Audio/Video Transport WG S. Wenger
Internet Draft Y.-K. Wang
Intended status: Standards track Nokia
Expires: March 2009 T. Schierl
Fraunhofer HHI
A. Eleftheriadis
Vidyo
September 26, 2008
RTP Payload Format for SVC Video
draft-ietf-avt-rtp-svc-14.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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Abstract
This memo describes an RTP payload format for Scalable Video Coding
(SVC) as defined in_Annex G of ITU-T Recommendation H.264, which is
technically identical to Amendment 3 of ISO/IEC International
Standard 14496-10. The RTP payload format allows for packetization
of one or more Network Abstraction Layer (NAL) units in each RTP
packet payload, as well as fragmentation of a NAL unit in multiple
RTP packets. Furthermore, it supports transmission of an SVC stream
over a single as well as multiple RTP sessions. For single-session
transmission the packetization modes of RFC 3984 are used. For
multi-session transmission four different modes are defined in this
memo. 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.
Specifically, decoding order recovery is performed using either
timestamp alignment or Cross-Session Decoding Order Numbers (CS-
DON), although in one of the modes both schemes are used in order to
allow receivers to use their preferred method. The multi-session
transmission modes use the packetization modes defined in RFC 3984
as each individual session still uses a packetization mode defined
in RFC 3984. The packetization modes defined in RFC 3984 are
slightly extended such that the three new NAL unit types defined in
this memo can be included in the RTP packet streams. The payload
format defines a new media subtype name "H264-SVC", but is still
backwards compatible to RFC 3984 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 3984.
The payload format has wide applicability in videoconferencing,
Internet video streaming, and high bit-rate entertainment-quality
video, among others.
Table of Contents
Status of this Memo...............................................1
Copyright Notice..................................................1
Abstract..........................................................2
Table of Contents.................................................2
1 . Introduction..................................................4
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
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1.2.2 Transmission Modes and Packetization Modes...........12
1.2.3 New Payload Structures...............................14
2 . Conventions..................................................15
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.............................................21
4 . RTP Payload Format...........................................21
4.1 RTP Header Usage..........................................21
4.2 NAL Unit Extension and Header Usage.......................22
4.2.1 NAL Unit Extension...................................22
4.2.2 NAL Unit Header Usage................................23
4.3 Payload Structures........................................24
4.4 Transmission Modes........................................27
4.5 Packetization Modes.......................................27
4.5.1 Packetization Modes for Single-Session Transmission..27
4.5.2 Packetization Modes for Multi-Session Transmission...28
4.6 Single NAL Unit Packets...................................32
4.7 Aggregation Packets.......................................32
4.7.1 Non-Interleaved Multi-Time Aggregation Packets (NI-
MTAPs).....................................................32
4.8 Fragmentation Units (FUs).................................34
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...............................................44
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..............................51
5.2.4 Packetization Rules for Non-VCL NAL Units............51
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..........................56
6.2.2 Decoding Order Recovery for the NI-C, NI-TC and I-C
Modes......................................................58
7 . Payload Format Parameters....................................60
7.1 Media Type Registration...................................61
7.2 SDP Parameters............................................71
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7.2.1 Mapping of Payload Type Parameters to SDP............71
7.2.2 Usage with the SDP Offer/Answer Model................72
7.2.3 Usage with Multi-Source Transmission.................77
7.2.4 Usage in Declarative Session Descriptions............77
7.3 Examples..................................................78
7.3.1 Example for Offering A Single SVC Session............78
7.3.2 Example for Offering Session Multiplexing............78
7.4 Parameter Set Considerations..............................79
8 . Security Considerations......................................79
9 . Congestion Control...........................................80
10 . IANA Consideration..........................................81
11 . Informative Appendix: Application Examples..................81
11.1 Introduction.............................................81
11.2 Layered Multicast........................................81
11.3 Streaming................................................82
11.4 Videoconferencing (Unicast to MANE, Unicast to Endpoints)83
11.5 Mobile TV (Multicast to MANE, Unicast to Endpoint).......83
11.6 SSRC Multiplexing........................................84
12 . References..................................................85
12.1 Normative References.....................................85
12.2 Informative References...................................86
13 . Authors' Addresses..........................................87
Intellectual Property Statement..................................88
Disclaimer of Validity...........................................88
Copyright Statement..............................................89
Acknowledgement..................................................89
14 . Open Issues.................................................89
15 . Changes Log.................................................89
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 [MPEG4-10],
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 HDTV broadcasting, and even Digital
Cinema that requires nearly lossless coding and hundreds of Mbps.
The scalability features that SVC adds to H.264/AVC enable several
system-level functionalities related to the ability of a system to
adapt the signal to different system conditions with no or minimal
processing. The adaptation relates both to the capabilities of
potentially heterogeneous receivers (differing in screen resolution,
processing speed, etc.), as well as differing or time-varying
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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 signaling parameter
("avc-ready", Section 7.2) that indicates whether or not the
stream can be converted to one compliant to RFC 3984 [RFC3984]
simply by simply eliminating RTP packets.
This memo defines two basic modes for transmission of SVC data,
single session-transmission (SST) and multi session-transmission
(MST). In SST, a single RTP session is used for the transmission of
all scalability layers comprising an SVC bitstream, whereas in MST
the scalability layers are transported on different RTP sessions.
In SST, packetization is a straightforward extension of RFC 3984.
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-DON). One of the MST
modes supports both decoding order recovery techniques, so that
receivers can select their preferred technique. More details can be
found in Section 1.2.2.
This memo further defines three new NAL unit types. The first type
is the Payload Content Scalability Information (PACSI) NAL unit,
which is used to provide an informative summary of the scalability
information of the data contained in an RTP packet, as well as
ancillary data (e.g., CS-DON values). The second and third new NAL
unit types are the Empty NAL unit and the Non-Interleaved Multi-time
Aggregation Packet (NI-MTAP) NAL unit. The Empty NAL unit is used to
ensure inter-session timestamp alignment required for decoding order
recovery in MST. The NI-MTAP is used as a new payload structure
allowing the grouping of NAL units of different time instances in
decoding order. More details about the new packet structures can be
found in Section 1.2.3.
This memo also defines the signaling support for SVC transport over
RTP, including a new media subtype name (H264-SVC).
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An overview of the SVC codec and the payload is given in the
remainder of this section.
1.1. The SVC Codec
1.1.1. Overview
SVC defines a coded video representation in which a given bitstream
offers representations of the source material at different levels of
fidelity (hence the term "scalable"). Scalable video coding
bitstreams, or scalable bitstreams, are constructed in a pyramidal
fashion: the coding process creates bitstream components that
improve the fidelity of hierarchically lower components.
The fidelity dimensions offered by SVC are spatial (picture size),
quality (or Signal-to-Noise Ratio - SNR), as well as temporal
(pictures per second). Bitstream components associated with a given
level of spatial, quality, and temporal fidelity are identified
using corresponding parameters in the bitstream: dependency_id,
quality_id, and temporal_id (see also Section 1.1.3). The fidelity
identifiers have integer values, where higher values designate
components that are higher in the hierarchy. It is noted that SVC
offers significant flexibility in terms of how an encoder may choose
to structure the dependencies between the various components.
Decoding of a particular component requires the availability of all
the components it depends upon, either directly, or indirectly. An
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.
SVC maintains the bitstream organization introduced in H.264/AVC.
Specifically, all bitstream components are encapsulated in Network
Abstraction Layer (NAL) units which are organized as Access Units
(AU). An AU is associated with a single sampling instance in time.
A subset of the NAL unit types correspond to the Video Coding Layer
(VCL), and contain the coded picture data associated with the source
content. Non-VCL NAL units carry ancillary data that may be
necessary for decoding (e.g., parameter sets as explained below), or
that facilitate certain system operations but are not needed by the
decoding process itself. Coded picture data at the various fidelity
dimensions are organized in slices. Within one AU, a coded picture
of an operation point consists of all the coded slices required for
decoding up to the particular combination of dependency_id and
quality_id values at the time instance corresponding to the AU.
It is noted that the concept of temporal scalability is already
present in H.264/AVC, as profiles defined in Annex A of [H.264]
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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 14), 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 20), 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
profile specified in Annex A of [H.264].
Within an AU, the VCL NAL units associated with a given
dependency_id and quality_id are referred to as a "layer
representation". The layer representation corresponding to the
lowest values of dependency_id and quality_id (i.e., zero for both)
is compliant by design to H.264/AVC. The set of VCL and associated
non-VCL NAL units across all AUs in a bitstream associated with a
particular combination of values of dependency_id and quality_id,
and regardless of the value of temporal_id, is conceptually a
scalable layer. For backwards compatibility with H.264/AVC, it is
important to differentiate, however, whether or not SVC-specific NAL
units are present in a given bitstream or not. This is particularly
important for the lowest fidelity values in terms of dependency_id
and quality_id (zero for both), as the corresponding VCL data are
compliant to H.264/AVC, and may or may not be accompanied by
associated prefix NAL units. This memo therefore uses the term "AVC
base layer" to designate the layer that does not contain SVC-
specific NAL units, and "SVC base layer" to designate the same layer
but with the addition of the associated SVC prefix NAL units. Note
that the SVC specification uses the term "base layer" for what in
this memo will be referred to as "AVC base layer". Similarly, it is
also important to be able to differentiate, within a layer, the
temporal fidelity components it contains. This memo uses the term
"T0" to indicate, within a particular layer, the subset that
contains the NAL units associated with temporal_id equal to 0.
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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.
SNR scalability in SVC is offered in two different ways. In what is
called Coarse-Grained Scalability (CGS), scalability is provided by
including or excluding a complete layer when decoding a particular
bitstream. In contrast, in Medium-Grained Scalability (MGS),
scalability is provided by selectively omitting the decoding of
specific NAL units belonging to MGS layers. The selection of the
NAL units to omit can be based on fixed length fields present in the
NAL unit header (see also Sections 1.1.3 and 4.2).
1.1.2. Parameter Sets
SVC maintains the parameter sets concept in H.264/AVC and introduces
a new type of sequence parameter set, referred to as subset sequence
parameter set [H.264]. Subset sequence parameter sets have NAL unit
type equal to 15, which is different from the NAL unit type value
(7) of sequence parameter sets. VCL NAL units of NAL unit type 1 to
5 must only (indirectly) refer to sequence parameter sets, while VCL
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
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change at IDR access units for any layer. At any time instant
during the decoding process there may be one active picture
parameter set (for the layer representation with the highest value
of DQId) and one or more active layer picture parameter set(s) (for
layer representations with lower values of DQId). The active
picture parameter set or an active layer picture parameter set
remains unchanged throughout a layer representation in which the
active picture parameter set or active layer picture parameter set
is referred to, but may change from one AU to the next.
1.1.3. NAL Unit Header
SVC extends the one-byte H.264/AVC NAL unit header by three
additional octets for NAL units of type 14 and 20. The header
indicates the type of the NAL unit, the (potential) presence of bit
errors or syntax violations in the NAL unit payload, information
regarding the relative importance of the NAL unit for the decoding
process, the layer identification information, and other fields as
discussed below.
The syntax and semantics of the NAL unit header are specified in
[H.264], but the essential properties of the NAL unit header are
summarized below for convenience.
The first byte of the NAL unit header has the following format (the
bit fields are the same as defined for the one-byte H.264/AVC NAL
unit header, while the semantics of some fields have changed
slightly, in a backwards compatible way):
+---------------+
|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
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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 should be ignored.
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).
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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
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MUST be equal to "11" (in binary form). The value of RR should
be ignored.
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 RFC 3984, this payload format can only be used to carry
the raw NAL unit stream over RTP and not the byte stream format
specified in Annex B of [H.264].
The design principles, transmission modes, packetization modes as
well as new payload structures are summarized in this section. It
is assumed that the reader is familiar with the terminology and
concepts defined in RFC 3984.
1.2.1 Design Principles
The following design principles have been observed for this payload
format:
o Backward compatibility with [RFC3984] 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 [RFC3984]. This ensures that such an RTP
stream can be understood by RFC 3984 receivers.
o Media-Aware Network Elements (MANEs) as defined in [RFC3984] 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
Receiver Reports.
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
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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 RFC 3984. 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;
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 RFC 3984 must be used, thus ensuring
compatibility with RFC 3984 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 RFC 3984 packetization modes, namely Single NAL Unit Mode,
Non-Interleaved Mode, and Interleaved Mode, can be used.
MST should be used in a multicast session when different receivers
may request different layers of the scalable bitstream. An
operation point for an SVC bit stream, as defined in this memo,
corresponds to a set of layers that together conform to one of the
profiles defined in Annex A or G of [H.264] and, when decoded, offer
a representation of the original video at a certain fidelity. The
number of streams used in MST should be at least equal to the number
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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 RFC 3984 must be used, such that RFC 3984 receivers
can be part of the MST and receive only this session. For MST, this
memo defines four different MST specific packetization modes, namely
Non-Interleaved Timestamp based Mode (NI-T), Non-Interleaved Cross-
Layer Decoding Order Number (CS-DON) based Mode (NI-C), Non-
Interleaved Combined Timestamp and CS-DON Mode (NI-TC), and
Interleaved CS-DON based Mode (I-C) (detailed in Section 4.5.2).
The modes differ depending on whether the SVC data are allowed to be
interleaved, i.e., to be transmitted in an order different than the
intended decoding order, and they also differ in the mechanisms
provided in order to recover the correct decoding order of the NAL
units across the multiple RTP sessions. These four MST modes re-use
the packetization modes introduced in RFC 3984 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 RFC 3984 receivers with
implementation of the Non-Interleaved Mode specified in RFC 3984 can
join a multi-session transmission of SVC, to receive the base RTP
session encapsulated according to RFC 3984.
1.2.3 New Payload Structures
RFC 3984 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
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NAL units. Each NAL unit of a type specified in [H.264] (i.e., 1 to
23, inclusive) may be carried in its entirety in a single NAL unit
packet, may be aggregated in an aggregation packet, or may be
fragmented and carried in a number of fragmentation unit packets.
To enable aggregation or fragmentation of NAL units while still
ensuring that the RTP packet payload is only comprised of NAL units,
RFC 3984 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 RFC 3984.
Furthermore, three new types of NAL units are defined: namely
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].
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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
3.1.1 Definitions from the SVC Specification
This document uses the terms and definitions of [H.264]. The
following terms are relevant to this memo, and their definitions are
copied here from [H.264] for convenience.
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.
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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[SVC].
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.
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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 to. However, some
layer representations, succeeding an anchor layer representation
in decoding order but preceding the anchor layer representation
in output order, may refer to earlier layer representations for
inter prediction, and hence the decoding may be incorrect if
random access is performed at the anchor layer representation.
AVC base layer: The subset of the SVC base layer in which all
prefix NAL units (type 14) are removed. Note that this is
equivalent to the term "base layer" as defined in Annex G of
[H.264].
base RTP session: When multi-session transmission is used, the
RTP session that carries the RTP stream containing the T0 AVC
base layer or the T0 SVC base layer, and zero or more enhancement
layers. This RTP session does not depend on any other RTP
session as indicated by mechanisms defined in [I-D.ietf-mmusic-
decoding-dependency]. The base RTP session may carry NAL units
of NAL unit type equal to 14 and 15.
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
[I-D.ietf-mmusic-decoding-dependency]. A lower RTP session to an
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enhancement RTP session is an RTP session which the enhancement
RTP session depends on. The lowest RTP session for a receiver is
the RTP session that does not depend on any other RTP session
received by the receiver. The highest RTP session for a receiver
is the RTP session which no other RTP session received by the
receiver depends on.
cross-session decoding order number (CS-DON): A derived variable
indicating NAL unit decoding order number over all NAL units
within all the session-multiplexed RTP sessions that carry the
same SVC bitstream.
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 types 1, 5, or 12). A layer may
contain NAL units associated with more than one values of
temporal_id.
multi-session transmission: The transmission mode in which the
SVC bitstream is transmitted over multiple RTP sessions, with
each stream having the same SSRC. These multiple RTP streams can
be associated using the RTCP CNAME, or explicit signalling of the
SSRC used. Dependency between RTP sessions MUST be signaled
according to [I-D.ietf-mmusic-decoding-dependency] and this memo.
operation point: An operation point is identified by a set of
values of temporal_id, dependency_id, and quality_id. A
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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 Annex G of [H.264], and offers a
representation of the original video signal at a certain
fidelity.
Informative Note: Additional NAL units may be removed (with
lower dependency_id or same dependency_id but lower
quality_id) if they are not required for decoding the
bitstream at the particular operation point. The resulting
bitstream, however, may no longer conform to any of the
profiles defined in Annex A or G of [H.264].
operation point representation: The set of all NAL units of an
operation point within the same access unit.
RTP packet stream: A sequence of RTP packets with increasing
sequence numbers (except for wrap-around), identical PT and
identical SSRC (Synchronization Source), carried in one RTP
session. Within the scope of this memo, one RTP packet stream is
utilized to transport one or more layers.
single-session transmission: The transmission mode in which the
SVC bitstream is transmitted over a single RTP session, with a
single SSRC and separate timestamp and sequence number spaces.
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.
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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.
3.2 Abbreviations
In addition to the abbreviations defined in [RFC3984], 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 [RFC3984] 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.
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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 RFC 3984 as the value that the RTP
timestamp would have if that NAL unit would be transported in its
own RTP packet.
o Setting of the SSRC:
For both SST and MST, the SSRC values MUST be set according to [RFC
3550].
4.2 NAL Unit Extension and Header Usage
4.2.1 NAL Unit Extension
This memo specifies a NAL unit extension mechanism to allow for
introduction of new types of NAL units, beyond the three NAL unit
types left undefined in RFC 3984 (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 SHOULD ignore the entire NAL unit when Subtype is equal to
any of these reserved values.
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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 [RFC3984] 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 [RFC3984], the semantics
specified in Section 5.3 of [RFC3984] apply, i.e., NRI also
indicates the relative importance of NAL units. For a bitstream
conforming to one of the profiles defined in Annex G of [H.264] and
transported using this memo, in addition to the semantics specified
in Annex G of [H.264], NRI also indicates the relative importance of
NAL units within a layer.
For I, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to protect
NAL units with I equal to 1 better than NAL units with I equal to 0.
MANEs MAY also utilize information of NAL units with I equal to 1 to
decide when to forward more packets for an RTP packet stream. For
example, when it is detected that spatial layer switching has
happened such that the operation point has changed to a higher value
of DID, MANEs MAY start to forward NAL units with the higher value
of DID only after forwarding a NAL unit with I equal to 1 with the
higher value of DID.
Note that, in the context of this section, "protecting a NAL unit"
means any RTP or network transport mechanism that could improve the
probability of successful delivery of the packet conveying the NAL
unit, including applying a QoS-enabled network, Forward Error
Correction (FEC), retransmissions, and advanced scheduling behavior,
whenever possible.
For PRID, the semantics specified in Annex G of [H.264] apply. Note
that MANEs implementing unequal error protection MAY use this
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information to protect NAL units with smaller PRID values better
than those with larger PRID values, for example by including only
the more important NAL units in an FEC protection mechanism. The
importance for the decoding process decreases as the PRID value
increases.
For DID, QID, TID, in addition to the semantics specified in Annex G
of [H.264], according to this memo, values of DID, QID, or TID
indicate the relative importance in their respective dimension. A
lower value of DID, QID, or TID indicates a higher importance if the
other two components are identical. MANEs MAY use this information
to protect more important NAL units better than less important NAL
units.
For U, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to protect
NAL units with U equal to 1 better than NAL units with U equal to 0.
For D, in addition to the semantics specified in Annex G of [H.264],
according to this memo, MANEs MAY use this information to determine
whether a given NAL unit is required for successfully decoding a
certain Operation Point of the SVC bitstream, hence to decide
whether to forward the NAL unit.
4.3 Payload Structures
The NAL unit structure is central to H.264/AVC, RFC 3984, 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 RFC
3984, 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.
RFC 3984 specifies three basic payload structures (in Section 5.2 of
[RFC3984]): Single NAL Unit Packet, Aggregation Packet, and Fragment
Unit, and six new types (24 to 29) of NAL units. The value of the
Type field of the RTP packet payload header (i.e., the first byte of
the payload) may be equal to any value from 1 to 23 for a Single
NAL Unit Packet, any value from 24 to 27 for an Aggregation Packet,
and 28 or 29 for a Fragmentation Unit.
In addition to the NAL unit types defined originally for H.264/AVC,
SVC defines three new NAL unit types specifically for SVC: coded
slice in scalable extension NAL units (type 20), prefix NAL units
(type 14), and subset sequence parameter set NAL units (type 15), as
described in Section 1.1.
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This memo further introduces three new types of NAL units, PACSI NAL
unit (NAL unit type 30) as specified in Section 4.9, Empty NAL unit
(type 31, subtype 1) as specified in Section 4.10, and NI-MTAP NAL
unit (type 31, subtype 2) as specified in Section 4.7.1.
The RTP packet payload structure in RFC 3984 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 RFC 3984, and the other
two, Single NAL Unit Packet and Aggregation Packet, are extended as
follows. The value of the Type field of the payload header may be
equal to any value from 1 to 23, inclusive, and 30 to 31, inclusive,
for a Single NAL Unit Packet, and any value from 24 to 27,
inclusive, and 31, for an Aggregation Packet. When the Type field
of the payload header is equal to 31 and the Subtype field of the
payload header is equal to 2, the packet is an Aggregation Packet
(containing a NI-MTAP NAL unit). When the Type field of the payload
header is equal to 31 and the Subtype field of the payload header is
equal to 1, the packet is a Single NAL Unit Packet (containing an
Empty NAL unit).
Note that, in this memo, the length of the payload header varies
depending on the value of the Type field in the first byte of the
RTP packet payload. If the value is equal to 14, 20, or 30, the
first four bytes of the packet payload form the payload header;
otherwise if the value is equal to 31, the first two bytes of the
payload form the payload header; otherwise, the payload header is
the first byte of the packet payload.
Table 1 lists the NAL unit types introduced in SVC and this memo and
where they are described in this memo. Table 2 summarizes the basic
payload structure types for all NAL unit types when they are
directly used as RTP packet payloads according to this memo.
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Table 1. NAL unit types introduced in SVC and this memo
Type Subtype NAL Unit Name Section Numbers
-----------------------------------------------------------
14 - Prefix NAL unit 1.1
15 - Subset sequence parameter set 1.1
20 - Coded slice in scalable extension 1.1
30 - PACSI NAL unit 4.9
31 0 reserved 4.2.1
31 1 Empty NAL unit 4.10
31 2 NI-MTAP 4.7.1
31 3-31 reserved 4.2.1
Table 2. Basic payload structure types for all NAL unit
types when they are directly used as RTP packet payloads
Type Subtype Basic Payload Structure
------------------------------------------
0 - reserved
1-23 - Single NAL Unit Packet
24-27 - Aggregation Packet
28-29 - Fragmentation Unit
30 - Single NAL Unit Packet
31 0 reserved
31 1 Single NAL Unit Packet
31 2 Aggregation Packet
31 3-31 reserved
Table 3 summarizes the NAL unit types allowed to be aggregated
(i.e., used as aggregation units in aggregation packets) or
fragmented (i.e., carried in fragmentation units) according to this
memo.
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Table 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 RFC 3984 applies with the
following modifications.
The packetization modes specified in Section 5.4 of RFC 3984, 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.
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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 RFC 3984. 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).
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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 RFC 3984.
The NI-T mode uses timestamps to recover the decoding order of NAL
units, whereas the NI-C and I-C modes both use the CS-DON mechanism
(explained later on) to do so. The NI-TC mode provides both
timestamps and the CS-DON method; receivers in this case may choose
to use either method for performing decoding order recovery
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
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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 RFC 3984. 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 - -
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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.
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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 [RFC3984] applies with the following modifications.
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 [RFC3984], 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. NI-MTAPs used in MST MUST always guarantee packets in the
corresponding RTP sessions with the same RTP timestamp as the NI-
MTAP.
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".
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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 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.
A PACSI NAL unit contained in an NI-MTAP MUST NOT have the DONC
field.
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 RFC
3984 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)
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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 3 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 [RFC3984] applies.
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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 [RFC3984]). 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.
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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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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, C, S, and E are specified only if the bit X is equal
to 1. 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).
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o The Type field MUST be set to 30.
o The R bit MUST be set to 1. Receivers SHOULD 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).
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o The U bit MUST be set to 1 if the U bit of at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the U bit of the next non-PACSI NAL unit in transmission order is
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the U bit MUST be set to 0.
o The D bit MUST be set to 1 if the D value of all the remaining
NAL unit in the aggregation packet is equal to 1 (when the PACSI
NAL unit is included in an aggregation packet) or if the D bit of
the next non-PACSI NAL unit in transmission order is equal to 1
(when the PACSI NAL unit is included in a single NAL unit
packet). Otherwise, the D bit MUST be set to 0.
o The O bit MUST be set to 1 if the O bit of at least one of the
remaining NAL units in the aggregation packet is equal to 1 (when
the PACSI NAL unit is included in an aggregation packet) or if
the O bit of the next non-PACSI NAL unit in transmission order is
equal to 1 (when the PACSI NAL unit is included in a single NAL
unit packet). Otherwise, the O bit MUST be set to 0.
o The RR field MUST be set to "11" (in binary form). Receivers
SHOULD 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 SHOULD 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 SHOULD 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.
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 with
the J field equal to 1.
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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.
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o The S bit MUST be set to 1, if the first NAL unit following the
PACSI NAL unit in an aggregation packet is the first VCL NAL
unit, in decoding order, of a layer representation (when the
PACSI NAL unit is included in an aggregation packet) or if the
next non-PACSI NAL unit in transmission order is the first VCL
NAL unit, in decoding order, of a layer representation(when the
PACSI NAL unit is included in a single NAL unit packet).
Otherwise, the S bit MUST be set to 0.
o The E bit MUST be set to 1, if the last NAL unit following the
PACSI NAL unit in an aggregation packet is the last VCL NAL unit,
in decoding order, of a layer representation (when the PACSI NAL
unit is included in an aggregation packet) or if the next non-
PACSI NAL unit in transmission order is the last VCL NAL unit, in
decoding order, of a layer representation (when the PACSI NAL
unit is included in a single NAL unit packet). Otherwise, the E
field MUST be set to 0.
Informative note: In an aggregation packet it is always possible
to detect the beginning or end of a layer representation by
detecting changes in the values of dependency_id, quality_id, and
temporal_id in NAL unit headers, except from the first and last
NAL units of a packet. The S or E bits are used to provide this
information, for both signal 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).
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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
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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 RFC 3984 and is used to recover the
decoding order when interleaving is used within a single session.
Section 5.5 of [RFC3984] 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.
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In addition to the timestamp mechanism described later on, the CS-
DON mechanism is an extension of the DON facility that can be used
for this purpose, and is defined in the following section.
4.11.1 Cross-Session DON (CS-DON) for Multi-Session Transmission
The Cross-Session Decoding Order Number (CS-DON) is a number that
indicates the decoding order of NAL units across all RTP sessions
involved in MST. It is similar to the DON concept in RFC 3984, but
contrary to RFC 3984 where the DON was used only for interleaved
packetization, in this memo it is used not only in the interleaved
MST mode (I-C) but also in two of the non-interleaved MST modes as
well (NI-C and NI-TC).
When the NI-C or NI-TC MST modes are in use, the packetization of
each session MUST be as specified in Section 5.2.2. In PACSI NAL
units the CS-DON value is explicitly coded in the field DONC. For
non-PACSI NAL units the CS-DON value is derived as follows. Let SN
indicate the RTP sequence number of a packet.
o For each non-PACSI NAL unit carried in a session using the single
NAL unit session packetization mode, the CS-DON value of the NAL
unit is equal to (DONC_prev_PACSI + SN_diff - 1) % 65536, wherein
"%" is the modulo operation, DONC_prev_PACSI is the DONC value of
the previous PACSI NAL unit with the same NALU-time as the
current NAL unit, and SN_diff is calculated as follows:
if SN1 > SN2, SN_diff = SN1 - SN2
else SN_diff = SN2 + 65536 - SN1
where SN1 and SN2 are the SNs of the current NAL unit and the
previous PACSI NAL unit with the same NALU-time, respectively.
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;
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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 RFC 3984 for all the NAL units in each of the
RTP sessions MUST indicate CS-DON values.
5. Packetization Rules
Section 6 of [RFC3984] 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 RFC 3984. 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
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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 of [RFC3984].
Informative note: The non-interleaved mode does allow an
application to encapsulate a single NAL unit in a single RTP
packet. Historically, the single NAL unit mode has been included
into [RFC3984] 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, STAP-A and FU-A implementation is required.
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 RFC 3984. This way, an RFC 3984 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
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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
which depends on RTP session A.
Informative note: This rules may require the insertion of NAL
units, typically when temporal scalability is used, i.e., an
enhancement RTP session does not contain any NAL units for an
access unit with a particular NTP timestamp (media timestamp),
which however is present in a lower enhancement RTP session or
the base RTP session. There are two ways to insert additional NAL
units in order to satisfy this rule:
- One option for adding additional NAL units is to use Empty NAL
units (defined in Section 4.10), which can be used by the process
described in Section 6.2.1 for the access unit re-ordering
process.
- Additional NAL units may also be added by the encoder itself,
for example by transmitting coded data that simply instruct the
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decoder to repeat the previous picture. This option, however,
may be difficult to use with pre-encoded content.
If a packet must be inserted for satisfying the above rule, the NTP
timestamp of such an inserted packet must be set equal to the NTP
timestamp of a packet of the access unit present in any lower
enhancement RTP session and 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. [Ed. (AE): What if there is no such
previous report?] [Ed. (AE): Should we require that if one session
sends an SR, then all sessions must transmit an SR at the same time
even at that time there is no RTP packet to transmit for some of the
sessions? This would solve this problem.]
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 Calculation of RTP timestamp for packet insertion
in an enhancement layer RTP session
The vertical bars ("|")in the NTP timeline in the figure above
indicate that access unit data is present in at least one of the
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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.
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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.
If the FU-A packet is the start of the fragmented NAL unit,
the following applies;
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;
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.
Otherwise if the FU-A packet is the end of the fragmented NAL
unit, the following applies.
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.
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.
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5.2.3 I-C Packetization Rules
When the I-C MST packetization mode is in use, the following
applies.
o When a PACSI NAL unit is present, the T bit MUST be equal to 0,
i.e., the DONC field is not present, and the Y bit MUST be equal
to 0, i.e., the TL0PICIDX and IDRPICID are not present.
5.2.4 Packetization Rules for Non-VCL NAL Units
NAL units which do not directly encode video slices are known in
H.264 as non-VCL NAL units. Non-VCL units that are only used by, or
only relevant to, enhancement RTP sessions SHOULD be sent in the
lowest session to which they are relevant.
Some senders, however, such as those sending pre-encoded data, may
be unable to easily determine which non-VCL units are relevant to
which session. Thus, non-VCL NAL units MAY, instead, be sent in a
session that the session using these non-VCL NAL units depends on
(e.g., the base RTP session).
If a non-VCL unit is relevant to more than one RTP session, neither
of which depends on the other(s), the NAL unit MAY be sent in
another session which all these sessions depend on.
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 RFC 3984, 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 [RFC3984]
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.
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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 [I-D.ietf-mmusic-decoding-dependency]. An enhancement
RTP session typically contains an RTP stream that depends on at
least one other RTP session, as indicated by mechanisms defined in
[I-D.ietf-mmusic-decoding-dependency]. A lower RTP session to an
enhancement RTP session is an RTP session which the enhancement RTP
session depends on. The lowest RTP session for a receiver is the
base RTP session, which does not depend on any other RTP session
received by the receiver. The highest RTP session for a receiver is
the RTP session which no other RTP session received by the receiver
depends on.
For each of the RTP sessions, the RTP reception process as specified
in RFC 3550 is applied. Then the received packets are passed into
the payload de-packetization process as defined in this memo.
The decoding order of the NAL units carried in all the associated
RTP sessions is then recovered by applying one of the following
subsections, depending on which of the MST packetization modes is in
use.
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, e.g., 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.
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Informative note: It is possible to use the media timestamp TS
directly (i.e., without having to first convert it to an NTP
timestamp), if every RTP session uses the same random initial
value for the timestamp [Lennox].
For NI-MTAP packets the NALU-time is derived for each contained NAL
unit by using the "TS offset" value in the NI-MTAP packet as defined
in Section 4.10, and is used instead of the RTP packet timestamp to
derive the media timestamp, e.g., using the NTP wall clock as
provided via RTCP sender reports. NAL units contained in
fragmentation packets are handled as defragmented, entire NAL units
with their own media timestamps. All NAL units associated with the
same value of media timestamp TS are part of the same access unit
AU(TS).Any Empty NAL units SHOULD be kept as, effectively, access
unit indicators in the re-ordering process. Empty NAL units and
PACSI NAL units SHOULD be removed before passing access unit data to
the decoder.
Informative note: These Empty NAL units are used to associate
NAL units present in other RTP sessions with RTP sessions not
containing any data for an access unit of a particular time
instance. They act as access unit indicators in sessions that
would otherwise contain no data for the particular access
unit. The presence of these NAL units is ensured by the
packetization rules in Section 5.2.1.
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:
o The process should be started with the NAL units received in
the highest RTP session with the first media timestamp TS
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).
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o Place the collected NAL units in the order of session
dependency as derived by the dependency indication provided
by mechanisms described 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.
Informative example:
The example shown in Figure 6 refers to three RTP sessions A, B and
C containing an SVC bitstream transmitted as 3 sources. In the
example, the dependency signaling (described in Section 7.2.3)
indicates that session A is the base RTP session, B is the first
enhancement RTP session and depends on A, and C is the second
enhancement RTP session and depends on A and B. A hierarchical
picture coding prediction structure is used, in which Session A has
the lowest frame rate and Session B and C have the same but higher
frame rate.
The figure shows NAL units contained in RTP packets which are stored
in the de-jittering buffer at the receiver for session de-
packetization. The NAL units are already re-ordered according to
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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 (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
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"|" - indicates corresponding NAL units of the
same access unit AU(TS[..]) in the RTP
sessions
Integer values in "[]" - media timestamp (TS), sampling time
as, e.g., derived from NTP timestamp
associated to 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 which are
discarded by an implementation's decoding process do not need to be
reordered.
In this algorithm, NAL units within an access unit are first ordered
by NAL unit type, in the order specified in Table 12 below, except
from NAL unit type 14 which is handled specially as described in the
table. NAL units of the same type are then ordered as specified for
the type, if necessary.
For the purposes of this algorithm, "session order" is the order of
NAL units implied by their transmission order within an RTP session.
For the Non-Interleaved and Single NAL unit modes, this is the RTP
sequence number order coupled with the order of NAL units within an
aggregation unit.
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Table 12 Ordering of NAL unit types within in Access Unit
Type Description / Comments
-----------------------------------------------------------
9 Access unit delimiter
7 Sequence parameter set
13 Sequence parameter set extension
15 Subset sequence parameter set
8 Picture parameter set
16-18 Reserved
6 Supplemental enhancement information (SEI)
If an SEI message with a first payload of 0 (Buffering
Period) is present, it must be the first SEI.
If SEI messages with a Scalable Nesting (30) payload and
a nested payload of 0 (Buffering Period) are present,
these then follow. Such an SEI message with the
all_layer_representations_in_au_flag equal to 1 is
placed first, followed by any others, sorted in DQId
order by the highest DQId mentioned.
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.)
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14 Prefix NAL unit in scalable extension
12 Filler data
Any order of filler data units within an access unit is
valid.
If NAL units of type 14 are present, every filler data
NAL unit is prefixed by a NAL unit of type 14.
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 DQId order, based on the
dependency_id and quality_id values in the slice's NAL
unit header extension. Within each DQId, they are
placed in session order. (Note: SVC slices with a given
DQId value will be sent on 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 remultiplexing buffer.
It is RECOMMENDED to set the size of the remultiplexing buffer (in
bytes) equal to or greater than the value of the sprop-remux-buf-req
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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 RFC 3984.
o Function don_diff is specified in Section 5.5 of RFC 3984.
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 remultiplexing 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.
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The NAL units to be removed from the remultiplexing buffer are
determined as follows:
o If the remultiplexing buffer contains at least N VCL NAL units,
NAL units are removed from the remultiplexing 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
remultiplexing 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
remultiplexing 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.
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Some parameters provide a receiver with the properties of the stream
that will be sent. The names of all these parameters start with
"sprop" for stream properties. Some of these "sprop" parameters are
limited by other payload or codec configuration parameters. For
example, the sprop-parameter-sets parameter is constrained by the
profile-level-id parameter. The media sender selects all "sprop"
parameters rather than the receiver. This uncommon characteristic
of the "sprop" parameters may be incompatible with some signaling
protocol concepts, in which case the use of these parameters SHOULD
be avoided.
7.1 Media Type Registration
The media subtype for the SVC codec is allocated from the IETF tree.
The receiver SHOULD ignore any unspecified parameter.
Informative note: Requiring that the receiver ignores unspecified
parameters allows for backward compatibility of future
extensions. For example, if a future specification that is
backward compatible to this specification specifies some new
parameters, then a receiver according to this specification is
capable of receiving data per the new payload but ignoring those
parameters newly specified in the new payload specification.
This provision is also present in RFC 3984.
Media Type name: video
Media subtype name: H264-SVC
[Ed. (TS): Text on "H264" must go into different section, see
Colin's comments sent on 10 June 2008]
The media subtype "H264" MUST be used for RTP streams using RFC
3984, i.e., not using any of the new features introduced by this
specification compared to RFC 3984. [Ed. (YkW): The new features
are to be listed herein.]
For RTP streams using any of the new features introduced by this
specification compared to RFC 3984, the media subtype "H264-SVC"
SHOULD be used, and the media subtype "H264" MAY be used. Use of
the media subtype "H264" for RTP streams using the new features
allows for RFC 3984 receivers to negotiate and receive H.264/AVC or
SVC streams packetized according to this specification, but to
ignore media parameters and NAL unit types it does not recognize.
Required parameters: none
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OPTIONAL parameters:
[Ed. (YkW): Check whether the current semantics of each of the
parameters from RFC 3984 apply accurately for both SST and MST. For
example, for packetization-mode, some additional constraints of the
value depending on the value of mst-mode should be added.]
profile-level-id:
A base16 [RFC3548] (hexadecimal) representation of the
following three bytes in the sequence parameter set NAL unit
specified in [H.264]: 1) profile_idc, 2) a byte herein
referred to as profile-iop, composed of the values of
constraint_set0_flag, constraint_set1_flag,
constraint_set2_flag, constraint_set3_flag, and
reserved_zero_4bits in bit-significance order, starting from
the most significant bit, and 3) level_idc. Note that
reserved_zero_4bits is required to be equal to 0 in [H.264],
but other values for it may be specified in the future by ITU-
T or ISO/IEC.
If the profile-level-id parameter is used to indicate
properties of a NAL unit stream, it indicates the profile and
level that a decoder has to support in order to comply with
[H.264] when it decodes the NAL unit stream. The profile-iop
byte indicates whether the NAL unit stream also obeys all the
constraints as specified in subsection G.7.4.2.1.1 of [H.264].
Herein the NAL unit stream refers to the one consisting of all
NAL units conveyed in the current RTP session, and all NAL
units conveyed in other RTP sessions, if present, that the
current RTP session depends on. The current RTP session MAY
depend on other RTP sessions when a scalable bitstream is
transported with more than one RTP session and the current
session is not an independent RTP session.
If the profile-level-id parameter is used for capability
exchange or session setup, it indicates the profile that the
codec supports and the highest level supported for the
signaled profile. The profile-iop byte indicates whether the
codec has additional limitations whereby only the common
subset of the algorithmic features and limitations signaled
with the profile-iop byte is supported by the codec. For
example, if a codec supports only the common subset of the
coding tools of the Baseline profile and the Main profile at
level 2.1 and below, the profile-level-id becomes 42E015, in
which 42 stands for the Baseline profile, E0 indicates that
only the common subset for all profiles is supported, and 15
indicates level 2.1.
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Informative note: Capability exchange and session setup
procedures should provide means to list the capabilities
for each supported codec profile separately. For example,
the one-of-N codec selection procedure of the SDP
Offer/Answer model can be used (section 10.2 of [RFC4566]).
If no profile-level-id is present, the Baseline Profile
without additional constraints at Level 1 MUST be implied.
max-mbps, max-fs, max-cpb, max-dpb, and max-br:
The common property of these parameters is as specified in RFC
3984.
max-mbps: This parameter is as specified in RFC 3984.
max-fs: This parameter is as specified in RFC 3984.
max-cpb: The value of max-cpb is an integer indicating the
maximum coded picture buffer size in units of 1000 bits for
the VCL HRD parameters (see A.3.1 item i or G.10.2.2 item g of
[H.264]) and in units of 1200 bits for the NAL HRD parameters
(see A.3.1 item j or G.10.2.2 item h of [H.264]). The max-cpb
parameter signals that the receiver has more memory than the
minimum amount of coded picture buffer memory required by the
signaled level conveyed in the value of the profile-level-id
parameter. When max-cpb is signaled, the receiver MUST be
able to decode NAL unit streams that conform to the signaled
level, with the exception that the MaxCPB value in Table A-1
of [H.264] for the signaled level is replaced with the value
of max-cpb. The value of max-cpb MUST be greater than or
equal to the value of MaxCPB for the level given in Table A-1
of [H.264]. Senders MAY use this knowledge to construct coded
video streams with greater variation of bit rate than can be
achieved with the MaxCPB value in Table A-1 of [H.264].
Informative note: The coded picture buffer is used in the
Hypothetical Reference Decoder (HRD, Annex C) of [H.264].
The use of the HRD is recommended in SVC encoders to verify
that the produced bitstream conforms to the standard and to
control the output bit rate. Thus, the coded picture
buffer is conceptually independent of any other potential
buffers in the receiver, including remultiplexing 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
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for video decoder can be integrated with the remultiplexing
and de-jitter buffers of the receiver.
max-dpb: This parameter is as specified in RFC 3984.
max-br: The value of max-br is an integer indicating the maximum
video bit rate in units of 1000 bits per second for the VCL
HRD parameters (see A.3.1 item i or G.10.2.2 item g of
[H.264]) and in units of 1200 bits per second for the NAL HRD
parameters (see A.3.1 item j or G.10.2.2 item h of [H.264]).
The max-br parameter signals that the video decoder of the
receiver is capable of decoding video at a higher bit rate
than is required by the signaled level conveyed in the value
of the profile-level-id 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 level, conveyed in the profile-level-id parameter,
with the following exceptions in the limits specified by the
level:
o The value of max-br replaces the MaxBR value of the
signaled level (in Table A-1 of [H.264]).
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 level).
For example, if a receiver signals capability for Level 1.2
with max-br equal to 1550, this indicates a maximum video
bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum
video bitrate of 1860 kbits/sec for NAL HRD parameters, and a
CPB size of 4036458 bits (1550000 / 384000 * 1000 * 1000).
The value of max-br MUST be greater than or equal to the value
MaxBR for the signaled level given in Table A-1 of [H.264].
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
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that the network is capable of handling such bit rates at
any given time. In particular, no conclusion can be drawn
that the signaled bit rate is possible under congestion
control constraints.
redundant-pic-cap:
This parameter is as specified in RFC 3984.
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 MUST be placed in the NAL unit stream to precede
any other NAL units in decoding order by the receiver. The
parameter MUST NOT be used to indicate codec capability in any
capability exchange procedure. The value of the parameter is
the base64 [RFC3548] representation of the initial 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]. The parameter sets are conveyed in
decoding order, and no framing of the parameter set NAL units
takes place. A comma is used to separate any pair of
parameter sets in the list. 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 double-buffer
all sprop-parameter-sets and make them available to the
decoder instance that decodes a certain payload type.
parameter-add:
This parameter is as specified in RFC 3984.
packetization-mode:
This parameter is as specified in RFC 3984.
sprop-interleaving-depth:
This parameter is as specified in RFC 3984.
sprop-deint-buf-req:
This parameter is as specified in RFC 3984.
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deint-buf-cap:
This parameter is as specified in RFC 3984.
sprop-init-buf-time:
This parameter is as specified in RFC 3984.
sprop-max-don-diff:
This parameter is as specified in RFC 3984.
max-rcmd-nalu-size:
This parameter is as specified in RFC 3984.
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
carried in the current RTP session and the RTP sessions the
current RTP session depends on. 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.
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
sessionand all the RTP sessions the current RTP session
depends on.
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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
carried in the current RTP session and the RTP sessions the
current RTP session depends on. It MUST be set to a value one
less than the minimum remultiplexing 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
remultiplexing buffer for the NAL unit stream carried in the
current RTP session and the RTP sessions the current RTP
session depends on. 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
remultiplexing buffer size is of at least the value of sprop-
remux-buf-req in terms 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 signals the capabilities of a receiver
implementation and indicates the amount of remultiplexing
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.
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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 a NAL
unit stream carried in the current RTP session and the RTP
sessions the current RTP session depends on. 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 a NAL
unit stream carried in the current RTP session and the RTP
sessions the current RTP session depends on. 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 RFC
3984, 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
representation of the NAL unit containing the scalability
information SEI message. If present, the NAL unit MUST
contain only a scalability information SEI message.
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This parameter MAY be used in an offering or declarative SDP
message to indicate what Layers can be provided. A receiver
MAY indicate its choice of one Layer using the optional media
type parameter scalable-layer-id.
sprop-layer-range:
This parameter MAY be used to signal two sets of the layer
identification values of the lowest and highest operation
points conveyed in the RTP session. Each set is a base16
representation of a three-character value, with the first
character representing DID, the second character representing
QID, and the third character representing TID. The two sets
are comma separated. Let DIDl and DIDh be the lowest DID
value and the highest DID value, respectively, among all the
NAL units conveyed in the RTP session. Let QIDl and TIDl be
the lowest QID value and the lowest TID value, respectively,
among all the NAL units that are conveyed in the RTP session
and that have DID equal to DIDl. Let QIDh and TIDh be the
highest QID value and the highest TID value, respectively,
among all the NAL units that are conveyed in the RTP session
and that have DID equal to DIDh. The first set indicates the
DID, QID and TID values of the lowest operation point, for
which the DID, QID and TID values are equal to DIDl, QIDl, and
TIDl, respectively. The second set indicates the DID, QID and
TID values of the highest operation point, for which the DID,
QID and TID values are equal to DIDh, QIDh, and TIDh,
respectively.
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. 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].
[Ed. (TS): That is, a SDP capable receiver/middle-box must
(YK: Not must, but may, only when desired) decode the sprop-
scalability-info syntax, which is not specified in this memo,
to select a scalable-layer-id. This is currently not
addressed in the offer answer section!]
sprop-frame-size:
This parameter MAY be used to signal the pixel dimensions of
decoded frames of the lowest and the highest operation points
conveyed in the current RTP session. The value is a base16
representation of the width and height of decoded frames of
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the lowest operation point, followed by the width and height
of decoded frames of the highest operation point, in pixels.
The four values of width or height are separated by three
commas.
sprop-bit-rate:
This parameter MAY be used to signal the bitrate values of the
lowest and the highest operation points conveyed in the
current RTP session. The value is a base16 representation of
the bitrate values of the lowest and the highest operation
points, in bits/s, separated by a comma.
sprop-frame-rate:
This parameter MAY be used to signal the frame rate values of
the lowest and the highest operation points conveyed in the
current RTP session. The value is a base16 representation of
the frame rate values of the lowest and the highest operation
points, in frames/s, multiplied by 1000. The two frame rate
values are separated by a comma.
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Security considerations:
See Section 8 of RFC XXXX.
Public specification:
Please refer to Section 14 of RFC XXXX.
Additional information:
None
File extensions: none
Macintosh file type code: none
Object identifier or OID: none
Person & email address to contact for further information:
Intended usage: COMMON
Author:
Change controller:
IETF Audio/Video Transport working group delegated from the
IESG.
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7.2 SDP Parameters
[Ed. (YkW): For agreeing on a Layer or OP in unicast, an SDP can
contain multiple m lines with bit rate, frame rate and spatial
resolution parameters available, in addition to sprop-scalability-
info. The receive can select one of the m lines, or, for operation
points that are not included in the m lines, one of the "scalable
layers" specified by sprop-scalability-info using scalable-layer-id.
For layered multicast, then the grouping signaling in I-D.ietf-
mmusic-decoding-dependency is needed.
The above would conveniently support also the normal ROI use cases
(with a few ROIs each indicated as a "scalable layer") but not the
interactive ROI use cases. The quality layer using priority_id use
cases are not supported either. That would need one more optional
media type parameter, to identify a quality layer. The lightweight
transcoding use cases should be supported well by using (multiple)
normal AVC SDP offering messages.]
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-mbps", "max-fs",
"max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
parameter-sets", "parameter-add", "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", "sprop-layer-
range", "sprop-scalability-info", "scalable-layer-id", "sprop-
frame-size", "sprop-bit-rate", and "sprop-frame-rate", when
present, MUST be included in the "a=fmtp" line of SDP. These
parameters are expressed as a media type string, in the form of a
semicolon separated list of parameter=value pairs.
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7.2.2 Usage with the SDP Offer/Answer Model
When 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, if required by
"packetization-mode", "sprop-deint-buf-req". [Ed. (AE): mst-mode
should be added?] These three parameters MUST be used
symmetrically; 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.
Informative note: The requirement for symmetric use applies
only for the above three parameters and not for the other
stream properties and capability parameters.
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 ("profile-level-id", "packetization-mode", and, if
present, "sprop-deint-buf-req") is the same as in the offer.
Informative note: An offerer, when receiving the answer, has
to compare payload types not declared in the offer based on
media type (i.e., video/H264-SVC) and the above three
parameters with any payload types it has already declared, in
order to determine whether the configuration in question is
new or equivalent to a configuration already offered.
An answerer MAY select from the layers offered in the "sprop-
scalability-information" parameter by including "scalable-layer-
id" or "sprop-layer-range" in the answer. [Ed (YK): do we need to
additionally define behavior with snd/rcvonly parameter? Also,
add example.]
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o The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
"sprop-interleaving-depth", "sprop-max-don-diff", "sprop-init-
buf-time", "sprop-scalability-information", "sprop-layer-range"
describe the properties of the NAL unit stream that the offerer
or answerer is sending for this 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 then being
bound to the payload type, the values may have to be applied
to another payload type when being sent, as they apply for the
configuration.
o The capability parameters ("max-mbps", "max-fs", "max-cpb",
"max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size")
MAY be used to declare further capabilities. Their
interpretation depends on the direction attribute. When the
direction attribute is sendonly, then the parameters describe the
limits of the RTP packets and the NAL unit stream that the sender
is capable of producing. When the direction attribute is
sendrecv or recvonly, then the parameters describe the
limitations of what the receiver accepts.
o As specified above, an offerer has to include the size of the
deinterleaving buffer in the offer for an interleaved SVC stream.
To enable the offerer and answerer to inform each other about
their capabilities for deinterleaving buffering, both parties are
RECOMMENDED to include "deint-buf-cap". This information MAY be
used when the value for "sprop-deint-buf-req" is selected in a
second round of offer and answer. For interleaved streams, it is
also RECOMMENDED to consider offering multiple payload types with
different buffering requirements when the capabilities of the
receiver are unknown.
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o The "sprop-parameter-sets" parameter is used as described above.
In addition, an answerer MUST maintain all parameter sets
received in the offer in its answer. Depending on the value of
the "parameter-add" parameter, different rules apply: If
"parameter-add" is false (0), the answer MUST NOT add any
additional parameter sets. If "parameter-add" is true (1), the
answerer, in its answer, MAY add additional parameter sets to the
"sprop-parameter-sets" parameter. The answerer MUST also,
independent of the value of "parameter-add", accept to receive a
video stream using the sprop-parameter-sets it declared in the
answer.
Informative note: care must be taken when parameter sets are
added not to cause overwriting of already transmitted
parameter sets by using conflicting parameter set identifiers.
For streams being delivered over multicast, the following rules
apply in addition:
o The stream properties parameters ("sprop-parameter-sets", "sprop-
deint-buf-req", "sprop-interleaving-depth", "sprop-max-don-diff",
"sprop-init-buf-time", "sprop-scalability-information", and
"sprop-layer-range") MUST NOT be changed by the answerer. Thus,
a payload type can either be accepted unaltered or removed.
o The receiver capability parameters "max-mbps", "max-fs", "max-
cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be
supported by the answerer for all streams declared as sendrecv or
recvonly; otherwise, one of the following actions MUST be
performed: the media format is removed, or the session rejected.
o The receiver capability parameter redundant-pic-cap SHOULD be
supported by the answerer for all streams declared as sendrecv or
recvonly as follows: The answerer SHOULD NOT include redundant
coded pictures in the transmitted stream if the offerer indicated
redundant-pic-cap equal to 0. Otherwise (when redundant_pic_cap
is equal to 1), it is beyond the scope of this memo to recommend
how the answerer should use redundant coded pictures.
Below are the complete lists of how the different parameters shall
be interpreted in the different combinations of offer or answer and
direction attribute.
o In offers and answers for which "a=sendrecv" or no direction
attribute is used, or in offers and answers for which
"a=recvonly" is used, the following interpretation of the
parameters MUST be used.
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Declaring actual configuration or properties for receiving:
- profile-level-id
- packetization-mode
Declaring actual properties of the stream to be sent (applicable
only when "a=sendrecv" or no direction attribute is used):
- sprop-deint-buf-req
- sprop-interleaving-depth
- sprop-parameter-sets
- sprop-max-don-diff
- sprop-init-buf-time
- sprop-scalability-information
- sprop-layer-range
- scalable-layer-id
Declaring receiver implementation capabilities:
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- redundant-pic-cap
- deint-buf-cap
- max-rcmd-nalu-size
Declaring how Offer/Answer negotiation shall be performed:
- parameter-add
o In an offer or answer for which the direction attribute
"a=sendonly" is included for the media stream, the following
interpretation of the parameters MUST be used:
Declaring actual configuration and properties of stream proposed
to be sent:
- profile-level-id
- packetization-mode
- sprop-deint-buf-req
- sprop-max-don-diff
- sprop-init-buf-time
- sprop-parameter-sets
- sprop-interleaving-depth
- sprop-scalability-information
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- sprop-layer-range
- sprop-spatial-resolution
Declaring how Offer/Answer negotiation shall be performed:
- parameter-add
Furthermore, the following considerations are necessary:
o 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. "sprop-parameter-sets" MUST NOT be used in a sender's
declaration of its capabilities, as the limits of the values that
are carried inside the parameter sets are implicit with the
profile and level used.
o Parameters declaring a configuration point are not downgradable,
with the exception of the level part of the "profile-level-id"
parameter. This expresses values a receiver expects to be used
and must be used verbatim on the sender side.
o 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. 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.
o A receiver SHOULD understand all MIME 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
receiver of the offer.
o 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
properties 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.
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o If an offerer wishes to have non-symmetric capabilities between
sending and receiving, the offerer has to offer different RTP
sessions; i.e., different media lines declared as "recvonly" and
"sendonly", respectively. This may have further implications on
the system.
7.2.3 Usage with Multi-Source Transmission
If MST is used, the rules on signaling media decoding dependency in
SDP as defined in [I-D.ietf-mmusic-decoding-dependency] apply.
7.2.4 Usage in Declarative Session Descriptions
When SVC over RTP is offered with SDP in a declarative style, as in
RTSP [RFC2326] or SAP [RFC2974], the following considerations are
necessary.
o All parameters capable of indicating the properties of both a NAL
unit stream and a receiver are used to indicate the properties of
a NAL unit stream. For example, in this case, the parameter
"profile-level-id" declares the values used by the stream,
instead of the capabilities of the sender. This results in that
the following interpretation of the parameters MUST be used:
Declaring actual configuration or properties:
- profile-level-id
- sprop-parameter-sets
- packetization-mode
- sprop-interleaving-depth
- sprop-deint-buf-req
- sprop-max-don-diff
- sprop-init-buf-time
- sprop-layer-range
- sprop-spatial-resolution
- sprop-scalability-info
Not usable:
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- redundant-pic-cap
- max-rcmd-nalu-size
- parameter-add
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- deint-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
7.3.1 Example for Offering A Single SVC Session
Offerer -> Answerer SDP message:
v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.12
s=SVC SDP example
i=SVC Scalable Video Coding session
t=2873397496 2873404696
m=video 20000 RTP/AVP 96 97 98
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=1;
sprop-parameter-sets=Z01ACprLFicg,aP4Eag==;
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,aGvgRiA=;
a=rtpmap:98 H264-SVC/90000
a=fmtp:98 profile-level-id=53000c; packetization-mode=2;
init-buf-time=156320;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,aGvgRiA=;
7.3.2 Example for Offering Session Multiplexing
Offerer -> Answerer SDP message:
v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.12
s=SVC Scalable Video Coding session
i=SDP is an Offer for a session offered by a transcoding entity
t=2873397496 2873404696
a=group:DDP 1 2 3
m=video 20000 RTP/AVP 96 97 98
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4d400a; packetization-mode=0;
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mst-mode=NI-T; sprop-parameter-sets=Z01ACprLFicg,aP4Eag==;
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
mst-mode=NI-TC; sprop-parameter-sets=Z01ACprLFicg,
Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgRqA=,aGvgRiA=;
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=53000c; packetization-mode=2;
mst-mode=I-C; init-buf-time=156320;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,aGvgRiA=;
a=mid:1
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-TC; sprop-parameter-sets=Z01ACprLFicg,
Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgRqA=,aGvgRiA=;
a=rtpmap:100 H264-SVC/90000
a=fmtp:100 profile-level-id=53000c; packetization-mode=2;
mst-mode=I-C; sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,
aP4Eag==,aEvgRqA=,aGvgRiA=;
a=mid:2
a=depend:99 lay 1:96,97; 100 lay 1:98
m=video 20004 RTP/AVP 101
a=rtpmap:101 H264-SVC/90000
a=fmtp:101 profile-level-id=53000c; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets=Z01ACprLFicg,
Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgRqA=,aGvgRiA=;
a=mid:3
a=depend:101 lay 1:96,97 2:99
7.4 Parameter Set Considerations
Please see Section 8.4 of [RFC3984].
8. Security Considerations
Section 9 of [RFC3984] applies. Additionally, the following
applies.
Decoders MUST exercise caution with respect to the handling of
reserved NAL unit types and reserved SEI messages, particularly if
they contain active elements, and MUST restrict their domain of
applicability to the presentation containing the stream. The safest
way is to simply discard these NAL units and SEI messages.
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When integrity protection is applied, 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.
Informative note: Other security aspects, including
confidentiality, authentication, and denial-of-service threat,
for SVC are similar as H.264/AVC, as discussed in Section 9 of
[RFC3984].
9. Congestion Control
Within any given RTP session carrying payload according to this
specification, the provisions of section 12 of [RFC3984] apply.
Reducing the session bandwidth is possible by one or more of the
following means, listed in an order that, in most cases, will assure
the least negative impact to the user experience:
a) within the highest Layer identified by the DID field, utilize the
TID and/or QID fields in the NAL unit header to drop NAL units
with lower importance for the decoding process or human
perception.
b) drop all NAL units belonging to the highest enhancement Layer as
identified by the highest DID value.
c) dropping NAL units according to their importance for the decoding
process, as indicated by the fields in the NAL unit header of the
NAL units or in the prefix NAL units.
d) dropping NAL units or entire packets not according to the
aforementioned rules (media-unaware stream thinning). This
results in the reception of a non-compliant bitstream and, most
likely, in very annoying artifacts.
Informative note: The discussion above is centered on NAL units
and not on packets, primarily because that is the level where
senders can meaningfully manipulate the scalable bitstream. The
mapping of NAL units to RTP packets is fairly flexible when using
aggregation packets. Depending on the nature of the congestion
control algorithm, the "dimension" of congestion measurement
(packet count or bitrate) and reaction to it (reducing packet
count or bitrate or both) can be adjusted accordingly.
All aforementioned means are available to the RTP sender, regardless
whether that sender is located in the sending endpoint or in a mixer
based MANE.
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When a translator-based MANE is employed, then the MANE MAY
manipulate the session only on the MANE's outgoing path, so that the
sensed end-to-end congestion falls within the permissible envelope.
As all translators, in this case the MANE needs to rewrite RTCP RRs
to reflect the manipulations it has performed on the session.
Informative note: Applications MAY also implement, in addition or
separately, other congestion control mechanisms, e.g., as
described in [RFC3450] and [Yan].
10. IANA Consideration
[Ed. (YkW): A new media type should be registered from 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. Dozens of scenarios have been
studied. While some of the scenarios appear not to follow the most
basic design principles of the Internet, e.g., as discussed in
section 11.6 -- and are therefore not appropriate for IETF
standardization -- others are clearly in the scope of IETF work. Of
these, this draft chooses the following subset for immediate
consideration. The MPEG and VCEG requirement documents are
available in [JVT-N026] and [JVT-N027], respectively.
With these remarks, we now introduce three main application
scenarios that we 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
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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 we 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.
We 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 sthe ending bit rate
by choosing fewer layers when composing the layered stream; see
section 9. SVC is designed to gracefully support both bandwidth
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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)
[Ed. (AE): TBD]
11.5 Mobile TV (Multicast to MANE, Unicast to Endpoint)
This scenario is a bit more complex, and designed to optimize the
network traffic in a core network, while still requiring only a
single pinhole in the endpoint's firewall. One of its key
applications is the mobile TV market.
Consider a large private IP network, e.g., the core network of 3GPP.
Streaming servers within this core network can be assumed to be
professionally maintained. We assume that these servers can have
many ports open to the network and that layered multicast is a real
option. Therefore, we assume that the streaming server multicasts
SVC scalable layers, instead of simulcasting different
representations of the same content at different bit rates.
Also consider many endpoints of different classes. Some of these
endpoints may lack the processing power or the display size to
meaningfully decode all layers; others may have these capabilities.
Users of some endpoints may wish not to pay for high quality and are
happy with a base service, which may be cheaper or even free. Other
users are willing to pay for high quality. Finally, some connected
users may have a bandwidth problem in that they can't receive the
bandwidth they would want to receive -- be it through congestion,
connectivity, change of service quality, or for whatever other
reasons. However, all these users have in common that they don't
want to be exposed too much, and therefore the number of firewall
pinholes needs to be small.
This situation can be handled best by introducing middleboxes close
to the edge of the core network, which receive the layered multicast
streams and compose the single SVC scalable bit stream according to
the needs of the endpoint connected. These middleboxes are called
MANEs throughout this specification. In practice, we 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
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and media traffic necessarily are multiplexed on the same physical
link. This is why we do not worry too much about decomposition
aspects of the MANE as such.
MANEs necessarily need to be fairly complex devices. They certainly
need to understand the signaling, so, for example, to associate the
PT octet in the RTP header with the SVC payload type.
A MANE may aggregate multiple RTP streams, possibly from multiple
RTP sessions, thus to reduce the number of firewall pinholes
required at the endpoints. This type of MANEs is conceptually easy
to implement and can offer powerful features, primarily because it
necessarily can "see" the payload (including the RTP payload
headers), utilize the wealth of layering information available
therein, and manipulate it.
While such an MANE operation in its most trivial form (combining
multiple RTP packet streams into a single one) can be implemented
comparatively simply -- reordering the incoming packets according to
the DON and sending them in the appropriate order -- more complex
forms can also be envisioned. For example, a MANE can be optimizing
the outgoing RTP stream to the MTU size of the outgoing path by
utilizing the aggregation and fragmentation mechanisms of this memo.
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. In particular, SVC and/or this specification
contain means to easily generate the correct inter-layer decoding
order of NAL units. No serious bit-oriented processing is required
and no significant state information (beyond that of the signaling
and perhaps the SVC sequence parameter sets) need to be kept.
11.6 SSRC Multiplexing
The authors have considered the possibility of introducing SSRC
multiplexing, i.e., allowing sending multiple RTP packet streams
containing layers in the same RTP session, differentiated by SSRC
values. The intention was to minimize the number of firewall
pinholes in an endpoint to one, by using MANEs to aggregate multiple
outgoing sessions stemming from a server into a single session (with
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SSRC-multiplexed packet streams). It was hoped that this would be
feasible even with encrypted packets in an SRTP context.
While an implementation along these lines indeed appears to be
feasible for the forward media path, the RTCP RR rewrite cannot be
implemented in the way necessary for this scheme to work. This
relates to the need to authenticate the RTCP RRs as per SRTP
[RFC3711]. While the RTCP RR itself does not need to be rewritten
by the scheme envisioned, its transport addresses need to be
manipulated. This, in turn, is incompatible with the mandatory
authentication of RTCP RRs. As a result, there would be a
requirement that a MANE needs to be in the RTCP security context of
the sessions, which was not envisioned in the use case at hand.
Therefore this memo does provide support for SSRC multiplexing.
12. References
12.1 Normative References
[H.264] ITU-T Recommendation H.264, "Advanced video coding for rd generic audiovisual services", 3 Edition, November 2007.
[I-D.ietf-mmusic-decoding-dependency] Schierl, T., and Wenger, S.,
"Signaling media decoding dependency in Session
Description Protocol (SDP)", draft-ietf-mmusic-decoding-
dependency-02 (work in progress), May 2008.
[MPEG4-10] 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.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and Jacobson,
V., "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3984] Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund,
M., and Singer, D., "RTP Payload Format for H.264 Video",
RFC 3984, February 2005. [Ed. (YkW): Update this to
RFC3984bis.]
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[RFC4566] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
12.2 Informative References
[DVB-H] DVB - Digital Video Broadcasting (DVB); DVB-H
Implementation Guidelines, ETSI TR 102 377, 2005.
[H.241] ITU-T Rec. H.241, "Extended video procedures and control
signals for H.300-series terminals", May 2006.
[IGMP] Cain, B., Deering S., Kovenlas, I., Fenner, B., and
Thyagarajan, A., "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[JVT-N026] Ohm J.-R., Koenen, R., and Chiariglione, L. (ed.), "SVC
requirements specified by MPEG (ISO/IEC JTC1 SC29 WG11)",
JVT-N026, available from http://ftp3.itu.ch/av-arch/jvt-
site/2005_01_HongKongGeneva/JVT-N026.doc, Hong Kong,
China, January 2005.
[JVT-N027] Sullivan, G., and Wiegand, T. (ed.), "SVC requirements
specified by VCEG (ITU-T SG16 Q.6)", JVT-N027, available
from http://ftp3.itu.ch/av-arch/jvt-
site/2005_01_HongKongGeneva/JVT-N027.doc, Hong Kong,
China, January 2005.
[Lennox] Lennox, J., Schierl, T., and Ganesan, S, "Real-Time
Transport Protocol (RTP) Timestamps for Layered
Encodings", draft-lennox-avt-rtp-layered-encoding-
timestamps-00 (work in progress), June 2,2008.
[McCanne] McCanne, S., Jacobson, V., and Vetterli, M., "Receiver-
driven layered multicast", in Proc. of ACM SIGCOMM'96,
pages 117--130, Stanford, CA, August 1996.
[MBMS] 3GPP - Technical Specification Group Services and System
Aspects; Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs (Release 6), December 2005.
[MPEG2] ISO/IEC International Standard 13818-2:1993.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
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[RFC3450] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and
Crowcroft, J., "Asynchronous layered coding (ALC) protocol
instantiation", RFC 3450, December 2002.
[RFC3711] Baugher, M., McGrew, D, Naslund, M., Carrara, E., and
Norrman, K., "The secure real-time transport protocol
(SRTP)", RFC 3711, March 2004.
[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.
13. Authors' Addresses
Stephan Wenger
Nokia
955 Page Mill Road
Palo Alto, CA 94304
USA
Phone: +1-650-862-7368
EMail: stewe@stewe.org
Ye-Kui Wang
Nokia Research Center
P.O. Box 100
33721 Tampere
Finland
Phone: +358-50-466-7004
EMail: ye-kui.wang@nokia.com
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
Email: schierl@hhi.fhg.de
Alex Eleftheriadis
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Vidyo, Inc.
433 Hackensack Ave.
Hackensack, NJ 07601
USA
Phone: +1-201-467-5135
Email: alex@vidyo.com
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This document and the information contained herein are provided on
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FOR A PARTICULAR PURPOSE.
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Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society. Further, the author Thomas Schierl of Fraunhofer
HHI is sponsored by the European Commission under the contract
number FP7-ICT-214063, project SEA. The authors want to thank
Jonathan Lennox for his valuable comments and input to the draft.
[Ed. (YkW): Thanks to many other contributors and reviewers TBD.]
14. Open Issues
1) Regarding signaling:
A) Can mst-mode be used to signal receiver capabilities?
B) Is it possible to use offer/answer in MST? Is it possible to
use MST for unicast? If no, the MST related new parameters do not
need to be mentioned in the offer/answer section. For examples,
the decoder configuration defined for offer/answer then does not
include mst-mode, and the second SDP example for MST needs to be
removed from the offer/answer section but as an example of the
declarative usage of the parameters.
C) What offer/answer examples are needed? Current we only have two
offer examples with no answer, one for SST and one for MST.
D) It seemed that the SDP exchange process in unicast streaming
applications is not called offer/answer. Should we include any
example for such SDP exchange (e.g. the use of sprop-scalability-
info and scalable-layer-id)? Is there a good name (like
offer/answer for conversational applications) for this?
E) Signaling support for on-the-fly change of codec modes (YK:
what does codec mode mean?). This must be done in syn with
3984bis. The goal is to avoid a new offer/answer round.
15. Changes Log
From draft-ietf-avt-rtp-svc-13 to draft-ietf-avt-rtp-svc-14
29 July - 1 August 2008: YkW
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- Updated the abstract to mention both new packetization modes
for MST and new NAL unit types.
- Restructured the document and addressed some major comments
asking for introductions of the new packetization modes and the
new NAL unit types in Section 1.
- Added the definition of dummy NAL unit in the definition
section (3.1).
- Added text for the NAL unit type extension mechanism.
- Added the description of payload structures in a new section
(4.3).
- Added the description of SST and MST transmission modes in a
new section (4.4).
- Added text explaining the handling of new NAL unit types for
SST in Section 4.5.1.
- Updated the description of MST packetization modes (4.5.2) with
all the tables changed and new tables added.
- Added description of single NAL unit packets in new section
(4.6).
- Updated the PACSI text (4.9) taking into account the use in NI-
MTAP.
- Replaced the possibility of using NI-MTAP and PACSI with dummy
NAL units for decoder order recovery for NI-T and NI-TC
(5.2.1).
- Addressed all the minor comments Dave Singer sent to AVT
mailing list on 18 July
- Other minor changes throughout the document.
- Added some editing comments to point out new open issues.
21 August 2008: AE
- Editorial corrections throughout.
- Changed smart quotes to plain quotes everywhere.
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- Introduced DQid in 1.1.2, to avoid continuous references to
dependency_id*16+quality_id.
- The reference to "outside the same RTP packet stream" in the
beginning of 1.2.2 is confusing, as MST has not been introduced
yet. I reordered the paragraphs so that it is easier to follow,
and also made some changes that were necessary after that.
- In 4.2.1, renamed bit0, bit1, and bit2 to J, K, and L. Also
indicated that their semantics depend on the subtype - this was
the original proposal in Hannover. The idea is that, for each
subtype, you get 3 one-bit fields to use as flags, so that you
don't have to waste another byte. I changed the order so that
subtypes are indicated in increasing values. I indicate that
the entire NAL unit should be ignored if the ST is not 1 or 2.
I also changed the ordering of the fields to put the flags at
the end.
- For the Y bit, in 4.9, I changed "SHOULD be identical" to "MUST
be identical".
- In 5.1, changed SHOULD to MUST for the first bullet of "When
SSE is used" (MUST be encapsulated according to RFC 3984).
- Implemented corrections based on Peter Amon's feedback of
August 21, 2008. (Not all of them - suggestion to use H.264/AVC
in some places was not correct.)
- The worlds SHOULD/MUST in the introduction were changed to
small case, since they are not mandated by this memo but
another memo/specification. I originally just flagged this, but
decided to implement the change after seeing that Peter too
suggested this format.
27 August 2008, TS
- Added text in introduction and abstract giving an basic
overview on transmission modes and new packet structures.
- Changed Dummy NAL unit to Empty NAL unit
- Added text to "RTP Header Usage" section on setting M-bit for
NI-MTAP and on setting RTP timestamp for Empty NAL unit
- Extended NI-MTAP definitions, including header, DON-field,
PACSI and timestamp alignments
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- Added definition of Empty NAL unit
- Corrected tables in section section 4.3 4.5
- Added some text on NI-T mode in PACSI section
- Clarified text on "sprop-remux-buf-req" "remux-buf-cap"
- Added text to section 5.2.1 on the presence of SDP parameters
28 August - 1 September 2008, YkW
- Changed (targeting an improvement) in the abstract the
sentences for introduction of the four MST packetization modes,
and highlighted some places in the beginning paragraphs in the
introduction section that require improvements.
- Added the requirement of using RFC 3984 for the session
carrying an H.264/AVC compatible subset in MST.
- Clarified the text for RTP header usage, added timestamp
setting for single NAL unit packet containing a PACSI NAL unit,
and setting of SSRC for SST and MST.
- Made some changes to the new text defining NI-MTAP, and added
some comments for discussion.
- Added an informative note regarding where to put the SVC NAL
unit header extension bytes in fragments of an SVC NAL unit
with four-byte NAL unit header.
- Made some changes to the text defining empty NAL units.
- In section 6.1.2, added an informative note regarding the cases
wherein CS-DON values cannot be derived for some NAL units due
to some particular ways of packet losses.
- Corrected the semantics of mst-mode, sprop-mst-remux-buf-size
(which was sprop-mst-interleave-depth) and sprop-mst-max-don-
diff with regard to the absence of mst-mode, and the semantics
of sprop-remux-buf-req, remux-buf-cap and sprop-remux-init-buf-
time with regard to their presence.
- Added optional and declarative media type parameters sprop-mst-
csdon-always-present, sprop-frame-rate, sprop-bit-rate, and
sprop-frame-rate.
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- Other editorial changes throughout the document, and removed a
few obsolete comments and editors' notes.
- Updated the open issues section.
5 - 25 September 2008, AE
General: reworked text removing all editorial correction
annotations, and leaving the text in the form that I think is the
best. You will notice that there are now very few Word edits, making
the text very clean, and what remained is mostly Word comments. All
[Ed.] entries now have Word comments. You can review outstanding
issues by browsing the comments.
At this point I suggest we NO LONGER make editorial corrections -
there has been some flip-flopping with changes. We need to stop in
order to finalize the memo.
The following identifies editorial changes that you should be aware
of.
- Reworked abstract and introduction. Note that it is no longer
allowed to signal the H264 media subtype for an SVC stream.
PLEASE DO NOT MAKE ANY FURTHER CHANGES unless you really want
to spend the time doing so.
This section also references the new 'avc-ready' signalling
parameter. Contrary to what was discussed in the 9/3/08
conference call, it's now an sprop, as discussed in the
Editor's call of 9/24/08.
- In 4.1, regarding the setting of the RTP timestamp in Empty NAL
units, I changed the text to require that the timestamp is set
according to 4.10, and I added text in 4.10 to require an
associated type 1, 5, or 20 NAL (from which to get the actual
time to use).
- Re-titled 4.2 to "NAL Unit Extension and Header Usage", from
"NAL Unit Header Extension and Usage". SVC already has an SVC
header extension, and this would be confusing. Plus we are not
extending the header, but the NAL unit type space.
- In Table 2, I opted for the word "reserved" rather than
"undefined", for NAL unit type 0. The same word should be used
in RFC3984bis (per YK's comment).
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- Reviewed Tables 4, 7, and 9 with the editors and changed some
wrong entries for the Empty NAL unit (it was disallowed in
Single NAL unit mode).
- Added new Table 4 to show the session packetization modes
allowed in SST. Pedantic, but we thought it's useful (all yes).
Also, 6.2.1.1 has wrong table number (7). It's now fixed (12).
- In 4.7.1, I specified that the J field in NI-MTAP must be 0 for
SST (no CS-DONs in SST).
- Fixed incorrect references to J field in NI-MTAP ("bit2", "L")
in 4.9 (PACSI).
- In 4.9, PACSI, changed the semantics of S and E flags to
indicate if the first and last NAL units are the first or last
NAL units of a layer representation. This is more useful. Also
updated informative note. Furthermore, I added informative note
text for TL0PICIDX to indicate how it is handled in NI-MTAP.
Finally, I disallowed this scheme for I-C by specifiing in
5.2.3 that the Y bit MUST be set to 0.
- I noticed that references to restrictions for Empty NAL, NI-
MTPA, and PACSI are in several places. Even if they are all
correct, it may be dangerous or confusing. Maybe they should be
in only one place. In general, it's good if the normative words
MUST etc. appear in a single place for each rule.
- In 4.10, fixed subtype to be 1 instead of 2. 2 is the NI-MTAP.
nd - In 5.2.1, in the 2 way of creating NAL units for time
aligment, I say that you can transmit coded data that instructs
the decoder to repeat the previous picture. I also merged the
two back-to-back informative notes.
- 5.2.1 uses inconsistent identation between the first line, the
first bullet, and what follows after the informative note. It
is not clear if the text after the note is part of the bullet,
or it is top-level material. I removed the bullets as they did
not appear to be useful.
- The example in 5.2.1 was wrong. The notation was confusing, and
the time alignment appears to be wrong. Some of the notation
was also wrong (e.g., TS_B2 is shown in the figure but not used
in the text, whereas TS_B1 is NOT shown in the figure. Also,
A(TS_B4) should probably be P(TS_B3) or something. I rewrote
the example completely.
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- Added Section 6.1, De-Packetization Process for SST, so that it
shows in the ToC. Updated all references to original 6.1.*.
- In (new) 6.2.1, I removed the use of empty NI-MTAP and single
PACSI, since we now have the Empty NAL for the same purpose.
- Added reference to [Lennox] and informative note in 6.2.1 about
directly using TSs rather than first obtaining NTP values.
- The reordering example of Figure 6 now reads well. The only
problem is the need to establish "order of dependency". What if
there is AU data for a layer that is not part of the top
layer's dependency path? How will ths data get ordered.
Example: base A, enh B depends on A, enh C depends on A, and
target layer is C. You don't know where to put the B data,
before or after C.
- Changed all "SHALL" to "MUST".
26 September 2008, YkW
- In Abstract and Introduction, changed all "RTP timestamp
alignment" to "timestamp alignment".
- In Introduction, changed "The Empty NAL unit is used to ensure
inter-session RTP timestamp alignment in MST when temporal
scalability is used" to "The Empty NAL unit is used to ensure
inter-session timestamp alignment required for decoding order
recovery in MST", because use of temporal scalability may or
may not require the use of Empty NAL units.
- In introduction, changed the wording of "two new NAL unit
types" to "three new NAL unit types", to be consistent with the
Abstract, section 1.2.3, as well as other sections.
- In section 2 "conventions", changed back "SHALL" to "MUST" in
two places.
- In section 4.7.1, removed the first Word comment and the first
editing comment, which are obsolete. It is clearly said that,
when used in NI-C or NI-TC mode, the J field MUST be equal to 1
(i.e. the DON field is present).
- In section 4.9, avoided the wording "may not", which has been
considered most confusing wording by some :-)
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- In section 4.11.1, added a bullet item on how CS-DON value is
"derived" for a non-PACSI NAL unit in an NI-MTAP packet, and
removed the comment.
- In section 5.1, removed the packetization rule "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.", as it is redundant - it must be the case
anyway according to the PACSI semantics.
- In the last bullet item of section 5.2.2, added NI-MTAP, and
removed the comment.
- Some typo corrections here and there, and removed a few other
obsolete comments.
- Updated the list of open issues (basically removed all except
for the big one on signaling).
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