idnits 2.17.1 draft-ietf-fecframe-framework-08.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 seems to contain a disclaimer for pre-RFC5378 work, and may have content which was first submitted before 10 November 2008. The disclaimer is necessary when there are original authors that you have been unable to contact, or if some do not wish to grant the BCP78 rights to the IETF Trust. If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (June 1, 2010) is 5071 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 1279 -- Looks like a reference, but probably isn't: '255' on line 1279 ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 FEC Framework Working Group M. Watson 3 Internet-Draft Qualcomm, Inc. 4 Intended status: Standards Track June 1, 2010 5 Expires: December 3, 2010 7 Forward Error Correction (FEC) Framework 8 draft-ietf-fecframe-framework-08 10 Abstract 12 This document describes a framework for using forward error 13 correction (FEC) codes with applications in public and private IP 14 networks to provide protection against packet loss. The framework 15 supports applying Forward Error Correction to arbitrary packet flows 16 over unreliable transport and is primarily intended for real-time, or 17 streaming, media. This framework can be used to define Content 18 Delivery Protocols that provide Forward Error Correction for 19 streaming media delivery or other packet flows. Content Delivery 20 Protocols defined using this framework can support any FEC Scheme 21 (and associated FEC codes) which is compliant with various 22 requirements defined in this document. Thus, Content Delivery 23 Protocols can be defined which are not specific to a particular FEC 24 Scheme and FEC Schemes can be defined which are not specific to a 25 particular Content Delivery Protocol. 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 December 3, 2010. 44 Copyright Notice 46 Copyright (c) 2010 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 This document may contain material from IETF Documents or IETF 60 Contributions published or made publicly available before November 61 10, 2008. The person(s) controlling the copyright in some of this 62 material may not have granted the IETF Trust the right to allow 63 modifications of such material outside the IETF Standards Process. 64 Without obtaining an adequate license from the person(s) controlling 65 the copyright in such materials, this document may not be modified 66 outside the IETF Standards Process, and derivative works of it may 67 not be created outside the IETF Standards Process, except to format 68 it for publication as an RFC or to translate it into languages other 69 than English. 71 Table of Contents 73 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 74 2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 6 75 3. Requirements notation . . . . . . . . . . . . . . . . . . . . 9 76 4. Architecture Overview . . . . . . . . . . . . . . . . . . . . 10 77 5. Procedural overview . . . . . . . . . . . . . . . . . . . . . 14 78 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 14 79 5.2. Sender Operation . . . . . . . . . . . . . . . . . . . . . 16 80 5.3. Receiver Operation . . . . . . . . . . . . . . . . . . . . 18 81 6. Protocol Specification . . . . . . . . . . . . . . . . . . . . 22 82 6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 22 83 6.2. Structure of the source block . . . . . . . . . . . . . . 22 84 6.3. Packet format for FEC Source packets . . . . . . . . . . . 22 85 6.3.1. Generic Explicit Source FEC Payload Id . . . . . . . . 24 86 6.4. Packet Format for FEC Repair packets . . . . . . . . . . . 24 87 6.4.1. Packet Format for FEC Repair packets over RTP . . . . 24 88 6.5. FEC Framework Configuration Information . . . . . . . . . 25 89 6.6. FEC Scheme requirements . . . . . . . . . . . . . . . . . 27 90 7. Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 91 8. Transport Protocols . . . . . . . . . . . . . . . . . . . . . 31 92 9. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 32 93 9.1. Normative requirements . . . . . . . . . . . . . . . . . . 33 94 10. Security Considerations . . . . . . . . . . . . . . . . . . . 35 95 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 96 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37 97 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 98 13.1. Normative references . . . . . . . . . . . . . . . . . . . 38 99 13.2. Informative references . . . . . . . . . . . . . . . . . . 38 100 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40 102 1. Introduction 104 Many applications have a requirement to transport a continuous stream 105 of packetised data from a source (sender) to one or more destinations 106 (receivers) over networks which do not provide guaranteed packet 107 delivery. Primary examples are real-time, or streaming, media 108 applications such as broadcast, multicast or on-demand audio, video 109 or multimedia. 111 Forward Error Correction is a well-known technique for improving 112 reliability of packet transmission over networks which do not provide 113 guaranteed packet delivery, especially in multicast and broadcast 114 applications. The FEC Building Block defined in [RFC5052] provides a 115 framework for definition of Content Delivery Protocols (CDPs) for 116 object delivery (including, primarily, file delivery) which make use 117 of separately defined FEC Schemes. Any CDP defined according to the 118 requirements of the FEC Building Block can then easily be used with 119 any FEC Scheme which is also defined according to the requirements of 120 the FEC Building Block. (Note that the term "Forward Erasure 121 Correction" is sometimes used, 'erasures' being a type of error in 122 which data is lost and this loss can be detected, rather than being 123 received in corrupted form - the focus of this document is strictly 124 on erasures, however the term Forward Error Correction is more widely 125 used). 127 This document defines a framework for the definition of CDPs which 128 provide for FEC protection of arbitrary packet flows over unreliable 129 transports such as UDP. As such, this document complements the FEC 130 Building Block of [RFC5052], by providing for the case of arbitrary 131 packet flows over unreliable transport, the same kind of framework as 132 that document provides for object delivery. This document does not 133 define a complete Content Delivery Protocol, but rather defines only 134 those aspects that are expected to be common to all Content Delivery 135 Protocols based on this framework. 137 This framework does not define how the flows to be protected are 138 determined, nor how the details of the protected flows and the FEC 139 streams which protect them are communicated from sender to receiver. 140 It is expected that any complete Content Delivery Protocol 141 specification which makes use of this framework will address these 142 signalling requirements. However, this document does specify the 143 information which is required by the FEC Framework at the sender and 144 receiver - for example details of the flows to be FEC protected, the 145 flow(s) that will carry the FEC protection data and an opaque 146 container for FEC-Scheme-specific information. 148 FEC Schemes designed for use with this framework must fulfil a number 149 of requirements defined in this document. Note that these 150 requirements are different from those defined in [RFC5052] for FEC 151 Schemes for object delivery. However there is a great deal of 152 commonality and FEC Schemes defined for object delivery may be easily 153 adapted for use with the framework defined here. 155 Since the RTP protocol layer is used over UDP, this framework can be 156 applied to RTP flows as well. FEC repair packets may be sent 157 directly over UDP or over RTP. The latter approach has the advantage 158 that RTP instrumentation, based on RTCP, can be used for the repair 159 flow. Additionally, the post-repair RTCP extended report [RFC5725] 160 may be used to obtain information about the loss rate after FEC 161 recovery. 163 The use of RTP for repair flows is defined for each FEC Scheme by 164 defining an RTP Payload Format for that particular FEC Scheme 165 (possibly in the same document). 167 2. Definitions/Abbreviations 169 'FEC' Forward Error Correction. 171 'AL-FEC' Application Layer Forward Error Correction 173 'FEC Framework' A protocol framework for definition of Content 174 Delivery Protocols using FEC, such as the framework defined in 175 this document. 177 'Source data flow' The packet flow or flows to which FEC protection 178 is to be applied. A source data flow consists of ADUs. 180 'Repair data flow' The packet flow or flows carrying forward error 181 correction data 183 'Source protocol' A protocol used for the source data flow being 184 protected - e.g. RTP. 186 'Transport protocol' The protocol used for transport of the source 187 and repair data flows - e.g. UDP, DCCP. 189 'Application Data Unit' The unit of source data provided as payload 190 to the transport layer 192 'ADU Flow' A sequence of ADUs associated with a transport layer flow 193 identifier (such as the standard 5-tuple { Source IP Address, 194 Source Transport Port, Destination IP Address, Destination 195 Transport Port, Transport Protocol } in the case of UDP) 197 'Application protocol' Control protocol used to establish and 198 control the source data flow being protected - e.g. RTSP. 200 'FEC Code' An algorithm for encoding data such that the encoded data 201 flow is resiliant to data loss (Note: in general FEC Codes may 202 also be used to make a data flow resiliant to corruption, but that 203 is not considered here). 205 'FEC Scheme' A specification which defines the additional protocol 206 aspects required to use a particular FEC code with the FEC 207 Framework, or, in the context of RMT, with the RMT FEC Building 208 Block. 210 'Protection amount' The relative increase in data sent due to the 211 use of FEC. 213 'FEC Framework Configuration Information' Information which controls 214 the operation of the FEC Framework. 216 'FEC Source Packet' At a sender (resp. receiver) a payload submitted 217 to (resp. received from) the Transport protocol containing an ADU 218 along with an optional Source FEC Payload ID. 220 'FEC Repair Packet' At a sender (resp. receiver) a payload submitted 221 to (resp. received from) the Transport protocol containing one or 222 more repair symbols along with a Repair FEC Payload ID and 223 possibly an RTP header. 225 'FEC Payload ID' Information which identifies the contents of a 226 packet with respect to the FEC Scheme. 228 'Source FEC Payload ID' An FEC Payload ID specifically for use with 229 source packets. 231 'Repair FEC Payload ID' An FEC Payload ID specifically for use with 232 repair packets. 234 'Content Delivery Protocol (CDP)' A complete application protocol 235 specification which, through the use of the framework defined in 236 this document, is able to make use of FEC Schemes to provide 237 Forward Error Correction capabilities 239 The following definitions are aligned with [RFC5052] 241 'Source symbol' unit of data used during the encoding process. 243 'Encoding symbol' unit of data generated by the encoding process. 244 With systematic codes, source symbols are part of the encoding 245 symbols. 247 'Repair symbol' encoding symbol that is not a source symbol. 249 'Code rate' the k/n ratio, i.e., the ratio between the number of 250 source symbols and the number of encoding symbols. By definition, 251 the code rate is such that: 0 < code rate <= 1. A code rate close 252 to 1 indicates that a small number of repair symbols have been 253 produced during the encoding process. 255 'Systematic code' FEC code in which the source symbols are part of 256 the encoding symbols. The Reed-Solomon codes introduced in this 257 document are systematic. 259 'Source Block' group of ADUs which are to be FEC protected as a 260 single block. 262 'Packet Erasure Channel' a communication path where packets are 263 either dropped (e.g., by a congested router, or because the number 264 of transmission errors exceeds the correction capabilities of the 265 physical layer codes) or received. When a packet is received, it 266 is assumed that this packet is not corrupted. 268 3. Requirements notation 270 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 271 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 272 document are to be interpreted as described in [RFC2119]. 274 4. Architecture Overview 276 The FEC Framework is described in terms of an additional layer 277 between the transport layer (e.g. UDP or DCCP) and protocols running 278 over this transport layer. Examples of such protocols are RTP, RTCP, 279 etc. As such, the data path interface between the FEC Framework and 280 both underlying and overlying layers can be thought of as being the 281 same as the standard interface to the transport layer - i.e. the data 282 exchanged consists of datagram payloads each associated with a single 283 ADU flow identified (in the case of UDP) by the standard 5-tuple { 284 Source IP Address, Source Transport Port, Destination IP Address, 285 Destination Transport Port, Transport Protocol }. In the case that 286 RTP is used for the repair flows, the source and repair data may be 287 multiplexed using RTP onto a single UDP flow and must consequently be 288 demultiplexed at the receiver. There are various ways in which this 289 multiplexing can be done, for example as described in [RFC4588]. 291 It is important to understand that the main purpose of the FEC 292 Framework architecture is to allocate fuctional responsibilities to 293 separately documented components in such a way that specific 294 instances of the components can be combined in different ways to 295 describe different protocols. 297 The FEC Framework makes use of an FEC Scheme, in a similar sense to 298 that defined in [RFC5052] and uses the terminology of that document. 299 The FEC Scheme defines the FEC encoding and decoding and defines the 300 protocol fields and procedures used to identify packet payload data 301 in the context of the FEC Scheme. The interface between the FEC 302 Framework and an FEC Scheme, which is described in this document, is 303 a logical one, which exists for specification purposes only. At an 304 encoder, the FEC Framework passes ADUs to the FEC Scheme for FEC 305 encoding. The FEC Scheme returns repair symbols with their 306 associated Repair FEC Payload IDs, and in some case Source FEC 307 Payload IDs, depending on the FEC Scheme. At a decoder, the FEC 308 Framework passes transport packet payloads (source and repair) to the 309 FEC Scheme and the FEC Scheme returns additional recovered source 310 packet payloads. 312 This document defines certain FEC Framework Configuration Information 313 which MUST be available to both sender and receiver(s). For example, 314 this information includes the specification of the ADU flows which 315 are to be FEC protected, specification of the ADU flow(s) which will 316 carry the FEC protection (repair) data and the relationship(s) 317 between these source and repair flows (i.e. which source flow(s) are 318 protected by each repair flow. The FEC Framework Configuration 319 Information also includes information fields which are specific to 320 the FEC Scheme. This information is analagous to the FEC Object 321 Transmission Information defined in [RFC5052]. 323 The FEC Framework does not define how the FEC Framework Configuration 324 Information for the stream is communicated from sender to receiver. 325 This must be defined by any Content Delivery Protocol specification 326 as described in the following sections. 328 In this architecture we assume that the interface to the transport 329 layer supports the concepts of data units (referred to here as 330 Application Data Units) to be transported and identification of ADU 331 flows on which those data units are transported. Since this is an 332 interface internal to the architecture, we do not specify this 333 interface explicitly, except to say that ADU flows which are distinct 334 from the transport layer point of view (for example, distinct UDP 335 flows as identified by the UDP source/destination ports/addresses) 336 are also distinct on the interface between the transport layer and 337 the FEC Framework. 339 As noted above, RTP flows are a specific example of ADU flows which 340 might be protected by the FEC Framework. From the FEC Framework 341 point of view, RTP source flows are ADU flows like any other, with 342 the RTP header included within the ADU. 344 Depending on the FEC Scheme, RTP may also be used as a transport for 345 repair packet flows. In this case an FEC Scheme must define an RTP 346 Payload Format for the repair data. 348 The architecture outlined above is illustrated in the Figure 1. In 349 this architecture, two RTP instances are shown, for the source and 350 repair data respectively. This is because the use of RTP for the 351 source data is separate from and independent of the use of RTP for 352 the repair data. The appearance of two RTP instances is more natural 353 when you consider that in many FEC codes, the repair payload contains 354 repair data calculated across the RTP headers of the source packets. 355 Thus a repair packet carried over RTP starts with an RTP header of 356 its own which is followed (after the Repair Payload ID) by repair 357 data containing bytes which protect the source RTP headers (as well 358 as repair data for the source RTP payloads). 360 +--------------------------------------------+ 361 | Application | 362 +--------------------------------------------+ 363 | 364 | 365 | 366 + - - - - - - - - - - - - - - - - - - - - - - - -+ 367 | +--------------------------------------------+ | 368 | Application Layer | 369 | +--------------------------------------------+ | 370 | | 371 | + -- -- -- -- -- -- -- -- -- -- --+ | | 372 | RTP (optional) | | 373 | | | |-Configuration/Coordination 374 +- -- -- -- -- -- -- -- -- -- -- -+ | 375 | | | | 376 | ADU flows | 377 | | v | 378 +--------------------------------------------+ +----------------+ 379 | | FEC Framework (this document) |<--->| FEC Scheme | 380 +--------------------------------------------+ +----------------+ 381 | | | | 382 Source | Repair | 383 | | | | 384 +-- -- -- -- --|-- --+ -- -- -- -- -- + -- --+ 385 | | RTP | | RTP processing | |<--- Optional 386 | | +-- -- -- |- -- -+ | - dependent on 387 | | +-- -- -- -- -- -- -- |--+ | | FEC Scheme 388 | | RTP (de)multiplexing | | 389 | +-- -- -- --- -- -- -- -- -- -- -- -- -- -- -+ | 390 | 391 | +--------------------------------------------+ | 392 | Transport Layer (e.g. UDP) | 393 | +--------------------------------------------+ | 394 | 395 | +--------------------------------------------+ | 396 | IP | 397 | +--------------------------------------------+ | 398 Content Delivery Protocol 399 + - - - - - - - - - - - - - - - - - - - - - - - + 401 Figure 1: FEC Framework Architecture 403 The contents of the transport payload for repair packets is fully 404 defined by the FEC Scheme. For a specific FEC Scheme, a means MAY be 405 defined for repair data to be carried over RTP, in which case the 406 repair packet payload format starts with the RTP header. This 407 corresponds to defining an RTP Payload Format for the specific FEC 408 Scheme. Guidelines for writers of RTP Payload Formats are provided 409 in [RFC2736]. 411 The use of RTP for repair packets is independent of the protocols 412 used for source packets: if RTP is used for source packets then 413 repair packets may or may not use RTP and vice versa (although it is 414 unlikely that there are useful scenarios where non-RTP source flows 415 are protected by RTP repair flows). FEC Schemes are expected to 416 recover entire transport payloads for recovered source packets in all 417 cases. For example if RTP is used for source flows, the FEC Scheme 418 is expected to recover the entire UDP payload, including the RTP 419 header. 421 5. Procedural overview 423 5.1. General 425 The mechanism defined in this document does not place any 426 restrictions on the Application Data Units which can be protected 427 together, except that the Application Data Unit is carried over a 428 supported transport protocol (See Section 8). The data may be from 429 multiple Source Data Flows that are protected jointly. The FEC 430 framework handles the Source Data Flows as a sequence of 'source 431 blocks' each consisting of a set of Application Data Units, possibly 432 from multiple Source Data Flows which are to be protected together. 433 For example, each source block may be constructed from those 434 Application Data Units related to a particular segment in time of the 435 flow. 437 At the sender, the FEC Framework passes the payloads for a given 438 block to the FEC Scheme for FEC encoding. The FEC Scheme performs 439 the FEC encoding operation and returns the following information: 441 o optionally, FEC Payload IDs for each of the source payloads 442 (encoded according to an FEC-Scheme-specific format) 444 o one or more FEC repair packet payloads 446 o FEC Payload IDs for each of the repair packet payloads (encoded 447 according to an FEC-Scheme-specific format) 449 The FEC framework then performs two operations: Firstly, it appends 450 the FEC payload IDs, if provided, to each of the Application Data 451 Units, and sends the resulting packets, known as 'FEC source 452 packets', to the receiver and secondly it places the provided 'FEC 453 repair packet payloads' and corresponding 'FEC Repair Payload IDs' 454 appropriately to construct 'FEC repair packets' and send them to the 455 receiver. Note that FEC repair packets MAY be sent to a different 456 multicast group or groups from the source packets. 458 This document does not define how the sender determines which 459 Application Data Units are included in which source blocks or the 460 sending order and timing of FEC source and FEC repair packets. A 461 specific Content Delivery Protocol MAY define this mapping or it MAY 462 be left as implementation dependent at the sender. However, a CDP 463 specification MUST define how a receiver determines a mimimum length 464 of time that it should wait to receive FEC repair packets for any 465 given source block. FEC Schemes MAY define limitations on this 466 mapping, such as maximum size of source blocks, but SHOULD NOT 467 attempt to define specific mappings. The sequence of operations at 468 the sender is described in more detail in Section 5.2. 470 At the receiver, original Application Data Units are recovered by the 471 FEC Framework directly from any FEC Source Packets received simply by 472 removing the Source FEC Payload ID, if present. The receiver also 473 passes the contents of the received Application Data Units, plus 474 their FEC Payload IDs to the FEC Scheme for possible decoding. 476 If any Application Data Units related to a given source block have 477 been lost, then the FEC Scheme may perform FEC decoding to recover 478 the missing Application Data Units (assuming sufficient FEC Source 479 and FEC Repair packets related to that source block have been 480 received). 482 Note that the receiver may need to buffer received source packets to 483 allow time for the FEC Repair packets to arrive and FEC decoding to 484 be performed before some or all of the received or recovered packets 485 are passed to the application. If such a buffer is not provided, 486 then the application must be able to deal with the severe re-ordering 487 of packets that may occur. However, such buffering is Content 488 Delivery Protocol and/or implementation-specific and is not specified 489 here. The receiver operation is described in more detail in 490 Section 5.3 492 The FEC Source packets MUST contain information which identifies the 493 source block and the position within the source block (in terms 494 specific to the FEC Scheme) occupied by the Application Data Unit. 495 This information is known as the 'Source FEC Payload ID'. The FEC 496 Scheme is responsible for defining and interpreting this information. 497 This information MAY be encoded into a specific field within the FEC 498 Source packet format defined in this specification, called the 499 Explicit Source FEC Payload ID field. The exact contents and format 500 of the Explicit Source FEC Payload ID field are defined by the FEC 501 Scheme. Alternatively, the FEC Scheme MAY define how the Source FEC 502 Payload ID is derived from other fields within the source packets. 503 This document defines the way that the Explicit Source FEC Payload ID 504 field is appended to source packets to form FEC Source packets. 506 The FEC Repair packets MUST contain information which identifies the 507 source block and the relationship between the contained repair 508 payloads and the original source block. This is known as the 'Repair 509 FEC Payload ID'. This information MUST be encoded into a specific 510 field, the Repair FEC Payload ID field, the contents and format of 511 which are defined by the FEC Scheme. 513 The FEC Scheme MAY use different FEC Payload ID field formats for FEC 514 Source packets and FEC Repair packets. 516 5.2. Sender Operation 518 It is assumed that the sender has constructed or received original 519 data packets for the session. These may be RTP, RTCP, MIKEY or 520 indeed any other type of packet. The following operations, 521 illustrated in Figure 2, for the case of UDP repair flows and 522 Figure 3 for the case of RTP repair flows, describe a possible way to 523 generate compliant FEC Source packet and FEC repair packet streams: 525 1. Application Data Units are provided by the application. 527 2. A source block is constructed as specified in Section 6.2. 529 3. The source block is passed to the FEC Scheme for FEC encoding. 530 The Source FEC Payload ID information of each Source packet is 531 determined by the FEC Scheme. If required by the FEC Scheme the 532 Source FEC Payload ID is encoded into the Explicit Source FEC 533 Payload ID field. 535 4. The FEC Scheme performs FEC Encoding, generating repair packet 536 payloads from a source block and a Repair FEC Payload ID field for 537 each repair payload. 539 5. The Explicit Source FEC Payload IDs (if used), Repair FEC 540 Payload IDs and repair packet payloads are provided back from the 541 FEC Scheme to the FEC Framework. 543 6. The FEC Framework constructs FEC Source packets according to 544 Section 6.3 and FEC Repair packets according to Section 6.4 using 545 the FEC Payload IDs and repair packet payloads provided by the FEC 546 Scheme. 548 7. The FEC Source and FEC Repair packets are sent using normal 549 transport layer procedures. The port(s) and multicast group(s) to 550 be used for FEC Repair packets are defined in the FEC Framework 551 Configuration Information. The FEC Source packets are sent using 552 the same ADU flow identification information as would have been 553 used for the original source packets if the FEC Framework were not 554 present (for example, in the UDP case, the UDP source and 555 destination addresses and ports on the IP datagram carrying the 556 Source Packet will be the same whether or not the FEC Framework is 557 applied). 559 +----------------------+ 560 | Application | 561 +----------------------+ 562 | 563 | (1) Application Data Units 564 | 565 v 566 +----------------------+ +------------------+ 567 | FEC Framework | | | 568 | |-------------------------->| FEC Scheme | 569 |(2) Construct source | (3) Source Block | | 570 | blocks | | (4) FEC Encoding | 571 |(6) Construct FEC src |<--------------------------| | 572 | packets and FEC | | | 573 | repair packets |(5) Ex src FEC Payload Ids,| | 574 +----------------------+ Repair FEC Payload Ids,+------------------+ 575 | Repair symbols 576 | 577 | (7) FEC Source packets and FEC repair packets 578 v 579 +----------------------+ 580 | Transport Layer | 581 | (e.g. UDP ) | 582 +----------------------+ 584 Figure 2: Sender operation 586 +----------------------+ 587 | Application | 588 +----------------------+ 589 | 590 | (1) Application Data Units 591 v 592 +----------------------+ +------------------+ 593 | FEC Framework | | | 594 | |-------------------------->| FEC Scheme | 595 |(2) Construct source | (3) Source Block | | 596 | blocks | | (4) FEC Encoding | 597 |(6) Construct FEC src |<--------------------------| | 598 | packets and FEC | | | 599 | repair payloads |(5) Ex src FEC Payload Ids,| | 600 +----------------------+ Repair FEC Payload Ids,+------------------+ 601 | | Repair symbols 602 |(7) Source | 603 | |(7') Repair RTP payloads 604 | + -- -- -- -- -+ 605 | | RTP | 606 | +-- -- -- -- --+ 607 v v 608 +----------------------+ 609 | Transport Layer | 610 | (e.g. UDP ) | 611 +----------------------+ 613 Figure 3: Sender operation with RTP repair flows 615 5.3. Receiver Operation 617 The following describes a possible receiver algorithm, illustrated in 618 Figure 4 and Figure 5 for the case of RTP repair flows, when 619 receiving an FEC source or repair packet: 621 1. FEC Source Packets and FEC Repair packets are received and 622 passed to the FEC Framework. The type of packet (Source or 623 Repair) and the Source Data Flow to which it belongs (in the case 624 of source packets) is indicated by the ADU flow information which 625 identifies the flow at the transport layer (for example source and 626 destination ports and addresses in the case of UDP). 628 1a. In the special case that RTP is used for repair packets and 629 source and repair packets are multiplexed onto the same UDP flow, 630 then RTP demultiplexing is required to demultiplex source and 631 repair flows. However, RTP processing is applied only to the 632 repair packets at this stage: source packets continue to be 633 handled as UDP payloads (i.e. including their RTP headers). 635 2. The FEC Framework extracts the Explicit Source FEC Payload ID 636 field (if present) from FEC Source Packets and the Repair FEC 637 Payload ID from FEC Repair Packets. 639 3. The Explicit Source FEC Payload IDs (if present), Repair FEC 640 Payload IDs, FEC Source payloads and FEC Repair payloads are 641 passed to the FEC Scheme. 643 4. The FEC Scheme uses the received FEC Payload IDs (and derived 644 FEC Source Payload IDs in the case that the Explicit Source FEC 645 Payload ID field is not used) to group source and repair packets 646 into source blocks. If at least one source packet is missing from 647 a source block, and at least one repair packet has been received 648 for the same source block then FEC decoding may be performed in 649 order to recover missing source payloads. The FEC Scheme 650 determines whether source packets have been lost and whether 651 enough data for decoding of any or all of the missing source 652 payloads in the source block has been received. 654 5. The FEC Scheme returns the Application Data Units to the FEC 655 Framework in the form of source blocks containing received and 656 decoded Application Data Units and indications of any Application 657 Data Units which were missing and could not be decoded. 659 6. The FEC Framework passes the received and recovered 660 Application Data Units to the application. 662 Note that the description above defines functionality 663 responsibilities but does not imply a specific set of timing 664 relationships. For example, ADUs may eb provided to the application 665 as soon as they are received or recovered (and hence potentially out- 666 of-order) or they may be buffered are delivered to the application 667 in-order. 669 +----------------------+ 670 | Application | 671 +----------------------+ 672 ^ 673 | (6) Application Data Units 674 | 675 +----------------------+ +------------------+ 676 | FEC Framework | | | 677 | |<---------------------------| FEC Scheme | 678 |(2)Extract FEC Payload| (5) Application Data Units | | 679 | IDs and pass IDs & | | (4) FEC Decoding | 680 | Payloads to FEC |--------------------------->| | 681 | Scheme | (3) Ex src FEC Payload IDs,| | 682 +----------------------+ FEC Repair Payload IDs,+------------------+ 683 ^ FEC Source Payloads, 684 | FEC Repair Payloads 685 | 686 | (1) FEC Source packets and FEC repair packets 687 | 688 +----------------------+ 689 | Transport Layer | 690 | (e.g. UDP ) | 691 +----------------------+ 693 Figure 4: Receiver Operation 695 +----------------------+ 696 | Application | 697 +----------------------+ 698 ^ 699 | (6) Application Data Units 700 | 701 +----------------------+ +------------------+ 702 | FEC Framework | | | 703 | |<---------------------------| FEC Scheme | 704 |(2)Extract FEC Payload| (5) Application Data Units | | 705 | IDs and pass IDs & | | (4) FEC Decoding | 706 | Payloads to FEC |--------------------------->| | 707 | Scheme | (3) Ex src FEC Payload IDs,| | 708 +----------------------+ FEC Repair Payload IDs,+------------------+ 709 ^ ^ FEC Source Payloads, 710 | | FEC Repair Payloads 711 |Source pkts | 712 | |(1a) FEC repair payloads 713 +-- |- -- -- -- -- -- -+ 714 |RTP| | RTP processing | 715 | | +-- -- -- --|-- -+ 716 | +-- -- -- -- -- |--+ | 717 | | RTP demux | | 718 +-- -- -- -- -- -- -- -+ 719 | (1) FEC Source packets and FEC repair packets 720 +----------------------+ 721 | Transport Layer | 722 | (e.g. UDP ) | 723 +----------------------+ 725 Figure 5: Receiver Operation 727 Note that the above procedure may result in a situation in which not 728 all ADUs are recovered. 730 Source packets which are correctly received and those which are 731 reconstructed MAY be delivered to the application out of order and in 732 a different order from the order of arrival at the receiver. 733 Alternatively, buffering and packet re-ordering MAY be applied to re- 734 order received and reconstructed source packets into the order they 735 were placed into the source block, if that is necessary according to 736 the application. 738 6. Protocol Specification 740 6.1. General 742 This section specifies the protocol elements for the FEC Framework. 743 Three components of the protocol are defined in this document and are 744 described in the following sections: 746 1. Construction of a source block from Application Data Units. 747 The FEC code will be applied to this source block to produce the 748 repair payloads. 750 2. A format for packets containing source data. 752 3. A format for packets containing repair data. 754 The operation of the FEC Framework is governed by certain FEC 755 Framework Configuation Information. This configuration information 756 is also defined in this section. A complete protocol specification 757 that uses this framework MUST specify the means to determine and 758 communicate this information between sender and receiver. 760 6.2. Structure of the source block 762 The FEC Framework and FEC Scheme exchange Application Data Units in 763 the form of source blocks. A source block is generated by the FEC 764 Framework from an ordered sequence of Application Data Units. The 765 allocation of Application Data Units to blocks is dependent on the 766 application. Note that some Application Data Units may not be 767 included in any block. Each Source Block provided to the FEC scheme 768 consists of an ordered sequence of Application Data Units where the 769 following information is provided for each ADU: 771 o A description of the Source Data Flow with which the Application 772 Data Unit is associated (See 6.5) 774 o The Application Data Unit itself 776 o The length of the Application Data Unit 778 6.3. Packet format for FEC Source packets 780 The packet format for FEC Source packets MUST be used to transport 781 the payload of an original source packet. As depicted in Figure 6, 782 it consists of the original packet, optionally followed by the 783 Explicit Source FEC Payload ID field. The FEC Scheme determines 784 whether the Explicit Source FEC Payload ID field is required. This 785 determination is specific to each ADU flow. 787 +------------------------------------+ 788 | IP header | 789 +------------------------------------+ 790 | Transport header | 791 +------------------------------------+ 792 | Application Data Unit | 793 +------------------------------------+ 794 | Explicit Source FEC Payload ID | 795 +------------------------------------+ 797 Figure 6: Structure of the FEC packet format for FEC Source packets 799 The FEC Source packets MUST be sent using the same ADU flow as would 800 have been used for the original source packets if the FEC Framework 801 were not present. The transport payload of the FEC Source packet 802 MUST consist of the Application Data Unit followed by the Explicit 803 Source FEC Payload ID field, if required. 805 The Explicit Source FEC Payload ID field contains information 806 required to associate the source packet with a source block and for 807 the operation of the FEC algorithm and is defined by the FEC Scheme. 808 The format of the Source FEC Payload ID field is defined by the FEC 809 Scheme. Note that in the case that the FEC Scheme or CDP defines a 810 means to derive the Source FEC Payload ID from other information in 811 the packet (for example the a sequence number of some kind used by 812 the application protocol), then the Source FEC Payload ID field is 813 not included in the packet. In this case the original source packet 814 and FEC Source Packet are identical. 816 Since the addition of the Explicit Source FEC Payload ID increases 817 the packet length, then in applications where avoidance of IP packet 818 fragmentation is a goal, Content Delivery Protocols SHOULD consider 819 the Explicit Source FEC Payload ID size when determining the size of 820 Application Data Units that will be delivered using the FEC 821 Framework. 823 Note: The Explicit Source FEC Payload ID is placed at the end of the 824 packet so that in the case that Robust Header Compression [RFC3095] 825 or other header compression mechanisms are used and in the case that 826 a ROHC profile is defined for the protocol carried within the 827 transport payload (for example RTP), then ROHC will still be applied 828 for the FEC Source packets. Applications that may be used with this 829 Framework should consider that FEC Schemes may add this Explicit 830 Source FEC Payload ID and thereby increase the packet size. 832 In many applications, support for Forward Error Correction is added 833 to a pre-existing protocol and in this case use of the Explicit 834 Source FEC Payload ID may break backwards compatibility, since source 835 packets are modified. 837 6.3.1. Generic Explicit Source FEC Payload Id 839 In order to apply FEC protection using multiple FEC Schemes to a 840 single source flow all schemes must use the same Explicit Source FEC 841 Payload Id format. In order to enable this, it is RECOMMENDED that 842 FEC Schemes support the Generic Explicit Source FEC Payload Id format 843 described below. 845 The Generic Explicit Source FEC Payload Id has length 2 bytes and 846 consists of an unsigned packet sequence number in network byte order. 847 The allocation of sequence numbers to packets is independent of any 848 FEC Scheme and of the Source Block contruction, except that the use 849 of this sequence number places a constraint on source block 850 construction source packets within a given source block MUST have 851 consecutive sequence numbers (where consecutive includes wrap-around 852 from 65535 to 0). Sequence numbers SHOULD NOT be reused until all 853 values in the sequence number space have been used. 855 6.4. Packet Format for FEC Repair packets 857 The packet format for FEC Repair packets is shown in Figure 7. The 858 transport payload consists of a Repair FEC Payload ID field followed 859 by repair data generated in the FEC encoding process. 861 +------------------------------------+ 862 | IP header | 863 +------------------------------------+ 864 | Transport header | 865 +------------------------------------+ 866 | Repair FEC Payload ID | 867 +------------------------------------+ 868 | Repair Symbols | 869 +------------------------------------+ 871 Figure 7: Packet format for repair packets 873 The Repair FEC Payload ID field contains information required for the 874 operation of the FEC algorithm at the receiver. This information is 875 defined by the FEC Scheme. The format of the Repair FEC Payload ID 876 field is defined by the FEC Scheme. 878 6.4.1. Packet Format for FEC Repair packets over RTP 880 For FEC Schemes which specify the use of RTP for repair packets, the 881 packet format for repair packets includes an RTP header as shown in 882 Figure 8. 884 +------------------------------------+ 885 | IP header | 886 +------------------------------------+ 887 | Transport header (UDP) | 888 +------------------------------------+ 889 | RTP Header | 890 +------------------------------------+ 891 | Repair FEC Payload ID | 892 +------------------------------------+ 893 | Repair Symbols | 894 +------------------------------------+ 896 Figure 8: Packet format for repair packets 898 6.5. FEC Framework Configuration Information 900 The FEC Framework Configuration Information is information that the 901 FEC Framework needs in order to apply FEC protection to the ADU 902 flows. A complete Content Delivery Protocol specification that uses 903 the framework specified here MUST include details of how this 904 information is derived and communicated between sender and receiver. 906 The FEC Framework Configuration Information includes identification 907 of the set of Source Data Flows. For example, in the case of UDP, 908 each Source Data Flow is uniquely identified by a tuple { Source IP 909 Address, Destination IP Address, Source UDP port, Destination UDP 910 port }. Note that in some applications some of these fields may be 911 wildcarded, so that the flow is identified by a subset of the fields 912 and in particular in many applications the limited tuple { 913 Destination IP Address, Destination UDP port } is sufficient. 915 A single instance of the FEC Framework provides FEC protection for 916 packets of the specified set of Source Data Flows, by means of one or 917 more packet flows consisting of repair packets. The FEC Framework 918 Configuation Information includes, for each instance of the FEC 919 Framework: 921 1. Identification of the packet flow(s) carrying FEC Repair 922 packets, known as the FEC repair flow(s). 924 2. For each Source Data Flow protected by the FEC repair flow(s): 926 a. Defintion of the Source Data Flow carrying source packets 927 (for example, by means of a tuple as describe above for UDP). 929 b. An integer identifier for this flow definition (i.e. 930 tuple). This identifier MUST be unique amongst all Source Data 931 Flows which are protected by the same FEC repair flow. 933 3. The FEC Encoding ID, identifying the FEC Scheme 935 4. The length of the Explicit Source FEC Payload Id, in bytes 937 5. Zero or more FEC-Scheme-specific information elements, each 938 consisting of a name and a value where the valid element names and 939 value ranges are defined by the FEC Scheme 941 Multiple instances of the FEC Framework, with separate and 942 independent FEC Framework Configuration Information, may be present 943 at a sender or receiver. A single instance of the FEC Framework 944 protects packets of the Source Data Flows identified in (2) above 945 i.e. all packets sent on those flows MUST be FEC Source packets as 946 defined in Section 6.3. A single Source Data Flow may be protected 947 by multiple instances of the FEC Framework. 949 The integer flow identifier identified in 2(b) is a "shorthand" to 950 identify source flows between the FEC Framework and the FEC Scheme. 951 The reason for defining this as an integer, and including it in the 952 FEC Framework Configuration Information is so that the FEC Scheme at 953 the sender and receiver may use it to identify the source flow with 954 which a recovered packet is associated. The integer flow identifier 955 may therefore take the place of the complete flow description (e.g. 956 UDP 4-tuple). 958 Whether and how this flow identifier is used is defined by the FEC 959 Scheme. Since source packets are directly associated with a flow by 960 virtue of their packet headers, this identifier need not be carried 961 in source packets. Since repair packets may provide protection for 962 multiple source flows, repair packets would either not carry the 963 identifier at all or may carry multiple identifiers. However, in any 964 case, the flow identifier associated with a particular source packet 965 may be recovered from the repair packets as part of an FEC decoding 966 operation. Integer flow identifiers SHOULD be allocated starting 967 from zero and increasing by one for each flow. 969 A single FEC repair flow provides repair packets for a single 970 instance of the FEC Framework. Other packets MUST NOT be sent within 971 this flow i.e. all packets in the FEC repair flow MUST be FEC repair 972 packets as defined in Section 6.4 and MUST relate to the same FEC 973 Framework instance. 975 In the case that RTP is used for repair packets, the identification 976 of the repair packet flow MAY also include the RTP Payload Type to be 977 used for repair packets. 979 FEC Scheme-specific information elements MAY be encoded into a text 980 string for transport within Content Delivery Protocols as according 981 to the following ABNF [RFC5234]: 983 scheme-specific-info = [ element *( ',' element ) ] 984 element = name ':' value 985 name = token 986 token = 1* 987 value = * 988 separators = "(" | ")" | "<" | ">" | "@" 989 | "," | ";" | ":" | "\" | <"> 990 | "/" | "[" | "]" | "?" | "=" 991 | "{" | "}" | SP | HT 993 6.6. FEC Scheme requirements 995 In order to be used with this framework, an FEC Scheme MUST be 996 capable of processing data arranged into blocks of Application Data 997 Units (source blocks). 999 A specification for a new FEC scheme MUST include the following 1000 things: 1002 1. The FEC Encoding ID value that uniquely identifies the FEC 1003 scheme. This value MUST be registered with IANA as described in 1004 Section 11. 1006 2. The type, semantics and encoding format of the Repair FEC Payload 1007 ID. 1009 3. The name, type, semantics and text value encoding rules for zero 1010 or more FEC Scheme-specific FEC Framework Configuration 1011 Information elements. Names must conform to the 1012 "name"__production and values encodings to the "value" __ 1013 production defined in Section 6.5 1015 4. A full specification of the FEC code. 1017 This specification MUST precisely define the valid FEC-Scheme- 1018 Specific FEC Framework Configuration Information values, the 1019 valid FEC Payload ID values and the valid packet payload sizes 1020 (where packet payload refers to the space within a packet 1021 dedicated to carrying encoding symbol bytes). 1023 Furthermore, given a source block as defined in Section 6.2, 1024 valid values of the FEC-Scheme-Specific FEC Framework 1025 Configuration Information, a valid Repair FEC Payload ID value 1026 and a valid packet payload size, the specification MUST uniquely 1027 define the values of the encoding symbol bytes to be included in 1028 the repair packet payload of a packet with the given Repair FEC 1029 Payload ID value. 1031 A common and simple way to specify the FEC code to the required 1032 level of detail is to provide a precise specification of an 1033 encoding algorithm which, given a source block, valid values of 1034 the FEC-Scheme-Specific FEC Framework Configuration Information, 1035 a valid Repair FEC Payload ID value and a valid packet payload 1036 size as input produces the exact value of the encoding symbol 1037 bytes as output. 1039 5. A description of practical encoding and decoding algorithms. 1041 This description need not be to the same level of detail as for 1042 the encoding above, however it must be sufficient to demonstrate 1043 that encoding and decoding of the code is both possible and 1044 practical. 1046 FEC scheme specifications MAY additionally define the following: 1048 1. Type, semantics and encoding format of an Explicit Source FEC 1049 Payload ID. 1051 Whenever an FEC scheme specification defines an 'encoding format' for 1052 an element, this must be defined in terms of a sequence of bytes 1053 which can be embedded within a protocol. The length of the encoding 1054 format MUST either be fixed or it must be possible to derive the 1055 length from examining the encoded bytes themselves. For example, the 1056 initial bytes may include some kind of length indication. 1058 FEC scheme specifications SHOULD use the terminology defined in this 1059 document and SHOULD follow the following format: 1061 1. Introduction 1064 2. Formats and Codes 1066 2.1 Source FEC Payload ID(s) 1070 2.2 Repair FEC Payload Id 1073 2.3 FEC Framework Configuration Information 1077 3. Procedures 1082 4. FEC code specification 1085 Specifications MAY include additional sections, for example, 1086 examples. 1088 Each FEC scheme MUST be specified independently of all other FEC 1089 schemes; for example, in a separate specification or a completely 1090 independent section of larger specification (except, of course, a 1091 specification of one FEC Scheme may include portions of another by 1092 reference). 1094 Where an RTP Payload Format is defined for repair data for a specific 1095 FEC Scheme, the RTP Payload Format and the FEC Scheme MAY be 1096 specified within the same document. 1098 7. Feedback 1100 Many applications require some kind of feedback on transport 1101 performance: how much data arrived at the receiver, at what rate, 1102 when etc. When FEC is added to such applications, feedback 1103 mechanisms may also need to be enhanced to report on the performance 1104 of the FEC (for example how much lost data was recovered by the FEC). 1106 When used to provide instrumentation for engineering purposes, it is 1107 important to remember that FEC is generally applied to relatively 1108 small blocks of data (in time) and so feedback information averaged 1109 over longer periods of time than the FEC block size will likely not 1110 provide sufficient information for engineering purposes. For example 1111 see [RFC5725]. 1113 Applications which used feedback for congestion control purposes MUST 1114 calculate such feedback on the basis of packets received before FEC 1115 recovery is applied. If this requirement conflicts with other uses 1116 of the feedback information then the application MUST be enhanced to 1117 support both information calculated pre- and post- FEC recovery. 1118 This is to ensure that congestion control mechanisms operate 1119 correctly based on congestion indications received from the network, 1120 rather than on post-FEC recovery information which would give an 1121 inaccurate picture of congestion conditions. 1123 New applications which require such feedback SHOULD use RTP/RTCP 1124 [RFC3550]. 1126 8. Transport Protocols 1128 The following transport protocols are supported: 1130 o User Datagram Protocol (UDP) 1132 o Datagram Congestion Control Protocol (DCCP) 1134 9. Congestion Control 1136 This section starts with a informative section on the motivation of 1137 the normative requirements for congestion control, which are spelled 1138 out in Section 9.1. 1140 Informative Note: The enforcement of Congestion Control (CC) 1141 principles has gained a lot of momentum in the IETF over the 1142 recent years. While the need of CC over the open Internet is 1143 unquestioned, and the goal of TCP friendliness is generally agreed 1144 for most (but not all) applications, the subject of congestion 1145 detection and measurement in heterogenous networks can hardly be 1146 considered as solved. Most congestion control algorithms detect 1147 and measure congestion by taking (primarily or exclusively) the 1148 packet loss rate into account. This appears to be inappropriate 1149 in environments where a large percentage of the packet losses are 1150 the result link-layer errors and independent of the network load. 1151 Note that such environments exist in the "open Internet", as well 1152 as in "closed" IP based networks. An example for the former would 1153 be the use of IP/UDP/RTP based streaming from an Internet- 1154 connected streaming server to a device attached to the Internet 1155 using cellular technology. 1157 The authors of this draft are primarily interested in applications 1158 where the application reliability requirements and end-to-end 1159 reliability of the network differ, such that it warrants higher 1160 layer protection of the packet stream - for example due to the 1161 presence of unreliable links in the end-to-end path - and where 1162 real-time, scalability or other constraints prohibit the use of 1163 higher layer (transport or application) feedback. A typical 1164 example for such applications is multicast and broadcast streaming 1165 or multimedia transmission over heterogenous networks. In other 1166 cases, application reliability requirements may be so high that 1167 the required end-to-end reliability is difficult to achieve even 1168 over wired networks. Furthermore the end-to-end network 1169 reliability may not be known in advance. 1171 This FEC framework is not proposed, nor intended, as a QoS 1172 enhancement tool to combat losses resulting from highly congested 1173 networks. It should not be used for such purposes. 1175 In order to prevent such mis-use, one approach would be to leave 1176 standardisation to bodies most concerned with the problem 1177 described above. However, the IETF defines base standards used by 1178 several bodies, including DVB, 3GPP, 3GPP2, all of which appear to 1179 share the environment and the problem described. 1181 Another approach would be to write a clear applicability statement 1182 - for example restricting use of the framework to networks with 1183 wireless links. However, there may be applications where the use 1184 of FEC may be justified to combat congestion-induced packet losses 1185 - particularly in lightly loaded networks, where congestion is the 1186 result of relatively rare random peaks in instantaneous traffic 1187 load - thereby intentionally violating congestion control 1188 principles. One possible example for such an application could be 1189 a no-matter-what, brute-force FEC protection of traffic generated 1190 as an emergency signal. 1192 We propose a third approach, which is to require at a minimum that 1193 the use of this framework with any given application, in any given 1194 environment, does not cause congestion issues which the 1195 application alone would not itself cause i.e. the use of this 1196 framework must not make things worse. 1198 Taking above considerations into account, the normative text of 1199 this section implements a small set of constraints for the FEC, 1200 which are mandatory for all senders compliant with this FEC 1201 framework. Further restrictions may be imposed for certain 1202 Content Delivery Protocols. In this it follows the spirit of the 1203 congestion control section of RTP and its Audio-Visual Profile 1204 (RFC3550/STD64 and RFC3551/STD65). 1206 One of the constraints effectively limits the bandwidth for the 1207 FEC protected packet stream to be no more than roughly twice as 1208 high as the original, non-FEC protected packet stream. This 1209 disallows the (static or dynamic) use of excessively strong FEC to 1210 combat high packet loss rates, which may otherwise be chosen by 1211 naively implemented dynamic FEC-strength selection mechanisms. We 1212 acknowledge that there may be a few exotic applications, e.g. IP 1213 traffic from space-based senders, or senders in certain hardened 1214 military devices, which would warrant a higher FEC strength. 1215 However, in this specification we give preference to the overall 1216 stability and network friendliness of the average application, and 1217 for those a factor of 2 appears to be appropriate. 1219 A second constraint requires that the FEC protected packet stream 1220 be in compliance with the congestion control in use for the 1221 application and network in question. 1223 9.1. Normative requirements 1225 The bandwidth of FEC Repair packet flows MUST NOT exceed the 1226 bandwidth of the source packet flows being protected. In addition, 1227 whenever the source packet flow bandwidth is adapted due to the 1228 operation of congestion control mechanisms, the FEC repair packet 1229 flow bandwidth MUST be similarly adapted. 1231 10. Security Considerations 1233 The application of FEC protection to a stream does not provide any 1234 kind of security protection. 1236 If security services are required for the stream, then they MUST 1237 either be applied to the original source data before FEC protection 1238 is applied, or to both the source and repair data, after FEC 1239 protection has been applied. 1241 If integrity protection is applied to source packets before FEC 1242 protection is applied, and no further integrity protection is applied 1243 to repair packets, then a denial of service attack is possible if an 1244 attacker is in a position to inject fake repair transport payloads. 1245 If received by a receiver, such fake repair transport payloads could 1246 cause incorrect FEC decoding resulting in incorrect Application Data 1247 Units being passed up to the application protocol. A similar attack 1248 may be possible if an attacker is in a position to inject fack FEC 1249 Framework Configuration Information or fake FEC Payload IDs. Such 1250 incorrect decoded Application Data Units would then be detected by 1251 the source integrity protection and discarded, resulting in partial 1252 or complete denial of service. Therefore, in such environments, 1253 integrity protection MUST also be applied to the FEC repair transport 1254 payloads, FEC Framework Configuration Information and FEC Payload 1255 IDs, for example using IPsec to integrity protect all packets. 1256 Receivers MUST also verify the integrity of source symbols before 1257 including the source symbols into the source block for FEC purposes. 1259 It is possible that multiple streams with different confidentiality 1260 requirements (for example, the streams may be visible to different 1261 sets of users) can be FEC protected by a single repair stream. This 1262 scenario is not recommended, since resources will be used to 1263 distribute and decode data which cannot then be decrypted by at least 1264 some receivers. However, in this scenario, confidentiality 1265 protection MUST be applied before FEC encoding of the streams, 1266 otherwise repair transport payload may be used by a receiver to 1267 decode unencrypted versions of source streams which they do not have 1268 permissionions to view. 1270 11. IANA Considerations 1272 FEC Schemes for use with this framework may be identified in 1273 protocols using FEC Encoding IDs. Values of FEC Encoding IDs are 1274 subject to IANA registration. They are in the registry named "FEC 1275 Framework (FECFRAME) FEC Encoding IDs" located at time of publication 1276 at . 1278 The values that can be assigned within the FEC Framework (FECFRAME) 1279 FEC Encoding ID registry are numeric indexes in the range [0, 255], 1280 boundaries included. Assignment requests are granted on a "IETF 1281 Consensus" basis as defined in[RFC5226] . Section 6.6 defines 1282 explicit requirements that documents defining new FEC Encoding IDs 1283 should meet. 1285 12. Acknowledgments 1287 This document is based in part on [I-D.watson-tsvwg-fec-sf] and so 1288 thanks are due to the additional authors of that document, Mike Luby, 1289 Magnus Westerlund and Stephan Wenger. That document was in turn 1290 based on the FEC streaming protocol defined by 3GPP in [MBMSTS] and 1291 thus thanks are also due to the participants in 3GPP TSG SA working 1292 group 4. Further thanks are due to the members of the FECFRAME 1293 working group for their comments and review. 1295 13. References 1297 13.1. Normative references 1299 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1300 Requirement Levels", BCP 14, RFC 2119, March 1997. 1302 [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., 1303 Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, 1304 K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., 1305 Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header 1306 Compression (ROHC): Framework and four profiles: RTP, UDP, 1307 ESP, and uncompressed", RFC 3095, July 2001. 1309 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 1310 Correction (FEC) Building Block", RFC 5052, August 2007. 1312 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1313 Jacobson, "RTP: A Transport Protocol for Real-Time 1314 Applications", STD 64, RFC 3550, July 2003. 1316 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1317 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1318 May 2008. 1320 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1321 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1323 13.2. Informative references 1325 [I-D.watson-tsvwg-fec-sf] 1326 Watson, M., "Forward Error Correction (FEC) Streaming 1327 Framework", draft-watson-tsvwg-fec-sf-00 (work in 1328 progress), July 2005. 1330 [RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE 1331 Report Block Type for RTP Control Protocol (RTCP) Extended 1332 Reports (XRs)", RFC 5725, February 2010. 1334 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 1335 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 1336 July 2006. 1338 [RFC2736] Handley, M. and C. Perkins, "Guidelines for Writers of RTP 1339 Payload Format Specifications", BCP 36, RFC 2736, 1340 December 1999. 1342 [MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 1343 Protocols and codecs", 3GPP TS 26.346, April 2005. 1345 Author's Address 1347 Mark Watson 1348 Qualcomm, Inc. 1349 3165 Kifer Road 1350 Santa Clara, CA 95051 1351 U.S.A. 1353 Email: watson@qualcomm.com