Network Working Group S. Wenger
Internet-Draft Y.-K. Wang
Intended status: Standards Track Nokia
Expires: July 7, 2008 T. Schierl
Fraunhofer HHI
January 8, 2008
RTP Payload Format for SVC Video
draft-ietf-avt-rtp-svc-05.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This memo describes an RTP payload format for scalable video coding
(SVC) defined in_Annex G of the ITU-T Recommendation H.264 video
codec which is technically identical to Amendment 3 of ISO/IEC
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International Standard 14496-10. The RTP payload format allows for
packetization of one or more Network Abstraction Layer (NAL) units,
produced by the video encoder, in each RTP packet payload. The
payload format has wide applicability, such as low bit-rate
conversational, Internet video streaming, or high bit-rate
entertainment quality video.
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Table of Contents
RTP Payload Format for SVC Video...................................1
1. Introduction .................................................5
2. Conventions ..................................................5
3. The SVC Codec ................................................5
3.1. Overview ..................................................5
3.2. Parameter Set Concept .....................................7
3.3. Network Abstraction Layer Unit Header .....................8
4. Scope .......................................................11
5. Definitions and Abbreviations ...............................11
5.1. Definitions ..............................................11
5.1.1. Definitions per SVC specification ........................11
5.1.2. Definitions local to this memo ...........................12
5.2. Abbreviations ............................................14
6. RTP Payload Format ..........................................14
6.1. Design Principles ........................................14
6.2. RTP Header Usage .........................................15
6.3. Common Structure of the RTP Payload Format ...............15
6.4. NAL Unit Header Usage ....................................15
6.5. Packetization Modes ......................................16
6.6. Decoding Order Number (DON) ..............................16
6.7. Aggregation Packets ......................................17
6.8. Fragmentation Units (FUs) ................................17
6.9. Payload Content Scalability Information (PACSI) NAL Unit .17
7. Packetization Rules .........................................23
8. De-Packetization Process (Informative) ......................25
8.1. De-Packetization Process for NAL Units Conveyed using
Session Multiplexing..............................................25
8.1.1. De-Packetization Process for Session Multiplexing using
non-interleaved mode or Single NAL unit mode without the use of CL-
DON ..........................................................26
8.1.2. De-Packetization Process for Session Multiplexing using CL-
DON ..........................................................28
9. Payload Format Parameters ...................................31
9.1. Media Type Registration ..................................31
9.2. SDP Parameters ...........................................48
9.2.1. Mapping of Payload Type Parameters to SDP ................48
9.2.2. Usage with the SDP Offer/Answer Model.................... 49
9.2.3. Usage with Session Multiplexing ..........................54
9.2.4. Usage in Declarative Session Descriptions................ 54
9.3. Examples .................................................55
9.3.1. Example for offering a single SVC session................ 55
9.3.2. Example for offering session multiplexing................ 56
9.4. Parameter Set Considerations .............................56
10. Security Considerations .....................................56
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11. Congestion Control ..........................................56
12. IANA Consideration ..........................................57
13. Informative Appendix: Application Examples ..................58
13.1. Introduction .............................................58
13.2. Layered Multicast ........................................58
13.3. Streaming of an SVC scalable stream ......................59
13.4. Multicast to MANE, SVC scalable stream to endpoint .......59
13.5. Scenarios currently not considered for being unaligned with
IP philosophy.....................................................61
13.6. SSRC Multiplexing ........................................62
14. References ..................................................63
14.1. Normative References .....................................63
14.2. Informative References ...................................64
15. Author's Addresses ..........................................64
16. Copyright Statement .........................................65
17. Disclaimer of Validity ......................................65
18. Intellectual Property Statement .............................65
19. Acknowledgement .............................................66
20. RFC Editor Considerations ...................................66
21. Open Issues .................................................66
22. Changes Log .................................................66
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1. Introduction
This memo specifies an RTP [RFC3550] payload format for the Scalable
Video Coding (SVC) extension of the H.264/AVC video coding standard.
Formally, SVC takes the form of Amendment 3 to ISO/IEC 14496 Part 10
[MPEG4-10], and Annex G of ITU-T Rec. H.264 [H.264]. The
specification of SVC is available in [SVC].
SVC covers the whole application ranges of H.264/AVC, starting with
low bit-rate Internet streaming applications to HDTV broadcast and
Digital Cinema with nearly lossless coding and requiring dozens or
hundreds of MBit/s.
This memo tries to follow a backward compatible enhancement
philosophy similar to what the video coding standardization
committees implement, by keeping as close an alignment to the
H.264/AVC payload format [RFC3984] as possible. It documents the
enhancements relevant from an RTP transport viewpoint, and defines
signaling support for SVC, including a new media subtype name.
This memo includes two processes for recovery of NAL unit decoding
order of NAL units transported using multiple RTP sessions, when
using of the interleaved mode is not required. The first uses the
cross-layer decoding order number, as specified in 8.1.2. The second
uses timestamp etc., as specified in 8.1.1. The first process has
been agreed by the editors, but having the second process in
addition has not been agreed by the editors. The editors therefore
request the AVT to make a decision whether to have the second
process in addition.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
This specification uses the notion of setting and clearing a bit
when bit fields are handled. Setting a bit is the same as assigning
that bit the value of 1 (On). Clearing a bit is the same as
assigning that bit the value of 0 (Off).
3. The SVC Codec
3.1. Overview
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SVC provides scalable video bitstreams. In SVC, a scalable video
bitstream contains a base layer conforming to at least one of the
profiles of H.264/AVC as defined in Annex A of [H.264], and one or
more enhancement layers, collectively denoted as Layers. A Layer
may be the base Layer or enhance the temporal resolution (i.e. the
frame rate), the spatial resolution, or the quality of the video
content, relative to the quality represented without the Layer.
Note, that the definition of Layer in this memo encompasses
temporal, spatial and fidelity enhancements.
Each RTP session can carry NAL units belonging to one or more
Layers. The NAL unit headers include information associating a
given NAL unit to a Layer. Therefore, extracting individual Layers
from an RTP session containing more than one Layer is a lightweight
operation, involving only fixed length bit fields in the header, as
documented in this memo and in [SVC].
Multiple RTP sessions, regardless of whether each carries a single
Layer or multiple Layers as discussed above, can be used to
transport the whole scalable bitstream, or Operation Points thereof.
An Operation Point consists of only those Layers necessary to
reconstruct a given quality (in temporal, spatial and fidelity
dimensions).
The concept of video coding layer (VCL) and network abstraction
layer (NAL) is inherited from H.264/AVC. The VCL contains the
signal processing functionality of the codec; mechanisms such as
transform, quantization, motion-compensated prediction, loop
filtering and inter-layer prediction. A coded picture in H.264/AVC
consists of one or more slices. Within one access unit, a coded
picture of an Operation Point consists of all the coded slices
required for decoding up to a particular Layer at the time instance
corresponding to the access unit. The Network Abstraction Layer
(NAL) encapsulates each slice generated by the VCL into one or more
Network Abstraction Layer Units (NAL units). Please consult RFC
3984 for a more in-depth discussion of the NAL unit concept. SVC
specifies the decoding order of NAL units.
"Layer" in the terms "Video Coding Layer" and "Network Abstraction
Layer" refers to a conceptual distinction, and is closely related to
syntax layers (block, macroblock, slice, ... layers). "Layer" here
describes a syntax level of the bitstream in contrast to a part of
the layered bitstream, which may be discarded. It should not be
confused with base and enhancement Layers.
The concept of temporal scalability is not newly introduced by SVC,
as profiles defined in Annex A of [H.264] already support it. In
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[H.264], sub-sequences have been introduced in order to allow
optional use of temporal layers. SVC extends this approach by
advertising the temporal scalability information within the NAL unit
header, or prefix NAL units, as discussed in section 3.3 of this
memo and in [SVC].
The concept of scaling the visual content quality in the granularity
of complete enhancement Layers, i.e. through omitting the transport
and decoding of entire Layers, is denoted as spatial scalability or
Signal-to-Noise Ratio (SNR) scalability, the latter is also know as
Coarse-Grained Scalability (CGS). This is what is commonly
understood as scalability in the IETF community. In addition, SVC
also offers the concept of another type of SNR scalability, the
Medium-Grained Scalability (MGS). MGS involves selectively omitting
the decoding of NAL units belonging to MGS layers. The selection of
the NAL units to omit can be based on fixed length fields in the NAL
unit header.
3.2. Parameter Set Concept
The parameter set concept is inherited from [H.264]. Please refer
to section 1.2 of RFC 3984 for more details.
SVC introduced a new type of sequence parameter set, referred to as
subset sequence parameter set. Subset sequence parameter sets have
NAL unit type equal to 15, which is different from the NAL unit type
value (7) of sequence parameter set. 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. Subset sequence parameter sets
use a separate identifier value space than sequence parameter sets.
In SVC, pictures from different Layers, defined as layer
representations in [SVC] (Note: A layer representation in [SVC] is
identified by a single combination of dependency_id and quality_id
values), may use the same sequence or picture parameter set, but may
also use different sequence or picture parameter sets. If different
sequence parameter sets are used, then, 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
sequence parameter set(s) (for layer representations with lower
values of (dependency_id * 16 + quality_id)). Any specific active
sequence parameter set or active layer sequence parameter set
remains unchanged throughout a coded video sequence in the Layer in
which the active sequence parameter set or active layer sequence
parameter set is referred to. This means that the referred sequence
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parameter set or subset sequence parameter set can only change at
IDR access units for any Layer. [Ed. May need to have a "layer"
definition to be used here, such as "dependency layer" identified by
dependency_id, or "quality layer", identified by quality_id. One
issue with the current Layer definition is that, a Layer of
temporal_id greater than 0 would not contain an IDR access unit. And
the SPS application scope includes all temporal Layers.] The active
picture parameter set remains unchanged within a layer
representation.
3.3. Network Abstraction Layer Unit Header
An SVC NAL unit consists of a header of four octets and the payload
byte string. SVC NAL units of type 20 encapsulate VCL data as
defined in Annex G of [SVC]. A special type of an SVC NAL unit is
the prefix NAL unit (type 14) that includes descriptive information
of the associated H.264/AVC VCL NAL unit (type 1 or 5) that
immediately follows the prefix NAL unit.
SVC extends the one-byte H.264/AVC NAL unit header by three
additional octets. 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.
This RTP payload specification is designed to be unaware of the
octet string in the NAL unit payload. The NAL unit header co-serves
as the payload header of this RTP payload format. The payload of a
NAL unit follows immediately.
The syntax and semantics of the NAL unit header are formally
specified in [SVC], but the essential properties of the NAL unit
header are summarized below.
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 |
+---------------+
F: 1 bit
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forbidden_zero_bit. H.264/AVC declares a value of 1 as a syntax
violation.
NRI: 2 bits
nal_ref_idc. A value of '00' (in binary form) indicates that the
content of the NAL unit is not used to reconstruct reference
pictures for future prediction. Such NAL units can be discarded
without risking the integrity of the reference pictures in the same
Layer. A value greater than '00' indicates that the decoding of the
NAL unit is required to maintain the integrity of reference pictures
in the same Layer, or that the NAL unit contains parameter sets.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit type as
defined in table 7-1 of [SVC], 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 [SVC].
In H.264/AVC, NAL unit types 14, 15 and 20 are reserved for future
extensions. SVC uses these three NAL unit types. NAL unit type 14
is used for prefix NAL unit, NAL unit type 15 is used for subset
sequence parameter set and NAL unit type 20 is used for coded slice
in scalable extension (see section 7.4.1 in [SVC]). 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 discard NAL units with R equal to 0.
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
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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 denotes the inter-layer coding
dependency hierarchy. At any access unit, a layer representation
with a less dependency_id may be used for inter-layer prediction for
coding of a layer representation with a greater dependency_id, while
a layer representation with a greater dependency_id shall not be
used for inter-layer prediction for coding of a layer representation
with a less dependency_id.
QID: 4 bits
quality_id. This component designates the quality level hierarchy
of a MGS layer representation. At any access unit and with
identical dependency_id value, 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 layer (or frame
rate) hierarchy. Informally put, a layer consisting of layer
representations with a less temporal_id corresponds to a lower frame
rate. A given temporal layer typically depends on the lower
temporal layers (i.e. the temporal layers with less temporal_id) but
never depends on any higher temporal layer.
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 greater dependency_id than
the current NAL unit in the current and all subsequent access units.
Such NAL units can be discarded without risking the integrity of
higher layers with greater dependency_id. discardable_flag equal to
0 indicates that the decoding of the NAL unit is required to
maintain the integrity of higher layers with greater dependency_id.
O: 1 bit
output_flag: Affects the decoded picture output process as defined
in Annex C of [SVC].
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RR: 2 bits
reserved_three_2bits. Reserved bits for future extension. RR MUST
be equal to '11' (in binary form). Receivers SHOULD discard NAL
units with RR not equal to '11'.
This memo reuses the same additional NAL unit types introduced in
RFC 3984, which are presented in section 6.3. In addition, this
memo introduces one OPTIONAL NAL unit type, 30, as specified in
section 6.9. These NAL unit types are marked as unspecified in [SVC]
and intentionally reserved for use in systems specifications like
this memo. Moreover, this specification extends the semantics of F,
NRI, I, PRID, DID, QID, TID, U, and D as described in section 6.4.
4. Scope
This payload specification can only be used to carry the "naked" NAL
unit stream over RTP, and not the byte stream format according to
Annex B of [SVC]. Likely, the applications of this specification
will be in the IP based multimedia communications fields including
conversational multimedia, video telephony or video conferencing,
Internet streaming and TV over IP.
This specification allows, in a given RTP session, to encapsulate
NAL units belonging to
o the base Layer only, detailed specification in [RFC3984], or
o one or more enhancement Layers, or
o the base Layer and one or more enhancement Layers
5. Definitions and Abbreviations
5.1. Definitions
5.1.1. Definitions per SVC specification
This document uses the definitions of [SVC]. The following terms,
defined in [SVC], are summed up for convenience:
access unit: A set of NAL units pertaining to a certain temporal
location. An access unit includes the coded slices of all the
scalable layers at that temporal location and possibly other
associated data, e.g. supplemental enhancement information (SEI)
messages and parameter sets.
coded video sequence: A sequence of access units that consists, in
decoding order, of an instantaneous decoding refresh (IDR) access
unit followed by zero or more non-IDR access units including all
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subsequent access units up to but not including any subsequent IDR
access unit. The coded video sequence is a Layer specific concept.
See below the definition of IDR access unit.
IDR access unit: An access unit in which the coded picture is an
IDR picture. For a certain SVC bitstream, an access unit may be an
IDR access unit for a Layer A but not an IDR access unit for Layer
B, subject to the maximum present value of dependency_id within the
access unit, which depends on which Layer is decoded.
IDR picture: A coded picture in which all slices with the maximum
present value of dependency_id 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 identical values of dependency_id and
quality_id, which are provided as part of the VCL NAL unit header or
by an associated prefix NAL unit.
prefix NAL unit: A NAL unit with nal_unit_type equal to 14 that
immediately precedes a NAL unit with nal_unit_type equal to 1, 5,
or 12. The NAL unit that succeeds the prefix NAL unit is also
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 decoded representation of an access unit
that is only used for inter prediction reference but not for output.
A reference base picture is constructed only when the
store_base_rep_flag as specified in the SVC specification is equal
to 1.
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.
5.1.2. Definitions local to this memo
anchor layer representation: An anchor layer representation is such
a layer representation that, if decoding of the Layer starts from
the layer representation, all the following layer representations of
the Layer, in output order, can be correctly decoded. An anchor
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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, hence may not be correctly decoded if random access is
performed at the anchor layer representation.
base Layer: The base Layer is typically representing the minimal
spatial resolution, the minimal fidelity, and the minimal frame rate
of an SVC bitstream. In other words, the base Layer consists of all
the VCL NAL units with dependency_id, quality_id and temporal_id
equal to 0 and the associated non-VCL NAL units. The base Layer is
independently decodable without the requirement of using any other
Layer of the SVC bitstream. Note that this definition is different
from the definition of "base layer" in Annex G of [SVC].
cross-layer decoding order number (CL-DON): An OPTIONAL field in the
payload structure, or a derived variable indicating NAL unit
decoding order over all the NAL units transported in all the RTP
sessions for transporting the SVC bitstream.
enhancement Layer: An SVC enhancement Layer is a Layer with any of
temporal_id, dependency_id, and quality_id greater than 0.
full base Layer: The bitstream containing the base Layer and the
temporal enhancement Layers with dependency_id and quality_id both
equal to 0. The full base Layer must conform to one of the profiles
defined in Annex A of [H.264]. In SVC context each slice NAL unit
in the full base Layer is associated with a prefix NAL unit, which
has a four bytes NAL unit header containing all the syntax elements
described in section 3.3. The full base layer is equivalent to the
definition of "base layer" in Annex G of [SVC].
intra layer representation: A layer representation that contains
only intra coded slices hence does not refer to any earlier layer
representation in decoding order in the same layer for inter
prediction. However, an intra layer representation may use inter-
layer prediction for its decoding.
Layer: A Layer may be the base Layer or an enhancement Layer that
enhances the temporal resolution (i.e. the frame rate), the spatial
resolution, or the quality of the video content, relative to the
quality represented without the Layer.
Operation Point: An Operation Point of an SVC bitstream represents
a certain level of temporal, spatial and quality scalability. An
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Operation Point contains only those NAL units required for a valid
bitstream (conforming to at least one of the profiles defined in
Annex A or Annex G of [SVC]) to represent a certain quality. The
Operation Point is described by the maximum present value of
dependency_id, and, within that maximum present value of
dependency_id, by the maximum quality_id and temporal_id, within the
bitstream subset representing the Operation Point.
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 an integer number of SVC Layers.
Session multiplexing: The scalable SVC bitstream is distributed
onto different RTP sessions, whereby each RTP session carries a
single RTP packet stream. Each RTP session requires a separate
signaling and has a separate Timestamp, Sequence Number, and SSRC
space. Dependency between sessions MUST be signaled according to
[I-D.ietf-mmusic-decoding-dependency] and this memo.
SVC NAL unit: A NAL unit of NAL unit type 14 or 20 as specified in
Annex G of [SVC]. An SVC NAL unit has a four-byte NAL unit header.
5.2. Abbreviations
In addition to the abbreviations defined in [RFC3984], the following
ones are defined.
CGS: Coarse-Grain Scalability
CL-DON: Cross-Layer Decoding Order Number
MGS: Medium-Grain Scalability
PACSI: Payload Content Scalability Information
SVC: Scalable Video Coding
6. RTP Payload Format
6.1. Design Principles
The following design principles have been observed:
o Backward compatibility with [RFC3984] wherever possible.
o As the SVC full base Layer is H.264/AVC compatible, the full base
Layer or any subset, when transmitted in its own session, MUST be
encapsulated using [RFC3984]. Requiring this has the desirable
side effect that it can be used by [RFC3984] legacy devices.
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o MANEs 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. 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 3.3. 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
applies herein.
For NRI, for the 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] are applicable, i.e., NRI also
indicates the relative importance of NAL units. In SVC context, in
addition to the semantics specified in Annex G of [SVC] are
applicable, NRI also indicate the relative importance of NAL units
within a Layer.
For I, in addition to the semantics specified in Annex G of [SVC],
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,
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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, FEC,
retransmissions, and advanced scheduling behavior, whenever
possible.
For PRID, the semantics specified in Annex G of [SVC] 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 a forward error correction (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 [SVC], 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 [SVC],
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 [SVC],
according to this memo, MANEs MAY use this information to determine
whether a given NAL unit is required for successfully decoding a
certain Operation Point of the SVC bitstream, hence to decide
whether to forward the NAL unit.
6.5. Packetization Modes
Please see section 5.4 of [RFC3984].
6.6. Decoding Order Number (DON)
Please see section 5.5 of [RFC3984]. The following applies in
addition.
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If different Layers of a SVC bitstream are transported in more than
one RTP session, the DON values of all the NAL units in the RTP
sessions using interleaved mode MUST indicate CL-DON values.
When Session multiplexing is used and at least one STAP-A packet is
present in any of the RTP sessions, the following applies.
- A PACSI NAL unit MUST be present in each STAP-A packet.
- A CL-DON field MUST be present in the PACSI NAL unit included in
each STAP-A.
- The DON values for the NAL units in each STAP-A packet MUST be
derived as follows and indicate CL-DON values.
o The CL-DON field in the PACSI NAL unit specifies the value
of DON for the first NAL unit in the STAP-A in transmission
order. For each successive NAL unit in appearance order in
the STAP-A, the value of DON is equal to (the value of DON
of the previous NAL unit in the STAP-A + 1) % 65536, wherein
'%' stands for modulo operation.
6.7. Aggregation Packets
Please see section 5.7 of [RFC3984].
6.8. Fragmentation Units (FUs)
Please see section 5.8 of [RFC3984].
6.9. Payload Content Scalability Information (PACSI) NAL Unit
A new NAL unit type is specified in this memo, and referred to as
payload content scalability information (PACSI) NAL unit. The
OPTIONAL PACSI NAL unit, if present, MUST be the first NAL unit in
an aggregation packet, and it MUST NOT be present in other types of
packets. The PACSI NAL unit 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. PACSI NAL unit
makes it easier for MANEs to decide whether to
forward/process/discard the aggregation packet containing the PACSI
NAL unit. Other reasons to use PACSI NAL units are explained later
when specifying 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. Note that the
NAL unit type for the PACSI NAL unit is selected among those values
that are unspecified in [SVC] and [RFC3984].
When the first aggregation unit of an aggregation packet contains a
PACSI NAL unit, there MUST be at least one additional aggregation
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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.
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 as
discussed in section 3.3. They are followed by one always present
octet, five optional octets, and zero or more SEI NAL units, each
SEI NAL unit 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 fields TL0PICIDX and IDRPICID MUST NOT be
present if the bit Y is equal to 0. The field CL-DON 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.
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.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CL-DON (o.) | NAL unit size 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SEI NAL unit 1 |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | NAL unit size 2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| SEI NAL unit 2 |
| +-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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 is equal to 1. Otherwise, the
F bit MUST be set to 0.
o The NRI field MUST be set to the highest value of NRI field among
all the remaining NAL units in the payload.
o The Type field MUST be set to 30.
o The R bit MUST be set to 1.
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 is equal to 1. 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.
o The N bit MUST be set to 1 if the N bit of all the remaining NAL
units in the payload is equal to 1. 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.
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.
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.
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 is equal to 1. 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. Otherwise, the D bit MUST be
set to 0.
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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 is equal to 1. Otherwise, the
O bit MUST be set to 0.
o The RR field MUST be set to '11' (in binary form).
o If the X bit is equal to 1, the bits A, P, C, S, and E are
specified as in below. Otherwise, the bits A, P, C, S, and E are
unspecified, and receivers MUST ignore these bits. The X bit SHOULD
be identical for all the PACSI NAL units involved in all the RTP
sessions conveying an SVC bitstream.
o If the Y bit is equal to 1, the OPTIONAL fields TL0PICIDX and
IDRPICID MUST be present and specified as in below. Otherwise, the
fields TL0PICIDX and IDRPICID MUST NOT be present. The Y bit SHOULD
be identical for all the PACSI NAL units involved in all the RTP
sessions conveying an SVC bitstream.
o If the T bit is equal to 1, the OPTIONAL field CL-DON MUST be
present and specified as in below. Otherwise, the field CL-DON 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 target NAL units are such NAL units contained in the
aggregation packet, but not included in the PACSI NAL unit, that are
within the access unit to which the first NAL unit following the
PACSI NAL unit in the aggregation packet belongs. 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 the coded pattern like IBBP
is in use, 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 recovery point SEI message.
However, with this A bit it is much easier to parse than to parse
the recovery point SEI message, which may even be buried deeply
in an SEI NAL unit. Furthermore, the SEI message may not be
present in the bitstream.
o The T bit MUST be set to 1 if all the target NAL units (as defined
above) belong to temporal scalable layer switching points.
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Otherwise, the T bit MUST be set to 0. The T 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 T bit indicates whether switching to the
next higher temporal layer (i.e. upgrading of frame rate) can be
performed at the layer representation. SVC specifies temporal
layer switching point SEI message for signaling of temporal layer
switching points when needed. However, with this T bit it is
much easier to parse than to parse the recovery point SEI
message, which may even 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 target NAL units (as defined
above) are with redundant_pic_cnt greater than 0, i.e. the slices
are redundant slices. Otherwise, the P bit MUST be set to 0. The P
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 P bit indicates whether the packet can be
discarded because it contains redundant slice NAL units. Without
this bit, the corresponding information can be concluded from the
syntax element redundant_pic_cnt, which is buried in the slice
header, which is not in the fixed-length coded NAL unit header.
o The C bit MUST be set to 1 if the target NAL units (as defined
above) belong to an access unit for which the layer representation
having the greatest value of dependency_id among all the layer
representations containing the target NAL units is an intra layer
representation. 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 the packet contains
intra slices which may be the only packets to be forwarded for a
fast forward playback, e.g. when the network condition is
extremely bad.
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. 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. Otherwise, the E field MUST be set to 0.
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Informative note: The S or E bit indicates whether the first or
last slice, in decoding order, of a layer representation is in
the 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 an efficient playout buffer handling
similarly as the M bit 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 [SVC] for the layer
representation containing the first NAL unit following the PACSI NAL
unit in the aggregation packet.
o When present, the IDRPICID field MUST be set to equal to
effective_idr_pic_id as specified in Annex G of [SVC] for the layer
representation containing the first NAL unit following the PACSI NAL
unit in the aggregation packet.
Informative note: The TL0PICIDX and IDRPICID fields enable the
detection of the loss of layer representations in the most
important temporal layer, by receivers as well as MANEs. SVC
includes a solution by using SEI messages, which are harder to
parse and may not be present in the bitstream at all.
o When present, the field CL-DON indicates the cross-layer decoding
order number for the first NAL unit in the STAP-A in transmission
order.
SEI NAL units included in the PACSI NAL unit, if any, MUST contain a
subset of the SEI messages associated with the access unit of the
first NAL unit following the PACSI NAL unit within the aggregation
packet.
Informative note: Senders may repeat such SEI NAL units in the
PACSI NAL unit the presence of which in more than one packet is
essential for packet loss robustness. 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 packet of those packets conveying an access
unit.
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An SEI message SHOULD NOT be included in a PACSI NAL unit and
included in one of the remaining NAL units contained in the same
aggregation packet at the same time.
7. Packetization Rules
Please see section 6 of [RFC3984]. The following rules apply in
addition.
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 single NAL unit
packetization mode SHOULD NOT be used whenever possible, because
encapsulating NAL units of small sizes, e.g. small NAL units
containing parameter sets or SEI messages, in their own packets is
typically less efficient because of the relatively big overhead.
All receivers MUST support the non-interleaved mode of [RFC3984].
Informative note: The non-interleaved mode allows an application
to encapsulate a single NAL unit in a single RTP packet.
Historically, the single NAL unit mode has been included into
[RFC3984] only for compatibility with ITU-T Rec. H.241 Annex A
[H.241]. There is no point in carrying this historic ballast
towards a new application space such as the one provided with SVC.
More technically speaking, the implementation complexity increase
for providing the additional mechanisms of the non-interleaved
mode (namely STAP-A) is so minor, and the benefits are so great,
that STAP-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 SHOULD be aggregated to the same packet as the
associated NAL unit following the prefix NAL unit in decoding order.
Informative note: When either the prefix NAL unit or the
associated NAL unit containing an H.264/AVC coded slice is lost,
the remaining one would be hardly useful in SVC context, wherein
the prefix NAL unit must be available for decoded picture buffer
management operations of the decoding process.
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.
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Non-VCL NAL units SHOULD be conveyed in the same session as
the associated VCL NAL units. To meet this, SEI messages
that are contained in scalable nesting SEI message and are
applicable to more than one session SHOULD be separated and
contained into multiple scalable nesting SEI messages. The
CL-DON values MUST indicate the cross-layer decoding order
number values as if all these SEI messages were in separate
scalable nesting SEI messages and contained in the beginning
of the corresponding access units as specified in [SVC].
When Session multiplexing is used, the following applies. The two
options I. and II. are available:
I. In all RTP Sessions non-interleaved mode or
single NAL unit mode MUST be used and CL-DON MUST NOT be present:
a. If an access unit of sampling time instance X is present
in RTP session A, this access unit MUST be also present in
any RTP session, which depends on RTP session A.
b. RTP sessions MUST have a one-dimensional dependency
structure, i.e. an RTP session can be enhanced by exactly
one other RTP session only.
c. M-bit of the RTP header SHALL be set according to
[RFC3550], i.e. the end of an access unit MUST be
indicated in each of the RTP sessions.
d. All RTP sessions MUST use the same RTP Timestamp scale and
MUST use the same RTP timestamp for packets in the
different RTP session containing NAL units of the same
sampling time instance.
e. At least one of the following parameters MUST be present
in the RTP sessions indicating buffering values for each
RTP session of the RTP session multiplex:
- sprop-prebuf-size
- sprop-prebuf-time
Informative note: Restriction a. allows only for multiplexing
of RTP session with the same frame rate or requires in RTP
sessions with higher frame rate also NAL units of the access
units, which are also present in RTP sessions which the
session in question depends on. An example algorithm for
packet and NAL unit reordering is given in 8.1.1.
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II. CL-DON MUST be used
a. An RTP session that does not use interleaved mode MUST be
constrained as follows.
i. Non-interleaved mode MUST be used.
ii. STAP-A MUST be used, and any other type of packets
MUST NOT be used.
iii. Each STAP-A MUST contain a PACSI NAL unit and the CL-
DON field MUST be present in the PACSI NAL unit.
Informative note: The motivation for these constraints is to
allow the use of non-interleaved mode for the session
conveying the H.264/AVC compatible (full) base layer, such
that RFC 3984 receivers without interleaved mode
implementation can subscribe to the (full) base layer
session.
8. De-Packetization Process (Informative)
For a single RTP session, the de-packetization process specified in
section 7 of [RFC3984] applies [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 receiving more than one of multiple RTP sessions conveying a
scalable bitstream, an example of a suitable implementation of the
de-packetization process is specified in section 8.1.
8.1. De-Packetization Process for NAL Units Conveyed using Session
Multiplexing
As for a single RTP session, the general concept behind these de-
packetization rules is to reorder NAL units from transmission order
to the NAL unit decoding order.
In this section, "the RTP sessions" refer to the RTP sessions for
which the NAL units are de-packetized.
The receiver includes a receiver buffer, which is used to compensate
for different session initiation times, transmission delay jitter
and to reorder NAL units from transmission order to the NAL unit
decoding order. In this section, the receiver operation is
described under the assumption that all the RTP sessions initiate at
the same time, and there is no transmission delay jitter. However,
receivers SHOULD also prepare for both different session initiation
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times and transmission delay jitter. Receivers can either reserve
separate buffers for session initiation variation buffering,
transmission delay jitter buffering, and de-session-multiplexing
buffering, or use a receiver buffer for all the aforementioned
purposes. Moreover, receivers SHOULD take session initiation
variation and transmission delay jitter into account in the
buffering operation; e.g., by additional initial buffering before
starting of decoding and playback.
8.1.1. De-Packetization Process for Session Multiplexing using non-
interleaved mode or Single NAL unit mode without the use of CL-DON
In this section, NAL unit reordering is described for the
constraints of section 7 using the non-interleaved mode or Single
NAL unit mode for all RTP sessions, i.e. no CL-DON is used within
any of these sessions. The reordering process is based on RTP
session dependency, RTP sequence numbers, RTP timestamp and RTP
header marker bit. In the following, in-session packet reordering
refers to the process of reordering RTP packets according to their
sequence number within the receiver buffer of an RTP session.
Inter-session packet reordering refers to the process of NAL unit
reordering between sessions in case of using Session multiplexing.
The following example is used to explain the reordering process.
The example refers to three RTP sessions A, B and C as shown in
Figure 2. In the example, the dependency signaling as described in
9.2.3, indicates, that Session A does not depend on any other of the
sessions; B depends on A; C depends on A and B as restricted in
section 7. Session A has the lowest frame rate and Session B and C
have the same, but a higher frame rate.
Informative note: The reordering process described in this
subsection can be applied on RTP session with the same or different
frame rates. The latter case is only valid, if NAL units of the same
time instances of an RTP session are also present in the RTP session
which depends on this RTP session. For describing the reordering
process no packet loss is assumed.
For each of the RTP sessions a receiver buffer according to one of
the parameters: sprop-prebuf-size or sprop-prebuf-time is used to
buffer RTP packets for each session and in-session packet reordering
is applied according to the RTP sequence numbers before starting
inter-session packet reordering. sprop-prebuf-size and sprop-
prebuf-time should be selected such that buffering each of the
sessions according to these parameters allows for an inter-session
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packet reordering with detection of at least two S0 Synchronization
Points, as defined later, in each session.
Inter-session packet reordering is started from the lowest RTP
session S0. This is the session with the lowest number of
dependencies in the one-dimensional dependency tree (in the example:
RTP session A). This session is referred to as the lowest session.
The session with the next higher number of dependencies is called
the next higher session Sn, where n is in the integer rage of 1 and
m-1. The highest session Sm-1 is the session with the highest
number of dependencies in the one-dimensional dependency tree, where
m is the number of RTP sessions in the session multiplex.
In the following, a packet loss free transmission is assumed.
Starting from Session S0, the first RTP timestamp in the receiver
buffer after in-session packet reordering referred to as TS_S0_0_is
searched in all the RTP sessions Sn starting with the first packet
in each receiver buffer. If TS_S0_0_is found in all sessions, this
point is referred to as the S0 Synchronization Point (see Figure 2)
If such a point is not found all packets with TS_S0_0 are removed
from the receiver buffer of session S0 only. The search as
described above is repeated with TS_S0_i until the first S0
Synchronization Point is found, where i gives the counter of RTP
timestamps within a RTP session's receiver buffer in RTP sequence
number order.
Informative note: The RTP timestamp order following the RTP packet
order is not the same as the order of increasing RTP timestamp
value.
A Synchronization Point of session Sn is called the Sn
Synchronization Point and defined by matching RTP timestamps in all
sessions Sy >= Sn, i.e. TS_Sy_i must be same over the RTP session Sy
>= Sn. If the an Sn Synchronization Point is found, that access
unit with RTP timestamp TS_Sn_i is restored by depacketizing RTP
packets following the rules of [RFC3984] starting from session Sn up
to Sm-1. The restored NAL units of the sessions are reassembled and
place into access unit with RTP timestamp TS_Sn_i in the order of
the RTP sessions Sn to Sm-1. Note: Sm-1 does not necessarily have
to be the highest available RTP session, but the highest subscribed
session using Layered Multicast. When reassembling an access unit
for TS_Sn_i Synchronization Point, the packets with TS_Sn_i are
removed in RTP sessions Sy >= Sn.
After finding the very first S0 Synchronization Point, all RTP
packets preceding the TS_S0_i packets in RTP sequence order are
removed from the receiver buffers in all sessions.
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After reassembling a Synchronization Point, the process described
above is repeated.
Before proceeding to a Synchronization Point in any RTP session
starting from the lowest session:
If a Synchronization Point is found in any higher session, the
access unit represented by such a Synchronization Point has to be
reassembled first.
Informative note: In case of packet loss, the essential connection
between RTP sessions and packets cannot be kept, for that reason, we
propose the reduce the reordering process only up to the session
which the packet loss contains. After finding a S0 Synchronization
point, the reordering can be applied without restrictions.
Decoding order and dependency of NAL units per RTP session:
C: -(1)---(2)--(3,4)-(5,6)--(7)---(8)-(9,10)(11,12)-(13)--(14)----
| | : : | | : : | |
B: -(1,2)-(3,4)-(5)---(6)--(7,8)-(9,10)-(11)-(12)--(13,14)(15,15)-
| | | | | |
A: -(1)---(2)---------------(3)---(4)---------------(5)----(6)----
------------------------------------------------------------------->
TS: [4] [2] [1] [3] [8] [6] [5] [7] [12] [10]
Key:
A, B, C - RTP sessions
Integer values in '()' - NAL unit decoding order per RTP session
'( )' - groups the NAL units of an access unit in
an RTP session
'|' - indicates layer dependency and the S0
Synchronization Points
':' - indicates layer dependency and the S1
Synchronization Points
Integer values in '[]' - RTP Timestamp (TS), sampling time
Figure 2. Synchronization Points in Session multiplexing without CL-
DON
8.1.2. De-Packetization Process for Session Multiplexing using CL-
DON
As present option in section 7, when more than one RTP session is
used to convey an SVC Bitstream, for each NAL unit a CL-DON value
can be derived. This enables a NAL unit decoding order recovery
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process without individual deinterleaving process for each RTP
session, irrespective of whether any of the sessions uses interleave
mode. Excluding the session initiation variation buffer and the
transmission delay jitter buffer, the receiver buffer is called the
de-session-multiplexing buffer.
The de-packetization process for NAL units conveyed in multiple RTP
sessions is similar to the single RTP session de-packetization
process for interleaved mode as specified in subsection 7.2 of RFC
3984.
It is RECOMMENDED to set the size of the de-session-multiplexing
buffer, in terms of number of bytes, equal to or greater than the
value of the sprop-deint-buf-req media type parameter of the RTP
session conveying the SVC Layer for which the decoding requires the
presence of the SVC Layers conveyed in all the other RTP sessions,
referred to the highest RTP session.
There are two buffering states in the receiver: initial buffering
and buffering while playing. Initial buffering occurs when the RTP
sessions are initialized. After initial buffering, decoding and
playback are started, and the buffering-while-playing mode is used.
Regardless of the buffering state, the receiver stores incoming NAL
units, in reception order, in the de-session-multiplexing buffer as
follows. NAL units of aggregation packets are stored in the de-
session-multiplexing buffer individually. The value of DON (i.e.
CL-DON) 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 9.1 of this
specification.
o Function don_diff is specified in section 5.5 of RFC 3984.
o Constant N is the value of the OPTIONAL sprop-interleaving-
depth media type parameter of the highest RTP session incremented by
1.
Initial buffering lasts until one of the following conditions is
fulfilled:
o There are N or more VCL NAL units in the de-session-
multiplexing buffer.
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o If sprop-max-don-diff of the highest RTP session is present,
don_diff(m,n) is greater than the value of sprop-max-don-diff of the
highest RTP session, in which 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-init-buf-time media
type parameter of the highest RTP session.
The NAL units to be removed from the de-session-multiplexing buffer
are determined as follows:
o If the de-session-multiplexing buffer contains at least N VCL
NAL units, NAL units are removed from the de-session-multiplexing
buffer and passed to the decoder in the order specified below until
the buffer contains N-1 VCL NAL units.
o If sprop-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 de-session-
multiplexing buffer and passed to the decoder in the order specified
below. Herein, n corresponds to the NAL unit having the greatest
value of AbsDON among the NAL units in the de-session-multiplexing
buffer.
The order in which NAL units are passed to the decoder is specified
as follows:
o Let PDON be a variable that is initialized to 0 at the
beginning of the RTP sessions.
o For each NAL unit associated with a value of DON, a DON
distance is calculated as follows. If the value of DON of the NAL
unit is larger than the value of PDON, the DON distance is equal to
DON - PDON. Otherwise, the DON distance is equal to 65535 - PDON +
DON + 1.
o NAL units are delivered to the decoder in ascending order of
DON distance. If several NAL units share the same value of DON
distance, they can be passed to the decoder in any order.
o When a desired number of NAL units have been passed to the
decoder, the value of PDON is set to the value of DON for the last
NAL unit passed to the decoder.
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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 ignoring unspecified parameter 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 sentence is also present in
RFC 3984.
Media Type name: video
Media subtype name: H264-SVC or H264
The media subtype "H264" MUST be used for RTP streams using RFC
3984, i.e. not using any of the new features introduced by this
specification compared to RFC 3984. [Ed. 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
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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 [SVC]: 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 [SVC], 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 [SVC] 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 7.4.2.1.1 of [SVC]. 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 procedure,
it indicates the profile that the codec
supports and the highest level
supported for the signaled profile. The
profile-iop byte indicates whether the codec
has additional limitations whereby only the
common subset of the algorithmic features and
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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:
[Ed. If these parameters may also be used to signal properties of
a NAL unit stream, as in 8.2.2 of RFC 3984, which is
contradictory with the semantics, then we need to say that the
NAL unit stream is the one containing also those from the RTP
sessions (if present) the current depends on. Furthermore, then
for max-br, it might be useful to have two versions, one for the
current session only, and one for the current session and the
sessions it depends on.]
These parameters MAY be used to signal the
capabilities of a receiver or a sender
implementation.
These parameters MUST NOT be used for any other
purpose. The profile-level-id parameter MUST
be present in the same receiver capability
description that contains any of these
parameters. The level conveyed in the value of
the profile-level-id parameter MUST be such
that the receiver is fully capable of
supporting. max-mbps, max-fs, max-cpb, max-
dpb, and max-br MAY be used to indicate
capabilities of the receiver that extend the
required capabilities of the signaled level, as
specified below.
When more than one parameter from the set (max-
mbps, max-fs, max-cpb, max-dpb, max-br) is
present, the receiver MUST support all signaled
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capabilities simultaneously. For example, if
both max-mbps and max-br are present, the
signaled level with the extension of both the
frame rate and bit rate is supported. That is,
the receiver is able to decode NAL unit
streams in which the macroblock processing rate
is up to max-mbps (inclusive), the bit rate is
up to max-br (inclusive), the coded picture
buffer size is derived as specified in the
semantics of the max-br parameter below, and
other properties comply with the level
specified in the value of the profile-level-id
parameter.
A receiver MUST NOT signal values of max-
mbps, max-fs, max-cpb, max-dpb, and max-br that
meet the requirements of a higher level,
referred to as level A herein, compared to the
level specified in the value of the profile-
level-id parameter, if the receiver can support
all the properties of level A.
Informative note: When the OPTIONAL media
type parameters are used to signal the
properties of a NAL unit stream, max-mbps,
max-fs, max-cpb, max-dpb, and max-br are
not present, and the value of profile-
level-id must always be such that the NAL
unit stream complies fully with the
specified profile and level.
max-mbps: The value of max-mbps is an integer indicating
the maximum macroblock processing rate in units
of macroblocks per second. The max-mbps
parameter signals that the receiver is capable
of decoding video at a higher rate than is
required by the signaled level conveyed in the
value of the profile-level-id parameter. When
max-mbps is signaled, the receiver MUST be able
to decode NAL unit streams that conform to the
signaled level, with the exception that the
MaxMBPS value in Table A-1 or Table G-n of [SVC]
for the
signaled level is replaced with the value of
max-mbps. The value of max-mbps MUST be
greater than or equal to the value of MaxMBPS
for the level given in Table A-1 or Table G-n of
[SVC].
Senders MAY use this knowledge to send pictures
of a given size at a higher picture rate than
is indicated in the signaled level.
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max-fs: The value of max-fs is an integer indicating
the maximum frame size in units of macroblocks.
The max-fs parameter signals that the receiver
is capable of decoding larger picture sizes
than are required by the signaled level conveyed
in the value of the profile-level-id parameter.
When max-fs is signaled, the receiver MUST be
able to decode NAL unit streams that conform to
the signaled level, with the exception that the
MaxFS value in Table A-1 or Table G-n of [SVC]
for the
signaled level is replaced with the value of
max-fs. The value of max-fs MUST be greater
than or equal to the value of MaxFS for the
level given in Table A-1 or Table G-n of [SVC].
Senders MAY
use this knowledge to send larger pictures at a
proportionally lower frame rate than is
indicated in the signaled level.
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.n item m of [SVC]) and in units
of 1200 bits
for the NAL HRD parameters (see A.3.1 item j or
G.n item m of
[SVC]). 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 or Table G-n of [SVC]
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 or Table G-n of [SVC].
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 or Table G-n of [SVC].
Informative note: The coded picture buffer
is used in the hypothetical reference
decoder (Annex C) of SVC. The use of the
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hypothetical reference decoder is
recommended in SVC encoders to verify
that the produced bitstream conforms to the
standard and to control the output bitrate.
Thus, the coded picture buffer is
conceptually independent of any other
potential buffers in the receiver,
including de-interleaving and de-jitter
buffers. The coded picture buffer need not
be implemented in decoders as specified in
Annex C of SVC, but rather 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 de-
interleaving and de-jitter buffers of the
receiver.
max-dpb: The value of max-dpb is an integer indicating
the maximum decoded picture buffer size in
units of 1024 bytes. The max-dpb parameter
signals that the receiver has more memory than
the minimum amount of decoded picture buffer
memory required by the signaled level conveyed
in the value of the profile-level-id parameter.
When max-dpb is signaled, the receiver MUST be
able to decode NAL unit streams that conform to
the signaled level, with the exception that the
MaxDPB value in Table A-1 or Table G-n of [SVC]
for the
signaled level is replaced with the value of
max-dpb. Consequently, a receiver that signals
max-dpb MUST be capable of storing the
following number of decoded frames,
complementary field pairs, and non-paired
fields in its decoded picture buffer:
Min(1024 * max-dpb / ( PicWidthInMbs *
FrameHeightInMbs * 256 * ChromaFormatFactor ),
16)
PicWidthInMbs, FrameHeightInMbs, and
ChromaFormatFactor are defined in [SVC].
The value of max-dpb MUST be greater than or
equal to the value of MaxDPB for the level
given in Table A-1 or Table G-n of [SVC].
Senders MAY use
this knowledge to construct coded video streams
with improved compression.
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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. The
decoded picture buffer stores reconstructed
samples. There is no relationship
between the size of the decoded picture
buffer and the buffers used in RTP,
especially de-interleaving and de-jitter
buffers.
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.n item m of [SVC]) and in units
of 1200 bits
per second for the NAL HRD parameters (see
A.3.1 item j or G.n item m of [SVC]).
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 or Table
G-n of [SVC]).
o When the max-cpb parameter is not present,
the result of the following formula replaces
the value of MaxCPB in Table A-1 or Table G-n
of [SVC]:
(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
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equal to the value MaxBR for the signaled level
given in Table A-1 or Table G-n of [SVC].
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 signals the capabilities of a
receiver implementation. When equal to 0, the
parameter indicates that the receiver makes no
attempt to use redundant coded pictures to
correct incorrectly decoded primary coded
pictures. When equal to 0, the receiver is not
capable of using redundant slices; therefore, a
sender SHOULD avoid sending redundant slices to
save bandwidth. When equal to 1, the receiver
is capable of decoding any such redundant slice
that covers a corrupted area in a primary
decoded picture (at least partly), and therefore
a sender MAY send redundant slices. When the
parameter is not present, then a value of 0
MUST be used for redundant-pic-cap. When
present, the value of redundant-pic-cap MUST be
either 0 or 1.
When the profile-level-id parameter is present
in the same capability signaling as the
redundant-pic-cap parameter, and the profile
indicated in profile-level-id is such that it
disallows the use of redundant coded pictures
(e.g., Main Profile), the value of redundant-
pic-cap MUST be equal to 0. When a receiver
indicates redundant-pic-cap equal to 0, the
received stream SHOULD NOT contain redundant
coded pictures.
Informative note: Even if redundant-pic-cap
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is equal to 0, the decoder is able to
ignore redundant codec pictures provided
that the decoder supports such a profile
(Baseline, Extended) in which redundant
coded pictures are allowed.
Informative note: Even if redundant-pic-cap
is equal to 1, the receiver may also choose
other error concealment strategies to
replace or complement decoding of redundant
slices.
sprop-parameter-sets:
This parameter MAY be used to convey
any sequence 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.7.3.2.1.3 of
[SVC]. 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 MAY be used to signal whether
the receiver of this parameter is allowed to
add parameter sets in its signaling response
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using the sprop-parameter-sets media parameter.
The value of this parameter is either 0 or 1.
0 is equal to false; i.e., it is not allowed to
add parameter sets. 1 is equal to true; i.e.,
it is allowed to add parameter sets. If the
parameter is not present, its value MUST be 1.
packetization-mode:
This parameter signals the properties of an
RTP payload type or the capabilities of a
receiver implementation. Only a single
configuration point can be indicated; thus,
when capabilities to support more than one
packetization-mode are declared, multiple
configuration points (RTP payload types) must
be used.
When the value of packetization-mode is equal
to 0 or packetization-mode is not present, the
single NAL mode, as defined in section 6.2 of
RFC 3984, MUST be used. This mode is in use in
standards using ITU-T Recommendation H.241
[H.241] (see section 12.1 of RFC 3984). When the
value of
packetization-mode is equal to 1, the non-
interleaved mode, as defined in section 6.3 of
RFC 3984, MUST be used. When the value of
packetization-mode is equal to 2, the
interleaved mode, as defined in section 6.4 of
RFC 3984, MUST be used. The value of
packetization mode MUST be an integer in the
range of 0 to 2, inclusive.
sprop-interleaving-depth:
This parameter MUST NOT be present when the
current RTP session does not depend on any other
RTP session, and packetization-mode is not present
or the value of packetization-mode is equal to 0
or 1. This parameter MUST be present when the
the current RTP session depends on any other RTP
session or the value of packetization-mode is
equal to 2.
This parameter signals the properties of a NAL
unit stream. It specifies the maximum number
of VCL NAL units that precede any VCL NAL unit
in the NAL unit stream in transmission order
and follow the VCL NAL unit in decoding order.
Consequently, it is guaranteed that receivers
can reconstruct NAL unit decoding order when
the buffer size for NAL unit decoding order
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recovery is at least the value of sprop-
interleaving-depth + 1 in terms of VCL NAL
units. 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 value of sprop-interleaving-depth MUST be
an integer in the range of 0 to 32767,
inclusive.
sprop-deint-buf-req:
This parameter MUST NOT be present when the
current RTP session does not depend on any other
RTP session, and packetization-mode is not present
or the value of packetization-mode is equal to 0
or 1. This parameter MUST be present when the
the current RTP session depends on any other RTP
session or the value of packetization-mode is
equal to 2.
sprop-deint-buf-req signals the required size
of the deinterleaving buffer for the NAL unit
stream. The value of the parameter MUST be
greater than or equal to the maximum buffer
occupancy (in units of bytes) required in such
a deinterleaving buffer that is specified in
section 8 of this specification. It is
guaranteed that receivers can perform the
deinterleaving of
interleaved NAL units into NAL unit decoding
order, when the deinterleaving buffer size is
at least the value of sprop-deint-buf-req in
terms of bytes. 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 value of sprop-deint-buf-req MUST be an
integer in the range of 0 to 4294967295,
inclusive.
Informative note: sprop-deint-buf-req
indicates the required size of the
deinterleaving 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
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the sessions initiates at different time, the
session initiation variation has also to be
compensated by an appropriately sized buffer.
deint-buf-cap:
This parameter signals the capabilities of a
receiver implementation and indicates the
amount of deinterleaving buffer space in units
of bytes that the receiver has available for
reconstructing the NAL unit decoding order, and
that the receiver is able to handle any stream
for which
the value of the sprop-deint-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 deint-buf-cap. The value
of deint-buf-cap MUST be an integer in the
range of 0 to 4294967295, inclusive.
Informative note: deint-buf-cap indicates
the maximum possible size of the
deinterleaving buffer of the receiver only.
When network jitter can occur, an
appropriately sized jitter buffer has to
be provisioned for as well.
sprop-init-buf-time:
This parameter MAY be used to signal the
properties of a NAL unit stream. 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 parameter signals the initial buffering
time for a receiver before
starting to recover the NAL unit
decoding order from the transmission order.
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.
An example of specifying the value of sprop-
init-buf-time follows. A NAL unit stream is
sent in the following interleaved order, in
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which the value corresponds to the decoding
time and the transmission order is from left to
right:
0 2 1 3 5 4 6 8 7 ...
Assuming a steady transmission rate of NAL
units, the transmission times are:
0 1 2 3 4 5 6 7 8 ...
Subtracting the decoding time from the
transmission time column-wise results in the
following series:
0 -1 1 0 -1 1 0 -1 1 ...
Thus, in terms of intervals of NAL unit
transmission times, the value of
sprop-init-buf-time in this
example is 1.
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-init-
buf-time MUST be an integer in the range of 0
to 4294967295, inclusive.
In addition to the signaled sprop-init-buf-
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.
sprop-max-don-diff:
This parameter MAY be used to signal the
properties of a NAL unit stream. It MUST NOT
be used to signal transmitter or receiver or
codec capabilities. sprop-max-don-diff is an
integer in the range of 0 to 32767, inclusive.
If sprop-max-don-diff is not present, the value
of the parameter is unspecified. Herein the NAL
unit stream refers to the one consisting of all
NAL units conveyed in the current RTP session,
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and all NAL units conveyed in other RTP sessions,
if present, the current RTP session depends on.
sprop-max-don-diff is calculated as follows:
sprop-max-don-diff = max{AbsDON(i) -
AbsDON(j)},
for any i and any j>i,
where i and j indicate the index of the NAL
unit in the transmission order and AbsDON
denotes a decoding order number of the NAL
unit that does not wrap around to 0 after
65535. In other words, AbsDON is calculated as
follows: Let m and n be consecutive NAL units
in transmission order. For the very first NAL
unit in transmission order (whose index is 0),
AbsDON(0) = DON(0). For other NAL units,
AbsDON is calculated as follows:
If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
If (DON(m) < DON(n) and DON(n) - DON(m) <
32768),
AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
If (DON(m) > DON(n) and DON(m) - DON(n) >=
32768),
AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
If (DON(m) < DON(n) and DON(n) - DON(m) >=
32768),
AbsDON(n) = AbsDON(m) - (DON(m) + 65536 -
DON(n))
If (DON(m) > DON(n) and DON(m) - DON(n) <
32768),
AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))
where DON(i) is the decoding order number of
the NAL unit having index i in the transmission
order. The decoding order number is specified
in section 6.6 of this specification.
Informative note: Receivers may use sprop-
max-don-diff to trigger which NAL units in
the receiver buffer can be passed to the
decoder.
max-rcmd-nalu-size:
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This parameter MAY be used to signal the
capabilities of a receiver. The parameter MUST
NOT be used for any other purposes. The value
of the parameter indicates the largest NALU
size in bytes that the receiver can handle
efficiently. The parameter value is a
recommendation, not a strict upper boundary.
The sender MAY create larger NALUs but must be
aware that the handling of these may come at a
higher cost than NALUs conforming to the
limitation.
The value of max-rcmd-nalu-size MUST be an
integer in the range of 0 to 4294967295,
inclusive. If this parameter is not specified,
no known limitation to the NALU size exists.
Senders still have to consider the MTU size
available between the sender and the receiver
and SHOULD run MTU discovery for this purpose.
This parameter is motivated by, for example, an
IP to H.223 video telephony gateway, where
NALUs smaller than the H.223 transport data
unit will be more efficient. A gateway may
terminate IP; thus, MTU discovery will normally
not work beyond the gateway.
Informative note: Setting this parameter to
a lower than necessary value may have a
negative impact.
sprop-prebuf-size:
With this parameter, CL-DON MUST NOT be present in the current RTP
session.
This parameter MUST be present when the the current RTP session
depends on any other RTP session.
sprop-prebuf-size signals the required size of the receiver buffer
for the NAL unit stream. It is guaranteed that receivers can
perform the inter-session packet reordering as described in section
8.1.1 into NAL unit decoding order, 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.
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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.
sprop-prebuf-time:
With this parameter, CL-DON MUST NOT be present in the current RTP
session. 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.
The parameter signals the initial buffering time is used for a
receiver before starting to recover the NAL unit decoding order from
the transmission order. 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.
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
[SVC]. 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 least DID value and the greatest DID value,
respectively, among all the NAL units conveyed in the RTP session.
Let QIDl and TIDl be the least QID value and the least 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
greatest QID value and the great 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 [SVC].
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.]
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.
Encoding considerations:
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This type is only defined for transfer
via RTP (RFC 3550).
Security considerations:
See section 10 of RFC XXXX.
Public specification:
Please refer to RFC XXXX and its section 14.
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.
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
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The media type video/H264-SVC string is mapped to fields in the
Session Description Protocol (SDP) as follows:
* The media name in the "m=" line of SDP MUST be video.
* The encoding name in the "a=rtpmap" line of SDP MUST be H264-SVC
(the media subtype).
* The clock rate in the "a=rtpmap" line MUST be 90000.
* 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", "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.
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.
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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 "scalable-layer-id" in the answer.[Ed. 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.
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
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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.
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.
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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
- 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:
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- 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 the capabilities of the sender when it receives a stream:
- max-mbps
- max-fs
- max-cpb
- max-dpb
- max-br
- redundant-pic-cap
- deint-buf-cap
- max-rcmd-nalu-size
Declaring how Offer/Answer negotiation shall be performed:
- parameter-add
Furthermore, the following considerations are necessary:
o Parameters used for declaring receiver capabilities are in
general downgradable; i.e., they express the upper limit for a
sender's possible behavior. Thus a sender MAY select to set its
encoder using only lower/lesser or equal values of these parameters.
"sprop-parameter-sets" MUST NOT be used in a sender's declaration of
its capabilities, as the limits of the values that are carried
inside the parameter sets are implicit with the profile and level
used.
o Parameters declaring a configuration point are not downgradable,
with the exception of the level part of the "profile-level-id"
parameter. This expresses values a receiver expects to be used and
must be used verbatim on the sender side.
o When a sender's capabilities are declared, and non-downgradable
parameters are used in this declaration, then these parameters
express a configuration that is acceptable. In order to achieve
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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 Session Multiplexing
If Session multiplexing 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.
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:
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- 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
m = video 20000 RTP/AVP 96 97 98
a = rtpmap:96 AVC/90000
a = fmtp:96 profile-level-id=4d400a; packetization-mode=1; \
sprop-parameter-sets=Z01ACprLFicg,aP4Eag= =;
a = rtpmap:97 SVC/90000
a = fmtp:97 profile-level-id=53000c; packetization-mode=1; \
sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==, \
aEvgRqA=,aGvgRiA=;
a = rtpmap:98 SVC/90000
a = fmtp:98 profile-level-id=53000c; packetization-mode=2; \
init-buf-time=156320; sprop-parameter-sets=Z01ACprLFicg, \
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Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
9.3.2. Example for offering session multiplexing
m = video 20000 RTP/AVP 96 97
a = rtpmap:96 H264/90000
a = fmtp:96 profile-level-id=4d400a; packetization-mode=2; \
init-buf-time=156320; \
sprop-parameter-sets=Z01ACprLFicg,aP4Eag==;
a = rtpmap:97 SVC/90000
a = fmtp:97 profile-level-id=53000c; packetization-mode=2; \
init-buf-time=156320; sprop-parameter-sets=Z01ACprLFicg, \
Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
a = mid:1
m = video 20002 RTP/AVP 98
a = rtpmap:98 SVC/90000
a = fmtp:98 profile-level-id=53000c; packetization-mode=2; \
init-buf-time=156320; sprop-parameter-sets=Z01ACprLFicg, \
Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
a = mid:2
a = depend:98 lay 1:96
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
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Within any given RTP session carrying payload according to this
specification, the provisions of section 12 of [RFC3984] apply.
Reducing the session bandwidth is possible by one or more of the
following means, listed in an order that, in most cases, will assure
the least negative impact to the user experience:
a) within the highest Layer identified by the DID field, utilize the
TID and/or QID fields in the NAL unit header to drop NAL units
with lower importance for the decoding process or human
perception.
b) drop all NAL units belonging to the highest enhancement Layer as
identified by the highest DID value.
c) dropping NAL units according to their importance for the decoding
process, as indicated by the fields in the NAL unit header of the
NAL units or in the prefix NAL units.
d) dropping NAL units or entire packets not according to the
aforementioned rules (media-unaware stream thinning). This
results in the reception of a non-compliant bitstream and, most
likely, in very annoying artifacts
Informative note: The discussion above is centered on NAL
units and not on packets, primarily because that is the level
where senders can meaningfully manipulate the scalable
bitstream. The mapping of NAL units to RTP packets is fairly
flexible when using aggregation packets. Depending on the
nature of the congestion control algorithm, the "dimension"
of congestion measurement (packet count or bitrate) and
reaction to it (reducing packet count or bitrate or both) can
be adjusted accordingly.
All aforementioned means are available to the RTP sender, regardless
whether that sender is located in the sending endpoint or in a mixer
based MANE.
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
13.1. Introduction
Scalable video coding is a concept that has been around at least
since 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.
MPEG and JVT, respectively, performed a requirement analysis before
the SVC project was launched. 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. Note that we do not reference the MPEG and JVT
documents directly; partly, because at least the MPEG documents have
a limited lifespan and are not publicly available, and partly
because the language used in these documents is inappropriately
video centric and imprecise, when it comes to protocol matters.
With these remarks, we now introduce three main application
scenarios that we consider as 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].
Layered Multicast has the great advantage of simplicity and easy
implementation. However, it has also the great disadvantage of
utilizing many different transport addresses. While we consider
this not to be a major problem for a professionally maintained
content server, receiving client endpoints need to open many ports
to IP multicast addresses in their firewalls. This is a practical
problem from a firewall and network address translation (NAT)
viewpoint. Furthermore, even today IP multicast is not as widely
deployed as many wish.
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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.
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
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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
necessarily can "see" the payload (including the RTP payload
headers), utilize the wealth of layering information available
therein, and manipulate it.
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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 for being unaligned with
IP philosophy
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
endpoint. It has been requested that this payload specification
contains protocol elements facilitating such an operation, and the
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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):
- 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.
- 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:
- 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.
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
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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
generic audiovisual services", Version 4, July 2005.
[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-00 (work in
progress), November 2007.
[MPEG4-10] ISO/IEC International Standard 14496-10:2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
With Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and
Jacobson, V., "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3984] Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund,M.,
and Singer, D., "RTP Payload Format for H.264 Video", RFC
3984, February 2005.
[RFC4566] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
[SVC] Joint Video Team, "Joint Draft 11 of SVC Amendment",
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available from http://ftp3.itu.ch/av-arch/jvt-site
/2007_06_Geneva/JVT-X201.zip, Geneva, Switzerland, June
2007.
14.2. Informative References
[DVB-H] DVB - Digital Video Broadcasting (DVB); DVB-H
Implementation Guidelines, ETSI TR 102 377, 2005.
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.
[McCanne] McCanne, S., Jacobson, V., and Vetterli, M., "Receiver-
driven layered multicast", in Proc. of ACM SIGCOMM'96,
pages 117--130, Stanford, CA, August 1996.
[MBMS] 3GPP - Technical Specification Group Services and System
Aspects; Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs (Release 6), December 2005.
[MPEG2] ISO/IEC International Standard 13818-2:1993.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[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. Author's 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
FIN-33721 Tampere
Finland
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Phone: +358-50-486-7004
Email: ye-kui.wang@nokia.com
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
Email: schierl@hhi.fhg.de
16. Copyright Statement
Full 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.
17. Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
18. Intellectual Property Statement
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
19. Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
20. RFC Editor Considerations
none
21. Open Issues
1. Packetization rules need work.
2. Alignment with the SVC specification (ongoing)
22. Changes Log
Version 00
- 29.08.2005, YkW: Initial version
- 29.09.2005, Miska: Reviewed and commented throughout the document
- 05.10.2006, StW: Editorial changes through the document, and
formatted the document in RFC payload format style
From -00 to -01
- 04.02.2006, StW: Added details to scope
- 04.02.2006, StW: Added short subsection 6.1 "Design Principles"
- 04.02.2006, StW: Added section 15, "Application Examples"
- 06.02 - 03.03.2006, YkW: Various modifications throughout the
document
- 13.02.2006 - 03.03.2006 , ThS: Added definitions and additional
information to section 3.3, 5.1, 7 and 8, parameters in section 9.1 and
added section 14 for NAL unit re-ordering for layered multicast.
Further modifications throughout the document
From -01 to -02
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- 06.03.2006, StW: Editorial improvements
- 26.05.2006, YkW: Updated NAL unit header syntax and semantics
according to the latest draft SVC spec
- 20.06.2006, Miska/YkW: Added section 6.10 "Payload Content
Scalability Information (PACSI) NAL Unit"
- 20.06.2006, YkW: Updated the NAL unit reordering process for layered
multicast (removed the old section 14 "Informative Appendix: NAL Unit
Re-ordering for Layered Multicast" and added the new section 13 "NAL
Unit Reordering for Layered Multicast")
From -02 to -03
- 05.09.2006, YkW: Updated the NAL unit header syntax, definitions,
etc., according to the foreseen July JVT output. Updated possible MANE
adaptation operations according to SPID, TL, DID and QL. Clarified the
removal of single NAL unit packetiztaion mode. Added the support of
SSRC multiplexing in layered multicast.
- 08.09.2006, StW: Editorial changes throughout the document
- 08.09.2006, YkW: Added the packetization rule for suffix NAL unit.
- 19.09.2006, YkW: Moved/updated SSRC multiplexing support to section
6.2 ``RTP header usage''. Moved/updated the cross layer DON constraint
to Section 6.6 ``Decoding order number''. Moved/updated the
packetization rule when a SVC bistream is transported over more than
one RTP session to Section 7 ``Packetization rules''. Removed Section
13 "Support of layered multicast".
- 16.10, TS: Added detailed four-byte NAL unit header description.
Change "AVC" to "H.264" conforming to 3984. Modifications throughout
the document. Extended description of 3rd byte of PACSI NAL unit.
Corrected terms RTP session and RTP packet stream in case of SSRC
multiplexing. Added terms in definition section on RTP multiplexing.
Constraints on optional media type parameters of 3984 for cross-layer
DON (DON section and media type parameters). Copied parts of SI paper
regarding mixer, translator and SSRC mux with SRTP to section
application examples. Added section on SDP usage with Session and SSRC
multiplexing. Added points in Design principles on translator/mixer and
RTP multiplexing. Added additional founding information in Ack-
section. Corrected reference for SVC and added reference for generic
signaling.
17.10, StW: Fixed many editorials, clarified MANE, mixer, translator
and RTP packet stream throughout doc (hopefully consistently)
18.10., removed comments, clarified B-Bit, changed definition of base-
layer (do not need to be of the lowest temporal resolution),
From -03 to draft-ietf-avt-rtp-svc-00
- 23.11.06, StW: Editorials throughout the memo
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- 23.11.06, StW: removed all occurrences of the security
discussions, as they are incorrect. When using SRTP, the RTCP is
authenticated, implying that a translator cannot rewrite RTCP
RRs, implying that RRs would be incorrect as soon as the session
is modified (i.e. packets are being removed), implying that SSRC-
mux does not work in multicast.
- 23.11.06, StW: rewrote congestion control
- 23.11.06, StW: removed application scenario related to SRTP, as
this does not work (see above
- 23.11.06, StW: added informative reference to H.241
- 27/29.11.06, YkW: editorial changes throughout the document
- 27/29.11.06, YkW: alignment with the SVC specification
- 19.12.06, TS:
TS: [SVC] is now the complete Joint Draft of H.264
TS: Removed SSRC Multiplexing
TS: Changed use cases for MANE as a translator
TS: Editorials throughout the document, alignment with SVC spec.
- 20-28.12.06, StW/TS/YkW: editorial changes throughout the
document
From draft-ietf-avt-rtp-svc-00 to draft-ietf-avt-rtp-svc-01
- 23.02.07, YkW/Miska Hannuksela: Added enhancements to PACSI NAL
unit
- 01.03.07, Jonathan Lennox/YkW: Added recommendatory packetization
rules for SEI messages and non-VCL NAL units
- 05.03.07, Thomas Wiegand/YkW: Added the fields of picture start,
picture end, and Tl0PicIdx to PACSI NAL unit
- 05.03.07, TS: Draft conforms to new I-D style
From draft-ietf-avt-rtp-svc-01 to draft-ietf-avt-rtp-svc-02
25-June-2007: TS
Clarified definitions Layer, Operation Points,
Removed FGS
Aligned with JVT-W201 spec
Use of DON in de-packetization
Congestion control
25-June-2007: YkW
Edit throughout the spec, aligned with JVT-X201 SVC spec
09-July-2007: TS
Further modifications and alignments with JVT-X201.
05-Dec-2007: TS
Formatting corrected, ref to signaling draft corrected
From draft-ietf-avt-rtp-svc-02 to draft-ietf-avt-rtp-svc-03
- 21-Aug-2007 to 24-Sep-2007: YkW
1) Resolved most of the comments sent to the AVT reflector and to
the editors
2) Updated the intro text for parameter sets
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3) Reordered the definitions according to alphabetical order and
added some definitions
4) Added the NAL unit order recovery process for layered multicast
using CL-DON in the PACSI NAL unit, thus to allow for layered
multicast without requiring the non-interleaved packetization
mode. The detailed NAL unit order recovery process added to
section 8.
5) Added some packetization rules. Some of these were to resolve
the "single NAL unit mode deprecation" issue.
6) Added semantics of the media type parameters inherited from RFC
3984, and added a couple of new parameters for negotiation of
operation point.
7) Other edits throughout the document.
- 16 to 18 November 2007: TS
1) Added the NAL unit order recovery process for layered multicast
without using CL-DON, thus to allow for layered multicast without
requiring the non-interleaved packetization mode.
2) Added the usages of the media type parameters, including SDP
usage with offer/answer model, declarative usage, and examples.
- 08 to 19 November 2007: YkW
1) Aligned the spec with the final version of the SVC spec.
2) Updated the congestion control part according to Colin Perkins'
comment.
3) Checked the parameter set considerations and confirmed that the
text in RFC 3984 is OK.
4) Updated the security considerations part.
5) Added justifications for some fields in the PACSI NAL units.
From draft-ietf-avt-rtp-svc-03 to draft-ietf-avt-rtp-svc-04
- 18 December 2007: TS
1) Updated formatting in the Media Type Registration section
2) Updated the semantics of sprop-layer-range
3) Updated Open issues according to Roni's email
4) Corrected usage of "depend" in SDP example
From draft-ietf-avt-rtp-svc-04 to draft-ietf-avt-rtp-svc-05
08 January 2008: TS
1) Back to original word source document for draft-ietf-avt-rtp-
svc-03.
2) Changed/corrected formatting of document:
a. Word source document margins
b. Footer/Header adjustment
c. Formatting of RFC default sections
d. Formatting of Author's section
e. Formatting of Reference sections
f. Corrected formatting of Media Type section
After TXT document generation, document should be readable by
Internet Draft submission tool.
3) Fixed SDP example (fmtp:97 to 96) [Mike's comment]
Wenger, Wang, Schierl Expires July 7, 2008 [Page 69]
Internet-Draft RTP Payload Format for SVC Video January 2008
4) Changed "sprop-layer-range" definition according to Ye-Kui's
internal proposal.
5) Corrected usage of "depend" in SDP example
Wenger, Wang, Schierl Expires July 7, 2008 [Page 70]
