Internet Engineering Task Force Avaro-France Telecom
Internet Draft Basso-AT&T
Casner-Packet Design
Civanlar-AT&T
Gentric-Philips
Herpel-Thomson
Lifshitz-Optibase
Lim-mp4cast
Perkins-ISI
van der Meer-Philips
July 2001
Expires Jan. 2002
Document: draft-ietf-avt-mpeg4-multisl-01.txt
RTP Payload Format for MPEG-4 Streams
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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six months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet- Drafts
as reference material or to cite them other than as "work in
progress."
This specification is a product of the Audio/Video Transport working
group within the Internet Engineering Task Force and ISO/IEC MPEG-4
ad hoc group on MPEG-4 over Internet. Comments are solicited and
should be addressed to the working group's mailing list at
avt@ietf.org and/or the authors.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document contains a MIME type registration form that is
intended to be taken as-is and therefore makes reference to this
document, using the temporary placeholder: <self-reference-to-this>.
Abstract
This document describes a payload format for transporting MPEG-4
encoded data using RTP. MPEG-4 is a recent standard from ISO/IEC for
the coding of natural and synthetic audio-visual data. Several
services provided by RTP are beneficial for MPEG-4 encoded data
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RTP Payload Format for MPEG-4 Streams July 2001
transport over the Internet. Additionally, the use of RTP makes it
possible to synchronize MPEG-4 data with other real-time data types.
1. Introduction
MPEG-4 is a recent standard from ISO/IEC for the coding of natural
and synthetic audio-visual data in the form of audiovisual objects
that are arranged into an audiovisual scene by means of a scene
description [1][2][3][4]. This draft specifies an RTP [5] payload
format for transporting MPEG-4 encoded data streams.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [6].
The benefits of using RTP for MPEG-4 data stream transport include:
i. Ability to synchronize MPEG-4 streams with other RTP payloads
ii. Monitoring MPEG-4 delivery performance through RTCP
iii. Combining MPEG-4 and other real-time data streams received from
multiple end-systems into a set of consolidated streams through RTP
mixers
iv. Converting data types, etc. through the use of RTP translators.
1.1 Overview of MPEG-4 End-System Architecture
Fig. 1 below shows the layered architecture of a terminal which
implements the complete MPEG-4 systems model. The Compression Layer
processes individual audio-visual media streams. The MPEG-4
compression schemes are defined in the ISO/IEC specifications 14496-
2 [2] and 14496-3 [3]. The compression schemes in MPEG-4 achieve
efficient encoding over a bandwidth ranging from several kbps to
many Mbps. The audio-visual content compressed by this layer is
organized into Elementary Streams (ESs).
The MPEG-4 standard specifies MPEG-4 compliant streams. Within the
constraint of this compliance the compression layer is unaware of a
specific delivery technology, but it can be made to react to the
characteristics of a particular delivery layer such as the path-MTU
or loss characteristics. Also, some compressors can be designed to
be delivery specific for implementation efficiency. In such cases
the compressor may work in a non-optimal fashion with delivery
technologies that are different than the one it is specifically
designed to operate with.
The hierarchical relations, location and properties of ESs in a
presentation are described by a dynamic set of Object Descriptors
(ODs). Each OD groups one or more ES Descriptors referring to a
single content item (audio-visual object). Hence, multiple
alternative or hierarchical representations of each content item are
possible.
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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
stream level. The resource description may itself be hierarchical,
i.e. an ES conveying an OD may describe other ESs conveying other
ODs.
The session description is accompanied by a dynamic scene
description, Binary Format for Scene (BIFS), again conveyed through
one or more ESs. At this level, content is identified in terms of
audio-visual objects. The spatio-temporal location of each object is
defined by BIFS. The audio-visual content of those objects that are
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
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RTP Payload Format for MPEG-4 Streams July 2001
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
so that SL packets are reduced to the media (compressed) data and in
that case implementations do not need to be aware of the SL at all.
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 +-----------------------------------------+
delivery unaware | COMPRESSION LAYER |
14496-2 Visual |streams from as low as Kbps to multi-Mbps|
14496-3 Audio +-----------------------------------------+
Elementary
Stream
===================================================Interface
(ESI)
+-------------------------------------------+
media and | SYNC LAYER |
delivery unaware | manages elementary streams, their synch- |
14496-1 Systems | ronization and hierarchical relations |
+-------------------------------------------+
DMIF
Application
====================================================Interface
(DAI)
+-------------------------------------------+
delivery aware | DELIVERY LAYER |
media unaware |provides transparent access to and delivery|
14496-6 DMIF | of content irrespective of delivery |
| technology |
+-------------------------------------------+
Figure 1: Conceptual MPEG-4 terminal architecture
1.2 MPEG-4 Elementary Stream Data Packetization
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.
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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.
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
information, is not carried by the media stream itself but is rather
transported via an ObjectDescriptor Stream encoded using the MPEG-4
Object Description framework. This can be done in a separate stream
using this payload format (see section 5.2 for details). The
SLConfigDescriptor MAY also be transported by other means (for
example as a parameter, see section 4.1). Finally streams for which
the SL packet headers are completely empty (or fully map into the
RTP headers) can also be transported using this payload format; in
these cases the Synch Layer can be seen as a purely conceptual
construction that does not have to be implemented at all. Since only
the knowledge of the decoder configuration is then needed it MAY
also be transported as a parameter, as described in section 4.1.
2. Analysis of the carriage of MPEG-4 over IP
When transporting MPEG-4 audio and video, applications may or may
not require the use of MPEG-4 systems. To achieve the highest level
of interoperability between all MPEG-4 applications, it is desirable
that (a) in both cases the same MPEG-4 transport format can be used
and that (b) receivers that have no MPEG-4 system knowledge can
easily skip the MPEG-4 system specific information, if any.
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
that both RTP and MPEG-4 systems contain a synchronization layer.
In particular, the RTP header duplicates some of the information
provided in SL packet headers such as the composition timestamps
(CTSs) and the marker bit that signals the end of access units.
To avoid unnecessary overhead and potential interoperability risks
when transporting MPEG-4 systems, it is desirable to remove the
redundancy between the SL packet header and the RTP packet header.
To be independent on the use of MPEG-4 systems, synchronization can
rely on the parameters provided in the RTP header.
In case SL headers are used, the redundant fields are removed from
the SL header, producing "reduced SL headers".
The remaining information from the SL header, if any, is contained
inside the RTP packet payload, together with the SL packet payload.
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The combination of RTP packet headers and reduced SL packet headers
can be used to logically map the RTP packets to complete SL packets.
Some of the information contained in the reduced SL headers is also
useful for transport over RTP when MPEG-4 systems is not used.
For that reason the information in the "reduced" SL headers is split
into "general useful information" and "MPEG-4 systems only
information".
The "general useful information" hereinafter called Mapped SL Packet
Header (MSLH) is carried by a number of fields configurable using
parameters defined in section 4.1; all receivers MUST parse these
fields.
The "MPEG-4 systems only information", if any, is contained in a
reduced SL header, hereinafter called Remaining SL Packet Header
(RSLH), also configured using parameters (see section 4.1) and
preceded by a length field, so that non-MPEG-4-system devices MAY
skip this information.
This is depicted in figure 2.
<----------SL Packet-------->
+---------------------------+
| SL Packet | SL Packet |
| Header | Payload |
+---------------------------+
| |
| |
+-------------+----------+---+ |
| | | |
V V V V
+-----------+ +-----------+ +-------------+ +-----------+
|RTP Packet | | Mapped SL | | Remaining SL| | SL Packet |
| Header | | Header | | Header | | Payload |
+-----------+ +-----------+ +-------------+ +-----------+
<----RTP Packet Payload------------------->
Figure 2: Mapping of SL Packet into RTP packet
When the configuration is such that SL packet headers map directly
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
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RTP Payload Format for MPEG-4 Streams July 2001
Appendix for examples) can be transported without explicit use of
the Synch Layer.
3. Payload Format
The RTP Payload corresponds to an integer number of SL packets.
If multiple SL packets are transported in each RTP packet, they MUST
be in decoding order, i.e:
i) decodingTimeStamp order, if present
ii) packetSequenceNumber order, if present
iii) Implicit decoding order in all other cases.
The SL Packet Headers are transformed into RSLH with some fields
extracted to be mapped in the RTP header and others extracted to be
mapped in the corresponding MSLH. The SL Packet Payload is
unchanged.
This payload format has two modes. The "SingleSL" mode is a mode
where a single SL packet is transported per RTP packet. The
"MultipleSL" mode is a mode where possibly more than one SL packet
are transported per RTP packet. The default mode is the Single-SL
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
above). In case of interleaving the first SL packet 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: [0,3,6][1,2][4,5]
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: [1,3,5][0,2,4]
This sequence is prohibited: [0,3,6][2,5][1,4]
The size (or number) of the SL packet(s) SHOULD be adjusted such
that the resulting RTP packet is not larger than the path-MTU. To
handle larger packets, this payload format relies on lower layers
for fragmentation, which may not be desirable.
3.1 RTP Header Fields Usage
Payload Type (PT): The assignment of an RTP payload type for this
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
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
range shall be chosen.
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.
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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
M bit is set to 1 for RTP packets that contain either:
. A single complete Access Unit
. The last fragment of an Access Unit
. Several complete Access Units
. Several 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
the M bit is 1 for all RTP packets.
Extension (X) bit: Defined by the RTP profile used.
Sequence Number: The RTP sequence number should be generated by the
sender with a constant random offset and does not have to be
correlated to any (optional) MPEG-4 SL sequence numbers.
Timestamp: Set to the value in the compositionTimeStamp field of the
first SL packet in the RTP packet, if present. If
compositionTimeStamp has less than 32 bits length, the MSBs of
timestamp MUST be set to zero.
Although it is available from the SL configuration data, the
resolution of the timestamp may need to be conveyed explicitly
through some out-of-band means to be used by network elements that
are not MPEG-4 aware.
If compositionTimeStamp has more than 32 bits length, this payload
format cannot be used.
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
packet, but has been present in a previous SL packet the reason is
that this is the same Access Unit that has been fragmented,
therefore the same timestamp value MUST be taken as RTP timestamp.
If compositionTimeStamp is never present in SL packets for this
stream, the RTP packetizer SHOULD convey a reading of a local clock
at the time the RTP packet is created.
According to RFC1889 [5, Section 5.1] timestamps are recommended to
start at a random value for security reasons. However then, a
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
applications where streams from multiple sources are to be
synchronized. Therefore the usage of such a random offset SHOULD be
avoided.
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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
expressed as difference to the RTP time stamp. Since this
subtraction may lead to negative values, the offset MUST be encoded
as a two's complement signed integer in network byte order. Note
these offsets (delta) typically require much fewer bits to be
encoded than the original length, which is another justification.
When startCompositionTimeStamp is signaled in the SLConfigDescriptor
the RTP time stamps MUST start with this value.
SSRC, CC and CSRC fields are used as described 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
The packet payload structure consists of 3 byte-aligned sections.
The first section is the MSLHSection and contains Mapped SL Packet
Headers (MSLH). The MSLH structure is described in 3.3. In the
Single-SL mode this section is empty by default.
The second section is the RSLHSection and contains Remaining SL
Headers (RSLH). The RSLH structure is described in 3.5. By default
this section is empty.
The last section (SLPPSection) contains the SL packet payloads. This
section is never empty.
The Nth MSLH in the MSLHSection, the Nth RSLH in the RSLHSection and
the Nth SL packet payload in the SLPPSection correspond to the Nth
SL packet transported by the RTP packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: contributing source (CSRC) identifiers :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
| MSLHSection (byte aligned) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| RSLHSection (byte aligned) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ |
| |
| SLPPSection (byte aligned) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: An RTP packet for MPEG-4
3.3 MSLHSection structure
If the MSLHSection consumes a non-integer number of bytes, up to 7
zero-valued padding bits MUST be inserted at the end in order to
achieve byte-alignment.
In the Single-SL mode the MSLHSection consists of a single MSLH.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSLH (x bits ) : padding bits|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: MSLHSection structure in Single-SL mode
In the Multiple-SL mode this section consist of a 2 bytes field
giving the size in bits (in network byte order) of the following
block of bit-wise concatenated MSLHs.
This size field is absent in the Single-SL mode not because it is
not needed (which would be a minor gain) but for compatibility with
RFC 3016.
This size field is also absent when the value would always be zero
because the MSLH is always empty, which may happen when a constant
size in signaled using ConstantSize.
0 1 2 3
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| as many bit-wise concatenated MSLHs |
| as SL packets in this RTP packet |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : padding bits|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: MSLHSection structure in Multiple-SL mode
3.4 MSLH structure
The Mapped SL Packet Header content depends on parameters (as
described in section 4.1); by default it is empty for the Single-SL
mode and, except when ConstantSize is signaled, contains at least
the PayloadSize field in the Multiple-SL mode.
When all options are used the MSLH structure is given in figure 6.
+============================+
|PayloadSize |
+----------------------------+
|Index or IndexDelta |
+----------------------------+
|CTSFlag |
+----------------------------+
|CTSDelta |
+----------------------------+
|DTSFlag |
+----------------------------+
|DTSDelta |
+============================+
Figure 6: Mapped SL Packet Header (MSLH) structure
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
PayloadSize: Indicates the size in bytes of the associated SL Packet
Payload, which can be found in the SLPPSection 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: when the RTP packet contains a single
SL packet the PayloadSize field SHALL contain the size of the entire
corresponding Access Unit, for two reasons, firstly the size of the
fragment is not needed when there is only one fragment, secondly
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this is useful in order to detect that a full Access Unit has been
received after the loss of a packet carrying M bit set to 1.
Index, IndexDelta: Encodes the packetSequenceNumber (serial number)
of the SL Packet. When making streams specifically for transport
with this payload format IndexDelta is useful for interleaving (see
section 3.8). Since a mapping of packetSequenceNumber to RTP
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
in a RTP packet.
The length in bits of the Index field is defined by the IndexLength
parameter (see section 4.1).
IndexDelta is optional and -if present- appears for subsequent (non-
first) SL packets in a RTP packet.
The length in bits of the IndexDelta field is defined by the
IndexDeltaLength parameter (see section 4.1).
If the parameter IndexDeltaLength is defined, non-first SL packets
inside a RTP packet have their packetSequenceNumber encoded as a
difference (thus the name IndexDelta). This difference is relative
to the previous SL packet in the RTP packet according to (with
i>=0):
packetSequenceNumber(0) = Index(0)
packetSequenceNumber(i+1) = packetSequenceNumber(i) +
IndexDelta(i+1) + 1
If the parameter IndexDeltaLength is not defined the default value
is zero and then the IndexDelta field is not present for non-first
SL packets. Nevertheless receivers SHALL then apply the above
formula with IndexDelta equal to zero. In other words by default
packetSequenceNumber is incremented by 1 for each SL packet in one
RTP packet.
CTSFlag (1 bit): Indicates whether the CTSDelta field is present. A
value of 1 indicates that the CTSDelta field is present, a value of
0 that it is not present.
If CTSDeltaLength is not zero, CTSFlag is present in all MSLH
regardless of whether the SL packet is an Access Unit start or not.
CTSDelta (CTSDeltaLength bits): Specifies the value of the CTS as a
2-complement offset (delta) from the timestamp in the RTP header of
the RTP packet. The length in bits of each CTSDelta field is
specified by the CTSDeltaLength parameter (see section 4.1).
The CTSDelta field is present if CTSFlag is 1.
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For the first MSLH of each RTP packet CTSFlag is always 0, since the
composition time stamp of the first SL packet in the RTP packet is
mapped to the RTP time stamp. In all cases the sender MUST remove
the compositionTimeStamp from the RSLH.
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
is not present.
If DTSDeltaLength is not zero, DTSFlag is present in all MSLH
regardless of whether the SL packet is an Access Unit start or not;
the receiver needs this flag in order to reconstruct the
decodingTimeStampFlag of SL Headers.
DTSDelta (DTSDeltaLength bits): encodes (compositionTimeStamp -
decodingTimeStamp) for the same SL packet (always positive). The
length in bits of each DTSDelta field is specified by the
DTSDeltaLength parameter (see section 4.1).
The DTSDelta field appears when DTSFlag is 1. The sender MUST always
remove the decodingTimeStamp from the RSLH.
If DTSDelta is zero i.e. if decodingTimeStamp equals
compositionTimeStamp then DTSFlag MUST be set to 0 and no DTSDelta
field SHALL be present.
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:
+===========================+=================================+
| 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
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This section consists of a field (RSLHSectionSize) giving the size
in bits of the following block of bit-wise concatenated RSLHs.
If the section consumes a non-integer number of bytes, up to 7 zero
padding bits MUST be inserted at the end in order to achieve byte-
alignment.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSLHSectionSize (RSLHSectionSizeLength bits)| RSLH (variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| number of bits) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | RSLH (variable number of bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| etc |
| as many bit-wise concatenated RSLHs |
| as SL Packets in this RTP packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSLH (variable number of bits) |
| +-+-+-+-+-+-+-+
| : padding bits|
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: RSLHSection structure
The length in bits of the RSLHSectionSize field is
RSLHSectionSizeLength and is specified with a default value of zero
indicating that the whole RSLHSection is absent. Compatibility with
RFC 3016 requires that the RSLHSection should be empty, including
the RSLHSectionSize field. This is the reason why there is such a
variable length with a default value indicating absence of the
RSLHSectionSize field.
+=================================+===============================+
| Fields of RSLHSection | Number of bits |
+=================================+===============================+
| RSLHSectionSize | RSLHSectionSizeLength |
+---------------------------------+-------------------------------+
| all bit-wise concatenated RSLHs | RSLHSectionSize |
+---------------------------------+-------------------------------+
Table 2: Sizes in bits inside RSLHSection
Parsing of the bit-wise concatenated RSLHs requires MPEG-4 system
awareness, specifically it requires to understand the MPEG-4
Synchronization Layer (SL) syntax and the modifications to this
syntax described in the next section.
However thanks to the RSLHSectionSize field non-MPEG-4-system
receivers MAY skip this part by rounding up RSLPHSize/8 to the next
integer number of bytes.
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RTP Payload Format for MPEG-4 Streams July 2001
3.6 RSLH structure
A Remaining SL Packet Header (RSLH) is what remains of an SL header
after modifications for mapping into this payload format.
The following modifications of the SL packet header MUST be applied.
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
below.
3.6.1 Removal of fields
The following SL Packet Header fields -if present- are removed since
they are mapped either in the RTP header or in the corresponding
MSLH:
. compositionTimeStampFlag
. compositionTimeStamp
. decodingTimeStampFlag
. decodingTimeStamp
. packetSequenceNumber
. AccessUnitEndFlag (in Single-SL mode only)
The AccessUnitEndFlag, when present for a given stream, MUST be
removed from every RSLH when using the Single-SL mode since it has
the same meaning as the Marker bit (and for compatibility with RFC
3016). However when using the Multiple-SL mode, AccessUnitEndFlag
MUST NOT be removed since it is useful to signal individual AU ends.
3.6.2 Mapping of OCR
Furthermore if the SL Packet header contains an OCR, then this field
is encoded in the RSLH as a 2-complement difference (delta) exactly
like a compositionTimeStamp or a decodingTimeStamp in the MSLH. 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
Time Stamps.
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.
3.6.3 Degradation Priority
For streams that use the optional degradationPriority field in the
SL Packet Headers, only SL packets with the same degradation
priority SHALL be transported by one RTP packet so that components
may dispatch the RTP packets according to appropriate QOS or
protection schemes. Furthermore only the first RSLH of one RTP
packet SHALL contain the degradationPriority field since it would be
otherwise redundant.
3.7 SLPPSection structure
Gentric et al. Expires January 2002 15
RTP Payload Format for MPEG-4 Streams July 2001
The SLPPSection (SL Packet Payload Section) contains the
concatenated SL Packet Payloads. By definition SL Packet Payloads
are byte aligned.
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.
However in the Multiple-SL mode the size of each SL packet payload
MUST be available to the receiver.
If the SL packet payload size is constant for a stream, the size
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
packet payload MUST be indicated in the corresponding MSLH. In order
to do so the MSLH MUST contain a PayloadSize field. The number of
bits on which this PayloadSize field is encoded MUST be indicated
using the SizeLength parameter (see section 4.1).
The absence of either ConstantSize or SizeLength indicates the
Single-SL mode i.e. that a single SL packet is transported in each
RTP packet for that stream.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLPP (variable number of bytes) |
| |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | SLPP (variable number of bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| etc |
| as many byte-wise concatenated SLPPs |
| as SL Packets in this RTP packet |
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: SLPPSection structure
3.8 Interleaving
SL Packets MAY be interleaved. Senders MAY perform interleaving.
Receivers MUST support interleaving.
When interleaving of SL packets is used it SHALL be implemented
using the Index and IndexDelta fields of MSLH.
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RTP Payload Format for MPEG-4 Streams July 2001
The conjunction of RTP sequence number and Index, IndexDelta can
produce a quasi-unique identifier for each SL packet so that a
receiver can unambiguously reconstruct the original order even in
case of out-of-order packets, packet loss or duplication.
However implementors of receivers must take care that when
IndexLength is small, Index will rollover often; for that reason
timestamps SHOULD be used as a basis for implementation of de-
interleaving, i.e. the reordering algorithm should consider
timestamps and IndexDelta first and use Index only when CTS are not
available. Symmetrically senders MUST either use properly large
values for IndexLength or use small values only when CTS are either
present in MSLH or can be otherwise unambiguously computed for each
SL packet (for example audio streams as in Appendix.5).
The AUSequenceNumber field of the SL header MUST NOT be used for
interleaving since firstly it may collide with the Scene Description
Carousel usage described in section 4.1 and secondly it is not
visible to non-MPEG-4 system receivers.
3.9 Fragmentation Rules
This section specifies rules for senders in order to prevent media
decoding difficulties at the receiver end.
MPEG-4 Access Units are the default fragments for MPEG-4 bitstreams
and SHOULD be mapped directly into RTP packets of this format with
two exceptions:
- Access Units larger than the MTU
- 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
fragmentation.
Some MPEG-4 codecs define optional syntax for Access Units sub-
entities (fragments) that are independently decodable for error
resilience purposes. Examples are Video Packets for video and Error
Sensitivity Categories (ESC) for audio. This always corresponds to
specific bitstream syntax, which is signaled in the
DecoderSpecificInfo inside the DecoderConfig in SLConfig, and/or
using the corresponding parameters as described in section 4.1.
Therefore encoders and decoders are both aware whether they are
operating in such a mode or not (however since this codec
configuration is an opaque data block this is not explicitly
signaled by this payload format).
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.
Similarly decoder implementations that do not implement robust
decoding of Access Units fragments have to discard all packets after
a packet loss until an Access Unit start is received. In the same
Gentric et al. Expires January 2002 17
RTP Payload Format for MPEG-4 Streams July 2001
way decoder implementations that do not implement re-synchronization
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
all obvious things that a good implementation would do.
However serious problems would arise for decoder implementations
that try to restart decoding after a packet loss if independently
decodable fragments are signaled (in the decoder configuration) but
the fragments actually received are not independently decodable
because the RTP sender has made RTP packets on different boundaries
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
sender component itself), because the decoder has in general no way
to detect such a faulty fragment.
For this reason the following rules must apply to SL streams that
are specifically made for transport with this payload format:
SL packets SHOULD be codec-semantic entities in the spirit of ALF
i.e. either complete Access Units or fragments of Access Units that
are independently decodable. Specifically when a given codec has an
independently decodable Access Unit fragments optional syntax this
option SHOULD be used.
Furthermore when streams are generated using independently decodable
Access Units fragments these Access Units fragments MUST be mapped
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
NOT split one ESC across 2 RTP packets.
This rule is relaxed when using MPEG-4 Video Packets for two
reasons: firstly Video Packets can be much larger than typical MTU
and secondly all Video Packets start with a specific
resynchronization marker that can be unambiguously detected.
Therefore for video streams using the Video Packet syntax Video
Packets MAY be split across several SL packets although it is
strongly RECOMMENDED to always adapt the Video Packet size to fit
the MTU. A Video Packet start MUST always be aligned with a SL
packet start, except when a GOV is present, in which case the GOV
and the first Video Packet of the following VOP MUST be included in
the same SL packet.
4. Types and Names
This section describes the MIME types and names associated with this
payload format. Section 4.1 is intended for registration with IANA
as in RFC 2048.
This format may require additional information about the mapping to
be made available to the receiver. This is done using parameters
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RTP Payload Format for MPEG-4 Streams July 2001
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
zero. The absence of any such parameters resolves into a default
"basic" configuration compatible with RFC3016 for MPEG-4 video.
In the MPEG-4 framework the SL stream configuration information is
carried using the Object Descriptor. For compatibility with
receivers that do not implement the full MPEG-4 system specification
this information MAY also be signaled using parameters described
here. When such information is present both in an Object Descriptor
and as a parameter of this payload format it MUST be exactly the
same.
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
also possible to transport information on the profile and level of
the stream and on the decoder configuration. This is also described
in the next section.
Finally this MIME type also defines a mode parameter and a profile
parameter that are intended for future derivations of this payload
format.
4.1 MIME type registration
MIME media type name: "video" or "audio" or "application"
"video" SHOULD be used for MPEG-4 Visual streams (i.e. video as
defined in ISO/IEC 14496-2 [2] and/or graphics as defined in ISO/IEC
14496-1 [1]) or MPEG-4 Systems streams that convey information
needed for an audio/visual presentation.
"audio" SHOULD be used for MPEG-4 Audio streams (ISO/IEC 14496-3) or
MPEG-4 Systems streams that convey information needed for an audio
only presentation.
"application" SHOULD be used for MPEG-4 Systems streams
(ISO/IEC14496-1) that serve other purposes than audio/visual
presentation, e.g. in some cases when MPEG-J streams are
transmitted.
MIME subtype name: mpeg4-generic
Required parameters: none
Optional parameters:
Mode:
The mode in which this specification is used. This specification
itself defines only the default mode (Mode=default). When the mode
parameter is not present the default mode SHALL be assumed. In the
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
Gentric et al. Expires January 2002 19
RTP Payload Format for MPEG-4 Streams July 2001
subset of this specification. Specifically when defining a mode care
MUST be taken that an implementation of this specification can
decode the payload format corresponding to this new mode. For this
reason a mode MUST NOT specify new default values for MIME
parameters and MIME parameters MUST be present (unless they have the
default value) even if it is redundant in case the mode assigns
fixed values. A mode may define additionally that some MIME
parameters are required instead of optional, that some MIME
parameters have fixed values (or ranges), and that there are rules
restricting the usage (for example forbidding the carriage of
multiple AU fragments in the same RTP packet).
Profile:
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
mode, for example the maximum delay (and therefore the size of
buffers) induced by the usage of interleaving. Implementations of
this specification can ignore this parameter.
DTSDeltaLength:
The number of bits on which the DTSDelta field is encoded in MSLH.
The default value is zero and indicates the absence of DTSFlag and
DTSDelta in MSLH (the stream does not transport decodingTimeStamps).
A value larger than zero indicates that there is a DTSFlag in each
MSLH. Since decodingTimeStamp -if present- must be encoded as a
difference to the RTP time stamp, the DTSDeltaLength parameter MUST
be present in order to transport decodingTimeStamps with this
payload format.
CTSDeltaLength:
The number of bits on which the CTSDelta field is encoded in (non-
first) MSLH. The default value is zero and indicates the absence of
the CTSFlag and CTSDelta fields in MSLH. Non-zero values MUST NOT be
signaled in the Single-SL mode. Since compositionTimeStamps ûif
present- must be encoded as a difference to the RTP time stamp, the
CTSDeltaLength parameter MUST be present in order to transport
compositionTimeStamps using this payload format (in the Multiple-SL
mode). However CTSDeltaLength SHOULD be set to zero (or not
signaled) for streams that have a constant Access Unit duration
(which can be explicitly signaled using the DurationFlag and
AccessUnitDuration field of SLConfigDescriptor).
OCRDeltaLength:
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
stream. Since objectClockReference -if present- must be encoded as a
difference to the RTP time stamp, the OCRDeltaLength parameter MUST
be present in order to transport objectClockReferences with this
payload format.
SizeLength:
The number of bits on which the PayloadSize field of MSLH is
encoded. The default value is zero and indicates the Single-SL mode
Gentric et al. Expires January 2002 20
RTP Payload Format for MPEG-4 Streams July 2001
(unless ConstantSize is present). Simultaneous presence of this
parameter and ConstantSize is illegal. Either the SizeLength or
ConstantSize parameter MUST be present in order to signal the
Multiple-SL mode of this payload format.
ConstantSize:
The constant size in bytes of each SL Packet Payload for this
stream. The default value is zero and indicates variable SL Packet
Payload size (or the Single-SL mode if SizeLength is 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-SL mode of this payload format. When
ConstantSize is present the PayloadSize of MSLH in the RTP packets
MUST NOT be present.
IndexLength:
The number of bits on which the Index is encoded in the first MSLH.
The default value is zero and indicates the absence of Index and
IndexDelta for all MSLHs. Since packetSequenceNumber -if present-
must be mapped in MSLH, the IndexLength parameter MUST be present in
order to transport packetSequenceNumber with this payload format.
IndexDeltaLength:
The number of bits on which the IndexDelta are encoded in any non-
first MSLH. The default value is zero and indicates that
packetSequenceNumber MUST be incremented by one for each SL packet
in the RTP packet (see section 3.5). IndexDeltaLength parameter MUST
be present when using interleaving with this payload format.
RSLHSectionSizeLength:
The number of bits that is used to encode the RSLHSectionSize field.
The default value is zero and indicates the absence of the whole
RSLHSection for all RTP packets of this stream.
SLConfigDescriptor:
A base-64 encoding of the SLConfigDescriptor. This SHALL be the
original SLConfigDescriptor and it SHALL be the same as the one
transported by the OD framework, if any.
profile-level-id:
A decimal representation of the MPEG-4 Profile Level indication
value. For audio this parameter indicates which MPEG-4 Audio tool
subsets are applied to encode the audio stream and is defined in
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
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
indicate MPEG-4 Profile and Level combination of which the relevant
MPEG-4 media codec is capable. If this parameter is not specified
its default value is 1 (Simple Profile/Level 1) for video (for
compatibility with RFC 3016) and otherwise 0xFE (defined in ISO/IEC
14496-1 [1] as being the generic default value).
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RTP Payload Format for MPEG-4 Streams July 2001
Config:
A hexadecimal representation of an octet string that expresses the
media payload configuration. Configuration data is mapped onto 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.
In the last octet, zero-valued padding bits, if necessary, shall
follow the configuration data. For audio streams, config is the
audio object type specific decoder configuration data
AudioSpecificConfig() as defined in ISO/IEC 14496-3 [3]. For video
this expresses the MPEG-4 Visual configuration information, as
defined in subclause 6.2.1 Start codes of ISO/IEC14496-2 [2] and the
configuration information indicated by this parameter SHALL be the
same as the configuration information in the corresponding MPEG-4
Visual stream, except for first-half-vbv-occupancy and latter-half-
vbv-occupancy, if it exists, which may vary in the repeated
configuration information inside an MPEG-4 Visual stream (See 6.2.1
Start codes of ISO/IEC14496-2).
StreamType:
The integer value that indicates the type of MPEG-4 stream that is
carried; its coding corresponds to the values of the streamType as
defined for the DecoderConfigDescriptor in ISO/IEC 14496-1.
Encoding considerations:
System bitstreams MUST be generated according to MPEG-4 System
specifications (ISO/IEC 14496-1). Video bitstreams MUST be generated
according to MPEG-4 Visual specifications (ISO/IEC 14496-2). Audio
bitstreams MUST be generated according to MPEG-4 Visual
specifications (ISO/IEC 14496-3). All SL streams MUST be generated
according to MPEG-4 Sync Layer specifications (ISO/IEC 14496-1
section 10), in order to read this format the SLConfigDescriptor may
be required. These bitstream are binary data and MUST be encoded for
non-binary transport (for Email, the Base64 encoding is sufficient).
This type is also defined for transfer via RTP. The RTP packets
MUST be packetized according to the RTP payload format defined in
RFC <self-reference-to-this>.
Security considerations:
As in RFC <self-reference-to-this>.
Interoperability considerations:
MPEG-4 provides a large and rich set of tools for the coding of
visual objects. For effective implementation of the standard,
subsets of the MPEG-4 tool sets have been provided for use in
specific applications. These subsets, called 'Profiles', limit the
size of the tool set a decoder is required to implement. In order to
restrict computational complexity, one or more 'Levels' are set for
each Profile. A Profile@Level combination allows:
. a codec builder to implement only the subset of the standard he
needs, while maintaining interoperability with other MPEG-4 devices
included in the same combination, and
. checking whether MPEG-4 devices comply with the standard
('conformance testing').
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RTP Payload Format for MPEG-4 Streams July 2001
A stream SHALL be compliant with the MPEG-4 Profile@Level specified
by the parameter "profile-level-id". Interoperability between a
sender and a receiver may be achieved by specifying the parameter
"profile-level-id" in MIME content, or by arranging in the
capability exchange/announcement procedure to set this parameter
mutually to the same value.
Published specification:
The specifications for MPEG-4 streams are presented in ISO/IEC
14469-1, 14469-2, and 14469-3. The RTP payload format is described
in RFC <self-reference-to-this>.
Applications that use this media type:
Multimedia streaming and conferencing tools, Internet messaging and
Email applications. Also trans-galactic supra-relativistic
elementary particle hyperspace tunneling communication devices :-)
Additional information: none
Magic number(s): none
File extension(s):
None. A file format with the extension .mp4 has been defined for
MPEG-4 content but is not directly correlated with this MIME type
which sole purpose is RTP transport.
Macintosh File Type Code(s): none
Person & email address to contact for further information:
Authors of RFC <self-reference-to-this>.
Intended usage: COMMON
Author/Change controller:
Authors of RFC <self-reference-to-this>.
4.2 Concatenation of parameters
Multiple parameters SHOULD be expressed as a MIME media type string,
in the form of a semicolon-separated list of parameter=value pairs
(see examples in Appendix).
4.3 Usage of SDP
4.3.1 The a=fmtp keyword
It is assumed that one typical way to transport the above-described
parameters associated with this payload format is via an SDP [10]
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)
keyword MUST be used as described in RFC 2327 [10, section 6]. The
syntax being then:
Gentric et al. Expires January 2002 23
RTP Payload Format for MPEG-4 Streams July 2001
a=fmtp:<format> <parameter name>=<value>
4.3.2 SDP example
The following is an example of SDP syntax for the description of a
session containing one MPEG-4 audio stream, one MPEG-4 video and
three MPEG-4 system streams, the first one being BIFS, the second
one OD and the third one IPMP. All are transported using this format
and the AVP profile [12]. Note that the video stream DTSDelta are
encoded on 4 bits in this example. See the Appendix for more
examples.
o= ....
I= ....
c=IN IP4 123.234.71.112
m=video 1034 RTP/AVP 97
a=fmtp:97 StreamType=4;DTSDeltaLength=4
a=rtpmap:97 mpeg4-sl
m=audio 810 RTP/AVP 98
a=fmtp:98 StreamType=5; profile-level-id=1; config=7866E7E6EF
a=rtpmap:98 mpeg4-sl
m=application 1234 RTP/AVP 99
a=rtpmap:99 mpeg4-sl
a=fmtp:99 StreamType=3;
m=application 1236 RTP/AVP 99
a=rtpmap:99 mpeg4-sl
a=fmtp:99 StreamType=1;
m=application 1238 RTP/AVP 99
a=rtpmap:99 mpeg4-sl
a=fmtp:99 StreamType=7;
5. Other issues
5.1 SL packetized stream reconstruction
The purpose of this section is to document how a receiver can
reconstruct a valid SL packetized stream. Since this format directly
transports SL packets this reconstruction is performed by reversing
the payload structure rules (section 3). We explicitly describe here
the most complex transformations.
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
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
MSLH.IndexDelta using:
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RTP Payload Format for MPEG-4 Streams July 2001
If ( IndexLength == 0) { // or is absent
if ( SLConfig.packetSeqNumLength == 0 ) {
// this stream does not have SL packet sequence number
}
else {
// illegal, normally the sender MUST map
// SLPacketHeader.packetSequenceNumber in MSLH
// and set a relevant IndexLength value;
// otherwise it is unfortunately impossible for the receiver
// to reconstruct the correct sequence
}
}
else { // IndexLength is not zero
if ( SLConfig.packetSeqNumLength == 0 ) {
// the original SL stream does not have SL packet
// sequence numbers, typically the sender inserted them
// in order to implement interleaving at the RTP level;
// they must be ignored for SL stream reconstruction
}
else {
if (i == 0){ // first SL packet in RTP packet
SLPacketHeader.packetSequenceNumber(0) = MSLH.Index(0);
}
else { // remaining SL packets
SLPacketHeader.packetSequenceNumber(i+1)=
SLPacketHeader.packetSequenceNumber(i)
+ MSLH.IndexDelta(i+1)
+1;
}
}
All time stamps (CTS, DTS, OCR), when present, are restored from the
delta values. Time stamps flags (CTSFlag, DTSFlag) in MSLH are used
to reconstruct respectively the compositionTimeStampFlag and
decodingTimeStampFlag of SLPacketHeader.
if ( CTSDeltaLength == 0) { // or CTSDeltaLength is absent
// CTS is not transported for this RTP stream
if (i == 0){ // first SL packet in RTP packet
if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.compositionTimeStampFlag(0) = 1;
SLPacketHeader.compositionTimeStamp(0) = RTP TimeStamp;
}
else {
// ignore
}
}
else {
// empty
}
}
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RTP Payload Format for MPEG-4 Streams July 2001
else { // non-first SL packets in RTP packet
if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.compositionTimeStampFlag(i) = 0;
}
else {
// ignore
}
}
else {
// empty
}
}
}
else { // CTSDeltaLength is not zero
// CTS is transported for this stream
if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.compositionTimeStampFlag(i) =
MSLH.CTSFlag(i);
SLPacketHeader.compositionTimeStamp(i) =
RTP TimeStamp + MSLH.CTSDelta(i);
}
else {
// ignore CTSFlag (which must be zero)
}
else {
// this is strange and sub-optimal at best
// a receiver should ignore this
}
}
if ( DTSDeltaLength == 0) { // or DTSDeltaLength is absent
// DTS is not transported for this stream
if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.decodingTimeStampFlag(i) = 0;
}
else {
// ignore
}
}
else {
// empty
}
}
else {
// DTS is transported for this stream
if ( SLConfig.useTimeStamps == 1 ) {
if ( SLPacketHeader.accessUnitStartFlag == 1 ) {
SLPacketHeader.decodingTimeStampFlag(i) =
MSLH.DTSFlag(i);
SLPacketHeader.decodingTimeStamp(i) =
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RTP Payload Format for MPEG-4 Streams July 2001
RTP TimeStamp + MSLH.DTSDelta(i);
}
else {
// ignore DTSFlag (which must be zero)
}
}
else {
// this is strange and sub-optimal at best
// a receiver should ignore this
}
}
if ( OCRDeltaLength == 0) { // or OCRDeltaLength is absent
// the RTP stream does not transport any OCR
if ( SLConfig.OCRLenght == 0 ) {
// this stream does not have any OCR
}
else {
// illegal, normally the sender MUST detect
// OCRs, replace them with OCRDelta and set
// a relevant OCRDeltaLength value
}
}
else {
if ( SLConfig.OCRLenght == 0 ) {
// this is strange and sub-optimal at best
// a receiver should ignore this
}
else {
SLPacketHeader.OCRflag(i) = RSLH.OCRFlag(i);
if ( SLPacketHeader.OCRflag(i) == 1) {
SLPacketHeader.objectClockReference(i) =
RTP TimeStamp + RSLH.OCRDelta(i);
}
}
}
In the SingleSL mode the AccessUnitEndFlag, if needed, is restored
from the M bit, as follows:
if ( SLConfig.useAccessUnitEndFlag == 0 ) {
// this SL stream does not signal access unit ends
else {
SLPacketHeader.AccessUnitEndFlag = M bit;
}
In the multipleSL mode the AccessUnitEndFlag is untouched 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
original SL packetized stream requires SL-awareness. However this
Gentric et al. Expires January 2002 27
RTP Payload Format for MPEG-4 Streams July 2001
payload format allows in all cases a receiver that does not know
about the SL syntax to reconstruct the semantic of SL for the
following very useful features:
- Packet order (decoding order)
- Access Unit boundaries (using the M bit)
- Access Unit fragments (i.e. SL packet boundaries using
MSLH.PayloadSize)
- Composition Time Stamps (using the RTP Time Stamp and
MSLH.CTSDelta)
- Decoding Time Stamps (using the RTP Time Stamp and MSLH.DTSDelta)
- Packet sequence number (using the RTP Time Sequence number and
MSLH.Index)
5.2 Handling of scene description streams
MPEG-4 introduces new stream types as described in section 1 namely
Object Descriptors and BIFS. In the following both OD and BIFS are
discussed on the same basis i.e. as "scene description".
Considering scene description as a "stream-able" type of content is
a rather new concept and for that reasons some specific comments are
needed.
Typically scene descriptions are encoded in such a way that
information loss would in the general case cripple the presentation
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
available via reliable channels to the client and then played. This
payload format is not intended for this type of applications for
which download of MPEG-4 interchange (.mp4) files is typical.
However it can also be used if the RTP packets are transported using
TCP or any other reliable protocol.
On the other hand MPEG-4 has introduced the possibility to
dynamically change the scene description by sending animation
information (changes in parameters) and structural change
information (updates). Since this information has to be sent in a
timely fashion MPEG-4 has defined a number of techniques in order to
encode the scene description in a manner that makes it behave
similarly to other temporal encoding schemes such as audio and
video. This payload format is intended for this usage.
Note that in many cases the application will consist of first the
reliable transmission of a static initial scene followed by the
streaming of animations and updates. For this reason the usage of
this payload format is attractive since it offers a unique solution.
Senders must be aware that suitable schemes should be used when
scene description streams transport sensitive configuration
information. For example in case the RTP packet transporting an OD-
update command would be lost, the corresponding media stream would
not be accessible by the receiver.
Gentric et al. Expires January 2002 28
RTP Payload Format for MPEG-4 Streams July 2001
Redundancy is a possibility and may either be added by tools
hierarchically higher than this payload format, e.g. by packet based
FEC, re-transmission, or similar tools. In such a case, the general
congestion control principles have to be observed.
Since BIFS and OD streams may be modified during the session with
update commands, there is a need to send both update commands and
full BIFS/OD refresh. For that reason MPEG-4 defines Random Access
Points (RAP) for scene description streams (OD and BIFS) where by
definition a decoder can restart decoding i.e. receives a "full
update" of the scene. This mechanism is called Scene and Object
Description Carrousel. The AU Sequence Number field of SL Packet
Header is used to support this behavior at the Synchronization
Layer. When two access units are sent consecutively with the same AU
Sequence Number, the second one is assumed to be a semantic
repetition of the first. If a receiver starts to listen in the
middle of a session or has detected losses, it can skip all received
Access Units until 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
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
individual RTP stream may not always be practical. Allocating and
controlling hundreds of destination addresses for each MPEG-4
session may pose insurmountable session administration problems.
The input/output processing overhead at the end-points will be
extremely high also. Additionally, low delay transmission of low
bitrate data streams, e.g. facial animation parameters, results in
extremely high header overheads.
To solve these problems, MPEG-4 data transport requires a
multiplexing scheme that allows selective bundling of several ESs.
This is beyond the scope of the payload format defined here.
The MPEG-4's Flexmux multiplexing scheme may be used for this
purpose and a specific RTP payload format is being developed [11].
Another approach may be to develop a generic RTP multiplexing scheme
usable for MPEG-4 data. The multiplexing scheme reported in [8] may
be a candidate for this approach.
For MPEG-4 applications, the multiplexing technique needs to address
the following requirements:
i. The ESs multiplexed in one stream can change frequently during a
session. Consequently, the coding type, individual packet size and
temporal relationships between the multiplexed data units must be
handled dynamically.
Gentric et al. Expires January 2002 29
RTP Payload Format for MPEG-4 Streams July 2001
ii. The multiplexing scheme should have a mechanism to determine the
ES identifier (ES_ID) for each of the multiplexed packets. ES_ID is
not a part of the SL header.
iii. In general, an SL packet does not contain information about its
size. The multiplexing scheme should be able to delineate the
multiplexed packets whose lengths may vary from a few bytes to close
to the path-MTU.
5.5 Overlap with RFC 3016
This payload format has been designed to have a (large) overlap with
RFC 3016 [7]. The conditions for this overlap are:
Conditions for RFC 3016:
i. MPEG-4 video elementary streams only
ii. There MUST be a single VOP or Video Packet per RTP packet (only
recommended in RFC 3016)
iii. The decoder configuration MUST be signaled out-of-band either
using the Config mime parameter or using the OD framework
Conditions for this payload format:
i. No structural parameters defined (or all set to zero), i.e.
Single-SL mode with empty MSLH and empty RSLH.
ii. Receivers MUST be ready to accept (and ignore) video
configuration headers (e.g. VOSH, VO and VOL) and visual-object-
sequence-end-code transported in-band.
6. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [5]. This implies that confidentiality of the media
streams is achieved by encryption. Because the data compression used
with this payload format is applied end-to-end, encryption may be
performed on the compressed data so there is no conflict between the
two operations. The packet processing complexity of this payload
type (i.e. excluding media data processing) does not exhibit any
significant non-uniformity in the receiver side to cause a denial-
of-service threat.
However, it is possible to inject non-compliant MPEG streams (Audio,
Video, and Systems) to overload the receiver/decoder's buffers which
might compromise the functionality of the receiver or even crash it.
This is especially true for end-to-end systems like MPEG where the
buffer models are precisely defined.
MPEG-4 Systems supports stream types including commands that are
executed on the terminal like OD commands, BIFS commands, etc. and
programmatic content like MPEG-J (Java(TM) Byte Code) and
ECMAScript. It is possible to use one or more of the above in a
manner non-compliant to MPEG to crash or temporarily make the
receiver unavailable.
Gentric et al. Expires January 2002 30
RTP Payload Format for MPEG-4 Streams July 2001
Authentication mechanisms can be used to validate of the sender and
the data to prevent security problems due to non-compliant malignant
MPEG-4 streams.
A security model is defined in MPEG-4 Systems streams carrying MPEG-
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
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
model. According to this security model, downloaded byte code is
forbidden to load libraries, define native methods, start programs,
read or write files, or read system properties.
Receivers can implement intelligent filters to validate the buffer
requirements or parametric (OD, BIFS, etc.) or programmatic (MPEG-J,
ECMAScript) commands in the streams. However, this can increase the
complexity significantly.
7. Acknowledgements
This document evolved across several years thanks to contributions
from a large number of people since it is based on work within the
IETF AVT working group and various ISO MPEG working groups,
especially the 4-on-IP ad-hoc group in the last stages. The authors
wish to thank Guido Fransceschini, Art Howarth, Dave Mackie, Dave
Singer, and Stephan Wenger for their valuable comments.
8. References
[1] ISO/IEC 14496-1:2001 MPEG-4 Systems
[2] ISO/IEC 14496-2:2001 MPEG-4 Visual
[3] ISO/IEC 14496-3:2001 MPEG-4 Audio
[4] ISO/IEC 14496-6:2001 Delivery Multimedia Integration Framework.
[5] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A
Transport Protocol for Real Time Applications, RFC 1889, Internet
Engineering Task Force, January 1996.
[6] S. Bradner, Key words for use in RFCs to Indicate Requirement
Levels, RFC 2119, Internet Engineering Task Force, March 1997.
[7] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, H. Kimata, RTP
payload format for MPEG-4 Audio/Visual streams, Internet Engineering
Task Force, RFC 3016.
[8] B. Thompson, T. Koren, D. Wing, Tunneling multiplexed Compressed
RTP ("TCRTP"), work in progress, draft-ietf-avt-tcrtp-02.txt,
November 2000.
Gentric et al. Expires January 2002 31
RTP Payload Format for MPEG-4 Streams July 2001
[9] D. Singer, Y Lim, A Framework for the delivery of MPEG-4 over
IP-based Protocols, work in progress, draft-singer-mpeg4-ip-02.txt,
May 2001.
[10] M. Handley, V. Jacobson, SDP: Session Description Protocol, RFC
2327, Internet Engineering Task Force, April 1998.
[11] C.Roux & al, RTP Payload Format for MPEG-4 FlexMultiplexed
Streams, work in progress, draft-curet-avt-rtp-mpeg4-flexmux-00.txt,
February 2001.
[12] H. Schulzrinne, RTP Profile for Audio and Video Conferences
with Minimal Control, RFC 1890, Internet Engineering Task Force,
January 1996.
[13] H. Schulzrinne, A. Rao, R. Lanphier, Real Time Streaming
Protocol, RFC 2326, Internet Engineering Task Force, April 1998.
[14] M. Handley, C. Perkins, E. Whelan, Session Announcement
Protocol, RFC 2974, Internet Engineering Task Force, October 2000.
9. Authors' Addresses
Olivier Avaro
France Telecom
35 A Schutzenhuttenweg
60598 Frankfurt am Main
Deutschland
e-mail: olivier.avaro@francetelecom.fr
Andrea Basso
AT&T Labs Research
200 Laurel Avenue
Middletown, NJ 07748
USA
e-mail: basso@research.att.com
Stephen L. Casner
Packet Design, Inc.
66 Willow Place
Menlo Park, CA 94025
USA
e-mail: casner@acm.org
M. Reha Civanlar
AT&T Labs - Research
100 Schultz Drive
Red Bank, NJ 07701
USA
e-mail: civanlar@research.att.com
Philippe Gentric
Gentric et al. Expires January 2002 32
RTP Payload Format for MPEG-4 Streams July 2001
Philips Digital Networks û MP4Net
51 rue Carnot
92156 Suresnes
France
e-mail: philippe.gentric@philips.com
Carsten Herpel
THOMSON multimedia
Karl-Wiechert-Allee 74
30625 Hannover
Germany
e-mail: herpelc@thmulti.com
Zvi Lifshitz
Optibase Ltd.
7 Shenkar St.
Herzliya 46120
Israel
e-mail: zvil@optibase.com
Young-kwon Lim
mp4cast (MPEG-4 Internet Broadcasting Solution Consortium)
1001-1 Daechi-Dong Gangnam-Gu
Seoul, 305-333,
Korea
e-mail : young@techway.co.kr
Colin Perkins
USC Information Sciences Institute
4350 N. Fairfax Drive #620
Arlington, VA 22203
USA
e-mail : csp@isi.edu
Jan van der Meer
Philips Digital Networks
Cederlaan 4
5600 JB Eindhoven
Netherlands
e-mail : jan.vandermeer@philips.com
APPENDIX: Examples of usage
This payload format has been designed to transport efficiently a
very versatile packetization scheme: the MPEG-4 Synch Layer; as a
result its complexity is larger than the average RTP payload format.
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RTP Payload Format for MPEG-4 Streams July 2001
For this reason this section describes a number of key examples of
how this payload format can be used.
A C++-like syntax called SDL (Syntactic Description Language)
defined in [1, section 14] is used to economically describe MPEG-4
system data structures.
However, as discussed in section 2, this payload format can also be
used without explicit knowledge of SL (logically equivalent to
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)
This is an example of a video stream where the SL is configured to
produce RTP packets compatible with RFC 3016.
SLConfigDescriptor
In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag {
bit(8) predefined;
if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 0
bit(1) useAccessUnitEndFlag; = 1
bit(1) useRandomAccessPointFlag; = 0
bit(1) hasRandomAccessUnitsOnlyFlag; = 0
bit(1) usePaddingFlag; = 0
bit(1) useTimeStampsFlag; = 0
bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 0
bit(32) timeStampResolution; = 0
bit(32) OCRResolution; = 0
bit(8) timeStampLength; = 0
bit(8) OCRLength; = 0
bit(8) AU_Length; = 0
bit(8) instantBitrateLength; = 0
bit(4) degradationPriorityLength; = 0
bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 0
bit(2) reserved=0b11;
}
if (durationFlag) {
bit(32) timeScale; // NOT USED
bit(16) accessUnitDuration; // NOT USED
bit(16) compositionUnitDuration; // NOT USED
}
if (!useTimeStampsFlag) {
Gentric et al. Expires January 2002 34
RTP Payload Format for MPEG-4 Streams July 2001
bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0
}
}
SL Packet Header structure
With this configuration we have the following SL packet header
structure:
aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
if (SL.useAccessUnitEndFlag) {
bit(1) accessUnitEndFlag; // 1 bit
}
}
In this case this payload produces RTP packets that are exactly
conformant to RFC 3016 and the Synch Layer is reduced to a purely
logical construction that neither sender nor receiver need to
implement.
Parameters
This configuration is the default one; no parameters are required.
RTP packet structure
Note that accessUnitEndFlag is mapped to the RTP header M bit.
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
| SL packet payload | 1400 bytes |
+-----------------------------------------+-------------+
Overhead
In this example we have an RTP overhead of 40 bytes for 1400 bytes
of payload i.e. 3 % overhead.
Appendix.2 MPEG-4 Video with SL
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
split in several SL packets (typically above 300 kb/s).
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
MTU aware and the MTU is 1500 bytes. We assume furthermore that this
stream contains B frames and that decodingTimeStamps are present.
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RTP Payload Format for MPEG-4 Streams July 2001
SLConfigDescriptor
In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag {
bit(8) predefined;
if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 1
bit(1) useAccessUnitEndFlag; = 0
bit(1) useRandomAccessPointFlag; = 1
bit(1) hasRandomAccessUnitsOnlyFlag; = 0
bit(1) usePaddingFlag; = 0
bit(1) useTimeStampsFlag; = 1
bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 0
bit(32) timeStampResolution; = 30
bit(32) OCRResolution; = 0
bit(8) timeStampLength; = 32
bit(8) OCRLength; = 0
bit(8) AU_Length; = 0
bit(8) instantBitrateLength; = 0
bit(4) degradationPriorityLength; = 0
bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 0
bit(2) reserved=0b11;
}
if (durationFlag) {
bit(32) timeScale; // NOT USED
bit(16) accessUnitDuration; // NOT USED
bit(16) compositionUnitDuration; // NOT USED
}
if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; // NOT USED
bit(timeStampLength) startCompositionTimeStamp; // NOT USED
}
}
The useRandomAccessPointFlag is set so that the
randomAccessPointFlag can indicate that the corresponding SL packet
contains a GOV and the first Video Packet of an Intra coded frame.
SL Packet Header structure
With this configuration we have the following SL packet header
structure:
aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
bit(1) accessUnitStartFlag; // 1 bit
if (accessUnitStartFlag) {
bit(1) randomAccessPointFlag; // 1 bit
bit(1) decodingTimeStampFlag; // 1 bit
bit(1) compositionTimeStampFlag; // 1 bit
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RTP Payload Format for MPEG-4 Streams July 2001
if (decodingTimeStampFlag) {
bit(SL.timeStampLength) decodingTimeStamp;
}
if (compositionTimeStampFlag) {
bit(SL.timeStampLength) compositionTimeStamp;
}
}
Parameters
decodingTimeStamps are encoded on 32 bits, which is much more than
needed for delta. Therefore the sender will use DTSDeltaLength to
signal that only 7 bits are used for the coding of relative DTS in
the RTP packet.
The RSLHSectionSize cannot exceed 2 bits, which is encoded on 2 bits
and signaled by RSLHSectionSizeLength. The resulting concatenated
fmtp line is:
a=fmtp:<format> DTSDeltaLength=7;RSLHSectionSizeLength=3
RTP packet structure
Two cases can occur; for packets that transport first fragments of
Access Units we have:
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
| DTSFlag = 1 | 1 bit |
+-----------------------------------------+-------------+
| DTSDelta | 7 bits |
+-----------------------------------------+-------------+
| bits to byte alignment | 0 bits |
+-----------------------------------------+-------------+
| RSLHSectionSize = 4 | 3 bits |
+-----------------------------------------+-------------+
| accessUnitStartFlag = 1 | 1 bit |
+-----------------------------------------+-------------+
| randomAccessPointFlag | 1 bit |
+-----------------------------------------+-------------+
| decodingTimeStampFlag | 1 bit |
+-----------------------------------------+-------------+
| compositionTimeStampFlag | 1 bit |
+-----------------------------------------+-------------+
| bits to byte alignment | 1 bit |
+-----------------------------------------+-------------+
| SL packet payload | N bytes |
+-----------------------------------------+-------------+
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RTP Payload Format for MPEG-4 Streams July 2001
For packets that transport non-first fragments of Access Units we
have:
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
| DTSFlag = 0 | 1 bit |
+-----------------------------------------+-------------+
| bits to byte alignment | 7 bits |
+-----------------------------------------+-------------+
| RSLHSectionSize = 1 | 3 bits |
+-----------------------------------------+-------------+
| accessUnitStartFlag = 0 | 1 bit |
+-----------------------------------------+-------------+
| bits to byte alignment | 4 bits |
+-----------------------------------------+-------------+
| SL packet payload | N bytes |
+-----------------------------------------+-------------+
Overhead estimation
In this example we have a RTP overhead of 40 + 2 bytes for 1400
bytes of payload i.e. 3 % overhead.
Appendix.3 Low delay MPEG-4 Audio (no SL)
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
packet contains a complete Access Unit.
SLConfigDescriptor
Since CTS=DTS and Access Unit duration is constant signaling of
MPEG-4 time stamps is not needed (the durationFlag of SLConfig is
set)
We also assume here an audio Object Type for which all Access Units
are Random Access Points, which is signaled using the
hasRandomAccessUnitsOnlyFlag in the SLConfigDescriptor.
We assume furthermore a mode where the Access Unit size is constant
and equal to 5 bytes (which is signaled with AU_Length).
In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag {
bit(8) predefined;
if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 0
bit(1) useAccessUnitEndFlag; = 0
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RTP Payload Format for MPEG-4 Streams July 2001
bit(1) useRandomAccessPointFlag; = 0
bit(1) hasRandomAccessUnitsOnlyFlag; = 1
bit(1) usePaddingFlag; = 0
bit(1) useTimeStampsFlag; = 0
bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 1 // signals constant AU duration
bit(32) timeStampResolution; = 0
bit(32) OCRResolution; = 0
bit(8) timeStampLength; = 0
bit(8) OCRLength; = 0
bit(8) AU_Length; = 5
bit(8) instantBitrateLength; = 0
bit(4) degradationPriorityLength; = 0
bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 0
bit(2) reserved=0b11;
}
if (durationFlag) {
bit(32) timeScale; = 1000 // for milliseconds
bit(16) accessUnitDuration; = 10 // ms
bit(16) compositionUnitDuration; = 10 // ms
}
if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0
}
}
SL packet header
With this configuration the SL packet header is empty. The Synch
Layer is reduced to a purely logical construction that neither
sender nor receiver need to implement.
Parameters
No parameters are required.
RTP packet structure
Note that the RTP header M bit should be always set to 1.
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
| SL packet payload | 5 bytes |
+-----------------------------------------+-------------+
Overhead estimation
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RTP Payload Format for MPEG-4 Streams July 2001
The overhead is extremely large i.e. more than 800 %, since 40 bytes
of headers are required to transport 5 bytes of data. Note however
that RTP header compression would work well since time stamps
increments are constant.
Appendix.4 Media delivery MPEG-4 Audio (no SL)
This example is for a media delivery service where delay is not an
issue but efficiency is. In this case several SL Packets are
transported in each RTP packet.
SLConfigDescriptor
Similar to previous example.
SL packet header
With this configuration the SL packet header is empty. The Synch
Layer is reduced to a purely logical construction that neither
sender nor receiver need to implement.
Parameters
The absence of RSLHSectionSizeLength indicates that the RSLHSection
is empty.
The size of SL Packets (which are all complete Access Units in this
case) is constant and is indicated with:
a=fmtp:<format> ConstantSize=5
This also indicates to the receiver that the Multiple-SL mode will
be used, the 2 bytes field that would give the size of the
MSLHSection is ommited since in this case this field always contains
zero (the MSLHSection is always empty).
RTP packet structure
Note that the RTP header M bit is always set to 1, which indicates
to the receiver that only complete Access Units are transported.
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
| SL packet payload | 5 bytes |
+-----------------------------------------+-------------+
| SL packet payload | 5 bytes |
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RTP Payload Format for MPEG-4 Streams July 2001
+-----------------------------------------+-------------+
| etc, until MTU is reached |
+-----------------------------------------+-------------+
| SL packet payload | 5 bytes |
+-----------------------------------------+-------------+
Overhead estimation
The overhead is 3% i.e. minimal.
Appendix.5 AAC with interleaving (no SL)
Let us consider AAC at 128 kb/s where each Access Unit is in the
average 320 bytes. Interleaving is applied with a continuous
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.
IndexDelta is constant and equal to 2 (since +1 is automatically
added); it is encoded on 3 bits.
Index (being encoded on 3 bits) rolls over very fast and is not very
useful for reordering. However this a case as explained in section
3.8 where time stamps should be used for de-interleaving; receivers
know that each SL packet is a complete Access Unit because all RTP
packets have the M bit set to 1 and therefore, since Access Unit
duration is constant, Access Unit timestamps can be computed from
RTP timestamps and IndexDelta values; this can be used for de-
interleaving even in case of losses.
+-----------------------------------------------------------------+
| RTP packet | RTP Timestamp | Aus | Index,IndexDelta |
+-----------------------------------------------------------------+
| 1 | CTS(AU1) | 1 | 1 |
+-----------------------------------------------------------------+
| 2 | CTS(AU2) | 2, 5 | 2,2 |
+-----------------------------------------------------------------+
| 3 | CTS(AU3) | 3, 6, 9 | 3,2,2 |
+-----------------------------------------------------------------+
| 4 | CTS(AU4) | 4, 7,10,13 | 4,2,2,2 |
+-----------------------------------------------------------------+
| 5 | CTS(AU8) | 8,11,14,17 | 0,2,2,2 |
+-----------------------------------------------------------------+
| 6 | CTS(AU12) | 12,15,18,21 | 4,2,2,2 |
+-----------------------------------------------------------------+
| 7 | CTS(AU16) | 16,19,22,25 | 0,2,2,2 |
+----------------------------------------------------------------+
| 8 | CTS(AU20) | 20,23,26,29 | 4,2,2,2 |
+-----------------------------------------------------------------+
| 9 | CTS(AU24) | 24,27,30,33 | 0,2,2,2 |
+-----------------------------------------------------------------+
| 10 | CTS(AU28) | 28,31,34,37 | 4,2,2,2 |
Gentric et al. Expires January 2002 41
RTP Payload Format for MPEG-4 Streams July 2001
+-----------------------------------------------------------------+
| etc |
+-----------------------------------------------------------------+
SLConfigDescriptor
Similar to previous example.
SL Packet Header
Similar to previous example (empty).
Parameters
The resulting concatenated fmtp line is:
a=fmtp:<format> SizeLength=13;IndexLength=3;IndexDeltaLength=3
RTP packet structure
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
MSLHSection
+=========================================+=============+
| MSLHSection size in bits = 135 | 2 bytes |
+-----------------------------------------+-------------+
| PayloadSize | 13 bits |
+-----------------------------------------+-------------+
| Index | 3 bits |
+-----------------------------------------+-------------+
| PayloadSize | 13 bits |
+-----------------------------------------+-------------+
| IndexDelta | 3 bits |
+-----------------------------------------+-------------+
| PayloadSize | 13 bits |
+-----------------------------------------+-------------+
| IndexDelta | 3 bits |
+-----------------------------------------+-------------+
| PayloadSize | 13 bits |
+-----------------------------------------+-------------+
| IndexDelta | 3 bits |
+-----------------------------------------+-------------+
| bits to byte alignment | 0 bits |
+-----------------------------------------+-------------+
SLPPSection
+=========================================+=============+
| AAC Access Unit | x bytes |
+-----------------------------------------+-------------+
| AAC Access Unit | x bytes |
+-----------------------------------------+-------------+
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| AAC Access Unit | x bytes |
+-----------------------------------------+-------------+
| AAC Access Unit | x bytes |
+-----------------------------------------+-------------+
Overhead estimation
The MSLHSection is 8 bytes; in this example we have therefore a RTP
overhead of 40 + 8 bytes for 1400 bytes (approx) of payload i.e.
around 4 % overhead.
Appendix.6 A more complex case: AAC with interleaving and SL
Let us consider AAC around 130 kb/s where each Access Unit is split
in 4 SL packets corresponding to Error Sensitivity Categories (ESC)
of maximum 90 bytes for which interleaving is very useful in terms
of error resilience. We thus use an interleaving scheme where 15 SL
Packets (extracted from 15 consecutive Access Units) are used to
construct each RTP packet in order to match a MTU of 1500 bytes.
Note that since ESC fragments are not byte aligned we also use the
paddingFlag and paddingBits features of the Synch Layer.
The interleaving sequence is 4 RTP packets and 350 ms long, which is
too long for conferencing but perfectly OK for Internet radio.
Since the sequence contains 60 SL packets, the sequence number can
be encoded on 6 bits. However 2 bits are actually enough if the
sender always resets the SL packet sequence number to zero at the
start of each sequence, since only the first MSLH in each of the 4
RTP packets in the sequence carries an absolute sequence number
value (0,1,2,3).
2 bits are also enough for IndexDelta, which is constant and equal
to 3 (since +1 is automatically added).
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
consecutive Access Units.
With this scheme a sender (for example upon reception of RTCP
reports indicating high loss rates) can (for example) choose to
duplicate for each interleaving sequence the first RTP packet that
contains the most useful data in terms of ESC or apply other error
protection techniques, with due care to congestion issues.
In this example we will also show several other SL features (OCR, AU
boundary flags, padding, as detailed below).
One feature demonstrated by this example is the degradation
priority. We assume degradation priority can take 4 different
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RTP Payload Format for MPEG-4 Streams July 2001
values, mapped to Error Sensitivity Categories, and is encoded on 2
bits. This interleaving scheme makes sure that only SL packets of
identical degradation priorities are grouped in the same RTP packet
(3.6.3) and that only the first RSLH of each RTP packet transports
the degradation priority.
We also assume that for each last SL packet of each RTP packet the
server inserts an OCR.
SLConfigDescriptor
In this example the SLConfigDescriptor is:
class SLConfigDescriptor extends BaseDescriptor : bit(8)
tag=SLConfigDescrTag {
bit(8) predefined;
if (predefined==0) {
bit(1) useAccessUnitStartFlag; = 1
bit(1) useAccessUnitEndFlag; = 1
bit(1) useRandomAccessPointFlag; = 0
bit(1) hasRandomAccessUnitsOnlyFlag; = 1
bit(1) usePaddingFlag; = 1 // we need to signal padding bits
bit(1) useTimeStampsFlag; = 0
bit(1) useIdleFlag; = 0
bit(1) durationFlag; = 1
bit(32) timeStampResolution; = 0
bit(32) OCRResolution; = 30
bit(8) timeStampLength; = 0
bit(8) OCRLength; = 32
bit(8) AU_Length; = 0
bit(8) instantBitrateLength; = 0
bit(4) degradationPriorityLength; = 2
bit(5) AU_seqNumLength; = 0
bit(5) packetSeqNumLength; = 6
bit(2) reserved=0b11;
}
if (durationFlag) {
bit(32) timeScale; = 1000// milliseconds
bit(16) accessUnitDuration; = 23.22 // ms
bit(16) compositionUnitDuration; = 23.22 // ms
}
if (!useTimeStampsFlag) {
bit(timeStampLength) startDecodingTimeStamp; = 0
bit(timeStampLength) startCompositionTimeStamp; = 0
}
}
SL Packet Header structure
With this configuration we have the following SL packet header
structure:
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aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
bit(1) accessUnitStartFlag;
bit(1) accessUnitEndFlag;
bit(1) OCRflag;
bit(1) paddingFlag;
if (paddingFlag) bit(3) paddingBits;
bit(SL.packetSeqNumLength) packetSequenceNumber;
bit(1) DegPrioflag;
if (DegPrioflag) {
bit(SL.degradationPriorityLength) degradationPriority;}
if (OCRflag) {
bit(SL.OCRLength) objectClockReference;}
}
}
Parameters
The RSLHSectionSize cannot exceed 2 bits, which is encoded on 2 bits
and signaled by RSLHSectionSizeLength.
The resulting concatenated fmtp line is:
a=fmtp:<format>
SizeLength=6;RSLHSectionSizeLength=2;IndexLength=2;IndexDeltaLength=
2;OCRDeltaLength=16
RTP packet structure
+=========================================+=============+
| Field | size |
+=========================================+=============+
| RTP header | - |
+-----------------------------------------+-------------+
MSLHSection
+=========================================+=============+
| MSLHSection size in bits = 135 | 2 bytes |
+-----------------------------------------+-------------+
| PayloadSize | 7 bits |
+-----------------------------------------+-------------+
| Index = 0 or 1 or 2 or 3 | 2 bits |
+-----------------------------------------+-------------+
| PayloadSize | 7 bits |
+-----------------------------------------+-------------+
| IndexDelta = 3 | 2 bits |
+-----------------------------------------+-------------+
| etc + 12 times 9 bits |
+-----------------------------------------+-------------+
| PayloadSize | 7 bits |
+-----------------------------------------+-------------+
| IndexDelta = 3 | 2 bits |
+-----------------------------------------+-------------+
| bits to byte alignment | 7 bits |
+-----------------------------------------+-------------+
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RSLHSection
+=========================================+=============+
| RSLHSectionSize | 6 bits |
+-----------------------------------------+-------------+
| accessUnitStartFlag | 1 bit |
+-----------------------------------------+-------------+
| accessUnitEndFlag | 1 bit |
+-----------------------------------------+-------------+
| OCRFlag = 0 | 1 bit |
+-----------------------------------------+-------------+
| paddingFlag = 1 | 1 bit |
+-----------------------------------------+-------------+
| paddingBits | 3 bits |
+-----------------------------------------+-------------+
| DegPrioflag = 1 | 1 bit |
+-----------------------------------------+-------------+
| degradationPriority | 2 bits |
+-----------------------------------------+-------------+
| accessUnitStartFlag | 1 bit |
+-----------------------------------------+-------------+
| accessUnitEndFlag | 1 bit |
+-----------------------------------------+-------------+
| OCRFlag = 0 | 1 bit |
+-----------------------------------------+-------------+
| paddingFlag = 1 | 1 bit |
+-----------------------------------------+-------------+
| paddingBits | 3 bits |
+-----------------------------------------+-------------+
| DegPrioflag = 0 | 1 bit |
+-----------------------------------------+-------------+
| etc + 12 times 8 bits |
+-----------------------------------------+-------------+
| accessUnitStartFlag | 1 bit |
+-----------------------------------------+-------------+
| accessUnitEndFlag | 1 bit |
+-----------------------------------------+-------------+
| OCRFlag = 1 | 1 bit |
+-----------------------------------------+-------------+
| OCRDelta | 16 bits |
+-----------------------------------------+-------------+
| paddingFlag = 0 | 1 bit |
+-----------------------------------------+-------------+
| DegPrioflag = 0 | 1 bit |
+-----------------------------------------+-------------+
| bits to byte alignment | 5 bits |
+-----------------------------------------+-------------+
SLPPSection
+=========================================+=============+
| SL packet payload |max 90 bytes |
+-----------------------------------------+-------------+
| etc + 13 SL packets |
+-----------------------------------------+-------------+
| SL packet payload |max 90 bytes |
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RTP Payload Format for MPEG-4 Streams July 2001
+-----------------------------------------+-------------+
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
paddingFlag is set to zero and the paddingBits field is omitted.
Overhead estimation
The MSLHSection is 19 bytes, the RSLHSection is 16 bytes; in this
example we have therefore a RTP overhead of 40 + 35 bytes for 1350
bytes (max) of payload i.e. around 6 % overhead.
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