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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: Standards Track B. Adamson 5 Expires: April 21, 2012 Naval Research Laboratory 6 October 19, 2011 8 FCAST: Scalable Object Delivery for the ALC and NORM Protocols 9 draft-ietf-rmt-fcast-05 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 21, 2012. 35 Copyright Notice 37 Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4 54 1.2. Definitions, Notations and Abbreviations . . . . . . . . . 5 55 2. FCAST Data Formats . . . . . . . . . . . . . . . . . . . . . . 6 56 2.1. Compound Object Format . . . . . . . . . . . . . . . . . . 6 57 2.2. Carousel Instance Descriptor Format . . . . . . . . . . . 8 58 3. FCAST Principles . . . . . . . . . . . . . . . . . . . . . . . 11 59 3.1. FCAST Content Delivery Service . . . . . . . . . . . . . . 11 60 3.2. Meta-Data Transmission . . . . . . . . . . . . . . . . . . 12 61 3.3. Meta-Data Content . . . . . . . . . . . . . . . . . . . . 13 62 3.4. Carousel Transmission . . . . . . . . . . . . . . . . . . 14 63 3.5. Carousel Instance Descriptor Special Object . . . . . . . 14 64 3.6. Compound Object Identification . . . . . . . . . . . . . . 15 65 3.7. FCAST Sender Behavior . . . . . . . . . . . . . . . . . . 17 66 3.8. FCAST Receiver Behavior . . . . . . . . . . . . . . . . . 18 67 4. Security Considerations . . . . . . . . . . . . . . . . . . . 18 68 4.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 18 69 4.2. Attacks Against the Data Flow . . . . . . . . . . . . . . 19 70 4.2.1. Access to Confidential Objects . . . . . . . . . . . . 19 71 4.2.2. Object Corruption . . . . . . . . . . . . . . . . . . 19 72 4.3. Attacks Against the Session Control Parameters and 73 Associated Building Blocks . . . . . . . . . . . . . . . . 21 74 4.3.1. Attacks Against the Session Description . . . . . . . 21 75 4.3.2. Attacks Against the FCAST CID . . . . . . . . . . . . 22 76 4.3.3. Attacks Against the Object Meta-Data . . . . . . . . . 22 77 4.3.4. Attacks Against the ALC/LCT and NORM Parameters . . . 22 78 4.3.5. Attacks Against the Associated Building Blocks . . . . 23 79 4.4. Other Security Considerations . . . . . . . . . . . . . . 23 80 4.5. Minimum Security Recommendations . . . . . . . . . . . . . 24 81 5. Requirements for Compliant Implementations . . . . . . . . . . 24 82 5.1. Requirements Related to the Object Meta-Data . . . . . . . 24 83 5.2. Requirements Related to the Carousel Instance 84 Descriptor (CID) Mechanism . . . . . . . . . . . . . . . . 26 85 6. Operational Considerations . . . . . . . . . . . . . . . . . . 26 86 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 87 7.1. Namespace declaration for Object Meta-Data Format . . . . 27 88 7.1.1. Object Meta-Data Format registration . . . . . . . . . 27 89 7.2. Namespace declaration for Object Meta-Data Encoding . . . 28 90 7.2.1. Object Meta-Data Encoding registration . . . . . . . . 28 91 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 92 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 93 9.1. Normative References . . . . . . . . . . . . . . . . . . . 28 94 9.2. Informative References . . . . . . . . . . . . . . . . . . 29 95 Appendix A. FCAST Examples . . . . . . . . . . . . . . . . . . . 30 96 A.1. Regular Compound Object Example . . . . . . . . . . . . . 30 97 A.2. Carousel Instance Descriptor Example . . . . . . . . . . . 31 99 Appendix B. Additional Meta-Data Transmission Mechanisms . . . . 32 100 B.1. Supporting Additional Mechanisms . . . . . . . . . . . . . 32 101 B.2. Using NORM_INFO Messages with FCAST/NORM . . . . . . . . . 33 102 B.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 33 103 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35 105 1. Introduction 107 This document introduces the FCAST reliable and scalable object 108 (e.g., file) delivery application. Two variants of FCAST exist: 110 o FCAST/ALC that relies on the Asynchronous Layer Coding (ALC) 111 [RFC5775] and the Layered Coding Transport (LCT) [RFC5651] 112 reliable multicast transport protocol, and 114 o FCAST/NORM that relies on the NACK-Oriented Reliable Multicast 115 (NORM) [RFC5740] reliable multicast transport protocol. 117 Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM. 118 FCAST is not a new protocol specification per se. Instead it is a 119 set of data format specifications and instructions on how to use ALC 120 and NORM to implement a file-casting service. 122 A design goal behind FCAST is to define a streamlined solution, in 123 order to enable lightweight implementations of the protocol stack, 124 and limit the operational processing and storage requirements. A 125 consequence of this choice is that FCAST cannot be considered as a 126 versatile application, capable of addressing all the possible use- 127 cases. On the contrary, FCAST has some intrinsic limitations. From 128 this point of view it differs from FLUTE [RMT-FLUTE] which favors 129 flexibility at the expense of some additional complexity. 131 A good example of the design choices meant to favor simplicity is the 132 way FCAST manages the object meta-data: by default, the meta-data and 133 the object content are sent together, in a compound object. This 134 solution has many advantages in terms of simplicity as will be 135 described later on. However this solution has an intrinsic 136 limitation since it does not enable a receiver to decide in advance, 137 before beginning the reception of the compound object, whether the 138 object is of interest or not, based on the information that may be 139 provided in the meta-data. Therefore this document discusses 140 additional techniques that may be used to mitigate this limitation. 141 When use-cases require that each receiver download the whole set of 142 objects sent in the session (e.g., with mirroring tools), this 143 limitation is not considered a problem. 145 1.1. Requirements notation 147 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 148 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 149 document are to be interpreted as described in [RFC2119]. 151 1.2. Definitions, Notations and Abbreviations 153 This document uses the following definitions: 155 FCAST/ALC: denotes the FCAST application running on top of the ALC/ 156 LCT reliable transport protocol; 158 FCAST/NORM: denotes the FCAST application running on top of the NORM 159 reliable transport protocol; 161 FCAST: denotes either FCAST/ALC or FCAST/NORM; 163 Compound Object: denotes an ALC or NORM transport object composed of 164 the Compound Object Header (Section 2.1), including any meta- 165 data, and the content of the original application object 166 (e.g., a file); 168 Carousel: denotes the process of sending Compound Objects 169 implemented by a FCAST sender; 171 Carousel Instance: denotes a fixed set of registered Compound 172 Objects that are sent by the carousel during a certain number 173 of cycles. Whenever Compound Objects need to be added or 174 removed, a new Carousel Instance is defined; 176 Carousel Instance Descriptor (CID): denotes a special object that 177 lists the Compound Objects that comprise a given Carousel 178 Instance; 180 Carousel Instance IDentifier (CIID): numeric value that identifies a 181 Carousel Instance. 183 Carousel Cycle: denotes a transmission round within which all the 184 Compound Objects registered in the Carousel Instance are 185 transmitted a certain number of times. By default, Compound 186 Objects are transmitted once per cycle, but higher values are 187 possible, that might differ on a per-object basis; 189 Transport Object Identifier (TOI): denotes the numeric identifier 190 associated to a specific object by the underlying transport 191 protocol. In the case of ALC, this corresponds to the TOI 192 described in [RFC5651]. In the case of NORM, this corresponds 193 to the NormTransportId described in [RFC5740]. 195 FEC Object Transmission Information (FEC OTI): FEC information 196 associated with an object and that is essential for the FEC 197 decoder to decode a specific object. 199 2. FCAST Data Formats 201 This section details the various data formats used by FCAST. 203 2.1. Compound Object Format 205 In an FCAST session, Compound Objects are constructed by prepending 206 the Compound Object Header (which contains the meta-data of the 207 object) before the original object data content (see Section 3.2). 208 Figure 1 illustrates the associated Compound Object header format. 210 0 1 2 3 211 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 1 212 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 213 |Ver| Resvd |G|C| MDFmt | MDEnc | Checksum | | 214 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 215 | Compound Object Header Length | h 216 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| d 217 | Object Meta-Data (variable length) | r 218 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 219 | | Padding (optional) | | 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 221 | | 222 . Object Data (optional, variable length) . 223 . . 224 . . 226 Figure 1: Compound Object Format. 228 The Compound Object Header fields are: 230 +------------+------------------------------------------------------+ 231 | Field | Description | 232 +------------+------------------------------------------------------+ 233 | Version | 2-bit field that MUST be set to 0 in this | 234 | | specification and indicates the protocol version | 235 | | number. | 236 | Reserved | 4-bit field that MUST be set to 0 in this | 237 | | specification and is reserved for future use. | 238 | | Receivers MUST ignore this field. | 239 | G | 1-bit field that, when set to 1, indicates that the | 240 | | checksum encompasses the whole Compound Object | 241 | | (Global checksum). When set to 0, this field | 242 | | indicates that the checksum encompasses only the | 243 | | Compound Object header. | 244 | C | 1-bit field that, when set to 1, indicates the | 245 | | object is a Carousel Instance Descriptor (CID). | 246 | | When set to 0, this field indicates that the | 247 | | transported object is a standard object. | 248 | Meta-Data | 4-bit field that defines the format of the object | 249 | Format | meta-data (see Section 7). An HTTP/1.1 | 250 | (MDFmt) | metainformation format [RFC2616] MUST be supported | 251 | | and is associated to value 0. Other formats (e.g., | 252 | | XML) MAY be defined in the future. | 253 | Meta-Data | 4-bit field that defines the optional encoding of | 254 | Encoding | the Object Meta-Data field (see Section 7). By | 255 | (MDEnc) | default, a plain text encoding is used and is | 256 | | associated to value 0. GZIP encoding MUST also be | 257 | | supported and is associated to value 1. Other | 258 | | encodings MAY be defined in the future. | 259 | Checksum | 16-bit field that contains the checksum computed | 260 | | over either the whole Compound Object (when G is set | 261 | | to 1), or over the Compound Object header (when G is | 262 | | set to 0), using the Internet checksum algorithm | 263 | | specified in [RFC1071]. More precisely, the | 264 | | checksum field is the 16-bit one's complement of the | 265 | | one's complement sum of all 16-bit words to be | 266 | | considered. If a segment contains an odd number of | 267 | | octets to be checksummed, the last octet is padded | 268 | | on the right with zeros to form a 16-bit word for | 269 | | checksum purposes (this pad is not transmitted). | 270 | | While computing the checksum, the checksum field | 271 | | itself is set to zero. | 272 | Compound | 32-bit field indicating total length (in bytes) of | 273 | Object | all fields of the Compound Object Header, except the | 274 | Header | optional padding. A header length field set to | 275 | Length | value 8 means that there is no meta-data included. | 276 | | When this size is not multiple to 32-bits words and | 277 | | when the Compound Object Header is followed by a non | 278 | | null Compound Object Data, padding MUST be added. | 279 | | It should be noted that the meta-data field maximum | 280 | | size is equal to (2^32 - 8) bytes. | 281 | Object | Variable length field that contains the meta-data | 282 | Meta-Data | associated to the object. The format and encoding | 283 | | of this field are defined respectively by the MDFmt | 284 | | and MDEnc fields. With the default HTTP/1.1 format | 285 | | and plain text encoding, the Meta-Data is | 286 | | NULL-terminated plain text that follows the "TYPE" | 287 | | ":" "VALUE" "" format used in HTTP/1.1 for | 288 | | metainformation [RFC2616]. The various meta-data | 289 | | items can appear in any order. The associated | 290 | | string, when non empty, MUST be NULL-terminated. | 291 | | When no meta-data is communicated, this field MUST | 292 | | be empty and the Compound Object Header Length MUST | 293 | | be equal to 8. | 294 | Padding | Optional, variable length field of zero-value bytes | 295 | | to align the start of the Object Data to 32-bit | 296 | | boundary. Padding is only used when the Compound | 297 | | Object Header Length value, in bytes, is not | 298 | | multiple of 4 and when the Compound Object Header is | 299 | | followed by non null Compound Object Data. | 300 +------------+------------------------------------------------------+ 302 The Compound Object Header is then followed by the Object Data, i.e., 303 the original object possibly encoded by FCAST. Note that the length 304 of this content is the transported object length (e.g., as specified 305 by the FEC OTI) minus the Compound Object Header Length and optional 306 padding if any. 308 2.2. Carousel Instance Descriptor Format 310 In an FCAST session, a Carousel Instance Descriptor (CID) MAY be sent 311 in order to carry the list of Compound Objects that are part of a 312 given Carousel Instance (see Section 3.5). The format of the CID, 313 that is sent as a special Compound Object, is given in Figure 2. 314 Being a special case of Compound Object, this format is in line with 315 the format described in Section 2.1. 317 0 1 2 3 318 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 1 319 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ 320 |Ver| Resvd |G|C| MDFmt | MDEnc | Checksum | | 321 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 322 | Compound Object Header Length | h 323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| d 324 | Object Meta-Data (variable length) | r 325 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 326 | | Padding (optional) | | 327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 328 . . ^ 329 . Object List (variable length) . | 330 . . o 331 . +-+-+-+-+-+-+-+-+ b 332 . | j 333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v 335 Figure 2: Carousel Instance Descriptor Format. 337 Because the CID is transmitted as a special Compound Object, the 338 following CID-specific meta-data entries are defined: 340 o Fcast-CID-Complete: when set to 1, it indicates that no new object 341 in addition to the ones whose TOI are specified in this CID, or 342 the ones that have been specified in the previous CID(s), will be 343 sent in the future. Otherwise it MUST be set to 0. This entry is 344 optional. If absent, a receiver MUST conclude that the session is 345 complete. 347 o Fcast-CID-ID: this entry contains the Carousel Instance 348 IDentifier, or CIID. It starts from 0 and is incremented by 1 for 349 each new carousel instance. This entry is optional if the FCAST 350 session consists of a single, complete, carousel instance. In all 351 other cases, this entry MUST be defined. In particular, the CIID 352 is used by the TOI equivalence mechanism thanks to which any 353 object is uniquely identified, even if the TOI is updated (e.g., 354 after re-enqueuing the object with NORM). The Fcast-CID-ID value 355 can also be useful to detect possible gaps in the Carousel 356 Instances, for instance caused by long disconnection periods. 357 Finally, it can also be useful to avoid problems when TOI wrapping 358 to 0 takes place to differentiate the various incarnations of the 359 TOIs if need be. 361 The motivation for making the Fcast-CID-Complete and Fcast-CID-ID 362 entries optional is to simplify the simple case of a session 363 consisting of a single, complete, carousel instance, with an Object 364 List given in plain text, without any content encoding. In that 365 case, the CID does not need to contain any meta-data entry. 367 Additionally, the following standard meta-data entries are often used 368 (Section 3.3): 370 o Content-Length: it specifies the size of the object list, before 371 any content encoding (if any). 373 o Content-Encoding: it specifies the optional encoding of the object 374 list, performed by FCAST. For instance: 375 Content-Encoding: gzip 376 indicates that the Object List field has been encoded with GZIP 377 [RFC1952]. If there is no Content-Encoding entry, the receiver 378 MUST assume that the Object List field is plain text (default). 379 The support of GZIP encoding, or any other solution, remains 380 optional. 382 An empty Object List is valid and indicates that the current carousel 383 instance does not include any object (Section 3.5). This can be 384 specified by using the following meta-data entry: 385 Content-Length: 0 386 or simply by leaving the Object List empty. In both cases, padding 387 MUST NOT be used and consequently the transported object length 388 (e.g., as specified by the FEC OTI) minus the Compound Object Header 389 Length equals zero. 391 The non-encoded (i.e., plain text) Object List, when non empty, is a 392 NULL-terminated ASCII string. It can contain two things: 394 o a list of TOI values, and 396 o a list of TOI equivalences; 398 First of all, this string can contain the list of TOIs included in 399 the current carousel instance, specified either as the individual 400 TOIs of each object, or as TOI intervals, or any combination. The 401 format of the ASCII string is a comma-separated list of individual 402 "TOI" values or "TOI_a-TOI_b" elements. This latter case means that 403 all values between TOI_a and TOI_b, inclusive, are part of the list. 404 In that case TOI_a MUST be strictly inferior to TOI_b. If a TOI 405 wrapping to 0 occurs in an interval, then two TOI intervals MUST be 406 specified, TOI_a-MAX_TOI and 0-TOI_b. 408 This string can also contain the TOI equivalences, if any. The 409 format is a comma-separated list of "(" newTOI "=" 1stTOI "/" 1stCIID 410 ")" elements. Each element says that the new TOI, for the current 411 Carousel Instance, is equivalent to (i.e., refers to the same object 412 as) the provided identifier, 1stTOI, for the Carousel Instance of ID 413 1stCIID. 415 The ABNF specification is the following: 416 cid-list = *(list-elem *( "," list-elem)) 417 list-elem = toi-elem / toieq-elem 418 toi-elem = toi-value / toi-interval 419 toi-value = 1*DIGIT 420 toi-interval = toi-value "-" toi-value 421 ; additionally, the first toi-value MUST be 422 ; strictly inferior to the second toi-value 423 toieq-elem = "(" toi-value "=" toi-value "/" ciid-value ")" 424 ciid-value = 1*DIGIT 425 DIGIT = %x30-39 426 ; a digit between O and 9, inclusive 428 For readability purposes, it is RECOMMENDED that all the TOI values 429 in the list be given in increasing order. However a receiver MUST be 430 able to handle non-monotonically increasing values. It is also 431 RECOMMENDED to group the TOI equivalence elements together, at the 432 end of the list, in increasing newTOI order. However a receiver MUST 433 be able to handle lists of mixed TOI and TOI equivalence elements. 434 Specifying a TOI equivalence for a given newTOI relieves the sender 435 from specifying newTOI explicitly in the TOI list. However a 436 receiver MUST be able to handle situations where the same TOI appears 437 both in the TOI value and TOI equivalence lists. Finally, a given 438 TOI value or TOI equivalence item MUST NOT be included multiple times 439 in either list. 441 For instance, the following object list specifies that the current 442 Carousel Instance is composed of 8 objects, and that TOIs 100 to 104 443 are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and 444 refer to the same objects: 446 97,98,99,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 448 or equivalently: 450 97-104,(100=10/2),(101=11/2),(102=12/2),(103=13/2),(104=14/2) 452 3. FCAST Principles 454 This section details the principles of FCAST. 456 3.1. FCAST Content Delivery Service 458 The basic goal of FCAST is to transmit objects to a group of 459 receivers in a reliable way, where the receiver set may be restricted 460 to a single receiver or may include possibly a very large number of 461 receivers. FCAST supports two forms of operation: 463 1. FCAST/ALC, where the FCAST application works on top of the ALC/ 464 LCT reliable multicast transport protocol, without any feedback 465 from the receivers, and 467 2. FCAST/NORM, where the FCAST application works on top of the NORM 468 reliable multicast transport protocol, that requires positive/ 469 negative acknowledgements from the receivers. 471 This specification is designed such that both forms of operation 472 share as much commonality as possible. Section 6 discusses some 473 operational aspects and the content delivery service that is provided 474 by FCAST for a given use-case. 476 3.2. Meta-Data Transmission 478 FCAST carries meta-data elements by prepending them to the object 479 they refer to. As a result, a Compound Object is created that is 480 composed of a header followed by the original object data. This 481 header is itself composed of the meta-data as well as several fields, 482 for instance to indicate the boundaries between the various parts of 483 this Compound Object (Figure 3). 485 <------------------------ Compound Object -----------------------> 486 +-------------------------+--------------------------------------+ 487 | Compound Object Header | Object Data | 488 | (can include meta-data) | (can be encoded by FCAST) | 489 +-------------------------+--------------------------------------+ 491 Figure 3: Compound Object composition. 493 Attaching the meta-data to the object is an efficient solution, since 494 it guaranties that meta-data are received along with the associated 495 object, and it allows the transport of the meta-data to benefit from 496 any transport-layer erasure protection of the Compound Object (e.g., 497 using FEC encoding and/or NACK-based repair). However a limit of 498 this scheme is that a client does not know the meta-data of an object 499 before beginning its reception, and in case of erasures affecting the 500 meta-data, not until the object decoding is completed. The details 501 of course depend upon the transport protocol and the FEC code used. 503 Appendix B describes extensions that provide additional means to 504 carry meta-data, e.g., to communicate meta-data ahead of time. 506 3.3. Meta-Data Content 508 A compliant FCAST implementation MUST support at least the following 509 items: 511 o Content-Location: the URI of the object, which gives the name and 512 location of the object; 514 o Content-Type: a string that contains the MIME type of the object; 516 o Content-Length: an unsigned 64-bit integer that contains the size 517 of the initial object, before any content encoding (if any) and 518 without considering the Compound Object header. Note that the use 519 of certain FEC schemes MAY further limit the maximum value of the 520 object; 522 o Content-Encoding: a string that contains the optional encoding of 523 the object performed by FCAST. If there is no Content-Encoding 524 entry, the receiver MUST assume that the object is not encoded 525 (default). The support of GZIP encoding, or any other solution, 526 remains optional. 528 o Content-MD5: a binary content coded as "base64" that contains the 529 MD5 message digest of the object in order to check its integrity. 530 This digest is meant to protect from transmission and processing 531 errors, not from deliberate attacks by an intelligent attacker 532 (see Section 4). This digest only protects the object, not the 533 header, and therefore not the meta-data. A separate checksum is 534 provided to that purpose (Section 2.1); 536 This list is not limited and new meta-data information can be added. 537 For instance, when dealing with very large objects (e.g., that 538 largely exceed the working memory of a receiver), it can be 539 interesting to split this object into several sub-objects (or 540 slices). When this happens, the meta-data associated to each sub- 541 object MUST include the following entries: 543 o Fcast-Obj-Slice-Nb: an unsigned 32-bit integer that contains the 544 total number of slices. A value greater than 1 indicates that 545 this object is the result of a split of the original object; 547 o Fcast-Obj-Slice-Idx: an unsigned 32-bit integer that contains the 548 slice index (in the {0 .. SliceNb - 1} interval); 550 o Fcast-Obj-Slice-Offset: an unsigned 64-bit integer that contains 551 the offset at which this slice starts within the original object; 553 3.4. Carousel Transmission 555 A set of FCAST Compound Objects scheduled for transmission are 556 considered a logical "Carousel". A given "Carousel Instance" is 557 comprised of a fixed set of Compound Objects. Whenever the FCAST 558 application needs to add new Compound Objects to, or remove old 559 Compound Objects from the transmission set, a new Carousel Instance 560 is defined since the set of Compound Objects changes. Because of the 561 native object multiplexing capability of both ALC and NORM, sender 562 and receiver(s) are both capable to multiplex and demultiplex FCAST 563 Compound Objects. 565 For a given Carousel Instance, one or more transmission cycles are 566 possible. During each cycle, all of the Compound Objects comprising 567 the Carousel are sent. By default, each object is transmitted once 568 per cycle. However, in order to allow different levels of priority, 569 some objects MAY be transmitted more often that others during a 570 cycle, and/or benefit from higher FEC protection than others. This 571 can be the case for instance for the CID objects (Section 3.5). For 572 some FCAST usage (e.g., a unidirectional "push" mode), a Carousel 573 Instance may be associated to a single transmission cycle. In other 574 cases it may be associated to a large number of transmission cycles 575 (e.g., in "on-demand" mode, where objects are made available for 576 download during a long period of time). 578 3.5. Carousel Instance Descriptor Special Object 580 The FCAST sender CAN transmit an OPTIONAL Carousel Instance 581 Descriptor (CID). The CID carries the list of the Compound Objects 582 that are part of a given Carousel Instance, by specifying their 583 respective Transmission Object Identifiers (TOI). However the CID 584 does not describe the objects themselves (i.e., there is no meta- 585 data). Additionally, the CID MAY include a "Complete" flag that is 586 used to indicate that no further modification to the enclosed list 587 will be done in the future. Finally, the CID MAY include a Carousel 588 Instance ID that identifies the Carousel Instance it pertains to. 589 These aspects are discussed in Section 2.2. 591 There is no reserved TOI value for the CID Compound Object itself, 592 since this special object is regarded by ALC/LCT or NORM as a 593 standard object. On the contrary, the nature of this object (CID) is 594 indicated by means of a specific Compound Object header field (the 595 "I" flag) so that it can be recognized and processed by the FCAST 596 application as needed. A direct consequence is the following: since 597 a receiver does not know in advance which TOI will be used for the 598 following CID (in case of a dynamic session), he MUST NOT filter out 599 packets that are not in the current CID's TOI list. Said 600 differently, the goal of CID is not to setup ALC or NORM packet 601 filters (this mechanism would not be secure in any case). 603 The use of a CID remains OPTIONAL. If it is not used, then the 604 clients progressively learn what files are part of the carousel 605 instance by receiving ALC or NORM packets with new TOIs. However 606 using a CID has several benefits: 608 o When the "Complete" flag is set (if ever), the receivers know when 609 they can leave the session, i.e., when they have received all the 610 objects that are part of the last carousel instance of this 611 delivery session; 613 o In case of a session with a dynamic set of objects, the sender can 614 reliably inform the receivers that some objects have been removed 615 from the carousel with the CID. This solution is more robust than 616 the "Close Object flag (B)" of ALC/LCT since a client with an 617 intermittent connectivity might loose all the packets containing 618 this B flag. And while NORM provides a robust object cancellation 619 mechanism in the form of its NORM_CMD(SQUELCH) message in response 620 to receiver NACK repair requests, the use of the CID provides an 621 additional means for receivers to learn of objects for which it is 622 futile to request repair; 624 o The TOI equivalence (Section 3.6) can be signaled with the CID. 625 This is often preferable to the alternative solution where the 626 equivalence is identified by examining the object meta-data, 627 especially in case of erasures. 629 During idle periods, when the carousel instance does not contain any 630 object, a CID with an empty TOI list MAY be transmitted. In that 631 case, a new carousel instance ID MUST be used to differentiate this 632 (empty) carousel instance from the other ones. This mechanism can be 633 useful to inform the receivers that: 635 o all the previously sent objects have been removed from the 636 carousel. It therefore improves the FCAST robustness even during 637 "idle" period; 639 o the session is still active even if there is currently no content 640 being sent. Said differently, it can be used as a heartbeat 641 mechanism. If the "Complete" flag has not been set, it implicitly 642 informs the receivers that new objects MAY be sent in the future; 644 3.6. Compound Object Identification 646 The FCAST Compound Objects are directly associated with the object- 647 based transport service that the ALC and NORM protocols provide. In 648 each of these protocols, the messages containing transport object 649 content are labeled with a numeric transport object identifier (i.e., 650 the ALC TOI and the NORM NormTransportId). For the purposes of this 651 document, this identifier in either case (ALC or NORM) is referred to 652 as the TOI. 654 There are several differences between ALC and NORM: 656 o the ALC use of TOI is rather flexible, since several TOI field 657 sizes are possible (from 16 to 112 bits), since this size can be 658 changed at any time, on a per-packet basis, and since the TOI 659 management is totally free as long as each object is associated to 660 a unique TOI (if no wraparound happened); 662 o the NORM use of TOI is more directive, since the TOI field is 16 663 bit long and since TOIs MUST be managed sequentially; 665 In both NORM and ALC, it is possible that the transport 666 identification space may eventually wrap for long-lived sessions 667 (especially with NORM where this phenomenon is expected to happen 668 more frequently). This can possibly introduce some ambiguity in 669 FCAST object identification if a sender retains some older objects in 670 newer Carousel Instances with updated object sets. To avoid 671 ambiguity the active TOIs (i.e., the TOIs corresponding to objects 672 being transmitted) can only occupy half of the TOI sequence space. 673 If an old object, whose TOI has fallen outside the current window, 674 needs to be transmitted again, a new TOI must be used for it. In 675 case of NORM, this constraint limits to 32768 the maximum number of 676 objects that can be part of any carousel instance. In order to allow 677 receivers to properly combine the transport packets with a newly- 678 assigned TOI to those of associated to the previously-assigned TOI, a 679 mechanism is required to equate the objects with the new and the old 680 TOIs. 682 The preferred mechanism consists in signaling, within the CID, that 683 the newly assigned TOI, for the current Carousel Instance, is 684 equivalent to the TOI used within a previous Carousel Instance. By 685 convention, the reference tuple for any object is the {TOI; CI ID} 686 tuple used for its first transmission within a Carousel Instance. 687 This tuple MUST be used whenever a TOI equivalence is provided. 689 An alternative solution, when meta-data can be processed rapidly 690 (e.g., by using NORM-INFO messages), consists for the receiver in 691 identifying that both objects are the same, after examining the meta- 692 data. The receiver can then take appropriate measures. 694 3.7. FCAST Sender Behavior 696 The following operations MAY take place at a sender: 698 1. The user (or another application) selects a set of objects (e.g., 699 files) to deliver and submits them, along with their meta-data, 700 to the FCAST application; 702 2. For each object, FCAST creates the Compound Object and registers 703 this latter in the carousel instance; 705 3. The user then informs FCAST that all the objects of the set have 706 been submitted. If the user knows that no new object will be 707 submitted in the future (i.e., if the session's content is now 708 complete), the user informs FCAST. Finally, the user specifies 709 how many transmission cycles are desired (this number may be 710 infinite); 712 4. At this point, the FCAST application knows the full list of 713 Compound Objects that are part of the Carousel Instance and can 714 create a CID if desired, possibly with the complete flag set; 716 5. The FCAST application can now define a transmission schedule of 717 these Compound Objects, including the optional CID. This 718 schedule defines in which order the packets of the various 719 Compound Objects should be sent. This document does not specify 720 any scheme. This is left to the developer within the provisions 721 of the underlying ALC or NORM protocol used and the knowledge of 722 the target use-case. 724 6. The FCAST application then starts the carousel transmission, for 725 the number of cycles specified. Transmissions take place until: 727 * the desired number of transmission cycles has been reached, or 729 * the user wants to prematurely stop the transmissions, or 731 * the user wants to add one or several new objects to the 732 carousel, or on the contrary wants to remove old objects from 733 the carousel. In that case a new carousel instance must be 734 created. 736 7. If the session is not finished, then continue at Step 1 above; 738 3.8. FCAST Receiver Behavior 740 The following operations MAY take place at a receiver: 742 1. The receiver joins the session and collects incoming packets; 744 2. If the header portion of a Compound Object is entirely received 745 (which may happen before receiving the entire object with some 746 ALC/NORM configurations), or if the meta-data is sent by means of 747 another mechanism prior to the object, the receiver processes the 748 meta-data and chooses to continue to receive the object content 749 or not; 751 3. When a Compound Object has been entirely received, the receiver 752 processes the header, retrieves the object meta-data, perhaps 753 decodes the meta-data, and processes the object accordingly; 755 4. When a CID is received, which is indicated by the 'C' flag set in 756 the Compound Object header, the receiver decodes the CID, and 757 retrieves the list of objects that are part of the current 758 carousel instance. This list CAN be used to remove objects sent 759 in a previous carousel instance that might not have been totally 760 decoded and that are no longer part of the current carousel 761 instance; 763 5. When a CID is received, the receiver also retrieves the list of 764 TOI equivalences, if any, and takes appropriate measures, for 765 instance by informing the transport layer; 767 6. When a receiver has received a CID with the "Complete" flag set, 768 and has successfully received all the objects of the current 769 carousel instance, it can safely exit from the current FCAST 770 session; 772 7. Otherwise continue at Step 2 above. 774 4. Security Considerations 776 4.1. Problem Statement 778 A content delivery system is potentially subject to attacks. Attacks 779 may target: 781 o the network (to compromise the routing infrastructure, e.g., by 782 creating congestion), 784 o the Content Delivery Protocol (CDP) (e.g., to compromise the 785 normal behavior of FCAST), or 787 o the content itself (e.g., to corrupt the objects being 788 transmitted). 790 These attacks can be launched either: 792 o against the data flow itself (e.g., by sending forged packets), 794 o against the session control parameters (e.g., by corrupting the 795 session description, the CID, the object meta-data, or the ALC/LCT 796 control parameters), that are sent either in-band or out-of-band, 797 or 799 o against some associated building blocks (e.g., the congestion 800 control component). 802 In the following sections we provide more details on these possible 803 attacks and sketch some possible counter-measures. We finally 804 provide recommendations in Section 4.5. 806 4.2. Attacks Against the Data Flow 808 Let us consider attacks against the data flow first. At least, the 809 following types of attacks exist: 811 o attacks that are meant to give access to a confidential object 812 (e.g., in case of a non-free content) and 814 o attacks that try to corrupt the object being transmitted (e.g., to 815 inject malicious code within an object, or to prevent a receiver 816 from using an object, which is a kind of Denial of Service (DoS)). 818 4.2.1. Access to Confidential Objects 820 Access control to the object being transmitted is typically provided 821 by means of encryption. This encryption can be done over the whole 822 object (e.g., by the content provider, before submitting the object 823 to FCAST), or be done on a packet per packet basis (e.g., when IPsec/ 824 ESP is used [RFC4303], see Section 4.5). If confidentiality is a 825 concern, it is RECOMMENDED that one of these solutions be used. 827 4.2.2. Object Corruption 829 Protection against corruptions (e.g., if an attacker sends forged 830 packets) is achieved by means of a content integrity verification/ 831 sender authentication scheme. This service can be provided at the 832 object level, but in that case a receiver has no way to identify 833 which symbol(s) is(are) corrupted if the object is detected as 834 corrupted. This service can also be provided at the packet level. 835 In this case, after removing all corrupted packets, the file may be 836 in some cases recovered. Several techniques can provide this content 837 integrity/sender authentication service: 839 o at the object level, the object can be digitally signed, for 840 instance by using RSASSA-PKCS1-v1_5 [RFC3447]. This signature 841 enables a receiver to check the object integrity, once this latter 842 has been fully decoded. Even if digital signatures are 843 computationally expensive, this calculation occurs only once per 844 object, which is usually acceptable; 846 o at the packet level, each packet can be digitally signed 847 [RMT-SIMPLE-AUTH]. A major limitation is the high computational 848 and transmission overheads that this solution requires. To avoid 849 this problem, the signature may span a set of packets (instead of 850 a single one) in order to amortize the signature calculation. But 851 if a single packets is missing, the integrity of the whole set 852 cannot be checked; 854 o at the packet level, a Group Message Authentication Code (MAC) 855 [RFC2104][RMT-SIMPLE-AUTH] scheme can be used, for instance by 856 using HMAC-SHA-256 with a secret key shared by all the group 857 members, senders and receivers. This technique creates a 858 cryptographically secured digest of a packet that is sent along 859 with the packet. The Group MAC scheme does not create prohibitive 860 processing load nor transmission overhead, but it has a major 861 limitation: it only provides a group authentication/integrity 862 service since all group members share the same secret group key, 863 which means that each member can send a forged packet. It is 864 therefore restricted to situations where group members are fully 865 trusted (or in association with another technique as a pre-check); 867 o at the packet level, Timed Efficient Stream Loss-Tolerant 868 Authentication (TESLA) [RFC4082][RFC5776] is an attractive 869 solution that is robust to losses, provides a true authentication/ 870 integrity service, and does not create any prohibitive processing 871 load or transmission overhead. Yet checking a packet requires a 872 small delay (a second or more) after its reception; 874 o at the packet level, IPsec/ESP [RFC4303] can be used to check the 875 integrity and authenticate the sender of all the packets being 876 exchanged in a session (see Section 4.5). 878 Techniques relying on public key cryptography (digital signatures and 879 TESLA during the bootstrap process, when used) require that public 880 keys be securely associated to the entities. This can be achieved by 881 a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by 882 pre-distributing securely the public keys of each group member. 884 Techniques relying on symmetric key cryptography (Group MAC) require 885 that a secret key be shared by all group members. This can be 886 achieved by means of a group key management protocol, or simply by 887 pre-distributing securely the secret key (but this manual solution 888 has many limitations). 890 It is up to the developer and deployer, who know the security 891 requirements and features of the target application area, to define 892 which solution is the most appropriate. In any case, whenever there 893 is any concern of the threat of file corruption, it is RECOMMENDED 894 that at least one of these techniques be used. 896 4.3. Attacks Against the Session Control Parameters and Associated 897 Building Blocks 899 Let us now consider attacks against the session control parameters 900 and the associated building blocks. The attacker has at least the 901 following opportunities to launch an attack: 903 o the attack can target the session description, 905 o the attack can target the FCAST CID, 907 o the attack can target the meta-data of an object, 909 o the attack can target the ALC/LCT parameters, carried within the 910 LCT header or 912 o the attack can target the FCAST associated building blocks, for 913 instance the multiple rate congestion control protocol. 915 The consequences of these attacks are potentially serious, since they 916 can compromise the behavior of content delivery system or even 917 compromise the network itself. 919 4.3.1. Attacks Against the Session Description 921 An FCAST receiver may potentially obtain an incorrect Session 922 Description for the session. The consequence of this is that 923 legitimate receivers with the wrong Session Description are unable to 924 correctly receive the session content, or that receivers 925 inadvertently try to receive at a much higher rate than they are 926 capable of, thereby possibly disrupting other traffic in the network. 928 To avoid these problems, it is RECOMMENDED that measures be taken to 929 prevent receivers from accepting incorrect Session Descriptions. One 930 such measure is the sender authentication to ensure that receivers 931 only accept legitimate Session Descriptions from authorized senders. 932 How these measures are achieved is outside the scope of this document 933 since this session description is usually carried out-of-band. 935 4.3.2. Attacks Against the FCAST CID 937 Corrupting the FCAST CID is one way to create a Denial of Service 938 attack. For example, the attacker can set the "Complete" flag to 939 make the receivers believe that no further modification will be done. 941 It is therefore RECOMMENDED that measures be taken to guarantee the 942 integrity and to check the sender's identity of the CID. To that 943 purpose, one of the counter-measures mentioned above (Section 4.2.2) 944 SHOULD be used. These measures will either be applied on a packet 945 level, or globally over the whole CID object. When there is no 946 packet level integrity verification scheme, it is RECOMMENDED to 947 digitally sign the CID. 949 4.3.3. Attacks Against the Object Meta-Data 951 Corrupting the object meta-data is another way to create a Denial of 952 Service attack. For example, the attacker changes the MD5 sum 953 associated to a file. This possibly leads a receiver to reject the 954 files received, no matter whether the files have been correctly 955 received or not. When the meta-data are appended to the object, 956 corrupting the meta-data means that the Compound Object will be 957 corrupted. 959 It is therefore RECOMMENDED that measures be taken to guarantee the 960 integrity and to check the sender's identity of the Compound Object. 961 To that purpose, one of the counter-measures mentioned above 962 (Section 4.2.2) SHOULD be used. These measures will either be 963 applied on a packet level, or globally over the whole Compound 964 Object. When there is no packet level integrity verification scheme, 965 it is RECOMMENDED to digitally sign the Compound Object. 967 4.3.4. Attacks Against the ALC/LCT and NORM Parameters 969 By corrupting the ALC/LCT header (or header extensions) one can 970 execute attacks on the underlying ALC/LCT implementation. For 971 example, sending forged ALC packets with the Close Session flag (A) 972 set to one can lead the receiver to prematurely close the session. 973 Similarly, sending forged ALC packets with the Close Object flag (B) 974 set to one can lead the receiver to prematurely give up the reception 975 of an object. The same comments can be made for NORM. 977 It is therefore RECOMMENDED that measures be taken to guarantee the 978 integrity and to check the sender's identity of each ALC or NORM 979 packet received. To that purpose, one of the counter-measures 980 mentioned above (Section 4.2.2) SHOULD be used. 982 4.3.5. Attacks Against the Associated Building Blocks 984 Let us first focus on the congestion control building block that may 985 be used in an ALC or NORM session. A receiver with an incorrect or 986 corrupted implementation of the multiple rate congestion control 987 building block may affect the health of the network in the path 988 between the sender and the receiver. That may also affect the 989 reception rates of other receivers who joined the session. 991 When congestion control is applied with FCAST, it is therefore 992 RECOMMENDED that receivers be required to identify themselves as 993 legitimate before they receive the Session Description needed to join 994 the session. If authenticating a receiver does not prevent this 995 latter to launch an attack, it will enable the network operator to 996 identify him and to take counter-measures. This authentication can 997 be made either toward the network operator or the session sender (or 998 a representative of the sender) in case of NORM. The details of how 999 it is done are outside the scope of this document. 1001 When congestion control is applied with FCAST, it is also RECOMMENDED 1002 that a packet level authentication scheme be used, as explained in 1003 Section 4.2.2. Some of them, like TESLA, only provide a delayed 1004 authentication service, whereas congestion control requires a rapid 1005 reaction. It is therefore RECOMMENDED [RFC5775] that a receiver 1006 using TESLA quickly reduces its subscription level when the receiver 1007 believes that a congestion did occur, even if the packet has not yet 1008 been authenticated. Therefore TESLA will not prevent DoS attacks 1009 where an attacker makes the receiver believe that a congestion 1010 occurred. This is an issue for the receiver, but this will not 1011 compromise the network. Other authentication methods that do not 1012 feature this delayed authentication could be preferred, or a group 1013 MAC scheme could be used in parallel to TESLA to prevent attacks 1014 launched from outside of the group. 1016 4.4. Other Security Considerations 1018 Lastly, we note that the security considerations that apply to, and 1019 are described in, ALC [RFC5775], LCT [RFC5651], NORM [RFC5740] and 1020 FEC [RFC5052] also apply to FCAST as FCAST builds on those 1021 specifications. In addition, any security considerations that apply 1022 to any congestion control building block used in conjunction with 1023 FCAST also applies to FCAST. Finally, the security discussion of 1024 [RMT-SEC] also applies here. 1026 4.5. Minimum Security Recommendations 1028 We now introduce a mandatory to implement but not necessarily to use 1029 security configuration, in the sense of [RFC3365]. Since FCAST/ALC 1030 relies on ALC/LCT, it inherits the "baseline secure ALC operation" of 1031 [RFC5775]. Similarly, since FCAST/NORM relies on NORM, it inherits 1032 the "baseline secure NORM operation" of [RFC5740]. More precisely, 1033 in both cases security is achieved by means of IPsec/ESP in transport 1034 mode. [RFC4303] explains that ESP can be used to potentially provide 1035 confidentiality, data origin authentication, content integrity, anti- 1036 replay and (limited) traffic flow confidentiality. [RFC5775] 1037 specifies that the data origin authentication, content integrity and 1038 anti-replay services SHALL be used, and that the confidentiality 1039 service is RECOMMENDED. If a short lived session MAY rely on manual 1040 keying, it is also RECOMMENDED that an automated key management 1041 scheme be used, especially in case of long lived sessions. 1043 Therefore, the RECOMMENDED solution for FCAST provides per-packet 1044 security, with data origin authentication, integrity verification and 1045 anti-replay. This is sufficient to prevent most of the in-band 1046 attacks listed above. If confidentiality is required, a per-packet 1047 encryption SHOULD also be used. 1049 5. Requirements for Compliant Implementations 1051 This section lists the features that any compliant FCAST/ALC or 1052 FCAST/NORM implementation MUST support, and those that remain 1053 OPTIONAL, e.g., in order to enable some optimizations for a given 1054 use-case, at a receiver. 1056 5.1. Requirements Related to the Object Meta-Data 1058 Meta-data transmission mechanisms: 1060 +-----------------------+-------------------------------------------+ 1061 | Feature | Status | 1062 +-----------------------+-------------------------------------------+ 1063 | meta-data | An FCAST sender MUST send meta-data with | 1064 | transmission using | the in-band mechanism provided by FCAST, | 1065 | FCAST's in-band | i.e., within the Compound Object header. | 1066 | mechanism | All the FCAST receivers MUST be able to | 1067 | | process meta-data sent with this FCAST | 1068 | | in-band mechanism. | 1069 | meta-data | In addition to the FCAST in-band | 1070 | transmission using | transmission of meta-data, an FCAST | 1071 | other mechanisms | sender MAY send a subset or all of the | 1072 | | meta-data using another mechanism. | 1073 | | Supporting this mechanism in a compliant | 1074 | | FCAST receiver is OPTIONAL, and its use | 1075 | | is OPTIONAL too. An FCAST receiver MAY | 1076 | | support this mechanism and take advantage | 1077 | | of the meta-data sent in this way. If it | 1078 | | is not the case, the FCAST receiver will | 1079 | | anyway receive and process meta-data sent | 1080 | | in-bound. See Annex Appendix B. | 1081 +-----------------------+-------------------------------------------+ 1083 Meta-data format and encoding: 1085 +-----------------------+-------------------------------------------+ 1086 | Feature | Status | 1087 +-----------------------+-------------------------------------------+ 1088 | Meta-Data Format | All FCAST implementations MUST support an | 1089 | (MDFmt field) | HTTP/1.1 metainformation format | 1090 | | [RFC2616]. Other formats (e.g., XML) MAY | 1091 | | be defined in the future. | 1092 | Meta-Data Encoding | All FCAST implementations MUST support | 1093 | (MDEnc field) | both a plain text and a GZIP encoding | 1094 | | [RFC1952] of the Object Meta-Data field. | 1095 | | Other encodings MAY be defined in the | 1096 | | future. | 1097 +-----------------------+-------------------------------------------+ 1099 Meta-data items (Section 3.3): 1101 +------------------------+---------------------+ 1102 | Feature | Status | 1103 +------------------------+---------------------+ 1104 | Content-Location | MUST be supported | 1105 | Content-Type | MUST be supported | 1106 | Content-Length | MUST be supported | 1107 | Content-Encoding | MUST be supported | 1108 | Content-MD5 | MUST be supported | 1109 | Fcast-Obj-Slice-Nb | SHOULD be supported | 1110 | Fcast-Obj-Slice-Idx | SHOULD be supported | 1111 | Fcast-Obj-Slice-Offset | SHOULD be supported | 1112 +------------------------+---------------------+ 1114 5.2. Requirements Related to the Carousel Instance Descriptor (CID) 1115 Mechanism 1117 Any compliant FCAST implementation MUST support the CID mechanism, in 1118 order to list the Compound Objects that are part of a given Carousel 1119 Instance. However its use is OPTIONAL. 1121 6. Operational Considerations 1123 FCAST is compatible with any congestion control protocol designed for 1124 ALC/LCT or NORM. However, depending on the use-case, the data flow 1125 generated by the FCAST application might not be constant, but instead 1126 be bursty in nature. Similarly, depending on the use-case, an FCAST 1127 session might be very short. Whether and how this will impact the 1128 congestion control protocol is out of the scope of the present 1129 document. 1131 FCAST is compatible with any security mechanism designed for ALC/LCT 1132 or NORM. The use of a security scheme is strongly RECOMMENDED (see 1133 Section 4). 1135 FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM. 1136 Whether FEC is used or not, and the kind of FEC scheme used, is to 1137 some extent transparent to FCAST. 1139 FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST 1140 specification has any implication on the source or destination IP 1141 address type. 1143 The delivery service provided by FCAST might be fully reliable, or 1144 only partially reliable depending upon: 1146 o the way ALC or NORM is used (e.g., whether FEC encoding and/or 1147 NACK-based repair requests are used or not), 1149 o the way the FCAST carousel is used (e.g., whether the objects are 1150 made available for a long time span or not), and 1152 o the way in which FCAST itself is employed (e.g., whether there is 1153 a session control application that might automatically extend an 1154 existing FCAST session until all receivers have received the 1155 transmitted content). 1157 The receiver set MAY be restricted to a single receiver or MAY 1158 include possibly a very large number of receivers. While the choice 1159 of the underlying transport protocol (i.e., ALC or NORM) and its 1160 parameters may limit the practical receiver group size, nothing in 1161 FCAST itself limits it. For instance, if FCAST/ALC is used on top of 1162 purely unidirectional transport channels, with no feedback 1163 information at all, which is the default mode of operation, then the 1164 scalability is maximum since neither FCAST, nor ALC, UDP or IP 1165 generates any feedback message. On the contrary, the FCAST/NORM 1166 scalability is typically limited by NORM scalability itself. 1167 Similarly, if FCAST is used along with a session control application 1168 that collects reception information from the receivers, then this 1169 session control application may limit the scalability of the global 1170 object delivery system. This situation can of course be mitigated by 1171 using a hierarchy of feedback message aggregators or servers. The 1172 details of this are out of the scope of the present document. 1174 The content of a carousel instance MAY be described by means of an 1175 OPTIONAL Carousel Instance Descriptor (CID) (Section 3.5). The 1176 decisions of whether a CID should be used or not, how often and when 1177 a CID should be sent, are left to the sender and depend on many 1178 parameters, including the target use case and the session dynamics. 1179 For instance it may be appropriate to send a CID at the beginning of 1180 each new carousel instance, and then periodically. These operational 1181 aspects are out of the scope of the present document. 1183 7. IANA Considerations 1185 7.1. Namespace declaration for Object Meta-Data Format 1187 This document requires a IANA registration for the following name- 1188 space: "Object Meta-Data Format" (MDFmt). Values in this namespace 1189 are 4-bit positive integers between 0 and 15 inclusive and they 1190 define the format of the object meta-data ((see Section 2.1). 1192 Initial values for the LCT Header Extension Type registry are defined 1193 in Section 7.1.1. Future assignments are to be made through Expert 1194 Review [RFC5226]. 1196 7.1.1. Object Meta-Data Format registration 1198 This document registers one value in the "Object Meta-Data Format" 1199 namespace as follows: 1201 +----------------------------------------+-------------+ 1202 | format name | Value | 1203 +----------------------------------------+-------------+ 1204 | as per HTTP/1.1 metainformation format | 0 (default) | 1205 +----------------------------------------+-------------+ 1207 7.2. Namespace declaration for Object Meta-Data Encoding 1209 This document requires a IANA registration for the following name- 1210 space: "Object Meta-Data Encoding" (MDEnc). Values in this namespace 1211 are 4-bit positive integers between 0 and 15 inclusive and they 1212 define the optional encoding of the Object Meta-Data field (see 1213 Section 2.1). 1215 Initial values for the LCT Header Extension Type registry are defined 1216 in Section 7.2.1. Future assignments are to be made through Expert 1217 Review [RFC5226]. 1219 7.2.1. Object Meta-Data Encoding registration 1221 This document registers two values in the "Object Meta-Data Encoding" 1222 namespace as follows: 1224 +------------+-------------+ 1225 | Name | Value | 1226 +------------+-------------+ 1227 | plain text | 0 (default) | 1228 | GZIP | 1 | 1229 +------------+-------------+ 1231 8. Acknowledgments 1233 The authors are grateful to the authors of [ALC-00] for specifying 1234 the first version of FCAST/ALC. The authors are also grateful to 1235 David Harrington, Gorry Fairhurst and Lorenzo Vicisano for their 1236 valuable comments. 1238 9. References 1240 9.1. Normative References 1242 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1243 Requirement Levels", BCP 14, RFC 2119, March 1997. 1245 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1246 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1247 May 2008. 1249 [RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer, 1250 "Computing the Internet checksum", RFC 1071, 1251 September 1988. 1253 [RFC5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding 1254 Transport (LCT) Building Block", RFC 5651, October 2009. 1256 [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, 1257 "NACK-Oriented Reliable Multicast (NORM) Transport 1258 Protocol", RFC 5740, November 2009. 1260 [RFC5775] Luby, M., Watson, M., and L. Vicisano, "Asynchronous 1261 Layered Coding (ALC) Protocol Instantiation", RFC 5775, 1262 April 2010. 1264 [RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. 1265 Randers-Pehrson, "GZIP file format specification version 1266 4.3", RFC 1952, May 1996. 1268 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1269 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1270 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1272 9.2. Informative References 1274 [ALC-00] Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J., and B. 1275 Lueckenhoff, "Asynchronous Layered Coding: a Scalable 1276 Reliable Multicast Protocol", March 2000. 1278 [RMT-FLUTE] 1279 Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen, 1280 "FLUTE - File Delivery over Unidirectional Transport", 1281 Work in Progress, February 2011. 1283 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 1284 Engineering Task Force Standard Protocols", BCP 61, 1285 RFC 3365, August 2002. 1287 [RMT-SEC] Roca, V., Adamson, B., and H. Asaeda, "Security and 1288 Reliable Multicast Transport Protocols: Discussions and 1289 Guidelines", Work in progress, 1290 draft-ietf-rmt-sec-discussion-06.txt, March 2011. 1292 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1293 Hashing for Message Authentication", RFC 2104, 1294 February 1997. 1296 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 1297 Standards (PKCS) #1: RSA Cryptography Specifications 1298 Version 2.1", RFC 3447, February 2003. 1300 [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. 1302 Briscoe, "Timed Efficient Stream Loss-Tolerant 1303 Authentication (TESLA): Multicast Source Authentication 1304 Transform Introduction", RFC 4082, June 2005. 1306 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", 1307 RFC 4303, December 2005. 1309 [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error 1310 Correction (FEC) Building Block", RFC 5052, August 2007. 1312 [RFC5510] Lacan, J., Roca, V., Peltotalo, J., and S. Peltotalo, 1313 "Reed-Solomon Forward Error Correction (FEC) Schemes", 1314 RFC 5510, April 2009. 1316 [RFC5776] Roca, V., Francillon, A., and S. Faurite, "Use of Timed 1317 Efficient Stream Loss-Tolerant Authentication (TESLA) in 1318 the Asynchronous Layered Coding (ALC) and NACK-Oriented 1319 Reliable Multicast (NORM) Protocols", RFC 5776, 1320 April 2010. 1322 [RMT-SIMPLE-AUTH] 1323 Roca, V., "Simple Authentication Schemes for the ALC and 1324 NORM Protocols", Work in 1325 progress draft-ietf-rmt-simple-auth-for-alc-norm-05.txt, 1326 September 2011. 1328 Appendix A. FCAST Examples 1330 A.1. Regular Compound Object Example 1331 0 1 2 3 1332 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 1333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1334 | 0 | 0 |1|0| 0 | 0 | Checksum | 1335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1336 | 41 | 1337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 1338 . . 1339 . meta-data ASCII null terminated string (33 bytes) . 1340 . . 1341 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1342 | | Padding | 1343 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1344 . . 1345 . Object data . 1346 . . 1347 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1349 Figure 4: Compound Object Example. 1351 Figure 4 shows a regular Compound Object where the meta-data ASCII 1352 string, in HTTP/1.1 meta-information format (MDFmt=0) contains: 1354 Content-Location: example.txt 1356 This string is 33 bytes long, including the NULL-termination 1357 character. There is no GZIP encoding of the meta-data (MDEnc=0) and 1358 there is no Content-Length information either since this length can 1359 easily be calculated by the receiver as the FEC OTI transfer length 1360 minus the header length. Finally, the checksum encompasses the whole 1361 Compound Object (G=1). 1363 A.2. Carousel Instance Descriptor Example 1365 Figure 5 shows an example CID object, in the case of a static FCAST 1366 session, i.e., a session where the set of objects is set once and for 1367 all. There is no meta-data in this example since Fcast-CID-Complete 1368 and Fcast-CID-ID are both implicit. 1370 0 1 2 3 1371 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 1372 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1373 | 0 | 0 |1|1| 0 | 0 | Checksum | 1374 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1375 | 8 | 1376 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 1377 . . 1378 . Object List string . 1379 . . 1380 . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1381 . | 1382 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1384 Figure 5: Example of CID, in case of a static session. 1386 The object list contains the following 26 byte long string, including 1387 the NULL-termination character: 1389 1,2,3,100-104,200-203,299 1391 There are therefore a total of 3+5+4+1 = 13 objects in the carousel 1392 instance, and therefore in the FCAST session. There is no meta-data 1393 associated to this CID. The session being static and composed of a 1394 single Carousel Instance, the sender did not feel the necessity to 1395 carry a Carousel Instance ID meta-data. 1397 Appendix B. Additional Meta-Data Transmission Mechanisms 1399 B.1. Supporting Additional Mechanisms 1401 In certain use-cases, FCAST MAY take advantage of another in-band 1402 (e.g., via NORM INFO messages Appendix B.2) or out-of-band signaling 1403 mechanism. This additional signaling scheme MAY be used to carry the 1404 whole meta-data, or a subset of it, ahead of time, before the 1405 associated compound object. Therefore a receiver may be able to 1406 decide in advance, before beginning the reception of the compound 1407 object, whether the object is of interest or not, based on the 1408 information retrieved by this way, which mitigates FCAST limitations. 1409 If out-of-band techniques are out of the scope of this document, we 1410 explain below how this may be achieved in case of FCAST/NORM. 1412 Supporting one of these additional mechanisms is OPTIONAL in FCAST 1413 implementations. An FCAST/NORM sender MUST continue to send all the 1414 required meta-data in the compound object, even if the whole meta- 1415 data, or a subset of it, is sent by another mechanism (Section 5). 1416 Additionally, when meta-data is sent several times, there MUST NOT be 1417 any contradiction in the information provided by the different 1418 mechanisms. In case a mismatch is detected, the meta-data contained 1419 in the Compound Object MUST be privileged. 1421 When meta-data elements are communicated out-of-band, in advance of 1422 data transmission, the following piece of information MAY be useful: 1424 o TOI: a positive integer that contains the Transmission Object 1425 Identifier (TOI) of the object, in order to enable a receiver to 1426 easily associate the meta-data to the object. The valid range for 1427 TOI values is discussed in Section 3.6; 1429 B.2. Using NORM_INFO Messages with FCAST/NORM 1431 The NORM_INFO message of NORM can convey "out-of-band" content with 1432 respect to a given transport object. With FCAST, this message MAY be 1433 used as an additional mechanism to transmit meta-data. In that case, 1434 the NORM_INFO message carries a new Compound Object that contains all 1435 the meta-data of the original object, or a subset of it. The 1436 NORM_INFO Compound Object MUST NOT contain any Object Data field 1437 (i.e., it is only composed of the header), it MUST feature a non 1438 global checksum, and it MUST NOT include any padding field. Finally, 1439 note that the availability of NORM_INFO for a given object is 1440 signaled through the use of a dedicated flag in the NORM_DATA message 1441 header. Along with NORM's NACK-based repair request signaling, it 1442 allows a receiver to quickly (and independently) request an object's 1443 NORM_INFO content. However, a limitation here is that the NORM_INFO 1444 Compound Object header MUST fit within the byte size limit defined by 1445 the NORM sender's configured "segment size" (typically a little less 1446 than the network MTU); 1448 B.2.1. Example 1450 In case of FCAST/NORM, the FCAST Compound Object meta-data (or a 1451 subset of it) can be carried as part of a NORM_INFO message, as a new 1452 Compound Object that does not contain any Compound Object Data. In 1453 the following example we assume that the whole meta-data is carried 1454 in such a message for a certain Compound Object. Figure 6 shows an 1455 example NORM_INFO message that contains the FCAST Compound Object 1456 Header and meta-data as its payload. In this example, the first 16 1457 bytes are the NORM_INFO base header, the next 12 bytes are a NORM 1458 EXT_FTI header extension containing the FEC Object Transport 1459 Information for the associated object, and the remaining bytes are 1460 the FCAST Compound Object Header and meta-data. Note that "padding" 1461 MUST NOT be used and that the FCAST checksum only encompasses the 1462 Compound Object Header (G=0). 1464 0 1 2 3 1465 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 1466 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -- 1467 |version| type=1| hdr_len = 7 | sequence | ^ 1468 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1469 | source_id | n 1470 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o 1471 | instance_id | grtt |backoff| gsize | r 1472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ m 1473 | flags | fec_id = 5 | object_transport_id | v 1474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -- 1475 | HET = 64 | HEL = 3 | | ^ 1476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + f 1477 | Transfer Length = 41 | t 1478 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i 1479 | Encoding Symbol Length (E) | MaxBlkLen (B) | max_n | v 1480 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -- 1481 | 0 | 0 |0|0| 0 | 0 | Checksum | ^ 1482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1483 | 41 | f 1484 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| c 1485 . . a 1486 . meta-data ASCII null terminated string (33 bytes) . s 1487 . . t 1488 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1489 | | v 1490 +-+-+-+-+-+-+-+-+ -- 1492 Figure 6: NORM_INFO containing an EXT_FTI header extension and an 1493 FCAST Compound Object Header 1495 The NORM_INFO message shown in Figure 6 contains the EXT_FTI header 1496 extension to carry the FEC OTI. In this example, the FEC OTI format 1497 is that of the Reed-Solomon FEC coding scheme for fec_id = 5 as 1498 described in [RFC5510]. Other alternatives for providing the FEC OTI 1499 would have been to either include it directly in the meta-data of the 1500 FCAST Compound Header, or to include an EXT_FTI header extension to 1501 all NORM_DATA packets (or a subset of them). Note that the NORM 1502 "Transfer_Length" is the total length of the associated FCAST 1503 Compound Object, i.e., 41 bytes. 1505 The FCAST Compound Object in this example does contain the same meta- 1506 data and is formatted as in the example of Figure 4. With the 1507 combination of the FEC_OTI and the FCAST meta-data, the NORM protocol 1508 and FCAST application have all of the information needed to reliably 1509 receive and process the associated object. Indeed, the NORM protocol 1510 provides rapid (NORM_INFO has precedence over the associated object 1511 content), reliable delivery of the NORM_INFO message and its payload, 1512 the FCAST Compound Object. 1514 Authors' Addresses 1516 Vincent Roca 1517 INRIA 1518 655, av. de l'Europe 1519 Inovallee; Montbonnot 1520 ST ISMIER cedex 38334 1521 France 1523 Email: vincent.roca@inria.fr 1524 URI: http://planete.inrialpes.fr/people/roca/ 1526 Brian Adamson 1527 Naval Research Laboratory 1528 Washington, DC 20375 1529 USA 1531 Email: adamson@itd.nrl.navy.mil 1532 URI: http://cs.itd.nrl.navy.mil