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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: July 17, 2019 callstats.io 6 A. Begen 7 Networked Media 8 G. Mandyam 9 Qualcomm Inc. 10 January 13, 2019 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-15 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 complexity. The RTP payload formats that are defined in this 30 document address scalability issues experienced with the earlier 31 specifications, and offer several improvements. Due to these 32 changes, the new payload formats are not backward compatible with 33 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 July 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-Dimensionsal (1-D) Non-interleaved (Row) FEC 74 Protection . . . . . . . . . . . . . . . . . . . . . 6 75 1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 7 76 1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 8 77 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection 10 78 1.1.5. FEC Overhead Considerations . . . . . . . . . . . . . 12 79 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 13 80 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 13 81 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 13 82 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 14 83 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 14 84 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 14 85 4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 15 86 4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 16 87 4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 17 88 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 25 89 5.1. Media Type Registration - Parity Codes . . . . . . . . . 25 90 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 25 91 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 26 92 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 28 93 5.1.4. Registration of application/flexfec . . . . . . . . . 29 94 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 31 95 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 31 96 5.2.2. Declarative Considerations . . . . . . . . . . . . . 32 98 6. Protection and Recovery Procedures - Parity Codes . . . . . . 32 99 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 32 100 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 33 101 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 34 102 6.3.1. Associating the Source and Repair Packets . . . . . . 35 103 6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 37 104 6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 38 105 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 106 Protection . . . . . . . . . . . . . . . . . . . . . 38 107 7. Signaling Requirements . . . . . . . . . . . . . . . . . . . 42 108 7.1. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 43 109 7.1.1. Example SDP for Flexible FEC Protection with in-band 110 SSRC mapping . . . . . . . . . . . . . . . . . . . . 43 111 7.1.2. Example SDP for Flexible FEC Protection with explicit 112 signalling in the SDP . . . . . . . . . . . . . . . . 44 113 7.2. On the Use of the RTP Stream Identifier Source 114 Description . . . . . . . . . . . . . . . . . . . . . . . 44 115 8. Congestion Control Considerations . . . . . . . . . . . . . . 45 116 9. Security Considerations . . . . . . . . . . . . . . . . . . . 45 117 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 118 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 46 119 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 46 120 12.1. Normative References . . . . . . . . . . . . . . . . . . 46 121 12.2. Informative References . . . . . . . . . . . . . . . . . 47 122 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49 124 1. Introduction 126 This document defines new RTP payload formats for the Forward Error 127 Correction (FEC) that is generated by the non-interleaved and 128 interleaved parity codes from a source media encapsulated in RTP 129 [RFC3550]. The type of the source media protected by these parity 130 codes can be audio, video, text or application. The FEC data are 131 generated according to the media type parameters, which are 132 communicated out-of-band (e.g., in SDP). Furthermore, the 133 associations or relationships between the source and repair RTP 134 streams may be communicated in-band or out-of-band. The in-band 135 mechanism is advantageous when the endpoint is adapting the FEC 136 parameters. The out-of-band mechanism may be preferable when the FEC 137 parameters are fixed. While this document fully defines the use of 138 FEC to protect RTP streams, it also leverages several definitions 139 along with the basic source/repair header description from [RFC6363] 140 in their application to the parity codes defined here. 142 The Redundancy RTP Stream [RFC7656] repair packets proposed in this 143 document protect the Source RTP Stream packets that belong to the 144 same RTP session. 146 The RTP payload formats that are defined in this document address the 147 scalability issues experienced with the formats defined in earlier 148 specifications including [RFC2733], [RFC5109] and [SMPTE2022-1]. 150 1.1. Parity Codes 152 Both the non-interleaved and interleaved parity codes use the 153 eXclusive OR (XOR) operation to generate the repair packets. The 154 following steps take place: 156 1. The sender determines a set of source packets to be protected by 157 FEC based on the media type parameters. 159 2. The sender applies the XOR operation on the source packets to 160 generate the required number of repair packets. 162 3. The sender sends the repair packet(s) along with the source 163 packets, in different RTP streams, to the receiver(s). The 164 repair packets may be sent proactively or on-demand based on RTCP 165 feedback messages such as NACK [RFC4585]. 167 At the receiver side, if all of the source packets are successfully 168 received, there is no need for FEC recovery and the repair packets 169 are discarded. However, if there are missing source packets, the 170 repair packets can be used to recover the missing information. 171 Figure 1 and Figure 2 describe example block diagrams for the 172 systematic parity FEC encoder and decoder, respectively. 174 +------------+ 176 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 178 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 180 | Encoder | 182 | (Sender) | --> +==+ +==+ 184 +------------+ +==+ +==+ 186 Source Packet: +--+ Repair Packet: +==+ 188 +--+ +==+ 190 Figure 1: Block diagram for systematic parity FEC encoder 192 +------------+ 194 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 196 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 198 | Decoder | 200 +==+ +==+ --> | (Receiver) | 202 +==+ +==+ +------------+ 204 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 206 +--+ +==+ 208 Figure 2: Block diagram for systematic parity FEC decoder 210 In Figure 2, it is clear that the FEC repair packets have to be 211 received by the endpoint within a certain amount of time for the FEC 212 recovery process to be useful. The repair window is defined as the 213 time that spans a FEC block, which consists of the source packets and 214 the corresponding repair packets. At the receiver side, the FEC 215 decoder SHOULD buffer source and repair packets at least for the 216 duration of the repair window, to allow all the repair packets to 217 arrive. The FEC decoder can start decoding the already received 218 packets sooner; however, it should not register a FEC decoding 219 failure until it waits at least for the duration of the repair 220 window. 222 1.1.1. One-Dimensionsal (1-D) Non-interleaved (Row) FEC Protection 224 Consider a group of D x L source packets that have sequence numbers 225 starting from 1 running to D x L, and a repair packet is generated by 226 applying the XOR operation to every L consecutive packets as sketched 227 in Figure 3. This process is referred to as 1-D non-interleaved FEC 228 protection. As a result of this process, D repair packets are 229 generated, which are referred to as non-interleaved (or row) FEC 230 repair packets. In general D and L represent values that describe 231 how packets are grouped together from a depth and length perspective 232 (respectively) when interleaving all D x L source packets. 234 +--------------------------------------------------+ --- +===+ 236 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 238 +--------------------------------------------------+ --- +===+ 240 +--------------------------------------------------+ --- +===+ 242 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 244 +--------------------------------------------------+ --- +===+ 246 . . . . . . 248 . . . . . . 250 . . . . . . 252 +--------------------------------------------------+ --- +===+ 254 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 256 +--------------------------------------------------+ --- +===+ 258 Figure 3: Generating non-interleaved (row) FEC repair packets 260 1.1.2. 1-D Interleaved (Column) FEC Protection 262 If the XOR operation is applied to the group of the source packets 263 whose sequence numbers are L apart from each other, as sketched in 264 Figure 4. In this case the endpoint generates L repair packets. 265 This process is referred to as 1-D interleaved FEC protection, and 266 the resulting L repair packets are referred to as interleaved (or 267 column) FEC repair packets. 269 +-------------+ +-------------+ +-------------+ +-------+ 271 | S_1 | | S_2 | | S3 | ... | S_L | 273 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 275 | . | | . | | | | | 277 | . | | . | | | | | 279 | . | | . | | | | | 281 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 283 +-------------+ +-------------+ +-------------+ +-------+ 285 + + + + 287 ------------- ------------- ------------- ------- 289 | XOR | | XOR | | XOR | ... | XOR | 291 ------------- ------------- ------------- ------- 293 = = = = 295 +===+ +===+ +===+ +===+ 297 |C_1| |C_2| |C_3| ... |C_L| 299 +===+ +===+ +===+ +===+ 301 Figure 4: Generating interleaved (column) FEC repair packets 303 1.1.3. Use Cases for 1-D FEC Protection 305 A sender may generate one non-interleaved repair packet out of L 306 consecutive source packets or one interleaved repair packet out of D 307 non-consecutive source packets. Regardless of whether the repair 308 packet is a non-interleaved or an interleaved one, it can provide a 309 full recovery of the missing information if there is only one packet 310 missing among the corresponding source packets. This implies that 311 1-D non-interleaved FEC protection performs better when the source 312 packets are randomly lost. However, if the packet losses occur in 313 bursts, 1-D interleaved FEC protection performs better provided that 314 L is chosen large enough, i.e., L-packet duration is not shorter than 315 the observed burst duration. If the sender generates non-interleaved 316 FEC repair packets and a burst loss hits the source packets, the 317 repair operation fails. This is illustrated in Figure 5. 319 +---+ +---+ +===+ 321 | 1 | X X | 4 | |R_1| 323 +---+ +---+ +===+ 325 +---+ +---+ +---+ +---+ +===+ 327 | 5 | | 6 | | 7 | | 8 | |R_2| 329 +---+ +---+ +---+ +---+ +===+ 331 +---+ +---+ +---+ +---+ +===+ 333 | 9 | | 10| | 11| | 12| |R_3| 335 +---+ +---+ +---+ +---+ +===+ 337 Figure 5: Example scenario where 1-D non-interleaved FEC protection 338 fails error recovery (Burst Loss) 340 The sender may generate interleaved FEC repair packets to combat with 341 the bursty packet losses. However, two or more random packet losses 342 may hit the source and repair packets in the same column. In that 343 case, the repair operation fails as well. This is illustrated in 344 Figure 6. Note that it is possible that two burst losses may occur 345 back-to-back, in which case interleaved FEC repair packets may still 346 fail to recover the lost data. 348 +---+ +---+ +---+ 350 | 1 | X | 3 | | 4 | 352 +---+ +---+ +---+ 354 +---+ +---+ +---+ 356 | 5 | X | 7 | | 8 | 358 +---+ +---+ +---+ 360 +---+ +---+ +---+ +---+ 362 | 9 | | 10| | 11| | 12| 364 +---+ +---+ +---+ +---+ 366 +===+ +===+ +===+ +===+ 368 |C_1| |C_2| |C_3| |C_4| 370 +===+ +===+ +===+ +===+ 372 Figure 6: Example scenario where 1-D interleaved FEC protection fails 373 error recovery (Periodic Loss) 375 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection 377 In networks where the source packets are lost both randomly and in 378 bursts, the sender ought to generate both non-interleaved and 379 interleaved FEC repair packets. This type of FEC protection is known 380 as 2-D parity FEC protection. At the expense of generating more FEC 381 repair packets, thus increasing the FEC overhead, 2-D FEC provides 382 superior protection against mixed loss patterns. However, it is 383 still possible for 2-D parity FEC protection to fail to recover all 384 of the lost source packets if a particular loss pattern occurs. An 385 example scenario is illustrated in Figure 7. 387 +---+ +---+ +===+ 389 | 1 | X X | 4 | |R_1| 391 +---+ +---+ +===+ 393 +---+ +---+ +---+ +---+ +===+ 395 | 5 | | 6 | | 7 | | 8 | |R_2| 397 +---+ +---+ +---+ +---+ +===+ 399 +---+ +---+ +===+ 401 | 9 | X X | 12| |R_3| 403 +---+ +---+ +===+ 405 +===+ +===+ +===+ +===+ 407 |C_1| |C_2| |C_3| |C_4| 409 +===+ +===+ +===+ +===+ 411 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 412 error recovery 414 2-D parity FEC protection also fails when at least two rows are 415 missing a source and the FEC packet and the missing source packets 416 (in at least two rows) are aligned in the same column. An example 417 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 418 protection cannot repair all missing source packets when at least two 419 columns are missing a source and the FEC packet and the missing 420 source packets (in at least two columns) are aligned in the same row. 422 +---+ +---+ +---+ 424 | 1 | | 2 | X | 4 | X 426 +---+ +---+ +---+ 428 +---+ +---+ +---+ +---+ +===+ 430 | 5 | | 6 | | 7 | | 8 | |R_2| 432 +---+ +---+ +---+ +---+ +===+ 434 +---+ +---+ +---+ 436 | 9 | | 10| X | 12| X 438 +---+ +---+ +---+ 440 +===+ +===+ +===+ +===+ 442 |C_1| |C_2| |C_3| |C_4| 444 +===+ +===+ +===+ +===+ 446 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 447 error recovery 449 1.1.5. FEC Overhead Considerations 451 The overhead is defined as the ratio of the number of bytes belonging 452 to the repair packets to the number of bytes belonging to the 453 protected source packets. 455 Generally, repair packets are larger in size compared to the source 456 packets. Also, not all the source packets are necessarily equal in 457 size. However, assuming that each repair packet carries an equal 458 number of bytes as carried by a source packet, the overhead for 459 different FEC protection methods can be computed as follows: 461 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 462 o 1-D Interleaved FEC Protection: Overhead = 1/D 464 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 466 where L and D are the number of columns and rows in the source block, 467 respectively. 469 2. Requirements Notation 471 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 472 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 473 "OPTIONAL" in this document are to be interpreted as described in BCP 474 14 [RFC2119] [RFC8174] when, and only when, they appear in all 475 capitals, as shown here. 477 3. Definitions and Notations 479 3.1. Definitions 481 This document uses a number of definitions from [RFC6363]. 483 1-D Non-interleaved Row FEC: A protection scheme that operates on 484 consecutive source packets in the source block, able to recover a 485 single lost source packet per row of the source block. 487 1-D Interleaved Column FEC: A protection scheme that operates on 488 interleaved source packets in the source block, able to recover a 489 single lost source packet per column of the source block. 491 2-D FEC: A protection scheme that combines row and column FEC. 493 Source Block: A set of source packets that are protected by a set 494 of 1-D or 2-D FEC repair packets. 496 FEC Block: A source block and its corresponding FEC repair 497 packets. 499 Repair Window: The time that spans a FEC block, which consists of 500 the source packets and the corresponding FEC repair packets. 502 XOR Parity Codes: A FEC code which uses the eXclusive OR (XOR) 503 parity operation to encode a set of source packets to form a FEC 504 repair packet. 506 3.2. Notations 508 L: Number of columns of the source block (length of each row). 510 D: Number of rows of the source block (depth of each column). 512 bitmask: A 15-bit, 46-bit, or 110-bit mask indicating which source 513 packets are protected by a FEC repair packet. If the bit i in the 514 mask is set to 1, the source packet number N + i is protected by 515 this FEC repair packet, where N is the sequence number base 516 indicated in the FEC repair packet. The most significant bit of 517 the mask corresponds to i=0. The least signficant bit of the mask 518 corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit mask, 519 or i=109 in the 110-bit mask. 521 4. Packet Formats 523 This section describes the formats of the source packets and defines 524 the formats of the FEC repair packets. 526 4.1. Source Packets 528 The source packets contain the information that identifies the source 529 block and the position within the source block occupied by the 530 packet. Since the source packets that are carried within an RTP 531 stream already contain unique sequence numbers in their RTP headers 532 [RFC3550], the source packets can be identified in a straightforward 533 manner and there is no need to append additional field(s). The 534 primary advantage of not modifying the source packets in any way is 535 that it provides backward compatibility for the receivers that do not 536 support FEC at all. In multicast scenarios, this backward 537 compatibility becomes quite useful as it allows the non-FEC-capable 538 and FEC-capable receivers to receive and interpret the same source 539 packets sent in the same multicast session. 541 The source packets are transmitted as usual without altering them. 542 They are used along with the FEC repair packets to recover any 543 missing source packets, making this scheme a systematic code. 545 The source packets are full RTP packets with optional CSRC list, RTP 546 header extension, and padding. If any of these optional elements are 547 present in the source RTP packet, and that source packet is lost, 548 they are recovered by the FEC repair operation, which recovers the 549 full source RTP packet including these optional elements. 551 4.2. FEC Repair Packets 553 The FEC repair packets will contain information that identifies the 554 source block they pertain to and the relationship between the 555 contained repair packets and the original source block. For this 556 purpose, the RTP header of the repair packets is used, as well as 557 another header within the RTP payload, called the FEC header, as 558 shown in Figure 9. 560 Note that all the source stream packets that are protected by a 561 particular FEC packet need to be in the same RTP session. 563 +------------------------------+ 565 | IP Header | 567 +------------------------------+ 569 | Transport Header | 571 +------------------------------+ 573 | RTP Header | 575 +------------------------------+ ---+ 577 | FEC Header | | 579 +------------------------------+ | RTP Payload 581 | Repair "Payload" | | 583 +------------------------------+ ---+ 585 Figure 9: Format of FEC repair packets 587 The Repair "Payload", which follows the FEC Header, includes repair 588 of everything following the fixed 12-byte RTP header of each source 589 packet, including any CSRC identifier list and header extensions if 590 present. 592 4.2.1. RTP Header of FEC Repair Packets 594 The RTP header is formatted according to [RFC3550] with some further 595 clarifications listed below: 597 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this 598 specification requires all source RTP packets and all FEC repair 599 packets to use RTP version 2. The reason for this restriction is 600 the first 2 bits of the FEC header contain other information (R 601 and F bits) rather than recovering the RTP version field. 603 Padding (P) bit: Source packets can have optional RTP padding, 604 which can be recovered. FEC repair packets can have optional RTP 605 padding, which is independent of the RTP padding of the source 606 pakcets. 608 Extension (X) bit: Source packets can have optional RTP header 609 extensions, which can be recovered. FEC repair packets can have 610 optional RTP header extensions, which are independent of the RTP 611 header extensions of the source packets. 613 CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: 614 Source packets can have an optional CSRC list and count, which can 615 be recovered. FEC repair packets MUST use the CSRC list and count 616 to specify the SSRC(s) of the source RTP stream(s) protected by 617 this FEC repair packet. 619 Marker (M) bit: This bit is not used for this payload type, and 620 SHALL be set to 0 by senders, and SHALL be ignored by receivers. 622 Payload Type: The (dynamic) payload type for the FEC repair 623 packets is determined through out-of-band means. Note that this 624 document registers new payload formats for the repair packets 625 (Refer to Section 5 for details). According to [RFC3550], an RTP 626 receiver that cannot recognize a payload type must discard it. 627 This provides backward compatibility. If a non-FEC-capable 628 receiver receives a repair packet, it will not recognize the 629 payload type, and hence, will discard the repair packet. 631 Sequence Number (SN): The sequence number follows the standard 632 definition provided in [RFC3550]. definition. Therefore it must 633 be one higher than the sequence number in the previously 634 transmitted repair packet, and the initial value of the sequence 635 number should be random (i.e. unpredictable). 637 Timestamp (TS): The timestamp SHALL be set to a time corresponding 638 to the repair packet's transmission time. Note that the timestamp 639 value has no use in the actual FEC protection process and is 640 usually useful for jitter calculations. 642 Synchronization Source (SSRC): The SSRC value for each repair 643 stream SHALL be randomly assigned as per the guidelines provided 644 in Section 8 of [RFC3550]. This allows the sender to multiplex 645 the source and repair RTP streams in the same RTP session, or 646 multiplex multiple repair streams in an RTP session. The repair 647 streams' SSRC's CNAME SHOULD be identical to the CNAME of the 648 source RTP stream(s) that this repair stream protects. An FEC 649 stream that protects multiple source RTP streams with different 650 CNAME's uses the CNAME associated with the entity generating the 651 FEC stream or the CNAME of the entity on whose behalf it performs 652 the protection operation. In cases when the repair stream covers 653 packets from multiple source RTP streams with different CNAME 654 values, any of these CNAME values MAY be used. 656 In some networks, the RTP Source, which produces the source 657 packets and the FEC Source, which generates the repair packets 658 from the source packets may not be the same host. In such 659 scenarios, using the same CNAME for the source and repair RTP 660 streams means that the RTP Source and the FEC Source will share 661 the same CNAME (for this specific source-repair stream 662 association). A common CNAME may be produced based on an 663 algorithm that is known both to the RTP and FEC Source [RFC7022]. 664 This usage is compliant with [RFC3550]. 666 Note that due to the randomness of the SSRC assignments, there is 667 a possibility of SSRC collision. In such cases, the collisions 668 must be resolved as described in [RFC3550]. 670 4.2.2. FEC Header of FEC Repair Packets 672 The format of the FEC header has 3 variants, depending on the values 673 in the first 2 bits (R and F bits) as shown in Figure 10. Two of 674 these variants are meant to describe different methods for deriving 675 the source data from a source packet for a repair packet. This 676 allows for customizing the FEC method to allow for robustness against 677 different levels of burst errors and random packet losses. The third 678 variant is for a straight retransmission of the source packet. 680 0 1 2 3 682 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 1 684 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 686 |R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... | 688 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 690 | | 692 | ...varies depending on R/F... | 694 | | 696 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 698 : Repair "Payload" follows FEC Header : 700 : : 702 Figure 10: FEC Header 704 The Repair "Payload", which follows the FEC Header, includes repair 705 of everything following the fixed 12-byte RTP header of each source 706 packet, including any CSRC identifier list and header extensions if 707 present. 709 +---+---+----------------------------------------------------------+ 711 | R | F | FEC Header variant | 713 +---+---+----------------------------------------------------------+ 715 | 0 | 0 | Flexible FEC Mask fields indicate source packets | 717 | 0 | 1 | Fixed FEC L/D (cols/rows) fields indicate source packets | 719 | 1 | 0 | Retransmission of a single source packet | 721 | 1 | 1 | Invalid, MUST NOT send, MUST ignore if received | 723 +---+---+----------------------------------------------------------+ 725 Figure 11: R and F bit values for FEC Header variants 727 The first variant, when R=0 and F=0, has a mask to signal protected 728 source packets, as shown in Figure 12. 730 The second variant, when R=0 and F=1, has a number of columns (L) and 731 rows (D) to signal protected source packets, as shown in Figure 13. 733 The final variant, when R=1 and F=0, is a retransmission format as 734 shown in Figure 15. 736 No variant uses R=1 and F=1, which is invalid, and MUST NOT be sent 737 by senders, and MUST be ignored by receivers. 739 The FEC header for all variants consists of the following common 740 fields: 742 o The R bit MUST be set to 1 to indicate a retransmission packet, 743 and MUST be set to 0 for FEC repair packets. 745 o The F bit indicates the type of FEC repair packets, as shown in 746 Figure 11, when the R bit is 0. The F bit MUST be set to 0 when 747 the R bit is 1 for retransmission packets. 749 o The P, X, CC, M and PT recovery fields are used to determine the 750 corresponding fields of the recovered packets. 752 4.2.2.1. FEC Header with Flexible Mask 754 When R=0 and F=0, the FEC Header includes flexible mask fields. 756 0 1 2 3 758 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 1 760 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 762 |0|0|P|X| CC |M| PT recovery | length recovery | 764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 766 | TS recovery | 768 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 770 | SN base_i |k| Mask [0-14] | 772 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 774 |k| Mask [15-45] (optional) | 776 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 778 | Mask [46-109] (optional) | 780 | | 782 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 784 | ... next SN base and Mask for CSRC_i in CSRC list ... | 786 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 788 : Repair "Payload" follows FEC Header : 790 : : 792 Figure 12: FEC Header for F=0 794 o The Length recovery (16 bits) field is used to determine the 795 length of the recovered packets. This length includes all octets 796 following the fixed 12-byte RTP header of source packets, 797 including CSRC list and optional header extension(s) if present. 798 It excludes the fixed 12-byte RTP header of source packets. 800 o The TS recovery (32 bits) field is used to determine the timestamp 801 of the recovered packets. 803 o The CSRC_i (32 bits) field in the RTP Header (not FEC Header) 804 describes the SSRC of the source packets protected by this 805 particular FEC packet. If a FEC packet protects multiple SSRCs 806 (indicated by the CSRC Count > 1 in the RTP Header), there will be 807 multiple blocks of data containing the SN base and Mask fields. 809 o The SN base_i (16 bits) field indicates the lowest sequence 810 number, taking wrap around into account, of the source packets for 811 a particular SSRC (indicated in CSRC_i) protected by this repair 812 packet. 814 o The Mask fields indicate a bitmask of which source packets are 815 protected by this FEC repair packet, where bit j of the mask set 816 to 1 indicates that the source packet with sequence number (SN 817 base_i + j) is protected by this FEC repair packet, where j=0 is 818 the most significant bit in the mask. 820 o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110 821 bits. k=1 denotes that another mask follows, and k=0 denotes that 822 it is the last block of mask. 824 o The Repair "Payload", which follows the FEC Header, includes 825 repair of everything following the fixed 12-byte RTP header of 826 each source packet, including any CSRC identifier list and header 827 extensions if present. 829 4.2.2.2. FEC Header with Fixed L Columns and D Rows 831 When R=0 and F=1, the FEC Header includes L and D fields for fixed 832 columns and rows. The other fields are the same as the prior 833 section. As in the previous section, the CSRC_i (32 bits) field in 834 the RTP Header (not FEC Header) describes the SSRC of the source 835 packets protected by this particular FEC packet. If there are 836 multiple SSRC's protected by the FEC packet, then there will be 837 multiple blocks of data containing an SN base along with L and D 838 fields. 840 0 1 2 3 842 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 1 844 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 846 |0|1|P|X| CC |M| PT recovery | length recovery | 848 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 850 | TS recovery | 852 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 854 | SN base_i | L (columns) | D (rows) | 856 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 858 | ... next SN base and L/D for CSRC_i in CSRC list ... | 860 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 862 : Repair "Payload" follows FEC Header : 864 : : 866 Figure 13: FEC Header for F=1 868 Consequently, the following conditions occur for L and D values: 870 If L=0, D=0, use the optional payload format parameters for L and D. 872 If L>0, D=0, indicates Row FEC, and no column FEC will follow. 874 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L. 876 If L>0, D=1, indicates Row FEC, and column FEC will follow. 878 Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L will be 880 produced for each row. 882 Then FEC = SN, SN+L, SN+2L, ..., SN+(D-1)L will be produced 884 for each column. 886 After all row FEC's have been sent, then the column FEC's 888 will be sent. 890 If L>0, D>1, indicates column FEC of every L packet 892 in a group of D packets starting at SN base. 894 Hence, FEC = SN+(Lx0), SN+(Lx1), ... , SN+(LxD). 896 Figure 14: Interpreting the L and D field values 898 It should be noted that the flexible mask-based approach may be 899 inefficient for protecting a large number of source packets, or 900 impossible to signal if larger than the largest mask size. In such 901 cases, the fixed columns and rows variant may be more useful. 903 4.2.2.3. FEC Header for Retransmissions 905 When R=1 and F=0, the FEC packet is a retransmission of a single 906 source packet. Note that the layout of this retransmission packet is 907 different from other FEC repair packets. The sequence number (SN 908 base_i) replaces the length recovery in the FEC header, since the 909 length is already known for a single packet. There are no L, D or 910 Mask fields, since only a single packet is retransmitted, identified 911 by the sequence number in the FEC header. The source packet SSRC is 912 included in the FEC header for retransmissions, not in the RTP header 913 CSRC list as in the FEC header variants with R=0. When performing 914 retransmissions, a single repair packet stream (SSRC) MAY be used for 915 retransmitting packets from multiple source packet streams (SSRCs), 916 as well as transmitting FEC repair packets that protect multiple 917 source packet streams (SSRCs). 919 This FEC header layout is identical to the source RTP (version 2) 920 packet, starting with its RTP header, where the retransmission 921 "payload" is everything following the fixed 12-byte RTP header of the 922 source packet, including CSRC list and extensions if present. 923 Therefore, the only operation needed for sending retransmissions is 924 to prepend a new RTP header to the source packet. 926 0 1 2 3 928 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 1 930 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 932 |1|0| P|X| CC |M| Payload Type| Sequence Number | 934 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 936 | Timestamp | 938 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 940 | SSRC | 942 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 944 : Retransmission "Payload" follows FEC Header : 946 : : 948 Figure 15: FEC Header for Retransmission 950 5. Payload Format Parameters 952 This section provides the media subtype registration for the non- 953 interleaved and interleaved parity FEC. The parameters that are 954 required to configure the FEC encoding and decoding operations are 955 also defined in this section. If no specific FEC code is specified 956 in the subtype, then the FEC code defaults to the parity code defined 957 in this specification. 959 5.1. Media Type Registration - Parity Codes 961 This registration is done using the template defined in [RFC6838] and 962 following the guidance provided in [RFC4855] along with [RFC4856]. 964 Note to the RFC Editor: In the following sections, please replace 965 "XXXX" with the number of this document prior to publication as an 966 RFC. 968 5.1.1. Registration of audio/flexfec 970 Type name: audio 972 Subtype name: flexfec 974 Required parameters: 976 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 977 than 1000 Hz to provide sufficient resolution to RTCP operations. 978 However, it is RECOMMENDED to select the rate that matches the 979 rate of the protected source RTP stream. 981 o repair-window: The time that spans the source packets and the 982 corresponding repair packets. The size of the repair window is 983 specified in microseconds. 985 Optional parameters: 987 o L: indicates the number of columns of the source block that are 988 protected by this FEC block and it applies to all the source 989 SSRCs. L is a positive integer. 991 o D: indicates the number of rows of the source block that are 992 protected by this FEC block and it applies to all the source 993 SSRCs. D is a positive integer. 995 o ToP: indicates the type of protection applied by the sender: 0 for 996 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 997 protection, 2 for 2-D parity FEC protection, and 3 for 998 retransmission. There can only be one value listed for ToP. The 999 absence of the ToP field means that all protection types are 1000 allowed. 1002 Note that both L and D in the optional parameters should follow the 1003 value pairings stated in Section 4.2.2.2 if included. 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: Varun 1023 Singh and IETF Audio/Video Transport Payloads Working 1024 Group (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 5.1.2. Registration of video/flexfec 1038 Type name: video 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. 1047 However, it is RECOMMENDED to select the rate that matches the 1048 rate of the protected source RTP stream. 1050 o repair-window: The time that spans the source packets and the 1051 corresponding repair packets. The size of the repair window is 1052 specified in microseconds. 1054 Optional parameters: 1056 o L: indicates the number of columns of the source block that are 1057 protected by this FEC block and it applies to all the source 1058 SSRCs. L is a positive integer. 1060 o D: indicates the number of rows of the source block that are 1061 protected by this FEC block and it applies to all the source 1062 SSRCs. D is a positive integer. 1064 o ToP: indicates the type of protection applied by the sender: 0 for 1065 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 1066 protection, 2 for 2-D parity FEC protection, and 3 for 1067 retransmission. There can only be one value listed for ToP. The 1068 absence of the ToP field means that all protection types are 1069 allowed. 1071 Note that both L and D in the optional parameters should follow the 1072 value pairings stated in Section 4.2.2.2 if included. 1074 Encoding considerations: This media type is framed (See Section 4.8 1075 in the template document [RFC6838]) and contains binary data. 1077 Security considerations: See Section 9 of [RFCXXXX]. 1079 Interoperability considerations: None. 1081 Published specification: [RFCXXXX]. 1083 Applications that use this media type: Multimedia applications that 1084 want to improve resiliency against packet loss by sending redundant 1085 data in addition to the source media. 1087 Fragment identifier considerations: None. 1089 Additional information: None. 1091 Person & email address to contact for further information: Varun 1092 Singh and IETF Audio/Video Transport Payloads Working 1093 Group (or it's successor as delegated by the IESG). 1095 Intended usage: COMMON. 1097 Restriction on usage: This media type depends on RTP framing, and 1098 hence, is only defined for transport via RTP [RFC3550]. 1100 Author: Varun Singh . 1102 Change controller: IETF Audio/Video Transport Payloads Working Group 1103 delegated from the IESG (or it's successor as delegated by the IESG). 1105 5.1.3. Registration of text/flexfec 1107 Type name: text 1109 Subtype name: flexfec 1111 Required parameters: 1113 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 1114 than 1000 Hz to provide sufficient resolution to RTCP operations. 1115 However, it is RECOMMENDED to select the rate that matches the 1116 rate of the protected source RTP stream. 1118 o repair-window: The time that spans the source packets and the 1119 corresponding repair packets. The size of the repair window is 1120 specified in microseconds. 1122 Optional parameters: 1124 o L: indicates the number of columns of the source block that are 1125 protected by this FEC block and it applies to all the source 1126 SSRCs. L is a positive integer. 1128 o D: indicates the number of rows of the source block that are 1129 protected by this FEC block and it applies to all the source 1130 SSRCs. D is a positive integer. 1132 o ToP: indicates the type of protection applied by the sender: 0 for 1133 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 1134 protection, 2 for 2-D parity FEC protection, and 3 for 1135 retransmission. There can only be one value listed for ToP. The 1136 absence of the ToP field means that all protection types are 1137 allowed. 1139 Note that both L and D in the optional parameters should follow the 1140 value pairings stated in Section 4.2.2.2 if included. 1142 Encoding considerations: This media type is framed (See Section 4.8 1143 in the template document [RFC6838]) and contains binary data. 1145 Security considerations: See Section 9 of [RFCXXXX]. 1147 Interoperability considerations: None. 1149 Published specification: [RFCXXXX]. 1151 Applications that use this media type: Multimedia applications that 1152 want to improve resiliency against packet loss by sending redundant 1153 data in addition to the source media. 1155 Fragment identifier considerations: None. 1157 Additional information: None. 1159 Person & email address to contact for further information: Varun 1160 Singh and IETF Audio/Video Transport Payloads Working 1161 Group (or it's successor as delegated by the IESG). 1163 Intended usage: COMMON. 1165 Restriction on usage: This media type depends on RTP framing, and 1166 hence, is only defined for transport via RTP [RFC3550]. 1168 Author: Varun Singh . 1170 Change controller: IETF Audio/Video Transport Payloads Working Group 1171 delegated from the IESG (or it's successor as delegated by the IESG). 1173 5.1.4. Registration of application/flexfec 1175 Type name: application 1177 Subtype name: flexfec 1179 Required parameters: 1181 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 1182 than 1000 Hz to provide sufficient resolution to RTCP operations. 1183 However, it is RECOMMENDED to select the rate that matches the 1184 rate of the protected source RTP stream. 1186 o repair-window: The time that spans the source packets and the 1187 corresponding repair packets. The size of the repair window is 1188 specified in microseconds. 1190 Optional parameters: 1192 o L: indicates the number of columns of the source block that are 1193 protected by this FEC block and it applies to all the source 1194 SSRCs. L is a positive integer. 1196 o D: indicates the number of rows of the source block that are 1197 protected by this FEC block and it applies to all the source 1198 SSRCs. D is a positive integer. 1200 o ToP: indicates the type of protection applied by the sender: 0 for 1201 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 1202 protection, 2 for 2-D parity FEC protection, and 3 for 1203 retransmission. There can only be one value listed for ToP. The 1204 absence of the ToP field means that all protection types are 1205 allowed. 1207 Note that both L and D in the optional parameters should follow the 1208 value pairings stated in Section 4.2.2.2 if included. 1210 Encoding considerations: This media type is framed (See Section 4.8 1211 in the template document [RFC6838]) and contains binary data. 1213 Security considerations: See Section 9 of [RFCXXXX]. 1215 Interoperability considerations: None. 1217 Published specification: [RFCXXXX]. 1219 Applications that use this media type: Multimedia applications that 1220 want to improve resiliency against packet loss by sending redundant 1221 data in addition to the source media. 1223 Fragment identifier considerations: None. 1225 Additional information: None. 1227 Person & email address to contact for further information: Varun 1228 Singh and IETF Audio/Video Transport Payloads Working 1229 Group (or it's successor as delegated by the IESG). 1231 Intended usage: COMMON. 1233 Restriction on usage: This media type depends on RTP framing, and 1234 hence, is only defined for transport via RTP [RFC3550]. 1236 Author: Varun Singh . 1238 Change controller: IETF Audio/Video Transport Payloads Working Group 1239 delegated from the IESG (or it's successor as delegated by the IESG). 1241 5.2. Mapping to SDP Parameters 1243 Applications that use the RTP transport commonly use Session 1244 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1245 The information that is used to specify the media types in an RTP 1246 session has specific mappings to the fields in an SDP description. 1247 This section provides these mappings for the media subtypes 1248 registered by this document. Note that if an application does not 1249 use SDP to describe the RTP sessions, an appropriate mapping must be 1250 defined and used to specify the media types and their parameters for 1251 the control/description protocol employed by the application. 1253 The mapping of the media type specification for "non-interleaved- 1254 parityfec" and "interleaved-parityfec" and their parameters in SDP is 1255 as follows: 1257 o The media type (e.g., "application") goes into the "m=" line as 1258 the media name. 1260 o The media subtype goes into the "a=rtpmap" line as the encoding 1261 name. The RTP clock rate parameter ("rate") also goes into the 1262 "a=rtpmap" line as the clock rate. 1264 o The remaining required payload-format-specific parameters go into 1265 the "a=fmtp" line by copying them directly from the media type 1266 string as a semicolon-separated list of parameter=value pairs. 1268 SDP examples are provided in Section 7.1. 1270 5.2.1. Offer-Answer Model Considerations 1272 When offering 1-D interleaved parity FEC over RTP using SDP in an 1273 Offer/Answer model [RFC3264], the following considerations apply: 1275 o Each combination of the L and D parameters produces a different 1276 FEC data and is not compatible with any other combination. A 1277 sender application may desire to offer multiple offers with 1278 different sets of L and D values as long as the parameter values 1279 are valid. The receiver SHOULD choose the offer that has a 1280 sufficient amount of interleaving. If multiple such offers exist, 1281 the receiver may choose the offer that has the lowest overhead or 1282 the one that requires the smallest amount of buffering. The 1283 selection depends on the application requirements. 1285 o The value for the repair-window parameter depends on the L and D 1286 values and cannot be chosen arbitrarily. More specifically, L and 1287 D values determine the lower limit for the repair-window size. 1288 The upper limit of the repair-window size does not depend on the L 1289 and D values. 1291 o Although combinations with the same L and D values but with 1292 different repair-window sizes produce the same FEC data, such 1293 combinations are still considered different offers. The size of 1294 the repair-window is related to the maximum delay between the 1295 transmission of a source packet and the associated repair packet. 1296 This directly impacts the buffering requirement on the receiver 1297 side and the receiver must consider this when choosing an offer. 1299 o Any unknown option in the offer MUST be ignored and deleted from 1300 the answer. If FEC is not desired by the receiver, it can be 1301 deleted from the answer. 1303 5.2.2. Declarative Considerations 1305 In declarative usage, like SDP in the Real-time Streaming Protocol 1306 (RTSP, for RTSP 1.0 see [RFC2326] and for RTSP 2.0 see [RFC7826]) or 1307 the Session Announcement Protocol (SAP) [RFC2974], the following 1308 considerations apply: 1310 o The payload format configuration parameters are all declarative 1311 and a participant MUST use the configuration that is provided for 1312 the session. 1314 o More than one configuration may be provided (if desired) by 1315 declaring multiple RTP payload types. In that case, the receivers 1316 should choose the repair stream that is best for them. 1318 6. Protection and Recovery Procedures - Parity Codes 1320 This section provides a complete specification of the 1-D and 2-D 1321 parity codes and their RTP payload formats. It does not apply to the 1322 single packet retransmission format (R=1 in the FEC Header). 1324 6.1. Overview 1326 The following sections specify the steps involved in generating the 1327 repair packets and reconstructing the missing source packets from the 1328 repair packets. 1330 6.2. Repair Packet Construction 1332 The RTP Header of a repair packet is formed based on the guidelines 1333 given in Section 4.2. 1335 The FEC Header and Repair "Payload" of repair packets are formed by 1336 applying the XOR operation on the bit strings that are generated from 1337 the individual source packets protected by this particular repair 1338 packet. The set of the source packets that are associated with a 1339 given repair packet can be computed by the formula given in 1340 Section 6.3.1. 1342 The bit string is formed for each source packet by concatenating the 1343 following fields together in the order specified: 1345 o The first 16 bits of the RTP header (16 bits). 1347 o Unsigned network-ordered 16-bit representation of the source 1348 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1349 the sum of the lengths of all the following if present: the CSRC 1350 list, extension header, RTP payload and RTP padding (16 bits). 1352 o The timestamp of the RTP header (32 bits). 1354 o All octets after the fixed 12-byte RTP header. (Note the SSRC 1355 field is skipped.) 1357 The FEC bit string is generated by applying the parity operation on 1358 the bit strings produced from the source packets. The FEC header is 1359 generated from the FEC bit string as follows: 1361 o The first (most significant) 2 bits in the FEC bit string, which 1362 contain the RTP version field, are skipped. The R and F bits in 1363 the FEC header are set to the appropriate value, i.e., it depends 1364 on the chosen format variant. As a consequence of overwriting the 1365 RTP version field with the R and F bits, this payload format only 1366 supports RTP version 2. 1368 o The next bit in the FEC bit string is written into the P recovery 1369 bit in the FEC header. 1371 o The next bit in the FEC bit string is written into the X recovery 1372 bit in the FEC header. 1374 o The next 4 bits of the FEC bit string are written into the CC 1375 recovery field in the FEC header. 1377 o The next bit is written into the M recovery bit in the FEC header. 1379 o The next 7 bits of the FEC bit string are written into the PT 1380 recovery field in the FEC header. 1382 o The next 16 bits are written into the length recovery field in the 1383 FEC header. 1385 o The next 32 bits of the FEC bit string are written into the TS 1386 recovery field in the FEC header. 1388 o The lowest Sequence Number of the source packets protected by this 1389 repair packet is written into the Sequence Number Base field in 1390 the FEC header. This needs to be repeated for each SSRC that has 1391 packets included in the source block. 1393 o Depending on the chosen FEC header variant, the mask(s) are set 1394 when F=0, or the L and D values are set when F=1. This needs to 1395 be repeated for each SSRC that has packets included in the source 1396 block. 1398 o The rest of the FEC bit string, which contains everything after 1399 the fixed 12-byte RTP header of the source packet, is written into 1400 the Repair "Payload" following the FEC header, where "Payload" 1401 refers to everything after the fixed 12-byte RTP header, including 1402 extensions, CSRC list, true payloads, and padding. 1404 If the lengths of the source packets are not equal, each shorter 1405 packet MUST be padded to the length of the longest packet by adding 1406 octet 0's at the end. 1408 Due to this possible padding and mandatory FEC header, a repair 1409 packet has a larger size than the source packets it protects. This 1410 may cause problems if the resulting repair packet size exceeds the 1411 Maximum Transmission Unit (MTU) size of the path over which the 1412 repair stream is sent. 1414 6.3. Source Packet Reconstruction 1416 This section describes the recovery procedures that are required to 1417 reconstruct the missing source packets. The recovery process has two 1418 steps. In the first step, the FEC decoder determines which source 1419 and repair packets should be used in order to recover a missing 1420 packet. In the second step, the decoder recovers the missing packet, 1421 which consists of an RTP header and RTP payload. 1423 The following describes the RECOMMENDED algorithms for the first and 1424 second steps. Based on the implementation, different algorithms MAY 1425 be adopted. However, the end result MUST be identical to the one 1426 produced by the algorithms described below. 1428 Note that the same algorithms are used by the 1-D parity codes, 1429 regardless of whether the FEC protection is applied over a column or 1430 a row. The 2-D parity codes, on the other hand, usually require 1431 multiple iterations of the procedures described here. This iterative 1432 decoding algorithm is further explained in Section 6.3.4. 1434 6.3.1. Associating the Source and Repair Packets 1436 Before associating source and repair packets, the receiver must know 1437 in which RTP sessions the source and repair respectively are being 1438 sent. After this is established by the reciever the first step is 1439 associating the source and repair packets. This association can be 1440 via flexible bitmasks, or fixed L and D offsets which can be in the 1441 FEC header or signaled in SDP in optional payload format parameters 1442 when L=D=0 in the FEC header. 1444 6.3.1.1. Using Bitmasks 1446 To use flexible bitmasks, the first two FEC header bits MUST have R=0 1447 and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source 1448 packets are protected by a FEC repair packet. If the bit i in the 1449 mask is set to 1, the source packet number N + i is protected by this 1450 FEC repair packet, where N is the sequence number base indicated in 1451 the FEC header. The most significant bit of the mask corresponds to 1452 i=0. The least signficant bit of the mask corresponds to i=14 in the 1453 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit mask. 1455 The bitmasks are able to represent arbitrary protection patterns, for 1456 example, 1-D interleaved, 1-D non-interleaved, 2-D. 1458 6.3.1.2. Using L and D Offsets 1460 Denote the set of the source packets associated with repair packet p* 1461 by set T(p*). Note that in a source block whose size is L columns by 1462 D rows, set T includes D source packets plus one repair packet for 1463 the FEC protection applied over a column, and L source packets plus 1464 one repair packet for the FEC protection applied over a row. Recall 1465 that 1-D interleaved and non-interleaved FEC protection can fully 1466 recover the missing information if there is only one source packet 1467 missing per column or row in set T. If there are more than one 1468 source packets missing per column or row in set T, 1-D FEC protection 1469 may fail to recover all the missing information. 1471 When value of L is non-zero, the 8-bit fields indicate the offset of 1472 packets protected by an interleaved (D>0) or non-interleaved (D=0) 1473 FEC packet. Using a combination of interleaved and non-interleaved 1474 FEC repair packets can form 2-D protection patterns. 1476 Mathematically, for any received repair packet, p*, the sequence 1477 numbers of the source packets that are protected by this repair 1478 packet are determined as follows, where p*_snb is the sequence number 1479 base in the FEC header: 1481 When D = 0: 1483 p*_snb, p*_snb+1,..., p*_snb+L 1485 When D > 0: 1487 p*_snb, p*_snb+(Lx1), p*_snb+(Lx2),..., p*_snb+(LxD) 1489 6.3.1.3. Signaled in SDP 1491 If the endpoint relies entirely on out-of-band signaling (R=0, F=1, 1492 L=0, D=0 in the FEC header), then this information may be inferred 1493 from the media type parameters specified in the SDP description. 1494 Furthermore, the payload type field in the RTP header assists the 1495 receiver to distinguish an interleaved or non-interleaved FEC packet. 1497 Mathematically, for any received repair packet, p*, the sequence 1498 numbers of the source packets that are protected by this repair 1499 packet are determined as follows: 1501 p*_snb + i * X_1 (modulo 65536) 1503 where p*_snb denotes the value in the SN base field of p*'s FEC 1504 header, X_1 is set to L and 1 for the interleaved and non-interleaved 1505 FEC repair packets, respectively, and 1507 0 <= i < X_2 1509 where X_2 is set to D and L for the interleaved and non-interleaved 1510 FEC repair packets, respectively. 1512 6.3.2. Recovering the RTP Header 1514 For a given set T, the procedure for the recovery of the RTP header 1515 of the missing packet, whose sequence number is denoted by SEQNUM, is 1516 as follows: 1518 1. For each of the source packets that are successfully received in 1519 T, compute the 80-bit string by concatenating the first 64 bits 1520 of their RTP header and the unsigned network-ordered 16-bit 1521 representation of their length in bytes minus 12. 1523 2. For the repair packet in T, compute the FEC bit string from the 1524 first 80 bits of the FEC header. 1526 3. Calculate the recovered bit string as the XOR of the bit strings 1527 generated from all source packets in T and the FEC bit string 1528 generated from the repair packet in T. 1530 4. Create a new packet with the standard 12-byte RTP header and no 1531 payload. 1533 5. Set the version of the new packet to 2. Skip the first 2 bits 1534 in the recovered bit string. 1536 6. Set the Padding bit in the new packet to the next bit in the 1537 recovered bit string. 1539 7. Set the Extension bit in the new packet to the next bit in the 1540 recovered bit string. 1542 8. Set the CC field to the next 4 bits in the recovered bit string. 1544 9. Set the Marker bit in the new packet to the next bit in the 1545 recovered bit string. 1547 10. Set the Payload type in the new packet to the next 7 bits in the 1548 recovered bit string. 1550 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 1551 bits in the recovered bit string. 1553 12. Set the TS field in the new packet to the next 32 bits in the 1554 recovered bit string. 1556 13. Take the next 16 bits of the recovered bit string and set the 1557 new variable Y to whatever unsigned integer this represents 1558 (assuming network order). Convert Y to host order. Y 1559 represents the length of the new packet in bytes minus 12 (for 1560 the fixed RTP header), i.e., the sum of the lengths of all the 1561 following if present: the CSRC list, header extension, RTP 1562 payload and RTP padding. 1564 14. Set the SSRC of the new packet to the SSRC of the missing source 1565 RTP stream. 1567 This procedure recovers the header of an RTP packet up to (and 1568 including) the SSRC field. 1570 6.3.3. Recovering the RTP Payload 1572 Following the recovery of the RTP header, the procedure for the 1573 recovery of the RTP "payload" is as follows, where "payload" refers 1574 to everything following the fixed 12-byte RTP header, including 1575 extensions, CSRC list, true payload and padding. 1577 1. Append Y bytes to the new packet. 1579 2. For each of the source packets that are successfully received in 1580 T, compute the bit string from the Y octets of data starting with 1581 the 13th octet of the packet. If any of the bit strings 1582 generated from the source packets has a length shorter than Y, 1583 pad them to that length. The zero-padding octets MUST be added 1584 at the end of the bit string. Note that the information of the 1585 first 8 octets are protected by the FEC header. 1587 3. For the repair packet in T, compute the FEC bit string from the 1588 repair packet payload, i.e., the Y octets of data following the 1589 FEC header. Note that the FEC header may be different sizes 1590 depending on the variant and bitmask size. 1592 4. Calculate the recovered bit string as the XOR of the bit strings 1593 generated from all source packets in T and the FEC bit string 1594 generated from the repair packet in T. 1596 5. Append the recovered bit string (Y octets) to the new packet 1597 generated in Section 6.3.2. 1599 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1601 In 2-D parity FEC protection, the sender generates both non- 1602 interleaved and interleaved FEC repair packets to combat with the 1603 mixed loss patterns (random and bursty). At the receiver side, these 1604 FEC packets are used iteratively to overcome the shortcomings of the 1605 1-D non-interleaved/interleaved FEC protection and improve the 1606 chances of full error recovery. 1608 The iterative decoding algorithm runs as follows: 1610 1. Set num_recovered_until_this_iteration to zero 1612 2. Set num_recovered_so_far to zero 1614 3. Recover as many source packets as possible by using the non- 1615 interleaved FEC repair packets as outlined in Section 6.3.2 and 1616 Section 6.3.3, and increase the value of num_recovered_so_far by 1617 the number of recovered source packets. 1619 4. Recover as many source packets as possible by using the 1620 interleaved FEC repair packets as outlined in Section 6.3.2 and 1621 Section 6.3.3, and increase the value of num_recovered_so_far by 1622 the number of recovered source packets. 1624 5. If num_recovered_so_far > num_recovered_until_this_iteration 1625 ---num_recovered_until_this_iteration = num_recovered_so_far 1626 ---Go to step 3 1627 Else 1628 ---Terminate 1630 The algorithm terminates either when all missing source packets are 1631 fully recovered or when there are still remaining missing source 1632 packets but the FEC repair packets are not able to recover any more 1633 source packets. For the example scenarios when the 2-D parity FEC 1634 protection fails full recovery, refer to Section 1.1.4. Upon 1635 termination, variable num_recovered_so_far has a value equal to the 1636 total number of recovered source packets. 1638 Example: 1640 Suppose that the receiver experienced the loss pattern sketched in 1641 Figure 16. 1643 +---+ +---+ +===+ 1645 X X | 3 | | 4 | |R_1| 1647 +---+ +---+ +===+ 1649 +---+ +---+ +---+ +---+ +===+ 1651 | 5 | | 6 | | 7 | | 8 | |R_2| 1653 +---+ +---+ +---+ +---+ +===+ 1655 +---+ +---+ +===+ 1657 | 9 | X X | 12| |R_3| 1659 +---+ +---+ +===+ 1661 +===+ +===+ +===+ +===+ 1663 |C_1| |C_2| |C_3| |C_4| 1665 +===+ +===+ +===+ +===+ 1667 Figure 16: Example loss pattern for the iterative decoding algorithm 1669 The receiver executes the iterative decoding algorithm and recovers 1670 source packets #1 and #11 in the first iteration. The resulting 1671 pattern is sketched in Figure 17. 1673 +---+ +---+ +---+ +===+ 1675 | 1 | X | 3 | | 4 | |R_1| 1677 +---+ +---+ +---+ +===+ 1679 +---+ +---+ +---+ +---+ +===+ 1681 | 5 | | 6 | | 7 | | 8 | |R_2| 1683 +---+ +---+ +---+ +---+ +===+ 1685 +---+ +---+ +---+ +===+ 1687 | 9 | X | 11| | 12| |R_3| 1689 +---+ +---+ +---+ +===+ 1691 +===+ +===+ +===+ +===+ 1693 |C_1| |C_2| |C_3| |C_4| 1695 +===+ +===+ +===+ +===+ 1697 Figure 17: The resulting pattern after the first iteration 1699 Since the if condition holds true, the receiver runs a new iteration. 1700 In the second iteration, source packets #2 and #10 are recovered, 1701 resulting in a full recovery as sketched in Figure 18. 1703 +---+ +---+ +---+ +---+ +===+ 1705 | 1 | | 2 | | 3 | | 4 | |R_1| 1707 +---+ +---+ +---+ +---+ +===+ 1709 +---+ +---+ +---+ +---+ +===+ 1711 | 5 | | 6 | | 7 | | 8 | |R_2| 1713 +---+ +---+ +---+ +---+ +===+ 1715 +---+ +---+ +---+ +---+ +===+ 1717 | 9 | | 10| | 11| | 12| |R_3| 1719 +---+ +---+ +---+ +---+ +===+ 1721 +===+ +===+ +===+ +===+ 1723 |C_1| |C_2| |C_3| |C_4| 1725 +===+ +===+ +===+ +===+ 1727 Figure 18: The resulting pattern after the second iteration 1729 7. Signaling Requirements 1731 Out-of-band signaling should be designed to enable the receiver to 1732 identify the RTP streams associated with source packets and repair 1733 packets, respectively. At a minimum, the signaling must be designed 1734 to allow the receiver to 1736 o Determine whether one or more source RTP streams will be sent. 1738 o Determine whether one or more repair RTP streams will be sent. 1740 o Associate the appropriate SSRC's to both source and repair 1741 streams. 1743 o Clearly identify which SSRC's are associated with each source 1744 block. 1746 o Clearly identify which repair packets correspond to which source 1747 blocks. 1749 o Make use of repair packets to recover source data associated with 1750 specific SSRC's. 1752 This section provides several Sesssion Description Protocol (SDP) 1753 examples to demonstrate how these requirements can be met. 1755 7.1. SDP Examples 1757 This section provides two SDP [RFC4566] examples. The examples use 1758 the FEC grouping semantics defined in [RFC5956]. 1760 7.1.1. Example SDP for Flexible FEC Protection with in-band SSRC 1761 mapping 1763 In this example, we have one source video stream and one FEC repair 1764 stream. The source and repair streams are multiplexed on different 1765 SSRCs. The repair window is set to 200 ms. 1767 v=0 1769 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1771 s=FlexFEC minimal SDP signalling Example 1773 t=0 0 1775 m=video 30000 RTP/AVP 96 98 1777 c=IN IP4 233.252.0.1/127 1779 a=rtpmap:96 VP8/90000 1781 a=rtpmap:98 flexfec/90000 1783 a=fmtp:98; repair-window=200ms 1785 7.1.2. Example SDP for Flexible FEC Protection with explicit signalling 1786 in the SDP 1788 This example shows one source video stream (ssrc:1234) and one FEC 1789 repair streams (ssrc:2345). One FEC group is formed with the 1790 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1791 are multiplexed on different SSRCs. The repair window is set to 200 1792 ms. 1794 v=0 1796 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1798 s=2-D Parity FEC with no in band signalling Example 1800 t=0 0 1802 m=video 30000 RTP/AVP 100 110 1804 c=IN IP4 192.0.2.0/24 1806 a=rtpmap:100 MP2T/90000 1808 a=rtpmap:110 flexfec/90000 1810 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 1812 a=ssrc:1234 1814 a=ssrc:2345 1816 a=ssrc-group:FEC-FR 1234 2345 1818 7.2. On the Use of the RTP Stream Identifier Source Description 1820 The RTP Stream Identifier Source Description [I-D.ietf-avtext-rid] is 1821 a format that can be used to identify a single RTP source stream 1822 along with an associated repair stream. However, this specification 1823 already defines a method of source and repair stream identification 1824 that can enable protection of multiple source streams with a single 1825 repair stream. Therefore the RTP Stream Idenfifer Source Description 1826 SHOULD NOT be used for the Flexible FEC payload format 1828 8. Congestion Control Considerations 1830 FEC is an effective approach to provide applications resiliency 1831 against packet losses. However, in networks where the congestion is 1832 a major contributor to the packet loss, the potential impacts of 1833 using FEC should be considered carefully before injecting the repair 1834 streams into the network. In particular, in bandwidth-limited 1835 networks, FEC repair streams may consume a significant part of the 1836 available bandwidth and consequently may congest the network. In 1837 such cases, the applications MUST NOT arbitrarily increase the amount 1838 of FEC protection since doing so may lead to a congestion collapse. 1839 If desired, stronger FEC protection MAY be applied only after the 1840 source rate has been reduced. 1842 In a network-friendly implementation, an application should avoid 1843 sending/receiving FEC repair streams if it knows that sending/ 1844 receiving those FEC repair streams would not help at all in 1845 recovering the missing packets. It is RECOMMENDED that the amount 1846 and type (row, column, or both) of FEC protection is adjusted 1847 dynamically based on the packet loss rate and burst loss length 1848 observed by the applications. 1850 In multicast scenarios, it may be difficult to optimize the FEC 1851 protection per receiver. If there is a large variation among the 1852 levels of FEC protection needed by different receivers, it is 1853 RECOMMENDED that the sender offers multiple repair streams with 1854 different levels of FEC protection and the receivers join the 1855 corresponding multicast sessions to receive the repair stream(s) that 1856 is best for them. 1858 9. Security Considerations 1860 RTP packets using the payload format defined in this specification 1861 are subject to the security considerations discussed in the RTP 1862 specification [RFC3550] and in any applicable RTP profile. The main 1863 security considerations for the RTP packet carrying the RTP payload 1864 format defined within this memo are confidentiality, integrity and 1865 source authenticity. Confidentiality is achieved by encrypting the 1866 RTP payload. Integrity of the RTP packets is achieved through a 1867 suitable cryptographic integrity protection mechanism. Such a 1868 cryptographic system may also allow the authentication of the source 1869 of the payload. A suitable security mechanism for this RTP payload 1870 format should provide confidentiality, integrity protection, and at 1871 least source authentication capable of determining if an RTP packet 1872 is from a member of the RTP session. 1874 Note that the appropriate mechanism to provide security to RTP and 1875 payloads following this memo may vary. It is dependent on the 1876 application, transport and signaling protocol employed. Therefore, a 1877 single mechanism is not sufficient, although if suitable, using the 1878 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1879 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1880 Layer Security (TLS, see [RFC8446]) (RTP over TCP); other 1881 alternatives may exist. 1883 Given that FLEX FEC enables the protection of multiple source 1884 streams, there exists the possibility that multiple source buffers 1885 may be created that may not be used. An attacker could leverage 1886 unused source buffers to as a means of occupying memory in a FLEX FEC 1887 endpoint. Moreover the application source data may not be perfectly 1888 matched with FLEX FEC source partitioning. If this is the case, 1889 there is a possibility for unprotected source data if, for instance, 1890 the FLEX FEC implementation discards data that does not fit perfectly 1891 into its source processing requirements. 1893 10. IANA Considerations 1895 New media subtypes are subject to IANA registration. For the 1896 registration of the payload formats and their parameters introduced 1897 in this document, refer to Section 5. 1899 11. Acknowledgments 1901 Some parts of this document are borrowed from [RFC5109]. Thus, the 1902 author would like to thank the editor of [RFC5109] and those who 1903 contributed to [RFC5109]. 1905 Thanks to Stephen Botzko , Bernard Aboba , Rasmus Brandt , Brian 1906 Baldino , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus 1907 Westerlund for providing valuable feedback on earlier versions of 1908 this draft. 1910 12. References 1912 12.1. Normative References 1914 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1915 Requirement Levels", BCP 14, RFC 2119, 1916 DOI 10.17487/RFC2119, March 1997, 1917 . 1919 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1920 with Session Description Protocol (SDP)", RFC 3264, 1921 DOI 10.17487/RFC3264, June 2002, 1922 . 1924 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1925 Jacobson, "RTP: A Transport Protocol for Real-Time 1926 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1927 July 2003, . 1929 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1930 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1931 July 2006, . 1933 [RFC4855] Casner, S., "Media Type Registration of RTP Payload 1934 Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, 1935 . 1937 [RFC4856] Casner, S., "Media Type Registration of Payload Formats in 1938 the RTP Profile for Audio and Video Conferences", 1939 RFC 4856, DOI 10.17487/RFC4856, February 2007, 1940 . 1942 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1943 the Session Description Protocol", RFC 5956, 1944 DOI 10.17487/RFC5956, September 2010, 1945 . 1947 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1948 Correction (FEC) Framework", RFC 6363, 1949 DOI 10.17487/RFC6363, October 2011, 1950 . 1952 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1953 Specifications and Registration Procedures", BCP 13, 1954 RFC 6838, DOI 10.17487/RFC6838, January 2013, 1955 . 1957 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1958 "Guidelines for Choosing RTP Control Protocol (RTCP) 1959 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1960 September 2013, . 1962 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1963 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1964 May 2017, . 1966 12.2. Informative References 1968 [I-D.ietf-avtext-rid] 1969 Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream 1970 Identifier Source Description (SDES)", draft-ietf-avtext- 1971 rid-09 (work in progress), October 2016. 1973 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1974 Streaming Protocol (RTSP)", RFC 2326, 1975 DOI 10.17487/RFC2326, April 1998, 1976 . 1978 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1979 for Generic Forward Error Correction", RFC 2733, 1980 DOI 10.17487/RFC2733, December 1999, 1981 . 1983 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1984 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1985 October 2000, . 1987 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1988 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1989 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1990 . 1992 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1993 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1994 December 2005, . 1996 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1997 "Extended RTP Profile for Real-time Transport Control 1998 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1999 DOI 10.17487/RFC4585, July 2006, 2000 . 2002 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 2003 Correction", RFC 5109, DOI 10.17487/RFC5109, December 2004 2007, . 2006 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 2007 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 2008 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 2009 DOI 10.17487/RFC7656, November 2015, 2010 . 2012 [RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 2013 and M. Stiemerling, Ed., "Real-Time Streaming Protocol 2014 Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December 2015 2016, . 2017 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2018 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2019 . 2021 [SMPTE2022-1] 2022 SMPTE 2022-1-2007, "Forward Error Correction for Real-Time 2023 Video/Audio Transport over IP Networks", 2007. 2025 Authors' Addresses 2027 Mo Zanaty 2028 Cisco 2029 Raleigh, NC 2030 USA 2032 Email: mzanaty@cisco.com 2034 Varun Singh 2035 CALLSTATS I/O Oy 2036 Runeberginkatu 4c A 4 2037 Helsinki 00100 2038 Finland 2040 Email: varun.singh@iki.fi 2041 URI: http://www.callstats.io/ 2043 Ali Begen 2044 Networked Media 2045 Konya 2046 Turkey 2048 Email: ali.begen@networked.media 2050 Giridhar Mandyam 2051 Qualcomm Inc. 2052 5775 Morehouse Drive 2053 San Diego, CA 92121 2054 USA 2056 Phone: +1 858 651 7200 2057 Email: mandyam@qti.qualcomm.com