Internet Engineering Task Force J. van der Meer Internet Draft Philips Electronics D. Mackie Cisco Systems Inc. V. Swaminathan Sun Microsystems Inc. D. Singer Apple Computer P. Gentric Philips Electronics
JuneJuly 2002 Expires December 2002January 2003 Document: draft-ietf-avt-mpeg4-simple-03.txtdraft-ietf-avt-mpeg4-simple-04.txt 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 RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- 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 email@example.com 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Carriage of MPEG-4 elementary streams over RTP . . . . . . . 4 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. MPEG Access Units . . . . . . . . . . . . . . . . . . . . 4 2.3. Concatenation of Access Units . . . . . . . . . . . . . . 4 2.4. Fragmentation of Access Units . . . . . . . . . . . . . . 5 2.5. Interleaving . . . . . . . . . . . . . . . . . . . . . . . 5 2.6. Time stamp information . . . . . . . . . . . . . . . . . . 6 2.7. Random Access IndicationState indication of MPEG-4 system streams . . . . . . . . 6 2.8. Random Access Indication . . . . . . . . . 6 2.8. State indication of MPEG-4 system streams. . . . . . . . 6 2.9. Carriage of auxiliary information . . . . . . . . . . . . 7 2.10. MIME format parameters and configuring conditional field . 7 2.11. Global structure of payload format . . . . . . . . . . . . 7 2.12. Modes to transport MPEG-4 streams . . . . . . . . . . . . 8 2.13. Alignment with RFC 3016 . . . . . . . . . . . . . . . . . 8 3. Payload format . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Usage of RTP header fields and RTCP . . . . . . . . . . . 9 3.2. RTP payload structure . . . . . . . . . . . . . . . . . . 10 3.2.1. The AU Header Section . . . . . . . . . . . . . . . . . 10 220.127.116.11. The AU-header . . . . . . . . . . . . . . . . . . . . 10 3.2.2. The Auxiliary Section . . . . . . . . . . . . . . . . . 1312 3.2.3. The Access Unit Data Section . . . . . . . . . . . . . . 13 18.104.22.168. Fragmentation . . . . . . . . . . . . . . . . . . . . 14 22.214.171.124. Interleaving . . . . . . . . . . . . . . . . . . . . . 14 126.96.36.199. Constraints for interleaving . . . . . . . . . . . . . 15 188.8.131.52. Crucial and non-crucial AUs with MPEG-4 System data . 16 3.3. Usage of this specification . . . . . . . . . . . . . . . 1617 3.3.1. General . . . . . . . . . . . . . . . . . . . . . . . . 1617 3.3.2. The generic mode . . . . . . . . . . . . . . . . . . . . 1617 3.3.3. Constant bit rate CELP . . . . . . . . . . . . . . . . . 1718 3.3.4. Variable bit rate CELP . . . . . . . . . . . . . . . . . 18 3.3.5. Low bit rate AAC . . . . . . . . . . . . . . . . . . . . 19 3.3.6. High bit rate AAC . . . . . . . . . . . . . . . . . . . 1920 3.3.7. Additional modes . . . . . . . . . . . . . . . . . . . . 2021 4. IANA considerations . . . . . . . . . . . . . . . . . . . . 2122 4.1. MIME type registration . . . . . . . . . . . . . . . . . . 2122 4.2. Registration of mode definitions with IANA . . . . . . . . 27 4.3. Concatenation of parameters . . . . . . . . . . . . . . . 26 4.3.27 4.4. Usage of SDP . . . . . . . . . . . . . . . . . . . . . . . 26 184.108.40.206 4.4.1. The a=fmtp keyword . . . . . . . . . . . . . . . . . . . 2628 5. Security considerations . . . . . . . . . . . . . . . . . . 2728 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 2829 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 2829 8. Author addresses . . . . . . . . . . . . . . . . . . . . . . 2930 APPENDIX: Usage of this payload format . . . . . . . . . . . 3031 A. Examples of delay analysis with interleave . . . . . . . 3031 A.1 Group interleave . . . . . . . . . . . . . . . . . . . . 3031 A.2 Continuous interleave . . . . . . . . . . . . . . . . . 3132 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 . 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 audiovisual 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. Though the RTP payload format defined in this document is capable to transportof transporting any MPEG-4 stream, other, more dedicatedspecific, formats may exist, such as RFC 3016 for transport of MPEG-4 video (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 SL. 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 . 2. Carriage of MPEG-4 elementary streams over RTP 2.1 Introduction 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  message or by other means.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 in receivers to parse all MIME format parameters. The applied mode MUST be signalled.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 byteoctet aligned. If for example an audio frame is not byteoctet aligned, up to 7 zero-padding bits MUST be inserted at the end of the frame to achieve a byte-alignedthe octet-aligned Access Unit.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 AAC 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. 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. 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 so as to avoid IP layer fragmentation. 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 Access Unit.Unit (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. The RTP sender is free to choose the interleaving pattern without propagating this information 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 rendering queue. An example of interleaving is given below. Assume that an RTP packet contains 3 AUs, and that the AUs are numbered 1, 2, 3, 4, etc. If an interleaving group length of 9 is chosen, then RTP packet(i) contains the following AU(n): RTP packet(1): AU(1), AU(4), AU(7) RTP packet(2): AU(2), AU(5), AU(8) RTP packet(3): AU(3), AU(6), AU(9) RTP packet(4): AU(10), AU(13), AU(16) RTP packet(5): AU(11), AU(14), AU(17) Etc. 2.6 Time stamp information The RTP time stamp MUST carry the sampling instance 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 and 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 latterfirst AU in the RTP packet is carried by the RTP time stamp. MPEG-4 defines two type of time stamps, the composition time stamp (CTS) and the decoding time stamp (DTS). The CTS represents the sampling instance of an AU, and hence the CTS is equivalent to the RTP time stamp. The DTS may be used only 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. Since RTP time stamps may be re-stamped by RTP devices, each time stamp contained in the RTP payload is coded differentially, the CTS from the RTP time stamp, and the DTS from the CTS, so as to avoid extensive parsing by re-stamping devices. 2.7 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. 2.8State 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. The indication of streamStream states is particularlyare 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. 2.9 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 as2.8 Random access indication Random access to facilitate skipping ofthe data without parsing it. The codingcontent of the auxiliary data isMPEG-4 elementary streams may be possible at some but not definedall 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. 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, but is left to the discretion of applications. 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  messages or through other means. 2.11 Global structure of payload format The RTP payload following the RTP header, contains three byteoctet 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 one Access Unit or one or more complete Access Units. The Access Unit Data Section is neverMUST 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 signalled. Signallingsignaled. 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 future, new RFCs are expected to specify additional modes of using this specification. New modes can be defined as deemed appropriate, typically by specifications that are hierarchically higher than this payload format. However, each mode MUST be in full compliance with this specification. 2.13 Alignment with RFC 3016 This payload can be configured to be nearly identical to the payload format defined in RFC 3016  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 recommendedRECOMMENDED to make the video decoder configuration available as a MIME parameter. 3. Payload Format 3.1 Usage of RTP Header Fields and RTCP Payload Type (PT): The assignment of an RTP payload type for this RTP packet format is outside the scope of this document, and will not be specified here. It is expected that the RTP profile for a particular class of applications will assign a payload type for this encoding, or if that is not done, then a payload type in the dynamic range shall be chosen. Marker (M) bit: The M bit is set to 1 to indicate that the RTP packet payload includes the end of each Access Unit of which data is contained in this RTP packet. As the payload either carries one or more complete Access Units or a single fragment of an Access Unit, the M bit is alwaysusually set to 1, except when the packet carries a single fragment of an Access Unit that is not the last one. 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 instance of the first AU contained in the RTP payload. This sampling instance 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  (section 5.1), RTP time stamps are recommendedRECOMMENDED to start at a random value for security reasons. This is not an issue for synchronization of multiple RTP streams. However, in applications whereWhen, however, streams from multiple sources are to be synchronized (for example one stream from local storage, another from aan RTP streaming server), synchronization may become impossible. To also enable synchronizationimpossible if the receiver only knows the original time stamp relationships. Synchronization in such cases, itmay be necessaryrequire to provide the requiredcorrect 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 RFC1889 . CC and CSRC fields are used as described in RFC 1889 . RTCP SHOULD be used as defined in RFC 1889 . 3.2 RTP Payload Structure 3.2.1 The AU Header Section When present, the AU Header Section consists of the AU-header-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 notNOT 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 byte-alignmentoctet-alignment of the AU Header Section. 220.127.116.11 The AU-header TheEach AU-header containsmay contain the fields given in figure 3. The length in bits of the above 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. 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. +---------------------------------------+ | 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. 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. If each AU-Index field is coded with the value 0, the serial number of the AU (fragment) is not specified, and in that case receivers MAY ignore the AU-Index field. 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 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 signalledsignaled 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 signalledsignaled to be larger than zero. The DTS-flag field SHOULD be 0 for any non-first fragment 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. 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 thean AU of an MPEG-4 system stream. For statesstream; each state is identified by a value of a modulo counter. In ISO/IEC 14496-1, MPEG-4 system streams see ISO/IEC 14496-1. The stream state is set either to 0 oruse the AU_SequenceNumber to 1. A change ofsignal stream states. When the stream state changes, the value (either from 1 to 0 or from 0 to 1) indicates another stateof the stream. At an AU that provides a random access point, as signalled by the RAP-flag, a change in the stream statestream-state MUST occur, unless the AU is a repeated random access point. Hence, receivers MAY ignore AUs with the RAP-flag set to 1 if the stream state does not change. Receivers that don't ignore a repeated random access point SHOULD take care that such processing does not disrupt the decoding process.be incremented by one. Note: no relation is required between stream-states of different streams. 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 signalled by the value of the CTS-flag and DTS-flag, respectively.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 byte-alignment.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. 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 not be desirable. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |AU(1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |AU(2) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+ | | | | -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | AU(n) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ||AU(n) continued| |-+-+-+-+-+-+-+-+ Figure 5: Access Unit Data Section; each AU is byteoctet 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 signalledsignaled 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. 18.104.22.168 Fragmentation A packet SHALL carry either one or more 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. 22.214.171.124 Interleaving Access Units MAY be interleaved. Senders MAY perform interleaving. Receivers MUST support interleaving. When interleaving of Access Units is used it SHALL be implemented using the AU-Index and AU-Index-delta fields in the AU-header. Based on the RTP sequence number, the RTP time stamp, the AU-Index and the AU-Index-delta, a receiver can unambiguously reconstruct the original order even in case of out-of-order packets, packet loss or duplication. Note that for this purpose the AU-Index is redundant when the RTP time stamp and the AU-Index-delta values are sufficient for placing the AUs correctly in time. In such cases receivers MAY ignore the AU-Index value and senders MAY code the AU-Index field with the value 0, but only if they code each AU-Index field with that value. 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 fixed 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 = T Timestamp[i, i > 0] = T +(Sum(for k=1 to i of (AU-Index-delta[k] + 1))) * access-unit-duration 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 not needed in this calculation. Hence in cases where the Access-unit-durationaccess-unit-duration has a fixed and known value, the AU-Index does not need to provide index information and can be coded with the value 0. See also the semantics of the AU-Index field in 126.96.36.199. If the Access Units are not fixed duration, the AU-Index is not redundant, and MUST provide the index information required for re-ordering. 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. Note that the CTS-delta may be required to compute the correct time stamp for each AU. When an RTP packet arrives (after any reordering has been done), receivers may 'flush' all Access Units from the interleave buffer which have aif the time stamp of each Access Units in the interleave buffer is strictly less than the time stamp of the arriving packet. Similarly the first Access Unit of every arriving packet can always be flushed (as no following packet can provide an earlier Access Unit), and any Access Units which are consecutive with it which have already been received.Access Units should also be flushed in time to be played; this can be important if there is loss before end-of-stream, before a silence interval, or before a large drop-out. 188.8.131.52 Constraints for interleaving The size of the packets should be suitably chosen to be appropriate to both the path MTU and the duration and capacity of the receiver's de-interleave buffer. The maximum packet size for a session shouldSHOULD be chosen not to exceed the path MTU. In order to control receiver latency and mitigate the effects of loss, there are profile-based limits on the size of the packet. This is expressed as a duration: it is calculated from the duration of the Access Units contained within a packet. Note that this duration is NOT the difference between the time stamps of the first and last Access Unit in a packet. No matter what interleaving scheme is used, the scheme must be analyzed to calculate the minimum number of frames a receiver has to buffer in order to de-interleave. Three profiles are defined to constrain the latency when interleaving. The applied profile is signalledsignaled by the MIME format parameter "Profile", indicating the decimal number of the profile. The maximum de-interleave buffer required at the receiver can be determined if the maximum packet duration is known. The maximum packet duration in milliseconds for the three profiles, shall notSHALL NOT exceed: Profile 0 -- 200 milliseconds Profile 1 -- 500 milliseconds Profile 2 -- 1500 milliseconds When interleaving is applied, the applied profile MUST be signalledsignaled by the MIME format parameter "Profile"; see section 4.1. Note that for low bit-rate material, this duration limit may make packets shorter than the MTU size. 3.3 Usage of this specification 3.3.1 General Usage of this specification requires definition of a mode. A mode defines how184.108.40.206. 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 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. 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". 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. In any mode compliant to this specification the same requirements apply for the rtpmap attributes. 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) 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 signalledsignaled by mode=generic. An example is given below for transport of a BIFS stream. In this example carriage of multiple BIFS 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 1410 and 15,16, respectively. The AU-header also contains the RAP-flag and the Stream-state, bothStream-state of 14 bits. This results in an AU-header with a Totaltotal size of two or four octets per BIFS AU. The RTP time stamp uses a 1 kHz clock. Note that the media type name is video, because the BIFS stream is part of an audiovisual 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=257; mode=generic; ObjectType=2; config=BIFSConfiguration(); SizeLength=15;SizeLength=10; CTSDeltaLength=16; RandomAccessIndication=1; StreamStateIndication=1 Note thatStreamStateIndication=4 Note: The a=fmtp line has been wrapped to fit the page, it comprises a single line in the SDP file. BIFSConfiguration() is the hexadecimal string as defined in ISO/IEC 14496-1; for the description of MIME parameters see section 4.1. 3.3.3 Constant bit-rate CELP This mode is signalledsignaled by mode=CELP-cbr. In this mode one or more fixed size CELP frames can be transported in one RTP packet; there is no support for interleaving. The RTP payload consist of one or more concatenated CELP frames, each of the same size. Both the AU Header Section and the Auxiliary Section areMUST 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/44100/2 a=fmtp:96 streamtype=5; profile-level-id=15; mode=CELP-cbr; config= AudioSpecificConfig(); ConstantSize=xxx; Note: The AudioSpecificConfig(),a=fmtp line has been wrapped to fit the page, it comprises a single line in the SDP file. AudioSpecificConfig() is the haxadecimal string as defined in ISO/IEC 14496-3,14496-3. AudioSpecificConfig() specifies that the audio stream type is CELP. For the description of MIME parameters see section 4.1. 3.3.4 Variable bit-rate CELP This mode is signalledsignaled by mode=CELP-vbr. With this mode one or more variable size CELP frames can be transported in one RTP packet with optional interleaving. As the largest possible frame size in this mode is greater than the maximum CELP frame size, there is no support for fragmentation of CELP frames. In this mode the RTP payload consists of the AU Header Section, followed by one or more concatenated CELP frames. The Auxiliary Section isMUST be empty. For each CELP frame contained in the payload there isMUST 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. 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. When interleaving is applied (AU-Index-delta coded with a value larger than 0), the parameter Profile MUST also be present. For example: m=audio 49230 RTP/AVP 96 a=rtpmap:96 mpeg4-generic/44100/2 a=fmtp:96 streamtype=5; profile-level-id=15; mode=CELP-vbr; config= AudioSpecificConfig(); SizeLength=6; IndexLength=2; IndexDeltaLength=2; Profile=1 Note: The AudioSpecificConfig(),a=fmtp line has been wrapped to fit the page, it comprises a single line in the SDP file. AudioSpecificConfig() is the hexadecimal string as defined in ISO/IEC 14496-3, AudioSpecificConfig() specifies that the audio stream type is CELP. For the description of MIME parameters see section 4.1. 3.3.5 Low bit-rate AAC This mode is signalledsignaled by mode=AAC-lbr. This mode supports transport of one or more variable size AAC frames with optional support for interleaving and fragmenting. The maximum size of an AAC frame (fragment) in this mode is 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 isMUST be empty. For each AAC frame contained in the payload the one octet AU-header provides: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. In the AU-header, the AU-size isMUST be coded with 6 bits and the AU-Index(-delta) with 2 bits; the AU-Index field MUST have the value 0 in each AU-header. In the AU-header Section, the concatenated AU-headers areMUST 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. When interleaving is applied (AU-Index-delta coded with a value larger than 0), also the parameter Profile MUST be present. For example: m=audio 49230 RTP/AVP 96 a=rtpmap:96 mpeg4-generic/44100/2 a=fmtp:96 streamtype=5; profile-level-id=15; mode=AAC-lbr; config= AudioSpecificConfig(); SizeLength=6; IndexLength=2; IndexDeltaLength=2; Profile=1 Note: The AudioSpecificConfig(),a=fmtp line has been wrapped to fit the page, it comprises a single line in the SDP file. AudioSpecificConfig() is the hexadecimal string as defined in ISO/IEC 14496-3,14496-3. AudioSpecificConfig() specifies that the audio stream type is AAC. For the description of MIME parameters see section 4.1. 3.3.6 High bit-rate AAC This mode is signalledsignaled by mode=AAC-hbr. This mode supports transport of one or more large variable size AAC frames in one RTP packet with optional support for interleaving and fragmenting. The maximum size of an AAC frame (fragment) in this mode is 8191 octets. In this mode the RTP payload consists of the AU Header Section, followed by one or more concatenated AAC frames. The Auxiliary Section isMUST be empty. For each AAC frame contained in the payload there isMUST 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 areMUST 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. When interleaving is applied (AU-Index-delta coded with a value larger than 0), also the parameter Profile MUST be present. For example: m=audio 49230 RTP/AVP 96 a=rtpmap:96 mpeg4-generic/44100/2 a=fmtp:96 streamtype=5; profile-level-id=15; mode=AAC-hbr; config=AudioSpecificConfig(); SizeLength=13; IndexLength=3; IndexDeltaLength=3; Profile=1 Note: The AudioSpecificConfig(),a=fmtp line has been wrapped to fit the page, it comprises a single line in the SDP file. AudioSpecificConfig() is the hexadecimal string as defined in ISO/IEC 14496-3,14496-3. AudioSpecificConfig() specifies that the audio stream type is AAC. 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. When defining a new mode care MUST be taken that an implementation of all 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. 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. 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 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 (objectTypeIndication Values) in ISO/IEC 14496-1. Note that the StreamType allows signallingsignaling of an MPEG-7 stream; this RTP payload format is not designed to carry an MPEG-7 stream, and may not be suitable for transport of MPEG-7 streams. 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 byteoctet 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 Stuctured Audio, the AudioSpecificConfig()may be conveyed by other means, not defined by this specification. If the AudioSpecificConfig() is conveyed by other means for Stuctured 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. 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 220.127.116.11, and for version 2 in section 18.104.22.168. 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 22.214.171.124 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 signalled: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 126.96.36.199.3.7 and 4.2 of RFCxxxx. 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 the ObjectType 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. ConstantSize: The constant size in octets of each Access Unit for this stream. Simultaneous presence of ConstantSize and the SizeLength parameters is not permitted. Profile: The decimal representation of the applied profile to constrain the latency when interleaving; see section 188.8.131.52. Absence of this parameter signals that the profile is not specified. This parameter MUST be present when interleaving is applied. Optional configuration parameters: SizeLength: The number of bits on which the AU-size field is encoded in the AU-header. Simultaneous presence of SizeLength and the ConstantSize parameter is not permitted. 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 and AU-Index-delta fields in each AU-header. IndexDeltaLength: The number of bits on which the AU-Index-delta field is encoded in any 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. StreamStateIndication: A decimal valueThe number of zero or one, indicating whetherbits on which the Stream-state field is presentencoded in the AU-header. The decimal value of one indicates presence of the Stream-state field, the default value zero its absence.This parameter MAY be present when transporting MPEG-4 system streams, and SHALL NOT be present 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 parameters 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). Receivers MUST tolerate the presence of such additional parameters, but these parameters SHALL notNOT impact the decoding of receivers that comply to this specification. Encoding considerations: 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. 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, Internet messaging and Email applications. 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 signalled 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). 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). 4.34.4 Usage of SDP 184.108.40.206.1 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  for example transported to the client in reply to a RTSP DESCRIBE or via SAP. In that case the (a=fmtp) keyword MUST be used as described in RFC 2327 , 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 . This implies that confidentiality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed on the compressed data so there is no conflict between the two operations. The packet processing complexity of this payload type (i.e. excluding media data processing) does not exhibit any significant non-uniformity in the receiver side to cause a denial- of-service threat. However, it is possible to inject non-compliant MPEG streams (Audio, Video, and Systems) to overload the receiver/decoder's buffers, which might compromise the functionality of the receiver or even crash it. This is especially true for end-to-end systems like MPEG where the buffer models are precisely defined. MPEG-4 Systems supports stream types including commands that are executed on the terminal like OD commands, BIFS commands, etc. and programmatic content like MPEG-J (Java(TM) Byte Code) and ECMAScript. It is possible to use one or more of the above in a manner non-compliant to MPEG to crash or temporarily make the receiver unavailable. Senders SHOULD ensure that packet loss does not cause severe problems in application execution when the packet carries OD commands, BIFS commands, or programmatic content such as MPEG-J and ECMAScript. For example, the reliability can be improved by re-transmission, or by using the carousel mechanism as defined by MPEG in ISO/IEC 14496-1, while observing the general congestion control principles. When such measures are deemed unsufficiently adequate, instead of this payload format applications SHOULD use more reliable means to transport the information, for example by applying an FEC scheme for RTP (such as in RFC 2733), or by using RTP over TCP (such as in RFC 2326, section 10.12), while giving due consideration to congestion control. For a general description of methods to repair streaming media see RFC 2354. 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 receiver within the defined security model. According to this security model, downloaded byte code is forbidden to load libraries, define native methods, start programs, read or write files, or read system properties. Receivers can implement intelligent filters to validate the buffer requirements or parametric (OD, BIFS, etc.) or programmatic (MPEG-J, ECMAScript) commands in the streams. However, this can increase the complexity significantly. 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 John Lazarro, Alex MacAulay, Bill May, Colin Perkins, Dorairaj V and Stephan Wenger for their valuable comments and support. 7. References  ISO/IEC International Standard 14496 (MPEG-4); "Information technology - Coding of audio-visual objects", January 2000  Schulzrinne, Casner, Frederick, Jacobson RTP, "A Transport Protocol for Real Time Applications", RFC 1889, Internet Engineering Task Force, January 1996.  S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.  D. Hoffman, G. Fernando, V. Goyal, M. Civanlar, "RTP payload format for MPEG1/MPEG2 Video", RFC 2250, January 1998.  Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, H. Kimata, "RTP payload format for MPEG-4 Audio/Visual streams", RFC 3016.  Handley, Jacobson, "SDP: Session Description Protocol", RFC 2327, Internet Engineering Task Force, April 1998. 8. Author Adresses Jan van der Meer Philips Digital Networks Cederlaan 4 5600 JB Eindhoven Netherlands Email : firstname.lastname@example.org David Mackie Cisco Systems Inc. 170 West Tasman Dr. San Jose, CA 95134 Email: email@example.com Viswanathan Swaminathan Sun Microsystems Inc. 901 San Antonio Road, M/S UMPK15-214 Palo Alto, CA 94303 Email: firstname.lastname@example.org David Singer Apple Computer, Inc. One Infinite Loop, MS:302-3MT Cupertino CA 95014 Email: email@example.com Philippe Gentric Philips Digital Networks, MP4Net 51 rue Carnot 92156 Suresnes France e-mail: firstname.lastname@example.org Full Copyright Statement "Copyright (C) The Internet Society (date). 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. APPENDIX: Usage of this payload format Appendix A. Examples of delay analysis with interleave A.1 Group interleave An example of regular interleave is when packets are formed into groups. If the number of packets in a group is N, for example packet 0 could contain frame 0, frame N, frame 2N, and so on; packet 1 could contain frame 1, frame 1+N, 1+2N, and so on. The AU-Index field is used to document the sequence of the packet within the group (or the first frame in the packet, which is the same thing in this scheme), and all the AU-Index-delta fields contain N-1. Because each subsequent frame in the packet has a higher time stamp than the preceding frame, receivers can tell when a new interleave group is starting, by noting that the computed time stamp of the first frame in a packet is later than any previously computed time stamp. In that case the time stamps of all frames contained in the packet are higher than any previously computed time stamp, and hence interleaving with any previously received frame is not possible. In conclusion, a new group has been started. If the group size is 3, then packets can be formed as follows: Packet Time stamp Frame Numbers AU-Index, AU-Index-delta 0 T 0, 3, 6 0, 2, 2 1 T 1, 4, 7 0, 2, 2 2 T 2, 5, 8 0, 2, 2 3 T 9,12,15 0, 2, 2 In this case, the receiver would have to buffer 4 frames at least from packets 0 and 1, and can flush all frames when packet 2 arrives. (Frame 0 can be flushed as packet 0 arrives, since it is the earliest frame we hold, and likewise frame 1 from packet 1; we are therefore holding 3,4,6,7 until packet 2 arrives). If there is loss, then the receiver may wait longer than is strictly necessary before it emits frames. For example, say packet 1 is lost from the above example. Packet 0 allows frame 0 to be emitted, and then packet 2 arrives, allowing us to notice the loss of frame 1, and emit frame 2 and 3. Then it is not until the arrival of packet 3 (which has a time-stamp beyond the times of all the frames seen so far), that we can finish dealing with the loss, even though the first group has, in fact, ended. (This is in contrast to schemes which signal the group size explicitly; if the receiver knows that this is packet 3 of 3, then even if 2 of 3 is missing, it can de-interleave this group without waiting for the next one to start). In the above example the AU-Index is coded with the value 0, as required for the modes defined in this document. To reconstruct the original order, the RTP time stamp and the AU-Index-delta are used. See also section 220.127.116.11. 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 RPT packets does not cause the loss of two subsequent audio frames. Note that in this example the RTP time stamps of packets 3 and 4 are earlier than the RTP time stamps of packets 1 and 2.2, respectively. Packet Time stamp Frame Numbers AU-Index, AU-Index-delta 0 T 0, 5, 10, 15 0, 5, 5, 5 1 T 2, 7, 12, 17 0, 5, 5, 5 2 T 4, 9, 14, 19 0, 5, 5, 5 3 T 1, 6, 11, 16 0, 5, 5, 5 4 T 3, 8, 13, 18 0, 5, 5, 5 5 T 20, 25, 30, 35 0, 5, 5, 5 and so on .. A.2 Continuous interleave In continuous interleave, once the scheme is 'primed', the number of frames 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 frames, but with 4 frames per packet, for a run of 20 frames. This shows both how the scheme 'starts up' and how it finishes. Packet Time-stamp Frame Numbers AU-Index, AU-Index-delta 0 T 0 0 1 T 1 4 0 2 2 T 2 5 8 0 2 2 3 T 3 6 9 12 0 2 2 2 4 T 7 10 13 16 0 2 2 2 5 T 11 14 17 20 0 2 2 2 6 T 15 18 0 2 7 T 19 0 In this case, the receiver has to buffer only 3 frames, not 4. Say we are waiting for packet 4. We can flush frames 0, 1, 2, 3, 4, 5, 6; we are holding therefore 8, 9, 12. Packet 4 arrives, allowing us to emit 7,8,9,10, and we are holding 12,13,16. Each arriving packet contains 4 frames, and allows 4 frames to be flushed. In the above example the AU-Index is coded with the value 0, as required for the modes defined in this document. To reconstruct the original order, the RTP time stamp and the AU-Index-delta are used. See also 18.104.22.168. If there is loss, again the receiver has to wait to emit the erasure frames. In this case, say packet 3 is lost. We were holding frames 4, 5, and 8. On the arrival of packet 4, (time-stamp of frame 7), we now know frame 3 was lost, we can emit frames 4,5, and we know 6 must be lost, and emit 7, which is in the packet that arrived. Then on the arrival of packet 5 (time-stamp 11) we can emit 8, indicate loss of 9, and emit 10 and 11. Finally, the arrival of packet 6 (time-stamp 15) indicates that 12 must be lost; we have now detected all the lost frames.