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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 FEC Framework A. Begen 3 Internet-Draft Cisco Systems 4 Intended status: Standards Track April 29, 2009 5 Expires: October 31, 2009 7 RTP Payload Format for 1-D Interleaved Parity FEC 8 draft-ietf-fecframe-interleaved-fec-scheme-04 10 Status of this Memo 12 This Internet-Draft is submitted to IETF in full conformance with the 13 provisions of BCP 78 and BCP 79. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This Internet-Draft will expire on October 31, 2009. 33 Copyright Notice 35 Copyright (c) 2009 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents in effect on the date of 40 publication of this document (http://trustee.ietf.org/license-info). 41 Please review these documents carefully, as they describe your rights 42 and restrictions with respect to this document. 44 Abstract 46 This document defines a new RTP payload format for the Forward Error 47 Correction (FEC) that is generated by the 1-D interleaved parity code 48 from a source media encapsulated in RTP. The 1-D interleaved parity 49 code is a systematic code, where a number of repair symbols are 50 generated from a set of source symbols and sent in a repair flow 51 separate from the source flow that carries the source symbols. The 52 1-D interleaved parity code offers a good protection against bursty 53 packet losses at a cost of decent complexity. The new payload format 54 defined in this document is used (with some exceptions) as a part of 55 the DVB Application-layer FEC specification. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6 61 1.2. Overhead Computation . . . . . . . . . . . . . . . . . . . 8 62 1.3. Relation to Existing Specifications . . . . . . . . . . . 8 63 1.3.1. RFC 2733 and RFC 3009 . . . . . . . . . . . . . . . . 8 64 1.3.2. SMPTE 2022-1 . . . . . . . . . . . . . . . . . . . . . 8 65 1.3.3. ETSI TS 102 034 . . . . . . . . . . . . . . . . . . . 9 66 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 10 67 3. Definitions, Notations and Abbreviations . . . . . . . . . . . 10 68 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 69 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 70 3.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 11 71 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 11 72 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . . 11 73 4.2. Repair Packets . . . . . . . . . . . . . . . . . . . . . . 11 74 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 15 75 5.1. Media Type Registration . . . . . . . . . . . . . . . . . 15 76 5.1.1. Registration of audio/1d-interleaved-parityfec . . . . 15 77 5.1.2. Registration of video/1d-interleaved-parityfec . . . . 16 78 5.1.3. Registration of text/1d-interleaved-parityfec . . . . 18 79 5.1.4. Registration of 80 application/1d-interleaved-parityfec . . . . . . . . . 19 81 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 20 82 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 21 83 5.2.2. Declarative Considerations . . . . . . . . . . . . . . 21 84 6. Protection and Recovery Procedures . . . . . . . . . . . . . . 22 85 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 22 86 6.2. Repair Packet Construction . . . . . . . . . . . . . . . . 22 87 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . . 24 88 6.3.1. Associating the Source and Repair Packets . . . . . . 24 89 6.3.2. Recovering the RTP Header and Payload . . . . . . . . 25 90 7. Session Description Protocol (SDP) Signaling . . . . . . . . . 26 91 8. Congestion Control Considerations . . . . . . . . . . . . . . 27 92 9. Security Considerations . . . . . . . . . . . . . . . . . . . 28 93 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 94 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 95 12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 29 96 12.1. draft-ietf-fecframe-interleaved-fec-scheme-04 . . . . . . 29 97 12.2. draft-ietf-fecframe-interleaved-fec-scheme-03 . . . . . . 29 98 12.3. draft-ietf-fecframe-interleaved-fec-scheme-02 . . . . . . 29 99 12.4. draft-ietf-fecframe-interleaved-fec-scheme-01 . . . . . . 29 100 12.5. draft-ietf-fecframe-interleaved-fec-scheme-00 . . . . . . 29 101 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 102 13.1. Normative References . . . . . . . . . . . . . . . . . . . 30 103 13.2. Informative References . . . . . . . . . . . . . . . . . . 30 104 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31 106 1. Introduction 108 This document extends the Forward Error Correction (FEC) header 109 defined in [RFC2733] and uses this new FEC header for the FEC that is 110 generated by the 1-D interleaved parity code from a source media 111 encapsulated in RTP [RFC3550]. The resulting new RTP payload format 112 is registered by this document. 114 The type of the source media protected by the 1-D interleaved parity 115 code can be audio, video, text or application. The FEC data are 116 generated according to the media type parameters that are 117 communicated through out-of-band means. The associations/ 118 relationships between the source and repair flows are also 119 communicated through out-of-band means. 121 The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation 122 to generate the repair symbols. In a nutshell, the following steps 123 take place: 125 1. The sender determines a set of source packets to be protected 126 together based on the media type parameters. 128 2. The sender applies the XOR operation on the source symbols to 129 generate the required number of repair symbols. 131 3. The sender packetizes the repair symbols and sends the repair 132 packet(s) along with the source packets to the receiver(s) (in 133 different flows). The repair packets MAY be sent proactively or 134 on-demand. 136 Note that the sender MUST transmit the source and repair packets in 137 different source and repair flows, respectively to offer backward 138 compatibility (See Section 4). At the receiver side, if all of the 139 source packets are successfully received, there is no need for FEC 140 recovery and the repair packets are discarded. However, if there are 141 missing source packets, the repair packets can be used to recover the 142 missing information. Block diagrams for the systematic parity FEC 143 encoder and decoder are sketched in Figure 1 and Figure 2, 144 respectively. 146 +------------+ 147 +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 148 +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 149 | Encoder | 150 | (Sender) | --> +==+ +==+ 151 +------------+ +==+ +==+ 153 Source Packet: +--+ Repair Packet: +==+ 154 +--+ +==+ 156 Figure 1: Block diagram for systematic parity FEC encoder 158 +------------+ 159 +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ 160 +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ 161 | Decoder | 162 +==+ +==+ --> | (Receiver) | 163 +==+ +==+ +------------+ 165 Source Packet: +--+ Repair Packet: +==+ Lost Packet: X 166 +--+ +==+ 168 Figure 2: Block diagram for systematic parity FEC decoder 170 Suppose that we have a group of D x L source packets that have 171 sequence numbers starting from 1 running to D x L. If we apply the 172 XOR operation to the group of the source packets whose sequence 173 numbers are L apart from each other as sketched in Figure 3, we 174 generate L repair packets. This process is referred to as 1-D 175 interleaved FEC protection, and the resulting L repair packets are 176 referred to as interleaved (or column) FEC packets. 178 +-------------+ +-------------+ +-------------+ +-------+ 179 | S_1 | | S_2 | | S3 | ... | S_L | 180 | S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL | 181 | . | | . | | | | | 182 | . | | . | | | | | 183 | . | | . | | | | | 184 | S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL | 185 +-------------+ +-------------+ +-------------+ +-------+ 186 + + + + 187 ------------- ------------- ------------- ------- 188 | XOR | | XOR | | XOR | ... | XOR | 189 ------------- ------------- ------------- ------- 190 = = = = 191 +===+ +===+ +===+ +===+ 192 |C_1| |C_2| |C_3| ... |C_L| 193 +===+ +===+ +===+ +===+ 195 Figure 3: Generating interleaved (column) FEC packets 197 In Figure 3, S_n and C_m denote the source packet with a sequence 198 number n and the interleaved (column) FEC packet with a sequence 199 number m, respectively. 201 1.1. Use Cases 203 We generate one interleaved FEC packet out of D non-consecutive 204 source packets. This repair packet can provide a full recovery of 205 the missing information if there is only one packet missing among the 206 corresponding source packets. This implies that 1-D interleaved FEC 207 protection performs well under bursty loss conditions provided that L 208 is chosen large enough, i.e., L-packet duration SHOULD NOT be shorter 209 than the duration of the burst that is intended to be repaired. 211 For example, consider the scenario depicted in Figure 4 where the 212 sender generates interleaved FEC packets and a bursty loss hits the 213 source packets. Since the number of columns is larger than the 214 number of packets lost due to the bursty loss, the repair operation 215 succeeds. 217 +---+ 218 | 1 | X X X 219 +---+ 221 +---+ +---+ +---+ +---+ 222 | 5 | | 6 | | 7 | | 8 | 223 +---+ +---+ +---+ +---+ 225 +---+ +---+ +---+ +---+ 226 | 9 | | 10| | 11| | 12| 227 +---+ +---+ +---+ +---+ 229 +===+ +===+ +===+ +===+ 230 |C_1| |C_2| |C_3| |C_4| 231 +===+ +===+ +===+ +===+ 233 Figure 4: Example scenario where 1-D interleaved FEC protection 234 succeeds error recovery 236 The sender may generate interleaved FEC packets to combat with the 237 bursty packet losses. However, two or more random packet losses may 238 hit the source and repair packets in the same column. In that case, 239 the repair operation fails. This is illustrated in Figure 5. Note 240 that it is possible that two or more bursty losses may occur in the 241 same source block, in which case interleaved FEC packets may still 242 fail to recover the lost data. 244 +---+ +---+ +---+ 245 | 1 | X | 3 | | 4 | 246 +---+ +---+ +---+ 248 +---+ +---+ +---+ 249 | 5 | X | 7 | | 8 | 250 +---+ +---+ +---+ 252 +---+ +---+ +---+ +---+ 253 | 9 | | 10| | 11| | 12| 254 +---+ +---+ +---+ +---+ 256 +===+ +===+ +===+ +===+ 257 |C_1| |C_2| |C_3| |C_4| 258 +===+ +===+ +===+ +===+ 260 Figure 5: Example scenario where 1-D interleaved FEC protection fails 261 error recovery 263 1.2. Overhead Computation 265 The overhead is defined as the ratio of the number of bytes belonging 266 to the repair packets to the number of bytes belonging to the 267 protected source packets. 269 Assuming that each repair packet carries an equal number of bytes 270 carried by a source packet, we can compute the overhead as follows: 272 Overhead = 1/D 274 where D is the number of rows in the source block. 276 1.3. Relation to Existing Specifications 278 This section discusses the relation of the current specification to 279 other existing specifications. 281 1.3.1. RFC 2733 and RFC 3009 283 The current specification extends the FEC header defined in [RFC2733] 284 and registers a new RTP payload format. This new payload format is 285 not backward compatible with the payload format that was registered 286 by [RFC3009]. 288 1.3.2. SMPTE 2022-1 290 In 2007, the Society of Motion Picture and Television Engineers 291 (SMPTE) - Technology Committee N26 on File Management and Networking 292 Technology - decided to revise the Pro-MPEG Code of Practice (CoP) #3 293 Release 2 specification, which (was initially produced by the Pro- 294 MPEG Forum in 2004) discussed the several aspects of the transmission 295 of MPEG-2 transport streams over IP networks. The new SMPTE 296 specification is referred to as [SMPTE2022-1]. 298 The Pro-MPEG CoP #3 r2 document was originally based on [RFC2733]. 299 SMPTE revised the document by extending the FEC header (by setting 300 the E bit) proposed in [RFC2733]. This extended header offers some 301 improvements. 303 For example, instead of utilizing the bitmap field used in [RFC2733], 304 [SMPTE2022-1] introduces separate fields to convey the number of rows 305 (D) and columns (L) of the source block as well as the type of the 306 repair packet (i.e., whether the repair packet is an interleaved FEC 307 packet computed over a column or a non-interleaved FEC packet 308 computed over a row). These fields plus the base sequence number 309 allow the receiver side to establish the associations between the 310 source and repair packets. Note that although the bitmap field is 311 not utilized, the FEC header of [SMPTE2022-1] inherently carries over 312 the bitmap field from [RFC2733]. 314 On the other hand, some parts of [SMPTE2022-1] are not in compliant 315 with RTP [RFC3550]. For example, [SMPTE2022-1] sets the SSRC field 316 to zero and does not use the timestamp field in the RTP headers of 317 the repair packets (Receivers ignore the timestamps of the repair 318 packets). Furthermore, [SMPTE2022-1] also sets the CC field in the 319 RTP header to zero and does not allow any Contributing Source (CSRC) 320 entry in the RTP header. 322 The current document adopts the extended FEC header of [SMPTE2022-1] 323 and registers a new RTP payload format. At the same time, this 324 document fixes the parts of [SMPTE2022-1] that are not compliant with 325 RTP [RFC3550], except the one discussed below. 327 The baseline header format first proposed in [RFC2733] does not have 328 fields to protect the P and X bits and the CC fields of the source 329 packets associated with a repair packet. Rather, the P bit, X bit 330 and CC field in the RTP header of the repair packet are used to 331 protect those bits and fields. This, however, may sometimes result 332 in failures when doing the RTP header validity checks as specified in 333 [RFC3550]. While this behavior has been fixed in [RFC5109] that 334 obsoleted [RFC2733], the RTP payload format defined in this document 335 still allows for this behavior for legacy purposes. Implementations 336 following this specification MUST be aware of this potential issue 337 when RTP header validity checks are applied. 339 1.3.3. ETSI TS 102 034 341 In 2007, the Digital Video Broadcasting (DVB) consortium published a 342 technical specification [ETSI-TS-102-034] through European 343 Telecommunications Standards Institute (ETSI). This specification 344 covers several areas related to the transmission of MPEG-2 transport 345 stream-based services over IP networks. 347 The Annex E of [ETSI-TS-102-034] defines an optional protocol for 348 Application-layer FEC (AL-FEC) protection of streaming media for 349 DVB-IP services carried over RTP [RFC3550] transport. AL-FEC 350 protocol uses two layers for protection: a base layer that is 351 produced by a packet-based interleaved parity code, and an 352 enhancement layer that is produced by a Raptor code. While the use 353 of the enhancement layer is optional, the use of the base layer is 354 mandatory wherever AL-FEC is used. The DVB AL-FEC protocol is also 355 described in [I-D.ietf-fecframe-dvb-al-fec]. 357 The interleaved parity code that is used in the base layer is a 358 subset of [SMPTE2022-1]. In particular, AL-FEC base layer uses only 359 the 1-D interleaved FEC protection from [SMPTE2022-1]. The new RTP 360 payload format that is defined and registered in this document (with 361 some exceptions listed in [I-D.ietf-fecframe-dvb-al-fec]) is used as 362 the AL-FEC base layer. 364 2. Requirements Notation 366 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 367 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 368 document are to be interpreted as described in [RFC2119]. 370 3. Definitions, Notations and Abbreviations 372 The definitions, notations and abbreviations commonly used in this 373 document are summarized in this section. 375 3.1. Definitions 377 This document uses the following definitions: 379 Source Flow: The packet flow(s) carrying the source data and to 380 which FEC protection is to be applied. 382 Repair Flow: The packet flow(s) carrying the repair data. 384 Symbol: A unit of data. Its size, in bytes, is referred to as the 385 symbol size. 387 Source Symbol: The smallest unit of data used during the encoding 388 process. 390 Repair Symbol: Repair symbols are generated from the source symbols. 392 Source Packet: Data packets that contain only source symbols. 394 Repair Packet: Data packets that contain only repair symbols. 396 Source Block: A block of source symbols that are considered together 397 in the encoding process. 399 3.2. Notations 401 o L: Number of columns of the source block. 403 o D: Number of rows of the source block. 405 3.3. Abbreviations 407 o XOR: Bitwise exclusive OR operation. 408 0 XOR 0 = 0 409 0 XOR 1 = 1 410 1 XOR 0 = 1 411 1 XOR 1 = 0 413 4. Packet Formats 415 This section defines the formats of the source and repair packets. 417 4.1. Source Packets 419 The source packets MUST contain the information that identifies the 420 source block and the position within the source block occupied by the 421 packet. Since the source packets that are carried within an RTP 422 stream already contain unique sequence numbers in their RTP headers 423 [RFC3550], we can identify the source packets in a straightforward 424 manner and there is no need to append additional field(s). The 425 primary advantage of not modifying the source packets in any way is 426 that it provides backward compatibility for the receivers that do not 427 support FEC at all. In multicast scenarios, this backward 428 compatibility becomes quite useful as it allows the non-FEC-capable 429 and FEC-capable receivers to receive and interpret the same source 430 packets sent in the same multicast session. 432 4.2. Repair Packets 434 The repair packets MUST contain information that identifies the 435 source block they pertain to and the relationship between the 436 contained repair symbols and the original source block. For this 437 purpose, we use the RTP header of the repair packets as well as 438 another header within the RTP payload, which we refer to as the FEC 439 header, as shown in Figure 6. 441 +------------------------------+ 442 | IP Header | 443 +------------------------------+ 444 | Transport Header | 445 +------------------------------+ 446 | RTP Header | __ 447 +------------------------------+ | 448 | FEC Header | \ 449 +------------------------------+ > RTP Payload 450 | Repair Symbols | / 451 +------------------------------+ __| 453 Figure 6: Format of repair packets 455 The RTP header is formatted according to [RFC3550] with some further 456 clarifications listed below: 458 o Version: The version field is set to 2. 460 o Padding (P) Bit: This bit is obtained by applying protection to 461 the corresponding P bits from the RTP headers of the source 462 packets protected by this repair packet. However, padding octets 463 are never present in a repair packet, independent of the value of 464 the P bit. 466 o Extension (X) Bit: This bit is obtained by applying protection to 467 the corresponding X bits from the RTP headers of the source 468 packets protected by this repair packet. However, an RTP header 469 extension is never present in a repair packet, independent of the 470 value of the X bit. 472 o CSRC Count (CC): This field is obtained by applying protection to 473 the corresponding CC values from the RTP headers of the source 474 packets protected by this repair packet. However, a CSRC list is 475 never present in a repair packet, independent of the value of the 476 CC field. 478 o Marker (M) Bit: This bit is obtained by applying protection to 479 the corresponding M bits from the RTP headers of the source 480 packets protected by this repair packet. 482 o Payload Type: The (dynamic) payload type for the repair packets 483 is determined through out-of-band means. Note that this document 484 registers a new payload format for the repair packets (Refer to 485 Section 5 for details). According to [RFC3550], an RTP receiver 486 that cannot recognize a payload type must discard it. This 487 provides backward compatibility. The FEC mechanisms can then be 488 used in a multicast group with mixed FEC-capable and non-FEC- 489 capable receivers. If a non-FEC-capable receiver receives a 490 repair packet, it will not recognize the payload type, and hence, 491 discards the repair packet. 493 o Sequence Number (SN): The sequence number has the standard 494 definition. It MUST be one higher than the sequence number in the 495 previously transmitted repair packet. The initial value of the 496 sequence number SHOULD be random (unpredictable) [RFC3550]. 498 o Timestamp (TS): The timestamp SHALL be set to a time 499 corresponding to the repair packet's transmission time. Note that 500 the timestamp value has no use in the actual FEC protection 501 process and is usually useful for jitter calculations. 503 o Synchronization Source (SSRC): The SSRC value SHALL be randomly 504 assigned as suggested by [RFC3550]. This allows the sender to 505 multiplex the source and repair flows on the same port, or 506 multiplex multiple repair flows on a single port. The repair 507 flows SHOULD use the RTCP CNAME field to associate themselves with 508 the source flow. 510 In some networks, the RTP Source, which produces the source 511 packets and the FEC Source, which generates the repair packets 512 from the source packets may not be the same host. In such 513 scenarios, using the same CNAME for the source and repair flows 514 means that the RTP Source and the FEC Source MUST share the same 515 CNAME (for this specific source-repair flow association). A 516 common CNAME may be produced based on an algorithm that is known 517 both to the RTP and FEC Source. This usage is compliant with 518 [RFC3550]. 520 Note that due to the randomness of the SSRC assignments, there is 521 a possibility of SSRC collision. In such cases, the collisions 522 MUST be resolved as described in [RFC3550]. 524 Note that the P bit, X bit, CC field and M bit of the source packets 525 are protected by the corresponding bits/fields in the RTP header of 526 the repair packet. On the other hand, the payload of a repair packet 527 protects the concatenation of (if present) the CSRC list, RTP 528 extension, payload and padding of the source RTP packets associated 529 with this repair packet. 531 The FEC header is 16 octets. The format of the FEC header is shown 532 in Figure 7. 534 0 1 2 3 535 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 1 536 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 537 | SN base low | Length recovery | 538 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 539 |E| PT recovery | Mask | 540 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 541 | TS recovery | 542 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 543 |N|D|Type |Index| Offset | NA | SN base ext | 544 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 Figure 7: Format of the FEC header 548 The FEC header consists of the following fields: 550 o The SN base low field is used to indicate the lowest sequence 551 number, taking wrap around into account, of those source packets 552 protected by this repair packet. 554 o The Length recovery field is used to determine the length of any 555 recovered packets. 557 o The E bit is the extension flag introduced in [RFC2733] and used 558 to extend the [RFC2733] FEC header. 560 o The PT recovery field is used to determine the payload type of the 561 recovered packets. 563 o The Mask field is not used. 565 o The TS recovery field is used to determine the timestamp of the 566 recovered packets. 568 o The N bit is the extension flag that is reserved for future uses. 570 o The D bit is not used. 572 o The Type field indicates the type of the error-correcting code 573 used. This document defines only one error-correcting code. 575 o The Index field is not used. 577 o The Offset and NA fields are used to indicate the number of 578 columns (L) and rows (D) of the source block, respectively. 580 o The SN base ext field is not used. 582 The details on setting the fields in the FEC header are provided in 583 Section 6.2. 585 It should be noted that a mask-based approach (similar to the one 586 specified in [RFC2733]) may not be very efficient to indicate which 587 source packets in the current source block are associated with a 588 given repair packet. In particular, for the applications that would 589 like to use large source block sizes, the size of the mask that is 590 required to describe the source-repair packet associations may be 591 prohibitively large. Instead, a systematic approach is inherently 592 more efficient. 594 5. Payload Format Parameters 596 This section provides the media subtype registration for the 1-D 597 interleaved parity FEC. The parameters that are required to 598 configure the FEC encoding and decoding operations are also defined 599 in this section. 601 5.1. Media Type Registration 603 This registration is done using the template defined in [RFC4288] and 604 following the guidance provided in [RFC3555]. 606 Note to the RFC Editor: In the following sections, please replace 607 "XXXX" with the number of this document prior to publication as an 608 RFC. 610 5.1.1. Registration of audio/1d-interleaved-parityfec 612 Type name: audio 614 Subtype name: 1d-interleaved-parityfec 616 Required parameters: 618 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 619 than 1000 Hz to provide sufficient resolution to RTCP operations. 620 However, it is RECOMMENDED to select the rate that matches the 621 rate of the protected source RTP stream. 623 o L: Number of columns of the source block. L is a positive 624 integer that is less than or equal to 255. 626 o D: Number of rows of the source block. D is a positive integer 627 that is less than or equal to 255. 629 o repair-window: The time that spans the source packets and the 630 corresponding repair packets. An FEC encoder processes a block of 631 source packets and generates a number of repair packets, which are 632 then transmitted within a certain duration. At the receiver, the 633 FEC decoder tries to decode all the packets received within the 634 repair window to recover the missing packets. Assuming that there 635 is no issue of delay variation, the FEC decoder SHOULD NOT wait 636 longer than the repair window since additional waiting would not 637 help the recovery process. The size of the repair window is 638 specified in microseconds. 640 Optional parameters: None. 642 Encoding considerations: This media type is framed (See Section 4.8 643 in the template document [RFC4288]) and contains binary data. 645 Security considerations: See Section 9 of [RFCXXXX]. 647 Interoperability considerations: None. 649 Published specification: [RFCXXXX]. 651 Applications that use this media type: Multimedia applications that 652 want to improve resiliency against packet loss by sending redundant 653 data in addition to the source media. 655 Additional information: None. 657 Person & email address to contact for further information: Ali Begen 658 and IETF Audio/Video Transport Working Group. 660 Intended usage: COMMON. 662 Restriction on usage: This media type depends on RTP framing, and 663 hence, is only defined for transport via RTP [RFC3550]. 665 Author: Ali Begen . 667 Change controller: IETF Audio/Video Transport Working Group 668 delegated from the IESG. 670 5.1.2. Registration of video/1d-interleaved-parityfec 672 Type name: video 674 Subtype name: 1d-interleaved-parityfec 676 Required parameters: 678 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 679 than 1000 Hz to provide sufficient resolution to RTCP operations. 680 However, it is RECOMMENDED to select the rate that matches the 681 rate of the protected source RTP stream. 683 o L: Number of columns of the source block. L is a positive 684 integer that is less than or equal to 255. 686 o D: Number of rows of the source block. D is a positive integer 687 that is less than or equal to 255. 689 o repair-window: The time that spans the source packets and the 690 corresponding repair packets. An FEC encoder processes a block of 691 source packets and generates a number of repair packets, which are 692 then transmitted within a certain duration. At the receiver, the 693 FEC decoder tries to decode all the packets received within the 694 repair window to recover the missing packets. Assuming that there 695 is no issue of delay variation, the FEC decoder SHOULD NOT wait 696 longer than the repair window since additional waiting would not 697 help the recovery process. The size of the repair window is 698 specified in microseconds. 700 Optional parameters: None. 702 Encoding considerations: This media type is framed (See Section 4.8 703 in the template document [RFC4288]) and contains binary data. 705 Security considerations: See Section 9 of [RFCXXXX]. 707 Interoperability considerations: None. 709 Published specification: [RFCXXXX]. 711 Applications that use this media type: Multimedia applications that 712 want to improve resiliency against packet loss by sending redundant 713 data in addition to the source media. 715 Additional information: None. 717 Person & email address to contact for further information: Ali Begen 718 and IETF Audio/Video Transport Working Group. 720 Intended usage: COMMON. 722 Restriction on usage: This media type depends on RTP framing, and 723 hence, is only defined for transport via RTP [RFC3550]. 725 Author: Ali Begen . 727 Change controller: IETF Audio/Video Transport Working Group 728 delegated from the IESG. 730 5.1.3. Registration of text/1d-interleaved-parityfec 732 Type name: text 734 Subtype name: 1d-interleaved-parityfec 736 Required parameters: 738 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 739 than 1000 Hz to provide sufficient resolution to RTCP operations. 740 However, it is RECOMMENDED to select the rate that matches the 741 rate of the protected source RTP stream. 743 o L: Number of columns of the source block. L is a positive 744 integer that is less than or equal to 255. 746 o D: Number of rows of the source block. D is a positive integer 747 that is less than or equal to 255. 749 o repair-window: The time that spans the source packets and the 750 corresponding repair packets. An FEC encoder processes a block of 751 source packets and generates a number of repair packets, which are 752 then transmitted within a certain duration. At the receiver, the 753 FEC decoder tries to decode all the packets received within the 754 repair window to recover the missing packets. Assuming that there 755 is no issue of delay variation, the FEC decoder SHOULD NOT wait 756 longer than the repair window since additional waiting would not 757 help the recovery process. The size of the repair window is 758 specified in microseconds. 760 Optional parameters: None. 762 Encoding considerations: This media type is framed (See Section 4.8 763 in the template document [RFC4288]) and contains binary data. 765 Security considerations: See Section 9 of [RFCXXXX]. 767 Interoperability considerations: None. 769 Published specification: [RFCXXXX]. 771 Applications that use this media type: Multimedia applications that 772 want to improve resiliency against packet loss by sending redundant 773 data in addition to the source media. 775 Additional information: None. 777 Person & email address to contact for further information: Ali Begen 778 and IETF Audio/Video Transport Working Group. 780 Intended usage: COMMON. 782 Restriction on usage: This media type depends on RTP framing, and 783 hence, is only defined for transport via RTP [RFC3550]. 785 Author: Ali Begen . 787 Change controller: IETF Audio/Video Transport Working Group 788 delegated from the IESG. 790 5.1.4. Registration of application/1d-interleaved-parityfec 792 Type name: application 794 Subtype name: 1d-interleaved-parityfec 796 Required parameters: 798 o rate: The RTP timestamp (clock) rate. The rate SHALL be larger 799 than 1000 Hz to provide sufficient resolution to RTCP operations. 800 However, it is RECOMMENDED to select the rate that matches the 801 rate of the protected source RTP stream. 803 o L: Number of columns of the source block. L is a positive 804 integer that is less than or equal to 255. 806 o D: Number of rows of the source block. D is a positive integer 807 that is less than or equal to 255. 809 o repair-window: The time that spans the source packets and the 810 corresponding repair packets. An FEC encoder processes a block of 811 source packets and generates a number of repair packets, which are 812 then transmitted within a certain duration. At the receiver, the 813 FEC decoder tries to decode all the packets received within the 814 repair window to recover the missing packets. Assuming that there 815 is no issue of delay variation, the FEC decoder SHOULD NOT wait 816 longer than the repair window since additional waiting would not 817 help the recovery process. The size of the repair window is 818 specified in microseconds. 820 Optional parameters: None. 822 Encoding considerations: This media type is framed (See Section 4.8 823 in the template document [RFC4288]) and contains binary data. 825 Security considerations: See Section 9 of [RFCXXXX]. 827 Interoperability considerations: None. 829 Published specification: [RFCXXXX]. 831 Applications that use this media type: Multimedia applications that 832 want to improve resiliency against packet loss by sending redundant 833 data in addition to the source media. 835 Additional information: None. 837 Person & email address to contact for further information: Ali Begen 838 and IETF Audio/Video Transport Working Group. 840 Intended usage: COMMON. 842 Restriction on usage: This media type depends on RTP framing, and 843 hence, is only defined for transport via RTP [RFC3550]. 845 Author: Ali Begen . 847 Change controller: IETF Audio/Video Transport Working Group 848 delegated from the IESG. 850 5.2. Mapping to SDP Parameters 852 Applications that are using RTP transport commonly use Session 853 Description Protocol (SDP) [RFC4566] to describe their RTP sessions. 854 The information that is used to specify the media types in an RTP 855 session has specific mappings to the fields in an SDP description. 856 In this section, we provide these mappings for the media subtype 857 registered by this document ("1d-interleaved-parityfec"). Note that 858 if an application does not use SDP to describe the RTP sessions, an 859 appropriate mapping must be defined and used to specify the media 860 types and their parameters for the control/description protocol 861 employed by the application. 863 The mapping of the media type specification for "1d-interleaved- 864 parityfec" and its parameters in SDP is as follows: 866 o The media type (e.g., "application") goes into the "m=" line as 867 the media name. 869 o The media subtype ("1d-interleaved-parityfec") goes into the 870 "a=rtpmap" line as the encoding name. The RTP clock rate 871 parameter ("rate") also goes into the "a=rtpmap" line as the clock 872 rate. 874 o The remaining required payload-format-specific parameters go into 875 the "a=fmtp" line by copying them directly from the media type 876 string as a semicolon-separated list of parameter=value pairs. 878 SDP examples are provided in Section 7. 880 5.2.1. Offer-Answer Model Considerations 882 When offering 1-D interleaved parity FEC over RTP using SDP in an 883 Offer/Answer model [RFC3264], the following considerations apply: 885 o Each combination of the L and D parameters produces a different 886 FEC data and is not compatible with any other combination. A 887 sender application may desire to offer multiple offers with 888 different sets of L and D values as long as the parameter values 889 are valid. The receiver SHOULD normally choose the offer that has 890 a sufficient amount of interleaving. If multiple such offers 891 exist, the receiver may choose the offer that has the lowest 892 overhead or the one that requires the smallest amount of 893 buffering. The selection depends on the application requirements. 895 o The value for the repair-window parameter depends on the L and D 896 values and cannot be chosen arbitrarily. More specifically, L and 897 D values determine the lower limit for the repair-window size. 898 The upper limit of the repair-window size does not depend on the L 899 and D values. 901 o Although combinations with the same L and D values but with 902 different repair-window sizes produce the same FEC data, such 903 combinations are still considered different offers. The size of 904 the repair-window is related to how fast the sender will send the 905 repair packets. This directly impacts the buffering requirement 906 on the receiver side and the receiver must consider this when 907 choosing an offer. 909 o There are no optional format parameters defined for this payload. 910 Any unknown option in the offer MUST be ignored and deleted from 911 the answer. If FEC is not desired by the receiver, it can be 912 deleted from the answer. 914 5.2.2. Declarative Considerations 916 In declarative usage, like SDP in the Real-time Streaming Protocol 917 (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) 918 [RFC2974], the following considerations apply: 920 o The payload format configuration parameters are all declarative 921 and a participant MUST use the configuration that is provided for 922 the session. 924 o More than one configuration may be provided (if desired) by 925 declaring multiple RTP payload types. In that case, the receivers 926 should choose the repair flow that is best for them. 928 6. Protection and Recovery Procedures 930 This section provides a complete specification of the 1-D interleaved 931 parity code. 933 6.1. Overview 935 The following sections specify the steps involved in generating the 936 repair packets and reconstructing the missing source packets from the 937 repair packets. 939 6.2. Repair Packet Construction 941 The RTP header of a repair packet is formed based on the guidelines 942 given in Section 4.2. 944 The FEC header includes 16 octets. It is constructed by applying the 945 XOR operation on the bit strings that are generated from the 946 individual source packets protected by this particular repair packet. 947 The set of the source packets that are associated with a given repair 948 packet can be computed by the formula given in Section 6.3.1. 950 The bit string is formed for each source packet by concatenating the 951 following fields together in the order specified: 953 o Padding bit (1 bit) (This is the most significant bit of the bit 954 string) 956 o Extension bit (1 bit) 958 o CC field (4 bits) 960 o Marker bit (1 bit) 962 o PT field (7 bits) 964 o Timestamp (32 bits) 965 o Unsigned network-ordered 16-bit representation of the source 966 packet length in bytes minus 12 (for the fixed RTP header), i.e., 967 the sum of the lengths of all the following if present: the CSRC 968 list, header extension, RTP payload and RTP padding (16 bits) 970 o If CC is nonzero, the CSRC list (variable length) 972 o If X is 1, the header extension (variable length) 974 o Payload (variable length) 976 o Padding, if present (variable length) 978 Note that if the payload lengths of the source packets are not equal, 979 each shorter packet MUST be padded to the length of the longest 980 packet by adding octet 0's at the end. Due to this possible padding 981 and mandatory FEC header, a repair packet usually has a larger size 982 than the source packets it protects. This may cause problems if the 983 resulting repair packet size exceeds the Maximum Transmission Unit 984 (MTU) size of the path over which the repair flow is sent. 986 By applying the parity operation on the bit strings produced from the 987 source packets, we generate the FEC bit string. Some parts of the 988 RTP header and the FEC header of the repair packet are generated from 989 the FEC bit string as follows: 991 o The first (most significant) bit in the FEC bit string is written 992 into the Padding bit in the RTP header of the repair packet. 994 o The next bit in the FEC bit string is written into the Extension 995 bit in the RTP header of the repair packet. 997 o The next 4 bits of the FEC bit string are written into the CC 998 field in the RTP header of the repair packet. 1000 o The next bit of the FEC bit string is written into the Marker bit 1001 in the RTP header of the repair packet. 1003 o The next 7 bits of the FEC bit string are written into the PT 1004 recovery field in the FEC header. 1006 o The next 32 bits of the FEC bit string are written into the TS 1007 recovery field in the FEC header. 1009 o The next 16 bits are written into the Length recovery field in the 1010 FEC header. This allows the FEC procedure to be applied even when 1011 the lengths of the protected source packets are not identical. 1013 o The remaining bits are set to be the payload of the repair packet. 1015 The remaining parts of the FEC header are set as follows: 1017 o The SN base low field MUST be set to the lowest sequence number, 1018 taking wrap around into account, of those source packets protected 1019 by this repair packet. 1021 o The E bit MUST be set to 1 to extend the [RFC2733] FEC header. 1023 o The Mask field SHALL be set to 0 and ignored by the receiver. 1025 o The N bit SHALL be set to 0 and ignored by the receiver. 1027 o The D bit SHALL be set to 0 and ignored by the receiver. 1029 o The Type field MUST be set to 0. 1031 o The Index field SHALL be set to 0 and ignored by the receiver. 1033 o The Offset field MUST be set to the number of columns of the 1034 source block (L). 1036 o The NA field MUST be set to the number of rows of the source block 1037 (D). 1039 o The SN base ext field SHALL be set to 0 and ignored by the 1040 receiver. 1042 6.3. Source Packet Reconstruction 1044 This section describes the recovery procedures that are required to 1045 reconstruct the missing packets. The recovery process has two steps. 1046 In the first step, the FEC decoder determines which source and repair 1047 packets should be used in order to recover a missing packet. In the 1048 second step, the decoder recovers the missing packet, which consists 1049 of an RTP header and RTP payload. 1051 In the following, we describe the RECOMMENDED algorithms for the 1052 first and second steps. Based on the implementation, different 1053 algorithms MAY be adopted. However, the end result MUST be identical 1054 to the one produced by the algorithms described below. 1056 6.3.1. Associating the Source and Repair Packets 1058 The first step is to associate the source and repair packets. The SN 1059 base low field in the FEC header shows the lowest sequence number of 1060 the source packets that form the particular column. In addition, the 1061 information of how many source packets are available in each column 1062 and row is available from the media type parameters specified in the 1063 SDP description. This set of information uniquely identifies all of 1064 the source packets associated with a given repair packet. 1066 Mathematically, for any received repair packet, p*, we can determine 1067 the sequence numbers of the source packets that are protected by this 1068 repair packet as follows: 1070 p*_snb + i * L 1072 where p*_snb denotes the value in the SN base low field of p*'s FEC 1073 header, L is the number of columns of the source block and 1075 0 <= i < D 1077 where D is the number of rows of the source block. 1079 We denote the set of the source packets associated with repair packet 1080 p* by set T(p*). Note that in a source block whose size is L columns 1081 by D rows, set T includes D source packets. Recall that 1-D 1082 interleaved FEC protection can fully recover the missing information 1083 if there is only one source packet is missing in set T. If the repair 1084 packet that protects the source packets in set T is missing, or the 1085 repair packet is available but two or more source packets are 1086 missing, then missing source packets in set T cannot be recovered by 1087 1-D interleaved FEC protection. 1089 6.3.2. Recovering the RTP Header and Payload 1091 For a given set T, the procedure for the recovery of the RTP header 1092 of the missing packet, whose sequence number is denoted by SEQNUM, is 1093 as follows: 1095 1. For each of the source packets that are successfully received in 1096 set T, compute the bit string as described in Section 6.2. 1098 2. For the repair packet associated with set T, compute the bit 1099 string in the same fashion except use the PT recovery field 1100 instead of the PT field and TS recovery field instead of the 1101 Timestamp field, and set the CSRC list, header extension and 1102 padding to null regardless of the values of the CC field, X bit 1103 and P bit. 1105 3. If any of the bit strings generated from the source packets are 1106 shorter than the bit string generated from the repair packet, 1107 pad them to be the same length as the bit string generated from 1108 the repair packet. For padding, the padding of octet 0 MUST be 1109 added at the end of the bit string. 1111 4. Calculate the recovered bit string as the XOR of the bit strings 1112 generated from all source packets in set T and the FEC bit 1113 string generated from the repair packet associated with set T. 1115 5. Create a new packet with the standard 12-byte RTP header and no 1116 payload. 1118 6. Set the version of the new packet to 2. 1120 7. Set the Padding bit in the new packet to the first bit in the 1121 recovered bit string. 1123 8. Set the Extension bit in the new packet to the next bit in the 1124 recovered bit string. 1126 9. Set the CC field to the next 4 bits in the recovered bit string. 1128 10. Set the Marker bit in the new packet to the next bit in the 1129 recovered bit string. 1131 11. Set the Payload type in the new packet to the next 7 bits in the 1132 recovered bit string. 1134 12. Set the SN field in the new packet to SEQNUM. 1136 13. Set the TS field in the new packet to the next 32 bits in the 1137 recovered bit string. 1139 14. Take the next 16 bits of the recovered bit string and set Y to 1140 whatever unsigned integer this represents (assuming network- 1141 order). Take Y bytes from the recovered bit string and append 1142 them to the new packet. Y represents the length of the new 1143 packet in bytes minus 12 (for the fixed RTP header), i.e., the 1144 sum of the lengths of all the following if present: the CSRC 1145 list, header extension, RTP payload and RTP padding. 1147 15. Set the SSRC of the new packet to the SSRC of the source RTP 1148 stream. 1150 This procedure completely recovers both the header and payload of an 1151 RTP packet. 1153 7. Session Description Protocol (SDP) Signaling 1155 This section provides an SDP [RFC4566] example. The following 1156 example uses the FEC grouping semantics [RFC4756]. 1158 In this example, we have one source video stream (mid:S1) and one FEC 1159 repair stream (mid:R1). We form one FEC group with the "a=group:FEC 1160 S1 R1" line. The source and repair streams are sent to the same port 1161 on different multicast groups. The repair window is set to 200 ms. 1163 v=0 1164 o=ali 1122334455 1122334466 IN IP4 fec.example.com 1165 s=Interleaved Parity FEC Example 1166 t=0 0 1167 a=group:FEC S1 R1 1168 m=video 30000 RTP/AVP 100 1169 c=IN IP4 224.1.1.1/127 1170 a=rtpmap:100 MP2T/90000 1171 a=mid:S1 1172 m=application 30000 RTP/AVP 110 1173 c=IN IP4 224.1.2.1/127 1174 a=rtpmap:110 1d-interleaved-parityfec/90000 1175 a=fmtp:110 L:5; D:10; repair-window: 200000 1176 a=mid:R1 1178 8. Congestion Control Considerations 1180 FEC is an effective approach to provide applications resiliency 1181 against packet losses. However, in networks where the congestion is 1182 a major contributor to the packet loss, the potential impacts of 1183 using FEC SHOULD be considered carefully before injecting the repair 1184 flows into the network. In particular, in bandwidth-limited 1185 networks, FEC repair flows may consume most or all of the available 1186 bandwidth and may consequently congest the network. In such cases, 1187 the applications MUST NOT arbitrarily increase the amount of FEC 1188 protection since doing so may lead to a congestion collapse. If 1189 desired, stronger FEC protection MAY be applied only after the source 1190 rate has been reduced. 1192 In a network-friendly implementation, an application SHOULD NOT send/ 1193 receive FEC repair flows if it knows that sending/receiving those FEC 1194 repair flows would not help at all in recovering the missing packets. 1195 Such a practice helps reduce the amount of wasted bandwidth. It is 1196 RECOMMENDED that the amount of FEC protection is adjusted dynamically 1197 based on the packet loss rate observed by the applications. 1199 In multicast scenarios, it may be difficult to optimize the FEC 1200 protection per receiver. If there is a large variation among the 1201 levels of FEC protection needed by different receivers, it is 1202 RECOMMENDED that the sender offers multiple repair flows with 1203 different levels of FEC protection and the receivers join the 1204 corresponding multicast sessions to receive the repair flow(s) that 1205 is best for them. 1207 9. Security Considerations 1209 RTP packets using the payload format defined in this specification 1210 are subject to the security considerations discussed in the RTP 1211 specification [RFC3550] and in any applicable RTP profile. The main 1212 security considerations for the RTP packet carrying the RTP payload 1213 format defined within this memo are confidentiality, integrity and 1214 source authenticity. Confidentiality is achieved by encrypting the 1215 RTP payload. Integrity of the RTP packets is achieved through a 1216 suitable cryptographic integrity protection mechanism. Such a 1217 cryptographic system may also allow the authentication of the source 1218 of the payload. A suitable security mechanism for this RTP payload 1219 format should provide confidentiality, integrity protection, and at 1220 least source authentication capable of determining if an RTP packet 1221 is from a member of the RTP session. 1223 Note that the appropriate mechanism to provide security to RTP and 1224 payloads following this memo may vary. It is dependent on the 1225 application, transport and signaling protocol employed. Therefore, a 1226 single mechanism is not sufficient, although if suitable, using the 1227 Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. 1228 Other mechanisms that may be used are IPsec [RFC4301] and Transport 1229 Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may 1230 exist. 1232 10. IANA Considerations 1234 New media subtypes are subject to IANA registration. For the 1235 registration of the payload format and its parameters introduced in 1236 this document, refer to Section 5. 1238 11. Acknowledgments 1240 A major part of this document is borrowed from [RFC2733] and 1241 [SMPTE2022-1]. Thus, the author would like to thank the authors and 1242 editors of these earlier specifications. The author also thanks 1243 Colin Perkins for his constructive suggestions for this document. 1245 12. Change Log 1247 12.1. draft-ietf-fecframe-interleaved-fec-scheme-04 1249 The following are the major changes compared to version 03: 1251 o Further comments from AVT WG have been addressed. 1253 12.2. draft-ietf-fecframe-interleaved-fec-scheme-03 1255 The following are the major changes compared to version 02: 1257 o Comments from WGLC have been addressed. 1259 12.3. draft-ietf-fecframe-interleaved-fec-scheme-02 1261 The following are the major changes compared to version 01: 1263 o Some details were added regarding the use of CNAME field. 1265 o Offer-Answer and Declarative Considerations sections have been 1266 completed. 1268 o Security Considerations section has been completed. 1270 12.4. draft-ietf-fecframe-interleaved-fec-scheme-01 1272 The following are the major changes compared to version 00: 1274 o The timestamp field definition has changed. 1276 12.5. draft-ietf-fecframe-interleaved-fec-scheme-00 1278 This is the initial version, which is based on an earlier individual 1279 submission. The following are the major changes compared to that 1280 document: 1282 o Per the discussion in the WG, references to the FEC Framework have 1283 been removed and the document has been turned into a pure RTP 1284 payload format specification. 1286 o A new section is added for congestion control considerations. 1288 o Editorial changes to clarify a few points. 1290 13. References 1291 13.1. Normative References 1293 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1294 Requirement Levels", BCP 14, RFC 2119, March 1997. 1296 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1297 Jacobson, "RTP: A Transport Protocol for Real-Time 1298 Applications", STD 64, RFC 3550, July 2003. 1300 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1301 Description Protocol", RFC 4566, July 2006. 1303 [RFC4756] Li, A., "Forward Error Correction Grouping Semantics in 1304 Session Description Protocol", RFC 4756, November 2006. 1306 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and 1307 Registration Procedures", BCP 13, RFC 4288, December 2005. 1309 [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP 1310 Payload Formats", RFC 3555, July 2003. 1312 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1313 with Session Description Protocol (SDP)", RFC 3264, 1314 June 2002. 1316 13.2. Informative References 1318 [I-D.ietf-fecframe-dvb-al-fec] 1319 Begen, A. and T. Stockhammer, "DVB Application-Layer 1320 Hybrid FEC Protection", draft-ietf-fecframe-dvb-al-fec-01 1321 (work in progress), January 2009. 1323 [RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format 1324 for Generic Forward Error Correction", RFC 2733, 1325 December 1999. 1327 [RFC3009] Rosenberg, J. and H. Schulzrinne, "Registration of 1328 parityfec MIME types", RFC 3009, November 2000. 1330 [RFC5109] Li, A., "RTP Payload Format for Generic Forward Error 1331 Correction", RFC 5109, December 2007. 1333 [ETSI-TS-102-034] 1334 ETSI TS 102 034 V1.3.1, "Transport of MPEG 2 TS Based DVB 1335 Services over IP Based Networks", October 2007. 1337 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 1338 Streaming Protocol (RTSP)", RFC 2326, April 1998. 1340 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1341 Announcement Protocol", RFC 2974, October 2000. 1343 [SMPTE2022-1] 1344 SMPTE 2022-1-2007, "Forward Error Correction for Real-Time 1345 Video/Audio Transport over IP Networks", 2007. 1347 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 1348 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 1349 RFC 3711, March 2004. 1351 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 1352 Internet Protocol", RFC 4301, December 2005. 1354 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1355 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1357 Author's Address 1359 Ali Begen 1360 Cisco Systems 1361 170 West Tasman Drive 1362 San Jose, CA 95134 1363 USA 1365 Email: abegen@cisco.com