idnits 2.17.1 draft-ietf-avt-rtp-rfc3984bis-01.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** It looks like you're using RFC 3978 boilerplate. You should update this to the boilerplate described in the IETF Trust License Policy document (see https://trustee.ietf.org/license-info), which is required now. -- Found old boilerplate from RFC 3978, Section 5.1 on line 17. -- Found old boilerplate from RFC 3978, Section 5.5, updated by RFC 4748 on line 4235. -- Found old boilerplate from RFC 3979, Section 5, paragraph 1 on line 4212. -- Found old boilerplate from RFC 3979, Section 5, paragraph 2 on line 4219. -- Found old boilerplate from RFC 3979, Section 5, paragraph 3 on line 4225. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust Copyright Line does not match the current year == The document seems to use 'NOT RECOMMENDED' as an RFC 2119 keyword, but does not include the phrase in its RFC 2119 key words list. -- The exact meaning of the all-uppercase expression 'NOT REQUIRED' is not defined in RFC 2119. If it is intended as a requirements expression, it should be rewritten using one of the combinations defined in RFC 2119; otherwise it should not be all-uppercase. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (November 3, 2008) is 5653 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: '4' is defined on line 4081, but no explicit reference was found in the text -- Possible downref: Non-RFC (?) normative reference: ref. '1' -- Possible downref: Non-RFC (?) normative reference: ref. '2' -- Possible downref: Non-RFC (?) normative reference: ref. '3' ** Obsolete normative reference: RFC 2327 (ref. '6') (Obsoleted by RFC 4566) ** Obsolete normative reference: RFC 3548 (ref. '7') (Obsoleted by RFC 4648) -- Obsolete informational reference (is this intentional?): RFC 2429 (ref. '10') (Obsoleted by RFC 4629) -- Obsolete informational reference (is this intentional?): RFC 2733 (ref. '17') (Obsoleted by RFC 5109) -- Obsolete informational reference (is this intentional?): RFC 2326 (ref. '26') (Obsoleted by RFC 7826) -- Obsolete informational reference (is this intentional?): RFC 5117 (ref. '28') (Obsoleted by RFC 7667) Summary: 3 errors (**), 0 flaws (~~), 4 warnings (==), 15 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Audio/Video Transport WG Y.-K. Wang 2 Internet Draft Nokia 3 Intended status: Standards track R. Even 4 Expires: May 2009 Self-employed 5 T. Kristensen 6 Tandberg 7 November 3, 2008 9 RTP Payload Format for H.264 Video 10 draft-ietf-avt-rtp-rfc3984bis-01.txt 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html 35 This Internet-Draft will expire on May 3, 2009. 37 Copyright Notice 39 Copyright (C) The IETF Trust (2008). 41 Abstract 43 This memo describes an RTP Payload format for the ITU-T 44 Recommendation H.264 video codec and the technically identical 45 ISO/IEC International Standard 14496-10 video codec, excluding the 46 Scalable Video Coding (SVC) extension and the Multivew Video Coding 47 extension, for which the RTP payload formats are defined elsewhere. 49 The RTP payload format allows for packetization of one or more 50 Network Abstraction Layer Units (NALUs), produced by an H.264 video 51 encoder, in each RTP payload. The payload format has wide 52 applicability, as it supports applications from simple low bit-rate 53 conversational usage, to Internet video streaming with interleaved 54 transmission, to high bit-rate video-on-demand. 56 This memo intends to obsolete RFC 3984. Changes from RFC 3984 are 57 summarized in section 17. Issues on backward compatibility to RFC 58 3984 are discussed in section 16. 60 Table of Contents 62 1. Introduction...................................................4 63 1.1. The H.264 Codec...........................................4 64 1.2. Parameter Set Concept.....................................5 65 1.3. Network Abstraction Layer Unit Types......................6 66 2. Conventions....................................................7 67 3. Scope..........................................................7 68 4. Definitions and Abbreviations..................................7 69 4.1. Definitions...............................................7 70 4.2. Abbreviations.............................................9 71 5. RTP Payload Format............................................10 72 5.1. RTP Header Usage.........................................10 73 5.2. Payload Structures.......................................13 74 5.3. NAL Unit Header Usage....................................14 75 5.4. Packetization Modes......................................16 76 5.5. Decoding Order Number (DON)..............................17 77 5.6. Single NAL Unit Packet...................................20 78 5.7. Aggregation Packets......................................21 79 5.7.1. Single-Time Aggregation Packet......................23 80 5.7.2. Multi-Time Aggregation Packets (MTAPs)..............25 81 5.7.3. Fragmentation Units (FUs)...........................29 82 6. Packetization Rules...........................................33 83 6.1. Common Packetization Rules...............................33 84 6.2. Single NAL Unit Mode.....................................34 85 6.3. Non-Interleaved Mode.....................................34 86 6.4. Interleaved Mode.........................................34 87 7. De-Packetization Process......................................35 88 7.1. Single NAL Unit and Non-Interleaved Mode.................35 89 7.2. Interleaved Mode.........................................35 90 7.2.1. Size of the De-interleaving Buffer..................36 91 7.2.2. De-interleaving Process.............................36 92 7.3. Additional De-Packetization Guidelines...................38 93 8. Payload Format Parameters.....................................39 94 8.1. Media Type Registration..................................39 95 8.2. SDP Parameters...........................................55 96 8.2.1. Mapping of Payload Type Parameters to SDP...........55 97 8.2.2. Usage with the SDP Offer/Answer Model...............56 98 8.2.3. Usage in Declarative Session Descriptions...........64 99 8.3. Examples.................................................65 100 8.4. Parameter Set Considerations.............................70 101 8.5. Decoder Refresh Point Procedure using In-Band Transport of 102 Parameter Sets (Informative)..................................73 103 8.5.1. IDR Procedure to Respond to a Request for a Decoder 104 Refresh Point..............................................73 105 8.5.2. Gradual Recovery Procedure to Respond to a Request for a 106 Decoder Refresh Point......................................74 107 9. Security Considerations.......................................74 108 10. Congestion Control...........................................75 109 11. IANA Consideration...........................................76 110 12. Informative Appendix: Application Examples...................76 111 12.1. Video Telephony according to ITU-T Recommendation H.241 112 Annex A.......................................................76 113 12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit 114 Aggregation...................................................77 115 12.3. Video Telephony, Interleaved Packetization Using NAL Unit 116 Aggregation...................................................77 117 12.4. Video Telephony with Data Partitioning..................78 118 12.5. Video Telephony or Streaming with FUs and Forward Error 119 Correction....................................................78 120 12.6. Low Bit-Rate Streaming..................................81 121 12.7. Robust Packet Scheduling in Video Streaming.............81 122 13. Informative Appendix: Rationale for Decoding Order Number....82 123 13.1. Introduction............................................82 124 13.2. Example of Multi-Picture Slice Interleaving.............83 125 13.3. Example of Robust Packet Scheduling.....................84 126 13.4. Robust Transmission Scheduling of Redundant Coded Slices88 127 13.5. Remarks on Other Design Possibilities...................89 128 14. Acknowledgements.............................................89 129 15. References...................................................90 130 15.1. Normative References....................................90 131 15.2. Informative References..................................90 132 Authors' Addresses...............................................92 133 Intellectual Property Statement..................................93 134 Disclaimer of Validity...........................................93 135 Acknowledgement..................................................93 136 16. Backward Compatibility to RFC 3984...........................94 137 17. Changes from RFC 3984........................................95 138 18. Open issues..................................................96 140 1. Introduction 142 This memo intends to obsolete RFC 3984. Changes from RFC 3984 are 143 summarized in section 17. Issues on backward compatibility to RFC 144 3984 are discussed in section 16. 146 1.1. The H.264 Codec 148 This memo specifies an RTP payload specification for the video coding 149 standard known as ITU-T Recommendation H.264 [1] and ISO/IEC 150 International Standard 14496 Part 10 [2] (both also known as Advanced 151 Video Coding, or AVC). In this memo the H.264 acronym is used for 152 the codec and the standard, but the memo is equally applicable to the 153 ISO/IEC counterpart of the coding standard. 155 The H.264 video codec has a very broad application range that covers 156 all forms of digital compressed video from, low bit-rate Internet 157 streaming applications to HDTV broadcast and Digital Cinema 158 applications with nearly lossless coding. Compared to the current 159 state of technology, the overall performance of H.264 is such that 160 bit rate savings of 50% or more are reported. Digital Satellite TV 161 quality, for example, was reported to be achievable at 1.5 Mbit/s, 162 compared to the current operation point of MPEG 2 video at around 3.5 163 Mbit/s [9]. 165 The codec specification [1] itself distinguishes conceptually between 166 a video coding layer (VCL) and a network abstraction layer (NAL). 167 The VCL contains the signal processing functionality of the codec; 168 mechanisms such as transform, quantization, and motion compensated 169 prediction; and a loop filter. It follows the general concept of 170 most of today's video codecs, a macroblock-based coder that uses 171 inter picture prediction with motion compensation and transform 172 coding of the residual signal. The VCL encoder outputs slices: a bit 173 string that contains the macroblock data of an integer number of 174 macroblocks, and the information of the slice header (containing the 175 spatial address of the first macroblock in the slice, the initial 176 quantization parameter, and similar information). Macroblocks in 177 slices are arranged in scan order unless a different macroblock 178 allocation is specified, by using the so-called Flexible Macroblock 179 Ordering syntax. In-picture prediction is used only within a slice. 180 More information is provided in [9]. 182 The Network Abstraction Layer (NAL) encoder encapsulates the slice 183 output of the VCL encoder into Network Abstraction Layer Units (NAL 184 units), which are suitable for transmission over packet networks or 185 use in packet oriented multiplex environments. Annex B of H.264 186 defines an encapsulation process to transmit such NAL units over 187 byte-stream oriented networks. In the scope of this memo, Annex B is 188 not relevant. 190 Internally, the NAL uses NAL units. A NAL unit consists of a one- 191 byte header and the payload byte string. The header indicates the 192 type of the NAL unit, the (potential) presence of bit errors or 193 syntax violations in the NAL unit payload, and information regarding 194 the relative importance of the NAL unit for the decoding process. 195 This RTP payload specification is designed to be unaware of the bit 196 string in the NAL unit payload. 198 One of the main properties of H.264 is the complete decoupling of the 199 transmission time, the decoding time, and the sampling or 200 presentation time of slices and pictures. The decoding process 201 specified in H.264 is unaware of time, and the H.264 syntax does not 202 carry information such as the number of skipped frames (as is common 203 in the form of the Temporal Reference in earlier video compression 204 standards). Also, there are NAL units that affect many pictures and 205 that are, therefore, inherently timeless. For this reason, the 206 handling of the RTP timestamp requires some special considerations 207 for NAL units for which the sampling or presentation time is not 208 defined or, at transmission time, unknown. 210 1.2. Parameter Set Concept 212 One very fundamental design concept of H.264 is to generate self- 213 contained packets, to make mechanisms such as the header duplication 214 of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11] 215 unnecessary. This was achieved by decoupling information relevant to 216 more than one slice from the media stream. This higher layer meta 217 information should be sent reliably, asynchronously, and in advance 218 from the RTP packet stream that contains the slice packets. 219 (Provisions for sending this information in-band are also available 220 for applications that do not have an out-of-band transport channel 221 appropriate for the purpose.) The combination of the higher-level 222 parameters is called a parameter set. The H.264 specification 223 includes two types of parameter sets: sequence parameter set and 224 picture parameter set. An active sequence parameter set remains 225 unchanged throughout a coded video sequence, and an active picture 226 parameter set remains unchanged within a coded picture. The sequence 227 and picture parameter set structures contain information such as 228 picture size, optional coding modes employed, and macroblock to slice 229 group map. 231 To be able to change picture parameters (such as the picture size) 232 without having to transmit parameter set updates synchronously to the 233 slice packet stream, the encoder and decoder can maintain a list of 234 more than one sequence and picture parameter set. Each slice header 235 contains a codeword that indicates the sequence and picture parameter 236 set to be used. 238 This mechanism allows the decoupling of the transmission of parameter 239 sets from the packet stream, and the transmission of them by external 240 means (e.g., as a side effect of the capability exchange), or through 241 a (reliable or unreliable) control protocol. It may even be possible 242 that they are never transmitted but are fixed by an application 243 design specification. 245 1.3. Network Abstraction Layer Unit Types 247 Tutorial information on the NAL design can be found in [12], [13], 248 and [14]. 250 All NAL units consist of a single NAL unit type octet, which also co- 251 serves as the payload header of this RTP payload format. The payload 252 of a NAL unit follows immediately. 254 The syntax and semantics of the NAL unit type octet are specified in 255 [1], but the essential properties of the NAL unit type octet are 256 summarized below. The NAL unit type octet has the following format: 258 +---------------+ 259 |0|1|2|3|4|5|6|7| 260 +-+-+-+-+-+-+-+-+ 261 |F|NRI| Type | 262 +---------------+ 264 The semantics of the components of the NAL unit type octet, as 265 specified in the H.264 specification, are described briefly below. 267 F: 1 bit 268 forbidden_zero_bit. The H.264 specification declares a value of 269 1 as a syntax violation. 271 NRI: 2 bits 272 nal_ref_idc. A value of 00 indicates that the content of the NAL 273 unit is not used to reconstruct reference pictures for inter 274 picture prediction. Such NAL units can be discarded without 275 risking the integrity of the reference pictures. Values greater 276 than 00 indicate that the decoding of the NAL unit is required to 277 maintain the integrity of the reference pictures. 279 Type: 5 bits 280 nal_unit_type. This component specifies the NAL unit payload 281 type as defined in Table 7-1 of [1], and later within this memo. 282 For a reference of all currently defined NAL unit types and their 283 semantics, please refer to section 7.4.1 in [1]. 285 This memo introduces new NAL unit types, which are presented in 286 section 5.2. The NAL unit types defined in this memo are marked as 287 unspecified in [1]. Moreover, this specification extends the 288 semantics of F and NRI as described in section 5.3. 290 2. Conventions 292 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 293 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 294 document are to be interpreted as described in RFC-2119 [3]. 296 This specification uses the notion of setting and clearing a bit when 297 bit fields are handled. Setting a bit is the same as assigning that 298 bit the value of 1 (On). Clearing a bit is the same as assigning 299 that bit the value of 0 (Off). 301 3. Scope 303 This payload specification can only be used to carry the "naked" 304 H.264 NAL unit stream over RTP, and not the bitstream format 305 discussed in Annex B of H.264. Likely, the first applications of 306 this specification will be in the conversational multimedia field, 307 video telephony or video conferencing, but the payload format also 308 covers other applications, such as Internet streaming and TV over IP. 310 4. Definitions and Abbreviations 312 4.1. Definitions 314 This document uses the definitions of [1]. The following terms, 315 defined in [1], are summed up for convenience: 317 access unit: A set of NAL units always containing a primary coded 318 picture. In addition to the primary coded picture, an access 319 unit may also contain one or more redundant coded pictures or 320 other NAL units not containing slices or slice data partitions of 321 a coded picture. The decoding of an access unit always results 322 in a decoded picture. 324 coded video sequence: A sequence of access units that consists, 325 in decoding order, of an instantaneous decoding refresh (IDR) 326 access unit followed by zero or more non-IDR access units 327 including all subsequent access units up to but not including any 328 subsequent IDR access unit. 330 IDR access unit: An access unit in which the primary coded 331 picture is an IDR picture. 333 IDR picture: A coded picture containing only slices with I or SI 334 slice types that causes a "reset" in the decoding process. After 335 the decoding of an IDR picture, all following coded pictures in 336 decoding order can be decoded without inter prediction from any 337 picture decoded prior to the IDR picture. 339 primary coded picture: The coded representation of a picture to 340 be used by the decoding process for a bitstream conforming to 341 H.264. The primary coded picture contains all macroblocks of the 342 picture. 344 redundant coded picture: A coded representation of a picture or a 345 part of a picture. The content of a redundant coded picture 346 shall not be used by the decoding process for a bitstream 347 conforming to H.264. The content of a redundant coded picture 348 may be used by the decoding process for a bitstream that contains 349 errors or losses. 351 VCL NAL unit: A collective term used to refer to coded slice and 352 coded data partition NAL units. 354 In addition, the following definitions apply: 356 decoding order number (DON): A field in the payload structure, or 357 a derived variable indicating NAL unit decoding order. Values of 358 DON are in the range of 0 to 65535, inclusive. After reaching 359 the maximum value, the value of DON wraps around to 0. 361 NAL unit decoding order: A NAL unit order that conforms to the 362 constraints on NAL unit order given in section 7.4.1.2 in [1]. 364 NALU-time: The value that the RTP timestamp would have if the NAL 365 unit would be transported in its own RTP packet. 367 transmission order: The order of packets in ascending RTP 368 sequence number order (in modulo arithmetic). Within an 369 aggregation packet, the NAL unit transmission order is the same 370 as the order of appearance of NAL units in the packet. 372 media aware network element (MANE): A network element, such as a 373 middlebox or application layer gateway that is capable of parsing 374 certain aspects of the RTP payload headers or the RTP payload and 375 reacting to the contents. 377 Informative note: The concept of a MANE goes beyond normal 378 routers or gateways in that a MANE has to be aware of the 379 signaling (e.g., to learn about the payload type mappings of 380 the media streams), and in that it has to be trusted when 381 working with SRTP. The advantage of using MANEs is that they 382 allow packets to be dropped according to the needs of the 383 media coding. For example, if a MANE has to drop packets due 384 to congestion on a certain link, it can identify those packets 385 whose dropping has the smallest negative impact on the user 386 experience and remove them in order to remove the congestion 387 and/or keep the delay low. 389 static macroblock: A certain amount of macroblocks in the video 390 stream can be defined as static, as defined in section 8.3.2.8 in 391 [3]. Static macroblocks free up additional processing cycles for 392 the handling of non-static macroblocks. Based on a given amount 393 of video processing resources and a given resolution, a higher 394 number of static macroblocks enables a correspondingly higher 395 frame rate. 397 default sub-profile: The subset of coding tools, which may be all 398 coding tools of one profile or the common subset of coding tools 399 of more than one profile, indicated by the profile-level-id 400 parameter. In SDP Offer/Answer, the default sub-profile must be 401 used in a symmetric manner, i.e. the answer must either use the 402 same sub-profile as the offer or reject the offer. 404 default level: The level indicated by the profile-level-id 405 parameter. In SDP Offer/Answer, level is downgradable, i.e., the 406 answer may either use the default level or a lower level. 408 4.2. Abbreviations 410 DON: Decoding Order Number 411 DONB: Decoding Order Number Base 412 DOND: Decoding Order Number Difference 413 FEC: Forward Error Correction 414 FU: Fragmentation Unit 415 IDR: Instantaneous Decoding Refresh 416 IEC: International Electrotechnical Commission 417 ISO: International Organization for Standardization 418 ITU-T: International Telecommunication Union, 419 Telecommunication Standardization Sector 420 MANE: Media Aware Network Element 421 MTAP: Multi-Time Aggregation Packet 422 MTAP16: MTAP with 16-bit timestamp offset 423 MTAP24: MTAP with 24-bit timestamp offset 424 NAL: Network Abstraction Layer 425 NALU: NAL Unit 426 SAR: Sample Aspect Ratio 427 SEI: Supplemental Enhancement Information 428 STAP: Single-Time Aggregation Packet 429 STAP-A: STAP type A 430 STAP-B: STAP type B 431 TS: Timestamp 432 VCL: Video Coding Layer 433 VUI: Video Usability Information 435 5. RTP Payload Format 437 5.1. RTP Header Usage 439 The format of the RTP header is specified in RFC 3550 [5] and 440 reprinted in Figure 1 for convenience. This payload format uses the 441 fields of the header in a manner consistent with that specification. 443 When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP 444 payload format is specified in section 5.6. The RTP payload (and the 445 settings for some RTP header bits) for aggregation packets and 446 fragmentation units are specified in sections 5.7 and 5.8, 447 respectively. 449 0 1 2 3 450 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 1 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 |V=2|P|X| CC |M| PT | sequence number | 453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 454 | timestamp | 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | synchronization source (SSRC) identifier | 457 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 458 | contributing source (CSRC) identifiers | 459 | .... | 460 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 462 Figure 1 RTP header according to RFC 3550 464 The RTP header information to be set according to this RTP payload 465 format is set as follows: 467 Marker bit (M): 1 bit 468 Set for the very last packet of the access unit indicated by the 469 RTP timestamp, in line with the normal use of the M bit in video 470 formats, to allow an efficient playout buffer handling. For 471 aggregation packets (STAP and MTAP), the marker bit in the RTP 472 header MUST be set to the value that the marker bit of the last 473 NAL unit of the aggregation packet would have been if it were 474 transported in its own RTP packet. Decoders MAY use this bit as 475 an early indication of the last packet of an access unit, but 476 MUST NOT rely on this property. 478 Informative note: Only one M bit is associated with an 479 aggregation packet carrying multiple NAL units. Thus, if a 480 gateway has re-packetized an aggregation packet into several 481 packets, it cannot reliably set the M bit of those packets. 483 Payload type (PT): 7 bits 484 The assignment of an RTP payload type for this new packet format 485 is outside the scope of this document and will not be specified 486 here. The assignment of a payload type has to be performed 487 either through the profile used or in a dynamic way. 489 Sequence number (SN): 16 bits 490 Set and used in accordance with RFC 3550. For the single NALU 491 and non-interleaved packetization mode, the sequence number is 492 used to determine decoding order for the NALU. 494 Timestamp: 32 bits 495 The RTP timestamp is set to the sampling timestamp of the 496 content. A 90 kHz clock rate MUST be used. 498 If the NAL unit has no timing properties of its own (e.g., 499 parameter set and SEI NAL units), the RTP timestamp is set to the 500 RTP timestamp of the primary coded picture of the access unit in 501 which the NAL unit is included, according to section 7.4.1.2 of 502 [1]. 504 The setting of the RTP Timestamp for MTAPs is defined in section 505 5.7.2. 507 Receivers SHOULD ignore any picture timing SEI messages included 508 in access units that have only one display timestamp. Instead, 509 receivers SHOULD use the RTP timestamp for synchronizing the 510 display process. 512 RTP senders SHOULD NOT transmit picture timing SEI messages for 513 pictures that are not supposed to be displayed as multiple 514 fields. 516 If one access unit has more than one display timestamp carried in 517 a picture timing SEI message, then the information in the SEI 518 message SHOULD be treated as relative to the RTP timestamp, with 519 the earliest event occurring at the time given by the RTP 520 timestamp, and subsequent events later, as given by the 521 difference in SEI message picture timing values. Let tSEI1, 522 tSEI2, ..., tSEIn be the display timestamps carried in the SEI 523 message of an access unit, where tSEI1 is the earliest of all 524 such timestamps. Let tmadjst() be a function that adjusts the 525 SEI messages time scale to a 90-kHz time scale. Let TS be the 526 RTP timestamp. Then, the display time for the event associated 527 with tSEI1 is TS. The display time for the event with tSEIx, 528 where x is [2..n] is TS + tmadjst (tSEIx - tSEI1). 530 Informative note: Displaying coded frames as fields is needed 531 commonly in an operation known as 3:2 pulldown, in which film 532 content that consists of coded frames is displayed on a 533 display using interlaced scanning. The picture timing SEI 534 message enables carriage of multiple timestamps for the same 535 coded picture, and therefore the 3:2 pulldown process is 536 perfectly controlled. The picture timing SEI message 537 mechanism is necessary because only one timestamp per coded 538 frame can be conveyed in the RTP timestamp. 540 Informative note: Because H.264 allows the decoding order to 541 be different from the display order, values of RTP timestamps 542 may not be monotonically non-decreasing as a function of RTP 543 sequence numbers. Furthermore, the value for inter-arrival 544 jitter reported in the RTCP reports may not be a trustworthy 545 indication of the network performance, as the calculation 546 rules for inter-arrival jitter (section 6.4.1 of RFC 3550) 547 assume that the RTP timestamp of a packet is directly 548 proportional to its transmission time. 550 5.2. Payload Structures 552 The payload format defines three different basic payload structures. 553 A receiver can identify the payload structure by the first byte of 554 the RTP packet payload, which co-serves as the RTP payload header 555 and, in some cases, as the first byte of the payload. This byte is 556 always structured as a NAL unit header. The NAL unit type field 557 indicates which structure is present. The possible structures are as 558 follows: 560 Single NAL Unit Packet: Contains only a single NAL unit in the 561 payload. The NAL header type field will be equal to the original NAL 562 unit type; i.e., in the range of 1 to 23, inclusive. Specified in 563 section 5.6. 565 Aggregation Packet: Packet type used to aggregate multiple NAL units 566 into a single RTP payload. This packet exists in four versions, the 567 Single-Time Aggregation Packet type A (STAP-A), the Single-Time 568 Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet 569 (MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet 570 (MTAP) with 24-bit offset (MTAP24). The NAL unit type numbers 571 assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and 572 27, respectively. Specified in section 5.7. 574 Fragmentation Unit: Used to fragment a single NAL unit over multiple 575 RTP packets. Exists with two versions, FU-A and FU-B, identified 576 with the NAL unit type numbers 28 and 29, respectively. Specified in 577 section 5.8. 579 Informative note: This specification does not limit the size of 580 NAL units encapsulated in single NAL unit packets and 581 fragmentation units. The maximum size of a NAL unit encapsulated 582 in any aggregation packet is 65535 bytes. 584 Table 1 summarizes NAL unit types and the corresponding RTP packet 585 types when each of these NAL units is directly used a packet payload, 586 and where the types are described in this memo. 588 Table 1. Summary of NAL unit types and the corresponding packet 589 types 591 NAL Unit Packet Packet Type Name Section 592 Type Type 593 --------------------------------------------------------- 594 0 reserved - 595 1-23 NAL unit Single NAL unit packet 5.6 596 24 STAP-A Single-time aggregation packet 5.7.1 597 25 STAP-B Single-time aggregation packet 5.7.1 598 26 MTAP16 Multi-time aggregation packet 5.7.2 599 27 MTAP24 Multi-time aggregation packet 5.7.2 600 28 FU-A Fragmentation unit 5.8 601 29 FU-B Fragmentation unit 5.8 602 30-31 reserved - 604 5.3. NAL Unit Header Usage 606 The structure and semantics of the NAL unit header were introduced in 607 section 1.3. For convenience, the format of the NAL unit header is 608 reprinted below: 610 +---------------+ 611 |0|1|2|3|4|5|6|7| 612 +-+-+-+-+-+-+-+-+ 613 |F|NRI| Type | 614 +---------------+ 616 This section specifies the semantics of F and NRI according to this 617 specification. 619 F: 1 bit 620 forbidden_zero_bit. A value of 0 indicates that the NAL unit 621 type octet and payload should not contain bit errors or other 622 syntax violations. A value of 1 indicates that the NAL unit type 623 octet and payload may contain bit errors or other syntax 624 violations. 626 MANEs SHOULD set the F bit to indicate detected bit errors in the 627 NAL unit. The H.264 specification requires that the F bit is 628 equal to 0. When the F bit is set, the decoder is advised that 629 bit errors or any other syntax violations may be present in the 630 payload or in the NAL unit type octet. The simplest decoder 631 reaction to a NAL unit in which the F bit is equal to 1 is to 632 discard such a NAL unit and to conceal the lost data in the 633 discarded NAL unit. 635 NRI: 2 bits 636 nal_ref_idc. The semantics of value 00 and a non-zero value 637 remain unchanged from the H.264 specification. In other words, a 638 value of 00 indicates that the content of the NAL unit is not 639 used to reconstruct reference pictures for inter picture 640 prediction. Such NAL units can be discarded without risking the 641 integrity of the reference pictures. Values greater than 00 642 indicate that the decoding of the NAL unit is required to 643 maintain the integrity of the reference pictures. 645 In addition to the specification above, according to this RTP 646 payload specification, values of NRI indicate the relative 647 transport priority, as determined by the encoder. MANEs can use 648 this information to protect more important NAL units better than 649 they do less important NAL units. The highest transport priority 650 is 11, followed by 10, and then by 01; finally, 00 is the lowest. 652 Informative note: Any non-zero value of NRI is handled 653 identically in H.264 decoders. Therefore, receivers need not 654 manipulate the value of NRI when passing NAL units to the 655 decoder. 657 An H.264 encoder MUST set the value of NRI according to the H.264 658 specification (subclause 7.4.1) when the value of nal_unit_type 659 is in the range of 1 to 12, inclusive. In particular, the H.264 660 specification requires that the value of NRI SHALL be equal to 0 661 for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or 662 12. 664 For NAL units having nal_unit_type equal to 7 or 8 (indicating a 665 sequence parameter set or a picture parameter set, respectively), 666 an H.264 encoder SHOULD set the value of NRI to 11 (in binary 667 format). For coded slice NAL units of a primary coded picture 668 having nal_unit_type equal to 5 (indicating a coded slice 669 belonging to an IDR picture), an H.264 encoder SHOULD set the 670 value of NRI to 11 (in binary format). 672 For a mapping of the remaining nal_unit_types to NRI values, the 673 following example MAY be used and has been shown to be efficient 674 in a certain environment [13]. Other mappings MAY also be 675 desirable, depending on the application and the H.264/AVC Annex A 676 profile in use. 678 Informative note: Data Partitioning is not available in 679 certain profiles; e.g., in the Main or Baseline profiles. 680 Consequently, the NAL unit types 2, 3, and 4 can occur only if 681 the video bitstream conforms to a profile in which data 682 partitioning is allowed and not in streams that conform to the 683 Main or Baseline profiles. 685 Table 2. Example of NRI values for coded slices and coded slice data 686 partitions of primary coded reference pictures 688 NAL Unit Type Content of NAL unit NRI (binary) 689 ---------------------------------------------------------------- 690 1 non-IDR coded slice 10 691 2 Coded slice data partition A 10 692 3 Coded slice data partition B 01 693 4 Coded slice data partition C 01 695 Informative note: As mentioned before, the NRI value of non- 696 reference pictures is 00 as mandated by H.264/AVC. 698 An H.264 encoder SHOULD set the value of NRI for coded slice and 699 coded slice data partition NAL units of redundant coded reference 700 pictures equal to 01 (in binary format). 702 Definitions of the values for NRI for NAL unit types 24 to 29, 703 inclusive, are given in sections 5.7 and 5.8 of this memo. 705 No recommendation for the value of NRI is given for NAL units 706 having nal_unit_type in the range of 13 to 23, inclusive, because 707 these values are reserved for ITU-T and ISO/IEC. No 708 recommendation for the value of NRI is given for NAL units having 709 nal_unit_type equal to 0 or in the range of 30 to 31, inclusive, 710 as the semantics of these values are not specified in this memo. 712 5.4. Packetization Modes 714 This memo specifies three cases of packetization modes: 716 o Single NAL unit mode 718 o Non-interleaved mode 720 o Interleaved mode 722 The single NAL unit mode is targeted for conversational systems that 723 comply with ITU-T Recommendation H.241 [3] (see section 12.1). The 724 non-interleaved mode is targeted for conversational systems that may 725 not comply with ITU-T Recommendation H.241. In the non-interleaved 726 mode, NAL units are transmitted in NAL unit decoding order. The 727 interleaved mode is targeted for systems that do not require very low 728 end-to-end latency. The interleaved mode allows transmission of NAL 729 units out of NAL unit decoding order. 731 The packetization mode in use MAY be signaled by the value of the 732 OPTIONAL packetization-mode media type parameter. The used 733 packetization mode governs which NAL unit types are allowed in RTP 734 payloads. Table 3 summarizes the allowed packet payload types for 735 each packetization mode. Packetization modes are explained in more 736 detail in section 6. 738 Table 3. Summary of allowed NAL unit types for each packetization 739 mode (yes = allowed, no = disallowed, ig = ignore) 741 Payload Packet Single NAL Non-Interleaved Interleaved 742 Type Type Unit Mode Mode Mode 743 ------------------------------------------------------------- 744 0 reserved ig ig ig 745 1-23 NAL unit yes yes no 746 24 STAP-A no yes no 747 25 STAP-B no no yes 748 26 MTAP16 no no yes 749 27 MTAP24 no no yes 750 28 FU-A no yes yes 751 29 FU-B no no yes 752 30-31 reserved ig ig ig 754 Some NAL unit or payload type values (indicated as reserved in 755 Table 3) are reserved for future extensions. NAL units of those 756 types SHOULD NOT be sent by a sender (direct as packet payloads, or 757 as aggregation units in aggregation packets, or as fragmented units 758 in FU packets) and MUST be ignored by a receiver. For example, the 759 payload types 1-23, with the associated packet type "NAL unit", are 760 allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode", but 761 disallowed in "Interleaved Mode". However, NAL units of NAL unit 762 types 1-23 can be used in "Interleaved Mode" as aggregation units in 763 STAP-B, MTAP16 and MTAP14 packets as well as fragmented units in FU-A 764 and FU-B packets. Similarly, NAL units of NAL unit types 1-23 can 765 also be used in the "Non-Interleaved Mode" as aggregation units in 766 STAP-A packets or fragmented units in FU-A packets, in addition to 767 being directly used as packet payloads. 769 5.5. Decoding Order Number (DON) 771 In the interleaved packetization mode, the transmission order of NAL 772 units is allowed to differ from the decoding order of the NAL units. 773 Decoding order number (DON) is a field in the payload structure or a 774 derived variable that indicates the NAL unit decoding order. 776 Rationale and examples of use cases for transmission out of decoding 777 order and for the use of DON are given in section 13. 779 The coupling of transmission and decoding order is controlled by the 780 OPTIONAL sprop-interleaving-depth media type parameter as follows. 781 When the value of the OPTIONAL sprop-interleaving-depth media type 782 parameter is equal to 0 (explicitly or per default), the transmission 783 order of NAL units MUST conform to the NAL unit decoding order. When 784 the value of the OPTIONAL sprop-interleaving-depth media type 785 parameter is greater than 0, 787 o the order of NAL units in an MTAP16 and an MTAP24 is NOT REQUIRED 788 to be the NAL unit decoding order, and 790 o the order of NAL units generated by de-packetizing STAP-Bs, MTAPs, 791 and FUs in two consecutive packets is NOT REQUIRED to be the NAL 792 unit decoding order. 794 The RTP payload structures for a single NAL unit packet, an STAP-A, 795 and an FU-A do not include DON. STAP-B and FU-B structures include 796 DON, and the structure of MTAPs enables derivation of DON as 797 specified in section 5.7.2. 799 Informative note: When an FU-A occurs in interleaved mode, it 800 always follows an FU-B, which sets its DON. 802 Informative note: If a transmitter wants to encapsulate a single 803 NAL unit per packet and transmit packets out of their decoding 804 order, STAP-B packet type can be used. 806 In the single NAL unit packetization mode, the transmission order of 807 NAL units, determined by the RTP sequence number, MUST be the same as 808 their NAL unit decoding order. In the non-interleaved packetization 809 mode, the transmission order of NAL units in single NAL unit packets, 810 STAP-As, and FU-As MUST be the same as their NAL unit decoding order. 811 The NAL units within an STAP MUST appear in the NAL unit decoding 812 order. Thus, the decoding order is first provided through the 813 implicit order within a STAP, and second provided through the RTP 814 sequence number for the order between STAPs, FUs, and single NAL unit 815 packets. 817 Signaling of the value of DON for NAL units carried in STAP-B, MTAP, 818 and a series of fragmentation units starting with an FU-B is 819 specified in sections 5.7.1, 5.7.2, and 5.8, respectively. The DON 820 value of the first NAL unit in transmission order MAY be set to any 821 value. Values of DON are in the range of 0 to 65535, inclusive. 822 After reaching the maximum value, the value of DON wraps around to 0. 824 The decoding order of two NAL units contained in any STAP-B, MTAP, or 825 a series of fragmentation units starting with an FU-B is determined 826 as follows. Let DON(i) be the decoding order number of the NAL unit 827 having index i in the transmission order. Function don_diff(m,n) is 828 specified as follows: 830 If DON(m) == DON(n), don_diff(m,n) = 0 832 If (DON(m) < DON(n) and DON(n) - DON(m) < 32768), 833 don_diff(m,n) = DON(n) - DON(m) 835 If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768), 836 don_diff(m,n) = 65536 - DON(m) + DON(n) 838 If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768), 839 don_diff(m,n) = - (DON(m) + 65536 - DON(n)) 841 If (DON(m) > DON(n) and DON(m) - DON(n) < 32768), 842 don_diff(m,n) = - (DON(m) - DON(n)) 844 A positive value of don_diff(m,n) indicates that the NAL unit having 845 transmission order index n follows, in decoding order, the NAL unit 846 having transmission order index m. When don_diff(m,n) is equal to 0, 847 then the NAL unit decoding order of the two NAL units can be in 848 either order. A negative value of don_diff(m,n) indicates that the 849 NAL unit having transmission order index n precedes, in decoding 850 order, the NAL unit having transmission order index m. 852 Values of DON related fields (DON, DONB, and DOND; see section 5.7) 853 MUST be such that the decoding order determined by the values of DON, 854 as specified above, conforms to the NAL unit decoding order. If the 855 order of two NAL units in NAL unit decoding order is switched and the 856 new order does not conform to the NAL unit decoding order, the NAL 857 units MUST NOT have the same value of DON. If the order of two 858 consecutive NAL units in the NAL unit stream is switched and the new 859 order still conforms to the NAL unit decoding order, the NAL units 860 MAY have the same value of DON. For example, when arbitrary slice 861 order is allowed by the video coding profile in use, all the coded 862 slice NAL units of a coded picture are allowed to have the same value 863 of DON. Consequently, NAL units having the same value of DON can be 864 decoded in any order, and two NAL units having a different value of 865 DON should be passed to the decoder in the order specified above. 866 When two consecutive NAL units in the NAL unit decoding order have a 867 different value of DON, the value of DON for the second NAL unit in 868 decoding order SHOULD be the value of DON for the first, incremented 869 by one. 871 An example of the de-packetization process to recover the NAL unit 872 decoding order is given in section 7. 874 Informative note: Receivers should not expect that the absolute 875 difference of values of DON for two consecutive NAL units in the 876 NAL unit decoding order will be equal to one, even in error-free 877 transmission. An increment by one is not required, as at the 878 time of associating values of DON to NAL units, it may not be 879 known whether all NAL units are delivered to the receiver. For 880 example, a gateway may not forward coded slice NAL units of non- 881 reference pictures or SEI NAL units when there is a shortage of 882 bit rate in the network to which the packets are forwarded. In 883 another example, a live broadcast is interrupted by pre-encoded 884 content, such as commercials, from time to time. The first intra 885 picture of a pre-encoded clip is transmitted in advance to ensure 886 that it is readily available in the receiver. When transmitting 887 the first intra picture, the originator does not exactly know how 888 many NAL units will be encoded before the first intra picture of 889 the pre-encoded clip follows in decoding order. Thus, the values 890 of DON for the NAL units of the first intra picture of the pre- 891 encoded clip have to be estimated when they are transmitted, and 892 gaps in values of DON may occur. 894 5.6. Single NAL Unit Packet 896 The single NAL unit packet defined here MUST contain only one NAL 897 unit, of the types defined in [1]. This means that neither an 898 aggregation packet nor a fragmentation unit can be used within a 899 single NAL unit packet. A NAL unit stream composed by de-packetizing 900 single NAL unit packets in RTP sequence number order MUST conform to 901 the NAL unit decoding order. The structure of the single NAL unit 902 packet is shown in Figure 2. 904 Informative note: The first byte of a NAL unit co-serves as the 905 RTP payload header. 907 0 1 2 3 908 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 1 909 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 910 |F|NRI| Type | | 911 +-+-+-+-+-+-+-+-+ | 912 | | 913 | Bytes 2..n of a Single NAL unit | 914 | | 915 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 916 | :...OPTIONAL RTP padding | 917 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 919 Figure 2 RTP payload format for single NAL unit packet 921 5.7. Aggregation Packets 923 Aggregation packets are the NAL unit aggregation scheme of this 924 payload specification. The scheme is introduced to reflect the 925 dramatically different MTU sizes of two key target networks: wireline 926 IP networks (with an MTU size that is often limited by the Ethernet 927 MTU size; roughly 1500 bytes), and IP or non-IP (e.g., ITU-T H.324/M) 928 based wireless communication systems with preferred transmission unit 929 sizes of 254 bytes or less. To prevent media transcoding between the 930 two worlds, and to avoid undesirable packetization overhead, a NAL 931 unit aggregation scheme is introduced. 933 Two types of aggregation packets are defined by this specification: 935 o Single-time aggregation packet (STAP): aggregates NAL units with 936 identical NALU-time. Two types of STAPs are defined, one without 937 DON (STAP-A) and another including DON (STAP-B). 939 o Multi-time aggregation packet (MTAP): aggregates NAL units with 940 potentially differing NALU-time. Two different MTAPs are defined, 941 differing in the length of the NAL unit timestamp offset. 943 Each NAL unit to be carried in an aggregation packet is encapsulated 944 in an aggregation unit. Please see below for the four different 945 aggregation units and their characteristics. 947 The structure of the RTP payload format for aggregation packets is 948 presented in Figure 3. 950 0 1 2 3 951 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 1 952 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 953 |F|NRI| Type | | 954 +-+-+-+-+-+-+-+-+ | 955 | | 956 | one or more aggregation units | 957 | | 958 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 959 | :...OPTIONAL RTP padding | 960 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 962 Figure 3 RTP payload format for aggregation packets 964 MTAPs and STAPs share the following packetization rules: The RTP 965 timestamp MUST be set to the earliest of the NALU-times of all the 966 NAL units to be aggregated. The type field of the NAL unit type 967 octet MUST be set to the appropriate value, as indicated in Table 4. 968 The F bit MUST be cleared if all F bits of the aggregated NAL units 969 are zero; otherwise, it MUST be set. The value of NRI MUST be the 970 maximum of all the NAL units carried in the aggregation packet. 972 Table 4. Type field for STAPs and MTAPs 974 Type Packet Timestamp offset DON related fields 975 field length (DON, DONB, DOND) 976 (in bits) present 977 -------------------------------------------------------- 978 24 STAP-A 0 no 979 25 STAP-B 0 yes 980 26 MTAP16 16 yes 981 27 MTAP24 24 yes 983 The marker bit in the RTP header is set to the value that the marker 984 bit of the last NAL unit of the aggregated packet would have if it 985 were transported in its own RTP packet. 987 The payload of an aggregation packet consists of one or more 988 aggregation units. See sections 5.7.1 and 5.7.2 for the four 989 different types of aggregation units. An aggregation packet can 990 carry as many aggregation units as necessary; however, the total 991 amount of data in an aggregation packet obviously MUST fit into an IP 992 packet, and the size SHOULD be chosen so that the resulting IP packet 993 is smaller than the MTU size. An aggregation packet MUST NOT contain 994 fragmentation units specified in section 5.8. Aggregation packets 995 MUST NOT be nested; i.e., an aggregation packet MUST NOT contain 996 another aggregation packet. 998 5.7.1. Single-Time Aggregation Packet 1000 Single-time aggregation packet (STAP) SHOULD be used whenever NAL 1001 units are aggregated that all share the same NALU-time. The payload 1002 of an STAP-A does not include DON and consists of at least one 1003 single-time aggregation unit, as presented in Figure 4. The payload 1004 of an STAP-B consists of a 16-bit unsigned decoding order number 1005 (DON) (in network byte order) followed by at least one single-time 1006 aggregation unit, as presented in Figure 5. 1008 0 1 2 3 1009 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 1010 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1011 : | 1012 +-+-+-+-+-+-+-+-+ | 1013 | | 1014 | single-time aggregation units | 1015 | | 1016 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1017 | : 1018 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1020 Figure 4 Payload format for STAP-A 1022 0 1 2 3 1023 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 1024 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1025 : decoding order number (DON) | | 1026 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1027 | | 1028 | single-time aggregation units | 1029 | | 1030 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1031 | : 1032 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1034 Figure 5 Payload format for STAP-B 1036 The DON field specifies the value of DON for the first NAL unit in an 1037 STAP-B in transmission order. For each successive NAL unit in 1038 appearance order in an STAP-B, the value of DON is equal to (the 1039 value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in 1040 which '%' stands for the modulo operation. 1042 A single-time aggregation unit consists of 16-bit unsigned size 1043 information (in network byte order) that indicates the size of the 1044 following NAL unit in bytes (excluding these two octets, but 1045 including the NAL unit type octet of the NAL unit), followed by the 1046 NAL unit itself, including its NAL unit type byte. A single-time 1047 aggregation unit is byte aligned within the RTP payload, but it may 1048 not be aligned on a 32-bit word boundary. Figure 6 presents the 1049 structure of the single-time aggregation unit. 1051 0 1 2 3 1052 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 1053 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1054 : NAL unit size | | 1055 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1056 | | 1057 | NAL unit | 1058 | | 1059 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1060 | : 1061 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1063 Figure 6 Structure for single-time aggregation unit 1065 Figure 7 presents an example of an RTP packet that contains an STAP- 1066 A. The STAP contains two single-time aggregation units, labeled as 1 1067 and 2 in the figure. 1069 0 1 2 3 1070 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 1071 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1072 | RTP Header | 1073 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1074 |STAP-A NAL HDR | NALU 1 Size | NALU 1 HDR | 1075 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1076 | NALU 1 Data | 1077 : : 1078 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1079 | | NALU 2 Size | NALU 2 HDR | 1080 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1081 | NALU 2 Data | 1082 : : 1083 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1084 | :...OPTIONAL RTP padding | 1085 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1087 Figure 7 An example of an RTP packet including an STAP-A containing 1088 two single-time aggregation units 1090 Figure 8 presents an example of an RTP packet that contains an STAP- 1091 B. The STAP contains two single-time aggregation units, labeled as 1 1092 and 2 in the figure. 1094 0 1 2 3 1095 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 1096 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1097 | RTP Header | 1098 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1099 |STAP-B NAL HDR | DON | NALU 1 Size | 1100 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1101 | NALU 1 Size | NALU 1 HDR | NALU 1 Data | 1102 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1103 : : 1104 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1105 | | NALU 2 Size | NALU 2 HDR | 1106 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1107 | NALU 2 Data | 1108 : : 1109 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1110 | :...OPTIONAL RTP padding | 1111 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1113 Figure 8 An example of an RTP packet including an STAP-B containing 1114 two single-time aggregation units 1116 5.7.2. Multi-Time Aggregation Packets (MTAPs) 1118 The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding 1119 order number base (DONB) (in network byte order) and one or more 1120 multi-time aggregation units, as presented in Figure 9. DONB MUST 1121 contain the value of DON for the first NAL unit in the NAL unit 1122 decoding order among the NAL units of the MTAP. 1124 Informative note: The first NAL unit in the NAL unit decoding 1125 order is not necessarily the first NAL unit in the order in which 1126 the NAL units are encapsulated in an MTAP. 1128 0 1 2 3 1129 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 1130 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1131 : decoding order number base | | 1132 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1133 | | 1134 | multi-time aggregation units | 1135 | | 1136 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1137 | : 1138 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1140 Figure 9 NAL unit payload format for MTAPs 1142 Two different multi-time aggregation units are defined in this 1143 specification. Both of them consist of 16 bits unsigned size 1144 information of the following NAL unit (in network byte order), an 8- 1145 bit unsigned decoding order number difference (DOND), and n bits (in 1146 network byte order) of timestamp offset (TS offset) for this NAL 1147 unit, whereby n can be 16 or 24. The choice between the different 1148 MTAP types (MTAP16 and MTAP24) is application dependent: the larger 1149 the timestamp offset is, the higher the flexibility of the MTAP, but 1150 the overhead is also higher. 1152 The structure of the multi-time aggregation units for MTAP16 and 1153 MTAP24 are presented in Figures 10 and 11, respectively. The 1154 starting or ending position of an aggregation unit within a packet is 1155 NOT REQUIRED to be on a 32-bit word boundary. The DON of the NAL 1156 unit contained in a multi-time aggregation unit is equal to (DONB + 1157 DOND) % 65536, in which % denotes the modulo operation. This memo 1158 does not specify how the NAL units within an MTAP are ordered, but, 1159 in most cases, NAL unit decoding order SHOULD be used. 1161 The timestamp offset field MUST be set to a value equal to the value 1162 of the following formula: If the NALU-time is larger than or equal to 1163 the RTP timestamp of the packet, then the timestamp offset equals 1164 (the NALU-time of the NAL unit - the RTP timestamp of the packet). 1165 If the NALU-time is smaller than the RTP timestamp of the packet, 1166 then the timestamp offset is equal to the NALU-time + (2^32 - the RTP 1167 timestamp of the packet). 1169 0 1 2 3 1170 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 1171 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1172 : NAL unit size | DOND | TS offset | 1173 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1174 | TS offset | | 1175 +-+-+-+-+-+-+-+-+ NAL unit | 1176 | | 1177 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1178 | : 1179 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1181 Figure 10 Multi-time aggregation unit for MTAP16 1183 0 1 2 3 1184 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 1185 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1186 : NAL unit size | DOND | TS offset | 1187 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1188 | TS offset | | 1189 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1190 | NAL unit | 1191 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1192 | : 1193 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1195 Figure 11 Multi-time aggregation unit for MTAP24 1197 For the "earliest" multi-time aggregation unit in an MTAP the 1198 timestamp offset MUST be zero. Hence, the RTP timestamp of the MTAP 1199 itself is identical to the earliest NALU-time. 1201 Informative note: The "earliest" multi-time aggregation unit is 1202 the one that would have the smallest extended RTP timestamp among 1203 all the aggregation units of an MTAP if the NAL units contained 1204 in the aggregation units were encapsulated in single NAL unit 1205 packets. An extended timestamp is a timestamp that has more than 1206 32 bits and is capable of counting the wraparound of the 1207 timestamp field, thus enabling one to determine the smallest 1208 value if the timestamp wraps. Such an "earliest" aggregation 1209 unit may not be the first one in the order in which the 1210 aggregation units are encapsulated in an MTAP. The "earliest" 1211 NAL unit need not be the same as the first NAL unit in the NAL 1212 unit decoding order either. 1214 Figure 12 presents an example of an RTP packet that contains a multi- 1215 time aggregation packet of type MTAP16 that contains two multi-time 1216 aggregation units, labeled as 1 and 2 in the figure. 1218 0 1 2 3 1219 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 1220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1221 | RTP Header | 1222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1223 |MTAP16 NAL HDR | decoding order number base | NALU 1 Size | 1224 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1225 | NALU 1 Size | NALU 1 DOND | NALU 1 TS offset | 1226 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1227 | NALU 1 HDR | NALU 1 DATA | 1228 +-+-+-+-+-+-+-+-+ + 1229 : : 1230 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1231 | | NALU 2 SIZE | NALU 2 DOND | 1232 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1233 | NALU 2 TS offset | NALU 2 HDR | NALU 2 DATA | 1234 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1235 : : 1236 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1237 | :...OPTIONAL RTP padding | 1238 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1240 Figure 12 An RTP packet including a multi-time aggregation packet of 1241 type MTAP16 containing two multi-time aggregation units 1243 Figure 13 presents an example of an RTP packet that contains a multi- 1244 time aggregation packet of type MTAP24 that contains two multi-time 1245 aggregation units, labeled as 1 and 2 in the figure. 1247 0 1 2 3 1248 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 1249 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1250 | RTP Header | 1251 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1252 |MTAP24 NAL HDR | decoding order number base | NALU 1 Size | 1253 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1254 | NALU 1 Size | NALU 1 DOND | NALU 1 TS offs | 1255 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1256 |NALU 1 TS offs | NALU 1 HDR | NALU 1 DATA | 1257 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1258 : : 1259 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1260 | | NALU 2 SIZE | NALU 2 DOND | 1261 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1262 | NALU 2 TS offset | NALU 2 HDR | 1263 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1264 | NALU 2 DATA | 1265 : : 1266 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1267 | :...OPTIONAL RTP padding | 1268 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1270 Figure 13 An RTP packet including a multi-time aggregation packet of 1271 type MTAP24 containing two multi-time aggregation units 1273 5.7.3. Fragmentation Units (FUs) 1275 This payload type allows fragmenting a NAL unit into several RTP 1276 packets. Doing so on the application layer instead of relying on 1277 lower layer fragmentation (e.g., by IP) has the following advantages: 1279 o The payload format is capable of transporting NAL units bigger 1280 than 64 kbytes over an IPv4 network that may be present in pre- 1281 recorded video, particularly in High Definition formats (there is 1282 a limit of the number of slices per picture, which results in a 1283 limit of NAL units per picture, which may result in big NAL 1284 units). 1286 o The fragmentation mechanism allows fragmenting a single NAL unit 1287 and applying generic forward error correction as described in 1288 section 12.5. 1290 Fragmentation is defined only for a single NAL unit and not for any 1291 aggregation packets. A fragment of a NAL unit consists of an integer 1292 number of consecutive octets of that NAL unit. Each octet of the NAL 1293 unit MUST be part of exactly one fragment of that NAL unit. 1295 Fragments of the same NAL unit MUST be sent in consecutive order with 1296 ascending RTP sequence numbers (with no other RTP packets within the 1297 same RTP packet stream being sent between the first and last 1298 fragment). Similarly, a NAL unit MUST be reassembled in RTP sequence 1299 number order. 1301 When a NAL unit is fragmented and conveyed within fragmentation units 1302 (FUs), it is referred to as a fragmented NAL unit. STAPs and MTAPs 1303 MUST NOT be fragmented. FUs MUST NOT be nested; i.e., an FU MUST NOT 1304 contain another FU. 1306 The RTP timestamp of an RTP packet carrying an FU is set to the NALU- 1307 time of the fragmented NAL unit. 1309 Figure 14 presents the RTP payload format for FU-As. An FU-A 1310 consists of a fragmentation unit indicator of one octet, a 1311 fragmentation unit header of one octet, and a fragmentation unit 1312 payload. 1314 0 1 2 3 1315 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 1316 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1317 | FU indicator | FU header | | 1318 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1319 | | 1320 | FU payload | 1321 | | 1322 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1323 | :...OPTIONAL RTP padding | 1324 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1326 Figure 14 RTP payload format for FU-A 1328 Figure 15 presents the RTP payload format for FU-Bs. An FU-B 1329 consists of a fragmentation unit indicator of one octet, a 1330 fragmentation unit header of one octet, a decoding order number (DON) 1331 (in network byte order), and a fragmentation unit payload. In other 1332 words, the structure of FU-B is the same as the structure of FU-A, 1333 except for the additional DON field. 1335 0 1 2 3 1336 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 1337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1338 | FU indicator | FU header | DON | 1339 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 1340 | | 1341 | FU payload | 1342 | | 1343 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1344 | :...OPTIONAL RTP padding | 1345 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1347 Figure 15 RTP payload format for FU-B 1349 NAL unit type FU-B MUST be used in the interleaved packetization mode 1350 for the first fragmentation unit of a fragmented NAL unit. NAL unit 1351 type FU-B MUST NOT be used in any other case. In other words, in the 1352 interleaved packetization mode, each NALU that is fragmented has an 1353 FU-B as the first fragment, followed by one or more FU-A fragments. 1355 The FU indicator octet has the following format: 1357 +---------------+ 1358 |0|1|2|3|4|5|6|7| 1359 +-+-+-+-+-+-+-+-+ 1360 |F|NRI| Type | 1361 +---------------+ 1363 Values equal to 28 and 29 in the Type field of the FU indicator octet 1364 identify an FU-A and an FU-B, respectively. The use of the F bit is 1365 described in section 5.3. The value of the NRI field MUST be set 1366 according to the value of the NRI field in the fragmented NAL unit. 1368 The FU header has the following format: 1370 +---------------+ 1371 |0|1|2|3|4|5|6|7| 1372 +-+-+-+-+-+-+-+-+ 1373 |S|E|R| Type | 1374 +---------------+ 1376 S: 1 bit 1377 When set to one, the Start bit indicates the start of a 1378 fragmented NAL unit. When the following FU payload is not the 1379 start of a fragmented NAL unit payload, the Start bit is set to 1380 zero. 1382 E: 1 bit 1383 When set to one, the End bit indicates the end of a fragmented 1384 NAL unit, i.e., the last byte of the payload is also the last 1385 byte of the fragmented NAL unit. When the following FU payload 1386 is not the last fragment of a fragmented NAL unit, the End bit is 1387 set to zero. 1389 R: 1 bit 1390 The Reserved bit MUST be equal to 0 and MUST be ignored by the 1391 receiver. 1393 Type: 5 bits 1394 The NAL unit payload type as defined in Table 7-1 of [1]. 1396 The value of DON in FU-Bs is selected as described in section 5.5. 1398 Informative note: The DON field in FU-Bs allows gateways to 1399 fragment NAL units to FU-Bs without organizing the incoming NAL 1400 units to the NAL unit decoding order. 1402 A fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the 1403 Start bit and End bit MUST NOT both be set to one in the same FU 1404 header. 1406 The FU payload consists of fragments of the payload of the fragmented 1407 NAL unit so that if the fragmentation unit payloads of consecutive 1408 FUs are sequentially concatenated, the payload of the fragmented NAL 1409 unit can be reconstructed. The NAL unit type octet of the fragmented 1410 NAL unit is not included as such in the fragmentation unit payload, 1411 but rather the information of the NAL unit type octet of the 1412 fragmented NAL unit is conveyed in F and NRI fields of the FU 1413 indicator octet of the fragmentation unit and in the type field of 1414 the FU header. An FU payload MAY have any number of octets and MAY 1415 be empty. 1417 Informative note: Empty FUs are allowed to reduce the latency of 1418 a certain class of senders in nearly lossless environments. 1419 These senders can be characterized in that they packetize NALU 1420 fragments before the NALU is completely generated and, hence, 1421 before the NALU size is known. If zero-length NALU fragments 1422 were not allowed, the sender would have to generate at least one 1423 bit of data of the following fragment before the current fragment 1424 could be sent. Due to the characteristics of H.264, where 1425 sometimes several macroblocks occupy zero bits, this is 1426 undesirable and can add delay. However, the (potential) use of 1427 zero-length NALU fragments should be carefully weighed against 1428 the increased risk of the loss of at least a part of the NALU 1429 because of the additional packets employed for its transmission. 1431 If a fragmentation unit is lost, the receiver SHOULD discard all 1432 following fragmentation units in transmission order corresponding to 1433 the same fragmented NAL unit. 1435 A receiver in an endpoint or in a MANE MAY aggregate the first n-1 1436 fragments of a NAL unit to an (incomplete) NAL unit, even if fragment 1437 n of that NAL unit is not received. In this case, the 1438 forbidden_zero_bit of the NAL unit MUST be set to one to indicate a 1439 syntax violation. 1441 6. Packetization Rules 1443 The packetization modes are introduced in section 5.2. The 1444 packetization rules common to more than one of the packetization 1445 modes are specified in section 6.1. The packetization rules for the 1446 single NAL unit mode, the non-interleaved mode, and the interleaved 1447 mode are specified in sections 6.2, 6.3, and 6.4, respectively. 1449 6.1. Common Packetization Rules 1451 All senders MUST enforce the following packetization rules regardless 1452 of the packetization mode in use: 1454 o Coded slice NAL units or coded slice data partition NAL units 1455 belonging to the same coded picture (and thus sharing the same RTP 1456 timestamp value) MAY be sent in any order; however, for delay- 1457 critical systems, they SHOULD be sent in their original decoding 1458 order to minimize the delay. Note that the decoding order is the 1459 order of the NAL units in the bitstream. 1461 o Parameter sets are handled in accordance with the rules and 1462 recommendations given in section 8.4. 1464 o MANEs MUST NOT duplicate any NAL unit except for sequence or 1465 picture parameter set NAL units, as neither this memo nor the 1466 H.264 specification provides means to identify duplicated NAL 1467 units. Sequence and picture parameter set NAL units MAY be 1468 duplicated to make their correct reception more probable, but any 1469 such duplication MUST NOT affect the contents of any active 1470 sequence or picture parameter set. Duplication SHOULD be 1471 performed on the application layer and not by duplicating RTP 1472 packets (with identical sequence numbers). 1474 Senders using the non-interleaved mode and the interleaved mode MUST 1475 enforce the following packetization rule: 1477 o MANEs MAY convert single NAL unit packets into one aggregation 1478 packet, convert an aggregation packet into several single NAL unit 1479 packets, or mix both concepts, in an RTP translator. The RTP 1480 translator SHOULD take into account at least the following 1481 parameters: path MTU size, unequal protection mechanisms (e.g., 1482 through packet-based FEC according to RFC 2733 [17], especially 1483 for sequence and picture parameter set NAL units and coded slice 1484 data partition A NAL units), bearable latency of the system, and 1485 buffering capabilities of the receiver. 1487 Informative note: An RTP translator is required to handle RTCP 1488 as per RFC 3550. 1490 6.2. Single NAL Unit Mode 1492 This mode is in use when the value of the OPTIONAL packetization-mode 1493 media type parameter is equal to 0 or the packetization-mode is not 1494 present. All receivers MUST support this mode. It is primarily 1495 intended for low-delay applications that are compatible with systems 1496 using ITU-T Recommendation H.241 [3] (see section 12.1). Only single 1497 NAL unit packets MAY be used in this mode. STAPs, MTAPs, and FUs 1498 MUST NOT be used. The transmission order of single NAL unit packets 1499 MUST comply with the NAL unit decoding order. 1501 6.3. Non-Interleaved Mode 1503 This mode is in use when the value of the OPTIONAL packetization-mode 1504 media type parameter is equal to 1. This mode SHOULD be supported. 1505 It is primarily intended for low-delay applications. Only single NAL 1506 unit packets, STAP-As, and FU-As MAY be used in this mode. STAP-Bs, 1507 MTAPs, and FU-Bs MUST NOT be used. The transmission order of NAL 1508 units MUST comply with the NAL unit decoding order. 1510 6.4. Interleaved Mode 1512 This mode is in use when the value of the OPTIONAL packetization-mode 1513 media type parameter is equal to 2. Some receivers MAY support this 1514 mode. STAP-Bs, MTAPs, FU-As, and FU-Bs MAY be used. STAP-As and 1515 single NAL unit packets MUST NOT be used. The transmission order of 1516 packets and NAL units is constrained as specified in section 5.5. 1518 7. De-Packetization Process 1520 The de-packetization process is implementation dependent. Therefore, 1521 the following description should be seen as an example of a suitable 1522 implementation. Other schemes may be used as well as long as the 1523 output for the same input is the same as the process described below. 1524 The output is the same meaning that the number of NAL units and their 1525 order are both the identical. Optimizations relative to the 1526 described algorithms are likely possible. Section 7.1 presents the 1527 de-packetization process for the single NAL unit and non-interleaved 1528 packetization modes, whereas section 7.2 describes the process for 1529 the interleaved mode. Section 7.3 includes additional de- 1530 packetization guidelines for intelligent receivers. 1532 All normal RTP mechanisms related to buffer management apply. In 1533 particular, duplicated or outdated RTP packets (as indicated by the 1534 RTP sequences number and the RTP timestamp) are removed. To 1535 determine the exact time for decoding, factors such as a possible 1536 intentional delay to allow for proper inter-stream synchronization 1537 must be factored in. 1539 7.1. Single NAL Unit and Non-Interleaved Mode 1541 The receiver includes a receiver buffer to compensate for 1542 transmission delay jitter. The receiver stores incoming packets in 1543 reception order into the receiver buffer. Packets are de-packetized 1544 in RTP sequence number order. If a de-packetized packet is a single 1545 NAL unit packet, the NAL unit contained in the packet is passed 1546 directly to the decoder. If a de-packetized packet is an STAP-A, the 1547 NAL units contained in the packet are passed to the decoder in the 1548 order in which they are encapsulated in the packet. For all the FU-A 1549 packets containing fragments of a single NAL unit, the de-packetized 1550 fragments are concatenated in their sending order to recover the NAL 1551 unit, which is then passed to the decoder. 1553 Informative note: If the decoder supports Arbitrary Slice Order, 1554 coded slices of a picture can be passed to the decoder in any 1555 order regardless of their reception and transmission order. 1557 7.2. Interleaved Mode 1559 The general concept behind these de-packetization rules is to reorder 1560 NAL units from transmission order to the NAL unit decoding order. 1562 The receiver includes a receiver buffer, which is used to compensate 1563 for transmission delay jitter and to reorder NAL units from 1564 transmission order to the NAL unit decoding order. In this section, 1565 the receiver operation is described under the assumption that there 1566 is no transmission delay jitter. To make a difference from a 1567 practical receiver buffer that is also used for compensation of 1568 transmission delay jitter, the receiver buffer is here after called 1569 the de-interleaving buffer in this section. Receivers SHOULD also 1570 prepare for transmission delay jitter; i.e., either reserve separate 1571 buffers for transmission delay jitter buffering and de-interleaving 1572 buffering or use a receiver buffer for both transmission delay jitter 1573 and de-interleaving. Moreover, receivers SHOULD take transmission 1574 delay jitter into account in the buffering operation; e.g., by 1575 additional initial buffering before starting of decoding and 1576 playback. 1578 This section is organized as follows: subsection 7.2.1 presents how o 1579 calculate the size of the de-interleaving buffer. Subsection 7.2.2 1580 specifies the receiver process how to organize received NAL units to 1581 the NAL unit decoding order. 1583 7.2.1. Size of the De-interleaving Buffer 1585 When the SDP Offer/Answer model or any other capability exchange 1586 procedure is used in session setup, the properties of the received 1587 stream SHOULD be such that the receiver capabilities are not 1588 exceeded. In the SDP Offer/Answer model, the receiver can indicate 1589 its capabilities to allocate a de-interleaving buffer with the deint- 1590 buf-cap media type parameter. The sender indicates the requirement 1591 for the de-interleaving buffer size with the sprop-deint-buf-req 1592 media type parameter. It is therefore RECOMMENDED to set the de- 1593 interleaving buffer size, in terms of number of bytes, equal to or 1594 greater than the value of sprop-deint-buf-req media type parameter. 1595 See section 8.1 for further information on deint-buf-cap and sprop- 1596 deint-buf-req media type parameters and section 8.2.2 for further 1597 information on their use in the SDP Offer/Answer model. 1599 When a declarative session description is used in session setup, the 1600 sprop-deint-buf-req media type parameter signals the requirement for 1601 the de-interleaving buffer size. It is therefore RECOMMENDED to set 1602 the de-interleaving buffer size, in terms of number of bytes, equal 1603 to or greater than the value of sprop-deint-buf-req media type 1604 parameter. 1606 7.2.2. De-interleaving Process 1608 There are two buffering states in the receiver: initial buffering and 1609 buffering while playing. Initial buffering occurs when the RTP 1610 session is initialized. After initial buffering, decoding and 1611 playback are started, and the buffering-while-playing mode is used. 1613 Regardless of the buffering state, the receiver stores incoming NAL 1614 units, in reception order, in the de-interleaving buffer as follows. 1615 NAL units of aggregation packets are stored in the de-interleaving 1616 buffer individually. The value of DON is calculated and stored for 1617 each NAL unit. 1619 The receiver operation is described below with the help of the 1620 following functions and constants: 1622 o Function AbsDON is specified in section 8.1. 1624 o Function don_diff is specified in section 5.5. 1626 o Constant N is the value of the OPTIONAL sprop-interleaving-depth 1627 media type type parameter (see section 8.1) incremented by 1. 1629 Initial buffering lasts until one of the following conditions is 1630 fulfilled: 1632 o There are N or more VCL NAL units in the de-interleaving buffer. 1634 o If sprop-max-don-diff is present, don_diff(m,n) is greater than 1635 the value of sprop-max-don-diff, in which n corresponds to the NAL 1636 unit having the greatest value of AbsDON among the received NAL 1637 units and m corresponds to the NAL unit having the smallest value 1638 of AbsDON among the received NAL units. 1640 o Initial buffering has lasted for the duration equal to or greater 1641 than the value of the OPTIONAL sprop-init-buf-time media type 1642 parameter. 1644 The NAL units to be removed from the de-interleaving buffer are 1645 determined as follows: 1647 o If the de-interleaving buffer contains at least N VCL NAL units, 1648 NAL units are removed from the de-interleaving buffer and passed 1649 to the decoder in the order specified below until the buffer 1650 contains N-1 VCL NAL units. 1652 o If sprop-max-don-diff is present, all NAL units m for which 1653 don_diff(m,n) is greater than sprop-max-don-diff are removed from 1654 the de-interleaving buffer and passed to the decoder in the order 1655 specified below. Herein, n corresponds to the NAL unit having the 1656 greatest value of AbsDON among the NAL units in the de- 1657 interleaving buffer. 1659 The order in which NAL units are passed to the decoder is specified 1660 as follows: 1662 o Let PDON be a variable that is initialized to 0 at the beginning 1663 of the RTP session. 1665 o For each NAL unit associated with a value of DON, a DON distance 1666 is calculated as follows. If the value of DON of the NAL unit is 1667 larger than the value of PDON, the DON distance is equal to DON - 1668 PDON. Otherwise, the DON distance is equal to 65535 - PDON + DON 1669 + 1. 1671 o NAL units are delivered to the decoder in ascending order of DON 1672 distance. If several NAL units share the same value of DON 1673 distance, they can be passed to the decoder in any order. 1675 o When a desired number of NAL units have been passed to the 1676 decoder, the value of PDON is set to the value of DON for the last 1677 NAL unit passed to the decoder. 1679 7.3. Additional De-Packetization Guidelines 1681 The following additional de-packetization rules may be used to 1682 implement an operational H.264 de-packetizer: 1684 o Intelligent RTP receivers (e.g., in gateways) may identify lost 1685 coded slice data partitions A (DPAs). If a lost DPA is found, a 1686 gateway may decide not to send the corresponding coded slice data 1687 partitions B and C, as their information is meaningless for H.264 1688 decoders. In this way a MANE can reduce network load by 1689 discarding useless packets without parsing a complex bitstream. 1691 o Intelligent RTP receivers (e.g., in gateways) may identify lost 1692 FUs. If a lost FU is found, a gateway may decide not to send the 1693 following FUs of the same fragmented NAL unit, as their 1694 information is meaningless for H.264 decoders. In this way a MANE 1695 can reduce network load by discarding useless packets without 1696 parsing a complex bitstream. 1698 o Intelligent receivers having to discard packets or NALUs should 1699 first discard all packets/NALUs in which the value of the NRI 1700 field of the NAL unit type octet is equal to 0. This will 1701 minimize the impact on user experience and keep the reference 1702 pictures intact. If more packets have to be discarded, then 1703 packets with a numerically lower NRI value should be discarded 1704 before packets with a numerically higher NRI value. However, 1705 discarding any packets with an NRI bigger than 0 very likely leads 1706 to decoder drift and SHOULD be avoided. 1708 8. Payload Format Parameters 1710 This section specifies the parameters that MAY be used to select 1711 optional features of the payload format and certain features of the 1712 bitstream. The parameters are specified here as part of the media 1713 subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec. A 1714 mapping of the parameters into the Session Description Protocol (SDP) 1715 [6] is also provided for applications that use SDP. Equivalent 1716 parameters could be defined elsewhere for use with control protocols 1717 that do not use SDP. 1719 Some parameters provide a receiver with the properties of the stream 1720 that will be sent. The names of all these parameters start with 1721 "sprop" for stream properties. Some of these "sprop" parameters are 1722 limited by other payload or codec configuration parameters. For 1723 example, the sprop-parameter-sets parameter is constrained by the 1724 profile-level-id parameter. The media sender selects all "sprop" 1725 parameters rather than the receiver. This uncommon characteristic of 1726 the "sprop" parameters may not be compatible with some signaling 1727 protocol concepts, in which case the use of these parameters SHOULD 1728 be avoided. 1730 8.1. Media Type Registration 1732 The media subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is 1733 allocated from the IETF tree. 1735 The receiver MUST ignore any unspecified parameter. 1737 Media Type name: video 1739 Media subtype name: H264 1741 Required parameters: none 1743 OPTIONAL parameters: 1745 profile-level-id: 1746 A base16 [7] (hexadecimal) representation of the following 1747 three bytes in the sequence parameter set NAL unit specified 1748 in [1]: 1) profile_idc, 2) a byte herein referred to as 1749 profile-iop, composed of the values of constraint_set0_flag, 1750 constraint_set1_flag,constraint_set2_flag, 1751 constraint_set3_flag, and reserved_zero_4bits in bit- 1752 significance order, starting from the most significant bit, 1753 and 3) level_idc. Note that reserved_zero_4bits is required 1754 to be equal to 0 in [1], but other values for it may be 1755 specified in the future by ITU-T or ISO/IEC. 1757 The profile-level-id parameter indicates the default sub- 1758 profile, i.e. the subset of coding tools that may have been 1759 used to generate the stream or the receiver supports, and the 1760 default level of the stream or the receiver supports. 1762 The default sub-profile is indicated collectively by the 1763 profile_idc byte and some fields in the profile-iop byte. 1764 Depending on the values of the fields in the profile-iop byte, 1765 the default sub-profile may be the same set of coding tools 1766 supported by one profile, or a common subset of coding tools 1767 of multiple profiles, as specified in subsection 7.4.2.1.1 of 1768 [1]. The default level is indicated by the level_idc byte, 1769 and, when profile_idc is equal to 66, 77 or 88 (the Baseline, 1770 Main, or Extended profile) and level_idc is equal to 11, 1771 additionally by bit 4 (constraint_set3_flag) of the profile- 1772 iop byte. When profile_idc is equal to 66, 77 or 88 (the 1773 Baseline, Main, or Extended profile) and level_idc is equal to 1774 11, and bit 4 (constraint_set3_flag) of the profile-iop byte 1775 is equal to 1, the default level is level 1b. 1777 Table 5 lists all profiles defined in Annex A of [1] and, for 1778 each of the profiles, the possible combinations of profile_idc 1779 and profile-iop that represent the same sub-profile. 1781 Table 5. Combinations of profile_idc and profile-iop 1782 representing the same sub-profile corresponding to the full 1783 set of coding tools supported by one profile. In the 1784 following, x may be either 0 or 1, and other notions as 1785 follows. CB: Constrained Baseline profile, B: Baseline 1786 profile, M: Main profile, E: Extended profile, H: High 1787 profile, H10: High 10 profile, H42: High 4:2:2 profile, 1788 H44: High 4:4:4 Predictive profile, H10I: High 10 Intra 1789 profile, H42I: High 4:2:2 Intra profile, H44I: High 4:4:4 1790 Intra profile, and C44I: CAVLC 4:4:4 Intra profile. 1792 Profile profile_idc profile-iop 1793 (hexadecimal) (binary) 1795 CB 42 x1xx0000 1796 4D 1xxx0000 1797 58 11xx0000 1798 64, 6E, 7A or F4 1xx00000 1799 B 42 x0xx0000 1800 58 10xx0000 1801 M 4D 0x0x0000 1802 64,6E,7A or F4 01000000 1803 E 58 00xx0000 1804 H 64 00000000 1805 H10 6E 00000000 1806 H42 7A 00000000 1807 H44 F4 00000000 1808 H10I 64 00010000 1809 H42I 7A 00010000 1810 H44I F4 00010000 1811 C44I 2C 00010000 1813 Note that other combinations of profile_idc and profile-iop 1814 (note listed in Table 13) may represent a sub-profile 1815 equivalent to the common subset of coding tools for more than 1816 one profile. Note also that a decoder conforming to a certain 1817 profile may be able to decode bitstreams conforming to other 1818 profiles. For example, a decoder conforming to the High 4:4:4 1819 profile at certain level must be able to decode bitstreams 1820 confirming to the Constrained Baseline, Main, High, High 10 or 1821 High 4:2:2 profile at the same or a lower level. 1823 If the profile-level-id parameter is used to indicate 1824 properties of a NAL unit stream, it indicates that, to decode 1825 the stream, the minimum subset of coding tools a decoder has 1826 to support is the default sub-profile, and the lowest level 1827 the decoder has to support is the default level. 1829 If the profile-level-id parameter is used for capability 1830 exchange or session setup procedure, it indicates the subset 1831 of coding tools, which is equal to the default sub-profile, 1832 and the highest level, which is equal to the default level, 1833 that the codec supports. All levels lower than the default 1834 level are also supported by the codec. 1836 Informative note: Capability exchange and session setup 1837 procedures should provide means to list the capabilities 1838 for each supported sub-profile separately. For example, 1839 the one-of-N codec selection procedure of the SDP 1840 Offer/Answer model can be used (section 10.2 of [8]). The 1841 one-of-N codec selection procedure may also be used to 1842 provide different combinations of profile_idc and profile- 1843 iop that represent the same sub-profile. When there are a 1844 lot of different combinations of profile_idc and profile- 1845 iop that represent the same sub-profile, using the one-of-N 1846 codec selection procedure may result into large-sized SDP 1847 message. Therefore, a receiver should understand the 1848 different equivalent combinations of profile_idc and 1849 profile-iop that represent the same sub-profile, and be 1850 ready to accept an offer using any of the equivalent 1851 combinations. 1853 If no profile-level-id is present, the Baseline Profile 1854 without additional constraints at Level 1 MUST be implied. 1856 max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br: 1857 These parameters MAY be used to signal the capabilities of a 1858 receiver implementation. These parameters MUST NOT be used for 1859 any other purpose. The profile-level-id parameter MUST be 1860 present in the same receiver capability description that 1861 contains any of these parameters. The level conveyed in the 1862 value of the profile-level-id parameter MUST be such that the 1863 receiver is fully capable of supporting. max-mbps, max-smbps, 1864 max-fs, max-cpb, max-dpb, and max-br MAY be used to indicate 1865 capabilities of the receiver that extend the required 1866 capabilities of the signaled level, as specified below. 1868 When more than one parameter from the set (max-mbps, max-smbps 1869 , max-fs, max-cpb, max-dpb, max-br) is present, the receiver 1870 MUST support all signaled capabilities simultaneously. For 1871 example, if both max-mbps and max-br are present, the signaled 1872 level with the extension of both the frame rate and bit rate 1873 is supported. That is, the receiver is able to decode NAL 1874 unit streams in which the macroblock processing rate is up to 1875 max-mbps (inclusive), the bit rate is up to max-br 1876 (inclusive), the coded picture buffer size is derived as 1877 specified in the semantics of the max-br parameter below, and 1878 other properties comply with the level specified in the value 1879 of the profile-level-id parameter. 1881 If a receiver can support all the properties of level A, the 1882 level specified in the value of the profile-level-id MUST be 1883 level A (i.e. MUST NOT be lower than level A). In other 1884 words, a sender or receiver MUST NOT signal values of max- 1885 mbps, max-fs, max-cpb, max-dpb, and max-br that meet the 1886 requirements of a higher level compared to the level specified 1887 in the value of the profile-level-id parameter. 1889 Informative note: When the OPTIONAL media type parameters 1890 are used to signal the properties of a NAL unit stream, 1891 max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br 1892 are not present, and the value of profile-level-id must 1893 always be such that the NAL unit stream complies fully with 1894 the specified profile and level. 1896 max-mbps: The value of max-mbps is an integer indicating the 1897 maximum macroblock processing rate in units of macroblocks per 1898 second. The max-mbps parameter signals that the receiver is 1899 capable of decoding video at a higher rate than is required by 1900 the signaled level conveyed in the value of the profile-level- 1901 id parameter. When max-mbps is signaled, the receiver MUST be 1902 able to decode NAL unit streams that conform to the signaled 1903 level, with the exception that the MaxMBPS value in Table A-1 1904 of [1] for the signaled level is replaced with the value of 1905 max-mbps. The value of max-mbps MUST be greater than or equal 1906 to the value of MaxMBPS for the level given in Table A-1 of 1907 [1]. Senders MAY use this knowledge to send pictures of a 1908 given size at a higher picture rate than is indicated in the 1909 signaled level. 1911 max-smbps: The value of max-smbps is an integer indicating the 1912 maximum static macroblock processing rate in units of static 1913 macroblocks per second, under the hypothetical assumption that 1914 all macroblocks are static macroblocks. When max-smbps is 1915 signalled the MaxMBPS value in Table A-1 of [1] should be 1916 replaced with the result of the following computation: 1918 o If the parameter max-mbps is signalled, set a variable 1919 MaxMacroblocksPerSecond to the value of max-mbps. 1920 Otherwise, set MaxMacroblocksPerSecond equal to the value 1921 of MaxMBPS for the level in Table A-1 [1]. 1923 o Set a variable P_non-static to the proportion of non-static 1924 macroblocks in picture n. 1926 o Set a variable P_static to the proportion of static 1927 macroblocks in picture n. 1929 o The value of MaxMBPS in Table A-1 of [1] should be 1930 considered by the encoder to be equal to: 1932 MaxMacroblocksPerSecond * max-smbps / ( P_non-static * max- 1933 smbps + P_static * MaxMacroblocksPerSecond) 1935 The encoder should recompute this value for each picture. The 1936 value of max-smbps MUST be greater than the value of MaxMBPS 1937 for the level given in Table A-1 of [1]. Senders MAY use this 1938 knowledge to send pictures of a given size at a higher picture 1939 rate than is indicated in the signalled level. 1941 max-fs: The value of max-fs is an integer indicating the maximum 1942 frame size in units of macroblocks. The max-fs parameter 1943 signals that the receiver is capable of decoding larger 1944 picture sizes than are required by the signaled level conveyed 1945 in the value of the profile-level-id parameter. When max-fs 1946 is signaled, the receiver MUST be able to decode NAL unit 1947 streams that conform to the signaled level, with the exception 1948 that the MaxFS value in Table A-1 of [1] for the signaled 1949 level is replaced with the value of max-fs. The value of max- 1950 fs MUST be greater than or equal to the value of MaxFS for the 1951 level given in Table A-1 of [1]. Senders MAY use this 1952 knowledge to send larger pictures at a proportionally lower 1953 frame rate than is indicated in the signaled level. 1955 max-cpb: The value of max-cpb is an integer indicating the 1956 maximum coded picture buffer size in units of 1000 bits for 1957 the VCL HRD parameters (see A.3.1 item i of [1]) and in units 1958 of 1200 bits for the NAL HRD parameters (see A.3.1 item j of 1959 [1]). The max-cpb parameter signals that the receiver has 1960 more memory than the minimum amount of coded picture buffer 1961 memory required by the signaled level conveyed in the value of 1962 the profile-level-id parameter. When max-cpb is signaled, the 1963 receiver MUST be able to decode NAL unit streams that conform 1964 to the signaled level, with the exception that the MaxCPB 1965 value in Table A-1 of [1] for the signaled level is replaced 1966 with the value of max-cpb. The value of max-cpb MUST be 1967 greater than or equal to the value of MaxCPB for the level 1968 given in Table A-1 of [1]. Senders MAY use this knowledge to 1969 construct coded video streams with greater variation of bit 1970 rate than can be achieved with the MaxCPB value in Table A-1 1971 of [1]. 1973 Informative note: The coded picture buffer is used in the 1974 hypothetical reference decoder (Annex C) of H.264. The use 1975 of the hypothetical reference decoder is recommended in 1976 H.264 encoders to verify that the produced bitstream 1977 conforms to the standard and to control the output bitrate. 1978 Thus, the coded picture buffer is conceptually independent 1979 of any other potential buffers in the receiver, including 1980 de-interleaving and de-jitter buffers. The coded picture 1981 buffer need not be implemented in decoders as specified in 1982 Annex C of H.264, but rather standard-compliant decoders 1983 can have any buffering arrangements provided that they can 1984 decode standard-compliant bitstreams. Thus, in practice, 1985 the input buffer for video decoder can be integrated with 1986 de-interleaving and de-jitter buffers of the receiver. 1988 max-dpb: The value of max-dpb is an integer indicating the 1989 maximum decoded picture buffer size in units of 1024 bytes. 1990 The max-dpb parameter signals that the receiver has more 1991 memory than the minimum amount of decoded picture buffer 1992 memory required by the signaled level conveyed in the value of 1993 the profile-level-id parameter. When max-dpb is signaled, the 1994 receiver MUST be able to decode NAL unit streams that conform 1995 to the signaled level, with the exception that the MaxDPB 1996 value in Table A-1 of [1] for the signaled level is replaced 1997 with the value of max-dpb. Consequently, a receiver that 1998 signals max-dpb MUST be capable of storing the following 1999 number of decoded frames, complementary field pairs, and non- 2000 paired fields in its decoded picture buffer: 2002 Min(1024 * max-dpb / ( PicWidthInMbs * FrameHeightInMbs * 2003 256 * ChromaFormatFactor ), 16) 2005 PicWidthInMbs, FrameHeightInMbs, and ChromaFormatFactor are 2006 defined in [1]. 2008 The value of max-dpb MUST be greater than or equal to the 2009 value of MaxDPB for the level given in Table A-1 of [1]. 2010 Senders MAY use this knowledge to construct coded video 2011 streams with improved compression. 2013 Informative note: This parameter was added primarily to 2014 complement a similar codepoint in the ITU-T Recommendation 2015 H.245, so as to facilitate signaling gateway designs. The 2016 decoded picture buffer stores reconstructed samples. There 2017 is no relationship between the size of the decoded picture 2018 buffer and the buffers used in RTP, especially de- 2019 interleaving and de-jitter buffers. 2021 max-br: The value of max-br is an integer indicating the maximum 2022 video bit rate in units of 1000 bits per second for the VCL 2023 HRD parameters (see A.3.1 item i of [1]) and in units of 1200 2024 bits per second for the NAL HRD parameters (see A.3.1 item j 2025 of [1]). 2027 The max-br parameter signals that the video decoder of the 2028 receiver is capable of decoding video at a higher bit rate 2029 than is required by the signaled level conveyed in the value 2030 of the profile-level-id parameter. 2032 When max-br is signaled, the video codec of the receiver MUST 2033 be able to decode NAL unit streams that conform to the 2034 signaled level, conveyed in the profile-level-id parameter, 2035 with the following exceptions in the limits specified by the 2036 level: 2038 o The value of max-br replaces the MaxBR value of the signaled 2039 level (in Table A-1 of [1]). 2041 o When the max-cpb parameter is not present, the result of the 2042 following formula replaces the value of MaxCPB in Table A-1 2043 of [1]: (MaxCPB of the signaled level) * max-br / (MaxBR of 2044 the signaled level). 2046 For example, if a receiver signals capability for Level 1.2 2047 with max-br equal to 1550, this indicates a maximum video 2048 bitrate of 1550 kbits/sec for VCL HRD parameters, a maximum 2049 video bitrate of 1860 kbits/sec for NAL HRD parameters, and a 2050 CPB size of 4036458 bits (1550000 / 384000 * 1000 * 1000). 2052 The value of max-br MUST be greater than or equal to the value 2053 MaxBR for the signaled level given in Table A-1 of [1]. 2055 Senders MAY use this knowledge to send higher bitrate video as 2056 allowed in the level definition of Annex A of H.264, to 2057 achieve improved video quality. 2059 Informative note: This parameter was added primarily to 2060 complement a similar codepoint in the ITU-T Recommendation 2061 H.245, so as to facilitate signaling gateway designs. No 2062 assumption can be made from the value of this parameter 2063 that the network is capable of handling such bit rates at 2064 any given time. In particular, no conclusion can be drawn 2065 that the signaled bit rate is possible under congestion 2066 control constraints. 2068 redundant-pic-cap: 2069 This parameter signals the capabilities of a receiver 2070 implementation. When equal to 0, the parameter indicates that 2071 the receiver makes no attempt to use redundant coded pictures 2072 to correct incorrectly decoded primary coded pictures. When 2073 equal to 0, the receiver is not capable of using redundant 2074 slices; therefore, a sender SHOULD avoid sending redundant 2075 slices to save bandwidth. When equal to 1, the receiver is 2076 capable of decoding any such redundant slice that covers a 2077 corrupted area in a primary decoded picture (at least partly), 2078 and therefore a sender MAY send redundant slices. When the 2079 parameter is not present, then a value of 0 MUST be used for 2080 redundant-pic-cap. When present, the value of redundant-pic- 2081 cap MUST be either 0 or 1. 2083 When the profile-level-id parameter is present in the same 2084 signaling as the redundant-pic-cap parameter, and the profile 2085 indicated in profile-level-id is such that it disallows the 2086 use of redundant coded pictures (e.g., Main Profile), the 2087 value of redundant-pic-cap MUST be equal to 0. When a 2088 receiver indicates redundant-pic-cap equal to 0, the received 2089 stream SHOULD NOT contain redundant coded pictures. 2091 Informative note: Even if redundant-pic-cap is equal to 0, 2092 the decoder is able to ignore redundant codec pictures 2093 provided that the decoder supports such a profile 2094 (Baseline, Extended) in which redundant coded pictures are 2095 allowed. 2097 Informative note: Even if redundant-pic-cap is equal to 1, 2098 the receiver may also choose other error concealment 2099 strategies to replace or complement decoding of redundant 2100 slices. 2102 sprop-parameter-sets: 2103 This parameter MAY be used to convey any sequence and picture 2104 parameter set NAL units (herein referred to as the initial 2105 parameter set NAL units) that can be placed in the NAL unit 2106 stream to precede any other NAL units in decoding order. The 2107 parameter MUST NOT be used to indicate codec capability in any 2108 capability exchange procedure. The value of the parameter is 2109 the base64 [7] representation of the initial parameter set NAL 2110 units as specified in sections 7.3.2.1 and 7.3.2.2 of [1]. 2112 The parameter sets are conveyed in decoding order, and no 2113 framing of the parameter set NAL units takes place. A comma 2114 (',') is used to separate any pair of parameter sets in the 2115 list. Note that the number of bytes in a parameter set NAL 2116 unit is typically less than 10, but a picture parameter set 2117 NAL unit can contain several hundreds of bytes. 2119 Informative note: When several payload types are offered in 2120 the SDP Offer/Answer model, each with its own sprop- 2121 parameter-sets parameter, then the receiver cannot assume 2122 that those parameter sets do not use conflicting storage 2123 locations (i.e., identical values of parameter set 2124 identifiers). Therefore, a receiver should double-buffer 2125 all sprop-parameter-sets and make them available to the 2126 decoder instance that decodes a certain payload type. 2128 The "sprop-parameter-sets" parameter MUST only contain 2129 parameter sets that are conforming to the profile-level-id, 2130 i.e., the subset of coding tools indicated by any of the 2131 parameter sets MUST be equal to the default sub-profile, and 2132 the level indicated by any of the parameter sets MUST be equal 2133 to the default level. 2135 sprop-level-parameter-sets: 2136 This parameter MAY be used to convey any sequence and picture 2137 parameter set NAL units (herein referred to as the initial 2138 parameter set NAL units) that can be placed in the NAL unit 2139 stream to precede any other NAL units in decoding order and 2140 that are associated with one or more levels lower than the 2141 default level. The parameter MUST NOT be used to indicate 2142 codec capability in any capability exchange procedure. 2144 The sprop-level-parameter-sets parameter contains parameter 2145 sets for one or more levels which are lower than the default 2146 level. All parameter sets associated with one level are 2147 clustered and prefixed with a three-byte field which has the 2148 same syntax as profile-level-id. This enables the receiver to 2149 install the parameter sets for one level and discard the rest. 2150 The three-byte field is named PLId, and all parameter sets 2151 associated with one level are named PSL, which has the same 2152 syntax as sprop-parameter-sets. Parameter sets for each level 2153 are represented in the form of PLId:PSL, i.e., PLId followed 2154 by a colon (':') and the base64 [7] representation of the 2155 initial parameter set NAL units for the level. Each pair of 2156 PLId:PSL is also separated by a colon. Note that a PSL can 2157 contain multiple parameter sets for that level, separated with 2158 commas (','). 2160 The subset of coding tools indicated by each PLId field MUST 2161 be equal to the default sub-profile, and the level indicated 2162 by each PLId field MUST be lower than the default level. All 2163 sequence parameter sets contained in each PSL MUST have the 2164 three bytes from profile_idc to level_idc, inclusive, equal to 2165 the preceding PLId. 2167 Informative note: This parameter allows for efficient level 2168 downgrade in SDP Offer/Answer and out-of-band transport of 2169 parameter sets, simultaneously. 2171 use-level-parameter-sets: 2172 This parameter MAY be used to indicate a receiver capability. 2173 The value MAY be equal to either 0 or 1. When the parameter 2174 is not present, the value MUST be inferred to be equal to 0. 2175 The value 0 indicates that the receiver does not understand 2176 the sprop-level-parameter-sets parameter and will ignore 2177 sprop-level-parameter-sets when present. The value 1 2178 indicates that the receiver understands the sprop-level- 2179 parameter-sets parameter and is capable of using parameter 2180 sets contained therein. 2182 Informative note: An RFC 3984 receiver does not understand 2183 both sprop-level-parameter-sets and use-level-parameter- 2184 sets. Therefore, during SDP Offer/Answer, an RFC 3984 2185 receiver as the answerer will simply ignore sprop-level- 2186 parameter-sets, when present in an offer. Assume that the 2187 offered payload type was accepted at a level lower than the 2188 default level. If the offered payload type included sprop- 2189 level-parameter-sets, and the offerer sees that the 2190 answerer has not included use-level-parameter-sets equal to 2191 1 in the answer, the offerer gets to know that in-band 2192 transport of parameter sets is needed. 2194 sprop-ssrc: 2195 This parameter MAY be used to signal the properties of an RTP 2196 packet stream. It specifies the SSRC values in the RTP header 2197 of all RTP packets in the RTP packet stream. The syntax of 2198 this parameter is the same as the syntax of the SSRC field in 2199 the RTP header. 2201 Informative note: This parameter allows for out-of-band 2202 transport of parameter sets in topologies like Topo-Video- 2203 switch-MCU [28]. 2205 packetization-mode: 2206 This parameter signals the properties of an RTP payload type 2207 or the capabilities of a receiver implementation. Only a 2208 single configuration point can be indicated; thus, when 2209 capabilities to support more than one packetization-mode are 2210 declared, multiple configuration points (RTP payload types) 2211 must be used. 2213 When the value of packetization-mode is equal to 0 or 2214 packetization-mode is not present, the single NAL mode, as 2215 defined in section 6.2 of RFC 3984, MUST be used. This mode 2216 is in use in standards using ITU-T Recommendation H.241 [3] 2217 (see section 12.1). When the value of packetization-mode is 2218 equal to 1, the non-interleaved mode, as defined in section 2219 6.3 of RFC 3984, MUST be used. When the value of 2220 packetization-mode is equal to 2, the interleaved mode, as 2221 defined in section 6.4 of RFC 3984, MUST be used. The value 2222 of packetization-mode MUST be an integer in the range of 0 to 2223 2, inclusive. 2225 sprop-interleaving-depth: 2226 This parameter MUST NOT be present when packetization-mode is 2227 not present or the value of packetization-mode is equal to 0 2228 or 1. This parameter MUST be present when the value of 2229 packetization-mode is equal to 2. 2231 This parameter signals the properties of an RTP packet stream. 2232 It specifies the maximum number of VCL NAL units that precede 2233 any VCL NAL unit in the RTP packet stream in transmission 2234 order and follow the VCL NAL unit in decoding order. 2235 Consequently, it is guaranteed that receivers can reconstruct 2236 NAL unit decoding order when the buffer size for NAL unit 2237 decoding order recovery is at least the value of sprop- 2238 interleaving-depth + 1 in terms of VCL NAL units. 2240 The value of sprop-interleaving-depth MUST be an integer in 2241 the range of 0 to 32767, inclusive. 2243 sprop-deint-buf-req: 2244 This parameter MUST NOT be present when packetization-mode is 2245 not present or the value of packetization-mode is equal to 0 2246 or 1. It MUST be present when the value of packetization-mode 2247 is equal to 2. 2249 sprop-deint-buf-req signals the required size of the de- 2250 interleaving buffer for the RTP packet stream. The value of 2251 the parameter MUST be greater than or equal to the maximum 2252 buffer occupancy (in units of bytes) required in such a de- 2253 interleaving buffer that is specified in section 7.2 of RFC 2254 3984. It is guaranteed that receivers can perform the de- 2255 interleaving of interleaved NAL units into NAL unit decoding 2256 order, when the de-interleaving buffer size is at least the 2257 value of sprop-deint-buf-req in terms of bytes. 2259 The value of sprop-deint-buf-req MUST be an integer in the 2260 range of 0 to 4294967295, inclusive. 2262 Informative note: sprop-deint-buf-req indicates the 2263 required size of the de-interleaving buffer only. When 2264 network jitter can occur, an appropriately sized jitter 2265 buffer has to be provisioned for as well. 2267 deint-buf-cap: 2268 This parameter signals the capabilities of a receiver 2269 implementation and indicates the amount of de-interleaving 2270 buffer space in units of bytes that the receiver has available 2271 for reconstructing the NAL unit decoding order. A receiver is 2272 able to handle any stream for which the value of the sprop- 2273 deint-buf-req parameter is smaller than or equal to this 2274 parameter. 2276 If the parameter is not present, then a value of 0 MUST be 2277 used for deint-buf-cap. The value of deint-buf-cap MUST be an 2278 integer in the range of 0 to 4294967295, inclusive. 2280 Informative note: deint-buf-cap indicates the maximum 2281 possible size of the de-interleaving buffer of the receiver 2282 only. When network jitter can occur, an appropriately 2283 sized jitter buffer has to be provisioned for as well. 2285 sprop-init-buf-time: 2286 This parameter MAY be used to signal the properties of an RTP 2287 packet stream. The parameter MUST NOT be present, if the 2288 value of packetization-mode is equal to 0 or 1. 2290 The parameter signals the initial buffering time that a 2291 receiver MUST wait before starting decoding to recover the NAL 2292 unit decoding order from the transmission order. The 2293 parameter is the maximum value of (decoding time of the NAL 2294 unit - transmission time of a NAL unit), assuming reliable and 2295 instantaneous transmission, the same timeline for transmission 2296 and decoding, and that decoding starts when the first packet 2297 arrives. 2299 An example of specifying the value of sprop-init-buf-time 2300 follows. A NAL unit stream is sent in the following 2301 interleaved order, in which the value corresponds to the 2302 decoding time and the transmission order is from left to 2303 right: 2305 0 2 1 3 5 4 6 8 7 ... 2307 Assuming a steady transmission rate of NAL units, the 2308 transmission times are: 2310 0 1 2 3 4 5 6 7 8 ... 2312 Subtracting the decoding time from the transmission time 2313 column-wise results in the following series: 2315 0 -1 1 0 -1 1 0 -1 1 ... 2317 Thus, in terms of intervals of NAL unit transmission times, 2318 the value of sprop-init-buf-time in this example is 1. The 2319 parameter is coded as a non-negative base10 integer 2320 representation in clock ticks of a 90-kHz clock. If the 2321 parameter is not present, then no initial buffering time value 2322 is defined. Otherwise the value of sprop-init-buf-time MUST 2323 be an integer in the range of 0 to 4294967295, inclusive. 2325 In addition to the signaled sprop-init-buf-time, receivers 2326 SHOULD take into account the transmission delay jitter 2327 buffering, including buffering for the delay jitter caused by 2328 mixers, translators, gateways, proxies, traffic-shapers, and 2329 other network elements. 2331 sprop-max-don-diff: 2332 This parameter MAY be used to signal the properties of an RTP 2333 packet stream. It MUST NOT be used to signal transmitter or 2334 receiver or codec capabilities. The parameter MUST NOT be 2335 present if the value of packetization-mode is equal to 0 or 1. 2336 sprop-max-don-diff is an integer in the range of 0 to 32767, 2337 inclusive. If sprop-max-don-diff is not present, the value of 2338 the parameter is unspecified. sprop-max-don-diff is 2339 calculated as follows: 2341 sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)}, 2342 for any i and any j>i, 2344 where i and j indicate the index of the NAL unit in the 2345 transmission order and AbsDON denotes a decoding order number 2346 of the NAL unit that does not wrap around to 0 after 65535. 2347 In other words, AbsDON is calculated as follows: Let m and n 2348 be consecutive NAL units in transmission order. For the very 2349 first NAL unit in transmission order (whose index is 0), 2350 AbsDON(0) = DON(0). For other NAL units, AbsDON is calculated 2351 as follows: 2353 If DON(m) == DON(n), AbsDON(n) = AbsDON(m) 2355 If (DON(m) < DON(n) and DON(n) - DON(m) < 32768), 2356 AbsDON(n) = AbsDON(m) + DON(n) - DON(m) 2358 If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768), 2359 AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n) 2361 If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768), 2362 AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n)) 2364 If (DON(m) > DON(n) and DON(m) - DON(n) < 32768), 2365 AbsDON(n) = AbsDON(m) - (DON(m) - DON(n)) 2367 where DON(i) is the decoding order number of the NAL unit 2368 having index i in the transmission order. The decoding order 2369 number is specified in section 5.5 of RFC 3984. 2371 Informative note: Receivers may use sprop-max-don-diff to 2372 trigger which NAL units in the receiver buffer can be 2373 passed to the decoder. 2375 max-rcmd-nalu-size: 2376 This parameter MAY be used to signal the capabilities of a 2377 receiver. The parameter MUST NOT be used for any other 2378 purposes. The value of the parameter indicates the largest 2379 NALU size in bytes that the receiver can handle efficiently. 2380 The parameter value is a recommendation, not a strict upper 2381 boundary. The sender MAY create larger NALUs but must be 2382 aware that the handling of these may come at a higher cost 2383 than NALUs conforming to the limitation. 2385 The value of max-rcmd-nalu-size MUST be an integer in the 2386 range of 0 to 4294967295, inclusive. If this parameter is not 2387 specified, no known limitation to the NALU size exists. 2388 Senders still have to consider the MTU size available between 2389 the sender and the receiver and SHOULD run MTU discovery for 2390 this purpose. 2392 This parameter is motivated by, for example, an IP to H.223 2393 video telephony gateway, where NALUs smaller than the H.223 2394 transport data unit will be more efficient. A gateway may 2395 terminate IP; thus, MTU discovery will normally not work 2396 beyond the gateway. 2398 Informative note: Setting this parameter to a lower than 2399 necessary value may have a negative impact. 2401 sar-understood: 2402 This parameter MAY be used to indicate a receiver capability 2403 and not anything else. The parameter indicates the maximum 2404 value of aspect_ratio_idc (specified in [1]) smaller than 255 2405 that the receiver understands. Table E-1 of [1] specifies 2406 aspect_ratio_idc equal to 0 as "unspecified", 1 to 16, 2407 inclusive, as specific Sample Aspect Ratios (SARs), 17 to 254, 2408 inclusive, as "reserved", and 255 as the Extended SAR, for 2409 which SAR width and SAR height are explicitly signaled. 2410 Therefore, a receiver with a decoder according to [1] 2411 understands aspect_ratio_idc in the range of 1 to 16, 2412 inclusive and aspect_ratio_idc equal to 255, in the sense that 2413 the receiver knows what exactly the SAR is. For such a 2414 receiver, the value of sar-understood is 16. If in the future 2415 Table E-1 of [1] is extended, e.g., such that the SAR for 2416 aspect_ratio_idc equal to 17 is specified, then for a receiver 2417 with a decoder that understands the extension, the value of 2418 sar-understood is 17. For a receiver with a decoder according 2419 to the 2003 version of [1], the value of sar-understood is 13, 2420 as the minimum reserved aspect_ratio_idc therein is 14. 2422 When sar-understood is not present, the value MUST be inferred 2423 to be equal to 13. 2425 sar-supported: 2426 This parameter MAY be used to indicate a receiver capability 2427 and not anything else. The value of this parameter is an 2428 integer in the range of 1 to sar-understood, inclusive, equal 2429 to 255. The value of sar-supported equal to N smaller than 2430 255 indicates that the reciever supports all the SARs 2431 corresponding to H.264 aspect_ratio_idc values (see Table E-1 2432 of [1]) in the range from 1 to N, inclusive, without geometric 2433 distortion. The value of sar-supported equal to 255 indicates 2434 that the receiver supports all sample aspect ratios which are 2435 expressible using two 16-bit integer values as the numerator 2436 and denominator, i.e., those that are expressible using the 2437 H.264 aspect_ratio_idc value of 255 (Extended_SAR, see Table 2438 E-1 of [1]), without geometric distortion. 2440 H.264 compliant encoders SHOULD NOT send an aspect_ratio_idc 2441 equal to 0, or an aspect_ratio_idc larger than sar-understood 2442 and smaller than 255. H.264 compliant encoders SHOULD send an 2443 aspect_ratio_idc that the receiver is able to display without 2444 geometrical distortion. However, H.264 compliant encoders MAY 2445 choose to send pictures using any SAR. 2447 Note that the actual sample aspect ratio or extended sample 2448 aspect ratio, when present, of the stream is conveyed in the 2449 Video Usability Information (VUI) part of the sequence 2450 parameter set. 2452 Encoding considerations: 2453 This type is only defined for transfer via RTP (RFC 3550). 2455 Security considerations: 2456 See section 9 of RFC xxxx. 2458 Public specification: 2459 Please refer to RFC xxxx and its section 15. 2461 Additional information: 2462 None 2464 File extensions: none 2466 Macintosh file type code: none 2468 Object identifier or OID: none 2470 Person & email address to contact for further information: 2471 Ye-Kui Wang, ye-kui.wang@nokia.com 2473 Intended usage: COMMON 2475 Author: 2476 Ye-Kui Wang, ye-kui.wang@nokia.com 2478 Change controller: 2479 IETF Audio/Video Transport working group delegated from the 2480 IESG. 2482 8.2. SDP Parameters 2484 8.2.1. Mapping of Payload Type Parameters to SDP 2486 The media type video/H264 string is mapped to fields in the Session 2487 Description Protocol (SDP) [6] as follows: 2489 o The media name in the "m=" line of SDP MUST be video. 2491 o The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the 2492 media subtype). 2494 o The clock rate in the "a=rtpmap" line MUST be 90000. 2496 o The OPTIONAL parameters "profile-level-id", "max-mbps", "max- 2497 smbps", "max-fs", "max-cpb", "max-dpb", "max-br", "redundant-pic- 2498 cap", "sprop-parameter-sets", "sprop-level-parameter-sets", "use- 2499 level-parameter-sets", "sprop-ssrc", "packetization-mode", "sprop- 2500 interleaving-depth", "sprop-deint-buf-req", "deint-buf-cap", 2501 "sprop-init-buf-time", "sprop-max-don-diff", "max-rcmd-nalu-size", 2502 "sar-understood", and "sar-supported", when present, MUST be 2503 included in the "a=fmtp" line of SDP. These parameters are 2504 expressed as a media type string, in the form of a semicolon 2505 separated list of parameter=value pairs. 2507 An example of media representation in SDP is as follows (Baseline 2508 Profile, Level 3.0, some of the constraints of the Main profile may 2509 not be obeyed): 2511 m=video 49170 RTP/AVP 98 2512 a=rtpmap:98 H264/90000 2513 a=fmtp:98 profile-level-id=42A01E; 2514 packetization-mode=1; 2515 sprop-parameter-sets= 2517 8.2.2. Usage with the SDP Offer/Answer Model 2519 When H.264 is offered over RTP using SDP in an Offer/Answer model [8] 2520 for negotiation for unicast usage, the following limitations and 2521 rules apply: 2523 o The parameters identifying a media format configuration for H.264 2524 are "profile-level-id" and "packetization-mode", when present. 2525 These media format configuration parameters (except for the level 2526 part of "profile-level-id") MUST be used symmetrically; i.e., the 2527 answerer MUST either maintain all configuration parameters or 2528 remove the media format (payload type) completely, if one or more 2529 of the parameter values are not supported. Note that the level 2530 part of "profile-level-id" includes level_idc, and, for indication 2531 of level 1b when profile_idc is equal to 66, 77 or 88, bit 4 2532 (constraint_set3_flag) of profile-iop. The level part of 2533 "profile-level-id" is downgradable, i.e. the answerer MUST 2534 maintain the same or a lower level or remove the media format 2535 (payload type) completely. 2537 Informative note: The requirement for symmetric use applies 2538 only for the above media format configuration parameters 2539 excluding the level part of "profile-level-id", and not for 2540 the other stream properties and capability parameters. 2542 Informative note: In H.264 [1], all the levels except for 2543 level 1b are equal to the value of level_idc divided by 10. 2544 Level 1b is a level higher than level 1.0 but lower than level 2545 1.1, and is signaled in an ad-hoc manner, due to that the 2546 level was specified after level 1.0 and level 1.1. For the 2547 Baseline, Main and Extended profiles (with profile_idc equal 2548 to 66, 77 and 88, respectively), level 1b is indicated by 2549 level_idc equal to 11 (i.e. same as level 1.1) and 2550 constraint_set3_flag equal to 1. For other profiles, level 1b 2551 is indicated by level_idc equal to 9 (but note that level 1b 2552 for these profiles are still higher than level 1, which has 2553 level_idc equal to 10, and lower than level 1.1). In SDP 2554 Offer/Answer, an answer to an offer may indicate a level equal 2555 to or lower than the level indicated in the offer. Due to the 2556 ad-hoc indication of level 1b, offerers and answerers must 2557 check the value of bit 4 (constraint_set3_flag) of the middle 2558 octet of the parameter "profile-level-id", when profile_idc is 2559 equal to 66, 77 or 88 and level_idc is equal to 11. 2561 To simplify handling and matching of these configurations, the 2562 same RTP payload type number used in the offer SHOULD also be 2563 used in the answer, as specified in [8]. An answer MUST NOT 2564 contain a payload type number used in the offer unless the 2565 configuration is exactly the same as in the offer or the 2566 configuration in the answer only differs from that in the offer 2567 with a level lower than the default level offered. 2569 Informative note: An offerer, when receiving the answer, has 2570 to compare payload types not declared in the offer based on 2571 media type (i.e., video/H264) and the above media format 2572 configuration parameters with any payload types it has already 2573 declared, in order to determine whether the configuration in 2574 question is new or equivalent to a configuration already 2575 offered. 2577 o The parameters "sprop-deint-buf-req", "sprop-interleaving-depth", 2578 "sprop-max-don-diff", "sprop-init-buf-time", and "sprop-ssrc" 2579 describe the properties of the RTP packet stream that the offerer 2580 or answerer is sending for the media format configuration. This 2581 differs from the normal usage of the Offer/Answer parameters: 2582 normally such parameters declare the properties of the stream that 2583 the offerer or the answerer is able to receive. When dealing with 2584 H.264, the offerer assumes that the answerer will be able to 2585 receive media encoded using the configuration being offered. 2587 Informative note: The above parameters apply for any stream 2588 sent by the declaring entity with the same configuration; 2589 i.e., they are dependent on their source. Rather than being 2590 bound to the payload type, the values may have to be applied 2591 to another payload type when being sent, as they apply for the 2592 configuration. 2594 o The capability parameters ("max-mbps", "max-smbps", "max-fs", 2595 "max-cpb", "max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd- 2596 nalu-size", "sar-understood", "sar-supported") MAY be used to 2597 declare further capabilities of the offerer or answerer for 2598 receiving. These parameters can only be present when the 2599 direction attribute is sendrecv or recvonly, and the parameters 2600 describe the limitations of what the offerer or answerer accepts 2601 for receiving streams. 2603 o An offerer has to include the size of the de-interleaving buffer, 2604 "sprop-deint-buf-req", in the offer for an interleaved H.264 2605 stream. To enable the offerer and answerer to inform each other 2606 about their capabilities for de-interleaving buffering in 2607 receiving streams, both parties are RECOMMENDED to include "deint- 2608 buf-cap". For interleaved streams, it is also RECOMMENDED to 2609 consider offering multiple payload types with different buffering 2610 requirements when the capabilities of the receiver are unknown. 2612 o The "sprop-parameter-sets" or "sprop-level-parameter-sets" 2613 parameter, when present, is used for out-of-band transport of 2614 parameter sets. However, when out-of-band transport of parameter 2615 sets is used, parameter sets MAY still be additionally transported 2616 in-band. If neither "sprop-parameter-sets" nor "sprop-level- 2617 parameter-sets" is present, then only in-band transport of 2618 parameter sets is used. 2620 An offer MAY include either or both of "sprop-parameter-sets" and 2621 "sprop-level-parameter-sets". An answer MAY include "sprop- 2622 parameter-sets", and MUST NOT include "sprop-level-parameter- 2623 sets". 2625 When an offered payload type is accepted without level downgrade, 2626 i.e. the default level is accepted, the following applies. 2628 o The answerer MUST be prepared to use the parameter sets 2629 included in "sprop-parameter-sets", when present, for 2630 decoding the incoming NAL unit stream, and ignore "sprop- 2631 level-parameter-sets", when present. 2633 o When "sprop-parameter-sets" is not present, in-band 2634 transport of parameter sets MUST be used. 2636 When level downgrade is in use, i.e., a level lower than the 2637 default level offered is accepted, the following applies. 2639 o If "use-level-parameter-sets" is not present in the answer 2640 for the accepted payload type or the value is equal to 0 in 2641 the answer for the accepted payload type, the answerer MUST 2642 ignore "sprop-parameter-sets" and "sprop-level-parameter- 2643 sets", when present in the offer for the accepted payload 2644 type. 2646 o Otherwise (the "use-level-parameter-sets" is present in the 2647 answer for the accepted payload type and the value is equal 2648 to 1), the answerer MUST be prepared to use the parameter 2649 sets that are included in "sprop-level-parameter-sets" for 2650 the accepted level, when present, for decoding the incoming 2651 NAL unit stream, and ignore all other parameter sets 2652 included in "sprop-level-parameter-sets" and "sprop- 2653 parameter-sets", when present. 2655 o When no parameter sets for the accepted level are present in 2656 the "sprop-level-parameter-sets", in-band transport of 2657 parameter sets MUST be used. 2659 The answerer MAY or MAY not include "sprop-parameter-sets", i.e., 2660 the answerer MAY use either out-of-band or in-band transport of 2661 parameter sets for the stream it is sending, regardless of 2662 whether out-of-band parameter sets transport has been used in the 2663 offerer-to-answerer direction. All parameter sets included in 2664 the "sprop-parameter-sets", when present, for the accepted 2665 payload type in an answer MUST be associated with the accepted 2666 level, as indicated by the profile-level-id in the answer for the 2667 accepted payload type. 2669 Parameter sets included in "sprop-parameter-sets" in an answer 2670 are independent of those parameter sets included in the offer, as 2671 they are used for decoding two different video streams, one from 2672 the answerer to the offerer, and the other in the opposite 2673 direction. The offerer MUST be prepared to use the parameter 2674 sets included in the answer's "sprop-parameter-sets", when 2675 present, for decoding the incoming NAL unit stream. 2677 When "sprop-parameter-sets" or "sprop-level-parameter-sets" is 2678 present and "sprop-ssrc" is present, the receiver of the 2679 parameters MUST store the parameter sets included in the "sprop- 2680 parameter-sets" or "sprop-level-parameter-sets" for the accepted 2681 level and associate them to "sprop-ssrc". Parameter sets 2682 associated with one "sprop-ssrc" MUST only be used to decode NAL 2683 units conveyed in packets with SSRC equal to the associated 2684 "sprop-ssrc". The "sprop-ssrc" MAY be used in topologies like 2685 Topo-Video-switch-MCU [28] to enable out-of-band transport of 2686 parameter sets. When "sprop-ssrc" is used, and SSRC collision is 2687 detected, the connection needs to be renegotiated using a new 2688 random SSRC. 2690 For streams being delivered over multicast, the following rules 2691 apply: 2693 o The media format configuration is identified by the same 2694 parameters as above for unicast (i.e. "profile-level-id" and 2695 "packetization-mode", when present). These media format 2696 configuration parameters (including the level part of "profile- 2697 level-id", i.e. the level part of "profile-level-id" is not 2698 downgradable for Offer/Answer in multicast) MUST be used 2699 symmetrically; i.e., the answerer MUST either maintain all 2700 configuration parameters or remove the media format (payload type) 2701 completely. 2703 To simplify handling and matching of these configurations, the 2704 same RTP payload type number used in the offer SHOULD also be 2705 used in the answer, as specified in [8]. An answer MUST NOT 2706 contain a payload type number used in the offer unless the 2707 configuration is the same as in the offer. 2709 o Parameter sets received MUST be associated with the originating 2710 source, and MUST be only used in decoding the incoming NAL unit 2711 stream from the same source. 2713 o The rules for other parameters are the same as above for unicast. 2715 Below are the complete lists of how the different parameters shall be 2716 interpreted in the different combinations of offer or answer and 2717 direction attribute. 2719 o In offers and answers for which "a=sendrecv" or no direction 2720 attribute is used, the following interpretation of the parameters 2721 MUST be used. 2723 Declaring actual configuration for sending and receiving streams: 2725 - profile-level-id 2726 - packetization-mode 2728 Declaring actual properties of the stream to be sent: 2730 - sprop-deint-buf-req 2731 - sprop-interleaving-depth 2732 - sprop-max-don-diff 2733 - sprop-init-buf-time 2734 - sprop-ssrc 2736 Declaring receiver capabilities: 2738 - max-mbps 2739 - max-smbps 2740 - max-fs 2741 - max-cpb 2742 - max-dpb 2743 - max-br 2744 - redundant-pic-cap 2745 - deint-buf-cap 2746 - max-rcmd-nalu-size 2747 - sar-understood 2748 - sar-supported 2749 - use-level-parameter-sets 2751 Out-of-band transporting of parameter sets: 2753 - sprop-parameter-sets 2754 - sprop-level-parameter-sets 2756 o In offers and answers for which "a=recvonly" is used, the 2757 following interpretation of the parameters MUST be used. 2759 Declaring actual configuration for receiving streams: 2761 - profile-level-id 2762 - packetization-mode 2764 Declaring receiver capabilities: 2766 - max-mbps 2767 - max-smbps 2768 - max-fs 2769 - max-cpb 2770 - max-dpb 2771 - max-br 2772 - redundant-pic-cap 2773 - deint-buf-cap 2774 - max-rcmd-nalu-size 2775 - sar-understood 2776 - sar-supported 2777 - use-level-parameter-sets 2779 Not usable (when present, they SHOULD be ignored): 2781 - sprop-deint-buf-req 2782 - sprop-interleaving-depth 2783 - sprop-parameter-sets 2784 - sprop-level-parameter-sets 2785 - sprop-max-don-diff 2786 - sprop-init-buf-time 2787 - sprop-ssrc 2789 o In offers or answers for which "a=sendonly" is used, the following 2790 interpretation of the parameters MUST be used. 2792 Declaring actual configuration or properties for sending streams: 2794 - profile-level-id 2795 - packetization-mode 2796 - sprop-deint-buf-req 2797 - sprop-max-don-diff 2798 - sprop-init-buf-time 2799 - sprop-interleaving-depth 2800 - sprop-ssrc 2802 Out-of-band transporting of parameter sets: 2804 - sprop-parameter-sets 2805 - sprop-level-parameter-sets 2807 Not usable(when present, they SHOULD be ignored): 2809 - max-mbps 2810 - max-smbps 2811 - max-fs 2812 - max-cpb 2813 - max-dpb 2814 - max-br 2815 - redundant-pic-cap 2816 - deint-buf-cap 2817 - max-rcmd-nalu-size 2818 - sar-understood 2819 - sar-supported 2820 - use-level-parameter-sets 2822 Furthermore, the following considerations are necessary: 2824 o Parameters used for declaring receiver capabilities are in general 2825 downgradable; i.e., they express the upper limit for a sender's 2826 possible behavior. Thus a sender MAY select to set its encoder 2827 using only lower/less or equal values of these parameters. 2829 o Parameters declaring a configuration point are not downgradable, 2830 with the exception of the level part of the "profile-level-id" 2831 parameter for unicast usage. This expresses values a receiver 2832 expects to be used and must be used verbatim on the sender side. 2834 o When a sender's capabilities are declared, and non-downgradable 2835 parameters are used in this declaration, then these parameters 2836 express a configuration that is acceptable for the sender to 2837 receive streams. In order to achieve high interoperability 2838 levels, it is often advisable to offer multiple alternative 2839 configurations; e.g., for the packetization mode. It is 2840 impossible to offer multiple configurations in a single payload 2841 type. Thus, when multiple configuration offers are made, each 2842 offer requires its own RTP payload type associated with the offer. 2844 o A receiver SHOULD understand all media type parameters, even if it 2845 only supports a subset of the payload format's functionality. 2846 This ensures that a receiver is capable of understanding when an 2847 offer to receive media can be downgraded to what is supported by 2848 the receiver of the offer. 2850 o An answerer MAY extend the offer with additional media format 2851 configurations. However, to enable their usage, in most cases a 2852 second offer is required from the offerer to provide the stream 2853 properties parameters that the media sender will use. This also 2854 has the effect that the offerer has to be able to receive this 2855 media format configuration, not only to send it. 2857 o If an offerer wishes to have non-symmetric capabilities between 2858 sending and receiving, the offerer should offer different RTP 2859 sessions; i.e., different media lines declared as "recvonly" and 2860 "sendonly", respectively. This may have further implications on 2861 the system. 2863 8.2.3. Usage in Declarative Session Descriptions 2865 When H.264 over RTP is offered with SDP in a declarative style, as in 2866 RTSP [26] or SAP [27], the following considerations are necessary. 2868 o All parameters capable of indicating both stream properties and 2869 receiver capabilities are used to indicate only stream properties. 2870 For example, in this case, the parameter "profile-level-id" 2871 declares only the values used by the stream, not the capabilities 2872 for receiving streams. This results in that the following 2873 interpretation of the parameters MUST be used: 2875 Declaring actual configuration or stream properties: 2877 - profile-level-id 2878 - packetization-mode 2879 - sprop-interleaving-depth 2880 - sprop-deint-buf-req 2881 - sprop-max-don-diff 2882 - sprop-init-buf-time 2883 - sprop-ssrc 2885 Out-of-band transporting of parameter sets: 2887 - sprop-parameter-sets 2888 - sprop-level-parameter-sets 2890 Not usable(when present, they SHOULD be ignored): 2892 - max-mbps 2893 - max-smbps 2894 - max-fs 2895 - max-cpb 2896 - max-dpb 2897 - max-br 2898 - redundant-pic-cap 2899 - max-rcmd-nalu-size 2900 - deint-buf-cap 2901 - sar-understood 2902 - sar-supported 2903 - use-level-parameter-sets 2905 o A receiver of the SDP is required to support all parameters and 2906 values of the parameters provided; otherwise, the receiver MUST 2907 reject (RTSP) or not participate in (SAP) the session. It falls 2908 on the creator of the session to use values that are expected to 2909 be supported by the receiving application. 2911 8.3. Examples 2913 An SDP Offer/Answer exchange wherein both parties are expected to 2914 both send and receive could look like the following. Only the media 2915 codec specific parts of the SDP are shown. Some lines are wrapped 2916 due to text constraints. 2918 Offerer -> Answerer SDP message: 2920 m=video 49170 RTP/AVP 100 99 98 2921 a=rtpmap:98 H264/90000 2922 a=fmtp:98 profile-level-id=42A01E; packetization-mode=0; 2923 sprop-parameter-sets= 2924 a=rtpmap:99 H264/90000 2925 a=fmtp:99 profile-level-id=42A01E; packetization-mode=1; 2926 sprop-parameter-sets= 2927 a=rtpmap:100 H264/90000 2928 a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; 2929 sprop-parameter-sets=; 2930 sprop-interleaving-depth=45; sprop-deint-buf-req=64000; 2931 sprop-init-buf-time=102478; deint-buf-cap=128000 2933 The above offer presents the same codec configuration in three 2934 different packetization formats. PT 98 represents single NALU mode, 2935 PT 99 represents non-interleaved mode, and PT 100 indicates the 2936 interleaved mode. In the interleaved mode case, the interleaving 2937 parameters that the offerer would use if the answer indicates support 2938 for PT 100 are also included. In all three cases the parameter 2939 "sprop-parameter-sets" conveys the initial parameter sets that are 2940 required by the answerer when receiving a stream from the offerer 2941 when this configuration is accepted. Note that the value for "sprop- 2942 parameter-sets" could be different for each payload type. 2944 Answerer -> Offerer SDP message: 2946 m=video 49170 RTP/AVP 100 99 97 2947 a=rtpmap:97 H264/90000 2948 a=fmtp:97 profile-level-id=42A01E; packetization-mode=0; 2949 sprop-parameter-sets= 2950 a=rtpmap:99 H264/90000 2951 a=fmtp:99 profile-level-id=42A01E; packetization-mode=1; 2952 sprop-parameter-sets=; 2953 max-rcmd-nalu-size=3980 2954 a=rtpmap:100 H264/90000 2955 a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; 2956 sprop-parameter-sets=; 2957 sprop-interleaving-depth=60; 2958 sprop-deint-buf-req=86000; sprop-init-buf-time=156320; 2959 deint-buf-cap=128000; max-rcmd-nalu-size=3980 2961 As the Offer/Answer negotiation covers both sending and receiving 2962 streams, an offer indicates the exact parameters for what the offerer 2963 is willing to receive, whereas the answer indicates the same for what 2964 the answerer accepts to receive. In this case the offerer declared 2965 that it is willing to receive payload type 98. The answerer accepts 2966 this by declaring an equivalent payload type 97; i.e., it has 2967 identical values for the two parameters "profile-level-id" and 2968 "packetization-mode" (since "packetization-mode" is equal to 0, 2969 "sprop-deint-buf-req" is not present). As the offered payload type 2970 98 is accepted, the answerer needs to store parameter sets included 2971 in sprop-parameter-sets= in case the offer finally 2972 decides to use this configuration. In the answer, the answerer 2973 includes the parameter sets in sprop-parameter-sets= 2974 that the answerer would use in the stream sent from the answerer if 2975 this configuration is finally used. 2977 The answerer also accepts the reception of the two configurations 2978 that payload types 99 and 100 represent. Again, the answerer needs 2979 to store parameter sets included in sprop-parameter-sets= and sprop-parameter-sets= in case the offer 2981 finally decides to use either of these two configurations. The 2982 answerer provides the initial parameter sets for the answerer-to- 2983 offerer direction, i.e. the parameter sets in sprop-parameter- 2984 sets= and sprop-parameter-sets=, for 2985 payload types 99 and 100, respectively, that it will use to send the 2986 payload types. The answerer also provides the offerer with its 2987 memory limit for de-interleaving operations by providing a "deint- 2988 buf-cap" parameter. This is only useful if the offerer decides on 2989 making a second offer, where it can take the new value into account. 2990 The "max-rcmd-nalu-size" indicates that the answerer can efficiently 2991 process NALUs up to the size of 3980 bytes. However, there is no 2992 guarantee that the network supports this size. 2994 In the following example, the offer is accepted without level 2995 downgrading (i.e. the default level, 3.0, is accepted), and both 2996 "sprop-parameter-sets" and "sprop-level-parameter-sets" are present 2997 in the offer. The answerer must ignore sprop-level-parameter- 2998 sets= and store parameter sets in sprop-parameter- 2999 sets= for decoding the incoming NAL unit stream. The 3000 offerer must store the parameter sets in sprop-parameter-sets= in the answer for decoding the incoming NAL unit stream. 3002 Note that in this example, parameter sets in sprop-parameter- 3003 sets= must be associated with level 3.0. 3005 Offer SDP: 3007 m=video 49170 RTP/AVP 98 3008 a=rtpmap:98 H264/90000 3009 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3010 packetization-mode=1; 3011 sprop-parameter-sets=; 3012 sprop-level-parameter-sets= 3014 Answer SDP: 3016 m=video 49170 RTP/AVP 98 3017 a=rtpmap:98 H264/90000 3018 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3019 packetization-mode=1; 3020 sprop-parameter-sets= 3022 In the following example, the offer (Baseline profile, level 1.1) is 3023 accepted with level downgrading (the accepted level is 1b), and both 3024 "sprop-parameter-sets" and "sprop-level-parameter-sets" are present 3025 in the offer. The answerer must ignore sprop-parameter-sets= and all parameter sets not for the accepted level (level 1b) 3027 in sprop-level-parameter-sets=, and must store 3028 parameter sets for the accepted level (level 1b) in sprop-level- 3029 parameter-sets= for decoding the incoming NAL unit 3030 stream. The offerer must store the parameter sets in sprop- 3031 parameter-sets= in the answer for decoding the 3032 incoming NAL unit stream. Note that in this example, parameter sets 3033 in sprop-parameter-sets= must be associated with 3034 level 1b. 3036 Offer SDP: 3038 m=video 49170 RTP/AVP 98 3039 a=rtpmap:98 H264/90000 3040 a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1 3041 packetization-mode=1; 3042 sprop-parameter-sets=; 3043 sprop-level-parameter-sets= 3045 Answer SDP: 3047 m=video 49170 RTP/AVP 98 3048 a=rtpmap:98 H264/90000 3049 a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b 3050 packetization-mode=1; 3051 sprop-parameter-sets=; 3052 use-level-parameter-sets=1 3054 In the following example, the offer (Baseline profile, level 1.1) is 3055 accepted with level downgrading (the accepted level is 1b), and both 3056 "sprop-parameter-sets" and "sprop-level-parameter-sets" are present 3057 in the offer. However, the answerer is a legacy RFC 3984 3058 implementation and does not understand "sprop-level-parameter-sets", 3059 hence it does not include "use-level-parameter-sets" (which the 3060 answerer does not understand, either) in the answer. Therefore, the 3061 answerer must ignore both sprop-parameter-sets= and 3062 sprop-level-parameter-sets=, and the offerer must 3063 transport parameter sets in-band. 3065 Offer SDP: 3067 m=video 49170 RTP/AVP 98 3068 a=rtpmap:98 H264/90000 3069 a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1 3070 packetization-mode=1; 3071 sprop-parameter-sets=; 3072 sprop-level-parameter-sets= 3074 Answer SDP: 3076 m=video 49170 RTP/AVP 98 3077 a=rtpmap:98 H264/90000 3078 a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b 3079 packetization-mode=1 3081 In the following example, the offer is accepted without level 3082 downgrading, and "sprop-parameter-sets" is present in the offer. 3083 Parameter sets in sprop-parameter-sets= must be 3084 stored and used used by the encoder of the offerer and the decoder of 3085 the answerer, and parameter sets in sprop-parameter-sets=must be used by the encoder of the answerer and the decoder of 3087 the offerer. Note that sprop-parameter-sets= is 3088 basically independent of sprop-parameter-sets=. 3090 Offer SDP: 3092 m=video 49170 RTP/AVP 98 3093 a=rtpmap:98 H264/90000 3094 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3095 packetization-mode=1; 3096 sprop-parameter-sets= 3098 Answer SDP: 3100 m=video 49170 RTP/AVP 98 3101 a=rtpmap:98 H264/90000 3102 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3103 packetization-mode=1; 3104 sprop-parameter-sets= 3106 In the following example, the offer is accepted without level 3107 downgrading, and neither "sprop-parameter-sets" nor "sprop-level- 3108 parameter-sets" is present in the offer, meaning that there is no 3109 out-of-band transmission of parameter sets, which then have to be 3110 transported in-band. 3112 Offer SDP: 3114 m=video 49170 RTP/AVP 98 3115 a=rtpmap:98 H264/90000 3116 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3117 packetization-mode=1 3119 Answer SDP: 3121 m=video 49170 RTP/AVP 98 3122 a=rtpmap:98 H264/90000 3123 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3124 packetization-mode=1 3126 In the following example, the offer is accepted with level 3127 downgrading and "sprop-parameter-sets" is present in the offer. As 3128 sprop-parameter-sets= contains level_idc indicating 3129 Level 3.0, therefore cannot be used as the answerer wants Level 2.0 3130 and must be ignored by the answerer, and in-band parameter sets must 3131 be used. 3133 Offer SDP: 3135 m=video 49170 RTP/AVP 98 3136 a=rtpmap:98 H264/90000 3137 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3138 packetization-mode=1; 3139 sprop-parameter-sets= 3141 Answer SDP: 3143 m=video 49170 RTP/AVP 98 3144 a=rtpmap:98 H264/90000 3145 a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0 3146 packetization-mode=1 3148 In the following example, the offer is also accepted with level 3149 downgrading, and neither "sprop-parameter-sets" nor "sprop-level- 3150 parameter-sets" is present in the offer, meaning that there is no 3151 out-of-band transmission of parameter sets, which then have to be 3152 transported in-band. 3154 Offer SDP: 3156 m=video 49170 RTP/AVP 98 3157 a=rtpmap:98 H264/90000 3158 a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0 3159 packetization-mode=1 3161 Answer SDP: 3163 m=video 49170 RTP/AVP 98 3164 a=rtpmap:98 H264/90000 3165 a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0 3166 packetization-mode=1 3168 8.4. Parameter Set Considerations 3170 The H.264 parameter sets are a fundamental part of the video codec 3171 and vital to its operation; see section 1.2. Due to their 3172 characteristics and their importance for the decoding process, lost 3173 or erroneously transmitted parameter sets can hardly be concealed 3174 locally at the receiver. A reference to a corrupt parameter set has 3175 normally fatal results to the decoding process. Corruption could 3176 occur, for example, due to the erroneous transmission or loss of a 3177 parameter set NAL unit, but also due to the untimely transmission of 3178 a parameter set update. A parameter set update refers to a change of 3179 at least one parameter in a picture parameter set or sequence 3180 parameter set for which the picture parameter set or sequence 3181 parameter set identifier remains unchanged. Therefore, the following 3182 recommendations are provided as a guideline for the implementer of 3183 the RTP sender. 3185 Parameter set NALUs can be transported using three different 3186 principles: 3188 A. Using a session control protocol (out-of-band) prior to the actual 3189 RTP session. 3191 B. Using a session control protocol (out-of-band) during an ongoing 3192 RTP session. 3194 C. Within the RTP packet stream in the payload (in-band) during an 3195 ongoing RTP session. 3197 It is recommended to implement principles A and B within a session 3198 control protocol. SIP and SDP can be used as described in the SDP 3199 Offer/Answer model and in the previous sections of this memo. This 3200 section contains guidelines on how principles A and B should be 3201 implemented within session control protocols. It is independent of 3202 the particular protocol used. Principle C is supported by the RTP 3203 payload format defined in this specification. There are topologies 3204 like Topo-Video-switch-MCU [28] for which the use of principle C may 3205 be desirable. 3207 If in-band signaling of parameter sets is used, the picture and 3208 sequence parameter set NALUs SHOULD be transmitted in the RTP payload 3209 using a reliable method of delivering of RTP (see below), as a loss 3210 of a parameter set of either type will likely prevent decoding of a 3211 considerable portion of the corresponding RTP packet stream. 3213 If in-band signaling of parameter sets is used, the sender SHOULD 3214 take the error characteristics into account and use mechanisms to 3215 provide a high probability for delivering the parameter sets 3216 correctly. Mechanisms that increase the probability for a correct 3217 reception include packet repetition, FEC, and retransmission. The 3218 use of an unreliable, out-of-band control protocol has similar 3219 disadvantages as the in-band signaling (possible loss) and, in 3220 addition, may also lead to difficulties in the synchronization (see 3221 below). Therefore, it is NOT RECOMMENDED. 3223 Parameter sets MAY be added or updated during the lifetime of a 3224 session using principles B and C. It is required that parameter sets 3225 are present at the decoder prior to the NAL units that refer to them. 3226 Updating or adding of parameter sets can result in further problems, 3227 and therefore the following recommendations should be considered. 3229 - When parameter sets are added or updated, care SHOULD be taken to 3230 ensure that any parameter set is delivered prior to its usage. 3231 When new parameter sets are added, previously unused parameter set 3232 identifiers are used. It is common that no synchronization is 3233 present between out-of-band signaling and in-band traffic. If 3234 out-of-band signaling is used, it is RECOMMENDED that a sender 3235 does not start sending NALUs requiring the added or updated 3236 parameter sets prior to acknowledgement of delivery from the 3237 signaling protocol. 3239 - When parameter sets are updated, the following synchronization 3240 issue should be taken into account. When overwriting a parameter 3241 set at the receiver, the sender has to ensure that the parameter 3242 set in question is not needed by any NALU present in the network 3243 or receiver buffers. Otherwise, decoding with a wrong parameter 3244 set may occur. To lessen this problem, it is RECOMMENDED either 3245 to overwrite only those parameter sets that have not been used for 3246 a sufficiently long time (to ensure that all related NALUs have 3247 been consumed), or to add a new parameter set instead (which may 3248 have negative consequences for the efficiency of the video 3249 coding). 3251 Informative note: In some topologies like Topo-Video-switch- 3252 MCU [28] the origin of the whole set of parameter sets may 3253 come from multiple sources that may use non-unique parameter 3254 sets identifiers. In this case an offer may overwrite an 3255 existing parameter set if no other mechanism that enables 3256 uniqueness of the parameter sets in the out-of-band channel 3257 exists. 3259 - In a multiparty session, one participant MUST associate parameter 3260 sets coming from different sources with the source identification 3261 whenever possible, e.g. by using sprop-ssrc for out-of-band 3262 transported parameter sets, as different sources typically use 3263 independent parameter set identifier value spaces. 3265 - Adding or modifying parameter sets by using both principles B and 3266 C in the same RTP session may lead to inconsistencies of the 3267 parameter sets because of the lack of synchronization between the 3268 control and the RTP channel. Therefore, principles B and C MUST 3269 NOT both be used in the same session unless sufficient 3270 synchronization can be provided. 3272 In some scenarios (e.g., when only the subset of this payload format 3273 specification corresponding to H.241 is used) or topologies, it is 3274 not possible to employ out-of-band parameter set transmission. In 3275 this case, parameter sets have to be transmitted in-band. Here, the 3276 synchronization with the non-parameter-set-data in the bitstream is 3277 implicit, but the possibility of a loss has to be taken into account. 3278 The loss probability should be reduced using the mechanisms discussed 3279 above. In case a loss of a parameter set is detected, recovery may 3280 be achieved by using a Decoder Refresh Point procedure, for example, 3281 using RTCP feedback Full Intra Request (FIR) [29]. Two example 3282 Decoder Refresh Point procedures are provided in the informative 3283 Section 8.5. 3285 - When parameter sets are initially provided using principle A and 3286 then later added or updated in-band (principle C), there is a risk 3287 associated with updating the parameter sets delivered out-of-band. 3288 If receivers miss some in-band updates (for example, because of a 3289 loss or a late tune-in), those receivers attempt to decode the 3290 bitstream using out-dated parameters. It is therefore RECOMMENDED 3291 that parameter set IDs be partitioned between the out-of-band and 3292 in-band parameter sets. 3294 8.5. Decoder Refresh Point Procedure using In-Band Transport of 3295 Parameter Sets (Informative) 3297 When a sender with a video encoder according to [1] receives a 3298 request for a decoder refresh point, the encoder shall enter the fast 3299 update mode by using one of the procedures specified in Section 8.5.1 3300 or 8.5.2 below. The procedure in 8.5.1 is the preferred response in 3301 a lossless transmission environment. Both procedures satisfy the 3302 requirement to enter the fast update mode for H.264 video encoding. 3304 8.5.1. IDR Procedure to Respond to a Request for a Decoder Refresh Point 3306 This section gives one possible way to respond to a request for a 3307 decoder refresh point. 3309 The encoder shall, in the order presented here: 3311 1) Immediately prepare to send an IDR picture. 3313 2) Send a sequence parameter set to be used by the IDR picture to be 3314 sent. The encoder may optionally also send other sequence 3315 parameter sets. 3317 3) Send a picture parameter set to be used by the IDR picture to be 3318 sent. The encoder may optionally also send other picture parameter 3319 sets. 3321 4) Send the IDR picture. 3323 5) From this point forward in time, send any other sequence or 3324 picture parameter sets that have not yet been sent in this 3325 procedure, prior to their reference by any NAL unit, regardless of 3326 whether such parameter sets were previously sent prior to 3327 receiving the request for a decoder refresh point. As needed, 3328 such parameter sets may be sent in a batch, one at a time, or in 3329 any combination of these two methods. Parameter sets may be re- 3330 sent at any time for redundancy. Caution should be taken when 3331 parameter set updates are present, as described above in Section 3332 8.4. 3334 8.5.2. Gradual Recovery Procedure to Respond to a Request for a Decoder 3335 Refresh Point 3337 This section gives another possible way to respond to a request for a 3338 decoder refresh point. 3340 The encoder shall, in the order presented here: 3342 1) Send a recovery point SEI message (see Sections D.1.7 and D.2.7 of 3343 [1]). 3345 2) Repeat any sequence and picture parameter sets that were sent 3346 before the recovery point SEI message, prior to their reference by 3347 a NAL unit. 3349 The encoder shall ensure that the decoder has access to all reference 3350 pictures for inter prediction of pictures at or after the recovery 3351 point, which is indicated by the recovery point SEI message, in 3352 output order, assuming that the transmission from now on is error- 3353 free. 3355 The value of the recovery_frame_cnt syntax element in the recovery 3356 point SEI message should be small enough to ensure a fast recovery. 3358 As needed, such parameter sets may be re-sent in a batch, one at a 3359 time, or in any combination of these two methods. Parameter sets may 3360 be re-sent at any time for redundancy. Caution should be taken when 3361 parameter set updates are present, as described above in Section 8.4. 3363 9. Security Considerations 3365 RTP packets using the payload format defined in this specification 3366 are subject to the security considerations discussed in the RTP 3367 specification [5], and in any appropriate RTP profile (for example, 3368 [15]). This implies that confidentiality of the media streams is 3369 achieved by encryption; for example, through the application of SRTP 3371 [25]. Because the data compression used with this payload format is 3372 applied end-to-end, any encryption needs to be performed after 3373 compression. A potential denial-of-service threat exists for data 3374 encodings using compression techniques that have non-uniform 3375 receiver-end computational load. The attacker can inject 3376 pathological datagrams into the stream that are complex to decode and 3377 that cause the receiver to be overloaded. H.264 is particularly 3378 vulnerable to such attacks, as it is extremely simple to generate 3379 datagrams containing NAL units that affect the decoding process of 3380 many future NAL units. Therefore, the usage of data origin 3381 authentication and data integrity protection of at least the RTP 3382 packet is RECOMMENDED; for example, with SRTP [25]. 3384 Note that the appropriate mechanism to ensure confidentiality and 3385 integrity of RTP packets and their payloads is very dependent on the 3386 application and on the transport and signaling protocols employed. 3387 Thus, although SRTP is given as an example above, other possible 3388 choices exist. 3390 Decoders MUST exercise caution with respect to the handling of user 3391 data SEI messages, particularly if they contain active elements, and 3392 MUST restrict their domain of applicability to the presentation 3393 containing the stream. 3395 End-to-End security with either authentication, integrity or 3396 confidentiality protection will prevent a MANE from performing media- 3397 aware operations other than discarding complete packets. And in the 3398 case of confidentiality protection it will even be prevented from 3399 performing discarding of packets in a media aware way. To allow any 3400 MANE to perform its operations, it will be required to be a trusted 3401 entity which is included in the security context establishment. 3403 10. Congestion Control 3405 Congestion control for RTP SHALL be used in accordance with RFC 3550 3406 [5], and with any applicable RTP profile; e.g., RFC 3551 [15]. An 3407 additional requirement if best-effort service is being used is: users 3408 of this payload format MUST monitor packet loss to ensure that the 3409 packet loss rate is within acceptable parameters. Packet loss is 3410 considered acceptable if a TCP flow across the same network path, and 3411 experiencing the same network conditions, would achieve an average 3412 throughput, measured on a reasonable timescale that is not less than 3413 the RTP flow is achieving. This condition can be satisfied by 3414 implementing congestion control mechanisms to adapt the transmission 3415 rate (or the number of layers subscribed for a layered multicast 3416 session), or by arranging for a receiver to leave the session if the 3417 loss rate is unacceptably high. 3419 The bit rate adaptation necessary for obeying the congestion control 3420 principle is easily achievable when real-time encoding is used. 3421 However, when pre-encoded content is being transmitted, bandwidth 3422 adaptation requires the availability of more than one coded 3423 representation of the same content, at different bit rates, or the 3424 existence of non-reference pictures or sub-sequences [21] in the 3425 bitstream. The switching between the different representations can 3426 normally be performed in the same RTP session; e.g., by employing a 3427 concept known as SI/SP slices of the Extended Profile, or by 3428 switching streams at IDR picture boundaries. Only when non- 3429 downgradable parameters (such as the profile part of the 3430 profile/level ID) are required to be changed does it become necessary 3431 to terminate and re-start the media stream. This may be accomplished 3432 by using a different RTP payload type. 3434 MANEs MAY follow the suggestions outlined in section 7.3 and remove 3435 certain unusable packets from the packet stream when that stream was 3436 damaged due to previous packet losses. This can help reduce the 3437 network load in certain special cases. 3439 11. IANA Consideration 3441 IANA has registered one new media type; see section 8.1. 3443 12. Informative Appendix: Application Examples 3445 This payload specification is very flexible in its use, in order to 3446 cover the extremely wide application space anticipated for H.264. 3447 However, this great flexibility also makes it difficult for an 3448 implementer to decide on a reasonable packetization scheme. Some 3449 information on how to apply this specification to real-world 3450 scenarios is likely to appear in the form of academic publications 3451 and a test model software and description in the near future. 3452 However, some preliminary usage scenarios are described here as well. 3454 12.1. Video Telephony according to ITU-T Recommendation H.241 Annex A 3456 H.323-based video telephony systems that use H.264 as an optional 3457 video compression scheme are required to support H.241 Annex A [3] as 3458 a packetization scheme. The packetization mechanism defined in this 3459 Annex is technically identical with a small subset of this 3460 specification. 3462 When a system operates according to H.241 Annex A, parameter set NAL 3463 units are sent in-band. Only Single NAL unit packets are used. Many 3464 such systems are not sending IDR pictures regularly, but only when 3465 required by user interaction or by control protocol means; e.g., when 3466 switching between video channels in a Multipoint Control Unit or for 3467 error recovery requested by feedback. 3469 12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit 3470 Aggregation 3472 The RTP part of this scheme is implemented and tested (though not the 3473 control-protocol part; see below). 3475 In most real-world video telephony applications, picture parameters 3476 such as picture size or optional modes never change during the 3477 lifetime of a connection. Therefore, all necessary parameter sets 3478 (usually only one) are sent as a side effect of the capability 3479 exchange/announcement process, e.g., according to the SDP syntax 3480 specified in section 8.2 of this document. As all necessary 3481 parameter set information is established before the RTP session 3482 starts, there is no need for sending any parameter set NAL units. 3483 Slice data partitioning is not used, either. Thus, the RTP packet 3484 stream basically consists of NAL units that carry single coded 3485 slices. 3487 The encoder chooses the size of coded slice NAL units so that they 3488 offer the best performance. Often, this is done by adapting the 3489 coded slice size to the MTU size of the IP network. For small 3490 picture sizes, this may result in a one-picture-per-one-packet 3491 strategy. Intra refresh algorithms clean up the loss of packets and 3492 the resulting drift-related artifacts. 3494 12.3. Video Telephony, Interleaved Packetization Using NAL Unit 3495 Aggregation 3497 This scheme allows better error concealment and is used in H.263 3498 based designs using RFC 2429 packetization [10]. It has been 3499 implemented, and good results were reported [12]. 3501 The VCL encoder codes the source picture so that all macroblocks 3502 (MBs) of one MB line are assigned to one slice. All slices with even 3503 MB row addresses are combined into one STAP, and all slices with odd 3504 MB row addresses into another. Those STAPs are transmitted as RTP 3505 packets. The establishment of the parameter sets is performed as 3506 discussed above. 3508 Note that the use of STAPs is essential here, as the high number of 3509 individual slices (18 for a CIF picture) would lead to unacceptably 3510 high IP/UDP/RTP header overhead (unless the source coding tool FMO is 3511 used, which is not assumed in this scenario). Furthermore, some 3512 wireless video transmission systems, such as H.324M and the IP-based 3513 video telephony specified in 3GPP, are likely to use relatively small 3514 transport packet size. For example, a typical MTU size of H.223 AL3 3515 SDU is around 100 bytes [16]. Coding individual slices according to 3516 this packetization scheme provides further advantage in communication 3517 between wired and wireless networks, as individual slices are likely 3518 to be smaller than the preferred maximum packet size of wireless 3519 systems. Consequently, a gateway can convert the STAPs used in a 3520 wired network into several RTP packets with only one NAL unit, which 3521 are preferred in a wireless network, and vice versa. 3523 12.4. Video Telephony with Data Partitioning 3525 This scheme has been implemented and has been shown to offer good 3526 performance, especially at higher packet loss rates [12]. 3528 Data Partitioning is known to be useful only when some form of 3529 unequal error protection is available. Normally, in single-session 3530 RTP environments, even error characteristics are assumed; i.e., the 3531 packet loss probability of all packets of the session is the same 3532 statistically. However, there are means to reduce the packet loss 3533 probability of individual packets in an RTP session. A FEC packet 3534 according to RFC 2733 [17], for example, specifies which media 3535 packets are associated with the FEC packet. 3537 In all cases, the incurred overhead is substantial but is in the same 3538 order of magnitude as the number of bits that have otherwise been 3539 spent for intra information. However, this mechanism does not add 3540 any delay to the system. 3542 Again, the complete parameter set establishment is performed through 3543 control protocol means. 3545 12.5. Video Telephony or Streaming with FUs and Forward Error Correction 3547 This scheme has been implemented and has been shown to provide good 3548 performance, especially at higher packet loss rates [18]. 3550 The most efficient means to combat packet losses for scenarios where 3551 retransmissions are not applicable is forward error correction (FEC). 3552 Although application layer, end-to-end use of FEC is often less 3553 efficient than an FEC-based protection of individual links 3554 (especially when links of different characteristics are in the 3555 transmission path), application layer, end-to-end FEC is unavoidable 3556 in some scenarios. RFC 2733 [17] provides means to use generic, 3557 application layer, end-to-end FEC in packet-loss environments. A 3558 binary forward error correcting code is generated by applying the XOR 3559 operation to the bits at the same bit position in different packets. 3561 The binary code can be specified by the parameters (n,k) in which k 3562 is the number of information packets used in the connection and n is 3563 the total number of packets generated for k information packets; 3564 i.e., n-k parity packets are generated for k information packets. 3565 [Ed. (YkW): from Randell: References to RFC 2733 should be updated to 3566 (and checked against) RFC 5109. There are a lot of calculations and 3567 the like that should be checked. Also update [17] to RFC 5109. ] 3569 When a code is used with parameters (n,k) within the RFC 2733 3570 framework, the following properties are well known: 3572 a) If applied over one RTP packet, RFC 2733 provides only packet 3573 repetition. 3575 b) RFC 2733 is most bit rate efficient if XOR-connected packets have 3576 equal length. 3578 c) At the same packet loss probability p and for a fixed k, the 3579 greater the value of n is, the smaller the residual error 3580 probability becomes. For example, for a packet loss probability 3581 of 10%, k=1, and n=2, the residual error probability is about 1%, 3582 whereas for n=3, the residual error probability is about 0.1%. 3584 d) At the same packet loss probability p and for a fixed code rate 3585 k/n, the greater the value of n is, the smaller the residual error 3586 probability becomes. For example, at a packet loss probability of 3587 p=10%, k=1 and n=2, the residual error rate is about 1%, whereas 3588 for an extended Golay code with k=12 and n=24, the residual error 3589 rate is about 0.01%. 3591 For applying RFC 2733 in combination with H.264 baseline coded video 3592 without using FUs, several options might be considered: 3594 1) The video encoder produces NAL units for which each video frame is 3595 coded in a single slice. Applying FEC, one could use a simple 3596 code; e.g., (n=2, k=1). That is, each NAL unit would basically 3597 just be repeated. The disadvantage is obviously the bad code 3598 performance according to d), above, and the low flexibility, as 3599 only (n, k=1) codes can be used. 3601 2) The video encoder produces NAL units for which each video frame is 3602 encoded in one or more consecutive slices. Applying FEC, one 3603 could use a better code, e.g., (n=24, k=12), over a sequence of 3604 NAL units. Depending on the number of RTP packets per frame, a 3605 loss may introduce a significant delay, which is reduced when more 3606 RTP packets are used per frame. Packets of completely different 3607 length might also be connected, which decreases bit rate 3608 efficiency according to b), above. However, with some care and 3609 for slices of 1kb or larger, similar length (100-200 bytes 3610 difference) may be produced, which will not lower the bit 3611 efficiency catastrophically. 3613 3) The video encoder produces NAL units, for which a certain frame 3614 contains k slices of possibly almost equal length. Then, applying 3615 FEC, a better code, e.g., (n=24, k=12), can be used over the 3616 sequence of NAL units for each frame. The delay compared to that 3617 of 2), above, may be reduced, but several disadvantages are 3618 obvious. First, the coding efficiency of the encoded video is 3619 lowered significantly, as slice-structured coding reduces intra- 3620 frame prediction and additional slice overhead is necessary. 3621 Second, pre-encoded content or, when operating over a gateway, the 3622 video is usually not appropriately coded with k slices such that 3623 FEC can be applied. Finally, the encoding of video producing k 3624 slices of equal length is not straightforward and might require 3625 more than one encoding pass. 3627 Many of the mentioned disadvantages can be avoided by applying FUs in 3628 combination with FEC. Each NAL unit can be split into any number of 3629 FUs of basically equal length; therefore, FEC with a reasonable k and 3630 n can be applied, even if the encoder made no effort to produce 3631 slices of equal length. For example, a coded slice NAL unit 3632 containing an entire frame can be split to k FUs, and a parity check 3633 code (n=k+1, k) can be applied. However, this has the disadvantage 3634 that unless all created fragments can be recovered, the whole slice 3635 will be lost. Thus a larger section is lost than would be if the 3636 frame had been split into several slices. 3638 The presented technique makes it possible to achieve good 3639 transmission error tolerance, even if no additional source coding 3640 layer redundancy (such as periodic intra frames) is present. 3641 Consequently, the same coded video sequence can be used to achieve 3642 the maximum compression efficiency and quality over error-free 3643 transmission and for transmission over error-prone networks. 3644 Furthermore, the technique allows the application of FEC to pre- 3645 encoded sequences without adding delay. In this case, pre-encoded 3646 sequences that are not encoded for error-prone networks can still be 3647 transmitted almost reliably without adding extensive delays. In 3648 addition, FUs of equal length result in a bit rate efficient use of 3649 RFC 2733. 3651 If the error probability depends on the length of the transmitted 3652 packet (e.g., in case of mobile transmission [14]), the benefits of 3653 applying FUs with FEC are even more obvious. Basically, the 3654 flexibility of the size of FUs allows appropriate FEC to be applied 3655 for each NAL unit and unequal error protection of NAL units. 3657 When FUs and FEC are used, the incurred overhead is substantial but 3658 is in the same order of magnitude as the number of bits that have to 3659 be spent for intra-coded macroblocks if no FEC is applied. In [18], 3660 it was shown that the overall performance of the FEC-based approach 3661 enhanced quality when using the same error rate and same overall bit 3662 rate, including the overhead. 3664 12.6. Low Bit-Rate Streaming 3666 This scheme has been implemented with H.263 and non-standard RTP 3667 packetization and has given good results [19]. There is no technical 3668 reason why similarly good results could not be achievable with H.264. 3670 In today's Internet streaming, some of the offered bit rates are 3671 relatively low in order to allow terminals with dial-up modems to 3672 access the content. In wired IP networks, relatively large packets, 3673 say 500 - 1500 bytes, are preferred to smaller and more frequently 3674 occurring packets in order to reduce network congestion. Moreover, 3675 use of large packets decreases the amount of RTP/UDP/IP header 3676 overhead. For low bit-rate video, the use of large packets means 3677 that sometimes up to few pictures should be encapsulated in one 3678 packet. 3680 However, loss of a packet including many coded pictures would have 3681 drastic consequences for visual quality, as there is practically no 3682 other way to conceal a loss of an entire picture than to repeat the 3683 previous one. One way to construct relatively large packets and 3684 maintain possibilities for successful loss concealment is to 3685 construct MTAPs that contain interleaved slices from several 3686 pictures. An MTAP should not contain spatially adjacent slices from 3687 the same picture or spatially overlapping slices from any picture. 3688 If a packet is lost, it is likely that a lost slice is surrounded by 3689 spatially adjacent slices of the same picture and spatially 3690 corresponding slices of the temporally previous and succeeding 3691 pictures. Consequently, concealment of the lost slice is likely to 3692 be relatively successful. 3694 12.7. Robust Packet Scheduling in Video Streaming 3696 Robust packet scheduling has been implemented with MPEG-4 Part 2 and 3697 simulated in a wireless streaming environment [20]. There is no 3698 technical reason why similar or better results could not be 3699 achievable with H.264. 3701 Streaming clients typically have a receiver buffer that is capable of 3702 storing a relatively large amount of data. Initially, when a 3703 streaming session is established, a client does not start playing the 3704 stream back immediately. Rather, it typically buffers the incoming 3705 data for a few seconds. This buffering helps maintain continuous 3706 playback, as, in case of occasional increased transmission delays or 3707 network throughput drops, the client can decode and play buffered 3708 data. Otherwise, without initial buffering, the client has to freeze 3709 the display, stop decoding, and wait for incoming data. The 3710 buffering is also necessary for either automatic or selective 3711 retransmission in any protocol level. If any part of a picture is 3712 lost, a retransmission mechanism may be used to resend the lost data. 3713 If the retransmitted data is received before its scheduled decoding 3714 or playback time, the loss is recovered perfectly. Coded pictures 3715 can be ranked according to their importance in the subjective quality 3716 of the decoded sequence. For example, non-reference pictures, such 3717 as conventional B pictures, are subjectively least important, as 3718 their absence does not affect decoding of any other pictures. In 3719 addition to non-reference pictures, the ITU-T H.264 | ISO/IEC 14496- 3720 10 standard includes a temporal scalability method called sub- 3721 sequences [21]. Subjective ranking can also be made on coded slice 3722 data partition or slice group basis. Coded slices and coded slice 3723 data partitions that are subjectively the most important can be sent 3724 earlier than their decoding order indicates, whereas coded slices and 3725 coded slice data partitions that are subjectively the least important 3726 can be sent later than their natural coding order indicates. 3727 Consequently, any retransmitted parts of the most important slices 3728 and coded slice data partitions are more likely to be received before 3729 their scheduled decoding or playback time compared to the least 3730 important slices and slice data partitions. 3732 13. Informative Appendix: Rationale for Decoding Order Number 3734 13.1. Introduction 3736 The Decoding Order Number (DON) concept was introduced mainly to 3737 enable efficient multi-picture slice interleaving (see section 12.6) 3738 and robust packet scheduling (see section 12.7). In both of these 3739 applications, NAL units are transmitted out of decoding order. DON 3740 indicates the decoding order of NAL units and should be used in the 3741 receiver to recover the decoding order. Example use cases for 3742 efficient multi-picture slice interleaving and for robust packet 3743 scheduling are given in sections 13.2 and 13.3, respectively. 3744 Section 13.4 describes the benefits of the DON concept in error 3745 resiliency achieved by redundant coded pictures. Section 13.5 3746 summarizes considered alternatives to DON and justifies why DON was 3747 chosen to this RTP payload specification. 3749 13.2. Example of Multi-Picture Slice Interleaving 3751 An example of multi-picture slice interleaving follows. A subset of 3752 a coded video sequence is depicted below in output order. R denotes 3753 a reference picture, N denotes a non-reference picture, and the 3754 number indicates a relative output time. 3756 ... R1 N2 R3 N4 R5 ... 3758 The decoding order of these pictures from left to right is as 3759 follows: 3761 ... R1 R3 N2 R5 N4 ... 3763 The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a 3764 DON equal to 1, 2, 3, 4, and 5, respectively. 3766 Each reference picture consists of three slice groups that are 3767 scattered as follows (a number denotes the slice group number for 3768 each macroblock in a QCIF frame): 3770 0 1 2 0 1 2 0 1 2 0 1 3771 2 0 1 2 0 1 2 0 1 2 0 3772 1 2 0 1 2 0 1 2 0 1 2 3773 0 1 2 0 1 2 0 1 2 0 1 3774 2 0 1 2 0 1 2 0 1 2 0 3775 1 2 0 1 2 0 1 2 0 1 2 3776 0 1 2 0 1 2 0 1 2 0 1 3777 2 0 1 2 0 1 2 0 1 2 0 3778 1 2 0 1 2 0 1 2 0 1 2 3780 For the sake of simplicity, we assume that all the macroblocks of a 3781 slice group are included in one slice. Three MTAPs are constructed 3782 from three consecutive reference pictures so that each MTAP contains 3783 three aggregation units, each of which contains all the macroblocks 3784 from one slice group. The first MTAP contains slice group 0 of 3785 picture R1, slice group 1 of picture R3, and slice group 2 of picture 3786 R5. The second MTAP contains slice group 1 of picture R1, slice 3787 group 2 of picture R3, and slice group 0 of picture R5. The third 3788 MTAP contains slice group 2 of picture R1, slice group 0 of picture 3789 R3, and slice group 1 of picture R5. Each non-reference picture is 3790 encapsulated into an STAP-B. 3792 Consequently, the transmission order of NAL units is the following: 3794 R1, slice group 0, DON 1, carried in MTAP,RTP SN: N 3795 R3, slice group 1, DON 2, carried in MTAP,RTP SN: N 3796 R5, slice group 2, DON 4, carried in MTAP,RTP SN: N 3797 R1, slice group 1, DON 1, carried in MTAP,RTP SN: N+1 3798 R3, slice group 2, DON 2, carried in MTAP,RTP SN: N+1 3799 R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+1 3800 R1, slice group 2, DON 1, carried in MTAP,RTP SN: N+2 3801 R3, slice group 1, DON 2, carried in MTAP,RTP SN: N+2 3802 R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+2 3803 N2, DON 3, carried in STAP-B, RTP SN: N+3 3804 N4, DON 5, carried in STAP-B, RTP SN: N+4 3806 The receiver is able to organize the NAL units back in decoding order 3807 based on the value of DON associated with each NAL unit. 3809 If one of the MTAPs is lost, the spatially adjacent and temporally 3810 co-located macroblocks are received and can be used to conceal the 3811 loss efficiently. If one of the STAPs is lost, the effect of the 3812 loss does not propagate temporally. 3814 13.3. Example of Robust Packet Scheduling 3816 An example of robust packet scheduling follows. The communication 3817 system used in the example consists of the following components in 3818 the order that the video is processed from source to sink: 3820 o camera and capturing 3821 o pre-encoding buffer 3822 o encoder 3823 o encoded picture buffer 3824 o transmitter 3825 o transmission channel 3826 o receiver 3827 o receiver buffer 3828 o decoder 3829 o decoded picture buffer 3830 o display 3832 The video communication system used in the example operates as 3833 follows. Note that processing of the video stream happens gradually 3834 and at the same time in all components of the system. The source 3835 video sequence is shot and captured to a pre-encoding buffer. The 3836 pre-encoding buffer can be used to order pictures from sampling order 3837 to encoding order or to analyze multiple uncompressed frames for bit 3838 rate control purposes, for example. In some cases, the pre-encoding 3839 buffer may not exist; instead, the sampled pictures are encoded right 3840 away. The encoder encodes pictures from the pre-encoding buffer and 3841 stores the output; i.e., coded pictures, to the encoded picture 3842 buffer. The transmitter encapsulates the coded pictures from the 3843 encoded picture buffer to transmission packets and sends them to a 3844 receiver through a transmission channel. The receiver stores the 3845 received packets to the receiver buffer. The receiver buffering 3846 process typically includes buffering for transmission delay jitter. 3847 The receiver buffer can also be used to recover correct decoding 3848 order of coded data. The decoder reads coded data from the receiver 3849 buffer and produces decoded pictures as output into the decoded 3850 picture buffer. The decoded picture buffer is used to recover the 3851 output (or display) order of pictures. Finally, pictures are 3852 displayed. 3854 In the following example figures, I denotes an IDR picture, R denotes 3855 a reference picture, N denotes a non-reference picture, and the 3856 number after I, R, or N indicates the sampling time relative to the 3857 previous IDR picture in decoding order. Values below the sequence of 3858 pictures indicate scaled system clock timestamps. The system clock 3859 is initialized arbitrarily in this example, and time runs from left 3860 to right. Each I, R, and N picture is mapped into the same timeline 3861 compared to the previous processing step, if any, assuming that 3862 encoding, transmission, and decoding take no time. Thus, events 3863 happening at the same time are located in the same column throughout 3864 all example figures. 3866 A subset of a sequence of coded pictures is depicted below in 3867 sampling order. 3869 ... N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ... 3870 ... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ... 3871 ... 58 59 60 61 62 63 64 65 66 ... 128 129 130 131 ... 3873 Figure 16 Sequence of pictures in sampling order 3875 The sampled pictures are buffered in the pre-encoding buffer to 3876 arrange them in encoding order. In this example, we assume that the 3877 non-reference pictures are predicted from both the previous and the 3878 next reference picture in output order, except for the non-reference 3879 pictures immediately preceding an IDR picture, which are predicted 3880 only from the previous reference picture in output order. Thus, the 3881 pre-encoding buffer has to contain at least two pictures, and the 3882 buffering causes a delay of two picture intervals. The output of the 3883 pre-encoding buffering process and the encoding (and decoding) order 3884 of the pictures are as follows: 3886 ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ... 3887 ... -|---|---|---|---|---|---|---|---|- ... 3888 ... 60 61 62 63 64 65 66 67 68 ... 3890 Figure 17 Re-ordered pictures in the pre-encoding buffer 3892 The encoder or the transmitter can set the value of DON for each 3893 picture to a value of DON for the previous picture in decoding order 3894 plus one. 3896 For the sake of simplicity, let us assume that: 3898 o the frame rate of the sequence is constant, 3899 o each picture consists of only one slice, 3900 o each slice is encapsulated in a single NAL unit packet, 3901 o there is no transmission delay, and 3902 o pictures are transmitted at constant intervals (that is, 1 / 3903 (frame rate)). 3905 When pictures are transmitted in decoding order, they are received as 3906 follows: 3908 ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ... 3909 ... -|---|---|---|---|---|---|---|---|- ... 3910 ... 60 61 62 63 64 65 66 67 68 ... 3912 Figure 18 Received pictures in decoding order 3914 The OPTIONAL sprop-interleaving-depth media type parameter is set to 3915 0, as the transmission (or reception) order is identical to the 3916 decoding order. 3918 The decoder has to buffer for one picture interval initially in its 3919 decoded picture buffer to organize pictures from decoding order to 3920 output order as depicted below: 3922 ... N58 N59 I00 N01 N02 R03 N04 N05 R06 ... 3923 ... -|---|---|---|---|---|---|---|---|- ... 3924 ... 61 62 63 64 65 66 67 68 69 ... 3926 Figure 19 Output order 3928 The amount of required initial buffering in the decoded picture 3929 buffer can be signaled in the buffering period SEI message or with 3930 the num_reorder_frames syntax element of H.264 video usability 3931 information. num_reorder_frames indicates the maximum number of 3932 frames, complementary field pairs, or non-paired fields that precede 3933 any frame, complementary field pair, or non-paired field in the 3934 sequence in decoding order and that follow it in output order. For 3935 the sake of simplicity, we assume that num_reorder_frames is used to 3936 indicate the initial buffer in the decoded picture buffer. In this 3937 example, num_reorder_frames is equal to 1. 3939 It can be observed that if the IDR picture I00 is lost during 3940 transmission and a retransmission request is issued when the value of 3941 the system clock is 62, there is one picture interval of time (until 3942 the system clock reaches timestamp 63) to receive the retransmitted 3943 IDR picture I00. 3945 Let us then assume that IDR pictures are transmitted two frame 3946 intervals earlier than their decoding position; i.e., the pictures 3947 are transmitted as follows: 3949 ... I00 N58 N59 R03 N01 N02 R06 N04 N05 ... 3950 ... --|---|---|---|---|---|---|---|---|- ... 3951 ... 62 63 64 65 66 67 68 69 70 ... 3953 Figure 20 Interleaving: Early IDR pictures in sending order 3955 The OPTIONAL sprop-interleaving-depth media type parameter is set 3956 equal to 1 according to its definition. (The value of sprop- 3957 interleaving-depth in this example can be derived as follows: Picture 3958 I00 is the only picture preceding picture N58 or N59 in transmission 3959 order and following it in decoding order. Except for pictures I00, 3960 N58, and N59, the transmission order is the same as the decoding 3961 order of pictures. As a coded picture is encapsulated into exactly 3962 one NAL unit, the value of sprop-interleaving-depth is equal to the 3963 maximum number of pictures preceding any picture in transmission 3964 order and following the picture in decoding order.) 3966 The receiver buffering process contains two pictures at a time 3967 according to the value of the sprop-interleaving-depth parameter and 3968 orders pictures from the reception order to the correct decoding 3969 order based on the value of DON associated with each picture. The 3970 output of the receiver buffering process is as follows: 3972 ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ... 3973 ... -|---|---|---|---|---|---|---|---|- ... 3974 ... 63 64 65 66 67 68 69 70 71 ... 3976 Figure 21 Interleaving: Receiver buffer 3978 Again, an initial buffering delay of one picture interval is needed 3979 to organize pictures from decoding order to output order, as depicted 3980 below: 3982 ... N58 N59 I00 N01 N02 R03 N04 N05 ... 3983 ... -|---|---|---|---|---|---|---|- ... 3984 ... 64 65 66 67 68 69 70 71 ... 3986 Figure 22 Interleaving: Receiver buffer after reordering 3988 Note that the maximum delay that IDR pictures can undergo during 3989 transmission, including possible application, transport, or link 3990 layer retransmission, is equal to three picture intervals. Thus, the 3991 loss resiliency of IDR pictures is improved in systems supporting 3992 retransmission compared to the case in which pictures were 3993 transmitted in their decoding order. 3995 13.4. Robust Transmission Scheduling of Redundant Coded Slices 3997 A redundant coded picture is a coded representation of a picture or a 3998 part of a picture that is not used in the decoding process if the 3999 corresponding primary coded picture is correctly decoded. There 4000 should be no noticeable difference between any area of the decoded 4001 primary picture and a corresponding area that would result from 4002 application of the H.264 decoding process for any redundant picture 4003 in the same access unit. A redundant coded slice is a coded slice 4004 that is a part of a redundant coded picture. 4006 Redundant coded pictures can be used to provide unequal error 4007 protection in error-prone video transmission. If a primary coded 4008 representation of a picture is decoded incorrectly, a corresponding 4009 redundant coded picture can be decoded. Examples of applications and 4010 coding techniques using the redundant codec picture feature include 4011 the video redundancy coding [22] and the protection of "key pictures" 4012 in multicast streaming [23]. 4014 One property of many error-prone video communications systems is that 4015 transmission errors are often bursty. Therefore, they may affect 4016 more than one consecutive transmission packets in transmission order. 4017 In low bit-rate video communication, it is relatively common that an 4018 entire coded picture can be encapsulated into one transmission 4019 packet. Consequently, a primary coded picture and the corresponding 4020 redundant coded pictures may be transmitted in consecutive packets in 4021 transmission order. To make the transmission scheme more tolerant of 4022 bursty transmission errors, it is beneficial to transmit the primary 4023 coded picture and redundant coded picture separated by more than a 4024 single packet. The DON concept enables this. 4026 13.5. Remarks on Other Design Possibilities 4028 The slice header syntax structure of the H.264 coding standard 4029 contains the frame_num syntax element that can indicate the decoding 4030 order of coded frames. However, the usage of the frame_num syntax 4031 element is not feasible or desirable to recover the decoding order, 4032 due to the following reasons: 4034 o The receiver is required to parse at least one slice header per 4035 coded picture (before passing the coded data to the decoder). 4037 o Coded slices from multiple coded video sequences cannot be 4038 interleaved, as the frame number syntax element is reset to 0 in 4039 each IDR picture. 4041 o The coded fields of a complementary field pair share the same 4042 value of the frame_num syntax element. Thus, the decoding order 4043 of the coded fields of a complementary field pair cannot be 4044 recovered based on the frame_num syntax element or any other 4045 syntax element of the H.264 coding syntax. 4047 The RTP payload format for transport of MPEG-4 elementary streams 4048 [24] enables interleaving of access units and transmission of 4049 multiple access units in the same RTP packet. An access unit is 4050 specified in the H.264 coding standard to comprise all NAL units 4051 associated with a primary coded picture according to subclause 4052 7.4.1.2 of [1]. Consequently, slices of different pictures cannot be 4053 interleaved, and the multi-picture slice interleaving technique (see 4054 section 12.6) for improved error resilience cannot be used. 4056 14. Acknowledgements 4058 Stephan Wenger, Miska Hannuksela, Thomas Stockhammer, Magnus 4059 Westerlund, and David Singer are thanked as the authors of RFC 3984. 4060 Dave Lindbergh, Philippe Gentric, Gonzalo Camarillo, Gary Sullivan, 4061 Joerg Ott, and Colin Perkins are thanked for careful review during 4062 the development of RFC 3984. Randell Jesup, Stephen Botzko, and 4063 Magnus Westerlund are thanked for their valuable comments during the 4064 development of this RFC. 4066 This document was prepared using 2-Word-v2.0.template.dot. 4068 15. References 4070 15.1. Normative References 4072 [1] ITU-T Recommendation H.264, "Advanced video coding for generic 4073 audiovisual services", November 2007. [Ed. (YkW): This should 4074 be updated after a later version is approved.] 4076 [2] ISO/IEC International Standard 14496-10:2008. 4078 [3] ITU-T Recommendation H.241, "Extended video procedures and 4079 control signals for H.300 series terminals", May 2006. 4081 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement 4082 Levels", BCP 14, RFC 2119, March 1997. 4084 [5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, 4085 "RTP: A Transport Protocol for Real-Time Applications", STD 64, 4086 RFC 3550, July 2003. 4088 [6] Handley, M. and V. Jacobson, "SDP: Session Description 4089 Protocol", RFC 2327, April 1998. 4091 [7] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", 4092 RFC 3548, July 2003. 4094 [8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 4095 Session Description Protocol (SDP)", RFC 3264, June 2002. 4097 15.2. Informative References 4099 [9] Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special 4100 Issue on H.264/AVC. IEEE Transactions on Circuits and Systems 4101 on Video Technology, July 2003. 4103 [10] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C., 4104 Newell, D., Ott, J., Sullivan, G., Wenger, S., and C. Zhu, "RTP 4105 Payload Format for the 1998 Version of ITU-T Rec. H.263 Video 4106 (H.263+)", RFC 2429, October 1998. 4108 [11] ISO/IEC IS 14496-2. 4110 [12] Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and 4111 Systems for Video technology, Vol. 13, No. 7, July 2003. 4113 [13] Wenger, S., "H.26L over IP: The IP Network Adaptation Layer", 4114 Proceedings Packet Video Workshop 02, April 2002. 4116 [14] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT 4117 Coding Network Abstraction Layer and IP-based Transport" in 4118 Proc. ICIP 2002, Rochester, NY, September 2002. 4120 [15] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video 4121 Conferences with Minimal Control", STD 65, RFC 3551, July 2003. 4123 [16] ITU-T Recommendation H.223, "Multiplexing protocol for low bit 4124 rate multimedia communication", July 2001. 4126 [17] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for 4127 Generic Forward Error Correction", RFC 2733, December 1999. 4129 [18] Stockhammer, T., Wiegand, T., Oelbaum, T., and F. Obermeier, 4130 "Video Coding and Transport Layer Techniques for H.264/AVC- 4131 Based Transmission over Packet-Lossy Networks", IEEE 4132 International Conference on Image Processing (ICIP 2003), 4133 Barcelona, Spain, September 2003. 4135 [19] Varsa, V. and M. Karczewicz, "Slice interleaving in compressed 4136 video packetization", Packet Video Workshop 2000. 4138 [20] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for 4139 wireless video streaming," International Packet Video Workshop 4140 2002. 4142 [21] Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042, 4143 available http://ftp3.itu.int/av-arch/video-site/0201_Gen/JVT- 4144 B042.doc, anuary 2002. 4146 [22] Wenger, S., "Video Redundancy Coding in H.263+", 1997 4147 International Workshop on Audio-Visual Services over Packet 4148 Networks, September 1997. 4150 [23] Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error Resilient 4151 Video Coding Using Unequally Protected Key Pictures", in Proc. 4152 International Workshop VLBV03, September 2003. 4154 [24] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and 4155 P. Gentric, "RTP Payload Format for Transport of MPEG-4 4156 Elementary Streams", RFC 3640, November 2003. 4158 [25] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 4159 Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 4160 3711, March 2004. 4162 [26] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming 4163 Protocol (RTSP)", RFC 2326, April 1998. 4165 [27] Handley, M., Perkins, C., and E. Whelan, "Session Announcement 4166 Protocol", RFC 2974, October 2000. 4168 [28] Westerlund, M. and Wenger, S., "RTP Topologies", RFC 5117, 4169 January 2008. 4171 [29] Wenger, S., Chandra, U., and Westerlund, M., "Codec Control 4172 Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", 4173 RFC 5104, February 2008. 4175 Authors' Addresses 4177 Ye-Kui Wang 4178 Nokia Research Center 4179 P.O. Box 1000 4180 33721 Tampere 4181 Finland 4183 Phone: +358-50-466-7004 4184 EMail: ye-kui.wang@nokia.com 4186 Roni Even 4187 14 David Hamelech 4188 Tel Aviv 64953 4189 Israel 4191 Phone: +972-545481099 4192 Email:ron.even.tlv@gmail.com 4194 Tom Kristensen 4195 TANDBERG 4196 Philip Pedersens vei 22 4197 N-1366 Lysaker 4198 Norway 4200 Phone: +47 67125125 4201 Email: tom.kristensen@tandberg.com, tomkri@ifi.uio.no 4203 Intellectual Property Statement 4205 The IETF takes no position regarding the validity or scope of any 4206 Intellectual Property Rights or other rights that might be claimed to 4207 pertain to the implementation or use of the technology described in 4208 this document or the extent to which any license under such rights 4209 might or might not be available; nor does it represent that it has 4210 made any independent effort to identify any such rights. Information 4211 on the procedures with respect to rights in RFC documents can be 4212 found in BCP 78 and BCP 79. 4214 Copies of IPR disclosures made to the IETF Secretariat and any 4215 assurances of licenses to be made available, or the result of an 4216 attempt made to obtain a general license or permission for the use of 4217 such proprietary rights by implementers or users of this 4218 specification can be obtained from the IETF on-line IPR repository at 4219 http://www.ietf.org/ipr. 4221 The IETF invites any interested party to bring to its attention any 4222 copyrights, patents or patent applications, or other proprietary 4223 rights that may cover technology that may be required to implement 4224 this standard. Please address the information to the IETF at 4225 ietf-ipr@ietf.org. 4227 Disclaimer of Validity 4229 This document and the information contained herein are provided on an 4230 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 4231 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 4232 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 4233 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 4234 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 4235 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 4237 Copyright Statement 4239 Copyright (C) The IETF Trust (2008). 4241 This document is subject to the rights, licenses and restrictions 4242 contained in BCP 78, and except as set forth therein, the authors 4243 retain all their rights. 4245 Acknowledgement 4247 Funding for the RFC Editor function is currently provided by the 4248 Internet Society. 4250 16. Backward Compatibility to RFC 3984 4252 The current document is a revision of RFC 3984 and intends to 4253 obsolete it. This section addresses the backward compatibility 4254 issues. 4256 The technical changes are listed in section 17. 4258 Items 1), 2), 3), 7), 8), 9), 11), 12) are bug-fix type of changes, 4259 and do not incur any backward compatibility issues. 4261 Item 4), addition of six new media type parameters, does not incur 4262 any backward compatibility issues for SDP Offer/Answer based 4263 applications, as legacy RFC 3984 receivers ignore these parameters, 4264 and it is fine for legacy RFC 3984 senders not to use these 4265 parameters as they are optional. However, there is a backward 4266 compatibility issue for SDP declarative usage based applications, 4267 e.g. those using RTSP and SAP, because the SDP receiver per RFC 3984 4268 cannot accept a session for which the SDP includes an unrecognized 4269 parameter. Therefore, the RTSP or SAP server may have to prepare two 4270 sets of streams, one for legacy RFC 3984 receivers and one for 4271 receivers according to this memo. 4273 Items 5), 6) and 10) are related to out-of-band transport of 4274 parameter sets. When a sender according to this memo is 4275 communicating with a legacy receiver according to RFC 3984, there is 4276 no backward compatibility issue. When the legacy receiver sees an SDP 4277 message with no parameter-add the value of parameter-add is inferred 4278 to be equal to 1 by the legacy receiver (related to change item 5)). 4279 As RFC 3984 allows inclusion of any parameter sets in sprop- 4280 parameter-sets, it is fine to the legacy receiver to include 4281 parameter sets only for the default level in sprop-parameter-sets 4282 (related to change item 6)). When there are new parameters e.g. 4283 sprop-level-parameter-sets present, the legacy receiver simply 4284 ignores them (related to change item 10)). When a legacy sender 4285 according to RFC 3984 is communicating with a receiver according to 4286 this memo, there is one backward compatibility issue. When the 4287 legacy sender includes parameter sets for a level different than the 4288 default level indicated by profile-level-id to sprop-parameter-sets, 4289 the parameter value of sprop-parameter-sets is invalid to the 4290 receiver and therefore the session may be rejected. In SDP 4291 Offer/Answer between a legacy offerer according to RFC 3984 and an 4292 answerer according to this memo, when the answerer includes in the 4293 answer parameter sets that are not a superset of the parameter sets 4294 included in the offer, the parameter value of sprop-parameter-sets is 4295 invalid to offerer and the session may not be initiated properly 4296 (related to change item 10)). 4298 Item 13) removed that use of out-of-band transport of parameter sets 4299 is recommended. As out-of-band transport of parameter sets is still 4300 allowed, this change does not incur any backward compatibility 4301 issues. 4303 Item 14) does not incur any backward compatibility issues as the 4304 added subsection 8.5 is informative. 4306 17. Changes from RFC 3984 4308 Following is the list of technical changes (including bug fixes) from 4309 RFC 3984. Besides this list of technical changes, numerous editorial 4310 changes have been made, but not documented in this memo. 4312 1) In subsections 5.4, 5.5, 6.2, 6,3 and 6.4, removed that the 4313 packetization mode in use may be signaled by external means. 4315 2) In subsection 7.2.2, changed the sentence 4317 There are N VCL NAL units in the deinterleaving buffer. 4319 to 4321 There are N or more VCL NAL units in the de-interleaving buffer. 4323 3) In subsection 8.1, the semantics of sprop-init-buf-time, paragraph 4324 2, changed the sentence 4326 The parameter is the maximum value of (transmission time of a NAL 4327 unit - decoding time of the NAL unit), assuming reliable and 4328 instantaneous transmission, the same timeline for transmission 4329 and decoding, and that decoding starts when the first packet 4330 arrives. 4332 to 4334 The parameter is the maximum value of (decoding time of the NAL 4335 unit - transmission time of a NAL unit), assuming reliable and 4336 instantaneous transmission, the same timeline for transmission 4337 and decoding, and that decoding starts when the first packet 4338 arrives. 4340 4) Added six new media type parameters, namely max-smbps, sprop- 4341 level-parameter-sets, use-level-parameter-sets, sprop-ssrc, sar- 4342 understood and sar-supported. 4344 5) In subsection 8.1, removed the specification of parameter-add. 4345 Other descriptions of parameter-add (in subsections 8.2 and 8.4) 4346 are also removed. 4348 6) In subsection 8.1, added a constraint to sprop-parameter-sets such 4349 that it can only contain parameter sets for the same profile and 4350 level as indicated by profile-level-id. 4352 7) In subsection 8.2.2, removed sprop-deint-buf-req from being part 4353 of the media format configuration in usage with the SDP 4354 Offer/Answer model. 4356 8) In subsection 8.2.2, made it clear that level is downgradable in 4357 the SDP Offer/Answer model, i.e. the use of the level part of 4358 "profile-level-id" does not need to be symmetric (the level 4359 included in the answer can be lower than or equal to the level 4360 included in the offer). 4362 9) In subsection 8.2.2, removed that the capability parameters may be 4363 used to declare encoding capabilities. 4365 10)In subsection 8.2.2, added rules on how to use sprop-parameter- 4366 sets and sprop-level-parameter-sets for out-of-band transport of 4367 parameter sets, with or without level downgrading. 4369 11)In subsection 8.2.2, clarified the rules of using the media type 4370 parameters with SDP Offer/Answer for multicast. 4372 12)In subsection 8.2.2, completed and corrected the list of how 4373 different media type parameters shall be interpreted in the 4374 different combinations of offer or answer and direction attribute. 4376 13)In subsection 8.4, changed the text such that both out-of-band and 4377 in-band transport of parameter sets are allowed and neither is 4378 recommended or required. 4380 14)Added subsection 8.5 (informative) providing example methods for 4381 decoder refresh to handle parameter set losses. 4383 18. Open issues 4385 The issues remaining open are: 4387 1) (From Randell) References to RFC 2733 should be updated to (and 4388 checked against) RFC 5109. There are a lot of calculations and 4389 the like that should be checked. Also update [17] to RFC 5109.