One document matched: draft-wenger-avt-rtp-svc-01.txt
Differences from draft-wenger-avt-rtp-svc-00.txt
Network Working Group S. Wenger
Internet Draft Y.-K. Wang
Document: draft-wenger-avt-rtp-svc-01.txt T. Schierl
Expires: September 2006
March 2006
RTP Payload Format for SVC Video
Status of this Memo
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This Internet-Draft will expire on September 5, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo describes an RTP Payload format for the scalable extension
of the ITU-T Recommendation H.264 video codec which is the
technically identical to ISO/IEC International Standard 14496-10
video codec. The RTP payload format allows for packetization of one
or more Network Abstraction Layer Units (NALUs), produced by the
video encoder, in each RTP payload. The payload format has wide
applicability, as it supports applications from simple low bit-rate
conversational usage, to Internet video streaming with interleaved
transmission, to high bit-rate video-on-demand.
INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006
Table of Content
RTP Payload Format for SVC Video...............................1
1. Introduction............................................4
1.1. SVC - the scalable extensions of H.264/AVC................4
2. Conventions.............................................4
3. The SVC Codec ...........................................4
3.1. Overview..............................................4
3.2. Parameter Set Concept...................................5
3.3. Network Abstraction Layer Unit Header ....................5
4. Scope...................................................8
5. Definitions and Abbreviations.............................8
5.1. Definitions............................................8
5.2. Abbreviations..........................................9
6. RTP Payload Format.......................................9
6.1. Design Principles......................................9
6.2. RTP Header Usage......................................10
6.3. Common Structure of the RTP Payload Format...............10
6.4. NAL Unit Header Usage..................................10
6.5. Packetization Modes....................................11
6.6. Decoding Order Number (DON)............................11
6.7. Single NAL Unit Packet.................................11
6.8. Aggregation Packets....................................11
6.9. Fragmentation Units (FUs)..............................11
7. Packetization Rules.....................................11
8. De-Packetization Process (Informative)....................11
9. Payload Format Parameters................................12
9.1. MIME Registration.....................................12
9.2. SDP Parameters........................................13
9.2.1. Mapping of MIME Parameters to SDP......................13
9.2.2. Usage with the SDP Offer/Answer Model..................14
9.2.3. Usage in Declarative Session Descriptions..............14
9.3. Examples.............................................14
9.4. Parameter Set Considerations ...........................14
10. Security Considerations .................................14
11. Congestion Control......................................14
12. IANA Consideration......................................15
13. Informative Appendix: Application Examples ................15
13.1. Introduction..........................................15
13.2. Layered Multicast.....................................15
13.3. Streaming of an SVC scalable stream.....................16
13.4. Multicast to MANE, SVC scalable stream to endpoint........17
13.5. Scenarios currently not considered for complexity reasons..18
13.6. Scenarios currently not considered for being unaligned with
IP philosophy...............................................18
14. Informative Appendix: NAL Unit Re-ordering for Layered
Multicast...................................................19
14.1. Examples.............................................19
14.2. Discussion: Using enhanced DON over different RTP sessions.24
15. Acknowledgements........................................24
16. References.............................................24
16.1. Normative References...................................24
16.2. Informative References.................................25
17. Author's Addresses......................................25
18. Intellectual Property Statement..........................25
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19. Disclaimer of Validity..................................26
20. Copyright Statement.....................................26
21. RFC Editor Considerations................................26
22. Open Issues............................................26
23. Changes Log............................................26
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1. Introduction
1.1. SVC - the scalable extensions of H.264/AVC
This memo specifies an RTP [RFC3550] payload format for a
forthcoming new mode of the H.264/AVC video codec, known as Scalable
Video Coding (SVC). Formally, SVC will take the form of an Amendment
to ISO/IEC 14496 Part 10 [MPEG4-10], and likely as one or more new
Annexes of ITU-T Rec. H.264 [H.264]. It is planned to keep the
technical alignment between the two mentioned specifications, as
well as backward compatibility with previous versions of H.264/AVC.
The current working draft of SVC is available for public review
[SVC]. Technical maturity will be reached perhaps around mid 2006.
In this memo, SVC is used as an acronym for the mentioned scalable
extensions of H.264/AVC.
SVC covers all of H.264/AVC's applications, ranging from all forms
of digital compressed video from, low bit-rate Internet streaming
applications to HDTV broadcast and Digital Cinema applications with
nearly lossless coding.
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 RFC [RFC3984] as possible. It basically documents
the enhancements relevant from an RTP transport viewpoint, defines
signaling support for SVC, and deprecates the single NAL unit mode
of RFC 3984.
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
SVC provides scalable video bitstreams. A scalable video bitstream
contains a base layer and one or more enhancement layers. An
enhancement layer may enhance the temporal resolution (i.e. the
frame rate), the spatial resolution, or the quality of the video
content represented by the lower layer or part thereof. The
scalable layers can be aggregated to a single RTP stream, or
transported independently.
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The concept of video coding layer (VCL) and network abstraction
layer (NAL) is inherited from 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 of a base or enhancement
layer consists of one or more slices. 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 these NAL units.
The term "Layer" in Video Coding Layer and Network Abstraction Layer
refers to a conceptual distinction, and is closely related to syntax
layers (block, macroblock, slice, ... layers). It should not be
confused with base and enhancement layers.
The concept of scaling the visual content quality by omitting the
transport and decoding of entire enhancement layers is denoted as
coarse-grained scalability (CGS).
In some cases, the bit rate of a given enhancement layer can be
reduced by truncating bits from individual NAL units. Truncation
leads to a graceful degradation of the video quality of the
reproduced enhancement layer. This concept is known as Fine
Granularity Scalability (FGS).
3.2. Parameter Set Concept
The parameter set concept is inherited from AVC. In SVC, pictures
from different layers may use the same sequence or picture parameter
set and 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 more than one
active sequence picture parameter set. Any specific active sequence
parameter set remains unchanged throughout a coded video sequence in
the layer in which the active sequence parameter set is referred to.
The active picture parameter set remains unchanged within a coded
picture.
3.3. Network Abstraction Layer Unit Header
An SVC NAL unit consists of a header of one, two or three bytes and
the payload byte string. 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, and (optionally, when the
header is of three bytes) the scalable layer decoding dependency
information. This RTP payload specification is designed to be
unaware of the bit 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.
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The syntax and semantics of the NAL unit header are 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 in H.264/AVC and RFC 3984, while the
semantics are slightly different, in a backward compatible way):
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
F: 1 bit
forbidden_zero_bit. The H.264 specification declares a value of 1
as a syntax violation.
NRI: 2 bits
nal_ref_idc. A value of 00 indicates that the content of the NAL
unit is not used to reconstruct reference pictures for inter picture
prediction. Such NAL units can be discarded without risking the
integrity of the reference pictures in the same layer. Values
greater than 00 indicate that the decoding of the NAL unit is
required to maintain the integrity of the reference pictures. For a
slice or slice data partitioning NAL unit, a NRI value of 11
indicates that the NAL unit contains data of a key picture, as
specified in [SVC].
Informative Note: The concept of a key picture has been introduced
in SVC, and no assumption should be made that any pictures in bit
streams compliant with the 2003 and 2005 versions of H.264 follow
this rule.
Type: 5 bits
nal_unit_type. This component specifies the NAL unit payload 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].
Previously, NAL unit types 20 and 21 (among others) have been
reserved for future extensions. SVC is using these two NAL unit
types. They indicate the presence of one more byte that is helpful
from a transport viewpoint. The additional byte(s), described
below, is called transport priority indicator.
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
| PRID |D|E|
+---------------+
PRID: 6 bits
simple_priority_id. This component specifies a priority identifier
for the NAL unit. When extension_flag (E) is equal to 0,
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simple_priority_id is used for inferring the values of
temporal_level (TL), dependency_id (DID), , and quality_level (QL).
When simple_priority_id is not present, it shall be inferred to be
equal to 0.
D: 1 bit
discardable_flag. A value of 1 indicates that the content of the
NAL unit with dependency_id equal to currDependencyId is not used in
the decoding process of NAL units with dependency_id larger than
currDependencyId. Such NAL units can be discarded without risking
the integrity of higher scalable layers with larger values of
dependency_id. discardable_flag equal to 0 indicates that the
decoding of the NAL unit is required to maintain the integrity of
higher scalable layers with larger values of dependency_id.
E: 1 bit
extension_flag. A value of 1 indicates that the third byte of the
NAL unit header is present.
When the E-bit of the second byte is 1, then the NAL unit header
extends to a third byte:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
| TL | DID | QL|
+---------------+
TL: 3 bits
temporal_level indicates the temporal layer (or frame rate)
hierarchy. A layer consisted of pictures of a smaller temporal_level
value has a smaller frame rate.
DID: 3 bits
dependency_id denotes the inter-layer coding dependency hierarchy.
At any temporal location, a picture of a smaller dependency_id value
may be used for inter-layer prediction for coding of a picture of a
larger dependency_id value, while a picture of a larger
dependency_id value is disallowed to be used for inter-layer
prediction for coding of a picture of a smaller dependency_id value.
QL: 2 bits
quality_level designates the quality level hierarchy of a
progressive refinement slice. At any temporal location and with
identical dependency_id value, a quality enhancement of a picture
with quality_level value equal to ql uses the quality enhancement or
base quality information (the non-quality enhancement information of
the slice when ql = 1) of the slice with quality_level value equal
to ql-1 for inter-layer prediction. When quality_level is larger
than 0, the NAL unit contains a progressive refinement slice or part
thereof.
This memo introduces new NAL unit types, which are presented in
section 5.2. The NAL unit types defined in this memo are marked as
unspecified in [SVC]. Moreover, this specification extends the
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semantics of F, NRI, PRID, D, TL, DID and QL as described in section
5.3.
4. Scope
This payload specification can only be used to carry the "naked" SVC
NAL unit stream over RTP, and not the bitstream format according to
in 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 belong to
o the base layer, 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
This document uses the definitions of [SVC] and [H.264]. The
following terms, defined in [SVC], are summed up for convenience:
scalable bitstream: an SVC compliant bit stream containing a base
layer and at least one enhancement layer.
base layer: The base layer is typically representing the minimal
temporal and, or spatial resolution and, or minimal quality of an
SVC bitstream. The base layer may be fully complying with [H.264].
The base layer is independently decodable without the requirement of
using any other layer of the SVC bitstream. If the base layer
contains NAL units fully conforming to [H.264] only, the layer is
called H.264/AVC base layer. For such a layer the ability of
signaling transport priority (simple_priority_id or temporal_level,
dependency_id and quality_level) per NAL unit may not be given.
operation point: A operation of a SVC bitstream represents a certain
level of temporal, spatial and quality scalability. An operation
point contains all NAL units required for successfully decoding a
certain SVC enhancement layer, which represents the highest value of
temporal and, or spatial and, or quality of the operation point.
scalable enhancement layer: an SVC enhancement layer is identified
by a certain NAL unit header value (transport priority) of
simple_priority_id or, if present, by a combination of
temporal_level, dependency_id, quality_level as defined in [SVC] and
summarized in section 3.3.
access unit: A set of NAL units pertaining to a certain temporal
location. An access unit includes the slice data of the pictures of
all scalable layers at that temporal location and possibly other
associated data e.g. SEI messages and parameter sets.
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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
subsequent access units up to but not including any subsequent IDR
access unit.
IDR access unit: An access unit in which all the primary coded
pictures are IDR pictures.
[Edt. note: This needs to be updated according to the new adoption
of the enhancement-layer IDR (EIDR) concept in January 2006. At the
time of writing, the SVC spec update for the January JVT meeting has
not yet been available.]
IDR picture: A coded picture with the property that the decoding of
this coded picture and all the following coded pictures in decoding
order, in the same layer (i.e. with the same values of dependency_id
and quality_level, respectively), can be performed without inter
prediction from any picture prior to the coded picture in decoding
order in the same layer. An IDR picture causes a "reset" in the
decoding process of the scalable layer containing the IDR picture.
[Edt. note: This needs to be updated according to the new adoption
of the enhancement-layer IDR (EIDR) concept in January 2006. At the
time of writing, the SVC spec update for the January JVT meeting has
not yet been available.]
progressive refinement slice: A progressive refinement slice [SVC]
is contained in an SVC NAL unit and may be signaled, if
extension_flag equal to one, by a quality_level not equal to zero.
Such slices can be truncated byte-wise from the end in NAL unit
payload byte-string order for bit-rate and quality reduction. This
ability is also known as Fine Granularity Scalability (FGS).
5.2. Abbreviations
In addition to the abbreviations defined in [RFC3984], the following
ones are defined.
CGS: Coarse Granularity Scalability
FGS: Fine Granularity Scalability
6. RTP Payload Format
6.1. Design Principles
The authors tried to follow design principles as follows:
o Backward compatibility with RFC 3984 wherever possible.
o As we expect the SVC base layer to be H.264/AVC compatible, we
assume the base layer (when transmitted in its own session) to be
encapsulated using RFC 3984. 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.
In other words, MANEs have state.
o MANEs terminate RTP sessions, and create different RTP sessions
with perhaps modified content.
Edt. Note: need to clarify this wrt. Translators and Mixers in the
spirit of PV06 paper.
o MANEs are within the security context of the RTP session.
o Packet integrity needs to be preserved end-to-end (whereby
end-to-end can mean endpoint to endpoint but also endpoint to
MANE.
o others?
6.2. RTP Header Usage
Please see section 5.1 of RFC3984 [RFC3984].
6.3. Common Structure of the RTP Payload Format
Please see section 5.2 of RFC3984 [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,
PRID, D, TL, DID and QL according to this specification.
The semantics of F specified in section 5.3 of [RFC3984] also
applies herein.
For NRI, for the bitstream that is compliant with AVC, the semantics
specified in section 5.3 of [H.264] are applicable, otherwise only
the semantics specified in SVC [SVC] is applicable.
For PRID, in addition to the semantics specified in [SVC], according
to this RTP payload specification, values of PRID indicate the
relative transport priority, as determined by the sender, which is
typically increasing from a layer of lower to a layer of higher
importance. MANEs implementing unequal error protection can use
this information to protect more important NAL units better than
less important ones, for example by including only the more
important NAL units in a FEC protection mechanism. The transport
priority increases as the PRID value increases.
For D, MANEs can use this information to protect NAL units with D
equal to 0 better than NAL units with D equal to 1. Furthermore a
MANE can determine whether the transmission of a NAL unit is
required for successfully decoding a certain operation point of the
SVC bitstream.
For TL, DID and QL, in addition to the semantics specified in [SVC],
according to this RTP payload specification, values of TL, DID or QL
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indicate the relative transport priority. MANEs can use this
information to protect more important NAL units better than less
important NAL units. A higher value of TL, DID or QL indicates a
higher priority if the other two components are identical
correspondingly.
Informative note: Using of PRID, D, TL, DID and QL in combination
may better indicate the relative transport priority. [Edt. note:
such examples may be provided in Informative Appendix 13 in
future versions.]
6.5. Packetization Modes
Please see section 5.4 of RFC3984 [RFC3984]. The single NAL unit
mode SHALL NOT be used.
6.6. Decoding Order Number (DON)
Please see section 5.5 of RFC3984 [RFC3984].
6.7. Single NAL Unit Packet
Please see section 5.6 of RFC3984 [RFC3984].
6.8. Aggregation Packets
Please see section 5.7 of RFC3984 [RFC3984].
6.9. Fragmentation Units (FUs)
Please see section 5.8 of RFC3984 [RFC3984].
7. Packetization Rules
Please see section 6 of RFC3984 [RFC3984]. The following rules
apply in addition.
The single NAL unit mode SHALL NOT be used.
In an RTP session, the first NAL unit of an aggregation packet SHALL
have a two- or three-byte NAL unit header containing the transport
priority indicator, as described in section 3.3. Non-VCL NAL units
SHALL be transmitted out-of-band or in a separate session for the
current state of this specification. If aggregating NAL units of
different layers within one aggregation packet, the first NAL unit
of the packet MUST have the highest transport priority of all NAL
units contained in the packet. The order of NAL units within a
packet is the same as the decoding order.
8. De-Packetization Process (Informative)
Please see section 7 of RFC3984 [RFC3984]. The following rules
apply in addition.
The single NAL unit mode SHALL NOT be used.
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Layered multicast is supported by this specification. An
informative appendix on recovering NAL unit decoding order in
layered multicast can be found in section 14.
9. Payload Format Parameters
[Edt. note: this section 9 and its subsections will be updated
according to the changes listed below, a little later in the
process. For now, we just list the adjustments necessary, so not to
bury any new information in the RFC 3984 text.]
Section 8 of [RFC3984] applies with the following modification.
The sentence
"The parameters are specified here as part of the MIME subtype
registration for the ITU-T H.264 | ISO/IEC 14496-10 codec."
is replaced with
"The parameters are specified here as part of the MIME subtype
registration for the SVC codec."
9.1. MIME Registration
The MIME subtype for the SVC codec is allocated from the IETF tree.
The receiver MUST ignore any unspecified parameter.
Media Type name: video
Media subtype name: H.264-SVC
Required parameters: none
OPTIONAL parameters:
The optional MIME parameters specified in [RFC3984] apply, in
addition to the following.
sprop-scalability-info:
This parameter MAY be used to convey the NAL unit containing the
scalability information SEI message that MUST precede any other NAL
units in decoding order. 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 as specified in
[SVC].
sprop-transport-priority:
This parameter MAY be used to signal the transport priority
indicator value(s) in terms of the one or two byte SVC NAL unit
header extension of one or more SVC layer(s) of one RTP session. A
transport priority indicator is base64 coded. If more than one
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layer is transmitted within one RTP session, the transport priority
indicator value of each layer MUST be itemized with decreasing
importance for decoding and MUST be comma-separated.
If a H.264/AVC base layer is part of the RTP session, this parameter
SHALL not be used.
Encoding considerations:
This type is only defined for transfer via
RTP (RFC 3550).
Security considerations:
See section 9 of this specification.
Public specification:
Please refer to section 15 of this
specification.
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
9.2.1. Mapping of MIME Parameters to SDP
The MIME media type video/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 SVC (the
MIME 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-transport-priority", and "sprop-scalability-info",
when present, MUST be included in the "a=fmtp" line of SDP. These
parameters are expressed as a MIME media type string, in the form
of a semicolon separated list of parameter=value pairs.
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9.2.2. Usage with the SDP Offer/Answer Model
TBD.
9.2.3. Usage in Declarative Session Descriptions
TBD.
9.3. Examples
TBD.
9.4. Parameter Set Considerations
Please see section 10 of RFC3984 [RFC3984].
10. Security Considerations
Please see section 11 of RFC3984 [RFC3984].
11. Congestion Control
Within any given RTP session carrying payload according to this
specification, the provisions of section 12 of RFC3984 [RFC3984]
apply.
One key motivation for the recent attention to scalable codecs has
been the increasing awareness of media codec designers to network
congestion. While CGS scalability cannot reduce congestion for the
transport path of a given RTP session, MANEs and layered multicast
technologies can be used to alleviate congestion on a larger scale.
FGS scalability can be helpful to reduce session bandwidth both end-
to-end (with pre-coded content) and in network segments, again
assuming the use of MANEs.
MANEs MAY alleviate congestion on their outgoing network path by
a) removing the NAL units belonging to hierarchically "highest"
enhancement layer (or set of enhancement layers) from an RTP
stream carrying base and enhancement layers.
b) removing some or all bits of a given FGS NAL unit as long as the
remaining bits still form a conforming SVC NAL unit.
Edt. note: In the following paragraph, "translator" and "mixer" are
not used consistently with RFC 3550. What we think we would need is
a "mixer" that mixes only a single input in a single output (as a
mixer terminates sessions). A "Translator" (that does not terminate
the RTP session) carries certain unnecessary baggage which appears
to make it undesirable for MANEs. The following paragraph can
either be fixed into RFC 3550 style and logic (thereby removing an
operation point we consider desirable), or we would need to explain
in detail what we want to do (not really congestion control related
and long). Perhaps we refer to the detailed discussions in the CCM
draft... Added to open issues.
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In both cases, the incoming RTP session is terminated in the MANE,
and a second RTP session originates at the MANE. The MANE acts as
an RTP translator. The concept of scalability keeps the
implementation and computational effort within the MANE low, and
avoids expensive and delay-intensive full transcoding (in the sense
of reconstruction and re-encoding).
When scalable layers are transported in their own RTP sessions, an
RTP receiver SHOULD unsubscribe to one or more enhancement layers
when it senses congestion, similar to what has been described in
[McCanne/Vetterli]. This behavior could perhaps be sufficient to
ease the network load to an acceptable level of congestion.
Nevertheless, it MUST follow the mechanisms described in section 12
of [RFC3984].
12. IANA Consideration
[Edt. note: A new MIME type should be registered from IANA.]
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 -- 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/Vetterli] implies that all layers are individually conveyed
in their own RTP session using their own IP multicast address.
Receivers "tune" into the layers by subscribing to the IP multicast,
normally by using IGMP [IGMP]. Optimization forms could be
envisioned in which a number of layers are sent combined in a single
Wenger, Wang, Schierl Standards Track [page 15]
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RTP session; but these optimizations are currently not considered in
this document.
Layered Multicast has the great advantage of simplicity and easy
implementation. However, it has also the great disadvantage of
utilizing many different ports. 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/NAT viewpoint. Furthermore, even today IP multicast is not
as widely deployed as many wish.
We consider layered multicast an important application scenario for
three reasons. First, it is well understood and the implementation
constraints are well known. There may well by 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]. Finally, when one base
and one enhancement layer is in use and are being conveyed
separately, that represents one operation point of layered
multicast.
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 and connectivity of the client(s), the
streaming server generates a scalable stream. This scalable stream
is served to the client(s). 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 different audiences.
As every endpoint receives only a single SVC RTP session, the number
of firewall pinholes can be optimized. In fact, only a single
firewall pinhole is required.
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 adaptivity 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. 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, both for CGS
and FGS layers. While, in theory, a transcoding step could achieve
a similar dynamic range, the computational demands are impractically
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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 final scenario is a bit more complex, and designed to optimize
the network traffic in a core network, while still requiring only a
single pinhole in the endpoint's firewall. One of its key
applications is the mobile TV market.
Consider a large 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; other 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,
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.
Edt. note: In the following paragraph, Mixers and Translators need
to be clarified.
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. Furthermore,
they terminate the multicasted layered RTP sessions coming in from
the core network side, and create new RTP sessions (perhaps even
multicast sessions) to the endpoints connected to them. In RTP
terminology, it appears that MANEs necessarily are mixers AND
translators; a MANE first mixes the content of one or more incoming
RTP streams, and then "translates" it into the outgoing stream
(which may involve pruning FGS coded NAL units and similar tasks).
Wenger, Wang, Schierl Standards Track [page 17]
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While the implementation complexity 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. It is
also simple to identify the fine granularity scalable bits in a
given NAL unit. 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.
Finally, another scenario with very similar properties could be
implemented in which the streaming server would send a single SVC
scalable stream (containing basically all available scalable layers)
to the MANE, and the MANE de-layers this scalable bit stream into
its individual layers, before further processing.
13.5. Scenarios currently not considered for complexity reasons
-- vacat --
13.6. 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
consider important. In particular, their idea is 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 call
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 RFC3984 and in this
specification, let's call it a 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
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):
Wenger, Wang, Schierl Standards Track [page 18]
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- 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
patches 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).
- An MDfH also can't "prune" FGS packets. Again, doing so would not
be compatible with meta payloads, and would mess up RTCP RRs and
congestion control (if the congestion control is based on octet
count and not on packet count; there are discussions related to
the former at least in the context of TFRC).
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.
14. Informative Appendix: NAL Unit Re-ordering for Layered Multicast
14.1. Examples
In layered multicast, the base layer, one or more enhancement
layers, or the base layer and one or more enhancement layers may be
transmitted within a separate RTP session, i.e. the NAL units
required for decoding an access unit of a certain operation point of
the scalable bitstream may be distributed in different RTP sessions.
After receiving NAL units from different RTP sessions, restoring of
the decoding order of NAL units is required. Since SVC typically
exploits temporal frame re-ordered structures for increased coding
efficiency, the decoding order of access units may not match their
presentation order. If the interleaved packetization mode is used
in any RTP session, then de-interleaving within that RTP session
must be first processed.
Wenger, Wang, Schierl Standards Track [page 19]
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1) Example for 3 RTP sessions, each carrying a different set of
layers of the SVC bitstream without NAL unit interleaving
An example for temporal re-ordering of SVC access units and
transmission of 11 different possible operation points within 3 RTP
sessions is given below. A, B and C represent the RTP sessions
carrying SVC layers for different operation points. 'A' contains
the base layer (DID = 0) with the second lowest temporal resolution
and its FGS quality enhancement (TL-DID-QL values: 0,0,0; 0,0,1;
1,0,0; 1,0,1), 'B' contains the second layer (DID = 1) with a higher
temporal level than 'A' (TL-DID-QL values: 0,1,0; 1,1,0; 2,1,0), 'C'
contains a temporal enhancement to the layer contained in 'B' and a
FGS quality enhancement to this layer 'C' (TL-DID-QL values: 1,1,1;
2,1,1; 3,1,0; 3,1,1).
Tree of the SVC stream showing dependencies of operation points
identified by the TL-DID-QL values per RTP session:
A:^ 000
| / | \
| / 100 001
| / / \ /
v / / 101
B:^ / /
| 010 /
| \ /
| 110
| / \
| / 210
v / / \
C:^ 111 / \
| \ / 310
| 211 /
| \ /
v 311
Figure 1. SVC bistream dependency tree
Decoding order and dependency of NAL units per RTP session:
A: -(1,2)-(3,4)---------------------------------------(5,6)--(7,8)-
| | | |
B: -(1)---(2)--(3)---(4)------------------------------(5)----(6)---
| | | | | |
C: -(1)---(2)--(3)---(4)--(5,6)-(7,8)-(9,10)-(11,12)--(13)---(14)--
------------------------------------------------------------------->
TL: <0> <1> <2> <2> <3> <3> <3> <3> <0> <1>
TS: [8] [4] [2] [6] [1] [3] [5] [7] [16] [12]
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
Wenger, Wang, Schierl Standards Track [page 20]
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an RTP session
'|' - indicates layer dependency
Integer values in '[]' - (Presentation) Timestamp (TS)
Integer values in '<>' - Temporal Level (TL)
Figure 2. Distribution of SVC NAL units among different RTP sessions
in Layered Mulicast transmission
The re-ordered decoding order for all operation points of RTP
session C is the following:
A(1,2)B(1)C(1), A(3,4)B(2)C(2), B(3)C(3), B(4)C(4), C(5,6), C(7,8),
C(9,10), C(11,12), A(5,6)B(5)C(13), A(7,8)B(6)C(14).
The decoding order of NAL units received from RTP sessions A, B and
C has to be restored after reception. Therefore an initial
buffering of NAL units received per RTP session is required. NAL
units belonging to the same access unit are identified by having
identical timestamps. The timestamps of different sessions are
aligned beforehand. Therefore NAL units of the same instance of
time are re-assembled to access units. While keeping the decoding
order of NAL units per RTP session, the NAL units with the same time
stamp are re-ordered to access units. The dependency information of
the sprop-scalability-info and sprop-transport-priority parameters
may be required for this operation. Note: The decoding order,
presentation order and transmission order of NAL units may vary from
each other, i.e. time stamps are not monotonically increasing with
the transmission (and decoding) order of the NAL units.
In case of using the non-interleaved mode, the decoding order of NAL
units within a RTP session is given by the transmission order, which
is indicated by the RTP sequence number. If an amount of NAL units
is received and initially buffered for each RTP session, re-ordering
of NAL units can be applied. Alternatively, an initial buffering
time is waited before NAL unit reordering is applied for all the RTP
sessions of the layered multicast transmission. In any case,
initially buffer amount of NAL units or the initial buffering time
shall guarantee correct NAL unit re-ordering with all valid
combinations of operation points of the scalable stream. The
initial buffering time for each RTP session is defined as the
maximum value of (transmission time of the NAL unit - decoding time
of an NAL unit) in terms of RTP timestamp time scale, assuming
reliable and instantaneous transmission and the same timeline for
transmission and decoding.
The re-combining of layers transported in different RTP sessions to
operation points of the scalable bitstream may be applied by using
the information provided by the sprop-scalability-info and the
sporp-transport-priority parameters in order to maintain integrity
of the resulting SVC bitstream. See also note at end of this
example 1).
Summarized re-ordering process for layered multicast:
Wenger, Wang, Schierl Standards Track [page 21]
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o Timestamp values are aligned for all the RTP sessions.
o Decoding order within a RTP session is derived from the
transmission order if using the non-interleaved mode. If using the
Interleaved mode, the Decoding Order Number (DON) must be used to
recover decoding order from transmission order in a RTP session.
o After reception of a safe amount of RTP packets or after a certain
initial-buffering time per RTP session, re-ordering process of NAL
units to decoding order can be started.
o NAL units belonging to one access unit are identified by an
identical timestamp value.
o The dependency of layers contained in various RTP sessions may be
derived form the sprop-scalability-info and the
sprop-transport-priority parameters.
Note:
If layers of different operation points are combined to one RTP
session, which do not directly or indirectly reference a layer
contained in this session, NAL unit re-ordering may be applied by
using the transport priority indicator of each NAL unit, which could
be very painful. This may be the case, if different hierarchical
dependencies of the operation points are possible, as shown in the
following example with RTP sessions U, V and W. All layers indicated
by their TL-DID-QL values are part of the same time instance:
U:^ 000
v / \
V:^ 010 \
v \ \
W:^ \ 001
| \ /
v 011
RTP session U contains the base layer (DID=0), session V the spatial
enhancement (DID=1) and session W quality enhancement for both
layers. Re-ordering session U and V is simple as described before.
For inserting session W into sessions U and V each NAL unit header
may be parsed for identifying correct decoding order.
A starting point of a discussion on a possible solution for this
issue can be found in 14.2.
2) Example for 3 RTP sessions, each carrying a different set of
layers of the SVC bitstream without NAL unit interleaving but
with packet losses
If packet loss is present, NAL unit re-ordering may become
complicated. Let us assume NAL units B(3), B(4) and B(5) are lost
(indicated by XXX) in the following scheme.
Wenger, Wang, Schierl Standards Track [page 22]
INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006
A: -(1,2)-(3,4)---------------------------------------(5,6)--(7,8)-
| | | |
B: -(1)---(2)---XXX------------------------------------------(6)---
| | | | | |
C: -(1)---(2)--(3)---(4)--(5,6)-(7,8)-(9,10)-(11,12)--(13)---(14)--
------------------------------------------------------------------->
TL: <0> <1> <2> <2> <3> <3> <3> <3> <0> <1>
TS: [8] [4] [2] [6] [1] [3] [5] [7] [16] [12]
Figure 3. Packet loss in Layered Multicast
The re-ordered decoding order for all operation points of RTP
session C is the following:
A(1,2)B(1)C(1), A(3,4)B(2)C(2), XXX C(3), XXX C(4), C(5,6), C(7,8),
C(9,10), C(11,12), A(5,6) XXX C(13), A(7,8)B(6)C(14).
In this case the receiver would not be able to correctly decode the
access units of timestamps [2] and [6]. Additionally the access
units (following in decoding order) of timestamps [1], [3], [5] and
[7] would not be correctly decode-able, although these access units
are not directly affected by the loss. Further the complete
operation point contained in session B and C must be discarded
following NAL unit B(5), since B(5) is also missing. Therefore a re-
ordering algorithm must determine the transport priority of each
received NAL unit following the packet loss. It cannot be determined
how many NAL units are missing, thus the integrity of each re-
constructed access unit must be verified with the decoding
dependency information of the sprop-scalability-info.
Note: The issue described above is especially important, if the
receiving node is a MANE, which intends to combine different streams
to new RTP sessions containing valid operation points.
3) Example for 3 RTP sessions, each carrying a different set of
layers of the SVC bitstream with NAL unit interleaving
The example is similar to example 1, but transmission order of the
NAL units of RTP session A and B has changed, e.g. for increasing
error robustness.
A: -(1,2)-(5,6)-(3,4)-(7,8)-----------------------------------------
| | | |
B: -(1)---(5)---(2)---(6)---(3)---(4)-------------------------------
| | | | | |
C: -(1)---(14)--(2)---(13)--(3)---(4)--(5,6)-(7,8)-(9,10)-(11,12)--
------------------------------------------------------------------->
TL: <0> <0> <1> <1> <2> <2> <3> <3> <3> <3>
TS: [8] [16] [4] [12] [2] [6] [1] [3] [5] [7]
Figure 4. NAL unit interleaving in Layered Multicast
Wenger, Wang, Schierl Standards Track [page 23]
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The re-ordered decoding order for all operation points of RTP
session C is the following and the same as in example 1:
A(1,2)B(1)C(1), A(3,4)B(2)C(2), B(3)C(3), B(4)C(4), C(5,6), C(7,8),
C(9,10), C(11,12), A(5,6)B(5)C(13), A(7,8)B(6)C(14).
In this case first the decoding order of RTP sessions A and B must
be restored by using the Decoding Order Number (DON) of the
interleaved packetization mode. After the de-interleaving process a
process equal to example 1 can be applied in order to restore the
decoding order of NAL units received from the different RTP
sessions. Using the interleaved mode in some or all RTP sessions is
unproblematic in layered multicast.
14.2. Discussion: Using enhanced DON over different RTP sessions
NAL unit re-ordering over different RTP sessions can lead to
complicated search operations in each receiver-buffer of these RTP
sessions for recovering decoding order, i.e. analyzing timestamps
and decoding dependency (transport priority) of the NAL units may be
required, as mentioned in section 14.1. This problem could be
solved by using an extended Decoding Order Number (DON) [RFC3984]
value, which is increased with NAL unit decoding order over
different RTP sessions. Such an extended DON may save much of the
complexity of the re-ordering process in the receiving node at the
cost of the additional signaling overhead.
15. Acknowledgements
Funding for the RFC Editor function is currently provided by the
Internet Society.
16. References
16.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[MPEG4-10] ISO/IEC International Standard 14496-10:2003.
[H.264] ITU-T Recommendation H.264, "Advanced video coding for
generic audiovisual services", May 2003.
[SVC] Joint Video Team, "Joint Scalable Video Model JSVM-4 Annex
G", available from http://ftp3.itu.ch/av-arch/jvt-site/
2005_10_Nice/JVT-Q202.zip., October 2005
[RFC3984] Wenger, S., Hannuksela, M, Stockhammer, T, Westerlund, M,
Singer, D, "RTP Payload Format for H.264 Video", RFC 3984,
February 2005
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Wenger, Wang, Schierl Standards Track [page 24]
INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006
16.2. Informative References
[DVB-H] DVB - Digital Video Broadcasting (DVB); DVB-H
Implementation Guidelines, ETSI TR 102 377, 2005
[IGMP] Cain, B., Deering S., Kovenlas, I., Fenner, B. and
Thyagarajan, A., "Internet Group Management Protocol, Version 3", RFC
3376, October 2002.
[McCanne/Vetterli] V. Jacobson, S. McCanne and M. Vetterli. 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.
17. Author's Addresses
Stephan Wenger Phone: +358-50-486-0637
Nokia Research Center Email: stewe@stewe.org
P.O. Box 100
FIN-33721 Tampere
Finland
Ye-Kui Wang Phone: +358-50-486-7004
Nokia Research Center Email: ye-kui.wang@nokia.com
P.O. Box 100
FIN-33721 Tampere
Finland
Thomas Schierl Phone: +49-30-31002-227
Fraunhofer HHI Email: schierl@hhi.fhg.de
Einsteinufer 37
D-10587 Berlin
Germany
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none
22. Open Issues
1. Signaling: Guidance from AVT mailing list: try to come up with
media independent signaling for layered codecs. Needs to go into a
new draft in MMUSIC, as it looks.
2. Cross-Layer DON, see 14.2 Is that acceptable? It would solve
many problems, but at the expense of cross-session fields in a
payload header. Also, DON has known IPR.
3. Need to clarify MANE, Mixers, and Translators throughout the
document (consistently with RFC 3550).
4. Packetization rules need work ones 3) is addressed
5. Alignment with JVT spec (ongoing)
23. Changes Log
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
06.03.2006, StW: Editorial improvements
Wenger, Wang, Schierl Standards Track [page 26]
| PAFTECH AB 2003-2026 | 2026-04-24 05:50:59 |