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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 PAYLOAD M. Zanaty 3 Internet-Draft Cisco 4 Intended status: Standards Track V. Singh 5 Expires: June 10, 2019 callstats.io 6 A. Begen 7 Networked Media 8 G. Mandyam 9 Qualcomm Inc. 10 December 7, 2018 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-12 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 source media encapsulated in RTP. 20 These parity codes are systematic codes, where a number of FEC repair 21 packets are generated from a set of source packets from one or more 22 source RTP streams. These FEC repair packets are sent in a 23 redundancy RTP stream separate from the source RTP stream(s) that 24 carries the source packets. RTP source packets that were lost in 25 transmission can be reconstructed using the source and repair packets 26 that were received. The non-interleaved and interleaved parity codes 27 which are defined in this specification offer a good protection 28 against random and bursty packet losses, respectively, at a cost of 29 decent complexity. The RTP payload formats that are defined in this 30 document address the scalability issues experienced with the earlier 31 specifications including RFC 2733, RFC 5109 and SMPTE 2022-1, and 32 offer several improvements. Due to these changes, the new payload 33 formats are not backward compatible with the earlier specifications, 34 but endpoints that do not implement this specification can still work 35 by simply ignoring the FEC repair packets. 37 Status of This Memo 39 This Internet-Draft is submitted in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at https://datatracker.ietf.org/drafts/current/. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 52 This Internet-Draft will expire on June 10, 2019. 54 Copyright Notice 56 Copyright (c) 2018 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents 61 (https://trustee.ietf.org/license-info) in effect on the date of 62 publication of this document. Please review these documents 63 carefully, as they describe your rights and restrictions with respect 64 to this document. Code Components extracted from this document must 65 include Simplified BSD License text as described in Section 4.e of 66 the Trust Legal Provisions and are provided without warranty as 67 described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 1.1. Parity Codes . . . . . . . . . . . . . . . . . . . . . . 4 73 1.1.1. 1-D Non-interleaved (Row) FEC Protection . . . . . . 5 74 1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 5 75 1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 6 76 1.1.4. 2-D (Row and Column) FEC Protection . . . . . . . . . 8 77 1.1.5. FEC Overhead Considerations . . . . . . . . . . . . . 9 78 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 9 79 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 10 80 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 81 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 82 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 11 83 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 11 84 4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 11 85 4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 12 86 4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 14 87 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 18 88 5.1. Media Type Registration - Parity Codes . . . . . . . . . 19 89 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 19 90 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 20 91 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 22 92 5.1.4. Registration of application/flexfec . . . . . . . . . 23 93 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 25 94 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 25 95 5.2.2. Declarative Considerations . . . . . . . . . . . . . 26 96 6. Protection and Recovery Procedures - Parity Codes . . . . . . 26 97 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 26 98 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 26 99 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 28 100 6.3.1. Associating the Source and Repair Packets . . . . . . 29 101 6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 30 102 6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 31 103 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 104 Protection . . . . . . . . . . . . . . . . . . . . . 32 105 7. Signaling Requirements . . . . . . . . . . . . . . . . . . . 34 106 7.1. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 35 107 7.1.1. Example SDP for Flexible FEC Protection with in-band 108 SSRC mapping . . . . . . . . . . . . . . . . . . . . 35 109 7.1.2. Example SDP for Flexible FEC Protection with explicit 110 signalling in the SDP . . . . . . . . . . . . . . . . 36 111 7.2. On the Use of the RTP Stream Identifier Source 112 Description . . . . . . . . . . . . . . . . . . . . . . . 36 113 8. Congestion Control Considerations . . . . . . . . . . . . . . 36 114 9. Security Considerations . . . . . . . . . . . . . . . . . . . 37 115 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 116 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38 117 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 118 12.1. Normative References . . . . . . . . . . . . . . . . . . 38 119 12.2. Informative References . . . . . . . . . . . . . . . . . 39 120 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 122 1. Introduction 124 This document defines new RTP payload formats for the Forward Error 125 Correction (FEC) that is generated by the non-interleaved and 126 interleaved parity codes from a source media encapsulated in RTP 127 [RFC3550]. The type of the source media protected by these parity 128 codes can be audio, video, text or application. The FEC data are 129 generated according to the media type parameters, which are 130 communicated out-of-band (e.g., in SDP). Furthermore, the 131 associations or relationships between the source and repair RTP 132 streams may be communicated in-band or out-of-band. The in-band 133 mechanism is advantageous when the endpoint is adapting the FEC 134 parameters. The out-of-band mechanism may be preferable when the FEC 135 parameters are fixed. 137 The Redunadncy RTP Stream [RFC7656] repair packets proposed in this 138 document protect the Source RTP Stream packets that belong to the 139 same RTP session. 141 1.1. Parity Codes 143 Both the non-interleaved and interleaved parity codes use the 144 eXclusive OR (XOR) operation to generate the repair packets. The 145 following steps take place: 147 1. The sender determines a set of source packets to be protected by 148 FEC based on the media type parameters. 150 2. The sender applies the XOR operation on the source packets to 151 generate the required number of repair packets. 153 3. The sender sends the repair packet(s) along with the source 154 packets, in different RTP streams, to the receiver(s). The 155 repair packets may be sent proactively or on-demand based on RTCP 156 feedback messages such as NACK [RFC4585]. 158 At the receiver side, if all of the source packets are successfully 159 received, there is no need for FEC recovery and the repair packets 160 are discarded. However, if there are missing source packets, the 161 repair packets can be used to recover the missing information. 162 Figure 1 and Figure 2 describe example block diagrams for the 163 systematic parity FEC encoder and decoder, respectively. 165 +------------+ 166 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 167 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 168 | Encoder | 169 | (Sender) | --> +==+ +==+ 170 +------------+ +==+ +==+ 172 Source Packet: +--+ Repair Packet: +==+ 173 +--+ +==+ 175 Figure 1: Block diagram for systematic parity FEC encoder 177 +------------+ 178 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 179 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 180 | Decoder | 181 +==+ +==+ --> | (Receiver) | 182 +==+ +==+ +------------+ 184 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 185 +--+ +==+ 187 Figure 2: Block diagram for systematic parity FEC decoder 189 In Figure 2, it is clear that the FEC repair packets have to be 190 received by the endpoint within a certain amount of time for the FEC 191 recovery process to be useful. The repair window is defined as the 192 time that spans a FEC block, which consists of the source packets and 193 the corresponding repair packets. At the receiver side, the FEC 194 decoder SHOULD buffer source and repair packets at least for the 195 duration of the repair window, to allow all the repair packets to 196 arrive. The FEC decoder can start decoding the already received 197 packets sooner; however, it should not register a FEC decoding 198 failure until it waits at least for the duration of the repair 199 window. 201 1.1.1. 1-D Non-interleaved (Row) FEC Protection 203 Consider a group of D x L source packets that have sequence numbers 204 starting from 1 running to D x L, and a repair packet is generated by 205 applying the XOR operation to every L consecutive packets as sketched 206 in Figure 3. This process is referred to as 1-D non-interleaved FEC 207 protection. As a result of this process, D repair packets are 208 generated, which are referred to as non-interleaved (or row) FEC 209 repair packets. 211 +--------------------------------------------------+ --- +===+ 212 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 213 +--------------------------------------------------+ --- +===+ 214 +--------------------------------------------------+ --- +===+ 215 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 216 +--------------------------------------------------+ --- +===+ 217 . . . . . . 218 . . . . . . 219 . . . . . . 220 +--------------------------------------------------+ --- +===+ 221 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 222 +--------------------------------------------------+ --- +===+ 224 Figure 3: Generating non-interleaved (row) FEC repair packets 226 1.1.2. 1-D Interleaved (Column) FEC Protection 228 If the XOR operation is applied to the group of the source packets 229 whose sequence numbers are L apart from each other, as sketched in 230 Figure 4. In this case the endpoint generates L repair packets. 231 This process is referred to as 1-D interleaved FEC protection, and 232 the resulting L repair packets are referred to as interleaved (or 233 column) FEC repair packets. 235 +-------------+ +-------------+ +-------------+ +-------+ 236 | S_1 | | S_2 | | S3 | ... | S_L | 237 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 238 | . | | . | | | | | 239 | . | | . | | | | | 240 | . | | . | | | | | 241 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 242 +-------------+ +-------------+ +-------------+ +-------+ 243 + + + + 244 ------------- ------------- ------------- ------- 245 | XOR | | XOR | | XOR | ... | XOR | 246 ------------- ------------- ------------- ------- 247 = = = = 248 +===+ +===+ +===+ +===+ 249 |C_1| |C_2| |C_3| ... |C_L| 250 +===+ +===+ +===+ +===+ 252 Figure 4: Generating interleaved (column) FEC repair packets 254 1.1.3. Use Cases for 1-D FEC Protection 256 A sender may generate one non-interleaved repair packet out of L 257 consecutive source packets or one interleaved repair packet out of D 258 non-consecutive source packets. Regardless of whether the repair 259 packet is a non-interleaved or an interleaved one, it can provide a 260 full recovery of the missing information if there is only one packet 261 missing among the corresponding source packets. This implies that 262 1-D non-interleaved FEC protection performs better when the source 263 packets are randomly lost. However, if the packet losses occur in 264 bursts, 1-D interleaved FEC protection performs better provided that 265 L is chosen large enough, i.e., L-packet duration is not shorter than 266 the observed burst duration. If the sender generates non-interleaved 267 FEC repair packets and a burst loss hits the source packets, the 268 repair operation fails. This is illustrated in Figure 5. 270 +---+ +---+ +===+ 271 | 1 | X X | 4 | |R_1| 272 +---+ +---+ +===+ 274 +---+ +---+ +---+ +---+ +===+ 275 | 5 | | 6 | | 7 | | 8 | |R_2| 276 +---+ +---+ +---+ +---+ +===+ 278 +---+ +---+ +---+ +---+ +===+ 279 | 9 | | 10| | 11| | 12| |R_3| 280 +---+ +---+ +---+ +---+ +===+ 282 Figure 5: Example scenario where 1-D non-interleaved FEC protection 283 fails error recovery (Burst Loss) 285 The sender may generate interleaved FEC repair packets to combat with 286 the bursty packet losses. However, two or more random packet losses 287 may hit the source and repair packets in the same column. In that 288 case, the repair operation fails as well. This is illustrated in 289 Figure 6. Note that it is possible that two burst losses may occur 290 back-to-back, in which case interleaved FEC repair packets may still 291 fail to recover the lost data. 293 +---+ +---+ +---+ 294 | 1 | X | 3 | | 4 | 295 +---+ +---+ +---+ 297 +---+ +---+ +---+ 298 | 5 | X | 7 | | 8 | 299 +---+ +---+ +---+ 301 +---+ +---+ +---+ +---+ 302 | 9 | | 10| | 11| | 12| 303 +---+ +---+ +---+ +---+ 305 +===+ +===+ +===+ +===+ 306 |C_1| |C_2| |C_3| |C_4| 307 +===+ +===+ +===+ +===+ 309 Figure 6: Example scenario where 1-D interleaved FEC protection fails 310 error recovery (Periodic Loss) 312 1.1.4. 2-D (Row and Column) FEC Protection 314 In networks where the source packets are lost both randomly and in 315 bursts, the sender ought to generate both non-interleaved and 316 interleaved FEC repair packets. This type of FEC protection is known 317 as 2-D parity FEC protection. At the expense of generating more FEC 318 repair packets, thus increasing the FEC overhead, 2-D FEC provides 319 superior protection against mixed loss patterns. However, it is 320 still possible for 2-D parity FEC protection to fail to recover all 321 of the lost source packets if a particular loss pattern occurs. An 322 example scenario is illustrated in Figure 7. 324 +---+ +---+ +===+ 325 | 1 | X X | 4 | |R_1| 326 +---+ +---+ +===+ 328 +---+ +---+ +---+ +---+ +===+ 329 | 5 | | 6 | | 7 | | 8 | |R_2| 330 +---+ +---+ +---+ +---+ +===+ 332 +---+ +---+ +===+ 333 | 9 | X X | 12| |R_3| 334 +---+ +---+ +===+ 336 +===+ +===+ +===+ +===+ 337 |C_1| |C_2| |C_3| |C_4| 338 +===+ +===+ +===+ +===+ 340 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 341 error recovery 343 2-D parity FEC protection also fails when at least two rows are 344 missing a source and the FEC packet and the missing source packets 345 (in at least two rows) are aligned in the same column. An example 346 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 347 protection cannot repair all missing source packets when at least two 348 columns are missing a source and the FEC packet and the missing 349 source packets (in at least two columns) are aligned in the same row. 351 +---+ +---+ +---+ 352 | 1 | | 2 | X | 4 | X 353 +---+ +---+ +---+ 355 +---+ +---+ +---+ +---+ +===+ 356 | 5 | | 6 | | 7 | | 8 | |R_2| 357 +---+ +---+ +---+ +---+ +===+ 359 +---+ +---+ +---+ 360 | 9 | | 10| X | 12| X 361 +---+ +---+ +---+ 363 +===+ +===+ +===+ +===+ 364 |C_1| |C_2| |C_3| |C_4| 365 +===+ +===+ +===+ +===+ 367 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 368 error recovery 370 1.1.5. FEC Overhead Considerations 372 The overhead is defined as the ratio of the number of bytes belonging 373 to the repair packets to the number of bytes belonging to the 374 protected source packets. 376 Generally, repair packets are larger in size compared to the source 377 packets. Also, not all the source packets are necessarily equal in 378 size. However, assuming that each repair packet carries an equal 379 number of bytes carried by a source packet, the overhead for 380 different FEC protection methods can be computed as follows: 382 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 384 o 1-D Interleaved FEC Protection: Overhead = 1/D 386 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 388 where L and D are the number of columns and rows in the source block, 389 respectively. 391 2. Requirements Notation 393 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 394 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 395 document are to be interpreted as described in [RFC2119]. 397 3. Definitions and Notations 399 3.1. Definitions 401 This document uses a number of definitions from [RFC6363]. 403 1-D Non-interleaved Row FEC: A protection scheme that operates on 404 consecutive source packets in the source block, able to recover a 405 single lost source packet per row of the source block. 407 1-D Interleaved Column FEC: A protection scheme that operates on 408 interleaved source packets in the source block, able to recover a 409 single lost source packet per column of the source block. 411 2-D FEC: A protection scheme that combines row and column FEC. 413 Source Block: A set of source packets that are protected by a set 414 of 1-D or 2-D FEC repair packets. 416 FEC Block: A source block and its corresponding FEC repair 417 packets. 419 Repair Window: The time that spans a FEC block, which consists of 420 the source packets and the corresponding FEC repair packets. 422 XOR Parity Codes: A FEC code which uses the eXclusive OR (XOR) 423 parity operation to encode a set of source packets to form a FEC 424 repair packet. 426 3.2. Notations 428 L: Number of columns of the source block (length of each row). 430 D: Number of rows of the source block (depth of each column). 432 bitmask: A 15-bit, 46-bit, or 110-bit mask indicating which source 433 packets are protected by a FEC repair packet. If the bit i in the 434 mask is set to 1, the source packet number N + i is protected by 435 this FEC repair packet, where N is the sequence number base 436 indicated in the FEC repair packet. The most significant bit of 437 the mask corresponds to i=0. The least signficant bit of the mask 438 corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit mask, 439 or i=109 in the 110-bit mask. 441 4. Packet Formats 443 This section describes the formats of the source packets and defines 444 the formats of the FEC repair packets. 446 4.1. Source Packets 448 The source packets contain the information that identifies the source 449 block and the position within the source block occupied by the 450 packet. Since the source packets that are carried within an RTP 451 stream already contain unique sequence numbers in their RTP headers 452 [RFC3550], the source packets can be identified in a straightforward 453 manner and there is no need to append additional field(s). The 454 primary advantage of not modifying the source packets in any way is 455 that it provides backward compatibility for the receivers that do not 456 support FEC at all. In multicast scenarios, this backward 457 compatibility becomes quite useful as it allows the non-FEC-capable 458 and FEC-capable receivers to receive and interpret the same source 459 packets sent in the same multicast session. 461 The source packets are transmitted as usual without altering them. 462 They are used along with the FEC repair packets to recover any 463 missing source packets, making this scheme a systematic code. 465 The source packets are full RTP packets with optional CSRC list, RTP 466 header extension, and padding. If any of these optional elements are 467 present in the source RTP packet, and that source packet is lost, 468 they are recovered by the FEC repair operation, which recovers the 469 full source RTP packet including these optional elements. 471 4.2. FEC Repair Packets 473 The FEC repair packets MUST contain information that identifies the 474 source block they pertain to and the relationship between the 475 contained repair packets and the original source block. For this 476 purpose, the RTP header of the repair packets is used, as well as 477 another header within the RTP payload, called the FEC header, as 478 shown in Figure 9. 480 Note that all the source stream packets that are protected by a 481 particular FEC packet need to be in the same RTP session. 483 +------------------------------+ 484 | IP Header | 485 +------------------------------+ 486 | Transport Header | 487 +------------------------------+ 488 | RTP Header | 489 +------------------------------+ ---+ 490 | FEC Header | | 491 +------------------------------+ | RTP Payload 492 | Repair "Payload" | | 493 +------------------------------+ ---+ 495 Figure 9: Format of FEC repair packets 497 Repair "Payload", which follows the FEC Header, includes repair of 498 everything following the fixed 12-byte RTP header of the source 499 packet, including any CSRC list and header extensions if present. 501 4.2.1. RTP Header of FEC Repair Packets 503 The RTP header is formatted according to [RFC3550] with some further 504 clarifications listed below: 506 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this 507 specification requires all source RTP packets and all FEC repair 508 packets to use RTP version 2. The reason for this restriction is 509 the first 2 bits of the FEC header contain other information (R 510 and F bits) rather than recovering the RTP version field. 512 Padding (P) bit: Source packets can have optional RTP padding, 513 which can be recovered. FEC repaire packets can have optional RTP 514 padding, which is independent of the RTP padding of the source 515 pakcets. 517 Extension (X) bit: Source packets can have optional RTP header 518 extensions, which can be recovered. FEC repair packets can have 519 optional RTP header extensions, which are independent of the RTP 520 header extensions of the source packets. 522 CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: 523 Source packets can have an optional CSRC list and count, which can 524 be recovered. FEC repair packets MUST use the CSRC list and count 525 to specify the SSRC(s) of the source RTP stream(s) protected by 526 this FEC repair packet. 528 Marker (M) bit: This bit is not used for this payload type, and 529 SHALL be set to 0 by senders, and SHALL be ignored by receivers. 531 Payload Type: The (dynamic) payload type for the FEC repair 532 packets is determined through out-of-band means. Note that this 533 document registers new payload formats for the repair packets 534 (Refer to Section 5 for details). According to [RFC3550], an RTP 535 receiver that cannot recognize a payload type must discard it. 536 This provides backward compatibility. If a non-FEC-capable 537 receiver receives a repair packet, it will not recognize the 538 payload type, and hence, will discard the repair packet. 540 Sequence Number (SN): The sequence number has the standard 541 definition. It MUST be one higher than the sequence number in the 542 previously transmitted repair packet. The initial value of the 543 sequence number SHOULD be random (unpredictable, based on 544 [RFC3550]). 546 Timestamp (TS): The timestamp SHALL be set to a time corresponding 547 to the repair packet's transmission time. Note that the timestamp 548 value has no use in the actual FEC protection process and is 549 usually useful for jitter calculations. 551 Synchronization Source (SSRC): The SSRC value for each repair 552 stream SHALL be randomly assigned as suggested by [RFC3550]. This 553 allows the sender to multiplex the source and repair RTP streams 554 in the same RTP session, or multiplex multiple repair streams in 555 an RTP session. The repair streams' SSRC's CNAME SHOULD be 556 identical to the CNAME of the source RTP stream(s) that this 557 repair stream protects. In cases when the repair stream covers 558 packets from multiple source RTP streams with different CNAME 559 values, any of these CNAME values MAY be used. 561 In some networks, the RTP Source, which produces the source 562 packets and the FEC Source, which generates the repair packets 563 from the source packets may not be the same host. In such 564 scenarios, using the same CNAME for the source and repair RTP 565 streams means that the RTP Source and the FEC Source MUST share 566 the same CNAME (for this specific source-repair stream 567 association). A common CNAME may be produced based on an 568 algorithm that is known both to the RTP and FEC Source [RFC7022]. 569 This usage is compliant with [RFC3550]. 571 Note that due to the randomness of the SSRC assignments, there is 572 a possibility of SSRC collision. In such cases, the collisions 573 MUST be resolved as described in [RFC3550]. 575 4.2.2. FEC Header of FEC Repair Packets 577 The format of the FEC header has 3 variants, depending on the values 578 in the first 2 bits (R and F bits) as shown in Figure 10. 580 0 1 2 3 581 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 1 582 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 583 |R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... | 584 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 585 | | 586 | ...varies depending on R/F... | 587 | | 588 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 589 : Repair "Payload" follows FEC Header : 590 : : 592 Figure 10: FEC Header 594 Repair "Payload", which follows the FEC Header, includes repair of 595 everything following the fixed 12-byte RTP header of the source 596 packet, including any CSRC list and header extensions if present. 598 +---+---+----------------------------------------------------------+ 599 | R | F | FEC Header variant | 600 +---+---+----------------------------------------------------------+ 601 | 0 | 0 | Flexible FEC Mask fields indicate source packets | 602 | 0 | 1 | Fixed FEC L/D (cols/rows) fields indicate source packets | 603 | 1 | 0 | Retransmission of a single source packet | 604 | 1 | 1 | Invalid, MUST NOT send, MUST ignore if received | 605 +---+---+----------------------------------------------------------+ 607 Figure 11: R and F bit values for FEC Header variants 609 The first variant, when R=0 and F=0, has a mask to signal protected 610 source packets, as shown in Figure 12. 612 The second variant, when R=0 and F=1, has a number of columns (L) and 613 rows (D) to signal protected source packets, as shown in Figure 13. 615 The final variant, when R=1 and F=0, is a retransmission format as 616 shown in Figure 15. 618 No variant uses R=1 and F=1, which is invalid, and MUST NOT be sent 619 by senders, and MUST be ignored by receivers. 621 The FEC header for all variants consists of the following common 622 fields: 624 o The R bit MUST be set to 1 to indicate a retransmission packet, 625 and MUST be set to 0 for FEC repair packets. 627 o The F bit indicates the type of FEC repair packets, as shown in 628 Figure 11, when the R bit is 0. The F bit MUST be set to 0 when 629 the R bit is 1 for retransmission packets. 631 o The P, X, CC, M and PT recovery fields are used to determine the 632 corresponding fields of the recovered packets. 634 4.2.2.1. FEC Header with Flexible Mask 636 When R=0 and F=0, the FEC Header includes flexible mask fields. 638 0 1 2 3 639 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 1 640 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 641 |0|0|P|X| CC |M| PT recovery | length recovery | 642 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 643 | TS recovery | 644 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 645 | SN base_i |k| Mask [0-14] | 646 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 647 |k| Mask [15-45] (optional) | 648 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 649 | Mask [46-109] (optional) | 650 | | 651 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 652 | ... next SN base and Mask for CSRC_i in CSRC list ... | 653 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 654 : Repair "Payload" follows FEC Header : 655 : : 657 Figure 12: FEC Header for F=0 659 o The Length recovery (16 bits) field is used to determine the 660 length of the recovered packets. This length includes all octets 661 following the fixed 12-byte RTP header of source packets, 662 including CSRC list and optional header extension(s) if present. 663 It excludes the fixed 12-byte RTP header of source packets. 665 o The TS recovery (32 bits) field is used to determine the timestamp 666 of the recovered packets. 668 o The CSRC_i (32 bits) field in the RTP Header (not FEC Header) 669 describes the SSRC of the source packets protected by this 670 particular FEC packet. If a FEC packet protects multiple SSRCs 671 (indicated by the CSRC Count > 1 in the RTP Header), there will be 672 multiple blocks of data containing the SN base and Mask fields. 674 o The SN base_i (16 bits) field indicates the lowest sequence 675 number, taking wrap around into account, of the source packets for 676 a particular SSRC (indicated in CSRC_i) protected by this repair 677 packet. 679 o The Mask fields indicate a bitmask of which source packets are 680 protected by this FEC repair packet, where bit j of the mask set 681 to 1 indicates that the source packet with sequence number (SN 682 base_i + j) is protected by this FEC repair packet, where j=0 is 683 the most significant bit in the mask. 685 o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110 686 bits. k=1 denotes that another mask follows, and k=0 denotes that 687 it is the last block of mask. 689 o Repair "Payload", which follows the FEC Header, includes repair of 690 everything following the fixed 12-byte RTP header of the source 691 packet, including any CSRC list and header extensions if present. 693 4.2.2.2. FEC Header with Fixed L Columns and D Rows 695 When R=0 and F=1, the FEC Header includes L and D fields for fixed 696 columns and rows. The other fields are the same as the prior 697 section. As in the previous section, the CSRC_i (32 bits) field in 698 the RTP Header (not FEC Header) describes the SSRC of the source 699 packets protected by this particular FEC packet. If there are 700 multiple SSRC's protected by the FEC packet, then there will be 701 multiple blocks of data containing an SN base along with L and D 702 fields. 704 0 1 2 3 705 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 1 706 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 707 |0|1|P|X| CC |M| PT recovery | length recovery | 708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 709 | TS recovery | 710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 711 | SN base_i | L (columns) | D (rows) | 712 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 713 | ... next SN base and L/D for CSRC_i in CSRC list ... | 714 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 715 : Repair "Payload" follows FEC Header : 716 : : 718 Figure 13: FEC Header for F=1 720 Consequently, the following conditions occur for L and D values: 722 If L=0, D=0, use the optional payload format parameters for L and D. 724 If L>0, D=0, indicates Row FEC, and no column FEC will follow. 725 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L. 727 If L>0, D=1, indicates Row FEC, and column FEC will follow. 728 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L will be 729 produced for each row. 730 Then FEC = SN, SN+L, SN+2L, ..., SN+(D-1)L will be produced 731 for each column. 732 After all row FEC's have been sent, then the column FEC's 733 will be sent. 735 If L>0, D>1, indicates column FEC of every L packet 736 in a group of D packets starting at SN base. 737 Hence, FEC = SN+(Lx0), SN+(Lx1), ... , SN+(LxD). 739 Figure 14: Interpreting the L and D field values 741 It should be noted that the flexible mask-based approach may be 742 inefficient for protecting a large number of source packets, or 743 impossible to signal if larger than the largest mask size. In such 744 cases, the fixed columns and rows variant may be more useful. 746 4.2.2.3. FEC Header for Retransmissions 748 When R=1 and F=0, the FEC packet is a retransmission of a single 749 source packet. Note that the layout of this retransmission packet is 750 different from other FEC repair packets. The sequence number (SN 751 base_i) replaces the length recovery in the FEC header, since the 752 length is already known for a single packet. There are no L, D or 753 Mask fields, since only a single packet is retransmitted, identified 754 by the sequence number in the FEC header. The source packet SSRC is 755 included in the FEC header for retransmissions, not in the RTP header 756 CSRC list as in the FEC header variants with R=0. When performing 757 retransmissions, a single repair packet stream (SSRC) MAY be used for 758 retransmitting packets from multiple source packet streams (SSRCs), 759 as well as transmitting FEC repair packets that protect multiple 760 source packet streams (SSRCs). 762 This FEC header layout is identical to the source RTP (version 2) 763 packet, starting with its RTP header, where the retransmission 764 "payload" is everything following the fixed 12-byte RTP header of the 765 source packet, including CSRC list and extensions if present. 766 Therefore, the only operation needed for sending retransmissions is 767 to prepend a new RTP header to the source packet. 769 0 1 2 3 770 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 1 771 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 772 |1|0| P|X| CC |M| Payload Type| Sequence Number | 773 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 774 | Timestamp | 775 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 776 | SSRC | 777 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 778 : Retransmission "Payload" follows FEC Header : 779 : : 781 Figure 15: FEC Header for Retransmission 783 5. Payload Format Parameters 785 This section provides the media subtype registration for the non- 786 interleaved and interleaved parity FEC. The parameters that are 787 required to configure the FEC encoding and decoding operations are 788 also defined in this section. If no specific FEC code is specified 789 in the subtype, then the FEC code defaults to the parity code defined 790 in this specification. 792 5.1. Media Type Registration - Parity Codes 794 This registration is done using the template defined in [RFC6838] and 795 following the guidance provided in [RFC3555]. 797 Note to the RFC Editor: In the following sections, please replace 798 "XXXX" with the number of this document prior to publication as an 799 RFC. 801 5.1.1. Registration of audio/flexfec 803 Type name: audio 805 Subtype name: flexfec 807 Required parameters: 809 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 810 than 1000 Hz to provide sufficient resolution to RTCP operations. 811 However, it is RECOMMENDED to select the rate that matches the 812 rate of the protected source RTP stream. 814 o repair-window: The time that spans the source packets and the 815 corresponding repair packets. The size of the repair window is 816 specified in microseconds. 818 Optional parameters: 820 o L: indicates the number of columns of the source block that are 821 protected by this FEC block and it applies to all the source 822 SSRCs. L is a positive integer. 824 o D: indicates the number of rows of the source block that are 825 protected by this FEC block and it applies to all the source 826 SSRCs. D is a positive integer. 828 o ToP: indicates the type of protection applied by the sender: 0 for 829 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 830 protection, 2 for 2-D parity FEC protection, and 3 for 831 retransmission. There can only be one value listed for ToP. The 832 absence of the ToP field means that all protection types are 833 allowed. 835 Note that both L and D in the optional parameters should follow the 836 value pairings stated in Section 4.2.2.2 if included. 838 Encoding considerations: This media type is framed (See Section 4.8 839 in the template document [RFC6838]) and contains binary data. 841 Security considerations: See Section 9 of [RFCXXXX]. 843 Interoperability considerations: None. 845 Published specification: [RFCXXXX]. 847 Applications that use this media type: Multimedia applications that 848 want to improve resiliency against packet loss by sending redundant 849 data in addition to the source media. 851 Fragment identifier considerations: None. 853 Additional information: None. 855 Person & email address to contact for further information: Varun 856 Singh and IETF Audio/Video Transport Payloads 857 Working Group. 859 Intended usage: COMMON. 861 Restriction on usage: This media type depends on RTP framing, and 862 hence, is only defined for transport via RTP [RFC3550]. 864 Author: Varun Singh . 866 Change controller: IETF Audio/Video Transport Working Group delegated 867 from the IESG. 869 Provisional registration? (standards tree only): Yes. 871 5.1.2. Registration of video/flexfec 873 Type name: video 875 Subtype name: flexfec 877 Required parameters: 879 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 880 than 1000 Hz to provide sufficient resolution to RTCP operations. 881 However, it is RECOMMENDED to select the rate that matches the 882 rate of the protected source RTP stream. 884 o repair-window: The time that spans the source packets and the 885 corresponding repair packets. The size of the repair window is 886 specified in microseconds. 888 Optional parameters: 890 o L: indicates the number of columns of the source block that are 891 protected by this FEC block and it applies to all the source 892 SSRCs. L is a positive integer. 894 o D: indicates the number of rows of the source block that are 895 protected by this FEC block and it applies to all the source 896 SSRCs. D is a positive integer. 898 o ToP: indicates the type of protection applied by the sender: 0 for 899 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 900 protection, 2 for 2-D parity FEC protection, and 3 for 901 retransmission. There can only be one value listed for ToP. The 902 absence of the ToP field means that all protection types are 903 allowed. 905 Note that both L and D in the optional parameters should follow the 906 value pairings stated in Section 4.2.2.2 if included. 908 Encoding considerations: This media type is framed (See Section 4.8 909 in the template document [RFC6838]) and contains binary data. 911 Security considerations: See Section 9 of [RFCXXXX]. 913 Interoperability considerations: None. 915 Published specification: [RFCXXXX]. 917 Applications that use this media type: Multimedia applications that 918 want to improve resiliency against packet loss by sending redundant 919 data in addition to the source media. 921 Fragment identifier considerations: None. 923 Additional information: None. 925 Person & email address to contact for further information: Varun 926 Singh and IETF Audio/Video Transport Payloads 927 Working Group. 929 Intended usage: COMMON. 931 Restriction on usage: This media type depends on RTP framing, and 932 hence, is only defined for transport via RTP [RFC3550]. 934 Author: Varun Singh . 936 Change controller: IETF Audio/Video Transport Working Group delegated 937 from the IESG. 939 Provisional registration? (standards tree only): Yes. 941 5.1.3. Registration of text/flexfec 943 Type name: text 945 Subtype name: flexfec 947 Required parameters: 949 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 950 than 1000 Hz to provide sufficient resolution to RTCP operations. 951 However, it is RECOMMENDED to select the rate that matches the 952 rate of the protected source RTP stream. 954 o repair-window: The time that spans the source packets and the 955 corresponding repair packets. The size of the repair window is 956 specified in microseconds. 958 Optional parameters: 960 o L: indicates the number of columns of the source block that are 961 protected by this FEC block and it applies to all the source 962 SSRCs. L is a positive integer. 964 o D: indicates the number of rows of the source block that are 965 protected by this FEC block and it applies to all the source 966 SSRCs. D is a positive integer. 968 o ToP: indicates the type of protection applied by the sender: 0 for 969 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 970 protection, 2 for 2-D parity FEC protection, and 3 for 971 retransmission. There can only be one value listed for ToP. The 972 absence of the ToP field means that all protection types are 973 allowed. 975 Note that both L and D in the optional parameters should follow the 976 value pairings stated in Section 4.2.2.2 if included. 978 Encoding considerations: This media type is framed (See Section 4.8 979 in the template document [RFC6838]) and contains binary data. 981 Security considerations: See Section 9 of [RFCXXXX]. 983 Interoperability considerations: None. 985 Published specification: [RFCXXXX]. 987 Applications that use this media type: Multimedia applications that 988 want to improve resiliency against packet loss by sending redundant 989 data in addition to the source media. 991 Fragment identifier considerations: None. 993 Additional information: None. 995 Person & email address to contact for further information: Varun 996 Singh and IETF Audio/Video Transport Payloads 997 Working Group. 999 Intended usage: COMMON. 1001 Restriction on usage: This media type depends on RTP framing, and 1002 hence, is only defined for transport via RTP [RFC3550]. 1004 Author: Varun Singh . 1006 Change controller: IETF Audio/Video Transport Working Group delegated 1007 from the IESG. 1009 Provisional registration? (standards tree only): Yes. 1011 5.1.4. Registration of application/flexfec 1013 Type name: application 1015 Subtype name: flexfec 1017 Required parameters: 1019 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 1020 than 1000 Hz to provide sufficient resolution to RTCP operations. 1021 However, it is RECOMMENDED to select the rate that matches the 1022 rate of the protected source RTP stream. 1024 o repair-window: The time that spans the source packets and the 1025 corresponding repair packets. The size of the repair window is 1026 specified in microseconds. 1028 Optional parameters: 1030 o L: indicates the number of columns of the source block that are 1031 protected by this FEC block and it applies to all the source 1032 SSRCs. L is a positive integer. 1034 o D: indicates the number of rows of the source block that are 1035 protected by this FEC block and it applies to all the source 1036 SSRCs. D is a positive integer. 1038 o ToP: indicates the type of protection applied by the sender: 0 for 1039 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 1040 protection, 2 for 2-D parity FEC protection, and 3 for 1041 retransmission. There can only be one value listed for ToP. The 1042 absence of the ToP field means that all protection types are 1043 allowed. 1045 Note that both L and D in the optional parameters should follow the 1046 value pairings stated in Section 4.2.2.2 if included. 1048 Encoding considerations: This media type is framed (See Section 4.8 1049 in the template document [RFC6838]) and contains binary data. 1051 Security considerations: See Section 9 of [RFCXXXX]. 1053 Interoperability considerations: None. 1055 Published specification: [RFCXXXX]. 1057 Applications that use this media type: Multimedia applications that 1058 want to improve resiliency against packet loss by sending redundant 1059 data in addition to the source media. 1061 Fragment identifier considerations: None. 1063 Additional information: None. 1065 Person & email address to contact for further information: Varun 1066 Singh and IETF Audio/Video Transport Payloads 1067 Working Group. 1069 Intended usage: COMMON. 1071 Restriction on usage: This media type depends on RTP framing, and 1072 hence, is only defined for transport via RTP [RFC3550]. 1074 Author: Varun Singh . 1076 Change controller: IETF Audio/Video Transport Working Group delegated 1077 from the IESG. 1079 Provisional registration? (standards tree only): Yes. 1081 5.2. Mapping to SDP Parameters 1083 Applications that are using RTP transport commonly use Session 1084 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1085 The information that is used to specify the media types in an RTP 1086 session has specific mappings to the fields in an SDP description. 1087 This section provides these mappings for the media subtypes 1088 registered by this document. Note that if an application does not 1089 use SDP to describe the RTP sessions, an appropriate mapping must be 1090 defined and used to specify the media types and their parameters for 1091 the control/description protocol employed by the application. 1093 The mapping of the media type specification for "non-interleaved- 1094 parityfec" and "interleaved-parityfec" and their parameters in SDP is 1095 as follows: 1097 o The media type (e.g., "application") goes into the "m=" line as 1098 the media name. 1100 o The media subtype goes into the "a=rtpmap" line as the encoding 1101 name. The RTP clock rate parameter ("rate") also goes into the 1102 "a=rtpmap" line as the clock rate. 1104 o The remaining required payload-format-specific parameters go into 1105 the "a=fmtp" line by copying them directly from the media type 1106 string as a semicolon-separated list of parameter=value pairs. 1108 SDP examples are provided in Section 7.1. 1110 5.2.1. Offer-Answer Model Considerations 1112 When offering 1-D interleaved parity FEC over RTP using SDP in an 1113 Offer/Answer model [RFC3264], the following considerations apply: 1115 o Each combination of the L and D parameters produces a different 1116 FEC data and is not compatible with any other combination. A 1117 sender application may desire to offer multiple offers with 1118 different sets of L and D values as long as the parameter values 1119 are valid. The receiver SHOULD normally choose the offer that has 1120 a sufficient amount of interleaving. If multiple such offers 1121 exist, the receiver may choose the offer that has the lowest 1122 overhead or the one that requires the smallest amount of 1123 buffering. The selection depends on the application requirements. 1125 o The value for the repair-window parameter depends on the L and D 1126 values and cannot be chosen arbitrarily. More specifically, L and 1127 D values determine the lower limit for the repair-window size. 1129 The upper limit of the repair-window size does not depend on the L 1130 and D values. 1132 o Although combinations with the same L and D values but with 1133 different repair-window sizes produce the same FEC data, such 1134 combinations are still considered different offers. The size of 1135 the repair-window is related to the maximum delay between the 1136 transmission of a source packet and the associated repair packet. 1137 This directly impacts the buffering requirement on the receiver 1138 side and the receiver must consider this when choosing an offer. 1140 o Any unknown option in the offer MUST be ignored and deleted from 1141 the answer. If FEC is not desired by the receiver, it can be 1142 deleted from the answer. 1144 5.2.2. Declarative Considerations 1146 In declarative usage, like SDP in the Real-time Streaming Protocol 1147 (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) 1148 [RFC2974], the following considerations apply: 1150 o The payload format configuration parameters are all declarative 1151 and a participant MUST use the configuration that is provided for 1152 the session. 1154 o More than one configuration may be provided (if desired) by 1155 declaring multiple RTP payload types. In that case, the receivers 1156 should choose the repair stream that is best for them. 1158 6. Protection and Recovery Procedures - Parity Codes 1160 This section provides a complete specification of the 1-D and 2-D 1161 parity codes and their RTP payload formats. It does not apply to the 1162 single packet retransmission format (R=1 in the FEC Header). 1164 6.1. Overview 1166 The following sections specify the steps involved in generating the 1167 repair packets and reconstructing the missing source packets from the 1168 repair packets. 1170 6.2. Repair Packet Construction 1172 The RTP Header of a repair packet is formed based on the guidelines 1173 given in Section 4.2. 1175 The FEC Header and Repair "Payload" of repair packets are formed by 1176 applying the XOR operation on the bit strings that are generated from 1177 the individual source packets protected by this particular repair 1178 packet. The set of the source packets that are associated with a 1179 given repair packet can be computed by the formula given in 1180 Section 6.3.1. 1182 The bit string is formed for each source packet by concatenating the 1183 following fields together in the order specified: 1185 o The first 16 bits of the RTP header (16 bits). 1187 o Unsigned network-ordered 16-bit representation of the source 1188 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1189 the sum of the lengths of all the following if present: the CSRC 1190 list, extension header, RTP payload and RTP padding (16 bits). 1192 o The timestamp of the RTP header (32 bits). 1194 o All octets after the fixed 12-byte RTP header. (Note the SSRC 1195 field is skipped.) 1197 The FEC bit string is generated by applying the parity operation on 1198 the bit strings produced from the source packets. The FEC header is 1199 generated from the FEC bit string as follows: 1201 o The first (most significant) 2 bits in the FEC bit string, which 1202 contain the RTP version field, are skipped. The R and F bits in 1203 the FEC header are set to the appropriate value, i.e., it depends 1204 on the chosen format variant. As a consequence of overwriting the 1205 RTP version field with the R and F bits, this payload format only 1206 supports RTP version 2. 1208 o The next bit in the FEC bit string is written into the P recovery 1209 bit in the FEC header. 1211 o The next bit in the FEC bit string is written into the X recovery 1212 bit in the FEC header. 1214 o The next 4 bits of the FEC bit string are written into the CC 1215 recovery field in the FEC header. 1217 o The next bit is written into the M recovery bit in the FEC header. 1219 o The next 7 bits of the FEC bit string are written into the PT 1220 recovery field in the FEC header. 1222 o The next 16 bits are written into the length recovery field in the 1223 FEC header. 1225 o The next 32 bits of the FEC bit string are written into the TS 1226 recovery field in the FEC header. 1228 o The lowest Sequence Number of the source packets protected by this 1229 repair packet is written into the Sequence Number Base field in 1230 the FEC header. This needs to be repeated for each SSRC that has 1231 packets included in the source block. 1233 o Depending on the chosen FEC header variant, the mask(s) are set 1234 when F=0, or the L and D values are set when F=1. This needs to 1235 be repeated for each SSRC that has packets included in the source 1236 block. 1238 o The rest of the FEC bit string, which contains everything after 1239 the fixed 12-byte RTP header of the source packet, is written into 1240 the Repair "Payload" following the FEC header, where "Payload" 1241 refers to everything after the fixed 12-byte RTP header, including 1242 extensions, CSRC list, true payloads, and padding. 1244 If the lengths of the source packets are not equal, each shorter 1245 packet MUST be padded to the length of the longest packet by adding 1246 octet 0's at the end. 1248 Due to this possible padding and mandatory FEC header, a repair 1249 packet has a larger size than the source packets it protects. This 1250 may cause problems if the resulting repair packet size exceeds the 1251 Maximum Transmission Unit (MTU) size of the path over which the 1252 repair stream is sent. 1254 6.3. Source Packet Reconstruction 1256 This section describes the recovery procedures that are required to 1257 reconstruct the missing source packets. The recovery process has two 1258 steps. In the first step, the FEC decoder determines which source 1259 and repair packets should be used in order to recover a missing 1260 packet. In the second step, the decoder recovers the missing packet, 1261 which consists of an RTP header and RTP payload. 1263 The following describes the RECOMMENDED algorithms for the first and 1264 second steps. Based on the implementation, different algorithms MAY 1265 be adopted. However, the end result MUST be identical to the one 1266 produced by the algorithms described below. 1268 Note that the same algorithms are used by the 1-D parity codes, 1269 regardless of whether the FEC protection is applied over a column or 1270 a row. The 2-D parity codes, on the other hand, usually require 1271 multiple iterations of the procedures described here. This iterative 1272 decoding algorithm is further explained in Section 6.3.4. 1274 6.3.1. Associating the Source and Repair Packets 1276 Before associating source and repair packets, the receiver must know 1277 in which RTP sessions the source and repair respectively are being 1278 sent. After this is established by the reciever the first step is 1279 associating the source and repair packets. This association can be 1280 via flexible bitmasks, or fixed L and D offsets which can be in the 1281 FEC header or signaled in SDP in optional payload format parameters 1282 when L=D=0 in the FEC header. 1284 6.3.1.1. Using Bitmasks 1286 To use flexible bitmasks, the first two FEC header bits MUST have R=0 1287 and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source 1288 packets are protected by a FEC repair packet. If the bit i in the 1289 mask is set to 1, the source packet number N + i is protected by this 1290 FEC repair packet, where N is the sequence number base indicated in 1291 the FEC header. The most significant bit of the mask corresponds to 1292 i=0. The least signficant bit of the mask corresponds to i=14 in the 1293 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit mask. 1295 The bitmasks are able to represent arbitrary protection patterns, for 1296 example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase. 1298 6.3.1.2. Using L and D Offsets 1300 Denote the set of the source packets associated with repair packet p* 1301 by set T(p*). Note that in a source block whose size is L columns by 1302 D rows, set T includes D source packets plus one repair packet for 1303 the FEC protection applied over a column, and L source packets plus 1304 one repair packet for the FEC protection applied over a row. Recall 1305 that 1-D interleaved and non-interleaved FEC protection can fully 1306 recover the missing information if there is only one source packet 1307 missing per column or row in set T. If there are more than one 1308 source packets missing per column or row in set T, 1-D FEC protection 1309 may fail to recover all the missing information. 1311 When value of L is non-zero, the 8-bit fields indicate the offset of 1312 packets protected by an interleaved (D>0) or non-interleaved (D=0) 1313 FEC packet. Using a combination of interleaved and non-interleaved 1314 FEC repair packets can form 2-D protection patterns. 1316 Mathematically, for any received repair packet, p*, the sequence 1317 numbers of the source packets that are protected by this repair 1318 packet are determined as follows, where p*_snb is the sequence number 1319 base in the FEC header: 1321 When D = 0: 1322 p*_snb, p*_snb+1,..., p*_snb+L 1323 When D > 0: 1324 p*_snb, p*_snb+(Lx1), p*_snb+(Lx2),..., p*_snb+(LxD) 1326 6.3.1.3. Signaled in SDP 1328 If the endpoint relies entirely on out-of-band signaling (R=0, F=1, 1329 L=0, D=0 in the FEC header), then this information may be inferred 1330 from the media type parameters specified in the SDP description. 1331 Furthermore, the payload type field in the RTP header assists the 1332 receiver to distinguish an interleaved or non-interleaved FEC packet. 1334 Mathematically, for any received repair packet, p*, the sequence 1335 numbers of the source packets that are protected by this repair 1336 packet are determined as follows: 1338 p*_snb + i * X_1 (modulo 65536) 1340 where p*_snb denotes the value in the SN base field of p*'s FEC 1341 header, X_1 is set to L and 1 for the interleaved and non-interleaved 1342 FEC repair packets, respectively, and 1344 0 <= i < X_2 1346 where X_2 is set to D and L for the interleaved and non-interleaved 1347 FEC repair packets, respectively. 1349 6.3.2. Recovering the RTP Header 1351 For a given set T, the procedure for the recovery of the RTP header 1352 of the missing packet, whose sequence number is denoted by SEQNUM, is 1353 as follows: 1355 1. For each of the source packets that are successfully received in 1356 T, compute the 80-bit string by concatenating the first 64 bits 1357 of their RTP header and the unsigned network-ordered 16-bit 1358 representation of their length in bytes minus 12. 1360 2. For the repair packet in T, compute the FEC bit string from the 1361 first 80 bits of the FEC header. 1363 3. Calculate the recovered bit string as the XOR of the bit strings 1364 generated from all source packets in T and the FEC bit string 1365 generated from the repair packet in T. 1367 4. Create a new packet with the standard 12-byte RTP header and no 1368 payload. 1370 5. Set the version of the new packet to 2. Skip the first 2 bits 1371 in the recovered bit string. 1373 6. Set the Padding bit in the new packet to the next bit in the 1374 recovered bit string. 1376 7. Set the Extension bit in the new packet to the next bit in the 1377 recovered bit string. 1379 8. Set the CC field to the next 4 bits in the recovered bit string. 1381 9. Set the Marker bit in the new packet to the next bit in the 1382 recovered bit string. 1384 10. Set the Payload type in the new packet to the next 7 bits in the 1385 recovered bit string. 1387 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 1388 bits in the recovered bit string. 1390 12. Set the TS field in the new packet to the next 32 bits in the 1391 recovered bit string. 1393 13. Take the next 16 bits of the recovered bit string and set the 1394 new variable Y to whatever unsigned integer this represents 1395 (assuming network order). Convert Y to host order. Y 1396 represents the length of the new packet in bytes minus 12 (for 1397 the fixed RTP header), i.e., the sum of the lengths of all the 1398 following if present: the CSRC list, header extension, RTP 1399 payload and RTP padding. 1401 14. Set the SSRC of the new packet to the SSRC of the missing source 1402 RTP stream. 1404 This procedure recovers the header of an RTP packet up to (and 1405 including) the SSRC field. 1407 6.3.3. Recovering the RTP Payload 1409 Following the recovery of the RTP header, the procedure for the 1410 recovery of the RTP "payload" is as follows, where "payload" refers 1411 to everything following the fixed 12-byte RTP header, including 1412 extensions, CSRC list, true payload and padding. 1414 1. Append Y bytes to the new packet. 1416 2. For each of the source packets that are successfully received in 1417 T, compute the bit string from the Y octets of data starting with 1418 the 13th octet of the packet. If any of the bit strings 1419 generated from the source packets has a length shorter than Y, 1420 pad them to that length. The padding of octet 0 MUST be added at 1421 the end of the bit string. Note that the information of the 1422 first 8 octets are protected by the FEC header. 1424 3. For the repair packet in T, compute the FEC bit string from the 1425 repair packet payload, i.e., the Y octets of data following the 1426 FEC header. Note that the FEC header may be different sizes 1427 depending on the variant and bitmask size. 1429 4. Calculate the recovered bit string as the XOR of the bit strings 1430 generated from all source packets in T and the FEC bit string 1431 generated from the repair packet in T. 1433 5. Append the recovered bit string (Y octets) to the new packet 1434 generated in Section 6.3.2. 1436 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1438 In 2-D parity FEC protection, the sender generates both non- 1439 interleaved and interleaved FEC repair packets to combat with the 1440 mixed loss patterns (random and bursty). At the receiver side, these 1441 FEC packets are used iteratively to overcome the shortcomings of the 1442 1-D non-interleaved/interleaved FEC protection and improve the 1443 chances of full error recovery. 1445 The iterative decoding algorithm runs as follows: 1447 1. Set num_recovered_until_this_iteration to zero 1449 2. Set num_recovered_so_far to zero 1451 3. Recover as many source packets as possible by using the non- 1452 interleaved FEC repair packets as outlined in Section 6.3.2 and 1453 Section 6.3.3, and increase the value of num_recovered_so_far by 1454 the number of recovered source packets. 1456 4. Recover as many source packets as possible by using the 1457 interleaved FEC repair packets as outlined in Section 6.3.2 and 1458 Section 6.3.3, and increase the value of num_recovered_so_far by 1459 the number of recovered source packets. 1461 5. If num_recovered_so_far > num_recovered_until_this_iteration 1462 ---num_recovered_until_this_iteration = num_recovered_so_far 1463 ---Go to step 3 1464 Else 1465 ---Terminate 1467 The algorithm terminates either when all missing source packets are 1468 fully recovered or when there are still remaining missing source 1469 packets but the FEC repair packets are not able to recover any more 1470 source packets. For the example scenarios when the 2-D parity FEC 1471 protection fails full recovery, refer to Section 1.1.4. Upon 1472 termination, variable num_recovered_so_far has a value equal to the 1473 total number of recovered source packets. 1475 Example: 1477 Suppose that the receiver experienced the loss pattern sketched in 1478 Figure 16. 1480 +---+ +---+ +===+ 1481 X X | 3 | | 4 | |R_1| 1482 +---+ +---+ +===+ 1484 +---+ +---+ +---+ +---+ +===+ 1485 | 5 | | 6 | | 7 | | 8 | |R_2| 1486 +---+ +---+ +---+ +---+ +===+ 1488 +---+ +---+ +===+ 1489 | 9 | X X | 12| |R_3| 1490 +---+ +---+ +===+ 1492 +===+ +===+ +===+ +===+ 1493 |C_1| |C_2| |C_3| |C_4| 1494 +===+ +===+ +===+ +===+ 1496 Figure 16: Example loss pattern for the iterative decoding algorithm 1498 The receiver executes the iterative decoding algorithm and recovers 1499 source packets #1 and #11 in the first iteration. The resulting 1500 pattern is sketched in Figure 17. 1502 +---+ +---+ +---+ +===+ 1503 | 1 | X | 3 | | 4 | |R_1| 1504 +---+ +---+ +---+ +===+ 1506 +---+ +---+ +---+ +---+ +===+ 1507 | 5 | | 6 | | 7 | | 8 | |R_2| 1508 +---+ +---+ +---+ +---+ +===+ 1510 +---+ +---+ +---+ +===+ 1511 | 9 | X | 11| | 12| |R_3| 1512 +---+ +---+ +---+ +===+ 1514 +===+ +===+ +===+ +===+ 1515 |C_1| |C_2| |C_3| |C_4| 1516 +===+ +===+ +===+ +===+ 1518 Figure 17: The resulting pattern after the first iteration 1520 Since the if condition holds true, the receiver runs a new iteration. 1521 In the second iteration, source packets #2 and #10 are recovered, 1522 resulting in a full recovery as sketched in Figure 18. 1524 +---+ +---+ +---+ +---+ +===+ 1525 | 1 | | 2 | | 3 | | 4 | |R_1| 1526 +---+ +---+ +---+ +---+ +===+ 1528 +---+ +---+ +---+ +---+ +===+ 1529 | 5 | | 6 | | 7 | | 8 | |R_2| 1530 +---+ +---+ +---+ +---+ +===+ 1532 +---+ +---+ +---+ +---+ +===+ 1533 | 9 | | 10| | 11| | 12| |R_3| 1534 +---+ +---+ +---+ +---+ +===+ 1536 +===+ +===+ +===+ +===+ 1537 |C_1| |C_2| |C_3| |C_4| 1538 +===+ +===+ +===+ +===+ 1540 Figure 18: The resulting pattern after the second iteration 1542 7. Signaling Requirements 1544 Out-of-band signaling should be designed to enable the receiver to 1545 identify the RTP streams associated with source packets and repair 1546 packets, respectively. At a minimum, the signaling must be designed 1547 to allow the receiver to 1548 o Determine whether one or more source RTP streams will be sent. 1550 o Determine whether one or more repair RTP streams will be sent. 1552 o Associate the appropriate SSRC's to both source and repair 1553 streams. 1555 o Clearly identify which SSRC's are associated with each source 1556 block. 1558 o Clearly identify which repair packets correspond to which source 1559 blocks. 1561 o Make use of repair packets to recover source data associated with 1562 specific SSRC's. 1564 This section provides several Sesssion Description Protocol (SDP) 1565 examples to demonstrate how these requirements can be met. 1567 7.1. SDP Examples 1569 This section provides two SDP [RFC4566] examples. The examples use 1570 the FEC grouping semantics defined in [RFC5956]. 1572 7.1.1. Example SDP for Flexible FEC Protection with in-band SSRC 1573 mapping 1575 In this example, we have one source video stream and one FEC repair 1576 stream. The source and repair streams are multiplexed on different 1577 SSRCs. The repair window is set to 200 ms. 1579 v=0 1580 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1581 s=FlexFEC minimal SDP signalling Example 1582 t=0 0 1583 m=video 30000 RTP/AVP 96 98 1584 c=IN IP4 143.163.151.157 1585 a=rtpmap:96 VP8/90000 1586 a=rtpmap:98 flexfec/90000 1587 a=fmtp:98; repair-window=200ms 1589 7.1.2. Example SDP for Flexible FEC Protection with explicit signalling 1590 in the SDP 1592 This example shows one source video stream (ssrc:1234) and one FEC 1593 repair streams (ssrc:2345). One FEC group is formed with the 1594 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1595 are multiplexed on different SSRCs. The repair window is set to 200 1596 ms. 1598 v=0 1599 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1600 s=2-D Parity FEC with no in band signalling Example 1601 t=0 0 1602 m=video 30000 RTP/AVP 100 110 1603 c=IN IP4 233.252.0.1/127 1604 a=rtpmap:100 MP2T/90000 1605 a=rtpmap:110 flexfec/90000 1606 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 1607 a=ssrc:1234 1608 a=ssrc:2345 1609 a=ssrc-group:FEC-FR 1234 2345 1611 7.2. On the Use of the RTP Stream Identifier Source Description 1613 The RTP Stream Identifier Source Description [I-D.roach-avtext-rid] 1614 is a format that can be used to identify a single RTP source stream 1615 along with an associated repair stream. However, this specification 1616 already defines a method of source and repair stream identification 1617 that can enable protection of multiple source streams with a single 1618 repair stream. Therefore the RTP Stream Idenfifer Source Description 1619 SHOULD NOT be used for the Flexible FEC payload format 1621 8. Congestion Control Considerations 1623 FEC is an effective approach to provide applications resiliency 1624 against packet losses. However, in networks where the congestion is 1625 a major contributor to the packet loss, the potential impacts of 1626 using FEC MUST be considered carefully before injecting the repair 1627 streams into the network. In particular, in bandwidth-limited 1628 networks, FEC repair streams may consume a significant part of the 1629 available bandwidth and consequently may congest the network. In 1630 such cases, the applications MUST NOT arbitrarily increase the amount 1631 of FEC protection since doing so may lead to a congestion collapse. 1632 If desired, stronger FEC protection MAY be applied only after the 1633 source rate has been reduced. 1635 In a network-friendly implementation, an application SHOULD NOT send/ 1636 receive FEC repair streams if it knows that sending/receiving those 1637 FEC repair streams would not help at all in recovering the missing 1638 packets. It is RECOMMENDED that the amount and type (row, column, or 1639 both) of FEC protection is adjusted dynamically based on the packet 1640 loss rate and burst loss length observed by the applications. 1642 In multicast scenarios, it may be difficult to optimize the FEC 1643 protection per receiver. If there is a large variation among the 1644 levels of FEC protection needed by different receivers, it is 1645 RECOMMENDED that the sender offers multiple repair streams with 1646 different levels of FEC protection and the receivers join the 1647 corresponding multicast sessions to receive the repair stream(s) that 1648 is best for them. 1650 9. Security Considerations 1652 RTP packets using the payload format defined in this specification 1653 are subject to the security considerations discussed in the RTP 1654 specification [RFC3550] and in any applicable RTP profile. The main 1655 security considerations for the RTP packet carrying the RTP payload 1656 format defined within this memo are confidentiality, integrity and 1657 source authenticity. Confidentiality is achieved by encrypting the 1658 RTP payload. Integrity of the RTP packets is achieved through a 1659 suitable cryptographic integrity protection mechanism. Such a 1660 cryptographic system may also allow the authentication of the source 1661 of the payload. A suitable security mechanism for this RTP payload 1662 format should provide confidentiality, integrity protection, and at 1663 least source authentication capable of determining if an RTP packet 1664 is from a member of the RTP session. 1666 Note that the appropriate mechanism to provide security to RTP and 1667 payloads following this memo may vary. It is dependent on the 1668 application, transport and signaling protocol employed. Therefore, a 1669 single mechanism is not sufficient, although if suitable, using the 1670 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1671 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1672 Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may 1673 exist. 1675 Given that FLEX FEC enables the protection of multiple source 1676 streams, there exists the possibility that multiple source buffers 1677 may be created that may not be used. In addition, the interaction 1678 between a FLEX FEC implementation and higher-layer applications may 1679 be affected by non-uniform processing requirements of the FEC scheme. 1681 10. IANA Considerations 1683 New media subtypes are subject to IANA registration. For the 1684 registration of the payload formats and their parameters introduced 1685 in this document, refer to Section 5. 1687 11. Acknowledgments 1689 Some parts of this document are borrowed from [RFC5109]. Thus, the 1690 author would like to thank the editor of [RFC5109] and those who 1691 contributed to [RFC5109]. 1693 Thanks to Stephen Botzko , Bernard Aboba , Rasmus Brandt , Brian 1694 Baldino , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus 1695 Westerlund for providing valuable feedback on earlier versions of 1696 this draft. 1698 12. References 1700 12.1. Normative References 1702 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1703 Requirement Levels", BCP 14, RFC 2119, 1704 DOI 10.17487/RFC2119, March 1997, 1705 . 1707 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1708 with Session Description Protocol (SDP)", RFC 3264, 1709 DOI 10.17487/RFC3264, June 2002, 1710 . 1712 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1713 Jacobson, "RTP: A Transport Protocol for Real-Time 1714 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1715 July 2003, . 1717 [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP 1718 Payload Formats", RFC 3555, DOI 10.17487/RFC3555, July 1719 2003, . 1721 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1722 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1723 July 2006, . 1725 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1726 the Session Description Protocol", RFC 5956, 1727 DOI 10.17487/RFC5956, September 2010, 1728 . 1730 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1731 Correction (FEC) Framework", RFC 6363, 1732 DOI 10.17487/RFC6363, October 2011, 1733 . 1735 [RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design 1736 Considerations for Protocol Extensions", RFC 6709, 1737 DOI 10.17487/RFC6709, September 2012, 1738 . 1740 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1741 Specifications and Registration Procedures", BCP 13, 1742 RFC 6838, DOI 10.17487/RFC6838, January 2013, 1743 . 1745 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1746 "Guidelines for Choosing RTP Control Protocol (RTCP) 1747 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1748 September 2013, . 1750 12.2. Informative References 1752 [I-D.roach-avtext-rid] 1753 Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream 1754 Identifier (RID) Source Description (SDES)", draft-roach- 1755 avtext-rid-02 (work in progress), February 2016. 1757 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1758 Streaming Protocol (RTSP)", RFC 2326, 1759 DOI 10.17487/RFC2326, April 1998, 1760 . 1762 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1763 for Generic Forward Error Correction", RFC 2733, 1764 DOI 10.17487/RFC2733, December 1999, 1765 . 1767 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1768 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1769 October 2000, . 1771 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1772 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1773 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1774 . 1776 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1777 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1778 December 2005, . 1780 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1781 "Extended RTP Profile for Real-time Transport Control 1782 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1783 DOI 10.17487/RFC4585, July 2006, 1784 . 1786 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 1787 Correction", RFC 5109, DOI 10.17487/RFC5109, December 1788 2007, . 1790 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1791 (TLS) Protocol Version 1.2", RFC 5246, 1792 DOI 10.17487/RFC5246, August 2008, 1793 . 1795 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 1796 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 1797 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 1798 DOI 10.17487/RFC7656, November 2015, 1799 . 1801 [SMPTE2022-1] 1802 SMPTE 2022-1-2007, "Forward Error Correction for Real-Time 1803 Video/Audio Transport over IP Networks", 2007. 1805 Authors' Addresses 1807 Mo Zanaty 1808 Cisco 1809 Raleigh, NC 1810 USA 1812 Email: mzanaty@cisco.com 1814 Varun Singh 1815 CALLSTATS I/O Oy 1816 Runeberginkatu 4c A 4 1817 Helsinki 00100 1818 Finland 1820 Email: varun.singh@iki.fi 1821 URI: http://www.callstats.io/ 1822 Ali Begen 1823 Networked Media 1824 Konya 1825 Turkey 1827 Email: ali.begen@networked.media 1829 Giridhar Mandyam 1830 Qualcomm Inc. 1831 5775 Morehouse Drive 1832 San Diego, CA 92121 1833 USA 1835 Phone: +1 858 651 7200 1836 Email: mandyam@qti.qualcomm.com