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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: The value for the repair window duration is related to the maximum L and D values that are expected during a FLEX FEC session and therefore cannot be chosen arbitrarily. Repair packets that include L and D values larger than the repair window MUST not be sent. The rate of the source streams should also be considered, as the repair window duration should ideally span several packetization intervals in order to leverage the error correction capabilities of the parity code. -- The document date (May 16, 2019) is 1807 days in the past. Is this intentional? 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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: November 17, 2019 callstats.io 6 A. Begen 7 Networked Media 8 G. Mandyam 9 Qualcomm Inc. 10 May 16, 2019 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-20 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 (Flexible FEC, or "FLEX 21 FEC"), where a number of FEC repair packets are generated from a set 22 of source packets from one or more source RTP streams. These FEC 23 repair packets are sent in a redundancy RTP stream separate from the 24 source RTP stream(s) that carries the source packets. RTP source 25 packets that were lost in transmission can be reconstructed using the 26 source and repair packets that were received. The non-interleaved 27 and interleaved parity codes which are defined in this specification 28 offer a good protection against random and bursty packet losses, 29 respectively, at a cost of complexity. The RTP payload formats that 30 are defined in this document address scalability issues experienced 31 with the earlier specifications, and offer several improvements. Due 32 to these changes, the new payload formats are not backward compatible 33 with earlier specifications, but endpoints that do not implement this 34 specification can still work by simply ignoring the FEC repair 35 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 November 17, 2019. 54 Copyright Notice 56 Copyright (c) 2019 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. One-Dimensional (1-D) Non-interleaved (Row) FEC 74 Protection . . . . . . . . . . . . . . . . . . . . . 5 75 1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 6 76 1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 7 77 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection 8 78 1.1.5. FEC Protection with Flexible Mask . . . . . . . . . . 10 79 1.1.6. FEC Overhead Considerations . . . . . . . . . . . . . 10 80 1.1.7. FEC Protection with Retransmission . . . . . . . . . 10 81 1.1.8. Repair Window Considerations . . . . . . . . . . . . 11 82 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 11 83 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 11 84 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 11 85 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 12 86 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 12 87 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 12 88 4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 13 89 4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 13 90 4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 15 91 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 20 92 5.1. Media Type Registration - Parity Codes . . . . . . . . . 20 93 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 21 94 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 22 95 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 23 96 5.1.4. Registration of application/flexfec . . . . . . . . . 24 98 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 25 99 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 25 100 5.2.2. Declarative Considerations . . . . . . . . . . . . . 26 101 6. Protection and Recovery Procedures - Parity Codes . . . . . . 26 102 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 26 103 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 26 104 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 28 105 6.3.1. Associating the Source and Repair Packets . . . . . . 28 106 6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 30 107 6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 31 108 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 109 Protection . . . . . . . . . . . . . . . . . . . . . 31 110 7. Signaling Requirements . . . . . . . . . . . . . . . . . . . 34 111 7.1. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 35 112 7.1.1. Example SDP for Flexible FEC Protection with in-band 113 SSRC mapping . . . . . . . . . . . . . . . . . . . . 35 114 7.1.2. Example SDP for Flexible FEC Protection with explicit 115 signalling in the SDP . . . . . . . . . . . . . . . . 35 116 7.2. On the Use of the RTP Stream Identifier Source 117 Description . . . . . . . . . . . . . . . . . . . . . . . 36 118 8. Congestion Control Considerations . . . . . . . . . . . . . . 36 119 9. Security Considerations . . . . . . . . . . . . . . . . . . . 37 120 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 121 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38 122 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 123 12.1. Normative References . . . . . . . . . . . . . . . . . . 38 124 12.2. Informative References . . . . . . . . . . . . . . . . . 39 125 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 127 1. Introduction 129 This document defines new RTP payload formats for the Forward Error 130 Correction (FEC) that is generated by the non-interleaved and 131 interleaved parity codes from a source media encapsulated in RTP 132 [RFC3550]. The type of the source media protected by these parity 133 codes can be audio, video, text or application. The FEC data are 134 generated according to the media type parameters, which are 135 communicated out-of-band (e.g., in SDP). Furthermore, the 136 associations or relationships between the source and repair RTP 137 streams may be communicated in-band or out-of-band. The in-band 138 mechanism is advantageous when the endpoint is adapting the FEC 139 parameters. The out-of-band mechanism may be preferable when the FEC 140 parameters are fixed. While this document fully defines the use of 141 FEC to protect RTP streams, it also leverages several definitions 142 along with the basic source/repair header description from [RFC6363] 143 in their application to the parity codes defined here. 145 The Redundancy RTP Stream [RFC7656] repair packets proposed in this 146 document protect the Source RTP Stream packets that belong to the 147 same RTP session. 149 The RTP payload formats that are defined in this document address the 150 scalability issues experienced with the formats defined in earlier 151 specifications including [RFC2733], [RFC5109] and [SMPTE2022-1]. 153 1.1. Parity Codes 155 Both the non-interleaved and interleaved parity codes use the 156 eXclusive OR (XOR) operation to generate the repair packets. The 157 following steps take place: 159 1. The sender determines a set of source packets to be protected by 160 FEC based on the media type parameters. 162 2. The sender applies the XOR operation on the source packets to 163 generate the required number of repair packets. 165 3. The sender sends the repair packet(s) along with the source 166 packets, in different RTP streams, to the receiver(s). The 167 repair packets may be sent proactively or on-demand based on RTCP 168 feedback messages such as NACK [RFC4585]. 170 At the receiver side, if all of the source packets are successfully 171 received, there is no need for FEC recovery and the repair packets 172 are discarded. However, if there are missing source packets, the 173 repair packets can be used to recover the missing information. 174 Figure 1 and Figure 2 describe example block diagrams for the 175 systematic parity FEC encoder and decoder, respectively. 177 +------------+ 178 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 179 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 180 | Encoder | 181 | (Sender) | --> +==+ +==+ 182 +------------+ +==+ +==+ 184 Source Packet: +--+ Repair Packet: +==+ 185 +--+ +==+ 187 Figure 1: Block diagram for systematic parity FEC encoder 188 +------------+ 189 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 190 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 191 | Decoder | 192 +==+ +==+ --> | (Receiver) | 193 +==+ +==+ +------------+ 195 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 196 +--+ +==+ 198 Figure 2: Block diagram for systematic parity FEC decoder 200 In Figure 2, it is clear that the FEC repair packets have to be 201 received by the endpoint within a certain amount of time for the FEC 202 recovery process to be useful. The repair window is defined as the 203 time that spans a FEC block, which consists of the source packets and 204 the corresponding repair packets. At the receiver side, the FEC 205 decoder SHOULD buffer source and repair packets at least for the 206 duration of the repair window, to allow all the repair packets to 207 arrive. The FEC decoder can start decoding the already received 208 packets sooner; however, it should not register a FEC decoding 209 failure until it waits at least for the duration of the repair 210 window. 212 1.1.1. One-Dimensional (1-D) Non-interleaved (Row) FEC Protection 214 Consider a group of D x L source packets that have sequence numbers 215 starting from 1 running to D x L, and a repair packet is generated by 216 applying the XOR operation to every L consecutive packets as sketched 217 in Figure 3. This process is referred to as 1-D non-interleaved FEC 218 protection. As a result of this process, D repair packets are 219 generated, which are referred to as non-interleaved (or row) FEC 220 repair packets. In general D and L represent values that describe 221 how packets are grouped together from a depth and length perspective 222 (respectively) when interleaving all D x L source packets. 224 +--------------------------------------------------+ --- +===+ 225 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 226 +--------------------------------------------------+ --- +===+ 227 +--------------------------------------------------+ --- +===+ 228 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 229 +--------------------------------------------------+ --- +===+ 230 . . . . . . 231 . . . . . . 232 . . . . . . 233 +--------------------------------------------------+ --- +===+ 234 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 235 +--------------------------------------------------+ --- +===+ 237 Figure 3: Generating non-interleaved (row) FEC repair packets 239 1.1.2. 1-D Interleaved (Column) FEC Protection 241 If the XOR operation is applied to the group of the source packets 242 whose sequence numbers are L apart from each other, as sketched in 243 Figure 4. In this case the endpoint generates L repair packets. 244 This process is referred to as 1-D interleaved FEC protection, and 245 the resulting L repair packets are referred to as interleaved (or 246 column) FEC repair packets. 248 +-------------+ +-------------+ +-------------+ +-------+ 249 | S_1 | | S_2 | | S3 | ... | S_L | 250 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 251 | . | | . | | | | | 252 | . | | . | | | | | 253 | . | | . | | | | | 254 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 255 +-------------+ +-------------+ +-------------+ +-------+ 256 + + + + 257 ------------- ------------- ------------- ------- 258 | XOR | | XOR | | XOR | ... | XOR | 259 ------------- ------------- ------------- ------- 260 = = = = 261 +===+ +===+ +===+ +===+ 262 |C_1| |C_2| |C_3| ... |C_L| 263 +===+ +===+ +===+ +===+ 265 Figure 4: Generating interleaved (column) FEC repair packets 267 1.1.3. Use Cases for 1-D FEC Protection 269 A sender may generate one non-interleaved repair packet out of L 270 consecutive source packets or one interleaved repair packet out of D 271 non-consecutive source packets. Regardless of whether the repair 272 packet is a non-interleaved or an interleaved one, it can provide a 273 full recovery of the missing information if there is only one packet 274 missing among the corresponding source packets. This implies that 275 1-D non-interleaved FEC protection performs better when the source 276 packets are randomly lost. However, if the packet losses occur in 277 bursts, 1-D interleaved FEC protection performs better provided that 278 L is chosen large enough, i.e., L-packet duration is not shorter than 279 the observed burst duration. If the sender generates non-interleaved 280 FEC repair packets and a burst loss hits the source packets, the 281 repair operation fails. This is illustrated in Figure 5. 283 +---+ +---+ +===+ 284 | 1 | X X | 4 | |R_1| 285 +---+ +---+ +===+ 287 +---+ +---+ +---+ +---+ +===+ 288 | 5 | | 6 | | 7 | | 8 | |R_2| 289 +---+ +---+ +---+ +---+ +===+ 291 +---+ +---+ +---+ +---+ +===+ 292 | 9 | | 10| | 11| | 12| |R_3| 293 +---+ +---+ +---+ +---+ +===+ 295 Figure 5: Example scenario where 1-D non-interleaved FEC protection 296 fails error recovery (Burst Loss) 298 The sender may generate interleaved FEC repair packets to combat with 299 the bursty packet losses. However, two or more random packet losses 300 may hit the source and repair packets in the same column. In that 301 case, the repair operation fails as well. This is illustrated in 302 Figure 6. Note that it is possible that two burst losses may occur 303 back-to-back, in which case interleaved FEC repair packets may still 304 fail to recover the lost data. 306 +---+ +---+ +---+ 307 | 1 | X | 3 | | 4 | 308 +---+ +---+ +---+ 310 +---+ +---+ +---+ 311 | 5 | X | 7 | | 8 | 312 +---+ +---+ +---+ 314 +---+ +---+ +---+ +---+ 315 | 9 | | 10| | 11| | 12| 316 +---+ +---+ +---+ +---+ 318 +===+ +===+ +===+ +===+ 319 |C_1| |C_2| |C_3| |C_4| 320 +===+ +===+ +===+ +===+ 322 Figure 6: Example scenario where 1-D interleaved FEC protection fails 323 error recovery (Periodic Loss) 325 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection 327 In networks where the source packets are lost both randomly and in 328 bursts, the sender ought to generate both non-interleaved and 329 interleaved FEC repair packets. This type of FEC protection is known 330 as 2-D parity FEC protection. At the expense of generating more FEC 331 repair packets, thus increasing the FEC overhead, 2-D FEC provides 332 superior protection against mixed loss patterns. However, it is 333 still possible for 2-D parity FEC protection to fail to recover all 334 of the lost source packets if a particular loss pattern occurs. An 335 example scenario is illustrated in Figure 7. 337 +---+ +---+ +===+ 338 | 1 | X X | 4 | |R_1| 339 +---+ +---+ +===+ 341 +---+ +---+ +---+ +---+ +===+ 342 | 5 | | 6 | | 7 | | 8 | |R_2| 343 +---+ +---+ +---+ +---+ +===+ 345 +---+ +---+ +===+ 346 | 9 | X X | 12| |R_3| 347 +---+ +---+ +===+ 349 +===+ +===+ +===+ +===+ 350 |C_1| |C_2| |C_3| |C_4| 351 +===+ +===+ +===+ +===+ 353 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 354 error recovery 356 2-D parity FEC protection also fails when at least two rows are 357 missing a source and the FEC packet and the missing source packets 358 (in at least two rows) are aligned in the same column. An example 359 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 360 protection cannot repair all missing source packets when at least two 361 columns are missing a source and the FEC packet and the missing 362 source packets (in at least two columns) are aligned in the same row. 364 +---+ +---+ +---+ 365 | 1 | | 2 | X | 4 | X 366 +---+ +---+ +---+ 368 +---+ +---+ +---+ +---+ +===+ 369 | 5 | | 6 | | 7 | | 8 | |R_2| 370 +---+ +---+ +---+ +---+ +===+ 372 +---+ +---+ +---+ 373 | 9 | | 10| X | 12| X 374 +---+ +---+ +---+ 376 +===+ +===+ +===+ +===+ 377 |C_1| |C_2| |C_3| |C_4| 378 +===+ +===+ +===+ +===+ 380 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 381 error recovery 383 1.1.5. FEC Protection with Flexible Mask 385 It is possible to define FEC protection for selected packets in the 386 source stream. This would enable differential protection, i.e. 387 application of FEC selectively to packets that require a higher level 388 of reliability then the other packets in the source stream. The 389 sender will be required to send a bitmap indicating the packets to be 390 protected, i.e. a "mask", to the receiver. Since the mask can be 391 modified during an RTP session ("flexible mask"), this kind of FEC 392 protection can also be used to implement FEC dynamically (e.g. for 393 adaptation to different types of traffic during the RTP session). 395 1.1.6. FEC Overhead Considerations 397 The overhead is defined as the ratio of the number of bytes belonging 398 to the repair packets to the number of bytes belonging to the 399 protected source packets. 401 Generally, repair packets are larger in size compared to the source 402 packets. Also, not all the source packets are necessarily equal in 403 size. However, assuming that each repair packet carries an equal 404 number of bytes as carried by a source packet, the overhead for 405 different FEC protection methods can be computed as follows: 407 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 409 o 1-D Interleaved FEC Protection: Overhead = 1/D 411 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 413 where L and D are the number of columns and rows in the source block, 414 respectively. 416 1.1.7. FEC Protection with Retransmission 418 This specification supports both forward error correction, i.e. 419 before any loss is reported, as well as retransmission of source 420 packets after loss is reported. The retransmission includes the RTP 421 header of the source packet in addition to the payload. If a peer 422 supporting both FLEX FEC and other RTP retransmission methods (see 423 [RFC4588]) receives an Offer including both FLEX FEC and another RTP 424 retransmission method, it MUST respond with an Answer containing only 425 FLEX FEC. 427 1.1.8. Repair Window Considerations 429 The value for the repair window duration is related to the maximum L 430 and D values that are expected during a FLEX FEC session and 431 therefore cannot be chosen arbitrarily. Repair packets that include 432 L and D values larger than the repair window MUST not be sent. The 433 rate of the source streams should also be considered, as the repair 434 window duration should ideally span several packetization intervals 435 in order to leverage the error correction capabilities of the parity 436 code. 438 Since the FEC configuration can change with each repair packet (see 439 Section 4.2.2), for any given repair packet the FLEX FEC receiver 440 MUST support all possible L and D combinations (both 1-D and 2-D 441 interleaved over all source flows) and all flexible mask 442 configurations (over all source flows) within the repair window to 443 which it has agreed (e.g. through SDP or out-of-band signaling) for a 444 FLEX FEC RTP session. In addition, the FLEX FEC receiver MUST 445 support receipt of a retransmission of any source flow packet within 446 the repair window to which it has agreed. 448 2. Requirements Notation 450 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 451 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 452 "OPTIONAL" in this document are to be interpreted as described in BCP 453 14 [RFC2119] [RFC8174] when, and only when, they appear in all 454 capitals, as shown here. 456 3. Definitions and Notations 458 3.1. Definitions 460 This document uses a number of definitions from [RFC6363]. 462 1-D Non-interleaved Row FEC: A protection scheme that operates on 463 consecutive source packets in the source block, able to recover a 464 single lost source packet per row of the source block. 466 1-D Interleaved Column FEC: A protection scheme that operates on 467 interleaved source packets in the source block, able to recover a 468 single lost source packet per column of the source block. 470 2-D FEC: A protection scheme that combines row and column FEC. 472 Source Block: A set of source packets that are protected by a set 473 of 1-D or 2-D FEC repair packets. 475 FEC Block: A source block and its corresponding FEC repair 476 packets. 478 Repair Window: The time that spans a FEC block, which consists of 479 the source packets and the corresponding FEC repair packets. 481 XOR Parity Codes: A FEC code which uses the eXclusive OR (XOR) 482 parity operation to encode a set of source packets to form a FEC 483 repair packet. 485 3.2. Notations 487 L: Number of columns of the source block (length of each row). 489 D: Number of rows of the source block (depth of each column). 491 bitmask: A 15-bit, 46-bit, or 110-bit mask indicating which source 492 packets are protected by a FEC repair packet. If the bit i in the 493 mask is set to 1, the source packet number N + i is protected by 494 this FEC repair packet, where N is the sequence number base 495 indicated in the FEC repair packet. The most significant bit of 496 the mask corresponds to i=0. The least significant bit of the 497 mask corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit 498 mask, or i=109 in the 110-bit mask. 500 4. Packet Formats 502 This section describes the formats of the source packets and defines 503 the formats of the FEC repair packets. 505 4.1. Source Packets 507 The source packets contain the information that identifies the source 508 block and the position within the source block occupied by the 509 packet. Since the source packets that are carried within an RTP 510 stream already contain unique sequence numbers in their RTP headers 511 [RFC3550], the source packets can be identified in a straightforward 512 manner and there is no need to append additional field(s). The 513 primary advantage of not modifying the source packets in any way is 514 that it provides backward compatibility for the receivers that do not 515 support FEC at all. In multicast scenarios, this backward 516 compatibility becomes quite useful as it allows the non-FEC-capable 517 and FEC-capable receivers to receive and interpret the same source 518 packets sent in the same multicast session. 520 The source packets are transmitted as usual without altering them. 521 They are used along with the FEC repair packets to recover any 522 missing source packets, making this scheme a systematic code. 524 The source packets are full RTP packets with optional CSRC list, RTP 525 header extension, and padding. If any of these optional elements are 526 present in the source RTP packet, and that source packet is lost, 527 they are recovered by the FEC repair operation, which recovers the 528 full source RTP packet including these optional elements. 530 4.2. FEC Repair Packets 532 The FEC repair packets will contain information that identifies the 533 source block they pertain to and the relationship between the 534 contained repair packets and the original source block. For this 535 purpose, the RTP header of the repair packets is used, as well as 536 another header within the RTP payload, called the FEC header, as 537 shown in Figure 9. 539 Note that all the source stream packets that are protected by a 540 particular FEC packet need to be in the same RTP session. 542 +------------------------------+ 543 | IP Header | 544 +------------------------------+ 545 | Transport Header | 546 +------------------------------+ 547 | RTP Header | 548 +------------------------------+ ---+ 549 | FEC Header | | 550 +------------------------------+ | RTP Payload 551 | Repair "Payload" | | 552 +------------------------------+ ---+ 554 Figure 9: Format of FEC repair packets 556 The Repair "Payload", which follows the FEC Header, includes repair 557 of everything following the fixed 12-byte RTP header of each source 558 packet, including any CSRC identifier list and header extensions if 559 present. 561 4.2.1. RTP Header of FEC Repair Packets 563 The RTP header is formatted according to [RFC3550] with some further 564 clarifications listed below: 566 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this 567 specification requires all source RTP packets and all FEC repair 568 packets to use RTP version 2. 570 Padding (P) bit: Source packets can have optional RTP padding, 571 which can be recovered. FEC repair packets can have optional RTP 572 padding, which is independent of the RTP padding of the source 573 packets. 575 Extension (X) bit: Source packets can have optional RTP header 576 extensions, which can be recovered. FEC repair packets can have 577 optional RTP header extensions, which are independent of the RTP 578 header extensions of the source packets. 580 CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: 581 Source packets can have an optional CSRC list and count, which can 582 be recovered. FEC repair packets MUST use the CSRC list and count 583 to specify the SSRC(s) of the source RTP stream(s) protected by 584 this FEC repair packet. 586 Marker (M) bit: This bit is not used for this payload type, and 587 SHALL be set to 0 by senders, and SHALL be ignored by receivers. 589 Payload Type: The (dynamic) payload type for the FEC repair 590 packets is determined through out-of-band means (e.g. SDP). Note 591 that this document registers new payload formats for the repair 592 packets (Refer to Section 5 for details). According to [RFC3550], 593 an RTP receiver that cannot recognize a payload type must discard 594 it. This provides backward compatibility. If a non-FEC-capable 595 receiver receives a repair packet, it will not recognize the 596 payload type, and hence, will discard the repair packet. 598 Sequence Number (SN): The sequence number follows the standard 599 definition provided in [RFC3550]. Therefore it must be one higher 600 than the sequence number in the previously transmitted repair 601 packet, and the initial value of the sequence number should be 602 random (i.e. unpredictable). 604 Timestamp (TS): The timestamp SHALL be set to a time corresponding 605 to the repair packet's transmission time. Note that the timestamp 606 value has no use in the actual FEC protection process and is 607 usually useful for jitter calculations. 609 Synchronization Source (SSRC): The SSRC value for each repair 610 stream SHALL be randomly assigned as per the guidelines provided 611 in Section 8 of [RFC3550]. This allows the sender to multiplex 612 the source and repair RTP streams in the same RTP session, or 613 multiplex multiple repair streams in an RTP session. The repair 614 streams' SSRC's CNAME SHOULD be identical to the CNAME of the 615 source RTP stream(s) that this repair stream protects. An FEC 616 stream that protects multiple source RTP streams with different 617 CNAME's uses the CNAME associated with the entity generating the 618 FEC stream or the CNAME of the entity on whose behalf it performs 619 the protection operation. In cases when the repair stream covers 620 packets from multiple source RTP streams with different CNAME 621 values and none of these CNAME values can be associated with the 622 entity generating the FEC stream, any of these CNAME values MAY be 623 used. 625 In some networks, the RTP Source, which produces the source 626 packets and the FEC Source, which generates the repair packets 627 from the source packets may not be the same host. In such 628 scenarios, using the same CNAME for the source and repair RTP 629 streams means that the RTP Source and the FEC Source will share 630 the same CNAME (for this specific source-repair stream 631 association). A common CNAME may be produced based on an 632 algorithm that is known both to the RTP and FEC Source [RFC7022]. 633 This usage is compliant with [RFC3550]. 635 Note that due to the randomness of the SSRC assignments, there is 636 a possibility of SSRC collision. In such cases, the collisions 637 must be resolved as described in [RFC3550]. 639 4.2.2. FEC Header of FEC Repair Packets 641 The format of the FEC header has 3 variants, depending on the values 642 in the first 2 bits (R and F bits) as shown in Figure 10. Note that 643 R and F stand for "retransmit" and "fixed block", respectively. Two 644 of these variants are meant to describe different methods for 645 deriving the source data from a source packet for a repair packet. 646 This allows for customizing the FEC method to allow for robustness 647 against different levels of burst errors and random packet losses. 648 The third variant is for a straight retransmission of the source 649 packet. 651 0 1 2 3 652 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 1 653 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 654 |R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... | 655 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 656 | | 657 | ...varies depending on R/F... | 658 | | 659 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 660 : Repair "Payload" follows FEC Header : 661 : : 663 Figure 10: FEC Header 665 The Repair "Payload", which follows the FEC Header, includes repair 666 of everything following the fixed 12-byte RTP header of each source 667 packet, including any CSRC identifier list and header extensions if 668 present. An overview on how the repair payload can be used to 669 recover source packets is provided Section 6. 671 +---+---+-----------------------------------------------------+ 672 | R | F | FEC Header variant | 673 +---+---+-----------------------------------------------------+ 674 | 0 | 0 | Flexible FEC Mask fields indicate source packets | 675 | 0 | 1 | Fixed FEC L/D (cols/rows) indicate source packets | 676 | 1 | 0 | Retransmission of a single source packet | 677 | 1 | 1 | Reserved for future use, MUST NOT send, MUST ignore | 678 +---+---+-----------------------------------------------------+ 680 Figure 11: R and F bit values for FEC Header variants 682 The first variant, when R=0 and F=0, has a mask to signal protected 683 source packets, as shown in Figure 12. 685 The second variant, when R=0 and F=1, has a number of columns (L) and 686 rows (D) to signal protected source packets, as shown in Figure 13. 688 The final variant, when R=1 and F=0, is a retransmission format as 689 shown in Figure 15. 691 No variant presently uses R=1 and F=1, which is reserved for future 692 use. Current FLEX FEC implementations MUST NOT send packets with 693 this variant, and receivers MUST ignore these packets. Future FLEX 694 FEC implementations may use this by updating the media type 695 registration. 697 The FEC header for all variants consists of the following common 698 fields: 700 o The R bit MUST be set to 1 to indicate a retransmission packet, 701 and MUST be set to 0 for FEC repair packets. 703 o The F bit indicates the type of FEC repair packets, as shown in 704 Figure 11, when the R bit is 0. The F bit MUST be set to 0 when 705 the R bit is 1 for retransmission packets. 707 o The P, X, CC, M and PT recovery fields are used to determine the 708 corresponding fields of the recovered packets (see also 709 Section 6.3.2). 711 4.2.2.1. FEC Header with Flexible Mask 713 When R=0 and F=0, the FEC Header includes flexible mask fields. 715 0 1 2 3 716 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 1 717 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 718 |0|0|P|X| CC |M| PT recovery | length recovery | 719 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 720 | TS recovery | 721 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 722 | SN base_i |k| Mask [0-14] | 723 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 724 |k| Mask [15-45] (optional) | 725 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 726 | Mask [46-109] (optional) | 727 | | 728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 729 | ... next SN base and Mask for CSRC_i in CSRC list ... | 730 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 731 : Repair "Payload" follows FEC Header : 732 : : 734 Figure 12: FEC Header for F=0 736 o The Length recovery (16 bits) field is used to determine the 737 length of the recovered packets. This length includes all octets 738 following the fixed 12-byte RTP header of source packets, 739 including CSRC list and optional header extension(s) if present. 740 It excludes the fixed 12-byte RTP header of source packets. 742 o The TS recovery (32 bits) field is used to determine the timestamp 743 of the recovered packets. 745 o The CSRC_i (32 bits) field in the RTP Header (not FEC Header) 746 describes the SSRC of the source packets protected by this 747 particular FEC packet. If a FEC packet protects multiple SSRCs 748 (indicated by the CSRC Count > 1 in the RTP Header), there will be 749 multiple blocks of data containing the SN base and Mask fields. 751 o The SN base_i (16 bits) field indicates the lowest sequence 752 number, taking wrap around into account, of the source packets for 753 a particular SSRC (indicated in CSRC_i) protected by this repair 754 packet. 756 o The Mask fields indicate a bitmask of which source packets are 757 protected by this FEC repair packet, where bit j of the mask set 758 to 1 indicates that the source packet with sequence number (SN 759 base_i + j) is protected by this FEC repair packet, where j=0 is 760 the most significant bit in the mask. 762 o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110 763 bits. k=1 denotes that another mask follows, and k=0 denotes that 764 it is the last block of mask. 766 o The Repair "Payload", which follows the FEC Header, includes 767 repair of everything following the fixed 12-byte RTP header of 768 each source packet, including any CSRC identifier list and header 769 extensions if present. 771 4.2.2.2. FEC Header with Fixed L Columns and D Rows 773 When R=0 and F=1, the FEC Header includes L and D fields for fixed 774 columns and rows. The other fields are the same as the prior 775 section. As in the previous section, the CSRC_i (32 bits) field in 776 the RTP Header (not FEC Header) describes the SSRC of the source 777 packets protected by this particular FEC packet. If there are 778 multiple SSRC's protected by the FEC packet, then there will be 779 multiple blocks of data containing an SN base along with L and D 780 fields. 782 0 1 2 3 783 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 1 784 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 785 |0|1|P|X| CC |M| PT recovery | length recovery | 786 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 787 | TS recovery | 788 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 789 | SN base_i | L (columns) | D (rows) | 790 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 791 | ... next SN base and L/D for CSRC_i in CSRC list ... | 792 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 793 : Repair "Payload" follows FEC Header : 794 : : 796 Figure 13: FEC Header for F=1 798 Consequently, the following conditions occur for L and D values: 800 If L=0, D=0, reserved for future use, 801 MUST NOT send, MUST ignore if received. 803 If L>0, D=0, indicates row FEC, and no column FEC will follow (1D). 804 Source packets for each row: SN, SN+1, ..., SN+(L-1) 806 If L>0, D=1, indicates row FEC, and column FEC will follow (2D). 807 Source packets for each row: SN, SN+1, ..., SN+(L-1) 808 Source packets for each col: SN, SN+L, ..., SN+(D-1)*L 809 After all row FEC packets have been sent, 810 then the column FEC packets will be sent. 812 If L>0, D>1, indicates column FEC of every L packet, D times. 813 Source packets for each col: SN, SN+L, ..., SN+(D-1)*L 815 Figure 14: Interpreting the L and D field values 817 Given the 8-bit limit on L and D (as depicted in Figure 13), the 818 maximum value of either parameter is 255. If L=0 and D=0 are in a 819 packet, then the repair packet MUST be ignored by the receiver. In 820 addition when L=1 and D=0, the repair packet becomes a retransmission 821 of a corresponding source packet. 823 The values of L and D for a given block of recovery data will 824 correspond to the type of recovery in use for that block of data. In 825 particular, for 2-D repair, the (L,D) values may not be constant 826 across all packets for a given SSRC being repaired. Similarly, the L 827 and D values can differ across different blocks of repair data 828 (repairing different SSRCs) in a single packet. If the values of L 829 and D result in a repair packet that exceed the repair window of the 830 FLEX FEC session, then the repair packet MUST be ignored. 832 It should be noted that the flexible mask-based approach may be 833 inefficient for protecting a large number of source packets, or 834 impossible to signal if larger than the largest mask size. In such 835 cases, the fixed columns and rows variant may be more useful. 837 4.2.2.3. FEC Header for Retransmissions 839 When R=1 and F=0, the FEC packet is a retransmission of a single 840 source packet. Note that the layout of this retransmission packet is 841 different from other FEC repair packets. The sequence number (SN 842 base_i) replaces the length recovery in the FEC header, since the 843 length is already known for a single packet. There are no L, D or 844 Mask fields, since only a single packet is retransmitted, identified 845 by the sequence number in the FEC header. The source packet SSRC is 846 included in the FEC header for retransmissions, not in the RTP header 847 CSRC list as in the FEC header variants with R=0. When performing 848 retransmissions, a single repair packet stream (SSRC) MAY be used for 849 retransmitting packets from multiple source packet streams (SSRCs), 850 as well as transmitting FEC repair packets that protect multiple 851 source packet streams (SSRCs). 853 This FEC header layout is identical to the source RTP (version 2) 854 packet, starting with its RTP header, where the retransmission 855 "payload" is everything following the fixed 12-byte RTP header of the 856 source packet, including CSRC list and extensions if present. 857 Therefore, the only operation needed for sending retransmissions is 858 to prepend a new RTP header to the source packet. 860 0 1 2 3 861 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 1 862 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 863 |1|0|P|X| CC |M| Payload Type| Sequence Number | 864 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 865 | Timestamp | 866 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 867 | SSRC | 868 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 869 : Retransmission "Payload" follows FEC Header : 870 : : 872 Figure 15: FEC Header for Retransmission 874 5. Payload Format Parameters 876 This section provides the media subtype registration for the non- 877 interleaved and interleaved parity FEC. The parameters that are 878 required to configure the FEC encoding and decoding operations are 879 also defined in this section. If no specific FEC code is specified 880 in the subtype, then the FEC code defaults to the parity code defined 881 in this specification. 883 5.1. Media Type Registration - Parity Codes 885 This registration is done using the template defined in [RFC6838] and 886 following the guidance provided in [RFC4855] along with [RFC4856]. 888 Note to the RFC Editor: In the following sections, please replace 889 "XXXX" with the number of this document prior to publication as an 890 RFC. 892 5.1.1. Registration of audio/flexfec 894 Type name: audio 896 Subtype name: flexfec 898 Required parameters: 900 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 901 than 1000 Hz to provide sufficient resolution to RTCP operations. 902 However, it is RECOMMENDED to select the rate that matches the 903 rate of the protected source RTP stream. 905 o repair-window: The time that spans the source packets and the 906 corresponding repair packets. The size of the repair window is 907 specified in microseconds. 909 Encoding considerations: This media type is framed (See Section 4.8 910 in the template document [RFC6838]) and contains binary data. 912 Security considerations: See Section 9 of [RFCXXXX]. 914 Interoperability considerations: None. 916 Published specification: [RFCXXXX]. 918 Applications that use this media type: Multimedia applications that 919 want to improve resiliency against packet loss by sending redundant 920 data in addition to the source media. 922 Fragment identifier considerations: None. 924 Additional information: None. 926 Person & email address to contact for further information: IESG 927 and IETF Audio/Video Transport Payloads Working Group 928 (or it's successor as delegated by the IESG). 930 Intended usage: COMMON. 932 Restriction on usage: This media type depends on RTP framing, and 933 hence, is only defined for transport via RTP [RFC3550]. 935 Author: Varun Singh . 937 Change controller: IETF Audio/Video Transport Payloads Working Group 938 delegated from the IESG (or it's successor as delegated by the IESG). 940 5.1.2. Registration of video/flexfec 942 Type name: video 944 Subtype name: flexfec 946 Required parameters: 948 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 949 than 1000 Hz to provide sufficient resolution to RTCP operations. 950 However, it is RECOMMENDED to select the rate that matches the 951 rate of the protected source RTP stream. 953 o repair-window: The time that spans the source packets and the 954 corresponding repair packets. The size of the repair window is 955 specified in microseconds. 957 Encoding considerations: This media type is framed (See Section 4.8 958 in the template document [RFC6838]) and contains binary data. 960 Security considerations: See Section 9 of [RFCXXXX]. 962 Interoperability considerations: None. 964 Published specification: [RFCXXXX]. 966 Applications that use this media type: Multimedia applications that 967 want to improve resiliency against packet loss by sending redundant 968 data in addition to the source media. 970 Fragment identifier considerations: None. 972 Additional information: None. 974 Person & email address to contact for further information: IESG 975 and IETF Audio/Video Transport Payloads Working Group 976 (or it's successor as delegated by the IESG). 978 Intended usage: COMMON. 980 Restriction on usage: This media type depends on RTP framing, and 981 hence, is only defined for transport via RTP [RFC3550]. 983 Author: Varun Singh . 985 Change controller: IETF Audio/Video Transport Payloads Working Group 986 delegated from the IESG (or it's successor as delegated by the IESG). 988 5.1.3. Registration of text/flexfec 990 Type name: text 992 Subtype name: flexfec 994 Required parameters: 996 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 997 than 1000 Hz to provide sufficient resolution to RTCP operations. 998 However, it is RECOMMENDED to select the rate that matches the 999 rate of the protected source RTP stream. 1001 o repair-window: The time that spans the source packets and the 1002 corresponding repair packets. The size of the repair window is 1003 specified in microseconds. 1005 Encoding considerations: This media type is framed (See Section 4.8 1006 in the template document [RFC6838]) and contains binary data. 1008 Security considerations: See Section 9 of [RFCXXXX]. 1010 Interoperability considerations: None. 1012 Published specification: [RFCXXXX]. 1014 Applications that use this media type: Multimedia applications that 1015 want to improve resiliency against packet loss by sending redundant 1016 data in addition to the source media. 1018 Fragment identifier considerations: None. 1020 Additional information: None. 1022 Person & email address to contact for further information: IESG 1023 and IETF Audio/Video Transport Payloads Working Group 1024 (or it's successor as delegated by the IESG). 1026 Intended usage: COMMON. 1028 Restriction on usage: This media type depends on RTP framing, and 1029 hence, is only defined for transport via RTP [RFC3550]. 1031 Author: Varun Singh . 1033 Change controller: IETF Audio/Video Transport Payloads Working Group 1034 delegated from the IESG (or it's successor as delegated by the IESG). 1036 5.1.4. Registration of application/flexfec 1038 Type name: application 1040 Subtype name: flexfec 1042 Required parameters: 1044 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 1045 than 1000 Hz to provide sufficient resolution to RTCP operations. 1046 However, it is RECOMMENDED to select the rate that matches the 1047 rate of the protected source RTP stream. 1049 o repair-window: The time that spans the source packets and the 1050 corresponding repair packets. The size of the repair window is 1051 specified in microseconds. 1053 Encoding considerations: This media type is framed (See Section 4.8 1054 in the template document [RFC6838]) and contains binary data. 1056 Security considerations: See Section 9 of [RFCXXXX]. 1058 Interoperability considerations: None. 1060 Published specification: [RFCXXXX]. 1062 Applications that use this media type: Multimedia applications that 1063 want to improve resiliency against packet loss by sending redundant 1064 data in addition to the source media. 1066 Fragment identifier considerations: None. 1068 Additional information: None. 1070 Person & email address to contact for further information: IESG 1071 and IETF Audio/Video Transport Payloads Working Group 1072 (or it's successor as delegated by the IESG). 1074 Intended usage: COMMON. 1076 Restriction on usage: This media type depends on RTP framing, and 1077 hence, is only defined for transport via RTP [RFC3550]. 1079 Author: Varun Singh . 1081 Change controller: IETF Audio/Video Transport Payloads Working Group 1082 delegated from the IESG (or it's successor as delegated by the IESG). 1084 5.2. Mapping to SDP Parameters 1086 Applications that use the RTP transport commonly use Session 1087 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1088 The information that is used to specify the media types in an RTP 1089 session has specific mappings to the fields in an SDP description. 1090 This section provides these mappings for the media subtypes 1091 registered by this document. Note that if an application does not 1092 use SDP to describe the RTP sessions, an appropriate mapping must be 1093 defined and used to specify the media types and their parameters for 1094 the control/description protocol employed by the application. 1096 The mapping of the media type specification for "flexfec" and its 1097 associated parameters in SDP is as follows: 1099 o The media type (e.g., "application") goes into the "m=" line as 1100 the media name. 1102 o The media subtype goes into the "a=rtpmap" line as the encoding 1103 name. The RTP clock rate parameter ("rate") also goes into the 1104 "a=rtpmap" line as the clock rate. 1106 o The remaining required payload-format-specific parameters go into 1107 the "a=fmtp" line by copying them directly from the media type 1108 string as a semicolon-separated list of parameter=value pairs. 1110 SDP examples are provided in Section 7.1. 1112 5.2.1. Offer-Answer Model Considerations 1114 When offering parity FEC over RTP using SDP in an Offer/Answer model 1115 [RFC3264], the following considerations apply: 1117 o A sender application will indicate a repair window consistent with 1118 the desired amount of protection. Note that since the sender can 1119 change the FEC configuration on a packet-by-packet basis, the 1120 receiver must support any valid FLEX FEC configuration within the 1121 repair window associated with the offer (see Section 4.2.2). If 1122 the receiver cannot support the offered repair window it MUST 1123 reject the offer. 1125 o The size of the repair-window is related to the maximum delay 1126 between the transmission of a source packet and the associated 1127 repair packet. This directly impacts the buffering requirement on 1128 the receiver side and the receiver must consider this when 1129 choosing an offer. 1131 o Any unknown option in the offer must be ignored and deleted from 1132 the answer (see Section 6 of [RFC3264]). If FEC is not desired by 1133 the receiver, it can be deleted from the answer. 1135 5.2.2. Declarative Considerations 1137 In declarative usage, like SDP in the Real-time Streaming Protocol 1138 (RTSP, for RTSP 1.0 see [RFC2326] and for RTSP 2.0 see [RFC7826]) or 1139 the Session Announcement Protocol (SAP) [RFC2974], the following 1140 considerations apply: 1142 o The payload format configuration parameters are all declarative 1143 and a participant MUST use the configuration that is provided for 1144 the session. 1146 o More than one configuration may be provided (if desired) by 1147 declaring multiple RTP payload types. In that case, the receivers 1148 should choose the repair stream that is best for them. 1150 6. Protection and Recovery Procedures - Parity Codes 1152 This section provides a complete specification of the 1-D and 2-D 1153 parity codes and their RTP payload formats. It does not apply to the 1154 single packet retransmission format (R=1 in the FEC Header). 1156 6.1. Overview 1158 The following sections specify the steps involved in generating the 1159 repair packets and reconstructing the missing source packets from the 1160 repair packets. 1162 6.2. Repair Packet Construction 1164 The RTP Header of a repair packet is formed based on the guidelines 1165 given in Section 4.2. 1167 The FEC Header and Repair "Payload" of repair packets are formed by 1168 applying the XOR operation on the bit strings that are generated from 1169 the individual source packets protected by this particular repair 1170 packet. The set of the source packets that are associated with a 1171 given repair packet can be computed by the formula given in 1172 Section 6.3.1. 1174 The bit string is formed for each source packet by concatenating the 1175 following fields together in the order specified: 1177 o The first 16 bits of the RTP header (16 bits), though the first 1178 two (version) bits will be ignored by the recovery procedure. 1180 o Unsigned network-ordered 16-bit representation of the source 1181 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1182 the sum of the lengths of all the following if present: the CSRC 1183 list, extension header, RTP payload and RTP padding (16 bits). 1185 o The timestamp of the RTP header (32 bits). 1187 o All octets after the fixed 12-byte RTP header. (Note the SSRC 1188 field is skipped.) 1190 The FEC bit string is generated by applying the parity operation on 1191 the bit strings produced from the source packets. The FEC header is 1192 generated from the FEC bit string as follows: 1194 o The first (most significant) 2 bits in the FEC bit string, which 1195 contain the RTP version field, are skipped. The R and F bits in 1196 the FEC header are set to the appropriate value, i.e., it depends 1197 on the chosen format variant. As a consequence of overwriting the 1198 RTP version field with the R and F bits, this payload format only 1199 supports RTP version 2. 1201 o The next bit in the FEC bit string is written into the P recovery 1202 bit in the FEC header. 1204 o The next bit in the FEC bit string is written into the X recovery 1205 bit in the FEC header. 1207 o The next 4 bits of the FEC bit string are written into the CC 1208 recovery field in the FEC header. 1210 o The next bit is written into the M recovery bit in the FEC header. 1212 o The next 7 bits of the FEC bit string are written into the PT 1213 recovery field in the FEC header. 1215 o The next 16 bits are written into the length recovery field in the 1216 FEC header. 1218 o The next 32 bits of the FEC bit string are written into the TS 1219 recovery field in the FEC header. 1221 o The lowest Sequence Number of the source packets protected by this 1222 repair packet is written into the Sequence Number Base field in 1223 the FEC header. This needs to be repeated for each SSRC that has 1224 packets included in the source block. 1226 o Depending on the chosen FEC header variant, the mask(s) are set 1227 when F=0, or the L and D values are set when F=1. This needs to 1228 be repeated for each SSRC that has packets included in the source 1229 block. 1231 o The rest of the FEC bit string, which contains everything after 1232 the fixed 12-byte RTP header of the source packet, is written into 1233 the Repair "Payload" following the FEC header, where "Payload" 1234 refers to everything after the fixed 12-byte RTP header, including 1235 extensions, CSRC list, true payloads, and padding. 1237 If the lengths of the source packets are not equal, each shorter 1238 packet MUST be padded to the length of the longest packet by adding 1239 octet 0's at the end. 1241 Due to this possible padding and mandatory FEC header, a repair 1242 packet has a larger size than the source packets it protects. This 1243 may cause problems if the resulting repair packet size exceeds the 1244 Maximum Transmission Unit (MTU) size of the path over which the 1245 repair stream is sent. 1247 6.3. Source Packet Reconstruction 1249 This section describes the recovery procedures that are required to 1250 reconstruct the missing source packets. The recovery process has two 1251 steps. In the first step, the FEC decoder determines which source 1252 and repair packets should be used in order to recover a missing 1253 packet. In the second step, the decoder recovers the missing packet, 1254 which consists of an RTP header and RTP payload. 1256 The following describes the RECOMMENDED algorithms for the first and 1257 second steps. Based on the implementation, different algorithms MAY 1258 be adopted. However, the end result MUST be identical to the one 1259 produced by the algorithms described below. 1261 Note that the same algorithms are used by the 1-D parity codes, 1262 regardless of whether the FEC protection is applied over a column or 1263 a row. The 2-D parity codes, on the other hand, usually require 1264 multiple iterations of the procedures described here. This iterative 1265 decoding algorithm is further explained in Section 6.3.4. 1267 6.3.1. Associating the Source and Repair Packets 1269 Before associating source and repair packets, the receiver must know 1270 in which RTP sessions the source and repair respectively are being 1271 sent. After this is established by the receiver the first step is 1272 associating the source and repair packets. This association can be 1273 via flexible bitmasks, or fixed L and D offsets which can be in the 1274 FEC header or signaled in SDP in optional payload format parameters 1275 when L=D=0 in the FEC header. 1277 6.3.1.1. Using Bitmasks 1279 To use flexible bitmasks, the first two FEC header bits MUST have R=0 1280 and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source 1281 packets are protected by a FEC repair packet. If the bit i in the 1282 mask is set to 1, the source packet number N + i is protected by this 1283 FEC repair packet, where N is the sequence number base indicated in 1284 the FEC header. The most significant bit of the mask corresponds to 1285 i=0. The least significant bit of the mask corresponds to i=14 in 1286 the 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit 1287 mask. 1289 The bitmasks are able to represent arbitrary protection patterns, for 1290 example, 1-D interleaved, 1-D non-interleaved, 2-D. 1292 6.3.1.2. Using L and D Offsets 1294 Denote the set of the source packets associated with repair packet p* 1295 by set T(p*). Note that in a source block whose size is L columns by 1296 D rows, set T includes D source packets plus one repair packet for 1297 the FEC protection applied over a column, and L source packets plus 1298 one repair packet for the FEC protection applied over a row. Recall 1299 that 1-D interleaved and non-interleaved FEC protection can fully 1300 recover the missing information if there is only one source packet 1301 missing per column or row in set T. If there are more than one 1302 source packets missing per column or row in set T, 1-D FEC protection 1303 may fail to recover all the missing information. 1305 When value of L is non-zero, the 8-bit fields indicate the offset of 1306 packets protected by an interleaved (D>0) or non-interleaved (D=0) 1307 FEC packet. Using a combination of interleaved and non-interleaved 1308 FEC repair packets can form 2-D protection patterns. 1310 Mathematically, for any received repair packet, p*, the sequence 1311 numbers of the source packets that are protected by this repair 1312 packet are determined as follows, where SN is the sequence number 1313 base in the FEC header: 1315 For each SSRC (in CSRC list): 1316 When D <= 1: Source packets for each row: SN, SN+1, ..., SN+(L-1) 1317 When D > 1: Source packets for each col: SN, SN+L, ..., SN+(D-1)*L 1319 6.3.2. Recovering the RTP Header 1321 For a given set T, the procedure for the recovery of the RTP header 1322 of the missing packet, whose sequence number is denoted by SEQNUM, is 1323 as follows: 1325 1. For each of the source packets that are successfully received in 1326 T, compute the 80-bit string by concatenating the first 64 bits 1327 of their RTP header and the unsigned network-ordered 16-bit 1328 representation of their length in bytes minus 12. 1330 2. For the repair packet in T, extract the FEC bit string as the 1331 first 80 bits of the FEC header. 1333 3. Calculate the recovered bit string as the XOR of the bit strings 1334 generated from all source packets in T and the FEC bit string 1335 generated from the repair packet in T. 1337 4. Create a new packet with the standard 12-byte RTP header and no 1338 payload. 1340 5. Set the version of the new packet to 2. Skip the first 2 bits 1341 in the recovered bit string. 1343 6. Set the Padding bit in the new packet to the next bit in the 1344 recovered bit string. 1346 7. Set the Extension bit in the new packet to the next bit in the 1347 recovered bit string. 1349 8. Set the CC field to the next 4 bits in the recovered bit string. 1351 9. Set the Marker bit in the new packet to the next bit in the 1352 recovered bit string. 1354 10. Set the Payload type in the new packet to the next 7 bits in the 1355 recovered bit string. 1357 11. Set the SN field in the new packet to SEQNUM. 1359 12. Take the next 16 bits of the recovered bit string and set the 1360 new variable Y to whatever unsigned integer this represents 1361 (assuming network order). Convert Y to host order. Y 1362 represents the length of the new packet in bytes minus 12 (for 1363 the fixed RTP header), i.e., the sum of the lengths of all the 1364 following if present: the CSRC list, header extension, RTP 1365 payload and RTP padding. 1367 13. Set the TS field in the new packet to the next 32 bits in the 1368 recovered bit string. 1370 14. Set the SSRC of the new packet to the SSRC of the missing source 1371 RTP stream. 1373 This procedure recovers the header of an RTP packet up to (and 1374 including) the SSRC field. 1376 6.3.3. Recovering the RTP Payload 1378 Following the recovery of the RTP header, the procedure for the 1379 recovery of the RTP "payload" is as follows, where "payload" refers 1380 to everything following the fixed 12-byte RTP header, including 1381 extensions, CSRC list, true payload and padding. 1383 1. Allocate Y additional bytes for the new packet generated in 1384 Section 6.3.2. 1386 2. For each of the source packets that are successfully received in 1387 T, compute the bit string from the Y octets of data starting with 1388 the 13th octet of the packet. If any of the bit strings 1389 generated from the source packets has a length shorter than Y, 1390 pad them to that length. The zero-padding octets MUST be added 1391 at the end of the bit string. Note that the information of the 1392 first 8 octets are protected by the FEC header. 1394 3. For the repair packet in T, compute the FEC bit string from the 1395 repair packet payload, i.e., the Y octets of data following the 1396 FEC header. Note that the FEC header may be different sizes 1397 depending on the variant and bitmask size. 1399 4. Calculate the recovered bit string as the XOR of the bit strings 1400 generated from all source packets in T and the FEC bit string 1401 generated from the repair packet in T. 1403 5. Set the last Y octets in the new packet to the recovered bit 1404 string. 1406 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1408 In 2-D parity FEC protection, the sender generates both non- 1409 interleaved and interleaved FEC repair packets to combat with the 1410 mixed loss patterns (random and bursty). At the receiver side, these 1411 FEC packets are used iteratively to overcome the shortcomings of the 1412 1-D non-interleaved/interleaved FEC protection and improve the 1413 chances of full error recovery. 1415 The iterative decoding algorithm runs as follows: 1417 1. Set num_recovered_until_this_iteration to zero 1419 2. Set num_recovered_so_far to zero 1421 3. Recover as many source packets as possible by using the non- 1422 interleaved FEC repair packets as outlined in Section 6.3.2 and 1423 Section 6.3.3, and increase the value of num_recovered_so_far by 1424 the number of recovered source packets. 1426 4. Recover as many source packets as possible by using the 1427 interleaved FEC repair packets as outlined in Section 6.3.2 and 1428 Section 6.3.3, and increase the value of num_recovered_so_far by 1429 the number of recovered source packets. 1431 5. If num_recovered_so_far > num_recovered_until_this_iteration 1432 ---num_recovered_until_this_iteration = num_recovered_so_far 1433 ---Go to step 3 1434 Else 1435 ---Terminate 1437 The algorithm terminates either when all missing source packets are 1438 fully recovered or when there are still remaining missing source 1439 packets but the FEC repair packets are not able to recover any more 1440 source packets. For the example scenarios when the 2-D parity FEC 1441 protection fails full recovery, refer to Section 1.1.4. Upon 1442 termination, variable num_recovered_so_far has a value equal to the 1443 total number of recovered source packets. 1445 Example: 1447 Suppose that the receiver experienced the loss pattern sketched in 1448 Figure 16. 1450 +---+ +---+ +===+ 1451 X X | 3 | | 4 | |R_1| 1452 +---+ +---+ +===+ 1454 +---+ +---+ +---+ +---+ +===+ 1455 | 5 | | 6 | | 7 | | 8 | |R_2| 1456 +---+ +---+ +---+ +---+ +===+ 1458 +---+ +---+ +===+ 1459 | 9 | X X | 12| |R_3| 1460 +---+ +---+ +===+ 1462 +===+ +===+ +===+ +===+ 1463 |C_1| |C_2| |C_3| |C_4| 1464 +===+ +===+ +===+ +===+ 1466 Figure 16: Example loss pattern for the iterative decoding algorithm 1468 The receiver executes the iterative decoding algorithm and recovers 1469 source packets #1 and #11 in the first iteration. The resulting 1470 pattern is sketched in Figure 17. 1472 +---+ +---+ +---+ +===+ 1473 | 1 | X | 3 | | 4 | |R_1| 1474 +---+ +---+ +---+ +===+ 1476 +---+ +---+ +---+ +---+ +===+ 1477 | 5 | | 6 | | 7 | | 8 | |R_2| 1478 +---+ +---+ +---+ +---+ +===+ 1480 +---+ +---+ +---+ +===+ 1481 | 9 | X | 11| | 12| |R_3| 1482 +---+ +---+ +---+ +===+ 1484 +===+ +===+ +===+ +===+ 1485 |C_1| |C_2| |C_3| |C_4| 1486 +===+ +===+ +===+ +===+ 1488 Figure 17: The resulting pattern after the first iteration 1490 Since the if condition holds true, the receiver runs a new iteration. 1491 In the second iteration, source packets #2 and #10 are recovered, 1492 resulting in a full recovery as sketched in Figure 18. 1494 +---+ +---+ +---+ +---+ +===+ 1495 | 1 | | 2 | | 3 | | 4 | |R_1| 1496 +---+ +---+ +---+ +---+ +===+ 1498 +---+ +---+ +---+ +---+ +===+ 1499 | 5 | | 6 | | 7 | | 8 | |R_2| 1500 +---+ +---+ +---+ +---+ +===+ 1502 +---+ +---+ +---+ +---+ +===+ 1503 | 9 | | 10| | 11| | 12| |R_3| 1504 +---+ +---+ +---+ +---+ +===+ 1506 +===+ +===+ +===+ +===+ 1507 |C_1| |C_2| |C_3| |C_4| 1508 +===+ +===+ +===+ +===+ 1510 Figure 18: The resulting pattern after the second iteration 1512 7. Signaling Requirements 1514 Out-of-band signaling should be designed to enable the receiver to 1515 identify the RTP streams associated with source packets and repair 1516 packets, respectively. At a minimum, the signaling must be designed 1517 to allow the receiver to 1519 o Determine whether one or more source RTP streams will be sent. 1521 o Determine whether one or more repair RTP streams will be sent. 1523 o Associate the appropriate SSRC's to both source and repair 1524 streams. 1526 o Clearly identify which SSRC's are associated with each source 1527 block. 1529 o Clearly identify which repair packets correspond to which source 1530 blocks. 1532 o Make use of repair packets to recover source data associated with 1533 specific SSRC's. 1535 This section provides several Session Description Protocol (SDP) 1536 examples to demonstrate how these requirements can be met. 1538 7.1. SDP Examples 1540 This section provides two SDP [RFC4566] examples. The examples use 1541 the FEC grouping semantics defined in [RFC5956]. 1543 7.1.1. Example SDP for Flexible FEC Protection with in-band SSRC 1544 mapping 1546 In this example, we have one source video stream and one FEC repair 1547 stream. The source and repair streams are multiplexed on different 1548 SSRCs. The repair window is set to 200 ms. 1550 v=0 1551 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1552 s=FlexFEC minimal SDP signalling Example 1553 t=0 0 1554 m=video 30000 RTP/AVP 96 98 1555 c=IN IP4 233.252.0.1/127 1556 a=rtpmap:96 VP8/90000 1557 a=rtpmap:98 flexfec/90000 1558 a=fmtp:98; repair-window=200000 1560 7.1.2. Example SDP for Flexible FEC Protection with explicit signalling 1561 in the SDP 1563 This example shows one source video stream (ssrc:1234) and one FEC 1564 repair streams (ssrc:2345). One FEC group is formed with the 1565 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1566 are multiplexed on different SSRCs. The repair window is set to 200 1567 ms. 1569 v=0 1570 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1571 s=2-D Parity FEC with no in band signalling Example 1572 t=0 0 1573 m=video 30000 RTP/AVP 100 110 1574 c=IN IP4 192.0.2.0/24 1575 a=rtpmap:100 MP2T/90000 1576 a=rtpmap:110 flexfec/90000 1577 a=fmtp:110; repair-window:200000 1578 a=ssrc:1234 1579 a=ssrc:2345 1580 a=ssrc-group:FEC-FR 1234 2345 1582 7.2. On the Use of the RTP Stream Identifier Source Description 1584 The RTP Stream Identifier Source Description [I-D.ietf-avtext-rid] is 1585 a format that can be used to identify a single RTP source stream 1586 along with an associated repair stream. However, this specification 1587 already defines a method of source and repair stream identification 1588 that can enable protection of multiple source streams with a single 1589 repair stream. Therefore the RTP Stream Idenfifer Source Description 1590 SHOULD NOT be used for the Flexible FEC payload format 1592 8. Congestion Control Considerations 1594 FEC is an effective approach to provide applications resiliency 1595 against packet losses. However, in networks where the congestion is 1596 a major contributor to the packet loss, the potential impacts of 1597 using FEC should be considered carefully before injecting the repair 1598 streams into the network. In particular, in bandwidth-limited 1599 networks, FEC repair streams may consume a significant part of the 1600 available bandwidth and consequently may congest the network. In 1601 such cases, the applications MUST NOT arbitrarily increase the amount 1602 of FEC protection since doing so may lead to a congestion collapse. 1603 If desired, stronger FEC protection MAY be applied only after the 1604 source rate has been reduced. 1606 In a network-friendly implementation, an application should avoid 1607 sending/receiving FEC repair streams if it knows that sending/ 1608 receiving those FEC repair streams would not help at all in 1609 recovering the missing packets. Examples of where FEC would not be 1610 beneficial are: (1) if the successful recovery rate as determined by 1611 RTCP feedback is low (see [RFC5725] and [RFC7509]), and (2) the 1612 application has a smaller latency requirement than the repair window 1613 adopted by the FEC configuration based on the expected burst loss 1614 duration and the target FEC overhead. It is RECOMMENDED that the 1615 amount and type (row, column, or both) of FEC protection is adjusted 1616 dynamically based on the packet loss rate and burst loss length 1617 observed by the applications. 1619 In multicast scenarios, it may be difficult to optimize the FEC 1620 protection per receiver. If there is a large variation among the 1621 levels of FEC protection needed by different receivers, it is 1622 RECOMMENDED that the sender offers multiple repair streams with 1623 different levels of FEC protection and the receivers join the 1624 corresponding multicast sessions to receive the repair stream(s) that 1625 is best for them. 1627 9. Security Considerations 1629 RTP packets using the payload format defined in this specification 1630 are subject to the security considerations discussed in the RTP 1631 specification [RFC3550] and in any applicable RTP profile. The main 1632 security considerations for the RTP packet carrying the RTP payload 1633 format defined within this memo are confidentiality, integrity and 1634 source authenticity. Confidentiality can be provided by encrypting 1635 the RTP payload. Integrity of the RTP packets is achieved through a 1636 suitable cryptographic integrity protection mechanism. Such a 1637 cryptographic system may also allow the authentication of the source 1638 of the payload. A suitable security mechanism for this RTP payload 1639 format should provide confidentiality, integrity protection, and at 1640 least source authentication capable of determining if an RTP packet 1641 is from a member of the RTP session. 1643 Note that the appropriate mechanism to provide security to RTP and 1644 payloads following this memo may vary. It is dependent on the 1645 application, transport and signaling protocol employed. Therefore, a 1646 single mechanism is not sufficient, although if suitable, using the 1647 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1648 Other mechanisms that may be used are IPsec [RFC4301] and Datagram 1649 Transport Layer Security (DTLS, see [RFC6347]) (RTP over UDP); other 1650 alternatives may exist. 1652 Given that FLEX FEC enables the protection of multiple source 1653 streams, there exists the possibility that multiple source buffers 1654 may be created that may not be used. An attacker could leverage 1655 unused source buffers to as a means of occupying memory in a FLEX FEC 1656 endpoint. In addition, an attack against the FEC parameters 1657 themselves (e.g. repair window, D or L values) can result in a 1658 receiver having to allocate source buffer space that may also lead to 1659 excessive consumption of resources. Similarly, a network attacker 1660 could modify the recovery fields corresponding to packet lengths 1661 (assuming there are no message integrity mechanisms) which in turn 1662 could force unnecessarily large memory allocations at the receiver. 1663 Moreover the application source data may not be perfectly matched 1664 with FLEX FEC source partitioning. If this is the case, there is a 1665 possibility for unprotected source data if, for instance, the FLEX 1666 FEC implementation discards data that does not fit perfectly into its 1667 source processing requirements. 1669 10. IANA Considerations 1671 New media subtypes are subject to IANA registration. For the 1672 registration of the payload formats and their parameters introduced 1673 in this document, refer to Section 5.1. 1675 11. Acknowledgments 1677 Some parts of this document are borrowed from [RFC5109]. Thus, the 1678 author would like to thank the editor of [RFC5109] and those who 1679 contributed to [RFC5109]. 1681 Thanks to Stephen Botzko , Bernard Aboba , Rasmus Brandt , Brian 1682 Baldino , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus 1683 Westerlund for providing valuable feedback on earlier versions of 1684 this draft. 1686 12. References 1688 12.1. Normative References 1690 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1691 Requirement Levels", BCP 14, RFC 2119, 1692 DOI 10.17487/RFC2119, March 1997, 1693 . 1695 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1696 with Session Description Protocol (SDP)", RFC 3264, 1697 DOI 10.17487/RFC3264, June 2002, 1698 . 1700 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1701 Jacobson, "RTP: A Transport Protocol for Real-Time 1702 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1703 July 2003, . 1705 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1706 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1707 July 2006, . 1709 [RFC4855] Casner, S., "Media Type Registration of RTP Payload 1710 Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, 1711 . 1713 [RFC4856] Casner, S., "Media Type Registration of Payload Formats in 1714 the RTP Profile for Audio and Video Conferences", 1715 RFC 4856, DOI 10.17487/RFC4856, February 2007, 1716 . 1718 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1719 the Session Description Protocol", RFC 5956, 1720 DOI 10.17487/RFC5956, September 2010, 1721 . 1723 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1724 Correction (FEC) Framework", RFC 6363, 1725 DOI 10.17487/RFC6363, October 2011, 1726 . 1728 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1729 Specifications and Registration Procedures", BCP 13, 1730 RFC 6838, DOI 10.17487/RFC6838, January 2013, 1731 . 1733 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1734 "Guidelines for Choosing RTP Control Protocol (RTCP) 1735 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1736 September 2013, . 1738 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1739 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1740 May 2017, . 1742 12.2. Informative References 1744 [I-D.ietf-avtext-rid] 1745 Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream 1746 Identifier Source Description (SDES)", draft-ietf-avtext- 1747 rid-09 (work in progress), October 2016. 1749 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1750 Streaming Protocol (RTSP)", RFC 2326, 1751 DOI 10.17487/RFC2326, April 1998, 1752 . 1754 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1755 for Generic Forward Error Correction", RFC 2733, 1756 DOI 10.17487/RFC2733, December 1999, 1757 . 1759 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1760 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1761 October 2000, . 1763 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1764 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1765 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1766 . 1768 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1769 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1770 December 2005, . 1772 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1773 "Extended RTP Profile for Real-time Transport Control 1774 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1775 DOI 10.17487/RFC4585, July 2006, 1776 . 1778 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 1779 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 1780 DOI 10.17487/RFC4588, July 2006, 1781 . 1783 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 1784 Correction", RFC 5109, DOI 10.17487/RFC5109, December 1785 2007, . 1787 [RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE 1788 Report Block Type for RTP Control Protocol (RTCP) Extended 1789 Reports (XRs)", RFC 5725, DOI 10.17487/RFC5725, February 1790 2010, . 1792 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1793 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1794 January 2012, . 1796 [RFC7509] Huang, R. and V. Singh, "RTP Control Protocol (RTCP) 1797 Extended Report (XR) for Post-Repair Loss Count Metrics", 1798 RFC 7509, DOI 10.17487/RFC7509, May 2015, 1799 . 1801 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 1802 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 1803 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 1804 DOI 10.17487/RFC7656, November 2015, 1805 . 1807 [RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 1808 and M. Stiemerling, Ed., "Real-Time Streaming Protocol 1809 Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December 1810 2016, . 1812 [SMPTE2022-1] 1813 "Forward Error Correction for Real-Time Video/Audio 1814 Transport over IP Networks", 2007. 1816 Authors' Addresses 1818 Mo Zanaty 1819 Cisco 1820 Raleigh, NC 1821 USA 1823 Email: mzanaty@cisco.com 1825 Varun Singh 1826 CALLSTATS I/O Oy 1827 Runeberginkatu 4c A 4 1828 Helsinki 00100 1829 Finland 1831 Email: varun.singh@iki.fi 1832 URI: http://www.callstats.io/ 1834 Ali Begen 1835 Networked Media 1836 Konya 1837 Turkey 1839 Email: ali.begen@networked.media 1841 Giridhar Mandyam 1842 Qualcomm Inc. 1843 5775 Morehouse Drive 1844 San Diego, CA 92121 1845 USA 1847 Phone: +1 858 651 7200 1848 Email: mandyam@qti.qualcomm.com