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Adamson 5 Expires: April 28, 2011 Naval Research Laboratory 6 October 25, 2010 8 FCAST: Scalable Object Delivery for the ALC and NORM Protocols 9 draft-ietf-rmt-fcast-02 11 Abstract 13 This document introduces the FCAST object (e.g., file) delivery 14 application on top of the ALC and NORM reliable multicast protocols. 15 FCAST is a highly scalable application that provides a reliable 16 object delivery service. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on April 28, 2011. 35 Copyright Notice 37 Copyright (c) 2010 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 54 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 55 3. Definitions, Notations and Abbreviations . . . . . . . . . . . 5 56 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5 57 3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 6 58 4. FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 6 59 4.1. FCAST Content Delivery Service . . . . . . . . . . . . . . 6 60 4.2. Meta-Data Transmission . . . . . . . . . . . . . . . . . . 7 61 4.3. Meta-Data Content . . . . . . . . . . . . . . . . . . . . 7 62 4.4. Carousel Transmission . . . . . . . . . . . . . . . . . . 9 63 4.5. Carousel Instance Descriptor Special Object . . . . . . . 9 64 4.6. Compound Object Identification . . . . . . . . . . . . . . 11 65 4.7. FCAST/ALC Additional Specificities . . . . . . . . . . . . 12 66 4.8. FCAST/NORM Additional Specificities . . . . . . . . . . . 12 67 4.9. FCAST Sender Behavior . . . . . . . . . . . . . . . . . . 13 68 4.10. FCAST Receiver Behavior . . . . . . . . . . . . . . . . . 14 69 5. FCAST Data Formats . . . . . . . . . . . . . . . . . . . . . . 15 70 5.1. Compound Object Header Format . . . . . . . . . . . . . . 15 71 5.2. Carousel Instance Descriptor Format . . . . . . . . . . . 17 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 73 6.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 20 74 6.2. Attacks Against the Data Flow . . . . . . . . . . . . . . 20 75 6.2.1. Access to Confidential Objects . . . . . . . . . . . . 21 76 6.2.2. Object Corruption . . . . . . . . . . . . . . . . . . 21 77 6.3. Attacks Against the Session Control Parameters and 78 Associated Building Blocks . . . . . . . . . . . . . . . . 22 79 6.3.1. Attacks Against the Session Description . . . . . . . 23 80 6.3.2. Attacks Against the FCAST CID . . . . . . . . . . . . 23 81 6.3.3. Attacks Against the Object Meta-Data . . . . . . . . . 23 82 6.3.4. Attacks Against the ALC/LCT and NORM Parameters . . . 24 83 6.3.5. Attacks Against the Associated Building Blocks . . . . 24 84 6.4. Other Security Considerations . . . . . . . . . . . . . . 25 85 6.5. Minimum Security Recommendations . . . . . . . . . . . . . 25 86 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 87 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 88 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 89 9.1. Normative References . . . . . . . . . . . . . . . . . . . 26 90 9.2. Informative References . . . . . . . . . . . . . . . . . . 27 91 Appendix A. FCAST Examples . . . . . . . . . . . . . . . . . . . 28 92 A.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . . 28 93 A.2. FCAST/NORM with NORM_INFO Examples . . . . . . . . . . . . 30 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 96 1. Introduction 98 This document introduces the FCAST reliable and scalable object 99 (e.g., file) delivery application. Two variants of FCAST exist: 101 o FCAST/ALC that relies on the Asynchronous Layer Coding (ALC) 102 [RFC5775] and the Layered Coding Transport (LCT) [RFC5651] 103 reliable multicast transport protocol, and 105 o FCAST/NORM that relies on the NACK-Oriented Reliable Multicast 106 (NORM) [RFC5740] reliable multicast transport protocol. 108 Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM. 109 FCAST is not a new protocol specification per se. Instead it is a 110 set of data format specifications and instructions on how to use ALC 111 and NORM to implement a file-casting service. 113 Depending on the target use case, the delivery service provided by 114 FCAST is more or less reliable. For instance, with FCAST/ALC used in 115 ON-DEMAND mode over a time period that largely exceeds the typical 116 download time, the service can be considered as fully reliable. 117 Similarly, when FCAST is used along with a session control 118 application that collects reception information and takes appropriate 119 corrective measures (e.g., a direct point-to-point retransmission of 120 missing packets, or a new multicast recovery session), then the 121 service can be considered as fully reliable. On the opposite, if 122 FCAST operates in PUSH mode, then the service is usually only 123 partially reliable, and a receiver that is disconnected during a 124 sufficient time will perhaps not have the possibility to download the 125 object. 127 Depending on the target use case, the FCAST scalability is more or 128 less important. For instance, if FCAST/ALC is used on top of purely 129 unidirectional transport channels, with no feedback information at 130 all, which is the default mode of operation, then the scalability is 131 maximum since neither FCAST, nor ALC, UDP or IP generates any 132 feedback message. On the opposite, the FCAST/NORM scalability is 133 typically limited by NORM scalability itself. Similarly, if FCAST is 134 used along with a session control application that collects reception 135 information from the receivers, then this session control application 136 may limit the scalability of the global object delivery system. This 137 situation can of course be mitigated by using a hierarchy of feedback 138 message aggregators or servers. The details of this are out of the 139 scope of the present document. 141 A design goal behind FCAST is to define a streamlined solution, in 142 order to enable lightweight implementations of the protocol stack, 143 and limit the operational processing and storage requirements. A 144 consequence of this choice is that FCAST cannot be considered as a 145 versatile application, capable of addressing all the possible use- 146 cases. On the opposite, FCAST has some intrinsic limitations. From 147 this point of view it differs from FLUTE [RMT-FLUTE] which favors 148 flexibility at the expense of some additional complexity. 150 A good example of the design choices meant to favor the simplicity is 151 the way FCAST manages the object meta-data: by default, the meta-data 152 and the object content are sent together, in a compound object. This 153 solution has many advantages in terms of simplicity as will be 154 described later on. However, as such, it also has an intrinsic 155 limitation since it does not enable a receiver to decide in advance, 156 before beginning the reception of the compound object, whether the 157 object is of interest or not, based on the information that may be 158 provided in the meta-data. Therefore this document defines 159 additional techniques that may be used to mitigate this limitation. 160 It is also possible that some use-cases require that each receiver 161 download the whole set of objects sent in the session (e.g., with 162 mirroring tools). When this is the case, the above limitation is no 163 longer be a problem. 165 1.1. Applicability 167 FCAST is compatible with any congestion control protocol designed for 168 ALC/LCT or NORM. However, depending on the use-case, the data flow 169 generated by the FCAST application might not be constant, but instead 170 be bursty in nature. Similarly, depending on the use-case, an FCAST 171 session might be very short. Whether and how this will impact the 172 congestion control protocol is out of the scope of the present 173 document. 175 FCAST is compatible with any security mechanism designed for ALC/LCT 176 or NORM. The use of a security scheme is strongly RECOMMENDED (see 177 Section 6). 179 FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM. 180 Whether FEC is used or not, and the kind of FEC scheme used, is to 181 some extent transparent to FCAST. 183 FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST 184 specification has any implication on the source or destination IP 185 address. 187 2. Requirements notation 189 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 190 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 191 document are to be interpreted as described in [RFC2119]. 193 3. Definitions, Notations and Abbreviations 195 3.1. Definitions 197 This document uses the following definitions: 199 FCAST/ALC: denotes the FCAST application running on top of the ALC/ 200 LCT reliable transport protocol; 202 FCAST/NORM: denotes the FCAST application running on top of the NORM 203 reliable transport protocol; 205 FCAST: denotes either FCAST/ALC or FCAST/NORM; 207 Compound Object: denotes an ALC or NORM transport object composed of 208 the Compound Object Header (Section 5.1), including any meta- 209 data, and the content of the original application object 210 (e.g., a file); 212 Carousel: denotes the process of sending Compound Objects 213 implemented by a FCAST sender; 215 Carousel Instance: denotes a fixed set of registered Compound 216 Objects that are sent by the carousel during a certain number 217 of cycles. Whenever Compound Objects need to be added or 218 removed, a new Carousel Instance is defined; 220 Carousel Instance Descriptor (CID): denotes a special object that 221 lists the Compound Objects that comprise a given Carousel 222 Instance; 224 Carousel Cycle: denotes a transmission round within which all the 225 Compound Objects registered in the Carousel Instance are 226 transmitted a certain number of times. By default, Compound 227 Objects are transmitted once per cycle, but higher values are 228 possible, that might differ on a per-object basis; 230 Transmission Object Identifier (TOI): denotes the numeric identifier 231 associated to a specific object by the underlying transport 232 layer. In the case of ALC, this corresponds to the TOI 233 described in that specification while for the NORM 234 specification this corresponds to the NormTransportId 235 described there. 237 3.2. Abbreviations 239 This document uses the following abbreviations: 241 CID: Carousel Instance Descriptor 243 CIID: Carousel Instance IDentifier 245 FEC OTI: FEC Object Transmission Information 247 TOI: Transmission Object Identifier 249 4. FCAST Principles 251 4.1. FCAST Content Delivery Service 253 The basic goal of FCAST is to transmit objects to a group of 254 receivers in a reliable way. The receiver set MAY be restricted to a 255 single receiver or MAY include possibly a very large number of 256 receivers. FCAST is specified to support two forms of operation: 258 1. FCAST/ALC: where the FCAST application is meant to run on top of 259 the ALC/LCT reliable multicast transport protocol, and 261 2. FCAST/NORM: where the FCAST application is meant to run on top of 262 the NORM reliable multicast transport protocol. 264 This specification is designed such that both forms of operation 265 share as much commonality as possible. 267 While the choice of the underlying transport protocol (i.e., ALC or 268 NORM) and its parameters may limit the practical receiver group size, 269 nothing in FCAST itself limits it. The transmission might be fully 270 reliable, or only partially reliable depending upon the way ALC or 271 NORM is used (e.g., whether FEC encoding and/or NACK-based repair 272 requests are used or not), the way the FCAST carousel is used (e.g., 273 whether the objects are made available for a long time span or not), 274 and the way in which FCAST itself is employed (e.g., whether there is 275 a session control application that might automatically extend an 276 existing FCAST session until all receivers have received the 277 transmitted content). 279 FCAST is designed to be as self-sufficient as possible, in particular 280 in the way object meta-data is attached to object data content. 281 However, for some uses, meta-data MAY also be communicated by an out- 282 of-band mechanism that is out of the scope of the present document. 284 4.2. Meta-Data Transmission 286 FCAST usually carries meta-data elements by prepending them to the 287 object it refers to. As a result, a Compound Object is created that 288 is composed of a header followed by the original object data. This 289 header is itself composed of the meta-data as well as several fields, 290 for instance to indicate the boundaries between the various parts of 291 this Compound Object (Figure 1). 293 <------------------------ Compound Object -----------------------> 294 +-------------------------+--------------------------------------+ 295 | Compound Object Header | Object Data | 296 | (can include meta-data) | (can be encoded by FCAST) | 297 +-------------------------+--------------------------------------+ 299 Figure 1: Compound Object composition. 301 Attaching the meta-data to the object is an efficient solution, since 302 it guaranties that meta-data be received along with the associated 303 object, and it allows the transport of the meta-data to benefit from 304 any transport-layer erasure protection of the Compound Object (e.g., 305 using FEC encoding and/or NACK-based repair). However a limit of 306 this scheme, as such, is that a client does not know the meta-data of 307 an object before beginning its reception, and in case of erasures 308 affecting the meta-data, not until the object decoding is completed. 309 The details of course depend upon the transport protocol and the FEC 310 code used. 312 In certain use-cases, FCAST can also be associated to another in-band 313 (e.g., via NORM INFO messages, Section 4.8) or out-of-band signaling 314 mechanism. In that case, this mechanism can be used in order to 315 carry the whole meta-data (or a subset of it), possibly ahead of 316 time. 318 4.3. Meta-Data Content 320 The meta-data associated to an object can be composed of, but are not 321 limited to: 323 o Content-Location: the URI of the object, which gives the name and 324 location of the object; 326 o Content-Type: the MIME type of the object; 328 o Content-Length: the size of the initial object, before any content 329 encoding (if any). Note that this content length does not include 330 the meta-data nor the fixed size Compound Object header; 332 o Content-Encoding: the optional encoding of the object performed by 333 FCAST. If there is no Content-Encoding entry, the receiver MUST 334 assume that the object is not encoded (default). The support of 335 gzip encoding, or any other solution, remains optional. 337 o Content-MD5: the MD5 message digest of the object in order to 338 check its integrity. Note that this digest is meant to protect 339 from transmission and processing errors, not from deliberate 340 attacks by an intelligent attacker. Note also that this digest 341 only protects the object, not the header, and therefore not the 342 meta-data. A separate checksum is provided to that purpose 343 (Section 5.1); 345 o a digital signature for this object; 347 This list is not limited and new meta-data information can be added. 348 For instance, when dealing with very large objects (e.g., that 349 largely exceed the working memory of a receiver), it can be 350 interesting to split this object into several sub-objects (or 351 slices). When this happens, the meta-data associated to each sub- 352 object MUST include the following entries: 354 o Fcast-Obj-Slice-Nb: the total number of slices. A value strictly 355 greater than 1 indicates that this object is the result of a split 356 of the original object; 358 o Fcast-Obj-Slice-Idx: the slice index (in the {0 .. SliceNb - 1} 359 interval); 361 o Fcast-Obj-Slice-Offset: the offset at which this slice starts 362 within the original object; 364 When meta-data elements are communicated out-of-band, in advance of 365 data transmission, the following pieces of information MAY also be 366 useful: 368 o TOI: the Transmission Object Identifier (TOI) of the object 369 (Section 4.6), in order to enable a receiver to easily associate 370 the meta-data to the object; 372 o FEC Object Transmission Information (FEC OTI). In this case the 373 FCAST sender does not need to use the optional EXT_FTI mechanism 374 of the ALC or NORM protocols. 376 4.4. Carousel Transmission 378 A set of FCAST Compound Objects scheduled for transmission are 379 considered a logical "Carousel". A given "Carousel Instance" is 380 comprised of a fixed set of Compound Objects. Whenever the FCAST 381 application needs to add new Compound Objects to, or remove old 382 Compound Objects from the transmission set, a new Carousel Instance 383 is defined since the set of Compound Objects changes. Because of the 384 native object multiplexing capability of both ALC and NORM, sender 385 and receiver(s) are both capable to multiplex and demultiplex FCAST 386 Compound Objects. 388 For a given Carousel Instance, one or more transmission cycles are 389 possible. During each cycle, all of the Compound Objects comprising 390 the Carousel are sent. By default, each object is transmitted once 391 per cycle. However, in order to allow different levels of priority, 392 some objects MAY be transmitted more often that others during a 393 cycle, and/or benefit from higher FEC protection than others. This 394 can be the case for instance for the CID objects (Section 4.5). For 395 some FCAST usage (e.g., a unidirectional "push" mode), a Carousel 396 Instance may be associated to a single transmission cycle. In other 397 cases it may be associated to a large number of transmission cycles 398 (e.g., in "on-demand" mode, where objects are made available for 399 download during a long period of time). 401 4.5. Carousel Instance Descriptor Special Object 403 The FCAST sender CAN transmit an OPTIONAL Carousel Instance 404 Descriptor (CID). The CID carries the list of the Compound Objects 405 that are part of a given Carousel Instance, by specifying their 406 respective Transmission Object Identifiers (TOI). However the CID 407 does not describe the objects themselves (i.e., there is no meta- 408 data). Additionally, the CID MAY include a "Complete" flag that is 409 used to indicate that no further modification to the enclosed list 410 will be done in the future. Finally, the CID MAY include a Carousel 411 Instance ID that identifies the Carousel Instance it pertains to. 412 These aspects are discussed in Section 5.2. 414 There is no reserved TOI value for the CID Compound Object itself, 415 since this special object is regarded by ALC/LCT or NORM as a 416 standard object. On the opposite, the nature of this object (CID) is 417 indicated by means of a specific Compound Object header field (the 418 "I" flag) so that it can be recognized and processed by the FCAST 419 application as needed. A direct consequence is the following: since 420 a receiver does not know in advance which TOI will be used for the 421 following CID (in case of a dynamic session), he MUST NOT filter out 422 packets that are not in the current CID's TOI list. Said 423 differently, the goal of CID is not to setup ALC or NORM packet 424 filters (this mechanism would not be secure in any case). 426 The use of a CID remains optional. If it is not used, then the 427 clients progressively learn what files are part of the carousel 428 instance by receiving ALC or NORM packets with new TOIs. However 429 using a CID has several benefits: 431 o When the "Complete" flag is set (if ever), the receivers know when 432 they can leave the session, i.e., when they have received all the 433 objects that are part of the last carousel instance of this 434 delivery session; 436 o In case of a session with a dynamic set of objects, the sender can 437 reliably inform the receivers that some objects have been removed 438 from the carousel with the CID. This solution is more robust than 439 the "Close Object flag (B)" of ALC/LCT since a client with an 440 intermittent connectivity might loose all the packets containing 441 this B flag. And while NORM provides a robust object cancellation 442 mechanism in the form of its NORM_CMD(SQUELCH) message in response 443 to receiver NACK repair requests, the use of the CID provides an 444 additional means for receivers to learn of objects for which it is 445 futile to request repair; 447 o The TOI equivalence (Section 4.6) can be signaled with the CID. 448 This is often preferable to the alternative solution where the 449 equivalence is identified by examining the object meta-data, 450 especially in case of erasures. 452 During idle periods, when the carousel instance does not contain any 453 object, a CID with an empty TOI list MAY be transmitted. In that 454 case, a new carousel instance ID MUST be used to differentiate this 455 (empty) carousel instance from the other ones. This mechanism can be 456 useful to inform the receivers that: 458 o all the previously sent objects have been removed from the 459 carousel. It therefore improves the FCAST robustness even during 460 "idle" period; 462 o the session is still active even if there is currently no content 463 being sent. Said differently, it can be used as a heartbeat 464 mechanism. If the "Complete" flag has not been set, it implicitly 465 informs the receivers that new objects MAY be sent in the future; 467 The decisions of whether a CID should be used or not, how often and 468 when a CID should be sent, are left to the sender and depend on many 469 parameters, including the target use case and the session dynamics. 470 For instance it may be appropriate to send a CID at the beginning of 471 each new carousel instance, and then periodically. These operational 472 aspects are out of the scope of the present document. 474 4.6. Compound Object Identification 476 The FCAST Compound Objects are directly associated with the object- 477 based transport service that the ALC and NORM protocols provide. In 478 each of these protocols, the messages containing transport object 479 content are labeled with a numeric transport object identifier (i.e., 480 the ALC TOI and the NORM NormTransportId). For the purposes of this 481 document, this identifier in either case (ALC or NORM) is referred to 482 as the TOI. 484 There are several differences between ALC and NORM: 486 o the ALC use of TOI is rather flexible, since several TOI field 487 sizes are possible (from 16 to 112 bits), since this size can be 488 changed at any time, on a per-packet basis, and since the TOI 489 management is totally free as long as each object is associated to 490 a unique TOI (if no wraparound happened); 492 o the NORM use of TOI is more directive, since the TOI field is 16 493 bit long and since TOIs MUST be managed sequentially; 495 In both NORM and ALC, it is possible that the transport 496 identification space may eventually wrap for long-lived sessions 497 (especially with NORM where this phenomenon is expected to happen 498 more frequently). This can possibly introduce some ambiguity in 499 FCAST object identification if a sender retains some older objects in 500 newer Carousel Instances with updated object sets. To avoid 501 ambiguity the active TOIs (i.e., the TOIs corresponding to objects 502 being transmitted) can only occupy half of the TOI sequence space. 503 If an old object, whose TOI has fallen outside the current window, 504 needs to be transmitted again, a new TOI must be used for it. In 505 case of NORM, this constraint limits to 32768 the maximum number of 506 objects that can be part of any carousel instance. In order to allow 507 receivers to properly combine the transport packets with a newly- 508 assigned TOI to those of associated to the previously-assigned TOI, a 509 mechanism is required to equate the objects with the new and the old 510 TOIs. 512 The preferred mechanism consists in signaling, within the CID, that 513 the newly assigned TOI, for the current Carousel Instance, is 514 equivalent to the TOI used within a previous Carousel Instance. By 515 convention, the reference tuple for any object is the {TOI; CI ID} 516 tuple used for its first transmission within a Carousel Instance. 517 This tuple MUST be used whenever a TOI equivalence is provided. 519 An alternative solution, when meta-data can be processed rapidly 520 (e.g., by using NORM-INFO messages), consists for the receiver in 521 identifying that both objects are the same, after examining the meta- 522 data. The receiver can then take appropriate measures. 524 4.7. FCAST/ALC Additional Specificities 526 There are no additional detail or option for FCAST/ALC operation. 528 4.8. FCAST/NORM Additional Specificities 530 The NORM Protocol provides a few additional capabilities that can be 531 used to specifically support FCAST operation: 533 1. The NORM_INFO message can convey "out-of-band" content with 534 respect to a given transport object. With FCAST, it MAY be used 535 to provide to the receivers a new, associated, Compound Object 536 which contains the main Compound Object meta-data, or a subset of 537 it. In that case the NORM_INFO Compound Object MUST NOT contain 538 any Object Data field (i.e., it is only composed of the header), 539 it MUST feature a non global checksum, and it MUST NOT include 540 any padding field. The main Compound Object MUST in any case 541 contain the whole meta-data (e.g., because a receiver MAY not 542 support the NORM_INFO facility). Additionally, the meta-data 543 entries contained in the NORM_INFO MUST be identical to the same 544 entries in the main Compound Object. Finally, note that the 545 availability of NORM_INFO for a given object is signaled through 546 the use of a dedicated flag in the NORM_DATA message header. 547 Along with NORM's NACK-based repair request signaling, it allows 548 a receiver to quickly (and independently) request an object's 549 NORM_INFO content. However, a limitation here is that the 550 NORM_INFO Compound Object header MUST fit within the byte size 551 limit defined by the NORM sender's configured "segment size" 552 (typically a little less than the network MTU); 554 2. The NORM_CMD(SQUELCH) messages are used by the NORM protocol 555 sender to inform receivers of objects that have been canceled 556 when receivers make repair requests for such invalid objects. 557 Along with the CID mechanism, a receiver has two efficient and 558 reliable ways to discover old objects that have been removed from 559 the carousel instance; 561 3. NORM also supports an optional positive acknowledgment mechanism 562 that can be used for small-scale multicast receiver group sizes. 563 Also, it may be possible in some cases for the sender to infer, 564 after some period without receiving NACKs at the end of its 565 transmission that the receiver set has fully received the 566 transmitted content. In particular, if the sender completes its 567 end-of-transmission series of NORM_CMD(FLUSH) messages without 568 receiving repair requests from the group, it may have some 569 assurance that the receiver set has received the content prior to 570 that point. These mechanisms are likely to help FCAST in 571 achieving fully reliable transmissions; 573 It should be noted that the NORM_INFO message header may carry the 574 EXT_FTI extension. The reliable delivery of the NORM_INFO content 575 allows the individual objects' FEC Transmission Information to be 576 provided to the receivers without burdening every packet (i.e. 577 NORM_DATA messages) with this additional, but important, content. 578 Examples are provided in Appendix A. 580 4.9. FCAST Sender Behavior 582 The following operations MAY take place at a sender: 584 1. The user (or another application) selects a set of objects (e.g., 585 files) to deliver and submits them, along with their meta-data, 586 to the FCAST application; 588 2. For each object, FCAST creates the Compound Object and registers 589 this latter in the carousel instance; 591 3. The user then informs FCAST that all the objects of the set have 592 been submitted. If the user knows that no new object will be 593 submitted in the future (i.e., if the session's content is now 594 complete), the user informs FCAST. Finally, the user specifies 595 how many transmission cycles are desired (this number may be 596 infinite); 598 4. At this point, the FCAST application knows the full list of 599 Compound Objects that are part of the Carousel Instance and can 600 create a CID if desired, possibly with the complete flag set; 602 5. The FCAST application can now define a transmission schedule of 603 these Compound Objects, including the optional CID. This 604 schedule defines in which order the packets of the various 605 Compound Objects should be sent. This document does not specify 606 any scheme. This is left to the developer within the provisions 607 of the underlying ALC or NORM protocol used and the knowledge of 608 the target use-case. 610 6. The FCAST application then starts the carousel transmission, for 611 the number of cycles specified. Transmissions take place until: 613 * the desired number of transmission cycles has been reached, or 614 * the user wants to prematurely stop the transmissions, or 616 * the user wants to add one or several new objects to the 617 carousel, or on the opposite wants to remove old objects from 618 the carousel. In that case a new carousel instance must be 619 created. 621 7. If the session is not finished, then continue at Step 1 above; 623 4.10. FCAST Receiver Behavior 625 The following operations MAY take place at a receiver: 627 1. The receiver joins the session and collects incoming packets; 629 2. If the header portion of a Compound Object is entirely received 630 (which may happen before receiving the entire object with some 631 ALC/NORM configurations), or if the meta-data is sent by means of 632 another mechanism prior to the object, the receiver processes the 633 meta-data and chooses to continue to receive the object content 634 or not; 636 3. When a Compound Object has been entirely received, the receiver 637 processes the header, retrieves the object meta-data, perhaps 638 decodes the meta-data, and processes the object accordingly; 640 4. When a CID is received, which is indicated by the 'C' flag set in 641 the Compound Object header, the receiver decodes the CID, and 642 retrieves the list of objects that are part of the current 643 carousel instance. This list CAN be used to remove objects sent 644 in a previous carousel instance that might not have been totally 645 decoded and that are no longer part of the current carousel 646 instance; 648 5. When a CID is received, the receiver also retrieves the list of 649 TOI equivalences, if any, and takes appropriate measures, for 650 instance by informing the transport layer; 652 6. When a receiver has received a CID with the "Complete" flag set, 653 and has successfully received all the objects of the current 654 carousel instance, it can safely exit from the current FCAST 655 session; 657 7. Otherwise continue at Step 2 above. 659 5. FCAST Data Formats 661 This section details the various data formats used by FCAST. 663 5.1. Compound Object Header Format 665 In an FCAST session, Compound Objects are constructed by prepending 666 the Compound Object Header (which may include meta-data) before the 667 original object data content (Figure 2). 669 0 1 2 3 670 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 671 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 672 |rsv|G|C|MDF|MDE| Compound Object Header Length | | 673 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 674 | Checksum | | h 675 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d 676 | Object Meta-Data (optional, variable length) | r 677 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 678 | | Padding (optional) | | 679 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 680 | | 681 . Object Data (optional, variable length) . 682 . . 683 . . 685 Figure 2: Compound Object Header with Meta-Data. 687 The Compound Object Header fields are: 689 +------------+------------------------------------------------------+ 690 | Field | Description | 691 +------------+------------------------------------------------------+ 692 | Reserved | 2-bit field set that MUST be set to 0 in this | 693 | | specification and is reserved for future use. | 694 | | Receivers MUST ignore this field. | 695 | G | 1-bit field that, when set to 1, indicates that the | 696 | | checksum encompasses the whole Compound Object | 697 | | (Global checksum). When set to 0, this field | 698 | | indicates that the checksum encompasses only the | 699 | | Compound Object header. | 700 | C | 1-bit field that, when set to 1, indicates the | 701 | | object is a Carousel Instance Descriptor (CID). | 702 | | When set to 0, this field indicates that the | 703 | | transported object is a standard object. | 704 | Meta-Data | 2-bit field that defines the format of the object | 705 | Format | meta-data (see Section 7). An HTTP/1.1 | 706 | (MDFmt) | metainformation format [RFC2616] MUST be supported | 707 | | and is associated to value 0. Other formats (e.g., | 708 | | XML) MAY be defined in the future. | 709 | Meta-Data | 2-bit field that defines the optional encoding of | 710 | Encoding | the Object Meta-Data field (see Section 7). By | 711 | (MDEnc) | default, a plain text encoding is used and is | 712 | | associated to value 0. Gzip encoding MUST also be | 713 | | supported and is associated to value 1. Other | 714 | | encodings MAY be defined in the future. | 715 | Compound | 24-bit field indicating total length (in bytes) of | 716 | Object | all fields of the Compound Object Header, except the | 717 | Header | optional padding. A header length field set to | 718 | Length | value 6 means that there is no meta-data included. | 719 | | When this size is not multiple to 32-bits words and | 720 | | when the Compound Object Header is followed by a non | 721 | | null Compound Object Data, padding MUST be added. | 722 | | It should be noted that the meta-data field maximum | 723 | | size is equal to (2^24 - 6) bytes. | 724 | Checksum | 16-bit field that contains the checksum computed | 725 | | over either the whole Compound Object (when G is set | 726 | | to 1), or over the Compound Object header (when G is | 727 | | set to 0), using the Internet checksum algorithm | 728 | | specified in [RFC1071]. More precisely, the | 729 | | checksum field is the 16-bit one's complement of the | 730 | | one's complement sum of all 16-bit words to be | 731 | | considered. If a segment contains an odd number of | 732 | | octets to be checksummed, the last octet is padded | 733 | | on the right with zeros to form a 16-bit word for | 734 | | checksum purposes (this pad is not transmitted). | 735 | | While computing the checksum, the checksum field | 736 | | itself is set to zero. | 737 | Object | Optional, variable length field that contains the | 738 | Meta-Data | meta-data associated to the object. The format and | 739 | | encoding of this field is defined by the MDFmt MDEnc | 740 | | fields. With the default HTTP/1.1 format and plain | 741 | | text encoding, the Meta-Data is NULL-terminated | 742 | | plain text that follows the "TYPE" ":" "VALUE" | 743 | | "" format used in HTTP/1.1 for | 744 | | metainformation [RFC2616]. The various meta-data | 745 | | items can appear in any order. The associated | 746 | | string, when non empty, MUST be NULL-terminated. | 747 | | When no meta-data is communicated, this field MUST | 748 | | be empty and the Compound Object Header Length MUST | 749 | | be equal to 6. | 750 | Padding | Optional, variable length field of zero-value bytes | 751 | | to align the start of the Object Data to 32-bit | 752 | | boundary. Padding is only used when the Compound | 753 | | Object Header Length value, in bytes, is not | 754 | | multiple of 4 and when the Compound Object Header is | 755 | | followed by non null Compound Object Data. | 756 +------------+------------------------------------------------------+ 758 The Compound Object Header is then followed by the Object Data, i.e., 759 the original object possibly encoded by FCAST. Note that the length 760 of this content is the transported object length (e.g., as specified 761 by the FEC OTI) minus the Compound Object Header Length and optional 762 padding if any. 764 5.2. Carousel Instance Descriptor Format 766 The format of the CID, which is a special Compound Object, is given 767 in Figure 3. 769 0 1 2 3 770 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 771 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 772 |rsv|G|1|MDF|MDE| Compound Object Header Length | | 773 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 774 | Checksum | | h 775 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d 776 | Object Meta-Data (optional, variable length) | r 777 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 778 | | Padding (optional) | | 779 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 780 . . ^ 781 . Object List (variable length) . | 782 . . o 783 . +-+-+-+-+-+-+-+-+ b 784 . | j 785 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 787 Figure 3: Carousel Instance Object Format. 789 Because the CID is transmitted as a special Compound Object, the 790 following CID-specific meta-data entries are defined: 792 o Fcast-CID-Complete: when set to 1, it indicates that no new 793 objects in addition to the ones whose TOI are specified in this 794 CID, or the ones that have been specified in the previous CID(s), 795 will be sent in the future. Otherwise it MUST be set to 0. This 796 entry is optional. If absent, a receiver MUST conclude that the 797 session is complete. 799 o Fcast-CID-ID: this entry contains the Carousel Instance 800 IDentifier, or CIID. It starts from 0 and is incremented by 1 for 801 each new carousel instance. This entry is optional if the FCAST 802 session consists of a single, complete, carousel instance. In all 803 other cases, this entry MUST be defined. In particular, the CIID 804 is used by the TOI equivalence mechanism thanks to which any 805 object is uniquely identified, even if the TOI is updated (e.g., 806 after re-enqueuing the object with NORM). The Fcast-CID-ID value 807 can also be useful to detect possible gaps in the Carousel 808 Instances, for instance caused by long disconnection periods. 809 Finally, it can also be useful to avoid problems when TOI wrapping 810 to 0 takes place to differentiate the various incarnations of the 811 TOIs if need be. 813 The motivation for making the Fcast-CID-Complete and Fcast-CID-ID 814 entries optional is to simplify the simple case of a session 815 consisting of a single, complete, carousel instance, with an Object 816 List given in plain text, without any content encoding. In that 817 case, the CID does not need to contain any meta-data entry. 819 Additionally, the following standard meta-data entries are often used 820 (Section 4.3): 822 o Content-Length: it specifies the size of the object list, before 823 any content encoding (if any). 825 o Content-Encoding: it specifies the optional encoding of the object 826 list, performed by FCAST. For instance: 827 Content-Encoding: gzip 828 indicates that the Object List field has been encoded with gzip 829 [RFC1952]. If there is no Content-Encoding entry, the receiver 830 MUST assume that the Object List field is plain text (default). 831 The support of gzip encoding, or any other solution, remains 832 optional. 834 An empty Object List is valid and indicates that the current carousel 835 instance does not include any object (Section 4.5). This can be 836 specified by using the following meta-data entry: 837 Content-Length: 0 838 or simply by leaving the Object List empty. In both cases, padding 839 MUST NOT be used and consequently the transported object length 840 (e.g., as specified by the FEC OTI) minus the Compound Object Header 841 Length equals zero. 843 The non-encoded (i.e., plain text) Object List, when non empty, is a 844 NULL-terminated ASCII string. It can contain two things: 846 o a list of TOI values, and 848 o a list of TOI equivalences; 850 First of all, this string can contain the list of TOIs included in 851 the current carousel instance, specified either as the individual 852 TOIs of each object, or as TOI intervals, or any combination. The 853 format of the ASCII string is a comma-separated list of individual 854 "TOI" values or "TOI_a-TOI_b" elements. This latter case means that 855 all values between TOI_a and TOI_b, inclusive, are part of the list. 856 In that case TOI_a MUST be strictly inferior to TOI_b. If a TOI 857 wrapping to 0 occurs in an interval, then two TOI intervals MUST be 858 specified, TOI_a-MAX_TOI and 0-TOI_b. 860 This string can also contain the TOI equivalences, if any. The 861 format is a comma-separated list of "(" newTOI "=" 1stTOI "/" 1stCIID 862 ")" elements. Each element says that the new TOI, for the current 863 Carousel Instance, is equivalent to (i.e., refers to the same object 864 as) the provided identifier, 1stTOI, for the Carousel Instance of ID 865 1stCIID. 867 The ABNF specification is the following: 868 cid-list = *(list-elem *( "," list-elem)) 869 list-elem = toi-elem / toieq-elem 870 toi-elem = toi-value / toi-interval 871 toi-value = 1*DIGIT 872 toi-interval = toi-value "-" toi-value 873 ; additionally, the first toi-value MUST be 874 ; strictly inferior to the second toi-value 875 toieq-elem = "(" toi-value "=" toi-value "/" ciid-value ")" 876 ciid-value = 1*DIGIT 877 DIGIT = %x30-39 878 ; a digit between O and 9, inclusive 880 For readability purposes, it is RECOMMENDED that all the TOI values 881 in the list be given in increasing order. However a receiver MUST be 882 able to handle non-monotonically increasing values. It is also 883 RECOMMENDED to group the TOI equivalence elements together, at the 884 end of the list, in increasing newTOI order. However a receiver MUST 885 be able to handle lists of mixed TOI and TOI equivalence elements. 886 Specifying a TOI equivalence for a given newTOI relieves the sender 887 from specifying newTOI explicitly in the TOI list. However a 888 receiver MUST be able to handle situations where the same TOI appears 889 both in the TOI value and TOI equivalence lists. Finally, a given 890 TOI value or TOI equivalence item MUST NOT be included multiple times 891 in either list. 893 For instance, the following object list specifies that the current 894 Carousel Instance is composed of 8 objects, and that TOIs 100 to 104 895 are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and 896 refer to the same objects: 898 97,98,99,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 900 or equivalently: 902 97-104,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 904 6. Security Considerations 906 6.1. Problem Statement 908 A content delivery system is potentially subject to attacks. Attacks 909 may target: 911 o the network (to compromise the routing infrastructure, e.g., by 912 creating congestion), 914 o the Content Delivery Protocol (CDP) (e.g., to compromise the 915 normal behavior of FCAST), or 917 o the content itself (e.g., to corrupt the objects being 918 transmitted). 920 These attacks can be launched either: 922 o against the data flow itself (e.g., by sending forged packets), 924 o against the session control parameters (e.g., by corrupting the 925 session description, the CID, the object meta-data, or the ALC/LCT 926 control parameters), that are sent either in-band or out-of-band, 927 or 929 o against some associated building blocks (e.g., the congestion 930 control component). 932 In the following sections we provide more details on these possible 933 attacks and sketch some possible counter-measures. We finally 934 provide recommendations in Section 6.5. 936 6.2. Attacks Against the Data Flow 938 Let us consider attacks against the data flow first. At least, the 939 following types of attacks exist: 941 o attacks that are meant to give access to a confidential object 942 (e.g., in case of a non-free content) and 944 o attacks that try to corrupt the object being transmitted (e.g., to 945 inject malicious code within an object, or to prevent a receiver 946 from using an object, which is a kind of Denial of Service (DoS)). 948 6.2.1. Access to Confidential Objects 950 Access control to the object being transmitted is typically provided 951 by means of encryption. This encryption can be done over the whole 952 object (e.g., by the content provider, before submitting the object 953 to FCAST), or be done on a packet per packet basis (e.g., when IPSec/ 954 ESP is used [RFC4303], see Section 6.5). If confidentiality is a 955 concern, it is RECOMMENDED that one of these solutions be used. 957 6.2.2. Object Corruption 959 Protection against corruptions (e.g., if an attacker sends forged 960 packets) is achieved by means of a content integrity verification/ 961 sender authentication scheme. This service can be provided at the 962 object level, but in that case a receiver has no way to identify 963 which symbol(s) is(are) corrupted if the object is detected as 964 corrupted. This service can also be provided at the packet level. 965 In this case, after removing all corrupted packets, the file may be 966 in some cases recovered. Several techniques can provide this content 967 integrity/sender authentication service: 969 o at the object level, the object can be digitally signed, for 970 instance by using RSASSA-PKCS1-v1_5 [RFC3447]. This signature 971 enables a receiver to check the object integrity, once this latter 972 has been fully decoded. Even if digital signatures are 973 computationally expensive, this calculation occurs only once per 974 object, which is usually acceptable; 976 o at the packet level, each packet can be digitally signed 977 [RMT-SIMPLE-AUTH]. A major limitation is the high computational 978 and transmission overheads that this solution requires. To avoid 979 this problem, the signature may span a set of packets (instead of 980 a single one) in order to amortize the signature calculation. But 981 if a single packets is missing, the integrity of the whole set 982 cannot be checked; 984 o at the packet level, a Group Message Authentication Code (MAC) 985 [RFC2104][RMT-SIMPLE-AUTH] scheme can be used, for instance by 986 using HMAC-SHA-256 with a secret key shared by all the group 987 members, senders and receivers. This technique creates a 988 cryptographically secured digest of a packet that is sent along 989 with the packet. The Group MAC scheme does not create prohibitive 990 processing load nor transmission overhead, but it has a major 991 limitation: it only provides a group authentication/integrity 992 service since all group members share the same secret group key, 993 which means that each member can send a forged packet. It is 994 therefore restricted to situations where group members are fully 995 trusted (or in association with another technique as a pre-check); 997 o at the packet level, Timed Efficient Stream Loss-Tolerant 998 Authentication (TESLA) [RFC4082][RFC5776] is an attractive 999 solution that is robust to losses, provides a true authentication/ 1000 integrity service, and does not create any prohibitive processing 1001 load or transmission overhead. Yet checking a packet requires a 1002 small delay (a second or more) after its reception; 1004 o at the packet level, IPsec/ESP [RFC4303] can be used to check the 1005 integrity and authenticate the sender of all the packets being 1006 exchanged in a session (see Section 6.5). 1008 Techniques relying on public key cryptography (digital signatures and 1009 TESLA during the bootstrap process, when used) require that public 1010 keys be securely associated to the entities. This can be achieved by 1011 a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by 1012 pre-distributing securely the public keys of each group member. 1014 Techniques relying on symmetric key cryptography (Group MAC) require 1015 that a secret key be shared by all group members. This can be 1016 achieved by means of a group key management protocol, or simply by 1017 pre-distributing securely the secret key (but this manual solution 1018 has many limitations). 1020 It is up to the developer and deployer, who know the security 1021 requirements and features of the target application area, to define 1022 which solution is the most appropriate. In any case, whenever there 1023 is any concern of the threat of file corruption, it is RECOMMENDED 1024 that at least one of these techniques be used. 1026 6.3. Attacks Against the Session Control Parameters and Associated 1027 Building Blocks 1029 Let us now consider attacks against the session control parameters 1030 and the associated building blocks. The attacker has at least the 1031 following opportunities to launch an attack: 1033 o the attack can target the session description, 1035 o the attack can target the FCAST CID, 1036 o the attack can target the meta-data of an object, 1038 o the attack can target the ALC/LCT parameters, carried within the 1039 LCT header or 1041 o the attack can target the FCAST associated building blocks, for 1042 instance the multiple rate congestion control protocol. 1044 The consequences of these attacks are potentially serious, since they 1045 can compromise the behavior of content delivery system or even 1046 compromise the network itself. 1048 6.3.1. Attacks Against the Session Description 1050 An FCAST receiver may potentially obtain an incorrect Session 1051 Description for the session. The consequence of this is that 1052 legitimate receivers with the wrong Session Description are unable to 1053 correctly receive the session content, or that receivers 1054 inadvertently try to receive at a much higher rate than they are 1055 capable of, thereby possibly disrupting other traffic in the network. 1057 To avoid these problems, it is RECOMMENDED that measures be taken to 1058 prevent receivers from accepting incorrect Session Descriptions. One 1059 such measure is the sender authentication to ensure that receivers 1060 only accept legitimate Session Descriptions from authorized senders. 1061 How these measures are achieved is outside the scope of this document 1062 since this session description is usually carried out-of-band. 1064 6.3.2. Attacks Against the FCAST CID 1066 Corrupting the FCAST CID is one way to create a Denial of Service 1067 attack. For example, the attacker can set the "Complete" flag to 1068 make the receivers believe that no further modification will be done. 1070 It is therefore RECOMMENDED that measures be taken to guarantee the 1071 integrity and to check the sender's identity of the CID. To that 1072 purpose, one of the counter-measures mentioned above (Section 6.2.2) 1073 SHOULD be used. These measures will either be applied on a packet 1074 level, or globally over the whole CID object. When there is no 1075 packet level integrity verification scheme, it is RECOMMENDED to 1076 digitally sign the CID. 1078 6.3.3. Attacks Against the Object Meta-Data 1080 Corrupting the object meta-data is another way to create a Denial of 1081 Service attack. For example, the attacker changes the MD5 sum 1082 associated to a file. This possibly leads a receiver to reject the 1083 files received, no matter whether the files have been correctly 1084 received or not. When the meta-data are appended to the object, 1085 corrupting the meta-data means that the Compound Object will be 1086 corrupted. 1088 It is therefore RECOMMENDED that measures be taken to guarantee the 1089 integrity and to check the sender's identity of the Compound Object. 1090 To that purpose, one of the counter-measures mentioned above 1091 (Section 6.2.2) SHOULD be used. These measures will either be 1092 applied on a packet level, or globally over the whole Compound 1093 Object. When there is no packet level integrity verification scheme, 1094 it is RECOMMENDED to digitally sign the Compound Object. 1096 6.3.4. Attacks Against the ALC/LCT and NORM Parameters 1098 By corrupting the ALC/LCT header (or header extensions) one can 1099 execute attacks on the underlying ALC/LCT implementation. For 1100 example, sending forged ALC packets with the Close Session flag (A) 1101 set to one can lead the receiver to prematurely close the session. 1102 Similarly, sending forged ALC packets with the Close Object flag (B) 1103 set to one can lead the receiver to prematurely give up the reception 1104 of an object. The same comments can be made for NORM. 1106 It is therefore RECOMMENDED that measures be taken to guarantee the 1107 integrity and to check the sender's identity of each ALC or NORM 1108 packet received. To that purpose, one of the counter-measures 1109 mentioned above (Section 6.2.2) SHOULD be used. 1111 6.3.5. Attacks Against the Associated Building Blocks 1113 Let us first focus on the congestion control building block that may 1114 be used in an ALC or NORM session. A receiver with an incorrect or 1115 corrupted implementation of the multiple rate congestion control 1116 building block may affect the health of the network in the path 1117 between the sender and the receiver. That may also affect the 1118 reception rates of other receivers who joined the session. 1120 When congestion control is applied with FCAST, it is therefore 1121 RECOMMENDED that receivers be required to identify themselves as 1122 legitimate before they receive the Session Description needed to join 1123 the session. If authenticating a receiver does not prevent this 1124 latter to launch an attack, it will enable the network operator to 1125 identify him and to take counter-measures. This authentication can 1126 be made either toward the network operator or the session sender (or 1127 a representative of the sender) in case of NORM. The details of how 1128 it is done are outside the scope of this document. 1130 When congestion control is applied with FCAST, it is also RECOMMENDED 1131 that a packet level authentication scheme be used, as explained in 1132 Section 6.2.2. Some of them, like TESLA, only provide a delayed 1133 authentication service, whereas congestion control requires a rapid 1134 reaction. It is therefore RECOMMENDED [RFC5775] that a receiver 1135 using TESLA quickly reduces its subscription level when the receiver 1136 believes that a congestion did occur, even if the packet has not yet 1137 been authenticated. Therefore TESLA will not prevent DoS attacks 1138 where an attacker makes the receiver believe that a congestion 1139 occurred. This is an issue for the receiver, but this will not 1140 compromise the network. Other authentication methods that do not 1141 feature this delayed authentication could be preferred, or a group 1142 MAC scheme could be used in parallel to TESLA to prevent attacks 1143 launched from outside of the group. 1145 6.4. Other Security Considerations 1147 Lastly, we note that the security considerations that apply to, and 1148 are described in, ALC [RFC5775], LCT [RFC5651], NORM [RFC5740] and 1149 FEC [RFC5052] also apply to FCAST as FCAST builds on those 1150 specifications. In addition, any security considerations that apply 1151 to any congestion control building block used in conjunction with 1152 FCAST also applies to FCAST. Finally, the security discussion of 1153 [RMT-SEC] also applies here. 1155 6.5. Minimum Security Recommendations 1157 We now introduce a mandatory to implement but not necessarily to use 1158 security configuration, in the sense of [RFC3365]. Since FCAST 1159 relies on ALC/LCT, it inherits the "baseline secure ALC operation" of 1160 [RFC5775]. More precisely, security is achieved by means of IPsec/ 1161 ESP in transport mode. [RFC4303] explains that ESP can be used to 1162 potentially provide confidentiality, data origin authentication, 1163 content integrity, anti-replay and (limited) traffic flow 1164 confidentiality. [RFC5775] specifies that the data origin 1165 authentication, content integrity and anti-replay services SHALL be 1166 used, and that the confidentiality service is RECOMMENDED. If a 1167 short lived session MAY rely on manual keying, it is also RECOMMENDED 1168 that an automated key management scheme be used, especially in case 1169 of long lived sessions. 1171 Therefore, the RECOMMENDED solution for FCAST provides per-packet 1172 security, with data origin authentication, integrity verification and 1173 anti-replay. This is sufficient to prevent most of the in-band 1174 attacks listed above. If confidentiality is required, a per-packet 1175 encryption SHOULD also be used. 1177 7. IANA Considerations 1179 This document requires a IANA registration for the following 1180 attributes: 1182 Object meta-data format (MDFmt): All implementations MUST support 1183 format 0 (default). 1185 +----------------------------------------+-------------+ 1186 | format name | Value | 1187 +----------------------------------------+-------------+ 1188 | as per HTTP/1.1 metainformation format | 0 (default) | 1189 +----------------------------------------+-------------+ 1191 Object Meta-Data Encoding (MDENC): All implementations MUST support 1192 value 0 (plain-text, default) and value 1 (gzip). 1194 +------------+-------------+ 1195 | Name | Value | 1196 +------------+-------------+ 1197 | plain text | 0 (default) | 1198 | gzip | 1 | 1199 +------------+-------------+ 1201 8. Acknowledgments 1203 The authors are grateful to the authors of [ALC-00] for specifying 1204 the first version of FCAST/ALC. The authors are also grateful to 1205 Gorry Fairhurst and Lorenzo Vicisano for their valuable comments. 1207 9. References 1209 9.1. Normative References 1211 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1212 Requirement Levels", BCP 14, RFC 2119, March 1997. 1214 [RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer, 1215 "Computing the Internet checksum", RFC 1071, 1216 September 1988. 1218 [RFC5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding 1219 Transport (LCT) Building Block", RFC 5651, October 2009. 1221 [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, 1222 "NACK-Oriented Reliable Multicast (NORM) Transport 1223 Protocol", RFC 5740, November 2009. 1225 [RFC5775] Luby, M., Watson, M., and L. Vicisano, "Asynchronous 1226 Layered Coding (ALC) Protocol Instantiation", RFC 5775, 1227 April 2010. 1229 9.2. Informative References 1231 [ALC-00] Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J., and B. 1232 Lueckenhoff, "Asynchronous Layered Coding: a Scalable 1233 Reliable Multicast Protocol", March 2000. 1235 [RMT-FLUTE] 1236 Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1237 "FLUTE - File Delivery over Unidirectional Transport", 1238 Work in Progress, March 2010. 1240 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 1241 Engineering Task Force Standard Protocols", BCP 61, 1242 RFC 3365, August 2002. 1244 [RMT-SEC] Roca, V., Adamson, B., and H. Asaeda, "Security and 1245 Reliable Multicast Transport Protocols: Discussions and 1246 Guidelines", Work in progress, 1247 draft-ietf-rmt-sec-discussion-05.txt, May 2010. 1249 [RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. 1250 Randers-Pehrson, "GZIP file format specification version 1251 4.3", RFC 1952, May 1996. 1253 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1254 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1255 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1257 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1258 Hashing for Message Authentication", RFC 2104, 1259 February 1997. 1261 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 1262 Standards (PKCS) #1: RSA Cryptography Specifications 1263 Version 2.1", RFC 3447, February 2003. 1265 [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. 1266 Briscoe, "Timed Efficient Stream Loss-Tolerant 1267 Authentication (TESLA): Multicast Source Authentication 1268 Transform Introduction", RFC 4082, June 2005. 1270 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", 1271 RFC 4303, December 2005. 1273 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 1274 Correction (FEC) Building Block", RFC 5052, August 2007. 1276 [RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, 1277 "Reed-Solomon Forward Error Correction (FEC) Schemes", 1278 RFC 5510, April 2009. 1280 [RFC5776] Roca, V., Francillon, A., and S. Faurite, "Use of Timed 1281 Efficient Stream Loss-Tolerant Authentication (TESLA) in 1282 the Asynchronous Layered Coding (ALC) and NACK-Oriented 1283 Reliable Multicast (NORM) Protocols", RFC 5776, 1284 April 2010. 1286 [RMT-SIMPLE-AUTH] 1287 Roca, V., "Simple Authentication Schemes for the ALC and 1288 NORM Protocols", Work in 1289 progress draft-ietf-rmt-simple-auth-for-alc-norm-03.txt, 1290 July 2010. 1292 Appendix A. FCAST Examples 1294 A.1. Basic Examples 1295 0 1 2 3 1296 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1297 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1298 | 0 |1|0| 0 | 0 | 39 | 1299 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1300 | Checksum | | 1301 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1302 . . 1303 . meta-data ASCII null terminated string (33 bytes) . 1304 . . 1305 + +-+-+-+-+-+-+-+-+ 1306 | | Padding | 1307 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1308 . . 1309 . Object data . 1310 . . 1311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1313 Figure 4: Compound Object Example. 1315 Figure 4 shows a regular Compound Object where the meta-data ASCII 1316 string, in HTTP/1.1 meta-information format (MDFmt=0) contains: 1318 Content-Location: example.txt 1320 This string is 33 bytes long, including the NULL-termination 1321 character. There is no gzip encoding of the meta-data (MDEnc=0) and 1322 there is no Content-Length information either since this length can 1323 easily be calculated by the receiver as the FEC OTI transfer length 1324 minus the header length. Finally, the checksum encompasses the whole 1325 Compound Object (G=1). 1326 0 1 2 3 1327 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1329 | 0 |1|0| 0 | 0 | 6 | 1330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1331 | Checksum | Padding | 1332 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1333 . . 1334 . Object data . 1335 . . 1336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1338 Figure 5: Compound Object Example with no Meta-Data. 1340 Figure 5 shows a Compound Object without any meta-data. The fact 1341 there is no meta-data is indicated by the value 6 of the Compound 1342 Object Header Length field. Two bytes of padding are required in 1343 that case for 32-bit word alignment purposes. 1345 Figure 6 shows an example CID object, in the case of a static FCAST 1346 session, i.e., a session where the set of objects is set once and for 1347 all. There is no meta-data in this example since Fcast-CID-Complete 1348 and Fcast-CID-ID are both implicit. 1349 0 1 2 3 1350 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1351 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1352 | 0 |1|1| 0 | 0 | 6 | 1353 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1354 | Checksum | Padding | 1355 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1356 . . 1357 . Object List string . 1358 . . 1359 . +-+-+-+-+-+-+-+-+ 1360 . | 1361 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1363 Figure 6: Example of CID, in case of a static session. 1365 The object list contains the following string: 1367 1,2,3,100-104,200-203,299 1369 There are therefore a total of 3+5+4+1 = 13 objects in the carousel 1370 instance, and therefore in the FCAST session. There is no meta-data 1371 associated to this CID. The session being static and composed of a 1372 single Carousel Instance, the sender did not feel the necessity to 1373 carry a Carousel Instance ID meta-data. 1375 A.2. FCAST/NORM with NORM_INFO Examples 1377 In case of FCAST/NORM, the FCAST Compound Object meta-data (or a 1378 subset of it) can be carried as part of a NORM_INFO message, as a new 1379 Compound Object that does not contain any Compound Object Data. In 1380 the following example we assume that the whole meta-data is carried 1381 in such a message for a certain Compound Object. Figure 7 shows an 1382 example NORM_INFO message that contains the FCAST Compound Object 1383 Header and meta-data as its payload. In this example, the first 16 1384 bytes are the NORM_INFO base header, the next 12 bytes are a NORM 1385 EXT_FTI header extension containing the FEC Object Transport 1386 Information for the associated object, and the remaining bytes are 1387 the FCAST Compound Object Header and meta-data. Note that "padding" 1388 MUST NOT be used and that the FCAST checksum only encompasses the 1389 Compound Object Header (G=0). 1391 0 1 2 3 1392 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1393 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1394 |version| type=1| hdr_len = 7 | sequence | 1395 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1396 | source_id | 1397 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1398 | instance_id | grtt |backoff| gsize | 1399 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1400 | flags | fec_id = 5 | object_transport_id | 1401 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1402 | HET = 64 | HEL = 3 | | 1403 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1404 | Transfer Length (L) | 1405 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1406 | Encoding Symbol Length (E) | MaxBlkLen (B) | max_n | 1407 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1408 | 0 |0|0| 0 | 0 | 39 | 1409 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1410 | Checksum | | 1411 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1412 . . 1413 . meta-data ASCII null terminated string (33 bytes) . 1414 . . 1415 + +-+-+-+-+-+-+-+-+ 1416 | | 1417 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1419 Figure 7: NORM_INFO containing FCAST Compound Object Header 1421 The NORM_INFO message shown in Figure 7 contains the EXT_FTI header 1422 extension to carry the FEC OTI. In this example, the FEC OTI format 1423 is that of the Reed-Solomon FEC coding scheme for fec_id = 5 as 1424 described in [RFC5510]. Other alternatives for providing the FEC OTI 1425 would have been to either include it directly in the meta-data of the 1426 FCAST Compound Header, or to include an EXT_FTI header extension to 1427 all NORM_DATA packets (or a subset of them). Note that the NORM 1428 "Transfer_Length" is the total length of the associated FCAST 1429 Compound Object. 1431 The FCAST Compound Object in this example does contain the same meta- 1432 data and is formatted as in the example of Figure 4. With the 1433 combination of the FEC_OTI and the FCAST meta-data, the NORM protocol 1434 and FCAST application have all of the information needed to reliably 1435 receive and process the associated object. Indeed, the NORM protocol 1436 provides rapid (NORM_INFO has precedence over the associated object 1437 content), reliable delivery of the NORM_INFO message and its payload, 1438 the FCAST Compound Object Header. 1440 Authors' Addresses 1442 Vincent Roca 1443 INRIA 1444 655, av. de l'Europe 1445 Inovallee; Montbonnot 1446 ST ISMIER cedex 38334 1447 France 1449 Email: vincent.roca@inria.fr 1450 URI: http://planete.inrialpes.fr/people/roca/ 1452 Brian Adamson 1453 Naval Research Laboratory 1454 Washington, DC 20375 1455 USA 1457 Email: adamson@itd.nrl.navy.mil 1458 URI: http://cs.itd.nrl.navy.mil