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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 PAYLOAD V. Singh 3 Internet-Draft callstats.io 4 Intended status: Standards Track A. Begen 5 Expires: September 22, 2016 Networked Media 6 M. Zanaty 7 Cisco 8 G. Mandyam 9 Qualcomm Innovation Center 10 March 21, 2016 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-02 15 Abstract 17 This document defines new RTP payload formats for the Forward Error 18 Correction (FEC) packets that are generated by the non-interleaved 19 and interleaved parity codes from a source media encapsulated in RTP. 20 These parity codes are systematic codes, where a number of repair 21 symbols are generated from a set of source symbols. These repair 22 symbols are sent in a repair flow separate from the source flow that 23 carries the source symbols. The non-interleaved and interleaved 24 parity codes which are defined in this specification offer a good 25 protection against random and bursty packet losses, respectively, at 26 a cost of decent complexity. Moreover, alternate FEC codes may be 27 used with the payload formats presented. The RTP payload formats 28 that are defined in this document address the scalability issues 29 experienced with the earlier specifications including RFC 2733, RFC 30 5109 and SMPTE 2022-1, and offer several improvements. Due to these 31 changes, the new payload formats are not backward compatible with the 32 earlier specifications, but endpoints that do not implement the 33 scheme can still work by simply ignoring the FEC packets. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at http://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on September 22, 2016. 51 Copyright Notice 53 Copyright (c) 2016 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (http://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 1.1. Parity Codes . . . . . . . . . . . . . . . . . . . . . . 3 70 1.1.1. Use Cases for 1-D FEC Protection . . . . . . . . . . 6 71 1.1.2. Use Cases for 2-D Parity FEC Protection . . . . . . . 8 72 1.1.3. Overhead Computation . . . . . . . . . . . . . . . . 9 73 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 9 74 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 10 75 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 76 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 77 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 10 78 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 10 79 4.2. Repair Packets . . . . . . . . . . . . . . . . . . . . . 10 80 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 16 81 5.1. Media Type Registration - Parity Codes . . . . . . . . . 17 82 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 17 83 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 18 84 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 19 85 5.1.4. Registration of application/flexfec . . . . . . . . . 21 86 5.2. Media Type Registration - Non-Parity Codes . . . . . . . 22 87 5.2.1. Registration of application/flexfec-raptor . . . . . 22 88 5.3. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 23 89 5.3.1. Offer-Answer Model Considerations . . . . . . . . . . 24 90 5.3.2. Declarative Considerations . . . . . . . . . . . . . 25 91 6. Protection and Recovery Procedures - Parity Codes . . . . . . 25 92 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 25 93 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 25 94 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 27 95 6.3.1. Associating the Source and Repair Packets . . . . . . 27 96 6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 29 97 6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 30 98 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 99 Protection . . . . . . . . . . . . . . . . . . . . . 30 100 7. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 33 101 7.1. Example SDP for Flexible FEC Protection with in-band SSRC 102 mapping . . . . . . . . . . . . . . . . . . . . . . . . . 33 103 7.2. Example SDP for Flex FEC Protection with explicit 104 signalling in the SDP . . . . . . . . . . . . . . . . . . 33 105 8. Congestion Control Considerations . . . . . . . . . . . . . . 34 106 9. Security Considerations . . . . . . . . . . . . . . . . . . . 35 107 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 108 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 109 12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 35 110 12.1. draft-ietf-payload-flexible-fec-scheme-02 . . . . . . . 36 111 12.2. draft-ietf-payload-flexible-fec-scheme-01 . . . . . . . 36 112 12.3. draft-ietf-payload-flexible-fec-scheme-00 . . . . . . . 36 113 12.4. draft-singh-payload-1d2d-parity-scheme-00 . . . . . . . 36 114 12.5. draft-ietf-fecframe-1d2d-parity-scheme-00 . . . . . . . 36 115 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 116 13.1. Normative References . . . . . . . . . . . . . . . . . . 37 117 13.2. Informative References . . . . . . . . . . . . . . . . . 38 118 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 120 1. Introduction 122 This document defines new RTP payload formats for the Forward Error 123 Correction (FEC) that is generated by the non-interleaved and 124 interleaved parity codes from a source media encapsulated in RTP 125 [RFC3550]. These payload formats may also be used for other types of 126 FEC codes. The type of the source media protected by these parity 127 codes can be audio, video, text or application. The FEC data are 128 generated according to the media type parameters, which are 129 communicated out-of-band (e.g., in SDP). Furthermore, the 130 associations or relationships between the source and repair flows may 131 be communicated in-band or out-of-band. Situations where adaptivitiy 132 of FEC parameters is desired, the endpoint can use the in-band 133 mechanism, whereas when the FEC parameters are fixed, the endpoint 134 may prefer to negotiate them out-of-band. 136 The repair packets proposed in this document protect the source 137 stream packets that belong to the same RTP session. 139 1.1. Parity Codes 141 Both the non-interleaved and interleaved parity codes use the 142 eXclusive OR (XOR) operation to generate the repair symbols. In a 143 nutshell, the following steps take place: 145 1. The sender determines a set of source packets to be protected by 146 FEC based on the media type parameters. 148 2. The sender applies the XOR operation on the source symbols to 149 generate the required number of repair symbols. 151 3. The sender packetizes the repair symbols and sends the repair 152 packet(s) along with the source packets to the receiver(s) (in 153 different flows). The repair packets may be sent proactively or 154 on-demand. 156 Note that the source and repair packets belong to different source 157 and repair flows, and the sender must provide a way for the receivers 158 to demultiplex them, even in the case they are sent in the same 159 5-tuple (i.e., same source/destination address/port with UDP). This 160 is required to offer backward compatibility for endpoints that do not 161 understand the FEC packets (See Section 4). At the receiver side, if 162 all of the source packets are successfully received, there is no need 163 for FEC recovery and the repair packets are discarded. However, if 164 there are missing source packets, the repair packets can be used to 165 recover the missing information. Figure 1 and Figure 2 describe 166 example block diagrams for the systematic parity FEC encoder and 167 decoder, respectively. 169 +------------+ 170 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 171 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 172 | Encoder | 173 | (Sender) | --> +==+ +==+ 174 +------------+ +==+ +==+ 176 Source Packet: +--+ Repair Packet: +==+ 177 +--+ +==+ 179 Figure 1: Block diagram for systematic parity FEC encoder 181 +------------+ 182 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 183 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 184 | Decoder | 185 +==+ +==+ --> | (Receiver) | 186 +==+ +==+ +------------+ 188 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 189 +--+ +==+ 191 Figure 2: Block diagram for systematic parity FEC decoder 193 In Figure 2, it is clear that the FEC packets have to be received by 194 the endpoint within a certain amount of time for the FEC recovery 195 process to be useful. In this document, we refer to the time that 196 spans a FEC block, which consists of the source packets and the 197 corresponding repair packets, as the repair window. At the receiver 198 side, the FEC decoder should wait at least for the duration of the 199 repair window after getting the first packet in a FEC block, to allow 200 all the repair packets to arrive. (The waiting time can be adjusted 201 if there are missing packets at the beginning of the FEC block.) The 202 FEC decoder can start decoding the already received packets sooner; 203 however, it should not register a FEC decoding failure until it waits 204 at least for the duration of the repair window. 206 Suppose that we have a group of D x L source packets that have 207 sequence numbers starting from 1 running to D x L, and a repair 208 packet is generated by applying the XOR operation to every L 209 consecutive packets as sketched in Figure 3. This process is 210 referred to as 1-D non-interleaved FEC protection. As a result of 211 this process, D repair packets are generated, which we refer to as 212 non-interleaved (or row) FEC packets. 214 +--------------------------------------------------+ --- +===+ 215 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 216 +--------------------------------------------------+ --- +===+ 217 +--------------------------------------------------+ --- +===+ 218 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 219 +--------------------------------------------------+ --- +===+ 220 . . . . . . 221 . . . . . . 222 . . . . . . 223 +--------------------------------------------------+ --- +===+ 224 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 225 +--------------------------------------------------+ --- +===+ 227 Figure 3: Generating non-interleaved (row) FEC packets 229 If we apply the XOR operation to the group of the source packets 230 whose sequence numbers are L apart from each other, as sketched in 231 Figure 4. In this case the endpoint generates L repair packets. 232 This process is referred to as 1-D interleaved FEC protection, and 233 the resulting L repair packets are referred to as interleaved (or 234 column) FEC packets. 236 +-------------+ +-------------+ +-------------+ +-------+ 237 | S_1 | | S_2 | | S3 | ... | S_L | 238 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 239 | . | | . | | | | | 240 | . | | . | | | | | 241 | . | | . | | | | | 242 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 243 +-------------+ +-------------+ +-------------+ +-------+ 244 + + + + 245 ------------- ------------- ------------- ------- 246 | XOR | | XOR | | XOR | ... | XOR | 247 ------------- ------------- ------------- ------- 248 = = = = 249 +===+ +===+ +===+ +===+ 250 |C_1| |C_2| |C_3| ... |C_L| 251 +===+ +===+ +===+ +===+ 253 Figure 4: Generating interleaved (column) FEC packets 255 1.1.1. Use Cases for 1-D FEC Protection 257 We generate one non-interleaved repair packet out of L consecutive 258 source packets or one interleaved repair packet out of D non- 259 consecutive source packets. Regardless of whether the repair packet 260 is a non-interleaved or an interleaved one, it can provide a full 261 recovery of the missing information if there is only one packet 262 missing among the corresponding source packets. This implies that 263 1-D non-interleaved FEC protection performs better when the source 264 packets are randomly lost. However, if the packet losses occur in 265 bursts, 1-D interleaved FEC protection performs better provided that 266 L is chosen large enough, i.e., L-packet duration is not shorter than 267 the observed burst duration. If the sender generates non-interleaved 268 FEC packets and a burst loss hits the source packets, the repair 269 operation fails. This is illustrated in Figure 5. 271 +---+ +---+ +===+ 272 | 1 | X X | 4 | |R_1| 273 +---+ +---+ +===+ 275 +---+ +---+ +---+ +---+ +===+ 276 | 5 | | 6 | | 7 | | 8 | |R_2| 277 +---+ +---+ +---+ +---+ +===+ 279 +---+ +---+ +---+ +---+ +===+ 280 | 9 | | 10| | 11| | 12| |R_3| 281 +---+ +---+ +---+ +---+ +===+ 283 Figure 5: Example scenario where 1-D non-interleaved FEC protection 284 fails error recovery (Burst Loss) 286 The sender may generate interleaved FEC packets to combat with the 287 bursty packet losses. However, two or more random packet losses may 288 hit the source and repair packets in the same column. In that case, 289 the repair operation fails as well. This is illustrated in Figure 6. 290 Note that it is possible that two burst losses may occur back-to- 291 back, in which case interleaved FEC packets may still fail to recover 292 the lost data. 294 +---+ +---+ +---+ 295 | 1 | X | 3 | | 4 | 296 +---+ +---+ +---+ 298 +---+ +---+ +---+ 299 | 5 | X | 7 | | 8 | 300 +---+ +---+ +---+ 302 +---+ +---+ +---+ +---+ 303 | 9 | | 10| | 11| | 12| 304 +---+ +---+ +---+ +---+ 306 +===+ +===+ +===+ +===+ 307 |C_1| |C_2| |C_3| |C_4| 308 +===+ +===+ +===+ +===+ 310 Figure 6: Example scenario where 1-D interleaved FEC protection fails 311 error recovery (Periodic Loss) 313 1.1.2. Use Cases for 2-D Parity FEC Protection 315 In networks where the source packets are lost both randomly and in 316 bursts, the sender ought to generate both non-interleaved and 317 interleaved FEC packets. This type of FEC protection is known as 2-D 318 parity FEC protection. At the expense of generating more FEC 319 packets, thus increasing the FEC overhead, 2-D FEC provides superior 320 protection against mixed loss patterns. However, it is still 321 possible for 2-D parity FEC protection to fail to recover all of the 322 lost source packets if a particular loss pattern occurs. An example 323 scenario is illustrated in Figure 7. 325 +---+ +---+ +===+ 326 | 1 | X X | 4 | |R_1| 327 +---+ +---+ +===+ 329 +---+ +---+ +---+ +---+ +===+ 330 | 5 | | 6 | | 7 | | 8 | |R_2| 331 +---+ +---+ +---+ +---+ +===+ 333 +---+ +---+ +===+ 334 | 9 | X X | 12| |R_3| 335 +---+ +---+ +===+ 337 +===+ +===+ +===+ +===+ 338 |C_1| |C_2| |C_3| |C_4| 339 +===+ +===+ +===+ +===+ 341 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 342 error recovery 344 2-D parity FEC protection also fails when at least two rows are 345 missing a source and the FEC packet and the missing source packets 346 (in at least two rows) are aligned in the same column. An example 347 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 348 protection cannot repair all missing source packets when at least two 349 columns are missing a source and the FEC packet and the missing 350 source packets (in at least two columns) are aligned in the same row. 352 +---+ +---+ +---+ 353 | 1 | | 2 | X | 4 | X 354 +---+ +---+ +---+ 356 +---+ +---+ +---+ +---+ +===+ 357 | 5 | | 6 | | 7 | | 8 | |R_2| 358 +---+ +---+ +---+ +---+ +===+ 360 +---+ +---+ +---+ 361 | 9 | | 10| X | 12| X 362 +---+ +---+ +---+ 364 +===+ +===+ +===+ +===+ 365 |C_1| |C_2| |C_3| |C_4| 366 +===+ +===+ +===+ +===+ 368 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 369 error recovery 371 1.1.3. Overhead Computation 373 The overhead is defined as the ratio of the number of bytes belonging 374 to the repair packets to the number of bytes belonging to the 375 protected source packets. 377 Generally, repair packets are larger in size compared to the source 378 packets. Also, not all the source packets are necessarily equal in 379 size. However, if we assume that each repair packet carries an equal 380 number of bytes carried by a source packet, we can compute the 381 overhead for different FEC protection methods as follows: 383 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 385 o 1-D Interleaved FEC Protection: Overhead = 1/D 387 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 389 where L and D are the number of columns and rows in the source block, 390 respectively. 392 2. Requirements Notation 394 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 395 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 396 document are to be interpreted as described in [RFC2119]. 398 3. Definitions and Notations 400 3.1. Definitions 402 This document uses a number of definitions from [RFC6363]. 404 3.2. Notations 406 o L: Number of columns of the source block. 408 o D: Number of rows of the source block. 410 o bitmask: Run-length encoding of packets protected by a FEC packet. 411 If the bit i in the mask is set to 1, the source packet number N + 412 i is protected by this FEC packet. Here, N is the sequence number 413 base, which is indicated in the FEC packet as well. 415 4. Packet Formats 417 This section defines the formats of the source and repair packets. 419 4.1. Source Packets 421 The source packets MUST contain the information that identifies the 422 source block and the position within the source block occupied by the 423 packet. Since the source packets that are carried within an RTP 424 stream already contain unique sequence numbers in their RTP headers 425 [RFC3550], we can identify the source packets in a straightforward 426 manner and there is no need to append additional field(s). The 427 primary advantage of not modifying the source packets in any way is 428 that it provides backward compatibility for the receivers that do not 429 support FEC at all. In multicast scenarios, this backward 430 compatibility becomes quite useful as it allows the non-FEC-capable 431 and FEC-capable receivers to receive and interpret the same source 432 packets sent in the same multicast session. 434 4.2. Repair Packets 436 The repair packets MUST contain information that identifies the 437 source block they pertain to and the relationship between the 438 contained repair symbols and the original source block. For this 439 purpose, we use the RTP header of the repair packets as well as 440 another header within the RTP payload, which we refer to as the FEC 441 header, as shown in Figure 9. 443 Note that all the source stream packets that are protected by a 444 particular FEC packet need to be in the same RTP session. 446 +------------------------------+ 447 | IP Header | 448 +------------------------------+ 449 | Transport Header | 450 +------------------------------+ 451 | RTP Header | __ 452 +------------------------------+ | 453 | FEC Header | \ 454 +------------------------------+ > RTP Payload 455 | Repair Symbols | / 456 +------------------------------+ __| 458 Figure 9: Format of repair packets 460 The RTP header is formatted according to [RFC3550] with some further 461 clarifications listed below: 463 o Marker (M) Bit: This bit is not used for this payload type, and 464 SHALL be set to 0. 466 o Payload Type: The (dynamic) payload type for the repair packets is 467 determined through out-of-band means. Note that this document 468 registers new payload formats for the repair packets (Refer to 469 Section 5 for details). According to [RFC3550], an RTP receiver 470 that cannot recognize a payload type must discard it. This 471 provides backward compatibility. If a non-FEC-capable receiver 472 receives a repair packet, it will not recognize the payload type, 473 and hence, will discard the repair packet. 475 o Sequence Number (SN): The sequence number has the standard 476 definition. It MUST be one higher than the sequence number in the 477 previously transmitted repair packet. The initial value of the 478 sequence number SHOULD be random (unpredictable, based on 479 [RFC3550]). 481 o Timestamp (TS): The timestamp SHALL be set to a time corresponding 482 to the repair packet's transmission time. Note that the timestamp 483 value has no use in the actual FEC protection process and is 484 usually useful for jitter calculations. 486 o Synchronization Source (SSRC): The SSRC value SHALL be randomly 487 assigned as suggested by [RFC3550]. This allows the sender to 488 multiplex the source and repair flows on the same port, or 489 multiplex multiple repair flows on a single port. The repair 490 flows SHOULD use the RTCP CNAME field to associate themselves with 491 the source flow. 493 In some networks, the RTP Source, which produces the source 494 packets and the FEC Source, which generates the repair packets 495 from the source packets may not be the same host. In such 496 scenarios, using the same CNAME for the source and repair flows 497 means that the RTP Source and the FEC Source MUST share the same 498 CNAME (for this specific source-repair flow association). A 499 common CNAME may be produced based on an algorithm that is known 500 both to the RTP and FEC Source [RFC7022]. This usage is compliant 501 with [RFC3550]. 503 Note that due to the randomness of the SSRC assignments, there is 504 a possibility of SSRC collision. In such cases, the collisions 505 MUST be resolved as described in [RFC3550]. 507 The format of the FEC header is shown in Figure 10. 509 0 1 2 3 510 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 1 511 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 512 |F|R| P|X| CC |M| PT recovery | length recovery | 513 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 514 | TS recovery | 515 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 516 | SSRCCount | reserved | 517 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 518 | SSRC_i | 519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 520 | SN base_i |k| Mask [0-14] | 521 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 522 |k| Mask [15-45] (optional) | 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 524 |k| | 525 +-+ Mask [46-108] (optional) | 526 | | 527 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 528 | ... next in SSRC_i ... | 529 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 531 Figure 10: Format of the FEC header 533 The FEC header consists of the following fields: 535 o The F field (1 bit) indicates the type of the mask. Namely: 537 +---------------+-------------------------------------+ 538 | F bit | Use | 539 +---------------+-------------------------------------+ 540 | 0 | flexible mask | 541 | 1 | packets indicated by offset M and N | 542 +---------------+-------------------------------------+ 544 Figure 11: F-bit values 546 o The R bit MUST be set to 1 to indicate a retransmission packet, 547 and MUST be set to 0 for repair packets. 549 o The P, X, CC, M and PT recovery fields are used to determine the 550 corresponding fields of the recovered packets. 552 o The Length recovery (16 bits) field is used to determine the 553 length of the recovered packets. 555 o The TS recovery (32 bits) field is used to determine the timestamp 556 of the recovered packets. 558 o The SSRC count (8 bits) field describes the number of SSRCs 559 protected by the FEC packet. 0 is not a valid value, and the 560 packet MUST be ignored. 562 o The Reserved (24 bits) field are reserved for future use. It MUST 563 be set to zero by senders and ignored by receivers (see [RFC6709], 564 Section 4.2). 566 o The SSRC_i (32 bits) field describes the SSRC of the packets 567 protected by this particular FEC packet. If a FEC packet contains 568 protects multiple SSRCs (indicated by the SSRC Count > 1), there 569 will be multiple blocks of data containing the SSRC, SN base and 570 Mask fields. 572 o The SN base_i (16 bits) field indicates the lowest sequence 573 number, taking wrap around into account, of the source packets for 574 a particular SSSRC (indicated in SSRC_i) protected by this repair 575 packet. 577 o If the F-bit is set to 0, it represents that the source packets of 578 all the SSRCs protected by this particular repair packet are 579 indicated by using a flexible bitmask. Mask is a run-length 580 encoding of packets for a particular SSRC_i protected by the FEC 581 packet. Where a bit j set to 1 indicates that the source packet 582 with sequence number (SN base_i + j + 1) is protected by this FEC 583 packet. 585 o The k-bit in the bitmasks indicates if it is 15-, 46-, or a 586 109-bitmask. k=0 denotes that there is one more k-bit set, and 587 k=1 denotes that it is the last block of bit mask. While parsing 588 the header, the current count of the number of k-bit gives the 589 size of the bit mask v = count(k). Size of next bitmask = 590 2^(v+3)-1. 592 0 1 2 3 593 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 1 594 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 595 |0|0| P|X| CC |M| PT recovery | length recovery | 596 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 597 | TS recovery | 598 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 599 | SSRCCount | reserved | 600 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 601 | SSRC_i | 602 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 603 | SN base_i |k| Mask [0-14] | 604 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 605 |k| Mask [15-45] (optional) | 606 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 607 |k| | 608 +-+ Mask [46-108] (optional) | 609 | | 610 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 611 | ... next in SSRC_i ... | 612 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 614 Figure 12: Protocol format for F=0 616 o If the F-bit is set to 1, it represents that the source packets of 617 all the SSRCs protected by this particular repair packet are 618 indicated by using fixed offsets. 620 0 1 2 3 621 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 1 622 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 623 |1|0| P|X| CC |M| PT recovery | length recovery | 624 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 625 | TS recovery | 626 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 627 | SSRCCount | reserved | 628 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 629 | SSRC_i | 630 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 631 | SN base_i | M (columns) | N (rows) | 632 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 634 Figure 13: Protocol format for F=1 636 Consequently, the following conditions occur for M and N values: 638 If M>0, N=0, is Row FEC, and no column FEC will follow 639 Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. 641 If M>0, N=1, is Row FEC, and column FEC will follow. 642 Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. 643 and more to come 645 If M>0, N>1, indicates column FEC of every M packet 646 in a group of N packets starting at SN base. 647 Hence, FEC = SN+(Mx0), SN+(Mx1), ... , SN+(MxN). 649 Figure 14: Interpreting the M and N field values 651 By setting F to 1, R=1, SSRC count to 1, M=0, and N=0, the FEC 652 protects only one packet, i.e., the FEC payload carries just the 653 packet indicated by SN Base_i, which is effectively retransmitting 654 the packet. 656 0 1 2 3 657 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 1 658 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 659 |1|1| P|X| CC |M| PT recovery | length recovery | 660 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 661 | TS recovery | 662 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 663 | SSRCCount=1 | reserved | 664 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 665 | SSRC_i | 666 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 667 | SN base_i |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| 668 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 670 Editor's note: this can surely be optimized. 672 Figure 15: Protocol format for Retransmission 674 The details on setting the fields in the FEC header are provided in 675 Section 6.2. 677 It should be noted that a mask-based approach (similar to the ones 678 specified in [RFC2733] and [RFC5109]) may not be very efficient to 679 indicate which source packets in the current source block are 680 associated with a given repair packet. In particular, for the 681 applications that would like to use large source block sizes, the 682 size of the mask that is required to describe the source-repair 683 packet associations may be prohibitively large. The 8-bit fields 684 proposed in [SMPTE2022-1] indicate a systematized approach. Instead 685 the approach in this document uses the 8-bit fields to indicate 686 packet offsets protected by the FEC packet. The approach in 687 [SMPTE2022-1] is inherently more efficient for regular patterns, it 688 does not provide flexibility to represent other protection patterns 689 (e.g., staircase). 691 5. Payload Format Parameters 693 This section provides the media subtype registration for the non- 694 interleaved and interleaved parity FEC. The parameters that are 695 required to configure the FEC encoding and decoding operations are 696 also defined in this section. If no specific FEC code is specified 697 in the subtype, then the FEC code defaults to the parity code defined 698 in this specification. 700 5.1. Media Type Registration - Parity Codes 702 This registration is done using the template defined in [RFC6838] and 703 following the guidance provided in [RFC3555]. 705 Note to the RFC Editor: In the following sections, please replace 706 "XXXX" with the number of this document prior to publication as an 707 RFC. 709 5.1.1. Registration of audio/flexfec 711 Type name: audio 713 Subtype name: flexfec 715 Required parameters: 717 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 718 than 1000 Hz to provide sufficient resolution to RTCP operations. 719 However, it is RECOMMENDED to select the rate that matches the 720 rate of the protected source RTP stream. 722 o repair-window: The time that spans the source packets and the 723 corresponding repair packets. The size of the repair window is 724 specified in microseconds. 726 Optional parameters: 728 o L: indicates the number of columns of the source block that are 729 protected by this FEC block and it applies to all the source 730 SSRCs. L is a positive integer. 732 o D: indicates the number of rows of the source block that are 733 protected by this FEC block and it applies to all the source 734 SSRCs. D is a positive integer. 736 o ToP: indicates the type of protection applied by the sender: 0 for 737 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 738 protection, and 2 for 2-D parity FEC protection. The ToP value of 739 3 is reserved for future uses. 741 Encoding considerations: This media type is framed (See Section 4.8 742 in the template document [RFC6838]) and contains binary data. 744 Security considerations: See Section 9 of [RFCXXXX]. 746 Interoperability considerations: None. 748 Published specification: [RFCXXXX]. 750 Applications that use this media type: Multimedia applications that 751 want to improve resiliency against packet loss by sending redundant 752 data in addition to the source media. 754 Fragment identifier considerations: None. 756 Additional information: None. 758 Person & email address to contact for further information: Varun 759 Singh and IETF Audio/Video Transport Payloads 760 Working Group. 762 Intended usage: COMMON. 764 Restriction on usage: This media type depends on RTP framing, and 765 hence, is only defined for transport via RTP [RFC3550]. 767 Author: Varun Singh . 769 Change controller: IETF Audio/Video Transport Working Group delegated 770 from the IESG. 772 Provisional registration? (standards tree only): Yes. 774 5.1.2. Registration of video/flexfec 776 Type name: video 778 Subtype name: flexfec 780 Required parameters: 782 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 783 than 1000 Hz to provide sufficient resolution to RTCP operations. 784 However, it is RECOMMENDED to select the rate that matches the 785 rate of the protected source RTP stream. 787 o repair-window: The time that spans the source packets and the 788 corresponding repair packets. The size of the repair window is 789 specified in microseconds. 791 Optional parameters: 793 o L: indicates the number of columns of the source block that are 794 protected by this FEC block and it applies to all the source 795 SSRCs. L is a positive integer. 797 o D: indicates the number of rows of the source block that are 798 protected by this FEC block and it applies to all the source 799 SSRCs. D is a positive integer. 801 o ToP: indicates the type of protection applied by the sender: 0 for 802 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 803 protection, and 2 for 2-D parity FEC protection. The ToP value of 804 3 is reserved for future uses. 806 Encoding considerations: This media type is framed (See Section 4.8 807 in the template document [RFC6838]) and contains binary data. 809 Security considerations: See Section 9 of [RFCXXXX]. 811 Interoperability considerations: None. 813 Published specification: [RFCXXXX]. 815 Applications that use this media type: Multimedia applications that 816 want to improve resiliency against packet loss by sending redundant 817 data in addition to the source media. 819 Fragment identifier considerations: None. 821 Additional information: None. 823 Person & email address to contact for further information: Varun 824 Singh and IETF Audio/Video Transport Payloads 825 Working Group. 827 Intended usage: COMMON. 829 Restriction on usage: This media type depends on RTP framing, and 830 hence, is only defined for transport via RTP [RFC3550]. 832 Author: Varun Singh . 834 Change controller: IETF Audio/Video Transport Working Group delegated 835 from the IESG. 837 Provisional registration? (standards tree only): Yes. 839 5.1.3. Registration of text/flexfec 841 Type name: text 843 Subtype name: flexfec 844 Required parameters: 846 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 847 than 1000 Hz to provide sufficient resolution to RTCP operations. 848 However, it is RECOMMENDED to select the rate that matches the 849 rate of the protected source RTP stream. 851 o repair-window: The time that spans the source packets and the 852 corresponding repair packets. The size of the repair window is 853 specified in microseconds. 855 Optional parameters: 857 o L: indicates the number of columns of the source block that are 858 protected by this FEC block and it applies to all the source 859 SSRCs. L is a positive integer. 861 o D: indicates the number of rows of the source block that are 862 protected by this FEC block and it applies to all the source 863 SSRCs. D is a positive integer. 865 o ToP: indicates the type of protection applied by the sender: 0 for 866 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 867 protection, and 2 for 2-D parity FEC protection. The ToP value of 868 3 is reserved for future uses. 870 Encoding considerations: This media type is framed (See Section 4.8 871 in the template document [RFC6838]) and contains binary data. 873 Security considerations: See Section 9 of [RFCXXXX]. 875 Interoperability considerations: None. 877 Published specification: [RFCXXXX]. 879 Applications that use this media type: Multimedia applications that 880 want to improve resiliency against packet loss by sending redundant 881 data in addition to the source media. 883 Fragment identifier considerations: None. 885 Additional information: None. 887 Person & email address to contact for further information: Varun 888 Singh and IETF Audio/Video Transport Payloads 889 Working Group. 891 Intended usage: COMMON. 893 Restriction on usage: This media type depends on RTP framing, and 894 hence, is only defined for transport via RTP [RFC3550]. 896 Author: Varun Singh . 898 Change controller: IETF Audio/Video Transport Working Group delegated 899 from the IESG. 901 Provisional registration? (standards tree only): Yes. 903 5.1.4. Registration of application/flexfec 905 Type name: application 907 Subtype name: flexfec 909 Required parameters: 911 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 912 than 1000 Hz to provide sufficient resolution to RTCP operations. 913 However, it is RECOMMENDED to select the rate that matches the 914 rate of the protected source RTP stream. 916 o repair-window: The time that spans the source packets and the 917 corresponding repair packets. The size of the repair window is 918 specified in microseconds. 920 Optional parameters: 922 o L: indicates the number of columns of the source block that are 923 protected by this FEC block and it applies to all the source 924 SSRCs. L is a positive integer. 926 o D: indicates the number of rows of the source block that are 927 protected by this FEC block and it applies to all the source 928 SSRCs. D is a positive integer. 930 o ToP: indicates the type of protection applied by the sender: 0 for 931 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 932 protection, and 2 for 2-D parity FEC protection. The ToP value of 933 3 is reserved for future uses. 935 Encoding considerations: This media type is framed (See Section 4.8 936 in the template document [RFC6838]) and contains binary data. 938 Security considerations: See Section 9 of [RFCXXXX]. 940 Interoperability considerations: None. 942 Published specification: [RFCXXXX]. 944 Applications that use this media type: Multimedia applications that 945 want to improve resiliency against packet loss by sending redundant 946 data in addition to the source media. 948 Fragment identifier considerations: None. 950 Additional information: None. 952 Person & email address to contact for further information: Varun 953 Singh and IETF Audio/Video Transport Payloads 954 Working Group. 956 Intended usage: COMMON. 958 Restriction on usage: This media type depends on RTP framing, and 959 hence, is only defined for transport via RTP [RFC3550]. 961 Author: Varun Singh . 963 Change controller: IETF Audio/Video Transport Working Group delegated 964 from the IESG. 966 Provisional registration? (standards tree only): Yes. 968 5.2. Media Type Registration - Non-Parity Codes 970 This registration is done using the template defined in [RFC6838] and 971 following the guidance provided in [RFC3555]. The media type 972 registration follows the "flexfec-XXXX" paradigm, with the Raptor 973 code provided here. Only the application media type is required, as 974 it is assumed the existing source payload registration types are 975 still applicable. Other FEC codes with specified RTP media types can 976 be defined in a similar manner. 978 Note to the RFC Editor: In the following sections, please replace 979 "XXXX" with the number of this document prior to publication as an 980 RFC. 982 5.2.1. Registration of application/flexfec-raptor 984 Type name: application 986 Subtype name: flexfec-raptor 988 Required parameters: 990 See Sec. 6.1.1 of [RFC6682]. 992 Optional parameters: 994 See Sec. 6.1.1 of [RFC6682]. 996 Encoding considerations: This media type is framed (See Section 4.8 997 in the template document [RFC6838]) and contains binary data. 999 Security considerations: See Section 9 of [RFCXXXX]. 1001 Interoperability considerations: None. 1003 Published specification: [RFCXXXX]. 1005 Applications that use this media type: Multimedia applications that 1006 want to improve resiliency against packet loss by sending redundant 1007 data in addition to the source media. 1009 Fragment identifier considerations: None. 1011 Additional information: None. 1013 Person & email address to contact for further information: Varun 1014 Singh and IETF Audio/Video Transport Payloads 1015 Working Group. 1017 Intended usage: COMMON. 1019 Restriction on usage: This media type depends on RTP framing, and 1020 hence, is only defined for transport via RTP [RFC3550]. 1022 Author: Varun Singh . 1024 Change controller: IETF Audio/Video Transport Working Group delegated 1025 from the IESG. 1027 Provisional registration? (standards tree only): Yes. 1029 5.3. Mapping to SDP Parameters 1031 Applications that are using RTP transport commonly use Session 1032 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1033 The information that is used to specify the media types in an RTP 1034 session has specific mappings to the fields in an SDP description. 1035 In this section, we provide these mappings for the media subtypes 1036 registered by this document. Note that if an application does not 1037 use SDP to describe the RTP sessions, an appropriate mapping must be 1038 defined and used to specify the media types and their parameters for 1039 the control/description protocol employed by the application. 1041 The mapping of the media type specification for "non-interleaved- 1042 parityfec" and "interleaved-parityfec" and their parameters in SDP is 1043 as follows: 1045 o The media type (e.g., "application") goes into the "m=" line as 1046 the media name. 1048 o The media subtype goes into the "a=rtpmap" line as the encoding 1049 name. The RTP clock rate parameter ("rate") also goes into the 1050 "a=rtpmap" line as the clock rate. 1052 o The remaining required payload-format-specific parameters go into 1053 the "a=fmtp" line by copying them directly from the media type 1054 string as a semicolon-separated list of parameter=value pairs. 1056 SDP examples are provided in Section 7. 1058 5.3.1. Offer-Answer Model Considerations 1060 When offering 1-D interleaved parity FEC over RTP using SDP in an 1061 Offer/Answer model [RFC3264], the following considerations apply: 1063 o Each combination of the L and D parameters produces a different 1064 FEC data and is not compatible with any other combination. A 1065 sender application may desire to offer multiple offers with 1066 different sets of L and D values as long as the parameter values 1067 are valid. The receiver SHOULD normally choose the offer that has 1068 a sufficient amount of interleaving. If multiple such offers 1069 exist, the receiver may choose the offer that has the lowest 1070 overhead or the one that requires the smallest amount of 1071 buffering. The selection depends on the application requirements. 1073 o The value for the repair-window parameter depends on the L and D 1074 values and cannot be chosen arbitrarily. More specifically, L and 1075 D values determine the lower limit for the repair-window size. 1076 The upper limit of the repair-window size does not depend on the L 1077 and D values. 1079 o Although combinations with the same L and D values but with 1080 different repair-window sizes produce the same FEC data, such 1081 combinations are still considered different offers. The size of 1082 the repair-window is related to the maximum delay between the 1083 transmission of a source packet and the associated repair packet. 1084 This directly impacts the buffering requirement on the receiver 1085 side and the receiver must consider this when choosing an offer. 1087 o There are no optional format parameters defined for this payload. 1088 Any unknown option in the offer MUST be ignored and deleted from 1089 the answer. If FEC is not desired by the receiver, it can be 1090 deleted from the answer. 1092 5.3.2. Declarative Considerations 1094 In declarative usage, like SDP in the Real-time Streaming Protocol 1095 (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) 1096 [RFC2974], the following considerations apply: 1098 o The payload format configuration parameters are all declarative 1099 and a participant MUST use the configuration that is provided for 1100 the session. 1102 o More than one configuration may be provided (if desired) by 1103 declaring multiple RTP payload types. In that case, the receivers 1104 should choose the repair flow that is best for them. 1106 6. Protection and Recovery Procedures - Parity Codes 1108 This section provides a complete specification of the 1-D and 2-D 1109 parity codes and their RTP payload formats. 1111 6.1. Overview 1113 The following sections specify the steps involved in generating the 1114 repair packets and reconstructing the missing source packets from the 1115 repair packets. 1117 6.2. Repair Packet Construction 1119 The RTP header of a repair packet is formed based on the guidelines 1120 given in Section 4.2. 1122 The FEC header includes 12 octets (or upto 28 octets when the longer 1123 optional masks are used). It is constructed by applying the XOR 1124 operation on the bit strings that are generated from the individual 1125 source packets protected by this particular repair packet. The set 1126 of the source packets that are associated with a given repair packet 1127 can be computed by the formula given in Section 6.3.1. 1129 The bit string is formed for each source packet by concatenating the 1130 following fields together in the order specified: 1132 o The first 64 bits of the RTP header (64 bits). 1134 o Unsigned network-ordered 16-bit representation of the source 1135 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1136 the sum of the lengths of all the following if present: the CSRC 1137 list, extension header, RTP payload and RTP padding (16 bits). 1139 By applying the parity operation on the bit strings produced from the 1140 source packets, we generate the FEC bit string. The FEC header is 1141 generated from the FEC bit string as follows: 1143 o The first (most significant) 2 bits in the FEC bit string are 1144 skipped. The MSK bits in the FEC header are set to the 1145 appropriate value, i.e., it depends on the chosen bitmask length. 1147 o The next bit in the FEC bit string is written into the P recovery 1148 bit in the FEC header. 1150 o The next bit in the FEC bit string is written into the X recovery 1151 bit in the FEC header. 1153 o The next 4 bits of the FEC bit string are written into the CC 1154 recovery field in the FEC header. 1156 o The next bit is written into the M recovery bit in the FEC header. 1158 o The next 7 bits of the FEC bit string are written into the PT 1159 recovery field in the FEC header. 1161 o The next 16 bits are skipped. 1163 o The next 32 bits of the FEC bit string are written into the TS 1164 recovery field in the FEC header. 1166 o The next 16 bits are written into the length recovery field in the 1167 FEC header. 1169 o Depending on the chosen MSK value, the bit mask of appropriate 1170 length will be set to the appropriate values. 1172 As described in Section 4.2, the SN base field of the FEC header MUST 1173 be set to the lowest sequence number of the source packets protected 1174 by this repair packet. When MSK represents a bitmask (MSK=00,01,10), 1175 the SN base field corresponds to the lowest sequence number indicated 1176 in the bitmask. When MSK=11, the following considerations apply: 1) 1177 for the interleaved FEC packets, this corresponds to the lowest 1178 sequence number of the source packets that forms the column, 2) for 1179 the non-interleaved FEC packets, the SN base field MUST be set to the 1180 lowest sequence number of the source packets that forms the row. 1182 The repair packet payload consists of the bits that are generated by 1183 applying the XOR operation on the payloads of the source RTP packets. 1184 If the payload lengths of the source packets are not equal, each 1185 shorter packet MUST be padded to the length of the longest packet by 1186 adding octet 0's at the end. 1188 Due to this possible padding and mandatory FEC header, a repair 1189 packet has a larger size than the source packets it protects. This 1190 may cause problems if the resulting repair packet size exceeds the 1191 Maximum Transmission Unit (MTU) size of the path over which the 1192 repair flow is sent. 1194 6.3. Source Packet Reconstruction 1196 This section describes the recovery procedures that are required to 1197 reconstruct the missing source packets. The recovery process has two 1198 steps. In the first step, the FEC decoder determines which source 1199 and repair packets should be used in order to recover a missing 1200 packet. In the second step, the decoder recovers the missing packet, 1201 which consists of an RTP header and RTP payload. 1203 In the following, we describe the RECOMMENDED algorithms for the 1204 first and second steps. Based on the implementation, different 1205 algorithms MAY be adopted. However, the end result MUST be identical 1206 to the one produced by the algorithms described below. 1208 Note that the same algorithms are used by the 1-D parity codes, 1209 regardless of whether the FEC protection is applied over a column or 1210 a row. The 2-D parity codes, on the other hand, usually require 1211 multiple iterations of the procedures described here. This iterative 1212 decoding algorithm is further explained in Section 6.3.4. 1214 6.3.1. Associating the Source and Repair Packets 1216 We denote the set of the source packets associated with repair packet 1217 p* by set T(p*). Note that in a source block whose size is L columns 1218 by D rows, set T includes D source packets plus one repair packet for 1219 the FEC protection applied over a column, and L source packets plus 1220 one repair packet for the FEC protection applied over a row. Recall 1221 that 1-D interleaved and non-interleaved FEC protection can fully 1222 recover the missing information if there is only one source packet 1223 missing in set T. If there are more than one source packets missing 1224 in set T, 1-D FEC protection will not work. 1226 6.3.1.1. Signaled in SDP 1228 The first step is associating the source and repair packets. If the 1229 endpoint relies entirely on out-of-band signaling (MSK=11, and 1230 M=N=0), then this information may be inferred from the media type 1231 parameters specified in the SDP description. Furthermore, the 1232 payload type field in the RTP header, assists the receiver 1233 distinguish an interleaved or non-interleaved FEC packet. 1235 Mathematically, for any received repair packet, p*, we can determine 1236 the sequence numbers of the source packets that are protected by this 1237 repair packet as follows: 1239 p*_snb + i * X_1 (modulo 65536) 1241 where p*_snb denotes the value in the SN base field of p*'s FEC 1242 header, X_1 is set to L and 1 for the interleaved and non-interleaved 1243 FEC packets, respectively, and 1245 0 <= i < X_2 1247 where X_2 is set to D and L for the interleaved and non-interleaved 1248 FEC packets, respectively. 1250 6.3.1.2. Using bitmasks 1252 When using fixed size bitmasks (16-, 48-, 112-bits), the SN base 1253 field in the FEC header indicates the lowest sequence number of the 1254 source packets that forms the FEC packet. Finally, the bits maked by 1255 "1" in the bitmask are offsets from the SN base and make up the rest 1256 of the packets protected by the FEC packet. The bitmasks are able to 1257 represent arbitrary protection patterns, for example, 1-D 1258 interleaved, 1-D non-interleaved, 2-D, staircase. 1260 6.3.1.3. Using M and N Offsets 1262 When value of M is non-zero, the 8-bit fields indicate the offset of 1263 packets protected by an interleaved (N>0) or non-interleaved (N=0) 1264 FEC packet. Using a combination of interleaved and non-interleaved 1265 FEC packets can form 2-D protection patterns. 1267 Mathematically, for any received repair packet, p*, we can determine 1268 the sequence numbers of the source packets that are protected by this 1269 repair packet are as follows: 1271 When N = 0: 1272 p*_snb, p*_snb+1,..., p*_snb+(M-1), p*_snb+M 1273 When N > 0: 1274 p*_snb, p*_snb+(Mx1), p*_snb+(Mx2),..., p*_snb+(Mx(N-1)), p*_snb+(MxN) 1276 6.3.2. Recovering the RTP Header 1278 For a given set T, the procedure for the recovery of the RTP header 1279 of the missing packet, whose sequence number is denoted by SEQNUM, is 1280 as follows: 1282 1. For each of the source packets that are successfully received in 1283 T, compute the 80-bit string by concatenating the first 64 bits 1284 of their RTP header and the unsigned network-ordered 16-bit 1285 representation of their length in bytes minus 12. 1287 2. For the repair packet in T, compute the FEC bit string from the 1288 first 80 bits of the FEC header. 1290 3. Calculate the recovered bit string as the XOR of the bit strings 1291 generated from all source packets in T and the FEC bit string 1292 generated from the repair packet in T. 1294 4. Create a new packet with the standard 12-byte RTP header and no 1295 payload. 1297 5. Set the version of the new packet to 2. Skip the first 2 bits 1298 in the recovered bit string. 1300 6. Set the Padding bit in the new packet to the next bit in the 1301 recovered bit string. 1303 7. Set the Extension bit in the new packet to the next bit in the 1304 recovered bit string. 1306 8. Set the CC field to the next 4 bits in the recovered bit string. 1308 9. Set the Marker bit in the new packet to the next bit in the 1309 recovered bit string. 1311 10. Set the Payload type in the new packet to the next 7 bits in the 1312 recovered bit string. 1314 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 1315 bits in the recovered bit string. 1317 12. Set the TS field in the new packet to the next 32 bits in the 1318 recovered bit string. 1320 13. Take the next 16 bits of the recovered bit string and set the 1321 new variable Y to whatever unsigned integer this represents 1322 (assuming network order). Convert Y to host order. Y 1323 represents the length of the new packet in bytes minus 12 (for 1324 the fixed RTP header), i.e., the sum of the lengths of all the 1325 following if present: the CSRC list, header extension, RTP 1326 payload and RTP padding. 1328 14. Set the SSRC of the new packet to the SSRC of the source RTP 1329 stream. 1331 This procedure recovers the header of an RTP packet up to (and 1332 including) the SSRC field. 1334 6.3.3. Recovering the RTP Payload 1336 Following the recovery of the RTP header, the procedure for the 1337 recovery of the RTP payload is as follows: 1339 1. Append Y bytes to the new packet. 1341 2. For each of the source packets that are successfully received in 1342 T, compute the bit string from the Y octets of data starting with 1343 the 13th octet of the packet. If any of the bit strings 1344 generated from the source packets has a length shorter than Y, 1345 pad them to that length. The padding of octet 0 MUST be added at 1346 the end of the bit string. Note that the information of the 1347 first 8 octets are protected by the FEC header. 1349 3. For the repair packet in T, compute the FEC bit string from the 1350 repair packet payload, i.e., the Y octets of data following the 1351 FEC header. Note that the FEC header may be 12, 16, 32 octets 1352 depending on the length of the bitmask. 1354 4. Calculate the recovered bit string as the XOR of the bit strings 1355 generated from all source packets in T and the FEC bit string 1356 generated from the repair packet in T. 1358 5. Append the recovered bit string (Y octets) to the new packet 1359 generated in Section 6.3.2. 1361 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1363 In 2-D parity FEC protection, the sender generates both non- 1364 interleaved and interleaved FEC packets to combat with the mixed loss 1365 patterns (random and bursty). At the receiver side, these FEC 1366 packets are used iteratively to overcome the shortcomings of the 1-D 1367 non-interleaved/interleaved FEC protection and improve the chances of 1368 full error recovery. 1370 The iterative decoding algorithm runs as follows: 1372 1. Set num_recovered_until_this_iteration to zero 1374 2. Set num_recovered_so_far to zero 1376 3. Recover as many source packets as possible by using the non- 1377 interleaved FEC packets as outlined in Section 6.3.2 and 1378 Section 6.3.3, and increase the value of num_recovered_so_far by 1379 the number of recovered source packets. 1381 4. Recover as many source packets as possible by using the 1382 interleaved FEC packets as outlined in Section 6.3.2 and 1383 Section 6.3.3, and increase the value of num_recovered_so_far by 1384 the number of recovered source packets. 1386 5. If num_recovered_so_far > num_recovered_until_this_iteration 1387 ---num_recovered_until_this_iteration = num_recovered_so_far 1388 ---Go to step 3 1389 Else 1390 ---Terminate 1392 The algorithm terminates either when all missing source packets are 1393 fully recovered or when there are still remaining missing source 1394 packets but the FEC packets are not able to recover any more source 1395 packets. For the example scenarios when the 2-D parity FEC 1396 protection fails full recovery, refer to Section 1.1.2. Upon 1397 termination, variable num_recovered_so_far has a value equal to the 1398 total number of recovered source packets. 1400 Example: 1402 Suppose that the receiver experienced the loss pattern sketched in 1403 Figure 16. 1405 +---+ +---+ +===+ 1406 X X | 3 | | 4 | |R_1| 1407 +---+ +---+ +===+ 1409 +---+ +---+ +---+ +---+ +===+ 1410 | 5 | | 6 | | 7 | | 8 | |R_2| 1411 +---+ +---+ +---+ +---+ +===+ 1413 +---+ +---+ +===+ 1414 | 9 | X X | 12| |R_3| 1415 +---+ +---+ +===+ 1417 +===+ +===+ +===+ +===+ 1418 |C_1| |C_2| |C_3| |C_4| 1419 +===+ +===+ +===+ +===+ 1421 Figure 16: Example loss pattern for the iterative decoding algorithm 1423 The receiver executes the iterative decoding algorithm and recovers 1424 source packets #1 and #11 in the first iteration. The resulting 1425 pattern is sketched in Figure 17. 1427 +---+ +---+ +---+ +===+ 1428 | 1 | X | 3 | | 4 | |R_1| 1429 +---+ +---+ +---+ +===+ 1431 +---+ +---+ +---+ +---+ +===+ 1432 | 5 | | 6 | | 7 | | 8 | |R_2| 1433 +---+ +---+ +---+ +---+ +===+ 1435 +---+ +---+ +---+ +===+ 1436 | 9 | X | 11| | 12| |R_3| 1437 +---+ +---+ +---+ +===+ 1439 +===+ +===+ +===+ +===+ 1440 |C_1| |C_2| |C_3| |C_4| 1441 +===+ +===+ +===+ +===+ 1443 Figure 17: The resulting pattern after the first iteration 1445 Since the if condition holds true, the receiver runs a new iteration. 1446 In the second iteration, source packets #2 and #10 are recovered, 1447 resulting in a full recovery as sketched in Figure 18. 1449 +---+ +---+ +---+ +---+ +===+ 1450 | 1 | | 2 | | 3 | | 4 | |R_1| 1451 +---+ +---+ +---+ +---+ +===+ 1453 +---+ +---+ +---+ +---+ +===+ 1454 | 5 | | 6 | | 7 | | 8 | |R_2| 1455 +---+ +---+ +---+ +---+ +===+ 1457 +---+ +---+ +---+ +---+ +===+ 1458 | 9 | | 10| | 11| | 12| |R_3| 1459 +---+ +---+ +---+ +---+ +===+ 1461 +===+ +===+ +===+ +===+ 1462 |C_1| |C_2| |C_3| |C_4| 1463 +===+ +===+ +===+ +===+ 1465 Figure 18: The resulting pattern after the second iteration 1467 7. SDP Examples 1469 This section provides two SDP [RFC4566] examples. The examples use 1470 the FEC grouping semantics defined in [RFC5956]. 1472 7.1. Example SDP for Flexible FEC Protection with in-band SSRC mapping 1474 In this example, we have one source video stream and one FEC repair 1475 stream. The source and repair streams are multiplexed on different 1476 SSRCs. The repair window is set to 200 ms. 1478 v=0 1479 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1480 s=FlexFEC minimal SDP signalling Example 1481 t=0 0 1482 m=video 30000 RTP/AVP 96 98 1483 c=IN IP4 143.163.151.157 1484 a=rtpmap:96 VP8/90000 1485 a=rtpmap:98 flexfec/90000 1486 a=fmtp:98; repair-window=200ms 1488 7.2. Example SDP for Flex FEC Protection with explicit signalling in 1489 the SDP 1491 In this example, we have one source video stream (ssrc:1234) and one 1492 FEC repair streams (ssrc:2345). We form one FEC group with the 1493 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1494 are multiplexed on different SSRCs. The repair window is set to 200 1495 ms. 1497 v=0 1498 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1499 s=2-D Parity FEC with no in band signalling Example 1500 t=0 0 1501 m=video 30000 RTP/AVP 100 110 1502 c=IN IP4 233.252.0.1/127 1503 a=rtpmap:100 MP2T/90000 1504 a=rtpmap:110 flexfec/90000 1505 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 1506 a=ssrc:1234 1507 a=ssrc:2345 1508 a=ssrc-group:FEC-FR 1234 2345 1510 8. Congestion Control Considerations 1512 FEC is an effective approach to provide applications resiliency 1513 against packet losses. However, in networks where the congestion is 1514 a major contributor to the packet loss, the potential impacts of 1515 using FEC SHOULD be considered carefully before injecting the repair 1516 flows into the network. In particular, in bandwidth-limited 1517 networks, FEC repair flows may consume most or all of the available 1518 bandwidth and consequently may congest the network. In such cases, 1519 the applications MUST NOT arbitrarily increase the amount of FEC 1520 protection since doing so may lead to a congestion collapse. If 1521 desired, stronger FEC protection MAY be applied only after the source 1522 rate has been reduced [I-D.singh-rmcat-adaptive-fec]. 1524 In a network-friendly implementation, an application SHOULD NOT send/ 1525 receive FEC repair flows if it knows that sending/receiving those FEC 1526 repair flows would not help at all in recovering the missing packets. 1527 However, it MAY still continue to use FEC if considered for bandwidth 1528 estimation instead of speculatively probe for additional capacity 1529 [Holmer13][Nagy14]. It is RECOMMENDED that the amount of FEC 1530 protection is adjusted dynamically based on the packet loss rate 1531 observed by the applications. 1533 In multicast scenarios, it may be difficult to optimize the FEC 1534 protection per receiver. If there is a large variation among the 1535 levels of FEC protection needed by different receivers, it is 1536 RECOMMENDED that the sender offers multiple repair flows with 1537 different levels of FEC protection and the receivers join the 1538 corresponding multicast sessions to receive the repair flow(s) that 1539 is best for them. 1541 Editor's note: Additional congestion control considerations regarding 1542 the use of 2-D parity codes should be added here. 1544 9. Security Considerations 1546 RTP packets using the payload format defined in this specification 1547 are subject to the security considerations discussed in the RTP 1548 specification [RFC3550] and in any applicable RTP profile. The main 1549 security considerations for the RTP packet carrying the RTP payload 1550 format defined within this memo are confidentiality, integrity and 1551 source authenticity. Confidentiality is achieved by encrypting the 1552 RTP payload. Integrity of the RTP packets is achieved through a 1553 suitable cryptographic integrity protection mechanism. Such a 1554 cryptographic system may also allow the authentication of the source 1555 of the payload. A suitable security mechanism for this RTP payload 1556 format should provide confidentiality, integrity protection, and at 1557 least source authentication capable of determining if an RTP packet 1558 is from a member of the RTP session. 1560 Note that the appropriate mechanism to provide security to RTP and 1561 payloads following this memo may vary. It is dependent on the 1562 application, transport and signaling protocol employed. Therefore, a 1563 single mechanism is not sufficient, although if suitable, using the 1564 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1565 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1566 Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may 1567 exist. 1569 10. IANA Considerations 1571 New media subtypes are subject to IANA registration. For the 1572 registration of the payload formats and their parameters introduced 1573 in this document, refer to Section 5. 1575 11. Acknowledgments 1577 Some parts of this document are borrowed from [RFC5109]. Thus, the 1578 author would like to thank the editor of [RFC5109] and those who 1579 contributed to [RFC5109]. 1581 12. Change Log 1583 Note to the RFC-Editor: please remove this section prior to 1584 publication as an RFC. 1586 12.1. draft-ietf-payload-flexible-fec-scheme-02 1588 FEC packet format changed as per discussions in IETF94, Tokyo. 1590 Added section on non-parity codes. 1592 Registration of application/flexfec-raptor. 1594 12.2. draft-ietf-payload-flexible-fec-scheme-01 1596 FEC packet format changed as per discussions in IETF93, Prague. 1598 Replaced non-interleaved-parityfec and interleaved-parity-fec with 1599 flexfec. 1601 SDP simplified for the case when association to RTP is made in the 1602 FEC header and not in the SDP. 1604 12.3. draft-ietf-payload-flexible-fec-scheme-00 1606 Initial WG version, based on draft-singh-payload-1d2d-parity-scheme- 1607 00. 1609 12.4. draft-singh-payload-1d2d-parity-scheme-00 1611 This is the initial version, which is based on draft-ietf-fecframe- 1612 1d2d-parity-scheme-00. The following are the major changes compared 1613 to that document: 1615 o Updated packet format with 16-, 48-, 112- bitmask. 1617 o Updated the sections on: repair packet construction, source packet 1618 construction. 1620 o Updated the media type registration and aligned to RFC6838. 1622 12.5. draft-ietf-fecframe-1d2d-parity-scheme-00 1624 o Some details were added regarding the use of CNAME field. 1626 o Offer-Answer and Declarative Considerations sections have been 1627 completed. 1629 o Security Considerations section has been completed. 1631 o The timestamp field definition has changed. 1633 13. References 1635 13.1. Normative References 1637 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1638 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 1639 RFC2119, March 1997, 1640 . 1642 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1643 with Session Description Protocol (SDP)", RFC 3264, DOI 1644 10.17487/RFC3264, June 2002, 1645 . 1647 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1648 Jacobson, "RTP: A Transport Protocol for Real-Time 1649 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1650 July 2003, . 1652 [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP 1653 Payload Formats", RFC 3555, DOI 10.17487/RFC3555, July 1654 2003, . 1656 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1657 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1658 July 2006, . 1660 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1661 the Session Description Protocol", RFC 5956, DOI 10.17487/ 1662 RFC5956, September 2010, 1663 . 1665 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1666 Correction (FEC) Framework", RFC 6363, DOI 10.17487/ 1667 RFC6363, October 2011, 1668 . 1670 [RFC6682] Watson, M., Stockhammer, T., and M. Luby, "RTP Payload 1671 Format for Raptor Forward Error Correction (FEC)", RFC 1672 6682, DOI 10.17487/RFC6682, August 2012, 1673 . 1675 [RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design 1676 Considerations for Protocol Extensions", RFC 6709, DOI 1677 10.17487/RFC6709, September 2012, 1678 . 1680 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1681 Specifications and Registration Procedures", BCP 13, RFC 1682 6838, DOI 10.17487/RFC6838, January 2013, 1683 . 1685 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1686 "Guidelines for Choosing RTP Control Protocol (RTCP) 1687 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1688 September 2013, . 1690 13.2. Informative References 1692 [Holmer13] 1693 Holmer, S., Shemer, M., and M. Paniconi, "Handling Packet 1694 Loss in WebRTC", Proc. of IEEE International Conference on 1695 Image Processing (ICIP 2013) , 9 2013. 1697 [I-D.singh-rmcat-adaptive-fec] 1698 Varun, V., Nagy, M., Ott, J., and L. Eggert, "Congestion 1699 Control Using FEC for Conversational Media", draft-singh- 1700 rmcat-adaptive-fec-03 (work in progress), March 2016. 1702 [Nagy14] Nagy, M., Singh, V., Ott, J., and L. Eggert, "Congestion 1703 Control using FEC for Conversational Multimedia 1704 Communication", Proc. of 5th ACM Internation Conference on 1705 Multimedia Systems (MMSys 2014) , 3 2014. 1707 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1708 Streaming Protocol (RTSP)", RFC 2326, DOI 10.17487/ 1709 RFC2326, April 1998, 1710 . 1712 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1713 for Generic Forward Error Correction", RFC 2733, DOI 1714 10.17487/RFC2733, December 1999, 1715 . 1717 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1718 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1719 October 2000, . 1721 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1722 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1723 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1724 . 1726 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1727 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1728 December 2005, . 1730 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 1731 Correction", RFC 5109, DOI 10.17487/RFC5109, December 1732 2007, . 1734 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1735 (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ 1736 RFC5246, August 2008, 1737 . 1739 [SMPTE2022-1] 1740 SMPTE 2022-1-2007, , "Forward Error Correction for Real- 1741 Time Video/Audio Transport over IP Networks", 2007. 1743 Authors' Addresses 1745 Varun Singh 1746 Nemu Dialogue Systems Oy 1747 Runeberginkatu 4c A 4 1748 Helsinki 00100 1749 Finland 1751 Email: varun.singh@iki.fi 1752 URI: http://www.callstats.io/ 1754 Ali Begen 1755 Networked Media 1756 Konya 1757 Turkey 1759 Email: ali.begen@networked.media 1761 Mo Zanaty 1762 Cisco 1763 Raleigh, NC 1764 USA 1766 Email: mzanaty@cisco.com 1767 Giridhar Mandyam 1768 Qualcomm Innovation Center 1769 5775 Morehouse Drive 1770 San Diego, CA 92121 1771 USA 1773 Phone: +1 858 651 7200 1774 Email: mandyam@quicinc.com