draft-ietf-avt-mpeg4-multisl-02.txt   draft-ietf-avt-mpeg4-multisl-03.txt 
Internet Engineering Task Force Basso-AT&T Internet Engineering Task Force Basso-AT&T
Internet Draft Civanlar-AT&T Internet Draft Civanlar-AT&T
Gentric-Philips Gentric-Philips
Herpel-Thomson Herpel-Thomson
Lifshitz-Optibase Lifshitz-Optibase
Lim-mp4cast Lim-mp4cast
Perkins-ISI Perkins-ISI
Van Der Meer-Philips Van Der Meer-Philips
September 2001 November 2001
Expires March 2002 Expires May 2002
Document: draft-ietf-avt-mpeg4-multisl-02.txt Document: draft-ietf-avt-mpeg4-multisl-03.txt
RTP Payload Format for MPEG-4 Streams RTP Payload Format for MPEG-4 Streams
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at line 83 skipping to change at line 83
ii. Monitoring MPEG-4 delivery performance through RTCP ii. Monitoring MPEG-4 delivery performance through RTCP
iii. Combining MPEG-4 and other real-time data streams received from iii. Combining MPEG-4 and other real-time data streams received from
multiple end-systems into a set of consolidated streams through RTP multiple end-systems into a set of consolidated streams through RTP
mixers mixers
iv. Converting data types, etc. through the use of RTP translators. iv. Converting data types, etc. through the use of RTP translators.
1.1 Overview of MPEG-4 End-System Architecture 1.1 Overview of MPEG-4 End-System Architecture
Two types of terminals can use this specification. One case is a
complete MPEG-4 terminal i.e. a terminal implementing the MPEG-4
system [1] specification and possibly also MPEG-4 video [2] and
audio [3]. Another possibility is a terminal implementing only a
part of this set of MPEG-4 specification; one example is a terminal
using MPEG-4 video [2] but not MPEG-4 systems as in RFC3016.
This document is structured so as to be understandable from both
points of view (with or without MPEG-4 systems). The target is also
that services deployed for one type of terminal can be adapted for
the other type thanks to minor session description change because
recorded streams are the same. Another key assumption is that the
properties of streams of various type (video, audio, scene
description) can be described with the same Elementary Stream model
so that this same payload format can transport any MPEG-4 stream.
1.1.1 The simplified MPEG-4 model
In the simplified MPEG-4 model MPEG-4 systems [1] is not used.
However the concept of Elementary Stream remains i.e. both MPEG-4
video [2] and MPEG-4 audio [3] describe how respectively audio and
video bit streams are fragmented into pieces that are called Access
Units. Each Access Unit has by definition a number of media
independent basic properties:
. composition time stamp
. framing
. possibly decoding time stamp
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Furthermore both the video [2] and audio [3] specification also
define how Access Units (AU) shall be themselves fragmented since in
the spirit of Application Level Framing AUs SHOULD be fragmented in
a way that decoders can process the packets immediately after a
packet loss. In this case the signaling of Access Unit fragment
boundaries is also required.
In order to be understandable from this point of view this payload
format is described in terms of Access Units (AU) and Access Units
fragments, without reference to media specific properties (but for a
few exceptions).
1.1.2 The complete MPEG-4 model
Fig. 1 below shows the layered architecture of a terminal, which Fig. 1 below shows the layered architecture of a terminal, which
implements the complete MPEG-4 systems model. The Compression Layer implements the complete MPEG-4 systems model. The Compression Layer
processes individual audio-visual media streams. The MPEG-4 processes individual audio-visual media streams. The MPEG-4
compression schemes are defined in the ISO/IEC specifications 14496- compression schemes are defined in the ISO/IEC specifications 14496-
2 [2] and 14496-3 [3]. The compression schemes in MPEG-4 achieve 2 [2] and 14496-3 [3]. The compression schemes in MPEG-4 achieve
efficient encoding over a bandwidth ranging from several kbps to efficient encoding over a bandwidth ranging from a few kbps to many
many Mbps. The audio-visual content compressed by this layer is Mbps. The audio-visual content compressed by this layer is organized
organized into Elementary Streams (ESs). into Elementary Streams (ESs).
The MPEG-4 standard specifies MPEG-4 compliant streams. Within the The MPEG-4 standard specifies MPEG-4 compliant streams. Within the
constraint of this compliance the compression layer is unaware of a constraint of this compliance the compression layer is unaware of a
specific delivery technology, but it can be made to react to the specific delivery technology, but it can be made to react to the
characteristics of a particular delivery layer such as the path-MTU characteristics of a particular delivery layer such as the path-MTU
or loss characteristics. Also, some compressors can be designed to or loss characteristics. Also, some compressors can be designed to
be delivery specific for implementation efficiency. In such cases be delivery specific for implementation efficiency. In such cases
the compressor may work in a non-optimal fashion with delivery the compressor may work in a non-optimal fashion with delivery
technologies that are different than the one it is specifically technologies that are different than the one it is specifically
designed to operate with. designed to operate with.
The hierarchical relations, location and properties of ESs in a The hierarchical relations, location and properties of ESs in a
presentation are described by a dynamic set of Object Descriptors presentation are described by a dynamic set of Object Descriptors
(ODs). Each OD groups one or more ES Descriptors referring to a (ODs). Each OD groups one or more ES Descriptors referring to a
single content item (audio-visual object). Hence, multiple single content item (audio-visual object). Hence, multiple
alternative or hierarchical representations of each content item are alternative or hierarchical representations of each content item are
possible. possible.
ODs are themselves conveyed through one or more ESs. A complete set ODs are themselves conveyed through one or more ESs. A complete set
of ODs can be seen as an MPEG-4 resource or session description at a of ODs can be seen as an MPEG-4 resource or session description at a
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RTP Payload Format for MPEG-4 Streams September 2001
stream level. The resource description may itself be hierarchical, stream level. The resource description may itself be hierarchical,
i.e. an ES conveying an OD may describe other ESs conveying other i.e. an ES conveying an OD may describe other ESs conveying other
ODs. ODs.
The session description is accompanied by a dynamic scene The session description is accompanied by a dynamic scene
description, Binary Format for Scene (BIFS), again conveyed through description, Binary Format for Scene (BIFS), again conveyed through
one or more ESs. At this level, content is identified in terms of one or more ESs. At this level, content is identified in terms of
audio-visual objects. The spatio-temporal location of each object is audio-visual objects. The spatio-temporal location of each object is
defined by BIFS. The audio-visual content of those objects that are defined by BIFS. The audio-visual content of those objects that are
synthetic and static are described by BIFS also. Natural and synthetic and static are described by BIFS also. Natural and
animated synthetic objects may refer to an OD that points to one or
more ESs that carries the coded representation of the object or its
animation data.
By conveying the session (or resource) description as well as the
scene (or content composition) description through their own ESs, it
is made possible to change portions of the content composition and
the number and properties of media streams that carry the audio-
visual content separately and dynamically at well known instants in
time.
One or more initial Scene Description streams and the corresponding
OD stream are pointed to by an initial object descriptor (IOD). In
this context the IOD needs to be made available to the receivers
through some out-of-band means that are out of scope of this payload
specification. However in the context of transport on IP networks it
is defined in a separate document [9]. Note that for applications
that only use audio and/or video this payload format can also be
used without IOD and OD streams (decoder configuration is then
transported as MIME parameters, see section 4.1).
The Compression Layer organizes the ESs in Access Units (AU), the
smallest elements that can be attributed individual timestamps. The
Access Units concept defines the boundary between media specific
processing and delivery specific processing. That is to say
transport should not depend on the nature of the media data but only
on AU properties.
The Sync Layer (SL) that primarily provides the synchronization
between streams defines a homogeneous encapsulation of ESs carrying
media or control data (ODs, BIFS). Integer or fractional AUs are
then encapsulated in SL packets and in the following we will
describe this payload format as transporting SL packets, although in
many cases SL packet payloads are actually (entire) Access Units
payloads i.e. encoded media frames. All consecutive data from one
stream is called an SL-packetized stream at this layer. The
interface between the compression layer and the SL is called the
Elementary Stream Interface (ESI). The ESI is informative i.e. it is
extremely useful in order to define concepts and mechanisms but does
not have to be implemented. For the same reason this draft describes
the transport of SL packets i.e. Access Units or fragments thereof.
It is important to note however that a SL stream can be configured
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so that SL packets are reduced to the media (compressed) data and in animated synthetic objects may refer to an OD that points to one or
that case implementations do not need to be aware of the SL at all. more ESs that carries the coded representation of the object or its
animation data.
The Delivery Layer in MPEG-4 consists of the Delivery Multimedia
Integration Framework defined in ISO/IEC 14496-6 [4]. This layer is
media unaware but delivery technology aware. It provides transparent
access to and delivery of content irrespective of the technologies
used. The interface between the SL and DMIF is called the DMIF
Application Interface (DAI). It offers content location independent
procedures for establishing MPEG-4 sessions and access to transport
channels. The specification of this payload format is considered as
a part of the MPEG-4 Delivery Layer.
media aware +-----------------------------------------+ media aware +-----------------------------------------+
delivery unaware | COMPRESSION LAYER | delivery unaware | COMPRESSION LAYER |
14496-2 Visual |streams from as low as Kbps to multi-Mbps| 14496-2 Visual |streams from as low as Kbps to multi-Mbps|
14496-3 Audio +-----------------------------------------+ 14496-3 Audio +-----------------------------------------+
Elementary Elementary
Stream Stream
===================================================Interface ===================================================Interface
skipping to change at line 213 skipping to change at line 204
(DAI) (DAI)
+-------------------------------------------+ +-------------------------------------------+
delivery aware | DELIVERY LAYER | delivery aware | DELIVERY LAYER |
media unaware |provides transparent access to and delivery| media unaware |provides transparent access to and delivery|
14496-6 DMIF | of content irrespective of delivery | 14496-6 DMIF | of content irrespective of delivery |
| technology | | technology |
+-------------------------------------------+ +-------------------------------------------+
Figure 1: Conceptual MPEG-4 terminal architecture Figure 1: Conceptual MPEG-4 terminal architecture
1.2 MPEG-4 Elementary Stream Data Packetization By conveying the session (or resource) description as well as the
scene (or content composition) description through their own ESs, it
is made possible to change portions of the content composition and
the number and properties of media streams that carry the audio-
visual content separately and dynamically at well known instants in
time.
The ESs from the encoders are fed into the SL with indications of AU One or more initial Scene Description streams and the corresponding
boundaries, random access points, desired composition time and the OD stream are pointed to by an initial object descriptor (IOD). In
current time. this context the IOD needs to be made available to the receivers
through some out-of-band means that are out of scope of this payload
specification. However in the context of transport on IP networks it
is defined in a separate document [9].
The Sync Layer fragments the ESs into SL packets, each containing a The Compression Layer organizes the ESs in Access Units (AU), the
header that encodes information conveyed through the ESI. If the AU smallest elements that can be attributed individual timestamps. The
is larger than a SL packet, subsequent packets containing remaining Access Units concept defines the boundary between media specific
processing and delivery specific processing. That is to say
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parts of the AU are generated with subset headers until the complete transport should not depend on the nature of the media data but only
AU is packetized. on AU properties.
The syntax of the Sync Layer is configurable and can be adapted to 1.1.3 The Sync Layer
the needs of the stream to be transported. This includes the
possibility to select the presence or absence of individual syntax The Sync Layer (SL) that primarily provides the synchronization
elements as well as configuration of their length in bits. The between streams defines a homogeneous encapsulation of ESs carrying
configuration for each individual stream is conveyed in a media or control data (ODs, BIFS). Integer or fractional AUs are
SLConfigDescriptor, which is an integral part of the ES Descriptor then encapsulated in SL packets.
All consecutive data from one stream is called an SL-packetized
stream. The interface between the compression layer and the SL is
called the Elementary Stream Interface (ESI). The ESI is informative
i.e. it is extremely useful in order to define concepts and
mechanisms but does not have to be implemented.
The Delivery Layer in MPEG-4 consists of the Delivery Multimedia
Integration Framework defined in ISO/IEC 14496-6 [4]. This layer is
media unaware but delivery technology aware. It provides transparent
access to and delivery of content irrespective of the technologies
used. The interface between the SL and DMIF is called the DMIF
Application Interface (DAI). It offers content location independent
procedures for establishing MPEG-4 sessions and access to transport
channels. This payload format can be used as an instance of the
MPEG-4 Delivery Layer but is otherwise not tied to DMIF.
The ESs from the encoders are fed into the SL with indications of AU
boundaries, random access points, desired composition time and the
current time. The Sync Layer fragments the ESs into SL packets, each
containing a header that encodes information conveyed through the
ESI. If the AU is larger than a SL packet, subsequent packets
containing remaining parts of the AU are generated with subset
headers until the complete AU is packetized. One SL packet describes
an Access Units or fragments thereof, the SL packet header contains
extended timing and framing information; the SL packet payload
contains the bit stream frame (AU) or fragment. For the complete
list of features of the Sync Layer refer to the MPEG-4 systems
specification [1]. The syntax of the Sync Layer is configurable and
can be adapted to the needs of the stream to be transported. This
includes the possibility to select the presence or absence of
individual syntax elements as well as configuration of their length
in bits. The configuration for each individual stream is conveyed in
a SLConfigDescriptor, which is an integral part of the ES Descriptor
for this stream. The MPEG-4 SLConfigDescriptor, being configuration for this stream. The MPEG-4 SLConfigDescriptor, being configuration
information, is not carried by the media stream itself but is rather information, is not carried by the media stream itself but is rather
transported via an ObjectDescriptor Stream encoded using the MPEG-4 transported via an ObjectDescriptor Stream encoded using the MPEG-4
Object Description framework. This can be done in a separate stream Object Description framework. This can be done in a separate stream
using this payload format (see section 5.2 for details). The using this payload format (see section 5.2 for details). The
SLConfigDescriptor MAY also be transported by other means (for SLConfigDescriptor MAY also be transported by other means (for
example as a parameter, see section 4.1). Finally streams for which example as a parameter, see section 4.1).
the SL packet headers are completely empty (or fully map into the
RTP headers) can also be transported using this payload format; in An important point is to note that this draft could just as well
these cases the Synch Layer can be seen as a purely conceptual have been entirely written in terms of SL packets instead of Access
construction that does not have to be implemented at all. Since only
the knowledge of the decoder configuration is then needed it MAY Gentric et al. Expires March 2002 5
also be transported as a parameter, as described in section 4.1. RTP Payload Format for MPEG-4 Streams September 2001
Units and Access Unit fragments. However this could have created
confusion for implementers who only need basic properties and do not
want to cope with the additional complexity of the Sync Layer.
Instead this specification refers to the Sync Layer only when
needed.
1.1.4 Where the two models meet
In basic cases an Elementary Stream is such that SL packets are
reduced to the media (compressed) data (empty headers) and in that
case implementations do not actually need to be aware of the Sync
Layer at all. In these cases it is logically equivalent to say that
the Sync Layer is not implemented or to say that the SL packet
headers are completely empty (or fully map into the RTP headers).
The Sync Layer can then be seen as a purely conceptual construction
that does not have to be implemented at all.
The above described MPEG-4 system model also deals with session
setup through Object Descriptors. In cases where the complete MPEG-4
system framework is not used a replacement for this key functionally
is required. In fact for simple (audio/video) systems only the
knowledge of the decoder configuration is needed; we will see how
this specification defines options so that decoder configuration can
also be signaled without MPEG-4 system.
In conclusion this payload format is intended to be capable of
transporting data formatted according to the Sync Layer
specification but is also useful without the Sync Layer, or when the
Sync Layer is invisible, which is equivalent to not using it.
2. Analysis of the carriage of MPEG-4 over IP 2. Analysis of the carriage of MPEG-4 over IP
When transporting MPEG-4 audio and video, applications may or may As explained above when transporting MPEG-4 audio and video,
not require the use of MPEG-4 systems. To achieve the highest level applications may or may not require the use of MPEG-4 systems. To
of interoperability between all MPEG-4 applications, it is desirable achieve the highest level of interoperability between all MPEG-4
that (a) in both cases the same MPEG-4 transport format can be used applications, it is desirable that (a) in both cases the same MPEG-4
and that (b) receivers that have no MPEG-4 system knowledge can transport format can be used and that (b) receivers that have no
easily skip the MPEG-4 system specific information, if any. MPEG-4 system knowledge can easily skip the MPEG-4 system specific
information, if any.
2.1 The Sync Layer point of view
RTP is perfectly suitable to transport MPEG-4 audio and MPEG-4 RTP is perfectly suitable to transport MPEG-4 audio and MPEG-4
video, but when using MPEG-4 systems a problem arises from the fact video, but when using MPEG-4 systems a problem arises from the fact
that both RTP and MPEG-4 systems contain a synchronization layer. that both RTP and MPEG-4 systems contain a synchronization layer.
In particular, the RTP header duplicates some of the information In particular, the RTP header duplicates some of the information
provided in SL packet headers such as the composition timestamps provided in SL packet headers such as the composition timestamps
(CTSs) and the marker bit that signals the end of access units. (CTSs) and Access Unit boundaries.
To avoid unnecessary overhead and potential interoperability risks To avoid unnecessary overhead and potential interoperability risks
when transporting MPEG-4 systems, it is desirable to remove the when transporting MPEG-4 systems, it is desirable to remove the
redundancy between the SL packet header and the RTP packet header. redundancy between the SL packet header and the RTP packet header.
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RTP Payload Format for MPEG-4 Streams September 2001
To be independent on the use of MPEG-4 systems, synchronization can To be independent on the use of MPEG-4 systems, synchronization can
rely on the parameters provided in the RTP header. rely on the parameters provided in the RTP header.
Another desired property is to have compatibility with RFC3016 for
MPEG-4 video transport.
In case SL headers are used, the redundant fields are removed from In case SL headers are used, the redundant fields are removed from
the SL header, producing "reduced SL headers". the SL header, producing "reduced SL headers". The remaining
The remaining information from the SL header, if any, is contained information from the SL header, if any, is contained inside the RTP
inside the RTP packet payload, together with the SL packet payload. packet payload, together with the SL packet payload.
The combination of RTP packet headers and reduced SL packet headers The combination of RTP packet headers and reduced SL packet headers
can be used to logically map the RTP packets to complete SL packets. can be used to logically map the RTP packets to complete SL packets.
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RTP Payload Format for MPEG-4 Streams September 2001
Some of the information contained in the reduced SL headers is also Some of the information contained in the reduced SL headers is also
useful for transport over RTP when MPEG-4 systems is not used. useful for transport over RTP when an MPEG-4 system is not used.
For that reason the information in the "reduced" SL headers is split For that reason the information in the "reduced" SL headers is split
into "general useful information" and "MPEG-4 systems only into "general useful information" and "MPEG-4 systems only
information". information".
The "general useful information" hereinafter called Mapped SL Packet The "general useful information" hereinafter called Payload Header
Header (MSLH) is carried by a number of fields configurable using is carried by a number of fields configurable using parameters
parameters defined in section 4.1; all receivers MUST parse these defined in section 4.1; all receivers MUST parse these fields.
fields.
The "MPEG-4 systems only information", if any, is contained in a The "MPEG-4 systems only information", if any, is contained in an
reduced SL header, hereinafter called Remaining SL Packet Header auxiliary header, hereinafter called Remaining SL Packet Header
(RSLH), also configured using parameters (see section 4.1) and (RSLH), also configured using parameters (see section 4.1) and
preceded by a length field, so that non-MPEG-4-system devices MAY preceded by a length field, so that non-MPEG-4-system devices MAY
skip this information. skip this information.
This is depicted in figure 2. This is depicted in figure 2a.
+------------+
extended framing and | AU or AU |
timing information | fragment |
+------------+
| |
| |
| |
| |
V V
<----------SL Packet--------> <----------SL Packet-------->
+---------------------------+ +---------------------------+
| SL Packet | SL Packet | | SL Packet | SL Packet |
| Header | Payload | | Header | Payload |
+---------------------------+ +---------------------------+
| | | |
| | | |
+-------------+----------+---+ | +-------------+----------+---+ |
| | | | | | | |
V V V V V V V V
+-----------+ +-----------+ +-------------+ +-----------+ +-----------+ +-----------+ +-------------+ +-----------+
|RTP Packet | | Mapped SL | | Remaining SL| | SL Packet | |RTP Packet | | Payload | | Remaining SL| | SL Packet |
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| Header | | Header | | Header | | Payload | | Header | | Header | | Header | | Payload |
+-----------+ +-----------+ +-------------+ +-----------+ +-----------+ +-----------+ +-------------+ +-----------+
<----RTP Packet Payload-------------------> <----RTP Packet Payload------------------->
Figure 2: Mapping of SL Packet into RTP packet Figure 2a: Mapping of ES into SL, then SL Packet into RTP packet
When the configuration is such that SL packet headers map directly 2.2 The Elementary Stream point of view
to RTP headers this process of mapping SL packet headers is purely
conceptual. For example this RTP payload format has been designed so
that it is by default configured to be identical to RFC 3016 for the
recommended MPEG-4 video configurations (see section 5.5). Hence
receivers that comply with this payload specification can decode
such RTP payload without knowledge about the Synch Layer (see the
example in Appendix.1). In a similar fashion MPEG-4 audio (see
Appendix for examples) can be transported without explicit use of
the Synch Layer.
Gentric et al. Expires March 2002 6 Another way to see the mapping of Elementary Streams (i.e. Access
Units or AU fragments) into RTP packets is depicted in figure 2.b.
In this view the "basic" timing and fragmentation information listed
in section 1.1.1 is obtained directly at the codec interfaces and
mapped into the RTP header or the RTP Payload Header.
For example this RTP payload format has been designed so that it is
by default configured to be identical to RFC 3016 for the
recommended MPEG-4 video configurations, specifically in this case
the Payload Header is empty. Hence receivers that comply with this
payload specification can decode such RTP payload without knowledge
about the Sync Layer (see the example in Appendix 1). In a similar
fashion but with non-empty Payload Headers, MPEG-4 audio (see
Appendix 3 and 4 for examples) can be transported without explicit
use of the Sync Layer.
+------------+
basic framing and | AU or AU |
timing information | fragment |
+------------+
| |
| |
+-------------+ |
| | |
V V V
+-----------+ +-----------+ +-----------+
|RTP Packet | | Payload | | |
| Header | | Header | | Payload |
+-----------+ +-----------+ +-----------+
<----RTP Packet Payload--->
Figure 2b: Direct mapping of Elementary Streams into RTP packet
2.3 How the two views reconcile
A simple concept enables to unify these apparently antagonistic
points of view: a "no-SL" terminals can skip (ignore) the Remaining
SL Header, if present.
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3. Payload Format 3. Payload Format
The RTP Payload corresponds to an integer number of SL packets. The RTP Payload corresponds to an integer number of Access Units or
Access Unit fragments.
If multiple SL packets are transported in each RTP packet, they MUST The RTP payload is composed of 3 sections:
be in decoding order, i.e: . a Payload Header section
i) decodingTimeStamp order, if present . a RSLH section
ii) packetSequenceNumber order, if present . a Payload Section.
iii) Implicit decoding order in all other cases.
The SL Packet Headers are transformed into RSLH with some fields The AU and AU fragment boundaries and timing information is
extracted to be mapped in the RTP header and others extracted to be transported in the Payload Header.
mapped in the corresponding MSLH. The SL Packet Payload is
unchanged.
This payload format has two modes. The "SingleSL" mode is a mode When transporting SL streams, SL Packet Headers are transformed into
where a single SL packet is transported per RTP packet. The Remaining SL Header (RSLH) with some fields extracted to be mapped
"MultipleSL" mode is a mode where possibly more than one SL packet in the RTP header and others extracted to be mapped in the
are transported per RTP packet. The default mode is the Single-SL corresponding Payload Header.
mode. The mode can be set to Multiple-SL by adding a non-zero
ConstantSize or SizeLength parameter (see section 4.1).
RTP Packets SHOULD be sent in the SL stream order (as defined The AU or AU fragment data (SL packet payload) i.e. Elementary
above). In case of interleaving the first SL packet of each RTP stream codec data is unchanged.
packet is used as reference as in the following examples of RTP
packets containing interleaved SL packets. This payload format has two modes. The "Single" mode is a mode where
a single AU or AU fragment is transported per RTP packet. The
"Multiple" mode is a mode where possibly more than one AU or AU
fragment are transported per RTP packet. The default mode is the
"Single" mode.
In the "Multiple" mode, AU or AU fragments MUST be in decoding order
inside one RTP packet. Decoding order is defined by the relevant
codec specification. Decoding order may be different than
presentation order, for example for video streams containing B
frames. According to the MPEG-4 system model this order is
quantified using decoding time stamps (DTS).
RTP Packets SHOULD be sent in the decoding order. In case of
interleaving the first AU or AU fragment of each RTP packet is used
as reference as in the following examples of RTP packets containing
interleaved SL packets.
This sequence is correct: [0,2,4][1,3,5] This sequence is correct: [0,2,4][1,3,5]
This sequence is correct: [0,3,6][1,2][4,5] This sequence is correct: [0,3,6][1,2][4,5]
This sequence is correct: [0,3,6][1,4][2,5] This sequence is correct: [0,3,6][1,4][2,5]
This sequence is prohibited: [0,4,2][1,5,3] This sequence is prohibited: [0,4,2][1,5,3]
This sequence is prohibited: [1,3,5][0,2,4] This sequence is prohibited: [1,3,5][0,2,4]
This sequence is prohibited: [0,3,6][2,5][1,4] This sequence is prohibited: [0,3,6][2,5][1,4]
In the multiple-SL modes senders MUST make sure that no fields In the "Multiple" mode senders MUST make sure that no fields undergo
undergo roll over inside one RTP packet. This may limit the number roll over inside one RTP packet. This may limit the number of SL
of SL packets inside one RTP packet and, when interleaving, may packets inside one RTP packet and, when interleaving, may limit the
limit the interleaving period. interleaving period as detailed below.
The size (or number) of the SL packet(s) SHOULD be adjusted such The size and/or number of the payload(s) SHOULD be adjusted such
that the resulting RTP packet is not larger than the path-MTU. To that the resulting RTP packet is not larger than the path-MTU. To
Gentric et al. Expires March 2002 9
RTP Payload Format for MPEG-4 Streams September 2001
handle larger packets, this payload format relies on lower layers handle larger packets, this payload format relies on lower layers
for fragmentation, which may not be desirable. for fragmentation, which may not be desirable.
3.1 RTP Header Fields Usage 3.1 RTP Header Fields Usage
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
new packet format is outside the scope of this document, and will new packet format is outside the scope of this document, and will
not be specified here. It is expected that the RTP profile for a not be specified here. It is expected that the RTP profile for a
particular class of applications will assign a payload type for this particular class of applications will assign a payload type for this
encoding, or if that is not done then a payload type in the dynamic encoding, or if that is not done then a payload type in the dynamic
range shall be chosen. range shall be chosen.
Gentric et al. Expires March 2002 7 Marker (M) bit: The M bit is set to 1 when all AU fragments in the
RTP Payload Format for MPEG-4 Streams September 2001 RTP packet are Access Units ends.
Marker (M) bit: The M bit is set to 1 when all SL packets in the RTP
packet are Access Units ends i.e. the M bit maps to the Synch Layer
accessUnitEndFlag.
Specifically the M bit is set to 0 when the RTP packet contains one Specifically the M bit is set to 0 when the RTP packet contains one
or more Access Unit fragments that are not Access Unit ends, and the or more AU fragments that are not Access Unit ends, and the M bit is
M bit is set to 1 for RTP packets that contain either: set to 1 for RTP packets that contain either:
. A single complete Access Unit . A single complete Access Unit
. The last fragment of an Access Unit . The last fragment of an Access Unit
. Several complete Access Units . Several complete Access Units
. Several last fragments of Access Units . Several last fragments of Access Units
. A mix of complete Access Units and last fragments of Access Units . A mix of complete Access Units and last fragments of Access Units
Therefore for streams where all SL packets are complete Access Units Therefore for streams where all SL packets are complete Access Units
the M bit is 1 for all RTP packets. the M bit is 1 for all RTP packets. Note also that in terms of Sync
Layer this means that the M bit is related to the accessUnitEndFlag.
Extension (X) bit: Defined by the RTP profile used. Extension (X) bit: Defined by the RTP profile used.
Sequence Number: The RTP sequence number should be generated by the Sequence Number: The RTP sequence number should be generated by the
sender with a constant random offset and does not have to be sender with a constant random offset.
correlated to any (optional) MPEG-4 SL sequence numbers.
Timestamp: Set to the value in the compositionTimeStamp field of the Timestamp: Set to the value in the compositionTimeStamp field of the
first SL packet in the RTP packet, if present. first AU or AU fragment in the RTP packet, if present.
If compositionTimeStamp has less than 32 bits length, the RTP If compositionTimeStamp has less than 32 bits length, the RTP
timestamp is incremented to extend it out to 32 bits. If timestamp is generated to extend it out to 32 bits. If
compositionTimeStamp has more than 32 bits length, the RTP timestamp compositionTimeStamp has more than 32 bits length, the RTP timestamp
uses the 32 LSB of it. The resolution of the timestamp uses the 32 LSB of it. When using the Sync Layer the resolution of
(timeStampLength) is available from the SL configuration data and the timestamp (timeStampLength) is available from the SL
shall be used by receivers to reconstruct compositionTimeStamps with configuration data and shall be used by receivers to reconstruct
the original bit length. When making SL streams specifically for compositionTimeStamps with the original bit length. In all other
usage with this payload format it is RECOMMENDED to use case it is RECOMMENDED to use timeStampLength=32.
timeStampLength=32.
In all cases, the sender SHALL always make sure that RTP time stamps
are identical only for RTP packets transporting fragments of the
same Access Unit.
In case compositionTimeStamp is not present in the current SL In case compositionTimeStamp is not present in the current SL
packet, but has been present in a previous SL packet the reason is packet, but has been present in a previous AU or AU fragmentthe
that this is the same Access Unit that has been fragmented, reason is that this is the same Access Unit that has been
therefore the same timestamp value MUST be taken as RTP timestamp. fragmented, therefore the same timestamp value MUST be taken as RTP
timestamp.
Gentric et al. Expires March 2002 10
RTP Payload Format for MPEG-4 Streams September 2001
If compositionTimeStamp is never present in SL packets for this If compositionTimeStamp is never present in SL packets for this
stream, the RTP packetizer SHOULD convey a reading of a local clock stream, the RTP packetizer SHOULD convey a reading of a local clock
at the time the RTP packet is created. at the time the RTP packet is created.
In all cases, the sender SHALL always make sure that RTP time stamps
are identical only for RTP packets transporting fragments of the
same Access Unit.
According to RFC1889 [5, Section 5.1] timestamps are recommended to According to RFC1889 [5, Section 5.1] timestamps are recommended to
start at a random value for security reasons. However then, a start at a random value for security reasons. However then, a
receiver is not in the general case able to reconstruct the original receiver is not in the general case able to reconstruct the original
MPEG-4 Time Stamps (CTS, DTS, OCR) which can be of use for MPEG-4 Time Stamps (CTS, DTS, OCR) which can be of use for
applications where streams from multiple sources are to be applications where streams from multiple sources are to be
synchronized (for example one stream from local storage, another
Gentric et al. Expires March 2002 8 from a streaming server). Therefore the usage of such a random
RTP Payload Format for MPEG-4 Streams September 2001 offset SHOULD be avoided.
synchronized. Therefore the usage of such a random offset SHOULD be
avoided.
Note that since RTP devices may re-stamp the stream, all time stamps Note that since RTP devices may re-stamp the stream, all time stamps
inside of the RTP payload (CTS and DTS in MSLH, OCR in RSLH) MUST be inside of the RTP payload (CTS and DTS in PayloadHeader, OCR in
expressed as difference to the RTP time stamp. Since this RSLH) MUST be expressed as difference to the RTP time stamp. Since
subtraction may lead to negative values, the offset MUST be encoded this subtraction may lead to negative values, the offset MUST be
as a two's complement signed integer in network byte order. Note encoded as a two's complement signed integer in network octet order.
these offsets (delta) typically require much fewer bits to be Note these offsets (delta) typically require much fewer bits to be
encoded than the original length, which is another justification. encoded than the original length, which is another justification.
When startCompositionTimeStamp is signaled in the SLConfigDescriptor When startCompositionTimeStamp is signaled in the SLConfigDescriptor
the RTP time stamps MUST start with this value. the RTP time stamps MUST start with this value.
SSRC, CC and CSRC fields are used as described in RFC 1889 [5]. SSRC, CC and CSRC fields are used as described in RFC 1889 [5].
RTCP SHOULD be used as defined in RFC 1889 [5]. RTCP SHOULD be used as defined in RFC 1889 [5].
RTP timestamps in RTCP SR packets: according to the RTP timing
model, the RTP timestamp that is carried into an RTCP SR packet is
the same as the compositionTimeStamp that would be applied to an RTP
packet for data that was sampled at the instant the SR packet is
being generated and sent. The RTP timestamp value is calculated from
the NTP timestamp for the current time, which also goes in the RTCP
SR packet. To perform that calculation, an implementation needs to
periodically establish a correspondence between the CTS value of a
data packet and the NTP time at which that data was sampled.
3.2 RTP payload structure 3.2 RTP payload structure
The packet payload structure consists of 3 byte-aligned sections. The packet payload structure consists of 3 octet-aligned sections.
The first section is the MSLHSection and contains Mapped SL Packet The first section is the Payload Header Section and contains Payload
Headers (MSLH). The MSLH structure is described in 3.3. In the Headers. Each Payload Header contains basic fragmentation and timing
Single-SL mode this section is empty by default. information for one AU or AU fragment. The Payload Header structure
is described in 3.3. In the "Single" mode this section is empty by
default.
The second section is the RSLHSection and contains Remaining SL The second section is the RSLHSection and contains Remaining SL
Headers (RSLH). The RSLH structure is described in 3.5. By default Headers (RSLH). The RSLH structure is described in 3.5. By default
this section is empty. this section is empty.
The last section (SLPPSection) contains the SL packet payloads. This The last section (Payload Section) contains the AU or AU fragment
section is never empty. codec bit stream fragments. This section is never empty.
The Nth MSLH in the MSLHSection, the Nth RSLH in the RSLHSection and The Nth Payload Header in the Payload Header Section, the Nth RSLH
the Nth SL packet payload in the SLPPSection correspond to the Nth in the RSLH Section and the Nth AU or AU fragment payload in the
SL packet transported by the RTP packet. Payload Section correspond to the Nth AU or AU fragment transported
by the RTP packet.
Gentric et al. Expires March 2002 11
RTP Payload Format for MPEG-4 Streams September 2001
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number | |V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Gentric et al. Expires March 2002 9
RTP Payload Format for MPEG-4 Streams September 2001
| timestamp | | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier | | synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: contributing source (CSRC) identifiers : : contributing source (CSRC) identifiers :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | | |
| MSLHSection (byte aligned) | | Payload Header Section (octet aligned) |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
| RSLHSection (byte aligned) | | RSLH Section (octet aligned) |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+ |
| | | |
| SLPPSection (byte aligned) | | Payload Section (octet aligned) |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding | | :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: An RTP packet for MPEG-4 Figure 3: An RTP packet for MPEG-4
3.3 MSLHSection structure 3.3 Payload Header Section structure
If the MSLHSection consumes a non-integer number of bytes, up to 7 If the Payload Header Section consumes a non-integer number of
zero-valued padding bits MUST be inserted at the end in order to octets, up to 7 zero-valued padding bits MUST be inserted at the end
achieve byte-alignment. in order to achieve octet-alignment. This size excludes the padding
bits, if any.
In the Single-SL mode the MSLHSection consists of a single MSLH. In the "Single" mode the Payload Header Section consists of a single
Payload Header.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSLH (x bits ) : padding bits| | Payload Header (x bits ) : padding bits|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: MSLHSection structure in Single-SL mode Figure 4: Payload Header Section structure in "Single" mode
In the Multiple-SL mode this section consist of a 2 bytes field Gentric et al. Expires March 2002 12
giving the size in bits (in network byte order) of the following RTP Payload Format for MPEG-4 Streams September 2001
block of bit-wise concatenated MSLHs.
This size field is absent in the Single-SL mode not because it is In the "Multiple" mode the Payload Header section consist of a 2
not needed (which would be a minor gain) but for compatibility with octets field giving the size in bits (in network octet order) of the
RFC 3016. following block of bit-wise concatenated PayloadHeaders.
This size field is also absent when the value would always be zero This size field is absent in the "Single" mode not because it is not
because the MSLH is always empty, which may happen when a constant needed (which would be a minor gain) but for compatibility with RFC
size in signaled using ConstantSize. 3016.
Gentric et al. Expires March 2002 10 This size field is also absent when the value would always be zero
RTP Payload Format for MPEG-4 Streams September 2001 because the Payload Header is always empty, which may happen when a
constant payload size in signaled using ConstantSize (see below).
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSLH section size in bits | MSLH | etc | | Payload Header section size in bits | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+ |
| as many bit-wise concatenated MSLHs | | as many bit-wise concatenated Payload Headers |
| as SL packets in this RTP packet | | as AU or AU fragments in this RTP packet |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : padding bits| | : padding bits|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: MSLHSection structure in Multiple-SL mode Figure 5: Payload Header Section structure in "Multiple" mode
3.4 MSLH structure 3.4 Payload Header structure
The Mapped SL Packet Header content depends on parameters (as The Payload Header content depends on parameters (as described in
described in section 4.1); by default it is empty for the Single-SL section 4.1); by default it is empty for the "Single" mode and,
mode and, except when ConstantSize is signaled, contains at least except when ConstantSize is signaled, contains at least the
the PayloadSize field in the Multiple-SL mode. PayloadSize field in the "Multiple" mode.
When all options are used the MSLH structure is given in figure 6. When all options are used the Payload Header structure and the
relationship with the related parameter is given in table 1.
+============================+ +===========================+=================================+
|PayloadSize | | Fields of MSLPH | Number of bits (parameters) |
+----------------------------+ +===========================+=================================+
|Index or IndexDelta | | PayloadSize | SizeLength |
+----------------------------+ +---------------------------+---------------------------------+
|CTSFlag | | Index | IndexLength |
+----------------------------+ +---------------------------+---------------------------------+
|CTSDelta | | IndexDelta | IndexDeltaLength |
+----------------------------+ +---------------------------+---------------------------------+
|DTSFlag | | CTSFlag | 1 If (CTSDeltaLength > 0) |
+----------------------------+ +---------------------------+---------------------------------+
|DTSDelta | | CTSDelta | CTSDeltaLength If (CTSFlag==1) |
+============================+ +---------------------------+---------------------------------+
| DTSFlag | 1 If (DTSDeltaLength > 0) |
+---------------------------+---------------------------------+
| DTSDelta | DTSDeltaLength If (DTSFlag==1) |
Figure 6: Mapped SL Packet Header (MSLH) structure Gentric et al. Expires March 2002 13
RTP Payload Format for MPEG-4 Streams September 2001
In the general case a receiver can only discover the size of a MSLH +---------------------------+---------------------------------+
by parsing it since for example the presence of CTSDelta is signaled
by the value of CTSFlag.
3.4.1 Fields of MSLH Table 1: Payload Header fields and parameters giving the sizes
PayloadSize: Indicates the size in bytes of the associated SL Packet In the general case a receiver can only discover the size of a
Payload, which can be found in the SLPPSection of the RTP packet. Payload Header by parsing it since for example the presence of
The length in bits of this field is signaled by the SizeLength CTSDelta is signaled by the value of CTSFlag.
parameter (see section 4.1).
There is an exception to that. In the case that the RTP packet 3.4.1 Fields of Payload Header
contains only one SL packet in the "Multiple SL mode", the
Gentric et al. Expires March 2002 11 PayloadSize: Indicates the size in octets of the associated Payload,
RTP Payload Format for MPEG-4 Streams September 2001 which can be found in the Payload Section of the RTP packet. The
length in bits of this field is signaled by the SizeLength parameter
(see section 4.1).
There is an exception to that. In the case that the RTP packet
contains only one AU or AU fragment in the "Multiple" mode, the
PayloadSize field SHALL contain the size of the entire corresponding PayloadSize field SHALL contain the size of the entire corresponding
Access Unit. There are two reasons, firstly the size of the fragment Access Unit. There are two reasons, firstly the size of the fragment
is not needed when there is only one fragment, secondly this is is not needed when there is only one fragment in the RTP packet,
useful in order to detect that a full Access Unit has been received secondly this is useful in order to detect if a full Access Unit has
after the loss of a packet carrying a M bit set to 1. been received after the loss of a packet carrying a M bit set to 1.
Index, IndexDelta: Encodes the packetSequenceNumber (serial number) Index, IndexDelta: encodes the serial number of the associated AU or
of the SL Packet. When making streams specifically for transport AU fragment. IndexDelta is useful for interleaving (see section
with this payload format IndexDelta is useful for interleaving (see 3.8). When transporting a SL stream, Index and IndexDelta SHALL be
section 3.8). Since a mapping of packetSequenceNumber to RTP used to encode the SL Packet Header packetSequenceNumber field.
sequence number is not possible in the Multiple-SL mode there is no
requirement for a correspondence.
Index is optional and -if present- appears for the first SL packet Index is optional and -if present- appears in the first Payload
in a RTP packet. Header of a RTP packet.
The length in bits of the Index field is defined by the IndexLength The length in bits of the Index field is defined by the IndexLength
parameter (see section 4.1). parameter (see section 4.1).
IndexDelta is optional and -if present- appears for subsequent (non- IndexDelta is optional and -if present- appears for subsequent (non-
first) SL packets in a RTP packet. first) Payload Headers of a RTP packet.
The length in bits of the IndexDelta field is defined by the The length in bits of the IndexDelta field is defined by the
IndexDeltaLength parameter (see section 4.1). IndexDeltaLength parameter (see section 4.1).
Both Index and IndexDelta MUST be incremented so that 2 different SL Both Index and IndexDelta MUST be incremented so that 2 consecutive
packets SHALL NOT have the same packetSequenceNumber. One exception AU or AU fragments SHALL be distinguishable. One exception for Index
for Index is described in 3.8.1. is described in 3.8.1.
If the parameter IndexDeltaLength is defined, non-first SL packets If the parameter IndexDeltaLength is defined, non-first AU or AU
inside a RTP packet have their packetSequenceNumber encoded as a fragments inside a RTP packet have their serial number encoded as a
difference (thus the name IndexDelta). This difference is relative difference (thus the name IndexDelta). This difference is relative
to the previous SL packet in the RTP packet according to (with to the previous AU or AU fragment in the RTP packet according to
i>=0): (with i>=0):
packetSequenceNumber(0) = Index(0) Serial number(0) = Index(0)
packetSequenceNumber(i+1) = packetSequenceNumber(i) + Serial number (i+1) = Serial number (i) + IndexDelta(i+1) + 1
IndexDelta(i+1) + 1
Gentric et al. Expires March 2002 14
RTP Payload Format for MPEG-4 Streams September 2001
If the parameter IndexDeltaLength is not defined the default value If the parameter IndexDeltaLength is not defined the default value
is zero and then the IndexDelta field is not present for non-first is zero and then the IndexDelta field is not present for non-first
SL packets. Nevertheless receivers SHALL then apply the above AU or AU fragments. Nevertheless receivers SHALL then apply the
formula with IndexDelta equal to zero. In other words by default above formula with IndexDelta equal to zero. In other words by
packetSequenceNumber is incremented by 1 for each SL packet in one default the serial number is incremented by 1 for each AU or AU
RTP packet. fragment in the RTP packet.
CTSFlag (1 bit): Indicates whether the CTSDelta field is present. A CTSFlag (1 bit): Indicates whether the CTSDelta field is present. A
value of 1 indicates that the CTSDelta field is present, a value of value of 1 indicates that the CTSDelta field is present, a value of
0 that it is not present. 0 that it is not present.
If CTSDeltaLength is not zero, CTSFlag is present in all MSLH If CTSDeltaLength is not zero, CTSFlag is present in all Payload
regardless of whether the SL packet is an Access Unit start or not. Headers regardless of whether the AU fragment is an Access Unit
start or not.
Gentric et al. Expires March 2002 12
RTP Payload Format for MPEG-4 Streams September 2001
CTSDelta (CTSDeltaLength bits): Specifies the value of the CTS as a CTSDelta (CTSDeltaLength bits): Specifies the value of the CTS as a
2-complement offset (delta) from the timestamp in the RTP header of 2-complement offset (delta) from the timestamp in the RTP header of
the RTP packet. The length in bits of each CTSDelta field is the RTP packet. The length in bits of each CTSDelta field is
specified by the CTSDeltaLength parameter (see section 4.1). specified by the CTSDeltaLength parameter (see section 4.1).
The CTSDelta field is present if CTSFlag is 1. The CTSDelta field is present if CTSFlag is 1.
For the first MSLH of each RTP packet CTSFlag is always 0, since the For the first Payload Header of each RTP packet CTSFlag is always 0,
composition time stamp of the first SL packet in the RTP packet is since the composition time stamp of the first AU or AU fragment in
mapped to the RTP time stamp. In all cases the sender MUST remove the RTP packet is mapped to the RTP time stamp. When using the Sync
the compositionTimeStamp from the RSLH. Layer the sender MUST remove the compositionTimeStamp from the RSLH.
Senders MUST NOT assemble RTP packets for which CTSDelta rolls over Senders MUST finish assembling a RTP packet for which CTSDelta would
inside the RTP packet. roll over since this would prevent the receiver from reconstructing
the correct CTS. This can result in sub optimal RTP packets (smaller
than the MTU) depending on the MTU, the AU or AU fragment sizes and
CTSDeltaLength.
DTSFlag (1 bit): Indicates whether the DTSDelta field is present. A DTSFlag (1 bit): Indicates whether the DTSDelta field is present. A
value of 1 indicates that DTSDelta is present, a value of 0 that it value of 1 indicates that DTSDelta is present, a value of 0 that it
is not present. is not present.
If DTSDeltaLength is not zero, DTSFlag is present in all MSLH If DTSDeltaLength is not zero, DTSFlag is present in all Payload
regardless of whether the SL packet is an Access Unit start or not; Headers regardless of whether the AU fragment is an Access Unit
the receiver needs this flag in order to reconstruct the start or not. When transporting SL streams the receiver needs this
decodingTimeStampFlag of SL Headers. flag in order to reconstruct the decodingTimeStampFlag of SL Packet
Headers.
DTSDelta (DTSDeltaLength bits): encodes (compositionTimeStamp - DTSDelta (DTSDeltaLength bits): encodes (compositionTimeStamp -
decodingTimeStamp) for the same SL packet (always positive). The decodingTimeStamp) for the same AU or AU fragment(always positive).
length in bits of each DTSDelta field is specified by the The length in bits of each DTSDelta field is specified by the
DTSDeltaLength parameter (see section 4.1). DTSDeltaLength parameter (see section 4.1).
Senders MUST NOT assemble RTP packets for which the difference Senders MUST make sure that DTSDeltaLength is large enough to encode
between compositionTimeStamp and decodingTimeStamp cannot be the difference between CTS and DTS (otherwise the DTS computed by
expressed on DTSDeltaLength bits. the receiver would be incorrect).
Gentric et al. Expires March 2002 15
RTP Payload Format for MPEG-4 Streams September 2001
The DTSDelta field appears when DTSFlag is 1. The sender MUST always The DTSDelta field appears when DTSFlag is 1. The sender MUST always
remove the decodingTimeStamp from the RSLH. remove the decodingTimeStamp from the RSLH.
If DTSDelta is zero i.e. if decodingTimeStamp equals If DTSDelta is zero i.e. if decodingTimeStamp equals
compositionTimeStamp then DTSFlag MUST be set to 0 and no DTSDelta compositionTimeStamp then DTSFlag MUST be set to 0 and no DTSDelta
field SHALL be present. field SHALL be present.
At the sender side the computation of DTSDelta MUST be performed by
taking into account roll over. For example for a SL stream with the
following (CTS, DTS) pairs (assuming timeStampLength=3):
(4,3), (5,4), (6,5), (7,6), (0,7); DTSDelta for the last pair is
logically (1) and not (-7) which would be illegal and could cause
receivers implemented following section 5.1 to fail.
3.4.2 Relationship between sizes of MSLH fields and parameters
The relationship between a Mapped SL Packet Header and the related
parameters is as follows:
+===========================+=================================+
Gentric et al. Expires March 2002 13
RTP Payload Format for MPEG-4 Streams September 2001
| Fields of MSLPH | Number of bits (parameters) |
+===========================+=================================+
| PayloadSize | SizeLength |
+---------------------------+---------------------------------+
| Index | IndexLength |
+---------------------------+---------------------------------+
| IndexDelta | IndexDeltaLength |
+---------------------------+---------------------------------+
| CTSFlag | 1 If (CTSDeltaLength > 0) |
+---------------------------+---------------------------------+
| CTSDelta | CTSDeltaLength If (CTSFlag==1) |
+---------------------------+---------------------------------+
| DTSFlag | 1 If (DTSDeltaLength > 0) |
+---------------------------+---------------------------------+
| DTSDelta | DTSDeltaLength If (DTSFlag==1) |
+---------------------------+---------------------------------+
Table 1: Relationship between MSLH field size and parameters
3.5 RSLHSection structure 3.5 RSLHSection structure
This section consists of a field (RSLHSectionSize) giving the size This section is present only when using the Sync Layer, and then,
in bits of the following block of bit-wise concatenated RSLHs. when the rules in the previous section have left remaining fields.
If the section consumes a non-integer number of bytes, up to 7 zero This section first consists of a field (RSLHSectionSize) giving the
padding bits MUST be inserted at the end in order to achieve byte- size in bits of the following block of bit-wise concatenated RSLHs
(this size does not include padding bits).
If the section consumes a non-integer number of octets, up to 7 zero
padding bits MUST be inserted at the end in order to achieve octet-
alignment. alignment.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSLHSectionSize (RSLHSectionSizeLength bits)| RSLH (variable | | RSLHSectionSize (RSLHSectionSizeLength bits)| RSLH (variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| number of bits) | | number of bits) |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | RSLH (variable number of bits) | | | RSLH (variable number of bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at line 775 skipping to change at line 872
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSLH (variable number of bits) | | RSLH (variable number of bits) |
| +-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+
| : padding bits| | : padding bits|
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: RSLHSection structure Figure 7: RSLHSection structure
The length in bits of the RSLHSectionSize field is The length in bits of the RSLHSectionSize field is
RSLHSectionSizeLength and is specified with a default value of zero RSLHSectionSizeLength and is specified with a default value of zero
indicating that the whole RSLHSection is absent. Compatibility with indicating that the whole RSLHSection is absent. Note that for
RFC 3016 requires that the RSLHSection should be empty, including compatibility with RFC 3016 we need to be able to make the
the RSLHSectionSize field. This is the reason why there is such a RSLHSection disappear completely, including the RSLHSectionSize
field. This is the reason why there is such a variable length with a
Gentric et al. Expires March 2002 14 zero default value indicating the absence of the RSLHSectionSize
RTP Payload Format for MPEG-4 Streams September 2001 field.
variable length with a default value indicating absence of the
RSLHSectionSize field.
+=================================+===============================+ +=================================+===============================+
| Fields of RSLHSection | Number of bits | | Fields of RSLHSection | Number of bits |
+=================================+===============================+ +=================================+===============================+
| RSLHSectionSize | RSLHSectionSizeLength | | RSLHSectionSize | RSLHSectionSizeLength |
Gentric et al. Expires March 2002 16
RTP Payload Format for MPEG-4 Streams September 2001
+---------------------------------+-------------------------------+ +---------------------------------+-------------------------------+
| all bit-wise concatenated RSLHs | RSLHSectionSize | | all bit-wise concatenated RSLHs | RSLHSectionSize |
+---------------------------------+-------------------------------+ +---------------------------------+-------------------------------+
Table 2: Sizes in bits inside RSLHSection Table 2: Sizes in bits inside RSLHSection
Parsing of the bit-wise concatenated RSLHs requires MPEG-4 system Parsing of the bit-wise concatenated RSLHs requires MPEG-4 system
awareness, specifically it requires to understand the MPEG-4 awareness, specifically it requires to understand the MPEG-4
Synchronization Layer (SL) syntax and the modifications to this Sync Layer (SL) syntax and the modifications to this syntax
syntax described in the next section. described in the next section.
However thanks to the RSLHSectionSize field non-MPEG-4-system However thanks to the RSLHSectionSize field non-MPEG-4-system
receivers MAY skip this part by rounding up RSLPHSize/8 to the next receivers CAN skip this part by rounding up RSLPHSize/8 to the next
integer number of bytes. integer number of octets. This means that receivers not implementing
the Sync Layer can process streams containing Sync Layer specific
items by simply ignoring the parts they would not be able to parse.
3.6 RSLH structure 3.6 RSLH structure
RSLH is present only when using the Sync Layer, and then, when the
rules in the previous section have left remaining fields.
A Remaining SL Packet Header (RSLH) is what remains of an SL header A Remaining SL Packet Header (RSLH) is what remains of an SL header
after modifications for mapping into this payload format. after modifications for mapping into this payload format.
The following modifications of the SL packet header MUST be applied. The following modifications of the SL Packet Header MUST be applied.
The other fields of the SL packet header MUST remain unchanged but The other fields of the SL Packet Header MUST remain unchanged but
are bit-shifted to fill in the gaps left by the operations specified are bit-shifted to fill in the gaps left by the operations specified
below. below.
3.6.1 Removal of fields 3.6.1 Removal of fields
The following SL Packet Header fields -if present- are removed since The following SL Packet Header fields -if present- are removed since
they are mapped either in the RTP header or in the corresponding they are mapped either in the RTP header or in the corresponding
MSLH: Payload Header:
. compositionTimeStampFlag . compositionTimeStampFlag
. compositionTimeStamp . compositionTimeStamp
. decodingTimeStampFlag . decodingTimeStampFlag
. decodingTimeStamp . decodingTimeStamp
. packetSequenceNumber . packetSequenceNumber
. AccessUnitEndFlag (in Single-SL mode only) . AccessUnitEndFlag (in "Single" mode only)
The AccessUnitEndFlag, when present for a given stream, MUST be The AccessUnitEndFlag, when present for a given stream, MUST be
removed from every RSLH when using the Single-SL mode since it has removed from every RSLH when using the "Single" mode since it has
the same meaning as the Marker bit (and for compatibility with RFC the same meaning as the Marker bit (and for compatibility with RFC
3016). However when using the Multiple-SL mode, AccessUnitEndFlag 3016). However when using the "Multiple" mode, AccessUnitEndFlag
MUST NOT be removed since it is useful to signal individual AU ends. MUST NOT be removed since it is useful to signal individual AU ends.
3.6.2 Mapping of OCR 3.6.2 Mapping of OCR
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RTP Payload Format for MPEG-4 Streams September 2001
Furthermore if the SL Packet header contains an OCR, then this field Furthermore if the SL Packet header contains an OCR, then this field
is encoded in the RSLH as a 2-complement difference (delta) exactly is encoded in the RSLH as a 2-complement difference (delta) exactly
like a compositionTimeStamp or a decodingTimeStamp in the MSLH. The like a compositionTimeStamp or a decodingTimeStamp in the
length in bit of this difference is indicated by the OCRDeltaLength
parameter (see section 4.1).
With this payload format OCRs MUST have the same clock resolution as Gentric et al. Expires March 2002 17
RTP Payload Format for MPEG-4 Streams September 2001
PayloadHeader. The length in bit of this difference is indicated by
the OCRDeltaLength parameter (see section 4.1).
With this payload format OCRs MUST have the same clock frequency as
Time Stamps. Time Stamps.
If compositionTimeStamp is not present for a SL packet that has OCR If compositionTimeStamp is not present for a SL packet that has OCR
then the OCR SHALL be encoded as a difference to the RTP time stamp. then the OCR SHALL be encoded as a difference to the RTP time stamp.
3.6.3 Degradation Priority 3.6.3 Degradation Priority
For streams that use the optional degradationPriority field in the For streams that use the optional degradationPriority field in the
SL Packet Headers, only SL packets with the same degradation SL Packet Headers, only SL packets with the same degradation
priority SHALL be transported by one RTP packet so that components priority SHALL be transported by one RTP packet so that components
may dispatch the RTP packets according to appropriate QoS or may dispatch the RTP packets according to appropriate QoS or
protection schemes. Furthermore only the first RSLH of one RTP protection schemes. Furthermore only the first RSLH of one RTP
packet SHALL contain the degradationPriority field since it would be packet SHALL contain the degradationPriority field since it would be
otherwise redundant. otherwise redundant.
3.7 SLPPSection structure 3.7 Payload Section structure
The SLPPSection (SL Packet Payload Section) contains the The Payload Section contains the concatenated AU or AU fragment
concatenated SL Packet Payloads. By definition SL Packet Payloads Payloads. By definition AU or AU fragment Payloads are octet
are byte aligned. aligned.
For efficiency SL packets do not carry their own payload size. This For efficiency SL packets do not carry their own payload size. This
is not an issue for RTP packets that contain a single SL Packet. is not an issue for RTP packets that contain a single SL Packet.
However in the "Multiple" mode the size of each AU or AU fragment
payload MUST be available to the receiver.
However in the Multiple-SL mode the size of each SL packet payload If the AU or AU fragment payload size is constant for a stream, the
MUST be available to the receiver. size information SHOULD NOT be transported in the RTP packet.
However in that case it MUST be signaled using the ConstantSize
If the SL packet payload size is constant for a stream, the size parameter (see section 4.1).
information SHOULD NOT be transported in the RTP packet. However in
that case it MUST be signaled using the ConstantSize parameter (see
section 4.1).
If the SL packet payload size is variable then the size of each SL If the AU or AU fragment payload size is variable then the size of
packet payload MUST be indicated in the corresponding MSLH. In order each AU or AU fragment payload MUST be indicated in the
to do so the MSLH MUST contain a PayloadSize field. The number of corresponding Payload Header. In order to do so the Payload Header
bits on which this PayloadSize field is encoded MUST be indicated MUST contain a PayloadSize field. The number of bits on which this
using the SizeLength parameter (see section 4.1). PayloadSize field is encoded MUST be indicated using the SizeLength
parameter (see section 4.1).
The absence of either ConstantSize or SizeLength indicates the The absence of either ConstantSize or SizeLength indicates the
Single-SL mode i.e. that a single SL packet is transported in each "Single" mode i.e. that a single AU or AU fragment is transported in
RTP packet for that stream. each RTP packet for that stream.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLPP (variable number of bytes) | | AU or AU fragment (variable number of octets) |
| | | |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | AU or AU fragment |
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RTP Payload Format for MPEG-4 Streams September 2001 RTP Payload Format for MPEG-4 Streams September 2001
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | SLPP (variable number of bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
| | | (variable number of octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| etc | | etc |
| as many byte-wise concatenated SLPPs | | as many octet-wise concatenated AU or AU fragment |
| as SL Packets in this RTP packet | | as required to finish RTP packet |
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: SLPPSection structure Figure 8: Payload Section structure
3.8 Interleaving 3.8 Interleaving
SL Packets MAY be interleaved. Senders MAY perform interleaving. SL Packets MAY be interleaved. Senders MAY perform interleaving.
Receivers MUST support interleaving. Receivers MUST support interleaving.
The AUSequenceNumber field of the SL header MUST NOT be used for Note for Sync Layer implementers: the AUSequenceNumber field of the
interleaving since firstly it may collide with the Scene Description SL Header MUST NOT be used for interleaving since firstly it may
Carousel usage described in section 5.2 and secondly it is not collide with the Scene Description Carousel usage described in
visible to non-MPEG-4 system receivers. section 5.2 and secondly it is not visible to receivers that do not
implement the Sync Layer and would skip the RSLH section
transporting AUSequenceNumber.
When interleaving of SL packets is used it SHALL be implemented When interleaving of AU or AU fragments is used it SHALL be
using the IndexDelta fields of MSLH. Senders MUST use properly large implemented using the IndexDelta fields of the Payload Header.
values for IndexDeltaLength, as required by the interleaving Senders MUST NOT make RTP packets for which IndexDelta rolls over.
algorithm. Therefore depending on the interleaving scheme (if any), the MTU and
the AU or AU fragment sizes, senders wishing to make optimally sized
RTP packets (i.e. close to the MTU) will need to set
IndexDeltaLength to a properly large value.
Senders SHALL use non zero values of IndexDeltaLength only for Senders SHALL use non zero values of IndexDeltaLength only for
streams that MAY exhibit interleaving, so that this CAN be streams that exhibit interleaving, so that this can be interpreted
interpreted by receivers as an indication that interleaving may be by receivers as an indication that interleaving is (maybe) present.
present.
There are, based on this, two ways for a receiver to implement de- There are, based on this, two ways for a receiver to implement de-
interleaving, using either Index or timestamps. This is signaled interleaving, using either Index or timestamps. This is signaled
using mime parameters as in the following table, where TSBI and IBI using mime parameters as in the following table, where TSBI and IBI
stand respectively for Time-Stamp-Based-Interleaving (see section stand respectively for Time-Stamp-Based-Interleaving (see section
3.8.1) and Index-Based-Interleaving (see section 3.8.2). 3.8.1) and Index-Based-Interleaving (see section 3.8.2). Note that
the need for two methods arises from two facts: firstly the time
stamp based method is more economical and in basic cases (no
multiple AU fragments, CTS always defined) simpler to implement.
Secondly, unfortunately this method does not always work as
explained below.
================================================================== ==================================================================
| | IndexDeltaLength = 0 | IndexDeltaLength != 0 | | | IndexDeltaLength = 0 | IndexDeltaLength != 0 |
------------------------------------------------------------------ ------------------------------------------------------------------
| IndexLength=0 | no interleaving | TSBI | | IndexLength=0 | no interleaving | TSBI |
------------------------------------------------------------------ ------------------------------------------------------------------
| IndexLength!=0 | no interleaving, | Index=0 | Index!=0 | | IndexLength!=0 | no interleaving, | Index=0 | Index!=0 |
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RTP Payload Format for MPEG-4 Streams September 2001
| | SL.packetSeqNum |------------------------- | | SL.packetSeqNum |-------------------------
| | transport | TSBI | IBI | | | transport | TSBI | IBI |
================================================================== ==================================================================
Gentric et al. Expires March 2002 17 3.8.1 Time stamp based interleaving (TSBI)
RTP Payload Format for MPEG-4 Streams September 2001
3.8.1 Time stamp based interleaving
The conjunction of RTP time stamp, IndexDelta and CTS may allow a The conjunction of RTP time stamp, IndexDelta and CTS may allow a
receiver to un-ambiguously re-order SL packets based on their time receiver to un-ambiguously re-order AU or AU fragments based on
stamps (CTS). their time stamps (CTS).
This is possible and efficient for streams where SL packets This is possible and efficient for streams where only complete
transport complete Access Units and receivers can always compute the Access Units are transported and receivers can always compute the
CTS of each Access Unit. time stamp of each Access Unit.
In case of Access Units of constant duration (e.g. audio streams) In case of Access Units of constant duration (e.g. audio streams)
the explicit presence of CTS in MSLH is not even required. the explicit presence of CTS in the Payload Header is not even
Indeed then we have (i being the index of SL packets in one RTP required; Indeed then we have (i being the index of one AU in one
packet): RTP packet):
CTS(0) = RTP-TS CTS(0) = RTP-TS
for (i >= 1): CTS(i) = CTS(i-1) + (IndexDelta(i)+1)*AU-duration for (i >= 1): CTS(i) = CTS(i-1) + (IndexDelta(i)+1)*AU-duration
AU-duration, when constant, can be either signaled in SLConfig or be AU-duration, when constant, can be either signaled in SLConfig or be
deduced from the decoder configuration (see the config MIME deduced from the decoder configuration (see the config MIME
parameter). parameter).
Senders MUST use either IndexLength=0 or set all Index values in all Senders MUST use either IndexLength=0 or set all Index values in all
packets to zero so that receivers CAN detect this as an indication packets to zero so that receivers CAN detect this as an indication
that de-interleaving SHOULD be performed using time stamps. that de-interleaving SHOULD be performed using time stamps.
In cases where CTS is transported in MSLH senders MUST use properly When using the Sync Layer and when interleaving senders MUST use for
large values for SL.timeStampLength when interleaving (in order to SL.timeStampLength values large enough to prevent the CTS from
prevent the CTS from rolling over). Pre-existing SL streams that do rolling over more often than a packet loss burst length. Pre-
not comply with this requirement cannot be interleaved using this existing SL streams that do not comply with this requirement cannot
payload format (or by using 3.8.2) be interleaved using this payload format (or by using 3.8.2)
3.8.2 Index based interleaving 3.8.2 Index based interleaving (IBI)
If the AU duration is not constant (SLConfigDescriptor.durationFlag The timestamp-based interleaving algorithm described in 3.8.1. does
= 0) and CTS is not signaled (SLConfigDescriptor.useTimeStampsFlag= not work when a CTS cannot always be computed for all AU or AU
0) or SL packets transport AU fragments, then the timestamp-based fragments (for example after a packet loss); this happens:
interleaving algorithm described in 3.8.1. would not work because a . If the AU duration is not constant (SL durationFlag = 0) and CTS
CTS cannot always be computed for all SL packets (for example after is not signaled (SL useTimeStampsFlag= 0).
a packet loss). . When interleaving AU fragments.
When interleaving, senders of such streams MUST use the index-based When interleaving, senders of such streams MUST use the index-based
technique described in this section. technique described in this section.
The conjunction of RTP sequence number, Index and IndexDelta can The conjunction of RTP sequence number, Index and IndexDelta can
produce a quasi-unique identifier for each SL packet so that a produce a quasi-unique identifier for each AU or AU fragment so that
receiver can unambiguously reconstruct the original order even in a receiver can unambiguously reconstruct the original order even in
case of out-of-order packets, packet loss or duplication (see the case of out-of-order packets, packet loss or duplication (see the
pseudo code in 3.4.1 and 5.1). pseudo code in 3.4.1 and 5.1).
This requires, however, that IndexLength is not too small. For that Gentric et al. Expires March 2002 20
reason senders MUST use properly large values for IndexLength when
interleaving in this fashion. Pre-existing SL streams that do not
comply with this requirement (specifically if SL.packetSeqNumLength
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RTP Payload Format for MPEG-4 Streams September 2001 RTP Payload Format for MPEG-4 Streams September 2001
is too small) cannot be interleaved using this payload format (or by This requires, however, that IndexLength is not too small. For that
using 3.8.1). reason senders when interleaving in this fashion MUST use for
IndexLength values large enough to prevent Index from rolling over
more often than a typical loss burst loss. Pre-existing SL streams
that do not comply with this requirement (specifically if
SL.packetSeqNumLength is too small) cannot be interleaved using this
payload format (or by using 3.8.1).
Receivers CAN interpret non-zero values in the Index field as an Receivers CAN interpret non-zero values in the Index field as an
indication that de-interleaving CAN be performed using Index and indication that de-interleaving CAN be performed using Index and
IndexDelta and CANNOT be performed using timestamps. IndexDelta and CANNOT be performed using timestamps.
3.8.3 SL streams that cannot be interleaved 3.8.3 SL streams that cannot be interleaved
SL streams for which both SL.timeStampLength and SL streams for which both SL.timeStampLength and
SL.packetSeqNumLength are too small cannot be interleaved with this SL.packetSeqNumLength are too small cannot be interleaved with this
payload format. payload format. Typically small values would cause a receiver to
drop a large part of the stream in case of packet loss. The actual
minimal value depends on network loss properties and on the expected
quality of service.
3.9 Fragmentation Rules 3.9 Fragmentation Rules
This section specifies rules for senders in order to prevent media This section specifies rules for senders in order to prevent media
decoding difficulties at the receiver end. decoding difficulties at the receiver end.
MPEG-4 Access Units are the default fragments for MPEG-4 bitstreams MPEG-4 Access Units are the default fragments for MPEG-4 bitstreams
and SHOULD be mapped directly into RTP packets of this format with and SHOULD be mapped directly into RTP packets of this format with
two exceptions: two exceptions:
- Access Units larger than the MTU - Access Units larger than the MTU
- When using interleaving for better packet loss resilience. - When using interleaving for better packet loss resilience.
In all cases Access Unit start MUST be aligned with SL packet start.
This section gives rules to apply when performing Access Unit This section gives rules to apply when performing Access Unit
fragmentation. fragmentation. Let us first explain the context before describing
the rules.
Some MPEG-4 codecs define optional syntax for Access Units sub- Some MPEG-4 codecs define optional syntax for Access Units sub-
entities (fragments) that are independently decodable for error entities (fragments) that are independently decodable for error
resilience purposes. Examples are Video Packets for video and Error resilience purposes. Examples are Video Packets for video and Error
Sensitivity Categories (ESC) for audio. This always corresponds to Sensitivity Categories (ESC) for audio. This always corresponds to
specific bitstream syntax, which is signaled in the specific bitstream syntax, which is signaled in the
DecoderSpecificInfo inside the DecoderConfig in SLConfig, and/or DecoderSpecificInfo inside the DecoderConfig in SLConfig, and/or
using the corresponding parameters as described in section 4.1. using the corresponding parameters as described in section 4.1.
Therefore encoders and decoders are both aware whether they are Thanks to that decoders are aware whether encoders are operating in
operating in such a mode or not (however since this codec such a mode or not (however since this codec configuration is an
configuration is an opaque data block this is not explicitly opaque data block this is not explicitly signaled by this payload
signaled by this payload format). format).
If not operating in such a mode it is obvious that the decoder has If not operating in such a mode it is obvious that the decoder has
to skip packets after a loss until an Access Unit start is received. to skip packets after a loss until an Access Unit start is received.
Similarly decoder implementations that do not implement robust Similarly decoder implementations that do not implement robust
decoding of Access Units fragments have to discard all packets after decoding of Access Units fragments have to discard all packets after
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RTP Payload Format for MPEG-4 Streams September 2001
a packet loss until an Access Unit start is received. In the same a packet loss until an Access Unit start is received. In the same
way decoder implementations that do not implement re-synchronization way decoder implementations that do not implement re-synchronization
at any Access Units start have to discard all packets after a packet at any Access Units start have to discard all packets after a packet
loss until a Random Access Point Access Unit is received. These are loss until a Random Access Point Access Unit is received. These are
all obvious things that a good implementation would do. all obvious things that a good implementation would do.
However serious problems would arise for decoder implementations However serious problems would arise for decoder implementations
that try to restart decoding after a packet loss if independently that try to restart decoding after a packet loss if independently
Gentric et al. Expires March 2002 19
RTP Payload Format for MPEG-4 Streams September 2001
decodable fragments are signaled (in the decoder configuration) but decodable fragments are signaled (in the decoder configuration) but
the fragments actually received are not independently decodable the fragments actually received are not independently decodable
because the RTP sender has made RTP packets on different boundaries because the RTP sender has made RTP packets on different boundaries
than the fragments provided by the encoder (so this issue applies to than the fragments provided by the encoder (so this issue applies to
the interface between the encoder and the RTP sender and to the RTP the interface between the encoder and the RTP sender and to the RTP
sender component itself), because the decoder has in general no way sender component itself), because the decoder has in general no way
to detect such a faulty fragment. to detect such a faulty fragment.
For this reason the following rules must apply to SL streams that For this reason the following rules must be applied:
are specifically made for transport with this payload format:
SL packets SHOULD be codec-semantic entities in the spirit of ALF In the spirit of ALF this payload format should transport either
i.e. either complete Access Units or fragments of Access Units that complete Access Units or fragments of Access Units that are
are independently decodable. Specifically when a given codec has an independently decodable. Specifically when a given codec has an
independently decodable Access Unit fragments optional syntax this independently decodable Access Unit fragments optional syntax this
option SHOULD be used. option SHOULD be used.
Furthermore when streams are generated using independently decodable Independently decodable Access Units fragments MUST NOT be split
Access Units fragments these Access Units fragments MUST be mapped across several RTP packets.
one-to-one into SL packets. Consequently independently decodable
Access Units fragments MUST NOT be split across several SL packets
and therefore MUST NOT be split across several RTP packets.
For example an MPEG-4 audio stream encoded using the ESC syntax MUST For example an MPEG-4 audio stream encoded using the ESC syntax MUST
NOT split one ESC across 2 RTP packets. NOT split one ESC across 2 RTP packets.
This rule is relaxed when using MPEG-4 Video Packets for two This rule is relaxed when using MPEG-4 Video Packets for two
reasons: firstly Video Packets can be much larger than typical MTU reasons: firstly Video Packets can be much larger than typical MTU
and secondly all Video Packets start with a specific and secondly all Video Packets start with a specific
resynchronization marker that can be unambiguously detected. resynchronization marker that can be unambiguously detected.
Therefore for video streams using the Video Packet syntax Video Therefore for video streams using the Video Packet syntax Video
Packets MAY be split across several SL packets although it is Packets MAY be split across several SL packets although it is
strongly RECOMMENDED to always adapt the Video Packet size to fit strongly RECOMMENDED to always adapt the Video Packet size to fit
the MTU. A Video Packet start MUST always be aligned with a SL the MTU. However a Video Packet start MUST always be aligned with an
packet start, except when a GOV is present, in which case the GOV AU fragment start, except when a GOV is present, in which case the
and the first Video Packet of the following VOP MUST be included in GOV and the first Video Packet of the following VOP MUST be included
the same SL packet. in the same SL packet.
4. Types and Names 4. Types and Names
This section describes the MIME types and names associated with this This section describes the MIME types and names associated with this
payload format. Section 4.1 is intended for registration with IANA payload format. Section 4.1 registers the MIME types, as per RFC
as in RFC 2048. 2048.
This format may require additional information about the mapping to This format may require additional information about the mapping to
be made available to the receiver. This is done using parameters be made available to the receiver. This is done using parameters
described in the next section. The absence of any of these fields is described in the next section. The absence of any of these fields is
equivalent to a field set to the default value, which is always equivalent to a field set to the default value, which is always
Gentric et al. Expires March 2002 22
RTP Payload Format for MPEG-4 Streams September 2001
zero. The absence of any such parameters resolves into a default zero. The absence of any such parameters resolves into a default
"basic" configuration compatible with RFC3016 for MPEG-4 video. "basic" configuration compatible with RFC3016 for MPEG-4 video.
In the MPEG-4 framework the SL stream configuration information is In the MPEG-4 framework the SL stream configuration information is
carried using the Object Descriptor. For compatibility with carried using the Object Descriptor. For compatibility with
Gentric et al. Expires March 2002 20
RTP Payload Format for MPEG-4 Streams September 2001
receivers that do not implement the full MPEG-4 system specification receivers that do not implement the full MPEG-4 system specification
this information MAY also be signaled using parameters described this information MAY also be signaled using parameters described
here. When such information is present both in an Object Descriptor here. When such information is present both in an Object Descriptor
and as a parameter of this payload format it MUST be exactly the and as a parameter of this payload format it MUST be exactly the
same. same.
For transport of MPEG-4 audio and video without the use of MPEG-4 For transport of MPEG-4 audio and video without the use of MPEG-4
systems, as well as to support non-MPEG-4 system receivers, it is systems, as well as to support non-MPEG-4 system receivers, it is
also possible to transport information on the profile and level of also possible to transport information on the profile and level of
the stream and on the decoder configuration. This is also described the stream and on the decoder configuration. This is also described
skipping to change at line 1158 skipping to change at line 1269
(ISO/IEC14496-1) that serve other purposes than audio/visual (ISO/IEC14496-1) that serve other purposes than audio/visual
presentation, e.g. in some cases when MPEG-J streams are presentation, e.g. in some cases when MPEG-J streams are
transmitted. transmitted.
MIME subtype name: mpeg4-generic MIME subtype name: mpeg4-generic
Required parameters: none Required parameters: none
Optional parameters: Optional parameters:
Mode: mode:
The mode in which this specification is used. This specification The mode in which this specification is used. This specification
itself defines only the default mode (Mode=default). When the mode itself defines only the default mode (Mode=default). When the mode
parameter is not present the default mode SHALL be assumed. In the parameter is not present the default mode SHALL be assumed. In the
default mode all parameters are optional and as defined here. Other default mode all parameters are optional and as defined here. Other
modes may be defined as needed in other RFCs. A mode MUST be a modes may be defined as needed in other RFCs. A mode MUST be a
subset of this specification. Specifically when defining a mode care subset of this specification. Specifically when defining a mode care
MUST be taken that an implementation of this specification can MUST be taken that an implementation of this specification can
Gentric et al. Expires March 2002 23
RTP Payload Format for MPEG-4 Streams September 2001
decode the payload format corresponding to this new mode. For this decode the payload format corresponding to this new mode. For this
reason a mode MUST NOT specify new default values for MIME reason a mode MUST NOT specify new default values for MIME
parameters and MIME parameters MUST be present (unless they have the parameters and MIME parameters MUST be present (unless they have the
default value) even if it is redundant in case the mode assigns default value) even if it is redundant in case the mode assigns
fixed values. A mode may define additionally that some MIME fixed values. A mode may define additionally that some MIME
Gentric et al. Expires March 2002 21
RTP Payload Format for MPEG-4 Streams September 2001
parameters are required instead of optional, that some MIME parameters are required instead of optional, that some MIME
parameters have fixed values (or ranges), and that there are rules parameters have fixed values (or ranges), and that there are rules
restricting the usage (for example forbidding the carriage of restricting the usage (for example forbidding the carriage of
multiple AU fragments in the same RTP packet). multiple AU fragments in the same RTP packet).
Profile: profile:
The meaning of this parameter may be defined by a mode. This is The meaning of this parameter may be defined by a mode. This is
meant to be used in order to define sub-configurations of a given meant to be used in order to define sub-configurations of a given
mode, for example the maximum delay (and therefore the size of mode, for example the maximum delay (and therefore the size of
buffers) induced by the usage of interleaving. Implementations of buffers) induced by the usage of interleaving. Implementations of
this specification can ignore this parameter. this specification can ignore this parameter.
DTSDeltaLength: DTSDeltaLength:
The number of bits on which the DTSDelta field is encoded in MSLH. The number of bits on which the DTSDelta field is encoded in each
The default value is zero and indicates the absence of DTSFlag and Payload Header. The default value is zero and indicates the absence
DTSDelta in MSLH (the stream does not transport decodingTimeStamps). of DTSFlag and DTSDelta in the Payload Header (the stream does not
A value larger than zero indicates that there is a DTSFlag in each transport decodingTimeStamps). A value larger than zero indicates
MSLH. Since decodingTimeStamp, if present, must be encoded as a that there is a DTSFlag in each Payload Header. Since
difference to the RTP time stamp, the DTSDeltaLength parameter MUST decodingTimeStamp, if present, must be encoded as a difference to
be present in order to transport decodingTimeStamps with this the RTP time stamp, the DTSDeltaLength parameter MUST be present in
payload format. order to transport decodingTimeStamps with this payload format.
CTSDeltaLength: CTSDeltaLength:
The number of bits on which the CTSDelta field is encoded in (non- The number of bits on which the CTSDelta field is encoded. The
first) MSLH. The default value is zero and indicates the absence of default value is zero and indicates the absence of the CTSFlag and
the CTSFlag and CTSDelta fields in MSLH. Non-zero values MUST NOT be CTSDelta fields in Payload Header. Non-zero values MUST NOT be
signaled in the Single-SL mode. Since compositionTimeStamps, if signaled in the "Single" mode. Since compositionTimeStamps, if
present, must be encoded as a difference to the RTP time stamp, the present, must be encoded as a difference to the RTP time stamp, the
CTSDeltaLength parameter MUST be present in order to transport CTSDeltaLength parameter MUST be present in order to transport
compositionTimeStamps using this payload format (in the Multiple-SL compositionTimeStamps using this payload format (in the "Multiple"
mode). However CTSDeltaLength SHOULD be set to zero (or not mode). However CTSDeltaLength SHOULD be set to zero (or not
signaled) for streams that have a constant Access Unit duration signaled) for streams that have a constant Access Unit duration
(which can be explicitly signaled using the DurationFlag and (which can be explicitly signaled using the DurationFlag and
AccessUnitDuration field of SLConfigDescriptor). AccessUnitDuration field of SLConfigDescriptor).
OCRDeltaLength: OCRDeltaLength:
The number of bits on which the OCRDelta field is encoded in RSLH. The number of bits on which the OCRDelta field is encoded in RSLH.
The default value is zero and indicates the absence of OCR for this The default value is zero and indicates the absence of OCR for this
stream. Since objectClockReference -if present- must be encoded as a stream. Since objectClockReference -if present- must be encoded as a
difference to the RTP time stamp, the OCRDeltaLength parameter MUST difference to the RTP time stamp, the OCRDeltaLength parameter MUST
be present in order to transport objectClockReferences with this be present in order to transport objectClockReferences with this
payload format. payload format.
SizeLength: SizeLength:
The number of bits on which the PayloadSize field of MSLH is The number of bits on which the PayloadSize field of a Payload
encoded. The default value is zero and indicates the Single-SL mode Header is encoded. The default value is zero and indicates the
(unless ConstantSize is present). Simultaneous presence of this "Single" mode (unless ConstantSize is present). Simultaneous
parameter and ConstantSize is illegal. Either the SizeLength or presence of this parameter and ConstantSize is illegal. Either the
ConstantSize parameter MUST be present in order to signal the
Multiple-SL mode of this payload format.
ConstantSize:
Gentric et al. Expires March 2002 22 Gentric et al. Expires March 2002 24
RTP Payload Format for MPEG-4 Streams September 2001 RTP Payload Format for MPEG-4 Streams September 2001
The constant size in bytes of each SL Packet Payload for this SizeLength or ConstantSize parameter MUST be present in order to
stream. The default value is zero and indicates variable SL Packet signal the "Multiple" mode of this payload format.
Payload size (or the Single-SL mode if SizeLength is absent).
Simultaneous presence of this parameter and SizeLength is illegal. ConstantSize:
Either the SizeLength or ConstantSize parameter MUST be present in The constant size in octets of each AU or AU fragment Payload for
order to signal the Multiple-SL mode of this payload format. When this stream. The default value is zero and indicates variable AU or
ConstantSize is present the PayloadSize of MSLH in the RTP packets AU fragment Payload size (or the "Single" mode if SizeLength is
MUST NOT be present. absent). Simultaneous presence of this parameter and SizeLength is
illegal. Either the SizeLength or ConstantSize parameter MUST be
present in order to signal the "Multiple" mode of this payload
format. When ConstantSize is present the PayloadSize field of the
Payload Header in the RTP packets MUST NOT be present.
IndexLength: IndexLength:
The number of bits on which the Index is encoded in the first MSLH. The number of bits on which the Index is encoded in the first
The default value is zero and indicates the absence of Index and Payload Header of a RTP packet. The default value is zero and
IndexDelta for all MSLHs. Since packetSequenceNumber -if present- indicates the absence of Index and IndexDelta for all Payload
must be mapped in MSLH, the IndexLength parameter MUST be present in Headers. Since SL.packetSequenceNumber -if present- must be mapped
order to transport packetSequenceNumber with this payload format. in PayloadHeader, the IndexLength parameter MUST be present in order
to transport SL.packetSequenceNumber with this payload format.
IndexDeltaLength: IndexDeltaLength:
The number of bits on which the IndexDelta are encoded in any non- The number of bits on which the IndexDelta are encoded in any non-
first MSLH. The default value is zero and indicates that first Payload Header. The default value is zero and indicates that
packetSequenceNumber MUST be incremented by one for each SL packet the serial number MUST be incremented by one for each AU or AU
in the RTP packet (see section 3.5). IndexDeltaLength parameter MUST fragment in the RTP packet (see section 3.5). IndexDeltaLength
be present when using interleaving with this payload format. parameter MUST be present when using interleaving with this payload
format.
RSLHSectionSizeLength: RSLHSectionSizeLength:
The number of bits that is used to encode the RSLHSectionSize field. The number of bits that is used to encode the RSLHSectionSize field.
The default value is zero and indicates the absence of the whole The default value is zero and indicates the absence of the whole
RSLHSection for all RTP packets of this stream. RSLHSection for all RTP packets of this stream.
SLConfigDescriptor: SLConfigDescriptor:
A base-64 encoding of the SLConfigDescriptor. This SHALL be the A base-64 encoding of the SLConfigDescriptor. This SHALL be the
original SLConfigDescriptor and it SHALL be the same as the one original SLConfigDescriptor and it SHALL be the same as the one
transported by the OD framework, if any. transported by the OD framework, if any.
skipping to change at line 1278 skipping to change at line 1391
ISO/IEC 14496-1 [1]. For video this parameter indicates which MPEG-4 ISO/IEC 14496-1 [1]. For video this parameter indicates which MPEG-4
Visual tool subsets are applied to encode the video stream and is Visual tool subsets are applied to encode the video stream and is
defined in Table G-1 of ISO/IEC 14496-2 [2]. This parameter MAY be defined in Table G-1 of ISO/IEC 14496-2 [2]. This parameter MAY be
used in the capability exchange or session setup procedure to used in the capability exchange or session setup procedure to
indicate MPEG-4 Profile and Level combination of which the relevant indicate MPEG-4 Profile and Level combination of which the relevant
MPEG-4 media codec is capable. If this parameter is not specified MPEG-4 media codec is capable. If this parameter is not specified
its default value is 1 (Simple Profile/Level 1) for video (for its default value is 1 (Simple Profile/Level 1) for video (for
compatibility with RFC 3016) and otherwise 0xFE (defined in ISO/IEC compatibility with RFC 3016) and otherwise 0xFE (defined in ISO/IEC
14496-1 [1] as being the generic default value). 14496-1 [1] as being the generic default value).
Config: Gentric et al. Expires March 2002 25
RTP Payload Format for MPEG-4 Streams September 2001
config:
A hexadecimal representation of an octet string that expresses the A hexadecimal representation of an octet string that expresses the
media payload configuration. Configuration data is mapped onto the media payload configuration. Configuration data is mapped onto the
octet string in an MSB-first basis. The first bit of the octet string in an MSB-first basis. The first bit of the
configuration data SHALL be located at the MSB of the first octet. configuration data SHALL be located at the MSB of the first octet.
In the last octet, zero-valued padding bits, if necessary, shall In the last octet, zero-valued padding bits, if necessary, shall
Gentric et al. Expires March 2002 23
RTP Payload Format for MPEG-4 Streams September 2001
follow the configuration data. For audio streams, config is the follow the configuration data. For audio streams, config is the
audio object type specific decoder configuration data audio object type specific decoder configuration data
AudioSpecificConfig() as defined in ISO/IEC 14496-3 [3]. For video AudioSpecificConfig() as defined in ISO/IEC 14496-3 [3]. For video
this expresses the MPEG-4 Visual configuration information, as this expresses the MPEG-4 Visual configuration information, as
defined in subclause 6.2.1 Start codes of ISO/IEC14496-2 [2] and the defined in subclause 6.2.1 Start codes of ISO/IEC14496-2 [2] and the
configuration information indicated by this parameter SHALL be the configuration information indicated by this parameter SHALL be the
same as the configuration information in the corresponding MPEG-4 same as the configuration information in the corresponding MPEG-4
Visual stream, except for first-half-vbv-occupancy and latter-half- Visual stream, except for first-half-vbv-occupancy and latter-half-
vbv-occupancy, if it exists, which may vary in the repeated vbv-occupancy, if it exists, which may vary in the repeated
configuration information inside an MPEG-4 Visual stream (See 6.2.1 configuration information inside an MPEG-4 Visual stream (See 6.2.1
skipping to change at line 1310 skipping to change at line 1422
StreamType: StreamType:
The integer value that indicates the type of MPEG-4 stream that is The integer value that indicates the type of MPEG-4 stream that is
carried; its coding corresponds to the values of the streamType as carried; its coding corresponds to the values of the streamType as
defined for the DecoderConfigDescriptor in ISO/IEC 14496-1. defined for the DecoderConfigDescriptor in ISO/IEC 14496-1.
Encoding considerations: Encoding considerations:
System bitstreams MUST be generated according to MPEG-4 System System bitstreams MUST be generated according to MPEG-4 System
specifications (ISO/IEC 14496-1). Video bitstreams MUST be generated specifications (ISO/IEC 14496-1). Video bitstreams MUST be generated
according to MPEG-4 Visual specifications (ISO/IEC 14496-2). Audio according to MPEG-4 Visual specifications (ISO/IEC 14496-2). Audio
bitstreams MUST be generated according to MPEG-4 Audio bitstreams MUST be generated according to MPEG-4 Audio
specifications (ISO/IEC 14496-3). All SL streams MUST be generated specifications (ISO/IEC 14496-3). If the Sync Layer is used SL
according to MPEG-4 Sync Layer specifications (ISO/IEC 14496-1 streams MUST be generated according to MPEG-4 Sync Layer
section 10), in order to read this format the SLConfigDescriptor may specifications (ISO/IEC 14496-1 section 10), then in order to read
be required. These bitstreams are binary data and MUST be encoded the RSLH parts of this format the SLConfigDescriptor is required.
for non-binary transport (for Email, the Base64 encoding is These bitstreams are binary data and MUST be encoded for non-binary
sufficient). This type is also defined for transfer via RTP. The transport (for Email, the Base64 encoding is sufficient). This type
RTP packets MUST be packetized according to the RTP payload format is also defined for transfer via RTP. The RTP packets MUST be
defined in RFC <self-reference-to-this>. packetized according to the RTP payload format defined in RFC <self-
reference-to-this>.
Security considerations: Security considerations:
As in RFC <self-reference-to-this>. As in RFC <self-reference-to-this>.
Interoperability considerations: Interoperability considerations:
MPEG-4 provides a large and rich set of tools for the coding of MPEG-4 provides a large and rich set of tools for the coding of
visual objects. For effective implementation of the standard, visual objects. For effective implementation of the standard,
subsets of the MPEG-4 tool sets have been provided for use in subsets of the MPEG-4 tool sets have been provided for use in
specific applications. These subsets, called 'Profiles', limit the specific applications. These subsets, called 'Profiles', limit the
size of the tool set a decoder is required to implement. In order to size of the tool set a decoder is required to implement. In order to
restrict computational complexity, one or more 'Levels' are set for restrict computational complexity, one or more 'Levels' are set for
each Profile. A Profile@Level combination allows: each Profile. A Profile@Level combination allows:
. a codec builder to implement only the subset of the standard he . a codec builder to implement only the subset of the standard he
needs, while maintaining interoperability with other MPEG-4 devices needs, while maintaining interoperability with other MPEG-4 devices
included in the same combination, and included in the same combination, and
Gentric et al. Expires March 2002 26
RTP Payload Format for MPEG-4 Streams September 2001
. checking whether MPEG-4 devices comply with the standard . checking whether MPEG-4 devices comply with the standard
('conformance testing'). ('conformance testing').
A stream SHALL be compliant with the MPEG-4 Profile@Level specified A stream SHALL be compliant with the MPEG-4 Profile@Level specified
by the parameter "profile-level-id". Interoperability between a by the parameter "profile-level-id". Interoperability between a
sender and a receiver may be achieved by specifying the parameter sender and a receiver may be achieved by specifying the parameter
"profile-level-id" in MIME content, or by arranging in the "profile-level-id" in MIME content, or by arranging in the
capability exchange/announcement procedure to set this parameter capability exchange/announcement procedure to set this parameter
mutually to the same value. mutually to the same value.
Gentric et al. Expires March 2002 24
RTP Payload Format for MPEG-4 Streams September 2001
Published specification: Published specification:
The specifications for MPEG-4 streams are presented in ISO/IEC The specifications for MPEG-4 streams are presented in ISO/IEC
14469-1, 14469-2, and 14469-3. The RTP payload format is described 14469-1, 14469-2, and 14469-3. The RTP payload format is described
in RFC <self-reference-to-this>. in RFC <self-reference-to-this>.
Applications that use this media type: Applications that use this media type:
Multimedia streaming and conferencing tools, Internet messaging and Multimedia streaming and conferencing tools, Internet messaging and
Email applications. Email applications.
Additional information: none Additional information: none
skipping to change at line 1390 skipping to change at line 1504
4.3.1 The a=fmtp keyword 4.3.1 The a=fmtp keyword
It is assumed that one typical way to transport the above-described It is assumed that one typical way to transport the above-described
parameters associated with this payload format is via an SDP [10] parameters associated with this payload format is via an SDP [10]
message for example transported to the client in reply to a RTSP message for example transported to the client in reply to a RTSP
[13] DESCRIBE message or via SAP [14]. In that case the (a=fmtp) [13] DESCRIBE message or via SAP [14]. In that case the (a=fmtp)
keyword MUST be used as described in RFC 2327 [10, section 6]. The keyword MUST be used as described in RFC 2327 [10, section 6]. The
syntax being then: syntax being then:
Gentric et al. Expires March 2002 27
RTP Payload Format for MPEG-4 Streams September 2001
a=fmtp:<format> <parameter name>=<value> a=fmtp:<format> <parameter name>=<value>
4.3.2 SDP example 4.3.2 SDP example
The following is an example of SDP syntax for the description of a The following is an example of SDP syntax for the description of a
session containing one MPEG-4 video, one MPEG-4 audio stream and session containing one MPEG-4 video, one MPEG-4 audio stream and
three MPEG-4 system streams, the first one being BIFS, the second three MPEG-4 system streams, the first one being BIFS, the second
Gentric et al. Expires March 2002 25
RTP Payload Format for MPEG-4 Streams September 2001
one OD and the third one IPMP. All are transported using this format one OD and the third one IPMP. All are transported using this format
and the AVP profile [12]. Note the usage of some MIME parameters: and the AVP profile [12]. Note the usage of some MIME parameters:
all stream display their streamtype; the video stream uses DTS with all stream display their streamtype; the video stream uses DTS with
DTSDelta encoded on 4 bits; the audio stream uses the multiple-SL DTSDelta encoded on 4 bits; the audio stream uses the "Multiple"
mode with 12 bits to describe the size of each SL packet payload. mode with 12 bits to describe the size of each AU or AU fragment
See the Appendix for more examples. payload. See the Appendix for more examples.
o= .... o= ....
I= .... I= ....
c=IN IP4 123.234.71.112 c=IN IP4 123.234.71.112
m=video 1034 RTP/AVP 97 m=video 1034 RTP/AVP 97
a=fmtp:97 StreamType=4;DTSDeltaLength=4 a=fmtp:97 StreamType=4;DTSDeltaLength=4
a=rtpmap:97 mpeg4-generic a=rtpmap:97 mpeg4-generic
m=audio 1810 RTP/AVP 98 m=audio 1810 RTP/AVP 98
a=fmtp:98 StreamType=5; SizeLength=12; profile-level-id=1; a=fmtp:98 StreamType=5; SizeLength=12; profile-level-id=1;
config=7866E7E6EF config=7866E7E6EF
a=rtpmap:98 mpeg4-generic a=rtpmap:98 mpeg4-generic
m=application 1234 RTP/AVP 99 m=application 1234 RTP/AVP 99
a=rtpmap:99 mpeg4-generic a=rtpmap:99 mpeg4-generic
a=fmtp:99 StreamType=3; a=fmtp:99 StreamType=3
m=application 1236 RTP/AVP 99 m=application 1236 RTP/AVP 99
a=rtpmap:99 mpeg4-generic a=rtpmap:99 mpeg4-generic
a=fmtp:99 StreamType=1; a=fmtp:99 StreamType=1
m=application 1238 RTP/AVP 99 m=application 1238 RTP/AVP 99
a=rtpmap:99 mpeg4-generic a=rtpmap:99 mpeg4-generic
a=fmtp:99 StreamType=7; a=fmtp:99 StreamType=7
5. Other issues 5. Other issues
5.1 SL packetized stream reconstruction 5.1 SL packetized stream reconstruction
The purpose of this section is to document how a receiver can The purpose of this section is to document how a receiver can
reconstruct a valid SL packetized stream. Since this format directly reconstruct a valid SL packetized stream. Since this format directly
transports SL packets this reconstruction is performed by reversing transports SL packets this reconstruction is performed by reversing
the payload structure rules (section 3). We explicitly describe here the payload structure rules (section 3). We explicitly describe here
the most complex transformations. the most complex transformations.
In the following let (i) be the index of SL packets inside one RTP In the following let (i) be the index of SL packets inside one RTP
packet (starting at zero for each RTP packet), let SLPacketHeader.x packet (starting at zero for each RTP packet), let SLPacketHeader.x
denote field x of the reconstructed SL packet header, let MSLH.x
denote field x of the received MSLH, etc.
SLPacketHeader.packetSequenceNumber is restored from MSLH.Index and Gentric et al. Expires March 2002 28
MSLH.IndexDelta using: RTP Payload Format for MPEG-4 Streams September 2001
denote field x of the reconstructed SL packet header, let
PayloadHeader.x denote field x of the received PayloadHeader, etc.
SLPacketHeader.packetSequenceNumber is restored from
PayloadHeader.Index and PayloadHeader.IndexDelta using:
If ( IndexLength == 0) { // or is absent If ( IndexLength == 0) { // or is absent
if ( SLConfig.packetSeqNumLength == 0 ) { if ( SLConfig.packetSeqNumLength == 0 ) {
// this stream does not have SL packet sequence number // this stream does not have SL packet sequence number
Gentric et al. Expires March 2002 26
RTP Payload Format for MPEG-4 Streams September 2001
} }
else { else {
// illegal, normally the sender MUST map // illegal, normally the sender MUST map
// SLPacketHeader.packetSequenceNumber in MSLH // SLPacketHeader.packetSequenceNumber in PayloadHeader
// and set a relevant IndexLength value; // and set a relevant IndexLength value;
// otherwise it is unfortunately impossible for the receiver // otherwise it is unfortunately impossible for the receiver
// to reconstruct the correct sequence // to reconstruct the correct sequence
} }
} }
else { // IndexLength is not zero else { // IndexLength is not zero
if ( SLConfig.packetSeqNumLength == 0 ) { if ( SLConfig.packetSeqNumLength == 0 ) {
// the original SL stream does not have SL packet // the original SL stream does not have SL packet
// sequence numbers, typically the sender inserted them // sequence numbers, typically the sender inserted them
// in order to implement interleaving at the RTP level; // in order to implement interleaving at the RTP level;
// they must be ignored for SL stream reconstruction // they must be ignored for SL stream reconstruction
} }
else { else {
if (i == 0){ // first SL packet in RTP packet if (i == 0){ // first SL packet in RTP packet
SLPacketHeader.packetSequenceNumber(0) = MSLH.Index(0); SLPacketHeader.packetSequenceNumber(0) =
PayloadHeader.Index(0);
} }
else { // remaining SL packets else { // remaining SL packets
SLPacketHeader.packetSequenceNumber(i+1)= SLPacketHeader.packetSequenceNumber(i+1)=
SLPacketHeader.packetSequenceNumber(i) SLPacketHeader.packetSequenceNumber(i)
+ MSLH.IndexDelta(i+1) + PayloadHeader.IndexDelta(i+1)
+1; +1;
} }
} }
All time stamps (CTS, DTS, OCR), when present, are restored from the All time stamps (CTS, DTS, OCR), when present, are restored from the
delta values. Time stamps flags (CTSFlag, DTSFlag) in MSLH are used delta values. Time stamps flags (CTSFlag, DTSFlag) in PayloadHeader
to reconstruct respectively the compositionTimeStampFlag and are used to reconstruct respectively the compositionTimeStampFlag
decodingTimeStampFlag of SLPacketHeader. The function corrected(x) and decodingTimeStampFlag of SLPacketHeader. The function
for the RTP time stamp transformation is the mapping from 32 bits to corrected(x) for the RTP time stamp transformation is the mapping
SLConfig.timeStampLength, which may be smaller or larger than 32 from 32 bits to SLConfig.timeStampLength, which may be smaller or
bits: larger than 32 bits:
If (timeStampLength < 32 ) { // short SL time stamps If (timeStampLength < 32 ) { // short SL time stamps
corrected(x) = LSB(x); // only the timeStampLength LSBits of x corrected(x) = LSB(x); // only the timeStampLength LSBits of x
} }
else If (timeStampLength > 32 ) { // long SL time stamps else If (timeStampLength > 32 ) { // long SL time stamps
corrected(x) = x + m; // start with m=0 corrected(x) = x + m; // start with m=0
if ( x(i) < x(i-1) ) { // 32 bits RTPTS roll over has occurred if ( x(i) < x(i-1) ) { // 32 bits RTPTS roll over has occurred
{ {
Gentric et al. Expires March 2002 29
RTP Payload Format for MPEG-4 Streams September 2001
m += 2^32; m += 2^32;
} }
} }
else If (timeStampLength = 32 ) { // recommended value else If (timeStampLength = 32 ) { // recommended value
corrected(x) = x; // direct mapping corrected(x) = x; // direct mapping
} }
if ( CTSDeltaLength == 0) { // or CTSDeltaLength is absent if ( CTSDeltaLength == 0) { // or CTSDeltaLength is absent
// CTS is not transported for this RTP stream // CTS is not transported for this RTP stream
Gentric et al. Expires March 2002 27
RTP Payload Format for MPEG-4 Streams September 2001
if (i == 0){ // first SL packet in RTP packet if (i == 0){ // first SL packet in RTP packet
if ( SLConfig.useTimeStamps == 1 ) { if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) { if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.compositionTimeStampFlag(0) = 1; SLPacketHeader.compositionTimeStampFlag(0) = 1;
SLPacketHeader.compositionTimeStamp(0) = SLPacketHeader.compositionTimeStamp(0) =
corrected(RTP TimeStamp); corrected(RTP TimeStamp);
} }
else { else {
// ignore // ignore
} }
skipping to change at line 1548 skipping to change at line 1662
else { else {
// empty // empty
} }
} }
} }
else { // CTSDeltaLength is not zero else { // CTSDeltaLength is not zero
// CTS is transported for this stream // CTS is transported for this stream
if ( SLConfig.useTimeStamps == 1 ) { if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) { if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.compositionTimeStampFlag(i) = SLPacketHeader.compositionTimeStampFlag(i) =
MSLH.CTSFlag(i); PayloadHeader.CTSFlag(i);
SLPacketHeader.compositionTimeStamp(i) = SLPacketHeader.compositionTimeStamp(i) =
corrected(RTP TimeStamp) + MSLH.CTSDelta(i); corrected(RTP TimeStamp) +
PayloadHeader.CTSDelta(i);
} }
else { else {
// ignore CTSFlag (which must be zero) // ignore CTSFlag (which must be zero)
} }
else { else {
Gentric et al. Expires March 2002 30
RTP Payload Format for MPEG-4 Streams September 2001
// this is strange and sub-optimal at best // this is strange and sub-optimal at best
// a receiver should ignore this // a receiver should ignore this
} }
} }
if ( DTSDeltaLength == 0) { // or DTSDeltaLength is absent if ( DTSDeltaLength == 0) { // or DTSDeltaLength is absent
// DTS is not transported for this stream // DTS is not transported for this stream
if ( SLConfig.useTimeStamps == 1 ) { if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) { if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.decodingTimeStampFlag(i) = 0; SLPacketHeader.decodingTimeStampFlag(i) = 0;
} }
Gentric et al. Expires March 2002 28
RTP Payload Format for MPEG-4 Streams September 2001
else { else {
// ignore // ignore
} }
} }
else { else {
// empty // empty
} }
} }
else { else {
// DTS is transported for this stream // DTS is transported for this stream
if ( SLConfig.useTimeStamps == 1 ) { if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) { if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.decodingTimeStampFlag(i) = SLPacketHeader.decodingTimeStampFlag(i) =
MSLH.DTSFlag(i); PayloadHeader.DTSFlag(i);
SLPacketHeader.decodingTimeStamp(i)= SLPacketHeader.decodingTimeStamp(i)=
SLPacketHeader.compositionTimeStamp(i) SLPacketHeader.compositionTimeStamp(i)
- MSLH.DTSDelta(i); // DTS <= CTS always - PayloadHeader.DTSDelta(i); // DTS <= CTS always
} }
else { else {
// ignore DTSFlag (which must be zero) // ignore DTSFlag (which must be zero)
} }
} }
else { else {
// this is strange and sub-optimal at best // this is strange and sub-optimal at best
// a receiver should ignore this // a receiver should ignore this
} }
} }
skipping to change at line 1613 skipping to change at line 1728
} }
else { else {
// illegal, normally the sender MUST detect // illegal, normally the sender MUST detect
// OCRs, replace them with OCRDelta and set // OCRs, replace them with OCRDelta and set
// a relevant OCRDeltaLength value // a relevant OCRDeltaLength value
} }
} }
else { else {
if ( SLConfig.OCRLenght == 0 ) { if ( SLConfig.OCRLenght == 0 ) {
// this is strange and sub-optimal at best // this is strange and sub-optimal at best
Gentric et al. Expires March 2002 31
RTP Payload Format for MPEG-4 Streams September 2001
// a receiver should ignore this // a receiver should ignore this
} }
else { else {
SLPacketHeader.OCRflag(i) = RSLH.OCRFlag(i); SLPacketHeader.OCRflag(i) = RSLH.OCRFlag(i);
if ( SLPacketHeader.OCRflag(i) == 1) { if ( SLPacketHeader.OCRflag(i) == 1) {
SLPacketHeader.objectClockReference(i) = SLPacketHeader.objectClockReference(i) =
corrected(RTP TimeStamp) + RSLH.OCRDelta(i); corrected(RTP TimeStamp) + RSLH.OCRDelta(i);
} }
} }
} }
Gentric et al. Expires March 2002 29 In the "Single" mode the AccessUnitEndFlag, if needed, is restored
RTP Payload Format for MPEG-4 Streams September 2001
In the SingleSL mode the AccessUnitEndFlag, if needed, is restored
from the M bit, as follows: from the M bit, as follows:
if ( SLConfig.useAccessUnitEndFlag == 0 ) { if ( SLConfig.useAccessUnitEndFlag == 0 ) {
// this SL stream does not signal access unit ends // this SL stream does not signal access unit ends
else { else {
SLPacketHeader.AccessUnitEndFlag = M bit; SLPacketHeader.AccessUnitEndFlag = M bit;
} }
In the multipleSL mode the AccessUnitEndFlag is untouched in RSLH. In the "Multiple" mode the AccessUnitEndFlag is untouched in RSLH.
The other SL packet header fields SHALL remain as found in RSLH. The other SL packet header fields SHALL remain as found in RSLH.
It is obvious that in the general case the reconstruction of the It is obvious that in the general case the reconstruction of the
original SL packetized stream requires SL-awareness. However this original SL packetized stream requires SL-awareness. However this
payload format allows in all cases a receiver that does not know payload format allows in all cases a receiver that does not know
about the SL syntax to reconstruct the semantic of SL for the about the SL syntax to reconstruct the semantic of Elementary
following very useful features: Streams for the following very useful features:
- Packet order (decoding order) - Packet order (decoding order)
- Access Unit boundaries (using the M bit) - Access Unit boundaries (using the M bit)
- Access Unit fragments (i.e. SL packet boundaries using - Access Unit fragments (fragment boundaries using PayloadSize)
MSLH.PayloadSize) - Composition Time Stamps, according to:
- Composition Time Stamps (using the RTP Time Stamp and compositionTimeStamp(i) = RTP TimeStamp + CTSDelta(i);
MSLH.CTSDelta) - Decoding Time Stamps, according to:
- Decoding Time Stamps (using the RTP Time Stamp and MSLH.DTSDelta) decodingTimeStamp(i) = compositionTimeStamp(i) - DTSDelta(i);
- Packet sequence number (using the RTP Time Sequence number and - Packet serial number, according to:
MSLH.Index) if (i == 0){ // first SL packet in RTP packet
packet serial number(0) = Index(0);
}
else { // remaining SL packets
packet serial number (i+1) = packet serial number (i)
+ IndexDelta(i+1) + 1;
}
5.2 Handling of scene description streams 5.2 Handling of scene description streams
MPEG-4 introduces new stream types as described in section 1 namely MPEG-4 introduces new stream types as described in section 1 namely
Object Descriptors and BIFS. In the following both OD and BIFS are Object Descriptors and BIFS. In the following both OD and BIFS are
discussed on the same basis i.e. as "scene description". discussed on the same basis i.e. as "scene description".
Gentric et al. Expires March 2002 32
RTP Payload Format for MPEG-4 Streams September 2001
Considering scene description as a "stream-able" type of content is Considering scene description as a "stream-able" type of content is
a rather new concept and for that reasons some specific comments are a rather new concept and for that reasons some specific comments are
needed. needed.
Typically scene descriptions are encoded in such a way that Typically scene descriptions are encoded in such a way that
information loss would in the general case cripple the presentation information loss would in the general case cripple the presentation
beyond any hope of repair by the receiver. Still this is well suited beyond any hope of repair by the receiver. Still this is well suited
for a number of multimedia applications were the scene is first made for a number of multimedia applications were the scene is first made
available via reliable channels to the client and then played. This available via reliable channels to the client and then played. This
payload format is not intended for this type of applications for payload format is not intended for this type of applications for
which download of MPEG-4 interchange (.mp4) files is typical. which download of MPEG-4 interchange (.mp4) files is typical.
However this payload format can also be used. It is then RECOMMENDED However this payload format can also be used. It is then RECOMMENDED
that the RTP packets should be transported using TCP (for example that the RTP packets should be transported using TCP (for example
inside RTSP as described in [13, section 10.12]) or any other inside RTSP as described in [13, section 10.12]) or any other
reliable protocol. reliable protocol.
On the other hand MPEG-4 has introduced the possibility to On the other hand MPEG-4 has introduced the possibility to
dynamically change the scene description by sending animation dynamically change the scene description by sending animation
Gentric et al. Expires March 2002 30
RTP Payload Format for MPEG-4 Streams September 2001
information (changes in parameters) and structural change information (changes in parameters) and structural change
information (updates). Since this information has to be sent in a information (updates). Since this information has to be sent in a
timely fashion MPEG-4 has defined a number of techniques in order to timely fashion MPEG-4 has defined a number of techniques in order to
encode the scene description in a manner that makes it behave encode the scene description in a manner that makes it behave
similarly to other temporal encoding schemes such as audio and similarly to other temporal encoding schemes such as audio and
video. This payload format is intended for this usage. video. This payload format is intended for this usage.
Note that in many cases the application will consist of first the Note that in many cases the application will consist of first the
reliable transmission of a static initial scene followed by the reliable transmission of a static initial scene followed by the
streaming of animations and updates. For this reason the usage of streaming of animations and updates. For this reason the usage of
skipping to change at line 1713 skipping to change at line 1834
FEC, re-transmission, or similar tools. In such a case, the general FEC, re-transmission, or similar tools. In such a case, the general
congestion control principles have to be observed. congestion control principles have to be observed.
Since BIFS and OD streams may be modified during the session with Since BIFS and OD streams may be modified during the session with
update commands, there is a need to send both update commands and update commands, there is a need to send both update commands and
full BIFS/OD refresh. For that reason MPEG-4 defines Random Access full BIFS/OD refresh. For that reason MPEG-4 defines Random Access
Points (RAP) for scene description streams (OD and BIFS) where by Points (RAP) for scene description streams (OD and BIFS) where by
definition a decoder can restart decoding i.e. receives a "full definition a decoder can restart decoding i.e. receives a "full
update" of the scene. This mechanism is called Scene and Object update" of the scene. This mechanism is called Scene and Object
Description Carousel. The AU Sequence Number field of SL Packet Description Carousel. The AU Sequence Number field of SL Packet
Header is used to support this behavior at the Synchronization Header is used to support this behavior at the Sync Layer. When two
Layer. When two access units are sent consecutively with the same AU access units are sent consecutively with the same AU Sequence
Sequence Number, the second one is assumed to be a semantic Number, the second one is assumed to be a semantic repetition of the
repetition of the first. If a receiver starts to listen in the first. If a receiver starts to listen in the middle of a session or
middle of a session or has detected losses, it can skip all received has detected losses, it can skip all received Access Units until
Access Units until such a RAP. The periodicity of transmission of
these RAPs should be chosen/adjusted depending on the application Gentric et al. Expires March 2002 33
and the network it is deployed on; i.e. exactly like Intra-coded RTP Payload Format for MPEG-4 Streams September 2001
frames for video, it is the responsibility of the sender to make
sure the periodicity of RAPs is suitable. such a RAP. The periodicity of transmission of these RAPs should be
chosen/adjusted depending on the application and the network it is
deployed on; i.e. exactly like Intra-coded frames for video, it is
the responsibility of the sender to make sure the periodicity of
RAPs is suitable.
5.3 Multiplexing 5.3 Multiplexing
An advanced MPEG-4 session may involve a large number of objects An advanced MPEG-4 session may involve a large number of objects
that may be as many as a few hundred, transporting each ES as an that may be as many as a few hundred, transporting each ES as an
individual RTP stream may not always be practical. Allocating and individual RTP stream may not always be practical. Allocating and
controlling hundreds of destination addresses for each MPEG-4 controlling hundreds of destination addresses for each MPEG-4
session may pose insurmountable session administration problems. session may pose insurmountable session administration problems.
The input/output processing overhead at the end-points will be The input/output processing overhead at the end-points will be
extremely high also. Additionally, low delay transmission of low extremely high also. Additionally, low delay transmission of low
bitrate data streams, e.g. facial animation parameters, results in bitrate data streams, e.g. facial animation parameters, results in
extremely high header overheads. extremely high header overheads.
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RTP Payload Format for MPEG-4 Streams September 2001
To solve these problems, MPEG-4 data transport requires a To solve these problems, MPEG-4 data transport requires a
multiplexing scheme that allows selective bundling of several ESs. multiplexing scheme that allows selective bundling of several ESs.
This is beyond the scope of the payload format defined here. This is beyond the scope of the payload format defined here.
The MPEG-4's Flexmux multiplexing scheme may be used for this The MPEG-4's Flexmux multiplexing scheme may be used for this
purpose and a specific RTP payload format is being developed [11]. purpose and a specific RTP payload format is being developed [11].
Another approach may be to develop a generic RTP multiplexing scheme Another approach may be to develop a generic RTP multiplexing scheme
usable for MPEG-4 data. The multiplexing scheme reported in [8] may usable for MPEG-4 data. The multiplexing scheme reported in [8] may
be a candidate for this approach. be a candidate for this approach.
skipping to change at line 1764 skipping to change at line 1886
session. Consequently, the coding type, individual packet size and session. Consequently, the coding type, individual packet size and
temporal relationships between the multiplexed data units must be temporal relationships between the multiplexed data units must be
handled dynamically. handled dynamically.
ii. The multiplexing scheme should have a mechanism to determine the ii. The multiplexing scheme should have a mechanism to determine the
ES identifier (ES_ID) for each of the multiplexed packets. ES_ID is ES identifier (ES_ID) for each of the multiplexed packets. ES_ID is
not a part of the SL header. not a part of the SL header.
iii. In general, an SL packet does not contain information about its iii. In general, an SL packet does not contain information about its
size. The multiplexing scheme should be able to delineate the size. The multiplexing scheme should be able to delineate the
multiplexed packets whose lengths may vary from a few bytes to close multiplexed packets whose lengths may vary from a few octets to
to the path-MTU. close to the path-MTU.
5.5 Overlap with RFC 3016 5.5 Overlap with RFC 3016
This payload format has been designed to have a (large) overlap with This payload format has been designed to have a (large) overlap with
RFC 3016 [7]. The conditions for this overlap are: RFC 3016 [7]. The conditions for this overlap are:
Conditions for RFC 3016: Conditions for RFC 3016:
i. MPEG-4 video elementary streams only i. MPEG-4 video elementary streams only
Gentric et al. Expires March 2002 34
RTP Payload Format for MPEG-4 Streams September 2001
ii. There MUST be a single VOP or Video Packet per RTP packet (only ii. There MUST be a single VOP or Video Packet per RTP packet (only
recommended in RFC 3016) recommended in RFC 3016)
iii. The decoder configuration MUST be signaled out-of-band either iii. The decoder configuration MUST be signaled out-of-band either
using the Config mime parameter or using the OD framework using the Config mime parameter or using the OD framework
Conditions for this payload format: Conditions for this payload format:
i. No structural parameters defined (or all set to zero), i.e. i. No structural parameters defined (or all set to zero), i.e.
Single-SL mode with empty MSLH and empty RSLH. "Single" mode with empty Payload Header and empty RSLH.
ii. Receivers MUST be ready to accept (and ignore) video ii. Receivers MUST be ready to accept (and ignore) video
configuration headers (e.g. VOSH, VO and VOL) and visual-object- configuration headers (e.g. VOSH, VO and VOL) and visual-object-
sequence-end-code transported in-band. sequence-end-code transported in-band.
6. Security Considerations 6. Security Considerations
RTP packets using the payload format defined in this specification RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP are subject to the security considerations discussed in the RTP
specification [5]. This implies that confidentiality of the media specification [5]. This implies that confidentiality of the media
streams is achieved by encryption. Because the data compression used streams is achieved by encryption. Because the data compression used
with this payload format is applied end-to-end, encryption may be with this payload format is applied end-to-end, encryption may be
performed on the compressed data so there is no conflict between the performed on the compressed data so there is no conflict between the
Gentric et al. Expires March 2002 32
RTP Payload Format for MPEG-4 Streams September 2001
two operations. The packet processing complexity of this payload two operations. The packet processing complexity of this payload
type (i.e. excluding media data processing) does not exhibit any type (i.e. excluding media data processing) does not exhibit any
significant non-uniformity in the receiver side to cause a denial- significant non-uniformity in the receiver side to cause a denial-
of-service threat. of-service threat.
However, it is possible to inject non-compliant MPEG streams (Audio, However, it is possible to inject non-compliant MPEG streams (Audio,
Video, and Systems) to overload the receiver/decoder's buffers which Video, and Systems) to overload the receiver/decoder's buffers which
might compromise the functionality of the receiver or even crash it. might compromise the functionality of the receiver or even crash it.
This is especially true for end-to-end systems like MPEG where the This is especially true for end-to-end systems like MPEG where the
buffer models are precisely defined. buffer models are precisely defined.
skipping to change at line 1829 skipping to change at line 1951
J access units which comprises Java(TM) classes and objects. MPEG-J J access units which comprises Java(TM) classes and objects. MPEG-J
defines a set of Java APIs and a secure execution model. MPEG-J defines a set of Java APIs and a secure execution model. MPEG-J
content can call this set of APIs and Java(TM) methods from a set of content can call this set of APIs and Java(TM) methods from a set of
Java packages supported in the receiver within the defined security Java packages supported in the receiver within the defined security
model. According to this security model, downloaded byte code is model. According to this security model, downloaded byte code is
forbidden to load libraries, define native methods, start programs, forbidden to load libraries, define native methods, start programs,
read or write files, or read system properties. read or write files, or read system properties.
Receivers can implement intelligent filters to validate the buffer Receivers can implement intelligent filters to validate the buffer
requirements or parametric (OD, BIFS, etc.) or programmatic (MPEG-J, requirements or parametric (OD, BIFS, etc.) or programmatic (MPEG-J,
Gentric et al. Expires March 2002 35
RTP Payload Format for MPEG-4 Streams September 2001
ECMAScript) commands in the streams. However, this can increase the ECMAScript) commands in the streams. However, this can increase the
complexity significantly. complexity significantly.
7. Acknowledgements 7. Acknowledgements
This document evolved across several years thanks to contributions This document evolved across several years thanks to contributions
from a large number of people since it is based on work within the from a large number of people since it is based on work within the
IETF AVT working group and various ISO MPEG working groups, IETF AVT working group and various ISO MPEG working groups,
especially the 4-on-IP ad-hoc group. The authors wish to thank especially the 4-on-IP ad-hoc group. The authors wish to thank
Olivier Avaro, Stephen Casner, Guido Fransceschini, Art Howarth, Olivier Avaro, Stephen Casner, Guido Fransceschini, Art Howarth,
Dave Mackie, Dave Singer, and Stephan Wenger for their valuable Dave Mackie, Dave Singer, and Stephan Wenger for their valuable
comments and support. Attentive readers and early implementers also comments and support. Attentive readers and early implementers also
found flaws and bugs, thank you all. found flaws and bugs, thank you all.
8. References 8. References
[1] ISO/IEC 14496-1:2001 MPEG-4 Systems [1] ISO/IEC 14496-1:2001 MPEG-4 Systems
[2] ISO/IEC 14496-2:2001 MPEG-4 Visual [2] ISO/IEC 14496-2:2001 MPEG-4 Visual
Gentric et al. Expires March 2002 33
RTP Payload Format for MPEG-4 Streams September 2001
[3] ISO/IEC 14496-3:2001 MPEG-4 Audio [3] ISO/IEC 14496-3:2001 MPEG-4 Audio
[4] ISO/IEC 14496-6:2001 Delivery Multimedia Integration Framework. [4] ISO/IEC 14496-6:2001 Delivery Multimedia Integration Framework.
[5] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A [5] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A
Transport Protocol for Real Time Applications, RFC 1889, Internet Transport Protocol for Real Time Applications, RFC 1889, Internet
Engineering Task Force, January 1996. Engineering Task Force, January 1996.
[6] S. Bradner, Key words for use in RFCs to Indicate Requirement [6] S. Bradner, Key words for use in RFCs to Indicate Requirement
Levels, RFC 2119, Internet Engineering Task Force, March 1997. Levels, RFC 2119, Internet Engineering Task Force, March 1997.
skipping to change at line 1886 skipping to change at line 2009
2327, Internet Engineering Task Force, April 1998. 2327, Internet Engineering Task Force, April 1998.
[11] C.Roux & al, RTP Payload Format for MPEG-4 FlexMultiplexed [11] C.Roux & al, RTP Payload Format for MPEG-4 FlexMultiplexed
Streams, work in progress, draft-curet-avt-rtp-mpeg4-flexmux-00.txt, Streams, work in progress, draft-curet-avt-rtp-mpeg4-flexmux-00.txt,
February 2001. February 2001.
[12] H. Schulzrinne, RTP Profile for Audio and Video Conferences [12] H. Schulzrinne, RTP Profile for Audio and Video Conferences
with Minimal Control, RFC 1890, Internet Engineering Task Force, with Minimal Control, RFC 1890, Internet Engineering Task Force,
January 1996. January 1996.
Gentric et al. Expires March 2002 36
RTP Payload Format for MPEG-4 Streams September 2001
[13] H. Schulzrinne, A. Rao, R. Lanphier, Real Time Streaming [13] H. Schulzrinne, A. Rao, R. Lanphier, Real Time Streaming
Protocol, RFC 2326, Internet Engineering Task Force, April 1998. Protocol, RFC 2326, Internet Engineering Task Force, April 1998.
[14] M. Handley, C. Perkins, E. Whelan, Session Announcement [14] M. Handley, C. Perkins, E. Whelan, Session Announcement
Protocol, RFC 2974, Internet Engineering Task Force, October 2000. Protocol, RFC 2974, Internet Engineering Task Force, October 2000.
9. Authors' Addresses 9. Authors' Addresses
Andrea Basso Andrea Basso
AT&T Labs Research AT&T Labs Research
200 Laurel Avenue 200 Laurel Avenue
Middletown, NJ 07748 Middletown, NJ 07748
USA USA
e-mail: basso@research.att.com e-mail: basso@research.att.com
M. Reha Civanlar M. Reha Civanlar
AT&T Labs - Research AT&T Labs - Research
200 Laurel Ave. South, A5 4D04 200 Laurel Ave. South, A5 4D04
Gentric et al. Expires March 2002 34
RTP Payload Format for MPEG-4 Streams September 2001
Middletown, NJ 07748 Middletown, NJ 07748
USA USA
e-mail: civanlar@research.att.com e-mail: civanlar@research.att.com
Philippe Gentric Philippe Gentric
Philips Digital Networks, MP4Net Philips Digital Networks, MP4Net
51 rue Carnot 51 rue Carnot
92156 Suresnes 92156 Suresnes
France France
e-mail: philippe.gentric@philips.com e-mail: philippe.gentric@philips.com
skipping to change at line 1941 skipping to change at line 2063
e-mail: zvil@optibase.com e-mail: zvil@optibase.com
Young-kwon Lim Young-kwon Lim
mp4cast (MPEG-4 Internet Broadcasting Solution Consortium) mp4cast (MPEG-4 Internet Broadcasting Solution Consortium)
1001-1 Daechi-Dong Gangnam-Gu 1001-1 Daechi-Dong Gangnam-Gu
Seoul, 305-333, Seoul, 305-333,
Korea Korea
e-mail : young@techway.co.kr e-mail : young@techway.co.kr
Colin Perkins Colin Perkins
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RTP Payload Format for MPEG-4 Streams September 2001
USC Information Sciences Institute USC Information Sciences Institute
4350 N. Fairfax Drive #620 3811 N. Fairfax Drive suite 200
Arlington, VA 22203 Arlington, VA 22203
USA USA
e-mail : csp@isi.edu e-mail : csp@isi.edu
Jan van der Meer Jan van der Meer
Philips Digital Networks Philips Digital Networks
Building WDB-1 Building WDB-1
Prof Holstlaan 4 Prof Holstlaan 4
5656 AA Eindhoven 5656 AA Eindhoven
Netherlands Netherlands
e-mail : jan.vandermeer@philips.com e-mail : jan.vandermeer@philips.com
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RTP Payload Format for MPEG-4 Streams September 2001
APPENDIX: Examples of usage APPENDIX: Examples of usage
This payload format has been designed to transport efficiently a This section describes a number of examples of how this payload
very versatile packetization scheme: the MPEG-4 Synch Layer; as a format can be used either with or without the Sync Layer. In all
result its complexity is larger than the average RTP payload format. examples however the Sync Layer syntax is given which shows how it
For this reason this section describes a number of key examples of becomes invisible in cases 1,3,4 and 5.
how this payload format can be used.
A C++-like syntax called SDL (Syntactic Description Language) A C++-like syntax called SDL (Syntactic Description Language)
defined in [1, section 14] is used to economically describe MPEG-4 defined in [1, section 14] is used to economically describe MPEG-4
system data structures. system data structures.
However, as discussed in section 2, this payload format can also be These examples assume that the (a=fmtp) SDP syntax is used to convey
used without explicit knowledge of SL (logically equivalent to the MIME parameters of the payload format.
configuring the SL headers as being empty), several examples
(Appendix 1,3,4,5) cover this case.
Furthermore these examples assume that the (a=fmtp) SDP syntax is
used to convey the MIME parameters of the payload format.
Appendix.1 RFC 3016 compatible MPEG-4 Video (no SL) Appendix.1 RFC 3016 compatible MPEG-4 Video (no SL)
This is an example of a video stream where the SL is configured to This is an example of a video stream where the SL is configured to
produce RTP packets compatible with RFC 3016. produce RTP packets compatible with RFC 3016.
SLConfigDescriptor SLConfigDescriptor
In this example the SLConfigDescriptor is: In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8) class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag { tag=SLConfigDescrTag {
bit(8) predefined; bit(8) predefined;
if (predefined==0) { if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 0 bit(1) useAccessUnitStartFlag; = 0
bit(1) useAccessUnitEndFlag; = 1 bit(1) useAccessUnitEndFlag; = 1
bit(1) useRandomAccessPointFlag; = 0 bit(1) useRandomAccessPointFlag; = 0
bit(1) hasRandomAccessUnitsOnlyFlag; = 0 bit(1) hasRandomAccessUnitsOnlyFlag; = 0
bit(1) usePaddingFlag; = 0 bit(1) usePaddingFlag; = 0
bit(1) useTimeStampsFlag; = 0 bit(1) useTimeStampsFlag; = 0
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RTP Payload Format for MPEG-4 Streams September 2001
bit(1) useIdleFlag; = 0 bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 0 bit(1) durationFlag; = 0
bit(32) timeStampResolution; = 0 bit(32) timeStampResolution; = 0
bit(32) OCRResolution; = 0 bit(32) OCRResolution; = 0
bit(8) timeStampLength; = 0 bit(8) timeStampLength; = 0
bit(8) OCRLength; = 0 bit(8) OCRLength; = 0
bit(8) AU_Length; = 0 bit(8) AU_Length; = 0
bit(8) instantBitrateLength; = 0 bit(8) instantBitrateLength; = 0
bit(4) degradationPriorityLength; = 0 bit(4) degradationPriorityLength; = 0
bit(5) AU_seqNumLength; = 0 bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 0 bit(5) packetSeqNumLength; = 0
bit(2) reserved=0b11; bit(2) reserved=0b11;
} }
if (durationFlag) { if (durationFlag) {
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RTP Payload Format for MPEG-4 Streams September 2001
bit(32) timeScale; // NOT USED bit(32) timeScale; // NOT USED
bit(16) accessUnitDuration; // NOT USED bit(16) accessUnitDuration; // NOT USED
bit(16) compositionUnitDuration; // NOT USED bit(16) compositionUnitDuration; // NOT USED
} }
if (!useTimeStampsFlag) { if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; = 0 bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0 bit(timeStampLength) startCompositionTimeStamp; = 0
} }
} }
skipping to change at line 2037 skipping to change at line 2154
With this configuration we have the following SL packet header With this configuration we have the following SL packet header
structure: structure:
aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) { aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
if (SL.useAccessUnitEndFlag) { if (SL.useAccessUnitEndFlag) {
bit(1) accessUnitEndFlag; // 1 bit bit(1) accessUnitEndFlag; // 1 bit
} }
} }
In this case this payload produces RTP packets that are exactly In this case this payload produces RTP packets that are exactly
conformant to RFC 3016 and the Synch Layer is reduced to a purely conformant to RFC 3016 and the SL is reduced to a purely logical
logical construction that neither sender nor receiver need to construction that neither sender nor receiver need to implement.
implement.
Parameters Parameters
This configuration is the default one; no parameters are required. This configuration is the default one; no parameters are required.
RTP packet structure RTP packet structure
Note that accessUnitEndFlag is mapped to the RTP header M bit. Note that accessUnitEndFlag is mapped to the RTP header M bit.
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | 1400 bytes | | Access Unit or AU fragment | 1400 octets |
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RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Overhead Overhead
In this example we have an RTP overhead of 40 bytes for 1400 bytes In this example we have an RTP overhead of 40 octets for 1400 octets
of payload i.e. 3 % overhead. of payload i.e. 3 % overhead.
Appendix.2 MPEG-4 Video with SL Appendix.2 MPEG-4 Video with SL
Let us consider the case of a 30 frames per second MPEG-4 video Let us consider the case of a 30 frames per second MPEG-4 video
stream which bit rate is high enough that Access Units have to be stream which bit rate is high enough that Access Units have to be
split in several SL packets (typically above 300 kb/s). split in several SL packets (typically above 300 kb/s).
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RTP Payload Format for MPEG-4 Streams September 2001
Let us assume also that the video codec generates in that case Video Let us assume also that the video codec generates in that case Video
Packets suitable to fit in one SL packet i.e that the video codec is Packets suitable to fit in one SL packet i.e that the video codec is
MTU aware and the MTU is 1500 bytes. We assume furthermore that this MTU aware and the MTU is 1500 octets. We assume furthermore that
stream contains B frames and that decodingTimeStamps are present. this stream contains B frames and that decodingTimeStamps are
present.
SLConfigDescriptor SLConfigDescriptor
In this example the SLConfigDescriptor is: In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8) class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag { tag=SLConfigDescrTag {
bit(8) predefined; bit(8) predefined;
if (predefined==0) { if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 1 bit(1) useAccessUnitStartFlag; = 1
skipping to change at line 2110 skipping to change at line 2228
bit(5) packetSeqNumLength; = 0 bit(5) packetSeqNumLength; = 0
bit(2) reserved=0b11; bit(2) reserved=0b11;
} }
if (durationFlag) { if (durationFlag) {
bit(32) timeScale; // NOT USED bit(32) timeScale; // NOT USED
bit(16) accessUnitDuration; // NOT USED bit(16) accessUnitDuration; // NOT USED
bit(16) compositionUnitDuration; // NOT USED bit(16) compositionUnitDuration; // NOT USED
} }
if (!useTimeStampsFlag) { if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; // NOT USED bit(timeStampLength) startDecodingTimeStamp; // NOT USED
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RTP Payload Format for MPEG-4 Streams September 2001
bit(timeStampLength) startCompositionTimeStamp; // NOT USED bit(timeStampLength) startCompositionTimeStamp; // NOT USED
} }
} }
The useRandomAccessPointFlag is set so that the The useRandomAccessPointFlag is set so that the
randomAccessPointFlag can indicate that the corresponding SL packet randomAccessPointFlag can indicate that the corresponding SL packet
contains a GOV and the first Video Packet of an Intra coded frame. contains a GOV and the first Video Packet of an Intra coded frame.
SL Packet Header structure SL Packet Header structure
With this configuration we have the following SL packet header With this configuration we have the following SL packet header
structure: structure:
aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) { aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
Gentric et al. Expires March 2002 38
RTP Payload Format for MPEG-4 Streams September 2001
bit(1) accessUnitStartFlag; // 1 bit bit(1) accessUnitStartFlag; // 1 bit
if (accessUnitStartFlag) { if (accessUnitStartFlag) {
bit(1) randomAccessPointFlag; // 1 bit bit(1) randomAccessPointFlag; // 1 bit
bit(1) decodingTimeStampFlag; // 1 bit bit(1) decodingTimeStampFlag; // 1 bit
bit(1) compositionTimeStampFlag; // 1 bit bit(1) compositionTimeStampFlag; // 1 bit
if (decodingTimeStampFlag) { if (decodingTimeStampFlag) {
bit(SL.timeStampLength) decodingTimeStamp; bit(SL.timeStampLength) decodingTimeStamp;
} }
if (compositionTimeStampFlag) { if (compositionTimeStampFlag) {
bit(SL.timeStampLength) compositionTimeStamp; bit(SL.timeStampLength) compositionTimeStamp;
skipping to change at line 2167 skipping to change at line 2285
Access Units we have: Access Units we have:
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| DTSFlag = (1) | 1 bit | | DTSFlag = (1) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| DTSDelta | 7 bits | | DTSDelta | 7 bits |
Gentric et al. Expires March 2002 41
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment | 0 bits | | bits to octet alignment | 0 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| RSLHSectionSize = (100) | 3 bits | | RSLHSectionSize = (100) | 3 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitStartFlag = (1) | 1 bit | | accessUnitStartFlag = (1) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| randomAccessPointFlag | 1 bit | | randomAccessPointFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| decodingTimeStampFlag | 1 bit | | decodingTimeStampFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| compositionTimeStampFlag | 1 bit | | compositionTimeStampFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment =(0) | 1 bit | | bits to octet alignment =(0) | 1 bit |
Gentric et al. Expires March 2002 39
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | N bytes | | SL packet payload | N octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
For packets that transport non-first fragments of Access Units we For packets that transport non-first fragments of Access Units we
have: have:
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| DTSFlag = 0 | 1 bit | | DTSFlag = 0 | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment = (0000000) | 7 bits | | bits to octet alignment = (0000000) | 7 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| RSLHSectionSize = (001) | 3 bits | | RSLHSectionSize = (001) | 3 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitStartFlag = (0) | 1 bit | | accessUnitStartFlag = (0) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment = (0000) | 4 bits | | bits to octet alignment = (0000) | 4 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | N bytes | | SL packet payload | N octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Overhead estimation Overhead estimation
In this example we have a RTP overhead of 40 + 2 bytes for 1400 In this example we have a RTP overhead of 40 + 2 octets for 1400
bytes of payload i.e. 3 % overhead. octets of payload i.e. 3 % overhead.
Appendix.3 Low delay MPEG-4 Audio (no SL) Appendix.3 Low delay MPEG-4 Audio (no SL)
This example is for a low delay audio service. For this reason a This example is for a low delay audio service. For this reason a
single SL packet is transported in each RTP packet. Actually each SL single Access Unit is transported in each RTP packet (in terms of
packet contains a complete Access Unit. Sync Layer each SL packet contains a complete Access Unit).
SLConfigDescriptor SLConfigDescriptor
Gentric et al. Expires March 2002 42
RTP Payload Format for MPEG-4 Streams September 2001
Since CTS=DTS and Access Unit duration is constant signaling of Since CTS=DTS and Access Unit duration is constant signaling of
MPEG-4 time stamps is not needed (the durationFlag of SLConfig is MPEG-4 time stamps is not needed (the durationFlag of SLConfig is
set) set)
We also assume here an audio Object Type for which all Access Units We also assume here an audio Object Type for which all Access Units
are Random Access Points, which is signaled using the are Random Access Points, which is signaled using the
hasRandomAccessUnitsOnlyFlag in the SLConfigDescriptor. hasRandomAccessUnitsOnlyFlag in the SLConfigDescriptor.
We assume furthermore a mode where the Access Unit size is constant We assume furthermore a mode where the Access Unit size is constant
and equal to 5 bytes (which is signaled with AU_Length). and equal to 5 octets (which is signaled with AU_Length).
In this example the SLConfigDescriptor is: In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8) class SLConfigDescriptor extends BaseDescriptor : bit(8)
Gentric et al. Expires March 2002 40
RTP Payload Format for MPEG-4 Streams September 2001
tag=SLConfigDescrTag { tag=SLConfigDescrTag {
bit(8) predefined; bit(8) predefined;
if (predefined==0) { if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 0 bit(1) useAccessUnitStartFlag; = 0
bit(1) useAccessUnitEndFlag; = 0 bit(1) useAccessUnitEndFlag; = 0
bit(1) useRandomAccessPointFlag; = 0 bit(1) useRandomAccessPointFlag; = 0
bit(1) hasRandomAccessUnitsOnlyFlag; = 1 bit(1) hasRandomAccessUnitsOnlyFlag; = 1
bit(1) usePaddingFlag; = 0 bit(1) usePaddingFlag; = 0
bit(1) useTimeStampsFlag; = 0 bit(1) useTimeStampsFlag; = 0
bit(1) useIdleFlag; = 0 bit(1) useIdleFlag; = 0
skipping to change at line 2276 skipping to change at line 2393
bit(16) compositionUnitDuration; = 10 // ms bit(16) compositionUnitDuration; = 10 // ms
} }
if (!useTimeStampsFlag) { if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; = 0 bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0 bit(timeStampLength) startCompositionTimeStamp; = 0
} }
} }
SL packet header SL packet header
With this configuration the SL packet header is empty. The Synch With this configuration the SL packet header is empty. The Sync
Layer is reduced to a purely logical construction that neither Layer is reduced to a purely logical construction that neither
sender nor receiver need to implement. sender nor receiver need to implement.
Gentric et al. Expires March 2002 43
RTP Payload Format for MPEG-4 Streams September 2001
Parameters Parameters
No parameters are required. No parameters are required.
RTP packet structure RTP packet structure
Note that the RTP header M bit should be always set to 1. Note that the RTP header M bit should be always set to 1.
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | 5 bytes | | Access Unit | 5 octets |
Gentric et al. Expires March 2002 41
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Overhead estimation Overhead estimation
The overhead is extremely large i.e. more than 800 %, since 40 bytes The overhead is extremely large i.e. more than 800 %, since 40
of headers are required to transport 5 bytes of data. Note however octets of headers are required to transport 5 octets of data. Note
that RTP header compression would work well since time stamps however that RTP header compression would work well since time
increments are constant. stamps increments are constant.
Appendix.4 Media delivery MPEG-4 Audio (no SL) Appendix.4 Media delivery MPEG-4 Audio (no SL)
This example is for a media delivery service where delay is not an This example is for a media delivery service where delay is not an
issue but efficiency is. In this case several SL Packets are issue but efficiency is. In this case several Access Units are
transported in each RTP packet. transported in each RTP packet.
SLConfigDescriptor SLConfigDescriptor
Similar to previous example. Similar to previous example.
SL packet header SL packet header
With this configuration the SL packet header is empty. The Synch With this configuration the SL packet header is empty. The Sync
Layer is reduced to a purely logical construction that neither Layer is reduced to a purely logical construction that neither
sender nor receiver need to implement. sender nor receiver need to implement.
Parameters Parameters
The absence of RSLHSectionSizeLength indicates that the RSLHSection The absence of RSLHSectionSizeLength indicates that the RSLHSection
is empty. is empty.
The size of SL Packets (which are all complete Access Units in this The size of SL Packets (which are all complete Access Units in this
case) is constant and is indicated with: case) is constant and is indicated with:
a=fmtp:<format> ConstantSize=5 a=fmtp:<format> ConstantSize=5
This also indicates to the receiver that the Multiple-SL mode will Gentric et al. Expires March 2002 44
be used, the 2 bytes field that would give the size of the RTP Payload Format for MPEG-4 Streams September 2001
MSLHSection is ommited since in this case this field always contains
zero (the MSLHSection is always empty due to the absence of any This also indicates to the receiver that the Multiple mode will be
other MIME parameter). used, the 2 octets field that would give the size of the
PayloadHeaderSection is ommited since in this case this field always
contains zero (the PayloadHeaderSection is always empty due to the
absence of any other MIME parameter).
RTP packet structure RTP packet structure
Note that the RTP header M bit is always set to 1, which indicates Note that the RTP header M bit is always set to 1, which indicates
to the receiver that only complete Access Units are transported. to the receiver that only complete Access Units are transported.
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
Gentric et al. Expires March 2002 42
RTP Payload Format for MPEG-4 Streams September 2001
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | 5 bytes | | Access Unit data | 5 octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | 5 bytes | | Access Unit data | 5 octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| etc, until MTU is reached | | etc, until MTU is reached |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload | 5 bytes | | Access Unit data | 5 octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Overhead estimation Overhead estimation
The overhead is 3% i.e. minimal. The overhead is 3% i.e. minimal.
Appendix.5 AAC with interleaving (no SL) Appendix.5 AAC with interleaving (no SL)
Let us consider AAC at 128 kb/s where each Access Unit is in the Let us consider AAC at 128 kb/s where each Access Unit is in the
average 320 bytes. Interleaving is applied with a continuous average 320 octets. Interleaving is applied with a continuous
interleaving scheme (see table below) where 4 Access Units are used interleaving scheme (see table below) where 4 Access Units are used
to construct each RTP packet in order to match a MTU of 1500 bytes. to construct each RTP packet in order to match a MTU of 1500 octets.
IndexDelta is constant and equal to 2 (since +1 is automatically IndexDelta is constant and equal to 2 (since +1 is automatically
added); it is encoded on 2 bits. added); it is encoded on 2 bits.
As explained in section 3.8 this is a time stamp based interleaving As explained in section 3.8 this is a time stamp based interleaving
scheme (IndexLength=0); indeed receivers know that each SL packet is (TSBI) scheme (IndexLength=0); indeed receivers know that each
a complete Access Unit because all RTP packets have the M bit set to payload is a complete Access Unit because all RTP packets have the M
1 and therefore, since Access Unit duration is constant, Access Unit bit set to 1 and therefore, since Access Unit duration is constant,
timestamps can be computed from RTP timestamps and IndexDelta Access Unit timestamps can be computed from RTP timestamps and
values; this can be used for de-interleaving even in case of losses. IndexDelta values; this can be used for de-interleaving even in case
of losses.
Note that it would also be possible to use IndexLength=2 so as to Note that it would also be possible to use IndexLength=2 so as to
maintain a byte alignement in the MSLH portions; in this case maintain a octet alignement in the Payload Header portions; in this
however the value of these two bits MUST be zero as stated in 3.8.1. case however the value of these two bits MUST be zero as stated in
3.8.1.
Gentric et al. Expires March 2002 45
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| RTP packet | RTP Timestamp | Aus | IndexDelta | | RTP packet | RTP Timestamp | Aus | IndexDelta |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 1 | CTS(AU1) | 1 | - | | 1 | CTS(AU1) | 1 | - |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 2 | CTS(AU2) | 2, 5 | -,2 | | 2 | CTS(AU2) | 2, 5 | -,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 3 | CTS(AU3) | 3, 6, 9 | -,2,2 | | 3 | CTS(AU3) | 3, 6, 9 | -,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 4 | CTS(AU4) | 4, 7,10,13 | -,2,2,2 | | 4 | CTS(AU4) | 4, 7,10,13 | -,2,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 5 | CTS(AU8) | 8,11,14,17 | -,2,2,2 | | 5 | CTS(AU8) | 8,11,14,17 | -,2,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 6 | CTS(AU12) | 12,15,18,21 | -,2,2,2 | | 6 | CTS(AU12) | 12,15,18,21 | -,2,2,2 |
Gentric et al. Expires March 2002 43
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 7 | CTS(AU16) | 16,19,22,25 | -,2,2,2 | | 7 | CTS(AU16) | 16,19,22,25 | -,2,2,2 |
+----------------------------------------------------------------+ +----------------------------------------------------------------+
| 8 | CTS(AU20) | 20,23,26,29 | -,2,2,2 | | 8 | CTS(AU20) | 20,23,26,29 | -,2,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 9 | CTS(AU24) | 24,27,30,33 | -,2,2,2 | | 9 | CTS(AU24) | 24,27,30,33 | -,2,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| 10 | CTS(AU28) | 28,31,34,37 | -,2,2,2 | | 10 | CTS(AU28) | 28,31,34,37 | -,2,2,2 |
+-----------------------------------------------------------------+ +-----------------------------------------------------------------+
| etc | | etc |
skipping to change at line 2438 skipping to change at line 2554
a=fmtp:<format> SizeLength=9; IndexDeltaLength=2; a=fmtp:<format> SizeLength=9; IndexDeltaLength=2;
RTP packet structure RTP packet structure
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
MSLHSection Payload Header Section
+=========================================+=============+ +=========================================+=============+
| MSLHSection size in bits = 42 bits | 2 bytes | | PayloadHeaderSection size = 42 bits | 2 octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Gentric et al. Expires March 2002 46
RTP Payload Format for MPEG-4 Streams September 2001
| PayloadSize | 9 bits | | PayloadSize | 9 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 9 bits | | PayloadSize | 9 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| IndexDelta | 2 bits | | IndexDelta | 2 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 9 bits | | PayloadSize | 9 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| IndexDelta | 2 bits | | IndexDelta | 2 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 9 bits | | PayloadSize | 9 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| IndexDelta | 2 bits | | IndexDelta | 2 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment = (000000) | 6 bits | | bits to octet alignment = (000000) | 6 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Payload Section
Gentric et al. Expires March 2002 44
RTP Payload Format for MPEG-4 Streams September 2001
SLPPSection
+=========================================+=============+ +=========================================+=============+
| AAC Access Unit | x bytes | | AAC Access Unit | x octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| AAC Access Unit | x bytes | | AAC Access Unit | x octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| AAC Access Unit | x bytes | | AAC Access Unit | x octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| AAC Access Unit | x bytes | | AAC Access Unit | x octets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Overhead estimation Overhead estimation
The MSLHSection is 8 bytes; in this example we have therefore a RTP The PayloadHeaderSection is 8 octets; in this example we have
overhead of 40 + 8 bytes for 1400 bytes (approx) of payload i.e. therefore a RTP overhead of 40 + 8 octets for 1400 octets (approx)
around 4 % overhead. of payload i.e. around 4 % overhead.
Appendix.6 AAC with Index-based interleaving and SL Appendix.6 AAC with Index-based interleaving and SL
Let us consider AAC around 130 kb/s where each Access Unit is split Let us consider AAC around 130 kb/s where each Access Unit is split
in 4 SL packets corresponding to Error Sensitivity Categories (ESC) in 4 SL packets corresponding to Error Sensitivity Categories (ESC)
of maximum 90 bytes for which interleaving is very useful in terms of maximum 90 octets for which interleaving is very useful in terms
of error resilience. We thus use an interleaving scheme where 15 SL of error resilience. We thus use an interleaving scheme where 15 SL
Packets (extracted from 15 consecutive Access Units) are used to Packets (extracted from 15 consecutive Access Units) are used to
construct each RTP packet in order to match a MTU of 1500 bytes. construct each RTP packet in order to match a MTU of 1500 octets.
Note that since ESC fragments are not byte aligned we also use the Note that since ESC fragments are not octet aligned we also use the
paddingFlag and paddingBits features of the Synch Layer. The paddingFlag and paddingBits features of the Sync Layer. The
interleaving sequence is 4 RTP packets and 350 ms long, which is too interleaving sequence is 4 RTP packets and 350 ms long, which is too
long for conferencing but perfectly OK for Internet radio. long for conferencing but perfectly OK for Internet radio.
Since the sequence contains 60 SL packets, IndexLength is set to 16 Since the sequence contains 60 SL packets, IndexLength is set to 16
bits so as to provide a safe margin in case of long loss bursts. bits so as to provide a safe margin in case of long loss bursts.
This will also indicate to the receiver that this is a Index-Based- This will also indicate to the receiver that this is a Index-Based-
Gentric et al. Expires March 2002 47
RTP Payload Format for MPEG-4 Streams September 2001
Interleaving scheme (indeed CTS cannot be computed for SL packets Interleaving scheme (indeed CTS cannot be computed for SL packets
that are not AU starts). that are not AU starts).
2 bits are enough for IndexDelta, which is constant and equal to 3 2 bits are enough for IndexDelta, which is constant and equal to 3
(since +1 is automatically added). (since +1 is automatically added).
Note that the 4th RTP packet in each sequence has its M bit set to 1 Note that the 4th RTP packet in each sequence has its M bit set to 1
since it contains 15 SL packets transporting the end of 15 since it contains 15 SL packets transporting the end of 15
consecutive Access Units. consecutive Access Units.
With this scheme a sender (for example upon reception of RTCP With this scheme a sender (for example upon reception of RTCP
reports indicating high loss rates) can (for example) choose to reports indicating high loss rates) can (for example) choose to
duplicate for each interleaving sequence the first RTP packet that duplicate for each interleaving sequence the first RTP packet that
contains the most useful data in terms of ESC or apply other error contains the most useful data in terms of ESC or apply other error
protection techniques, with due care to congestion issues. protection techniques, with due care to congestion issues.
Gentric et al. Expires March 2002 45
RTP Payload Format for MPEG-4 Streams September 2001
In this example we will also show several other SL features (OCR, AU In this example we will also show several other SL features (OCR, AU
boundary flags, padding, as detailed below). boundary flags, padding, as detailed below).
One feature demonstrated by this example is the degradation One feature demonstrated by this example is the degradation
priority. We assume degradation priority can take 4 different priority. We assume degradation priority can take 4 different
values, mapped to Error Sensitivity Categories, and is encoded on 2 values, mapped to Error Sensitivity Categories, and is encoded on 2
bits. This interleaving scheme makes sure that only SL packets of bits. This interleaving scheme makes sure that only SL packets of
identical degradation priorities are grouped in the same RTP packet identical degradation priorities are grouped in the same RTP packet
(3.6.3) and that only the first RSLH of each RTP packet transports (3.6.3) and that only the first RSLH of each RTP packet transports
the degradation priority. the degradation priority.
skipping to change at line 2550 skipping to change at line 2667
bit(1) usePaddingFlag; = 1 // we need to signal padding bits bit(1) usePaddingFlag; = 1 // we need to signal padding bits
bit(1) useTimeStampsFlag; = 0 bit(1) useTimeStampsFlag; = 0
bit(1) useIdleFlag; = 0 bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 1 bit(1) durationFlag; = 1
bit(32) timeStampResolution; = 0 bit(32) timeStampResolution; = 0
bit(32) OCRResolution; = 30 bit(32) OCRResolution; = 30
bit(8) timeStampLength; = 0 bit(8) timeStampLength; = 0
bit(8) OCRLength; = 32 bit(8) OCRLength; = 32
bit(8) AU_Length; = 0 bit(8) AU_Length; = 0
bit(8) instantBitrateLength; = 0 bit(8) instantBitrateLength; = 0
Gentric et al. Expires March 2002 48
RTP Payload Format for MPEG-4 Streams September 2001
bit(4) degradationPriorityLength; = 2 bit(4) degradationPriorityLength; = 2
bit(5) AU_seqNumLength; = 0 bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 6 bit(5) packetSeqNumLength; = 6
bit(2) reserved=0b11; bit(2) reserved=0b11;
} }
if (durationFlag) { if (durationFlag) {
bit(32) timeScale; = 1000// milliseconds bit(32) timeScale; = 1000// milliseconds
bit(16) accessUnitDuration; = 23.22 // ms bit(16) accessUnitDuration; = 23.22 // ms
bit(16) compositionUnitDuration; = 23.22 // ms bit(16) compositionUnitDuration; = 23.22 // ms
} }
if (!useTimeStampsFlag) { if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; = 0 bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0 bit(timeStampLength) startCompositionTimeStamp; = 0
} }
} }
Gentric et al. Expires March 2002 46
RTP Payload Format for MPEG-4 Streams September 2001
SL Packet Header structure SL Packet Header structure
With this configuration we have the following SL packet header With this configuration we have the following SL packet header
structure: structure:
aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) { aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
bit(1) accessUnitStartFlag; bit(1) accessUnitStartFlag;
bit(1) accessUnitEndFlag; bit(1) accessUnitEndFlag;
bit(1) OCRflag; bit(1) OCRflag;
bit(1) paddingFlag; bit(1) paddingFlag;
skipping to change at line 2603 skipping to change at line 2721
a=fmtp:<format> SizeLength=7; RSLHSectionSizeLength=8; a=fmtp:<format> SizeLength=7; RSLHSectionSizeLength=8;
IndexLength=16; IndexDeltaLength=2; OCRDeltaLength=16 IndexLength=16; IndexDeltaLength=2; OCRDeltaLength=16
RTP packet structure RTP packet structure
+=========================================+=============+ +=========================================+=============+
| Field | size | | Field | size |
+=========================================+=============+ +=========================================+=============+
| RTP header | - | | RTP header | - |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
MSLHSection Payload Header Section
+=========================================+=============+ +=========================================+=============+
| MSLHSection size in bits = 149 | 2 bytes | | Payload Header Section size = 149 bits | 2 octets |
Gentric et al. Expires March 2002 49
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 7 bits | | PayloadSize | 7 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| Index | 16 bits | | Index | 16 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 7 bits | | PayloadSize | 7 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| IndexDelta = (11) | 2 bits | | IndexDelta = (11) | 2 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| etc + 12 times 9 bits | | etc + 12 times 9 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| PayloadSize | 7 bits | | PayloadSize | 7 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| IndexDelta = (11) | 2 bits | | IndexDelta = (11) | 2 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment = (000) | 3 bits | | bits to octet alignment = (000) | 3 bits |
Gentric et al. Expires March 2002 47
RTP Payload Format for MPEG-4 Streams September 2001
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
RSLHSection RSLHSection
+=========================================+=============+ +=========================================+=============+
| RSLHSectionSize = (10000111) | 8 bits | | RSLHSectionSize = (10000111) | 8 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitStartFlag | 1 bit | | accessUnitStartFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitEndFlag | 1 bit | | accessUnitEndFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| OCRFlag = (0) | 1 bit | | OCRFlag = (0) | 1 bit |
skipping to change at line 2663 skipping to change at line 2781
| paddingBits | 3 bits | | paddingBits | 3 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| DegPrioflag = (0) | 1 bit | | DegPrioflag = (0) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| etc + 12 times 8 bits | | etc + 12 times 8 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitStartFlag | 1 bit | | accessUnitStartFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| accessUnitEndFlag | 1 bit | | accessUnitEndFlag | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Gentric et al. Expires March 2002 50
RTP Payload Format for MPEG-4 Streams September 2001
| OCRFlag = (1) | 1 bit | | OCRFlag = (1) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| OCRDelta | 16 bits | | OCRDelta | 16 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| paddingFlag = (0) | 1 bit | | paddingFlag = (0) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| DegPrioflag = (0) | 1 bit | | DegPrioflag = (0) | 1 bit |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| bits to byte alignment = (000) | 3 bits | | bits to octet alignment = (000) | 3 bits |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
SLPPSection Payload Section
+=========================================+=============+ +=========================================+=============+
| SL packet payload |max 90 bytes | | SL packet payload |max 90 octets|
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| etc + 13 SL packets | | etc + 13 SL packets |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
| SL packet payload |max 90 octets|
Gentric et al. Expires March 2002 48
RTP Payload Format for MPEG-4 Streams September 2001
| SL packet payload |max 90 bytes |
+-----------------------------------------+-------------+ +-----------------------------------------+-------------+
Note that in the above table the last SL packet in the RTP packet Note that in the above table the last SL packet in the RTP packet
has a payload that is byte-aligned (at the end). When this happens has a payload that is octet-aligned (at the end). When this happens
paddingFlag is set to zero and the paddingBits field is omitted. paddingFlag is set to zero and the paddingBits field is omitted.
Overhead estimation Overhead estimation
The MSLHSection is 19 bytes, the RSLHSection is 16 bytes; in this The PayloadHeaderSection is 19 octets, the RSLHSection is 16 octets;
example we have therefore a RTP overhead of 40 + 35 bytes for 1350 in this example we have therefore a RTP overhead of 40 + 35 octets
bytes of payload i.e. around 6 % overhead. for 1350 octets of payload i.e. around 6 % overhead.
Gentric et al. Expires March 2002 49 Gentric et al. Expires March 2002 51
 End of changes. 

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