| Audio/Video Transport Payloads | M.S. Schmidt |
| Internet-Draft | Dolby Laboratories |
| Obsoletes: 3016 (if approved) | F.d.B. de Bont |
| Intended status: Standards Track | Philips Electronics |
| Expires: December 08, 2011 | S.D. Doehla |
| Fraunhofer IIS | |
| Jaehwan. Kim | |
| LG Electronics Inc. | |
| June 06, 2011 |
RTP Payload Format for MPEG-4 Audio/Visual Streams
draft-ietf-payload-rfc3016bis-01.txt
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 Media Type registration and the use of Session Description Protocol (SDP). The audio payload format described in this document has some limitations related to the signaling of audio codec parameters for the required multiplexing format. Therefore, for new system designs [RFC3640] is preferred, which does not have these restrictions.
This document obsoletes RFC 3016. A revision of RFC 3016 was required because of some misalignments between RFC 3016 and the 3GPP PSS specification regarding the RTP payload format for MPEG-4 Audio. Changes from RFC 3016 are summarized in Section 11. Issues on backward compatibility to RFC 3016 are discussed in Section 1.3.
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The RTP payload formats described in this document specify how MPEG-4 Audio [14496-3] and MPEG-4 Visual streams [14496-2] 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 [14496-1]. 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 the MPEG-4 Systems Sync Layer. Other examples are SIP and RTSP where Media Type and SDP are used. Media Type and SDP usages of the RTP payload formats described in this document are defined to directly specify the attribute of Audio/Visual streams (e.g., media type, packetization format and codec configuration) without using MPEG-4 Systems. The obvious benefit is that these MPEG-4 Audio/Visual RTP payload formats can be handled in an unified way together with those formats defined for non-MPEG-4 codecs. The disadvantage is that interoperability with environments using MPEG-4 Systems may be difficult, hence, 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 resiliency tools provided inside the MPEG-4 Video stream.
MPEG-4 Visual is a visual coding standard with many features: high coding efficiency; high error resiliency; multiple, arbitrary shape object-based coding; etc. [14496-2]. It covers a wide range of bitrates from scores of Kbps to several Mbps. It also covers a wide variety of networks, ranging from those guaranteed to be almost error-free to mobile networks with high error rates.
With respect to the fragmentation rules for an MPEG-4 Visual bitstream defined in this document, since MPEG-4 Visual is used for a wide variety of networks, it is desirable not to apply too much restriction on fragmentation, and a fragmentation rule such as "a single video packet shall always be mapped on a single RTP packet" may be inappropriate. On the other hand, careless, media unaware fragmentation may cause degradation in error resiliency and bandwidth efficiency. The fragmentation rules described in this document are flexible but manage to define the minimum rules for preventing meaningless fragmentation while utilizing the error resiliency functionalities of MPEG-4 Visual.
The fragmentation rule "Different VOPs SHOULD be fragmented into different RTP packets" is made 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 5.2 and marker bit and timestamp in Section 5.1.)
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 resiliency 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.
MPEG-4 Audio is an 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 RTP packets without using MPEG-4 Systems.
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 [RFC5109] and Redundant Audio Data [RFC2198], MAY be applied to improve error resiliency.
Strictly speaking systems that support MPEG-4 Audio as specified in [RFC3016] are incompatible with systems supporting this document. However, all known existing systems do already comply with the specification in 3GPP PSS service [3GPP] and therefore there is no need to consider backward compatibility.
This document makes use of terms, specified in [14496-2], [14496-3], and [23003-1]. In addition, the following terms are used in this document and have specific meaning within the context of this document.
Abbreviations:
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 [RFC2119].
For MPEG-4 Audio [14496-3] streams the decoder output configuration can differ from the core codec configurating depending of use of the SBR and PS tools.
The core codec sampling rate is the default audio codec sampling rate. When SBR is used, typically the double value of the core codec sampling rate will be regarded as the definitive sampling rate (i.e., the decoder's output sampling rate)
Note: The exception is downsampled SBR mode in which case the SBR sampling rate and core codec sampling rate are identical.
The core codec channel configuration is the default audio codec channel configuration. When PS is used, the core codec channel configuration indicates one channel (i.e., mono) whereas the definitive channel configuration is two channels (i.e. stereo). When MPEG Surround is used, the definitive channel configuration depends on the output of the MPEG Surround decoder.
While LATM has several multiplexing features as follows;
in RTP transmission there is no need for the last two features. Therefore, these two features MUST NOT be used in applications based on RTP packetization specified by this document. Since LATM has been developed for only natural audio coding tools, i.e., not for synthesis tools, it seems difficult to transmit Structured Audio (SA) data and Text to Speech Interface (TTSI) data by LATM. Therefore, SA data and TTSI data MUST NOT be transported by the RTP packetization in this document.
For transmission of scalable streams, audio data of each layer SHOULD be packetized onto different RTP streams allowing for the different layers to be treated differently at the IP level, for example via some means of differentiated service. On the other hand, all configuration data of the scalable streams are contained in one LATM configuration data "StreamMuxConfig" and every scalable layer shares the StreamMuxConfig. The mapping between each layer and its configuration data is achieved by LATM header information attached to the audio data. In order to indicate the dependency information of the scalable streams, the signaling mechanism as specified in [RFC5583] SHOULD be used (see Section 6.2).
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 [14496-2]) The configuration information MAY additionally be specified by some out-of-band means. If needed by systems using Media Type parameters and SDP parameters, "e.g., SIP and RTSP", the optional parameter "config" MUST be used to specify the configuration information (see Section 7.1 and Section 7.2).
When the short video header mode is used, the RTP payload format for H.263 SHOULD be used (the format defined in [RFC4629] is RECOMMENDED, but the [RFC4628] format MAY be used for compatibility with older implementations).
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
Payload Type (PT): The assignment of an RTP payload type for this 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.
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 Media Type parameter as defined in Section 7).
Other header fields are used as described in [RFC3550].
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:
(1) Configuration information and Group_of_VideoObjectPlane() fields SHALL be placed at the beginning of the RTP payload (just after the RTP header) or just after the header of the syntactically upper layer function.
(2) If one or more headers exist in the RTP payload, the RTP payload SHALL begin with the header of the syntactically highest function. Note: The visual_object_sequence_end_code is regarded as the lowest function.
(3) A header SHALL NOT be split into a plurality of RTP packets.
(4) Different VOPs SHOULD be fragmented into different RTP packets so that one RTP packet consists of the data bytes associated with a unique 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 5.1.)
(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. If 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, 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().
Figure 2 shows examples of RTP packets generated based on the criteria described in Section 5.2
(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 Section 5.2 guarantees that the configuration information, starting with visual_object_sequence_start_code, is always placed at the beginning of the RTP payload, RTP receivers can detect the random access point by checking if the first 32-bit field of the RTP payload is visual_object_sequence_start_code.
(b) is another example of the RTP packet containing the configuration information. It differs from example (a) in that the RTP packet also contains a VOP header and 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 VOP can be packetized into a single RTP packet.
(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.
Figure 3 shows examples of RTP packets prohibited by the criteria of Section 5.2.
Fragmentation of a header into multiple RTP packets, as in (a), will not only increase the overhead of RTP/IP headers but also decrease the error resiliency. Therefore, it is prohibited by the criterion (3).
When concatenating more than one video packets into an RTP packet, VOP header or video_packet_header() are not allowed to 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
+------+-------------+ +------+------------+------------+
(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
This section specifies RTP packetization rules for MPEG-4 Audio bitstreams. MPEG-4 Audio streams MUST be formatted LATM (Low-overhead MPEG-4 Audio Transport Multiplex) [14496-3] streams, and the LATM-based streams are then mapped onto RTP packets as described in the sections below.
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.
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 out-of-band means. When SDP is utilized for this indication, the Media Type parameter "cpresent" corresponds to the muxConfigPresent information (see Section 7.3). The following restrictions apply:
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, Media Type parameter "config" is utilized (see Section 7.3).
Payload Type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document, and will only be restricted 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, the server SHALL assign a different value to each layer. The dependency relationships between the enhanced layer and the base layer MUST be signaled as specified in [RFC5583]. An example of the use of such signaling for scalable audio streams can be found in [RFC5691].
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 [RFC3550].
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 SHALL be fragmented and spread across multiple packets.
The following sections describe the Media Type registrations for MPEG-4 Audio/Visual streams, which are registered in accordance with [RFC4855] and uses the template of [RFC4288]. Media Type registration and SDP usage for the MPEG-4 Visual stream are described in Section 7.1 and Section 7.2, respectively, while Media Type registration and SDP usage for MPEG-4 Audio stream are described in Section 7.3 and Section 7.4, respectively.
The receiver MUST ignore any unspecified parameter, to ensure that additional parameters can be added in any future revision of this specification.
Type name: video
Subtype name: MP4V-ES
Required parameters: none
Optional parameters:
Published specification:
Encoding considerations:
Security considerations:
Interoperability considerations:
Applications which use this Media Type:
Additional information: none
Person and email address to contact for further information:
Intended usage: COMMON
Author:
Change controller:
Example usages for the profile-level-id parameter are:
1 : MPEG-4 Visual Simple Profile/Level 1
34 : MPEG-4 Visual Core Profile/Level 2
145: MPEG-4 Visual Advanced Real Time Simple Profile/Level 1
The Media Type video/MP4V-ES string is mapped to fields in the Session Description Protocol (SDP) [RFC4566], as follows:
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
The receiver MUST ignore any unspecified parameter, to ensure that additional parameters can be added in any future revision of this specification.
Type name: audio
Subtype name: MP4A-LATM
Required parameters:
Optional parameters:
Published specification:
Encoding considerations:
Security considerations:
Interoperability considerations:
Applications which use this media type:
Additional information: none
Personal and email address to contact for further information:
Intended usage: COMMON
Author:
Change controller:
Followings are some examples of the profile-level-id value:
1 : Main Audio Profile Level 1
9 : Speech Audio Profile Level 1
15: High Quality Audio Profile Level 2
30: Natural Audio Profile Level 1
44: High Efficiency AAC Profile Level 2
48: High Efficiency AAC v2 Profile Level 2
55: Baseline MPEG Surround Profile (see ISO/IEC 23003-1) Level 3
The Media Type audio/MP4A-LATM string is mapped to fields in the Session Description Protocol (SDP) [RFC4566], as follows:
The following sections contain some examples of the media representation in SDP.
Note that the a=fmtp line in some of the examples has been wrapped to fit the page; they would comprise a single line in the SDP file.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/90000
a=fmtp:96 object=2; cpresent=1
In this example the audio configuration data appears in the RTP payload exclusively (i.e., the MPEG-4 audio configuration is known when a StreamMuxConfig element appears within the RTP payload).
The "clock rate" is set to 90kHz. This is the default value and the real audio sampling rate is known when the audio configuration data is received.
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
6 kb/s CELP bitstreams (with an audio sampling rate of 8 kHz)
In this example 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.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000/2
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
object=2; config=400026203fc0
64 kb/s AAC LC stereo bitstream (with an audio sampling rate of 24 kHz)
In this example 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.
In this example, the presence of SBR can not be determined by the SDP parameter set. The clock rate represents the core codec sampling rate. An SBR enabled decoder can use the SBR tool to upsample the audio data if complexity and resulting output sampling rate permits.
These two examples are identical to the example above with the exception of the SBR-enabled parameter. The presence of SBR is not signaled by the SDP parameters object, profile-level-id and config, but instead the SBR-enabled parameter is present. The rate parameter and the StreamMuxConfig contain the core codec sampling rate.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000/2
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
SBR-enabled=0; config=400026203fc0
Example with "SBR-enabled=0", definitive and core codec sampling rate 24kHz:
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/24000/2
a=fmtp:96 profile-level-id=1; bitrate=64000; cpresent=0;
SBR-enabled=1; config=400026203fc0
Example with "SBR-enabled=1", core codec sampling rate 24kHz, definitive and SBR sampling rate 48kHz:
In this example, the clock rate is still 24000 and this information is used for RTP timestamp calculation. The value of 24000 is used to support old AAC decoders. This makes the decoder supporting only AAC understand the HE AAC coded data, although only plain AAC is supported. A HE AAC decoder is able to generate output data with the SBR sampling rate.
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/48000/2
a=fmtp:96 profile-level-id=44; bitrate=64000; cpresent=0;
config=40005623101fe0; SBR-enabled=1
When the presence of SBR is explicitly signaled 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.
This config string uses the explicit signaling mode 2.A (hierarchical signaling; See [14496-3]. This means that the AOT(Audio Object Type) is SBR(5) and SFI(Sampling Frequency Index) is 6(24000 Hz) which refers to the underlying core codec sampling frequency. CC(Channel Configuration) is stereo(2), and the ESFI(Extension Sampling Frequency Index)=3 (48000) is referring to the sampling frequency of the extension tool(SBR).
HE AAC v2 decoders are required to always produce a stereo signal from a mono signal. Hence, there is no parameter necessary to signal the presence of PS.
m=audio 49230 RTP/AVP 110
a=rtpmap:110 MP4A-LATM/24000/1
a=fmtp:110 profile-level-id=15; object=2; cpresent=0;
config=400026103fc0; SBR-enabled=1
Example with "SBR-enabled=1" and 1 channel signaled in the a=rtpmap line and within the config parameter. Core codec sampling rate is 24kHz, definitive and SBR sampling rate is 48kHz. Core codec channel configuration is mono, PS channel configuration is stereo.
m=audio 49230 RTP/AVP 110
a=rtpmap:110 MP4A-LATM/48000/2
a=fmtp:110 profile-level-id=48; cpresent=0; config=4001d613101fe0
Example: 48khz stereo audio input:
The config parameter indicates explicit hierarchical signaling of PS and SBR. This configuration method is not supported by legacy AAC an HE AAC decoders and these are therefore unable to decode the the coded data.
The following examples show how MPEG Surround configuration data can be signaled 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 (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]).
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=8FF8004192B11880FF0DDE3699F2408C00536C02313CF3CE0FF0
In this example the signaling is carried by using two different LATM layers. The MPEG surround payload is carried together with the AAC payload 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=44; bitrate=64000; cpresent=0;
config=40005623101fe0; MPS-profile-level-id=55;
MPS-asc=F1B4CF920442029B501185B6DA00;
The 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])].
The following example shows how MPEG Surround configuration data can be signaled using the SDP config parameter. The configuration is carried within the config string using a single layer. The general parameters in this example are: AudioMuxVersion=1; allStreamsSameTimeFraming=1; numSubFrames=0; numProgram=0; numLayer=0. The single layer describes the combination of HE AAC and MPEG Surround payload and signals the following parameters: ascLen=101; audioObjectType=2 (AAC LC); extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=7 (22.05kHz); extensionSamplingFrequencyIndex=7 (44.1kHz); channelConfiguration=2 (2.0 channels). A backward compatible extension according to [14496-3/Amd.1] signals the presence of MPEG surround payload data and specifies the following parameters: SpatialSpecificConfig=(44.1 kHz; 32 slots; 525 tree; ResCoding=0).
m=audio 49230 RTP/AVP 96
a=rtpmap:96 MP4A-LATM/44100
a=fmtp:96 profile-level-id=44; bitrate=64000; cpresent=0;
SBR-enabled=1; config=8FF8000652B920876A83A1F440884053620FF0;
MPS-profile-level-id=55
In this example the signaling is carried by using a single LATM layer. The MPEG surround payload is carried together with the HE AAC payload in a single layer.
This document updates the media subtypes "MP4A-LATM" and "MP4V-ES" from RFC 3016. The new registrations are in Section 7.1 and Section 7.3 of this document.
The authors would like to thank Yoshihiro Kikuchi, Yoshinori Matsui, Toshiyuki Nomura, Shigeru Fukunaga and Hideaki Kimata for their work on RFC 3016, and Ali Begen, Keith Drage, Roni Even and Qin Wu for their valuable input and comments on this document.
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550], and in any applicable RTP profile. The main security considerations for the RTP packet carrying the RTP payload format defined within this document are confidentiality, integrity, and source authenticity. Confidentiality is achieved by encryption of the RTP payload, and integrity of the RTP packets through a suitable cryptographic integrity protection mechanism. A cryptographic system may also allow the authentication of the source of the payload. A suitable security mechanism for this RTP payload format should provide confidentiality, integrity protection, and at least source authentication capable of determining whether or not an RTP packet is from a member of the RTP session.
Note that most MPEG-4 codecs define an extension mechanism to transmit extra data within a stream that is gracefully skipped by decoders that do not support this extra data. This covert channel may be used to transmit unwanted data in an otherwise valid stream.
The appropriate mechanism to provide security to RTP and payloads following this may vary. It is dependent on the application, the transport, and the signaling protocol employed. Therefore, a single mechanism is not sufficient, although if suitable, the usage of the Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. Other mechanisms that may be used are IPsec [RFC4301] and Transport Layer Security (TLS) [RFC5246] (e.g., for RTP over TCP), but other alternatives may also exist.
This RTP payload format and its media decoder do not exhibit any significant non-uniformity in the receiver-side computational complexity for packet processing, and thus are unlikely to pose a denial-of-service threat due to the receipt of pathological data. 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.
The RTP payload format for MPEG-4 Audio as specified in RFC 3016 is used by the 3GPP PSS service [3GPP]. However, there are some misalignments between RFC 3016 and the 3GPP PSS specification that are addressed by this update: [23003-1] was added.
Furthermore some comments have been addressed and signaling support for MPEG surround
Below a summary of the changes in requirements by this update: