idnits 2.17.1 draft-ietf-rmt-fcast-00.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** You're using the IETF Trust Provisions' Section 6.b License Notice from 12 Sep 2009 rather than the newer Notice from 28 Dec 2009. (See https://trustee.ietf.org/license-info/) 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, but was first submitted on or after 10 November 2008. The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, 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 (December 23, 2009) is 5232 days in the past. Is this intentional? Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Outdated reference: A later version (-08) exists of draft-ietf-rmt-sec-discussion-04 -- Obsolete informational reference (is this intentional?): RFC 2068 (Obsoleted by RFC 2616) -- Obsolete informational reference (is this intentional?): RFC 3447 (Obsoleted by RFC 8017) == Outdated reference: A later version (-06) exists of draft-ietf-rmt-simple-auth-for-alc-norm-02 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RMT V. Roca 3 Internet-Draft INRIA 4 Intended status: Experimental B. Adamson 5 Expires: June 26, 2010 Naval Research Laboratory 6 December 23, 2009 8 FCAST: Scalable Object Delivery for the ALC and NORM Protocols 9 draft-ietf-rmt-fcast-00 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 to IETF 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), its areas, and its working groups. Note that 25 other groups may also distribute working documents as Internet- 26 Drafts. 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 The list of current Internet-Drafts can be accessed at 34 http://www.ietf.org/ietf/1id-abstracts.txt. 36 The list of Internet-Draft Shadow Directories can be accessed at 37 http://www.ietf.org/shadow.html. 39 This Internet-Draft will expire on June 26, 2010. 41 Copyright Notice 43 Copyright (c) 2009 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the BSD License. 56 This document may contain material from IETF Documents or IETF 57 Contributions published or made publicly available before November 58 10, 2008. The person(s) controlling the copyright in some of this 59 material may not have granted the IETF Trust the right to allow 60 modifications of such material outside the IETF Standards Process. 61 Without obtaining an adequate license from the person(s) controlling 62 the copyright in such materials, this document may not be modified 63 outside the IETF Standards Process, and derivative works of it may 64 not be created outside the IETF Standards Process, except to format 65 it for publication as an RFC or to translate it into languages other 66 than English. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 71 1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 5 72 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5 73 3. Definitions, Notations and Abbreviations . . . . . . . . . . . 6 74 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 75 3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 6 76 4. FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 7 77 4.1. FCAST Content Delivery Service . . . . . . . . . . . . . . 7 78 4.2. Meta-Data Transmission . . . . . . . . . . . . . . . . . . 7 79 4.3. Meta-Data Content . . . . . . . . . . . . . . . . . . . . 8 80 4.4. Carousel Transmission . . . . . . . . . . . . . . . . . . 9 81 4.5. Carousel Instance Object . . . . . . . . . . . . . . . . . 10 82 4.6. Compound Object Identification . . . . . . . . . . . . . . 11 83 4.7. FCAST/ALC Additional Specificities . . . . . . . . . . . . 12 84 4.8. FCAST/NORM Additional Specificities . . . . . . . . . . . 13 85 4.9. FCAST Sender Behavior . . . . . . . . . . . . . . . . . . 14 86 4.10. FCAST Receiver Behavior . . . . . . . . . . . . . . . . . 15 87 5. FCAST Specifications . . . . . . . . . . . . . . . . . . . . . 15 88 5.1. Compound Object Header Format . . . . . . . . . . . . . . 15 89 5.2. Carousel Instance Object Format . . . . . . . . . . . . . 18 90 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 91 6.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 21 92 6.2. Attacks Against the Data Flow . . . . . . . . . . . . . . 21 93 6.2.1. Access to Confidential Objects . . . . . . . . . . . . 21 94 6.2.2. Object Corruption . . . . . . . . . . . . . . . . . . 22 95 6.3. Attacks Against the Session Control Parameters and 96 Associated Building Blocks . . . . . . . . . . . . . . . . 23 97 6.3.1. Attacks Against the Session Description . . . . . . . 24 98 6.3.2. Attacks Against the FCAST CIO . . . . . . . . . . . . 24 99 6.3.3. Attacks Against the Object Meta-Data . . . . . . . . . 24 100 6.3.4. Attacks Against the ALC/LCT and NORM Parameters . . . 25 101 6.3.5. Attacks Against the Associated Building Blocks . . . . 25 102 6.4. Other Security Considerations . . . . . . . . . . . . . . 26 103 6.5. Minimum Security Recommendations . . . . . . . . . . . . . 26 104 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 105 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 106 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 107 9.1. Normative References . . . . . . . . . . . . . . . . . . . 27 108 9.2. Informative References . . . . . . . . . . . . . . . . . . 27 109 Appendix A. FCAST Examples . . . . . . . . . . . . . . . . . . . 29 110 A.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . . 29 111 A.2. FCAST/NORM with NORM_INFO Examples . . . . . . . . . . . . 31 112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 114 1. Introduction 116 This document introduces the FCAST reliable and scalable object 117 (e.g., file) delivery application. Two versions of FCAST exist: 119 o FCAST/ALC that relies on the Asynchronous Layer Coding (ALC) 120 [RMT-PI-ALC] and the Layered Coding Transport (LCT) [RFC5651] 121 reliable multicast transport protocol, and 123 o FCAST/NORM that relies on the NACK-Oriented Reliable Multicast 124 (NORM) [RFC5740] reliable multicast transport protocol. 126 Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM. 128 Depending on the target use case, the delivery service provided by 129 FCAST is more or less reliable. For instance, with FCAST/ALC used in 130 ON-DEMAND mode over a time period that largely exceeds the typical 131 download time, the service can be considered as fully reliable. 132 Similarly, when FCAST is used along with a session control 133 application that collects reception information and takes appropriate 134 corrective measures (e.g., a direct point-to-point retransmission of 135 missing packets, or a new multicast recovery session), then the 136 service can be considered as fully reliable. On the opposite, if 137 FCAST operates in PUSH mode, then the service is usually only 138 partially reliable, and a receiver that is disconnected during a 139 sufficient time will perhaps not have the possibility to download the 140 object. 142 Depending on the target use case, the FCAST scalability is more or 143 less important. For instance, if FCAST/ALC is used on top of purely 144 unidirectional transport channels, with no feedback information at 145 all, which is the default mode of operation, then the scalability is 146 maximum since neither FCAST, nor ALC, UDP or IP generates any 147 feedback message. On the opposite, the FCAST/NORM scalability is 148 typically limited by NORM scalability itself. Similarly, if FCAST is 149 used along with a session control application that collects reception 150 information from the receivers, then this session control application 151 may limit the scalability of the global object delivery system. This 152 situation can of course be mitigated by using a hierarchy of feedback 153 message aggregators or servers. The details of this are out of the 154 scope of the present document. 156 A design goal behind FCAST is to define a streamlined solution, in 157 order to enable lightweight implementations of the protocol stack, 158 and limit the operational processing and storage requirements. A 159 consequence of this choice is that FCAST cannot be considered as a 160 versatile application, capable of addressing all the possible use- 161 cases. On the opposite, FCAST has some intrinsic limitations. From 162 this point of view it differs from FLUTE [RMT-FLUTE] which favors 163 flexibility at the expense of some additional complexity. 165 A good example of the design choices meant to favor the simplicity is 166 the way FCAST manages the object meta-data: by default, the meta-data 167 and the object content are sent together, in a compound object. This 168 solution has many advantages in terms of simplicity as will be 169 described later on. However, as such, it also has an intrinsic 170 limitation since it does not enable a receiver to decide in advance, 171 before beginning the reception of the compound object, whether the 172 object is of interest or not, based on the information that may be 173 provided in the meta-data. Therefore this document defines 174 additional techniques that may be used to mitigate this limitation. 175 It is also possible that some use-cases require that each receiver 176 download the whole set of objects sent in the session (e.g., with 177 mirroring tools). When this is the case, the above limitation is no 178 longer be a problem. 180 1.1. Applicability 182 FCAST is compatible with any congestion control protocol designed for 183 ALC/LCT or NORM. However, depending on the use-case, the data flow 184 generated by the FCAST application might not be constant, but instead 185 be bursty in nature. Similarly, depending on the use-case, an FCAST 186 session might be very short. Whether and how this will impact the 187 congestion control protocol is out of the scope of the present 188 document. 190 FCAST is compatible with any security mechanism designed for ALC/LCT 191 or NORM. The use of a security scheme is strongly RECOMMENDED (see 192 Section 6). 194 FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM. 195 Whether FEC is used or not, and the kind of FEC scheme used, is to 196 some extent transparent to FCAST. 198 FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST 199 specification has any implication on the source or destination IP 200 address. 202 2. Requirements notation 204 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 205 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 206 document are to be interpreted as described in [RFC2119]. 208 3. Definitions, Notations and Abbreviations 210 3.1. Definitions 212 This document uses the following definitions: 214 FCAST/ALC: denotes the FCAST application running on top of the ALC/ 215 LCT reliable transport protocol; 217 FCAST/NORM: denotes the FCAST application running on top of the NORM 218 reliable transport protocol; 220 FCAST: denotes either FCAST/ALC or FCAST/NORM; 222 Compound Object: denotes an ALC or NORM transport object composed of 223 the Compound Object Header (Section 5.1), including any meta- 224 data, and the content of the original application object 225 (e.g., a file); 227 Carousel: denotes the Compound Object transmission system of an 228 FCAST sender; 230 Carousel Instance: denotes a fixed set of registered Compound 231 Objects that are sent by the carousel during a certain number 232 of cycles. Whenever Compound Objects need to be added or 233 removed, a new Carousel Instance is defined; 235 Carousel Instance Object (CIO): denotes a specific object that lists 236 the Compound Objects that comprise a given Carousel Instance; 238 Carousel Cycle: denotes a transmission round within which all the 239 Compound Objects registered in the Carousel Instance are 240 transmitted a certain number of times. By default, Compound 241 Objects are transmitted once per cycle, but higher values are 242 possible, that might differ on a per-object basis; 244 Transmission Object Identifier (TOI): denotes the numeric identifier 245 associated to a specific object by the underlying transport 246 layer. In the case of ALC, this corresponds to the TOI 247 described in that specification while for the NORM 248 specification this corresponds to the NormTransportId 249 described there. 251 3.2. Abbreviations 253 This document uses the following abbreviations: 255 CIO: Carousel Instance Object 257 CIID: Carousel Instance IDentifier 259 FEC OTI: FEC Object Transmission Information 261 TOI: Transmission Object Identifier 263 4. FCAST Principles 265 4.1. FCAST Content Delivery Service 267 The basic goal of FCAST is to transmit objects to a group of 268 receivers in a reliable way. The receiver set MAY be restricted to a 269 single receiver or MAY include possibly a very large number of 270 receivers. FCAST is specified to support two forms of operation: 272 1. FCAST/ALC: where the FCAST application is meant to run on top of 273 the ALC/LCT reliable multicast transport protocol, and 275 2. FCAST/NORM: where the FCAST application is meant to run on top of 276 the NORM reliable multicast transport protocol. 278 This specification is designed such that both forms of operation 279 share as much commonality as possible. 281 While the choice of the underlying transport protocol (i.e., ALC or 282 NORM) and its parameters may limit the practical receiver group size, 283 nothing in FCAST itself limits it. The transmission might be fully 284 reliable, or only partially reliable depending upon the way ALC or 285 NORM is used (e.g., whether FEC encoding and/or NACK-based repair 286 requests are used or not), the way the FCAST carousel is used (e.g., 287 whether the objects are made available for a long time span or not), 288 and the way in which FCAST itself is employed (e.g., whether there is 289 a session control application that might automatically extend an 290 existing FCAST session until all receivers have received the 291 transmitted content). 293 FCAST is designed to be as self-sufficient as possible, in particular 294 in the way object meta-data is attached to object data content. 295 However, for some uses, meta-data MAY also be communicated by an out- 296 of-band mechanism that is out of the scope of the present document. 298 4.2. Meta-Data Transmission 300 FCAST usually carries meta-data elements by prepending them to the 301 object it refers to. As a result, a Compound Object is created that 302 is composed of a header followed by the original object data. This 303 header is itself composed of the meta-data as well as several fields, 304 for instance to indicate the boundaries between the various parts of 305 this Compound Object (Figure 1). 307 <------------------------ Compound Object -----------------------> 308 +-------------------------+--------------------------------------+ 309 | Compound Object Header | Object Data | 310 | (can include meta-data) | (can be encoded by FCAST) | 311 +-------------------------+--------------------------------------+ 313 Figure 1: Compound Object composition. 315 Attaching the meta-data to the object is an efficient solution, since 316 it guaranties that meta-data be received along with the associated 317 object, and it allows the transport of the meta-data to benefit from 318 any transport-layer erasure protection of the Compound Object (e.g., 319 using FEC encoding and/or NACK-based repair). However a limit of 320 this scheme, as such, is that a client does not know the meta-data of 321 an object before beginning its reception, and in case of erasures 322 affecting the meta-data, not until the object decoding is completed. 323 The details of course depend upon the transport protocol and the FEC 324 code used. 326 In certain use-cases, FCAST can also be associated to another in-band 327 (e.g., via NORM INFO messages, Section 4.8) or out-of-band signaling 328 mechanism. In that case, this mechanism can be used in order to 329 carry the whole meta-data (or a subset of it), possibly ahead of 330 time. 332 4.3. Meta-Data Content 334 The meta-data associated to an object can be composed of, but are not 335 limited to: 337 o Content-Location: the URI of the object, which gives the name and 338 location of the object; 340 o Content-Type: the MIME type of the object; 342 o Content-Length: the size of the initial object, before any content 343 encoding (if any). Note that this content length does not include 344 the meta-data nor the fixed size Compound Object header; 346 o Content-Encoding: the optional encoding of the object performed by 347 FCAST. If there is no Content-Encoding entry, the receiver MUST 348 assume that the object is not encoded (default). The support of 349 gzip encoding, or any other solution, remains optional. 351 o Content-MD5: the MD5 message digest of the object in order to 352 check its integrity. Note that this digest is meant to protect 353 from transmission and processing errors, not from deliberate 354 attacks by an intelligent attacker. Note also that this digest 355 only protects the object, not the header, and therefore not the 356 meta-data. A separate checksum is provided to that purpose 357 (Section 5.1); 359 o a digital signature for this object; 361 This list is not limited and new meta-data information can be added. 362 For instance, when dealing with very large objects (e.g., that 363 largely exceed the working memory of a receiver), it can be 364 interesting to split this object into several sub-objects (or 365 slices). When this happens, the meta-data associated to each sub- 366 object MUST include the following entries: 368 o Fcast-Obj-Slice-Nb: the total number of slices. A value strictly 369 greater than 1 indicates that this object is the result of a split 370 of the original object; 372 o Fcast-Obj-Slice-Idx: the slice index (in the {0 .. SliceNb - 1} 373 interval); 375 o Fcast-Obj-Slice-Offset: the offset at which this slice starts 376 within the original object; 378 When meta-data elements are communicated out-of-band, in advance of 379 data transmission, the following pieces of information MAY also be 380 useful: 382 o TOI: the Transmission Object Identifier (TOI) of the object 383 (Section 4.6), in order to enable a receiver to easily associate 384 the meta-data to the object; 386 o FEC Object Transmission Information (FEC OTI). In this case the 387 FCAST sender does not need to use the optional EXT_FTI mechanism 388 of the ALC or NORM protocols. 390 4.4. Carousel Transmission 392 A set of FCAST Compound Objects scheduled for transmission are 393 considered a logical "Carousel". A given "Carousel Instance" is 394 comprised of a fixed set of Compound Objects. Whenever the FCAST 395 application needs to add new Compound Objects to, or remove old 396 Compound Objects from the transmission set, a new Carousel Instance 397 is defined since the set of Compound Objects changes. 399 For a given Carousel Instance, one or more transmission cycles are 400 possible. During each cycle, all of the Compound Objects comprising 401 the Carousel are sent. By default, each object is transmitted once 402 per cycle. However, in order to allow different levels of priority, 403 some objects MAY be transmitted more often that others during a 404 cycle, and/or benefit from higher FEC protection than others. This 405 can be the case for instance of the CIO objects (Section 4.5). For 406 some FCAST usage (e.g., a unidirectional "push" mode), a Carousel 407 Instance may be associated to a single transmission cycle. In other 408 cases it may be associated to a large number of transmission cycles 409 (e.g., in "on-demand" mode, where objects are made available for 410 download during a long period of time). 412 4.5. Carousel Instance Object 414 The FCAST sender CAN transmit an OPTIONAL Carousel Instance Object 415 (CIO). The CIO carries the list of the Compound Objects that are 416 part of a given Carousel Instance, by specifying their respective 417 Transmission Object Identifiers (TOI). However the CIO does not 418 describe the objects themselves (i.e., there is no meta-data). 419 Additionally, the CIO MAY include a "Complete" flag that is used to 420 indicate that no further modification to the enclosed list will be 421 done in the future. Finally, the CIO MAY include a Carousel Instance 422 ID that identifies the Carousel Instance it pertains to. These 423 aspects are discussed in Section 5.2. 425 There is no reserved TOI value for the CIO itself, since this object 426 is regarded by ALC/LCT or NORM as a standard object. On the 427 opposite, the nature of this object (CIO) is indicated by means of a 428 specific Compound Object header field (the "I" flag) so that it can 429 be recognized and processed by the FCAST application as needed. A 430 direct consequence is the following: since a receiver does not know 431 in advance which TOI will be used for the following CIO (in case of a 432 dynamic session), he MUST NOT filter out packets that are not in the 433 current CIO's TOI list. Said differently, the goal of CIO is not to 434 setup ALC or NORM packet filters (this mechanism would not be secure 435 in any case). 437 The use of a CIO remains optional. If it is not used, then the 438 clients progressively learn what files are part of the carousel 439 instance by receiving ALC or NORM packets with new TOIs. However 440 using a CIO has several benefits: 442 o When the "Complete" flag is set (if ever), the receivers know when 443 they can leave the session, i.e., when they have received all the 444 objects that are part of the last carousel instance of this 445 delivery session; 447 o In case of a session with a dynamic set of objects, the sender can 448 reliably inform the receivers that some objects have been removed 449 from the carousel with the CIO. This solution is more robust than 450 the "Close Object flag (B)" of ALC/LCT since a client with an 451 intermittent connectivity might loose all the packets containing 452 this B flag. And while NORM provides a robust object cancellation 453 mechanism in the form of its NORM_CMD(SQUELCH) message in response 454 to receiver NACK repair requests, the use of the CIO provides an 455 additional means for receivers to learn of objects for which it is 456 futile to request repair; 458 o The TOI equivalence (Section 4.6) can be signaled with the CIO. 459 This is often preferable to the alternative solution where the 460 equivalence is identified by examining the object meta-data, 461 especially in case of erasures. 463 During idle periods, when the carousel instance does not contain any 464 object, a CIO with an empty TOI list MAY be transmitted. In that 465 case, a new carousel instance ID MUST be used to differentiate this 466 (empty) carousel instance from the other ones. This mechanism can be 467 useful to inform the receivers that: 469 o all the previously sent objects have been removed from the 470 carousel. It therefore improves the FCAST robustness even during 471 "idle" period; 473 o the session is still active even if there is currently no content 474 being sent. Said differently, it can be used as a heartbeat 475 mechanism. If the "Complete" flag has not been set, it implicitly 476 informs the receivers that new objects MAY be sent in the future; 478 The decisions of whether a CIO should be used or not, how often and 479 when a CIO should be sent, are left to the sender and depend on many 480 parameters, including the target use case and the session dynamics. 481 For instance it may be appropriate to send a CIO at the beginning of 482 each new carousel instance, and then periodically. These operational 483 aspects are out of the scope of the present document. 485 4.6. Compound Object Identification 487 The FCAST Compound Objects are directly associated with the object- 488 based transport service that the ALC and NORM protocols provide. In 489 each of these protocols, the messages containing transport object 490 content are labeled with a numeric transport object identifier (i.e., 491 the ALC TOI and the NORM NormTransportId). For the purposes of this 492 document, this identifier in either case (ALC or NORM) is referred to 493 as the TOI. 495 There are several differences between ALC and NORM: 497 o the ALC use of TOI is rather flexible, since several TOI field 498 sizes are possible (from 16 to 112 bits), since this size can be 499 changed at any time, on a per-packet basis, and since the TOI 500 management is totally free as long as each object is associated to 501 a unique TOI (if no wraparound happened); 503 o the NORM use of TOI is more directive, since the TOI field is 16 504 bit long and since TOIs MUST be managed sequentially; 506 In both NORM and ALC, it is possible that the transport 507 identification space may eventually wrap for long-lived sessions 508 (especially with NORM where this phenomenon is expected to happen 509 more frequently). This can possibly introduce some ambiguity in 510 FCAST object identification if a sender retains some older objects in 511 newer Carousel Instances with updated object sets. Thus, when an 512 updated object set, for a new Carousel Instance, would transport 513 identifiers that exceed one-half of the TOI sequence space (or 514 otherwise exceed the sender repair window capability in the case of 515 NORM), it MAY be necessary to re-enqueue old objects within the 516 Carousel with new TOI to stay within transport identifier limits. In 517 case of NORM, this constraint limits to 32768 the maximum number of 518 objects that can be part of any carousel instance. In order to allow 519 receivers to properly combine the transport packets with a newly- 520 assigned TOI to those of associated to the previously-assigned TOI, a 521 mechanism is required to equate the objects with the new and the old 522 TOIs. 524 The preferred mechanism consists in signaling, within the CIO, that 525 the newly assigned TOI, for the current Carousel Instance, is 526 equivalent to the TOI used within a previous Carousel Instance. By 527 convention, the reference tuple for any object is the {TOI; CI ID} 528 tuple used for its first transmission within a Carousel Instance. 529 This tuple MUST be used whenever a TOI equivalence is provided. 531 An alternative solution, when meta-data can be processed rapidly 532 (e.g., by using NORM-INFO messages), consists for the receiver in 533 identifying that both objects are the same, after examining the meta- 534 data. The receiver can then take appropriate measures. 536 4.7. FCAST/ALC Additional Specificities 538 There are no additional detail or option for FCAST/ALC operation. 540 4.8. FCAST/NORM Additional Specificities 542 The NORM Protocol provides a few additional capabilities that can be 543 used to specifically support FCAST operation: 545 1. The NORM_INFO message can convey "out-of-band" content with 546 respect to a given transport object. With FCAST, it MAY be used 547 to provide to the receivers a new, associated, Compound Object 548 which contains the main Compound Object meta-data, or a subset of 549 it. In that case the NORM_INFO Compound Object MUST NOT contain 550 any Object Data field (i.e., it is only composed of the header), 551 it MUST feature a non global checksum, and it MUST NOT include 552 any padding field. The main Compound Object MUST in any case 553 contain the whole meta-data (e.g., because a receiver MAY not 554 support the NORM_INFO facility). Additionally, the meta-data 555 entries contained in the NORM_INFO MUST be identical to the same 556 entries in the main Compound Object. Finally, note that the 557 availability of NORM_INFO for a given object is signaled through 558 the use of a dedicated flag in the NORM_DATA message header. 559 Along with NORM's NACK-based repair request signaling, it allows 560 a receiver to quickly (and independently) request an object's 561 NORM_INFO content. However, a limitation here is that the 562 NORM_INFO Compound Object header MUST fit within the byte size 563 limit defined by the NORM sender's configured "segment size" 564 (typically a little less than the network MTU); 566 2. The NORM_CMD(SQUELCH) messages are used by the NORM protocol 567 sender to inform receivers of objects that have been canceled 568 when receivers make repair requests for such invalid objects. 569 Along with the CIO mechanism, a receiver has two efficient and 570 reliable ways to discover old objects that have been removed from 571 the carousel instance; 573 3. NORM also supports an optional positive acknowledgment mechanism 574 that can be used for small-scale multicast receiver group sizes. 575 Also, it may be possible in some cases for the sender to infer, 576 after some period without receiving NACKs at the end of its 577 transmission that the receiver set has fully received the 578 transmitted content. In particular, if the sender completes its 579 end-of-transmission series of NORM_CMD(FLUSH) messages without 580 receiving repair requests from the group, it may have some 581 assurance that the receiver set has received the content prior to 582 that point. These mechanisms are likely to help FCAST in 583 achieving fully reliable transmissions; 585 It should be noted that the NORM_INFO message header may carry the 586 EXT_FTI extension. The reliable delivery of the NORM_INFO content 587 allows the individual objects' FEC Transmission Information to be 588 provided to the receivers without burdening every packet (i.e. 589 NORM_DATA messages) with this additional, but important, content. 590 Examples are provided in Appendix A. 592 4.9. FCAST Sender Behavior 594 The following operations MAY take place at a sender: 596 1. The user (or another application) selects a set of objects (e.g., 597 files) to deliver and submits them, along with their meta-data, 598 to the FCAST application; 600 2. For each object, FCAST creates the Compound Object and registers 601 this latter in the carousel instance; 603 3. The user then informs FCAST that all the objects of the set have 604 been submitted. If the user knows that no new object will be 605 submitted in the future (i.e., if the session's content is now 606 complete), the user informs FCAST. Finally, the user specifies 607 how many transmission cycles are desired (this number may be 608 infinite); 610 4. At this point, the FCAST application knows the full list of 611 Compound Objects that are part of the Carousel Instance and can 612 create a CIO if desired, possibly with the complete flag set; 614 5. The FCAST application can now define a transmission schedule of 615 these Compound Objects, including the optional CIO. This 616 schedule defines in which order the packets of the various 617 Compound Objects should be sent. This document does not specify 618 any scheme. This is left to the developer within the provisions 619 of the underlying ALC or NORM protocol used and the knowledge of 620 the target use-case. 622 6. The FCAST application then starts the carousel transmission, for 623 the number of cycles specified. Transmissions take place until: 625 * the desired number of transmission cycles has been reached, or 627 * the user wants to prematurely stop the transmissions, or 629 * the user wants to add one or several new objects to the 630 carousel, or on the opposite wants to remove old objects from 631 the carousel. In that case a new carousel instance must be 632 created. 634 7. If the session is not finished, then continue as Set 1 above; 636 4.10. FCAST Receiver Behavior 638 The following operations MAY take place at a receiver: 640 1. The receiver joins the session and collects incoming packets; 642 2. If the header portion of a Compound Object is entirely received 643 (which may happen before receiving the entire object with some 644 ALC/NORM configurations), or if the meta-data is sent by means of 645 another mechanism prior to the object, the receiver processes the 646 meta-data and chooses to continue to receive the object content 647 or not; 649 3. When a Compound Object has been entirely received, the receiver 650 processes the header, retrieves the object meta-data, perhaps 651 decodes the meta-data, and processes the object accordingly; 653 4. When a CIO is received, which is indicated by the 'I' flag set in 654 the Compound Object header, the receiver decodes the CIO, and 655 retrieves the list of objects that are part of the current 656 carousel instance. This list CAN be used to remove objects sent 657 in a previous carousel instance that might not have been totally 658 decoded and that are no longer part of the current carousel 659 instance; 661 5. When a CIO is received, the receiver also retrieves the list of 662 TOI equivalences, if any, and takes appropriate measures, for 663 instance by informing the transport layer; 665 6. When a receiver has received a CIO with the "Complete" flag set, 666 and has successfully received all the objects of the current 667 carousel instance, it can safely exit from the current FCAST 668 session; 670 7. Otherwise continue at Step 2 above. 672 5. FCAST Specifications 674 This section details the technical aspects of FCAST. 676 5.1. Compound Object Header Format 678 In an FCAST session, Compound Objects are constructed by prepending 679 the Compound Object Header (which may include meta-data) before the 680 original object data content (Figure 2). 682 0 1 2 3 683 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 1 684 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 685 |rsv|G|C|MDF|MDE| Header Length | | 686 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 687 | Checksum | | h 688 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d 689 | Object Meta-Data (optional, variable length) | r 690 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 691 | | Padding (optional) | | 692 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 693 | | 694 . Object Data (optional, variable length) . 695 . . 696 . . 698 Figure 2: Compound Object Header with Meta-Data. 700 The Compound Object Header fields are: 702 +------------+------------------------------------------------------+ 703 | Field | Description | 704 +------------+------------------------------------------------------+ 705 | Reserved | 2-bit field set to 0 in this specification and | 706 | | reserved for future use. | 707 | G | 1-bit field that, when set to 1, indicates that the | 708 | | checksum encompasses the whole Compound Object | 709 | | (Global checksum). When set to 0, this field | 710 | | indicates that the checksum encompasses only the | 711 | | Compound Object header. | 712 | C | 1-bit field that, when set to 1, indicates the | 713 | | object is a Carousel Instance Object (CIO). When | 714 | | set to 0, this field indicates that the transported | 715 | | object is a standard object. | 716 | Meta-Data | 2-bit field that defines the format of the object | 717 | Format | meta-data (see Section 7). An HTTP/1.1 | 718 | (MDFmt) | metainformation format [RFC2068] MUST be supported | 719 | | and is associated to value 0. Other formats (e.g., | 720 | | XML) MAY be defined in the future. | 721 | Meta-Data | 2-bit field that defines the optional encoding of | 722 | Encoding | the Object Meta-Data field (see Section 7). By | 723 | (MDEnc) | default, a plain text encoding is used and is | 724 | | associated to value 0. Gzip encoding MUST also be | 725 | | supported and is associated to value 1. Other | 726 | | encodings MAY be defined in the future. | 727 | Header | 24-bit field indicating total length (in bytes) of | 728 | Length | all fields of the Compound Object Header, except the | 729 | | optional padding. A header length field set to | 730 | | value 6 means that there is no meta-data included. | 731 | | When this size is not multiple to 32-bits words and | 732 | | when the Compound Object Header is followed by a non | 733 | | null Compound Object Data, padding MUST be added. | 734 | | It should be noted that the meta-data field maximum | 735 | | size is equal to 2^24 - 6 bytes. | 736 | Checksum | 16-bit field that contains the checksum computed | 737 | | over either the whole Compound Object (when G is set | 738 | | to 1), or over the Compound Object header (when G is | 739 | | set to 0), using the algorithm specified for TCP in | 740 | | RFC793. More precisely, the checksum field is the | 741 | | 16-bit one's complement of the one's complement sum | 742 | | of all 16-bit words to be considered. If a segment | 743 | | contains an odd number of octets to be checksummed, | 744 | | the last octet is padded on the right with zeros to | 745 | | form a 16-bit word for checksum purposes (this pad | 746 | | is not transmitted). While computing the checksum, | 747 | | the checksum field itself is set to zero. | 748 | Object | Optional, variable length field that contains the | 749 | Meta-Data | meta-data associated to the object, either in plain | 750 | | text or encoded, as specified by the MDEnc field. | 751 | | The Meta-Data is NULL-terminated plain text that | 752 | | follows the "TYPE" ":" "VALUE" "" format used | 753 | | in HTTP/1.1 for metainformation [RFC2068]. The | 754 | | various meta-data items can appear in any order. | 755 | | The associated string, when non empty, MUST be | 756 | | NULL-terminated. When no meta-data is communicated, | 757 | | this field MUST be empty and the Header Length MUST | 758 | | be equal to 6. | 759 | Padding | Optional, variable length field of zero-value bytes | 760 | | to align the start of the Object Data to 32-bit | 761 | | boundary. Padding is only used when the header | 762 | | length value, in bytes, is not multiple of 4 and | 763 | | when the Compound Object Header is followed by a non | 764 | | null Compound Object Data. | 765 +------------+------------------------------------------------------+ 767 The Compound Object Header is then followed by the Object Data, i.e., 768 the original object possibly encoded by FCAST. Note that the length 769 of this content is the transported object length (e.g., as specified 770 by the FEC OTI) minus the Header Length and optional padding if any. 772 5.2. Carousel Instance Object Format 774 The format of the CIO, which is a particular Compound Object, is 775 given in Figure 3. 777 0 1 2 3 778 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 1 779 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 780 |rsv|G|1|MDF|MDE| Header Length | | 781 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 782 | Checksum | | h 783 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d 784 | Object Meta-Data (optional, variable length) | r 785 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 786 | | Padding (optional) | | 787 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 788 . . ^ 789 . Object List (variable length) . | 790 . . o 791 . +-+-+-+-+-+-+-+-+ b 792 . | j 793 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 795 Figure 3: Carousel Instance Object Format. 797 Because the CIO is transmitted as a special Compound Object, the 798 following CIO-specific meta-data entries are defined: 800 o Fcast-CIO-Complete: when set to 1, it indicates that no new 801 objects in addition to the ones whose TOI are specified in this 802 CIO, or the ones that have been specified in the previous CIO(s), 803 will be sent in the future. Otherwise it MUST be set to 0. This 804 entry is optional. If absent, a receiver MUST conclude that the 805 session is complete. 807 o Fcast-CIO-ID: this value identifies the carousel instance. It 808 starts from 0 and is incremented by 1 for each new carousel 809 instance. This entry is optional if the FCAST session consists of 810 a single, complete, carousel instance. In all other cases, this 811 entry MUST be defined. In particular, the CIID is used by the TOI 812 equivalence mechanism thanks to which any object is uniquely 813 identified, even if the TOI is updated (e.g., after re-enqueuing 814 the object with NORM). The Fcast-CIO-ID value can also be useful 815 to detect possible gaps in the Carousel Instances, for instance 816 caused by long disconnection periods. Finally, it can also be 817 useful to avoid problems when TOI wrapping to 0 takes place to 818 differentiate the various incarnations of the TOIs if need be. 820 The motivation for making the Fcast-CIO-Complete and Fcast-CIO- 821 Complete entries optional is to simplify the simple case of a session 822 consisting of a single, complete, carousel instance. Indeed, the CIO 823 does not need to contain any meta-data entry then. 825 Additionally, the following standard meta-data entries are often used 826 (Section 4.3): 828 o Content-Length: it specifies the size of the object list, before 829 any content encoding (if any). 831 o Content-Encoding: it specifies the optional encoding of the object 832 list, performed by FCAST. For instance: 833 Content-Encoding: gzip 834 indicates that the Object List field has been encoded with gzip 835 [RFC1952]. If there is no Content-Encoding entry, the receiver 836 MUST assume that the Object List field is plain text (default). 837 The support of gzip encoding, or any other solution, remains 838 optional. 840 An empty Object List is valid and indicates that the current carousel 841 instance does not include any object (Section 4.5). This can be 842 specified by using the following meta-data entry: 843 Content-Length: 0 844 or simply by leaving the Object List empty. In both cases, padding 845 MUST NOT be used and consequently the transported object length 846 (e.g., as specified by the FEC OTI) minus the Header Length equals 847 zero. 849 The non-encoded (i.e., plain text) Object List, when non empty, is a 850 NULL-terminated ASCII string. It can contain two things: 852 o a list of TOI values, and 854 o a list of TOI equivalences; 856 First of all, this string can contain the list of TOIs included in 857 the current carousel instance, specified either as the individual 858 TOIs of each object, or as TOI intervals, or any combination. The 859 format of the ASCII string is a comma-separated list of individual 860 "TOI" values or "TOI_a-TOI_b" elements. This latter case means that 861 all values between TOI_a and TOI_b, inclusive, are part of the list. 862 In that case TOI_a MUST be strictly inferior to TOI_b. If a TOI 863 wrapping to 0 occurs in an interval, then two TOI intervals MUST be 864 specified, TOI_a-MAX_TOI and 0-TOI_b. 866 This string can also contain the TOI equivalences, if any. The 867 format is a comma-separated list of "(" newTOI "=" 1stTOI "/" 1stCIID 868 ")" elements. Each element says that the new TOI, for the current 869 Carousel Instance, is equivalent to (i.e., refers to the same object 870 as) the provided identifier, 1stTOI, for the Carousel Instance of ID 871 1stCIID. 873 The ABNF specification is the following: 874 cio-list = *(list-elem *( "," list-elem)) 875 list-elem = toi-elem / toieq-elem 876 toi-elem = toi-value / toi-interval 877 toi-value = 1*DIGIT 878 toi-interval = toi-value "-" toi-value 879 ; additionally, the first toi-value MUST be 880 ; strictly inferior to the second toi-value 881 toieq-elem = "(" toi-value "=" toi-value "/" ciid-value ")" 882 ciid-value = 1*DIGIT 883 DIGIT = %x30-39 884 ; a digit between O and 9, inclusive 886 For readability purposes, it is RECOMMENDED that all the TOI values 887 in the list be given in increasing order. However a receiver MUST be 888 able to handle non-monotonically increasing values. It is also 889 RECOMMENDED to group the TOI equivalence elements together, at the 890 end of the list, in increasing newTOI order. However a receiver MUST 891 be able to handle lists of mixed TOI and TOI equivalence elements. 892 Specifying a TOI equivalence for a given newTOI relieves the sender 893 from specifying newTOI explicitly in the TOI list. However a 894 receiver MUST be able to handle situations where the same TOI appears 895 both in the TOI value and TOI equivalence lists. Finally, a given 896 TOI value or TOI equivalence item MUST NOT be included multiple times 897 in either list. 899 For instance, the following object list specifies that the current 900 Carousel Instance is composed of 8 objects, and that TOIs 100 to 104 901 are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and 902 refer to the same objects: 904 97,98,99,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 906 or equivalently: 908 97-104,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 910 6. Security Considerations 911 6.1. Problem Statement 913 A content delivery system is potentially subject to attacks. Attacks 914 may target: 916 o the network (to compromise the routing infrastructure, e.g., by 917 creating congestion), 919 o the Content Delivery Protocol (CDP) (e.g., to compromise the 920 normal behavior of FCAST), or 922 o the content itself (e.g., to corrupt the objects being 923 transmitted). 925 These attacks can be launched either: 927 o against the data flow itself (e.g., by sending forged packets), 929 o against the session control parameters (e.g., by corrupting the 930 session description, the CIO, the object meta-data, or the ALC/LCT 931 control parameters), that are sent either in-band or out-of-band, 932 or 934 o against some associated building blocks (e.g., the congestion 935 control component). 937 In the following sections we provide more details on these possible 938 attacks and sketch some possible counter-measures. We finally 939 provide recommendations in Section 6.5. 941 6.2. Attacks Against the Data Flow 943 Let us consider attacks against the data flow first. At least, the 944 following types of attacks exist: 946 o attacks that are meant to give access to a confidential object 947 (e.g., in case of a non-free content) and 949 o attacks that try to corrupt the object being transmitted (e.g., to 950 inject malicious code within an object, or to prevent a receiver 951 from using an object, which is a kind of Denial of Service (DoS)). 953 6.2.1. Access to Confidential Objects 955 Access control to the object being transmitted is typically provided 956 by means of encryption. This encryption can be done over the whole 957 object (e.g., by the content provider, before submitting the object 958 to FCAST), or be done on a packet per packet basis (e.g., when IPSec/ 959 ESP is used [RFC4303], see Section 6.5). If confidentiality is a 960 concern, it is RECOMMENDED that one of these solutions be used. 962 6.2.2. Object Corruption 964 Protection against corruptions (e.g., if an attacker sends forged 965 packets) is achieved by means of a content integrity verification/ 966 sender authentication scheme. This service can be provided at the 967 object level, but in that case a receiver has no way to identify 968 which symbol(s) is(are) corrupted if the object is detected as 969 corrupted. This service can also be provided at the packet level. 970 In this case, after removing all corrupted packets, the file may be 971 in some cases recovered. Several techniques can provide this content 972 integrity/sender authentication service: 974 o at the object level, the object can be digitally signed, for 975 instance by using RSASSA-PKCS1-v1_5 [RFC3447]. This signature 976 enables a receiver to check the object integrity, once this latter 977 has been fully decoded. Even if digital signatures are 978 computationally expensive, this calculation occurs only once per 979 object, which is usually acceptable; 981 o at the packet level, each packet can be digitally signed 982 [RMT-SIMPLE-AUTH]. A major limitation is the high computational 983 and transmission overheads that this solution requires. To avoid 984 this problem, the signature may span a set of packets (instead of 985 a single one) in order to amortize the signature calculation. But 986 if a single packets is missing, the integrity of the whole set 987 cannot be checked; 989 o at the packet level, a Group Message Authentication Code (MAC) 990 [RFC2104][RMT-SIMPLE-AUTH] scheme can be used, for instance by 991 using HMAC-SHA-256 with a secret key shared by all the group 992 members, senders and receivers. This technique creates a 993 cryptographically secured digest of a packet that is sent along 994 with the packet. The Group MAC scheme does not create prohibitive 995 processing load nor transmission overhead, but it has a major 996 limitation: it only provides a group authentication/integrity 997 service since all group members share the same secret group key, 998 which means that each member can send a forged packet. It is 999 therefore restricted to situations where group members are fully 1000 trusted (or in association with another technique as a pre-check); 1002 o at the packet level, Timed Efficient Stream Loss-Tolerant 1003 Authentication (TESLA) [RFC4082][MSEC-TESLA] is an attractive 1004 solution that is robust to losses, provides a true authentication/ 1005 integrity service, and does not create any prohibitive processing 1006 load or transmission overhead. Yet checking a packet requires a 1007 small delay (a second or more) after its reception; 1009 o at the packet level, IPsec/ESP [RFC4303] can be used to check the 1010 integrity and authenticate the sender of all the packets being 1011 exchanged in a session (see Section 6.5). 1013 Techniques relying on public key cryptography (digital signatures and 1014 TESLA during the bootstrap process, when used) require that public 1015 keys be securely associated to the entities. This can be achieved by 1016 a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by 1017 pre-distributing securely the public keys of each group member. 1019 Techniques relying on symmetric key cryptography (Group MAC) require 1020 that a secret key be shared by all group members. This can be 1021 achieved by means of a group key management protocol, or simply by 1022 pre-distributing securely the secret key (but this manual solution 1023 has many limitations). 1025 It is up to the developer and deployer, who know the security 1026 requirements and features of the target application area, to define 1027 which solution is the most appropriate. In any case, whenever there 1028 is any concern of the threat of file corruption, it is RECOMMENDED 1029 that at least one of these techniques be used. 1031 6.3. Attacks Against the Session Control Parameters and Associated 1032 Building Blocks 1034 Let us now consider attacks against the session control parameters 1035 and the associated building blocks. The attacker has at least the 1036 following opportunities to launch an attack: 1038 o the attack can target the session description, 1040 o the attack can target the FCAST CIO, 1042 o the attack can target the meta-data of an object, 1044 o the attack can target the ALC/LCT parameters, carried within the 1045 LCT header or 1047 o the attack can target the FCAST associated building blocks, for 1048 instance the multiple rate congestion control protocol. 1050 The consequences of these attacks are potentially serious, since they 1051 can compromise the behavior of content delivery system or even 1052 compromise the network itself. 1054 6.3.1. Attacks Against the Session Description 1056 An FCAST receiver may potentially obtain an incorrect Session 1057 Description for the session. The consequence of this is that 1058 legitimate receivers with the wrong Session Description are unable to 1059 correctly receive the session content, or that receivers 1060 inadvertently try to receive at a much higher rate than they are 1061 capable of, thereby possibly disrupting other traffic in the network. 1063 To avoid these problems, it is RECOMMENDED that measures be taken to 1064 prevent receivers from accepting incorrect Session Descriptions. One 1065 such measure is the sender authentication to ensure that receivers 1066 only accept legitimate Session Descriptions from authorized senders. 1067 How these measures are achieved is outside the scope of this document 1068 since this session description is usually carried out-of-band. 1070 6.3.2. Attacks Against the FCAST CIO 1072 Corrupting the FCAST CIO is one way to create a Denial of Service 1073 attack. For example, the attacker can set the "Complete" flag to 1074 make the receivers believe that no further modification will be done. 1076 It is therefore RECOMMENDED that measures be taken to guarantee the 1077 integrity and to check the sender's identity of the CIO. To that 1078 purpose, one of the counter-measures mentioned above (Section 6.2.2) 1079 SHOULD be used. These measures will either be applied on a packet 1080 level, or globally over the whole CIO object. When there is no 1081 packet level integrity verification scheme, it is RECOMMENDED to 1082 digitally sign the CIO. 1084 6.3.3. Attacks Against the Object Meta-Data 1086 Corrupting the object meta-data is another way to create a Denial of 1087 Service attack. For example, the attacker changes the MD5 sum 1088 associated to a file. This possibly leads a receiver to reject the 1089 files received, no matter whether the files have been correctly 1090 received or not. When the meta-data are appended to the object, 1091 corrupting the meta-data means that the Compound Object will be 1092 corrupted. 1094 It is therefore RECOMMENDED that measures be taken to guarantee the 1095 integrity and to check the sender's identity of the Compound Object. 1096 To that purpose, one of the counter-measures mentioned above 1097 (Section 6.2.2) SHOULD be used. These measures will either be 1098 applied on a packet level, or globally over the whole Compound 1099 Object. When there is no packet level integrity verification scheme, 1100 it is RECOMMENDED to digitally sign the Compound Object. 1102 6.3.4. Attacks Against the ALC/LCT and NORM Parameters 1104 By corrupting the ALC/LCT header (or header extensions) one can 1105 execute attacks on the underlying ALC/LCT implementation. For 1106 example, sending forged ALC packets with the Close Session flag (A) 1107 set to one can lead the receiver to prematurely close the session. 1108 Similarly, sending forged ALC packets with the Close Object flag (B) 1109 set to one can lead the receiver to prematurely give up the reception 1110 of an object. The same comments can be made for NORM. 1112 It is therefore RECOMMENDED that measures be taken to guarantee the 1113 integrity and to check the sender's identity of each ALC or NORM 1114 packet received. To that purpose, one of the counter-measures 1115 mentioned above (Section 6.2.2) SHOULD be used. 1117 6.3.5. Attacks Against the Associated Building Blocks 1119 Let us first focus on the congestion control building block that may 1120 be used in an ALC or NORM session. A receiver with an incorrect or 1121 corrupted implementation of the multiple rate congestion control 1122 building block may affect the health of the network in the path 1123 between the sender and the receiver. That may also affect the 1124 reception rates of other receivers who joined the session. 1126 When congestion control is applied with FCAST, it is therefore 1127 RECOMMENDED that receivers be required to identify themselves as 1128 legitimate before they receive the Session Description needed to join 1129 the session. If authenticating a receiver does not prevent this 1130 latter to launch an attack, it will enable the network operator to 1131 identify him and to take counter-measures. This authentication can 1132 be made either toward the network operator or the session sender (or 1133 a representative of the sender) in case of NORM. The details of how 1134 it is done are outside the scope of this document. 1136 When congestion control is applied with FCAST, it is also RECOMMENDED 1137 that a packet level authentication scheme be used, as explained in 1138 Section 6.2.2. Some of them, like TESLA, only provide a delayed 1139 authentication service, whereas congestion control requires a rapid 1140 reaction. It is therefore RECOMMENDED [RMT-PI-ALC] that a receiver 1141 using TESLA quickly reduces its subscription level when the receiver 1142 believes that a congestion did occur, even if the packet has not yet 1143 been authenticated. Therefore TESLA will not prevent DoS attacks 1144 where an attacker makes the receiver believe that a congestion 1145 occurred. This is an issue for the receiver, but this will not 1146 compromise the network. Other authentication methods that do not 1147 feature this delayed authentication could be preferred, or a group 1148 MAC scheme could be used in parallel to TESLA to prevent attacks 1149 launched from outside of the group. 1151 6.4. Other Security Considerations 1153 Lastly, we note that the security considerations that apply to, and 1154 are described in, ALC [RMT-PI-ALC], LCT [RFC5651], NORM [RFC5740] and 1155 FEC [RFC5052] also apply to FCAST as FCAST builds on those 1156 specifications. In addition, any security considerations that apply 1157 to any congestion control building block used in conjunction with 1158 FCAST also applies to FCAST. Finally, the security discussion of 1159 [RMT-SEC] also applies here. 1161 6.5. Minimum Security Recommendations 1163 We now introduce a mandatory to implement but not necessarily to use 1164 security configuration, in the sense of [RFC3365]. Since FCAST 1165 relies on ALC/LCT, it inherits the "baseline secure ALC operation" of 1166 [RMT-PI-ALC]. More precisely, security is achieved by means of 1167 IPsec/ESP in transport mode. [RFC4303] explains that ESP can be used 1168 to potentially provide confidentiality, data origin authentication, 1169 content integrity, anti-replay and (limited) traffic flow 1170 confidentiality. [RMT-PI-ALC] specifies that the data origin 1171 authentication, content integrity and anti-replay services SHALL be 1172 used, and that the confidentiality service is RECOMMENDED. If a 1173 short lived session MAY rely on manual keying, it is also RECOMMENDED 1174 that an automated key management scheme be used, especially in case 1175 of long lived sessions. 1177 Therefore, the RECOMMENDED solution for FCAST provides per-packet 1178 security, with data origin authentication, integrity verification and 1179 anti-replay. This is sufficient to prevent most of the in-band 1180 attacks listed above. If confidentiality is required, a per-packet 1181 encryption SHOULD also be used. 1183 7. IANA Considerations 1185 This document requires a IANA registration for the following 1186 attributes: 1188 Object meta-data format (MDFmt): All implementations MUST support 1189 format 0 (default). 1191 +----------------------------------------+-------------+ 1192 | format name | Value | 1193 +----------------------------------------+-------------+ 1194 | as per HTTP/1.1 metainformation format | 0 (default) | 1195 +----------------------------------------+-------------+ 1197 Object Meta-Data Encoding (MDENC): All implementations MUST support 1198 value 0 (plain-text, default) and value 1 (gzip). 1200 +------------+-------------+ 1201 | Name | Value | 1202 +------------+-------------+ 1203 | plain text | 0 (default) | 1204 | gzip | 1 | 1205 +------------+-------------+ 1207 8. Acknowledgments 1209 The authors are grateful to the authors of [ALC-00] for specifying 1210 the first version of FCAST/ALC. The authors are also grateful to 1211 Gorry Fairhurst for his valuable comments. 1213 9. References 1215 9.1. Normative References 1217 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1218 Requirement Levels", BCP 14, RFC 2119, March 1997. 1220 [RFC5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding 1221 Transport (LCT) Building Block", RFC 5651, October 2009. 1223 [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, 1224 "NACK-Oriented Reliable Multicast (NORM) Transport 1225 Protocol", RFC 5740, November 2009. 1227 [RMT-PI-ALC] 1228 Luby, M., Watson, M., and L. Vicisano, "Asynchronous 1229 Layered Coding (ALC) Protocol Instantiation", Work 1230 in Progress, November 2009. 1232 9.2. Informative References 1234 [ALC-00] Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J., and B. 1235 Lueckenhoff, "Asynchronous Layered Coding: a Scalable 1236 Reliable Multicast Protocol", March 2000. 1238 [RMT-FLUTE] 1239 Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1240 "FLUTE - File Delivery over Unidirectional Transport", 1241 Work in Progress, December 2009. 1243 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 1244 Engineering Task Force Standard Protocols", BCP 61, 1245 RFC 3365, August 2002. 1247 [RMT-SEC] Roca, V., Adamson, B., and H. Asaeda, "Security and 1248 Reliable Multicast Transport Protocols: Discussions and 1249 Guidelines", Work in progress, 1250 draft-ietf-rmt-sec-discussion-04.txt, July 2009. 1252 [RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. 1253 Randers-Pehrson, "GZIP file format specification version 1254 4.3", RFC 1952, May 1996. 1256 [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. 1257 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", 1258 RFC 2068, January 1997. 1260 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1261 Hashing for Message Authentication", RFC 2104, 1262 February 1997. 1264 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 1265 Standards (PKCS) #1: RSA Cryptography Specifications 1266 Version 2.1", RFC 3447, February 2003. 1268 [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. 1269 Briscoe, "Timed Efficient Stream Loss-Tolerant 1270 Authentication (TESLA): Multicast Source Authentication 1271 Transform Introduction", RFC 4082, June 2005. 1273 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", 1274 RFC 4303, December 2005. 1276 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 1277 Correction (FEC) Building Block", RFC 5052, August 2007. 1279 [RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, 1280 "Reed-Solomon Forward Error Correction (FEC) Schemes", 1281 RFC 5510, April 2009. 1283 [MSEC-TESLA] 1284 Roca, V., Francillon, A., and S. Faurite, "Use of TESLA in 1285 the ALC and NORM Protocols", 1286 draft-ietf-msec-tesla-for-alc-norm-10.txt (work in 1287 progress), October 2009. 1289 [RMT-SIMPLE-AUTH] 1290 Roca, V., "Simple Authentication Schemes for the ALC and 1291 NORM Protocols", 1292 draft-ietf-rmt-simple-auth-for-alc-norm-02.txt (work in 1293 progress), October 2009. 1295 Appendix A. FCAST Examples 1297 A.1. Basic Examples 1298 0 1 2 3 1299 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 1300 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1301 | 0 |1|0| 0 | 0 | 39 | 1302 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1303 | Checksum | | 1304 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1305 . . 1306 . meta-data ASCII null terminated string (33 bytes) . 1307 . . 1308 + +-+-+-+-+-+-+-+-+ 1309 | | Padding | 1310 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1311 . . 1312 . Object data . 1313 . . 1314 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1316 Figure 4: Compound Object Example. 1318 Figure 4 shows a regular Compound Object where the meta-data ASCII 1319 string, in HTTP/1.1 meta-information format (MDFmt=0) contains: 1321 Content-Location: example.txt 1323 This string is 33 bytes long, including the NULL-termination 1324 character. There is no gzip encoding of the meta-data (MDEnc=0) and 1325 there is no Content-Length information either since this length can 1326 easily be calculated by the receiver as the FEC OTI transfer length 1327 minus the header length. Finally, the checksum encompasses the whole 1328 Compound Object (G=1). 1330 0 1 2 3 1331 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 1332 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1333 | 0 |1|0| 0 | 0 | 6 | 1334 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1335 | Checksum | Padding | 1336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1337 . . 1338 . Object data . 1339 . . 1340 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1342 Figure 5: Compound Object Example with no Meta-Data. 1344 Figure 5 shows a Compound Object without any meta-data. The fact 1345 there is no meta-data is indicated by the value 6 of the Header 1346 Length field. 1348 Figure 6 shows an example CIO object, in the case of a static FCAST 1349 session, i.e., a session where the set of objects is set once and for 1350 all. 1351 0 1 2 3 1352 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 1353 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1354 | 0 |1| 0 | 0 | 4 | 1355 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1356 . . 1357 . Object List string . 1358 . . 1359 . +-+-+-+-+-+-+-+-+ 1360 . | 1361 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1363 Figure 6: Example of CIO, 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 CIO. 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). 1390 0 1 2 3 1391 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 1392 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1393 |version| type=1| hdr_len = 7 | sequence | 1394 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1395 | source_id | 1396 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1397 | instance_id | grtt |backoff| gsize | 1398 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1399 | flags | fec_id = 5 | object_transport_id | 1400 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1401 | HET = 64 | HEL = 3 | | 1402 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1403 | Transfer Length (L) | 1404 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1405 | Encoding Symbol Length (E) | MaxBlkLen (B) | max_n | 1406 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1407 | 0 |0|0| 0 | 0 | 39 | 1408 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1409 | Checksum | | 1410 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1411 . . 1412 . meta-data ASCII null terminated string (33 bytes) . 1413 . . 1414 + +-+-+-+-+-+-+-+-+ 1415 | | 1416 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1418 Figure 7: NORM_INFO containing FCAST Compound Object Header 1420 The NORM_INFO message shown in Figure 7 contains the EXT_FTI header 1421 extension to carry the FEC OTI. In this example, the FEC OTI format 1422 is that of the Reed-Solomon FEC coding scheme for fec_id = 5 as 1423 described in [RFC5510]. Other alternatives for providing the FEC OTI 1424 would have been to either include it directly in the meta-data of the 1425 FCAST Compound Header, or to include an EXT_FTI header extension to 1426 all NORM_DATA packets (or a subset of them). Note that the NORM 1427 "Transfer_Length" is the total length of the associated FCAST 1428 Compound Object. 1430 The FCAST Compound Object in this example does contain the same meta- 1431 data and is formatted as in the example of Figure 4. With the 1432 combination of the FEC_OTI and the FCAST meta-data, the NORM protocol 1433 and FCAST application have all of the information needed to reliably 1434 receive and process the associated object. Indeed, the NORM protocol 1435 provides rapid (NORM_INFO has precedence over the associated object 1436 content), reliable delivery of the NORM_INFO message and its payload, 1437 the FCAST Compound Object Header. 1439 Authors' Addresses 1441 Vincent Roca 1442 INRIA 1443 655, av. de l'Europe 1444 Inovallee; Montbonnot 1445 ST ISMIER cedex 38334 1446 France 1448 Email: vincent.roca@inria.fr 1449 URI: http://planete.inrialpes.fr/people/roca/ 1451 Brian Adamson 1452 Naval Research Laboratory 1453 Washington, DC 20375 1454 USA 1456 Email: adamson@itd.nrl.navy.mil 1457 URI: http://cs.itd.nrl.navy.mil