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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 (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 TSVWG V. Roca 3 Internet-Draft INRIA 4 Updates: 6363 (if approved) A. Begen 5 Intended status: Standards Track Networked Media 6 Expires: March 11, 2019 September 7, 2018 8 Forward Error Correction (FEC) Framework Extension to Sliding Window 9 Codes 10 draft-ietf-tsvwg-fecframe-ext-04 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 March 11, 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. Summary of 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 lost (e.g., 93 because of a congested router, of a poor signal-to-noise ratio in a 94 wireless network, or because the number of bit errors exceeds the 95 correction capabilities of the physical-layer error correcting code) 96 or received by the transport protocol without any corruption (i.e., 97 the bit-errors, if any, have been fixed by the physical-layer error 98 correcting code and therefore are hidden to the upper layers). 100 For these use-cases, Forward Error Correction (FEC) applied within 101 the transport or application layer, is an efficient technique to 102 improve packet transmission robustness in presence of packet losses 103 (or "erasures"), without going through packet retransmissions that 104 create a delay often incompatible with real-time constraints. The 105 FEC Building Block defined in [RFC5052] provides a framework for the 106 definition of Content Delivery Protocols (CDPs) that make use of 107 separately defined FEC schemes. Any CDP defined according to the 108 requirements of the FEC Building Block can then easily be used with 109 any FEC Scheme that is also defined according to the requirements of 110 the FEC Building Block. 112 Then FECFRAME [RFC6363] provides a framework to define Content 113 Delivery Protocols (CDPs) that provide FEC protection for arbitrary 114 packet flows over an unreliable datagram service transports such as 115 UDP. It is primarily intended for real-time or streaming media 116 applications, using broadcast, multicast, or on-demand delivery. 118 However [RFC6363] only considers block FEC schemes defined in 119 accordance with the FEC Building Block [RFC5052] (e.g., [RFC6681], 120 [RFC6816] or [RFC6865]). These codes require the input flow(s) to be 121 segmented into a sequence of blocks. Then FEC encoding (at a sender 122 or an encoding middlebox) and decoding (at a receiver or a decoding 123 middlebox) are both performed on a per-block basis. This approach 124 has major impacts on FEC encoding and decoding delays. The data 125 packets of continuous media flow(s) may be passed to the transport 126 layer immediately, without delay. But the block creation time, that 127 depends on the number of source symbols in this block, impacts both 128 the FEC encoding delay (since encoding requires that all source 129 symbols be known), and mechanically the packet loss recovery delay at 130 a receiver (since no repair symbol for the current block can be 131 generated and therefore received before that time). Therefore a good 132 value for the block size is necessarily a balance between the maximum 133 FEC decoding latency at the receivers (which must be in line with the 134 most stringent real-time requirement of the protected flow(s), hence 135 an incentive to reduce the block size), and the desired robustness 136 against long loss bursts (which increases with the block size, hence 137 an incentive to increase this size). 139 This document extends [RFC6363] in order to also support FEC codes 140 based on a sliding encoding window (A.K.A. convolutional codes) 141 [RFC8406]. This encoding window, either of fixed or variable size, 142 slides over the set of source symbols. FEC encoding is launched 143 whenever needed, from the set of source symbols present in the 144 sliding encoding window at that time. This approach significantly 145 reduces FEC-related latency, since repair symbols can be generated 146 and passed to the transport layer on-the-fly, at any time, and can be 147 regularly received by receivers to quickly recover packet losses. 148 Using sliding window FEC codes is therefore highly beneficial to 149 real-time flows, one of the primary targets of FECFRAME. [RLC-ID] 150 provides an example of such FEC Scheme for FECFRAME, built upon the 151 simple sliding window Random Linear Codes (RLC). 153 This document is fully backward compatible with [RFC6363] that it 154 extends but does not replace. Indeed: 156 o this extension does not prevent nor compromise in any way the 157 support of block FEC codes. Both types of codes can nicely co- 158 exist, just like different block FEC schemes can co-exist; 160 o any receiver, for instance a legacy receiver that only supports 161 block FEC schemes, can easily identify the FEC Scheme used in a 162 FECFRAME session thanks to the associated SDP file and its FEC 163 Encoding ID information (i.e., the "encoding-id=" parameter of a 164 "fec-repair-flow" attribute, [RFC6364]). This mechanism is not 165 specific to this extension but is the basic approach for a 166 FECFRAME receiver to determine whether or not it supports the FEC 167 Scheme used in a given FECFRAME session; 169 This document leverages on [RFC6363] and re-uses its structure. It 170 proposes new sections specific to sliding window FEC codes whenever 171 required. The only exception is Section 3 that provides a quick 172 summary of FECFRAME in order to facilitate the understanding of this 173 document to readers not familiar with the concepts and terminology. 175 2. Definitions and Abbreviations 177 The following list of definitions and abbreviations is copied from 178 [RFC6363], adding only the Block/sliding window FEC Code and 179 Encoding/Decoding Window definitions (tagged with "ADDED"): 181 Application Data Unit (ADU): The unit of source data provided as 182 payload to the transport layer. 184 ADU Flow: A sequence of ADUs associated with a transport-layer flow 185 identifier (such as the standard 5-tuple {source IP address, 186 source port, destination IP address, destination port, transport 187 protocol}). 189 AL-FEC: Application-layer Forward Error Correction. 191 Application Protocol: Control protocol used to establish and control 192 the source flow being protected, e.g., the Real-Time Streaming 193 Protocol (RTSP). 195 Content Delivery Protocol (CDP): A complete application protocol 196 specification that, through the use of the framework defined in 197 this document, is able to make use of FEC schemes to provide FEC 198 capabilities. 200 FEC Code: An algorithm for encoding data such that the encoded data 201 flow is resilient to data loss. Note that, in general, FEC codes 202 may also be used to make a data flow resilient to corruption, but 203 that is not considered in this document. 205 Block FEC Code: (ADDED) An FEC Code that operates on blocks, i.e., 206 for which the input flow MUST be segmented into a sequence of 207 blocks, FEC encoding and decoding being performed independently 208 on a per-block basis. 210 Sliding Window FEC Code: (ADDED) An FEC Code that can generate 211 repair symbols on-the-fly, at any time, from the set of source 212 symbols present in the sliding encoding window at that time. 213 These codes are also known as convolutional codes. 215 FEC Framework: A protocol framework for the definition of Content 216 Delivery Protocols using FEC, such as the framework defined in 217 this document. 219 FEC Framework Configuration Information: Information that controls 220 the operation of the FEC Framework. 222 FEC Payload ID: Information that identifies the contents of a packet 223 with respect to the FEC Scheme. 225 FEC Repair Packet: At a sender (respectively, at a receiver), a 226 payload submitted to (respectively, received from) the transport 227 protocol containing one or more repair symbols along with a 228 Repair FEC Payload ID and possibly an RTP header. 230 FEC Scheme: A specification that defines the additional protocol 231 aspects required to use a particular FEC code with the FEC 232 Framework. 234 FEC Source Packet: At a sender (respectively, at a receiver), a 235 payload submitted to (respectively, received from) the transport 236 protocol containing an ADU along with an optional Explicit Source 237 FEC Payload ID. 239 Repair Flow: The packet flow carrying FEC data. 241 Repair FEC Payload ID: A FEC Payload ID specifically for use with 242 repair packets. 244 Source Flow: The packet flow to which FEC protection is to be 245 applied. A source flow consists of ADUs. 247 Source FEC Payload ID: A FEC Payload ID specifically for use with 248 source packets. 250 Source Protocol: A protocol used for the source flow being 251 protected, e.g., RTP. 253 Transport Protocol: The protocol used for the transport of the 254 source and repair flows, using an unreliable datagram service 255 such as UDP. 257 Encoding Window: (ADDED) Set of Source Symbols available at the 258 sender/coding node that are used to generate a repair symbol, 259 with a Sliding Window FEC Code. 261 Decoding Window: (ADDED) Set of received or decoded source and 262 repair symbols available at a receiver that are used to decode 263 erased source symbols, with a Sliding Window FEC Code. 265 Code Rate: The ratio between the number of source symbols and the 266 number of encoding symbols. By definition, the code rate is such 267 that 0 < code rate <= 1. A code rate close to 1 indicates that a 268 small number of repair symbols have been produced during the 269 encoding process. 271 Encoding Symbol: Unit of data generated by the encoding process. 272 With systematic codes, source symbols are part of the encoding 273 symbols. 275 Packet Erasure Channel: A communication path where packets are 276 either lost (e.g., in our case, by a congested router, or because 277 the number of transmission errors exceeds the correction 278 capabilities of the physical-layer code) or received. When a 279 packet is received, it is assumed that this packet is not 280 corrupted (i.e., in our case, the bit-errors, if any, are fixed 281 by the physical-layer code and therefore hidden to the upper 282 layers). 284 Repair Symbol: Encoding symbol that is not a source symbol. 286 Source Block: Group of ADUs that are to be FEC protected as a single 287 block. This notion is restricted to Block FEC Codes. 289 Source Symbol: Unit of data used during the encoding process. 291 Systematic Code: FEC code in which the source symbols are part of 292 the encoding symbols. 294 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 295 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 296 document are to be interpreted as described in [RFC2119]. 298 3. Summary of Architecture Overview 300 The architecture of [RFC6363], Section 3, equally applies to this 301 FECFRAME extension and is not repeated here. However we provide 302 hereafter a quick summary to facilitate the understanding of this 303 document to readers not familiar with the concepts and terminology. 305 +----------------------+ 306 | Application | 307 +----------------------+ 308 | 309 | (1) Application Data Units (ADUs) 310 | 311 v 312 +----------------------+ +----------------+ 313 | FEC Framework | | | 314 | |-------------------------->| FEC Scheme | 315 |(2) Construct source |(3) Source Block | | 316 | blocks | |(4) FEC Encoding| 317 |(6) Construct FEC |<--------------------------| | 318 | Source and Repair | | | 319 | Packets |(5) Explicit Source FEC | | 320 +----------------------+ Payload IDs +----------------+ 321 | Repair FEC Payload IDs 322 | Repair symbols 323 | 324 |(7) FEC Source and Repair Packets 325 v 326 +----------------------+ 327 | Transport Protocol | 328 +----------------------+ 330 Figure 1: FECFRAME architecture at a sender. 332 The FECFRAME architecture is illustrated in Figure 1 from the 333 sender's point of view, in case of a block FEC Scheme. It shows an 334 application generating an ADU flow (other flows, from other 335 applications, may co-exist). These ADUs, of variable size, must be 336 somehow mapped to source symbols of fixed size. This is the goal of 337 an ADU to symbols mapping process that is FEC Scheme specific (see 338 below). Once the source block is built, taking into account both the 339 FEC Scheme constraints (e.g., in terms of maximum source block size) 340 and the application's flow constraints (e.g., in terms of real-time 341 constraints), the associated source symbols are handed to the FEC 342 Scheme in order to produce an appropriate number of repair symbols. 343 FEC Source Packets (containing ADUs) and FEC Repair Packets 344 (containing one or more repair symbols each) are then generated and 345 sent using an appropriate transport protocol (more precisely 346 [RFC6363], Section 7, requires a transport protocol providing an 347 unreliable datagram service, such as UDP). In practice FEC Source 348 Packets may be passed to the transport layer as soon as available, 349 without having to wait for FEC encoding to take place. In that case 350 a copy of the associated source symbols needs to be kept within 351 FECFRAME for future FEC encoding purposes. 353 At a receiver (not shown), FECFRAME processing operates in a similar 354 way, taking as input the incoming FEC Source and Repair Packets 355 received. In case of FEC Source Packet losses, the FEC decoding of 356 the associated block may recover all (in case of successful decoding) 357 or a subset potentially empty (otherwise) of the missing source 358 symbols. After source symbol to ADU mapping, when lost ADUs are 359 recovered, they are then assigned to their respective flow (see 360 below). ADUs are returned to the application(s), either in their 361 initial transmission order (in that case ADUs received after an 362 erased one will be delayed until FEC decoding has taken place) or not 363 (in that case each ADU is returned as soon as it is received or 364 recovered), depending on the application requirements. 366 FECFRAME features two subtle mechanisms: 368 o ADUs to source symbols mapping: in order to manage variable size 369 ADUs, FECFRAME and FEC Schemes can use small, fixed size symbols 370 and create a mapping between ADUs and symbols. To each ADU this 371 mechanism prepends a length field (plus a flow identifier, see 372 below) and pads the result to a multiple of the symbol size. A 373 small ADU may be mapped to a single source symbol while a large 374 one may be mapped to multiple symbols. The mapping details are 375 FEC Scheme dependant and must be defined in the associated 376 document; 378 o Assignment of decoded ADUs to flows in multi-flow configurations: 379 when multiple flows are multiplexed over the same FECFRAME 380 instance, a problem is to assign a decoded ADU to the right flow 381 (UDP port numbers and IP addresses traditionally used to map 382 incoming ADUs to flows are not recovered during FEC decoding). To 383 make it possible, at the FECFRAME sending instance, each ADU is 384 prepended with a flow identifier (1 byte) during the ADU to source 385 symbols mapping (see above). The flow identifiers are also shared 386 between all FECFRAME instances as part of the FEC Framework 387 Configuration Information. This (flow identifier + length + 388 application payload + padding), called ADUI, is then FEC 389 protected. Therefore a decoded ADUI contains enough information 390 to assign the ADU to the right flow. 392 A few aspects are not covered by FECFRAME, namely: 394 o [RFC6363] section 8 does not detail any congestion control 395 mechanism, but only provides high level normative requirements; 397 o the possibility of having feedbacks from receiver(s) is considered 398 out of scope, although such a mechanism may exist within the 399 application (e.g., through RCTP control messages); 401 o flow adaptation at a FECFRAME sender (e.g., how to set the FEC 402 code rate based on transmission conditions) is not detailed, but 403 it needs to comply with the congestion control normative 404 requirements (see above). 406 4. Procedural Overview 408 4.1. General 410 The general considerations of [RFC6363], Section 4.1, that are 411 specific to block FEC codes are not repeated here. 413 With a Sliding Window FEC Code, the FEC Source Packet MUST contain 414 information to identify the position occupied by the ADU within the 415 source flow, in terms specific to the FEC Scheme. This information 416 is known as the Source FEC Payload ID, and the FEC Scheme is 417 responsible for defining and interpreting it. 419 With a Sliding Window FEC Code, the FEC Repair Packets MUST contain 420 information that identifies the relationship between the contained 421 repair payloads and the original source symbols used during encoding. 422 This information is known as the Repair FEC Payload ID, and the FEC 423 Scheme is responsible for defining and interpreting it. 425 The Sender Operation ([RFC6363], Section 4.2.) and Receiver Operation 426 ([RFC6363], Section 4.3) are both specific to block FEC codes and 427 therefore omitted below. The following two sections detail similar 428 operations for Sliding Window FEC codes. 430 4.2. Sender Operation with Sliding Window FEC Codes 432 With a Sliding Window FEC Scheme, the following operations, 433 illustrated in Figure 2 for the generic case (non-RTP repair flows), 434 and in Figure 3 for the case of RTP repair flows, describe a possible 435 way to generate compliant source and repair flows: 437 1. A new ADU is provided by the application. 439 2. The FEC Framework communicates this ADU to the FEC Scheme. 441 3. The sliding encoding window is updated by the FEC Scheme. The 442 ADU to source symbols mapping as well as the encoding window 443 management details are both the responsibility of the FEC Scheme 444 and MUST be detailed there. Appendix A provides non normative 445 hints about what FEC Scheme designers need to consider; 447 4. The Source FEC Payload ID information of the source packet is 448 determined by the FEC Scheme. If required by the FEC Scheme, 449 the Source FEC Payload ID is encoded into the Explicit Source 450 FEC Payload ID field and returned to the FEC Framework. 452 5. The FEC Framework constructs the FEC Source Packet according to 453 [RFC6363] Figure 6, using the Explicit Source FEC Payload ID 454 provided by the FEC Scheme if applicable. 456 6. The FEC Source Packet is sent using normal transport-layer 457 procedures. This packet is sent using the same ADU flow 458 identification information as would have been used for the 459 original source packet if the FEC Framework were not present 460 (e.g., the source and destination addresses and UDP port numbers 461 on the IP datagram carrying the source packet will be the same 462 whether or not the FEC Framework is applied). 464 7. When the FEC Framework needs to send one or several FEC Repair 465 Packets (e.g., according to the target Code Rate), it asks the 466 FEC Scheme to create one or several repair packet payloads from 467 the current sliding encoding window along with their Repair FEC 468 Payload ID. 470 8. The Repair FEC Payload IDs and repair packet payloads are 471 provided back by the FEC Scheme to the FEC Framework. 473 9. The FEC Framework constructs FEC Repair Packets according to 474 [RFC6363] Figure 7, using the FEC Payload IDs and repair packet 475 payloads provided by the FEC Scheme. 477 10. The FEC Repair Packets are sent using normal transport-layer 478 procedures. The port(s) and multicast group(s) to be used for 479 FEC Repair Packets are defined in the FEC Framework 480 Configuration Information. 482 +----------------------+ 483 | Application | 484 +----------------------+ 485 | 486 | (1) New Application Data Unit (ADU) 487 v 488 +---------------------+ +----------------+ 489 | FEC Framework | | FEC Scheme | 490 | |-------------------------->| | 491 | | (2) New ADU |(3) Update of | 492 | | | encoding | 493 | |<--------------------------| window | 494 |(5) Construct FEC | (4) Explicit Source | | 495 | Source Packet | FEC Payload ID(s) |(7) FEC | 496 | |<--------------------------| encoding | 497 |(9) Construct FEC | (8) Repair FEC Payload ID | | 498 | Repair Packet(s) | + Repair symbol(s) +----------------+ 499 +---------------------+ 500 | 501 | (6) FEC Source Packet 502 | (10) FEC Repair Packets 503 v 504 +----------------------+ 505 | Transport Protocol | 506 +----------------------+ 508 Figure 2: Sender Operation with Sliding Window FEC Codes 510 +----------------------+ 511 | Application | 512 +----------------------+ 513 | 514 | (1) New Application Data Unit (ADU) 515 v 516 +---------------------+ +----------------+ 517 | FEC Framework | | FEC Scheme | 518 | |-------------------------->| | 519 | | (2) New ADU |(3) Update of | 520 | | | encoding | 521 | |<--------------------------| window | 522 |(5) Construct FEC | (4) Explicit Source | | 523 | Source Packet | FEC Payload ID(s) |(7) FEC | 524 | |<--------------------------| encoding | 525 |(9) Construct FEC | (8) Repair FEC Payload ID | | 526 | Repair Packet(s) | + Repair symbol(s) +----------------+ 527 +---------------------+ 528 | | 529 |(6) Source |(10) Repair payloads 530 | packets | 531 | + -- -- -- -- -+ 532 | | RTP | 533 | +-- -- -- -- --+ 534 v v 535 +----------------------+ 536 | Transport Protocol | 537 +----------------------+ 539 Figure 3: Sender Operation with Sliding Window FEC Codes and RTP 540 Repair Flows 542 4.3. Receiver Operation with Sliding Window FEC Codes 544 With a Sliding Window FEC Scheme, the following operations, 545 illustrated in Figure 4 for the generic case (non-RTP repair flows), 546 and in Figure 5 for the case of RTP repair flows. The only 547 differences with respect to block FEC codes lie in steps (4) and (5). 548 Therefore this section does not repeat the other steps of [RFC6363], 549 Section 4.3, "Receiver Operation". The new steps (4) and (5) are: 551 4. The FEC Scheme uses the received FEC Payload IDs (and derived FEC 552 Source Payload IDs when the Explicit Source FEC Payload ID field 553 is not used) to insert source and repair packets into the 554 decoding window in the right way. If at least one source packet 555 is missing and at least one repair packet has been received and 556 the rank of the associated linear system permits it, then FEC 557 decoding can be performed in order to recover missing source 558 payloads. The FEC Scheme determines whether source packets have 559 been lost and whether enough repair packets have been received to 560 decode any or all of the missing source payloads. 562 5. The FEC Scheme returns the received and decoded ADUs to the FEC 563 Framework, along with indications of any ADUs that were missing 564 and could not be decoded. 566 +----------------------+ 567 | Application | 568 +----------------------+ 569 ^ 570 |(6) ADUs 571 | 572 +----------------------+ +----------------+ 573 | FEC Framework | | FEC Scheme | 574 | |<--------------------------| | 575 |(2)Extract FEC Payload|(5) ADUs |(4) FEC Decoding 576 | IDs and pass IDs & |-------------------------->| | 577 | payloads to FEC |(3) Explicit Source FEC +----------------+ 578 | scheme | Payload IDs 579 +----------------------+ Repair FEC Payload IDs 580 ^ Source payloads 581 | Repair payloads 582 |(1) FEC Source 583 | and Repair Packets 584 +----------------------+ 585 | Transport Protocol | 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 Protocol | 619 +----------------------+ 621 Figure 5: Receiver Operation with Sliding Window FEC Codes and RTP 622 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-bis) applies in 646 case of Sliding Window FEC schemes: 648 4-bis. 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 non normative hints about 670 what FEC Scheme designers need to consider; 672 o MUST define the management of the decoding window, consisting of a 673 system of linear equations (in case of a linear FEC code); 675 6. Feedback 677 The discussion of [RFC6363], Section 6, equally applies to this 678 FECFRAME extension and is not repeated here. 680 7. Transport Protocols 682 The discussion of [RFC6363], Section 7, equally applies to this 683 FECFRAME extension and is not repeated here. 685 8. Congestion Control 687 The discussion of [RFC6363], Section 8, equally applies to this 688 FECFRAME extension and is not repeated here. 690 9. Implementation Status 692 Editor's notes: RFC Editor, please remove this section motivated by 693 RFC 7942 before publishing the RFC. Thanks! 695 An implementation of FECFRAME extended to Sliding Window codes 696 exists: 698 o Organisation: Inria 700 o Description: This is an implementation of FECFRAME extended to 701 Sliding Window codes and supporting the RLC FEC Scheme [RLC-ID]. 702 It is based on: (1) a proprietary implementation of FECFRAME, made 703 by Inria and Expway for which interoperability tests have been 704 conducted; and (2) a proprietary implementation of RLC Sliding 705 Window FEC Codes. 707 o Maturity: the basic FECFRAME maturity is "production", the 708 FECFRAME extension maturity is "under progress". 710 o Coverage: the software implements a subset of [RFC6363], as 711 specialized by the 3GPP eMBMS standard [MBMSTS]. This software 712 also covers the additional features of FECFRAME extended to 713 Sliding Window codes, in particular the RLC FEC Scheme. 715 o Lincensing: proprietary. 717 o Implementation experience: maximum. 719 o Information update date: March 2018. 721 o Contact: vincent.roca@inria.fr 723 10. Security Considerations 725 This FECFRAME extension does not add any new security consideration. 726 All the considerations of [RFC6363], Section 9, apply to this 727 document as well. 729 11. Operations and Management Considerations 731 This FECFRAME extension does not add any new Operations and 732 Management Consideration. All the considerations of [RFC6363], 733 Section 10, apply to this document as well. 735 12. IANA Considerations 737 A FEC Scheme for use with this FEC Framework is identified via its 738 FEC Encoding ID. It is subject to IANA registration in the "FEC 739 Framework (FECFRAME) FEC Encoding IDs" registry. All the rules of 740 [RFC6363], Section 11, apply and are not repeated here. 742 13. Acknowledgments 744 The authors would like to thank David Black, Gorry Fairhurst, and 745 Emmanuel Lochin for their valuable feedbacks on this document. This 746 document being an extension to [RFC6363], the authors would also like 747 to thank Mark Watson as the main author this RFC. 749 14. References 751 14.1. Normative References 753 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 754 Requirement Levels", BCP 14, RFC 2119, 755 DOI 10.17487/RFC2119, March 1997, 756 . 758 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 759 Correction (FEC) Framework", RFC 6363, 760 DOI 10.17487/RFC6363, October 2011, 761 . 763 14.2. Informative References 765 [MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 766 Protocols and codecs", 3GPP TS 26.346, March 2009, 767 . 769 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 770 Correction (FEC) Building Block", RFC 5052, 771 DOI 10.17487/RFC5052, August 2007, 772 . 774 [RFC6364] Begen, A., "Session Description Protocol Elements for the 775 Forward Error Correction (FEC) Framework", RFC 6364, 776 DOI 10.17487/RFC6364, October 2011, 777 . 779 [RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor Forward 780 Error Correction (FEC) Schemes for FECFRAME", RFC 6681, 781 DOI 10.17487/RFC6681, August 2012, 782 . 784 [RFC6816] Roca, V., Cunche, M., and J. Lacan, "Simple Low-Density 785 Parity Check (LDPC) Staircase Forward Error Correction 786 (FEC) Scheme for FECFRAME", RFC 6816, 787 DOI 10.17487/RFC6816, December 2012, 788 . 790 [RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K. 791 Matsuzono, "Simple Reed-Solomon Forward Error Correction 792 (FEC) Scheme for FECFRAME", RFC 6865, 793 DOI 10.17487/RFC6865, February 2013, 794 . 796 [RFC8406] Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek, 797 F., Ghanem, S., Lochin, E., Masucci, A., Montpetit, M-J., 798 Pedersen, M., Peralta, G., Roca, V., Ed., Saxena, P., and 799 S. Sivakumar, "Taxonomy of Coding Techniques for Efficient 800 Network Communications", RFC 8406, DOI 10.17487/RFC8406, 801 June 2018, . 803 [RLC-ID] Roca, V. and B. Teibi, "Sliding Window Random Linear Code 804 (RLC) Forward Erasure Correction (FEC) Scheme for 805 FECFRAME", Work in Progress, Transport Area Working Group 806 (TSVWG) draft-ietf-tsvwg-rlc-fec-scheme (Work in 807 Progress), September 2018, . 810 Appendix A. About Sliding Encoding Window Management (non Normative) 812 The FEC Framework does not specify the management of the sliding 813 encoding window which is the responsibility of the FEC Scheme. This 814 annex only provides a few non normative hints. 816 Source symbols are added to the sliding encoding window each time a 817 new ADU is available at the sender, after the ADU to source symbol 818 mapping specific to the FEC Scheme. 820 Source symbols are removed from the sliding encoding window, for 821 instance: 823 o after a certain delay, when an "old" ADU of a real-time flow times 824 out. The source symbol retention delay in the sliding encoding 825 window should therefore be initialized according to the real-time 826 features of incoming flow(s) when applicable; 828 o once the sliding encoding window has reached its maximum size 829 (there is usually an upper limit to the sliding encoding window 830 size). In that case the oldest symbol is removed each time a new 831 source symbol is added. 833 Several considerations can impact the management of this sliding 834 encoding window: 836 o at the source flows level: real-time constraints can limit the 837 total time source symbols can remain in the encoding window; 839 o at the FEC code level: theoretical or practical limitations (e.g., 840 because of computational complexity) can limit the number of 841 source symbols in the encoding window; 843 o at the FEC Scheme level: signaling and window management are 844 intrinsically related. For instance, an encoding window composed 845 of a non sequential set of source symbols requires an appropriate 846 signaling to inform a receiver of the composition of the encoding 847 window, and the associated transmission overhead can limit the 848 maximum encoding window size. On the opposite, an encoding window 849 always composed of a sequential set of source symbols simplifies 850 signaling: providing the identity of the first source symbol plus 851 their number is sufficient, which creates a fixed and relatively 852 small transmission overhead. 854 Authors' Addresses 856 Vincent Roca 857 INRIA 858 Univ. Grenoble Alpes 859 France 861 EMail: vincent.roca@inria.fr 863 Ali Begen 864 Networked Media 865 Konya 866 Turkey 868 EMail: ali.begen@networked.media