Audio/Video Transport WG Y.-K. Wang Internet Draft S. Wenger Intended status: Standards track M.M. Hanuksela Expires: January 2009 Nokia T. Stockhammer Nomor Research M. Westerlund Ericsson D. Singer Apple July 14, 2008 RTP Payload Format for H.264 Video draft-wang-avt-rfc3984bis-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 Internet-Draft will expire on January 14, 2009. Copyright Notice Copyright (C) The IETF Trust (2008). Wang, et al Expires January 14, 2009 [Page 1] Internet-Draft RFC3984bis July 2008 Abstract This memo describes an RTP Payload format for the ITU-T Recommendation H.264 video codec and the technically identical ISO/IEC International Standard 14496-10 video codec. The RTP payload format allows for packetization of one or more Network Abstraction Layer Units (NALUs), produced by an H.264 video encoder, in each RTP payload. The payload format has wide applicability, as it supports applications from simple low bit-rate conversational usage, to Internet video streaming with interleaved transmission, to high bit- rate video-on-demand. This memo intends to obsolete RFC 3984. Table of Contents 1. Introduction...................................................4 1.1. The H.264 Codec...........................................4 1.2. Parameter Set Concept.....................................5 1.3. Network Abstraction Layer Unit Types......................6 2. Conventions....................................................7 3. Scope..........................................................7 4. Definitions and Abbreviations..................................7 4.1. Definitions...............................................7 4.2. Abbreviations.............................................9 5. RTP Payload Format.............................................9 5.1. RTP Header Usage..........................................9 5.2. Common Structure of the RTP Payload Format...............12 5.3. NAL Unit Octet Usage.....................................13 5.4. Packetization Modes......................................15 5.5. Decoding Order Number (DON)..............................16 5.6. Single NAL Unit Packet...................................19 5.7. Aggregation Packets......................................20 5.7.1. Single-Time Aggregation Packet......................22 5.7.2. Multi-Time Aggregation Packets (MTAPs)..............24 5.7.3. Fragmentation Units (FUs)...........................28 6. Packetization Rules...........................................32 6.1. Common Packetization Rules...............................32 6.2. Single NAL Unit Mode.....................................33 6.3. Non-Interleaved Mode.....................................33 6.4. Interleaved Mode.........................................33 7. De-Packetization Process......................................34 7.1. Single NAL Unit and Non-Interleaved Mode.................34 7.2. Interleaved Mode.........................................34 Wang, et al Expires January 14, 2009 [Page 2] Internet-Draft RFC3984bis July 2008 7.2.1. Size of the Deinterleaving Buffer...................35 7.2.2. Deinterleaving Process..............................35 7.3. Additional De-Packetization Guidelines...................37 8. Payload Format Parameters.....................................38 8.1. MIME Registration........................................38 8.2. SDP Parameters...........................................50 8.2.1. Mapping of MIME Parameters to SDP...................50 8.2.2. Usage with the SDP Offer/Answer Model...............50 8.2.3. Usage in Declarative Session Descriptions...........55 8.3. Examples.................................................56 8.4. Parameter Set Considerations.............................61 9. Security Considerations.......................................63 10. Congestion Control...........................................64 11. IANA Consideration...........................................65 12. Informative Appendix: Application Examples...................65 12.1. Video Telephony according to ITU-T Recommendation H.241 Annex A.......................................................65 12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit Aggregation...................................................65 12.3. Video Telephony, Interleaved Packetization Using NAL Unit Aggregation...................................................66 12.4. Video Telephony with Data Partitioning..................66 12.5. Video Telephony or Streaming with FUs and Forward Error Correction....................................................67 12.6. Low Bit-Rate Streaming..................................69 12.7. Robust Packet Scheduling in Video Streaming.............70 13. Informative Appendix: Rationale for Decoding Order Number....71 13.1. Introduction............................................71 13.2. Example of Multi-Picture Slice Interleaving.............71 13.3. Example of Robust Packet Scheduling.....................73 13.4. Robust Transmission Scheduling of Redundant Coded Slices77 13.5. Remarks on Other Design Possibilities...................77 14. Acknowledgements.............................................78 15. References...................................................78 15.1. Normative References....................................78 15.2. Informative References..................................79 Authors' Addresses...............................................80 Intellectual Property Statement..................................82 Disclaimer of Validity...........................................82 Acknowledgement..................................................83 16. Backward compatibility to RFC 3984...........................83 17. Changes from RFC 3984........................................83 17.1. Technical changes.......................................83 17.2. Editorial changes.......................................86 18. Open issues..................................................97 Wang, et al Expires January 14, 2009 [Page 3] Internet-Draft RFC3984bis July 2008 1. Introduction This memo intends to obsolete RFC 3984. 1.1. The H.264 Codec This memo specifies an RTP payload specification for the video coding standard known as ITU-T Recommendation H.264 [1] and ISO/IEC International Standard 14496 Part 10 [2] (both also known as Advanced Video Coding, or AVC). Recommendation H.264 was approved by ITU-T on May 2003, and the approved draft specification is available for public review [8]. In this memo the H.264 acronym is used for the codec and the standard, but the memo is equally applicable to the ISO/IEC counterpart of the coding standard. The H.264 video codec has a very broad application range that covers all forms of digital compressed video from, low bit-rate Internet streaming applications to HDTV broadcast and Digital Cinema applications with nearly lossless coding. Compared to the current state of technology, the overall performance of H.264 is such that bit rate savings of 50% or more are reported. Digital Satellite TV quality, for example, was reported to be achievable at 1.5 Mbit/s, compared to the current operation point of MPEG 2 video at around 3.5 Mbit/s [9]. The codec specification [1] itself distinguishes conceptually between a video coding layer (VCL) and a network abstraction layer (NAL). The VCL contains the signal processing functionality of the codec; mechanisms such as transform, quantization, and motion compensated prediction; and a loop filter. It follows the general concept of most of today's video codecs, a macroblock-based coder that uses inter picture prediction with motion compensation and transform coding of the residual signal. The VCL encoder outputs slices: a bit string that contains the macroblock data of an integer number of macroblocks, and the information of the slice header (containing the spatial address of the first macroblock in the slice, the initial quantization parameter, and similar information). Macroblocks in slices are arranged in scan order unless a different macroblock allocation is specified, by using the so-called Flexible Macroblock Ordering syntax. In-picture prediction is used only within a slice. More information is provided in [9]. The Network Abstraction Layer (NAL) encoder encapsulates the slice output of the VCL encoder into Network Abstraction Layer Units (NAL units), which are suitable for transmission over packet networks or use in packet oriented multiplex environments. Annex B of H.264 defines an encapsulation process to transmit such NAL units over Wang, et al Expires January 14, 2009 [Page 4] Internet-Draft RFC3984bis July 2008 byte-stream oriented networks. In the scope of this memo, Annex B is not relevant. Internally, the NAL uses NAL units. A NAL unit consists of a one- byte header and the payload byte string. The header indicates the type of the NAL unit, the (potential) presence of bit errors or syntax violations in the NAL unit payload, and information regarding the relative importance of the NAL unit for the decoding process. This RTP payload specification is designed to be unaware of the bit string in the NAL unit payload. One of the main properties of H.264 is the complete decoupling of the transmission time, the decoding time, and the sampling or presentation time of slices and pictures. The decoding process specified in H.264 is unaware of time, and the H.264 syntax does not carry information such as the number of skipped frames (as is common in the form of the Temporal Reference in earlier video compression standards). Also, there are NAL units that affect many pictures and that are, therefore, inherently timeless. For this reason, the handling of the RTP timestamp requires some special considerations for NAL units for which the sampling or presentation time is not defined or, at transmission time, unknown. 1.2. Parameter Set Concept One very fundamental design concept of H.264 is to generate self- contained packets, to make mechanisms such as the header duplication of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11] unnecessary. This was achieved by decoupling information relevant to more than one slice from the media stream. This higher layer meta information should be sent reliably, asynchronously, and in advance from the RTP packet stream that contains the slice packets. (Provisions for sending this information in-band are also available for applications that do not have an out-of-band transport channel appropriate for the purpose.) The combination of the higher-level parameters is called a parameter set. The H.264 specification includes two types of parameter sets: sequence parameter set and picture parameter set. An active sequence parameter set remains unchanged throughout a coded video sequence, and an active picture parameter set remains unchanged within a coded picture. The sequence and picture parameter set structures contain information such as picture size, optional coding modes employed, and macroblock to slice group map. To be able to change picture parameters (such as the picture size) without having to transmit parameter set updates synchronously to the slice packet stream, the encoder and decoder can maintain a list of Wang, et al Expires January 14, 2009 [Page 5] Internet-Draft RFC3984bis July 2008 more than one sequence and picture parameter set. Each slice header contains a codeword that indicates the sequence and picture parameter set to be used. This mechanism allows the decoupling of the transmission of parameter sets from the packet stream, and the transmission of them by external means (e.g., as a side effect of the capability exchange), or through a (reliable or unreliable) control protocol. It may even be possible that they are never transmitted but are fixed by an application design specification. 1.3. Network Abstraction Layer Unit Types Tutorial information on the NAL design can be found in [12], [13], and [14]. All NAL units consist of a single NAL unit type octet, which also co- serves as the payload header of this RTP payload format. The payload of a NAL unit follows immediately. The syntax and semantics of the NAL unit type octet are specified in [1], but the essential properties of the NAL unit type octet are summarized below. The NAL unit type octet has the following format: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |F|NRI| Type | +---------------+ The semantics of the components of the NAL unit type octet, as specified in the H.264 specification, are described briefly below. F: 1 bit forbidden_zero_bit. The H.264 specification declares a value of 1 as a syntax violation. NRI: 2 bits nal_ref_idc. A value of 00 indicates that the content of the NAL unit is not used to reconstruct reference pictures for inter picture prediction. Such NAL units can be discarded without risking the integrity of the reference pictures. Values greater than 00 indicate that the decoding of the NAL unit is required to maintain the integrity of the reference pictures. Type: 5 bits nal_unit_type. This component specifies the NAL unit payload Wang, et al Expires January 14, 2009 [Page 6] Internet-Draft RFC3984bis July 2008 type as defined in table 7-1 of [1], and later within this memo. For a reference of all currently defined NAL unit types and their semantics, please refer to section 7.4.1 in [1]. This memo introduces new NAL unit types, which are presented in section 5.2. The NAL unit types defined in this memo are marked as unspecified in [1]. Moreover, this specification extends the semantics of F and NRI as described in section 5.3. 2. Conventions 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 [3]. This specification uses the notion of setting and clearing a bit when bit fields are handled. Setting a bit is the same as assigning that bit the value of 1 (On). Clearing a bit is the same as assigning that bit the value of 0 (Off). 3. Scope This payload specification can only be used to carry the "naked" H.264 NAL unit stream over RTP, and not the bitstream format discussed in Annex B of H.264. Likely, the first applications of this specification will be in the conversational multimedia field, video telephony or video conferencing, but the payload format also covers other applications, such as Internet streaming and TV over IP. 4. Definitions and Abbreviations 4.1. Definitions This document uses the definitions of [1]. The following terms, defined in [1], are summed up for convenience: access unit: A set of NAL units always containing a primary coded picture. In addition to the primary coded picture, an access unit may also contain one or more redundant coded pictures or other NAL units not containing slices or slice data partitions of a coded picture. The decoding of an access unit always results in a decoded picture. coded video sequence: A sequence of access units that consists, in decoding order, of an instantaneous decoding refresh (IDR) access unit followed by zero or more non-IDR access units Wang, et al Expires January 14, 2009 [Page 7] Internet-Draft RFC3984bis July 2008 including all subsequent access units up to but not including any subsequent IDR access unit. IDR access unit: An access unit in which the primary coded picture is an IDR picture. IDR picture: A coded picture containing only slices with I or SI slice types that causes a "reset" in the decoding process. After the decoding of an IDR picture, all following coded pictures in decoding order can be decoded without inter prediction from any picture decoded prior to the IDR picture. primary coded picture: The coded representation of a picture to be used by the decoding process for a bitstream conforming to H.264. The primary coded picture contains all macroblocks of the picture. redundant coded picture: A coded representation of a picture or a part of a picture. The content of a redundant coded picture shall not be used by the decoding process for a bitstream conforming to H.264. The content of a redundant coded picture may be used by the decoding process for a bitstream that contains errors or losses. VCL NAL unit: A collective term used to refer to coded slice and coded data partition NAL units. In addition, the following definitions apply: decoding order number (DON): A field in the payload structure, or a derived variable indicating NAL unit decoding order. Values of DON are in the range of 0 to 65535, inclusive. After reaching the maximum value, the value of DON wraps around to 0. NAL unit decoding order: A NAL unit order that conforms to the constraints on NAL unit order given in section 7.4.1.2 in [1]. NALU-time: The value that the RTP timestamp would have if the NAL unit would be transported in its own RTP packet. transmission order: The order of packets in ascending RTP sequence number order (in modulo arithmetic). Within an aggregation packet, the NAL unit transmission order is the same as the order of appearance of NAL units in the packet. media aware network element (MANE): A network element, such as a middlebox or application layer gateway that is capable of parsing Wang, et al Expires January 14, 2009 [Page 8] Internet-Draft RFC3984bis July 2008 certain aspects of the RTP payload headers or the RTP payload and reacting to the contents. Informative note: The concept of a MANE goes beyond normal routers or gateways in that a MANE has to be aware of the signaling (e.g., to learn about the payload type mappings of the media streams), and in that it has to be trusted when working with SRTP. The advantage of using MANEs is that they allow packets to be dropped according to the needs of the media coding. For example, if a MANE has to drop packets due to congestion on a certain link, it can identify those packets whose dropping has the smallest negative impact on the user experience and remove them in order to remove the congestion and/or keep the delay low. 4.2. Abbreviations DON: Decoding Order Number DONB: Decoding Order Number Base DOND: Decoding Order Number Difference FEC: Forward Error Correction FU: Fragmentation Unit IDR: Instantaneous Decoding Refresh IEC: International Electrotechnical Commission ISO: International Organization for Standardization ITU-T: International Telecommunication Union, Telecommunication Standardization Sector MANE: Media Aware Network Element MTAP: Multi-Time Aggregation Packet MTAP16: MTAP with 16-bit timestamp offset MTAP24: MTAP with 24-bit timestamp offset NAL: Network Abstraction Layer NALU: NAL Unit SEI: Supplemental Enhancement Information STAP: Single-Time Aggregation Packet STAP-A: STAP type A STAP-B: STAP type B TS: Timestamp VCL: Video Coding Layer 5. RTP Payload Format 5.1. RTP Header Usage The format of the RTP header is specified in RFC 3550 [4] and reprinted in Figure 1 for convenience. This payload format uses the fields of the header in a manner consistent with that specification. Wang, et al Expires January 14, 2009 [Page 9] Internet-Draft RFC3984bis July 2008 When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP payload format is specified in section 5.6. The RTP payload (and the settings for some RTP header bits) for aggregation packets and fragmentation units are specified in sections 5.7 and 5.8, respectively. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1 RTP header according to RFC 3550 The RTP header information to be set according to this RTP payload format is set as follows: Marker bit (M): 1 bit Set for the very last packet of the access unit indicated by the RTP timestamp, in line with the normal use of the M bit in video formats, to allow an efficient playout buffer handling. For aggregation packets (STAP and MTAP), the marker bit in the RTP header MUST be set to the value that the marker bit of the last NAL unit of the aggregation packet would have been if it were transported in its own RTP packet. Decoders MAY use this bit as an early indication of the last packet of an access unit, but MUST NOT rely on this property. Informative note: Only one M bit is associated with an aggregation packet carrying multiple NAL units. Thus, if a gateway has re-packetized an aggregation packet into several packets, it cannot reliably set the M bit of those packets. Payload type (PT): 7 bits The assignment of an RTP payload type for this new packet format is outside the scope of this document and will not be specified here. The assignment of a payload type has to be performed either through the profile used or in a dynamic way. Wang, et al Expires January 14, 2009 [Page 10] Internet-Draft RFC3984bis July 2008 Sequence number (SN): 16 bits Set and used in accordance with RFC 3550. For the single NALU and non-interleaved packetization mode, the sequence number is used to determine decoding order for the NALU. Timestamp: 32 bits The RTP timestamp is set to the sampling timestamp of the content. A 90 kHz clock rate MUST be used. If the NAL unit has no timing properties of its own (e.g., parameter set and SEI NAL units), the RTP timestamp is set to the RTP timestamp of the primary coded picture of the access unit in which the NAL unit is included, according to section 7.4.1.2 of [1]. The setting of the RTP Timestamp for MTAPs is defined in section 5.7.2. Receivers SHOULD ignore any picture timing SEI messages included in access units that have only one display timestamp. Instead, receivers SHOULD use the RTP timestamp for synchronizing the display process. RTP senders SHOULD NOT transmit picture timing SEI messages for pictures that are not supposed to be displayed as multiple fields. If one access unit has more than one display timestamp carried in a picture timing SEI message, then the information in the SEI message SHOULD be treated as relative to the RTP timestamp, with the earliest event occurring at the time given by the RTP timestamp, and subsequent events later, as given by the difference in SEI message picture timing values. Let tSEI1, tSEI2, ..., tSEIn be the display timestamps carried in the SEI message of an access unit, where tSEI1 is the earliest of all such timestamps. Let tmadjst() be a function that adjusts the SEI messages time scale to a 90-kHz time scale. Let TS be the RTP timestamp. Then, the display time for the event associated with tSEI1 is TS. The display time for the event with tSEIx, where x is [2..n] is TS + tmadjst (tSEIx - tSEI1). Informative note: Displaying coded frames as fields is needed commonly in an operation known as 3:2 pulldown, in which film content that consists of coded frames is displayed on a display using interlaced scanning. The picture timing SEI message enables carriage of multiple timestamps for the same coded picture, and therefore the 3:2 pulldown process is Wang, et al Expires January 14, 2009 [Page 11] Internet-Draft RFC3984bis July 2008 perfectly controlled. The picture timing SEI message mechanism is necessary because only one timestamp per coded frame can be conveyed in the RTP timestamp. Informative note: Because H.264 allows the decoding order to be different from the display order, values of RTP timestamps may not be monotonically non-decreasing as a function of RTP sequence numbers. Furthermore, the value for interarrival jitter reported in the RTCP reports may not be a trustworthy indication of the network performance, as the calculation rules for interarrival jitter (section 6.4.1 of RFC 3550) assume that the RTP timestamp of a packet is directly proportional to its transmission time. 5.2. Common Structure of the RTP Payload Format The payload format defines three different basic payload structures. A receiver can identify the payload structure by the first byte of the RTP payload, which co-serves as the RTP payload header and, in some cases, as the first byte of the payload. This byte is always structured as a NAL unit header. The NAL unit type field indicates which structure is present. The possible structures are as follows: Single NAL Unit Packet: Contains only a single NAL unit in the payload. The NAL header type field will be equal to the original NAL unit type; i.e., in the range of 1 to 23, inclusive. Specified in section 5.6. Aggregation packet: Packet type used to aggregate multiple NAL units into a single RTP payload. This packet exists in four versions, the Single-Time Aggregation Packet type A (STAP-A), the Single-Time Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet (MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet (MTAP) with 24-bit offset (MTAP24). The NAL unit type numbers assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and 27, respectively. Specified in section 5.7. Fragmentation unit: Used to fragment a single NAL unit over multiple RTP packets. Exists with two versions, FU-A and FU-B, identified with the NAL unit type numbers 28 and 29, respectively. Specified in section 5.8. Informative note: This specification does not limit the size of NAL units encapsulated in single NAL unit packets and fragmentation units. The maximum size of a NAL unit encapsulated in any aggregation packet is 65535 bytes. Wang, et al Expires January 14, 2009 [Page 12] Internet-Draft RFC3984bis July 2008 Table 1 summarizes NAL unit types and the corresponding RTP packet types when each of these NAL units is directly used a packet payload, and where the types are described in this memo. Table 1. Summary of NAL unit types and the corresponding packet types NAL Unit Packet Packet Type Name Section Type Type --------------------------------------------------------- 0 undefined - 1-23 NAL unit Single NAL unit packet 5.6 24 STAP-A Single-time aggregation packet 5.7.1 25 STAP-B Single-time aggregation packet 5.7.1 26 MTAP16 Multi-time aggregation packet 5.7.2 27 MTAP24 Multi-time aggregation packet 5.7.2 28 FU-A Fragmentation unit 5.8 29 FU-B Fragmentation unit 5.8 30-31 undefined - 5.3. NAL Unit Octet Usage The structure and semantics of the NAL unit octet were introduced in section 1.3. For convenience, the format of the NAL unit type octet is reprinted below: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |F|NRI| Type | +---------------+ This section specifies the semantics of F and NRI according to this specification. F: 1 bit forbidden_zero_bit. A value of 0 indicates that the NAL unit type octet and payload should not contain bit errors or other syntax violations. A value of 1 indicates that the NAL unit type octet and payload may contain bit errors or other syntax violations. MANEs SHOULD set the F bit to indicate detected bit errors in the NAL unit. The H.264 specification requires that the F bit is equal to 0. When the F bit is set, the decoder is advised that bit errors or any other syntax violations may be present in the payload or in the NAL unit type octet. The simplest decoder Wang, et al Expires January 14, 2009 [Page 13] Internet-Draft RFC3984bis July 2008 reaction to a NAL unit in which the F bit is equal to 1 is to discard such a NAL unit and to conceal the lost data in the discarded NAL unit. NRI: 2 bits nal_ref_idc. The semantics of value 00 and a non-zero value remain unchanged from the H.264 specification. In other words, a value of 00 indicates that the content of the NAL unit is not used to reconstruct reference pictures for inter picture prediction. Such NAL units can be discarded without risking the integrity of the reference pictures. Values greater than 00 indicate that the decoding of the NAL unit is required to maintain the integrity of the reference pictures. In addition to the specification above, according to this RTP payload specification, values of NRI indicate the relative transport priority, as determined by the encoder. MANEs can use this information to protect more important NAL units better than they do less important NAL units. The highest transport priority is 11, followed by 10, and then by 01; finally, 00 is the lowest. Informative note: Any non-zero value of NRI is handled identically in H.264 decoders. Therefore, receivers need not manipulate the value of NRI when passing NAL units to the decoder. An H.264 encoder MUST set the value of NRI according to the H.264 specification (subclause 7.4.1) when the value of nal_unit_type is in the range of 1 to 12, inclusive. In particular, the H.264 specification requires that the value of NRI SHALL be equal to 0 for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or 12. For NAL units having nal_unit_type equal to 7 or 8 (indicating a sequence parameter set or a picture parameter set, respectively), an H.264 encoder SHOULD set the value of NRI to 11 (in binary format). For coded slice NAL units of a primary coded picture having nal_unit_type equal to 5 (indicating a coded slice belonging to an IDR picture), an H.264 encoder SHOULD set the value of NRI to 11 (in binary format). For a mapping of the remaining nal_unit_types to NRI values, the following example MAY be used and has been shown to be efficient in a certain environment [13]. Other mappings MAY also be desirable, depending on the application and the H.264/AVC Annex A profile in use. Wang, et al Expires January 14, 2009 [Page 14] Internet-Draft RFC3984bis July 2008 Informative note: Data Partitioning is not available in certain profiles; e.g., in the Main or Baseline profiles. Consequently, the NAL unit types 2, 3, and 4 can occur only if the video bitstream conforms to a profile in which data partitioning is allowed and not in streams that conform to the Main or Baseline profiles. Table 2. Example of NRI values for coded slices and coded slice data partitions of primary coded reference pictures NAL Unit Type Content of NAL unit NRI (binary) ---------------------------------------------------------------- 1 non-IDR coded slice 10 2 Coded slice data partition A 10 3 Coded slice data partition B 01 4 Coded slice data partition C 01 Informative note: As mentioned before, the NRI value of non- reference pictures is 00 as mandated by H.264/AVC. An H.264 encoder SHOULD set the value of NRI for coded slice and coded slice data partition NAL units of redundant coded reference pictures equal to 01 (in binary format). Definitions of the values for NRI for NAL unit types 24 to 29, inclusive, are given in sections 5.7 and 5.8 of this memo. No recommendation for the value of NRI is given for NAL units having nal_unit_type in the range of 13 to 23, inclusive, because these values are reserved for ITU-T and ISO/IEC. No recommendation for the value of NRI is given for NAL units having nal_unit_type equal to 0 or in the range of 30 to 31, inclusive, as the semantics of these values are not specified in this memo. 5.4. Packetization Modes This memo specifies three cases of packetization modes: o Single NAL unit mode o Non-interleaved mode o Interleaved mode The single NAL unit mode is targeted for conversational systems that comply with ITU-T Recommendation H.241 [15] (see section 12.1). The non-interleaved mode is targeted for conversational systems that may Wang, et al Expires January 14, 2009 [Page 15] Internet-Draft RFC3984bis July 2008 not comply with ITU-T Recommendation H.241. In the non-interleaved mode, NAL units are transmitted in NAL unit decoding order. The interleaved mode is targeted for systems that do not require very low end-to-end latency. The interleaved mode allows transmission of NAL units out of NAL unit decoding order. The packetization mode in use MAY be signaled by the value of the OPTIONAL packetization-mode MIME parameter. The used packetization mode governs which NAL unit types are allowed in RTP payloads. Table 3 summarizes the allowed packet payload types for each packetization mode. Packetization modes are explained in more detail in section 6. Table 3. Summary of allowed NAL unit types for each packetization mode (yes = allowed, no = disallowed, ig = ignore) Payload Packet Single NAL Non-Interleaved Interleaved Type Type Unit Mode Mode Mode ------------------------------------------------------------- 0 undefined ig ig ig 1-23 NAL unit yes yes no 24 STAP-A no yes no 25 STAP-B no no yes 26 MTAP16 no no yes 27 MTAP24 no no yes 28 FU-A no yes yes 29 FU-B no no yes 30-31 undefined ig ig ig Some UAL unit or payload type values (indicated as undefined in Table 3) are reserved for future extensions. NAL units of those types SHOULD NOT be sent by a sender (direct as packet payloads, or as aggregation units in aggregation packets, or as fragmented units in FU packets) and MUST be ignored by a receiver. For example, the payload types 1-23, with the associated packet type "NAL unit", are allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode", but disallowed in "Interleaved Mode". However, NAL units of NAL unit types 1-23 can be used in "Interleaved Mode" as aggregation units in STAP-B, MTAP16 and MTAP14 packets as well as fragmented units in FU-A and FU-B packets. Similarly, NAL units of NAL unit types 1-23 can also be used in the "Non-Interleaved Mode" as aggregation units in STAP-A packets or fragmented units in FU-A packets, in addition to being directly used as packet payloads. 5.5. Decoding Order Number (DON) In the interleaved packetization mode, the transmission order of NAL units is allowed to differ from the decoding order of the NAL units. Wang, et al Expires January 14, 2009 [Page 16] Internet-Draft RFC3984bis July 2008 Decoding order number (DON) is a field in the payload structure or a derived variable that indicates the NAL unit decoding order. Rationale and examples of use cases for transmission out of decoding order and for the use of DON are given in section 13. The coupling of transmission and decoding order is controlled by the OPTIONAL sprop-interleaving-depth MIME parameter as follows. When the value of the OPTIONAL sprop-interleaving-depth MIME parameter is equal to 0 (explicitly or per default), the transmission order of NAL units MUST conform to the NAL unit decoding order. When the value of the OPTIONAL sprop-interleaving-depth MIME parameter is greater than 0, o the order of NAL units in an MTAP16 and an MTAP24 is NOT REQUIRED to be the NAL unit decoding order, and o the order of NAL units generated by decapsulating STAP-Bs, MTAPs, and FUs in two consecutive packets is NOT REQUIRED to be the NAL unit decoding order. The RTP payload structures for a single NAL unit packet, an STAP-A, and an FU-A do not include DON. STAP-B and FU-B structures include DON, and the structure of MTAPs enables derivation of DON as specified in section 5.7.2. Informative note: When an FU-A occurs in interleaved mode, it always follows an FU-B, which sets its DON. Informative note: If a transmitter wants to encapsulate a single NAL unit per packet and transmit packets out of their decoding order, STAP-B packet type can be used. In the single NAL unit packetization mode, the transmission order of NAL units, determined by the RTP sequence number, MUST be the same as their NAL unit decoding order. In the non-interleaved packetization mode, the transmission order of NAL units in single NAL unit packets, STAP-As, and FU-As MUST be the same as their NAL unit decoding order. The NAL units within an STAP MUST appear in the NAL unit decoding order. Thus, the decoding order is first provided through the implicit order within a STAP, and second provided through the RTP sequence number for the order between STAPs, FUs, and single NAL unit packets. Signaling of the value of DON for NAL units carried in STAP-B, MTAP, and a series of fragmentation units starting with an FU-B is specified in sections 5.7.1, 5.7.2, and 5.8, respectively. The DON value of the first NAL unit in transmission order MAY be set to any Wang, et al Expires January 14, 2009 [Page 17] Internet-Draft RFC3984bis July 2008 value. Values of DON are in the range of 0 to 65535, inclusive. After reaching the maximum value, the value of DON wraps around to 0. The decoding order of two NAL units contained in any STAP-B, MTAP, or a series of fragmentation units starting with an FU-B is determined as follows. Let DON(i) be the decoding order number of the NAL unit having index i in the transmission order. Function don_diff(m,n) is specified as follows: If DON(m) == DON(n), don_diff(m,n) = 0 If (DON(m) < DON(n) and DON(n) - DON(m) < 32768), don_diff(m,n) = DON(n) - DON(m) If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768), don_diff(m,n) = 65536 - DON(m) + DON(n) If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768), don_diff(m,n) = - (DON(m) + 65536 - DON(n)) If (DON(m) > DON(n) and DON(m) - DON(n) < 32768), don_diff(m,n) = - (DON(m) - DON(n)) A positive value of don_diff(m,n) indicates that the NAL unit having transmission order index n follows, in decoding order, the NAL unit having transmission order index m. When don_diff(m,n) is equal to 0, then the NAL unit decoding order of the two NAL units can be in either order. A negative value of don_diff(m,n) indicates that the NAL unit having transmission order index n precedes, in decoding order, the NAL unit having transmission order index m. Values of DON related fields (DON, DONB, and DOND; see section 5.7) MUST be such that the decoding order determined by the values of DON, as specified above, conforms to the NAL unit decoding order. If the order of two NAL units in NAL unit decoding order is switched and the new order does not conform to the NAL unit decoding order, the NAL units MUST NOT have the same value of DON. If the order of two consecutive NAL units in the NAL unit stream is switched and the new order still conforms to the NAL unit decoding order, the NAL units MAY have the same value of DON. For example, when arbitrary slice order is allowed by the video coding profile in use, all the coded slice NAL units of a coded picture are allowed to have the same value of DON. Consequently, NAL units having the same value of DON can be decoded in any order, and two NAL units having a different value of DON should be passed to the decoder in the order specified above. When two consecutive NAL units in the NAL unit decoding order have a different value of DON, the value of DON for the second NAL unit in Wang, et al Expires January 14, 2009 [Page 18] Internet-Draft RFC3984bis July 2008 decoding order SHOULD be the value of DON for the first, incremented by one. An example of the decapsulation process to recover the NAL unit decoding order is given in section 7. Informative note: Receivers should not expect that the absolute difference of values of DON for two consecutive NAL units in the NAL unit decoding order will be equal to one, even in error-free transmission. An increment by one is not required, as at the time of associating values of DON to NAL units, it may not be known whether all NAL units are delivered to the receiver. For example, a gateway may not forward coded slice NAL units of non- reference pictures or SEI NAL units when there is a shortage of bit rate in the network to which the packets are forwarded. In another example, a live broadcast is interrupted by pre-encoded content, such as commercials, from time to time. The first intra picture of a pre-encoded clip is transmitted in advance to ensure that it is readily available in the receiver. When transmitting the first intra picture, the originator does not exactly know how many NAL units will be encoded before the first intra picture of the pre-encoded clip follows in decoding order. Thus, the values of DON for the NAL units of the first intra picture of the pre- encoded clip have to be estimated when they are transmitted, and gaps in values of DON may occur. 5.6. Single NAL Unit Packet The single NAL unit packet defined here MUST contain only one NAL unit, of the types defined in [1]. This means that neither an aggregation packet nor a fragmentation unit can be used within a single NAL unit packet. A NAL unit stream composed by decapsulating single NAL unit packets in RTP sequence number order MUST conform to the NAL unit decoding order. The structure of the single NAL unit packet is shown in Figure 2. Informative note: The first byte of a NAL unit co-serves as the RTP payload header. Wang, et al Expires January 14, 2009 [Page 19] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|NRI| Type | | +-+-+-+-+-+-+-+-+ | | | | Bytes 2..n of a Single NAL unit | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2 RTP payload format for single NAL unit packet 5.7. Aggregation Packets Aggregation packets are the NAL unit aggregation scheme of this payload specification. The scheme is introduced to reflect the dramatically different MTU sizes of two key target networks: wireline IP networks (with an MTU size that is often limited by the Ethernet MTU size; roughly 1500 bytes), and IP or non-IP (e.g., ITU-T H.324/M) based wireless communication systems with preferred transmission unit sizes of 254 bytes or less. To prevent media transcoding between the two worlds, and to avoid undesirable packetization overhead, a NAL unit aggregation scheme is introduced. Two types of aggregation packets are defined by this specification: o Single-time aggregation packet (STAP): aggregates NAL units with identical NALU-time. Two types of STAPs are defined, one without DON (STAP-A) and another including DON (STAP-B). o Multi-time aggregation packet (MTAP): aggregates NAL units with potentially differing NALU-time. Two different MTAPs are defined, differing in the length of the NAL unit timestamp offset. Each NAL unit to be carried in an aggregation packet is encapsulated in an aggregation unit. Please see below for the four different aggregation units and their characteristics. The structure of the RTP payload format for aggregation packets is presented in Figure 3. Wang, et al Expires January 14, 2009 [Page 20] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|NRI| Type | | +-+-+-+-+-+-+-+-+ | | | | one or more aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3 RTP payload format for aggregation packets MTAPs and STAPs share the following packetization rules: The RTP timestamp MUST be set to the earliest of the NALU-times of all the NAL units to be aggregated. The type field of the NAL unit type octet MUST be set to the appropriate value, as indicated in Table 4. The F bit MUST be cleared if all F bits of the aggregated NAL units are zero; otherwise, it MUST be set. The value of NRI MUST be the maximum of all the NAL units carried in the aggregation packet. Table 4. Type field for STAPs and MTAPs Type Packet Timestamp offset DON related fields field length (DON, DONB, DOND) (in bits) present -------------------------------------------------------- 24 STAP-A 0 no 25 STAP-B 0 yes 26 MTAP16 16 yes 27 MTAP24 24 yes The marker bit in the RTP header is set to the value that the marker bit of the last NAL unit of the aggregated packet would have if it were transported in its own RTP packet. The payload of an aggregation packet consists of one or more aggregation units. See sections 5.7.1 and 5.7.2 for the four different types of aggregation units. An aggregation packet can carry as many aggregation units as necessary; however, the total amount of data in an aggregation packet obviously MUST fit into an IP packet, and the size SHOULD be chosen so that the resulting IP packet is smaller than the MTU size. An aggregation packet MUST NOT contain fragmentation units specified in section 5.8. Aggregation packets MUST NOT be nested; i.e., an aggregation packet MUST NOT contain another aggregation packet. Wang, et al Expires January 14, 2009 [Page 21] Internet-Draft RFC3984bis July 2008 5.7.1. Single-Time Aggregation Packet Single-time aggregation packet (STAP) SHOULD be used whenever NAL units are aggregated that all share the same NALU-time. The payload of an STAP-A does not include DON and consists of at least one single-time aggregation unit, as presented in Figure 4. The payload of an STAP-B consists of a 16-bit unsigned decoding order number (DON) (in network byte order) followed by at least one single-time aggregation unit, as presented in Figure 5. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : | +-+-+-+-+-+-+-+-+ | | | | single-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4 Payload format for STAP-A 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : decoding order number (DON) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | single-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5 Payload format for STAP-B The DON field specifies the value of DON for the first NAL unit in an STAP-B in transmission order. For each successive NAL unit in appearance order in an STAP-B, the value of DON is equal to (the value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in which '%' stands for the modulo operation. A single-time aggregation unit consists of 16-bit unsigned size information (in network byte order) that indicates the size of the following NAL unit in bytes (excluding these two octets, but Wang, et al Expires January 14, 2009 [Page 22] Internet-Draft RFC3984bis July 2008 including the NAL unit type octet of the NAL unit), followed by the NAL unit itself, including its NAL unit type byte. A single-time aggregation unit is byte aligned within the RTP payload, but it may not be aligned on a 32-bit word boundary. Figure 6 presents the structure of the single-time aggregation unit. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | NAL unit | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6 Structure for single-time aggregation unit Figure 7 presents an example of an RTP packet that contains an STAP- A. The STAP contains two single-time aggregation units, labeled as 1 and 2 in the figure. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAP-A NAL HDR | NALU 1 Size | NALU 1 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Data | : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 Size | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 Data | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7 An example of an RTP packet including an STAP-A containing two single-time aggregation units Wang, et al Expires January 14, 2009 [Page 23] Internet-Draft RFC3984bis July 2008 Figure 8 presents an example of an RTP packet that contains an STAP- B. The STAP contains two single-time aggregation units, labeled as 1 and 2 in the figure. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAP-B NAL HDR | DON | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 HDR | NALU 1 Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 Size | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 Data | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8 An example of an RTP packet including an STAP-B containing two single-time aggregation units 5.7.2. Multi-Time Aggregation Packets (MTAPs) The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding order number base (DONB) (in network byte order) and one or more multi-time aggregation units, as presented in Figure 9. DONB MUST contain the value of DON for the first NAL unit in the NAL unit decoding order among the NAL units of the MTAP. Informative note: The first NAL unit in the NAL unit decoding order is not necessarily the first NAL unit in the order in which the NAL units are encapsulated in an MTAP. Wang, et al Expires January 14, 2009 [Page 24] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : decoding order number base | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | multi-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9 NAL unit payload format for MTAPs Two different multi-time aggregation units are defined in this specification. Both of them consist of 16 bits unsigned size information of the following NAL unit (in network byte order), an 8- bit unsigned decoding order number difference (DOND), and n bits (in network byte order) of timestamp offset (TS offset) for this NAL unit, whereby n can be 16 or 24. The choice between the different MTAP types (MTAP16 and MTAP24) is application dependent: the larger the timestamp offset is, the higher the flexibility of the MTAP, but the overhead is also higher. The structure of the multi-time aggregation units for MTAP16 and MTAP24 are presented in Figures 10 and 11, respectively. The starting or ending position of an aggregation unit within a packet is NOT REQUIRED to be on a 32-bit word boundary. The DON of the NAL unit contained in a multi-time aggregation unit is equal to (DONB + DOND) % 65536, in which % denotes the modulo operation. This memo does not specify how the NAL units within an MTAP are ordered, but, in most cases, NAL unit decoding order SHOULD be used. The timestamp offset field MUST be set to a value equal to the value of the following formula: If the NALU-time is larger than or equal to the RTP timestamp of the packet, then the timestamp offset equals (the NALU-time of the NAL unit - the RTP timestamp of the packet). If the NALU-time is smaller than the RTP timestamp of the packet, then the timestamp offset is equal to the NALU-time + (2^32 - the RTP timestamp of the packet). Wang, et al Expires January 14, 2009 [Page 25] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | DOND | TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TS offset | | +-+-+-+-+-+-+-+-+ NAL unit | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10 Multi-time aggregation unit for MTAP16 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | DOND | TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TS offset | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NAL unit | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11 Multi-time aggregation unit for MTAP24 For the "earliest" multi-time aggregation unit in an MTAP the timestamp offset MUST be zero. Hence, the RTP timestamp of the MTAP itself is identical to the earliest NALU-time. Informative note: The "earliest" multi-time aggregation unit is the one that would have the smallest extended RTP timestamp among all the aggregation units of an MTAP if the NAL units contained in the aggregation units were encapsulated in single NAL unit packets. An extended timestamp is a timestamp that has more than 32 bits and is capable of counting the wraparound of the timestamp field, thus enabling one to determine the smallest value if the timestamp wraps. Such an "earliest" aggregation unit may not be the first one in the order in which the aggregation units are encapsulated in an MTAP. The "earliest" NAL unit need not be the same as the first NAL unit in the NAL unit decoding order either. Wang, et al Expires January 14, 2009 [Page 26] Internet-Draft RFC3984bis July 2008 Figure 12 presents an example of an RTP packet that contains a multi- time aggregation packet of type MTAP16 that contains two multi-time aggregation units, labeled as 1 and 2 in the figure. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MTAP16 NAL HDR | decoding order number base | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 DOND | NALU 1 TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 HDR | NALU 1 DATA | +-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 SIZE | NALU 2 DOND | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 TS offset | NALU 2 HDR | NALU 2 DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12 An RTP packet including a multi-time aggregation packet of type MTAP16 containing two multi-time aggregation units Figure 13 presents an example of an RTP packet that contains a multi- time aggregation packet of type MTAP24 that contains two multi-time aggregation units, labeled as 1 and 2 in the figure. Wang, et al Expires January 14, 2009 [Page 27] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MTAP24 NAL HDR | decoding order number base | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 DOND | NALU 1 TS offs | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |NALU 1 TS offs | NALU 1 HDR | NALU 1 DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 SIZE | NALU 2 DOND | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 TS offset | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 DATA | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13 An RTP packet including a multi-time aggregation packet of type MTAP24 containing two multi-time aggregation units 5.7.3. Fragmentation Units (FUs) This payload type allows fragmenting a NAL unit into several RTP packets. Doing so on the application layer instead of relying on lower layer fragmentation (e.g., by IP) has the following advantages: o The payload format is capable of transporting NAL units bigger than 64 kbytes over an IPv4 network that may be present in pre- recorded video, particularly in High Definition formats (there is a limit of the number of slices per picture, which results in a limit of NAL units per picture, which may result in big NAL units). o The fragmentation mechanism allows fragmenting a single NAL unit and applying generic forward error correction as described in section 12.5. Fragmentation is defined only for a single NAL unit and not for any aggregation packets. A fragment of a NAL unit consists of an integer number of consecutive octets of that NAL unit. Each octet of the NAL unit MUST be part of exactly one fragment of that NAL unit. Wang, et al Expires January 14, 2009 [Page 28] Internet-Draft RFC3984bis July 2008 Fragments of the same NAL unit MUST be sent in consecutive order with ascending RTP sequence numbers (with no other RTP packets within the same RTP packet stream being sent between the first and last fragment). Similarly, a NAL unit MUST be reassembled in RTP sequence number order. When a NAL unit is fragmented and conveyed within fragmentation units (FUs), it is referred to as a fragmented NAL unit. STAPs and MTAPs MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU MUST NOT contain another FU. The RTP timestamp of an RTP packet carrying an FU is set to the NALU- time of the fragmented NAL unit. Figure 14 presents the RTP payload format for FU-As. An FU-A consists of a fragmentation unit indicator of one octet, a fragmentation unit header of one octet, and a fragmentation unit payload. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FU indicator | FU header | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | FU payload | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14 RTP payload format for FU-A Figure 15 presents the RTP payload format for FU-Bs. An FU-B consists of a fragmentation unit indicator of one octet, a fragmentation unit header of one octet, a decoding order number (DON) (in network byte order), and a fragmentation unit payload. In other words, the structure of FU-B is the same as the structure of FU-A, except for the additional DON field. Wang, et al Expires January 14, 2009 [Page 29] Internet-Draft RFC3984bis July 2008 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FU indicator | FU header | DON | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | FU payload | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 15 RTP payload format for FU-B NAL unit type FU-B MUST be used in the interleaved packetization mode for the first fragmentation unit of a fragmented NAL unit. NAL unit type FU-B MUST NOT be used in any other case. In other words, in the interleaved packetization mode, each NALU that is fragmented has an FU-B as the first fragment, followed by one or more FU-A fragments. The FU indicator octet has the following format: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |F|NRI| Type | +---------------+ Values equal to 28 and 29 in the Type field of the FU indicator octet identify an FU-A and an FU-B, respectively. The use of the F bit is described in section 5.3. The value of the NRI field MUST be set according to the value of the NRI field in the fragmented NAL unit. The FU header has the following format: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |S|E|R| Type | +---------------+ S: 1 bit When set to one, the Start bit indicates the start of a fragmented NAL unit. When the following FU payload is not the start of a fragmented NAL unit payload, the Start bit is set to zero. Wang, et al Expires January 14, 2009 [Page 30] Internet-Draft RFC3984bis July 2008 E: 1 bit When set to one, the End bit indicates the end of a fragmented NAL unit, i.e., the last byte of the payload is also the last byte of the fragmented NAL unit. When the following FU payload is not the last fragment of a fragmented NAL unit, the End bit is set to zero. R: 1 bit The Reserved bit MUST be equal to 0 and MUST be ignored by the receiver. Type: 5 bits The NAL unit payload type as defined in table 7-1 of [1]. The value of DON in FU-Bs is selected as described in section 5.5. Informative note: The DON field in FU-Bs allows gateways to fragment NAL units to FU-Bs without organizing the incoming NAL units to the NAL unit decoding order. A fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the Start bit and End bit MUST NOT both be set to one in the same FU header. The FU payload consists of fragments of the payload of the fragmented NAL unit so that if the fragmentation unit payloads of consecutive FUs are sequentially concatenated, the payload of the fragmented NAL unit can be reconstructed. The NAL unit type octet of the fragmented NAL unit is not included as such in the fragmentation unit payload, but rather the information of the NAL unit type octet of the fragmented NAL unit is conveyed in F and NRI fields of the FU indicator octet of the fragmentation unit and in the type field of the FU header. An FU payload MAY have any number of octets and MAY be empty. Informative note: Empty FUs are allowed to reduce the latency of a certain class of senders in nearly lossless environments. These senders can be characterized in that they packetize NALU fragments before the NALU is completely generated and, hence, before the NALU size is known. If zero-length NALU fragments were not allowed, the sender would have to generate at least one bit of data of the following fragment before the current fragment could be sent. Due to the characteristics of H.264, where sometimes several macroblocks occupy zero bits, this is undesirable and can add delay. However, the (potential) use of zero-length NALU fragments should be carefully weighed against Wang, et al Expires January 14, 2009 [Page 31] Internet-Draft RFC3984bis July 2008 the increased risk of the loss of at least a part of the NALU because of the additional packets employed for its transmission. If a fragmentation unit is lost, the receiver SHOULD discard all following fragmentation units in transmission order corresponding to the same fragmented NAL unit. A receiver in an endpoint or in a MANE MAY aggregate the first n-1 fragments of a NAL unit to an (incomplete) NAL unit, even if fragment n of that NAL unit is not received. In this case, the forbidden_zero_bit of the NAL unit MUST be set to one to indicate a syntax violation. 6. Packetization Rules The packetization modes are introduced in section 5.2. The packetization rules common to more than one of the packetization modes are specified in section 6.1. The packetization rules for the single NAL unit mode, the non-interleaved mode, and the interleaved mode are specified in sections 6.2, 6.3, and 6.4, respectively. 6.1. Common Packetization Rules All senders MUST enforce the following packetization rules regardless of the packetization mode in use: o Coded slice NAL units or coded slice data partition NAL units belonging to the same coded picture (and thus sharing the same RTP timestamp value) MAY be sent in any order; however, for delay- critical systems, they SHOULD be sent in their original decoding order to minimize the delay. Note that the decoding order is the order of the NAL units in the bitstream. o Parameter sets are handled in accordance with the rules and recommendations given in section 8.4. o MANEs MUST NOT duplicate any NAL unit except for sequence or picture parameter set NAL units, as neither this memo nor the H.264 specification provides means to identify duplicated NAL units. Sequence and picture parameter set NAL units MAY be duplicated to make their correct reception more probable, but any such duplication MUST NOT affect the contents of any active sequence or picture parameter set. Duplication SHOULD be performed on the application layer and not by duplicating RTP packets (with identical sequence numbers). Wang, et al Expires January 14, 2009 [Page 32] Internet-Draft RFC3984bis July 2008 Senders using the non-interleaved mode and the interleaved mode MUST enforce the following packetization rule: o MANEs MAY convert single NAL unit packets into one aggregation packet, convert an aggregation packet into several single NAL unit packets, or mix both concepts, in an RTP translator. The RTP translator SHOULD take into account at least the following parameters: path MTU size, unequal protection mechanisms (e.g., through packet-based FEC according to RFC 2733 [18], especially for sequence and picture parameter set NAL units and coded slice data partition A NAL units), bearable latency of the system, and buffering capabilities of the receiver. Informative note: An RTP translator is required to handle RTCP as per RFC 3550. 6.2. Single NAL Unit Mode This mode is in use when the value of the OPTIONAL packetization-mode MIME parameter is equal to 0 or the packetization-mode is not present. All receivers MUST support this mode. It is primarily intended for low-delay applications that are compatible with systems using ITU-T Recommendation H.241 [15] (see section 12.1). Only single NAL unit packets MAY be used in this mode. STAPs, MTAPs, and FUs MUST NOT be used. The transmission order of single NAL unit packets MUST comply with the NAL unit decoding order. 6.3. Non-Interleaved Mode This mode is in use when the value of the OPTIONAL packetization-mode MIME parameter is equal to 1. This mode SHOULD be supported. It is primarily intended for low-delay applications. Only single NAL unit packets, STAP-As, and FU-As MAY be used in this mode. STAP-Bs, MTAPs, and FU-Bs MUST NOT be used. The transmission order of NAL units MUST comply with the NAL unit decoding order. 6.4. Interleaved Mode This mode is in use when the value of the OPTIONAL packetization-mode MIME parameter is equal to 2. Some receivers MAY support this mode. STAP-Bs, MTAPs, FU-As, and FU-Bs MAY be used. STAP-As and single NAL unit packets MUST NOT be used. The transmission order of packets and NAL units is constrained as specified in section 5.5. Wang, et al Expires January 14, 2009 [Page 33] Internet-Draft RFC3984bis July 2008 7. De-Packetization Process The de-packetization process is implementation dependent. Therefore, the following description should be seen as an example of a suitable implementation. Other schemes may be used as well as long as the output for the same input is the same as the process described below. The output is the same meaning that the number of NAL units and their order are both the identical. Optimizations relative to the described algorithms are likely possible. Section 7.1 presents the de-packetization process for the single NAL unit and non-interleaved packetization modes, whereas section 7.2 describes the process for the interleaved mode. Section 7.3 includes additional decapsulation guidelines for intelligent receivers. All normal RTP mechanisms related to buffer management apply. In particular, duplicated or outdated RTP packets (as indicated by the RTP sequences number and the RTP timestamp) are removed. To determine the exact time for decoding, factors such as a possible intentional delay to allow for proper inter-stream synchronization must be factored in. 7.1. Single NAL Unit and Non-Interleaved Mode The receiver includes a receiver buffer to compensate for transmission delay jitter. The receiver stores incoming packets in reception order into the receiver buffer. Packets are decapsulated in RTP sequence number order. If a decapsulated packet is a single NAL unit packet, the NAL unit contained in the packet is passed directly to the decoder. If a decapsulated packet is an STAP-A, the NAL units contained in the packet are passed to the decoder in the order in which they are encapsulated in the packet. For all the FU-A packets containing fragments of a single NAL unit, the decapsulated fragments are concatenated in their sending order to recover the NAL unit, which is then passed to the decoder. Informative note: If the decoder supports Arbitrary Slice Order, coded slices of a picture can be passed to the decoder in any order regardless of their reception and transmission order. 7.2. Interleaved Mode The general concept behind these de-packetization rules is to reorder NAL units from transmission order to the NAL unit decoding order. The receiver includes a receiver buffer, which is used to compensate for transmission delay jitter and to reorder NAL units from transmission order to the NAL unit decoding order. In this section, Wang, et al Expires January 14, 2009 [Page 34] Internet-Draft RFC3984bis July 2008 the receiver operation is described under the assumption that there is no transmission delay jitter. To make a difference from a practical receiver buffer that is also used for compensation of transmission delay jitter, the receiver buffer is here after called the deinterleaving buffer in this section. Receivers SHOULD also prepare for transmission delay jitter; i.e., either reserve separate buffers for transmission delay jitter buffering and deinterleaving buffering or use a receiver buffer for both transmission delay jitter and deinterleaving. Moreover, receivers SHOULD take transmission delay jitter into account in the buffering operation; e.g., by additional initial buffering before starting of decoding and playback. This section is organized as follows: subsection 7.2.1 presents how o calculate the size of the deinterleaving buffer. Subsection 7.2.2 specifies the receiver process how to organize received NAL units to the NAL unit decoding order. 7.2.1. Size of the Deinterleaving Buffer When SDP Offer/Answer model or any other capability exchange procedure is used in session setup, the properties of the received stream SHOULD be such that the receiver capabilities are not exceeded. In the SDP Offer/Answer model, the receiver can indicate its capabilities to allocate a deinterleaving buffer with the deint- buf-cap MIME parameter. The sender indicates the requirement for the deinterleaving buffer size with the sprop-deint-buf-req MIME parameter. It is therefore RECOMMENDED to set the deinterleaving buffer size, in terms of number of bytes, equal to or greater than the value of sprop-deint-buf-req MIME parameter. See section 8.1 for further information on deint-buf-cap and sprop-deint-buf-req MIME parameters and section 8.2.2 for further information on their use in SDP Offer/Answer model. When a declarative session description is used in session setup, the sprop-deint-buf-req MIME parameter signals the requirement for the deinterleaving buffer size. It is therefore RECOMMENDED to set the deinterleaving buffer size, in terms of number of bytes, equal to or greater than the value of sprop-deint-buf-req MIME parameter. 7.2.2. Deinterleaving Process There are two buffering states in the receiver: initial buffering and buffering while playing. Initial buffering occurs when the RTP session is initialized. After initial buffering, decoding and playback are started, and the buffering-while-playing mode is used. Wang, et al Expires January 14, 2009 [Page 35] Internet-Draft RFC3984bis July 2008 Regardless of the buffering state, the receiver stores incoming NAL units, in reception order, in the deinterleaving buffer as follows. NAL units of aggregation packets are stored in the deinterleaving buffer individually. The value of DON is calculated and stored for each NAL unit. The receiver operation is described below with the help of the following functions and constants: o Function AbsDON is specified in section 8.1. o Function don_diff is specified in section 5.5. o Constant N is the value of the OPTIONAL sprop-interleaving-depth MIME type parameter (see section 8.1) incremented by 1. Initial buffering lasts until one of the following conditions is fulfilled: o There are N or more VCL NAL units in the deinterleaving buffer. o If sprop-max-don-diff is present, don_diff(m,n) is greater than the value of sprop-max-don-diff, in which n corresponds to the NAL unit having the greatest value of AbsDON among the received NAL units and m corresponds to the NAL unit having the smallest value of AbsDON among the received NAL units. o Initial buffering has lasted for the duration equal to or greater than the value of the OPTIONAL sprop-init-buf-time MIME parameter. The NAL units to be removed from the deinterleaving buffer are determined as follows: o If the deinterleaving buffer contains at least N VCL NAL units, NAL units are removed from the deinterleaving buffer and passed to the decoder in the order specified below until the buffer contains N-1 VCL NAL units. o If sprop-max-don-diff is present, all NAL units m for which don_diff(m,n) is greater than sprop-max-don-diff are removed from the deinterleaving buffer and passed to the decoder in the order specified below. Herein, n corresponds to the NAL unit having the greatest value of AbsDON among the NAL units in the deinterleaving buffer. The order in which NAL units are passed to the decoder is specified as follows: Wang, et al Expires January 14, 2009 [Page 36] Internet-Draft RFC3984bis July 2008 o Let PDON be a variable that is initialized to 0 at the beginning of the RTP session. o For each NAL unit associated with a value of DON, a DON distance is calculated as follows. If the value of DON of the NAL unit is larger than the value of PDON, the DON distance is equal to DON - PDON. Otherwise, the DON distance is equal to 65535 - PDON + DON + 1. o NAL units are delivered to the decoder in ascending order of DON distance. If several NAL units share the same value of DON distance, they can be passed to the decoder in any order. o When a desired number of NAL units have been passed to the decoder, the value of PDON is set to the value of DON for the last NAL unit passed to the decoder. 7.3. Additional De-Packetization Guidelines The following additional de-packetization rules may be used to implement an operational H.264 de-packetizer: o Intelligent RTP receivers (e.g., in gateways) may identify lost coded slice data partitions A (DPAs). If a lost DPA is found, a gateway may decide not to send the corresponding coded slice data partitions B and C, as their information is meaningless for H.264 decoders. In this way a MANE can reduce network load by discarding useless packets without parsing a complex bitstream. o Intelligent RTP receivers (e.g., in gateways) may identify lost FUs. If a lost FU is found, a gateway may decide not to send the following FUs of the same fragmented NAL unit, as their information is meaningless for H.264 decoders. In this way a MANE can reduce network load by discarding useless packets without parsing a complex bitstream. o Intelligent receivers having to discard packets or NALUs should first discard all packets/NALUs in which the value of the NRI field of the NAL unit type octet is equal to 0. This will minimize the impact on user experience and keep the reference pictures intact. If more packets have to be discarded, then packets with a numerically lower NRI value should be discarded before packets with a numerically higher NRI value. However, discarding any packets with an NRI bigger than 0 very likely leads to decoder drift and SHOULD be avoided. Wang, et al Expires January 14, 2009 [Page 37] Internet-Draft RFC3984bis July 2008 8. Payload Format Parameters This section specifies the parameters that MAY be used to select optional features of the payload format and certain features of the bitstream. The parameters are specified here as part of the MIME subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec. A mapping of the parameters into the Session Description Protocol (SDP) [5] is also provided for applications that use SDP. Equivalent parameters could be defined elsewhere for use with control protocols that do not use MIME or SDP. Some parameters provide a receiver with the properties of the stream that will be sent. The names of all these parameters start with "sprop" for stream properties. Some of these "sprop" parameters are limited by other payload or codec configuration parameters. For example, the sprop-parameter-sets parameter is constrained by the profile-level-id parameter. The media sender selects all "sprop" parameters rather than the receiver. This uncommon characteristic of the "sprop" parameters may not be compatible with some signaling protocol concepts, in which case the use of these parameters SHOULD be avoided. 8.1. MIME Registration The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is allocated from the IETF tree. The receiver MUST ignore any unspecified parameter. Media Type name: video Media subtype name: H264 Required parameters: none OPTIONAL parameters: profile-level-id: A base16 [6] (hexadecimal) representation of the following three bytes in the sequence parameter set NAL unit specified in [1]: 1) profile_idc, 2) a byte herein referred to as profile-iop, composed of the values of constraint_set0_flag, constraint_set1_flag,constraint_set2_flag, and reserved_zero_5bits in bit-significance order, starting from the most significant bit, and 3) level_idc. Note that reserved_zero_5bits is required to be equal to 0 in [1], but Wang, et al Expires January 14, 2009 [Page 38] Internet-Draft RFC3984bis July 2008 other values for it may be specified in the future by ITU-T or ISO/IEC. If the profile-level-id parameter is used to indicate properties of a NAL unit stream, it indicates the profile and level that a decoder has to support in order to comply with [1] when it decodes the stream. The profile-iop byte indicates whether the NAL unit stream also obeys all constraints of the indicated profiles as follows. If bit 7 (the most significant bit), bit 6, or bit 5 of profile-iop is equal to 1, all constraints of the Baseline profile, the Main profile, or the Extended profile, respectively, are obeyed in the NAL unit stream. If the profile-level-id parameter is used for capability exchange or session setup procedure, it indicates the profile that the codec supports and the highest level supported for the signaled profile. The profile-iop byte indicates whether the codec has additional limitations whereby only the common subset of the algorithmic features and limitations of the profiles signaled with the profile-iop byte and of the profile indicated by profile_idc is supported by the codec. For example, if a codec supports only the common subset of the coding tools of the Baseline profile and the Main profile at level 2.1 and below, the profile-level-id becomes 42E015, in which 42 stands for the Baseline profile, E0 indicates that only the common subset for all profiles is supported, and 15 indicates level 2.1. Informative note: Capability exchange and session setup procedures should provide means to list the capabilities for each supported codec profile separately. For example, the one-of-N codec selection procedure of the SDP Offer/Answer model can be used (section 10.2 of [7]). If no profile-level-id is present, the Baseline Profile without additional constraints at Level 1 MUST be implied. max-mbps, max-fs, max-cpb, max-dpb, and max-br: These parameters MAY be used to signal the capabilities of a receiver implementation. These parameters MUST NOT be used for any other purpose. The profile-level-id parameter MUST be present in the same receiver capability description that contains any of these parameters. The level conveyed in the value of the profile-level-id parameter MUST be such that the receiver is fully capable of supporting. max-mbps, max-fs, max-cpb, max-dpb, and max-br MAY be used to indicate capabilities of the receiver that extend the required capabilities of the signaled level, as specified below. Wang, et al Expires January 14, 2009 [Page 39] Internet-Draft RFC3984bis July 2008 When more than one parameter from the set (max-mbps, max-fs, max-cpb, max-dpb, max-br) is present, the receiver MUST support all signaled capabilities simultaneously. For example, if both max-mbps and max-br are present, the signaled level with the extension of both the frame rate and bit rate is supported. That is, the receiver is able to decode NAL unit streams in which the macroblock processing rate is up to max-mbps (inclusive), the bit rate is up to max-br (inclusive), the coded picture buffer size is derived as specified in the semantics of the max-br parameter below, and other properties comply with the level specified in the value of the profile-level-id parameter. If a receiver can support all the properties of level A, the level specified in the value of the profile-level-id MUST be level A (i.e. MUST NOT be lower than level A). In other words, a sender or receiver MUST NOT signal values of max- mbps, max-fs, max-cpb, max-dpb, and max-br that meet the requirements of a higher level compared to the level specified in the value of the profile-level-id parameter. Informative note: When the OPTIONAL MIME type parameters are used to signal the properties of a NAL unit stream, max-mbps, max-fs, max-cpb, max-dpb, and max-br are not present, and the value of profile-level-id must always be such that the NAL unit stream complies fully with the specified profile and level. max-mbps: The value of max-mbps is an integer indicating the maximum macroblock processing rate in units of macroblocks per second. The max-mbps parameter signals that the receiver is capable of decoding video at a higher rate than is required by the signaled level conveyed in the value of the profile-level- id parameter. When max-mbps is signaled, the receiver MUST be able to decode NAL unit streams that conform to the signaled level, with the exception that the MaxMBPS value in Table A-1 of [1] for the signaled level is replaced with the value of max-mbps. The value of max-mbps MUST be greater than or equal to the value of MaxMBPS for the level given in Table A-1 of [1]. Senders MAY use this knowledge to send pictures of a given size at a higher picture rate than is indicated in the signaled level. max-fs: The value of max-fs is an integer indicating the maximum frame size in units of macroblocks. The max-fs parameter signals that the receiver is capable of decoding larger picture sizes than are required by the signaled level conveyed Wang, et al Expires January 14, 2009 [Page 40] Internet-Draft RFC3984bis July 2008 in the value of the profile-level-id parameter. When max-fs is signaled, the receiver MUST be able to decode NAL unit streams that conform to the signaled level, with the exception that the MaxFS value in Table A-1 of [1] for the signaled level is replaced with the value of max-fs. The value of max- fs MUST be greater than or equal to the value of MaxFS for the level given in Table A-1 of [1]. Senders MAY use this knowledge to send larger pictures at a proportionally lower frame rate than is indicated in the signaled level. max-cpb: The value of max-cpb is an integer indicating the maximum coded picture buffer size in units of 1000 bits for the VCL HRD parameters (see A.3.1 item i of [1]) and in units of 1200 bits for the NAL HRD parameters (see A.3.1 item j of [1]). The max-cpb parameter signals that the receiver has more memory than the minimum amount of coded picture buffer memory required by the signaled level conveyed in the value of the profile-level-id parameter. When max-cpb is signaled, the receiver MUST be able to decode NAL unit streams that conform to the signaled level, with the exception that the MaxCPB value in Table A-1 of [1] for the signaled level is replaced with the value of max-cpb. The value of max-cpb MUST be greater than or equal to the value of MaxCPB for the level given in Table A-1 of [1]. Senders MAY use this knowledge to construct coded video streams with greater variation of bit rate than can be achieved with the MaxCPB value in Table A-1 of [1]. Informative note: The coded picture buffer is used in the hypothetical reference decoder (Annex C) of H.264. The use of the hypothetical reference decoder is recommended in H.264 encoders to verify that the produced bitstream conforms to the standard and to control the output bitrate. Thus, the coded picture buffer is conceptually independent of any other potential buffers in the receiver, including de-interleaving and de-jitter buffers. The coded picture buffer need not be implemented in decoders as specified in Annex C of H.264, but rather standard-compliant decoders can have any buffering arrangements provided that they can decode standard-compliant bitstreams. Thus, in practice, the input buffer for video decoder can be integrated with de-interleaving and de-jitter buffers of the receiver. max-dpb: The value of max-dpb is an integer indicating the maximum decoded picture buffer size in units of 1024 bytes. The max-dpb parameter signals that the receiver has more memory than the minimum amount of decoded picture buffer Wang, et al Expires January 14, 2009 [Page 41] Internet-Draft RFC3984bis July 2008 memory required by the signaled level conveyed in the value of the profile-level-id parameter. When max-dpb is signaled, the receiver MUST be able to decode NAL unit streams that conform to the signaled level, with the exception that the MaxDPB value in Table A-1 of [1] for the signaled level is replaced with the value of max-dpb. Consequently, a receiver that signals max-dpb MUST be capable of storing the following number of decoded frames, complementary field pairs, and non- paired fields in its decoded picture buffer: Min(1024 * max-dpb / ( PicWidthInMbs * FrameHeightInMbs * 256 * ChromaFormatFactor ), 16) PicWidthInMbs, FrameHeightInMbs, and ChromaFormatFactor are defined in [1]. The value of max-dpb MUST be greater than or equal to the value of MaxDPB for the level given in Table A-1 of [1]. Senders MAY use this knowledge to construct coded video streams with improved compression. Informative note: This parameter was added primarily to complement a similar codepoint in the ITU-T Recommendation H.245, so as to facilitate signaling gateway designs. The decoded picture buffer stores reconstructed samples. There is no relationship between the size of the decoded picture buffer and the buffers used in RTP, especially de- interleaving and de-jitter buffers. max-br: The value of max-br is an integer indicating the maximum video bit rate in units of 1000 bits per second for the VCL HRD parameters (see A.3.1 item i of [1]) and in units of 1200 bits per second for the NAL HRD parameters (see A.3.1 item j of [1]). The max-br parameter signals that the video decoder of the receiver is capable of decoding video at a higher bit rate than is required by the signaled level conveyed in the value of the profile-level-id parameter. When max-br is signaled, the video codec of the receiver MUST be able to decode NAL unit streams that conform to the signaled level, conveyed in the profile-level-id parameter, with the following exceptions in the limits specified by the level: Wang, et al Expires January 14, 2009 [Page 42] Internet-Draft RFC3984bis July 2008 o The value of max-br replaces the MaxBR value of the signaled level (in Table A-1 of [1]). o When the max-cpb parameter is not present, the result of the following formula replaces the value of MaxCPB in Table A-1 of [1]: (MaxCPB of the signaled level) * max-br / (MaxBR of the signaled level). For example, if a receiver signals capability for Level 1.2 with max-br equal to 1550, this indicates a maximum video bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum video bitrate of 1860 kbits/sec for NAL HRD parameters, and a CPB size of 4036458 bits (1550000 / 384000 * 1000 * 1000). The value of max-br MUST be greater than or equal to the value MaxBR for the signaled level given in Table A-1 of [1]. Senders MAY use this knowledge to send higher bitrate video as allowed in the level definition of Annex A of H.264, to achieve improved video quality. Informative note: This parameter was added primarily to complement a similar codepoint in the ITU-T Recommendation H.245, so as to facilitate signaling gateway designs. No assumption can be made from the value of this parameter that the network is capable of handling such bit rates at any given time. In particular, no conclusion can be drawn that the signaled bit rate is possible under congestion control constraints. redundant-pic-cap: This parameter signals the capabilities of a receiver implementation. When equal to 0, the parameter indicates that the receiver makes no attempt to use redundant coded pictures to correct incorrectly decoded primary coded pictures. When equal to 0, the receiver is not capable of using redundant slices; therefore, a sender SHOULD avoid sending redundant slices to save bandwidth. When equal to 1, the receiver is capable of decoding any such redundant slice that covers a corrupted area in a primary decoded picture (at least partly), and therefore a sender MAY send redundant slices. When the parameter is not present, then a value of 0 MUST be used for redundant-pic-cap. When present, the value of redundant-pic- cap MUST be either 0 or 1. When the profile-level-id parameter is present in the same capability signaling as the redundant-pic-cap parameter, and Wang, et al Expires January 14, 2009 [Page 43] Internet-Draft RFC3984bis July 2008 the profile indicated in profile-level-id is such that it disallows the use of redundant coded pictures (e.g., Main Profile), the value of redundant-pic-cap MUST be equal to 0. When a receiver indicates redundant-pic-cap equal to 0, the received stream SHOULD NOT contain redundant coded pictures. Informative note: Even if redundant-pic-cap is equal to 0, the decoder is able to ignore redundant codec pictures provided that the decoder supports such a profile (Baseline, Extended) in which redundant coded pictures are allowed. Informative note: Even if redundant-pic-cap is equal to 1, the receiver may also choose other error concealment strategies to replace or complement decoding of redundant slices. sprop-parameter-sets: This parameter MAY be used to convey any sequence and picture parameter set NAL units (herein referred to as the initial parameter set NAL units) that MUST precede any other NAL units in decoding order. The parameter MUST NOT be used to indicate codec capability in any capability exchange procedure. The value of the parameter is the base64 [6] representation of the initial parameter set NAL units as specified in sections 7.3.2.1 and 7.3.2.2 of [1]. The parameter sets are conveyed in decoding order, and no framing of the parameter set NAL units takes place. A comma is used to separate any pair of parameter sets in the list. Note that the number of bytes in a parameter set NAL unit is typically less than 10, but a picture parameter set NAL unit can contain several hundreds of bytes. Informative note: When several payload types are offered in the SDP Offer/Answer model, each with its own sprop- parameter-sets parameter, then the receiver cannot assume that those parameter sets do not use conflicting storage locations (i.e., identical values of parameter set identifiers). Therefore, a receiver should double-buffer all sprop-parameter-sets and make them available to the decoder instance that decodes a certain payload type. packetization-mode: This parameter signals the properties of an RTP payload type or the capabilities of a receiver implementation. Only a single configuration point can be indicated; thus, when capabilities to support more than one packetization-mode are Wang, et al Expires January 14, 2009 [Page 44] Internet-Draft RFC3984bis July 2008 declared, multiple configuration points (RTP payload types) must be used. When the value of packetization-mode is equal to 0 or packetization-mode is not present, the single NAL mode, as defined in section 6.2 of RFC 3984, MUST be used. This mode is in use in standards using ITU-T Recommendation H.241 [15] (see section 12.1). When the value of packetization-mode is equal to 1, the non-interleaved mode, as defined in section 6.3 of RFC 3984, MUST be used. When the value of packetization-mode is equal to 2, the interleaved mode, as defined in section 6.4 of RFC 3984, MUST be used. The value of packetization-mode MUST be an integer in the range of 0 to 2, inclusive. sprop-interleaving-depth: This parameter MUST NOT be present when packetization-mode is not present or the value of packetization-mode is equal to 0 or 1. This parameter MUST be present when the value of packetization-mode is equal to 2. This parameter signals the properties of an RTP packet stream. It specifies the maximum number of VCL NAL units that precede any VCL NAL unit in the RTP packet stream in transmission order and follow the VCL NAL unit in decoding order. Consequently, it is guaranteed that receivers can reconstruct NAL unit decoding order when the buffer size for NAL unit decoding order recovery is at least the value of sprop- interleaving-depth + 1 in terms of VCL NAL units. The value of sprop-interleaving-depth MUST be an integer in the range of 0 to 32767, inclusive. sprop-deint-buf-req: This parameter MUST NOT be present when packetization-mode is not present or the value of packetization-mode is equal to 0 or 1. It MUST be present when the value of packetization-mode is equal to 2. sprop-deint-buf-req signals the required size of the deinterleaving buffer for the RTP packet stream. The value of the parameter MUST be greater than or equal to the maximum buffer occupancy (in units of bytes) required in such a deinterleaving buffer that is specified in section 7.2 of RFC 3984. It is guaranteed that receivers can perform the deinterleaving of interleaved NAL units into NAL unit decoding Wang, et al Expires January 14, 2009 [Page 45] Internet-Draft RFC3984bis July 2008 order, when the deinterleaving buffer size is at least the value of sprop-deint-buf-req in terms of bytes. The value of sprop-deint-buf-req MUST be an integer in the range of 0 to 4294967295, inclusive. Informative note: sprop-deint-buf-req indicates the required size of the deinterleaving buffer only. When network jitter can occur, an appropriately sized jitter buffer has to be provisioned for as well. deint-buf-cap: This parameter signals the capabilities of a receiver implementation and indicates the amount of deinterleaving buffer space in units of bytes that the receiver has available for reconstructing the NAL unit decoding order. A receiver is able to handle any stream for which the value of the sprop- deint-buf-req parameter is smaller than or equal to this parameter. If the parameter is not present, then a value of 0 MUST be used for deint-buf-cap. The value of deint-buf-cap MUST be an integer in the range of 0 to 4294967295, inclusive. Informative note: deint-buf-cap indicates the maximum possible size of the deinterleaving buffer of the receiver only. When network jitter can occur, an appropriately sized jitter buffer has to be provisioned for as well. sprop-init-buf-time: This parameter MAY be used to signal the properties of an RTP packet stream. The parameter MUST NOT be present, if the value of packetization-mode is equal to 0 or 1. The parameter signals the initial buffering time that a receiver MUST wait before starting decoding to recover the NAL unit decoding order from the transmission order. The parameter is the maximum value of (decoding time of the NAL unit - transmission time of a NAL unit), assuming reliable and instantaneous transmission, the same timeline for transmission and decoding, and that decoding starts when the first packet arrives. An example of specifying the value of sprop-init-buf-time follows. A NAL unit stream is sent in the following interleaved order, in which the value corresponds to the Wang, et al Expires January 14, 2009 [Page 46] Internet-Draft RFC3984bis July 2008 decoding time and the transmission order is from left to right: 0 2 1 3 5 4 6 8 7 ... Assuming a steady transmission rate of NAL units, the transmission times are: 0 1 2 3 4 5 6 7 8 ... Subtracting the decoding time from the transmission time column-wise results in the following series: 0 -1 1 0 -1 1 0 -1 1 ... Thus, in terms of intervals of NAL unit transmission times, the value of sprop-init-buf-time in this example is 1. The parameter is coded as a non-negative base10 integer representation in clock ticks of a 90-kHz clock. If the parameter is not present, then no initial buffering time value is defined. Otherwise the value of sprop-init-buf-time MUST be an integer in the range of 0 to 4294967295, inclusive. In addition to the signaled sprop-init-buf-time, receivers SHOULD take into account the transmission delay jitter buffering, including buffering for the delay jitter caused by mixers, translators, gateways, proxies, traffic-shapers, and other network elements. sprop-max-don-diff: This parameter MAY be used to signal the properties of an RTP packet stream. It MUST NOT be used to signal transmitter or receiver or codec capabilities. The parameter MUST NOT be present if the value of packetization-mode is equal to 0 or 1. sprop-max-don-diff is an integer in the range of 0 to 32767, inclusive. If sprop-max-don-diff is not present, the value of the parameter is unspecified. sprop-max-don-diff is calculated as follows: sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)}, for any i and any j>i, where i and j indicate the index of the NAL unit in the transmission order and AbsDON denotes a decoding order number of the NAL unit that does not wrap around to 0 after 65535. In other words, AbsDON is calculated as follows: Let m and n be consecutive NAL units in transmission order. For the very Wang, et al Expires January 14, 2009 [Page 47] Internet-Draft RFC3984bis July 2008 first NAL unit in transmission order (whose index is 0), AbsDON(0) = DON(0). For other NAL units, AbsDON is calculated as follows: If DON(m) == DON(n), AbsDON(n) = AbsDON(m) If (DON(m) < DON(n) and DON(n) - DON(m) < 32768), AbsDON(n) = AbsDON(m) + DON(n) - DON(m) If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768), AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n) If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768), AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n)) If (DON(m) > DON(n) and DON(m) - DON(n) < 32768), AbsDON(n) = AbsDON(m) - (DON(m) - DON(n)) where DON(i) is the decoding order number of the NAL unit having index i in the transmission order. The decoding order number is specified in section 5.5 of RFC 3984. Informative note: Receivers may use sprop-max-don-diff to trigger which NAL units in the receiver buffer can be passed to the decoder. max-rcmd-nalu-size: This parameter MAY be used to signal the capabilities of a receiver. The parameter MUST NOT be used for any other purposes. The value of the parameter indicates the largest NALU size in bytes that the receiver can handle efficiently. The parameter value is a recommendation, not a strict upper boundary. The sender MAY create larger NALUs but must be aware that the handling of these may come at a higher cost than NALUs conforming to the limitation. The value of max-rcmd-nalu-size MUST be an integer in the range of 0 to 4294967295, inclusive. If this parameter is not specified, no known limitation to the NALU size exists. Senders still have to consider the MTU size available between the sender and the receiver and SHOULD run MTU discovery for this purpose. This parameter is motivated by, for example, an IP to H.223 video telephony gateway, where NALUs smaller than the H.223 transport data unit will be more efficient. A gateway may Wang, et al Expires January 14, 2009 [Page 48] Internet-Draft RFC3984bis July 2008 terminate IP; thus, MTU discovery will normally not work beyond the gateway. Informative note: Setting this parameter to a lower than necessary value may have a negative impact. spatial-resolution: This parameter MAY be used to indicate the maximum spatial resolution of a NAL unit stream or the preferred spatial resolution of a receiver. The value is a base16 [6] (hexadecimal) representation of the width and height of the spatial resolution, in pixels, separated by a comma. Encoding considerations: This type is only defined for transfer via RTP (RFC 3550). Security considerations: See section 9 of RFC xxxx. Public specification: Please refer to RFC xxxx and its section 15. Additional information: None File extensions: none Macintosh file type code: none Object identifier or OID: none Person & email address to contact for further information: Ye-Kui Wang, ye-kui.wang@nokia.com Intended usage: COMMON Author: Ye-Kui Wang, ye-kui.wang@nokia.com Change controller: IETF Audio/Video Transport working group delegated from the IESG. Wang, et al Expires January 14, 2009 [Page 49] Internet-Draft RFC3984bis July 2008 8.2. SDP Parameters 8.2.1. Mapping of MIME Parameters to SDP The MIME media type video/H264 string is mapped to fields in the Session Description Protocol (SDP) [5] as follows: o The media name in the "m=" line of SDP MUST be video. o The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the MIME subtype). o The clock rate in the "a=rtpmap" line MUST be 90000. o The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs", "max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop- parameter-sets", "packetization-mode", "sprop-interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req", "sprop-init-buf-time", "sprop-max-don-diff", "max-rcmd-nalu-size", and "spatial- resolution", when present, MUST be included in the "a=fmtp" line of SDP. These parameters are expressed as a MIME media type string, in the form of a semicolon separated list of parameter=value pairs. An example of media representation in SDP is as follows (Baseline Profile, Level 3.0, some of the constraints of the Main profile may not be obeyed): m=video 49170 RTP/AVP 98 a=rtpmap:98 H264/90000 a=fmtp:98 profile-level-id=42A01E; packetization-mode=1; spatial-resolution=704,576; sprop-parameter-sets= 8.2.2. Usage with the SDP Offer/Answer Model When H.264 is offered over RTP using SDP in an Offer/Answer model [7] for negotiation for unicast usage, the following limitations and rules apply: Wang, et al Expires January 14, 2009 [Page 50] Internet-Draft RFC3984bis July 2008 o The parameters identifying a media format configuration for H.264 are "profile-level-id", "packetization-mode", and, if required by "packetization-mode", "sprop-deint-buf-req". These media format configuration parameters (except for the level part of "profile- level-id") MUST be used symmetrically; i.e., the answerer MUST either maintain all configuration parameters or remove the media format (payload type) completely, if one or more of the parameter values are not supported. For the level part of "profile-level- id", the answerer MUST maintain the same or a lower level or remove the media format (payload type) completely. Informative note: The requirement for symmetric use applies only for the above media format configuration parameters and not for the other stream properties and capability parameters, including the level part of "profile-level-id". Informative note: In H.264, all the levels except for level 1b are equal to the value of level_idc divided by 10. Level 1b is a level higher than level 1.0 but lower than level 1.1, and is signaled in an ad-hoc manner, due to that the level was specified after level 1.0 and level 1.1. For the Baseline, Main and Extended profiles (with profile_idc equal to 66, 77 and 88, respectively), level 1b is indicated by level_idc equal to 11 (i.e. same as level 1.1) and constraint_set3_flag equal to 1. For other profiles, level 1b is indicated by level_idc equal to 9 (but note that level 1b for these profiles are still higher than level 1, which has level_idc equal to 10, and lower than level 1.1). To simplify handling and matching of these configurations, the same RTP payload type number used in the offer SHOULD also be used in the answer, as specified in [7]. An answer MUST NOT contain a payload type number used in the offer unless the configuration ("profile-level-id", "packetization-mode", and, if present, "sprop-deint-buf-req") is the same as in the offer or the configuration only differs from that in the offer with a lower level indicated by "profile-level-id". Informative note: An offerer, when receiving the answer, has to compare payload types not declared in the offer based on media type (i.e., video/h264) and the above media format configuration parameters with any payload types it has already declared, in order to determine whether the configuration in question is new or equivalent to a configuration already offered. Wang, et al Expires January 14, 2009 [Page 51]