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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (September 1, 2005) is 6810 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) == Missing Reference: '0' is mentioned on line 934, but not defined == Missing Reference: '255' is mentioned on line 950, but not defined == Missing Reference: '128' is mentioned on line 950, but not defined == Missing Reference: '127' is mentioned on line 934, but not defined == Unused Reference: '5' is defined on line 990, but no explicit reference was found in the text ** Obsolete normative reference: RFC 2434 (ref. '2') (Obsoleted by RFC 5226) -- Obsolete informational reference (is this intentional?): RFC 3452 (ref. '3') (Obsoleted by RFC 5052, RFC 5445) -- Obsolete informational reference (is this intentional?): RFC 3926 (ref. '8') (Obsoleted by RFC 6726) Summary: 4 errors (**), 0 flaws (~~), 7 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Reliable Multicast M. Watson 3 Internet-Draft M. Luby 4 Expires: March 5, 2006 Digital Fountain 5 L. Vicisano 6 Cisco Systems, Inc. 7 September 1, 2005 9 Forward Error Correction (FEC) Building Block 10 draft-ietf-rmt-fec-bb-revised-01 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on March 5, 2006. 37 Copyright Notice 39 Copyright (C) The Internet Society (2005). 41 Abstract 43 This document describes how to use Forward Error Correction (FEC) 44 codes to efficiently provide and/or augment reliability for data 45 transport. This document defines a framework for the definition of 46 the information that needs to be communicated in order to use an FEC 47 code for delivering content, in addition to the encoded data itself, 48 and for definition of formats and codes for communcation of that 49 information. Both information communicated with the encoded data 50 itself and information that needs to be communicated 'out-of-band' 51 are considered. The procedures for specifying new FEC codes, 52 defining the information communication requirements associated with 53 those codes and registering them with the Internet Assigned Numbers 54 Authority (IANA) are also described. The requirements on Content 55 Delivery Protocols which wish to use FEC codes defined within this 56 framework are also defined. The companion document titled, "The Use 57 of Forward Error Correction (FEC) in Reliable Multicast" describes 58 some applications of FEC codes for delivering content. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 4 64 3. Requirements notation . . . . . . . . . . . . . . . . . . . . 5 65 4. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 6 66 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 8 67 6. Functionality . . . . . . . . . . . . . . . . . . . . . . . . 9 68 6.1. FEC Schemes . . . . . . . . . . . . . . . . . . . . . . . 10 69 6.2. FEC Object Transmission Information . . . . . . . . . . . 12 70 6.2.1. Transport of FEC Object Transmission Information . . . 13 71 6.2.2. Opacity of FEC Object Transmission Information . . . . 14 72 6.2.3. Mandatory FEC Object Transmission Information 73 elements . . . . . . . . . . . . . . . . . . . . . . . 14 74 6.2.4. Common FEC Object Transmission Information elements . 15 75 6.2.5. Scheme-specific FEC Object Transmission 76 Information element . . . . . . . . . . . . . . . . . 15 77 6.3. FEC Payload ID . . . . . . . . . . . . . . . . . . . . . . 16 78 7. FEC scheme specifications . . . . . . . . . . . . . . . . . . 17 79 8. CDP specifications . . . . . . . . . . . . . . . . . . . . . . 20 80 9. Common algorithms . . . . . . . . . . . . . . . . . . . . . . 21 81 9.1. Block partitioning algorithm . . . . . . . . . . . . . . . 21 82 9.1.1. First Step . . . . . . . . . . . . . . . . . . . . . . 21 83 9.1.2. Second step . . . . . . . . . . . . . . . . . . . . . 22 84 10. Requirements from other building blocks . . . . . . . . . . . 24 85 11. Security Considerations . . . . . . . . . . . . . . . . . . . 25 86 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 87 12.1. Explicit IANA Assignment Guidelines . . . . . . . . . . . 26 88 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 89 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 90 14.1. Normative References . . . . . . . . . . . . . . . . . . . 29 91 14.2. Informative References . . . . . . . . . . . . . . . . . . 29 92 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 93 Intellectual Property and Copyright Statements . . . . . . . . . . 31 95 1. Introduction 97 This document describes how to use Forward Error Correction (FEC) 98 codes to provide support for reliable delivery of content within the 99 context of a Content Delivery Protocol (CDP). This document 100 describes a building block as defined in [10]. This document is a 101 product of the IETF RMT WG and follows the general guidelines 102 provided in [8]. 104 The purpose of this building block is to define a framework for 105 forward error correction such that: 107 1. CDPs can be designed to operate with a range of different FEC 108 codes/scheme, without needing to know details of the specific FEC 109 code/scheme that may be used 111 2. FEC schemes can be designed to operate with a range of different 112 CDPs, without needing to know details of the specific CDPs 114 Note that a 'CDP' in the context of this document may consist of 115 several distinct protocol mechanisms. 117 This document also provides detailed guidelines on how to write an 118 RFC for an FEC scheme corresponding to a new FEC Encoding ID (for 119 both Fully-Specified and Under-Specified FEC Schemes). 121 2. Definitions/Abbreviations 123 Object: An ordered sequence of bytes to be transferred by the 124 transport protocol. For example, a file or stream. 126 Symbol: A unit of data processed by the Forward Error Correction 127 code. A symbol is always considered as a unit i.e. it is either 128 completely received or completely lost. 130 Source symbol: A symbol containing information from the original 131 object. 133 Repair symbol: A symbol containing information generated by the FEC 134 code which can be used to recover lost source symbols. 136 Encoding symbol: A source symbol or a repair symbol. 138 Encoder: The FEC scheme specific functions required to transform a 139 object into FEC encoded data. i.e. the functions that produce 140 repair symbols using source symbols. 142 Decoder: The FEC scheme specific functions required to transform 143 received FEC encoded data into a copy of the original object 145 Receiver: A system supporting the receiving functions of a CDP and 146 FEC scheme according to this specification 148 Sender: A system supporting the sending functions of a CDP and FEC 149 scheme according to this specification 151 Source Block: A part of the object formed from a subset of the 152 object's source symbols 154 CDP: Content Delivery Protocol 156 FEC: Forward Error Correction 158 3. Requirements notation 160 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 161 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 162 document are to be interpreted as described in [1]. 164 4. Rationale 166 FEC codes are a valuable basic component of any CDP that is to 167 provide reliable delivery of an object. Using FEC codes is effective 168 in the context of IP multicast and reliable delivery because FEC 169 encoding symbols can be useful to all receivers for reconstructing an 170 object even when the receivers have received different encoding 171 symbols. Furthermore, FEC codes can ameliorate or even eliminate the 172 need for feedback from receivers to senders to request retransmission 173 of lost packets. 175 Central to this document is the concept of an 'FEC Scheme'. An FEC 176 scheme defines ancilliary information and procedures which, combined 177 with an FEC code specification, fully define how the FEC code can be 178 used with CDPs. An FEC scheme may be associated with a single 179 standardised FEC code (A 'Fully-Specified' FEC scheme) or may be 180 applicable to many FEC codes (An 'Under-Specified' FEC scheme). 182 This document describes a framework for the definition of FEC 183 schemes. Definition of actual FEC schemes is outside the scope of 184 this document. This document also defines requirements for reliable 185 CDPs that make use of FEC schemes. Any such CDP which is compliant 186 to the requirements specified in this document can make use of any 187 FEC scheme which is defined within the framework described here. 188 Note that FEC schemes may place restrictions on the types of CDP they 189 are intended to be used with. For example, some FEC schemes may be 190 specific to particular types of application, such as file delivery or 191 streaming. 193 The goal of the FEC building block is to describe functionality 194 directly related to FEC codes that is common to all reliable CDPs and 195 to all FEC schemes, and to leave out any additional functionality 196 that is specific to particular CDPs or particular FEC schemes. The 197 primary functionality described in this document that is common to 198 all such CDPs that use FEC codes is the definition and transport of 199 three kinds of information from sender to receiver(s): encoding 200 symbols themselves, anciliary information associated with encoding 201 symbols (or groups of such symbols, such as the group of symbols in a 202 single packet, or the group of symbols related to a single source 203 block) and anciliary information associated with the whole object 204 being transfered. It is important to note that this ancilliary 205 information is only required by the receiver if one or more of the 206 encoding symbols to which it relates are received. 208 This document for example does not describe how receivers may request 209 transmission of particular encoding symbols for an object. This is 210 because although there are CDPs where requests for transmission are 211 of use, there are also CDPs that do not require such requests. 213 The companion document [4] should be consulted for a full explanation 214 of the benefits of using FEC codes for reliable content delivery 215 using IP multicast. FEC codes are also useful in the context of 216 unicast, and thus the scope and applicability of this document is not 217 limited to IP multicast. 219 5. Applicability Statement 221 The FEC building block does not provide any support for congestion 222 control. Any complete multicast CDP MUST provide congestion control 223 that conforms to [6], and thus this MUST be provided by another 224 building block when the FEC building block is used in a CDP. 226 A more complete description of the applicability of FEC codes can be 227 found in the companion document [4]. 229 6. Functionality 231 This section describes FEC information that is either to be sent in 232 packets also containing FEC encoding symbols or 'out-of-band'. The 233 FEC information is associated with transmission of encoding symbols 234 related to a particular object. There are three classes of packets 235 that may contain FEC information: data packets, session-control 236 packets and feedback packets. They generally contain different kinds 237 of FEC information. Note that some CDPs may not use session-control 238 or feedback packets. 240 Data packets may sometimes serve as session-control packets as well; 241 both data and session-control packets generally travel downstream 242 from the sender towards receivers and are sent to a multicast channel 243 or to a specific receiver using unicast. Session-control packets may 244 additionally travel upstream from receivers to senders. 246 As a general rule, feedback packets travel upstream from receivers to 247 the sender. Sometimes, however, they might be sent to a multicast 248 channel or to another receiver or to some intermediate node or 249 neighboring router that provides recovery services. 251 This document specifies the FEC information that must be carried in 252 data packets and the other FEC information that must be communicated 253 from sender to receiver(s) either out-of-band or in data packets. 254 This document does not specify out-of-band methods nor does it 255 specify the way out-of-band FEC information is associated with FEC 256 information carried in data packets. These methods must be specified 257 in CDP specifications that use this FEC building block. 259 FEC information is classified as follows: 261 1. FEC information associated with an object 263 This is information that is essential for the FEC decoder to 264 decode a specific object. An example of this information is the 265 identity of the FEC scheme that is being used to encode the 266 object, in the form of the FEC Encoding ID. The FEC Encoding ID 267 is described further below. This information may also include 268 FEC scheme-specific parameters for the FEC decoder. 270 2. FEC information associated with specific encoding symbols for an 271 object 273 This is information which is associated with one or more encoding 274 symbols and is thus needed by the decoder whenever one or more of 275 those encoding symbols have been received. Depending on the FEC 276 scheme, information may be associated with individual symbols 277 and/or with groups of symbols. One common such grouping is the 278 group of symbols included within a single packet. Many FEC 279 schemes also segment the object being encoded into multiple 280 'source blocks', each of which is processed independently for FEC 281 purposes. Information about each source block is another type of 282 information associated with a group of encoding symbols, this 283 time the group of symbols which are related to a given source 284 block. 286 Two 'containers' are provided for communicating the FEC information 287 described above, but there is not necessarily a one-to-one 288 correspondance between the class of FEC information and the mechanism 289 used. The two mechanisms are: 291 a. FEC Object Transmission Information 293 CDPs must provide a reliable mechanism for communicating certain 294 FEC information from sender to receiver(s). This information is 295 known as 'FEC Object Transmission Information' and its contents 296 depend on the particular FEC scheme. It includes all information 297 of the first class above and may include information of the 298 second class. The FEC Object Transmission Information can be 299 sent to a receiver within the data packet headers, within session 300 control packets, or by some other means. 302 b. FEC Payload ID 304 CDPs must provide a mechanism for communicating information which 305 identifies (for FEC purposes) the encoding symbols carried by a 306 packet. This information is known as the FEC Payload ID and its 307 contents depend on the FEC scheme. It includes only information 308 of the second class above. A data packet that carries encoding 309 symbols MUST include an FEC Payload ID. 311 6.1. FEC Schemes 313 Two types of FEC scheme are defined by this document: 'Fully- 314 Specified' FEC schemes and 'Under-Specified' FEC schemes. An FEC 315 scheme is a Fully-Specified FEC scheme if the encoding scheme is 316 formally and Fully-Specified, in a way that independent implementors 317 can implement both encoder and decoder from a specification that is 318 an IETF RFC. 320 It is possible that an FEC scheme may not be a Fully-Specified FEC 321 scheme, because either a specification is simply not available or a 322 party exists that owns the encoding scheme and is not willing to 323 disclose the algorithm or specification. We refer to such an FEC 324 encoding scheme as an Under-Specified FEC scheme. 326 FEC schemes are identified by an FEC Encoding ID, which is an integer 327 identifier assigned by IANA. The FEC Encoding ID allows receivers to 328 select the appropriate FEC decoder. The value of the FEC Encoding ID 329 MUST be the same for all transmission of encoding symbols related to 330 a particular object, but MAY vary across different transmissions of 331 encoding symbols about different objects, even if transmitted to the 332 same set of multicast channels and/or using a single upper-layer 333 session. 335 The FEC Instance ID is an integer value that identifies a specific 336 instance of an Under-Specified FEC scheme. This value is not used 337 for Fully-Specified FEC schemes. The FEC Instance ID is scoped by 338 the FEC Encoding ID, and FEC Instance ID values are subject to IANA 339 registration. 341 The FEC Encoding ID for Fully-Specified FEC Schemes and both the FEC 342 Encoding ID and FEC Instance ID for Under-Specified FEc Schemes are 343 essential for the decoder to decode an object and thus are part of 344 the FEC Object Transmission Information. 346 The following requirements apply to all FEC schemes, whether Fully- 347 Specified or Under-Specified: 349 o The type, semantics and an encoding format for the FEC Payload ID 350 and the FEC Object Transmission Information MUST be defined. 352 o A value for the FEC Encoding ID MUST be reserved and associated 353 with the types, semantics and encoding format of the FEC Payload 354 ID and the the FEC Object Transmission Information. 356 The specification for an Under-Specified FEC Scheme MAY allocate a 357 sub-field within the Scheme-specific FEC Object Transmission 358 Information element which is for instance-specific information. Each 359 specific instance of the Under-Specified FEC Scheme may then use this 360 field in an instance-specific way. The FEC scheme should define the 361 scheme-specific FEC Object Transmission Information element in such a 362 way that receivers that do not support the received FEC Instance ID 363 can still parse and interpret the scheme-specific FEC Object 364 Transmission Information element with the exception of the instance- 365 specific field. 367 An already defined Under-Specified FEC Scheme (i.e. FEC Encoding ID 368 value) MUST be reused if the associated FEC Payload ID and FEC Object 369 Transmission Information have the required fields and encoding 370 formats for a new Under-Specified FEC scheme instance. 372 An instance of an Under-Specified FEC scheme is fully identified by 373 the tuple (FEC Encoding ID, FEC Instance ID). The tuple MUST 374 identify a single scheme instance that has at least one 375 implementation. The party that owns this tuple MUST be able to 376 provide information on how to obtain the the Under-Specified FEC 377 scheme instance identified by the tuple, e.g., a pointer to a 378 publicly available reference-implementation or the name and contacts 379 of a company that sells it, either separately or embedded in another 380 product. 382 This specification reserves the range 0-127 for the values of FEC 383 Encoding IDs for Fully-Specified FEC schemes and the range 128-255 384 for the values of Under-Specified FEC schemes. 386 6.2. FEC Object Transmission Information 388 The FEC Object Transmission Information contains information which is 389 essential to the decoder in order to decode the encoded object. It 390 may also contain information which is required to decode certain 391 groups of encoding symbols, for example individual Source Blocks 392 within the object. This information is communicated reliably by the 393 CDP to the receiver(s) as described in Section 8. 395 The FEC Object Transmission Information may consist of several 396 elements and each element may be one of three types, as follows: 398 Mandatory: These elements are defined in this specification and are 399 each mandatory for at least one of the two types of FEC Scheme. 400 Each FEC scheme specifies how the values of the Mandatory FEC 401 Object Transmission Information elements are determined and each 402 CDP specifies how this information is encoded and reliably 403 communicated to the receiver(s). The Mandatory FEC Object 404 Transmission Information includes the identification of the FEC 405 Scheme, which is needed by the receiver to determine whether it 406 supports the FEC Scheme. 408 Common: These elements are defined in this specification and are 409 optional to be used by an FEC scheme. Each FEC scheme specifies 410 which of the Common FEC Object Transmission Information elements 411 it uses and how the values of these elements are determined. Each 412 FEC scheme also specifies an encoding format for the information. 413 Each CDP must specify at least one of the following: 415 1. A means to reliably communicated the Common FEC Object 416 Transmission Information elements to the receiver(s) using the 417 encoding format defined by the FEC scheme. 419 2. An alternative, CDP specific, encoding format for each of the 420 Common FEC Object Transmission Information elements. 422 Scheme-specific: An FEC scheme may specify a single Scheme-specific 423 FEC Object Transmission Information element. The FEC scheme 424 specifies the type, semantics and encoding format of the Scheme- 425 specific FEC Object Transmission Information element. The 426 encoding format may assume that the receiver can determine the 427 length of the Scheme-specific FEC Object Transmission Information 428 element from information communicated outside that element. Each 429 CDP specifies how the Scheme-specific FEC Object Transmission 430 Element is communicated reliably to the receiver(s) i.e. exactly 431 where it shall be carried within packets of the CDP. CDPs also 432 specify how the length of the Scheme-specific FEC Object 433 Transmission Information can be determined by the receiver. Note 434 that although from the point of view of this specification and of 435 CDPs there is only a single Scheme-specific FEC Object 436 Transmission Information element, the FEC scheme may specify this 437 element to contain multiple distinct pieces of information. 439 The Mandatory and Common FEC Object Transmission Information elements 440 are defined in the sections below. 442 6.2.1. Transport of FEC Object Transmission Information 444 It is the responsibility of the CDP to reliably transport the FEC 445 Object Transmission Information to the receiver(s). 447 It is important to note that the encoding format of the Mandatory FEC 448 Object Transmission Information elements (The FEC Encoding ID and FEC 449 Instance ID) is defined by the CDP. This is so that the receiver can 450 identify the FEC Scheme to be used for interpreting the remaining FEC 451 Object Transmission Information elements. All CDPs must define 452 encoding formats for all the Mandatory FEC Object Transmission 453 Information elements. 455 Common FEC Object Transmission Information elements can be 456 transported in two different ways: (a) the FEC Scheme defines an 457 encoding format for each Common FEC Object Transmission Information 458 element that it uses and the CDP transports it, or (b) the CDP 459 defines an encoding format and transports the information in this 460 format. 462 An FEC Scheme MUST define encoding formats for the Common FEC Object 463 Transmission Information elements that it uses. A CDP MAY define 464 encoding formats for the Common FEC Object Transmission Information 465 elements. The CDP determines which way the Common FEC Object 466 Transmission Information elements shall be transported, (a) or (b). 468 Note that a CDP may provide support for one or both options. 470 In the case that the CDP uses the encoding formats specified by the 471 FEC scheme then it may simply concatenate the encoding formats 472 defined by the FEC scheme or it may carry each element in a seperate 473 field or wrapper within the CDP. The FEC scheme must define the 474 encoding formats of the Common FEC Object Transmission Information 475 elements that it uses in such a way that the length of each element 476 is either fixed or can be determined from the encoded data itself. 478 The encoding format of the Scheme-specific FEC Object Transmission 479 Information element is defined by the FEC scheme. CDPs specify only 480 how the resulting byte sequence is communicated. As with encoding 481 formats for the Common FEC Oject Transmission Information elements 482 the length of the Scheme-specific FEC Object Transmission Information 483 must either be fixed or it must be possible to determine the length 484 from the encoded data itself. 486 6.2.2. Opacity of FEC Object Transmission Information 488 The Scheme-specific FEC Object Transmission Information element is 489 opaque to the CDP in the sense that inspecting the contents of this 490 element can only be done if FEC scheme-specific logic is included in 491 the CDP. 493 The encoding formats defined by the FEC scheme for the Common FEC 494 Object Transmission Information elements are also opaque to the CDP 495 in the same sense. 497 The encoding formats defined by the CDP for the Common FEC Object 498 Transmission Information elements are not opaque in this sense, 499 although it must be considered that different FEC Schemes may use 500 different combinations of the Common FEC Object Transmission 501 Information elements. 503 6.2.3. Mandatory FEC Object Transmission Information elements 505 The Mandatory FEC Object Transmission Information elements are: 507 FEC Encoding ID: an integer between 0 and 255 inclusive identifying a 508 specific FEC scheme (Fully-Specified or Under-Specified.) 510 FEC Instance ID (if the FEC Encoding ID identifies an Under-Specified 511 FEC scheme): an integer between 0 and 65535 inclusive identifying an 512 instance of an Under-specifed FEC scheme 514 6.2.4. Common FEC Object Transmission Information elements 516 The Common FEC Object Transmission Information elements are described 517 below. Note that this specification does not provide complete 518 definitions of these fields. Instead only aspects of the abstract 519 type are defined. The precise type and semantics are defined for 520 each FEC scheme in the FEC scheme specification. 522 Transfer-Length: a non-negative integer indicating the length of the 523 object in bytes 525 Encoding-Symbol-Length: a non-negative integer indicating the length 526 of each encoding symbol in bytes 528 Maximum-Source-Block-Length: a non-negative integer indicating the 529 maximum number of source symbols in a source block 531 Max-Number-of-Encoding-Symbols: a non-negative integer indicating the 532 maximum number of encoding symbols (i.e. source plus repair 533 symbols in the case of a systematic code) 535 FEC Schemes define the precise type of those of the above elements 536 that they use and in particular may restrict the value range of each 537 element. FEC Schemes also define an encoding format for each of the 538 above elements that they use. CDPs may also provide an encoding 539 format for each element in which case this encoding format MUST be 540 capable of representing values up to (2^^48)-1 in the case of the 541 Transfer-Length and up to (2^^32)-1 for the other elements. CDPs may 542 additionally or alternatively provide a mechanism to transport these 543 elements encoded according to the encoding format defined by the FEC 544 scheme. For example, FLUTE [8] specifies an XML-based encoding 545 format for these elements, but can also transport FEC scheme-specific 546 encoding formats within the EXT-FTI header extension. 548 6.2.5. Scheme-specific FEC Object Transmission Information element 550 The Scheme-specific FEC Object Transmission Information element may 551 be used by an FEC Scheme to communicate information which is 552 essential to the decoder which cannot adequately be represented 553 within the Mandatory or Common FEC Object Transmission Information 554 elements. 556 From the point of view of a CDP, the Scheme-specific FEC Object 557 Transmission Information element is an opaque, variable length, 558 bitstring. The FEC Scheme defines the structure of this bitstring, 559 which may contain multiple distinct elements. 561 6.3. FEC Payload ID 563 The FEC Payload ID contains information which indicates to the FEC 564 decoder the relationships between the encoding symbols carried by a 565 particular packet and the FEC encoding transformation. For example 566 if the packet carries source symbols then the FEC Payload ID 567 indicates which source symbols of the object are carried by the 568 packet. If the packet carries repair symbols, then the FEC Payload 569 ID indicates how those repair symbols were constructed from the 570 object. 572 The FEC Payload ID may also contain information about larger groups 573 of encoding symbols of which those contained in the packet are part. 574 For example, the FEC Payload ID may contain information about the 575 source block the symbols are related to. 577 The FEC Payload ID for a given packet is essential to the decoder if 578 and only if the packet itself is received. Thus it must be possible 579 to obtain the FEC Payload ID from the recieved packet. Usually, the 580 FEC Payload ID is simply carried explicitly as a separate field 581 within each packet. Some FEC schemes may specify means for deriving 582 the relationship between the carried encoding symbols and the object 583 implicitly from other information within the packet, such as protocol 584 headers already present. Such FEC schemes could obviously only be 585 used with CDPs which provided the appropriate information from which 586 the FEC Payload ID could be derived. 588 The encoding format of the FEC Payload ID is defined by the FEC 589 Scheme. CDPs specify how the FEC Payload ID is carried within data 590 packets i.e. the position of the FEC Payload ID within the CDP packet 591 format and the how it is associated with encoding symbols. 593 FEC schemes for systematic FEC codes may specify two FEC Payload ID 594 formats, one for packets carrying only source symbols and another for 595 packets carrying at least one repair symbol. CDPs must include an 596 indication of which of the two FEC Payload ID formats is included in 597 each packet if they wish to support such FEC Schemes. 599 7. FEC scheme specifications 601 A specification for a new FEC scheme MUST include the following 602 things: 604 1. The FEC Encoding ID value that uniquely identifies the FEC 605 scheme. This value MUST be registered with IANA as described in 606 Section 12. 608 2. The type, semantics and encoding format of one or two FEC Payload 609 IDs. Where two FEC Payload ID formats are specified, then the 610 FEC scheme MUST be a systematic FEC code and one FEC Payload ID 611 format MUST be designated for use with packets carrying only 612 source symbols and the other FEC Payload ID format MUST be 613 designated for use with packets carrying at least one repair 614 symbol. 616 3. The type and semantics of the FEC Object Transmission 617 Information. The FEC Scheme MAY define additional restrictions 618 on the type (including value range) of the Common FEC Object 619 Transmission Information elements. 621 4. An encoding format for the Common FEC Object Transmission 622 Information elements used by the FEC Scheme. 624 Fully-Specified FEC schemes MUST further specify: 626 1. A full specification of the FEC code. 628 This specification MUST precisely define the valid FEC Object 629 Transmission Information values, the valid FEC Payload ID values 630 and the valid packet payload sizes for any given object (where 631 packet payload refers to the space - not necessarily contiguous - 632 within a packet dedicated to carrying encoding symbol bytes). 634 Furthermore, given an object, valid values for each of the FEC 635 Object Transmission Information elements used by the FEC Scheme, 636 a valid FEC Payload ID value and a valid packet payload size, the 637 specification MUST uniquely define the values of the encoding 638 symbol bytes to be included in the packet payload of a packet 639 with the given FEC Payload ID value. 641 A common and simple way to specify the FEC code to the required 642 level of detail is to provide a precise specification of an 643 encoding algorithm which, given an object, valid values for each 644 of the FEC Object Transmission Information elements used by the 645 FEC Scheme for the object, a valid FEC Payload ID and packet 646 payload length as input produces the exact value of the encoding 647 symbol bytes as output. 649 2. A description of practical encoding and decoding algorithms. 651 This description need not be to the same level of detail as for 652 (1) above, however it must be sufficient to demonstrate that 653 encoding and decoding of the code is both possible and practical. 655 FEC scheme specifications MAY additionally define the following: 657 1. Type, semantics and encoding format of a Scheme-specific FEC 658 Object Transmission Information element. 660 Note that if an FEC sheme does not define a Scheme-specific FEC 661 Object Transmission Information then such an element MUST NOT be 662 introduced in future versions of the FEC Scheme. This requirement is 663 included to ensure backwards-compatibility of CDPs designed to 664 support only FEC Schemes which do not use the Scheme-specific FEC 665 Object Transmission Information element. 667 Whenever an FEC scheme specification defines an 'encoding format' for 668 an element, this must be defined in terms of a sequence of bytes 669 which can be embedded within a protocol. The length of the encoding 670 format MUST either be fixed or it must be possible to derive the 671 length from examining the encoded bytes themselves. For example, the 672 initial bytes may include some kind of length indication. 674 FEC schemes SHOULD make use of the Common FEC Object Transmission 675 Information elements in preference to including infomation in a 676 Scheme-specific FEC Object Transmission Information element. 678 FEC scheme specifications SHOULD use the terminology defined in this 679 document and SHOULD follow the following format: 681 1. Introduction 684 686 2. Formats and Codes 688 2.1 FEC Payload ID(s) 691 2.2 FEC Object Transmission Information 693 2.2.1 Mandatory 696 700 2.2.2 Common 704 2.2.3 Scheme-Specific 708 3. Procedures 713 4. FEC code specification (for Fully-Specified FEC schemes 714 only) 716 Specifications MAY include additional sections, for example, 717 examples. 719 Each FEC scheme MUST be specified independently of all other FEC 720 schemes; for example, in a separate specification or a completely 721 independent section of larger specification. 723 8. CDP specifications 725 A specification for a CDP that uses this building block MUST include 726 the following things: 728 1. Definitions of encoding formats for the Mandatory FEC Object 729 Transmission Information elements. 731 2. A means to reliably communicate the Mandatory FEC Object 732 Transmission Information elements from sender to receiver(s) 733 using the encoding format defined in (1). 735 3. Means to reliably communicate the Common FEC Object Transmission 736 Information element from sender to receiver(s) using either or 737 both of (a) encoding formats defined by the FEC Scheme or (b) 738 encoding formats defined by the CDP 740 4. A means to reliably communicate the Scheme-specific FEC Object 741 Transmission Information element from sender to receiver(s) using 742 the encoding format of the Scheme-specific FEC Object 743 Transmission Information element defined by the FEC scheme. 745 5. A means to communicate the FEC Payload ID in association with a 746 data packet. Note that the encoding format of the FEC Payload ID 747 is defined by the FEC Scheme. 749 If option (b) of (3) above is used, then the CDP MUST specify an 750 encoding format for the Common FEC Object Transmission Information 751 elements. 753 CDPs MAY additionally specify the following things: 755 1. A means to indicate whether the FEC Payload ID within a packet is 756 encoded according to the format for packets including only source 757 symbols or according to the format for packets including at least 758 one repair symbol. 760 9. Common algorithms 762 This section describes certain algorithms which are expected to be 763 commonly required by FEC schemes or by CDPs. FEC Schemes and CDPs 764 SHOULD use these algorithms in preference to scheme or protocol 765 specific algorithms where appropriate. 767 9.1. Block partitioning algorithm 769 This algorithm computes a partitioning of a object into source blocks 770 so that all source blocks are as close to being equal length as 771 possible. A first number of source blocks are of the same larger 772 length, and the remaining second number of source blocks of the same 773 smaller length. 775 This algorithm is described in two steps, the second of which may be 776 useful in itself as an independent algorithm in some cases. In the 777 first step, the number of source symbols (T) and the number of source 778 blocks (N) are derived from the Object transfer length (L), Maximum 779 Source Block Length (B) and Symbol Length (E). 781 In the second step, the partitioning of the object is derived from 782 the number of source symbols (T) and the number of source blocks (N). 783 The partitioning is defined in terms of a first number of source 784 blocks (I), a second number of source blocks (N-I), the length of 785 each of the first source blocks (A_large) and the length of each of 786 the second source blocks (A_small). 788 This algorithm is identical to the one specified in Section 5.1.2.3 789 of FLUTE [8] 791 The following notation is used in the description below: 793 ceil[x] denotes x rounded up to the nearest integer. 795 floor[x] denotes x rounded down to the nearest integer. 797 9.1.1. First Step 799 Input: 801 B -- Maximum Source Block Length, i.e., the maximum number of source 802 symbols per source block 804 L -- Transfer Length in bytes 805 E -- Encoding Symbol Length in bytes 807 Output: 809 T -- the number of source symbols in the object. 811 N -- The number of source blocks into which the object shall be 812 partitioned. 814 Algorithm: 816 1. The number of source symbols in the transport object is computed 817 as T = ceil[L/E]. 819 2. The transport object shall be partitioned into N = ceil[T/B] 820 source blocks. 822 9.1.2. Second step 824 Input: 826 T -- the number of source symbols in the object. 828 N -- The number of source blocks into which the object is 829 partitioned. 831 Output: 833 I -- the number of larger source blocks. 835 A_large -- the length of each of the larger source blocks in symbols. 837 A_small -- the length of each of the smaller source blocks in 838 symbols. 840 Algorithm: 842 1. A_large = ceil[T/N] 844 2. A_small = floor[T/N] 846 3. I = T - A_small * N 848 Each of the first I source blocks then consists of A_large source 849 symbols, each source symbol is E bytes in length. Each of the 850 remaining N-I source blocks consist of A_small source symbols, each 851 source symbol is E bytes in length except that the last source symbol 852 of the last source block is L-(((L-1)/E) rounded down to the nearest 853 integer)*E bytes in length. 855 10. Requirements from other building blocks 857 The FEC building block does not provide any support for congestion 858 control. Any complete CDP MUST provide congestion control that 859 conforms to [6], and thus this MUST be provided by another building 860 block when the FEC building block is used in a CDP. 862 There are no other specific requirements from other building blocks 863 for the use of this FEC building block. However, any CDP that uses 864 the FEC building block may use other building blocks for example to 865 provide support for sending higher level session information within 866 data packets containing FEC encoding symbols. 868 11. Security Considerations 870 Data delivery can be subject to denial-of-service attacks by 871 attackers which send corrupted packets that are accepted as 872 legitimate by receivers. This is particularly a concern for 873 multicast delivery because a corrupted packet may be injected into 874 the session close to the root of the multicast tree, in which case 875 the corrupted packet will arrive to many receivers. This is 876 particularly a concern for the FEC building block because the use of 877 even one corrupted packet containing encoding data may result in the 878 decoding of an object that is completely corrupted and unusable. It 879 is thus RECOMMENDED that the decoded objects be checked for integrity 880 before delivering objects to an application. For example, an MD5 881 hash [7] of an object may be appended before transmission, and the 882 MD5 hash is computed and checked after the object is decoded but 883 before it is delivered to an application. Moreover, in order to 884 obtain strong cryptographic integrity protection a digital signature 885 verifiable by the receiver SHOULD be computed on top of such a hash 886 value. It is also RECOMMENDED that a packet authentication protocol 887 such as TESLA [9] be used to detect and discard corrupted packets 888 upon arrival. Furthermore, it is RECOMMENDED that Reverse Path 889 Forwarding checks be enabled in all network routers and switches 890 along the path from the sender to receivers to limit the possibility 891 of a bad agent successfully injecting a corrupted packet into the 892 multicast tree data path. 894 Another security concern is that some FEC information may be obtained 895 by receivers out-of-band in a session description, and if the session 896 description is forged or corrupted then the receivers will not use 897 the correct protocol for decoding content from received packets. To 898 avoid these problems, it is RECOMMENDED that measures be taken to 899 prevent receivers from accepting incorrect session descriptions, 900 e.g., by using source authentication to ensure that receivers only 901 accept legitimate session descriptions from authorized senders. 903 12. IANA Considerations 905 Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA 906 registration. FEC Encoding IDs and FEC Instance IDs are 907 hierarchical: FEC Encoding IDs scope independent ranges of FEC 908 Instance IDs. Only FEC Encoding IDs that correspond to Under- 909 Specified FEC schemes scope a corresponding set of FEC Instance IDs. 911 The FEC Encoding ID and FEC Instance IDs are non-negative integers. 912 In this document, the range of values for FEC Encoding IDs is 0 to 913 255. Values from 0 to 127 are reserved for Fully-Specified FEC 914 schemes and Values from 128 to 255 are reserved for Under-Specified 915 FEC schemes, as described in more detail in Section 6.1. 917 12.1. Explicit IANA Assignment Guidelines 919 This document defines a name-space for FEC Encoding IDs named: 920 ietf:rmt:fec:encoding 922 The values that can be assigned within the "ietf:rmt:fec:encoding" 923 name-space are numeric indexes in the range [0, 255], boundaries 924 included. Assignment requests are granted on a "IETF Consensus" 925 basis as defined in [2]. 927 This document also defines a name-space for FEC Instance IDs named: 928 ietf:rmt:fec:encoding:instance 930 The "ietf:rmt:fec:encoding:instance" name-space is a sub-name-space 931 associated with the "ietf:rmt:fec:encoding" name-space. Each value 932 of "ietf:rmt:fec:encoding" assigned in the range [128, 255] has a 933 separate "ietf:rmt:fec:encoding:instance" sub-name-space that it 934 scopes. Values of "ietf:rmt:fec:encoding" in the range [0, 127] do 935 not scope a "ietf:rmt:fec:encoding:instance" sub-name-space. 937 The values that can be assigned within each "ietf:rmt:fec:encoding: 938 instance" sub-name-space are non-negative integers less than 65536. 939 Assignment requests are granted on a "First Come First Served" basis 940 as defined in [2]. The same value of "ietf:rmt:fec:encoding: 941 instance" can be assigned within multiple distinct sub-name-spaces, 942 i.e., the same value of "ietf:rmt:fec:encoding:instance" can be used 943 for multiple values of "ietf:rmt:fec:encoding". 945 Requestors of "ietf:rmt:fec:encoding:instance" assignments MUST 946 provide the following information: 948 o The value of "ietf:rmt:fec:encoding" that scopes the "ietf:rmt: 949 fec:encoding:instance" sub-name-space. This must be in the range 950 [128, 255]. 952 o Point of contact information 954 o A pointer to publicly accessible documentation describing the 955 Under-Specified FEC scheme, associated with the value of "ietf: 956 rmt:fec:encoding:instance" assigned, and a way to obtain it (e.g., 957 a pointer to a publicly available reference-implementation or the 958 name and contacts of a company that sells it, either separately or 959 embedded in a product). 961 It is the responsibility of the requestor to keep all the above 962 information up to date. 964 13. Acknowledgments 966 This document is largely based on RFC3452 [3] and thus thanks are due 967 to the additional authors of that document: J. Gemmell, L. Rizzo, M. 968 Handley, J. Crowcroft. 970 14. References 972 14.1. Normative References 974 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 975 Levels", BCP 14, RFC 2119, March 1997. 977 [2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA 978 Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. 980 14.2. Informative References 982 [3] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., 983 and J. Crowcroft, "Forward Error Correction (FEC) Building 984 Block", RFC 3452, December 2002. 986 [4] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., 987 and J. Crowcroft, "The Use of Forward Error Correction (FEC) in 988 Reliable Multicast", RFC 3453, December 2002. 990 [5] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable 991 Multicast Transport (RMT) Building Blocks and Protocol 992 Instantiation documents", RFC 3269, April 2002. 994 [6] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF 995 Criteria for Evaluating Reliable Multicast Transport and 996 Application Protocols", RFC 2357, June 1998. 998 [7] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 999 April 1992. 1001 [8] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh, 1002 "FLUTE - File Delivery over Unidirectional Transport", 1003 RFC 3926, October 2004. 1005 [9] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, 1006 "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): 1007 Multicast Source Authentication Transform Introduction", 1008 RFC 4082, June 2005. 1010 [10] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S., 1011 and M. Luby, "Reliable Multicast Transport Building Blocks for 1012 One-to-Many Bulk-Data Transfer", RFC 3048, January 2001. 1014 Authors' Addresses 1016 Mark Watson 1017 Digital Fountain 1018 39141 Civic Center Drive 1019 Suite 300 1020 Fremont, CA 94538 1021 U.S.A. 1023 Email: mark@digitalfountain.com 1025 Michael Luby 1026 Digital Fountain 1027 39141 Civic Center Drive 1028 Suite 300 1029 Fremont, CA 94538 1030 U.S.A. 1032 Email: luby@digitalfountain.com 1034 Lorenzo Vicisano 1035 Cisco Systems, Inc. 1036 170 West Tasman Dr. 1037 San Jose, CA 95134 1038 U.S.A. 1040 Email: lorenzo@cisco.com 1042 Intellectual Property Statement 1044 The IETF takes no position regarding the validity or scope of any 1045 Intellectual Property Rights or other rights that might be claimed to 1046 pertain to the implementation or use of the technology described in 1047 this document or the extent to which any license under such rights 1048 might or might not be available; nor does it represent that it has 1049 made any independent effort to identify any such rights. 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