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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TSVWG V. Roca 3 Internet-Draft INRIA 4 Intended status: Standards Track A. Begen 5 Expires: January 26, 2019 Networked Media 6 July 25, 2018 8 Forward Error Correction (FEC) Framework Extension to Sliding Window 9 Codes 10 draft-ietf-tsvwg-fecframe-ext-03 12 Abstract 14 RFC 6363 describes a framework for using Forward Error Correction 15 (FEC) codes to provide protection against packet loss. The framework 16 supports applying FEC to arbitrary packet flows over unreliable 17 transport and is primarily intended for real-time, or streaming, 18 media. However FECFRAME as per RFC 6363 is restricted to block FEC 19 codes. The present document extends FECFRAME to support FEC Codes 20 based on a sliding encoding window, in addition to Block FEC Codes, 21 in a backward compatible way. During multicast/broadcast real-time 22 content delivery, the use of sliding window codes significantly 23 improves robustness in harsh environments, with less repair traffic 24 and lower FEC-related added latency. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on January 26, 2019. 43 Copyright Notice 45 Copyright (c) 2018 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 4 62 3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 7 63 4. Procedural Overview . . . . . . . . . . . . . . . . . . . . . 9 64 4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9 65 4.2. Sender Operation with Sliding Window FEC Codes . . . . . 10 66 4.3. Receiver Operation with Sliding Window FEC Codes . . . . 12 67 5. Protocol Specification . . . . . . . . . . . . . . . . . . . 14 68 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 14 69 5.2. FEC Framework Configuration Information . . . . . . . . . 15 70 5.3. FEC Scheme Requirements . . . . . . . . . . . . . . . . . 15 71 6. Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . 15 72 7. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 16 73 8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 16 74 9. Implementation Status . . . . . . . . . . . . . . . . . . . . 16 75 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 76 11. Operations and Management Considerations . . . . . . . . . . 17 77 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 78 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 79 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 80 14.1. Normative References . . . . . . . . . . . . . . . . . . 17 81 14.2. Informative References . . . . . . . . . . . . . . . . . 17 82 Appendix A. About Sliding Encoding Window Management (non 83 Normative) . . . . . . . . . . . . . . . . . . . . . 19 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 86 1. Introduction 88 Many applications need to transport a continuous stream of packetized 89 data from a source (sender) to one or more destinations (receivers) 90 over networks that do not provide guaranteed packet delivery. In 91 particular packets may be lost, which is strictly the focus of this 92 document: we assume that transmitted packets are either received 93 without any corruption or totally lost (e.g., because of a congested 94 router, of a poor signal-to-noise ratio in a wireless network, or 95 because the number of bit errors exceeds the correction capabilities 96 of a low-layer error correcting code). 98 For these use-cases, Forward Error Correction (FEC) applied within 99 the transport or application layer, is an efficient technique to 100 improve packet transmission robustness in presence of packet losses 101 (or "erasures"), without going through packet retransmissions that 102 create a delay often incompatible with real-time constraints. The 103 FEC Building Block defined in [RFC5052] provides a framework for the 104 definition of Content Delivery Protocols (CDPs) that make use of 105 separately defined FEC schemes. Any CDP defined according to the 106 requirements of the FEC Building Block can then easily be used with 107 any FEC Scheme that is also defined according to the requirements of 108 the FEC Building Block. 110 Then FECFRAME [RFC6363] provides a framework to define Content 111 Delivery Protocols (CDPs) that provide FEC protection for arbitrary 112 packet flows over unreliable transports such as UDP. It is primarily 113 intended for real-time or streaming media applications, using 114 broadcast, multicast, or on-demand delivery. 116 However [RFC6363] only considers block FEC schemes defined in 117 accordance with the FEC Building Block [RFC5052] (e.g., [RFC6681], 118 [RFC6816] or [RFC6865]). These codes require the input flow(s) to be 119 segmented into a sequence of blocks. Then FEC encoding (at a sender 120 or an encoding middlebox) and decoding (at a receiver or a decoding 121 middlebox) are both performed on a per-block basis. This approach 122 has major impacts on FEC encoding and decoding delays. The data 123 packets of continuous media flow(s) may be passed to the transport 124 layer immediately, without delay. But the block creation time, that 125 depends on the number k of source symbols in this block, impacts the 126 FEC encoding delay since encoding requires that all source symbols be 127 known. This block creation time also impacts the decoding delay a 128 receiver will experience in case of erasures, since no repair symbol 129 for the current block can be received before. Therefore a good value 130 for the block size is necessarily a balance between the maximum 131 decoding latency at the receivers (which must be in line with the 132 most stringent real-time requirement of the protected flow(s), hence 133 an incentive to reduce the block size), and the desired robustness 134 against long loss bursts (which increases with the block size, hence 135 an incentive to increase this size). 137 This document extends [RFC6363] in order to also support FEC codes 138 based on a sliding encoding window (A.K.A. convolutional codes). 139 This encoding window, either of fixed or variable size, slides over 140 the set of source symbols. FEC encoding is launched whenever needed, 141 from the set of source symbols present in the sliding encoding window 142 at that time. This approach significantly reduces FEC-related 143 latency, since repair symbols can be generated and passed to the 144 transport layer on-the-fly, at any time, and can be regularly 145 received by receivers to quickly recover packet losses. Using 146 sliding window FEC codes is therefore highly beneficial to real-time 147 flows, one of the primary targets of FECFRAME. [RLC-ID] provides an 148 example of such FEC Scheme for FECFRAME, built upon the simple 149 sliding window Random Linear Codes (RLC). 151 This document is fully backward compatible with [RFC6363] that it 152 extends but does not replace. Indeed: 154 o this extension does not prevent nor compromise in any way the 155 support of block FEC codes. Both types of codes can nicely co- 156 exist, just like different block FEC schemes can co-exist; 158 o any receiver, for instance a legacy receiver that only supports 159 block FEC schemes, can easily identify the FEC Scheme used in a 160 FECFRAME session thanks to the associated SDP file and its FEC 161 Encoding ID information (i.e., the "encoding-id=" parameter of a 162 "fec-repair-flow" attribute, [RFC6364]). This mechanism is not 163 specific to this extension but is the basic approach for a 164 FECFRAME receiver to determine whether or not it supports the FEC 165 Scheme used in a given FECFRAME session; 167 This document leverages on [RFC6363] and re-uses its structure. It 168 proposes new sections specific to sliding window FEC codes whenever 169 required. The only exception is Section Section 3 that provides a 170 quick summary of FECFRAME in order to facilitate the understanding of 171 this document to readers not familiar with the concepts and 172 terminology. 174 2. Definitions and Abbreviations 176 The following list of definitions and abbreviations is copied from 177 [RFC6363], adding only the Block/sliding window FEC Code and 178 Encoding/Decoding Window definitions (tagged with "ADDED"): 180 Application Data Unit (ADU): The unit of source data provided as 181 payload to the transport layer. 183 ADU Flow: A sequence of ADUs associated with a transport-layer flow 184 identifier (such as the standard 5-tuple {source IP address, 185 source port, destination IP address, destination port, transport 186 protocol}). 188 AL-FEC: Application-layer Forward Error Correction. 190 Application Protocol: Control protocol used to establish and control 191 the source flow being protected, e.g., the Real-Time Streaming 192 Protocol (RTSP). 194 Content Delivery Protocol (CDP): A complete application protocol 195 specification that, through the use of the framework defined in 196 this document, is able to make use of FEC schemes to provide FEC 197 capabilities. 199 FEC Code: An algorithm for encoding data such that the encoded data 200 flow is resilient to data loss. Note that, in general, FEC codes 201 may also be used to make a data flow resilient to corruption, but 202 that is not considered in this document. 204 Block FEC Code: (ADDED) An FEC Code that operates on blocks, i.e., 205 for which the input flow MUST be segmented into a sequence of 206 blocks, FEC encoding and decoding being performed independently 207 on a per-block basis. 209 Sliding Window FEC Code: (ADDED) An FEC Code that can generate 210 repair symbols on-the-fly, at any time, from the set of source 211 symbols present in the sliding encoding window at that time. 212 These codes are also known as convolutional codes. 214 FEC Framework: A protocol framework for the definition of Content 215 Delivery Protocols using FEC, such as the framework defined in 216 this document. 218 FEC Framework Configuration Information: Information that controls 219 the operation of the FEC Framework. 221 FEC Payload ID: Information that identifies the contents of a packet 222 with respect to the FEC Scheme. 224 FEC Repair Packet: At a sender (respectively, at a receiver), a 225 payload submitted to (respectively, received from) the transport 226 protocol containing one or more repair symbols along with a 227 Repair FEC Payload ID and possibly an RTP header. 229 FEC Scheme: A specification that defines the additional protocol 230 aspects required to use a particular FEC code with the FEC 231 Framework. 233 FEC Source Packet: At a sender (respectively, at a receiver), a 234 payload submitted to (respectively, received from) the transport 235 protocol containing an ADU along with an optional Explicit Source 236 FEC Payload ID. 238 Repair Flow: The packet flow carrying FEC data. 240 Repair FEC Payload ID: A FEC Payload ID specifically for use with 241 repair packets. 243 Source Flow: The packet flow to which FEC protection is to be 244 applied. A source flow consists of ADUs. 246 Source FEC Payload ID: A FEC Payload ID specifically for use with 247 source packets. 249 Source Protocol: A protocol used for the source flow being 250 protected, e.g., RTP. 252 Transport Protocol: The protocol used for the transport of the 253 source and repair flows, e.g., UDP and the Datagram Congestion 254 Control Protocol (DCCP). 256 Encoding Window: (ADDED) Set of Source Symbols available at the 257 sender/coding node that are used to generate a repair symbol, 258 with a Sliding Window FEC Code. 260 Decoding Window: (ADDED) Set of received or decoded source and 261 repair symbols available at a receiver that are used to decode 262 erased source symbols, with a Sliding Window FEC Code. 264 Code Rate: The ratio between the number of source symbols and the 265 number of encoding symbols. By definition, the code rate is such 266 that 0 < code rate <= 1. A code rate close to 1 indicates that a 267 small number of repair symbols have been produced during the 268 encoding process. 270 Encoding Symbol: Unit of data generated by the encoding process. 271 With systematic codes, source symbols are part of the encoding 272 symbols. 274 Packet Erasure Channel: A communication path where packets are 275 either lost (e.g., by a congested router, or because the number 276 of transmission errors exceeds the correction capabilities of the 277 physical-layer codes) or received. When a packet is received, it 278 is assumed that this packet is not corrupted. 280 Repair Symbol: Encoding symbol that is not a source symbol. 282 Source Block: Group of ADUs that are to be FEC protected as a single 283 block. This notion is restricted to Block FEC Codes. 285 Source Symbol: Unit of data used during the encoding process. 287 Systematic Code: FEC code in which the source symbols are part of 288 the encoding symbols. 290 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 291 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 292 document are to be interpreted as described in [RFC2119]. 294 3. Architecture Overview 296 The architecture of [RFC6363], Section 3, equally applies to this 297 FECFRAME extension and is not repeated here. However we provide 298 hereafter a quick summary to facilitate the understanding of this 299 document to readers not familiar with the concepts and terminology. 301 +----------------------+ 302 | Application | 303 +----------------------+ 304 | 305 | (1) Application Data Units (ADUs) 306 | 307 v 308 +----------------------+ +----------------+ 309 | FEC Framework | | | 310 | |-------------------------->| FEC Scheme | 311 |(2) Construct source |(3) Source Block | | 312 | blocks | |(4) FEC Encoding| 313 |(6) Construct FEC |<--------------------------| | 314 | Source and Repair | | | 315 | Packets |(5) Explicit Source FEC | | 316 +----------------------+ Payload IDs +----------------+ 317 | Repair FEC Payload IDs 318 | Repair symbols 319 | 320 |(7) FEC Source and Repair Packets 321 v 322 +----------------------+ 323 | Transport Layer | 324 | (e.g., UDP) | 325 +----------------------+ 327 Figure 1: FECFRAME architecture at a sender. 329 The FECFRAME architecture is illustrated in Figure 1 from the 330 sender's point of view, in case of a block FEC Scheme. It shows an 331 application generating an ADU flow (other flows, from other 332 applications, may co-exist). These ADUs, of variable size, must be 333 somehow mapped to source symbols of fixed size. This is the goal of 334 an ADU to symbols mapping process that is FEC Scheme specific (see 335 below). Once the source block is built, taking into account both the 336 FEC Scheme constraints (e.g., in terms of maximum source block size) 337 and the application's flow constraints (e.g., in terms of real-time 338 constraints), the associated source symbols are handed to the FEC 339 Scheme in order to produce an appropriate number of repair symbols. 340 FEC Source Packets (containing ADUs) and FEC Repair Packets 341 (containing one or more repair symbols each) are then generated and 342 sent using UDP (more precisely [RFC6363], Section 7, requires a 343 transport protocol providing an unreliable datagram service, like UDP 344 or DCCP). In practice FEC Source Packets may be passed to the 345 transport layer as soon as available, without having to wait for FEC 346 encoding to take place. In that case a copy of the associated source 347 symbols needs to be kept within FECFRAME for future FEC encoding 348 purposes. 350 At a receiver (not shown), FECFRAME processing operates in a similar 351 way, taking as input the incoming FEC Source and Repair Packets 352 received. In case of FEC Source Packet losses, the FEC decoding of 353 the associated block may recover all (in case of successful decoding) 354 or a subset potentially empty (otherwise) of the missing source 355 symbols. After source symbol to ADU mapping, when lost ADUs are 356 recovered, they are then assigned to their respective flow (see 357 below). ADUs are returned to the application(s), either in their 358 initial transmission order (in that case ADUs received after an 359 erased one will be delayed until FEC decoding has taken place) or not 360 (in that case each ADU is returned as soon as it is received or 361 recovered), depending on the application requirements. 363 FECFRAME features two subtle mechanisms: 365 o ADUs to source symbols mapping: in order to manage variable size 366 ADUs, FECFRAME and FEC Schemes can use small, fixed size symbols 367 and create a mapping between ADUs and symbols. To each ADU this 368 mechanism prepends a length field (plus a flow identifier, see 369 below) and pads the result to a multiple of the symbol size. A 370 small ADU may be mapped to a single source symbol while a large 371 one may be mapped to multiple symbols. The mapping details are 372 FEC Scheme dependant and must be defined in the associated 373 document; 375 o Assignment of decoded ADUs to flows in multi-flow configurations: 376 when multiple flows are multiplexed over the same FECFRAME 377 instance, a problem is to assign a decoded ADU to the right flow 378 (UDP port numbers and IP addresses traditionally used to map 379 incoming ADUs to flows are not recovered during FEC decoding). To 380 make it possible, at the FECFRAME sending instance, each ADU is 381 prepended with a flow identifier (1 byte) during the ADU to source 382 symbols mapping (see above). The flow identifiers are also shared 383 between all FECFRAME instances as part of the FEC Framework 384 Configuration Information. This (flow identifier + length + 385 application payload + padding), called ADUI, is then FEC 386 protected. Therefore a decoded ADUI contains enough information 387 to assign the ADU to the right flow. 389 A few aspects are not covered by FECFRAME, namely: 391 o [RFC6363] section 8 does not detail any congestion control 392 mechanism, but only provides high level normative requirements; 394 o the possibility of having feedbacks from receiver(s) is considered 395 out of scope, although such a mechanism may exist within the 396 application (e.g., through RCTP control messages); 398 o flow adaptation at a FECFRAME sender (e.g., how to set the FEC 399 code rate based on transmission conditions) is not detailed, but 400 it needs to comply with the congestion control normative 401 requirements (see above). 403 4. Procedural Overview 405 4.1. General 407 The general considerations of [RFC6363], Section 4.1, that are 408 specific to block FEC codes are not repeated here. 410 With a Sliding Window FEC Code, the FEC Source Packet MUST contain 411 information to identify the position occupied by the ADU within the 412 source flow, in terms specific to the FEC Scheme. This information 413 is known as the Source FEC Payload ID, and the FEC Scheme is 414 responsible for defining and interpreting it. 416 With a Sliding Window FEC Code, the FEC Repair Packets MUST contain 417 information that identifies the relationship between the contained 418 repair payloads and the original source symbols used during encoding. 419 This information is known as the Repair FEC Payload ID, and the FEC 420 Scheme is responsible for defining and interpreting it. 422 The Sender Operation ([RFC6363], Section 4.2.) and Receiver Operation 423 ([RFC6363], Section 4.3) are both specific to block FEC codes and 424 therefore omitted below. The following two sections detail similar 425 operations for Sliding Window FEC codes. 427 4.2. Sender Operation with Sliding Window FEC Codes 429 With a Sliding Window FEC Scheme, the following operations, 430 illustrated in Figure 2 for the case of UDP repair flows, and in 431 Figure 3 for the case of RTP repair flows, describe a possible way to 432 generate compliant source and repair flows: 434 1. A new ADU is provided by the application. 436 2. The FEC Framework communicates this ADU to the FEC Scheme. 438 3. The sliding encoding window is updated by the FEC Scheme. The 439 ADU to source symbols mapping as well as the encoding window 440 management details are both the responsibility of the FEC Scheme 441 and MUST be detailed there. Appendix A provides some hints on 442 the way it might be performed. 444 4. The Source FEC Payload ID information of the source packet is 445 determined by the FEC Scheme. If required by the FEC Scheme, 446 the Source FEC Payload ID is encoded into the Explicit Source 447 FEC Payload ID field and returned to the FEC Framework. 449 5. The FEC Framework constructs the FEC Source Packet according to 450 [RFC6363] Figure 6, using the Explicit Source FEC Payload ID 451 provided by the FEC Scheme if applicable. 453 6. The FEC Source Packet is sent using normal transport-layer 454 procedures. This packet is sent using the same ADU flow 455 identification information as would have been used for the 456 original source packet if the FEC Framework were not present 457 (for example, in the UDP case, the UDP source and destination 458 addresses and ports on the IP datagram carrying the source 459 packet will be the same whether or not the FEC Framework is 460 applied). 462 7. When the FEC Framework needs to send one or several FEC Repair 463 Packets (e.g., according to the target Code Rate), it asks the 464 FEC Scheme to create one or several repair packet payloads from 465 the current sliding encoding window along with their Repair FEC 466 Payload ID. 468 8. The Repair FEC Payload IDs and repair packet payloads are 469 provided back by the FEC Scheme to the FEC Framework. 471 9. The FEC Framework constructs FEC Repair Packets according to 472 [RFC6363] Figure 7, using the FEC Payload IDs and repair packet 473 payloads provided by the FEC Scheme. 475 10. The FEC Repair Packets are sent using normal transport-layer 476 procedures. The port(s) and multicast group(s) to be used for 477 FEC Repair Packets are defined in the FEC Framework 478 Configuration Information. 480 +----------------------+ 481 | Application | 482 +----------------------+ 483 | 484 | (1) New Application Data Unit (ADU) 485 v 486 +---------------------+ +----------------+ 487 | FEC Framework | | FEC Scheme | 488 | |-------------------------->| | 489 | | (2) New ADU |(3) Update of | 490 | | | encoding | 491 | |<--------------------------| window | 492 |(5) Construct FEC | (4) Explicit Source | | 493 | Source Packet | FEC Payload ID(s) |(7) FEC | 494 | |<--------------------------| encoding | 495 |(9) Construct FEC | (8) Repair FEC Payload ID | | 496 | Repair Packet(s) | + Repair symbol(s) +----------------+ 497 +---------------------+ 498 | 499 | (6) FEC Source Packet 500 | (10) FEC Repair Packets 501 v 502 +----------------------+ 503 | Transport Layer | 504 | (e.g., UDP) | 505 +----------------------+ 507 Figure 2: Sender Operation with Sliding Window FEC Codes 509 +----------------------+ 510 | Application | 511 +----------------------+ 512 | 513 | (1) New Application Data Unit (ADU) 514 v 515 +---------------------+ +----------------+ 516 | FEC Framework | | FEC Scheme | 517 | |-------------------------->| | 518 | | (2) New ADU |(3) Update of | 519 | | | encoding | 520 | |<--------------------------| window | 521 |(5) Construct FEC | (4) Explicit Source | | 522 | Source Packet | FEC Payload ID(s) |(7) FEC | 523 | |<--------------------------| encoding | 524 |(9) Construct FEC | (8) Repair FEC Payload ID | | 525 | Repair Packet(s) | + Repair symbol(s) +----------------+ 526 +---------------------+ 527 | | 528 |(6) Source |(10) Repair payloads 529 | packets | 530 | + -- -- -- -- -+ 531 | | RTP | 532 | +-- -- -- -- --+ 533 v v 534 +----------------------+ 535 | Transport Layer | 536 | (e.g., UDP) | 537 +----------------------+ 539 Figure 3: Sender Operation with RTP Repair Flows 541 4.3. Receiver Operation with Sliding Window FEC Codes 543 With a Sliding Window FEC Scheme, the following operations, 544 illustrated in Figure 4 for the case of UDP repair flows, and in 545 Figure 5 for the case of RTP repair flows. The only differences with 546 respect to block FEC codes lie in steps (4) and (5). Therefore this 547 section does not repeat the other steps of [RFC6363], Section 4.3, 548 "Receiver Operation". The new steps (4) and (5) are: 550 4. The FEC Scheme uses the received FEC Payload IDs (and derived FEC 551 Source Payload IDs when the Explicit Source FEC Payload ID field 552 is not used) to insert source and repair packets into the 553 decoding window in the right way. If at least one source packet 554 is missing and at least one repair packet has been received and 555 the rank of the associated linear system permits it, then FEC 556 decoding can be performed in order to recover missing source 557 payloads. The FEC Scheme determines whether source packets have 558 been lost and whether enough repair packets have been received to 559 decode any or all of the missing source payloads. 561 5. The FEC Scheme returns the received and decoded ADUs to the FEC 562 Framework, along with indications of any ADUs that were missing 563 and could not be decoded. 565 +----------------------+ 566 | Application | 567 +----------------------+ 568 ^ 569 |(6) ADUs 570 | 571 +----------------------+ +----------------+ 572 | FEC Framework | | FEC Scheme | 573 | |<--------------------------| | 574 |(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding 575 | IDs and pass IDs & |-------------------------->| | 576 | payloads to FEC |(3) Explicit Source FEC +----------------+ 577 | scheme | Payload IDs 578 +----------------------+ Repair FEC Payload IDs 579 ^ Source payloads 580 | Repair payloads 581 |(1) FEC Source 582 | and Repair Packets 583 +----------------------+ 584 | Transport Layer | 585 | (e.g., UDP) | 586 +----------------------+ 588 Figure 4: Receiver Operation with Sliding Window FEC Codes 590 +----------------------+ 591 | Application | 592 +----------------------+ 593 ^ 594 |(6) ADUs 595 | 596 +----------------------+ +----------------+ 597 | FEC Framework | | FEC Scheme | 598 | |<--------------------------| | 599 |(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding| 600 | IDs and pass IDs & |-------------------------->| | 601 | payloads to FEC |(3) Explicit Source FEC +----------------+ 602 | scheme | Payload IDs 603 +----------------------+ Repair FEC Payload IDs 604 ^ ^ Source payloads 605 | | Repair payloads 606 |Source pkts |Repair payloads 607 | | 608 +-- |- -- -- -- -- -- -+ 609 |RTP| | RTP Processing | 610 | | +-- -- -- --|-- -+ 611 | +-- -- -- -- -- |--+ | 612 | | RTP Demux | | 613 +-- -- -- -- -- -- -- -+ 614 ^ 615 |(1) FEC Source and Repair Packets 616 | 617 +----------------------+ 618 | Transport Layer | 619 | (e.g., UDP) | 620 +----------------------+ 622 Figure 5: Receiver Operation with RTP Repair Flows 624 5. Protocol Specification 626 5.1. General 628 This section discusses the protocol elements for the FEC Framework 629 specific to Sliding Window FEC schemes. The global formats of source 630 data packets (i.e., [RFC6363], Figure 6) and repair data packets 631 (i.e., [RFC6363], Figures 7 and 8) remain the same with Sliding 632 Window FEC codes. They are not repeated here. 634 5.2. FEC Framework Configuration Information 636 The FEC Framework Configuration Information considerations of 637 [RFC6363], Section 5.5, equally applies to this FECFRAME extension 638 and is not repeated here. 640 5.3. FEC Scheme Requirements 642 The FEC Scheme requirements of [RFC6363], Section 5.6, mostly apply 643 to this FECFRAME extension and are not repeated here. An exception 644 though is the "full specification of the FEC code", item (4), that is 645 specific to block FEC codes. The following item (4) applies in case 646 of Sliding Window FEC schemes: 648 4. A full specification of the Sliding Window FEC code 650 This specification MUST precisely define the valid FEC-Scheme- 651 Specific Information values, the valid FEC Payload ID values, and 652 the valid packet payload sizes (where packet payload refers to 653 the space within a packet dedicated to carrying encoding 654 symbols). 656 Furthermore, given valid values of the FEC-Scheme-Specific 657 Information, a valid Repair FEC Payload ID value, a valid packet 658 payload size, and a valid encoding window (i.e., a set of source 659 symbols), the specification MUST uniquely define the values of 660 the encoding symbol (or symbols) to be included in the repair 661 packet payload with the given Repair FEC Payload ID value. 663 Additionally, the FEC Scheme associated to a Sliding Window FEC Code: 665 o MUST define the relationships between ADUs and the associated 666 source symbols (mapping); 668 o MUST define the management of the encoding window that slides over 669 the set of ADUs. Appendix A provides a non normative example; 671 o MUST define the management of the decoding window, consisting of a 672 system of linear equations (in case of a linear FEC code); 674 6. Feedback 676 The discussion of [RFC6363], Section 6, equally applies to this 677 FECFRAME extension and is not repeated here. 679 7. Transport Protocols 681 The discussion of [RFC6363], Section 7, equally applies to this 682 FECFRAME extension and is not repeated here. 684 8. Congestion Control 686 The discussion of [RFC6363], Section 8, equally applies to this 687 FECFRAME extension and is not repeated here. 689 9. Implementation Status 691 Editor's notes: RFC Editor, please remove this section motivated by 692 RFC 7942 before publishing the RFC. Thanks! 694 An implementation of FECFRAME extended to Sliding Window codes 695 exists: 697 o Organisation: Inria 699 o Description: This is an implementation of FECFRAME extended to 700 Sliding Window codes and supporting the RLC FEC Scheme [RLC-ID]. 701 It is based on: (1) a proprietary implementation of FECFRAME, made 702 by Inria and Expway for which interoperability tests have been 703 conducted; and (2) a proprietary implementation of RLC Sliding 704 Window FEC Codes. 706 o Maturity: the basic FECFRAME maturity is "production", the 707 FECFRAME extension maturity is "under progress". 709 o Coverage: the software implements a subset of [RFC6363], as 710 specialized by the 3GPP eMBMS standard [MBMSTS]. This software 711 also covers the additional features of FECFRAME extended to 712 Sliding Window codes, in particular the RLC FEC Scheme. 714 o Lincensing: proprietary. 716 o Implementation experience: maximum. 718 o Information update date: March 2018. 720 o Contact: vincent.roca@inria.fr 722 10. Security Considerations 724 This FECFRAME extension does not add any new security consideration. 725 All the considerations of [RFC6363], Section 9, apply to this 726 document as well. 728 11. Operations and Management Considerations 730 This FECFRAME extension does not add any new Operations and 731 Management Consideration. All the considerations of [RFC6363], 732 Section 10, apply to this document as well. 734 12. IANA Considerations 736 A FEC Scheme for use with this FEC Framework is identified via its 737 FEC Encoding ID. It is subject to IANA registration in the "FEC 738 Framework (FECFRAME) FEC Encoding IDs" registry. All the rules of 739 [RFC6363], Section 11, apply and are not repeated here. 741 13. Acknowledgments 743 The authors would like to thank David Black, Gorry Fairhurst, and 744 Emmanuel Lochin for their valuable feedbacks on this document. This 745 document being an extension to [RFC6363], the authors would also like 746 to thank Mark Watson as the main author this RFC. 748 14. References 750 14.1. Normative References 752 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 753 Requirement Levels", BCP 14, RFC 2119, 754 DOI 10.17487/RFC2119, March 1997, 755 . 757 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 758 Correction (FEC) Framework", RFC 6363, 759 DOI 10.17487/RFC6363, October 2011, 760 . 762 14.2. Informative References 764 [MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 765 Protocols and codecs", 3GPP TS 26.346, March 2009, 766 . 768 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 769 Correction (FEC) Building Block", RFC 5052, 770 DOI 10.17487/RFC5052, August 2007, 771 . 773 [RFC6364] Begen, A., "Session Description Protocol Elements for the 774 Forward Error Correction (FEC) Framework", RFC 6364, 775 DOI 10.17487/RFC6364, October 2011, 776 . 778 [RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward 779 Error Correction (FEC) Schemes for FECFRAME", RFC 6681, 780 DOI 10.17487/RFC6681, August 2012, 781 . 783 [RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density 784 Parity Check (LDPC) Staircase Forward Error Correction 785 (FEC) Scheme for FECFRAME", RFC 6816, 786 DOI 10.17487/RFC6816, December 2012, 787 . 789 [RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K. 790 Matsuzono, "Simple Reed-Solomon Forward Error Correction 791 (FEC) Scheme for FECFRAME", RFC 6865, 792 DOI 10.17487/RFC6865, February 2013, 793 . 795 [RLC-ID] Roca, V. and B. Teibi, "Sliding Window Random Linear Code 796 (RLC) Forward Erasure Correction (FEC) Scheme for 797 FECFRAME", Work in Progress, Transport Area Working Group 798 (TSVWG) draft-ietf-tsvwg-rlc-fec-scheme (Work in 799 Progress), July 2018, . 802 Appendix A. About Sliding Encoding Window Management (non Normative) 804 The FEC Framework does not specify the management of the sliding 805 encoding window which is the responsibility of the FEC Scheme. This 806 annex only provides a few non normative hints. 808 Source symbols are added to the sliding encoding window each time a 809 new ADU is available at the sender, after the ADU to source symbol 810 mapping specific to the FEC Scheme. 812 Source symbols are removed from the sliding encoding window, for 813 instance: 815 o after a certain delay, when an "old" ADU of a real-time flow times 816 out. The source symbol retention delay in the sliding encoding 817 window should therefore be initialized according to the real-time 818 features of incoming flow(s) when applicable; 820 o once the sliding encoding window has reached its maximum size 821 (there is usually an upper limit to the sliding encoding window 822 size). In that case the oldest symbol is removed each time a new 823 source symbol is added. 825 Several considerations can impact the management of this sliding 826 encoding window: 828 o at the source flows level: real-time constraints can limit the 829 total time source symbols can remain in the encoding window; 831 o at the FEC code level: theoretical or practical limitations (e.g., 832 because of computational complexity) can limit the number of 833 source symbols in the encoding window; 835 o at the FEC Scheme level: signaling and window management are 836 intrinsically related. For instance, an encoding window composed 837 of a non sequential set of source symbols requires an appropriate 838 signaling to inform a receiver of the composition of the encoding 839 window, and the associated transmission overhead can limit the 840 maximum encoding window size. On the opposite, an encoding window 841 always composed of a sequential set of source symbols simplifies 842 signaling: providing the identity of the first source symbol plus 843 their number is sufficient, which creates a fixed and relatively 844 small transmission overhead. 846 Authors' Addresses 848 Vincent Roca 849 INRIA 850 Univ. Grenoble Alpes 851 France 853 EMail: vincent.roca@inria.fr 855 Ali Begen 856 Networked Media 857 Konya 858 Turkey 860 EMail: ali.begen@networked.media