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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: September 22, 2018 callstats.io 6 A. Begen 7 Networked Media 8 G. Mandyam 9 Qualcomm Innovation Center 10 March 21, 2018 12 RTP Payload Format for Flexible Forward Error Correction (FEC) 13 draft-ietf-payload-flexible-fec-scheme-07 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 September 22, 2018. 54 Copyright Notice 56 Copyright (c) 2018 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents 61 (https://trustee.ietf.org/license-info) in effect on the date of 62 publication of this document. Please review these documents 63 carefully, as they describe your rights and restrictions with respect 64 to this document. Code Components extracted from this document must 65 include Simplified BSD License text as described in Section 4.e of 66 the Trust Legal Provisions and are provided without warranty as 67 described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 1.1. Parity Codes . . . . . . . . . . . . . . . . . . . . . . 4 73 1.1.1. 1-D Non-interleaved (Row) FEC Protection . . . . . . 5 74 1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 5 75 1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 6 76 1.1.4. 2-D (Row and Column) FEC Protection . . . . . . . . . 8 77 1.1.5. FEC Overhead Considerations . . . . . . . . . . . . . 9 78 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 9 79 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 10 80 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 81 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 82 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 11 83 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 11 84 4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 11 85 4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 12 86 4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 13 87 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 17 88 5.1. Media Type Registration - Parity Codes . . . . . . . . . 17 89 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 18 90 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 19 91 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 20 92 5.1.4. Registration of application/flexfec . . . . . . . . . 22 93 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 23 94 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 24 95 5.2.2. Declarative Considerations . . . . . . . . . . . . . 24 96 6. Protection and Recovery Procedures - Parity Codes . . . . . . 25 97 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 25 98 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 25 99 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 27 100 6.3.1. Associating the Source and Repair Packets . . . . . . 27 101 6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 28 102 6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 30 103 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 104 Protection . . . . . . . . . . . . . . . . . . . . . 30 105 7. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 32 106 7.1. Example SDP for Flexible FEC Protection with in-band SSRC 107 mapping . . . . . . . . . . . . . . . . . . . . . . . . . 33 108 7.2. Example SDP for Flex FEC Protection with explicit 109 signalling in the SDP . . . . . . . . . . . . . . . . . . 33 110 8. Congestion Control Considerations . . . . . . . . . . . . . . 33 111 9. Security Considerations . . . . . . . . . . . . . . . . . . . 34 112 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 113 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 114 12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 35 115 12.1. draft-ietf-payload-flexible-fec-scheme-05 . . . . . . . 35 116 12.2. draft-ietf-payload-flexible-fec-scheme-03 . . . . . . . 35 117 12.3. draft-ietf-payload-flexible-fec-scheme-02 . . . . . . . 35 118 12.4. draft-ietf-payload-flexible-fec-scheme-01 . . . . . . . 35 119 12.5. draft-ietf-payload-flexible-fec-scheme-00 . . . . . . . 36 120 12.6. draft-singh-payload-1d2d-parity-scheme-00 . . . . . . . 36 121 12.7. draft-ietf-fecframe-1d2d-parity-scheme-00 . . . . . . . 36 122 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 36 123 13.1. Normative References . . . . . . . . . . . . . . . . . . 36 124 13.2. Informative References . . . . . . . . . . . . . . . . . 37 125 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 127 1. Introduction 129 This document defines new RTP payload formats for the Forward Error 130 Correction (FEC) that is generated by the non-interleaved and 131 interleaved parity codes from a source media encapsulated in RTP 132 [RFC3550]. The type of the source media protected by these parity 133 codes can be audio, video, text or application. The FEC data are 134 generated according to the media type parameters, which are 135 communicated out-of-band (e.g., in SDP). Furthermore, the 136 associations or relationships between the source and repair RTP 137 streams may be communicated in-band or out-of-band. For situations 138 where adaptivitiy of FEC parameters is desired, the endpoint can use 139 the in-band mechanism, whereas when the FEC parameters are fixed, the 140 endpoint may prefer to negotiate them out-of-band. 142 The Redunadncy 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 1.1. Parity Codes 148 Both the non-interleaved and interleaved parity codes use the 149 eXclusive OR (XOR) operation to generate the repair packets. The 150 following steps take place: 152 1. The sender determines a set of source packets to be protected by 153 FEC based on the media type parameters. 155 2. The sender applies the XOR operation on the source packets to 156 generate the required number of repair packets. 158 3. The sender sends the repair packet(s) along with the source 159 packets, in different RTP streams, to the receiver(s). The 160 repair packets may be sent proactively or on-demand based on RTCP 161 feedback messages such as NACK [RFC4585]. 163 At the receiver side, if all of the source packets are successfully 164 received, there is no need for FEC recovery and the repair packets 165 are discarded. However, if there are missing source packets, the 166 repair packets can be used to recover the missing information. 167 Figure 1 and Figure 2 describe example block diagrams for the 168 systematic parity FEC encoder and decoder, respectively. 170 +------------+ 171 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 172 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 173 | Encoder | 174 | (Sender) | --> +==+ +==+ 175 +------------+ +==+ +==+ 177 Source Packet: +--+ Repair Packet: +==+ 178 +--+ +==+ 180 Figure 1: Block diagram for systematic parity FEC encoder 182 +------------+ 183 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 184 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 185 | Decoder | 186 +==+ +==+ --> | (Receiver) | 187 +==+ +==+ +------------+ 189 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 190 +--+ +==+ 192 Figure 2: Block diagram for systematic parity FEC decoder 194 In Figure 2, it is clear that the FEC repair packets have to be 195 received by the endpoint within a certain amount of time for the FEC 196 recovery process to be useful. In this document, we refer to the 197 time that spans a FEC block, which consists of the source packets and 198 the corresponding repair packets, as the repair window. At the 199 receiver side, the FEC decoder SHOULD buffer source and repair 200 packets at least for the duration of the repair window, to allow all 201 the repair packets to arrive. The FEC decoder can start decoding the 202 already received packets sooner; however, it should not register a 203 FEC decoding failure until it waits at least for the duration of the 204 repair window. 206 1.1.1. 1-D Non-interleaved (Row) FEC Protection 208 Suppose that we have a group of D x L source packets that have 209 sequence numbers starting from 1 running to D x L, and a repair 210 packet is generated by applying the XOR operation to every L 211 consecutive packets as sketched in Figure 3. This process is 212 referred to as 1-D non-interleaved FEC protection. As a result of 213 this process, D repair packets are generated, which we refer to as 214 non-interleaved (or row) FEC repair packets. 216 +--------------------------------------------------+ --- +===+ 217 | S_1 S_2 S3 ... S_L | + |XOR| = |R_1| 218 +--------------------------------------------------+ --- +===+ 219 +--------------------------------------------------+ --- +===+ 220 | S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2| 221 +--------------------------------------------------+ --- +===+ 222 . . . . . . 223 . . . . . . 224 . . . . . . 225 +--------------------------------------------------+ --- +===+ 226 | S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D| 227 +--------------------------------------------------+ --- +===+ 229 Figure 3: Generating non-interleaved (row) FEC repair packets 231 1.1.2. 1-D Interleaved (Column) FEC Protection 233 If we apply the XOR operation to the group of the source packets 234 whose sequence numbers are L apart from each other, as sketched in 235 Figure 4. In this case the endpoint generates L repair packets. 236 This process is referred to as 1-D interleaved FEC protection, and 237 the resulting L repair packets are referred to as interleaved (or 238 column) FEC repair packets. 240 +-------------+ +-------------+ +-------------+ +-------+ 241 | S_1 | | S_2 | | S3 | ... | S_L | 242 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 243 | . | | . | | | | | 244 | . | | . | | | | | 245 | . | | . | | | | | 246 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 247 +-------------+ +-------------+ +-------------+ +-------+ 248 + + + + 249 ------------- ------------- ------------- ------- 250 | XOR | | XOR | | XOR | ... | XOR | 251 ------------- ------------- ------------- ------- 252 = = = = 253 +===+ +===+ +===+ +===+ 254 |C_1| |C_2| |C_3| ... |C_L| 255 +===+ +===+ +===+ +===+ 257 Figure 4: Generating interleaved (column) FEC repair packets 259 1.1.3. Use Cases for 1-D FEC Protection 261 A sender may generate one non-interleaved repair packet out of L 262 consecutive source packets or one interleaved repair packet out of D 263 non-consecutive source packets. Regardless of whether the repair 264 packet is a non-interleaved or an interleaved one, it can provide a 265 full recovery of the missing information if there is only one packet 266 missing among the corresponding source packets. This implies that 267 1-D non-interleaved FEC protection performs better when the source 268 packets are randomly lost. However, if the packet losses occur in 269 bursts, 1-D interleaved FEC protection performs better provided that 270 L is chosen large enough, i.e., L-packet duration is not shorter than 271 the observed burst duration. If the sender generates non-interleaved 272 FEC repair packets and a burst loss hits the source packets, the 273 repair operation fails. This is illustrated in Figure 5. 275 +---+ +---+ +===+ 276 | 1 | X X | 4 | |R_1| 277 +---+ +---+ +===+ 279 +---+ +---+ +---+ +---+ +===+ 280 | 5 | | 6 | | 7 | | 8 | |R_2| 281 +---+ +---+ +---+ +---+ +===+ 283 +---+ +---+ +---+ +---+ +===+ 284 | 9 | | 10| | 11| | 12| |R_3| 285 +---+ +---+ +---+ +---+ +===+ 287 Figure 5: Example scenario where 1-D non-interleaved FEC protection 288 fails error recovery (Burst Loss) 290 The sender may generate interleaved FEC repair packets to combat with 291 the bursty packet losses. However, two or more random packet losses 292 may hit the source and repair packets in the same column. In that 293 case, the repair operation fails as well. This is illustrated in 294 Figure 6. Note that it is possible that two burst losses may occur 295 back-to-back, in which case interleaved FEC repair packets may still 296 fail to recover the lost data. 298 +---+ +---+ +---+ 299 | 1 | X | 3 | | 4 | 300 +---+ +---+ +---+ 302 +---+ +---+ +---+ 303 | 5 | X | 7 | | 8 | 304 +---+ +---+ +---+ 306 +---+ +---+ +---+ +---+ 307 | 9 | | 10| | 11| | 12| 308 +---+ +---+ +---+ +---+ 310 +===+ +===+ +===+ +===+ 311 |C_1| |C_2| |C_3| |C_4| 312 +===+ +===+ +===+ +===+ 314 Figure 6: Example scenario where 1-D interleaved FEC protection fails 315 error recovery (Periodic Loss) 317 1.1.4. 2-D (Row and Column) FEC Protection 319 In networks where the source packets are lost both randomly and in 320 bursts, the sender ought to generate both non-interleaved and 321 interleaved FEC repair packets. This type of FEC protection is known 322 as 2-D parity FEC protection. At the expense of generating more FEC 323 repair packets, thus increasing the FEC overhead, 2-D FEC provides 324 superior protection against mixed loss patterns. However, it is 325 still possible for 2-D parity FEC protection to fail to recover all 326 of the lost source packets if a particular loss pattern occurs. An 327 example scenario is illustrated in Figure 7. 329 +---+ +---+ +===+ 330 | 1 | X X | 4 | |R_1| 331 +---+ +---+ +===+ 333 +---+ +---+ +---+ +---+ +===+ 334 | 5 | | 6 | | 7 | | 8 | |R_2| 335 +---+ +---+ +---+ +---+ +===+ 337 +---+ +---+ +===+ 338 | 9 | X X | 12| |R_3| 339 +---+ +---+ +===+ 341 +===+ +===+ +===+ +===+ 342 |C_1| |C_2| |C_3| |C_4| 343 +===+ +===+ +===+ +===+ 345 Figure 7: Example scenario #1 where 2-D parity FEC protection fails 346 error recovery 348 2-D parity FEC protection also fails when at least two rows are 349 missing a source and the FEC packet and the missing source packets 350 (in at least two rows) are aligned in the same column. An example 351 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC 352 protection cannot repair all missing source packets when at least two 353 columns are missing a source and the FEC packet and the missing 354 source packets (in at least two columns) are aligned in the same row. 356 +---+ +---+ +---+ 357 | 1 | | 2 | X | 4 | X 358 +---+ +---+ +---+ 360 +---+ +---+ +---+ +---+ +===+ 361 | 5 | | 6 | | 7 | | 8 | |R_2| 362 +---+ +---+ +---+ +---+ +===+ 364 +---+ +---+ +---+ 365 | 9 | | 10| X | 12| X 366 +---+ +---+ +---+ 368 +===+ +===+ +===+ +===+ 369 |C_1| |C_2| |C_3| |C_4| 370 +===+ +===+ +===+ +===+ 372 Figure 8: Example scenario #2 where 2-D parity FEC protection fails 373 error recovery 375 1.1.5. FEC Overhead Considerations 377 The overhead is defined as the ratio of the number of bytes belonging 378 to the repair packets to the number of bytes belonging to the 379 protected source packets. 381 Generally, repair packets are larger in size compared to the source 382 packets. Also, not all the source packets are necessarily equal in 383 size. However, if we assume that each repair packet carries an equal 384 number of bytes carried by a source packet, we can compute the 385 overhead for different FEC protection methods as follows: 387 o 1-D Non-interleaved FEC Protection: Overhead = 1/L 389 o 1-D Interleaved FEC Protection: Overhead = 1/D 391 o 2-D Parity FEC Protection: Overhead = 1/L + 1/D 393 where L and D are the number of columns and rows in the source block, 394 respectively. 396 2. Requirements Notation 398 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 399 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 400 document are to be interpreted as described in [RFC2119]. 402 3. Definitions and Notations 404 3.1. Definitions 406 This document uses a number of definitions from [RFC6363]. 408 1-D Non-interleaved Row FEC: A protection scheme that operates on 409 consecutive source packets in the source block, able to recover a 410 single lost source packet per row of the source block. 412 1-D Interleaved Column FEC: A protection scheme that operates on 413 interleaved source packets in the source block, able to recover a 414 single lost source packet per column of the source block. 416 2-D FEC: A protection scheme that combines row and column FEC. 418 Source Block: A set of source packets that are protected by a set 419 of 1-D or 2-D FEC repair packets. 421 FEC Block: A source block and its corresponding FEC repair 422 packets. 424 Repair Window: The time that spans a FEC block, which consists of 425 the source packets and the corresponding FEC repair packets. 427 XOR Parity Codes: A FEC code which uses the eXclusive OR (XOR) 428 parity operation to encode a set of source packets to form a FEC 429 repair packet. 431 3.2. Notations 433 L: Number of columns of the source block (length of each row). 435 D: Number of rows of the source block (depth of each column). 437 bitmask: A 15-bit, 46-bit, or 110-bit mask indicating which source 438 packets are protected by a FEC repair packet. If the bit i in the 439 mask is set to 1, the source packet number N + i is protected by 440 this FEC repair packet, where N is the sequence number base 441 indicated in the FEC repair packet. The most significant bit of 442 the mask corresponds to i=0. The least signficant bit of the mask 443 corresponds to i=14 in the 15-bit mask, i=45 in the 46-bit mask, 444 or i=109 in the 110-bit mask. 446 4. Packet Formats 448 This section describes the formats of the source packets and defines 449 the formats of the FEC repair packets. 451 4.1. Source Packets 453 The source packets contain the information that identifies the source 454 block and the position within the source block occupied by the 455 packet. Since the source packets that are carried within an RTP 456 stream already contain unique sequence numbers in their RTP headers 457 [RFC3550], we can identify the source packets in a straightforward 458 manner and there is no need to append additional field(s). The 459 primary advantage of not modifying the source packets in any way is 460 that it provides backward compatibility for the receivers that do not 461 support FEC at all. In multicast scenarios, this backward 462 compatibility becomes quite useful as it allows the non-FEC-capable 463 and FEC-capable receivers to receive and interpret the same source 464 packets sent in the same multicast session. 466 The source packets are transmitted as usual without altering them. 467 They are used along with the FEC repair packets to recover any 468 missing source packets, making this scheme a systematic code. 470 The source packets are full RTP packets with optional CSRC list, RTP 471 header extension, and padding. If any of these optional elements are 472 present in the source RTP packet, and that source packet is lost, 473 they are recovered by the FEC repair operation, which recovers the 474 full source RTP packet including these optional elements. 476 4.2. FEC Repair Packets 478 The FEC repair packets MUST contain information that identifies the 479 source block they pertain to and the relationship between the 480 contained repair packets and the original source block. For this 481 purpose, we use the RTP header of the repair packets as well as 482 another header within the RTP payload, which we refer to as the FEC 483 header, as shown in Figure 9. 485 Note that all the source stream packets that are protected by a 486 particular FEC packet need to be in the same RTP session. 488 +------------------------------+ 489 | IP Header | 490 +------------------------------+ 491 | Transport Header | 492 +------------------------------+ 493 | RTP Header | 494 +------------------------------+ ---+ 495 | FEC Header | | 496 +------------------------------+ | RTP Payload 497 | Repair Payload | | 498 +------------------------------+ ---+ 500 Figure 9: Format of FEC repair packets 502 4.2.1. RTP Header of FEC Repair Packets 504 The RTP header is formatted according to [RFC3550] with some further 505 clarifications listed below: 507 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this 508 specification requires all source RTP packets and all FEC repair 509 packets to use RTP version 2. 511 Padding (P) bit: Source packets can have optional RTP padding, 512 which can be recovered. FEC repaire packets can have optional RTP 513 padding, which is independent of the RTP padding of the source 514 pakcets. 516 Extension (X) bit: Source packets can have optional RTP header 517 extensions, which can be recovered. FEC repair packets can have 518 optional RTP header extensions, which are independent of the RTP 519 header extensions of the source packets. 521 CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: 522 Source packets can have an optional CSRC list and count, which can 523 be recovered. FEC repair packets MUST use the CSRC list and count 524 to specify the SSRC(s) of the source RTP stream(s) protected by 525 this FEC repair packet. 527 Marker (M) bit: This bit is not used for this payload type, and 528 SHALL be set to 0 by senders, and SHALL be ignored by receivers. 530 Payload Type: The (dynamic) payload type for the FEC repair 531 packets is determined through out-of-band means. Note that this 532 document registers new payload formats for the repair packets 533 (Refer to Section 5 for details). According to [RFC3550], an RTP 534 receiver that cannot recognize a payload type must discard it. 535 This provides backward compatibility. If a non-FEC-capable 536 receiver receives a repair packet, it will not recognize the 537 payload type, and hence, will discard the repair packet. 539 Sequence Number (SN): The sequence number has the standard 540 definition. It MUST be one higher than the sequence number in the 541 previously transmitted repair packet. The initial value of the 542 sequence number SHOULD be random (unpredictable, based on 543 [RFC3550]). 545 Timestamp (TS): The timestamp SHALL be set to a time corresponding 546 to the repair packet's transmission time. Note that the timestamp 547 value has no use in the actual FEC protection process and is 548 usually useful for jitter calculations. 550 Synchronization Source (SSRC): The SSRC value for each repair 551 stream SHALL be randomly assigned as suggested by [RFC3550]. This 552 allows the sender to multiplex the source and repair RTP streams 553 on the same port, or multiplex multiple repair streams on a single 554 port. The repair streams SHOULD use the RTCP CNAME field to 555 associate themselves with the source stream. 557 In some networks, the RTP Source, which produces the source 558 packets and the FEC Source, which generates the repair packets 559 from the source packets may not be the same host. In such 560 scenarios, using the same CNAME for the source and repair RTP 561 streams means that the RTP Source and the FEC Source MUST share 562 the same CNAME (for this specific source-repair stream 563 association). A common CNAME may be produced based on an 564 algorithm that is known both to the RTP and FEC Source [RFC7022]. 565 This usage is compliant with [RFC3550]. 567 Note that due to the randomness of the SSRC assignments, there is 568 a possibility of SSRC collision. In such cases, the collisions 569 MUST be resolved as described in [RFC3550]. 571 4.2.2. FEC Header of FEC Repair Packets 573 The format of the FEC header has 3 variants, depending on the values 574 in the first 2 bits (R and F bits) as shown in Figure 10. 576 The first variant, when R=0 and F=0, has a mask to signal protected 577 source packets, as shown in Figure 12. 579 The second variant, when R=0 and F=1, has a number of columns (M) and 580 rows (N) to signal protected source packets, as shown in Figure 13. 582 The final variant, when R=1, is a retransmission format as shown in 583 Figure 15. 585 0 1 2 3 586 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 1 587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 588 |R|F| P|X| CC |M| PT recovery | length recovery | 589 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 590 | TS recovery | 591 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 592 | SN base_i |k| Mask [0-14] | 593 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 594 |k| Mask [15-45] (optional) | 595 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 596 | | 597 + Mask [46-109] (optional) | 598 | | 599 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 600 | ... next SN base and Mask for CSRC_i in CSRC list ... | 601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 603 Figure 10: Format of the FEC header 605 The FEC header consists of the following fields: 607 o The R bit MUST be set to 1 to indicate a retransmission packet, 608 and MUST be set to 0 for repair packets. 610 o The F field (1 bit) indicates the type of the mask. Namely: 612 +---------------+-------------------------------------+ 613 | F bit | Use | 614 +---------------+-------------------------------------+ 615 | 0 | flexible mask | 616 | 1 | packets indicated by offset M and N | 617 +---------------+-------------------------------------+ 619 Figure 11: F-bit values 621 o The P, X, CC, M and PT recovery fields are used to determine the 622 corresponding fields of the recovered packets. 624 o The Length recovery (16 bits) field is used to determine the 625 length of the recovered packets. 627 o The TS recovery (32 bits) field is used to determine the timestamp 628 of the recovered packets. 630 o The CSRC_i (32 bits) field describes the SSRC of the packets 631 protected by this particular FEC packet. If a FEC packet contains 632 protects multiple SSRCs (indicated by the CSRC Count > 1), there 633 will be multiple blocks of data containing the SN base and Mask 634 fields. 636 o The SN base_i (16 bits) field indicates the lowest sequence 637 number, taking wrap around into account, of the source packets for 638 a particular SSSRC (indicated in CSRC_i) protected by this repair 639 packet. 641 o If the F-bit is set to 0, it represents that the source packets of 642 all the SSRCs protected by this particular repair packet are 643 indicated by using a flexible bitmask. Mask is a run-length 644 encoding of packets for a particular CSRC_i protected by the FEC 645 packet. Where a bit j set to 1 indicates that the source packet 646 with sequence number (SN base_i + j + 1) is protected by this FEC 647 packet. 649 o The k-bit in the bitmasks indicates if it is 15-, 46-, or a 650 110-bitmask. k=1 denotes that another mask follows, and k=0 651 denotes that it is the last block of bit mask. 653 o 655 0 1 2 3 656 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 1 657 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 658 |0|0| P|X| CC |M| PT recovery | length recovery | 659 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 660 | TS recovery | 661 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 662 | SN base_i |k| Mask [0-14] | 663 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 664 |k| Mask [15-45] (optional) | 665 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 666 | | 667 + Mask [46-109] (optional) | 668 | | 669 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 670 | ... next SN base and Mask for CSRC_i in CSRC list ... | 671 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 673 Figure 12: Protocol format for F=0 675 o If the F-bit is set to 1, it represents that the source packets of 676 all the SSRCs protected by this particular repair packet are 677 indicated by using fixed offsets. 679 0 1 2 3 680 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 1 681 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 682 |1|0| P|X| CC |M| PT recovery | length recovery | 683 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 684 | TS recovery | 685 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 686 | SN base_i | M (columns) | N (rows) | 687 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 689 Figure 13: Protocol format for F=1 691 Consequently, the following conditions occur for M and N values: 693 If M>0, N=0, is Row FEC, and no column FEC will follow 694 Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. 696 If M>0, N=1, is Row FEC, and column FEC will follow. 697 Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. 698 and more to come 700 If M>0, N>1, indicates column FEC of every M packet 701 in a group of N packets starting at SN base. 702 Hence, FEC = SN+(Mx0), SN+(Mx1), ... , SN+(MxN). 704 Figure 14: Interpreting the M and N field values 706 By setting R to 1, F to 1, this FEC protects only one packet, i.e., 707 the FEC payload carries just the packet indicated by SN Base_i, which 708 is effectively retransmitting the packet. 710 Note that the parsing of this packet is different. The sequence 711 number (SN base_i) replaces the length recovery in the FEC packet. 712 The CSRC Count (CC) which would be 1, M and N would be set to 0, and 713 the reserved bits from the FEC header are removed. By doing this, we 714 save 64 bits. 716 0 1 2 3 717 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 1 718 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 719 |1|1| P|X| CC |M| PT recovery | sequence number | 720 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 721 | timestamp | 722 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 723 | SSRC | 724 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 725 | Retransmission | 726 : payload : 727 | | 728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 730 Figure 15: Protocol format for Retransmission 732 The details on setting the fields in the FEC header are provided in 733 Section 6.2. 735 It should be noted that a mask-based approach (similar to the ones 736 specified in [RFC2733] and [RFC5109]) may not be very efficient to 737 indicate which source packets in the current source block are 738 associated with a given repair packet. In particular, for the 739 applications that would like to use large source block sizes, the 740 size of the mask that is required to describe the source-repair 741 packet associations may be prohibitively large. The 8-bit fields 742 proposed in [SMPTE2022-1] indicate a systematized approach. Instead 743 the approach in this document uses the 8-bit fields to indicate 744 packet offsets protected by the FEC packet. The approach in 745 [SMPTE2022-1] is inherently more efficient for regular patterns, it 746 does not provide flexibility to represent other protection patterns 747 (e.g., staircase). 749 5. Payload Format Parameters 751 This section provides the media subtype registration for the non- 752 interleaved and interleaved parity FEC. The parameters that are 753 required to configure the FEC encoding and decoding operations are 754 also defined in this section. If no specific FEC code is specified 755 in the subtype, then the FEC code defaults to the parity code defined 756 in this specification. 758 5.1. Media Type Registration - Parity Codes 760 This registration is done using the template defined in [RFC6838] and 761 following the guidance provided in [RFC3555]. 763 Note to the RFC Editor: In the following sections, please replace 764 "XXXX" with the number of this document prior to publication as an 765 RFC. 767 5.1.1. Registration of audio/flexfec 769 Type name: audio 771 Subtype name: flexfec 773 Required parameters: 775 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 776 than 1000 Hz to provide sufficient resolution to RTCP operations. 777 However, it is RECOMMENDED to select the rate that matches the 778 rate of the protected source RTP stream. 780 o repair-window: The time that spans the source packets and the 781 corresponding repair packets. The size of the repair window is 782 specified in microseconds. 784 Optional parameters: 786 o L: indicates the number of columns of the source block that are 787 protected by this FEC block and it applies to all the source 788 SSRCs. L is a positive integer. 790 o D: indicates the number of rows of the source block that are 791 protected by this FEC block and it applies to all the source 792 SSRCs. D is a positive integer. 794 o ToP: indicates the type of protection applied by the sender: 0 for 795 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 796 protection, and 2 for 2-D parity FEC protection. The ToP value of 797 3 is reserved for future uses. 799 Encoding considerations: This media type is framed (See Section 4.8 800 in the template document [RFC6838]) and contains binary data. 802 Security considerations: See Section 9 of [RFCXXXX]. 804 Interoperability considerations: None. 806 Published specification: [RFCXXXX]. 808 Applications that use this media type: Multimedia applications that 809 want to improve resiliency against packet loss by sending redundant 810 data in addition to the source media. 812 Fragment identifier considerations: None. 814 Additional information: None. 816 Person & email address to contact for further information: Varun 817 Singh and IETF Audio/Video Transport Payloads 818 Working Group. 820 Intended usage: COMMON. 822 Restriction on usage: This media type depends on RTP framing, and 823 hence, is only defined for transport via RTP [RFC3550]. 825 Author: Varun Singh . 827 Change controller: IETF Audio/Video Transport Working Group delegated 828 from the IESG. 830 Provisional registration? (standards tree only): Yes. 832 5.1.2. Registration of video/flexfec 834 Type name: video 836 Subtype name: flexfec 838 Required parameters: 840 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 841 than 1000 Hz to provide sufficient resolution to RTCP operations. 842 However, it is RECOMMENDED to select the rate that matches the 843 rate of the protected source RTP stream. 845 o repair-window: The time that spans the source packets and the 846 corresponding repair packets. The size of the repair window is 847 specified in microseconds. 849 Optional parameters: 851 o L: indicates the number of columns of the source block that are 852 protected by this FEC block and it applies to all the source 853 SSRCs. L is a positive integer. 855 o D: indicates the number of rows of the source block that are 856 protected by this FEC block and it applies to all the source 857 SSRCs. D is a positive integer. 859 o ToP: indicates the type of protection applied by the sender: 0 for 860 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 861 protection, and 2 for 2-D parity FEC protection. The ToP value of 862 3 is reserved for future uses. 864 Encoding considerations: This media type is framed (See Section 4.8 865 in the template document [RFC6838]) and contains binary data. 867 Security considerations: See Section 9 of [RFCXXXX]. 869 Interoperability considerations: None. 871 Published specification: [RFCXXXX]. 873 Applications that use this media type: Multimedia applications that 874 want to improve resiliency against packet loss by sending redundant 875 data in addition to the source media. 877 Fragment identifier considerations: None. 879 Additional information: None. 881 Person & email address to contact for further information: Varun 882 Singh and IETF Audio/Video Transport Payloads 883 Working Group. 885 Intended usage: COMMON. 887 Restriction on usage: This media type depends on RTP framing, and 888 hence, is only defined for transport via RTP [RFC3550]. 890 Author: Varun Singh . 892 Change controller: IETF Audio/Video Transport Working Group delegated 893 from the IESG. 895 Provisional registration? (standards tree only): Yes. 897 5.1.3. Registration of text/flexfec 899 Type name: text 901 Subtype name: flexfec 903 Required parameters: 905 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 906 than 1000 Hz to provide sufficient resolution to RTCP operations. 908 However, it is RECOMMENDED to select the rate that matches the 909 rate of the protected source RTP stream. 911 o repair-window: The time that spans the source packets and the 912 corresponding repair packets. The size of the repair window is 913 specified in microseconds. 915 Optional parameters: 917 o L: indicates the number of columns of the source block that are 918 protected by this FEC block and it applies to all the source 919 SSRCs. L is a positive integer. 921 o D: indicates the number of rows of the source block that are 922 protected by this FEC block and it applies to all the source 923 SSRCs. D is a positive integer. 925 o ToP: indicates the type of protection applied by the sender: 0 for 926 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 927 protection, and 2 for 2-D parity FEC protection. The ToP value of 928 3 is reserved for future uses. 930 Encoding considerations: This media type is framed (See Section 4.8 931 in the template document [RFC6838]) and contains binary data. 933 Security considerations: See Section 9 of [RFCXXXX]. 935 Interoperability considerations: None. 937 Published specification: [RFCXXXX]. 939 Applications that use this media type: Multimedia applications that 940 want to improve resiliency against packet loss by sending redundant 941 data in addition to the source media. 943 Fragment identifier considerations: None. 945 Additional information: None. 947 Person & email address to contact for further information: Varun 948 Singh and IETF Audio/Video Transport Payloads 949 Working Group. 951 Intended usage: COMMON. 953 Restriction on usage: This media type depends on RTP framing, and 954 hence, is only defined for transport via RTP [RFC3550]. 956 Author: Varun Singh . 958 Change controller: IETF Audio/Video Transport Working Group delegated 959 from the IESG. 961 Provisional registration? (standards tree only): Yes. 963 5.1.4. Registration of application/flexfec 965 Type name: application 967 Subtype name: flexfec 969 Required parameters: 971 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 972 than 1000 Hz to provide sufficient resolution to RTCP operations. 973 However, it is RECOMMENDED to select the rate that matches the 974 rate of the protected source RTP stream. 976 o repair-window: The time that spans the source packets and the 977 corresponding repair packets. The size of the repair window is 978 specified in microseconds. 980 Optional parameters: 982 o L: indicates the number of columns of the source block that are 983 protected by this FEC block and it applies to all the source 984 SSRCs. L is a positive integer. 986 o D: indicates the number of rows of the source block that are 987 protected by this FEC block and it applies to all the source 988 SSRCs. D is a positive integer. 990 o ToP: indicates the type of protection applied by the sender: 0 for 991 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC 992 protection, and 2 for 2-D parity FEC protection. The ToP value of 993 3 is reserved for future uses. 995 Encoding considerations: This media type is framed (See Section 4.8 996 in the template document [RFC6838]) and contains binary data. 998 Security considerations: See Section 9 of [RFCXXXX]. 1000 Interoperability considerations: None. 1002 Published specification: [RFCXXXX]. 1004 Applications that use this media type: Multimedia applications that 1005 want to improve resiliency against packet loss by sending redundant 1006 data in addition to the source media. 1008 Fragment identifier considerations: None. 1010 Additional information: None. 1012 Person & email address to contact for further information: Varun 1013 Singh and IETF Audio/Video Transport Payloads 1014 Working Group. 1016 Intended usage: COMMON. 1018 Restriction on usage: This media type depends on RTP framing, and 1019 hence, is only defined for transport via RTP [RFC3550]. 1021 Author: Varun Singh . 1023 Change controller: IETF Audio/Video Transport Working Group delegated 1024 from the IESG. 1026 Provisional registration? (standards tree only): Yes. 1028 5.2. Mapping to SDP Parameters 1030 Applications that are using RTP transport commonly use Session 1031 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 1032 The information that is used to specify the media types in an RTP 1033 session has specific mappings to the fields in an SDP description. 1034 In this section, we provide these mappings for the media subtypes 1035 registered by this document. Note that if an application does not 1036 use SDP to describe the RTP sessions, an appropriate mapping must be 1037 defined and used to specify the media types and their parameters for 1038 the control/description protocol employed by the application. 1040 The mapping of the media type specification for "non-interleaved- 1041 parityfec" and "interleaved-parityfec" and their parameters in SDP is 1042 as follows: 1044 o The media type (e.g., "application") goes into the "m=" line as 1045 the media name. 1047 o The media subtype goes into the "a=rtpmap" line as the encoding 1048 name. The RTP clock rate parameter ("rate") also goes into the 1049 "a=rtpmap" line as the clock rate. 1051 o The remaining required payload-format-specific parameters go into 1052 the "a=fmtp" line by copying them directly from the media type 1053 string as a semicolon-separated list of parameter=value pairs. 1055 SDP examples are provided in Section 7. 1057 5.2.1. Offer-Answer Model Considerations 1059 When offering 1-D interleaved parity FEC over RTP using SDP in an 1060 Offer/Answer model [RFC3264], the following considerations apply: 1062 o Each combination of the L and D parameters produces a different 1063 FEC data and is not compatible with any other combination. A 1064 sender application may desire to offer multiple offers with 1065 different sets of L and D values as long as the parameter values 1066 are valid. The receiver SHOULD normally choose the offer that has 1067 a sufficient amount of interleaving. If multiple such offers 1068 exist, the receiver may choose the offer that has the lowest 1069 overhead or the one that requires the smallest amount of 1070 buffering. The selection depends on the application requirements. 1072 o The value for the repair-window parameter depends on the L and D 1073 values and cannot be chosen arbitrarily. More specifically, L and 1074 D values determine the lower limit for the repair-window size. 1075 The upper limit of the repair-window size does not depend on the L 1076 and D values. 1078 o Although combinations with the same L and D values but with 1079 different repair-window sizes produce the same FEC data, such 1080 combinations are still considered different offers. The size of 1081 the repair-window is related to the maximum delay between the 1082 transmission of a source packet and the associated repair packet. 1083 This directly impacts the buffering requirement on the receiver 1084 side and the receiver must consider this when choosing an offer. 1086 o There are no optional format parameters defined for this payload. 1087 Any unknown option in the offer MUST be ignored and deleted from 1088 the answer. If FEC is not desired by the receiver, it can be 1089 deleted from the answer. 1091 5.2.2. Declarative Considerations 1093 In declarative usage, like SDP in the Real-time Streaming Protocol 1094 (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) 1095 [RFC2974], the following considerations apply: 1097 o The payload format configuration parameters are all declarative 1098 and a participant MUST use the configuration that is provided for 1099 the session. 1101 o More than one configuration may be provided (if desired) by 1102 declaring multiple RTP payload types. In that case, the receivers 1103 should choose the repair stream that is best for them. 1105 6. Protection and Recovery Procedures - Parity Codes 1107 This section provides a complete specification of the 1-D and 2-D 1108 parity codes and their RTP payload formats. 1110 6.1. Overview 1112 The following sections specify the steps involved in generating the 1113 repair packets and reconstructing the missing source packets from the 1114 repair packets. 1116 6.2. Repair Packet Construction 1118 The RTP header of a repair packet is formed based on the guidelines 1119 given in Section 4.2. 1121 The FEC header includes 12 octets (or upto 28 octets when the longer 1122 optional masks are used). It is constructed by applying the XOR 1123 operation on the bit strings that are generated from the individual 1124 source packets protected by this particular repair packet. The set 1125 of the source packets that are associated with a given repair packet 1126 can be computed by the formula given in Section 6.3.1. 1128 The bit string is formed for each source packet by concatenating the 1129 following fields together in the order specified: 1131 o The first 64 bits of the RTP header (64 bits). 1133 o Unsigned network-ordered 16-bit representation of the source 1134 packet length in bytes minus 12 (for the fixed RTP header), i.e., 1135 the sum of the lengths of all the following if present: the CSRC 1136 list, extension header, RTP payload and RTP padding (16 bits). 1138 By applying the parity operation on the bit strings produced from the 1139 source packets, we generate the FEC bit string. The FEC header is 1140 generated from the FEC bit string as follows: 1142 o The first (most significant) 2 bits in the FEC bit string are 1143 skipped. The MSK bits in the FEC header are set to the 1144 appropriate value, i.e., it depends on the chosen bitmask length. 1146 o The next bit in the FEC bit string is written into the P recovery 1147 bit in the FEC header. 1149 o The next bit in the FEC bit string is written into the X recovery 1150 bit in the FEC header. 1152 o The next 4 bits of the FEC bit string are written into the CC 1153 recovery field in the FEC header. 1155 o The next bit is written into the M recovery bit in the FEC header. 1157 o The next 7 bits of the FEC bit string are written into the PT 1158 recovery field in the FEC header. 1160 o The next 16 bits are skipped. 1162 o The next 32 bits of the FEC bit string are written into the TS 1163 recovery field in the FEC header. 1165 o The next 16 bits are written into the length recovery field in the 1166 FEC header. 1168 o Depending on the chosen MSK value, the bit mask of appropriate 1169 length will be set to the appropriate values. 1171 As described in Section 4.2, the SN base field of the FEC header MUST 1172 be set to the lowest sequence number of the source packets protected 1173 by this repair packet. When MSK represents a bitmask (MSK=00,01,10), 1174 the SN base field corresponds to the lowest sequence number indicated 1175 in the bitmask. When MSK=11, the following considerations apply: 1) 1176 for the interleaved FEC repair packets, this corresponds to the 1177 lowest sequence number of the source packets that forms the column, 1178 2) for the non-interleaved FEC repair packets, the SN base field MUST 1179 be set to the lowest sequence number of the source packets that forms 1180 the row. 1182 The repair packet payload consists of the bits that are generated by 1183 applying the XOR operation on the payloads of the source RTP packets. 1184 If the payload lengths of the source packets are not equal, each 1185 shorter packet MUST be padded to the length of the longest packet by 1186 adding octet 0's at the end. 1188 Due to this possible padding and mandatory FEC header, a repair 1189 packet has a larger size than the source packets it protects. This 1190 may cause problems if the resulting repair packet size exceeds the 1191 Maximum Transmission Unit (MTU) size of the path over which the 1192 repair stream is sent. 1194 6.3. Source Packet Reconstruction 1196 This section describes the recovery procedures that are required to 1197 reconstruct the missing source packets. The recovery process has two 1198 steps. In the first step, the FEC decoder determines which source 1199 and repair packets should be used in order to recover a missing 1200 packet. In the second step, the decoder recovers the missing packet, 1201 which consists of an RTP header and RTP payload. 1203 In the following, we describe the RECOMMENDED algorithms for the 1204 first and second steps. Based on the implementation, different 1205 algorithms MAY be adopted. However, the end result MUST be identical 1206 to the one produced by the algorithms described below. 1208 Note that the same algorithms are used by the 1-D parity codes, 1209 regardless of whether the FEC protection is applied over a column or 1210 a row. The 2-D parity codes, on the other hand, usually require 1211 multiple iterations of the procedures described here. This iterative 1212 decoding algorithm is further explained in Section 6.3.4. 1214 6.3.1. Associating the Source and Repair Packets 1216 We denote the set of the source packets associated with repair packet 1217 p* by set T(p*). Note that in a source block whose size is L columns 1218 by D rows, set T includes D source packets plus one repair packet for 1219 the FEC protection applied over a column, and L source packets plus 1220 one repair packet for the FEC protection applied over a row. Recall 1221 that 1-D interleaved and non-interleaved FEC protection can fully 1222 recover the missing information if there is only one source packet 1223 missing per column or row in set T. If there are more than one 1224 source packets missing per column or row in set T, 1-D FEC protection 1225 may fail to recover all the missing information. 1227 6.3.1.1. Signaled in SDP 1229 The first step is associating the source and repair packets. If the 1230 endpoint relies entirely on out-of-band signaling (MSK=11, and 1231 M=N=0), then this information may be inferred from the media type 1232 parameters specified in the SDP description. Furthermore, the 1233 payload type field in the RTP header, assists the receiver 1234 distinguish an interleaved or non-interleaved FEC packet. 1236 Mathematically, for any received repair packet, p*, we can determine 1237 the sequence numbers of the source packets that are protected by this 1238 repair packet as follows: 1240 p*_snb + i * X_1 (modulo 65536) 1242 where p*_snb denotes the value in the SN base field of p*'s FEC 1243 header, X_1 is set to L and 1 for the interleaved and non-interleaved 1244 FEC repair packets, respectively, and 1246 0 <= i < X_2 1248 where X_2 is set to D and L for the interleaved and non-interleaved 1249 FEC repair packets, respectively. 1251 6.3.1.2. Using bitmasks 1253 When using fixed size bitmasks (16-, 48-, 112-bits), the SN base 1254 field in the FEC header indicates the lowest sequence number of the 1255 source packets that forms the FEC packet. Finally, the bits maked by 1256 "1" in the bitmask are offsets from the SN base and make up the rest 1257 of the packets protected by the FEC packet. The bitmasks are able to 1258 represent arbitrary protection patterns, for example, 1-D 1259 interleaved, 1-D non-interleaved, 2-D, staircase. 1261 6.3.1.3. Using M and N Offsets 1263 When value of M is non-zero, the 8-bit fields indicate the offset of 1264 packets protected by an interleaved (N>0) or non-interleaved (N=0) 1265 FEC packet. Using a combination of interleaved and non-interleaved 1266 FEC repair packets can form 2-D protection patterns. 1268 Mathematically, for any received repair packet, p*, we can determine 1269 the sequence numbers of the source packets that are protected by this 1270 repair packet are as follows: 1272 When N = 0: 1273 p*_snb, p*_snb+1,..., p*_snb+(M-1), p*_snb+M 1274 When N > 0: 1275 p*_snb, p*_snb+(Mx1), p*_snb+(Mx2),..., p*_snb+(Mx(N-1)), p*_snb+(MxN) 1277 6.3.2. Recovering the RTP Header 1279 For a given set T, the procedure for the recovery of the RTP header 1280 of the missing packet, whose sequence number is denoted by SEQNUM, is 1281 as follows: 1283 1. For each of the source packets that are successfully received in 1284 T, compute the 80-bit string by concatenating the first 64 bits 1285 of their RTP header and the unsigned network-ordered 16-bit 1286 representation of their length in bytes minus 12. 1288 2. For the repair packet in T, compute the FEC bit string from the 1289 first 80 bits of the FEC header. 1291 3. Calculate the recovered bit string as the XOR of the bit strings 1292 generated from all source packets in T and the FEC bit string 1293 generated from the repair packet in T. 1295 4. Create a new packet with the standard 12-byte RTP header and no 1296 payload. 1298 5. Set the version of the new packet to 2. Skip the first 2 bits 1299 in the recovered bit string. 1301 6. Set the Padding bit in the new packet to the next bit in the 1302 recovered bit string. 1304 7. Set the Extension bit in the new packet to the next bit in the 1305 recovered bit string. 1307 8. Set the CC field to the next 4 bits in the recovered bit string. 1309 9. Set the Marker bit in the new packet to the next bit in the 1310 recovered bit string. 1312 10. Set the Payload type in the new packet to the next 7 bits in the 1313 recovered bit string. 1315 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 1316 bits in the recovered bit string. 1318 12. Set the TS field in the new packet to the next 32 bits in the 1319 recovered bit string. 1321 13. Take the next 16 bits of the recovered bit string and set the 1322 new variable Y to whatever unsigned integer this represents 1323 (assuming network order). Convert Y to host order. Y 1324 represents the length of the new packet in bytes minus 12 (for 1325 the fixed RTP header), i.e., the sum of the lengths of all the 1326 following if present: the CSRC list, header extension, RTP 1327 payload and RTP padding. 1329 14. Set the SSRC of the new packet to the SSRC of the source RTP 1330 stream. 1332 This procedure recovers the header of an RTP packet up to (and 1333 including) the SSRC field. 1335 6.3.3. Recovering the RTP Payload 1337 Following the recovery of the RTP header, the procedure for the 1338 recovery of the RTP payload is as follows: 1340 1. Append Y bytes to the new packet. 1342 2. For each of the source packets that are successfully received in 1343 T, compute the bit string from the Y octets of data starting with 1344 the 13th octet of the packet. If any of the bit strings 1345 generated from the source packets has a length shorter than Y, 1346 pad them to that length. The padding of octet 0 MUST be added at 1347 the end of the bit string. Note that the information of the 1348 first 8 octets are protected by the FEC header. 1350 3. For the repair packet in T, compute the FEC bit string from the 1351 repair packet payload, i.e., the Y octets of data following the 1352 FEC header. Note that the FEC header may be 12, 16, 32 octets 1353 depending on the length of the bitmask. 1355 4. Calculate the recovered bit string as the XOR of the bit strings 1356 generated from all source packets in T and the FEC bit string 1357 generated from the repair packet in T. 1359 5. Append the recovered bit string (Y octets) to the new packet 1360 generated in Section 6.3.2. 1362 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection 1364 In 2-D parity FEC protection, the sender generates both non- 1365 interleaved and interleaved FEC repair packets to combat with the 1366 mixed loss patterns (random and bursty). At the receiver side, these 1367 FEC packets are used iteratively to overcome the shortcomings of the 1368 1-D non-interleaved/interleaved FEC protection and improve the 1369 chances of full error recovery. 1371 The iterative decoding algorithm runs as follows: 1373 1. Set num_recovered_until_this_iteration to zero 1375 2. Set num_recovered_so_far to zero 1377 3. Recover as many source packets as possible by using the non- 1378 interleaved FEC repair packets as outlined in Section 6.3.2 and 1379 Section 6.3.3, and increase the value of num_recovered_so_far by 1380 the number of recovered source packets. 1382 4. Recover as many source packets as possible by using the 1383 interleaved FEC repair packets as outlined in Section 6.3.2 and 1384 Section 6.3.3, and increase the value of num_recovered_so_far by 1385 the number of recovered source packets. 1387 5. If num_recovered_so_far > num_recovered_until_this_iteration 1388 ---num_recovered_until_this_iteration = num_recovered_so_far 1389 ---Go to step 3 1390 Else 1391 ---Terminate 1393 The algorithm terminates either when all missing source packets are 1394 fully recovered or when there are still remaining missing source 1395 packets but the FEC repair packets are not able to recover any more 1396 source packets. For the example scenarios when the 2-D parity FEC 1397 protection fails full recovery, refer to Section 1.1.4. Upon 1398 termination, variable num_recovered_so_far has a value equal to the 1399 total number of recovered source packets. 1401 Example: 1403 Suppose that the receiver experienced the loss pattern sketched in 1404 Figure 16. 1406 +---+ +---+ +===+ 1407 X X | 3 | | 4 | |R_1| 1408 +---+ +---+ +===+ 1410 +---+ +---+ +---+ +---+ +===+ 1411 | 5 | | 6 | | 7 | | 8 | |R_2| 1412 +---+ +---+ +---+ +---+ +===+ 1414 +---+ +---+ +===+ 1415 | 9 | X X | 12| |R_3| 1416 +---+ +---+ +===+ 1418 +===+ +===+ +===+ +===+ 1419 |C_1| |C_2| |C_3| |C_4| 1420 +===+ +===+ +===+ +===+ 1422 Figure 16: Example loss pattern for the iterative decoding algorithm 1424 The receiver executes the iterative decoding algorithm and recovers 1425 source packets #1 and #11 in the first iteration. The resulting 1426 pattern is sketched in Figure 17. 1428 +---+ +---+ +---+ +===+ 1429 | 1 | X | 3 | | 4 | |R_1| 1430 +---+ +---+ +---+ +===+ 1432 +---+ +---+ +---+ +---+ +===+ 1433 | 5 | | 6 | | 7 | | 8 | |R_2| 1434 +---+ +---+ +---+ +---+ +===+ 1436 +---+ +---+ +---+ +===+ 1437 | 9 | X | 11| | 12| |R_3| 1438 +---+ +---+ +---+ +===+ 1440 +===+ +===+ +===+ +===+ 1441 |C_1| |C_2| |C_3| |C_4| 1442 +===+ +===+ +===+ +===+ 1444 Figure 17: The resulting pattern after the first iteration 1446 Since the if condition holds true, the receiver runs a new iteration. 1447 In the second iteration, source packets #2 and #10 are recovered, 1448 resulting in a full recovery as sketched in Figure 18. 1450 +---+ +---+ +---+ +---+ +===+ 1451 | 1 | | 2 | | 3 | | 4 | |R_1| 1452 +---+ +---+ +---+ +---+ +===+ 1454 +---+ +---+ +---+ +---+ +===+ 1455 | 5 | | 6 | | 7 | | 8 | |R_2| 1456 +---+ +---+ +---+ +---+ +===+ 1458 +---+ +---+ +---+ +---+ +===+ 1459 | 9 | | 10| | 11| | 12| |R_3| 1460 +---+ +---+ +---+ +---+ +===+ 1462 +===+ +===+ +===+ +===+ 1463 |C_1| |C_2| |C_3| |C_4| 1464 +===+ +===+ +===+ +===+ 1466 Figure 18: The resulting pattern after the second iteration 1468 7. SDP Examples 1470 This section provides two SDP [RFC4566] examples. The examples use 1471 the FEC grouping semantics defined in [RFC5956]. 1473 7.1. Example SDP for Flexible FEC Protection with in-band SSRC mapping 1475 In this example, we have one source video stream and one FEC repair 1476 stream. The source and repair streams are multiplexed on different 1477 SSRCs. The repair window is set to 200 ms. 1479 v=0 1480 o=mo 1122334455 1122334466 IN IP4 fec.example.com 1481 s=FlexFEC minimal SDP signalling Example 1482 t=0 0 1483 m=video 30000 RTP/AVP 96 98 1484 c=IN IP4 143.163.151.157 1485 a=rtpmap:96 VP8/90000 1486 a=rtpmap:98 flexfec/90000 1487 a=fmtp:98; repair-window=200ms 1489 7.2. Example SDP for Flex FEC Protection with explicit signalling in 1490 the SDP 1492 In this example, we have one source video stream (ssrc:1234) and one 1493 FEC repair streams (ssrc:2345). We form one FEC group with the 1494 "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams 1495 are multiplexed on different SSRCs. The repair window is set to 200 1496 ms. 1498 v=0 1499 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1500 s=2-D Parity FEC with no in band signalling Example 1501 t=0 0 1502 m=video 30000 RTP/AVP 100 110 1503 c=IN IP4 233.252.0.1/127 1504 a=rtpmap:100 MP2T/90000 1505 a=rtpmap:110 flexfec/90000 1506 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 1507 a=ssrc:1234 1508 a=ssrc:2345 1509 a=ssrc-group:FEC-FR 1234 2345 1511 8. Congestion Control Considerations 1513 FEC is an effective approach to provide applications resiliency 1514 against packet losses. However, in networks where the congestion is 1515 a major contributor to the packet loss, the potential impacts of 1516 using FEC SHOULD be considered carefully before injecting the repair 1517 streams into the network. In particular, in bandwidth-limited 1518 networks, FEC repair streams may consume most or all of the available 1519 bandwidth and consequently may congest the network. In such cases, 1520 the applications MUST NOT arbitrarily increase the amount of FEC 1521 protection since doing so may lead to a congestion collapse. If 1522 desired, stronger FEC protection MAY be applied only after the source 1523 rate has been reduced. 1525 In a network-friendly implementation, an application SHOULD NOT send/ 1526 receive FEC repair streams if it knows that sending/receiving those 1527 FEC repair streams would not help at all in recovering the missing 1528 packets. However, it MAY still continue to use FEC if considered for 1529 bandwidth estimation instead of speculatively probe for additional 1530 capacity [Holmer13][Nagy14]. It is RECOMMENDED that the amount of 1531 FEC protection is adjusted dynamically based on the packet loss rate 1532 observed by the applications. 1534 In multicast scenarios, it may be difficult to optimize the FEC 1535 protection per receiver. If there is a large variation among the 1536 levels of FEC protection needed by different receivers, it is 1537 RECOMMENDED that the sender offers multiple repair streams with 1538 different levels of FEC protection and the receivers join the 1539 corresponding multicast sessions to receive the repair stream(s) that 1540 is best for them. 1542 9. Security Considerations 1544 RTP packets using the payload format defined in this specification 1545 are subject to the security considerations discussed in the RTP 1546 specification [RFC3550] and in any applicable RTP profile. The main 1547 security considerations for the RTP packet carrying the RTP payload 1548 format defined within this memo are confidentiality, integrity and 1549 source authenticity. Confidentiality is achieved by encrypting the 1550 RTP payload. Integrity of the RTP packets is achieved through a 1551 suitable cryptographic integrity protection mechanism. Such a 1552 cryptographic system may also allow the authentication of the source 1553 of the payload. A suitable security mechanism for this RTP payload 1554 format should provide confidentiality, integrity protection, and at 1555 least source authentication capable of determining if an RTP packet 1556 is from a member of the RTP session. 1558 Note that the appropriate mechanism to provide security to RTP and 1559 payloads following this memo may vary. It is dependent on the 1560 application, transport and signaling protocol employed. Therefore, a 1561 single mechanism is not sufficient, although if suitable, using the 1562 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1563 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1564 Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may 1565 exist. 1567 10. IANA Considerations 1569 New media subtypes are subject to IANA registration. For the 1570 registration of the payload formats and their parameters introduced 1571 in this document, refer to Section 5. 1573 11. Acknowledgments 1575 Some parts of this document are borrowed from [RFC5109]. Thus, the 1576 author would like to thank the editor of [RFC5109] and those who 1577 contributed to [RFC5109]. 1579 Thanks to Bernard Aboba , Rasmus Brandt , Roni Even , Stefan Holmer , 1580 Jonathan Lennox , and Magnus Westerlund for providing valuable 1581 feedback on earlier versions of this draft. 1583 12. Change Log 1585 Note to the RFC-Editor: please remove this section prior to 1586 publication as an RFC. 1588 12.1. draft-ietf-payload-flexible-fec-scheme-05 1590 FEC packet format changed as per discussions in IETF97, Seoul. 1592 12.2. draft-ietf-payload-flexible-fec-scheme-03 1594 FEC packet format changed as per discussions in IETF96, Berlin. 1596 Removed section on non-parity codes and flexfec-raptor. 1598 12.3. draft-ietf-payload-flexible-fec-scheme-02 1600 FEC packet format changed as per discussions in IETF94, Tokyo. 1602 Added section on non-parity codes. 1604 Registration of application/flexfec-raptor. 1606 12.4. draft-ietf-payload-flexible-fec-scheme-01 1608 FEC packet format changed as per discussions in IETF93, Prague. 1610 Replaced non-interleaved-parityfec and interleaved-parity-fec with 1611 flexfec. 1613 SDP simplified for the case when association to RTP is made in the 1614 FEC header and not in the SDP. 1616 12.5. draft-ietf-payload-flexible-fec-scheme-00 1618 Initial WG version, based on draft-singh-payload-1d2d-parity-scheme- 1619 00. 1621 12.6. draft-singh-payload-1d2d-parity-scheme-00 1623 This is the initial version, which is based on draft-ietf-fecframe- 1624 1d2d-parity-scheme-00. The following are the major changes compared 1625 to that document: 1627 o Updated packet format with 16-, 48-, 112- bitmask. 1629 o Updated the sections on: repair packet construction, source packet 1630 construction. 1632 o Updated the media type registration and aligned to RFC6838. 1634 12.7. draft-ietf-fecframe-1d2d-parity-scheme-00 1636 o Some details were added regarding the use of CNAME field. 1638 o Offer-Answer and Declarative Considerations sections have been 1639 completed. 1641 o Security Considerations section has been completed. 1643 o The timestamp field definition has changed. 1645 13. References 1647 13.1. Normative References 1649 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1650 Requirement Levels", BCP 14, RFC 2119, 1651 DOI 10.17487/RFC2119, March 1997, 1652 . 1654 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1655 with Session Description Protocol (SDP)", RFC 3264, 1656 DOI 10.17487/RFC3264, June 2002, 1657 . 1659 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1660 Jacobson, "RTP: A Transport Protocol for Real-Time 1661 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1662 July 2003, . 1664 [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP 1665 Payload Formats", RFC 3555, DOI 10.17487/RFC3555, July 1666 2003, . 1668 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1669 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1670 July 2006, . 1672 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 1673 the Session Description Protocol", RFC 5956, 1674 DOI 10.17487/RFC5956, September 2010, 1675 . 1677 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 1678 Correction (FEC) Framework", RFC 6363, 1679 DOI 10.17487/RFC6363, October 2011, 1680 . 1682 [RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design 1683 Considerations for Protocol Extensions", RFC 6709, 1684 DOI 10.17487/RFC6709, September 2012, 1685 . 1687 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1688 Specifications and Registration Procedures", BCP 13, 1689 RFC 6838, DOI 10.17487/RFC6838, January 2013, 1690 . 1692 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 1693 "Guidelines for Choosing RTP Control Protocol (RTCP) 1694 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 1695 September 2013, . 1697 13.2. Informative References 1699 [Holmer13] 1700 Holmer, S., Shemer, M., and M. Paniconi, "Handling Packet 1701 Loss in WebRTC", Proc. of IEEE International Conference on 1702 Image Processing (ICIP 2013) , 9 2013. 1704 [Nagy14] Nagy, M., Singh, V., Ott, J., and L. Eggert, "Congestion 1705 Control using FEC for Conversational Multimedia 1706 Communication", Proc. of 5th ACM Internation Conference on 1707 Multimedia Systems (MMSys 2014) , 3 2014. 1709 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1710 Streaming Protocol (RTSP)", RFC 2326, 1711 DOI 10.17487/RFC2326, April 1998, 1712 . 1714 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1715 for Generic Forward Error Correction", RFC 2733, 1716 DOI 10.17487/RFC2733, December 1999, 1717 . 1719 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1720 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 1721 October 2000, . 1723 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1724 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1725 RFC 3711, DOI 10.17487/RFC3711, March 2004, 1726 . 1728 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1729 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 1730 December 2005, . 1732 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1733 "Extended RTP Profile for Real-time Transport Control 1734 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1735 DOI 10.17487/RFC4585, July 2006, 1736 . 1738 [RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error 1739 Correction", RFC 5109, DOI 10.17487/RFC5109, December 1740 2007, . 1742 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1743 (TLS) Protocol Version 1.2", RFC 5246, 1744 DOI 10.17487/RFC5246, August 2008, 1745 . 1747 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 1748 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 1749 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 1750 DOI 10.17487/RFC7656, November 2015, 1751 . 1753 [SMPTE2022-1] 1754 SMPTE 2022-1-2007, "Forward Error Correction for Real-Time 1755 Video/Audio Transport over IP Networks", 2007. 1757 Authors' Addresses 1759 Mo Zanaty 1760 Cisco 1761 Raleigh, NC 1762 USA 1764 Email: mzanaty@cisco.com 1766 Varun Singh 1767 CALLSTATS I/O Oy 1768 Runeberginkatu 4c A 4 1769 Helsinki 00100 1770 Finland 1772 Email: varun.singh@iki.fi 1773 URI: http://www.callstats.io/ 1775 Ali Begen 1776 Networked Media 1777 Konya 1778 Turkey 1780 Email: ali.begen@networked.media 1782 Giridhar Mandyam 1783 Qualcomm Innovation Center 1784 5775 Morehouse Drive 1785 San Diego, CA 92121 1786 USA 1788 Phone: +1 858 651 7200 1789 Email: mandyam@qti.qualcomm.com