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'12') (Obsoleted by RFC 4566) -- Obsolete informational reference (is this intentional?): RFC 2633 (ref. '18') (Obsoleted by RFC 3851) -- Obsolete informational reference (is this intentional?): RFC 2048 (ref. '20') (Obsoleted by RFC 4288, RFC 4289) -- Obsolete informational reference (is this intentional?): RFC 1521 (ref. '21') (Obsoleted by RFC 2045, RFC 2046, RFC 2047, RFC 2048, RFC 2049) Summary: 9 errors (**), 0 flaws (~~), 2 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 RMT T. Paila 2 Internet-Draft Nokia 3 Expires: December 4, 2004 M. Luby 4 Digital Fountain 5 R. Lehtonen 6 TeliaSonera 7 V. Roca 8 INRIA Rhone-Alpes 9 R. Walsh 10 Nokia 11 June 5, 2004 13 FLUTE - File Delivery over Unidirectional Transport 14 draft-ietf-rmt-flute-08.txt 16 Status of this Memo 18 This document is an Internet-Draft and is in full conformance with 19 all provisions of Section 10 of RFC2026. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that other 23 groups may also distribute working documents as Internet-Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at http:// 31 www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on December 4, 2004. 38 Copyright Notice 40 Copyright (C) The Internet Society (2004). All Rights Reserved. 42 Abstract 44 This document defines FLUTE, a protocol for the unidirectional 45 delivery of files over the Internet, which is particularly suited to 46 multicast networks. The specification builds on Asynchronous Layered 47 Coding, the base protocol designed for massively scalable multicast 48 distribution. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 53 1.1 Applicability Statement . . . . . . . . . . . . . . . . . . 4 54 1.1.1 The Target Application Space . . . . . . . . . . . . . . . . 4 55 1.1.2 The Target Scale . . . . . . . . . . . . . . . . . . . . . . 4 56 1.1.3 Intended Environments . . . . . . . . . . . . . . . . . . . 4 57 1.1.4 Weaknesses . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 2. Conventions used in this document . . . . . . . . . . . . . 5 59 3. File delivery . . . . . . . . . . . . . . . . . . . . . . . 5 60 3.1 File delivery session . . . . . . . . . . . . . . . . . . . 6 61 3.2 File Delivery Table . . . . . . . . . . . . . . . . . . . . 8 62 3.3 Dynamics of FDT Instances within file delivery session . . . 10 63 3.4 Structure of FDT Instance packets . . . . . . . . . . . . . 11 64 3.4.1 Format of FDT Instance Header . . . . . . . . . . . . . . . 12 65 3.4.2 Syntax of FDT Instance . . . . . . . . . . . . . . . . . . . 13 66 3.4.3 Content Encoding of FDT Instance . . . . . . . . . . . . . . 17 67 3.5 Multiplexing of files within a file delivery session . . . . 17 68 4. Channels, congestion control and timing . . . . . . . . . . 18 69 5. Delivering FEC Object Transmission Information . . . . . . . 19 70 5.1 Use of EXT_FTI for delivery of FEC Object Transmission 71 Information . . . . . . . . . . . . . . . . . . . . . . . . 20 72 5.1.1 General EXT_FTI format . . . . . . . . . . . . . . . . . . . 20 73 5.1.2 FEC Encoding ID specific formats for EXT_FTI . . . . . . . . 21 74 5.2 Use of FDT for delivery of FEC Object Transmission 75 Information . . . . . . . . . . . . . . . . . . . . . . . . 24 76 6. Describing file delivery sessions . . . . . . . . . . . . . 25 77 7. Security Considerations . . . . . . . . . . . . . . . . . . 26 78 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 28 79 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 80 Normative references . . . . . . . . . . . . . . . . . . . . 29 81 Informative references . . . . . . . . . . . . . . . . . . . 29 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 31 83 A. Receiver operation (informative) . . . . . . . . . . . . . . 32 84 B. Example of FDT Instance (informative) . . . . . . . . . . . 33 85 Intellectual Property and Copyright Statements . . . . . . . 34 87 1. Introduction 89 This document defines FLUTE version 1, a protocol for unidirectional 90 delivery of files over the Internet. The specification builds on 91 Asynchronous Layered Coding (ALC), version 1 [2], the base protocol 92 designed for massively scalable multicast distribution. ALC defines 93 transport of arbitrary binary objects. For file delivery applications 94 mere transport of objects is not enough, however. The end systems 95 need to know what do the objects actually represent. This document 96 specifies a technique called FLUTE - a mechanism for signaling and 97 mapping the properties of files to concepts of ALC in a way that 98 allows receivers to assign those parameters for received objects. 99 Consequently, throughout this document the term 'file' relates to an 100 'object' as discussed in ALC. Although this specification frequently 101 makes use of multicast addressing as an example, the techniques are 102 similarly applicable for use with unicast addressing. 104 This document defines a specific transport application of ALC, adding 105 the following specifications: 107 - Definition of a file delivery session built on top of ALC, 108 including transport details and timing constraints. 110 - In-band signalling of the transport parameters of the ALC session. 112 - In-band signalling of the properties of delivered files. 114 - Details associated with the multiplexing of multiple files within 115 a session. 117 This specification is structured as follows. Section 3 begins by 118 defining the concept of the file delivery session. Following that it 119 introduces the File Delivery Table that forms the core part of this 120 specification. Further, it discusses multiplexing issues of transport 121 objects within a file delivery session. Section 4 describes the use 122 of congestion control and channels with FLUTE. Section 5 defines how 123 the Forward Error Correction (FEC) Object Transmission Information is 124 to be delivered within a file delivery session. Section 6 defines the 125 required parameters for describing file delivery sessions in a 126 general case. Section 7 outlines security considerations regarding 127 file delivery with FLUTE. Last, there are two informative appendices. 128 The first appendix describes an envisioned receiver operation for the 129 receiver of the file delivery session. The second appendix gives an 130 example of File Delivery Table. 132 Statement of Intent 134 This memo contains part of the definitions necessary to fully 135 specify a Reliable Multicast Transport protocol in accordance with 136 RFC2357. As per RFC2357, the use of any reliable multicast 137 protocol in the Internet requires an adequate congestion control 138 scheme. 140 While waiting for such a scheme to be available, or for an 141 existing scheme to be proven adequate, the Reliable Multicast 142 Transport working group (RMT) publishes this Request for Comments 143 in the "Experimental" category. 145 It is the intent of RMT to re-submit this specification as an IETF 146 Proposed Standard as soon as the above condition is met. 148 1.1 Applicability Statement 150 1.1.1 The Target Application Space 152 FLUTE is applicable to the delivery of large and small files to many 153 hosts, using delivery sessions of several seconds or more. For 154 instance, FLUTE could be used for the delivery of large software 155 updates to many hosts simultaneously. It could also be used for 156 continuous, but segmented, data such as time-lined text for 157 subtitling - potentially leveraging its layering inheritance from ALC 158 and LCT to scale the richness of the session to the congestion status 159 of the network. It is also suitable for the basic transport of 160 metadata, for example SDP [12] files which enable user applications 161 to access multimedia sessions. 163 1.1.2 The Target Scale 165 Massive scalability is a primary design goal for FLUTE. IP multicast 166 is inherently massively scalable, but the best effort service that it 167 provides does not provide session management functionality, 168 congestion control or reliability. FLUTE provides all of this using 169 ALC and IP multicast without sacrificing any of the inherent 170 scalability of IP multicast. 172 1.1.3 Intended Environments 174 All of the environmental requirements and considerations that apply 175 to the ALC building block [2] and to any additional building blocks 176 that FLUTE uses also apply to FLUTE. 178 FLUTE can be used with both multicast and unicast delivery, but it's 179 primary application is for unidirectional multicast file delivery. 180 FLUTE requires connectivity between a sender and receivers but does 181 not require connectivity from receivers to a sender. FLUTE inherently 182 works with all types of networks, including LANs, WANs, Intranets, 183 the Internet, asymmetric networks, wireless networks, and satellite 184 networks. 186 FLUTE is compatible with both IPv4 or IPv6 as no part of the packet 187 is IP version specific. FLUTE works with both multicast models: 188 Any-Source Multicast (ASM) [13] and the Source-Specific Multicast 189 (SSM) [15]. 191 FLUTE is applicable for both Internet use, with a suitable congestion 192 control building block, and provisioned/controlled systems, such as 193 delivery over wireless broadcast radio systems. 195 1.1.4 Weaknesses 197 Some networks are not amenable to some congestion control protocols 198 that could be used with FLUTE. In particular, for a satellite or 199 wireless network, there may be no mechanism for receivers to 200 effectively reduce their reception rate since there may be a fixed 201 transmission rate allocated to the session. 203 FLUTE provides reliability using the FEC building block. This will 204 reduce the error rate as seen by applications. However, FLUTE does 205 not provide a method for senders to verify the reception success of 206 receivers, and the specification of such a method is outside the 207 scope of this document. 209 2. Conventions used in this document 211 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 212 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 213 document are to be interpreted as described in RFC 2119 [1]. 215 The terms "object" and "transport object" are consistent with the 216 definitions in ALC [2] and LCT [3]. The terms "file" and "source 217 object" are pseudonyms for "object". 219 3. File delivery 221 Asynchronous Layered Coding [2] is a protocol designed for delivery 222 of arbitrary binary objects. It is especially suitable for massively 223 scalable, unidirectional, multicast distribution. ALC provides the 224 basic transport for FLUTE, and thus FLUTE inherits the requirements 225 of ALC. 227 This specification is designed for the delivery of files. The core of 228 this specification is to define how the properties of the files are 229 carried in-band together with the delivered files. 231 As an example, let us consider a 5200 byte file referred to by 232 "http://www.example.com/docs/file.txt". Using the example, the 233 following properties describe the properties that need to be conveyed 234 by the file delivery protocol. 236 * Identifier of the file, expressed as a URI. This identifier may be 237 globally unique. The identifier may also provide a location for 238 the file. In the above example: "http://www.example.com/docs/ 239 file.txt". 241 * File name (usually, this can be concluded from the URI). In the 242 above example: "file.txt". 244 * File type, expressed as MIME media type (usually, this can also be 245 concluded from the extension of the file name). In the above 246 example: "text/plain". If an explicit value for the MIME type is 247 provided separately from the file extension and does not match the 248 MIME type of the file extension then the explicitly provided value 249 MUST be used as the MIME type. 251 * File size, expressed in bytes. In the above example: "5200". If 252 the file is content encoded then this is the file size before 253 content encoding. 255 * Content encoding of the file, within transport. In the above 256 example, the file could be encoded using ZLIB [10]. In this case 257 the size of the transport object carrying the file would probably 258 differ from the file size. The transport object size is delivered 259 to receivers as part of the FLUTE protocol. 261 * Security properties of the file such as digital signatures, 262 message digests, etc. For example, one could use S/MIME [18] as 263 the content encoding type for files with this authentication 264 wrapper, and one could use XML-DSIG [19] to digitally sign an FDT 265 Instance. 267 3.1 File delivery session 269 ALC is a protocol instantiation of Layered Coding Transport building 270 block (LCT) [3]. Thus ALC inherits the session concept of LCT. In 271 this document we will use the concept ALC/LCT session to collectively 272 denote the interchangeable terms ALC session and LCT session. 274 An ALC/LCT session consists of a set of logically grouped ALC/LCT 275 channels associated with a single sender sending packets with ALC/LCT 276 headers for one or more objects. An ALC/LCT channel is defined by the 277 combination of a sender and an address associated with the channel by 278 the sender. A receiver joins a channel to start receiving the data 279 packets sent to the channel by the sender, and a receiver leaves a 280 channel to stop receiving data packets from the channel. 282 One of the fields carried in the ALC/LCT header is the Transport 283 Session Identifier (TSI). The TSI is scoped by the source IP address, 284 and the (source IP address, TSI) pair uniquely identifies a session, 285 i.e., the receiver uses this pair carried in each packet to uniquely 286 identify from which session the packet was received. In case multiple 287 objects are carried within a session another field within the ALC/LCT 288 header, the Transport Object Identifier (TOI), identifies from which 289 object within the session the data in the packet was generated. Note 290 that each object is associated with a unique TOI within the scope of 291 a session. 293 If the sender is not assigned a permanent IP address accessible to 294 receivers, but instead packets that can be received by receivers 295 contain a temporary IP address for packets sent by the sender, then 296 the TSI is scoped by this temporary IP address of the sender for the 297 duration of the session. As an example, the sender may be behind a 298 Network Address Translation (NAT) device that temporarily assigns an 299 IP address for the sender that is accessible to receivers, and in 300 this case the TSI is scoped by the temporary IP address assigned by 301 the NAT that will appear in packets received by the receiver. As 302 another example, the sender may send its original packets using IPv6, 303 but some portions of the network may not be IPv6 capable and thus 304 there may be an IPv6 to IPv4 translator that changes the IP address 305 of the packets to a different IPv4 address. In this case, receivers 306 in the IPv4 portion of the network will receive packets containing 307 the IPv4 address, and thus the TSI for them is scoped by the IPv4 308 address. How the IP address of the sender to be used to scope the 309 session by receivers is delivered to receivers, whether it is a 310 permanent IP address or a temporary IP address, is outside the scope 311 of this document. 313 When FLUTE is used for file delivery over ALC the following rules 314 apply: 316 * The ALC/LCT session is called file delivery session. 318 * The ALC/LCT concept of 'object' denotes either a 'file' or a 'File 319 Delivery Table Instance' (section 3.2) 321 * The TOI field MUST be included in ALC packets sent within a FLUTE 322 session, with the exception that ALC packets sent in a FLUTE 323 session with the Close Session (A) flag set to 1 (signaling the 324 end of the session) and that contain no payload (carrying no 325 information for any file for FDT) SHALL NOT carry the TOI. See 326 Section 5.1 of RFC 3451 [3] for the LCT definition of the Close 327 Session flag, and see Section 4.2 of RFC 3450 [2] for an example 328 of its use within an ALC packet. 330 * The TOI value '0' is reserved for delivery of File Delivery Table 331 Instances. Each File Delivery Table Instance is uniquely 332 identified by an FDT Instance ID. 334 * Each file in a file delivery session MUST be associated with a TOI 335 (>0) in the scope of that session. 337 * Information carried in the headers and the payload of a packet is 338 scoped by the source IP address and the TSI. Information 339 particular to the object carried in the headers and the payload of 340 a packet is further scoped by the TOI for file objects, and is 341 further scoped by both the TOI and the FDT Instance ID for FDT 342 Instance objects. 344 3.2 File Delivery Table 346 The File Delivery Table (FDT) provides a means to describe various 347 attributes associated with files that are to be delivered within the 348 file delivery session. The following lists are examples of such 349 attributes, and are not intended to be mutually exclusive nor 350 exhaustive. 352 Attributes related to the delivery of file: 354 - TOI value that represents the file 356 - FEC Object Transmission Information (including the FEC Encoding ID 357 and, if relevant, the FEC Instance ID) 359 - Size of the transport object carrying the file 361 - Aggregate rate of sending packets to all channels 363 Attributes related to the file itself: 365 - Name, Identification and Location of file (specified by the URI) 366 - MIME media type of file 368 - Size of file 370 - Encoding of file 372 - Message digest of file 374 Some of these attributes MUST be included in the file description 375 entry for a file, others are optional, as defined in section 3.4.2. 377 Logically, the FDT is a set of file description entries for files to 378 be delivered in the session. Each file description entry MUST include 379 the TOI for the file that it describes and the URI identifying the 380 file. The TOI is included in each ALC/LCT data packet during the 381 delivery of the file, and thus the TOI carried in the file 382 description entry is how the receiver determines which ALC/LCT data 383 packets contain information about which file. Each file description 384 entry may also contain one or more descriptors that map the 385 above-mentioned attributes to the file. 387 Each file delivery session MUST have an FDT that is local to the 388 given session. The FDT MUST provide a file description entry mapped 389 to a TOI for each file appearing within the session. An object that 390 is delivered within the ALC session, but not described in the FDT, is 391 not considered a 'file' belonging to the file delivery session. 392 Handling of these unmapped TOIs (TOIs that are not resolved by the 393 FDT) is out of scope of this specification. 395 Within the file delivery session the FDT is delivered as FDT 396 Instances. An FDT Instance contains one or more file description 397 entries of the FDT. Any FDT Instance can be equal to, a subset of, a 398 superset of, or complement any other FDT Instance. A certain FDT 399 Instance may be repeated several times during a session, even after 400 subsequent FDT Instances (with higher FDT Instance ID numbers) have 401 been transmitted. Each FDT Instance contains at least a single file 402 description entry and at most the complete FDT of the file delivery 403 session. 405 A receiver of the file delivery session keeps an FDT database for 406 received file description entries. The receiver maintains the 407 database, for example, upon reception of FDT Instances. Thus, at any 408 given time the contents of the FDT database represent the receiver's 409 current view of the FDT of the file delivery session. Since each 410 receiver behaves independently of other receivers, it SHOULD NOT be 411 assumed that the contents of the FDT database are the same for all 412 the receivers of a given file delivery session. 414 Since FDT database is an abstract concept, the structure and the 415 maintaining of the FDT database are left to individual 416 implementations and are thus out of scope of this specification. 418 3.3 Dynamics of FDT Instances within file delivery session 420 The following rules define the dynamics of the FDT Instances within a 421 file delivery session: 423 * For every file delivered within a file delivery session there MUST 424 be a file description entry included in at least one FDT Instance 425 sent within the session. A file description entry contains at a 426 minimum the mapping between the TOI and the URI. 428 * An FDT Instance MAY appear in any part of the file delivery 429 session and packets for an FDT Instance MAY be interleaved with 430 packets for other files or other FDT Instances within a session. 432 * The TOI value of '0' MUST be reserved for delivery of FDT 433 Instances. The use of other TOI values for FDT Instances is 434 outside the scope of this specification. 436 * FDT Instance is identified by the use of a new fixed length LCT 437 Header Extension EXT_FDT (defined later in this section). Each FDT 438 Instance is uniquely identified within the file delivery session 439 by its FDT Instance ID. Any ALC/LCT packet carrying FDT Instance 440 (indicated by TOI = 0) MUST include EXT_FDT. 442 * It is RECOMMENDED that FDT Instance that contains the file 443 description entry for a file is sent prior to the sending of the 444 described file within a file delivery session. 446 * Within a file delivery session, any TOI MAY be described more than 447 once. An example: previous FDT Instance 0 describes TOI of value 448 '3'. Now, subsequent FDT Instances can either keep TOI '3' 449 unmodified on the table, not to include it or complement the 450 description. However, subsequent FDT Instances MUST NOT change the 451 parameters already described for a specific TOI. 453 * An FDT Instance is valid until its expiration time. The expiration 454 time is expressed within the FDT Instance payload as a 32 bit data 455 field. The value of the data field represents the 32 most 456 significant bits of a 64 bit Network Time Protocol (NTP) [5] time 457 value. These 32 bits provide an unsigned integer representing the 458 time in seconds relative to 0 hours 1 January 1900. Handling of 459 wraparound of the 32 bit time is outside the scope of NTP and 460 FLUTE. 462 * The receiver SHOULD NOT use a received FDT Instance to interpret 463 packets received beyond the expiration time of the FDT Instance. 465 * A sender MUST use an expiry time in the future upon creation of an 466 FDT Instance relative to its Sender Current Time (SCT). 468 * Any FEC Encoding ID MAY be used for the sending of FDT Instances. 469 The default is to use FEC Encoding ID 0 for the sending of FDT 470 Instances. (Note that since FEC Encoding ID 0 is the default for 471 FLUTE, this implies that Source Block Number and Encoding Symbol 472 ID lengths both default to 16 bits each.) 474 Generally, a receiver needs to receive an FDT Instance describing a 475 file before it is able to recover the file itself. In this sense FDT 476 Instances are of higher priority than files. Thus, it is RECOMMENDED 477 that FDT Instances describing a file be sent with at least as much 478 reliability within a session (more often or with more FEC protection) 479 as the files they describe. In particular, if FDT Instances are 480 generally longer than one packet payload in length it is RECOMMENDED 481 that an FEC code that can provide protection against loss be used for 482 delivering FDT Instances. How often the description of a file is sent 483 in an FDT Instance or how much FEC protection is provided for each 484 FDT Instance (if the FDT Instance is longer than one packet payload) 485 is dependent on the particular application and outside the scope of 486 this document. 488 3.4 Structure of FDT Instance packets 490 FDT Instances are carried in ALC packets with TOI = 0 and with an 491 additional REQUIRED LCT Header extension called the FDT Instance 492 Header. The FDT Instance Header (EXT_FDT) contains the FDT Instance 493 ID that uniquely identifies FDT Instances within a file delivery 494 session. The FDT Instance Header is placed in the same way as any 495 other LCT extension header. There MAY be other LCT extension headers 496 in use. 498 The LCT extension headers are followed by the FEC Payload ID, and 499 finally the Encoding Symbols for the FDT Instance which contains one 500 or more file description entries. The FDT Instance MAY span over 501 several ALC packets - the number of ALC packets is a function of the 502 FEC Object Transmission Information associated to this FDT Instance. 503 The FDT Instance Header is carried in each ALC packet carrying the 504 FDT Instance. The FDT Instance Header is identical for all ALC/LCT 505 packets for a particular FDT Instance. 507 The overall format of ALC/LCT packets carrying an FDT Instance is 508 depicted in the Figure 1 below. All integer fields are carried in 509 "big-endian" or "network order" format, that is, most significant 510 byte (octet) first. As defined in [2], all ALC/LCT packets are sent 511 using UDP. 513 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 514 | UDP header | 515 | | 516 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 517 | Default LCT header (with TOI = 0) | 518 | | 519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 520 | LCT header extensions (EXT_FDT, EXT_FTI, etc.) | 521 | | 522 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 523 | FEC Payload ID | 524 | | 525 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 526 | Encoding Symbol(s) for FDT Instance | 527 | ... | 528 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 530 Figure 1 - Overall FDT Packet 532 3.4.1 Format of FDT Instance Header 534 FDT Instance Header (EXT_FDT) is a new fixed length, ALC PI specific 535 LCT header extension [3]. The Header Extension Type (HET) for the 536 extension is 192. Its format is defined below: 538 0 1 2 3 539 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 1 540 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 541 | HET = 192 | V | FDT Instance ID | 542 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 544 Version of FLUTE (V), 4 bits: 546 This document specifies FLUTE version 1. Hence in any ALC packet that 547 carries FDT Instance and that belongs to the file delivery session as 548 specified in this specification MUST set this field to '1'. 550 FDT Instance ID, 20 bits: 552 For each file delivery session the numbering of FDT Instances starts 553 from '0' and is incremented by exactly one for each subsequent FDT 554 Instance. After reaching the maximum value (2^20-1), the numbering 555 starts again from '0'. When wraparound from 2^20-1 to 0 occurs, 0 is 556 considered higher than 2^20-1. A new FDT Instance reusing a previous 557 FDT Instance ID number, due to wraparound, may not implicitly expire 558 the previous FDT Instance with the same ID. It would be reasonable 559 for FLUTE Senders to only construct and deliver FDT Instances with 560 wraparound IDs after the previous FDT Instance using the same ID has 561 expired. However, mandatory receiver behavior for handling FDT 562 Instance ID wraparound and other special situations (for example, 563 missing FDT Instance IDs resulting in longer increments than one) is 564 left out of this specification to individual implementations of 565 FLUTE. 567 3.4.2 Syntax of FDT Instance 569 The FDT Instance contains file description entries that provide the 570 mapping functionality described in 3.2 above. 572 The FDT Instance is an XML structure that has a single root element 573 "FDT-Instance". The "FDT-Instance" element MUST contain "Expires" 574 attribute, which tells the expiry time of the FDT Instance. In 575 addition, the "FDT-Instance" element MAY contain the "Complete" 576 attribute (boolean), which, when TRUE, signals that no new data will 577 be provided in future FDT Instances within this session (i.e. that 578 either FDT Instances with higher ID numbers will not be used or if 579 they are used, will only provide identical file parameters to that 580 already given in this and previous FDT Instances). For example, this 581 may be used to provide a complete list of files in an entire FLUTE 582 session (a "complete FDT"). 584 The "FDT-Instance" element MAY contain attributes that give common 585 parameters for all files of an FDT Instance. These attributes MAY 586 also be provided for individual files in the "File" element. Where 587 the same attribute appears in both the "FDT-Instance" and the "File" 588 elements, the value of the attribute provided in the "File" element 589 takes precedence. 591 For each file to be declared in the given FDT Instance there is a 592 single file description entry in the FDT Instance. Each entry is 593 represented by element "File" which is a child element of the FDT 594 Instance structure. 596 The attributes of "File" element in the XML structure represent the 597 attributes given to the file that is delivered in the file delivery 598 session. The value of the XML attribute name corresponds to MIME 599 field name and the XML attribute value corresponds to the value of 600 the MIME field body. Each "File" element MUST contain at least two 601 attributes "TOI" and "Content-Location". "TOI" MUST be assigned a 602 valid TOI value as described in section 3.3 above. "Content-Location" 603 MUST be assigned a valid URI as defined in [6]. 605 In addition to mandatory attributes, the "FDT-Instance" element and 606 the "File" element MAY contain other attributes of which the 607 following are specifically pointed out. 609 * Where the MIME type is described, the attribute "Content-Type" 610 MUST be used for the purpose as defined in [6]. 612 * Where the length is described, the attribute "Content-Length" MUST 613 be used for the purpose as defined in [6]. The transfer length is 614 defined to be the length of the object transported in bytes. It is 615 often important to convey the transfer length to receivers, 616 because the source block structure needs to be known for the FEC 617 decoder to be applied to recover source blocks of the file, and 618 the transfer length is often needed to properly determine the 619 source block structure of the file. There generally will be a 620 difference between the length of the original file and the 621 transfer length if content encoding is applied to the file before 622 transport, and thus the "Content-Encoding" attribute is used. If 623 the file is not content encoded before transport (and thus the 624 "Content-Encoding" attribute is not used) then the transfer length 625 is the length of the original file, and in this case the 626 "Content-Length" is also the transfer length. However, if the file 627 is content encoded before transport (and thus the 628 "Content-Encoding" attribute is used), e.g. if compression is 629 applied before transport to reduce the number of bytes that need 630 to be transferred, then the transfer length is generally different 631 than the length of the original file, and in this case the 632 attribute "Transfer-Length" MAY be used to carry the transfer 633 length. 635 * Where the content encoding scheme is described, the attribute 636 "Content-Encoding" MUST be used for the purpose as defined in [6]. 638 * Where the MD5 message digest is described, the attribute 639 "Content-MD5" MUST be used for the purpose as defined in [6]. 641 * The FEC Object Transmission Information attributes as described in 642 section 5.2. 644 The following specifies the XML Schema [8][9] for FDT Instance: 646 647 651 652 653 654 655 656 659 662 665 668 671 674 677 680 683 686 689 692 693 694 695 696 699 702 705 708 711 714 717 720 723 724 725 726 728 Any valid FDT Instance must use the above XML Schema. This way FDT 729 provides extensibility to support private attributes within the file 730 description entries. Those could be, for example, the attributes 731 related to the delivery of the file (timing, packet transmission 732 rate, etc.). 734 In case the basic FDT XML Schema is extended in terms of new 735 descriptors, for attributes applying to a single file, those MUST be 736 placed within the attributes of the element "File", and for 737 attributes applying to all files described by the current FDT 738 Instance MUST be placed within the element "FDT-Instance". It is 739 RECOMMENDED that the new descriptors applied in the FDT are in the 740 format of MIME fields and are either defined in HTTP/1.1 741 specification [6] or otherwise well-known specification. 743 3.4.3 Content Encoding of FDT Instance 745 The FDT Instance itself MAY be content encoded, for example 746 compressed. This specification defines FDT Instance Content Encoding 747 Header (EXT_CENC). EXT_CENC is a new fixed length, ALC PI specific 748 LCT header extension [3]. The Header Extension Type (HET) for the 749 extension is 193. If the FDT Instance is content encoded, the 750 EXT_CENC MUST be used to signal the content encoding type. In that 751 case, EXT_CENC header extension MUST be used in all ALC packets 752 carrying the same FDT Instance ID. Consequently, when EXT_CENC header 753 is used, it MUST be used together with a proper FDT Instance Header 754 (EXT_FDT). Within a file delivery session, FDT Instances that are not 755 content encoded and FDT Instances that are content encoded MAY both 756 appear. If content encoding is not used for a given FDT Instance, the 757 EXT_CENC MUST NOT be used in any packet carrying the FDT Instance. 758 The format of EXT_CENC is defined below: 760 0 1 2 3 761 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 1 762 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 763 | HET = 193 | CENC | Reserved | 764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 766 Content Encoding Algorithm (CENC), 8 bits: 768 This field signals the content encoding algorithm used in the FDT 769 Instance payload. The definition of this field is outside the scope 770 of this specification. Applicable content encoding algorithms 771 include, for example, ZLIB [10], DEFLATE [16] and GZIP [17]. 773 Reserved, 16 bits: 775 This field MUST be set to all '0'. 777 3.5 Multiplexing of files within a file delivery session 779 The delivered files are carried as transport objects (identified with 780 TOIs) in the file delivery session. All these objects, including the 781 FDT Instances, MAY be multiplexed in any order and in parallel with 782 each other within a session, i.e., packets for one file MAY be 783 interleaved with packets for other files or other FDT Instances 784 within a session. 786 Multiple FDT Instances MAY be delivered in a single session using TOI 787 = 0. In this case, it is RECOMMENDED that the sending of a previous 788 FDT Instance SHOULD end before the sending of the next FDT Instance 789 starts. However, due to unexpected network conditions, packets for 790 the FDT Instances MAY be interleaved. A receiver can determine which 791 FDT Instance a packet contains information about since the FDT 792 Instances are uniquely identified by their FDT Instance ID carried in 793 the EXT_FDT headers. 795 4. Channels, congestion control and timing 797 ALC/LCT has a concept of channels and congestion control. There are 798 four scenarios FLUTE is envisioned to be applied. 800 (a) Use a single channel and a single-rate congestion control 801 protocol. 803 (b) Use multiple channels and a multiple-rate congestion control 804 protocol. In this case the FDT Instances MAY be delivered on more 805 than one channel. 807 (c) Use a single channel without congestion control supplied by ALC, 808 but only when in a controlled network environment where flow/ 809 congestion control is being provided by other means. 811 (d) Use multiple channels without congestion control supplied by ALC, 812 but only when in a controlled network environment where flow/ 813 congestion control is being provided by other means. In this case 814 the FDT Instances MAY be delivered on more than one channel. 816 When using just one channel for a file delivery session, as in (a) 817 and (c), the notion of 'prior' and 'after' are intuitively defined 818 for the delivery of objects with respect to their delivery times. 820 However, if multiple channels are used, as in (b) and (d), it is not 821 straightforward to state that an object was delivered 'prior' to the 822 other. An object may begin to be delivered on one or more of those 823 channels before the delivery of a second object begins. However, the 824 use of multiple channels/layers may complete the delivery of the 825 second object before the first. This is not a problem when objects 826 are delivered sequentially using a single channel. Thus, if the 827 application of FLUTE has a mandatory or critical requirement that the 828 first transport object must complete 'prior' to the second one, it is 829 RECOMMENDED that only a single channel is used for the file delivery 830 session. 832 Furthermore, if multiple channels are used then a receiver joined to 833 the session at a low reception rate will only be joined to the lower 834 layers of the session. Thus, since the reception of FDT Instances is 835 of higher priority than the reception of files (because the reception 836 of files depends on the reception of an FDT Instance describing it), 837 the following is RECOMMENDED: 839 1. The layers to which packets for FDT Instances are sent SHOULD NOT 840 be biased towards those layers to which lower rate receivers are 841 not joined. For example, it is ok to put all the packets for an 842 FDT Instance into the lowest layer (if this layer carries enough 843 packets to deliver the FDT to higher rate receivers in a 844 reasonable amount of time), but it is not ok to put all the 845 packets for an FDT Instance into the higher layers that only high 846 rate receivers will receive. 848 2. If FDT Instances are generally longer than one Encoding Symbol in 849 length and some packets for FDT Instances are sent to layers that 850 lower rate receivers do not receive, an FEC Encoding other than 851 FEC Encoding ID 0 SHOULD be used to deliver FDT Instances. This 852 is because in this case, even when there is no packet loss in the 853 network, a lower rate receiver will not receive all packets sent 854 for an FDT Instance. 856 5. Delivering FEC Object Transmission Information 858 FLUTE inherits the use of FEC building block [4] from ALC. When using 859 FLUTE for file delivery over ALC the FEC Object Transmission 860 Information MUST be delivered in-band within the file delivery 861 session. In this section, two methods are specified for FLUTE for 862 this purpose: the use of ALC specific LCT extension header EXT_FTI 863 [2] and the use of FDT. 865 The receiver of file delivery session MUST support delivery of FEC 866 Object Transmission Information using the EXT_FTI for the FDT 867 Instances carried using TOI value 0. For the TOI values other than 0 868 the receiver MUST support both methods: the use of EXT_FTI and the 869 use of FDT. 871 The FEC Object Transmission Information that needs to be delivered to 872 receivers MUST be exactly the same whether it is delivered using 873 EXT_FTI or using FDT (or both). Section 5.1 describes the required 874 FEC Object Transmission Information that MUST be delivered to 875 receivers for various FEC Encoding IDs. In addition, it describes the 876 delivery using EXT_FTI. Section 5.2 describes the delivery using FDT. 878 The FEC Object Transmission Information regarding a given TOI may be 879 available from several sources. In this case, it is RECOMMENDED that 880 the receiver of the file delivery session prioritizes the sources in 881 the following way (in the order of decreasing priority). 883 1. FEC Object Transmission Information that is available in EXT_FTI. 885 2. FEC Object Transmission Information that is available in the FDT. 887 5.1 Use of EXT_FTI for delivery of FEC Object Transmission Information 889 As specified in [2], the EXT_FTI header extension is intended to 890 carry the FEC Object Transmission Information for an object in-band. 891 It is left up to individual implementations to decide how frequently 892 and in which ALC packets the EXT_FTI header extension is included. In 893 environments with higher packet loss rate, the EXT_FTI might need to 894 be included more frequently in ALC packets than in environments with 895 low error probability. The EXT_FTI MUST be included in at least one 896 sent ALC packet for each FDT Instance. 898 The ALC specification does not define the format or the processing of 899 the EXT_FTI header extension. The following sections specify EXT_FTI 900 when used in FLUTE. 902 In FLUTE, the FEC Encoding ID (8 bits) is carried in the Codepoint 903 field of the ALC/LCT header. 905 5.1.1 General EXT_FTI format 907 The general EXT_FTI format specifies the structure and those 908 attributes of FEC Object Transmission Information that are applicable 909 to any FEC Encoding ID. 911 0 1 2 3 912 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 1 913 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 914 | HET = 64 | HEL | | 915 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 916 | Transfer Length | 917 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 918 | FEC Instance ID | FEC Enc. ID Specific Format | 919 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 921 Header Extension Type (HET), 8 bits: 923 64 as defined in [2]. 925 Header Extension Length (HEL), 8 bits: 927 The length of the whole Header Extension field, expressed in 928 multiples of 32-bit words. This length includes the FEC Encoding ID 929 specific format part. 931 Transfer Length, 48 bits: 933 The length of the transport object that carries the file in bytes. 934 (This is the same as the file length if the file is not content 935 encoded.) 937 FEC Instance ID, optional, 16 bits: 939 This field is used for FEC Instance ID. It is only present if the 940 value of FEC Encoding ID is in the range of 128-255. When the value 941 of FEC Encoding ID is in the range of 0-127, this field is set to 0. 943 FEC Encoding ID Specific Format: 945 Different FEC encoding schemes will need different sets of encoding 946 parameters. Thus, the structure and length of this field depends on 947 FEC Encoding ID. The next sections specify structure of this field 948 for FEC Encoding ID numbers 0, 128, 129 and 130. 950 5.1.2 FEC Encoding ID specific formats for EXT_FTI 952 5.1.2.1 FEC Encoding IDs 0, 128, and 130 954 FEC Encoding ID 0 is 'Compact No-Code FEC' (Fully-Specified) [7]. FEC 955 Encoding ID 128 is 'Small Block, Large Block and Expandable FEC' 956 (Under-Specified) [4]. FEC Encoding ID 130 is 'Compact FEC' 957 (Under-Specified) [7]. For these FEC Encoding IDs, the FEC Encoding 958 ID specific format of EXT_FTI is defined as follows. 960 0 1 2 3 961 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 1 962 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 963 General EXT_FTI format | Encoding Symbol Length | 964 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 965 | Maximum Source Block Length | 966 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 968 Encoding Symbol Length, 16 bits: 970 Length of Encoding Symbol in bytes. 972 All Encoding Symbols of a transport object MUST be equal to this 973 length, with the optional exception of the last source symbol of the 974 last source block (so that redundant padding is not mandatory in this 975 last symbol). This last source symbol MUST be logically padded out 976 with zeroes when another Encoding Symbol is computed based on this 977 source symbol to ensure the same interpretation of this Encoding 978 Symbol value by the sender and receiver. However, this padding need 979 not be actually sent with the data of the last source symbol. 981 Maximum Source Block Length, 32 bits: 983 The maximum number of source symbols per source block. 985 This EXT_FTI specification requires that an algorithm is known to 986 both sender and receivers for determining the size of all source 987 blocks of the transport object that carries the file identified by 988 the TOI (or within the FDT Instance identified by the TOI and the FDT 989 Instance ID). The algorithm SHOULD be the same for all files using 990 the same FEC Encoding ID within a session. 992 Section 5.1.2.3 describes an algorithm that is RECOMMENDED for this 993 use. 995 For the FEC Encoding IDs 0, 128 and 130, this algorithm is the only 996 well known way the receiver can determine the length of each source 997 block. Thus, the algorithm does two things: (a) it tells the receiver 998 the length of each particular source block as it is receiving packets 999 for that source block - this is essential to all of these FEC 1000 schemes; and, (b) it provides the source block structure immediately 1001 to the receiver so that the receiver can determine where to save 1002 recovered source blocks at the beginning - this is an optimization 1003 which is essential for some implementations. 1005 5.1.2.2 FEC Encoding ID 129 1007 Small Block Systematic FEC (Under-Specified). The FEC Encoding ID 1008 specific format of EXT_FTI is defined as follows. 1010 0 1 2 3 1011 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 1012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1013 General EXT_FTI format | Encoding Symbol Length | 1014 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1015 | Maximum Source Block Length | Max. Num. of Encoding Symbols | 1016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1018 Encoding Symbol Length, 16 bits: 1020 Length of Encoding Symbol in bytes. 1022 All Encoding Symbols of a transport object MUST be equal to this 1023 length, with the optional exception of the last source symbol of the 1024 last source block (so that redundant padding is not mandatory in this 1025 last symbol). This last source symbol MUST be logically padded out 1026 with zeroes when another Encoding Symbol is computed based on this 1027 source symbol to ensure the same interpretation of this Encoding 1028 Symbol value by the sender and receiver. However, this padding need 1029 not be actually sent with the data of the last source symbol. 1031 Maximum Source Block Length, 16 bits: 1033 The maximum number of source symbols per source block. 1035 Maximum Number of Encoding Symbols, 16 bits: 1037 Maximum number of Encoding Symbols that can be generated for a source 1038 block. 1040 This EXT_FTI specification requires that an algorithm is known to 1041 both sender and receivers for determining the size of all source 1042 blocks of the transport object that carries the file identified by 1043 the TOI (or within the FDT Instance identified by the TOI and the FDT 1044 Instance ID). The algorithm SHOULD be the same for all files using 1045 the same FEC Encoding ID within a session. 1047 Section 5.1.2.3 describes an algorithm that is RECOMMENDED for this 1048 use. For FEC Encoding ID 129 the FEC Payload ID in each data packet 1049 already contains the source block length for the source block 1050 corresponding to the Encoding Symbol carried in the data packet. 1051 Thus, the algorithm for computing source blocks for FEC Encoding ID 1052 129 could be to just use the source block lengths carried in data 1053 packets within the FEC Payload ID. However, the algorithm described 1054 in Section 5.1.2.3 is useful for the receiver to compute the source 1055 block structure at the beginning of the reception of data packets for 1056 the file. If the algorithm described in Section 5.1.2.3 is used then 1057 it MUST be the case that the source block lengths that appear in data 1058 packets agree with the source block lengths calculated by the 1059 algorithm. 1061 5.1.2.3 Algorithm for Computing Source Block Structure 1063 This algorithm computes a source block structure so that all source 1064 blocks are as close to being equal length as possible. A first number 1065 of source blocks are of the same larger length, and the remaining 1066 second number of source blocks are sent of the same smaller length. 1067 The total number of source blocks (N), the first number of source 1068 blocks (I), the second number of source blocks (N-I), the larger 1069 length (A_large) and the smaller length (A_small) are calculated 1070 thus, 1072 Input: 1074 B -- Maximum Source Block Length, i.e., the maximum number of 1075 source symbols per source block 1076 L -- Transfer Length in bytes 1077 E -- Encoding Symbol Length in bytes 1079 Output: 1080 N -- The number of source blocks into which the transport 1081 object is partitioned. 1083 The number and lengths of source symbols in each of the N 1084 source blocks. 1086 Algorithm: 1087 (a) The number of source symbols in the transport object is 1088 computed as T = L/E rounded up to the nearest integer. 1089 (b) The transport object is partitioned into N source blocks, 1090 where N = T/B rounded up to the nearest integer 1091 (c) The average length of a source block, A = T/N 1092 (this may be non-integer) 1093 (d) A_large = A rounded up to the nearest integer 1094 (it will always be the case that the value of A_large is at 1095 most B) 1096 (e) A_small = A rounded down to the nearest integer 1097 (if A is an integer A_small = A_large, 1098 and otherwise A_small = A_large - 1) 1099 (f) The fractional part of A, A_fraction = A - A_small 1100 (g) I = A_fraction * N 1101 (I is an integer between 0 and N-1) 1102 (h) Each of the first I source blocks consists of A_large source 1103 symbols, each source symbol is E bytes in length. Each of the 1104 remaining N-I source blocks consist of A_small source symbols, 1105 each source symbol is E bytes in length except that the last 1106 source symbol of the last source block is L-(((L-1)/E) rounded 1107 down to the nearest integer)*E bytes in length. 1109 Note, this algorithm does not imply implementation by floating point 1110 arithmetic and integer arithmetic may be used to avoid potential 1111 floating point rounding errors. 1113 5.2 Use of FDT for delivery of FEC Object Transmission Information 1115 The FDT delivers FEC Object Transmission Information for each file 1116 using an appropriate attribute within the "FDT-Instance" or the 1117 "File" element of the FDT structure. For future FEC Encoding IDs, if 1118 the attributes listed below do not fulfill the needs of describing 1119 the FEC Object Transmission Information then additional new 1120 attributes MAY be used. 1122 * "Transfer-Length" is semantically equivalent with the field 1123 "Transfer Length" of EXT_FTI. 1125 * "FEC-OTI-FEC-Encoding-ID" is semantically equivalent with the 1126 field "FEC Encoding ID" as carried in the Codepoint field of the 1127 ALC/LCT header. 1129 * "FEC-OTI-FEC-Instance-ID" is semantically equivalent with the 1130 field "FEC Instance ID" of EXT_FTI. 1132 * "FEC-OTI-Maximum-Source-Block-Length" is semantically equivalent 1133 with the field "Maximum Source Block Length" of EXT_FTI for FEC 1134 Encoding IDs 0, 128 and 130, and semantically equivalent with the 1135 field "Maximum Source Block Length" of EXT_FTI for FEC Encoding ID 1136 129. 1138 * "FEC-OTI-Encoding-Symbol-Length" is semantically equivalent with 1139 the field "Encoding Symbol Length" of EXT_FTI for FEC Encoding IDs 1140 0, 128, 129 and 130. 1142 * "FEC-OTI-Max-Number-of-Encoding-Symbols" is semantically 1143 equivalent with the field "Maximum Number of Encoding Symbols" of 1144 EXT_FTI for FEC Encoding ID 129. 1146 6. Describing file delivery sessions 1148 To start receiving a file delivery session, the receiver needs to 1149 know transport parameters associated with the session. Interpreting 1150 these parameters and starting the reception therefore represents the 1151 entry point from which thereafter the receiver operation falls into 1152 the scope of this specification. According to [2], the transport 1153 parameters of an ALC/LCT session that the receiver needs to know are: 1155 * The source IP address; 1157 * The number of channels in the session; 1159 * The destination IP address and port number for each channel in the 1160 session; 1162 * The Transport Session Identifier (TSI) of the session; 1164 * An indication that the session is a FLUTE session. The need to 1165 demultiplex objects upon reception is implicit in any use of 1166 FLUTE, and this fulfills the ALC requirement of an indication of 1167 whether or not a session carries packets for more than one object 1168 (all FLUTE sessions carry packets for more than one object); 1170 Optionally, the following parameters MAY be associated with the 1171 session (Note, the list is not exhaustive): 1173 * The start time and end time of the session; 1175 * FEC Encoding ID and FEC Instance ID when the default FEC Encoding 1176 ID 0 is not used for the delivery of FDT; 1178 * Content Encoding format if optional content encoding of FDT 1179 Instance is used, e.g., compression; 1181 * Some information that tells receiver, in the first place, that the 1182 session contains files that are of interest. 1184 It is envisioned that these parameters would be described according 1185 to some session description syntax (such as SDP [12] or XML based) 1186 and held in a file which would be acquired by the receiver before the 1187 FLUTE session begins by means of some transport protocol (such as 1188 Session Announcement Protocol [11], email, HTTP [6], SIP [22], manual 1189 pre-configuration, etc.) However, the way in which the receiver 1190 discovers the above-mentioned parameters is out of scope of this 1191 document, as it is for LCT and ALC. In particular, this specification 1192 does not mandate or exclude any mechanism. 1194 7. Security Considerations 1196 The security considerations that apply to, and are described in, ALC 1197 [2], LCT [3] and FEC [4] also apply to FLUTE. In addition, any 1198 security considerations that apply to any congestion control building 1199 block used in conjunction with FLUTE also apply to FLUTE. 1201 Because of the use of FEC, FLUTE is especially vulnerable to 1202 denial-of-service attacks by attackers that try to send forged 1203 packets to the session which would prevent successful reconstruction 1204 or cause inaccurate reconstruction of large portions of the FDT or 1205 file by receivers. Like ALC, FLUTE is particularly affected by such 1206 an attack because many receivers may receive the same forged packet. 1207 A malicious attacker may spoof file packets and cause incorrect 1208 recovery of a file. 1210 Even more damaging, a malicious forger may spoof FDT Instance 1211 packets, for example sending packets with erroneous FDT-Instance 1212 fields. Many attacks can follow this approach. For instance a 1213 malicious attacker may alter the Content-Location field of TOI 'n', 1214 to make it point to a system file or a user configuration file. 1215 Then, TOI 'n' can carry a Trojan Horse or some other type of virus. 1216 It is thus STRONGLY RECOMMENDED that the FLUTE delivery service at 1217 the receiver does not have write access to the system files or 1218 directories, or any other critical areas. As described for MIME 1219 [20][21], special consideration should be paid to the security 1220 implications of any MIME types that can cause the remote execution of 1221 any actions in the recipient's environment. Note, RFC 1521 [21] 1222 describes important security issues for this environment, even though 1223 its protocol is obsoleted by RFC 2048 [20]. 1225 Another example is generating a bad Content-MD5 sum, leading 1226 receivers to reject the associated file that will be declared 1227 corrupted. The Content-Encoding can also be modified, which also 1228 prevents the receivers to correctly handle the associated file. 1229 These examples show that the FDT information is critical to the FLUTE 1230 delivery service. 1232 At the application level, it is RECOMMENDED that an integrity check 1233 on the entire received object be done once the object is 1234 reconstructed to ensure it is the same as the sent object, especially 1235 for objects that are FDT Instances. Moreover, in order to obtain 1236 strong cryptographic integrity protection a digital signature 1237 verifiable by the receiver SHOULD be used to provide this application 1238 level integrity check. However, if even one corrupted or forged 1239 packet is used to reconstruct the object, it is likely that the 1240 received object will be reconstructed incorrectly. This will 1241 appropriately cause the integrity check to fail and in this case the 1242 inaccurately reconstructed object SHOULD be discarded. Thus, the 1243 acceptance of a single forged packet can be an effective denial of 1244 service attack for distributing objects, but an object integrity 1245 check at least prevents inadvertent use of inaccurately reconstructed 1246 objects. The specification of an application level integrity check 1247 of the received object is outside the scope of this document. 1249 At the packet level, it is RECOMMENDED that a packet level 1250 authentication be used to ensure that each received packet is an 1251 authentic and uncorrupted packet containing FEC data for the object 1252 arriving from the specified sender. Packet level authentication has 1253 the advantage that corrupt or forged packets can be discarded 1254 individually and the received authenticated packets can be used to 1255 accurately reconstruct the object. Thus, the effect of a denial of 1256 service attack that injects forged packets is proportional only to 1257 the number of forged packets, and not to the object size. Although 1258 there is currently no IETF standard that specifies how to do 1259 multicast packet level authentication, TESLA [14] is a known 1260 multicast packet authentication scheme that would work. 1262 In addition to providing protection against reconstruction of 1263 inaccurate objects, packet level authentication can also provide some 1264 protection against denial of service attacks on the multiple rate 1265 congestion control. Attackers can try to inject forged packets with 1266 incorrect congestion control information into the multicast stream, 1267 thereby potentially adversely affecting network elements and 1268 receivers downstream of the attack, and much less significantly the 1269 rest of the network and other receivers. Thus, it is also 1270 RECOMMENDED that packet level authentication be used to protect 1271 against such attacks. TESLA [14] can also be used to some extent to 1272 limit the damage caused by such attacks. However, with TESLA a 1273 receiver can only determine if a packet is authentic several seconds 1274 after it is received, and thus an attack against the congestion 1275 control protocol can be effective for several seconds before the 1276 receiver can react to slow down the session reception rate. 1278 Reverse Path Forwarding checks SHOULD be enabled in all network 1279 routers and switches along the path from the sender to receivers to 1280 limit the possibility of a bad agent injecting forged packets into 1281 the multicast tree data path. 1283 A receiver with an incorrect or corrupted implementation of the 1284 multiple rate congestion control building block may affect health of 1285 the network in the path between the sender and the receiver, and may 1286 also affect the reception rates of other receivers joined to the 1287 session. It is therefore RECOMMENDED that receivers be required to 1288 identify themselves as legitimate before they receive the Session 1289 Description needed to join the session. How receivers identify 1290 themselves as legitimate is outside the scope of this document. 1292 Another vulnerability of FLUTE is the potential of receivers 1293 obtaining an incorrect Session Description for the session. The 1294 consequences of this could be that legitimate receivers with the 1295 wrong Session Description are unable to correctly receive the session 1296 content, or that receivers inadvertently try to receive at a much 1297 higher rate than they are capable of, thereby disrupting traffic in 1298 portions of the network. To avoid these problems, it is RECOMMENDED 1299 that measures be taken to prevent receivers from accepting incorrect 1300 Session Descriptions, e.g., by using source authentication to ensure 1301 that receivers only accept legitimate Session Descriptions from 1302 authorized senders. How this is done is outside the scope of this 1303 document. 1305 8. IANA Considerations 1307 No information in this specification is directly subject to IANA 1308 registration. However, building blocks components used by ALC may 1309 introduce additional IANA considerations. In particular, the FEC 1310 building block used by FLUTE does require IANA registration of the 1311 FEC codec used. 1313 9. Acknowledgements 1315 The following persons have contributed to this specification: Brian 1316 Adamson, Mark Handley, Esa Jalonen, Roger Kermode, Juha-Pekka Luoma, 1317 Jani Peltotalo, Sami Peltotalo, Topi Pohjolainen and Lorenzo 1318 Vicisano. The authors would like to thank all the contributors for 1319 their valuable work in reviewing and providing feedback regarding 1320 this specification. 1322 Normative references 1324 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 1325 Levels", RFC 2119, BCP 14, March 1997. 1327 [2] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L. and J. Crowcroft, 1328 "Asynchronous Layered Coding (ALC) Protocol Instantiation", RFC 1329 3450, December 2002. 1331 [3] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M. and 1332 J. Crowcroft, "Layered Coding Transport (LCT) Building Block", 1333 RFC 3451, December 2002. 1335 [4] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M. and 1336 J. Crowcroft, "Forward Error Correction (FEC) Building Block", 1337 RFC 3452, December 2002. 1339 [5] Mills, D., "Network Time Protocol (Version 3), Specification, 1340 Implementation and Analysis", RFC 1305, March 1992. 1342 [6] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., 1343 Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- 1344 HTTP/1.1", RFC 2616, June 1999. 1346 [7] Luby, M. and L. Vicisano, "Compact Forward Error Correction 1347 (FEC) Schemes", RFC 3695, February 2004. 1349 [8] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML 1350 Schema Part 1: Structures", W3C Recommendation, May 2001. 1352 [9] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C 1353 Recommendation, May 2001. 1355 Informative references 1357 [10] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format 1358 Specification version 3.3", RFC 1950, May 1996. 1360 [11] Handley, M., Perkins, C. and E. Whelan, "Session Announcement 1361 Protocol", RFC 2974, October 2000. 1363 [12] Handley, M. and V. Jacobson, "Session Description Protocol", 1364 RFC 2327, April 1998. 1366 [13] Deering, S., "Host Extensions for IP Multicasting", RFC 1112, 1367 STD 5, August 1989. 1369 [14] Perrig, A., Canetti, R., Song, D. and J. Tygar, "Efficient and 1370 Secure Source Authentication for Multicast, Network and 1371 Distributed System Security Symposium, NDSS 2001, pp. 35-46.", 1372 February 2001. 1374 [15] Holbrook, H., "A Channel Model for Multicast, Ph.D. 1375 Dissertation, Stanford University, Department of Computer 1376 Science, Stanford, California", August 2001. 1378 [16] Deutsch, P., "DEFLATE Compressed Data Format Specification 1379 version 1.3", RFC 1951, May 1996. 1381 [17] Deutsch, P., "GZIP file format specification version 4.3", RFC 1382 1952, May 1996. 1384 [18] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 1385 2633, June 1999. 1387 [19] Eastlake, D., Reagle, J. and D. Solo, "(Extensible Markup 1388 Language) XML-Signature Syntax and Processing", RFC 3275, March 1389 2002. 1391 [20] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet 1392 Mail Extensions (MIME) Part Four: Registration Procedures", RFC 1393 2048, November 1996. 1395 [21] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part 1396 Three: Message Header Extensions for Non-ASCII Text", RFC 1521, 1397 November 1996. 1399 [22] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1400 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1401 session initiation protocol", RFC 3261, June 2002. 1403 Authors' Addresses 1405 Toni Paila 1406 Nokia 1407 Itamerenkatu 11-13 1408 Helsinki FIN-00180 1409 Finland 1411 EMail: toni.paila@nokia.com 1413 Michael Luby 1414 Digital Fountain 1415 39141 Civic Center Dr. 1416 Suite 300 1417 Fremont, CA 94538 1418 USA 1420 EMail: luby@digitalfountain.com 1422 Rami Lehtonen 1423 TeliaSonera 1424 Hatanpaan valtatie 18 1425 Tampere FIN-33100 1426 Finland 1428 EMail: rami.lehtonen@teliasonera.com 1430 Vincent Roca 1431 INRIA Rhone-Alpes 1432 655, av. de l'Europe 1433 Montbonnot 1434 St Ismier cedex 38334 1435 France 1437 EMail: vincent.roca@inrialpes.fr 1439 Rod Walsh 1440 Nokia 1441 Visiokatu 1 1442 Tampere FIN-33720 1443 Finland 1445 EMail: rod.walsh@nokia.com 1447 Appendix A. Receiver operation (informative) 1449 This section gives an example how the receiver of the file delivery 1450 session may operate. Instead of a detailed state-by-state 1451 specification the following should be interpreted as a rough sequence 1452 of an envisioned file delivery receiver. 1454 1. The receiver obtains the description of the file delivery session 1455 identified by the pair: (source IP address, Transport Session 1456 Identifier). The receiver also obtains the destination IP 1457 addresses and respective ports associated with the file delivery 1458 session. 1460 2. The receiver joins the channels in order to receive packets 1461 associated with the file delivery session. The receiver may 1462 schedule this join operation utilizing the timing information 1463 contained in a possible description of the file delivery session. 1465 3. The receiver receives ALC/LCT packets associated with the file 1466 delivery session. The receiver checks that the packets match the 1467 declared Transport Session Identifier. If not, packets are 1468 silently discarded. 1470 4. While receiving, the receiver demultiplexes packets based on their 1471 TOI and stores the relevant packet information in an appropriate 1472 area for recovery of the corresponding file. Multiple files can be 1473 reconstructed concurrently. 1475 5. Receiver recovers an object. An object can be recovered when an 1476 appropriate set of packets containing Encoding Symbols for the 1477 transport object have been received. An appropriate set of packets 1478 is dependent on the properties of the FEC Encoding ID and FEC 1479 Instance ID, and on other information contained in the FEC Object 1480 Transmission Information. 1482 6. If the recovered object was an FDT Instance with FDT Instance ID 1483 'N', the receiver parses the payload of the instance 'N' of FDT 1484 and updates its FDT database accordingly. The receiver identifies 1485 FDT Instances within a file delivery session by the EXT_FDT header 1486 extension. Any object that is delivered using EXT_FDT header 1487 extension is an FDT Instance, uniquely identified by the FDT 1488 Instance ID. Note that TOI '0' is exclusively reserved for FDT 1489 delivery. 1491 7. If the object recovered is not an FDT Instance but a file, the 1492 receiver looks up its FDT database to get the properties described 1493 in the database, and assigns file with the given properties. The 1494 receiver also checks that received content length matches with the 1495 description in the database. Optionally, if MD5 checksum has been 1496 used, the receiver checks that calculated MD5 matches with the 1497 description in the FDT database. 1499 8. The actions the receiver takes with imperfectly received files 1500 (missing data, mismatching digestive, etc.) is outside the scope 1501 of this specification. When a file is recovered before the 1502 associated file description entry is available, a possible 1503 behavior is to wait until an FDT Instance is received that 1504 includes the missing properties. 1506 9. If the file delivery session end time has not been reached go back 1507 to 3. Otherwise end. 1509 Appendix B. Example of FDT Instance (informative) 1511 1512 1516 1520 1528 1530 Intellectual Property Statement 1532 The IETF takes no position regarding the validity or scope of any 1533 intellectual property or other rights that might be claimed to 1534 pertain to the implementation or use of the technology described in 1535 this document or the extent to which any license under such rights 1536 might or might not be available; neither does it represent that it 1537 has made any effort to identify any such rights. Information on the 1538 IETF's procedures with respect to rights in standards-track and 1539 standards-related documentation can be found in BCP-11. 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