Audio/Video Transport WG S. Wenger
Internet Draft Y.-K. Wang
Intended status: Standards track Nokia
Expires: December 2008 T. Schierl
Fraunhofer HHI
A. Eleftheriadis
Vidyo
June 30, 2008
RTP Payload Format for SVC Video
draft-ietf-avt-rtp-svc-12.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 H.264 Network Abstraction Layer (NAL) units in each
RTP packet payload, supporting both single-session as well as multi-
session streams. For single-session streams the packetization modes
of RFC 3984 are used, whereas for multi-session streams four
different packetization modes are defined in this memo. The payload
format is backwards compatible to RFC 3984, and has wide
applicability in conversational applications such as
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
2. Conventions ................................................. 6
3. Scope ....................................................... 7
4. Definitions and Abbreviations ............................... 7
4.1 Definitions ............................................. 7
4.1.1 Definitions from the SVC Specification ............. 7
4.1.2 Definitions Specific to This Memo .................. 9
4.2 Abbreviations .......................................... 12
5. The SVC Codec .............................................. 12
5.1 Overview ............................................... 12
5.2 Parameter Sets ......................................... 15
5.3 Network Abstraction Layer Units ........................ 16
6. RTP Payload Format ......................................... 20
6.1 Design Principles ...................................... 20
6.2 RTP Header Usage ....................................... 20
6.3 Common Structure of the RTP Payload Format ............. 20
6.4 NAL Unit Header Usage .................................. 20
6.5 Packetization Modes .................................... 22
6.5.1 Packetization Modes for Single-Source
Transmission ...................................... 22
6.5.2 Packetization Modes for Multi-Source
Transmission ...................................... 22
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6.6 Aggregation Packets .................................... 25
6.7 Fragmentation Units (FUs) .............................. 25
6.8 Payload Content Scalability Information (PACSI)
NAL Unit ............................................... 25
6.9 Non-Interleaved Multi-Time Aggregation Packets
(NI-MTAPs) ............................................. 32
6.10 Decoding Order Number (DON) ........................... 34
6.10.1 Cross-Session DON (CS-DON) for Multi-Source
Transmission ............................................ 35
7. Packetization Rules ........................................ 36
7.1 Packetization Rules for Multi-Source Transmission ...... 37
7.1.1 NI-T / NI-TC Packetization Rules .................. 38
7.1.2 NI-C / NI-TC Packetization Rules .................. 38
7.1.3 I-C Packetization Rules ........................... 40
7.1.4 Packetization Rules for Non-VCL NAL Units ......... 40
7.1.5 Packetization Rules for Prefix NAL Units .......... 40
8. De-Packetization Process ................................... 41
8.1 De-Packetization Process for Multi-Source Transmission . 41
8.1.1 Decoding Order Recovery for the NI-T and
NI-TC Modes ....................................... 42
8.1.1.1 Informative Algorithm for NI-T Decoding Order
Recovery within an Access Unit ............... 45
8.1.2 Decoding Order Recovery for the NI-C, NI-TC and I-C
Modes ............................................. 48
9. Payload Format Parameters .................................. 50
9.1 Media Type Registration ................................ 50
9.2 SDP Parameters ......................................... 60
9.2.1 Mapping of Payload Type Parameters to SDP ......... 61
9.2.2 Usage with the SDP Offer/Answer Model.............. 61
9.2.3 Usage with Multi-Source Transmission .............. 66
9.2.4 Usage in Declarative Session Descriptions ......... 66
9.3 Examples ............................................... 67
9.3.1 Example for Offering A Single SVC Session ......... 67
9.3.2 Example for Offering Session Multiplexing ......... 68
9.4 Parameter Set Considerations ........................... 69
10. Security Considerations ................................... 69
11. Congestion Control ........................................ 69
12. IANA Consideration ........................................ 70
13. Informative Appendix: Application Examples................. 71
13.1 Introduction .......................................... 71
13.2 Layered Multicast ..................................... 71
13.3 Streaming of an SVC Scalable Stream ................... 72
13.4 Multicast to MANE, SVC Scalable Stream to Endpoint .... 73
13.5 Scenarios Currently Not Considered .................... 74
13.6 SSRC Multiplexing ..................................... 76
14. References ................................................ 76
14.1 Normative References................................... 76
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14.2 Informative References................................. 77
15. Authors' Addresses......................................... 78
Intellectual Property Statement ............................... 79
Disclaimer of Validity......................................... 79
Copyright Statement............................................ 80
Acknowledgement................................................ 80
16. Open Issues................................................ 80
17. Changes Log................................................ 81
From draft-ietf-avt-rtp-svc-08 to draft-ietf-avt-rtp-svc-09 ... 81
From draft-ietf-avt-rtp-svc-09 to draft-ietf-avt-rtp-svc-10 ... 82
From draft-ietf-avt-rtp-svc-10 to draft-ietf-avt-rtp-svc-11 ... 83
From draft-ietf-avt-rtp-svc-11 to draft-ietf-avt-rtp-svc-12 ... 83
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 Annex G of ITU-T Rec. H.264/AVC [H.264].
SVC covers the entire application range of H.264/AVC, from low
bitrate Internet streaming applications, to HDTV broadcasting, and
even Digital Cinema that requires nearly lossless coding and
hundreds of Mbps. The payload format specified in this memo is a
backwards compatible enhancement to the H264/AVC payload format
(H264, [RFC3984]), in which the specific features introduced by SVC
are taken into account. It is assumed that the reader is familiar
with the terminology and concepts defined in RFC 3984.
SVC provides a coded representation of a video signal as a set of
hierarchical components, composed of a base layer and one or more
enhancement layers, as explained in Section 5 All data produced by
an SVC encoder are structured in H.264 Network Abstraction Layer
(NAL) units. This payload specification can only be used to carry
the raw H.264 NAL unit stream over RTP, and not the byte stream
format specified in Annex B of [H.264].
Depending on the packetization mode used, one or more than one NAL
unit may be present in a single RTP packet. The base layer is, by
design, compatible to H264, but may be formatted either according to
RFC 3984 ("AVC base layer") or according to this memo ("SVC base
layer"). Furthermore, the base layer may have multiple temporal
components (i.e., supporting different frame rates). As a result,
we distinguish the lowest temporal component ("T0") of the base
layer (either AVC or SVC) as the starting point of the SVC bitstream
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hierarchy. The difference of an SVC base layer as compared to an
AVC base layer is that additional NAL unit types may be present in
the RTP stream in the SVC base layer case, which, however, are
ignored by a receiver conforming to RFC 3984.
This specification allows to encapsulate in a given RTP stream NAL
units belonging to either:
o the T0 AVC base layer or the T0 SVC base layer only;
o one or more enhancement layers; or
o the T0 SVC base layer, and one or more enhancement layers.
Furthermore, this specification allows the packetization of SVC data
for either single-source or multi-source transmission. In the case
of single-source transmission (SST) all SVC data are carried in a
single RTP session with the same SSRC. In the case of Multi-Source
Transmission (MST), two or more RTP sessions are used to carry the
SVC data, using distinct SSRC's, in accordance with the
packetization modes defined in this memo and in RFC 3984. Each RTP
session is associated with one RTP stream, which MAY carry one or
more layers, structured according to one of the three cases
indicated above.
When MST is not used, the following applies.
o When an H.264/AVC compatible subset of the SVC base layer is
transmitted, the subset SHOULD be carried in one RTP stream that
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.
o When a set of layers including one or more SVC enhancement layers
is transmitted, the set SHOULD be carried in one RTP stream that
SHOULD be encapsulated according to this memo.
When MST is used, this memo defines four different packetization
modes. The modes differ depending on if 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.
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MST SHOULD be used in a multicast session when different receivers
may request different layers of the scalable bitstream. An
operation point for an SVC bit stream, as defined in this memo,
corresponds to a set of layers that together conform to one of the
profiles defined in Annex A or G of [H.264] and, when decoded, offer
a representation of the original video at a certain fidelity. The
number of streams used in MST SHOULD be at least equal to the number
of operation points that may be requested by the receivers.
Depending on the application, this may result in each layer being
carried in its own RTP session, or in having multipe 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 13.2
This RTP payload specification is designed to be unaware of the NAL
unit payload defined in [H.264]. Similar to RFC 3984, this memo
introduces two new NAL unit types, using unit type numbers from the
space explicitly left unspecified in [H.264] and not used in RFC
3984. When the single NAL unit packetization mode is used, where
one NAL unit always corresponds to one RTP packet, the NAL unit
header defined in [H.264] co-serves as the payload header of this
RTP payload format. In this case, the payload of the NAL unit
follows immediately. In all other modes data from multiple NAL units
may be present in an RTP packet, either through nesting (a NAL unit
is contained in another one) or serialization (NAL units appear in
sequence in an RTP packet).
This memo also also defines signaling support for SVC, including a
new media subtype name (H264-SVC).
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
This specification uses the notion of setting and clearing a bit
when bit fields are handled. Setting a bit is the same as assigning
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that bit the value of 1 (On). Clearing a bit is the same as
assigning that bit the value of 0 (Off).
3. Scope
o The scalability features that SVC adds to the H.264 specification
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 heterogenous receivers
(screen resolution, processing speed, etc.), as well as differing
or time-varying network conditions. The adaptation can be
performed at the source, the destination, or in intermediate
media-aware network elements. This payload specification exposes
these system-level functionalities so that system designers can
take direct advantage of these features. The likely applications
of this specification are in the IP-based multimedia
communication fields, including conversational multimedia, video
telephony or video conferencing, Internet streaming, and IPTV.
4. Definitions and Abbreviations
4.1 Definitions
4.1.1 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.
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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 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 consist of one or more layer
representations.
IDR access unit: An access unit in which the primary coded
picture is an IDR picture.
IDR 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
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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.
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.
4.1.2 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. 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 may not be
correctly decoded 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-source 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-
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decoding-dependency]. The base RTP session may carry NAL units
of NAL unit type equal to 14 and 15.
effective NAL unit timestamp: The value that the RTP timestamp
would have if the particular NAL unit was transported in its own
RTP packet. (The NAL unit time is different than that actual RTP
timestamp of the packet containing the particular NAL unit in the
case of MTAPs.)
enhancement RTP session: When multi-source 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
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: A biststream subset that conforms 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 bistream 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
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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-source transmission: This specifies that the SVC bitstream
is distributed across multiple RTP sessions, with each stream
having a distinct SSRC, and consequently its own timestamp and
sequence number spaces. Those multiple streams can be associated
using the RTCP CNAME, or explicit signalling of the SSRC used.
[Ed. (AE): Is the single transport connection mode supported? It
does not appear to, as seen by the definitions of base and
enhancement RTP sessions, and the rest of the text. I modified
the definition so that it is not allowed.] 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 bistream
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.
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. 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. [Ed.Note(YkW): Need to check whether a bitstream subset
with those additional NAL units removed is a conforming
bitstream.]
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.
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
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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, 15 and 20.
SVC RTP session: Either the base RTP session or an enhancement
RTP session.
T0 AVC base layer: A subset of the AVC base layer constructed by
removing all VCL NAL units associated with temporal_id values
higher than 0.
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 their associated prefix NAL units.
4.2 Abbreviations
In addition to the abbreviations defined in [RFC3984], the following
abbrevations are used in this memo.
CGS: Coarse-Grain Scalability
CS-DON: Cross-Session Decoding Order Number
ETS: Effective Timestamp (of a NAL unit)
MGS: Medium-Grain Scalability
MST: Multi-Source Transmission
PACSI: Payload Content Scalability Information
SST: Single-Source Transmission
SNR: Signal-to-Noise Ratio
SVC: Scalable Video Coding
5. The SVC Codec
5.1 Overview
SVC [H.264]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
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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 5.3). 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 bistream
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-VLC 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.
The NAL encapsulates each slice generated by the VCL into one or
more NAL units. RFC 3984 provides a more in-depth discussion of the
NAL unit concept. SVC specifies the decoding order of NAL units.
It is noted that the concept of temporal scalability is already
present in H.264/AVC, as profiles defined in Annex A of [H.264]
already support it. Specifically, in [H.264] sub-sequences have
been introduced in order to allow optional use of temporal layers.
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. For coded picture data defined in
Annex G of [H.264] this is accomplished by using a new type of NAL
unit where the fidelity parameters are part of its header. For
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coded picture data that follow H.264/AVC, and to ensure
compatibility with existing H.264/AVC receivers, a new type of
"prefix" NAL unit has been defined to carry this header information.
This prefix NAL unit type is among those ignored by H.264/AVC
receivers as explained in [RFC3984].
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) is
referred to as the base layer representation and 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. Due to the 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 contains only H.264/AVC VCL
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.
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),
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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.
5.2 Parameter Sets
The parameter set concept is defined in [H.264]. Please refer to
Section 1.2 of RFC 3984 for more details.
SVC introduces a new type of sequence parameter set, referred to as
a 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
sequence parameter sets. Subset sequence parameter sets use a
separate identifier value space than sequence parameter sets. An
overview of the NAL unit and packet types used in this memo can be
found in Table 1 in Section 5.3.
In SVC, coded picture data from different layers may use the same or
different sequence and picture parameter sets. At any time instant
during the decoding process there may be one active sequence
parameter set (for the layer representation with the highest value
of (dependency_id * 16 + quality_id)) and one or more active layer
SVC sequence parameter set(s) (for layer representations with lower
values of (dependency_id * 16 + quality_id)). The active sequence
parameter set or an active layer SVC sequence parameter set remains
unchanged throughout a coded video sequence in the scalable layer in
which the active sequence parameter set or active layer SVC sequence
parameter set is referred to. This means that the referred sequence
parameter set or subset sequence parameter set can only change at
IDR access units for any layer. At any time instant during the
decoding process there may be one active picture parameter set (for
the layer representation with the highest value of (dependency_id *
16 + quality_id)) and one or more active layer picture parameter
set(s) (for layer representations with lower values of
(dependency_id * 16 + quality_id)). 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.
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5.3 Network Abstraction Layer Units
The NAL unit organization is central to [ H.264], RFC 3984, as well
as this memo. In addition to the NAL unit types defined originally
for H.264/AVC, [H.264]introduces two new NAL unit types for SVC
(among others): SVC VCL NAL units ("slice in scalable extension",
type 20), and prefix NAL units (type 14). SVC VCL NAL units
encapsulate VCL data as defined in Annex G of [H.264]. The prefix
NAL unithas no payload of its own, and instead includes descriptive
information of the associated H.264/AVC VCL NAL unit (type 1 or 5)
that immediately follows the prefix NAL unit.
In addition to the NAL unit types introduced for packetization
purposes in RFC 2984, this memo also introduces two new NAL unit
types to facilitate packetization (PACSI and NI-MTAP, specified in
detail later on). The following table gives an overview of NAL unit
and packet types used in this memo and also provides references to
the appropriate document and section where their use is defined.
Table 1. Summary of NAL unit and packet types used in this memo
Type Description Definition in: [RC3984] / this memo
--------------------------------------------------------------------
0 unspecified - / -
1-23 NAL unit per [H.264]/Single NAL unit packet 5.2 / 6.3
14 Prefix NAL unit per [SVC] - / 5.1
15 Subset sequence parameter set per [SVC] - / 5.2
20 Slice in scalable extensions per [SVC] - / 5.3
24 Single-time aggregation packet (STAP-A) 5.7.1 / 6.6
25 Single-time aggregation packet (STAP-B) 5.7.1 / 6.6
26 Multi-time aggregation packet (MTAP16) 5.7.2 / 6.6
27 Multi-time aggregation packet (MTAP24) 5.7.2 / 6.66.7
28 Fragmentation unit (FU-A) 5.8 / 6.7
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29 Fragmentation unit with DON (FU-B) 5.8 / 6.7
30 Payload Content Scalability Info. (PACSI) - / 6.8
31 unspecified - / -
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 backward compatible way):
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
The semantics of the components of the NAL unit type octet, as
specified in the H.264 specification, are described briefly below.
F: 1 bit
forbidden_zero_bit. H.264/AVC declares a value of 1 as a syntax
violation.
NRI: 2 bits
nal_ref_idc. A value of '00' (in binary form) indicates that the
content of the NAL unit is not used to reconstruct reference
pictures for future prediction. Such NAL units can be discarded
without risking the integrity of the reference pictures in the
same layer. A value greater than '00' indicates that the
decoding of the NAL unit is required to maintain the integrity of
reference pictures in the same layer, or that the NAL unit
contains parameter sets.
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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 data 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. Receivers SHOULD ignore the value of R.
I: 1 bit
idr_flag. This component specifies whether the layer
representation is an instantaneous decoding refresh (IDR) layer
representation (when equal to 1) or not (when equal to 0).
PRID: 6 bits
priority_id. This flag specifies a priority identifier for the
NAL unit. A lower value of PRID indicates a higher priority.
N: 1 bit
no_inter_layer_pred_flag. This flag specifies, when present in a
coded slice NAL unit, whether inter-layer prediction may be used
for decoding the coded slice (when equal to 1) or not (when equal
to 0).
DID: 3 bits
dependency_id. This component indicates the inter-layer coding
dependency level of a layer representation. At any access unit,
a layer representation with a given dependency_id may be used for
inter-layer prediction for coding of a layer representation with
a higher dependency_id, while a layer representation with a given
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dependency_id shall not be used for inter-layer prediction for
coding of a layer representation with a lower dependency_id.
QID: 4 bits
quality_id. This component indicates the quality level of an MGS
layer representation. At any access unit and for identical
dependency_id values, a layer representation with quality_id
equal to ql uses a layer representation with quality_id equal to
ql-1 for inter-layer prediction.
TID: 3 bits
temporal_id. This component indicates the temporal level of a
layer representation. The temporal_id is associated with the
frame rate, with lower values of _temporal_id corresponding to
lower frame rates. A layer representation at a given temporal_id
typically depends on layer representations with lower temporal_id
values, but it never depends on layer representations with higher
temporal_id values.
U: 1 bit
use_ref_base_pic_flag. A value of 1 indicates that only
reference base pictures are used during the inter prediction
process. A value of 0 indicates that the reference base pictures
are not used during the inter prediction process.
D: 1 bit
discardable_flag. A value of 1 indicates that the current NAL
unit is not used for decoding NAL units with values of
dependency_id higher than the one of the current NAL unit, in the
current and all subsequent access units. Such NAL units can be
discarded without risking the integrity of layers with higher
dependency_id values. discardable_flag equal to 0 indicates that
the decoding of the NAL unit is required to maintain the
integrity of layers with higher dependency_id.
O: 1 bit
output_flag: Affects the decoded picture output process as
defined in Annex C of [H.264].
RR: 2 bits
reserved_three_2bits. Reserved bits for future extension. RR
MUST be equal to '11' (in binary form). Receivers SHOULD ignore
the value of RR.
This specification extends the semantics of F, NRI, I, PRID, DID,
QID, TID, U, and D per [H.264] as described in Section 6.4.
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6. RTP Payload Format
6.1Design Principles
The following design principles have been observed:
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 and rely on signaling information. MANEs 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 from the incoming RTP
packet stream. This implies rewriting the RTP headers of the
outgoing packet stream and rewriting of RTCP Receiver Reports.
6.2 RTP Header Usage
Please see Section 5.1 of [RFC3984].
6.3 Common Structure of the RTP Payload Format
Please see Section 5.2 of [RFC3984].
6.4 NAL Unit Header Usage
The structure and semantics of the NAL unit header were introduced
in Section 5.3. This section specifies the semantics of F, NRI, I,
PRID, DID, QID, TID, U, and D according to this specification.
The semantics of F specified in Section 5.3 of [RFC3984] also apply
in this memo.
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
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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 sensed 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 success 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] applies.
Note, that MANEs implementing unequal error protection MAY use this
information to protect NAL units with smaller PRID values better
than those with larger PRID values, for example by including only
the more important NAL units in an FEC protection mechanism. The
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
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certain Operation Point of the SVC bitstream, hence to decide
whether to forward the NAL unit.
6.5 Packetization Modes
6.5.1Packetization Modes for Single-Source Transmission
Section 5.4 of RFC 3984 applies when using single-source
transmission. The packetization modes specified in Section 5.4 of
RFC 3984 are also referred to as session-specific packetization
modes.
6.5.2 Packetization Modes for Multi-Source Transmission
When multi-source transmission (MST) is used this memo specifies
four cases of 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
if they require that the NAL units are transmited in NAL unit
decoding order (non-interleaved) or if they allow them to be
transmitted in an arbitrary order (interleaved). Second, they differ
in the mechanisms they provide in order to recover the correct
decoding order of the NAL units across all RTP sessions involved.
The NI-T, NI-C, and NI-TC modes do not allow interleaving, and are
thus targeted for systems that require relatively low end-to-end
latency, e.g. conversational systems. The I-C mode allows
interleaving and is thus targeted for systems that do not require
very low end-to-end latency.
The NI-T and NI-TC modes use timestamps to recover the decoding
order of NAL units, whereas NI-TC, NI-C, and I-C all use the CS-DON
mechanism (explained later on) to do so. Note that the NI-TC mode
uses both timestamps and the CS-DON method; receivers in this case
may use either method for performing decoding order recovery.
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The MST packetization mode in use MAY be signaled by the value of
the OPTIONAL pmode media type parameter or by external means. When
the value of pmode is equal to "NI-T", the NI-T mode MUST be used.
When the value of pmode is equal to "NI-C", the NI-C mode MUST be
used. When the value of pmode is equal to "NI-TC" or pmode is not
present, the NI-TC mode MUST be used. When the value of pmode is
equal to "I-C", the I-C mode MUST be used. [Ed.Note(YkW): There MAY
be at most one global pmode present in the SDP common for all the
multiplexed RTP sessions. It is also possible to have pmode
session-specific in the SDP, but then all the multiplexed sessions
MUST have the same value of this parameter. When pmode is not
present, the NI-TC mode is implied.]
The used MST packetization mode governs which session-specific
packetization modes are allowed in the associated RTP sessions,
which in turn govern which NAL unit types are allowed as RTP
payloads.
Table 2 summarizes the allowed session-specific packetization modes
for the NI-T, NI-C and NI-TC packetization modes. Table 3
summarizes the allowed session-specific packetization modes for the
I-C packetization mode.
Table 2 Summary of allowed session-specific packetization modes for
the NI-T, NI-C and NI-TC packetization modes (yes = allowed, no =
disallowed)
Session-Specific Mode Base Session Enhancement Session
----------------------------------------------------------
Single NAL Unit Mode yes no
Non-Interleaved Mode yes yes
Interleaved Mode no no
Table 3 Summary of allowed session-specific packetization modes for
the I-C packetization mode (yes = allowed, no = disallowed)
Session-Specific Mode Base Session Enhancement Session
----------------------------------------------------------
Single NAL Unit Mode no no
Non-Interleaved Mode no no
Interleaved Mode yes yes
Table 4 summarizes the allowed NAL unit types for each allowed
session-specific packetization mode of the NI-T packetization mode.
Table 5 summarizes the allowed NAL unit types for each allowed
session-specific packetization mode of the NI-C and NI-TC
packetization modes. Table 6 summarizes the allowed NAL unit types
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for the only allowed session-specific packetization mode (i.e. the
interleaved mode) of the I-C packetization mode.
Table 4 Summary of allowed NAL unit types for each session-specific
packetization mode of the NI-T packetization mode (yes = allowed, no
= disallowed, ig = ignore)
Type Packet Single NAL Non-Interleaved
Unit Mode Mode
------------------------------------------------
0 undefined ig ig
1-23 NAL unit yes yes
24 STAP-A no yes
25 STAP-B no no
26 MTAP16 no no
27 MTAP24 no no
28 FU-A no yes
29 FU-B no no
30 PACSI no yes
31 NI-MTAP no yes
Table 5 Summary of allowed NAL unit types for each session-specific
packetization mode of the NI-C and NI-TC packetization modes (yes =
allowed, no = disallowed, ig = ignore)
Type Packet Single NAL Non-Interleaved
Unit Mode Mode
------------------------------------------------
0 undefined ig ig
1-23 NAL unit yes yes
24 STAP-A no yes
25 STAP-B no no
26 MTAP16 no no
27 MTAP24 no no
28 FU-A no yes
29 FU-B no no
30 PACSI yes yes
31 NI-MTAP no yes
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Table 6 Summary of allowed NAL unit types for the session-specific
packetization mode of the I-C packetization mode (yes = allowed, no
= disallowed, ig = ignore)
Type Packet Interleaved
Mode
------------------------------
0 undefined ig
1-23 NAL unit no
24 STAP-A no
25 STAP-B yes
26 MTAP16 yes
27 MTAP24 yes
28 FU-A yes
29 FU-B yes
30 PACSI yes
31 undefined no
The NAL unit type values indicated as undefined in Tables 3.3, 3.4
and 3.5 are reserved for future extensions. NAL units of those
types SHOULD NOT be sent by a sender and MUST be ignored by a
receiver. Note that NAL unit type 30 and 31 are indicated as
undefined in RFC 3984, therefore RFC 3984 receivers MUST ignore NAL
units of this type, if present.
6.6 Aggregation Packets
Please see Section 5.7 of [RFC3984].
6.7 Fragmentation Units (FUs)
Please see section 5.8 of [RFC3984].
6.8 Payload Content Scalability Information (PACSI) NAL Unit
One of the two new NAL unit types specified in this memo is the
Payload Content Scalability Information (PACSI) NAL unit. The
OPTIONAL PACSI NAL unit, if present, MUST be the first NAL unit in
an aggregation packet or the NAL unit in a single NAL unit packet,
and it MUST NOT be present in other types of packets. The PACSI NAL
unit, when included in an aggregation packet, indicates scalability
information and other characteristics that are common for all the
remaining NAL units in the payload of the aggregation packet.
Furthermore, a PACSI NAL unit MAY contain zero or more SEI NAL
units. The PACSI NAL unit makes it easier for MANEs to decide
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whether to forward/process/discard the aggregation packet containing
the PACSI NAL unit. Additional reasons to use PACSI NAL units are
indicated later on, in the specification of the semantics of the
fields. Senders MAY create PACSI NAL units and receivers MAY ignore
them, or use them as hints to enable efficient aggregation packet
processing or decoding order recovery in multi-source transmission.
Note that the NAL unit type for the PACSI NAL unit (type 30) is
among the types that are left unspecified in [H.264] and [RFC3984].
When the first aggregation unit of an aggregation packet contains a
PACSI NAL unit, there MUST be at least one additional aggregation
unit present in the same packet. The RTP header and payload header
fields of the aggregation packet are set according to the remaining
NAL units in the aggregation packet.
When a PACSI NAL unit is included in a multi-time aggregation packet
(MTAP), 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, the
RTP header and payload header fields of the packet are set according
to the next non-PACSI NAL unit in transmission order.
The structure of a PACSI NAL unit is as follows. The first four
octets are exactly the same as the four-byte SVC NAL unit header
discussed in Section 5.35.3. 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 type octet of the SEI NAL unit). Figure
1 illustrates the PACSI NAL unit structure and an example of a PACSI
NAL unit containing two SEI NAL units.
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 aggregation
packet containing the PACSI NAL unit is not an STAP-A packet.
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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 1 PACSI NAL unit structure. Fields suffixed by
"(o.)" are OPTIONAL.
The values of the fields in PACSI NAL unit MUST be set as follows.
The term "target NAL unit" is used in the semantics of some fields.
When the PACSI NAL unit is included in an aggregation packet, a
"target NAL unit" refers to one or more NAL units that are contained
in the aggregation packet, but not included in the PACSI NAL unit
itself, that are in the same access unit as the first NAL unit
following the PACSI NAL unit in the aggregation packet. When the
PACSI NAL unit is included in a single NAL unit packet, a "target
NAL unit" refers to the next non-PACSI NAL unit in transmission
order.
o The F bit MUST be set to 1 if the F bit in at least one of the
remaining NAL units in the payload of the aggregation packet is
equal to 1 (when the PACSI NAL unit is included in an aggregation
packet) or if the next non-PACSI NAL unit in transmission order
has the F bit equal to 1 (when the PACSI NAL unit is included in
a single NAL unit packet). Otherwise, the F bit MUST be set
to 0.
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o The NRI field MUST be set to the highest value of NRI field among
all the remaining NAL units in the payload of the aggregation
packet (when the PACSI NAL unit is included in an aggregation
packet) or the value of the NRI field of the next non-PACSI NAL
unit in transmission order (when the PACSI NAL unit is included
in a single NAL unit packet).
o The Type field MUST be set to 30.
o The R bit MUST be set to 1. Receivers 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 payload of 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 all the remaining NAL units in the payload of 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 payload of 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 all the remaining NAL units in the payload of 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 all the remaining NAL units with the lowest value of DID in
the payload of the aggregation packet (when the PACSI NAL unit is
included in an aggregation packet) or the QID value of the next
non-PACSI NAL unit in transmission order (when the PACSI NAL unit
is included in a single NAL unit packet).
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o The TID field MUST be set to the lowest value of the TID values
of all the remaining NAL units with the lowest value of DID in
the payload of the aggregation packet (when the PACSI NAL unit is
included in an aggregation packet) or the TID value of the next
non-PACSI NAL unit in transmission order (when the PACSI NAL unit
is included in a single NAL unit packet).
o The U bit MUST be set to 1 if the U bit of at least one of the
remaining NAL units in the payload of 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 payload 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 payload of 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 MUST ignore 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, whereas the S and E
bits are unspecified and receivers MUST ignore these bits. The Y
bit SHOULD be identical for all the PACSI NAL units in all the
RTP sessions carrying the same SVC bitstream.
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o If the T bit is equal to 1, the OPTIONAL field DONC MUST be
present and specified as below. Otherwise, the field DONC MUST
NOT be present.
o The A bit MUST be set to 1 if all the target NAL units belong to
anchor layer representations. Otherwise, the A bit MUST be set
to 0. The A bit SHOULD be identical for all the PACSI NAL units
for which the target NAL units belong to the same access unit.
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. When a picture coding structure
such as IBBP is in use, 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 not be present in the bitstream.
o The P bit MUST be set to 1 if all the remaining NAL units in the
payload of 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 containedin
the variable-length coded slice header.
o The C bit MUST be set to 1 if all the target NAL units belong to
intra layer representations. Otherwise, the C bit MUST be set to
0. The C bit SHOULD be identical for all the PACSI NAL units for
which the target NAL units belong to the same access unit.
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 VCL NAL unit, in
decoding order, of the layer representation containing the first
NAL unit following the PACSI NAL unit in the aggregation packet
is present in the payload (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 VCL NAL unit, in decoding
order, of the layer representation containing the first NAL unit
following the PACSI NAL unit in the aggregation packet is present
in the payload (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: The S or E bit indicates whether the first or
last slice, in transmission order, of a layer representation is
in a packet, to enable a MANE to detect slice loss and take
proper action such as requesting a retransmission as soon as
possible, as well as to allow efficient playout buffer handling
similarly to the M bit present in the RTP header. The M bit in
the RTP header still indicates the end of an access unit, not the
end of a layer representation.
o When present, the TL0PICIDX field MUST be set to equal to
tl0_dep_rep_idx as specified in Annex G of [H.264] for the layer
representation containing the first NAL unit following the PACSI
NAL unit in the aggregation packet (when the PACSI NAL unit is
included in an aggregation packet) or containing the next non-
PACSI NAL unit in transmission order (when the PACSI NAL unit is
included in a single NAL unit packet).
o When present, the IDRPICID field MUST be set to equal to
effective_idr_pic_id as specified in Annex G of [H.264] for the
layer representation containing the first NAL unit following the
PACSI NAL unit in the aggregation packet (when the PACSI NAL unit
is included in an aggregation packet) or containing the next non-
PACSI NAL unit in transmission order (when the PACSI NAL unit is
included in a single NAL unit packet).
Informative note: The TL0PICIDX and IDRPICID fields enable the
detection of the loss of layer representations in the most
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important temporal layer (0) by receivers as well as MANEs. SVC
provides a solution that uses SEI messages, which are harder to
parse and may not be present in the bitstream at all.
o When present, the field DONC indicates the Cross-Session Decoding
Order Number (CS-DON) for the first NAL unit 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). The CS-DON is further
discussed in Section 6.10.
The PACSI NAL unit SHALL include a subset (zero to all) of the SEI
NAL units associated with the access unit to which the target NAL
units belong, and SHALL NOT contain SEI NAL units associated with
any other access unit. [Ed. (AE): Is the intention here to say: if
the AU has SEI messages, then they must all be included in the
PACSI. Or to say that the PACSI MAY include one or more of the SEI
NAL units..., i.e., to make it an option? The Informative note below
seems to indicate the latter (it uses the word "may").]
Informative note: Senders may repeat such SEI NAL units in the
PACSI NAL unit, so that they are repeated in more than one packet
and thus increase robustness against packet loss. Receivers may
use the repeated SEI messages in place of missing SEI messages.
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 comprising an access unit.
When the PACSI NAL unit is included in an aggregation packet, an SEI
message SHOULD NOT be included in the PACSI NAL unit and included in
one of the remaining NAL units contained in the same aggregation
packet.
6.9 Non-Interleaved Multi-Time Aggregation Packets (NI-MTAPs)
The second new NAL unit type introduced in this memois the Non-
Interleaved Multi-Time Aggregation packet (NI-MTAP). An NI-MTAP
consists of zero or more non-interleaved multi-time aggregation
units, as shown in Figure 2.
Informative note: The rule above differs from the constraint on
aggregation packets present in [RFC3984], where at least one NAL
unit must be contained in the aggregation packet.
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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 this NAL unit.
The structure of the multi-time aggregation units for the NI-MTAP is
presented in Figure 2. The starting or ending position of an
aggregation unit within a packet MAY not be on a 32-bit word
boundary. The NAL units in the NI-MTAP are ordered in NAL unit
decoding order.
The term NAL unit Effective Timestamp (ETS) is defined as the value
that the RTP timestamp would have if the particular NAL unit was
transported in its own RTP packet. This value is different from the
actual RTP timestamp present in the packet carrying the particular
NAL units in MTAP packets.
Let ETS be the effective timestamp of a NAL unit and TS the actual
RTP timestamp of the packet carrying the NAL unit. The timestamp
offset field MUST be set to a value equal to the value of the
following formula: If ETS >= TS, then TS offset = ETS - TS. If ETS
< TS, then TS offset = ETS + (2^32 - TS).
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Non-interleaved Multi-time aggregation unit for
NI-MTAP
For the "earliest" multi-time aggregation unit in an MTAP the
timestamp offset MUST be zero. Hence, the RTP timestamp of the MTAP
itself is identical to the earliest NAL unit effective timestamp.
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 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
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the smallest value if the timestamp wraps. Such an "earliest"
aggregation unit 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 multi-time aggregation packet 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NI-MTAPNAL HDR | NALU 1 Size | NALU 1 TS Off.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 1 TS Off.| NALU 1 HDR | NALU 1 DATA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: :
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
| NALU 2 SIZE | NALU 2 TS Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NALU 2 HDR | NALU 2 DATA |
+-+-+-+-+-+-+-+-+ |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 An RTP packet including a NI-MTAP non-interleaved
multi-time aggregation packet and two non-interleaved multi-
time aggregation units
6.10 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.
In addition to the timestamp mechanism desribed 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.
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6.10.1 Cross-Session DON (CS-DON) for Multi-Source Transmission
The Cross-Session Decoding Order Number (CS-DON) is a number that
indicates the decoding order of NAL units across all sessions
involved in MST. It is similar to the DON concept in [H.264], 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
mode (I-C) but also in two of the non-interleaved modes as well (NI-
C and NI-TC).
When the NI-C or NI-TC MST packetization modes are in use, the
packetization of each session MUST be as specified in Section 7.1.
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, and
recall that the NAL unit effective timestamp (ETS) was defined in
Section 6.9 as the value that the RTP timestamp would have if that
NAL unit would be transported in its own RTP packet.
o For each non-PACSI NAL unit carried in a session using the single
NAL unit session-specific 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 ETS 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 ETS, respectively.
o For non-PACSI NAL units carried in a session using a non-
interleaved session-specific packetization mode (NI-TC, NI-C),
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 when the single NAL unit session-
specific packetization mode is in use;
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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
equal to: (the CS-DON value of the previous non-PACSI
NAL unit in decoding order + 1) % 65536, where '%' is
the modulo operation.
. 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 as above when the
single NAL unit session-specific packetization mode is in
use, with SN1 being the SN value of the first FU-A packet.
When the I-C MST packetization mode is in use, the DON values
derived according to RFC 3984 of all the NAL units in each of the
multiplexed RTP sessions MUST indicate CS-DON values.
7. Packetization Rules
Section 6 of [RFC3984] applies in this memo, with the following
additions.
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
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
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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 delimiter, parameter set, or SEI
NAL unit 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.
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 the first aggregation unit of an aggregation packet contains a
PACSI NAL unit, there MUST be at least one additional aggregation
unit present in the same packet.
7.1 Packetization Rules for Multi-Source Transmission
When MST is used, decoding order recovery for NAL units carried in
the associated RTP sessions is needed. The following packetization
rules ensure that decoding order of NAL units carried in the
sessions can be correctly recovered for each of the MST
packetization modes using the de-packetization process specified in
Section 8.1.
The NI-T and NI-TC modes rely on timestamps to recover the decoding
order. In order to be able to do so, it is necessary for the SVC
stream to contain data for all sampling instances of a given layer
in all enhancement layers that depend on the given layer. The NI-TC,
NI-C, and I-C modes do not have this limitation, and use the CS-DON
value as a means to explicitly indicate decoding order, either
direcly coded in PACSI NAL units, or inferred from them using the
packetization rules. It is noted that the NI-TC mode offers both
techniques and it is up to the receiver to select which one to use.
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7.1.1 NI-T / NI-TC Packetization Rules
When the NI-T or NI-TC packetization mode is in use, the following
applies.
o 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 of the same sampling time instance
t MUST be present in any enhancement RTP session which depends on
RTP session A.
Informative note: There are multiple ways to insert additional
NAL units in order to satisfy this rule:
- One option for adding additional NAL units is to place NI-
MTAP packets (defined in Section 6.9), and not include any
aggregation packet in the payload. Although empty, these
packets are used by the process described in Section 8.1.1 for
the access unit re-ordering process.
- Additional NAL units may also be added by repeating prefix
NAL units (NAL unit type 14). Before passing NAL units to the
decoder re-ordering of the access unit as described in Section
8.1.1 is needed. This may only be possible for access units
which contain base layer NAL units. [Ed. (TS): It may be
useful to indicate in the SDP parameters that additional NAL
unit re-ordering as specified in 7.1.4 is not required.][Ed.
(AE): I don't understand this comment.]
- Additional NAL units may also be added by placing single NAL
unit packets containing exactly one PACSI NAL unit in the
enhancement RTP sessions.
- Additional NAL units may also be added by the encoder
itself. This option, however, may not be available with pre-
encoded content.
o When not using NI-TC 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.
7.1.2 NI-C / NI-TC Packetization Rules
When the NI-C or NI-TC packetization mode is in use, the following
applies.
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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.
. If the ETS of the non-PACSI NAL unit is not equal to the ETS
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 ETS of the non-PACSI NAL unit is equal to the
ETS of the previous non-PACSI NAL unit in decoding order),
the previous packet MAY contain a PACSI NAL unit containing
the DONC field.
o For each STAP-A packet, the first NAL unit in the STAP-A packet,
if present, MUST be a PACSI NAL unit containing the DONC field.
[Ed. (AE): Is it possible to have an empty STAP-A? Or was the "if
present" superfluous?]
o For each FU-A packet, if present, the previous packet MUST have
the same RTP timestamp as the FU-A packet, and the following
applies. [Ed. (AE): See the previous comment for STAP-A,
regarding the "if present" part.]
. If the FU-A packet is the start of the fragmented NAL unit,
the following applies;
. If the ETS of the fragmented NAL unit is not equal to
the ETS 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 ETS of the fragmented NAL unit is equal
to the ETS 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 ETS
equal to the ETS 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.
[Ed. (AE): Does this mean I am inserting a single RTP
packet with just PACSI in it? Just to make sure.]
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. 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.
7.1.3 I-C Packetization Rules
When the I-C session-multiplexing 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 MUST NOT be present.[Ed. (AE): Why?
Revisit.]
7.1.4 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, might
not be able to easily determine which non-VCL units are relevant to
which session. Thus, essential non-VCL NAL units (parameter sets
sent in-band, i.e., NAL unit types 7, 8, 13, and 15) MAY, instead,
be sent in session that the one they are used by depends on (e.g.,
the base RTP session), and non-essential non-VCL NAL units MAY be
sent in any 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. Alternatively,
it MAY be repeated in all such sessions. In general, identical non-
VCL units MAY be sent in more than one session for redundancy. [Ed.
(JL): Can this cause issues with HRD timing?]
7.1.5 Packetization Rules for Prefix NAL Units
When the base layer is sent on an RTP session using the Non-
Interleaved or the Interleaved mode, prefix NAL units SHOULD be
aggregated (using STAP-A or STAP-B units) with the NAL unit they
prefix, unless this would violate session MTU constraints or if
fragmentation units are used.
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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.
8. De-Packetization Process
For single-source transmission where a single RTP session is used,
the de-packetization process specified in Section 7 of [RFC3984]
applies. [Ed. (??): with some fixes to section 7 of RFC 3984 and
some changes/additions to section 7.3 (Additional De-Packetization
Guidelines) of RFC 3984 - TBD]
For multi-source transmission, where more than one RTP sessions are
used to receive data from the same SVC bitstream, the de-
packetization process is specified in Section 8.1.
8.1 De-Packetization Process for Multi-Source Transmission
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 SHALL be identified by mechanisms
specified in [I-D.ietf-mmusic-decoding-dependency]. Enhancement RTP
sessions typically contain 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 in
increasing order of sequence number into the payload de-
packetization to NAL units as defined in this memo.
The decoding order of the NAL units carried in all the associated
RTP sessions is then recovered by applying one of the following
subsections, depending on which of the MST packetization modes is in
use.
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8.1.1 Decoding Order Recovery for the NI-T and NI-TC Modes
The following process SHALL 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 S is given by the ordering of sequence numbers SN of the RTP
packets the NAL units are contained in. In an aggregation packet
contained in an RTP packet the decoding order is given by the order
of appearance of the NAL units within the packet. The RTP session
identifier S gives the increasing order of dependency of the
received RTP sessions as indicated by mechanisms specified in
Section 9.2.3, where S equal to 0 identifies the base RTP session.
[Ed. (AE): Does the mmusic draft excplicitly order the session ID's
by dependency? I coulnd't find it in the text.]
Timing information according to the media timestamp TS(SN) derived
from the RTP packet timestamp of the RTP packet with sequence number
SN is associated with all NAL units contained in the same RTP packet
received in RTP session S. For NI-MTAP packets the effective
timstamp ETS is derived for each contained NAL unit by using the "TS
offset" value in the NI-MTAP packet as defined in 6.9, and is used
instead of the actual TS. NAL units contained in fragmentation
packets are handled as defragmented, entire NAL units with their own
timestamp. All NAL units associated with the same value of media
timestamp TS are part of the same access unit AU(TS).
Each NI-MTAP packet which does not contain any aggregation units or
each PACSI NAL unit in a single NAL unit packet SHOULD be kept as,
effectively, access unit indicators in the re-ordering process. NI-
MTAP or PACSI NAL units SHOULD be removed before passing access unit
data to the decoder.
Informative Note: These special (essentially, empty) NI-MTAP
and PACSI 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 7.1.1.
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The decoding order of NAL units from multiple RTP streams in
multiple RTP sessions SHALL be recovered into a single sequence of
NAL units, grouped into access units, by performing the following
steps:
o The process SHOULD be started with the NAL units received in
the highest RTP enhancement session with the earliest
timestamp TS available in the session's (de-jittering)
buffer.
o Collect all NAL units associated with the same value of
timestamp TS, starting from the highest RTP enhancement
session, from all the (de-jittering) buffers of the received
RTP sessions. The collected NAL units will be those
associated with the access unit AU(TS).
o Place the collected NAL units in increasing order of session
identifier S.
o Place the ordered collected NAL units in decoding order
within the particular access unit by satisfying the NAL unit
ordering rules for SVC access units, as specified in the
informative algorithm provided in Section 8.1.1.1.
o Remove NI-MTAP and any PACSI NAL units from the access unit
AU(TS).
o The access units MAY be transferred to the decoder. If
access units AU(TS) are transferred to the decoder, they
SHALL be passed in the order of appearance (given by the
order of RTP sequence numbers) of 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
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 the highest RTP session to be the largest
session identifier 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:
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The example shown in Figure 4 refers to three RTP sessions A, B and
C containing an SVC bitstream transmitted as 3 sources. In the
example, the dependency signaling as described in Section 9.2.3,
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 RTP
enhancement 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 indicates decoding order numbers for NAL units in
the various packets across the sessions, as well as the associated
media timestamps (TS[]). The example demonstrates decoding order
recovery when differenta amounts of jitter is present in each of the
sessions (i.e., i.e., at buffering startup not all packets with the
same timestamp are available in all the de-jittering buffers.
The process first proceeds to the NAL units associated with the
first timestamp TS[1] present in session C and removes/ignores all
preceding 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 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 timestamp TS[1]. Then the
next 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)----
------------------------------------------------------------------->
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
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in a RTP session
'|' - indicates corresponding NAL units of the
same access unit AU(TS[..]) in the RTP
sessions
Integer values in '[]' - media Timestamp (TS), sampling time as
derived from RTP timestamps associated to
the access unit AU(TS[..]).
Figure 4 Example of decoding order recovery in multi-source
transmission with different session jitter at startup.
8.1.1.1 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 the list below, except
from NAL unit type 14 which is handled specially as described. 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.
o 9 Access unit delimiter
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Only one access unit delimiter will be present within an
access unit.
o 7 Sequence parameter set
Any order of sequence parameter sets within an access unit is
valid.
o 13 Sequence parameter set extension
Any order of sequence parameter set extensions within an
access unit is valid.
o 15 Subset sequence parameter set
Any order of subset sequence parameter sets within an access
unit is valid.
o 8 Picture parameter set
Any order of picture parameter sets within an access unit is
valid.
o 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.
o 1 Coded slice of a non-IDR picture
o 5 Coded slice of an IDR picture
o 14 Prefix NAL unit in scalable extension
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.)
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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.) [Ed. (AE): Shouldn't this go before #5??]`
o 12 Filler data
o 14 Prefix NAL unit in scalable extension
Any order of filler data units within an access unit is valid.
[Ed. (AE): Shouldn't this move up, below #12?]
If NAL units of type 14 are present, every filler data NAL
unit is prefixed by a NAL unit of type 14.
o 2 Coded slice data partition A
o 3 Coded slice data partition B
o 4 Coded slice data partition C
These NAL units will be sent within only a single session for
any given access unit, and are placed in session order.
(Note: No current SVC profile uses slice data partitioning.)
o 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.
o 16-18 Reserved
o 21-23 Reserved
These are placed immediately following the non-reserved-type
VCL NAL unit they follow in session order.
o 20 Coded slice in scalable extension
These 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:
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SVC slices with a given DQId value will be sent on a single
session for any given access unit.)
o 10 End of sequence
Only one end of sequence will be present within an access
unit.
o 11 End of stream
Only one end of stream will be present within an access unit.
[Ed. (AE): This list needs reformatting as a table.]
8.1.2 Decoding Order Recovery for the NI-C, NI-TC and I-C Modes
The following process SHALL 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.
[Ed.Note(YkW): Add handling of some cases of packet losses when the
NI-C or NI-TC mode is in use, that discards some received NAL units
for which the CS-DON value cannot be derived.]
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
media type parameter of the highest RTP session the receiver
receives.
The CS-DON value is calculated and stored for each NAL unit.
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-interleave-
depth media type parameter of the highest RTP session incremented
by 1.
Initial buffering lasts until one of the following conditions is
fulfilled:
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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.
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.
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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.
9. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features of the
bitstream. The parameters are specified here as part of the media
type registration for the SVC codec. A mapping of the parameters
into the Session Description Protocol (SDP) [RFC4566] is also
provided for applications that use SDP. Equivalent parameters could
be defined elsewhere for use with control protocols that do not use
SDP.
Some parameters provide a receiver with the properties of the stream
that will be sent. The names of all these parameters start with
"sprop" for stream properties. Some of these "sprop" parameters are
limited by other payload or codec configuration parameters. For
example, the sprop-parameter-sets parameter is constrained by the
profile-level-id parameter. The media sender selects all "sprop"
parameters rather than the receiver. This uncommon characteristic
of the "sprop" parameters may not be compatible with some signaling
protocol concepts, in which case the use of these parameters SHOULD
be avoided.
9.1 Media Type Registration
The media subtype for the SVC codec is allocated from the IETF tree.
The receiver MUST ignore any unspecified parameter.
Informative note: Requiring that the receiver ignores unspecified
parameters allows for backward compatibility of future
extensions. For example, if a future specification that is
backward compatible to this specification specifies some new
parameters, then a receiver according to this specification is
capable of 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. (??): Text on "H264" must go into different section, see
Colin's comments sent on 10 June 2008]The media subtype "H264" MUST
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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.
[Edt. Note: 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
OPTIONAL parameters:
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, 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
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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.
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
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that the produced bitstream conforms to the standard and to
control the output bitrate. Thus, the coded picture buffer
is conceptually independent of any other potential buffers
in the receiver, including 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 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].
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Senders MAY use this knowledge to send higher bitrate video as
allowed in the level definition of SVC, to achieve improved
video quality.
Informative note: This parameter was added primarily to
complement a similar codepoint in the ITU-T Recommendation
H.245, so as to facilitate signaling gateway designs. No
assumption can be made from the value of this parameter
that the network is capable of handling such bit rates at
any given time. In particular, no conclusion can be drawn
that the signaled bit rate is possible under congestion
control constraints.
redundant-pic-cap:
This parameter is as specified in 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.
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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.
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.
pmode:
This parameter signals the properties of a NAL unit stream
carried in more than one RTP session using session
multiplexing or the capabilities of a receiver implementation.
When the value of pmode is equal to "NI-T", the NI-T mode MUST
be used. When the value of pmode is equal to "NI-C", the NI-C
mode MUST be used. When the value of pmode is equal to "NI-
TC" or pmode is not present, the NI-TC mode MUST be used.
When the value of pmode is equal to "I-C", the I-C mode MUST
be used. The value of pmode MUST have one of the following
tokens: "NI-T", "NI-C", "NI-TC", or "I-C". This parameter
MUST NOT be present, when "packetization-mode" is present.
sprop-mst-interleave-depth:
This parameter MUST NOT be present when the value of pmode is
equal to "NI-T". This parameter MUST be present when the
value of pmode is equal to "NI-C", "NI-TC", or "I-C" or pmode
is not present.
This parameter signals the properties of a 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 reconstruct NAL unit decoding order as specified
in Subsection 8.1.2 of this memo when the remultiplexing
buffer size is at least the value of sprop-mst-interleave-
depth + 1 in terms of VCL NAL units. [Ed. (AE): But you don't
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say what's the property explicitly. Only what it guarantees.
Is it interleaving depth?]
The value of sprop-mst-interleave-depth MUST be an integer in
the range of 0 to 32767, inclusive.
sprop-remux-buf-req:
This parameter MUST NOT be present when the value of pmode is
equal to "NI-T". It MUST be present when pmode is not present
or the value of pmode 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 8.1.2, when the
remultiplexing buffer size is 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 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 8.1.2. A receiver is able to handle any NAL unit
stream for which the value of the sprop-remux-buf-req
parameter is smaller than or equal to this parameter.
If the parameter is not present, then a value of 0 MUST be
used for remux-buf-cap. The value of remux-buf-cap MUST be an
integer in the range of 0 to 4294967295, inclusive.
sprop-remux-init-buf-time:
This parameter MAY be used to signal the properties of 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 pmode is not present or the value of
pmode is equal to "NI-C", "NI-TC", or "I-C".
The parameter signals the initial buffering time that a
receiver MUST wait before starting to recover the NAL unit
decoding order as specified in Subsection 8.1.2 of this memo.
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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 the value of pmode 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-prebuf-size:
This parameter MAY be present when the current RTP session
depends on any other RTP session. This parameter MUST NOT be
present when pmode is not present or the value of pmode is
equal to "NI-C", "NI-TC", or "I-C". sprop-prebuf-size MAY
signal the required size of the receiver buffer for the NAL
unit stream per RTP session. This parameter may be useful to
compensate the impact of inter-RTP session jitter, when the
receiver buffer size is at least the value of sprop-prebuf-
size in terms of bytes. Herein the NAL unit stream refers to
the one consisting of all NAL units conveyed in the current
RTP session.
The value of sprop-prebuf-size MUST be an integer in the range
of 0 to 4294967295, inclusive.
Informative note: sprop-prebuf-size indicates the required
size of the prebuffering receiver buffer only. When
network jitter can occur, an appropriately sized jitter
buffer has to be provisioned for as well. When a scalable
bitstream is conveyed in more than one RTP session, and the
sessions initiates at different time, the session
initiation variation has also to be compensated by an
appropriately sized buffer.
[Ed. (AE): None of the sprop-prebuf-* parameters are
mentioned anywhere else in the spec. Why are they needed?]
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sprop-prebuf-time:
This parameter MAY be used to signal the properties of a NAL
unit stream within a session multiplexing. Herein the NAL
unit stream refers to the one consisting of all NAL units
conveyed in the current RTP session. This parameter MUST NOT
be present when pmode is not present or the value of pmode is
equal to "NI-C", "NI-TC", or "I-C".
The parameter signals the initial buffering time used for a
receiver before starting to recover the NAL unit decoding
order for more than one RTP session. The parameter is the
maximum value of (transmission time of a NAL unit - decoding
time of the NAL unit), assuming reliable and instantaneous
transmission, the same timeline for transmission and decoding,
and that decoding starts when the first packet arrives.
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-prebuf-time MUST be
an integer in the range of 0 to 4294967295, inclusive.
In addition to the signaled sprop-prebuf-time, receivers
SHOULD take into account the transmission delay jitter
buffering, including buffering for the delay jitter caused by
mixers, translators, gateways, proxies, traffic-shapers, and
other network elements. Yet another aspect receivers SHOULD
take into account is the session initiation variation when a
scalable bitstream is conveyed in more than one session,
including buffering the variation.
[Ed. (YkW): Need to discuss how inter-RTP session jitter
should be handled in general, and how it works by using sprop-
prebuf-size and sprop-prebuf-time.]
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 [H.264].
[Ed. (TS): That is, a SDP capable receiver/middle-box must
decode the sprop-scalabiltiy-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-spatial-resolution:
[Ed. (??): I know that framerate and bitrate SDP parameters
are already available, but failed to find a spatial resolution
SDP parameter. It would be good if this is already defined.
Otherwise, it would be better to be defined somewhere else
because it is a generic parameter.]
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This parameter MAY be used to indicate the property of a
stream or the capability of a receiver or sender
implementation. The value is a base16 of the width and height
of the spatial resolution, in pixels, separated by a comma.
[Edt. Note (TS): Shouldn't this be a generic SDP parameter?]
Encoding considerations:
This type is only defined for transfer via RTP (RFC 3550).
Security considerations:
See Section 10 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.
9.2 SDP Parameters
[Ed. (??): For agreeing on a Layer or OP in unicast, an SDP can
contain multiple m lines with bitrate, framerate 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-scalabiltiy-info using scalable-layer-id.
For layered multicast, then the grouping signaling in I-D.ietf-
mmusic-decoding-dependency is needed.
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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.]
9.2.1 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", "pmode", "sprop-mst-interleave-depth", "sprop-remux-buf-
req", "remux-buf-cap", "sprop-remux-init-buf-time", "sprop-mst-
max-don-diff", "sprop-prebuf-size", "sprop-prebuf-time", "sprop-
layer-range", "sprop-scalability-info", "scalable-layer-id", and
"sprop-spatial-resolution", 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.
9.2.2 Usage with the SDP Offer/Answer Model
When H.264 or 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 H.264
or SVC are "profile-level-id", "packetization-mode", and, if
required by "packetization-mode", "sprop-deint-buf-req". 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.
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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.[Edt. Note: do we need
to additionally define behavior with snd/rcvonly parameter?]
o The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
"sprop-interleaving-depth", "sprop-max-don-diff", "sprop-init-
buf-time", "sprop-prebuf-size", "sprop-prebuf-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 H.264 or
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.
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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 H.264 or
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.
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-prebuf-size", "sprop-prebuf-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.
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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.
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-prebuf-size
- sprop-prebuf-time
- sprop-scalability-information
- sprop-layer-range
- scalable-layer-id
Declaring receiver implementation capabilities:
- max-mbps
- max-fs
- max-cpb
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- 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-prebuf-size
- sprop-prebuf-time
- sprop-scalability-information
- sprop-layer-range
- sprop-spatial-resoltuion
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.
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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.
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.
9.2.3 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.
9.2.4 Usage in Declarative Session Descriptions
When H.264 or SVC over RTP is offered with SDP in a declarative
style, as in RTSP [RFC2326] or SAP [RFC2974], the following
considerations are necessary.
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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-prebuf-size
- sprop-prebuf-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
- 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.
9.3 Examples
9.3.1 Example for Offering A Single SVC Session
Offerer -> Answerer SDP message:
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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=;
9.3.2 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;
sprop-parameter-sets=Z01ACprLFicg,aP4Eag==;
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,aGvgRiA=;
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=53000c; packetization-mode=2;
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; pmode=NI-T;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
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qA=,aGvgRiA=;
a=rtpmap:100 H264-SVC/90000
a=fmtp:100 profile-level-id=53000c; pmode=I-C;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,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; pmode=NI-TC;
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==,aEvgR
qA=,aGvgRiA=;
a=mid:3
a=depend:101 lay 1:96,97 2:99
9.4 Parameter Set Considerations
Please see Section 8.4 of [RFC3984].
10. 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.
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].
11. 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:
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a) within the highest Layer identified by the DIDfield, 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.
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].
12. IANA Consideration
[Edt. Note: A new media type should be registered from IANA.]
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13. Informative Appendix: Application Examples
[Ed. (AE): I think this whole section does not add any real value,
is outdated, and should be eliminated. In particular, the
application scenario of the only currently shipping SVC product is
not even listed here.]
13.1 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 13.5 -- 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.
13.2 Layered Multicast
This well-understood form of the use of layered coding [McCanne]
implies that all layers are individually conveyed in their own RTP
packet streams, each carried in its own RTP session using the IP
(multicast) address and port number as the single demultiplexing
point. Receivers "tune" into the layers by subscribing to the IP
multicast, normally by using IGMP [IGMP]. Depending on the
application scenario, it is also possible to convey a number of
layers in one RTP session, when finer operation points within the
subset of layers are not needed.
Layered multicast has the great advantage of simplicity and easy
implementation. However, it has also the great disadvantage of
utilizing many different transport addresses. While 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
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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].
13.3 Streaming of an SVC Scalable Stream
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 sending bitrate by
choosing fewer layers, when composing the layered stream; see
section 11. SVC is designed to gracefully support both bandwidth
rampdown and bandwidth rampup with a considerable dynamic range.
This payload format is designed to allow for bandwidth flexibility
in the mentioned sense. While, in theory, a transcoding step could
achieve a similar dynamic range, the computational demands are
impractically high and video quality is typically lowered --
therefore, few (if any) streaming servers implement full
transcoding.
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13.4 Multicast to MANE, SVC Scalable Stream 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 not have the processing power or the display size to
meaningfully decode all layers; others may have these capabilities.
Users of some endpoints may not wish 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 need 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
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
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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, so to adhere to congestion
control principles as discussed in section 11. 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.
13.5 Scenarios Currently Not Considered
Remarks have been made that the current draft does not take into
consideration at least one application scenario which some JVT folks
considered important. In particular, their idea was to make the RTP
payload format (or the media stream itself) self-contained enough
that a stateless, non-signaling-aware device can "thin" an RTP
session to meet the bandwidth demands of the endpoint. They called
this device a "Router" or "Gateway", and sometimes a MANE.
Obviously, it's not a Router or Gateway in the IETF sense. To
distinguish it from a MANE as defined in RFC 3984 and in this
specification, let's call it an MDfH (Magic Device from Heaven).
To simplify discussions, let's assume point-to-point traffic only.
The endpoint has a signaling relationship with the streaming server,
but it is known that the MDfH is somewhere in the media path (e.g.
because the physical network topology ensures this). It has been
requested, at least implicitly through MPEG's and JVT's requirements
document, that the MDfH should be capable to intercept the SVC
scalable bit stream, modify it by dropping packets or parts thereof,
and forwarding the resulting packet stream to the receiving
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endpoint. It has been requested that this payload specification
contains protocol elements facilitating such an operation, and the
argument has been made that the NRI field of RFC 3984 serves exactly
the same purpose.
The authors of this I-D do not consider the scenario above to be
aligned with the most basic design philosophies the IETF follows,
and therefore have not addressed the comments made (except through
this section). In particular, we see the following problems with
the MDfH approach):
o As the very minimum, the MDfH would need to know which RTP streams
are carrying SVC. We don't see how this could be accomplished but
by using a static payload type. None of the IETF defined RTP
profiles envision static payload types for SVC, and even the de-
facto profiles developed by some application standard
organizations (3GPP for example) do not use this outdated concept.
Therefore, the MDfH necessarily needs to be at least "listening"
to the signaling.
o If the RTP packet payload were encrypted, it would be impossible
to interpret the payload header and/or the first bytes of the
media stream. We understand that there are crypto schemes under
discussion that encrypt only the last n bytes of an RTP payload,
but we are more than unsure that this is fully in line with the
IETF's security vision.
Even if the above two problems would have been overcome through
standardization outside of the IETF, we still foresee serious design
flaws:
o An MDfH can't simply dump RTP packets it doesn't want to forward.
It either needs to act as a full RTP Translator (implying that it
rewrites RTCP RRs and such), or it needs to patch the RTP sequence
numbers to fulfill the RTP specification. Not doing either would,
for the receiver, look like the gaps in the sequence numbers
occurred due to unintentional erasures, which has interesting
effects on congestion control (if implemented), will break pretty
much every meta-payload ever developed, and so on. (Many more
points could be made here).
In summary, based on our current knowledge we are not willing to
specify protocol mechanisms that support an operation point that has
so little in common with classic RTP use.
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13.6 SSRC Multiplexing
The authors have played with the idea of introducing SSRC
multiplexing, i.e. allowing sending multiple RTP packet streams
containing layers in the same RTP session, differentiated by SSRC
values. Our 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
SSRC multiplexed packet streams). We were hoping that 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 we envisioned, its transport addresses needs 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 our use case.
As the envisioned use case cannot be implemented, we refrained to
add the considerable document complexity to support SSRC
multiplexing herein.
14. References
14.1 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.
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[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.
[RFC4566] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
14.2 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.
[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.
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[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.
[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.
15. 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
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Germany
Phone: +49-30-31002-227
Email: schierl@hhi.fhg.de
Alex Eleftheriadis
Vidyo, Inc.
433 Hackensack Ave.
Hackensack, NJ 07601
USA
Phone: +1-201-467-5135
Email: alex@vidyo.com
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WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
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FOR A PARTICULAR PURPOSE.
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 on and input to the draft.
16. Open Issues
1) There is a list of remaining issues for decoding order recovery in
session multiplexing, as documented in editing notes.
2) A lot of work needed for section9.2 (SDP parameters).
3) Bugs in RFC 3984 (see the BIS draft) need to be fixed also in the
memo.
4) Clarify the usage of the new parameters like sprop-scalability-
info, relation to SEI and usage in offer/answer. In the Design
Team's conference call on 6 May 2008, it was decided that Ye-Kui
would study and report.
5) Non-VCL NAL units, e.g. SEI messages and parameter sets, may be
needed by an enhancement layer but not the base layer. However,
according to SVC, within an access unit, these non-VCL NAL units
must precede VCL NAL units in decoding order. In session
multiplexing, should non-VCL NAL units be transported in the same
session as the layer that requires the non-VCL NAL unit, or should
they be always transported in the base session? It may be
impossible to find out without parsing details which session
respectively SPS/subset SPS a picture parameter set belongs to. It
may make sense for simplicity to allow a MANE to include all of
the non-VCL NAL units within all the sessions. In the Design
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Team's conference call on 6 May 2008, it was decided that
Jonathan/Alex would provide text change for review, including
handling of prefix NAL units.
6) sprop-spatial-resolution: do we need this for offering/answering
spatial scalable layers? In this draft or a more generic draft? In
the Design Team's conference call on 6 May 2008, it was decided
that Ye-Kui will study and report.
17. Changes Log
From draft-ietf-avt-rtp-svc-08 to draft-ietf-avt-rtp-svc-09
5-9 May 2008: YkW
- Added Alex as an editor. Welcome!
- Added text for session-multiplexing packetization modes
- Updated section6.5 (Packetization Modes) and added section6.5.1
- Updated section6.6 (DON) and added section6.6.1
- Updated PACSI introductory text and semantics (section6.9)
- Updated packetization rules for session multiplexing (section7.1)
- Updated the de-packetization process for session multiplexing
(section8.1)
- Updated semantics of existing media type parameters and added new
media type parameters ("pmode", "sprop-mst-interleave-depth",
"sprop-remux-buf-req", "remux-buf-cap", "sprop-remux-init-buf-
time", "sprop-mst-max-don-diff") in section9.1.
- Removed obsolete comments.
- Updated some definitions.
- Updated one design principle regarding cases that must use RFC
3984 encapsulation.
- Removed "(Informative)" from the title of section8 (De-
Packetization Process) - same to be proposed to RFC 3984 bis.
- Updated the open issues
- Removed earlier changes log that can be found from earlier
versions of the draft
13 May 2008: AE
- Corrected definition of "highest RTP session" in "enhancement RTP
session" definition.
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- Moved the triggering of the use of the S and E bits from the X
flag to the Y flag, so that they are triggered together with
TL0PICIDX.
- Other minor editorial corrections for language.
13 May 2008: YkW
- Removed some obsolete comments.
14 May 2008: AE
- Moved definition of lower/higher/lowest/highest RTP session from
definitions (part of "enhancement RTP session") to 8.1.1, right
before they are first needed. The text assumes that base is
lowest, and highest is the single RTP session which no other
session depends on.
- Modified definition of "operation point" to distinguish between
the OP and the associated bitstream, as there may be multiple ways
to construct it (i.e., removing or not removing unneeded NAL units
below the operation point).
- Renamed modes as follows: NITS->NI-T, NICD->NI-C, NICB->NI-TC,
and AINT->I-C. This way the interleaving and DON process are
evident from the acronym.
- Rephrased "target NAL unit" definition (in Section 6.9) to be a
bit more clear.
15 May 2008: YkW
- Added a comment regarding the definition of operation point.
- Updated open issues (removed the one on PACSI).
- Changed the Word template used to generation of this I-D.
From draft-ietf-avt-rtp-svc-09 to draft-ietf-avt-rtp-svc-10
30 May 2008: TS
- Improved text in 7.1 for NI-T
02 June 2008: TS
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- Improved text on NI-T and NI-TC in section 8.1.1
- Added new packet type NI-MTAP in section 6.10
- Added informative text on placeholder NAL units to section 7.1.1
- Added text on placeholder NAL units to 8.1.1
- Changed text regarding the order of RTP packets are delivered from
the RFC3550 process to the re-ordering process in section 8.
02 June 2008: JL
- Added non-VCL and prefix NAL packetization rules sent by Jonathan
Lennox in sections 7.1.4 and 7.1.5.
- Added informative reordering algorithm.
From draft-ietf-avt-rtp-svc-10 to draft-ietf-avt-rtp-svc-11
17 June 2008: TS
- Addressed the following comments sent by Colin on 10 June 2008 to
the mailing list:
- Inserted table on NAL unit and packet types into section 3.3
- Corrected text on NAL unit header ext. in 3.3
- Corrected text in 3.4
- Corrected text in 5.1.1 on base layer VCL Nal unit types
- Added reference to MANEs in section 6.1
- Used symbolic names for packetization modes
- Corrected SDP examples
- Removed H264 from media type section, TBD: text on "H264" needs to
be moved
- TBD: new name for session mux.
- Improved text on NI-T mode
From draft-ietf-avt-rtp-svc-11 to draft-ietf-avt-rtp-svc-12
30 June 2008: AE
The entire text has been edited. The following offers highlights of
the important changes.
- Moved Section 3 (The SVC Codec) after "Scope" (Sec. 4) and
"Definitions" (Sec. 5).
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- Changed section numbering style definitions in Word so that
section references are properly formed (without a trailing ".").
- Renumbered tables to run sequentially in the draft.
- Removed [SVC] from the normative references. All references to
[SVC] were converted to references to [H.264].
- Major rewrite of Introduction (and Abstract) to introduce concepts
and facilitate understanding. The text from Scope has been moved
there, and the Scope re-written to be inline with the intended
content.
- Definition of "session-multiplexing", is problematic. It states
that in all cases there is a single RTP session, which is
incorrect. The two cases are: 1) transmission on sepearate
transport addresses, 2) transmission on a single transport
address. In the former case there are clearly multiple RTP
sessions. It is also not clear to me if the single transport
address mode is supported in this memo or it should be explicitly
disallowed. I put a comment to that effect in the text.
- Session multiplexing: The text uses the term RTP base session and
RTP enhancement session throughout, as well as RTP stream.
Jonathan suggested multi-session stream, but that would create a
lot of problems with the terminology in this draft (which says,
e.g., that each RTP session carries an RTP stream). So I initially
opted for the term "multi-session transmission". This is in-line
with the RFC3550 definition of an "RTP session". We can easily
search/replace to convert to "mutli-stream" transmission" if
necessary (but it is not advisable, as the whole text needs to be
reviewed for consistency). But due to the fact the draft's
definition of "session multiplexing" explicitly says that it can
be over the same transport or different transport addresses (thus
using a single or multiple RTP sessions), I converged to "multi-
source transmission" to indicate that the SVC stream is
transmitted as streams coming from multiple sources. I think that
nails it.
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- Defined acronyms SST (Single-Source Transmission) and MST (Multi-
Source Transmission), and added in "Abbreviations".
- Added definition for "NALU time" as "NAL unit effective
timestamp". This is also inline with the concepts in 8.1.1.
- Added ETS in the abbreviations for Effective Timestamp of a NAL
unit.
- It is noted that FU-A and FU-B are also carrying one NAL unit per
RTP packet (since FU itself is defined as a NAL unit), so the
statement that only in the single NAL unit mode you have one NAL
unit per packet is, technically, incorrect. Since the intended use
is present in the name of the mode (single NAL unit mode) I
removed any dubious reference to this property.
- Table 3.3 (now Table 4), changed PACSI from 'no' to 'yes' (in the
non-interleaved column only).
- Table 3.5 (now Table 6), changed PACSI from 'no' to 'yes'. But in
7.1.3 (I-C packetization rules) PACSIs are not allowed to have
DONC. Why?
- Moved Section 6.5 DON to the end of Section 6, as 6.10. Part of
the reason is that the section requires that PACSI is already
introduced.
- "de-session-multiplexing buffer" was changed to "remultiplexing
buffer", properly reflecting what's going on.
- Changed all sprop-desemul-* to sprop-remux-*.
- Changed all sprop-semul-* to sprop-mst-*.
- Changed sprop-session-multiplexing-depth to sprop-mst-interleave-
depth.
- Added all acronyms encountered in the text in the "Abbreviations"
section.
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