Internet Engineering Task Force Yoshihiro Kikuchi - Toshiba
Internet Draft Toshiyuki Nomura - NEC
Document: draft-ietf-avt-rtp-mpeg4-es-05.txt Shigeru Fukunaga - Oki
Yoshinori Matsui - Matsushita
Hideaki Kimata - NTT
October 11, 2000
RTP payload format for MPEG-4 Audio/Visual streams
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026 [1].
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Abstract
This document describes RTP payload formats for carrying each of MPEG-4
Audio and MPEG-4 Visual bitstreams without using MPEG-4 Systems. For the
purpose of directly mapping MPEG-4 Audio/Visual bitstreams onto RTP
packets, it provides specifications for the use of RTP header fields and
also specifies fragmentation rules. It also provides specifications for
MIME type registrations and the use of SDP.
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RTP payload format for MPEG-4 Audio/Visual streams September 2000
1. Introduction
The RTP payload formats described in this document specify how MPEG-4
Audio [3][5] and MPEG-4 Visual streams [2][4] are to be fragmented and
mapped directly onto RTP packets.
These RTP payload formats enable transport of MPEG-4 Audio/Visual streams
without using the synchronization and stream management functionality of
MPEG-4 Systems [6]. Such RTP payload formats will be used in systems that
have intrinsic stream management functionality and thus require no such
functionality from MPEG-4 Systems. H.323 terminals are an example of such
a systems, where MPEG-4 Audio/Visual streams are not managed by MPEG-4
Systems Object Descriptors but by H.245. The streams are directly mapped
onto RTP packets without using MPEG-4 Systems Sync Layer. Other examples
are SIP and RTSP where MIME and SDP are used. MIME types and SDP usages
of the RTP payload formats described in this document are defined to
directly specify the attribute of Audio/Visual streams (e.g. media type,
packetization format and codec configuration) without using MPEG-4
Systems. The obvious benefit is that these MPEG-4 Audio/Visual RTP
payload formats can be handled in an unified way together with those
formats defined for non-MPEG-4 codecs. The disadvantage is that
interoperability with environments using MPEG-4 Systems may be difficult,
other payload formats may be better suited to those applications.
The semantics of RTP headers in such cases need to be clearly defined,
including the association with MPEG-4 Audio/Visual data elements. In
addition, it is beneficial to define the fragmentation rules of RTP
packets for MPEG-4 Video streams so as to enhance error resiliency by
utilizing the error resilience tools provided inside the MPEG-4 Video
stream.
1.1 MPEG-4 Visual RTP payload format
MPEG-4 Visual is a visual coding standard with many new features: high
coding efficiency; high error resiliency; multiple, arbitrary shape
object-based coding; etc. [2]. It covers a wide range of bitrates from
scores of Kbps to several Mbps. It also covers a wide variety of
networks, ranging from those guaranteed to be almost error-free to mobile
networks with high error rates.
With respect to the fragmentation rules for an MPEG-4 visual bitstream
defined in this document, since MPEG-4 Visual is used for a wide variety
of networks, it is desirable not to apply too much restriction on
fragmentation, and a fragmentation rule such as "a single video packet
shall always be mapped on a single RTP packet" may be inappropriate. On
the other hand, careless, media unaware fragmentation may cause
degradation in error resiliency and bandwidth efficiency. The
fragmentation rules described in this document are flexible but manage to
define the minimum rules for preventing meaningless fragmentation while
utilizing the error resilience functionalities of MPEG-4 Visual.
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The fragmentation rule recommends not to map more than one VOP in an RTP
packet so that the RTP timestamp uniquely indicates the VOP time framing.
On the other hand, MPEG-4 video may generate VOPs of very small size, in
cases with an empty VOP (vop_coded=0) containing only VOP header or an
arbitrary shaped VOP with a small number of coding blocks. To reduce the
overhead for such cases, the fragmentation rule permits concatenating
multiple VOPs in an RTP packet. (See fragmentation rule (4) in section
3.2 and marker bit and timestamp in section 3.1.)
While the additional media specific RTP header defined for such video
coding tools as H.261 or MPEG-1/2 is effective in helping to recover
picture headers corrupted by packet losses, MPEG-4 Visual has already
error resilience functionalities for recovering corrupt headers, and
these can be used on RTP/IP networks as well as on other networks
(H.223/mobile, MPEG-2/TS, etc.). Therefore, no extra RTP header fields
are defined in this MPEG-4 Visual RTP payload format.
1.2 MPEG-4 Audio RTP payload format
MPEG-4 Audio is a new kind of audio standard that integrates many
different types of audio coding tools. Low-overhead MPEG-4 Audio
Transport Multiplex (LATM) manages the sequences of audio data with
relatively small overhead. In audio-only applications, then, it is
desirable for LATM-based MPEG-4 Audio bitstreams to be directly mapped
onto the RTP packets without using MPEG-4 Systems.
While LATM has several multiplexing features as follows;
- Carrying configuration information with audio data,
- Concatenation of multiple audio frames in one audio stream,
- Multiplexing multiple objects (programs),
- Multiplexing scalable layers,
in RTP transmission there is no need for the last two features.
Therefore, these two features MUST NOT be used in applications based on
RTP packetization specified by this document. Since LATM has been
developed for only natural audio coding tools, i.e. not for synthesis
tools, it seems difficult to transmit Structured Audio (SA) data and Text
to Speech Interface (TTSI) data by LATM. Therefore, SA data and TTSI data
MUST NOT be transported by the RTP packetization in this document
For transmission of scalable streams, audio data of each layer should be
packetized onto different RTP packets allowing for the different layers
to be treated differently at the IP level, for example via some means of
differentiated service. On the other hand, all configuration data of the
scalable streams are contained in one LATM configuration data
"StreamMuxConfig" and every scalable layer shares the StreamMuxConfig.
The mapping between each layer and its configuration data is achieved by
LATM header information attached to the audio data. In order to indicate
the dependency information of the scalable streams, a restriction is
applied to the dynamic assignment rule of payload type (PT) values (see
section 4.2).
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For MPEG-4 Audio coding tools, as is true for other audio coders, if the
payload is a single audio frame, packet loss will not impair the
decodability of adjacent packets. Therefore, the additional media
specific header for recovering errors will not be required for MPEG-4
Audio. Existing RTP protection mechanisms, such as Generic Forward Error
Correction (RFC 2733) and Redundant Audio Data (RFC 2198), MAY be applied
to improve error resiliency.
2. Conventions used in this document
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 [7].
3. RTP Packetization of MPEG-4 Visual bitstream
This section specifies RTP packetization rules for MPEG-4 Visual content.
An MPEG-4 Visual bitstream is mapped directly onto RTP packets without
the addition of extra header fields or any removal of Visual syntax
elements. The Combined Configuration/Elementary stream mode MUST be used
so that configuration information will be carried to the same RTP port as
the elementary stream. (see 6.2.1 "Start codes" of ISO/IEC 14496-2
[2][9][4]) The configuration information MAY additionally be specified by
some out-of-band means. If needed for an H.323 terminal, H.245 codepoint
"decoderConfigurationInformation" MUST be used for this purpose. If
needed by systems using MIME content type and SDP parameters, e.g. SIP
and RTSP, the optional parameter "config" MUST be used to specify the
configuration information. (see 5.1 and 5.2)
When the short video header mode is used, the RTP payload format for
H.263 SHOULD be used (the format defined in RFC 2429 is RECOMMENDED, but
the RFC 2190 format MAY be used for compatibility with older
implementations).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number | RTP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp | Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | RTP
| MPEG-4 Visual stream (byte aligned) | Payload
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 - An RTP packet for MPEG-4 Visual stream
3.1 Use of RTP header fields for MPEG-4 Visual
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
by means of an out of band signaling protocol (e.g. H.245, SIP, etc).
Extension (X) bit: Defined by the RTP profile used.
Sequence Number: Incremented by one for each RTP data packet sent,
starting, for security reasons, with a random initial value.
Marker (M) bit: The marker bit is set to one to indicate the last RTP
packet (or only RTP packet) of a VOP. When multiple VOPs are carried in
the same RTP packet, the marker bit is set to 1.
Timestamp: The timestamp indicates the sampling instance of the VOP
contained in the RTP packet. A constant offset, which is random, is added
for security reasons.
- When multiple VOPs are carried in the same RTP packet, the timestamp
indicates the earliest of the VOP times within the VOPs carried in the
RTP packet. Timestamp information of the rest of the VOPs are derived
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from the timestamp fields in the VOP header (modulo_time_base and
vop_time_increment).
- If the RTP packet contains only configuration information and/or
Group_of_VideoObjectPlane() fields, the timestamp of the next VOP in
the coding order is used.
- If the RTP packet contains only visual_object_sequence_end_code
information, the timestamp of the immediately preceding VOP in the
coding order is used.
The resolution of the timestamp is set to its default value of 90KHz,
unless specified by an out-of-band means (e.g. SDP parameter or MIME
parameter as defined in section 5).
Other header fields are used as described in RFC 1889 [8].
3.2 Fragmentation of MPEG-4 Visual bitstream
A fragmented MPEG-4 Visual bitstream is mapped directly onto the RTP
payload without any addition of extra header fields or any removal of
Visual syntax elements. The Combined Configuration/Elementary streams
mode is used. The following rules apply for the fragmentation.
In the following header means one of the following:
- Configuration information (Visual Object Sequence Header, Visual Object
Header and Video Object Layer Header)
- visual_object_sequence_end_code
- The header of the entry point function for an elementary stream
(Group_of_VideoObjectPlane() or the header of VideoObjectPlane(),
video_plane_with_short_header(), MeshObject() or FaceObject())
- The video packet header (video_packet_header() excluding
next_resync_marker())
- The header of gob_layer()
See 6.2.1 "Start codes" of ISO/IEC 14496-2[2][9][4] for the definition of
the configuration information and the entry point functions.
(1) Configuration information and Group_of_VideoObjectPlane() fields
SHALL be placed at the beginning of the RTP payload (just after the RTP
header) or just after the header of the syntactically upper layer
function.
(2) If one or more headers exist in the RTP payload, the RTP payload
SHALL begin with the header of the syntactically highest function.
Note: The visual_object_sequence_end_code is regarded as the lowest
function.
(3) A header SHALL NOT be split into a plurality of RTP packets.
(4) Different VOPs SHOULD be fragmented into different RTP packets so
that one RTP packet consists of the data bytes associated with a unique
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RTP payload format for MPEG-4 Audio/Visual streams September 2000
VOP time instance (that is indicated in the timestamp field in the RTP
packet header), with the exception that multiple consecutive VOPs MAY be
carried within one RTP packet in the decoding order if the size of the
VOPs is small.
Note: When multiple VOPs are carried in one RTP payload, the timestamp of
the VOPs after the first one may be calculated by the decoder. This
operation is necessary only for RTP packets in which the marker bit
equals to one and the beginning of RTP payload corresponds to a start
code. (See timestamp and marker bit in section 3.1)
(5) It is RECOMMENDED that a single video packet is sent as a single RTP
packet. The size of a video packet SHOULD be adjusted in such a way that
the resulting RTP packet is not larger than the path-MTU.
Note: Rule (5) does not apply when the video packet is disabled by the
coder configuration (by setting resync_marker_disable in the VOL header
to 1), or in coding tools where the video packet is not supported. In
this case, a VOP MAY be split at arbitrary byte-positions.
The video packet starts with the VOP header or the video packet header,
followed by motion_shape_texture(), and ends with next_resync_marker() or
next_start_code().
3.3 Examples of packetized MPEG-4 Visual bitstream
Figure 2 shows examples of RTP packets generated based on the criteria
described in 3.2
(a) is an example of the first RTP packet or the random access point of
an MPEG-4 visual bitstream containing the configuration information.
According to criterion (1), the Visual Object Sequence Header(VS header)
is placed at the beginning of the RTP payload, preceding the Visual
Object Header and the Video Object Layer Header(VO header, VOL header).
Since the fragmentation rule defined in 3.2 guarantees that the
configuration information, starting with
visual_object_sequence_start_code, is always placed at the beginning of
the RTP payload, RTP receivers can detect the random access point by
checking if the first 32-bit field of the RTP payload is
visual_object_sequence_start_code.
(b) is another example of the RTP packet containing the configuration
information. It differs from example (a) in that the RTP packet also
contains a video packet in the VOP following the configuration
information. Since the length of the configuration information is
relatively short (typically scores of bytes) and an RTP packet containing
only the configuration information may thus increase the overhead, the
configuration information and the immediately following GOV and/or (a
part of) VOP can be packetized into a single RTP packet as in this
example.
(c) is an example of an RTP packet that contains
Group_of_VideoObjectPlane(GOV). Following criterion (1), the GOV is
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RTP payload format for MPEG-4 Audio/Visual streams September 2000
placed at the beginning of the RTP payload. It would be a waste of RTP/IP
header overhead to generate an RTP packet containing only a GOV whose
length is 7 bytes. Therefore, (a part of) the following VOP can be placed
in the same RTP packet as shown in (c).
(d) is an example of the case where one video packet is packetized into
one RTP packet. When the packet-loss rate of the underlying network is
high, this kind of packetization is recommended. Even when the RTP packet
containing the VOP header is discarded by a packet loss, the other RTP
packets can be decoded by using the HEC(Header Extension Code)
information in the video packet header. No extra RTP header field is
necessary.
(e) is an example of the case where more than one video packet is
packetized into one RTP packet. This kind of packetization is effective
to save the overhead of RTP/IP headers when the bit-rate of the
underlying network is low. However, it will decrease the packet-loss
resiliency because multiple video packets are discarded by a single RTP
packet loss. The optimal number of video packets in an RTP packet and the
length of the RTP packet can be determined considering the packet-loss
rate and the bit-rate of the underlying network.
(f) is an example of the case when the video packet is disabled by
setting resync_marker_disable in the VOL header to 1. In this case, a VOP
may be split into a plurality of RTP packets at arbitrary byte-positions.
For example, it is possible to split a VOP into fixed-length packets.
This kind of coder configuration and RTP packet fragmentation may be used
when the underlying network is guaranteed to be error-free. On the other
hand, it is not recommended to use it in error-prone environment since it
provides only poor packet loss resiliency.
Figure 3 shows examples of RTP packets prohibited by the criteria of 3.2.
Fragmentation of a header into multiple RTP packets, as in (a), will not
only increase the overhead of RTP/IP headers but also decrease the error
resiliency. Therefore, it is prohibited by the criterion (3).
When concatenating more than one video packets into an RTP packet, VOP
header or video_packet_header() shall not be placed in the middle of the
RTP payload. The packetization as in (b) is not allowed by criterion (2)
due to the aspect of the error resiliency. Comparing this example with
Figure 2(d), although two video packets are mapped onto two RTP packets
in both cases, the packet-loss resiliency is not identical. Namely, if
the second RTP packet is lost, both video packets 1 and 2 are lost in the
case of Figure 3(b) whereas only video packet 2 is lost in the case of
Figure 2(d).
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RTP payload format for MPEG-4 Audio/Visual streams September 2000
+------+------+------+------+
(a) | RTP | VS | VO | VOL |
|header|header|header|header|
+------+------+------+------+
+------+------+------+------+------------+
(b) | RTP | VS | VO | VOL |Video Packet|
|header|header|header|header| |
+------+------+------+------+------------+
+------+-----+------------------+
(c) | RTP | GOV |Video Object Plane|
|header| | |
+------+-----+------------------+
+------+------+------------+ +------+------+------------+
(d) | RTP | VOP |Video Packet| | RTP | VP |Video Packet|
|header|header| (1) | |header|header| (2) |
+------+------+------------+ +------+------+------------+
+------+------+------------+------+------------+------+------------+
(e) | RTP | VP |Video Packet| VP |Video Packet| VP |Video Packet|
|header|header| (1) |header| (2) |header| (3) |
+------+------+------------+------+------------+------+------------+
+------+------+------------+ +------+------------+
(f) | RTP | VOP |VOP fragment| | RTP |VOP fragment|
|header|header| (1) | |header| (2) | ___
+------+------+------------+ +------+------------+
Figure 2 - Examples of RTP packetized MPEG-4 Visual bitstream
+------+-------------+ +------+------------+------------+
(a) | RTP |First half of| | RTP |Last half of|Video Packet|
|header| VP header | |header| VP header | |
+------+-------------+ +------+------------+------------+
+------+------+----------+ +------+---------+------+------------+
(b) | RTP | VOP |First half| | RTP |Last half| VP |Video Packet|
|header|header| of VP(1) | |header| of VP(1)|header| (2) |
+------+------+----------+ +------+---------+------+------------+
Figure 3 - Examples of prohibited RTP packetization for MPEG-4 Visual
bitstream
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RTP payload format for MPEG-4 Audio/Visual streams September 2000
4. RTP Packetization of MPEG-4 Audio bitstream
This section specifies RTP packetization rules for MPEG-4 Audio
bitstreams. MPEG-4 Audio streams MUST be formatted by LATM (Low-overhead
MPEG-4 Audio Transport Multiplex) tool[5], and the LATM-based streams are
then mapped onto RTP packets as described the three sections below.
4.1 RTP Packet Format
LATM-based streams consist of a sequence of audioMuxElements that include
one or more audio frames. A complete audioMuxElement or a part of one
SHALL be mapped directly onto an RTP payload without any removal of
audioMuxElement syntax elements (see Figure 4). The first byte of each
audioMuxElement SHALL be located at the first payload location in an 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 |RTP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |RTP
: audioMuxElement (byte aligned) :Payload
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 - An RTP packet for MPEG-4 Audio
In order to decode the audioMuxElement, the following muxConfigPresent
information is required to be indicated by an out-of-band means. When SDP
is utilized for this indication, MIME parameter "cpresent" corresponds to
the muxConfigPresent information (see section 5.3).
muxConfigPresent: If this value is set to 1 (in-band mode), the
audioMuxElement SHALL include an indication bit "useSameStreamMux" and
MAY include the configuration information for audio compression
"StreamMuxConfig". The useSameStreamMux bit indicates whether the
StreamMuxConfig element in the previous frame is applied in the current
frame. If the useSameStreamMux bit indicates to use the StreamMuxConfig
from the previous frame, but if the previous frame has been lost, the
current frame may not be decodable. Therefore, in case of in-band mode,
the StreamMuxConfig element SHOULD be transmitted repeatedly depending on
the network condition. On the other hand, if muxConfigPresent is set to 0
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(out-band mode), the StreamMuxConfig element is required to be
transmitted by an out-of-band means. In case of SDP, MIME parameter
"config" is utilized (see section 5.3).
4.2 Use of RTP Header Fields for MPEG-4 Audio
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
by means of an out of band signaling protocol (e.g. H.245, SIP, etc). In
the dynamic assignment of RTP payload types for scalable streams, a
different value should be assigned to each layer. The assigned values
should be in order of enhance layer dependency, where the base layer has
the smallest value.
Marker (M) bit: The marker bit indicates audioMuxElement boundaries. It
is set to one to indicate that the RTP packet contains a complete
audioMuxElement or the last fragment of an audioMuxElement.
Timestamp: The timestamp indicates the sampling instance of the first
audio frame contained in the RTP packet. Timestamps are recommended to
start at a random value for security reasons.
Unless specified by an out-of-band means, the resolution of the timestamp
is set to its default value of 90 kHz.
Sequence Number: Incremented by one for each RTP packet sent, starting,
for security reasons, with a random value.
Other header fields are used as described in RFC 1889 [8].
4.3 Fragmentation of MPEG-4 Audio bitstream
It is RECOMMENDED to put one audioMuxElement in each RTP packet. If the
size of an audioMuxElement can be kept small enough that the size of the
RTP packet containing it does not exceed the size of the path-MTU, this
will be no problem. If it cannot, the audioMuxElement MAY be fragmented
and spread across multiple packets.
5. MIME type registration for MPEG-4 Audio/Visual streams
The following sections describe the MIME type registrations for MPEG-4
Audio/Visual streams. MIME type registration and SDP usage for the MPEG-4
Visual stream are described in Sections 5.1 and 5.2, respectively, while
MIME type registration and SDP usage for MPEG-4 Audio stream are
described in Sections 5.3 and 5.4, respectively.
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(In the following sections, the RFC number "XXXX" represents the RFC
number, which should be assigned for this document.)
5.1 MIME type registration for MPEG-4 Visual
MIME media type name: video
MIME subtype name: MP4V-ES
Required parameters: none
Optional parameters:
rate: This parameter is used only for RTP transport. It indicates the
resolution of the timestamp field in the RTP header. If this parameter
is not specified, its default value of 90000 (90KHz) is used.
profile-level-id: A decimal representation of MPEG-4 Visual Profile
Level indication value (profile_and_level_indication) defined in Table
G-1 of ISO/IEC 14496-2 [2][4]. This parameter MAY be used in the
capability exchange or session setup procedure to indicate MPEG-4
Visual Profile and Level combination of which the MPEG-4 Visual codec
is capable. If this parameter is not specified by the procedure, its
default value of 1 (Simple Profile/Level 1) is used.
config: This parameter SHALL be used to indicate the configuration of
the corresponding MPEG-4 visual bitstream. It SHALL NOT be used to
indicate the codec capability in the capability exchange procedure. It
is a hexadecimal representation of an octet string that expresses the
MPEG-4 Visual configuration information, as defined in subclause 6.2.1
Start codes of ISO/IEC14496-2[2][4][9]. The configuration information
is mapped onto the octet string in an MSB-first basis. The first bit
of the configuration information SHALL be located at the MSB of the
first octet. 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 exist,
which may vary in the repeated configuration information inside an
MPEG-4 Visual stream (See 6.2.1 Start codes of ISO/IEC14496-2).
Example usages for these parameters are:
- MPEG-4 Visual Simple Profile/Level 1:
Content-type: video/mp4v-es; profile-level-id=1
- MPEG-4 Visual Core Profile/Level 2:
Content-type: video/mp4v-es; profile-level-id=34
- MPEG-4 Visual Advanced Real Time Simple Profile/Level 1:
Content-type: video/mp4v-es; profile-level-id=145
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Published specification:
The specifications for MPEG-4 Visual streams are presented in ISO/IEC
14469-2[2][4][9]. The RTP payload format is described in RFCXXXX.
Encoding considerations:
Video bitstreams must be generated according to MPEG-4 Visual
specifications (ISO/IEC 14496-2). A video bitstream is 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 MPEG-4 Visual
RTP payload format defined in RFCXXXX.
Security considerations:
See section 6 of RFCXXXX.
Interoperability considerations:
MPEG-4 Visual 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 Visual 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:
o a codec builder to implement only the subset of the standard he
needs, while maintaining interworking with other MPEG-4 devices
included in the same combination, and
o checking whether MPEG-4 devices comply with the standard
('conformance testing').
The visual stream SHALL be compliant with the MPEG-4 Visual
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.
Applications which use this media type:
Audio and visual streaming and conferencing tools, Internet messaging
and Email applications.
Additional information: none
Person & email address to contact for further information:
The authors of RFCXXXX. (See section 8)
Intended usage: COMMON
Author/Change controller:
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The authors of RFCXXXX. (See section 8)
5.2 SDP usage of MPEG-4 Visual
The MIME media type video/MP4V-ES string is mapped to fields in the
Session Description Protocol (SDP), RFC 2327, as follows:
o The MIME type (video) goes in SDP "m=" as the media name.
o The MIME subtype (MP4V-ES) goes in SDP "a=rtpmap" as the encoding name.
o The optional parameter "rate" goes in "a=rtpmap" as the clock rate.
o The optional parameter "profile-level-id" and "config" go in the
"a=fmtp" line to indicate the coder capability and configuration,
respectively. These parameters are expressed as a MIME media type string,
in the form of as a semicolon separated list of parameter=value pairs.
The following are some examples of media representation in SDP:
Simple Profile/Level 1, rate=90000(90KHz), "profile-level-id" and
"config" are present in "a=fmtp" line:
m=video 49170/2 RTP/AVP 98
a=rtpmap:98 MP4V-ES/90000
a=fmtp:98 profile-level-id=1;config=000001B001000001B509000001000000012
0008440FA282C2090A21F
Core Profile/Level 2, rate=90000(90KHz), "profile-level-id" is present in
"a=fmtp" line:
m=video 49170/2 RTP/AVP 98
a=rtpmap:98 MP4V-ES/90000
a=fmtp:98 profile-level-id=34
Advance Real Time Simple Profile/Level 1, rate=25(25Hz), "profile-level-
id" is present in "a=fmtp" line:
m=video 49170/2 RTP/AVP 98
a=rtpmap:98 MP4V-ES/25
a=fmtp:98 profile-level-id=145
5.3 MIME type registration of MPEG-4 Audio
MIME media type name: audio
MIME subtype name: MP4A-LATM
Required parameters:
rate: the rate parameter indicates the RTP time stamp clock rate. The
default value is 90000. Other rates MAY be specified only if they are
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set to the same value as the audio sampling rate (number of samples
per second).
Optional parameters:
profile-level-id: a decimal representation of MPEG-4 Audio Profile
Level indication value defined in ISO/IEC 14496-1 [10]. This parameter
indicates which MPEG-4 Audio tool subsets the decoder is capable of
using. If this parameter is not specified in the capability exchange
or session setup procedure, its default value of 30 (Natural Audio
Profile/Level 1) is used.
object: a decimal representation of the MPEG-4 Audio Object Type value
defined in ISO/IEC 14496-3 [5]. This parameter specifies the tool to
be used by the coder. It CAN be used to limit the capability within
the specified "profile-level-id".
bitrate: the data rate for the audio bit stream.
cpresent: this parameter indicates whether audio payload configuration
data has been multiplexed into an RTP payload (see section 4.1). If
not specified, the default value is 1.
config: a hexadecimal representation of an octet string that expresses
the audio payload configuration data "StreamMuxConfig", as defined in
ISO/IEC 14496-3 [5] (see section 4.1). 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-padding bits, if necessary, shall follow the
configuration data.
ptime: RECOMMENDED duration of each packet in milliseconds.
Published specification:
Payload format specifications are described in this document. Encoding
specifications are provided in ISO/IEC 14496-3 [3][5].
Encoding considerations:
This type is only defined for transfer via RTP.
Security considerations:
See Section 6 of RFCXXXX.
Interoperability considerations:
MPEG-4 Audio provides a large and rich set of tools for the coding of
audio objects. For effective implementation of the standard, subsets of
the MPEG-4 Audio tool sets similar to those used in MPEG-4 Visual have
been provided (see section 5.1).
The audio stream SHALL be compliant with the MPEG-4 Audio
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
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arranging in the capability exchange procedure to set this parameter
mutually to the same value. Furthermore, the "object" parameter can be
used to limit the capability within the specified Profile@Level in
capability exchange.
Applications which use this media type:
Audio and video streaming and conferencing tools.
Additional information: none
Personal & email address to contact for further information:
See Section 8 of RFCXXXX.
Intended usage: COMMON
Author/Change controller:
See Section 8 of RFCXXXX.
5.4 SDP usage of MPEG-4 Audio
The MIME media type audio/MP4A-LATM string is mapped to fields in the
Session Description Protocol (SDP), RFC 2327, as follows:
o The MIME type (audio) goes in SDP "m=" as the media name.
o The MIME subtype (MP4A-LATM) goes in SDP "a=rtpmap" as the encoding
name.
o The required parameter "rate" goes in "a=rtpmap" as the clock rate.
o The optional parameter "ptime" goes in SDP "a=ptime" attribute.
o The optional parameter "profile-level-id" goes in the "a=fmtp" line to
indicate the coder capability. The "object" parameter goes in the
"a=fmtp" attribute. The payload-format-specific parameters "bitrate",
"cpresent" and "config" go in the "a=fmtp" line. These parameters are
expressed as a MIME media type string, in the form of as a semicolon
separated list of parameter=value pairs.
The following are some examples of the media representation in SDP:
For 6 kb/s CELP bitstreams (with an audio sampling rate of 8 kHz),
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/8000
a=fmtp:96 profile-level-id=9;object=8;cpresent=0;config=9128B1071070
a=ptime:20
For 64 kb/s AAC LC stereo bitstreams (with an audio sampling rate of 24
kHz),
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m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
config=9122620000
In the above two examples, audio configuration data is not multiplexed
into the RTP payload and is described only in SDP. Furthermore, the
"clock rate" is set to the audio sampling rate.
If the clock rate has been set to its default value and it is necessary
to obtain the audio sampling rate, this can be done by parsing the
"config" parameter (see the following example).
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/90000
a=fmtp:96 object=8; cpresent=0; config=9128B1071070
The following example shows that the audio configuration data appears in
the RTP payload.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/90000
a=fmtp:96 object=2; cpresent=1
6. Security Considerations
RTP packets using the payload format defined in this specification are
subject to the security considerations discussed in the RTP specification
[8]. 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 complete MPEG-4 system allows for transport of a wide range of
content, including Java applets (MPEG-J) and scripts. Since this payload
format is restricted to audio and video streams, it is not possible to
transport such active content in this format.
7. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9,
RFC 2026, October 1996.
2 ISO/IEC 14496-2:1999, "Information technology - Coding of audio-visual
objects - Part2: Visual", December 1999.
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3 ISO/IEC 14496-3:1999, "Information technology - Coding of audio-visual
objects - Part3: Audio", December 1999.
4 ISO/IEC 14496-2:1999/FDAM1:2000, December 1999.
5 ISO/IEC 14496-3:1999/FDAM1:2000, December 1999.
6 ISO/IEC 14496-1:1999, "Information technology - Coding of audio-visual
objects - Part1: Systems", December 1999.
7 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
8 H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson "RTP: A Transport
Protocol for Real Time Applications", RFC 1889, Internet Engineering
Task Force, January 1996.
9 ISO/IEC 14496-2:1999/COR1:2000, "Information technology - Coding of
audio-visual objects - Part2: Visual, Technical corrigendum 1", August
2000.
10 ISO/IEC 14496-1:1999/FDAM1:2000, December 1999.
8. Author's Addresses
Yoshihiro Kikuchi
Toshiba corporation
1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki, 212-8582, Japan
Email: yoshihiro.kikuchi@toshiba.co.jp
Yoshinori Matsui
Matsushita Electric Industrial Co., LTD.
1006, Kadoma, Kadoma-shi, Osaka, Japan
Email: matsui@drl.mei.co.jp
Toshiyuki Nomura
NEC Corporation
4-1-1,Miyazaki,Miyamae-ku,Kawasaki,JAPAN
Email: t-nomura@ccm.cl.nec.co.jp
Shigeru Fukunaga
Oki Electric Industry Co., Ltd.
1-2-27 Shiromi, Chuo-ku, Osaka 540-6025 Japan.
Email: fukunaga444@oki.co.jp
Hideaki Kimata
Nippon Telegraph and Telephone Corporation
1-1, Hikari-no-oka, Yokosuka-shi, Kanagawa, Japan
Email: kimata@nttvdt.hil.ntt.co.jp
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