU-T SG16 Q.6)", JVT-N027, available
from http://ftp3.itu.ch/av-arch/jvt-
site/2005_01_HongKongGeneva/JVT-N027.doc, Hong Kong, China,
January 2005.
[McCanne] McCanne, S., Jacobson, V., and Vetterli, M., "Receiver-
driven layered multicast", in Proc. of ACM SIGCOMM'96,
pages 117--130, Stanford, CA, August 1996.
[MBMS] 3GPP - Technical Specification Group Services and System
Aspects; Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs (Release 6), December 2005.
[MPEG2] ISO/IEC International Standard 13818-2:1993.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
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[RFC5117] Westerlund, M. and Wenger, S., "RTP Topologies", RFC 5117,
January 2008.
[RFC5775] Luby, M., Watson, M., and Vicisano, L., "Asynchronous
layered coding (ALC) protocol instantiation", RFC 5775,
April 2010.
[Yan] Yan, J., Katrinis, K., May, M., and Plattner, R., "Media-
and TCP-friendly congestion control for scalable video
streams", in IEEE Trans. Multimedia, pages 196--206, April
2006.
14. Authors' Addresses
Stephan Wenger
2400 Skyfarm Dr.
Hillsborough, CA 94010
USA
Phone: +1-415-713-5473
EMail: stewe@stewe.org
Ye-Kui Wang
Huawei Technologies
400 Crossing Blvd, 2nd Floor
Bridgewater, NJ 08807
USA
Phone: +1-908-541-3518
EMail: yekuiwang@huawei.com
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
Email: ts@thomas-schierl.de
Alex Eleftheriadis
Vidyo, Inc.
433 Hackensack Ave.
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Hackensack, NJ 07601
USA
Phone: +1-201-467-5135
Email: alex@vidyo.com
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