idnits 2.17.1 draft-ietf-payload-flexible-fec-scheme-09.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (October 22, 2018) is 2006 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: '0-14' is mentioned on line 643, but not defined == Missing Reference: '15-45' is mentioned on line 645, but not defined == Missing Reference: '46-109' is mentioned on line 647, but not defined == Missing Reference: 'RFCXXXX' is mentioned on line 1042, but not defined == Unused Reference: 'RFC6709' is defined on line 1714, but no explicit reference was found in the text == Unused Reference: 'RFC2733' is defined on line 1736, but no explicit reference was found in the text == Unused Reference: 'SMPTE2022-1' is defined on line 1775, but no explicit reference was found in the text ** Obsolete normative reference: RFC 3555 (Obsoleted by RFC 4855, RFC 4856) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) ** Downref: Normative reference to an Informational RFC: RFC 6709 -- Obsolete informational reference (is this intentional?): RFC 2326 (Obsoleted by RFC 7826) -- Obsolete informational reference (is this intentional?): RFC 2733 (Obsoleted by RFC 5109) -- Obsolete informational reference (is this intentional?): RFC 5246 (Obsoleted by RFC 8446) Summary: 3 errors (**), 0 flaws (~~), 9 warnings (==), 4 comments (--). 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: April 25, 2019 callstats.io 6 A. Begen 7 Networked Media 8 G. Mandyam 9 Qualcomm Inc. 10 October 22, 2018 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-09 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 April 25, 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 . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . 24 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 . . . . . . 28 101 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 Flex FEC Protection with explicit 110 signalling in the SDP . . . . . . . . . . . . . . . . 36 111 8. Congestion Control Considerations . . . . . . . . . . . . . . 36 112 9. Security Considerations . . . . . . . . . . . . . . . . . . . 37 113 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 114 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37 115 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 116 12.1. Normative References . . . . . . . . . . . . . . . . . . 38 117 12.2. Informative References . . . . . . . . . . . . . . . . . 39 118 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 120 1. Introduction 122 This document defines new RTP payload formats for the Forward Error 123 Correction (FEC) that is generated by the non-interleaved and 124 interleaved parity codes from a source media encapsulated in RTP 125 [RFC3550]. The type of the source media protected by these parity 126 codes can be audio, video, text or application. The FEC data are 127 generated according to the media type parameters, which are 128 communicated out-of-band (e.g., in SDP). Furthermore, the 129 associations or relationships between the source and repair RTP 130 streams may be communicated in-band or out-of-band. The in-band 131 mechanism is advantageous when the endpoint is adapting the FEC 132 parameters. The out-of-band mechanism may be preferable when the FEC 133 parameters are fixed. 135 The Redunadncy RTP Stream [RFC7656] repair packets proposed in this 136 document protect the Source RTP Stream packets that belong to the 137 same RTP session. 139 1.1. Parity Codes 141 Both the non-interleaved and interleaved parity codes use the 142 eXclusive OR (XOR) operation to generate the repair packets. The 143 following steps take place: 145 1. The sender determines a set of source packets to be protected by 146 FEC based on the media type parameters. 148 2. The sender applies the XOR operation on the source packets to 149 generate the required number of repair packets. 151 3. The sender sends the repair packet(s) along with the source 152 packets, in different RTP streams, to the receiver(s). The 153 repair packets may be sent proactively or on-demand based on RTCP 154 feedback messages such as NACK [RFC4585]. 156 At the receiver side, if all of the source packets are successfully 157 received, there is no need for FEC recovery and the repair packets 158 are discarded. However, if there are missing source packets, the 159 repair packets can be used to recover the missing information. 160 Figure 1 and Figure 2 describe example block diagrams for the 161 systematic parity FEC encoder and decoder, respectively. 163 +------------+ 164 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 165 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 166 | Encoder | 167 | (Sender) | --> +==+ +==+ 168 +------------+ +==+ +==+ 170 Source Packet: +--+ Repair Packet: +==+ 171 +--+ +==+ 173 Figure 1: Block diagram for systematic parity FEC encoder 175 +------------+ 176 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 177 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 178 | Decoder | 179 +==+ +==+ --> | (Receiver) | 180 +==+ +==+ +------------+ 182 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 183 +--+ +==+ 185 Figure 2: Block diagram for systematic parity FEC decoder 187 In Figure 2, it is clear that the FEC repair packets have to be 188 received by the endpoint within a certain amount of time for the FEC 189 recovery process to be useful. The repair window is defined as the 190 time that spans a FEC block, which consists of the source packets and 191 the corresponding repair packets. At the receiver side, the FEC 192 decoder SHOULD buffer source and repair packets at least for the 193 duration of the repair window, to allow all the repair packets to 194 arrive. The FEC decoder can start decoding the already received 195 packets sooner; however, it should not register a FEC decoding 196 failure until it waits at least for the duration of the repair 197 window. 199 1.1.1. 1-D Non-interleaved (Row) FEC Protection 201 Consider a group of D x L source packets that have sequence numbers 202 starting from 1 running to D x L, and a repair packet is generated by 203 applying the XOR operation to every L consecutive packets as sketched 204 in Figure 3. This process is referred to as 1-D non-interleaved FEC 205 protection. As a result of this process, D repair packets are 206 generated, which are referred to as non-interleaved (or row) FEC 207 repair packets. 209 +--------------------------------------------------+ --- +===+ 210 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 211 +--------------------------------------------------+ --- +===+ 212 +--------------------------------------------------+ --- +===+ 213 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 214 +--------------------------------------------------+ --- +===+ 215 . . . . . . 216 . . . . . . 217 . . . . . . 218 +--------------------------------------------------+ --- +===+ 219 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 220 +--------------------------------------------------+ --- +===+ 222 Figure 3: Generating non-interleaved (row) FEC repair packets 224 1.1.2. 1-D Interleaved (Column) FEC Protection 226 If the XOR operation is applied to the group of the source packets 227 whose sequence numbers are L apart from each other, as sketched in 228 Figure 4. In this case the endpoint generates L repair packets. 229 This process is referred to as 1-D interleaved FEC protection, and 230 the resulting L repair packets are referred to as interleaved (or 231 column) FEC repair packets. 233 +-------------+ +-------------+ +-------------+ +-------+ 234 | S_1 | | S_2 | | S3 | ... | S_L | 235 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 236 | . | | . | | | | | 237 | . | | . | | | | | 238 | . | | . | | | | | 239 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 240 +-------------+ +-------------+ +-------------+ +-------+ 241 + + + + 242 ------------- ------------- ------------- ------- 243 | XOR | | XOR | | XOR | ... | XOR | 244 ------------- ------------- ------------- ------- 245 = = = = 246 +===+ +===+ +===+ +===+ 247 |C_1| |C_2| |C_3| ... |C_L| 248 +===+ +===+ +===+ +===+ 250 Figure 4: Generating interleaved (column) FEC repair packets 252 1.1.3. Use Cases for 1-D FEC Protection 254 A sender may generate one non-interleaved repair packet out of L 255 consecutive source packets or one interleaved repair packet out of D 256 non-consecutive source packets. Regardless of whether the repair 257 packet is a non-interleaved or an interleaved one, it can provide a 258 full recovery of the missing information if there is only one packet 259 missing among the corresponding source packets. This implies that 260 1-D non-interleaved FEC protection performs better when the source 261 packets are randomly lost. However, if the packet losses occur in 262 bursts, 1-D interleaved FEC protection performs better provided that 263 L is chosen large enough, i.e., L-packet duration is not shorter than 264 the observed burst duration. If the sender generates non-interleaved 265 FEC repair packets and a burst loss hits the source packets, the 266 repair operation fails. This is illustrated in Figure 5. 268 +---+ +---+ +===+ 269 | 1 | X X | 4 | |R_1| 270 +---+ +---+ +===+ 272 +---+ +---+ +---+ +---+ +===+ 273 | 5 | | 6 | | 7 | | 8 | |R_2| 274 +---+ +---+ +---+ +---+ +===+ 276 +---+ +---+ +---+ +---+ +===+ 277 | 9 | | 10| | 11| | 12| |R_3| 278 +---+ +---+ +---+ +---+ +===+ 280 Figure 5: Example scenario where 1-D non-interleaved FEC protection 281 fails error recovery (Burst Loss) 283 The sender may generate interleaved FEC repair packets to combat with 284 the bursty packet losses. However, two or more random packet losses 285 may hit the source and repair packets in the same column. In that 286 case, the repair operation fails as well. This is illustrated in 287 Figure 6. Note that it is possible that two burst losses may occur 288 back-to-back, in which case interleaved FEC repair packets may still 289 fail to recover the lost data. 291 +---+ +---+ +---+ 292 | 1 | X | 3 | | 4 | 293 +---+ +---+ +---+ 295 +---+ +---+ +---+ 296 | 5 | X | 7 | | 8 | 297 +---+ +---+ +---+ 299 +---+ +---+ +---+ +---+ 300 | 9 | | 10| | 11| | 12| 301 +---+ +---+ +---+ +---+ 303 +===+ +===+ +===+ +===+ 304 |C_1| |C_2| |C_3| |C_4| 305 +===+ +===+ +===+ +===+ 307 Figure 6: Example scenario where 1-D interleaved FEC protection fails 308 error recovery (Periodic Loss) 310 1.1.4. 2-D (Row and Column) FEC Protection 312 In networks where the source packets are lost both randomly and in 313 bursts, the sender ought to generate both non-interleaved and 314 interleaved FEC repair packets. This type of FEC protection is known 315 as 2-D parity FEC protection. At the expense of generating more FEC 316 repair packets, thus increasing the FEC overhead, 2-D FEC provides 317 superior protection against mixed loss patterns. However, it is 318 still possible for 2-D parity FEC protection to fail to recover all 319 of the lost source packets if a particular loss pattern occurs. An 320 example scenario is illustrated in Figure 7. 322 +---+ +---+ +===+ 323 | 1 | X X | 4 | |R_1| 324 +---+ +---+ +===+ 326 +---+ +---+ +---+ +---+ +===+ 327 | 5 | | 6 | | 7 | | 8 | |R_2| 328 +---+ +---+ +---+ +---+ +===+ 330 +---+ +---+ +===+ 331 | 9 | X X | 12| |R_3| 332 +---+ +---+ +===+ 334 +===+ +===+ +===+ +===+ 335 |C_1| |C_2| |C_3| |C_4| 336 +===+ +===+ +===+ +===+ 338 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 339 error recovery 341 2-D parity FEC protection also fails when at least two rows are 342 missing a source and the FEC packet and the missing source packets 343 (in at least two rows) are aligned in the same column. An example 344 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 345 protection cannot repair all missing source packets when at least two 346 columns are missing a source and the FEC packet and the missing 347 source packets (in at least two columns) are aligned in the same row. 349 +---+ +---+ +---+ 350 | 1 | | 2 | X | 4 | X 351 +---+ +---+ +---+ 353 +---+ +---+ +---+ +---+ +===+ 354 | 5 | | 6 | | 7 | | 8 | |R_2| 355 +---+ +---+ +---+ +---+ +===+ 357 +---+ +---+ +---+ 358 | 9 | | 10| X | 12| X 359 +---+ +---+ +---+ 361 +===+ +===+ +===+ +===+ 362 |C_1| |C_2| |C_3| |C_4| 363 +===+ +===+ +===+ +===+ 365 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 366 error recovery 368 1.1.5. FEC Overhead Considerations 370 The overhead is defined as the ratio of the number of bytes belonging 371 to the repair packets to the number of bytes belonging to the 372 protected source packets. 374 Generally, repair packets are larger in size compared to the source 375 packets. Also, not all the source packets are necessarily equal in 376 size. However, assuming that each repair packet carries an equal 377 number of bytes carried by a source packet, the overhead for 378 different FEC protection methods can be computed as follows: 380 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 382 o 1-D Interleaved FEC Protection: Overhead = 1/D 384 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 386 where L and D are the number of columns and rows in the source block, 387 respectively. 389 2. Requirements Notation 391 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 392 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 393 document are to be interpreted as described in [RFC2119]. 395 3. Definitions and Notations 397 3.1. Definitions 399 This document uses a number of definitions from [RFC6363]. 401 1-D Non-interleaved Row FEC: A protection scheme that operates on 402 consecutive source packets in the source block, able to recover a 403 single lost source packet per row of the source block. 405 1-D Interleaved Column FEC: A protection scheme that operates on 406 interleaved source packets in the source block, able to recover a 407 single lost source packet per column of the source block. 409 2-D FEC: A protection scheme that combines row and column FEC. 411 Source Block: A set of source packets that are protected by a set 412 of 1-D or 2-D FEC repair packets. 414 FEC Block: A source block and its corresponding FEC repair 415 packets. 417 Repair Window: The time that spans a FEC block, which consists of 418 the source packets and the corresponding FEC repair packets. 420 XOR Parity Codes: A FEC code which uses the eXclusive OR (XOR) 421 parity operation to encode a set of source packets to form a FEC 422 repair packet. 424 3.2. Notations 426 L: Number of columns of the source block (length of each row). 428 D: Number of rows of the source block (depth of each column). 430 bitmask: A 15-bit, 46-bit, or 110-bit mask indicating which source 431 packets are protected by a FEC repair packet. If the bit i in the 432 mask is set to 1, the source packet number N + i is protected by 433 this FEC repair packet, where N is the sequence number base 434 indicated in the FEC repair packet. The most significant bit of 435 the mask corresponds to i=0. The least signficant bit of the mask 436 corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit mask, 437 or i=109 in the 110-bit mask. 439 4. Packet Formats 441 This section describes the formats of the source packets and defines 442 the formats of the FEC repair packets. 444 4.1. Source Packets 446 The source packets contain the information that identifies the source 447 block and the position within the source block occupied by the 448 packet. Since the source packets that are carried within an RTP 449 stream already contain unique sequence numbers in their RTP headers 450 [RFC3550], the source packets can be identified in a straightforward 451 manner and there is no need to append additional field(s). The 452 primary advantage of not modifying the source packets in any way is 453 that it provides backward compatibility for the receivers that do not 454 support FEC at all. In multicast scenarios, this backward 455 compatibility becomes quite useful as it allows the non-FEC-capable 456 and FEC-capable receivers to receive and interpret the same source 457 packets sent in the same multicast session. 459 The source packets are transmitted as usual without altering them. 460 They are used along with the FEC repair packets to recover any 461 missing source packets, making this scheme a systematic code. 463 The source packets are full RTP packets with optional CSRC list, RTP 464 header extension, and padding. If any of these optional elements are 465 present in the source RTP packet, and that source packet is lost, 466 they are recovered by the FEC repair operation, which recovers the 467 full source RTP packet including these optional elements. 469 4.2. FEC Repair Packets 471 The FEC repair packets MUST contain information that identifies the 472 source block they pertain to and the relationship between the 473 contained repair packets and the original source block. For this 474 purpose, the RTP header of the repair packets is used, as well as 475 another header within the RTP payload, called the FEC header, as 476 shown in Figure 9. 478 Note that all the source stream packets that are protected by a 479 particular FEC packet need to be in the same RTP session. 481 +------------------------------+ 482 | IP Header | 483 +------------------------------+ 484 | Transport Header | 485 +------------------------------+ 486 | RTP Header | 487 +------------------------------+ ---+ 488 | FEC Header | | 489 +------------------------------+ | RTP Payload 490 | Repair "Payload" | | 491 +------------------------------+ ---+ 493 Figure 9: Format of FEC repair packets 495 Repair "Payload", which follows the FEC Header, includes repair of 496 everything following the fixed 12-byte RTP header of the source 497 packet, including any CSRC list and header extensions if present. 499 4.2.1. RTP Header of FEC Repair Packets 501 The RTP header is formatted according to [RFC3550] with some further 502 clarifications listed below: 504 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this 505 specification requires all source RTP packets and all FEC repair 506 packets to use RTP version 2. The reason for this restriction is 507 the first 2 bits of the FEC header contain other information (R 508 and F bits) rather than recovering the RTP version field. 510 Padding (P) bit: Source packets can have optional RTP padding, 511 which can be recovered. FEC repaire packets can have optional RTP 512 padding, which is independent of the RTP padding of the source 513 pakcets. 515 Extension (X) bit: Source packets can have optional RTP header 516 extensions, which can be recovered. FEC repair packets can have 517 optional RTP header extensions, which are independent of the RTP 518 header extensions of the source packets. 520 CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: 521 Source packets can have an optional CSRC list and count, which can 522 be recovered. FEC repair packets MUST use the CSRC list and count 523 to specify the SSRC(s) of the source RTP stream(s) protected by 524 this FEC repair packet. 526 Marker (M) bit: This bit is not used for this payload type, and 527 SHALL be set to 0 by senders, and SHALL be ignored by receivers. 529 Payload Type: The (dynamic) payload type for the FEC repair 530 packets is determined through out-of-band means. Note that this 531 document registers new payload formats for the repair packets 532 (Refer to Section 5 for details). According to [RFC3550], an RTP 533 receiver that cannot recognize a payload type must discard it. 534 This provides backward compatibility. If a non-FEC-capable 535 receiver receives a repair packet, it will not recognize the 536 payload type, and hence, will discard the repair packet. 538 Sequence Number (SN): The sequence number has the standard 539 definition. It MUST be one higher than the sequence number in the 540 previously transmitted repair packet. The initial value of the 541 sequence number SHOULD be random (unpredictable, based on 542 [RFC3550]). 544 Timestamp (TS): The timestamp SHALL be set to a time corresponding 545 to the repair packet's transmission time. Note that the timestamp 546 value has no use in the actual FEC protection process and is 547 usually useful for jitter calculations. 549 Synchronization Source (SSRC): The SSRC value for each repair 550 stream SHALL be randomly assigned as suggested by [RFC3550]. This 551 allows the sender to multiplex the source and repair RTP streams 552 in the same RTP session, or multiplex multiple repair streams in 553 an RTP session. The repair streams' SSRC's CNAME SHOULD be 554 identical to the CNAME of the source RTP stream(s) that this 555 repair stream protects. In cases when the repair stream covers 556 packets from multiple source RTP streams with different CNAME 557 values, any of these CNAME values MAY be used. 559 In some networks, the RTP Source, which produces the source 560 packets and the FEC Source, which generates the repair packets 561 from the source packets may not be the same host. In such 562 scenarios, using the same CNAME for the source and repair RTP 563 streams means that the RTP Source and the FEC Source MUST share 564 the same CNAME (for this specific source-repair stream 565 association). A common CNAME may be produced based on an 566 algorithm that is known both to the RTP and FEC Source [RFC7022]. 567 This usage is compliant with [RFC3550]. 569 Note that due to the randomness of the SSRC assignments, there is 570 a possibility of SSRC collision. In such cases, the collisions 571 MUST be resolved as described in [RFC3550]. 573 4.2.2. FEC Header of FEC Repair Packets 575 The format of the FEC header has 3 variants, depending on the values 576 in the first 2 bits (R and F bits) as shown in Figure 10. 578 0 1 2 3 579 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 1 580 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 581 |R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... | 582 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 583 | | 584 | ...varies depending on R/F... | 585 | | 586 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 587 : Repair "Payload" follows FEC Header : 588 : : 590 Figure 10: FEC Header 592 Repair "Payload", which follows the FEC Header, includes repair of 593 everything following the fixed 12-byte RTP header of the source 594 packet, including any CSRC list and header extensions if present. 596 +---+---+----------------------------------------------------------+ 597 | R | F | FEC Header variant | 598 +---+---+----------------------------------------------------------+ 599 | 0 | 0 | Flexible FEC Mask fields indicate source packets | 600 | 0 | 1 | Fixed FEC L/D (cols/rows) fields indicate source packets | 601 | 1 | 0 | Retransmission of a single source packet | 602 | 1 | 1 | Invalid, MUST NOT send, MUST ignore if received | 603 +---+---+----------------------------------------------------------+ 605 Figure 11: R and F bit values for FEC Header variants 607 The first variant, when R=0 and F=0, has a mask to signal protected 608 source packets, as shown in Figure 12. 610 The second variant, when R=0 and F=1, has a number of columns (L) and 611 rows (D) to signal protected source packets, as shown in Figure 13. 613 The final variant, when R=1 and F=0, is a retransmission format as 614 shown in Figure 15. 616 No variant uses R=1 and F=1, which is invalid, and MUST NOT be sent 617 by senders, and MUST be ignored by receivers. 619 The FEC header for all variants consists of the following common 620 fields: 622 o The R bit MUST be set to 1 to indicate a retransmission packet, 623 and MUST be set to 0 for FEC repair packets. 625 o The F bit indicates the type of FEC repair packets, as shown in 626 Figure 11, when the R bit is 0. The F bit MUST be set to 0 when 627 the R bit is 1 for retransmission packets. 629 o The P, X, CC, M and PT recovery fields are used to determine the 630 corresponding fields of the recovered packets. 632 4.2.2.1. FEC Header with Flexible Mask 634 When R=0 and F=0, the FEC Header includes flexible mask fields. 636 0 1 2 3 637 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 1 638 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 639 |0|0|P|X| CC |M| PT recovery | length recovery | 640 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 641 | TS recovery | 642 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 643 | SN base_i |k| Mask [0-14] | 644 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 645 |k| Mask [15-45] (optional) | 646 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 647 | Mask [46-109] (optional) | 648 | | 649 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 650 | ... next SN base and Mask for CSRC_i in CSRC list ... | 651 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 652 : Repair "Payload" follows FEC Header : 653 : : 655 Figure 12: FEC Header for F=0 657 o The Length recovery (16 bits) field is used to determine the 658 length of the recovered packets. This length includes all octets 659 following the fixed 12-byte RTP header of source packets, 660 including CSRC list and optional header extension(s) if present. 661 It excludes the fixed 12-byte RTP header of source packets. 663 o The TS recovery (32 bits) field is used to determine the timestamp 664 of the recovered packets. 666 o The CSRC_i (32 bits) field in the RTP Header (not FEC Header) 667 describes the SSRC of the source packets protected by this 668 particular FEC packet. If a FEC packet protects multiple SSRCs 669 (indicated by the CSRC Count > 1 in the RTP Header), there will be 670 multiple blocks of data containing the SN base and Mask fields. 672 o The SN base_i (16 bits) field indicates the lowest sequence 673 number, taking wrap around into account, of the source packets for 674 a particular SSRC (indicated in CSRC_i) protected by this repair 675 packet. 677 o The Mask fields indicate a bitmask of which source packets are 678 protected by this FEC repair packet, where bit j of the mask set 679 to 1 indicates that the source packet with sequence number (SN 680 base_i + j) is protected by this FEC repair packet, where j=0 is 681 the most significant bit in the mask. 683 o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110 684 bits. k=1 denotes that another mask follows, and k=0 denotes that 685 it is the last block of mask. 687 o Repair "Payload", which follows the FEC Header, includes repair of 688 everything following the fixed 12-byte RTP header of the source 689 packet, including any CSRC list and header extensions if present. 691 4.2.2.2. FEC Header with Fixed L Columns and D Rows 693 When R=0 and F=1, the FEC Header includes L and D fields for fixed 694 columns and rows. The other fields are the same as the prior 695 section. As in the previous section, the CSRC_i (32 bits) field in 696 the RTP Header (not FEC Header) describes the SSRC of the source 697 packets protected by this particular FEC packet. If there are 698 multiple SSRC's protected by the FEC packet, then there will be 699 multiple blocks of data containing an SN base along with L and D 700 fields. 702 0 1 2 3 703 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 1 704 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 705 |0|1|P|X| CC |M| PT recovery | length recovery | 706 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 707 | TS recovery | 708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 709 | SN base_i | L (columns) | D (rows) | 710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 711 | ... next SN base and L/D for CSRC_i in CSRC list ... | 712 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 713 : Repair "Payload" follows FEC Header : 714 : : 716 Figure 13: FEC Header for F=1 718 Consequently, the following conditions occur for L and D values: 720 If L=0, D=0, use the optional payload format parameters for L and D. 722 If L>0, D=0, indicates Row FEC, and no column FEC will follow. 723 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L. 725 If L>0, D=1, indicates Row FEC, and column FEC will follow. 726 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L will be 727 produced for each row. 728 Then FEC = SN, SN+L, SN+2L, ..., SN+(D-1)L will be produced 729 for each column. 730 After all row FEC's have been sent, then the column FEC's 731 will be sent. 733 If L>0, D>1, indicates column FEC of every L packet 734 in a group of D packets starting at SN base. 735 Hence, FEC = SN+(Lx0), SN+(Lx1), ... , SN+(LxD). 737 Figure 14: Interpreting the L and D field values 739 It should be noted that the flexible mask-based approach may be 740 inefficient for protecting a large number of source packets, or 741 impossible to signal if larger than the largest mask size. In such 742 cases, the fixed columns and rows variant may be more useful. 744 4.2.2.3. FEC Header for Retransmissions 746 When R=1 and F=0, the FEC packet is a retransmission of a single 747 source packet. Note that the layout of this retransmission packet is 748 different from other FEC repair packets. The sequence number (SN 749 base_i) replaces the length recovery in the FEC header, since the 750 length is already known for a single packet. There are no L, D or 751 Mask fields, since only a single packet is retransmitted, identified 752 by the sequence number in the FEC header. The source packet SSRC is 753 included in the FEC header for retransmissions, not in the RTP header 754 CSRC list as in the FEC header variants with R=0. Note that the 755 retransmission packet corresponds only to a single source SSRC. 757 This FEC header layout is identical to the source RTP (version 2) 758 packet, starting with its RTP header, where the retransmission 759 "payload" is everything following the fixed 12-byte RTP header of the 760 source packet, including CSRC list and extensions if present. 761 Therefore, the only operation needed for sending retransmissions is 762 to prepend a new RTP header to the source packet. 764 0 1 2 3 765 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 1 766 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 767 |1|0| P|X| CC |M| Payload Type| Sequence Number | 768 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 769 | Timestamp | 770 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 771 | SSRC | 772 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 773 : Retransmission "Payload" follows FEC Header : 774 : : 776 Figure 15: FEC Header for Retransmission 778 5. Payload Format Parameters 780 This section provides the media subtype registration for the non- 781 interleaved and interleaved parity FEC. The parameters that are 782 required to configure the FEC encoding and decoding operations are 783 also defined in this section. If no specific FEC code is specified 784 in the subtype, then the FEC code defaults to the parity code defined 785 in this specification. 787 5.1. Media Type Registration - Parity Codes 789 This registration is done using the template defined in [RFC6838] and 790 following the guidance provided in [RFC3555]. 792 Note to the RFC Editor: In the following sections, please replace 793 "XXXX" with the number of this document prior to publication as an 794 RFC. 796 5.1.1. Registration of audio/flexfec 798 Type name: audio 800 Subtype name: flexfec 802 Required parameters: 804 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 805 than 1000 Hz to provide sufficient resolution to RTCP operations. 806 However, it is RECOMMENDED to select the rate that matches the 807 rate of the protected source RTP stream. 809 o repair-window: The time that spans the source packets and the 810 corresponding repair packets. The size of the repair window is 811 specified in microseconds. 813 Optional parameters: 815 o L: indicates the number of columns of the source block that are 816 protected by this FEC block and it applies to all the source 817 SSRCs. L is a positive integer. 819 o D: indicates the number of rows of the source block that are 820 protected by this FEC block and it applies to all the source 821 SSRCs. D is a positive integer. 823 o ToP: indicates the type of protection applied by the sender: 0 for 824 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 825 protection, 2 for 2-D parity FEC protection, and 3 for 826 retranmission. 828 Note that both L and D in the optional parameters should follow the 829 value pairings stated in Section 4.2.2.2 if included. 831 Encoding considerations: This media type is framed (See Section 4.8 832 in the template document [RFC6838]) and contains binary data. 834 Security considerations: See Section 9 of [RFCXXXX]. 836 Interoperability considerations: None. 838 Published specification: [RFCXXXX]. 840 Applications that use this media type: Multimedia applications that 841 want to improve resiliency against packet loss by sending redundant 842 data in addition to the source media. 844 Fragment identifier considerations: None. 846 Additional information: None. 848 Person & email address to contact for further information: Varun 849 Singh and IETF Audio/Video Transport Payloads 850 Working Group. 852 Intended usage: COMMON. 854 Restriction on usage: This media type depends on RTP framing, and 855 hence, is only defined for transport via RTP [RFC3550]. 857 Author: Varun Singh . 859 Change controller: IETF Audio/Video Transport Working Group delegated 860 from the IESG. 862 Provisional registration? (standards tree only): Yes. 864 5.1.2. Registration of video/flexfec 866 Type name: video 868 Subtype name: flexfec 870 Required parameters: 872 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 873 than 1000 Hz to provide sufficient resolution to RTCP operations. 874 However, it is RECOMMENDED to select the rate that matches the 875 rate of the protected source RTP stream. 877 o repair-window: The time that spans the source packets and the 878 corresponding repair packets. The size of the repair window is 879 specified in microseconds. 881 Optional parameters: 883 o L: indicates the number of columns of the source block that are 884 protected by this FEC block and it applies to all the source 885 SSRCs. L is a positive integer. 887 o D: indicates the number of rows of the source block that are 888 protected by this FEC block and it applies to all the source 889 SSRCs. D is a positive integer. 891 o ToP: indicates the type of protection applied by the sender: 0 for 892 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 893 protection, 2 for 2-D parity FEC protection, and 3 for 894 retranmission. 896 Note that both L and D in the optional parameters should follow the 897 value pairings stated in Section 4.2.2.2 if included. 899 Encoding considerations: This media type is framed (See Section 4.8 900 in the template document [RFC6838]) and contains binary data. 902 Security considerations: See Section 9 of [RFCXXXX]. 904 Interoperability considerations: None. 906 Published specification: [RFCXXXX]. 908 Applications that use this media type: Multimedia applications that 909 want to improve resiliency against packet loss by sending redundant 910 data in addition to the source media. 912 Fragment identifier considerations: None. 914 Additional information: None. 916 Person & email address to contact for further information: Varun 917 Singh and IETF Audio/Video Transport Payloads 918 Working Group. 920 Intended usage: COMMON. 922 Restriction on usage: This media type depends on RTP framing, and 923 hence, is only defined for transport via RTP [RFC3550]. 925 Author: Varun Singh . 927 Change controller: IETF Audio/Video Transport Working Group delegated 928 from the IESG. 930 Provisional registration? (standards tree only): Yes. 932 5.1.3. Registration of text/flexfec 934 Type name: text 936 Subtype name: flexfec 938 Required parameters: 940 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 941 than 1000 Hz to provide sufficient resolution to RTCP operations. 942 However, it is RECOMMENDED to select the rate that matches the 943 rate of the protected source RTP stream. 945 o repair-window: The time that spans the source packets and the 946 corresponding repair packets. The size of the repair window is 947 specified in microseconds. 949 Optional parameters: 951 o L: indicates the number of columns of the source block that are 952 protected by this FEC block and it applies to all the source 953 SSRCs. L is a positive integer. 955 o D: indicates the number of rows of the source block that are 956 protected by this FEC block and it applies to all the source 957 SSRCs. D is a positive integer. 959 o ToP: indicates the type of protection applied by the sender: 0 for 960 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 961 protection, 2 for 2-D parity FEC protection, and 3 for 962 retranmission. 964 Note that both L and D in the optional parameters should follow the 965 value pairings stated in Section 4.2.2.2 if included. 967 Encoding considerations: This media type is framed (See Section 4.8 968 in the template document [RFC6838]) and contains binary data. 970 Security considerations: See Section 9 of [RFCXXXX]. 972 Interoperability considerations: None. 974 Published specification: [RFCXXXX]. 976 Applications that use this media type: Multimedia applications that 977 want to improve resiliency against packet loss by sending redundant 978 data in addition to the source media. 980 Fragment identifier considerations: None. 982 Additional information: None. 984 Person & email address to contact for further information: Varun 985 Singh and IETF Audio/Video Transport Payloads 986 Working Group. 988 Intended usage: COMMON. 990 Restriction on usage: This media type depends on RTP framing, and 991 hence, is only defined for transport via RTP [RFC3550]. 993 Author: Varun Singh . 995 Change controller: IETF Audio/Video Transport Working Group delegated 996 from the IESG. 998 Provisional registration? (standards tree only): Yes. 1000 5.1.4. Registration of application/flexfec 1002 Type name: application 1004 Subtype name: flexfec 1006 Required parameters: 1008 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 1009 than 1000 Hz to provide sufficient resolution to RTCP operations. 1010 However, it is RECOMMENDED to select the rate that matches the 1011 rate of the protected source RTP stream. 1013 o repair-window: The time that spans the source packets and the 1014 corresponding repair packets. The size of the repair window is 1015 specified in microseconds. 1017 Optional parameters: 1019 o L: indicates the number of columns of the source block that are 1020 protected by this FEC block and it applies to all the source 1021 SSRCs. L is a positive integer. 1023 o D: indicates the number of rows of the source block that are 1024 protected by this FEC block and it applies to all the source 1025 SSRCs. D is a positive integer. 1027 o ToP: indicates the type of protection applied by the sender: 0 for 1028 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 1029 protection, 2 for 2-D parity FEC protection, and 3 for 1030 retranmission. 1032 Note that both L and D in the optional parameters should follow the 1033 value pairings stated in Section 4.2.2.2 if included. 1035 Encoding considerations: This media type is framed (See Section 4.8 1036 in the template document [RFC6838]) and contains binary data. 1038 Security considerations: See Section 9 of [RFCXXXX]. 1040 Interoperability considerations: None. 1042 Published specification: [RFCXXXX]. 1044 Applications that use this media type: Multimedia applications that 1045 want to improve resiliency against packet loss by sending redundant 1046 data in addition to the source media. 1048 Fragment identifier considerations: None. 1050 Additional information: None. 1052 Person & email address to contact for further information: Varun 1053 Singh and IETF Audio/Video Transport Payloads 1054 Working Group. 1056 Intended usage: COMMON. 1058 Restriction on usage: This media type depends on RTP framing, and 1059 hence, is only defined for transport via RTP [RFC3550]. 1061 Author: Varun Singh . 1063 Change controller: IETF Audio/Video Transport Working Group delegated 1064 from the IESG. 1066 Provisional registration? (standards tree only): Yes. 1068 5.2. Mapping to SDP Parameters 1070 Applications that are using RTP transport commonly use Session 1071 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1072 The information that is used to specify the media types in an RTP 1073 session has specific mappings to the fields in an SDP description. 1074 This section provides these mappings for the media subtypes 1075 registered by this document. Note that if an application does not 1076 use SDP to describe the RTP sessions, an appropriate mapping must be 1077 defined and used to specify the media types and their parameters for 1078 the control/description protocol employed by the application. 1080 The mapping of the media type specification for "non-interleaved- 1081 parityfec" and "interleaved-parityfec" and their parameters in SDP is 1082 as follows: 1084 o The media type (e.g., "application") goes into the "m=" line as 1085 the media name. 1087 o The media subtype goes into the "a=rtpmap" line as the encoding 1088 name. The RTP clock rate parameter ("rate") also goes into the 1089 "a=rtpmap" line as the clock rate. 1091 o The remaining required payload-format-specific parameters go into 1092 the "a=fmtp" line by copying them directly from the media type 1093 string as a semicolon-separated list of parameter=value pairs. 1095 SDP examples are provided in Section 7.1. 1097 5.2.1. Offer-Answer Model Considerations 1099 When offering 1-D interleaved parity FEC over RTP using SDP in an 1100 Offer/Answer model [RFC3264], the following considerations apply: 1102 o Each combination of the L and D parameters produces a different 1103 FEC data and is not compatible with any other combination. A 1104 sender application may desire to offer multiple offers with 1105 different sets of L and D values as long as the parameter values 1106 are valid. The receiver SHOULD normally choose the offer that has 1107 a sufficient amount of interleaving. If multiple such offers 1108 exist, the receiver may choose the offer that has the lowest 1109 overhead or the one that requires the smallest amount of 1110 buffering. The selection depends on the application requirements. 1112 o The value for the repair-window parameter depends on the L and D 1113 values and cannot be chosen arbitrarily. More specifically, L and 1114 D values determine the lower limit for the repair-window size. 1115 The upper limit of the repair-window size does not depend on the L 1116 and D values. 1118 o Although combinations with the same L and D values but with 1119 different repair-window sizes produce the same FEC data, such 1120 combinations are still considered different offers. The size of 1121 the repair-window is related to the maximum delay between the 1122 transmission of a source packet and the associated repair packet. 1124 This directly impacts the buffering requirement on the receiver 1125 side and the receiver must consider this when choosing an offer. 1127 o Any unknown option in the offer MUST be ignored and deleted from 1128 the answer. If FEC is not desired by the receiver, it can be 1129 deleted from the answer. 1131 5.2.2. Declarative Considerations 1133 In declarative usage, like SDP in the Real-time Streaming Protocol 1134 (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) 1135 [RFC2974], the following considerations apply: 1137 o The payload format configuration parameters are all declarative 1138 and a participant MUST use the configuration that is provided for 1139 the session. 1141 o More than one configuration may be provided (if desired) by 1142 declaring multiple RTP payload types. In that case, the receivers 1143 should choose the repair stream that is best for them. 1145 6. Protection and Recovery Procedures - Parity Codes 1147 This section provides a complete specification of the 1-D and 2-D 1148 parity codes and their RTP payload formats. It does not apply to the 1149 single packet retransmission format (R=1 in the FEC Header). 1151 6.1. Overview 1153 The following sections specify the steps involved in generating the 1154 repair packets and reconstructing the missing source packets from the 1155 repair packets. 1157 6.2. Repair Packet Construction 1159 The RTP Header of a repair packet is formed based on the guidelines 1160 given in Section 4.2. 1162 The FEC Header and Repair "Payload" of repair packets are formed by 1163 applying the XOR operation on the bit strings that are generated from 1164 the individual source packets protected by this particular repair 1165 packet. The set of the source packets that are associated with a 1166 given repair packet can be computed by the formula given in 1167 Section 6.3.1. 1169 The bit string is formed for each source packet by concatenating the 1170 following fields together in the order specified: 1172 o The first 16 bits of the RTP header (16 bits). 1174 o Unsigned network-ordered 16-bit representation of the source 1175 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1176 the sum of the lengths of all the following if present: the CSRC 1177 list, extension header, RTP payload and RTP padding (16 bits). 1179 o The timestamp of the RTP header (32 bits). 1181 o All octets after the fixed 12-byte RTP header. (Note the SSRC 1182 field is skipped.) 1184 The FEC bit string is generated by applying the parity operation on 1185 the bit strings produced from the source packets. The FEC header is 1186 generated from the FEC bit string as follows: 1188 o The first (most significant) 2 bits in the FEC bit string, which 1189 contain the RTP version field, are skipped. The R and F bits in 1190 the FEC header are set to the appropriate value, i.e., it depends 1191 on the chosen format variant. As a consequence of overwriting the 1192 RTP version field with the R and F bits, this payload format only 1193 supports RTP version 2. 1195 o The next bit in the FEC bit string is written into the P recovery 1196 bit in the FEC header. 1198 o The next bit in the FEC bit string is written into the X recovery 1199 bit in the FEC header. 1201 o The next 4 bits of the FEC bit string are written into the CC 1202 recovery field in the FEC header. 1204 o The next bit is written into the M recovery bit in the FEC header. 1206 o The next 7 bits of the FEC bit string are written into the PT 1207 recovery field in the FEC header. 1209 o The next 16 bits are written into the length recovery field in the 1210 FEC header. 1212 o The next 32 bits of the FEC bit string are written into the TS 1213 recovery field in the FEC header. 1215 o The lowest Sequence Number of the source packets protected by this 1216 repair packet is written into the Sequence Number Base field in 1217 the FEC header. This needs to be repeated for each SSRC that has 1218 packets included in the source block. 1220 o Depending on the chosen FEC header variant, the mask(s) are set 1221 when F=0, or the L and D values are set when F=1. This needs to 1222 be repeated for each SSRC that has packets included in the source 1223 block. 1225 o The rest of the FEC bit string, which contains everything after 1226 the fixed 12-byte RTP header of the source packet, is written into 1227 the Repair "Payload" following the FEC header, where "Payload" 1228 refers to everything after the fixed 12-byte RTP header, including 1229 extensions, CSRC list, true payloads, and padding. 1231 If the lengths of the source packets are not equal, each shorter 1232 packet MUST be padded to the length of the longest packet by adding 1233 octet 0's at the end. 1235 Due to this possible padding and mandatory FEC header, a repair 1236 packet has a larger size than the source packets it protects. This 1237 may cause problems if the resulting repair packet size exceeds the 1238 Maximum Transmission Unit (MTU) size of the path over which the 1239 repair stream is sent. 1241 6.3. Source Packet Reconstruction 1243 This section describes the recovery procedures that are required to 1244 reconstruct the missing source packets. The recovery process has two 1245 steps. In the first step, the FEC decoder determines which source 1246 and repair packets should be used in order to recover a missing 1247 packet. In the second step, the decoder recovers the missing packet, 1248 which consists of an RTP header and RTP payload. 1250 The following describes the RECOMMENDED algorithms for the first and 1251 second steps. Based on the implementation, different algorithms MAY 1252 be adopted. However, the end result MUST be identical to the one 1253 produced by the algorithms described below. 1255 Note that the same algorithms are used by the 1-D parity codes, 1256 regardless of whether the FEC protection is applied over a column or 1257 a row. The 2-D parity codes, on the other hand, usually require 1258 multiple iterations of the procedures described here. This iterative 1259 decoding algorithm is further explained in Section 6.3.4. 1261 6.3.1. Associating the Source and Repair Packets 1263 Before associating source and repair packets, the receiver must know 1264 in which RTP sessions the source and repair respectively are being 1265 sent. After this is established by the reciever the first step is 1266 associating the source and repair packets. This association can be 1267 via flexible bitmasks, or fixed L and D offsets which can be in the 1268 FEC header or signaled in SDP in optional payload format parameters 1269 when L=D=0 in the FEC header. 1271 6.3.1.1. Using Bitmasks 1273 To use flexible bitmasks, the first two FEC header bits MUST have R=0 1274 and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source 1275 packets are protected by a FEC repair packet. If the bit i in the 1276 mask is set to 1, the source packet number N + i is protected by this 1277 FEC repair packet, where N is the sequence number base indicated in 1278 the FEC header. The most significant bit of the mask corresponds to 1279 i=0. The least signficant bit of the mask corresponds to i=14 in the 1280 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit mask. 1282 The bitmasks are able to represent arbitrary protection patterns, for 1283 example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase. 1285 6.3.1.2. Using L and D Offsets 1287 Denote the set of the source packets associated with repair packet p* 1288 by set T(p*). Note that in a source block whose size is L columns by 1289 D rows, set T includes D source packets plus one repair packet for 1290 the FEC protection applied over a column, and L source packets plus 1291 one repair packet for the FEC protection applied over a row. Recall 1292 that 1-D interleaved and non-interleaved FEC protection can fully 1293 recover the missing information if there is only one source packet 1294 missing per column or row in set T. If there are more than one 1295 source packets missing per column or row in set T, 1-D FEC protection 1296 may fail to recover all the missing information. 1298 When value of L is non-zero, the 8-bit fields indicate the offset of 1299 packets protected by an interleaved (D>0) or non-interleaved (D=0) 1300 FEC packet. Using a combination of interleaved and non-interleaved 1301 FEC repair packets can form 2-D protection patterns. 1303 Mathematically, for any received repair packet, p*, the sequence 1304 numbers of the source packets that are protected by this repair 1305 packet are determined as follows, where p*_snb is the sequence number 1306 base in the FEC header: 1308 When D = 0: 1309 p*_snb, p*_snb+1,..., p*_snb+L 1310 When D > 0: 1311 p*_snb, p*_snb+(Lx1), p*_snb+(Lx2),..., p*_snb+(LxD) 1313 6.3.1.3. Signaled in SDP 1315 If the endpoint relies entirely on out-of-band signaling (R=0, F=1, 1316 L=0, D=0 in the FEC header), then this information may be inferred 1317 from the media type parameters specified in the SDP description. 1318 Furthermore, the payload type field in the RTP header assists the 1319 receiver to distinguish an interleaved or non-interleaved FEC packet. 1321 Mathematically, for any received repair packet, p*, the sequence 1322 numbers of the source packets that are protected by this repair 1323 packet are determined as follows: 1325 p*_snb + i * X_1 (modulo 65536) 1327 where p*_snb denotes the value in the SN base field of p*'s FEC 1328 header, X_1 is set to L and 1 for the interleaved and non-interleaved 1329 FEC repair packets, respectively, and 1331 0 <= i < X_2 1333 where X_2 is set to D and L for the interleaved and non-interleaved 1334 FEC repair packets, respectively. 1336 6.3.2. Recovering the RTP Header 1338 For a given set T, the procedure for the recovery of the RTP header 1339 of the missing packet, whose sequence number is denoted by SEQNUM, is 1340 as follows: 1342 1. For each of the source packets that are successfully received in 1343 T, compute the 80-bit string by concatenating the first 64 bits 1344 of their RTP header and the unsigned network-ordered 16-bit 1345 representation of their length in bytes minus 12. 1347 2. For the repair packet in T, compute the FEC bit string from the 1348 first 80 bits of the FEC header. 1350 3. Calculate the recovered bit string as the XOR of the bit strings 1351 generated from all source packets in T and the FEC bit string 1352 generated from the repair packet in T. 1354 4. Create a new packet with the standard 12-byte RTP header and no 1355 payload. 1357 5. Set the version of the new packet to 2. Skip the first 2 bits 1358 in the recovered bit string. 1360 6. Set the Padding bit in the new packet to the next bit in the 1361 recovered bit string. 1363 7. Set the Extension bit in the new packet to the next bit in the 1364 recovered bit string. 1366 8. Set the CC field to the next 4 bits in the recovered bit string. 1368 9. Set the Marker bit in the new packet to the next bit in the 1369 recovered bit string. 1371 10. Set the Payload type in the new packet to the next 7 bits in the 1372 recovered bit string. 1374 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 1375 bits in the recovered bit string. 1377 12. Set the TS field in the new packet to the next 32 bits in the 1378 recovered bit string. 1380 13. Take the next 16 bits of the recovered bit string and set the 1381 new variable Y to whatever unsigned integer this represents 1382 (assuming network order). Convert Y to host order. Y 1383 represents the length of the new packet in bytes minus 12 (for 1384 the fixed RTP header), i.e., the sum of the lengths of all the 1385 following if present: the CSRC list, header extension, RTP 1386 payload and RTP padding. 1388 14. Set the SSRC of the new packet to the SSRC of the missing source 1389 RTP stream. 1391 This procedure recovers the header of an RTP packet up to (and 1392 including) the SSRC field. 1394 6.3.3. Recovering the RTP Payload 1396 Following the recovery of the RTP header, the procedure for the 1397 recovery of the RTP "payload" is as follows, where "payload" refers 1398 to everything following the fixed 12-byte RTP header, including 1399 extensions, CSRC list, true payload and padding. 1401 1. Append Y bytes to the new packet. 1403 2. For each of the source packets that are successfully received in 1404 T, compute the bit string from the Y octets of data starting with 1405 the 13th octet of the packet. If any of the bit strings 1406 generated from the source packets has a length shorter than Y, 1407 pad them to that length. The padding of octet 0 MUST be added at 1408 the end of the bit string. Note that the information of the 1409 first 8 octets are protected by the FEC header. 1411 3. For the repair packet in T, compute the FEC bit string from the 1412 repair packet payload, i.e., the Y octets of data following the 1413 FEC header. Note that the FEC header may be different sizes 1414 depending on the variant and bitmask size. 1416 4. Calculate the recovered bit string as the XOR of the bit strings 1417 generated from all source packets in T and the FEC bit string 1418 generated from the repair packet in T. 1420 5. Append the recovered bit string (Y octets) to the new packet 1421 generated in Section 6.3.2. 1423 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1425 In 2-D parity FEC protection, the sender generates both non- 1426 interleaved and interleaved FEC repair packets to combat with the 1427 mixed loss patterns (random and bursty). At the receiver side, these 1428 FEC packets are used iteratively to overcome the shortcomings of the 1429 1-D non-interleaved/interleaved FEC protection and improve the 1430 chances of full error recovery. 1432 The iterative decoding algorithm runs as follows: 1434 1. Set num_recovered_until_this_iteration to zero 1436 2. Set num_recovered_so_far to zero 1438 3. Recover as many source packets as possible by using the non- 1439 interleaved FEC repair packets as outlined in Section 6.3.2 and 1440 Section 6.3.3, and increase the value of num_recovered_so_far by 1441 the number of recovered source packets. 1443 4. Recover as many source packets as possible by using the 1444 interleaved FEC repair packets as outlined in Section 6.3.2 and 1445 Section 6.3.3, and increase the value of num_recovered_so_far by 1446 the number of recovered source packets. 1448 5. If num_recovered_so_far > num_recovered_until_this_iteration 1449 ---num_recovered_until_this_iteration = num_recovered_so_far 1450 ---Go to step 3 1451 Else 1452 ---Terminate 1454 The algorithm terminates either when all missing source packets are 1455 fully recovered or when there are still remaining missing source 1456 packets but the FEC repair packets are not able to recover any more 1457 source packets. For the example scenarios when the 2-D parity FEC 1458 protection fails full recovery, refer to Section 1.1.4. Upon 1459 termination, variable num_recovered_so_far has a value equal to the 1460 total number of recovered source packets. 1462 Example: 1464 Suppose that the receiver experienced the loss pattern sketched in 1465 Figure 16. 1467 +---+ +---+ +===+ 1468 X X | 3 | | 4 | |R_1| 1469 +---+ +---+ +===+ 1471 +---+ +---+ +---+ +---+ +===+ 1472 | 5 | | 6 | | 7 | | 8 | |R_2| 1473 +---+ +---+ +---+ +---+ +===+ 1475 +---+ +---+ +===+ 1476 | 9 | X X | 12| |R_3| 1477 +---+ +---+ +===+ 1479 +===+ +===+ +===+ +===+ 1480 |C_1| |C_2| |C_3| |C_4| 1481 +===+ +===+ +===+ +===+ 1483 Figure 16: Example loss pattern for the iterative decoding algorithm 1485 The receiver executes the iterative decoding algorithm and recovers 1486 source packets #1 and #11 in the first iteration. The resulting 1487 pattern is sketched in Figure 17. 1489 +---+ +---+ +---+ +===+ 1490 | 1 | X | 3 | | 4 | |R_1| 1491 +---+ +---+ +---+ +===+ 1493 +---+ +---+ +---+ +---+ +===+ 1494 | 5 | | 6 | | 7 | | 8 | |R_2| 1495 +---+ +---+ +---+ +---+ +===+ 1497 +---+ +---+ +---+ +===+ 1498 | 9 | X | 11| | 12| |R_3| 1499 +---+ +---+ +---+ +===+ 1501 +===+ +===+ +===+ +===+ 1502 |C_1| |C_2| |C_3| |C_4| 1503 +===+ +===+ +===+ +===+ 1505 Figure 17: The resulting pattern after the first iteration 1507 Since the if condition holds true, the receiver runs a new iteration. 1508 In the second iteration, source packets #2 and #10 are recovered, 1509 resulting in a full recovery as sketched in Figure 18. 1511 +---+ +---+ +---+ +---+ +===+ 1512 | 1 | | 2 | | 3 | | 4 | |R_1| 1513 +---+ +---+ +---+ +---+ +===+ 1515 +---+ +---+ +---+ +---+ +===+ 1516 | 5 | | 6 | | 7 | | 8 | |R_2| 1517 +---+ +---+ +---+ +---+ +===+ 1519 +---+ +---+ +---+ +---+ +===+ 1520 | 9 | | 10| | 11| | 12| |R_3| 1521 +---+ +---+ +---+ +---+ +===+ 1523 +===+ +===+ +===+ +===+ 1524 |C_1| |C_2| |C_3| |C_4| 1525 +===+ +===+ +===+ +===+ 1527 Figure 18: The resulting pattern after the second iteration 1529 7. Signaling Requirements 1531 Out-of-band signaling should be designed to enable the receiver to 1532 identify the RTP streams associated with source packets and repair 1533 packets, respectively. At a minimum, the signaling must be designed 1534 to allow the receiver to 1535 o Determine whether one or more source RTP streams will be sent. 1537 o Determine whether one or more repair RTP streams will be sent. 1539 o Associate the appropriate SSRC's to both source and repair 1540 streams. 1542 o Clearly identify which SSRC's are associated with each source 1543 block. 1545 o Clearly identify which repair packets correspond to which source 1546 blocks. 1548 o Make use of repair packets to recover source data associated with 1549 specific SSRC's. 1551 This section provides several Sesssion Description Protocol (SDP) 1552 examples to demonstrate how these requirements can be met. Note that 1553 other approaches to RTP stream identification SHOULD NOT be used for 1554 the purposes of FLEX FEC. 1556 7.1. SDP Examples 1558 This section provides two SDP [RFC4566] examples. The examples use 1559 the FEC grouping semantics defined in [RFC5956]. 1561 7.1.1. Example SDP for Flexible FEC Protection with in-band SSRC 1562 mapping 1564 In this example, we have one source video stream and one FEC repair 1565 stream. The source and repair streams are multiplexed on different 1566 SSRCs. The repair window is set to 200 ms. 1568 v=0 1569 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1570 s=FlexFEC minimal SDP signalling Example 1571 t=0 0 1572 m=video 30000 RTP/AVP 96 98 1573 c=IN IP4 143.163.151.157 1574 a=rtpmap:96 VP8/90000 1575 a=rtpmap:98 flexfec/90000 1576 a=fmtp:98; repair-window=200ms 1578 7.1.2. Example SDP for Flex FEC Protection with explicit signalling in 1579 the SDP 1581 This example shows one source video stream (ssrc:1234) and one FEC 1582 repair streams (ssrc:2345). One FEC group is formed with the 1583 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1584 are multiplexed on different SSRCs. The repair window is set to 200 1585 ms. 1587 v=0 1588 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1589 s=2-D Parity FEC with no in band signalling Example 1590 t=0 0 1591 m=video 30000 RTP/AVP 100 110 1592 c=IN IP4 233.252.0.1/127 1593 a=rtpmap:100 MP2T/90000 1594 a=rtpmap:110 flexfec/90000 1595 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 1596 a=ssrc:1234 1597 a=ssrc:2345 1598 a=ssrc-group:FEC-FR 1234 2345 1600 8. Congestion Control Considerations 1602 FEC is an effective approach to provide applications resiliency 1603 against packet losses. However, in networks where the congestion is 1604 a major contributor to the packet loss, the potential impacts of 1605 using FEC MUST be considered carefully before injecting the repair 1606 streams into the network. In particular, in bandwidth-limited 1607 networks, FEC repair streams may consume a significant part of the 1608 available bandwidth and consequently may congest the network. In 1609 such cases, the applications MUST NOT arbitrarily increase the amount 1610 of FEC protection since doing so may lead to a congestion collapse. 1611 If desired, stronger FEC protection MAY be applied only after the 1612 source rate has been reduced. 1614 In a network-friendly implementation, an application SHOULD NOT send/ 1615 receive FEC repair streams if it knows that sending/receiving those 1616 FEC repair streams would not help at all in recovering the missing 1617 packets. It is RECOMMENDED that the amount and type (row, column, or 1618 both) of FEC protection is adjusted dynamically based on the packet 1619 loss rate and burst loss length observed by the applications. 1621 In multicast scenarios, it may be difficult to optimize the FEC 1622 protection per receiver. If there is a large variation among the 1623 levels of FEC protection needed by different receivers, it is 1624 RECOMMENDED that the sender offers multiple repair streams with 1625 different levels of FEC protection and the receivers join the 1626 corresponding multicast sessions to receive the repair stream(s) that 1627 is best for them. 1629 9. Security Considerations 1631 RTP packets using the payload format defined in this specification 1632 are subject to the security considerations discussed in the RTP 1633 specification [RFC3550] and in any applicable RTP profile. The main 1634 security considerations for the RTP packet carrying the RTP payload 1635 format defined within this memo are confidentiality, integrity and 1636 source authenticity. Confidentiality is achieved by encrypting the 1637 RTP payload. Integrity of the RTP packets is achieved through a 1638 suitable cryptographic integrity protection mechanism. Such a 1639 cryptographic system may also allow the authentication of the source 1640 of the payload. A suitable security mechanism for this RTP payload 1641 format should provide confidentiality, integrity protection, and at 1642 least source authentication capable of determining if an RTP packet 1643 is from a member of the RTP session. 1645 Note that the appropriate mechanism to provide security to RTP and 1646 payloads following this memo may vary. It is dependent on the 1647 application, transport and signaling protocol employed. Therefore, a 1648 single mechanism is not sufficient, although if suitable, using the 1649 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1650 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1651 Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may 1652 exist. 1654 Given that FLEX FEC enables the protection of multiple source 1655 streams, there exists the possibility that multiple source buffers 1656 may be created that may not be used. In addition, the interaction 1657 between a FLEX FEC implementation and higher-layer applications may 1658 be affected by non-uniform processing requirements of the FEC scheme. 1660 10. IANA Considerations 1662 New media subtypes are subject to IANA registration. For the 1663 registration of the payload formats and their parameters introduced 1664 in this document, refer to Section 5. 1666 11. Acknowledgments 1668 Some parts of this document are borrowed from [RFC5109]. Thus, the 1669 author would like to thank the editor of [RFC5109] and those who 1670 contributed to [RFC5109]. 1672 Thanks to Stephen Botzko , Bernard Aboba , Rasmus Brandt , Brian 1673 Baldino , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus 1674 Westerlund for providing valuable feedback on earlier versions of 1675 this draft. 1677 12. References 1679 12.1. Normative References 1681 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1682 Requirement Levels", BCP 14, RFC 2119, 1683 DOI 10.17487/RFC2119, March 1997, 1684 . 1686 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1687 with Session Description Protocol (SDP)", RFC 3264, 1688 DOI 10.17487/RFC3264, June 2002, 1689 . 1691 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1692 Jacobson, "RTP: A Transport Protocol for Real-Time 1693 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1694 July 2003, . 1696 [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP 1697 Payload Formats", RFC 3555, DOI 10.17487/RFC3555, July 1698 2003, . 1700 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1701 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1702 July 2006, . 1704 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1705 the Session Description Protocol", RFC 5956, 1706 DOI 10.17487/RFC5956, September 2010, 1707 . 1709 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1710 Correction (FEC) Framework", RFC 6363, 1711 DOI 10.17487/RFC6363, October 2011, 1712 . 1714 [RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design 1715 Considerations for Protocol Extensions", RFC 6709, 1716 DOI 10.17487/RFC6709, September 2012, 1717 . 1719 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1720 Specifications and Registration Procedures", BCP 13, 1721 RFC 6838, DOI 10.17487/RFC6838, January 2013, 1722 . 1724 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1725 "Guidelines for Choosing RTP Control Protocol (RTCP) 1726 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1727 September 2013, . 1729 12.2. Informative References 1731 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1732 Streaming Protocol (RTSP)", RFC 2326, 1733 DOI 10.17487/RFC2326, April 1998, 1734 . 1736 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1737 for Generic Forward Error Correction", RFC 2733, 1738 DOI 10.17487/RFC2733, December 1999, 1739 . 1741 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1742 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1743 October 2000, . 1745 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1746 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1747 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1748 . 1750 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1751 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1752 December 2005, . 1754 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1755 "Extended RTP Profile for Real-time Transport Control 1756 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1757 DOI 10.17487/RFC4585, July 2006, 1758 . 1760 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 1761 Correction", RFC 5109, DOI 10.17487/RFC5109, December 1762 2007, . 1764 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1765 (TLS) Protocol Version 1.2", RFC 5246, 1766 DOI 10.17487/RFC5246, August 2008, 1767 . 1769 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 1770 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 1771 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 1772 DOI 10.17487/RFC7656, November 2015, 1773 . 1775 [SMPTE2022-1] 1776 SMPTE 2022-1-2007, "Forward Error Correction for Real-Time 1777 Video/Audio Transport over IP Networks", 2007. 1779 Authors' Addresses 1781 Mo Zanaty 1782 Cisco 1783 Raleigh, NC 1784 USA 1786 Email: mzanaty@cisco.com 1788 Varun Singh 1789 CALLSTATS I/O Oy 1790 Runeberginkatu 4c A 4 1791 Helsinki 00100 1792 Finland 1794 Email: varun.singh@iki.fi 1795 URI: http://www.callstats.io/ 1797 Ali Begen 1798 Networked Media 1799 Konya 1800 Turkey 1802 Email: ali.begen@networked.media 1803 Giridhar Mandyam 1804 Qualcomm Inc. 1805 5775 Morehouse Drive 1806 San Diego, CA 92121 1807 USA 1809 Phone: +1 858 651 7200 1810 Email: mandyam@qti.qualcomm.com