One document matched: draft-schmidt-avt-rfc3016bis-00.txt
Audio Video Transport WG M.Schmidt et.al.
INTERNET DRAFT
Intended status: Standards Track see authors section
Expires: March 20, 2009 for full list of authors
September 19, 2008
RTP Payload Format for MPEG-4 Audio/Visual Streams
draft-schmidt-avt-rfc3016bis-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes Real-Time Transport Protocol (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
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Multipurpose Internet Mail Extensions (MIME) type registrations and
the use of Session Description Protocol (SDP).
Comments are solicited and should be addressed to the working group's
mailing list at avt@ietf.org and/or the author(s).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 MPEG-4 Visual RTP payload format . . . . . . . . . . . . . . . 3
1.2 MPEG-4 Audio RTP payload format . . . . . . . . . . . . . . . 4
2. Conventions Used in this Document . . . . . . . . . . . . . . 6
3. RTP Packetization of MPEG-4 Visual bitstream . . . . . . . . . 6
3.1 Use of RTP header fields for MPEG-4 Visual . . . . . . . . . . 7
3.2 Fragmentation of MPEG-4 Visual bitstream . . . . . . . . . . . 8
3.3 Examples of packetized MPEG-4 Visual bitstream . . . . . . . . 9
4. RTP Packetization of MPEG-4 Audio bitstream . . . . . . . . . 12
4.1 RTP Packet Format . . . . . . . . . . . . . . . . . . . . . . 12
4.2 Use of RTP Header Fields for MPEG-4 Audio . . . . . . . . . . 14
4.3 Fragmentation of MPEG-4 Audio bitstream . . . . . . . . . . . 14
5. MIME type registration for MPEG-4 Audio/Visual streams . . . . 14
5.1 MIME type registration for MPEG-4 Visual . . . . . . . . . . . 14
5.2 SDP usage of MPEG-4 Visual . . . . . . . . . . . . . . . . . . 18
5.3 MIME type registration of MPEG-4 Audio . . . . . . . . . . . . 18
5.4 SDP usage of MPEG-4 Audio . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26
9. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 28
1. Introduction
The RTP payload formats described in this document specify how MPEG-4
Audio [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 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
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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.
The RTP payload formats described in this document have been
specified in RFC 3016 and are used by the 3GPP PSS service. However
there are some misalignments between RFC 3016 and the 3GPP PSS
specification that are addressed by this update:
- The audio payload format (LATM) referenced in RFC 3016 is binary
incompatible to the format used in 3GPP.
- The audio signalling format (StreamMuxConfig) referenced in RFC
3016 is binary incompatible to the format used in 3GPP.
- The audio parameter "SBR-enabled" is not defined within RFC 3016
but used by 3GPP
- The rate parameter specification in ambiguous in the presence of
SBR (Spectral Band Replication)
Furthermore some comments have been addressed and signalling support
for MPEG surround was added. It should be noted that the audio
payload format described here has some known limitations. For new
system designs RFC 3640 [12] is recommended.
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"
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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.
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
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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
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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).
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 4629 is RECOMMENDED,
but the RFC 4628 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) | Pay-
| | load
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...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 one.
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
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the VOPs are derived 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 3550 [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.
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(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 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.
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(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
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.
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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).
+------+------+------+------+
(a) | RTP | VS | VO | VOL |
|header|header|header|header|
+------+------+------+------+
+------+------+------+------+------+------------+
(b) | RTP | VS | VO | VOL | VOP |Video Packet|
|header|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
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+------+-------------+ +------+------------+------------+
(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
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 PayloadMux Elements which carry the 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.
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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
: 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). The following restrictions apply:
o In the out-of-band signalling case the number of PayloadMux
Elements contained in each audioMuxElement can only be set once.
If values greater than one PayloadMux Element are used, special
care is required to ensure that the last RTP packet remains
decodable.
o In the in-band siganlling case the audio frames are in general
not byte aligned. Hinting RTP payload from MP4 file format
[10][11] is therefore not possible.
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 (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
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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 3550 [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.
5.1 MIME type registration for MPEG-4 Visual
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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 and 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
M.Schmidt, et al. Standards Track [Page 15]
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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
RFC 3016.
M.Schmidt, et al. Standards Track [Page 16]
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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 RFC 3016.
Security considerations:
See section 6 of RFC 3016.
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 RFC 3016. (See section 8.)
Intended usage: COMMON
Author/Change controller:
The authors of RFC 3016. (See section 8.)
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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 4566, 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=000001B001000001B50900000100000001
20008440FA282C2090A21F
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=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=145
5.3 MIME type registration of MPEG-4 Audio
MIME media type name: audio
MIME subtype name: MP4A-LATM
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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 set to the same value as the audio sampling rate (number
of samples per second).
In the presence of SBR (Spectral Band Replication) the sampling
rates for the core en-/decoder and the SBR tool differ in most
cases. This parameter shall therefore not be considered as the
definitive sampling rate. If this parameter is used the following
recommendations apply to servers:
o When the presence of SBR is not explicitly signalled by the
optional SDP parameters: object parameter, profile-level-id or
config string, this parameter shall be set to the core codec
sampling rate.
o When the presence of SBR is explicitly signalled by the
optional SDP parameters: object parameter, profile-level-id or
config string this parameter shall be set to the SBR sampling
rate.
Optional parameters:
profile-level-id: a decimal representation of MPEG-4 Audio Profile
Level indication value defined in ISO/IEC 14496-3 [5]. 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.
MPS-profile-level-id: a decimal representation of the MPEG
Surround Profile Level indication as defined in ISO/IEC 14496-3
[5]. This parameter indicates the MPEG Surround profile and level
that the decoder must be capable in order to decode the stream.
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: a boolean parameter indicates whether audio payload
configuration data has been multiplexed into an RTP payload (see
section 4.1). A 0 indicates the configuration data has not been
multiplexed into an RTP payload, a 1 indicates that it has. The
default if the parameter is omitted is 1.
config: a hexadecimal representation of an octet string that
expresses the audio payload configuration data "StreamMuxConfig",
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as defined in ISO/IEC 14496-3 [5]. 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.
MPS-asc: a hexadecimal representation of an octet string that
expresses audio payload configuration data "AudioSpecificConfig",
as defined in ISO/IEC 14496-3 [5]. If this parameter is not
present the relevant signalling is performed by other means (e.g.
in-band or contained in the config string).
The same mapping rules as for the config parameter apply.
ptime: RECOMMENDED duration of each packet in milliseconds.
SBR-enabled: a boolean parameter which indicates whether SBR-data
can be expected in the RTP-payload of a stream. This parameter is
relevant for an SBR-capable decoder if the presence of SBR can not
be detected from an out-of-band decoder configuration (e.g.
contained in the config string).
If this parameter is set to 0, a decoder SHALL expect that SBR is
not used. If this parameter is set to 1, a decoder SHOULD upsample
the audio data with the SBR tool, regardless whether SBR data is
present in the stream or not.
If the presence of SBR can not be detected from out-of-band
configuration and the SBR-enabled parameter is not present, the
parameter defaults to 1 for an SBR-capable decoder. If the
resulting output sampling rate or the computational complexity is
not supported, the SBR tool may be disabled or run in downsampled
mode.
Published specification:
Payload format specifications are described in this document.
Encoding specifications are provided in ISO/IEC 14496-3 [5].
Encoding considerations:
This type is only defined for transfer via RTP.
Security considerations:
See Section 6 of RFC 3016.
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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 parameters "profile-level-id" and
"MPS-profile-level-id". Interoperability between a sender and a
receiver may be achieved by specifying the parameters "profile-
level-id" and "MPS-profile-level-id" in MIME content, or by
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 RFC 3016.
Intended usage: COMMON
Author/Change controller:
See Section 8 of RFC 3016.
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 4566, 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 parameters "profile-level-id" and "MPS-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
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"bitrate", "cpresent", "config", "MPS-asc" and "SBR-enabled" 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=40008B18388380
a=ptime:20
For 64 kb/s AAC LC stereo bitstreams (with an audio sampling rate of
24 kHz),
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
config=400026203fc0
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=40008B18388380
In the examples above the presence of SBR can not be determined by
the SDP parameter set. If SBR is not present in the payload, the rate
parameter and/or the StreamMuxConfig contains the audio sampling
rate. If SBR is present in the payload the rate parameter and/or the
StreamMuxConfig contains the core codec sampling rate. The
StreamMuxConfig shall be considered definitive in both cases.
In this case the presence of SBR can not be detected in advance. An
SBR enabled decoder SHOULD use the SBR tool to upsample the audio
data if complexity and resulting output sampling rate permits.
In the following examples the presence of SBR is not signalled by the
SDP parameters object, profile-level-id and config string but the
SBR-enabled parameter is present. The rate parameter and the
StreamMuxConfig contain the core codec sampling rate. The
StreamMuxConfig shall be considered definitive. Receivers supporting
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SBR should set the output sampling rate to either the core AAC
sampling rate as indicated in the StreamMuxConfig (when "SBR-enabled"
is set to 0) or twice the indicated rate (when "SBR-enabled" is set
to 1).
Example with "SBR-enabled=0", sampling rate 24khz:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
SBR-enabled=0; config=400026203fc0
Example with "SBR-enabled=1", receivers supporting SBR should set the
sampling rate to 48khz:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
SBR-enabled=1; config=400026203fc0
When the presence of SBR is explicitly signalled by the SDP
parameters object, profile-level-id or the config string as in the
example below, the StreamMuxConfig contains both the core codec
sampling rate and the SBR sampling rate. The appropriate output
sampling rate may be chosen dependent on the support of SBR.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/48000
a=fmtp:96 profile-level-id=44; bitrate=64000; cpresent=0;
config=40005623101fe0
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
The following examples show how MPEG Surround configuration data can
be signalled using SDP. The configuration is carried within the
config string in the first example by using two different layers. The
general parameters in this example are: AudioMuxVersion=1;
allStreamsSameTimeFraming=1; numSubFrames=0; numProgram=0;
numLayer=1. The first layer describes the HE-AAC payload and signals
the following parameters: ascLen=25; audioObjectType=2 (AAC LC);
extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=6 (24kHz);
extensionSamplingFrequencyIndex=3 (48kHz); channelConfiguration=2
M.Schmidt, et al. Standards Track [Page 23]
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(2.0 channels). The second layer describes the MPEG surround payload
and specifies the following parameters: ascLen=110;
AudioObjectType=30 (MPEG Surround); samplingFrequencyIndex=3 (48kHz);
channelConfiguration=6 (5.1 channels); sacPayloadEmbedding=1;
SpatialSpecificConfig=(48 kHz; 32 slots; 525 tree; ResCoding=1;
ResBands=[7,7,7,7]).
In this example the signalling is carried by using two different LATM
layers. The MPEG surround payload is carried together with the AAC
playload in a single layer as indicated by the sacPayloadEmbedding
Flag.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/48000
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
SBR-enabled=1;
config=9FF8005192B11880FF2DDE3699F2408C00536C02313CF3CE0FF0
This following example is an extension of the configuration given
above by the MPEG Surround specific parameters. The MPS-asc parameter
specifies the MPEG Surround Baseline Profile at Level 3 (PLI55) and
the MPS-asc string contains the hexadecimal representation of the
MPEG Surround ASC [audioObjectType=30 (MPEG Surround);
samplingFrequencyIndex=0x3 (48kHz); channelConfiguration=6 (5.1
channels); sacPayloadEmbedding=1; SpatialSpecificConfig=(48 kHz; 32
slots; 525 tree; ResCoding=1; ResBands=[0,13,13,13])].
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/48000
a=fmtp:96 profile-level-id=44; bitrate=64000; cpresent=0;
config=40005623101fe0; MPS-profile-level-id=55;
MPS-asc=F1B4CF920442029B501185B6DA00;
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.
M.Schmidt, et al. Standards Track [Page 24]
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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".
[3] void
[4] ISO/IEC 14496-2:1999/Amd.1:2000, "Information technology -
Coding of audio-visual objects - Part 2: Visual, Amendment 1:
Visual extensions".
[5] ISO/IEC 14496-3:2005 and its amendments, "Information technology
- Coding of audio-visual objects - Part 3: Audio"
[6] ISO/IEC 14496-1, "Information technology - Coding of audio-
visual objects - Part1: Systems".
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson "RTP:
A Transport Protocol for Real Time Applications", RFC 3550,
January 1996.
[9] ISO/IEC 14496-2:1999/Cor.1:2000, "Information technology -
Coding of audio-visual objects - Part 2: Visual, Technical
corrigendum 1".
[10] ISO/IEC 14496-12, "Information technology - Coding of audio-
visual objects - Part 12: ISO base media file format".
[11] ISO/IEC 14496-14, "Information technology - Coding of audio-
visual objects - Part 14: MP4 file format".
[12] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and
P. Gentric, "RTP Payload Format for Transport of MPEG-4 Elementary
Streams", RFC 3640, November 2003.
M.Schmidt, et al. Standards Track [Page 25]
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8. Authors' 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
Malte Schmidt
Dolby Germany GmbH
Deutschherrnstr. 15-19, 90429 Nuernberg, Germany
EMail: malte.schmidt@dolby.com
M.Schmidt, et al. Standards Track [Page 26]
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Frans de Bont
Philips Electronics
High Tech Campus 5, 5656 AE Eindhoven, NL
Email: frans.de.bont@philips.com
M.Schmidt, et al. Standards Track [Page 27]
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9. Full Copyright Statement
Copyright (C) The IETF Trust (2008). All Rights Reserved.
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
M.Schmidt, et al. Standards Track [Page 28]
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