RTP Payload Format for Transport of MPEG-4 Elementary Streams
draft-ietf-avt-mpeg4-simple-08
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 3640.
|
|
|---|---|---|---|
| Authors | David Mackie , David Singer , Jan Van der Meer , Viswanathan Swaminathan , Philippe Gentric | ||
| Last updated | 2013-03-02 (Latest revision 2003-09-03) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 3640 (Proposed Standard) | |
| Action Holders |
(None)
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||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Allison J. Mankin | ||
| IESG note | ** No value found for 'doc.notedoc.note' ** | ||
| Send notices to | <csp@csperkins.org>, <magnus.westerlund@ericsson.com> |
draft-ietf-avt-mpeg4-simple-08
Internet Engineering Task Force J. van der Meer
Internet Draft Philips Electronics
D. Mackie
Apple Computer
V. Swaminathan
Sun Microsystems Inc.
D. Singer
Apple Computer
P. Gentric
Philips Electronics
August 2003
Expires February 2004
Document: draft-ietf-avt-mpeg4-simple-08.txt
RTP Payload Format for Transport of MPEG-4 Elementary Streams
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC 2026.
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-
Drafts. Internet-Drafts are draft documents valid for a maximum of
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."
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 specification is a product of the Audio/Video Transport working
group within the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing
list at avt@ietf.org and/or the authors.
<< Note for the RFC editor: xxxx should be replaced with the RFC
number that will be assigned. >>
Abstract
The MPEG Committee (ISO/IEC JTC1/SC29 WG11) is a working group in
ISO that produced the MPEG-4 standard. MPEG defines tools to
compress content such as audio-visual information into elementary
streams. This specification defines a simple, but generic RTP
payload format for transport of any non-multiplexed MPEG-4
elementary stream.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Carriage of MPEG-4 elementary streams over RTP . . . . . . . 6
2.1. Signaling by MIME format parameters . . . . . . . . . . . 6
2.2. MPEG Access Units . . . . . . . . . . . . . . . . . . . . 6
2.3. Concatenation of Access Units . . . . . . . . . . . . . . 6
2.4. Fragmentation of Access Units . . . . . . . . . . . . . . 7
2.5. Interleaving . . . . . . . . . . . . . . . . . . . . . . . 7
2.6. Time stamp information . . . . . . . . . . . . . . . . . . 8
2.7. State indication of MPEG-4 system streams . . . . . . . . 8
2.8. Random Access Indication . . . . . . . . . . . . . . . . . 8
2.9. Carriage of auxiliary information . . . . . . . . . . . . 9
2.10. MIME format parameters and configuring conditional field . 9
2.11. Global structure of payload format . . . . . . . . . . . . 9
2.12. Modes to transport MPEG-4 streams . . . . . . . . . . . . 10
2.13. Alignment with RFC 3016 . . . . . . . . . . . . . . . . . 10
3. Payload format . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. Usage of RTP header fields and RTCP . . . . . . . . . . . 11
3.2. RTP payload structure . . . . . . . . . . . . . . . . . . 12
3.2.1. The AU Header Section . . . . . . . . . . . . . . . . . 12
3.2.1.1. The AU-header . . . . . . . . . . . . . . . . . . . . 12
3.2.2. The Auxiliary Section . . . . . . . . . . . . . . . . . 15
3.2.3. The Access Unit Data Section . . . . . . . . . . . . . . 15
3.2.3.1. Fragmentation . . . . . . . . . . . . . . . . . . . . 16
3.2.3.2. Interleaving . . . . . . . . . . . . . . . . . . . . . 16
3.2.3.3. Constraints for interleaving . . . . . . . . . . . . . 18
3.2.3.4. Crucial and non-crucial AUs with MPEG-4 System data . 20
3.3. Usage of this specification . . . . . . . . . . . . . . . 22
3.3.1. General . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3.2. The generic mode . . . . . . . . . . . . . . . . . . . . 22
3.3.3. Constant bit rate CELP . . . . . . . . . . . . . . . . . 23
3.3.4. Variable bit rate CELP . . . . . . . . . . . . . . . . . 23
3.3.5. Low bit rate AAC . . . . . . . . . . . . . . . . . . . . 24
3.3.6. High bit rate AAC . . . . . . . . . . . . . . . . . . . 25
3.3.7. Additional modes . . . . . . . . . . . . . . . . . . . . 26
4. IANA considerations . . . . . . . . . . . . . . . . . . . . 27
4.1. MIME type registration . . . . . . . . . . . . . . . . . . 27
4.2. Registration of mode definitions with IANA . . . . . . . . 32
4.3. Concatenation of parameters . . . . . . . . . . . . . . . 32
4.4. Usage of SDP . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.1. The a=fmtp keyword . . . . . . . . . . . . . . . . . . . 33
5. Security considerations . . . . . . . . . . . . . . . . . . 33
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.1 Normative references . . . . . . . . . . . . . . . . . . . . 34
7.2 Informative references . . . . . . . . . . . . . . . . . . . 35
8. Author addresses . . . . . . . . . . . . . . . . . . . . . . 35
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APPENDIX: Usage of this payload format . . . . . . . . . . . 37
A. Examples of delay analysis with interleave . . . . . . . 37
A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 37
A.2 De-interleaving and error concealment . . . . . . . . . 37
A.3 Simple Group interleave . . . . . . . . . . . . . . . . 37
A.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . 37
A.3.2 Determining the de-interleave buffer size . . . . . . 38
A.3.3 Determining the maximum displacement . . . . . . . . . 38
A.4 More subtle group interleave . . . . . . . . . . . . . . 38
A.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . 38
A.4.2 Determining the de-interleave buffer size . . . . . . 39
A.4.3 Determining the maximum displacement . . . . . . . . . 39
A.5 Continuous interleave . . . . . . . . . . . . . . . . . 40
A.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . 40
A.5.2 Determining the de-interleave buffer size . . . . . . 40
A.5.3 Determining the maximum displacement . . . . . . . . . 41
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1. Introduction
The MPEG Committee is Working Group 11 (WG11) in ISO/IEC JTC1 SC29
that specified the MPEG-1, MPEG-2 and, more recently, the MPEG-4
standards [1]. The MPEG-4 standard specifies compression of
audio-visual data into for example an audio or video elementary
stream. In the MPEG-4 standard, these streams take the form of
audio-visual objects that may be arranged into an audio-visual scene
by means of a scene description. Each MPEG-4 elementary stream
consists of a sequence of Access Units; examples of an Access Unit
(AU) are an audio frame and a video picture.
This specification defines a general and configurable payload
structure to transport MPEG-4 elementary streams, in particular
MPEG-4 audio (including speech) streams, MPEG-4 video streams and
also MPEG-4 systems streams, such as BIFS (BInary Format for
Scenes), OCI (Object Content Information), OD (Object Descriptor)
and IPMP (Intellectual Property Management and Protection) streams.
The RTP payload defined in this document is simple to implement and
reasonably efficient. It allows for optional interleaving of Access
Units (such as audio frames) to increase error resiliency in packet
loss.
Some types of MPEG-4 elementary streams include "crucial"
information whose loss cannot be tolerated, but RTP does not provide
reliable transmission so receipt of that crucial information is not
assured. Section 3.2.3.4 specifies how stream state is conveyed so
that the receiver can detect the loss of crucial information and
cease decoding until the next random access point is received.
Applications transmitting streams that include crucial information,
such as OD commands, BIFS commands, or programmatic content such as
MPEG-J (Java) and ECMAScript, should include random access points
sufficiently often, depending upon the probability of loss, to
reduce stream corruption to an acceptable level. An example is the
carousel mechanism as defined by MPEG in ISO/IEC 14496-1.
Such applications may also employ additional protocols or services
to reduce the probability of loss. At the RTP layer, these measures
include payload formats and profiles for retransmission or forward
error correction (such as in RFC 2733 [10]), which must be employed
with due consideration to congestion control. Another solution that
may be appropriate for some applications is to carry RTP over TCP
(such as in RFC 2326 [8], section 10.12). At the network layer,
resource allocation or preferential service may be available to
reduce the probability of loss. For a general description of methods
to repair streaming media see RFC 2354 [9].
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Though the RTP payload format defined in this document is capable
of transporting any MPEG-4 stream, other, more specific, formats
may exist, such as RFC 3016 [12] for transport of MPEG-4 video
(ISO/IEC 14496 [1] part 2).
Configuration of the payload is provided to accommodate transport
of any MPEG-4 stream at any possible bit rate. However, for a
specific MPEG-4 elementary stream typically only very few
configurations are needed. So as to allow for the design of
simplified, but dedicated receivers, this specification requires
that specific modes are defined for transport of MPEG-4 streams.
This document defines modes for MPEG-4 CELP and AAC streams, as
well as a generic mode that can be used to transport any MPEG-4
stream. In the future new RFCs are expected to specify additional
modes for transport of MPEG-4 streams.
The RTP payload format defined in this document specifies carriage
of system-related information that is often equivalent to the
information that may be contained in the MPEG-4 Sync Layer (SL) as
defined in MPEG-4 Systems [1]. This document does not prescribe how
to transcode or map information from the SL to fields defined in
the RTP payload format. Such processing, if any, is left to the
discretion of the application. However, to anticipate the need for
transport of any additional system-related information in future,
an auxiliary field can be configured that may carry any such data.
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 [4].
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2. Carriage of MPEG-4 elementary streams over RTP
2.1 Signaling by MIME format parameters
With this payload format a single MPEG-4 elementary stream can be
transported. Information on the type of MPEG-4 stream carried in
the payload is conveyed by MIME format parameters, for example in
an SDP [5] message or by other means (see section 4). These MIME
format parameters specify the configuration of the payload. To
allow for simplified and dedicated receivers, a MIME format
parameter is available to signal a specific mode of using this
payload. A mode definition MAY include the type of MPEG-4
elementary stream as well as the applied configuration, so as to
avoid the need for receivers to parse all MIME format parameters.
The applied mode MUST be signaled.
2.2 MPEG Access Units
For carriage of compressed audio-visual data MPEG defines Access
Units. An MPEG Access Unit (AU) is the smallest data entity to
which timing information is attributed. In case of audio an Access
Unit may represent an audio frame and in case of video a picture.
MPEG Access Units are by definition octet-aligned. If for example
an audio frame is not octet-aligned, up to 7 zero-padding bits MUST
be inserted at the end of the frame to achieve the octet-aligned
Access Units, as required by the MPEG-4 specification. MPEG-4
decoders MUST be able to decode AUs in which such padding is
applied.
Consistent with the MPEG-4 specification, this document requires
that each MPEG-4 part 2 video Access Unit includes all the coded
data of a picture, any video stream headers that may precede the
coded picture data, and any video stream stuffing that may follow
it, up to, but not including the startcode indicating the start of
a new video stream or the next Access Unit.
2.3 Concatenation of Access Units
Frequently it is possible to carry multiple Access Units in one RTP
packet. This is particularly useful for audio; for example, when
AAC is used for encoding of a stereo signal at 64 kbits/sec, AAC
frames contain on average approximately 200 octets. On a LAN with a
1500 octet MTU this would allow on average 7 complete AAC frames to
be carried per RTP packet.
Access Units may have a fixed size in octets, but a variable size
is also possible. To facilitate parsing in case of multiple
concatenated AUs in one RTP packet, the size of each AU is made
known to the receiver. When concatenating in case of a constant AU
size, this size is communicated "out of band" through a MIME format
parameter. When concatenating in case of variable size AUs, the RTP
payload carries "in band" an AU size field for each contained AU.
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In combination with the RTP payload length the size information
allows the RTP payload to be split by the receiver back into the
individual AUs.
To simplify the implementation of RTP receivers, it is required
that when multiple AUs are carried in an RTP packet, each AU MUST
be complete, i.e. the number of AUs in an RTP packet MUST be
integral. In addition, an AU MUST NOT be repeated in other RTP
packets; hence repetition of an AU is only possible by using a
duplicate RTP packet.
2.4 Fragmentation of Access Units
MPEG allows for very large Access Units. Since most IP networks
have significantly smaller MTU sizes, this payload format allows
for the fragmentation of an Access Unit over multiple RTP packets.
Hence when an IP packet is lost after IP-level fragmentation, only an
AU fragment may get lost instead of the entire AU. To simplify the
implementation of RTP receivers, an RTP packet SHALL either carry
one or more complete Access Units or a single fragment of one AU,
i.e. packets MUST NOT contain fragments of multiple Access Units.
2.5 Interleaving
When an RTP packet carries a contiguous sequence of Access Units,
the loss of such a packet can result in a "decoding gap" for the
user. One method to alleviate this problem is to allow for the
Access Units to be interleaved in the RTP packets. For a modest
cost in latency and implementation complexity, significant error
resiliency to packet loss can be achieved.
To support optional interleaving of Access Units, this payload
format allows for index information to be sent for each Access Unit.
After informing receivers about buffer resources to allocate for
de-interleaving, the RTP sender is free to choose the interleaving
pattern without propagating this information a priori to the
receiver(s). Indeed the sender could dynamically adjust the
interleaving pattern based on the Access Unit size, error rates,
etc. The RTP receiver does not need to know the interleaving
pattern used, it only needs to extract the index information of the
Access Unit and insert the Access Unit into the appropriate
sequence in the decoding or rendering queue. An example of
interleaving is given below.
For example, if we assume that an RTP packet contains 3 AUs, and
that the AUs are numbered 0, 1, 2, 3, 4, and so forth, and if an
interleaving group length of 9 is chosen, then RTP packet(i)
contains the following AU(n):
RTP packet(0): AU(0), AU(3), AU(6)
RTP packet(1): AU(1), AU(4), AU(7)
RTP packet(2): AU(2), AU(5), AU(8)
RTP packet(3): AU(9), AU(12), AU(15)
RTP packet(4): AU(10), AU(13), AU(16) Etc.
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2.6 Time stamp information
The RTP time stamp MUST carry the sampling instant of the first AU
(fragment) in the RTP packet. When multiple AUs are carried within
an RTP packet, the time stamps of subsequent AUs can be calculated
if the frame period of each AU is known. For audio and video this
is possible if the frame rate is constant. However, in some cases
it is not possible to make such calculation. For example, for
variable frame rate video, or for MPEG-4 BIFS streams carrying
composition information. To support such cases, this payload format
can be configured to carry a time stamp in the RTP payload for each
contained Access Unit. A time stamp MAY be conveyed in the RTP
payload only for non-first AUs in the RTP packet, and SHALL NOT be
conveyed for the first AU (fragment), as the time stamp for the
first AU in the RTP packet is carried by the RTP time stamp.
MPEG-4 defines two types of time stamp: the composition time stamp
(CTS) and the decoding time stamp (DTS). The CTS represents the
sampling instant of an AU, and hence the CTS is equivalent to the
RTP time stamp. The DTS may be used in MPEG-4 video streams that
use bi-directional coding, i.e. when pictures are predicted in both
forward and backward direction by using either a reference picture
in the past, or a reference picture in the future. The DTS cannot
be carried in the RTP header. In some cases the DTS can be derived
from the RTP time stamp using frame rate information; this requires
deep parsing in the video stream, which may be considered
objectionable. But if the video frame rate is variable, the required
information may not even be present in the video stream. For both
reasons, the capability has been defined to optionally carry the
DTS in the RTP payload for each contained Access Unit.
To keep the coding of time stamps efficient, each time stamp
contained in the RTP payload is coded differentially, the CTS from
the RTP time stamp, and the DTS from the CTS.
2.7 State indication of MPEG-4 system streams
ISO/IEC 14496-1 defines states for MPEG-4 system streams. So as to
convey state information when transporting MPEG-4 system streams,
this payload format allows for the optional carriage in the RTP
payload of the stream state for each contained Access Unit. Stream
states are used to signal "crucial" AUs that carry information whose
loss cannot be tolerated and are also useful when repeating AUs
according to the carousel mechanism defined in ISO/IEC 14496-1.
2.8 Random access indication
Random access to the content of MPEG-4 elementary streams may be
possible at some but not all Access Units. To signal Access Units
where random access is possible, a random access point flag can
optionally be carried in the RTP payload for each contained Access
Unit. Carriage of random access points is particularly useful for
MPEG-4 system streams in combination with the stream state.
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2.9 Carriage of auxiliary information.
This payload format defines a specific field to carry auxiliary
data. The auxiliary data field is preceded by a field that specifies
the length of the auxiliary data, so as to facilitate skipping of
the data without parsing it. The coding of the auxiliary data is not
defined in this document; instead the format, meaning and signaling
of auxiliary information is expected to be specified in one or more
future RFCs. Auxiliary information MUST NOT be transmitted until its
format, meaning and signaling have been specified and its use has
been signaled. Receivers that have knowledge of the auxiliary data
MAY decode the auxiliary data, but receivers without knowledge of
such data MUST skip the auxiliary data field.
2.10 MIME format parameters and configuring conditional fields
To support the features described in the previous sections several
fields are defined for carriage in the RTP payload. However, their
use strongly depends on the type of MPEG-4 elementary stream that
is carried. Sometimes a specific field is needed with a certain
length, while in other cases such field is not needed at all. To be
efficient in either case, the fields to support these features are
configurable by means of MIME format parameters. In general, a MIME
format parameter defines the presence and length of the associated
field. A length of zero indicates absence of the field. As a
consequence, parsing of the payload requires knowledge of MIME
format parameters. The MIME format parameters are conveyed to the
receiver via SDP [5] messages, as specified in section 4.4.1, or
through other means.
2.11 Global structure of payload format
The RTP payload following the RTP header, contains three
octet-aligned data sections, of which the first two MAY be empty.
See figure 1.
+---------+-----------+-----------+---------------+
| RTP | AU Header | Auxiliary | Access Unit |
| Header | Section | Section | Data Section |
+---------+-----------+-----------+---------------+
<----------RTP Packet Payload----------->
Figure 1: Data sections within an RTP packet
The first data section is the AU (Access Unit) Header Section, that
contains one or more AU-headers; however, each AU-header MAY be
empty, in which case the entire AU Header Section is empty. The
second section is the Auxiliary Section, containing auxiliary data;
this section MAY also be configured empty. The third section is the
Access Unit Data Section, containing either a single fragment of
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one Access Unit or one or more complete Access Units. The Access
Unit Data Section MUST NOT be empty.
2.12 Modes to transport MPEG-4 streams
While it is possible to build fully configurable receivers capable
of receiving any MPEG-4 stream, this specification also allows for
the design of simplified, but dedicated receivers, that are capable
for example of receiving only one type of MPEG-4 stream. This
is achieved by requiring that specific modes be defined for using
this specification. Each mode may define constraints for transport
of one or more type of MPEG-4 streams, for instance on the payload
configuration.
The applied mode MUST be signaled. Signaling the mode is
particularly important for receivers that are only capable of
decoding one or more specific modes. Such receivers need to
determine whether the applied mode is supported, so as to avoid
problems with processing of payloads that are beyond the
capabilities of the receiver.
In this document several modes are defined for transport of MPEG-4
CELP and AAC streams, as well as a generic mode that can be used
for any MPEG-4 stream. In the future, new RFCs may specify other
modes of using this specification. However, each mode MUST be in
full compliance with this specification (see section 3.3.7).
2.13 Alignment with RFC 3016
This payload can be configured to be nearly identical to the
payload format defined in RFC 3016 [12] for the MPEG-4 video
configurations recommended in RFC 3016. Hence, receivers that
comply with RFC 3016 can decode such RTP payload, providing that
additional packets containing video decoder configuration (VO,
VOL, VOSH) are inserted in the stream, as required by RFC 3016.
Conversely, receivers that comply with the specification in this
document SHOULD be able to decode payloads, names and parameters
defined for MPEG-4 video in RFC 3016. In this respect it is
strongly RECOMMENDED to implement the ability to ignore "in band"
video decoder configuration packets in the RFC 3016 payload.
Note the "out of band" availability of the video decoder
configuration is optional in RFC 3016. To achieve maximum
interoperability with the RTP payload format defined in this
document, applications that use RFC 3016 to transport MPEG-4 video
(part 2) are recommended to make the video decoder configuration
available as a MIME parameter.
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3. Payload Format
3.1 Usage of RTP Header Fields and RTCP
Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is
specified by the RTP profile under which this payload format is
used, or signaled dynamically out-of-band (e.g. using SDP).
Marker (M) bit: The M bit is set to 1 to indicate that the RTP
packet payload contains either the final fragment of a fragmented
Access Unit or one or more complete Access Units.
Extension (X) bit: Defined by the RTP profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the
sender in the usual manner with a constant random offset.
Timestamp: Indicates the sampling instant of the first AU
contained in the RTP payload. This sampling instant is equivalent
to the CTS in the MPEG-4 time domain. When using SDP the clock rate
of the RTP time stamp MUST be expressed using the "rtpmap"
attribute. If an MPEG-4 audio stream is transported, the rate SHOULD
be set to the same value as the sampling rate of the audio stream.
If an MPEG-4 video stream is transported, it is RECOMMENDED to set
the rate to 90 kHz.
In all cases, the sender SHALL make sure that RTP time stamps
are identical only if the RTP time stamp refers to fragments of the
same Access Unit.
According to RFC 1889 [2] (section 5.1), RTP time stamps are
RECOMMENDED to start at a random value for security reasons. This
is not an issue for synchronization of multiple RTP streams. When,
however, streams from multiple sources are to be synchronized (for
example one stream from local storage, another from an RTP streaming
server), synchronization may become impossible if the receiver only
knows the original time stamp relationships. Synchronization in such
cases, may require to provide the correct relationship between time
stamps for obtaining synchronization by out of band means. The
format of such information as well as methods to convey such
information are beyond the scope of this specification.
SSRC: set as described in RFC 1889 [2].
CC and CSRC fields are used as described in RFC 1889 [2].
RTCP SHOULD be used as defined in RFC 1889 [2]. Note that time
stamps in RTCP Sender Reports may be used to synchronize multiple
MPEG-4 elementary streams and also to synchronize MPEG-4 streams
with non-MPEG-4 streams, in case the delivery of these streams uses
RTP.
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3.2 RTP Payload Structure
3.2.1 The AU Header Section
When present, the AU Header Section consists of the
AU-headers-length field, followed by a number of AU-headers. See
figure 2.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+
|AU-headers-length|AU-header|AU-header| |AU-header|padding|
| | (1) | (2) | | (n) | bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+
Figure 2: The AU Header Section
The AU-headers are configured using MIME format parameters and MAY
be empty. If the AU-header is configured empty, the
AU-headers-length field SHALL NOT be present and consequently the
AU Header Section is empty. If the AU-header is not configured
empty, then the AU-headers-length is a two octet field that
specifies the length in bits of the immediately following
AU-headers, excluding the padding bits.
Each AU-header is associated with a single Access Unit (fragment)
contained in the Access Unit Data Section in the same RTP packet.
For each contained Access Unit (fragment) there is exactly one
AU-header. Within the AU Header Section, the AU-headers are
bit-wise concatenated in the order in which the Access Units are
contained in the Access Unit Data Section. Hence, the n-th
AU-header refers to the n-th AU (fragment). If the concatenated
AU-headers consume a non-integer number of octets, up to 7
zero-padding bits MUST be inserted at the end in order to achieve
octet-alignment of the AU Header Section.
3.2.1.1 The AU-header
Each AU-header may contain the fields given in figure 3. The length
in bits of the fields, with the exception of the CTS-flag, the
DTS-flag and the RAP-flag fields is defined by MIME format
parameters; see section 4.1. If a MIME format parameter has the
default value of zero, then the associated field is not present.
The number of bits for fields that are present and that represent
the value of a parameter MUST be chosen large enough to correctly
encode the largest value of that parameter during the session.
If present, the fields MUST occur in the mutual order given in
figure 3. In the general case a receiver can only discover the size
of an AU-header by parsing it since the presence of the CTS-delta
and DTS-delta fields is signaled by the value of the CTS-flag and
DTS-flag, respectively.
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+---------------------------------------+
| AU-size |
+---------------------------------------+
| AU-Index / AU-Index-delta |
+---------------------------------------+
| CTS-flag |
+---------------------------------------+
| CTS-delta |
+---------------------------------------+
| DTS-flag |
+---------------------------------------+
| DTS-delta |
+---------------------------------------+
| RAP-flag |
+---------------------------------------+
| Stream-state |
+---------------------------------------+
Figure 3: The fields in the AU-header. If used, the AU-Index field
only occurs in the first AU-header within an AU Header
Section; in any other AU-header the AU-Index-delta field
occurs instead.
AU-size: Indicates the size in octets of the associated Access Unit
in the Access Unit Data Section in the same RTP packet. When
the AU-size is associated with an AU fragment, the AU size
indicates the size of the entire AU and not the size of the
fragment. In this case, the size of the fragment is known
from the size of the AU data section. This can be exploited
to determine whether a packet contains an entire AU or a
fragment, which is particularly useful after losing a packet
carrying the last fragment of an AU.
AU-Index: Indicates the serial number of the associated Access Unit
(fragment). For each (in decoding order) consecutive AU or AU
fragment, the serial number is incremented with 1. When
present, the AU-Index field occurs in the first AU-header in
the AU Header Section, but MUST NOT occur in any subsequent
(non-first) AU-header in that Section. To encode the serial
number in any such non-first AU-header, the AU-Index-delta
field is used.
AU-Index-delta: The AU-Index-delta field is an unsigned integer
that specifies the serial number of the associated AU as the
difference with respect to the serial number of the previous
Access Unit. Hence, for the n-th (n>1) AU the serial number
is found from:
AU-Index(n) = AU-Index(n-1) + AU-Index-delta(n) + 1
If the AU-Index field is present in the first AU-header in
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the AU Header Section, then the AU-Index-delta field MUST be
present in any subsequent (non-first) AU-header. When the
AU-Index-delta is coded with the value 0, it indicates that
the Access Units are consecutive in decoding order. An
AU-Index-delta value larger than 0 signals that interleaving
is applied.
CTS-flag: Indicates whether the CTS-delta field is present.
A value of 1 indicates that the field is present, a value
of 0 that it is not present.
The CTS-flag field MUST be present in each AU-header if the
length of the CTS-delta field is signaled to be larger than
zero. In that case, the CTS-flag field MUST have the value 0
in the first AU-header and MAY have the value 1 in all
non-first AU-headers. The CTS-flag field SHOULD be 0 for
any non-first fragment of an Access Unit.
CTS-delta: Encodes the CTS by specifying the value of CTS as a 2's
complement offset (delta) from the time stamp in the RTP
header of this RTP packet. The CTS MUST use the same clock
rate as the time stamp in the RTP header.
DTS-flag: Indicates whether the DTS-delta field is present. A value
of 1 indicates that DTS-delta is present, a value of 0 that
it is not present.
The DTS-flag field MUST be present in each AU-header if the
length of the DTS-delta field is signaled to be larger than
zero. The DTS-flag field MUST have the same value for all
fragments of an Access Unit.
DTS-delta: Specifies the value of the DTS as a 2's complement
offset (delta) from the CTS. The DTS MUST use the
same clock rate as the time stamp in the RTP header. The
DTS-delta field MUST have the same value for all fragments of
an Access Unit.
RAP-flag: Indicates when set to 1 that the associated Access Unit
provides a random access point to the content of the stream.
If an Access Unit is fragmented, the RAP flag, if present,
MUST be set to 0 for each non-first fragment of the AU.
Stream-state: Specifies the state of the stream for an AU of an
MPEG-4 system stream; each state is identified by a value of
a modulo counter. In ISO/IEC 14496-1, MPEG-4 system streams
use the AU_SequenceNumber to signal stream states. When the
stream state changes, the value of stream-state MUST be
incremented by one.
Note: no relation is required between stream-states of
different streams.
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3.2.2 The Auxiliary Section
The Auxiliary Section consists of the auxiliary-data-size field
followed by the auxiliary-data field. Receivers MAY (but are not
required to) parse the auxiliary-data field; to facilitate skipping
of the auxiliary-data field by receivers, the auxiliary-data-size
field indicates the length in bits of the auxiliary-data. If the
concatenation of the auxiliary-data-size and the auxiliary-data
fields consume a non-integer number of octets, up to 7 zero padding
bits MUST be inserted immediately after the auxiliary data in order
to achieve octet-alignment. See figure 4.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
| auxiliary-data-size | auxiliary-data |padding bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
Figure 4: The fields in the Auxiliary Section
The length in bits of the auxiliary-data-size field is configurable
by a MIME format parameter; see section 4.1. The default length of
zero indicates that the entire Auxiliary Section is absent.
auxiliary-data-size: specifies the length in bits of the immediately
following auxiliary-data field;
auxiliary-data: the auxiliary-data field contains data of a format
not defined by this specification.
3.2.3 The Access Unit Data Section
The Access Unit Data Section contains an integer number of complete
Access Units or a single fragment of one AU. The Access Unit Data
Section is never empty. If data of more than one Access Unit is
present, then the AUs are concatenated into a contiguous string
of octets. See figure 5. The AUs inside the Access Unit Data
Section MUST be in decoding order, though not necessarily contiguous
in the case of interleaving.
The size and number of Access Units 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 result in reduced performance.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AU(1) |
+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |AU(2) |
+-+-+-+-+-+-+-+-+ |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | AU(n) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AU(n) continued|
|-+-+-+-+-+-+-+-+
Figure 5: Access Unit Data Section; each AU is octet-aligned.
When multiple Access Units are carried, the size of each AU MUST be
made available to the receiver. If the AU size is variable then the
size of each AU MUST be indicated in the AU-size field of the
corresponding AU-header. However, if the AU size is constant for a
stream, this mechanism SHOULD NOT be used, but instead the fixed
size SHOULD be signaled by the MIME format parameter
"constantSize", see section 4.1.
The absence of both AU-size in the AU-header and the constantSize
MIME format parameter indicates carriage of a single AU (fragment),
i.e. that a single Access Unit (fragment) is transported in each
RTP packet for that stream.
3.2.3.1 Fragmentation
A packet SHALL carry either one or more complete Access Units, or
a single fragment of an Access Unit. Fragments of the same Access
Unit have the same time stamp but different RTP sequence numbers.
The marker bit in the RTP header is 1 on the last fragment of an
Access Unit, and 0 on all other fragments.
3.2.3.2 Interleaving
Unless prohibited by the signaled mode, a sender MAY interleave
Access Units. Receivers that are capable of receiving modes that
support interleaving, MUST be able to decode interleaved Access
Units.
When a sender interleaves Access Units, it needs to provide
sufficient information to enable a receiver to unambiguously
reconstruct the original order, even in case of out-of-order
packets, packet loss or duplication. The information that senders
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need to provide depends on whether or not the Access Units have a
constant time duration. Access Units have a constant time duration,
if:
TS(i+1) - TS(i) = constant, for any i, where
i indicates the index of the AU in original order
TS(i) denotes the time stamp of AU(i)
The MIME parameter "constantDuration" SHOULD be used to signal that
Access Units have a constant time duration, see section 4.1.
If the "constantDuration" parameter is present, the receiver can
reconstruct the original Access Unit timing based solely on the RTP
timestamp and AU-Index-delta. Accordingly, when transmitting Access
Units of constant duration, the AU-Index, if present, MUST be set
to the value 0. Receivers of constant duration Access Units MUST
use the RTP timestamp to determine the index of the first AU in the
RTP packet. The AU-Index-delta header and the signaled
"constantDuration" are used to reconstruct AU timing.
If the "constantDuration" parameter is not present, then Access
Units are assumed to have a variable duration, unless the AU-Index
is present and coded with the value 0 in each RTP packet. When
transmitting Access Units of variable duration, then the
"constantDuration" parameter MUST NOT be present, and the
transmitter MUST use the AU-Index to encode the index information
required for re-ordering, and the receiver MUST use that value to
determine the index of each AU in the RTP packet. The number of
bits of the AU-Index field MUST be chosen so that valid index
information is provided at the applied interleaving scheme, without
causing problems due to roll-over of the AU-Index field. In
addition, the CTS-delta MUST be coded in the AU header for each
non-first AU in the RTP packet, so that receivers can place the AUs
correctly in time.
When interleaving is applied, a de-interleave buffer is needed in
receivers to put the Access Units in their correct logical
consecutive decoding order. This requires the computation of the
time stamp for each Access Unit. In case of a constant time duration
per Access Unit, the time stamp of the i-th access unit in an RTP
packet with RTP time stamp T is calculated as follows:
Timestamp[0] = T
Timestamp[i, i > 0] = T +(Sum(for k=1 to i of (AU-Index-delta[k]
+ 1))) * access-unit-duration
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When AU-Index-delta is always 0, this reduces to T + i * (access-
unit-duration). This is the non-interleaved case, where the frames
are consecutive in decoding order. Note that the AU-Index field
(present for the first Access Unit) is indeed not needed in this
calculation.
3.2.3.3 Constraints for interleaving
The size of the packets should be suitably chosen to be appropriate
to both the path MTU and the capacity of the receiver's
de-interleave buffer. The maximum packet size for a session SHOULD
be chosen not to exceed the path MTU.
To allow receivers to allocate sufficient resources for
de-interleaving, senders MUST provide the information to receivers
as specified in this section.
AUs enter the decoder in decoding order. The de-interleave buffer
is used to re-order a stream of interleaved AUs back into decoding
order. When interleaving is applied, the decoding of "early" AUs
has to be postponed until all AUs that precede in decoding order
are present. Therefore these "early" AUs are stored in the
de-interleave buffer. As an example in figure 6 the interleaving
pattern from section 2.5 is considered.
+--+--+--+--+--+--+--+--+--+--+--+-
Interleaved AUs | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
+--+--+--+--+--+--+--+--+--+--+--+-
Storage of "early" AUs 3 3 3 3 3 3
6 6 6 6 6 6
4 4 4
7 7 7
12 12
Figure 6: Storage of "early" AUs in the de-interleave buffer per
interleaved AU.
AU(3) is to be delivered to the decoder after AU(0), AU(1)and
AU(2); of these AUs, AU(2) is most late and hence AU(3) needs to be
stored until AU(2) is present in the pattern. Similarly, AU(6) is
to be stored until AU(5) is present, while AU(4) and AU(7) are to
be stored until AU(2) and AU(5) are present, respectively. Note
that the fullness of the de-interleave buffer varies in time. In
figure 6, the de-interleave buffer contains at most 4, but often
less AUs.
So as to give a rough indication of the resources needed in the
receiver for de-interleaving, the maximum displacement in time of
an AU is defined. For any AU in the pattern it can be verified
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which AUs are not yet present. The maximum displacement in time of
an AU is the maximum difference between the time stamp of an AU in
the pattern and the time stamp of the earliest AU that is not yet
present. In other words, when considering a sequence of interleaved
AUs, then:
Maximum displacement = max{TS(i) - TS(j)}, for any i and any j>i,
where i and j indicate the index of the AU in the
interleaving pattern and TS denotes the time stamp
of the AU
As an example in figure 7 the interleaving pattern from section 2.5
is considered. For each AU in the pattern the earliest not yet
present AU is indicated. A "-" indicates that all previous AUs
are present. If the AU period is constant, the maximum displacement
equals 5 AU periods, as found for AU(6) and AU(7).
+--+--+--+--+--+--+--+--+--+--+--+-
Interleaved AUs | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
+--+--+--+--+--+--+--+--+--+--+--+-
Earliest not yet present AU - 1 1 - 2 2 - - - - 10
Figure 7: The earliest not yet present AU for each AU in the
interleaving pattern.
When interleaving, senders MUST signal the maximum displacement
in time during the session via the MIME format parameter
"maxDisplacement"; see section 4.1.
An estimate of the size of the de-interleave buffer is found by
multiplying the maximum displacement by the maximum bit rate:
size(de-interleave buffer) = {(maxDisplacement) * Rate(max)} / (RTP
clock frequency),
where Rate(max) is the maximum bit-rate of the transported stream.
Note that receivers can derive Rate(max) from the MIME format
parameters streamType, profile-level-id, and config.
However, this calculation estimates the size of the de-interleave
buffer and the really required size may differ from the calculated
value. If this calculation under-estimates the size of the
de-interleave buffer, then senders, when interleaving, MUST signal
a size of the de-interleave buffer via the MIME format parameter
"de-interleaveBufferSize"; see section 4.1. If the calculation
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over-estimates the size of the de-interleave buffer, then senders,
when interleaving, MAY signal a size of the de-interleave buffer
via the MIME format parameter "de-interleaveBufferSize".
The signaled size of the de-interleave buffer MUST be large enough
to contain all "early" AUs at any point in time during the session,
that is:
minimum de-interleave buffer size = max [sum {if TS(i) > TS(j) then
AU-size(i) else 0}] for any j
and any i<j, where
i and j indicate the index of an AU in the interleaving
pattern,
TS(i) denotes the time stamp of AU(i), and
AU-size(i) denotes the size of AU(i) in number of octets.
If the "de-interleaveBufferSize" parameter is present, then the
applied buffer for de-interleaving in a receiver MUST have a size
that is at least equal to the signaled size of the de-interleave
buffer, else a size that is at least equal to the calculated size
of the de-interleave buffer.
No matter what interleaving scheme is used, the scheme must be
analyzed to calculate the applicable maxDisplacement value, as well
as the required size of the de-interleave buffer. Senders SHOULD
signal values that are not larger than the strictly required
values; if larger values are signaled, the receiver will buffer
excessively.
Note that for low bit-rate material, the applied interleaving
may make packets shorter than the MTU size.
3.2.3.4 Crucial and non-crucial AUs with MPEG-4 System data
Some Access Units with MPEG-4 system data, called "crucial" AUs,
carry information whose loss cannot be tolerated, either in the
presentation or in the decoder. At each crucial AU in an MPEG-4
system stream, the stream state changes. The stream-state MAY
remain constant at non-crucial AUs. In ISO/IEC 14496-1, MPEG-4
system streams use the AU_SequenceNumber to signal stream states.
Example: Given three AUs, AU1 = "Insertion of node X", AU2 = "Set
position of node X", AU3 = "Set position of node X". AU1 is crucial,
since if it is lost, AU2 cannot be executed. However, AU2 is not
crucial, since AU3 can be executed even if AU2 is lost.
When a crucial AU is (possibly) lost, the stream is corrupted. For
example, when an AU is lost and the stream state has changed at the
next received AU, then it is possible that the lost AU was crucial.
Once corrupted, the stream remains corrupted until the next random
access point. Note that loss of non-crucial AUs does not corrupt the
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stream. When a decoder starts receiving a stream, the decoder MUST
consider the stream corrupted until an AU is received that provides
a random access point.
An AU that provides a random access point, as signaled by the
RAP-flag, may be crucial or not. Non-crucial RAP AUs provide a
"repeated" random access point for use by decoders that recently
joined the stream or that need to re-start decoding after a stream
corruption. Non-crucial RAP AUs MUST include all updates since the
last crucial RAP AU.
Upon receiving AUs, decoders are to react as follows:
a) if the RAP-flag is set to 1 and the stream-state changes, then
the AU is a crucial RAP AU, and the AU MUST be decoded.
b) if the RAP-flag is set to 1 and the stream state does not change,
then the AU is a non-crucial RAP AU, and the receiver SHOULD
decode it if the stream is corrupted. Otherwise, the decoder MUST
ignore the AU.
c) if the RAP-flag is set to 0, then the AU MUST be decoded, unless
the stream is corrupted, in which case the AU MUST be ignored.
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3.3 Usage of this specification
3.3.1 General
Usage of this specification requires definition of a mode. A mode
defines how to use this specification, as deemed appropriate.
Senders MUST signal the applied mode via the MIME format parameter
"mode", as specified in section 4.1. This specification defines a
generic mode that can be used for any MPEG-4 stream, as well as
specific modes for transport of MPEG-4 CELP and MPEG-4 AAC streams,
defined in ISO/IEC 14496-3.
When use of this payload format is signaled using SDP [5], an
"rtpmap" attribute is part of that signaling. The same requirements
apply for the rtpmap attribute in any mode compliant to this
specification. The general form of an rtpmap attribute is:
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
parameters>]
For audio streams, <encoding parameters> specifies the number of
audio channels: 2 for stereo material (see RFC 2327 [5]) and 1 for
mono. Provided no additional parameters are needed, this parameter
may be omitted for mono material, hence its default value is 1.
3.3.2 The generic mode
The generic mode can be used for any MPEG-4 stream. In this mode
no mode-specific constraints are applied; hence, in the generic
mode the full flexibility of this specification can be exploited.
The generic mode is signaled by mode=generic.
An example is given below for transport of a BIFS-Anim stream. In
this example carriage of multiple BIFS-Anim Access Units is allowed
in one RTP packet. The AU-header contains the AU-size field, the
CTS-flag and, if the CTS flag is set to 1, the CTS-delta field. The
number of bits of the AU-size and the CTS-delta fields is 10 and
16, respectively. The AU-header also contains the RAP-flag and the
Stream-state of 4 bits. This results in an AU-header with a
total size of two or four octets per BIFS-Anim AU. The RTP time
stamp uses a 1 kHz clock. Note that the media type name is video,
because the BIFS-Anim stream is part of an audio-visual
presentation. For conventions on media type names see section 4.1.
In detail:
m=video 49230 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/1000
a=fmtp:96 streamtype=3; profile-level-id=1807; mode=generic;
objectType=2; config=0842237F24001FB400094002C0; sizeLength=10;
CTSDeltaLength=16; randomAccessIndication=1;
streamStateIndication=4
Note: The a=fmtp line has been wrapped to fit the page, it comprises
a single line in the SDP file.
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The hexadecimal value of the "config" parameter is the
BIFSConfiguration() as defined in ISO/IEC 14496-1. The
BIFSConfiguration() specifies that the BIFS stream is a BIFS-Anim
stream. For the description of MIME parameters see section 4.1.
3.3.3 Constant bit-rate CELP
This mode is signaled by mode=CELP-cbr. In this mode one or more
complete CELP frames of fixed size can be transported in one RTP
packet; interleaving MUST NOT be used with this mode. The RTP
payload consists of one or more concatenated CELP frames, each of
the same size. CELP frames MUST NOT be fragmented when using this
mode. Both the AU Header Section and the Auxiliary Section MUST be
empty.
The MIME format parameter constantSize MUST be provided to specify
the length of each CELP frame.
For example:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/16000/1
a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-cbr; config=
440E00; constantSize=27; constantDuration=240
Note: The a=fmtp line has been wrapped to fit the page, it comprises
a single line in the SDP file.
The hexadecimal value of the "config" parameter is the
AudioSpecificConfig()as defined in ISO/IEC 14496-3.
AudioSpecificConfig() specifies a mono CELP stream with a sampling
rate of 16 kHz at a fixed bitrate of 14.4 kb/s and 6 sub-frames per
CELP frame. For the description of MIME parameters see section 4.1.
3.3.4 Variable bit-rate CELP
This mode is signaled by mode=CELP-vbr. With this mode one or more
complete CELP frames of variable size can be transported in one RTP
packet with OPTIONAL interleaving. As CELP frames are very small,
while the largest possible AU-size in this mode is greater than the
maximum CELP frame size, there is no support for fragmentation of
CELP frames. Hence CELP frames MUST NOT be fragmented when using
this mode.
In this mode the RTP payload consists of the AU Header Section,
followed by one or more concatenated CELP frames. The Auxiliary
Section MUST be empty. For each CELP frame contained in the payload
there MUST be a one octet AU-header in the AU Header Section to
provide:
(a) the size of each CELP frame in the payload and
(b) index information for computing the sequence (and hence timing)
of each CELP frame.
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Transport of CELP frames requires that the AU-size field is coded
with 6 bits. In this mode therefore 6 bits are allocated to the
AU-size field, and 2 bits to the AU-Index(-delta) field. Each
AU-Index field MUST be coded with the value 0. In the AU Header
Section, the concatenated AU-headers are preceded by the 16-bit
AU-headers-length field, as specified in section 3.2.1.
In addition to the required MIME format parameters, the following
parameters MUST be present: sizeLength, indexLength, and
indexDeltaLength. CELP frames have fixed time duration per Access
Unit; when interleaving in this mode, the applicable duration MUST
be signaled by the MIME format parameter constantDuration. In
addition, the parameter maxDisplacement MUST be present when
interleaving.
For example:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/16000/1
a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-vbr; config=
440F20; sizeLength=6; indexLength=2; indexDeltaLength=2;
constantDuration=160; maxDisplacement=5
Note: The a=fmtp line has been wrapped to fit the page, it comprises
a single line in the SDP file.
The hexadecimal value of the "config" parameter is the
AudioSpecificConfig()as defined in ISO/IEC 14496-3.
AudioSpecificConfig() specifies a mono CELP stream with a sampling
rate of 16 kHz at a bitrate that varies between 13.9 and 16.2 kb/s
and with 4 sub-frames per CELP frame. For the description of MIME
parameters see section 4.1.
3.3.5 Low bit-rate AAC
This mode is signaled by mode=AAC-lbr. This mode supports transport
of one or more complete AAC frames of variable size. In this mode
the AAC frames are allowed to be interleaved and hence receivers
MUST support de-interleaving. The maximum size of an AAC frame in
this mode is 63 octets. AAC frames MUST NOT be fragmented when
using this mode. Hence, when using this mode, encoders MUST ensure
that the size of each AAC frame is at most 63 octets.
The payload configuration in this mode is the same as in the
variable bit-rate CELP mode as defined in 3.3.4. The RTP payload
consists of the AU Header Section, followed by concatenated AAC
frames. The Auxiliary Section MUST be empty. For each AAC frame
contained in the payload the one octet AU-header MUST provide:
(a) the size of each AAC frame in the payload and
(b) index information for computing the sequence (and hence timing)
of each AAC frame.
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In the AU-header Section, the concatenated AU-headers MUST be
preceded by the 16-bit AU-headers-length field, as specified in
section 3.2.1.
In addition to the required MIME format parameters, the following
parameters MUST be present: sizeLength, indexLength, and
indexDeltaLength. AAC frames have fixed time duration per Access
Unit; when interleaving in this mode, the applicable duration MUST
be signaled by the MIME format parameter constantDuration. In
addition, the parameter maxDisplacement MUST be present when
interleaving.
For example:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/22050/1
a=fmtp:96 streamtype=5; profile-level-id=14; mode=AAC-lbr; config=
1388; sizeLength=6; indexLength=2; indexDeltaLength=2;
constantDuration=1024; maxDisplacement=5
Note: The a=fmtp line has been wrapped to fit the page, it comprises
a single line in the SDP file.
The hexadecimal value of the "config" parameter is the
AudioSpecificConfig() as defined in ISO/IEC 14496-3.
AudioSpecificConfig() specifies a mono AAC stream with a sampling
rate of 22.05 kHz. For the description of MIME parameters see
section 4.1.
3.3.6 High bit-rate AAC
This mode is signaled by mode=AAC-hbr. This mode supports transport
of variable size AAC frames. In one RTP packet either one or more
complete AAC frames are carried, or a single fragment of an AAC
frame. In this mode the AAC frames are allowed to be interleaved
and hence receivers MUST support de-interleaving. The maximum size
of an AAC frame in this mode is 8191 octets.
In this mode the RTP payload consists of the AU Header Section,
followed by either one AAC frame, several concatenated AAC frames
or one fragmented AAC frame. The Auxiliary Section MUST be empty.
For each AAC frame contained in the payload there MUST be an
AU-header in the AU Header Section to provide:
(a) the size of each AAC frame in the payload and
(b) index information for computing the sequence (and hence timing)
of each AAC frame.
To code the maximum size of an AAC frame requires 13 bits. Therefore
in this configuration 13 bits are allocated to the AU-size, and
3 bits to the AU-Index(-delta) field. Thus each AU-header has a size
of 2 octets. Each AU-Index field MUST be coded with the value 0. In
the AU Header Section, the concatenated AU-headers MUST be preceded
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by the 16-bit AU-headers-length field, as specified in
section 3.2.1.
In addition to the required MIME format parameters, the following
parameters MUST be present: sizeLength, indexLength, and
indexDeltaLength. AAC frames have fixed time duration per Access
Unit; when interleaving in this mode, the applicable duration MUST
be signaled by the MIME format parameter constantDuration. In
addition, the parameter maxDisplacement MUST be present when
interleaving.
For example:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/48000/6
a=fmtp:96 streamtype=5; profile-level-id=16; mode=AAC-hbr;
config=11B0; sizeLength=13; indexLength=3;
indexDeltaLength=3; constantDuration=1024
Note: The a=fmtp line has been wrapped to fit the page, it comprises
a single line in the SDP file.
The hexadecimal value of the "config" parameter is the
AudioSpecificConfig() as defined in ISO/IEC 14496-3.
AudioSpecificConfig() specifies a 5.1 channel AAC stream with a
sampling rate of 48 kHz. For the description of MIME parameters see
section 4.1.
3.3.7 Additional modes
This specification only defines the modes specified in sections
3.3.2 up to 3.3.6. Additional modes are expected to be defined in
future RFCs. Each additional mode MUST be in full compliance with
this specification.
Any new mode MUST be defined such that an implementation including
all the features 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. In particular, MIME
parameters that configure the RTP payload MUST be present (unless
they have the default value), even if its presence is redundant in
case the mode assigns a fixed value to a parameter. 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.
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4. IANA considerations
This section describes the MIME types and names associated with
this payload format. Section 4.1 registers the MIME types, as per
RFC 2048 [3].
This format may require additional information about the mapping to
be made available to the receiver. This is done using parameters
also described in the next section.
4.1 MIME type registration
MIME media type name: "video" or "audio" or "application"
"video" MUST be used for MPEG-4 Visual streams (ISO/IEC 14496-2)
or MPEG-4 Systems streams (ISO/IEC 14496-1) that convey information
needed for an audio/visual presentation.
"audio" MUST 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" MUST be used for MPEG-4 Systems streams (ISO/IEC
14496-1) that serve purposes other than audio/visual presentation,
e.g. in some cases when MPEG-J (Java) streams are transmitted.
Depending on the required payload configuration, MIME format
parameters need to be available to the receiver. This is done using
the parameters described in the next section. There are required
and optional parameters.
Optional parameters are of two types: general parameters and
configuration parameters. The configuration parameters are used to
configure the fields in the AU Header section and in the auxiliary
section. The absence of any configuration parameter is equivalent to
the associated field set to its default value, which is always zero.
The absence of all configuration parameters resolves into a default
"basic" configuration with an empty AU-header section and an empty
auxiliary section in each RTP packet.
MIME subtype name: mpeg4-generic
Required parameters:
MIME format parameters are not case dependent; however for clarity
both upper and lower case are used in the names of the parameters
described in this specification.
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 in Table 9 (streamType Values) in ISO/IEC
14496-1.
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profile-level-id:
A decimal representation of the MPEG-4 Profile Level indication.
This parameter MUST be used in the capability exchange or
session set-up procedure to indicate the MPEG-4 Profile and Level
combination of which the relevant MPEG-4 media codec is capable
of.
For MPEG-4 Audio streams, this parameter is the decimal value
from Table 5 (audioProfileLevelIndication Values) in ISO/IEC
14496-1, indicating which MPEG-4 Audio tool subsets are
required to decode the audio stream.
For MPEG-4 Visual streams, this parameter is the decimal value
from Table G-1 (FLC table for profile and level indication) of
ISO/IEC 14496-2, indicating which MPEG-4 Visual tool subsets
are required to decode the visual stream.
For BIFS streams, this parameter is the decimal value that is
obtained from (SPLI + 256*GPLI), where:
SPLI is the decimal value from Table 4 in ISO/IEC 14496-1 with
the applied sceneProfileLevelIndication;
GPLI is the decimal value from Table 7 in ISO/IEC 14496-1 with
the applied graphicsProfileLevelIndication.
For MPEG-J streams, this parameter is the decimal value from
table 13 (MPEGJProfileLevelIndication) in ISO/IEC 14496-1,
indicating the profile and level of the MPEG-J stream.
For OD streams, this parameter is the decimal value from table 3
(ODProfileLevelIndication) in ISO/IEC 14496-1, indicating the
profile and level of the OD stream.
For IPMP streams, this parameter has either the decimal value 0,
indicating an unspecified profile and level, or a value larger
than zero, indicating an MPEG-4 IPMP profile and level as
defined in a future MPEG-4 specification.
For Clock Reference streams and Object Content Info streams, this
parameter has the decimal value zero, indicating that profile
and level information is conveyed through the OD framework.
config:
A hexadecimal representation of an octet string that expresses
the media payload configuration. Configuration data is mapped
onto the hexadecimal 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, if necessary to achieve
octet-alignment, up to 7 zero-valued padding bits shall follow
the configuration data.
For MPEG-4 Audio streams, config is the audio object type
specific decoder configuration data AudioSpecificConfig() as
defined in ISO/IEC 14496-3. For Structured Audio, the
AudioSpecificConfig() may be conveyed by other means, not
defined by this specification. If the AudioSpecificConfig()
is conveyed by other means for Structured Audio, then the
config MUST be a quoted empty hexadecimal octet string, as
follows: config="".
Note that a future mode of using this RTP payload format for
Structured Audio may define such other means.
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For MPEG-4 Visual streams, config is the MPEG-4 Visual
configuration information as defined in subclause 6.2.1 Start
codes of ISO/IEC 14496-2. 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/IEC 14496-2).
For BIFS streams, this is the BIFSConfig() information as defined
in ISO/IEC 14496-1. For version 1, BIFSConfig is defined in
section 9.3.5.2, and for version 2 in section 9.3.5.3. The
MIME format parameter objectType signals the version of
BIFSConfig.
For IPMP streams, this is either a quoted empty hexadecimal octet
string, indicating the absence of any decoder configuration
information (config=""), or the IPMPConfiguration() as
defined in a future MPEG-4 IPMP specification.
For Object Content Info (OCI) streams, this is the
OCIDecoderConfiguration() information of the OCI stream, as
defined in section 8.4.2.4 in ISO/IEC 14496-1.
For OD streams, Clock Reference streams and MPEG-J streams, this
is a quoted empty hexadecimal octet string (config=""), as
no information on the decoder configuration is required.
mode:
The mode in which this specification is used. The following modes
can be signaled:
mode=generic,
mode=CELP-cbr,
mode=CELP-vbr,
mode=AAC-lbr and
mode=AAC-hbr.
Other modes are expected to be defined in future RFCs. See also
section 3.3.7 and 4.2 of RFC xxxx.
Optional general parameters:
objectType:
The decimal value from Table 8 in ISO/IEC 14496-1, indicating
the value of the objectTypeIndication of the transported stream.
For BIFS streams this parameter MUST be present to signal the
version of BIFSConfiguration(). Note that objectTypeIndication
may signal a non-MPEG-4 stream and that the RTP payload format
defined in this document may not be suitable to carry a stream
that is not defined by MPEG-4. The objectType parameter SHOULD
NOT be set to a value that signals a stream that cannot be
carried by this payload format.
constantSize:
The constant size in octets of each Access Unit for this stream.
The constantSize and the sizeLength parameters MUST NOT be
simultaneously present.
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constantDuration:
The constant duration of each Access Unit for this stream,
measured with the same units as the RTP time stamp.
maxDisplacement:
The decimal representation of the maximum displacement in time
of an interleaved AU, as defined in section 3.2.3.3, expressed
in units of the RTP time stamp clock.
This parameter MUST be present when interleaving is applied.
de-interleaveBufferSize:
The decimal representation in number of octets of the size of
the de-interleave buffer, described in section 3.2.3.3.
When interleaving, this parameter MUST be present if the
calculation of the de-interleave buffer size given in 3.2.3.3
and based on maxDisplacement and rate(max) under-estimates the
size of the de-interleave buffer. If this calculation does not
under-estimate the size of the de-interleave buffer, then the
de-interleaveBufferSize parameter SHOULD NOT be present.
Optional configuration parameters:
sizeLength:
The number of bits on which the AU-size field is encoded in the
AU-header. The sizeLength and the constantSize parameters MUST
NOT be simultaneously present.
indexLength:
The number of bits on which the AU-Index is encoded in the first
AU-header. The default value of zero indicates the absence of
the AU-Index field in each first AU-header.
indexDeltaLength:
The number of bits on which the AU-Index-delta field is encoded
in any non-first AU-header. The default value of zero indicates
the absence of the AU-Index-delta field in each non-first
AU-header.
CTSDeltaLength:
The number of bits on which the CTS-delta field is encoded in
the AU-header.
DTSDeltaLength:
The number of bits on which the DTS-delta field is encoded in
the AU-header.
randomAccessIndication:
A decimal value of zero or one, indicating whether the RAP-flag
is present in the AU-header. The decimal value of one indicates
presence of the RAP-flag, the default value zero its absence.
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streamStateIndication:
The number of bits on which the Stream-state field is encoded in
the AU-header. This parameter MAY be present when transporting
MPEG-4 system streams, and SHALL NOT be present for MPEG-4 audio
and MPEG-4 video streams.
auxiliaryDataSizeLength:
The number of bits that is used to encode the auxiliary-data-size
field.
Applications MAY use more parameters, in addition to those defined
above. Each additional parameter MUST be registered with IANA, to
ensure that there is no clash of names. Each additional parameter
MUST be accompanied by a specification in the form of an RFC, MPEG
standard, or other permanent and readily available reference (the
"Specification Required" policy defined in RFC 2434 [6]). Receivers
MUST tolerate the presence of such additional parameters, but these
parameters SHALL NOT impact the decoding of receivers that comply to
this specification.
Encoding considerations:
This MIME subtype is defined for RTP transport only. System
bitstreams MUST be generated according to MPEG-4 Systems
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 Audio
specifications (ISO/IEC 14496-3). The RTP packets MUST be packetized
according to the RTP payload format defined in RFC xxxx.
Security considerations:
As defined in section 5 of RFC xxxx.
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 interworking with other MPEG-4 devices
that implement the same combination, and
. checking whether MPEG-4 devices comply with the standard
('conformance testing').
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 is achieved by specifying the parameter
"profile-level-id" in MIME content. In the capability exchange /
announcement procedure this parameter may mutually be set to the
same value.
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Published specification:
The specifications for MPEG-4 streams are presented in ISO/IEC
14496-1, 14496-2, and 14496-3. The RTP payload format is described
in RFC xxxx.
Applications which use this media type:
Multimedia streaming and conferencing tools.
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
for which the sole purpose is RTP transport.
Macintosh File Type Code(s): none
Person & email address to contact for further information:
Authors of RFC xxxx, IETF Audio/Video Transport working group.
Intended usage: COMMON
Author/Change controller:
Authors of RFC xxxx, IETF Audio/Video Transport working group.
4.2 Registration of mode definitions with IANA
This specification can be used in a number of modes. The mode of
operation is signaled using the "mode" MIME parameter, with the
initial set of values specified in section 4.1. New modes may be
defined at any time, as described in section 3.3.7. These modes
MUST be registered with IANA, to ensure that there is no clash
of names.
A new mode registration MUST be accompanied by a specification in
the form of an RFC, MPEG standard, or other permanent and readily
available reference (the "Specification Required" policy defined
in RFC 2434 [6]).
4.3 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
(for parameter usage examples see sections 3.3.2 up to 3.3.6).
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4.4 Usage of SDP
4.4.1 The a=fmtp keyword
It is assumed that one typical way to transport the above-described
parameters associated with this payload format is via a SDP message
[5] for example transported to the client in reply to a RTSP
DESCRIBE [8] or via SAP [11]. In that case the (a=fmtp) keyword
MUST be used as described in RFC 2327 [5], section 6, the syntax
being then:
a=fmtp:<format> <parameter name>=<value>[; <parameter name>=<value>]
5. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [2]. 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 MPEG-4
scripts. It is possible to use one or more of the above in a
manner non-compliant to MPEG to crash the receiver or make it
temporarily unavailable. Senders that transport MPEG-4 content
SHOULD ensure that such content is MPEG compliant, as defined in the
compliance part of IEC/ISO 14496 [1]. Receivers that support MPEG-4
content should prevent malfunctioning of the receiver in case of
non MPEG compliant content.
Authentication mechanisms can be used to validate the sender and
the data to prevent security problems due to non-compliant malignant
MPEG-4 streams.
In ISO/IEC 14496-1 a security model is defined for MPEG-4 Systems
streams carrying MPEG-J access units which comprise 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
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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,
MPEG-4 scripts) commands in the streams. However, this can increase
the complexity significantly.
Implementors of MPEG-4 streaming over RTP who also implement MPEG-4
scripts (subset of ECMAScript) MUST ensure that the action of such
scripts is limited solely to the domain of the single presentation
in which they reside (thus disallowing session to session
communication, access to local resources and storage, etc). Though
loading static network-located resources (such as media) into the
presentation should be permitted, network access by scripts MUST be
restricted to such (media) download.
6. Acknowledgements
This document evolved through several revisions thanks to
contributions by people from the ISMA forum, from the IETF AVT
Working Group and from the 4-on-IP ad-hoc group within MPEG. The
authors wish to thank all involved people, and in particular Andrea
Basso, Stephen Casner, M. Reha Civanlar, Carsten Herpel, John
Lazaro, Zvi Lifshitz, Young-kwon Lim, Alex MacAulay, Bill May,
Colin Perkins, Dorairaj V and Stephan Wenger for their valuable
comments and support.
7. References
7.1 Normative references
[1] ISO/IEC International Standard 14496 (MPEG-4); "Information
technology - Coding of audio-visual objects", January 2000
[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson RTP, "A
Transport Protocol for Real Time Applications", RFC 1889, Internet
Engineering Task Force, January 1996.
[3] N. Freed, J. Klensin, J. Postel, " Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
Internet Engineering Task Force, November 1996.
[4] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[5] M. Handley, V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, Internet Engineering Task Force, April 1998.
[6] T. Narten, H. Alvestrand, " Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
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7.2 Informative references
[7] D. Hoffman, G. Fernando, V. Goyal, M. Civanlar, "RTP payload
format for MPEG1/MPEG2 Video", RFC 2250, January 1998.
[8] H. Schulzrinne, A. Rao, R. Lanphier, "RTSP: Real-Time Session
Protocol", RFC 2326, Internet Engineering Task Force, April 1998.
[9] C. Perkins, O. Hodson, "Options for Repair of Streaming Media"
RFC 2354, Internet Engineering Task Force, June 1998.
[10] H. Schulzrinne, J. Rosenberg, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, Internet Engineering
Task Force, December 1999.
[11] M. Handley, C. Perkins, E. Whelan, "SAP: Session Announcement
Protocol", RFC 2974, Internet Engineering Task Force, October 2000.
[12] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, H. Kimata, "RTP
payload format for MPEG-4 Audio/Visual streams", RFC 3016, Internet
Engineering Task Force, November 2000.
8. Author Addresses
Jan van der Meer
Philips Electronics, MP4Net
Prof Holstlaan 4
Building WDB-1
5600 JZ Eindhoven
Netherlands
Email : jan.vandermeer@philips.com
David Mackie
Apple Computer, Inc.
One Infinite Loop, MS:302-2LF
Cupertino CA 95014
Email: dmackie@apple.com
Viswanathan Swaminathan
Sun Microsystems Inc.
901 San Antonio Road, M/S UMPK15-214
Palo Alto, CA 94303
Email: viswanathan.swaminathan@sun.com
David Singer
Apple Computer, Inc.
One Infinite Loop, MS:302-3MT
Cupertino CA 95014
Email: singer@apple.com
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Philippe Gentric
Philips Electronics, MP4Net
51 rue Carnot
92156 Suresnes
France
e-mail: philippe.gentric@philips.com
Full Copyright Statement
Copyright (C) The Internet Society (August 2003). All Rights
Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied,
published and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the
purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards process
MUST be followed, or as required to translate it into languages
other than English.
The limited permissions granted above are perpetual and will
not be revoked by the Internet Society or its successors or
assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS 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.
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APPENDIX: Usage of this payload format
Appendix A. Interleave analysis
A.1 Introduction
In this appendix interleaving issues are discussed. Some general
notes are provided on de-interleaving and error concealment, while
a number of interleaving patterns are examined, in particular
for determining the maximum displacement in time and the size of
the de-interleave buffer. In these examples, the maximum
displacement is cited in terms of an access unit count, for ease of
reading. In actual streams, it is signaled in units of the RTP
time stamp clock.
A.2 De-interleaving and error concealment
This appendix does not describe any details on de-interleaving and
error concealment, as the control of the AU decoding and error
concealment process has little to do with interleaving. If the
next AU to be decoded is present and there is sufficient storage
available for the decoded AU, then decode it now. If not, wait.
When the decoding deadline is reached (i.e., the time when decoding
must begin in order to be completed by the time the AU is to be
presented), or if the decoder is some hardware that presents a
constant delay between initiation of decoding of an AU and
presentation of that AU, then decoding must begin at that deadline
time.
If the next AU to be decoded is not present when the decoding
deadline is reached, then that AU is lost so the receiver must take
whatever error concealment measures is deemed appropriate. The
play-out delay may need to be adjusted at that point (especially if
other AUs have also missed their deadline recently). Or, if it was
a momentary delay, and maintaining the latency is important, then
the receiver should minimize the glitch and continue processing
with the next AU.
A.3 Simple Group interleave
A.3.1 Introduction
An example of regular interleave is when packets are formed into
groups. If the 'stride' of the interleave (the distance between
interleaved AUs) is N, packet 0 could contain AU(0), AU(N), AU(2N),
and so on; packet 1 could contain AU(1), AU(1+N), AU(1+2N), and so
on. If there are M access units in a packet, then there are M*N
access units in the group.
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An example with N=M=3 follows; note that this is the same example
as given in section 2.5 and that a fixed time duration per Access
Unit is assumed:
Packet Time stamp Carried AUs AU-Index, AU-Index-delta
P(0) T[0] 0, 3, 6 0, 2, 2
P(1) T[1] 1, 4, 7 0, 2, 2
P(2) T[2] 2, 5, 8 0, 2, 2
P(3) T[9] 9,12,15 0, 2, 2
In this example the AU-Index is present in the first AU-header and
coded with the value 0, as required for fixed duration AUs. The
position of the first AU of each packet within the group is defined
by the RTP time stamp, while the AU-Index-delta field indicates the
position of subsequent AUs relative to the first AU in the packet.
All AU-Index-delta fields are coded with the value N-1, equal to 2
in this example. Hence the RTP time stamp and the AU-Index-delta are
used to reconstruct the original order. See also section 3.2.3.2.
A.3.2 Determining the de-interleave buffer size
For the regular pattern as in this example, figure 6 in section
3.2.3.3 shows that the de-interleave buffer stores at most 4 AUs. A
de-interleaveBufferSize value may be signaled that is at least
equal to the total number of octets of any 4 "early" AUs that are
stored at the same time.
A.3.3 Determining the maximum displacement
For the regular pattern as in this example, figure 7 in section 3.3
shows that the maximum displacement in time equals 5 AU periods.
Hence the minimum maxDisplacement value that must be signaled is 5
AU periods. In case each AU has the same size, this maxDisplacement
value over-estimates the de-interleave buffer size with one AU.
However, note that in case of variable AU sizes the total size of
any 4 "early" AUs that must be stored at the same time may exceed
maxDisplacement times the maximum bitrate, in which case the
de-interleaveBufferSize must be signaled.
A.4 More subtle group interleave
A.4.1 Introduction
Another example of forming packets with group interleave is given
below. In this example the packets are formed such that the loss of
two subsequent RTP packets does not cause the loss of two subsequent
AUs. Note that in this example the RTP time stamps of packet 3 and
packet 4 are earlier than the RTP time stamps of packets 1 and 2,
respectively; a fixed time duration per Access Unit is assumed.
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Packet Time stamp Carried AUs AU-Index, AU-Index-delta
0 T[0] 0, 5 0, 4
1 T[2] 2, 7 0, 4
2 T[4] 4, 9 0, 4
3 T[1] 1, 6 0, 4
4 T[3] 3, 8 0, 4
5 T[10] 10, 15 0, 4
and so on ..
In this example the AU-Index is present in the first AU-header and
coded with the value 0, as required for AUs with a fixed duration.
To reconstruct the original order, the RTP time stamp and the
AU-Index-delta (coded with the value 4) are used. See also
section 3.2.3.2.
A.4.2 Determining the de-interleave buffer size
From figure 8 it can be to determined that at most 5 "early" AUs
are to be stored. If the AUs are of constant size, then this value
equals 5 times the AU size. The minimum size of the de-interleave
buffer equals the maximum total number of octets of the "early" AUs
that are to be stored at the same time. This gives the minimum
value of the de-interleaveBufferSize that may be signaled.
+--+--+--+--+--+--+--+--+--+--+
Interleaved AUs | 0| 5| 2| 7| 4| 9| 1| 6| 3| 8|
+--+--+--+--+--+--+--+--+--+--+
- - 5 - 5 - 2 7 4 9
7 4 9 5
"Early" AUs 5 6
7 7
9 9
Figure 8: Storage of "early" AUs in the de-interleave buffer per
interleaved AU.
A.4.3 Determining the maximum displacement
From figure 9 it can be seen that the maximum displacement in time
equals 8 AU periods. Hence the minimum maxDisplacement value to be
signaled is 8 AU periods.
+--+--+--+--+--+--+--+--+--+--+
Interleaved AUs | 0| 5| 2| 7| 4| 9| 1| 6| 3| 8|
+--+--+--+--+--+--+--+--+--+--+
Earliest not yet present AU - 1 1 1 1 1 - 3 - -
Figure 9: The earliest not yet present AU for each AU in the
interleaving pattern.
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In case each AU has the same size, the found maxDisplacement value
over-estimates the de-interleave buffer size with three AUs.
However, in case of variable AU sizes the total size of any 5
"early" AUs stored at the same time may exceed maxDisplacement
times the maximum bitrate, in which case de-interleaveBufferSize
must be signaled.
A.5 Continuous interleave
A.5.1 Introduction
In continuous interleave, once the scheme is 'primed', the number
of AUs in a packet exceeds the 'stride' (the distance between
them). This shortens the buffering needed, smooths the data-flow,
and gives slightly larger packets -- and thus lower overhead -- for
the same interleave. For example, here is a continuous interleave
also over a stride of 3 AUs, but with 4 AUs per packet, for a run
of 20 AUs. This shows both how the scheme 'starts up' and how it
finishes. Once again, the example assumes fixed time duration per
Access Unit.
Packet Time-stamp Carried AUs AU-Index, AU-Index-delta
0 T[0] 0 0
1 T[1] 1 4 0 2
2 T[2] 2 5 8 0 2 2
3 T[3] 3 6 9 12 0 2 2 2
4 T[7] 7 10 13 16 0 2 2 2
5 T[11] 11 14 17 20 0 2 2 2
6 T[15] 15 18 0 2
7 T[19] 19 0
In this example the AU-Index is present in the first AU-header and
coded with the value 0, as required for AUs with a fixed duration.
To reconstruct the original order, the RTP time stamp and the
AU-Index-delta (coded with the value 2) are used. See also 3.2.3.2.
Note that this example has RTP time-stamps in increasing order.
A.5.2 Determining the de-interleave buffer size
For this example the de-interleave buffer size can be derived from
figure 10. The maximum number of "early" AUs is three. If the AUs
are of constant size, then this value equals 3 times the AU size.
Compared to the example in A.2, for constant size AUs the
de-interleave buffer size is reduced from 4 to 3 times the AU size,
while maintaining the same 'stride'.
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+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
Interleaved AUs | 0| 1| 4| 2| 5| 8| 3| 6| 9|12| 7|10|13|16|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
- - - 4 - - 4 8 - - 8 12 - -
5 9
"Early" AUs 8 12
Figure 10: Storage of "early" AUs in the de-interleave buffer per
interleaved AU.
A.5.3 Determining the maximum displacement
For this example the maximum displacement has a value of 5 AU
periods. See figure 11. Compared to the example in A.2, the maximum
displacement does not decrease, though in fact less de-interleave
buffering is required.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
Interleaved AUs | 0| 1| 4| 2| 5| 8| 3| 6| 9|12| 7|10|13|16|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
Earliest not yet
present AU - - 2 - 3 3 - - 7 7 - - 11 11
Figure 11: The earliest not yet present AU for each AU in the
interleaving pattern.
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