| Rfc | 6190 |
|---|
| Title | RTP Payload Format for Scalable Video Coding |
|---|
| Author | S. Wenger, Y.-K. Wang,
T. Schierl, A. Eleftheriadis |
|---|
| Date | May 2011 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) S. Wenger
Request for Comments: 6190 Independent
Category: Standards Track Y.-K. Wang
ISSN: 2070-1721 Huawei Technologies
T. Schierl
Fraunhofer HHI
A. Eleftheriadis
Vidyo
May 2011
RTP Payload Format for Scalable Video Coding
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 backward
compatible to RFC 6184 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 RFC 6184. The payload format
has wide applicability in videoconferencing, Internet video
streaming, and high-bitrate entertainment-quality video, among
others.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6190.
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Table of Contents
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 .............16
3.1.2. Definitions Specific to This Memo ..................18
3.2. Abbreviations .............................................22
4. RTP Payload Format .............................................23
4.1. RTP Header Usage ..........................................23
4.2. NAL Unit Extension and Header Usage .......................23
4.2.1. NAL Unit Extension .................................23
4.2.2. NAL Unit Header Usage ..............................24
4.3. Payload Structures ........................................25
4.4. Transmission Modes ........................................28
4.5. Packetization Modes .......................................28
4.5.1. Packetization Modes for Single-Session
Transmission .......................................28
4.5.2. Packetization Modes for Multi-Session
Transmission .......................................29
4.6. Single NAL Unit Packets ...................................32
4.7. Aggregation Packets .......................................33
4.7.1. Non-Interleaved Multi-Time Aggregation
Packets (NI-MTAPs) .................................33
4.8. Fragmentation Units (FUs) .................................35
4.9. Payload Content Scalability Information (PACSI) NAL Unit ..35
4.10. Empty NAL unit ...........................................43
4.11. Decoding Order Number (DON) ..............................43
4.11.1. Cross-Session DON (CS-DON) for
Multi-Session Transmission ........................43
5. Packetization Rules ............................................45
5.1. Packetization Rules for Single-Session Transmission .......45
5.2. Packetization Rules for Multi-Session Transmission ........46
5.2.1. NI-T/NI-TC Packetization Rules .....................47
5.2.2. NI-C/NI-TC Packetization Rules .....................49
5.2.3. I-C Packetization Rules ............................50
5.2.4. Packetization Rules for Non-VCL NAL Units ..........50
5.2.5. Packetization Rules for Prefix NAL Units ...........51
6. De-Packetization Process .......................................51
6.1. De-Packetization Process for Single-Session Transmission ..51
6.2. De-Packetization Process for Multi-Session Transmission ...51
6.2.1. Decoding Order Recovery for the NI-T and
NI-TC Modes ........................................52
6.2.1.1. Informative Algorithm for NI-T
Decoding Order Recovery within
an Access Unit ............................55
6.2.2. Decoding Order Recovery for the NI-C,
NI-TC, and I-C Modes ...............................57
7. Payload Format Parameters ......................................59
7.1. Media Type Registration ...................................60
7.2. SDP Parameters ............................................75
7.2.1. Mapping of Payload Type Parameters to SDP ..........75
7.2.2. Usage with the SDP Offer/Answer Model ..............76
7.2.3. Dependency Signaling in Multi-Session
Transmission .......................................84
7.2.4. Usage in Declarative Session Descriptions ..........85
7.3. Examples ..................................................86
7.3.1. Example for Offering a Single SVC Session ..........86
7.3.2. Example for Offering a Single SVC Session Using
scalable-layer-id ..................................87
7.3.3. Example for Offering Multiple Sessions in MST ......87
7.3.4. Example for Offering Multiple Sessions in
MST Including Operation with Answerer Using
scalable-layer-id ..................................89
7.3.5. Example for Negotiating an SVC Stream with
a Constrained Base Layer in SST ....................90
7.4. Parameter Set Considerations ..............................91
8. Security Considerations ........................................91
9. Congestion Control .............................................92
10. IANA Considerations ...........................................93
11. Informative Appendix: Application Examples ....................93
11.1. Introduction .............................................93
11.2. Layered Multicast ........................................93
11.3. Streaming ................................................94
11.4. Videoconferencing (Unicast to MANE, Unicast to
Endpoints) ...............................................95
11.5. Mobile TV (Multicast to MANE, Unicast to Endpoint) .......96
12. Acknowledgements ..............................................97
13. References ....................................................97
13.1. Normative References .....................................97
13.2. Informative References ...................................98
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/IEC14496-10] 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 megabits 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.), and to 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 with [RFC6184] by
eliminating RTP packets, and rewriting RTP Control Protocol (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; in MST, the
scalability layers are transported on different RTP sessions. In
SST, packetization is a straightforward extension of [RFC6184]. 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 alignment [RFC3550] or cross-session
decoding order numbers (CS-DONs). 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)), and 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 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
(AUs). 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
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
backward compatibility with H.264/AVC, it is important to
differentiate, however, whether or not SVC-specific NAL units are
present in a given bitstream. 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 with
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-grain scalability (CGS), scalability is provided by
including or excluding a complete layer when decoding a particular
bitstream. In contrast, in medium-grain 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 the 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 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
instantaneous decoding refresh (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 types 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 backward-compatible way):
+---------------+
|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.
+---------------+---------------+---------------+
|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 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 [RFC6184], this payload format can only be used to carry
the raw NAL unit stream over RTP and not the bytestream format
specified in Annex B of [H.264].
The design principles, transmission modes, and 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 [RFC6184].
1.2.1. Design Principles
The following design principles have been observed for this payload
format:
o Backward compatibility with [RFC6184] 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 [RFC6184]. This ensures that such an RTP
stream can be understood by [RFC6184] receivers.
o Media-aware network elements (MANEs) as defined in [RFC6184] 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
[RFC6184]. 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
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; or
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 [RFC6184] must be used, thus ensuring
compatibility with [RFC6184] 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 [RFC6184] 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 bitstream, 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
of the SVC base layer is transmitted in its own session in MST, the
packetization of [RFC6184] must be used, such that [RFC6184]
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 (NI-T) based mode, non-
interleaved CS-DON (NI-C) based mode, non-interleaved combined
timestamp and CS-DON mode (NI-TC), and interleaved CS-DON (I-C) based
mode (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 reuse the packetization modes
introduced in [RFC6184] 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 [RFC6184] receivers with
implementation of the non-interleaved mode specified in [RFC6184] can
join a multi-session transmission of SVC, to receive the base RTP
session encapsulated according to [RFC6184].
1.2.3. New Payload Structures
[RFC6184] 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 composed of NAL
units, [RFC6184] 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 [RFC6184].
Furthermore, three new types of NAL units are defined: payload
content scalability information (PACSI) NAL 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.
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
[RFC6184] 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 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.
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. 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 [RFC6184] 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 Section
7.2.3. A lower RTP session to an enhancement RTP session is an RTP
session on which the enhancement RTP session depends. 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 on which no other RTP session
received by the receiver depends.
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 Session Description Protocol (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 [RFC6184] 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 [RFC6184] 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,
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 type 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
[RFC6184] 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 Secure Real-
time Transport Protocol (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 [RFC6184] 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 [RFC6184] 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 profiles defined in Annex A
or 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 payload type 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.
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 [RFC6184] in exactly the same form.
3.2. Abbreviations
In addition to the abbreviations defined in [RFC6184], 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 [RFC6184], 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-
PACSI NAL unit in transmission order. Recall that the NALU-time of a
NAL unit in an MTAP is defined in [RFC6184] 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
[RFC3550].
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 [RFC6184] (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
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 [RFC6184] 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 [RFC6184], the semantics
specified in Section 5.3 of [RFC6184] 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 Quality of Service (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 a FEC protection mechanism. The
importance for the decoding process decreases as the PRID value
increases.
For DID, QID, or 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, [RFC6184], 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
[RFC6184], 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.
[RFC6184] specifies three basic payload structures (in Section 5.2 of
[RFC6184]): single NAL unit packet, aggregation packet, 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.
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 [RFC6184] 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 [RFC6184], 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
an 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.
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
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 [RFC6184] applies with the
following extensions.
The packetization modes specified in Section 5.4 of [RFC6184],
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
[RFC6184]. 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.
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 [RFC6184].
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) 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. 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
[RFC6184]. 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.
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 - -
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 [RFC6184] 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 [RFC6184], the following applies in
this memo.
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.
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
[RFC6184] 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.
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 [RFC6184] 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 [RFC6184]). 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:
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
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.
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).
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
units 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.
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.
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 bit
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.
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.
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
K MUST be equal to 0
L MUST be equal to 0
4.11. Decoding Order Number (DON)
The DON concept is introduced in [RFC6184] and is used to recover the
decoding order when interleaving is used within a single session.
Section 5.5 of [RFC6184] 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, 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 [RFC6184], but
contrary to [RFC6184] 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 (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.
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 [RFC6184] for all the NAL units in each of the RTP
sessions MUST indicate CS-DON values.
5. Packetization Rules
Section 6 of [RFC6184] 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 [RFC6184]. 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 [RFC6184] does allow
an application to encapsulate a single NAL unit in a single RTP
packet. Historically, the single NAL unit mode has been included
in [RFC6184] 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.
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 [RFC6184]. This way, an [RFC6184] 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 same
access unit MUST be present in any enhancement RTP session that
depends on RTP session A.
Informative note: 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: 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
reordering 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 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 time line 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
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.
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 that 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
on which the session using these non-VCL NAL units depends (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 on which all these sessions depend.
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 [RFC6184], 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 [RFC6184]
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 on
which the enhancement RTP session depends. 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 on which no other RTP session
received by the receiver depends.
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.
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 reordering 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.
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).
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 sessions B and C have the same but higher
frame rate.
The figure shows NAL units contained in RTP packets that are stored
in the de-jittering buffer at the receiver for session de-
packetization. The NAL units are already reordered 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 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.
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 that 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 an 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)
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.
(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:
o Function AbsDON is specified in Section 8.1 of [RFC6184].
o Function don_diff is specified in Section 5.5 of [RFC6184].
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.
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 has been allocated from the IETF
tree.
The receiver MUST ignore any unspecified parameter.
Informative note: Requiring that the receiver ignore 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 receiving
data per the new payload but ignoring those parameters newly
specified in the new payload specification. This provision is
also present in [RFC6184].
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, constraint_set4_flag,
constraint_set5_flag, and reserved_zero_2bits, in bit-
significance order, starting from the most-significant bit, and
3) level_idc. Note that reserved_zero_2bits 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 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 Annexes A and 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.
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
B 42 (B) x0xx0000
same as: 58 (E) 10xx0000
M 4D (M) 0x0x0000
E 58 00xx0000
H 64 00000000
H10 6E 00000000
H42 7A 00000000
H44 F4 00000000
H10I 6E 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 able to
decode bitstreams conforming to other profiles.
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 in 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
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
[RFC6184].
max-mbps: This parameter is as specified in [RFC6184].
max-fs: This parameter is as specified in [RFC6184].
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 and in units of 1200 bits for the NAL HRD
parameters. Note that this parameter does not use units of
cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of [H.264]).
The max-cpb parameter signals that the receiver has more memory
than the 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 (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed). The value of
max-cpb (after taking cpbBrVclFactor and cpbBrNALFactor into
consideration when needed) 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 bitrate 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 bitrate. 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 [RFC6184].
max-br: The value of max-br is an integer indicating the maximum
video bitrate in units of 1000 bits per second for the VCL HRD
parameters and in units of 1200 bits per second for the NAL HRD
parameters. Note that this parameter does not use units of
cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of [H.264]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bitrate 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 signaled
highest level, with the following exceptions in the limits
specified by the highest level:
o The value of max-br (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) 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 Main profile
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 (after taking cpbBrVclFactor and
cpbBrNALFactor into consideration when needed) 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 bitrates at any
given time. In particular, no conclusion can be drawn that
the signaled bitrate is possible under congestion control
constraints.
redundant-pic-cap:
This parameter is as specified in [RFC6184].
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 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 that 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 that 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.
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 [RFC6184]. 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 [RFC6184].
sprop-deint-buf-req:
This parameter is as specified in [RFC6184].
deint-buf-cap:
This parameter is as specified in [RFC6184].
sprop-init-buf-time:
This parameter is as specified in [RFC6184].
sprop-max-don-diff:
This parameter is as specified in [RFC6184].
max-rcmd-nalu-size:
This parameter is as specified in [RFC6184].
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 on which the current RTP session
depends.
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".
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".
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 [RFC6184], 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 that 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 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
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 bitrate 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 bitrate 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 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 parameter 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 [RFC6184] 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 6190.
Published specification:
Please refer to RFC 6190 and its Section 13.
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, yekui.wang@huawei.com
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing, and hence is only
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, yekui.wang@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.
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 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., the 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 is 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.
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.
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, and 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.
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.
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.
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:
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
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.
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).
7.2.4. Usage in Declarative Session Descriptions
When SVC over RTP is offered with SDP in a declarative style, as in
Real Time Streaming Protocol (RTSP) [RFC2326] or Session Announcement
Protocol (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
- 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=4de00a; 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=4de00a; packetization-mode=0;
sprop-parameter-sets={sps2},{pps2};
a=rtpmap:97 H264-SVC/90000
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=<1,0,0,0,4de00a,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-scalability-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 that are backward compatible with
[RFC6184], 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.
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=4de00a; packetization-mode=0;
mst-mode=NI-T; sprop-parameter-sets={sps0},{pps0};
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=4de00a; packetization-mode=1;
mst-mode=NI-TC; sprop-parameter-sets={sps0},{pps0};
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=4de00a; 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 use 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=4de00a; 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
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 that is backward
compatible with [RFC6184]. 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=4de00a; 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.
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=4de00a; 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.
7.4. Parameter Set Considerations
Section 8.4 of [RFC6184] 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 are 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 [RFC6184] apply. 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 that 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 [RFC6184] apply.
Reducing the session bitrate is possible by one or more of the
following means:
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
of 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 with all translators, in this case, the MANE needs to rewrite RTCP
RRs to reflect the manipulations it has performed on the session.
Informative note: Applications MAY also implement, in addition or
separately, other congestion control mechanisms, e.g., as
described in [RFC5775] and [Yan].
10. IANA Considerations
A new media type, as specified in Section 7.1 of this memo, has been
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
(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 bitrate 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.
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 endpoint(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 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 the
Third Generation Partnership Project (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 bitrates.
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 bitstream 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.
MANEs necessarily need to be fairly complex devices. They certainly
need to understand the signaling, so, for example, to associate the
payload type 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 MANE 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. Xiaohui
(Joanne) Wei helped improving Table 13 and the SDP examples.
The work of Thomas Schierl has been supported by the European
Commission under contract number FP7-ICT-248036, project COAST.
13. References
13.1. Normative References
[H.264] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services", March 2010.
[RFC6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184, May 2011.
[ISO/IEC14496-10]
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 V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "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 T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)", RFC
5583, July 2009.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
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., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "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_HongKong/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.int/av-arch/
jvt-site/2005_01_HongKong/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.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC5775] Luby, M., Watson, M., and L. Vicisano, "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.
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: yekui.wang@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.
Hackensack, NJ 07601
USA
Phone: +1-201-467-5135
EMail: alex@vidyo.com