idnits 2.17.1 draft-ietf-fecframe-ldpc-00.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (September 14, 2011) is 4609 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '0' on line 388 -- Looks like a reference, but probably isn't: '1' on line 390 -- Looks like a reference, but probably isn't: '2' on line 392 -- Looks like a reference, but probably isn't: '3' on line 394 Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 FecFrame V. Roca 3 Internet-Draft INRIA 4 Intended status: Standards Track M. Cunche 5 Expires: March 17, 2012 NICTA 6 J. Lacan 7 ISAE/LAAS-CNRS 8 September 14, 2011 10 Simple LDPC-Staircase Forward Error Correction (FEC) Scheme for FECFRAME 11 draft-ietf-fecframe-ldpc-00 13 Abstract 15 This document describes a fully-specified simple FEC scheme for LDPC- 16 Staircase codes that can be used to protect media streams along the 17 lines defined by the FECFRAME framework. These codes have many 18 interesting properties: they are systematic codes, they perform close 19 to ideal codes in many use-cases and they also feature very high 20 encoding and decoding throughputs. LDPC-Staircase codes are 21 therefore a good solution to protect a single high bitrate source 22 flow, or to protect globally several mid-rate flows within a single 23 FECFRAME instance. They are also a good solution whenever the 24 processing load of a software encoder or decoder must be kept to a 25 minimum. 27 Status of this Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on March 17, 2012. 44 Copyright Notice 46 Copyright (c) 2011 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 63 3. Definitions Notations and Abbreviations . . . . . . . . . . . 4 64 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 65 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 6 66 3.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 7 67 4. Common Procedures Related to the ADU Block and Source 68 Block Creation . . . . . . . . . . . . . . . . . . . . . . . . 7 69 4.1. Restrictions . . . . . . . . . . . . . . . . . . . . . . . 7 70 4.2. ADU Block Creation . . . . . . . . . . . . . . . . . . . . 7 71 4.3. Source Block Creation . . . . . . . . . . . . . . . . . . 8 72 5. LDPC-Staircase FEC Scheme for Arbitrary ADU Flows . . . . . . 10 73 5.1. Formats and Codes . . . . . . . . . . . . . . . . . . . . 10 74 5.1.1. FEC Framework Configuration Information . . . . . . . 10 75 5.1.2. Explicit Source FEC Payload ID . . . . . . . . . . . . 12 76 5.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 13 77 5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 13 78 5.3. FEC Code Specification . . . . . . . . . . . . . . . . . . 14 79 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 80 6.1. Attacks Against the Data Flow . . . . . . . . . . . . . . 14 81 6.1.1. Access to Confidential Content . . . . . . . . . . . . 14 82 6.1.2. Content Corruption . . . . . . . . . . . . . . . . . . 15 83 6.2. Attacks Against the FEC Parameters . . . . . . . . . . . . 15 84 6.3. When Several Source Flows are to be Protected Together . . 16 85 6.4. Baseline Secure FEC Framework Operation . . . . . . . . . 16 86 7. Operations and Management Considerations . . . . . . . . . . . 16 87 7.1. Operational Recommendations . . . . . . . . . . . . . . . 16 88 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 89 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 90 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 91 10.1. Normative References . . . . . . . . . . . . . . . . . . . 18 92 10.2. Informative References . . . . . . . . . . . . . . . . . . 18 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 95 1. Introduction 97 The use of Forward Error Correction (FEC) codes is a classic solution 98 to improve the reliability of unicast, multicast and broadcast 99 Content Delivery Protocols (CDP) and applications [RFC3453]. The 100 [RFC6363] document describes a generic framework to use FEC schemes 101 with media delivery applications, and for instance with real-time 102 streaming media applications based on the RTP real-time protocol. 103 Similarly the [RFC5052] document describes a generic framework to use 104 FEC schemes with with objects (e.g., files) delivery applications 105 based on the ALC [RFC5775] and NORM [RFC5740] reliable multicast 106 transport protocols. 108 More specifically, the [RFC5053] (Raptor) and [RFC5170] (LDPC- 109 Staircase and LDPC-Triangle) FEC schemes introduce erasure codes 110 based on sparse parity check matrices for object delivery protocols 111 like ALC and NORM. Similarly, the [RFC5510] document introduces 112 Reed-Solomon codes based on Vandermonde matrices for the same object 113 delivery protocols. All these codes are systematic codes, meaning 114 that the k source symbols are part of the n encoding symbols. 115 Additionally, the Reed-Solomon FEC codes belong to the class of 116 Maximum Distance Separable (MDS) codes that are optimal in terms of 117 erasure recovery capabilities. It means that a receiver can recover 118 the k source symbols from any set of exactly k encoding symbols out 119 of n. This is not the case with either Raptor or LDPC-Staircase 120 codes, and these codes require a certain number of encoding symbols 121 in excess to k. However, this number is small in practice when an 122 appropriate decoding scheme is used at the receiver [Cunche08]. 123 Another key difference is the high encoding/decoding complexity of 124 Reed-Solomon codecs compared to Raptor or LDPC-Staircase codes. A 125 difference of one or more orders of magnitude or more in terms of 126 encoding/decoding speed exists between the Reed-Solomon and LDPC- 127 Staircase software codecs [Cunche08][CunchePHD10]. Finally, Raptor 128 and LDPC-Staircase codes are large block FEC codes, in the sense of 129 [RFC3453], since they can efficiently deal with a large number of 130 source symbols. 132 The present document focuses on LDPC-Staircase codes, that belong to 133 the well-known class of "Low Density Parity Check" codes. Because of 134 their key features, these codes are a good solution in many 135 situations, as detailed in Section 7. 137 This documents inherits from [RFC5170] the specifications of the core 138 LDPC-Staircase codes. Therefore this document specifies only the 139 information specific to the FECFRAME context and refers to [RFC5170] 140 for the core specifications of the codes. To that purpose, the 141 present document introduces: 143 o the Fully-Specified FEC Scheme with FEC Encoding ID XXX that 144 specifies a simple way of using LDPC-Staircase codes in order to 145 protect arbitrary ADU flows. 147 Finally, publicly available reference implementations of these codes 148 are available [LDPC-codec] [LDPC-codec-OpenFEC]. 150 2. Terminology 152 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 153 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 154 document are to be interpreted as described in RFC 2119 [RFC2119]. 156 3. Definitions Notations and Abbreviations 158 3.1. Definitions 160 This document uses the following terms and definitions. Some of them 161 are FEC scheme specific and are in line with [RFC5052]: 162 Source symbol: unit of data used during the encoding process. In 163 this specification, there is always one source symbol per ADU. 164 Encoding symbol: unit of data generated by the encoding process. 165 With systematic codes, source symbols are part of the encoding 166 symbols. 167 Repair symbol: encoding symbol that is not a source symbol. 168 Code rate: the k/n ratio, i.e., the ratio between the number of 169 source symbols and the number of encoding symbols. By definition, 170 the code rate is such that: 0 < code rate <= 1. A code rate close 171 to 1 indicates that a small number of repair symbols have been 172 produced during the encoding process. 173 Systematic code: FEC code in which the source symbols are part of 174 the encoding symbols. The LDPC-Staircase codes introduced in this 175 document are systematic. 176 Source block: a block of k source symbols that are considered 177 together for the encoding. 178 Packet Erasure Channel: a communication path where packets are 179 either dropped (e.g., by a congested router, or because the number 180 of transmission errors exceeds the correction capabilities of the 181 physical layer codes) or received. When a packet is received, it 182 is assumed that this packet is not corrupted. 184 Some of them are FECFRAME framework specific and are in line with 185 [RFC6363]: 187 Application Data Unit (ADU): a unit of data coming from (sender) or 188 given to (receiver) the media delivery application. Depending on 189 the use-case, an ADU can use an RTP encapsulation. In this 190 specification, there is always one source symbol per ADU. 191 (Source) ADU Flow: a flow of ADUs from a media delivery application 192 and to which FEC protection is applied. Depending on the use- 193 case, several ADU flows can be protected together by the FECFRAME 194 framework. 195 ADU Block: a set of ADUs that are considered together by the 196 FECFRAME instance for the purpose of the FEC scheme. Along with 197 the F[], L[], and Pad[] fields, they form the set of source 198 symbols over which FEC encoding will be performed. 199 ADU Information (ADUI): a unit of data constituted by the ADU and 200 the associated Flow ID, Length and Padding fields (Section 4.3). 201 This is the unit of data that is used as source symbol. 202 FEC Framework Configuration Information: the FEC scheme specific 203 information that enables the synchronization of the FECFRAME 204 sender and receiver instances. 205 FEC Source Packet: a data packet submitted to (sender) or received 206 from (receiver) the transport protocol. It contains an ADU along 207 with its optional Explicit Source FEC Payload ID. 208 FEC Repair Packet: a repair packet submitted to (sender) or received 209 from (receiver) the transport protocol. It contains a repair 210 symbol along with its Repair FEC Payload ID. 212 The above terminology is illustrated in Figure 1 (sender's point of 213 view): 215 +----------------------+ 216 | Application | 217 +----------------------+ 218 | 219 ADU flow | (1) Application Data Unit (ADU) 220 v 221 +----------------------+ +----------------+ 222 | FEC Framework | | | 223 | |------------------------- >| FEC Scheme | 224 |(2) Construct an ADU | (4) Source Symbols for | | 225 | block | this Source Block |(5) Perform FEC | 226 |(3) Construct ADU Info| | Encoding | 227 |(7) Construct FEC Src |< -------------------------| | 228 | Packets and FEC |(6) Ex src FEC Payload Ids,| | 229 | Repair Packets | Repair FEC Payload Ids,| | 230 +----------------------+ Repair Symbols +----------------+ 231 | | 232 |(8) FEC Src |(8') FEC Repair 233 | packets | packets 234 v v 235 +----------------------+ 236 | Transport Layer | 237 | (e.g., UDP ) | 238 +----------------------+ 240 Figure 1: Terminology used in this document (sender). 242 3.2. Notations 244 This document uses the following notations: Some of them are FEC 245 scheme specific: 246 k denotes the number of source symbols in a source block. 247 max_k denotes the maximum number of source symbols for any source 248 block. 249 n denotes the number of encoding symbols generated for a source 250 block. 251 E denotes the encoding symbol length in bytes. 252 CR denotes the "code rate", i.e., the k/n ratio. 253 N1 denotes the target number of "1s" per column in the left side 254 of the parity check matrix. 255 N1m3 denotes the value N1 - 3. 256 a^^b denotes a raised to the power b. 258 Some of them are FECFRAME framework specific: 260 B denotes the number of ADUs per ADU block. 261 max_B denotes the maximum number of ADUs for any ADU block. 263 3.3. Abbreviations 265 This document uses the following abbreviations: 266 ADU stands for Application Data Unit. 267 ESI stands for Encoding Symbol ID. 268 FEC stands for Forward Error (or Erasure) Correction code. 269 FFCI stands for FEC Framework Configuration Information. 270 FSSI stands for FEC Scheme Specific Information. 271 LDPC stands for Low Density Parity Check. 272 MDS stands for Maximum Distance Separable code. 274 4. Common Procedures Related to the ADU Block and Source Block Creation 276 This section introduces the procedures that are used during the ADU 277 block and the related source block creation, for the FEC scheme 278 considered. 280 4.1. Restrictions 282 This specification has the following restrictions: 283 o there MUST be exactly one source symbol per ADUI, and therefore 284 per ADU; 285 o there MUST be exactly one repair symbol per FEC Repair Packet; 286 o there MUST be exactly one source block per ADU block; 287 o the use of the LDPC-Staircase scheme is such that there MUST be 288 exactly one encoding symbol per group, i.e., G MUST be equal to 1 289 [RFC5170]; 291 4.2. ADU Block Creation 293 Several aspects must be considered, that impact the ADU block 294 creation: 295 o the maximum source block size (max_k parameter); 296 o the potential real-time constraints, that impact the maximum ADU 297 block size, since the larger the block size, the larger the 298 decoding delay; 299 We now detail each of these aspects. 301 The maximum source block length in symbols, max_k, depends on several 302 parameters: the code rate (CR), the Encoding Symbol ID (ESI) field 303 length in the Explicit Source/Repair FEC Payload ID (16 bits), as 304 well as possible internal codec limitations. More specifically, 305 max_k cannot be larger than the following values, derived from the 306 ESI field size limitation, for a given code rate: 308 max1_k = 2^^(16 - ceil(Log2(1/CR))) 309 Some common max1_k values are: 310 o CR == 1 (no repair symbol): max1_k = 2^^16 = 65536 symbols 311 o 1/2 <= CR < 1: max1_k = 2^^15 = 32,768 symbols 312 o 1/4 <= CR < 1/2: max1_k = 2^^14 = 16,384 symbols 314 Additionally, a codec MAY impose other limitations on the maximum 315 block size, for instance, because of a limited working memory size. 316 This decision MUST be clarified at implementation time, when the 317 target use-case is known. This results in a max2_k limitation. 319 Then, max_k is given by: 320 max_k = min(max1_k, max2_k) 321 Note that this calculation is only required at the encoder (sender), 322 since the actual k parameter (k <= max_k) is communicated to the 323 decoder (receiver) through the Explicit Source/Repair FEC Payload ID. 325 The source ADU flows usually have real-time constraints. It means 326 that the maximum number of ADUs of an ADU block must not exceed a 327 certain threshold since it directly impacts the decoding delay. It 328 is the role of the developer, who knows the flow real-time features, 329 to define an appropriate upper bound to the ADU block size, max_rt. 331 If we take into account these constraints, we find: max_B = 332 min(max_k, max_rt). Then max_B gives an upper bound to the number of 333 ADUs that can constitute an ADU block. 335 4.3. Source Block Creation 337 In its most general form the FECFRAME framework and the LDPC- 338 Staircase FEC scheme are meant to protect a set of independent flows. 339 Since the flows have no relationship to one another, the ADU size of 340 each flow can potentially vary significantly. Even in the special 341 case of a single flow, the ADU sizes can largely vary (e.g., the 342 various frames of a "Group of Pictures (GOP) of an H.264 flow). This 343 diversity must be addressed since the LDPC-Staircase FEC scheme 344 requires a constant encoding symbol size (E parameter) per source 345 block. Since this specification requires that there is only one 346 source symbol per ADU, E must be large enough to contain all the ADUs 347 of an ADU block along with their prepended 3 bytes (see below). 349 In situations where E is determined per source block (default, 350 specified by the FFCI/FSSI with S = 0, Section 5.1.1.2), E is equal 351 to the size of the largest ADU of this source block plus three (for 352 the prepended 3 bytes, see below). In this case, upon receiving the 353 first FEC Repair Packet for this source block, since this packet MUST 354 contain a single repair symbol (Section 5.1.3), a receiver determines 355 the E parameter used for this source block. 357 In situations where E is fixed (specified by the FFCI/FSSI with S = 358 1, Section 5.1.1.2), then E must be greater or equal to the size of 359 the largest ADU of this source block plus three (for the prepended 3 360 bytes, see below). If this is not the case, an error is returned. 361 How to handle this error is use-case specific (e.g., a larger E 362 parameter may be communicated to the receivers in an updated FFCI 363 message, using an appropriate mechanism) and is not considered by 364 this specification. 366 The ADU block is always encoded as a single source block. There are 367 a total of B <= max_B ADUs in this ADU block. For the ADU i, with 0 368 <= i <= B-1, 3 bytes are prepended (Figure 2): 369 o The first byte, FID[i] (Flow ID), contains the integer identifier 370 associated to the source ADU flow to which this ADU belongs to. 371 It is assumed that a single byte is sufficient, or said 372 differently, that no more than 256 flows will be protected by a 373 single instance of the FECFRAME framework. 374 o The following two bytes, L[i] (Length), contain the length of this 375 ADU, in network byte order (i.e., big endian). This length is for 376 the ADU itself and does not include the FID[i], L[i], or Pad[i] 377 fields. 379 Then zero padding is added to ADU i (if needed) in field Pad[i], for 380 alignment purposes up to a size of exactly E bytes. The data unit 381 resulting from the ADU i and the F[i], L[i] and Pad[i] fields, is 382 called ADU Information (or ADUI). Each ADUI contributes to exactly 383 one source symbol to the source block. 385 Encoding Symbol Length (E) 386 < -------------------------------------------------------------- > 387 +----+----+-----------------------+------------------------------+ 388 |F[0]|L[0]| ADU[0] | Pad[0] | 389 +----+----+----------+------------+------------------------------+ 390 |F[1]|L[1]| ADU[1] | Pad[1] | 391 +----+----+----------+-------------------------------------------+ 392 |F[2]|L[2]| ADU[2] | 393 +----+----+------+-----------------------------------------------+ 394 |F[3]|L[3]|ADU[3]| Pad[3] | 395 +----+----+------+-----------------------------------------------+ 396 \_______________________________ _______________________________/ 397 \/ 398 simple FEC encoding 400 +----------------------------------------------------------------+ 401 | Repair 4 | 402 +----------------------------------------------------------------+ 403 . . 404 . . 405 +----------------------------------------------------------------+ 406 | Repair 7 | 407 +----------------------------------------------------------------+ 409 Figure 2: Source block creation, for code rate 1/2 (equal number of 410 source and repair symbols, 4 in this example), and S = 0. 412 Note that neither the initial 3 bytes nor the optional padding are 413 sent over the network. However, they are considered during FEC 414 encoding. It means that a receiver who lost a certain FEC source 415 packet (e.g., the UDP datagram containing this FEC source packet) 416 will be able to recover the ADUI if FEC decoding succeeds. Thanks to 417 the initial 3 bytes, this receiver will get rid of the padding (if 418 any) and identify the corresponding ADU flow. 420 5. LDPC-Staircase FEC Scheme for Arbitrary ADU Flows 422 5.1. Formats and Codes 424 5.1.1. FEC Framework Configuration Information 426 The FEC Framework Configuration Information (or FFCI) includes 427 information that MUST be communicated between the sender and 428 receiver(s). More specifically, it enables the synchronization of 429 the FECFRAME sender and receiver instances. It includes both 430 mandatory elements and scheme-specific elements, as detailed below. 432 5.1.1.1. Mandatory Information 434 FEC Encoding ID: the value assigned to this fully-specified FEC 435 scheme MUST be XXX, as assigned by IANA (Section 8). 436 When SDP is used to communicate the FFCI, this FEC Encoding ID is 437 carried in the 'encoding-id' parameter. 439 5.1.1.2. FEC Scheme-Specific Information 441 The FEC Scheme Specific Information (FSSI) includes elements that are 442 specific to the present FEC scheme. More precisely: 443 PRNG seed (seed): a non-negative 32 bit integer used as the seed of 444 the Pseudo Random Number Generator, as defined in [RFC5170]. 445 Encoding symbol length (E): a non-negative integer that indicates 446 either the length of each encoding symbol in bytes (strict mode, 447 i.e., if S = 1), or the maximum length of any encoding symbol 448 (i.e., if S = 0). 449 Strict (S) flag: when set to 1 this flag indicates that the E 450 parameter is the actual encoding symbol length value for each 451 block of the session (unless otherwise notified by an updated FFCI 452 if this possibility is considered by the use-case or CDP). When 453 set to 0 this flag indicates that the E parameter is the maximum 454 encoding symbol length value for each block of the session (unless 455 otherwise notified by an updated FFCI if this possibility is 456 considered by the use-case or CDP). 457 N1 minus 3 (n1m3): an integer between 0 (default) and 7, inclusive. 458 The number of "1s" per column in the left side of the parity check 459 matrix, N1, is then equal to N1m3 + 3, as specified in [RFC5170]. 460 These elements are required both by the sender (LDPC-Staircase 461 encoder) and the receiver(s) (LDPC-Staircase decoder). 463 When SDP is used to communicate the FFCI, this FEC scheme-specific 464 information is carried in the 'fssi' parameter in textual 465 representation as specified in [SDP_ELEMENTS]. For instance: 467 fssi = seed:1234,E:1400,S:0,n1m3:0 469 If another mechanism requires the FSSI to be carried as an opaque 470 octet string (for instance after a Base64 encoding), the encoding 471 format consists of the following 7 octets: 472 o PRNG seed (seed): 32 bit field. 473 o Encoding symbol length (E): 16 bit field. 474 o Strict (S) flag: 1 bit field. 475 o Reserved: a 4 bit field that MUST be set to zero. 476 o N1m3 parameter (n1m3): 3 bit field. 478 0 1 2 479 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 480 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 481 | PRNG seed (seed) | 482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 483 | Encoding Symbol Length (E) |S| resvd | n1m3| 484 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 486 Figure 3: FSSI encoding format. 488 5.1.2. Explicit Source FEC Payload ID 490 A FEC source packet MUST contain an Explicit Source FEC Payload ID 491 that is appended to the end of the packet as illustrated in Figure 4. 493 +--------------------------------+ 494 | IP Header | 495 +--------------------------------+ 496 | Transport Header | 497 +--------------------------------+ 498 | ADU | 499 +--------------------------------+ 500 | Explicit Source FEC Payload ID | 501 +--------------------------------+ 503 Figure 4: Structure of a FEC Source Packet with the Explicit Source 504 FEC Payload ID. 506 More precisely, the Explicit Source FEC Payload ID is composed of the 507 following fields (Figure 5): 508 Source Block Number (SBN) (16 bit field): this field identifies the 509 source block to which this FEC source packet belongs. 510 Encoding Symbol ID (ESI) (16 bit field): this field identifies the 511 source symbol contained in this FEC source packet. This value is 512 such that 0 <= ESI <= k - 1 for source symbols. 513 Source Block Length (k) (16 bit field): this field provides the 514 number of source symbols for this source block, i.e., the k 515 parameter. 517 0 1 2 3 518 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 1 519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 520 | Source Block Number (SBN) | Encoding Symbol ID (ESI) | 521 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 522 | Source Block Length (k) | 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 525 Figure 5: Source FEC Payload ID encoding format. 527 5.1.3. Repair FEC Payload ID 529 A FEC repair packet MUST contain a Repair FEC Payload ID that is 530 prepended to the repair symbol(s) as illustrated in Figure 6. There 531 MUST be a single repair symbol per FEC repair packet. 533 +--------------------------------+ 534 | IP Header | 535 +--------------------------------+ 536 | Transport Header | 537 +--------------------------------+ 538 | Repair FEC Payload ID | 539 +--------------------------------+ 540 | Repair Symbol | 541 +--------------------------------+ 543 Figure 6: Structure of a FEC Repair Packet with the Repair FEC 544 Payload ID. 546 More precisely, the Repair FEC Payload ID is composed of the 547 following fields: (Figure 7): 548 Source Block Number (SBN) (16 bit field): this field identifies the 549 source block to which the FEC repair packet belongs. 550 Encoding Symbol ID (ESI) (16 bit field) this field identifies the 551 repair symbol contained in this FEC repair packet. This value is 552 such that k <= ESI <= n - 1 for repair symbols. 553 Source Block Length (k) (16 bit field): this field provides the 554 number of source symbols for this source block, i.e., the k 555 parameter. 556 Number of Encoding Symbols (n) (16 bit field): this field provides 557 the number of encoding symbols for this source block, i.e., the n 558 parameter. 560 0 1 2 3 561 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 1 562 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 563 | Source Block Number (SBN) | Encoding Symbol ID (ESI) | 564 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 565 | Source Block Length (k) | Number Encoding Symbols (n) | 566 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 568 Figure 7: Repair FEC Payload ID encoding format. 570 5.2. Procedures 572 The following procedures apply: 574 o The source block creation procedures are specified in Section 4.3. 575 o The SBN value is incremented for each new source block, starting 576 at 0 for the first block of the ADU flow. Wrapping to zero will 577 happen for long sessions, after value 2^^16 - 1. 578 o The ESI of encoding symbols is managed sequentially, starting at 0 579 for the first symbol. The first k values (0 <= ESI <= k - 1) 580 identify source symbols, whereas the last n-k values (k <= ESI <= 581 n - 1) identify repair symbols. 582 o The FEC repair packet creation procedures are specified in 583 Section 5.1.3. 585 5.3. FEC Code Specification 587 The present document inherits from [RFC5170] the specification of the 588 core LDPC-Staircase codes for a packet erasure transmission channel. 590 Because of the requirement to have exactly one encoding symbol per 591 group, i.e., because G MUST be equal to 1 (Section 4.1), several 592 parts of [RFC5170] are useless. In particular, this is the case of 593 Section 5.6. "Identifying the G Symbols of an Encoding Symbol 594 Group". 596 6. Security Considerations 598 The FEC Framework document [RFC6363] provides a comprehensive 599 analysis of security considerations applicable to FEC schemes. 600 Therefore the present section follows the security considerations 601 section of [RFC6363] and only discusses topics that are specific to 602 the use of LDPC-Staircase codes. 604 6.1. Attacks Against the Data Flow 606 6.1.1. Access to Confidential Content 608 The LDPC-Staircase FEC Scheme specified in this document does not 609 change the recommendations of [RFC6363]. To summarize, if 610 confidentiality is a concern, it is RECOMMENDED that one of the 611 solutions mentioned in [RFC6363] is used, with special considerations 612 to the way this solution is applied (e.g., before versus after FEC 613 protection, and within the end-system versus in a middlebox), to the 614 operational constraints (e.g., performing FEC decoding in a protected 615 environment may be complicated or even impossible) and to the threat 616 model. 618 6.1.2. Content Corruption 620 The LDPC-Staircase FEC Scheme specified in this document does not 621 change the recommendations of [RFC6363]. To summarize, it is 622 RECOMMENDED that one of the solutions mentioned in [RFC6363] is used 623 on both the FEC Source and Repair Packets. 625 6.2. Attacks Against the FEC Parameters 627 The FEC Scheme specified in this document defines parameters that can 628 be the basis of several attacks. More specifically, the following 629 parameters of the FFCI may be modified by an attacker 630 (Section 5.1.1.2): 631 o FEC Encoding ID: changing this parameter leads the receiver to 632 consider a different FEC Scheme, which enables an attacker to 633 create a Denial of Service (DoS). 634 o Encoding symbol length (E): setting this E parameter to a value 635 smaller than the valid one enables an attacker to create a DoS 636 since the repair symbols and certain source symbols will be larger 637 than E, which is an incoherency for the receiver. Setting this E 638 parameter to a value larger than the valid one has similar impacts 639 when S=1 since the received repair symbol size will be smaller 640 than expected. On the opposite it will not lead to any 641 incoherency when S=0 since the actual symbol length value for the 642 block is determined by the size of any received repair symbol, as 643 long as this value is smaller than E. However setting this E 644 parameter to a larger value may have impacts on receivers that 645 pre-allocate memory space in advance to store incoming symbols. 646 o Strict (S) flag: flipping this S flag from 0 to 1 (i.e., E is now 647 considered as a strict value) enables an attacker to mislead the 648 receiver if the actual symbol size varies over different source 649 blocks. Flipping this S flag from 1 to 0 has no major 650 consequences unless the receiver requires to have a fixed E value 651 (e.g., because the receiver pre-allocates memory space). 652 o N1 minus 3 (n1m3): changing this parameter leads the receiver to 653 consider a different code, which enables an attacker to create a 654 DoS. 656 It is therefore RECOMMENDED that security measures are taken to 657 guarantee the FFCI integrity, as specified in [RFC6363]. How to 658 achieve this depends on the way the FFCI is communicated from the 659 sender to the receiver, which is not specified in this document. 661 Similarly, attacks are possible against the Explicit Source FEC 662 Payload ID and Repair FEC Payload ID: by modifying the Source Block 663 Number (SBN), or the Encoding Symbol ID (ESI), or the Source Block 664 Length (k), or the Number Encoding Symbols (n), an attacker can 665 easily corrupt the block identified by the SBN. Other consequences, 666 that are use-case and/or CDP dependant, may also happen. It is 667 therefore RECOMMENDED that security measures are taken to guarantee 668 the FEC Source and Repair Packets as stated in [RFC6363]. 670 6.3. When Several Source Flows are to be Protected Together 672 The LDPC-Staircase FEC Scheme specified in this document does not 673 change the recommendations of [RFC6363]. 675 6.4. Baseline Secure FEC Framework Operation 677 The LDPC-Staircase FEC Scheme specified in this document does not 678 change the recommendations of [RFC6363] concerning the use of the 679 IPsec/ESP security protocol as a mandatory to implement (but not 680 mandatory to use) security scheme. This is well suited to situations 681 where the only insecure domain is the one over which the FEC 682 Framework operates. 684 7. Operations and Management Considerations 686 The FEC Framework document [RFC6363] provides a comprehensive 687 analysis of operations and management considerations applicable to 688 FEC schemes. Therefore the present section only discusses topics 689 that are specific to the use of LDPC-Staircase codes as specified in 690 this document. 692 7.1. Operational Recommendations 694 LDPC-Staircase codes have excellent erasure recovery capabilities 695 with large source blocks, close to ideal MDS codes. For instance, 696 with a medium source block size k=1024, CR=2/3, N1=5, G=1, with a 697 hybrid ITerative/Maximum Likelihood (IT/ML) decoding approach (see 698 below) and when all symbols are sent in a random order (see below), 699 the average overhead amounts to 0.64% (corresponding to 6.5 symbols 700 in addition to k) and receiving 1043 symbols (corresponding to a 1.9% 701 overhead) is sufficient to reduce the decoding failure probability to 702 5.1*10^^-5. This is why these codes are a good solution to protect a 703 single high bitrate source flow as in [Matsuzono10], or to protect 704 globally several mid-rate source flows within a single FECFRAME 705 instance: in both cases the source block size can be assumed to be 706 equal to a few hundreds (or more) source symbols. 708 LDPC-Staircase codes are also a good solution whenever processing 709 requirements at a software encoder or decoder must be kept to a 710 minimum. This is true when the decoder uses an IT decoding 711 algorithm, or an ML algorithm (we use a Gaussian Elimination as the 712 ML algorithm) when this latter is carefully implemented and the 713 source block size kept reasonable, or a mixture of both techniques 714 which is the recommended solution [Cunche08][CunchePHD10]. For 715 instance an average decoding speed between 1.3 Gbps (corresponding to 716 a very bad channel, close to the theoretical decoding limit and 717 requiring an ML decoding) and 4.3 Gbps (corresponding to a medium 718 quality channel where IT decoding is sufficient) are easily achieved 719 with a source block size composed of k=1024 source symbols, a code 720 rate CR=2/3 (i.e., 512 repair symbols), 1024 byte long symbols, G=1, 721 and N1=5, on an Intel Xeon 5120/1.86GHz workstation running Linux/64 722 bits. Additionally, with a hybrid IT/ML approach, a receiver can 723 decide if and when ML decoding is used, depending on local criteria 724 (e.g., battery or CPU capabilities), independently from other 725 receivers. 727 As the source block size decreases, the erasure recovery capabilities 728 of LDPC codes in general also decrease. In the case of LDPC- 729 Staircase codes, in order to compensate this phenomenon, it is 730 recommended to increase the N1 parameter (e.g., experiments carried 731 out in [Matsuzono10] use N1=7 if k=170 symbols, and N1=5 otherwise) 732 and to use a hybrid IT/ML decoding approach. For instance, with a 733 small source block size k=256 symbols, CR=2/3, N1=7, and G=1, the 734 average overhead amounts to 0.67% (corresponding to 1.7 symbols in 735 addition to k), and receiving 267 symbols (corresponding to a 4.3% 736 overhead) is sufficient to reduce the decoding failure probability to 737 1.4*10^^-5. Using N1=9 further improves these results if need be, 738 which also enables to use LDPC-Staircase codes with k=100 symbols for 739 instance. 741 With very small source blocks (e.g., a few tens symbols), using for 742 instance Reed-Solomon codes [SIMPLE_RS] or 2D parity check codes MAY 743 be more appropriate. 745 The way the FEC Repair Packets are transmitted is of high importance. 746 A good strategy, that works well for any kind of channel loss model, 747 consists in sending FEC Repair Packets in random order (rather than 748 in sequence) while FEC Source Packets are sent first and in sequence. 749 Sending all packets in a random order is another possibility, but it 750 requires that all repair symbols for a source block be produced 751 first, which adds some extra delay at a sender. 753 8. IANA Considerations 755 Values of FEC Encoding IDs are subject to IANA registration. 756 [RFC6363] defines general guidelines on IANA considerations. In 757 particular it defines a registry called FEC Framework (FECFRAME) FEC 758 Encoding IDs whose values are granted on an IETF Consensus basis. 760 This document registers one value in the FEC Framework (FECFRAME) FEC 761 Encoding IDs registry as follows: 762 o XXX refers to the Simple LDPC-Staircase [RFC5170] FEC Scheme for 763 Arbitrary Packet Flows. 765 9. Acknowledgments 767 The authors want to thank K. Matsuzono, J. Detchart and H. Asaeda for 768 their contributions in evaluating the use of LDPC-Staircase codes in 769 the context of FECFRAME [Matsuzono10]. 771 10. References 773 10.1. Normative References 775 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 776 Requirement Levels", RFC 2119. 778 [RFC5170] Roca, V., Neumann, C., and D. Furodet, "Low Density Parity 779 Check (LDPC) Forward Error Correction", RFC 5170, 780 June 2008. 782 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 783 Correction (FEC) Framework", RFC 6363, September 2011. 785 [SDP_ELEMENTS] 786 Begen, A., "SDP Elements for FEC Framework", 787 draft-ietf-fecframe-sdp-elements-10 (Work in Progress), 788 October 2010. 790 10.2. Informative References 792 [RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, 793 M., and J. Crowcroft, "The Use of Forward Error Correction 794 (FEC) in Reliable Multicast", RFC 3453, December 2002. 796 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 797 Correction (FEC) Building Block", RFC 5052, August 2007. 799 [RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, 800 "Reed-Solomon Forward Error Correction (FEC) Schemes", 801 RFC 5510, April 2009. 803 [SIMPLE_RS] 804 Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K. 805 Matsuzono, "Simple Reed-Solomon Forward Error Correction 806 (FEC) Scheme for FECFRAME", 807 draft-ietf-fecframe-simple-rs-01 (Work in Progress), 808 September 2011. 810 [RFC5053] Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer, 811 "Raptor Forward Error Correction Scheme", RFC 5053, 812 June 2007. 814 [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, 815 "NACK-Oriented Reliable Multicast (NORM) Transport 816 Protocol", RFC 5740, November 2009. 818 [RFC5775] Luby, M., Watson, M., and L. Vicisano, "Asynchronous 819 Layered Coding (ALC) Protocol Instantiation", RFC 5775, 820 April 2010. 822 [Cunche08] 823 Cunche, M. and V. Roca, "Optimizing the Error Recovery 824 Capabilities of LDPC-Staircase Codes Featuring a Gaussian 825 Elimination Decoding Scheme", 10th IEEE International 826 Workshop on Signal Processing for Space Communications 827 (SPSC'08), October 2008. 829 [CunchePHD10] 830 Cunche, M., "High performances AL-FEC codes for the 831 erasure channel : variation around LDPC codes", PhD 832 dissertation (in 833 French) (http://tel.archives-ouvertes.fr/tel- 834 00451336/en/), June 2010. 836 [Matsuzono10] 837 Matsuzono, K., Detchart, J., Cunche, M., Roca, V., and H. 838 Asaeda, "Performance Analysis of a High-Performance Real- 839 Time Application with Several AL-FEC Schemes", 35th Annual 840 IEEE Conference on Local Computer Networks (LCN 2010), 841 October 2010. 843 [LDPC-codec] 844 Cunche, M., Roca, V., Neumann, C., and J. Laboure, "LDPC- 845 Staircase/LDPC-Triangle Codec Reference Implementation", 846 INRIA Rhone-Alpes and STMicroelectronics, 847 . 849 [LDPC-codec-OpenFEC] 850 "The OpenFEC project", . 852 Authors' Addresses 854 Vincent Roca 855 INRIA 856 655, av. de l'Europe 857 Inovallee; Montbonnot 858 ST ISMIER cedex 38334 859 France 861 Email: vincent.roca@inria.fr 862 URI: http://planete.inrialpes.fr/people/roca/ 864 Mathieu Cunche 865 NICTA 866 Australia 868 Email: mathieu.cunche@nicta.com.au 869 URI: http://mathieu.cunche.free.fr/ 871 Jerome Lacan 872 ISAE/LAAS-CNRS 873 1, place Emile Blouin 874 Toulouse 31056 875 France 877 Email: jerome.lacan@isae.fr 878 URI: http://dmi.ensica.fr/auteur.php3?id_auteur=5