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Consider using 'MUST NOT' instead (if that is what you mean). Found 'MAY NOT' in this paragraph: If more than one object is to be carried within a session then the Transmission Object Identifier (TOI) MUST be used in the LCT header to identify which packets are to be associated with which objects. In this case the receiver MUST use the TOI to associate received packets with objects. The TOI is scoped by the IP address of the sender and the TSI, i.e., the TOI is scoped by the session. The TOI for each object is REQUIRED to be unique within a session, but MAY NOT be unique across sessions. Furthermore, the same object MAY have a different TOI in different sessions. The mapping between TOIs and objects carried in a session is outside the scope of this document. == The expression 'MAY NOT', while looking like RFC 2119 requirements text, is not defined in RFC 2119, and should not be used. Consider using 'MUST NOT' instead (if that is what you mean). Found 'MAY NOT' in this paragraph: All senders and receivers implementing ALC MUST support the EXT_NOP Header Extension and MUST recognize EXT_AUTH, but MAY NOT be able to parse its content. The EXT_NOP and EXT_AUTH LCT Header Extensions are defined in [I-D.ietf-rmt-bb-lct-revised] -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (March 3, 2006) is 6628 days in the past. Is this intentional? 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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Reliable Multicast Transport (RMT) Luby 3 Working Group Watson 4 Internet-Draft Digital Fountain 5 Expires: September 4, 2006 Vicisano 6 Cisco Systems, Inc. 7 March 3, 2006 9 Asynchronous Layered Coding (ALC) Protocol Instantiation 10 draft-ietf-rmt-pi-alc-revised-02 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on September 4, 2006. 37 Copyright Notice 39 Copyright (C) The Internet Society (2006). 41 Abstract 43 This document describes the Asynchronous Layered Coding (ALC) 44 protocol, a massively scalable reliable content delivery protocol. 45 Asynchronous Layered Coding combines the Layered Coding Transport 46 (LCT) building block, a multiple rate congestion control building 47 block and the Forward Error Correction (FEC) building block to 48 provide congestion controlled reliable asynchronous delivery of 49 content to an unlimited number of concurrent receivers from a single 50 sender. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. Delivery service models . . . . . . . . . . . . . . . . . 4 56 1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 4 57 1.3. Environmental Requirements and Considerations . . . . . . 5 58 2. Architecture Definition . . . . . . . . . . . . . . . . . . . 6 59 2.1. LCT building block . . . . . . . . . . . . . . . . . . . . 7 60 2.2. Multiple rate congestion control building block . . . . . 8 61 2.3. FEC building block . . . . . . . . . . . . . . . . . . . . 9 62 2.4. Session Description . . . . . . . . . . . . . . . . . . . 11 63 2.5. Packet authentication building block . . . . . . . . . . . 12 64 3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 13 65 4. Functionality Definition . . . . . . . . . . . . . . . . . . . 14 66 4.1. Packet format used by ALC . . . . . . . . . . . . . . . . 14 67 4.2. LCT Header-Extension Fields . . . . . . . . . . . . . . . 15 68 4.3. Sender Operation . . . . . . . . . . . . . . . . . . . . . 16 69 4.4. Receiver Operation . . . . . . . . . . . . . . . . . . . . 16 70 5. Security Considerations . . . . . . . . . . . . . . . . . . . 19 71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 72 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 73 8. Changes from RFC3450 . . . . . . . . . . . . . . . . . . . . . 23 74 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 75 9.1. Normative references . . . . . . . . . . . . . . . . . . . 24 76 9.2. Informative references . . . . . . . . . . . . . . . . . . 24 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 78 Intellectual Property and Copyright Statements . . . . . . . . . . 27 80 1. Introduction 82 This document describes a massively scalable reliable content 83 delivery protocol, Asynchronous Layered Coding (ALC), for multiple 84 rate congestion controlled reliable content delivery. The protocol 85 is specifically designed to provide massive scalability using IP 86 multicast as the underlying network service. Massive scalability in 87 this context means the number of concurrent receivers for an object 88 is potentially in the millions, the aggregate size of objects to be 89 delivered in a session ranges from hundreds of kilobytes to hundreds 90 of gigabytes, each receiver can initiate reception of an object 91 asynchronously, the reception rate of each receiver in the session is 92 the maximum fair bandwidth available between that receiver and the 93 sender, and all of this can be supported using a single sender. 95 Because ALC is focused on reliable content delivery, the goal is to 96 deliver objects as quickly as possible to each receiver while at the 97 same time remaining network friendly to competing traffic. Thus, the 98 congestion control used in conjunction with ALC should strive to 99 maximize use of available bandwidth between receivers and the sender 100 while at the same time backing off aggressively in the face of 101 competing traffic. 103 The sender side of ALC consists of generating packets based on 104 objects to be delivered within the session and sending the 105 appropriately formatted packets at the appropriate rates to the 106 channels associated with the session. The receiver side of ALC 107 consists of joining appropriate channels associated with the session, 108 performing congestion control by adjusting the set of joined channels 109 associated with the session in response to detected congestion, and 110 using the packets to reliably reconstruct objects. All information 111 flow in an ALC session is in the form of data packets sent by a 112 single sender to channels that receivers join to receive data. 114 ALC does specify the Session Description needed by receivers before 115 they join a session, but the mechanisms by which receivers obtain 116 this required information is outside the scope of ALC. An 117 application that uses ALC may require that receivers report 118 statistics on their reception experience back to the sender, but the 119 mechanisms by which receivers report back statistics is outside the 120 scope of ALC. In general, ALC is designed to be a minimal protocol 121 instantiation that provides reliable content delivery without 122 unnecessary limitations to the scalability of the basic protocol. 124 This document is a product of the IETF RMT WG and follows the general 125 guidelines provided in [RFC3269]. 127 RFC3450 [RFC3450] contained a previous versions of the protocol 128 defined in this specification. RFC3450 was published in the 129 "Experimental" category. It was the stated intent of the RMT working 130 group to re-submit these specifications as an IETF Proposed Standard 131 in due course. 133 This Proposed Standard specification is thus based on and backwards 134 compatible with the protocol defined in RFC3450 [RFC3450] updated 135 according to accumulated experience and growing protocol maturity 136 since its original publication. Said experience applies both to this 137 specification itself and to congestion control strategies related to 138 the use of this specification. 140 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 141 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 142 document are to be interpreted as described in BCP 14, [RFC2119]. 144 1.1. Delivery service models 146 ALC can support several different reliable content delivery service 147 models as described in [I-D.ietf-rmt-bb-lct-revised]. 149 1.2. Scalability 151 Massive scalability is a primary design goal for ALC. IP multicast 152 is inherently massively scalable, but the best effort service that it 153 provides does not provide session management functionality, 154 congestion control or reliability. ALC provides all of this on top 155 of IP multicast without sacrificing any of the inherent scalability 156 of IP multicast. ALC has the following properties: 158 o To each receiver, it appears as if though there is a dedicated 159 session from the sender to the receiver, where the reception rate 160 adjusts to congestion along the path from sender to receiver. 162 o To the sender, there is no difference in load or outgoing rate if 163 one receiver is joined to the session or a million (or any number 164 of) receivers are joined to the session, independent of when the 165 receivers join and leave. 167 o No feedback packets are required from receivers to the sender. 169 o Almost all packets in the session that pass through a bottleneck 170 link are utilized by downstream receivers, and the session shares 171 the link with competing flows fairly in proportion to their 172 utility. 174 Thus, ALC provides a massively scalable content delivery transport 175 that is network friendly. 177 ALC intentionally omits any application specific features that could 178 potentially limit its scalability. By doing so, ALC provides a 179 minimal protocol that is massively scalable. Applications may be 180 built on top of ALC to provide additional features that may limit the 181 scalability of the application. Such applications are outside the 182 scope of this document. 184 1.3. Environmental Requirements and Considerations 186 All of the environmental requirements and considerations that apply 187 to the LCT building block [I-D.ietf-rmt-bb-lct-revised], the FEC 188 building block [I-D.ietf-rmt-fec-bb-revised], the multiple rate 189 congestion control building block and to any additional building 190 blocks that ALC uses also apply to ALC. 192 One issues that is specific to ALC with respect to the Any- Source 193 Multicast (ASM) model of IP multicast as defined in RFC 1112 194 [RFC1112] is the way the multiple rate congestion control building 195 block interacts with ASM. The congestion control building block may 196 use the measured difference in time between when a join to a channel 197 is sent and when the first packet from the channel arrives in 198 determining the receiver reception rate. The congestion control 199 building block may also uses packet sequence numbers per channel to 200 measure losses, and this is also used to determine the receiver 201 reception rate. These features raise two concerns with respect to 202 ASM: The time difference between when the join to a channel is sent 203 and when the first packet arrives can be significant due to the use 204 of Rendezvous Points (RPs) and the MSDP protocol, and packets can be 205 lost in the switch over from the (*,G) join to the RP and the (S,G) 206 join directly to the sender. Both of these issues could potentially 207 substantially degrade the reception rate of receivers. To ameliorate 208 these concerns, it is RECOMMENDED that the RP be as close to the 209 sender as possible. SSM does not share these same concerns. For a 210 fuller consideration of these issues, consult the multiple rate 211 congestion control building block. 213 2. Architecture Definition 215 ALC uses the LCT building block [I-D.ietf-rmt-bb-lct-revised] to 216 provide in-band session management functionality. ALC uses a 217 multiple rate congestion control building block that is compliant 218 with [RFC2357] to provide congestion control that is feedback free. 219 Receivers adjust their reception rates individually by joining and 220 leaving channels associated with the session. ALC uses the FEC 221 building block [I-D.ietf-rmt-fec-bb-revised] to provide reliability. 222 The sender generates encoding symbols based on the object to be 223 delivered using FEC codes and sends them in packets to channels 224 associated with the session. Receivers simply wait for enough 225 packets to arrive in order to reliably reconstruct the object. Thus, 226 there is no request for retransmission of individual packets from 227 receivers that miss packets in order to assure reliable reception of 228 an object, and the packets and their rate of transmission out of the 229 sender can be independent of the number and the individual reception 230 experiences of the receivers. 232 The definition of a session for ALC is the same as it is for LCT. An 233 ALC session comprises multiple channels originating at a single 234 sender that are used for some period of time to carry packets 235 pertaining to the transmission of one or more objects that can be of 236 interest to receivers. Congestion control is performed over the 237 aggregate of packets sent to channels belonging to a session. The 238 fact that an ALC session is restricted to a single sender does not 239 preclude the possibility of receiving packets for the same objects 240 from multiple senders. However, each sender would be sending packets 241 to a a different session to which congestion control is individually 242 applied. Although receiving concurrently from multiple sessions is 243 allowed, how this is done at the application level is outside the 244 scope of this document. 246 ALC is a protocol instantiation as defined in [RFC3048]. This 247 document describes version 1 of ALC which MUST use version 1 of LCT 248 described in [I-D.ietf-rmt-bb-lct-revised]. Like LCT, ALC is 249 designed to be used with the IP multicast network service. This 250 specification defines ALC as payload of the UDP transport protocol 251 [RFC0768] that supports IP multicast delivery of packets. Future 252 versions of this specification, or companion documents may extend ALC 253 to use the IP network layer service directly. ALC could be used as 254 the basis for designing a protocol that uses a different underlying 255 network service such as unicast UDP, but the design of such a 256 protocol is outside the scope of this document. 258 An ALC packet header immediately follows the UDP header and consists 259 of the default LCT header that is described in [I-D.ietf-rmt-bb-lct- 260 revised] followed by the FEC Payload ID that is described in 262 [I-D.ietf-rmt-fec-bb-revised]. The Congestion Control Information 263 field within the LCT header carries the required Congestion Control 264 Information that is described in the multiple rate congestion control 265 building block specified that is compliant with [RFC2357]. The 266 packet payload that follows the ALC packet header consists of 267 encoding symbols that are identified by the FEC Payload ID as 268 described in [I-D.ietf-rmt-fec-bb-revised]. 270 Each receiver is required to obtain a Session Description before 271 joining an ALC session. As described later, the Session Description 272 includes out-of-band information required for the LCT, FEC and the 273 multiple rate congestion control building blocks. The FEC Object 274 Transmission Information specified in the FEC building block 275 [I-D.ietf-rmt-fec-bb-revised] required for each object to be received 276 by a receiver can be communicated to a receiver either out-of-band or 277 in-band using a Header Extension. The means for communicating the 278 Session Description and the FEC Object Transmission Information to a 279 receiver is outside the scope of this document. 281 2.1. LCT building block 283 LCT requires receivers to be able to uniquely identify and 284 demultiplex packets associated with an LCT session, and ALC inherits 285 and strengthens this requirement. A Transport Session Identifier 286 (TSI) MUST be associated with each session and MUST be carried in the 287 LCT header of each ALC packet. The TSI is scoped by the sender IP 288 address, and the (sender IP address, TSI) pair MUST uniquely identify 289 the session. 291 The LCT header contains a Congestion Control Information (CCI) field 292 that MUST be used to carry the Congestion Control Information from 293 the specified multiple rate congestion control protocol. There is a 294 field in the LCT header that specifies the length of the CCI field, 295 and the multiple rate congestion control building block MUST uniquely 296 identify a format of the CCI field that corresponds to this length. 298 The LCT header contains a Codepoint field that MAY be used to 299 communicate to a receiver the settings for information that may vary 300 during a session. If used, the mapping between settings and 301 Codepoint values is to be communicated in the Session Description, 302 and this mapping is outside the scope of this document. For example, 303 the FEC Encoding ID that is part of the FEC Object Transmission 304 Information as specified in the FEC building block [I-D.ietf-rmt-fec- 305 bb-revised] could vary for each object carried in the session, and 306 the Codepoint value could be used to communicate the FEC Encoding ID 307 to be used for each object. The mapping between FEC Encoding IDs and 308 Codepoints could be, for example, the identity mapping. 310 If more than one object is to be carried within a session then the 311 Transmission Object Identifier (TOI) MUST be used in the LCT header 312 to identify which packets are to be associated with which objects. 313 In this case the receiver MUST use the TOI to associate received 314 packets with objects. The TOI is scoped by the IP address of the 315 sender and the TSI, i.e., the TOI is scoped by the session. The TOI 316 for each object is REQUIRED to be unique within a session, but MAY 317 NOT be unique across sessions. Furthermore, the same object MAY have 318 a different TOI in different sessions. The mapping between TOIs and 319 objects carried in a session is outside the scope of this document. 321 If only one object is carried within a session then the TOI MAY be 322 omitted from the LCT header. 324 The LCT header from version 1 of the LCT building block [I-D.ietf- 325 rmt-bb-lct-revised] MUST be used. 327 The LCT Header includes a two-bit Protocol Specific Indication (PSI) 328 field. These two bits are used by ALC as follows: 330 PSI bit 0 (LSB) - Source Packet Indicator (SPI) 332 PSI bit 1 (MSB) - Reserved 334 The Source Packet Indicator is used with systematic FEC Schemes which 335 define a different FEC Payload ID format for packets containing only 336 source data compared to the FEC Payload ID format for packets 337 containing repair data. For such FEC Schemes, then the SPI MUST be 338 set to 1 when the FEC Payload ID format for packets containing only 339 source data is used and the SPI MUST be set to zero, when the FEC 340 Payload ID for packerts containing repair data is used. In the case 341 of FEC Schemes which define only a single FEC Payload ID format, then 342 the SPI MUST be set to zero by the sender and MUST be ignored by the 343 receiver. 345 Support of two FEC Payload ID formats allows FEC Payload ID 346 information which is only of relevance when FEC decoding is to be 347 performed to be provided in the FEC Payload ID format for packets 348 containing repair data. This information need not be processed by 349 receivers which do not perform FEC decoding (either because no FEC 350 decoding is required or because the receiver does not support FEC 351 decoding). 353 2.2. Multiple rate congestion control building block 355 Implementors of ALC MUST implement a multiple rate feedback-free 356 congestion control building block that is in accordance to [RFC2357]. 357 Congestion control MUST be applied to all packets within a session 358 independently of which information about which object is carried in 359 each packet. Multiple rate congestion control is specified because 360 of its suitability to scale massively and because of its suitability 361 for reliable content delivery. The multiple rate congestion control 362 building block MUST specify in-band Congestion Control Information 363 (CCI) that MUST be carried in the CCI field of the LCT header. The 364 multiple rate congestion control building block MAY specify more than 365 one format, but it MUST specify at most one format for each of the 366 possible lengths 32, 64, 96 or 128 bits. The value of C in the LCT 367 header that determines the length of the CCI field MUST correspond to 368 one of the lengths for the CCI defined in the multiple rate 369 congestion control building block, this length MUST be the same for 370 all packets sent to a session, and the CCI format that corresponds to 371 the length as specified in the multiple rate congestion control 372 building block MUST be the format used for the CCI field in the LCT 373 header. 375 When using a multiple rate congestion control building block a sender 376 sends packets in the session to several channels at potentially 377 different rates. Then, individual receivers adjust their reception 378 rate within a session by adjusting which set of channels they are 379 joined to at each point in time depending on the available bandwidth 380 between the receiver and the sender, but independent of other 381 receivers. 383 2.3. FEC building block 385 The FEC building block [I-D.ietf-rmt-fec-bb-revised] provides 386 reliable object delivery within an ALC session. Each object sent in 387 the session is independently encoded using FEC codes as described in 388 [RFC3453], which provide a more in-depth description of the use of 389 FEC codes in reliable content delivery protocols. All packets in an 390 ALC session MUST contain an FEC Payload ID in a format that is 391 compliant with the FEC Scheme in use. The FEC Payload ID uniquely 392 identifies the encoding symbols that constitute the payload of each 393 packet, and the receiver MUST use the FEC Payload ID to determine how 394 the encoding symbols carried in the payload of the packet were 395 generated from the object as described in the FEC building block. 397 As described in [I-D.ietf-rmt-fec-bb-revised], a receiver is REQUIRED 398 to obtain the FEC Object Transmission Information for each object for 399 which data packets are received from the session. In the context of 400 ALC, the FEC Object Transmission Information includes: 402 o The FEC Encoding ID. 404 o If an Under-Specified FEC Encoding ID is used then the FEC 405 Instance ID associated with the FEC Encoding ID. 407 o For each object in the session, the transfer length of the object 408 in bytes. 410 Additional FEC Object Transmission Information may be required 411 depending on the FEC Scheme that is used (identified by the FEC 412 Encoding ID). 414 Some of the FEC Object Transmission Information MAY be implicit based 415 on the FEC Scheme and/or implementation. As an example, source block 416 lengths may be derived by a fixed algorithm from the object length. 417 As another example, it may be that all source blocks are the same 418 length and this is what is passed out-of-band to the receiver. As 419 another example, it could be that the full sized source block length 420 is provided and this is the length used for all but the last source 421 block, which is calculated based on the full source block length and 422 the object length. As another example, it could be that the same FEC 423 Encoding ID and FEC Instance ID are always used for a particular 424 application and thus the FEC Encoding ID and FEC Instance ID are 425 implicitly defined. 427 Sometimes the objects that will be sent in a session are completely 428 known before the receiver joins the session, in which case the FEC 429 Object Transmission Information for all objects in the session can be 430 communicated to receivers before they join the session. At other 431 times the objects may not know when the session begins, or receivers 432 may join a session in progress and may not be interested in some 433 objects for which transmission has finished, or receivers may leave a 434 session before some objects are even available within the session. 435 In these cases, the FEC Object Transmission Information for each 436 object may be dynamically communicated to receivers at or before the 437 time packets for the object are received from the session. This may 438 be accomplished using either an out-of-band mechanism, in-band using 439 the Codepoint field or a Header Extension, or any combination of 440 these methods. How the FEC Object Transmission Information is 441 communicated to receivers is outside the scope of this document. 443 If packets for more than one object are transmitted within a session 444 then a Transmission Object Identifier (TOI) that uniquely identifies 445 objects within a session MUST appear in each packet header. Portions 446 of the FEC Object Transmission Information could be the same for all 447 objects in the session, in which case these portions can be 448 communicated to the receiver with an indication that this applies to 449 all objects in the session. These portions may be implicitly 450 determined based on the application, e.g., an application may use the 451 same FEC Encoding ID for all objects in all sessions. If there is a 452 portion of the FEC Object Transmission Information that may vary from 453 object to object and if this FEC Object Transmission Information is 454 communicated to a receiver out-of-band then the TOI for the object 455 MUST also be communicated to the receiver together with the 456 corresponding FEC Object Transmission Information, and the receiver 457 MUST use the corresponding FEC Object Transmission Information for 458 all packets received with that TOI. How the TOI and corresponding 459 FEC Object Transmission Information is communicated out-of-band to 460 receivers is outside the scope of this document. 462 It is also possible that there is a portion of the FEC Object 463 Transmission Information that may vary from object to object that is 464 carried in-band, for example in the CodePoint field or in Header 465 Extensions. How this is done is outside the scope of this document. 466 In this case the FEC Object Transmission Information is associated 467 with the object identified by the TOI carried in the packet. 469 2.4. Session Description 471 The Session Description that a receiver is REQUIRED to obtain before 472 joining an ALC session MUST contain the following information: 474 o The multiple rate congestion control building block to be used for 475 the session; 477 o The sender IP address; 479 o The number of channels in the session; 481 o The address and port number used for each channel in the session; 483 o The Transport Session ID (TSI) to be used for the session; 485 o An indication of whether or not the session carries packets for 486 more than one object; 488 o If Header Extensions are to be used, the format of these Header 489 Extensions. 491 o Enough information to determine the packet authentication scheme 492 being used, if it is being used. 494 How the Session Description is communicated to receivers is outside 495 the scope of this document. 497 The Codepoint field within the LCT portion of the header CAN be used 498 to communicate in-band some of the dynamically changing information 499 within a session. To do this, a mapping between Codepoint values and 500 the different dynamic settings MUST be included within the Session 501 Description, and then settings to be used are communicated via the 502 Codepoint value placed into each packet. For example, it is possible 503 that multiple objects are delivered within the same session and that 504 a different FEC encoding algorithm is used for different types of 505 objects. Then the Session Description could contain the mapping 506 between Codepoint values and FEC Encoding IDs. As another example, 507 it is possible that a different packet authentication scheme is used 508 for different packets sent to the session. In this case, the mapping 509 between the packet authentication scheme and Codepoint values could 510 be provided in the Session Description. Combinations of settings can 511 be mapped to Codepoint values as well. For example, a particular 512 combination of a FEC Encoding ID and a packet authentication scheme 513 could be associated with a Codepoint value. 515 The Session Description could also include, but is not limited to: 517 o The mappings between combinations of settings and Codepoint 518 values; 520 o The data rates used for each channel; 522 o The length of the packet payload; 524 o Any information that is relevant to each object being transported, 525 such as the Object Transmission Information for each object, when 526 the object will be available within the session and for how long. 528 The Session Description could be in a form such as SDP as defined in 529 [RFC2327], or XML metadata as defined in [RFC3023], or HTTP/Mime 530 headers as defined in [RFC2616], etc. It might be carried in a 531 session announcement protocol such as SAP as defined in [RFC2974], 532 obtained using a proprietary session control protocol, located on a 533 web page with scheduling information, or conveyed via E-mail or other 534 out-of-band methods. Discussion of Session Description formats and 535 methods for communication of Session Descriptions to receivers is 536 beyond the scope of this document. 538 2.5. Packet authentication building block 540 It is RECOMMENDED that implementors of ALC use some packet 541 authentication scheme to protect the protocol from attacks. An 542 example of a possibly suitable scheme is described in [PER2001]. 543 Packet authentication in ALC, if used, is to be integrated through 544 the Header Extension support for packet authentication provided in 545 the LCT building block. 547 3. Conformance Statement 549 This Protocol Instantiation document, in conjunction with the LCT 550 building block [I-D.ietf-rmt-bb-lct-revised], the FEC building block 551 [I-D.ietf-rmt-fec-bb-revised] and with a multiple rate congestion 552 control building block completely specifies a working reliable 553 multicast transport protocol that conforms to the requirements 554 described in [RFC2357]. 556 4. Functionality Definition 558 This section describes the format and functionality of the data 559 packets carried in an ALC session as well as the sender and receiver 560 operations for a session. 562 4.1. Packet format used by ALC 564 The packet format used by ALC is the UDP header followed by the LCT 565 header followed by the FEC Payload ID followed by the packet payload. 566 The LCT header is defined in the LCT building block [I-D.ietf-rmt-bb- 567 lct-revised] and the FEC Payload ID is described in the FEC building 568 block [I-D.ietf-rmt-fec-bb-revised]. The Congestion Control 569 Information field in the LCT header contains the REQUIRED Congestion 570 Control Information that is described in the multiple rate congestion 571 control building block used. The packet payload contains encoding 572 symbols generated from an object. If more than one object is carried 573 in the session then the Transmission Object ID (TOI) within the LCT 574 header MUST be used to identify which object the encoding symbols are 575 generated from. Within the scope of an object, encoding symbols 576 carried in the payload of the packet are identified by the FEC 577 Payload ID as described in the FEC building block. 579 The version number of ALC specified in this document is 1. The 580 version number field of the LCT header MUST be interpreted as the ALC 581 version number field. This version of ALC implicitly makes use of 582 version 1 of the LCT building block defined in [I-D.ietf-rmt-bb-lct- 583 revised]. 585 The overall ALC packet format is depicted in Figure 1. The packet is 586 an IP packet, either IPv4 or IPv6, and the IP header precedes the UDP 587 header. The ALC packet format has no dependencies on the IP version 588 number. 590 0 1 2 3 591 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 1 592 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 593 | UDP header | 594 | | 595 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 596 | LCT header | 597 | | 598 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 599 | FEC Payload ID | 600 | | 601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 602 | Encoding Symbol(s) | 603 | ... | 604 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 606 Figure 1: Overall ALC packet format 608 In some special cases an ALC sender may need to produce ALC packets 609 that do not contain any payload. This may be required, for example, 610 to signal the end of a session or to convey congestion control 611 information. These data-less packets do not contain the FEC Payload 612 ID either, but only the LCT header fields. The total datagram 613 length, conveyed by outer protocol headers (e.g., the IP or UDP 614 header), enables receivers to detect the absence of the ALC payload 615 and FEC Payload ID. 617 For ALC the length of the TSI field within the LCT header is REQUIRED 618 to be non-zero. This implies that the sender MUST NOT set both the 619 LCT flags S and H to zero. 621 4.2. LCT Header-Extension Fields 623 All senders and receivers implementing ALC MUST support the EXT_NOP 624 Header Extension and MUST recognize EXT_AUTH, but MAY NOT be able to 625 parse its content. The EXT_NOP and EXT_AUTH LCT Header Extensions 626 are defined in [I-D.ietf-rmt-bb-lct-revised] 628 This specification defines a new LCT Header Extension, "EXT_FTI", for 629 the purpose of communicating the FEC Object Transmission Information 630 in association with data packets of an object. The LCT Header 631 Extension Type for EXT_FTI is 64. 633 The Header Extension Content (HEC) field of the EXT_FTI LCT Header 634 Extension contains the encoded FEC Object Transmission Information as 635 defined in [I-D.ietf-rmt-fec-bb-revised]. The format of the encoded 636 FEC Object Transmission Information is dependent on the FEC Scheme in 637 use and so is outside the scope of this document. 639 4.3. Sender Operation 641 The sender operation when using ALC includes all the points made 642 about the sender operation when using the LCT building block 643 [I-D.ietf-rmt-bb-lct-revised], the FEC building block [I-D.ietf-rmt- 644 fec-bb-revised] and the multiple rate congestion control building 645 block. 647 A sender using ALC MUST make available the required Session 648 Description as described in Section 2.4. A sender also MUST make 649 available the required FEC Object Transmission Information as 650 described in Section 2.3. 652 Within a session a sender transmits a sequence of packets to the 653 channels associated with the session. The ALC sender MUST obey the 654 rules for filling in the CCI field in the packet headers and MUST 655 send packets at the appropriate rates to the channels associated with 656 the session as dictated by the multiple rate congestion control 657 building block. 659 The ALC sender MUST use the same TSI for all packets in the session. 660 Several objects MAY be delivered within the same ALC session. If 661 more than one object is to be delivered within a session then the 662 sender MUST use the TOI field and each object MUST be identified by a 663 unique TOI within the session, and the sender MUST use corresponding 664 TOI for all packets pertaining to the same object. The FEC Payload 665 ID MUST correspond to the encoding symbol(s) for the object carried 666 in the payload of the packet. 668 It is RECOMMENDED that packet authentication be used. If packet 669 authentication is used then the Header Extensions described in 670 Section 4.2 MUST be used to carry the authentication. 672 4.4. Receiver Operation 674 The receiver operation when using ALC includes all the points made 675 about the receiver operation when using the LCT building block 676 [I-D.ietf-rmt-bb-lct-revised], the FEC building block [I-D.ietf-rmt- 677 fec-bb-revised] and the multiple rate congestion control building 678 block. 680 To be able to participate in a session, a receiver MUST obtain the 681 REQUIRED Session Description as listed in Section 2.4. How receivers 682 obtain a Session Description is outside the scope of this document. 684 To be able to be a receiver in a session, the receiver MUST be able 685 to process the ALC header. The receiver MUST be able to discard, 686 forward, store or process the other headers and the packet payload. 687 If a receiver is not able to process the ALC header, it MUST drop 688 from the session. 690 As described in Section 2.3, a receiver MUST obtain the required FEC 691 Object Transmission Information for each object for which the 692 receiver receives and processes packets. 694 Upon receipt of each packet the receiver proceeds with the following 695 steps in the order listed. 697 1. The receiver MUST parse the packet header and verify that it is a 698 valid header. If it is not valid then the packet MUST be 699 discarded without further processing. If multiple packets are 700 received that cannot be parsed then the receiver SHOULD leave the 701 session. 703 2. The receiver MUST verify that the sender IP address together with 704 the TSI carried in the header matches one of the (sender IP 705 address, TSI) pairs that was received in a Session Description 706 and that the receiver is currently joined to. If there is not a 707 match then the packet MUST be discarded without further 708 processing. If multiple packets are received with non-matching 709 (sender IP address, TSI) values then the receiver SHOULD leave 710 the session. If the receiver is joined to multiple ALC sessions 711 then the remainder of the steps are performed within the scope of 712 the (sender IP address, TSI) session of the received packet. 714 3. The receiver MUST process and act on the CCI field in accordance 715 with the multiple rate congestion control building block. 717 4. If more than one object is carried in the session, the receiver 718 MUST verify that the TOI carried in the LCT header is valid. If 719 the TOI is not valid, the packet MUST be discarded without 720 further processing. 722 5. The receiver SHOULD process the remainder of the packet, 723 including interpreting the other header fields appropriately, and 724 using the FEC Payload ID and the encoding symbol(s) in the 725 payload to reconstruct the corresponding object. 727 It is RECOMMENDED that packet authentication be used. If packet 728 authentication is used then it is RECOMMENDED that the receiver 729 immediately check the authenticity of a packet before proceeding with 730 step (3) above. If immediate checking is possible and if the packet 731 fails the check then the receiver MUST discard the packet and reduce 732 its reception rate to a minimum before continuing to regulate its 733 reception rate using the multiple rate congestion control. 735 Some packet authentication schemes such as TESLA [PER2001] do not 736 allow an immediate authenticity check. In this case the receiver 737 SHOULD check the authenticity of a packet as soon as possible, and if 738 the packet fails the check then it MUST be discarded before step (5) 739 above and reduce its reception rate to a minimum before continuing to 740 regulate its reception rate using the multiple rate congestion 741 control. 743 5. Security Considerations 745 The same security consideration that apply to the LCT, FEC and the 746 multiple rate congestion control building blocks also apply to ALC. 748 Because of the use of FEC, ALC is especially vulnerable to denial- 749 of-service attacks by attackers that try to send forged packets to 750 the session which would prevent successful reconstruction or cause 751 inaccurate reconstruction of large portions of the object by 752 receivers. ALC is also particularly affected by such an attack 753 because many receivers may receive the same forged packet. There are 754 two ways to protect against such attacks, one at the application 755 level and one at the packet level. It is RECOMMENDED that prevention 756 be provided at both levels. 758 At the application level, it is RECOMMENDED that an integrity check 759 on the entire received object be done once the object is 760 reconstructed to ensure it is the same as the sent object. Moreover, 761 in order to obtain strong cryptographic integrity protection a 762 digital signature verifiable by the receiver SHOULD be used to 763 provide this application level integrity check. However, if even one 764 corrupted or forged packet is used to reconstruct the object, it is 765 likely that the received object will be reconstructed incorrectly. 766 This will appropriately cause the integrity check to fail and in this 767 case the inaccurately reconstructed object SHOULD be discarded. 768 Thus, the acceptance of a single forged packet can be an effective 769 denial of service attack for distributing objects, but an object 770 integrity check at least prevents inadvertent use of inaccurately 771 reconstructed objects. The specification of an application level 772 integrity check of the received object is outside the scope of this 773 document. 775 At the packet level, it is RECOMMENDED that a packet level 776 authentication be used to ensure that each received packet is an 777 authentic and uncorrupted packet containing FEC data for the object 778 arriving from the specified sender. Packet level authentication has 779 the advantage that corrupt or forged packets can be discarded 780 individually and the received authenticated packets can be used to 781 accurately reconstruct the object. Thus, the effect of a denial of 782 service attack that injects forged packets is proportional only to 783 the number of forged packets, and not to the object size. Although 784 there is currently no IETF standard that specifies how to do 785 multicast packet level authentication, TESLA [PER2001] is a known 786 multicast packet authentication scheme that would work. 788 In addition to providing protection against reconstruction of 789 inaccurate objects, packet level authentication can also provide some 790 protection against denial of service attacks on the multiple rate 791 congestion control. Attackers can try to inject forged packets with 792 incorrect congestion control information into the multicast stream, 793 thereby potentially adversely affecting network elements and 794 receivers downstream of the attack, and much less significantly the 795 rest of the network and other receivers. Thus, it is also 796 RECOMMENDED that packet level authentication be used to protect 797 against such attacks. TESLA [PER2001] can also be used to some 798 extent to limit the damage caused by such attacks. However, with 799 TESLA a receiver can only determine if a packet is authentic several 800 seconds after it is received, and thus an attack against the 801 congestion control protocol can be effective for several seconds 802 before the receiver can react to slow down the session reception 803 rate. 805 Reverse Path Forwarding checks SHOULD be enabled in all network 806 routers and switches along the path from the sender to receivers to 807 limit the possibility of a bad agent injecting forged packets into 808 the multicast tree data path. 810 6. IANA Considerations 812 This specification registers the following LCT Header Extensions 813 Types in namespace ietf:rmt:lct:headerExtensionTypes:variableLength: 815 +-------+---------+--------------------+ 816 | Value | Name | Reference | 817 +-------+---------+--------------------+ 818 | 64 | EXT_FTI | This specification | 819 +-------+---------+--------------------+ 821 7. Acknowledgments 823 This specification is substantially based on RFC3450 [RFC3450] and 824 thus credit for the authorship of this document is primarily due to 825 the authors of RFC3450: Mike Luby, Jim Gemmel, Lorenzo Vicisano, 826 Luigi Rizzo and Jon Crowcroft. Vincent Roca, Justin Chapweske and 827 Roger Kermode also contributed to RFC3450. Additional thanks are due 828 to Vincent Roca and Rod Walsh for contributions to this update to 829 Proposed Standard. 831 8. Changes from RFC3450 833 This section summarises the changes that were made from the 834 Experimental version of this specification published as RFC3450 835 [RFC3450]: 837 o Update all references to the obsoleted RFC 2068 to RFC 2616 839 o Removed the 'Statement of Intent' from the introduction (The 840 statement of intent was meant to clarify the "Experimental" status 841 of RFC3450.) 843 o Removed the 'Intellectual Property Issues' Section and replaced 844 with a standard IPR Statement 846 o Remove material duplicated in LCT 848 o Update references for LCT and FEC Building Block to new versions 849 and align with changes in the FEC Building Block. 851 o Split normative and informative references 853 o Material applicable in a general LCT context, not just for ALC has 854 been moved to LCT 856 o The first bit of the "Protocol Specific Indication" in the LCT 857 Headert is defined as a "Source Packet Indication". This is used 858 in the case that an FEC Scheme defines two FEC Payload ID formats, 859 one of which is for packets containing only source symbols which 860 can be processed by receivers that do not support FEC Decoding. 862 o Definition and IANA registration of the EXT_FTI LCT Header 863 Extension 865 9. References 867 9.1. Normative references 869 [I-D.ietf-rmt-bb-lct-revised] 870 Luby, M., "Layered Coding Transport (LCT) Building Block", 871 draft-ietf-rmt-bb-lct-revised-01 (work in progress), 872 October 2005. 874 [I-D.ietf-rmt-fec-bb-revised] 875 Watson, M., "Forward Error Correction (FEC) Building 876 Block", draft-ietf-rmt-fec-bb-revised-03 (work in 877 progress), January 2006. 879 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 880 August 1980. 882 [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, 883 RFC 1112, August 1989. 885 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 886 3", BCP 9, RFC 2026, October 1996. 888 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 889 Requirement Levels", BCP 14, RFC 2119, March 1997. 891 [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description 892 Protocol", RFC 2327, April 1998. 894 [RFC2357] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF 895 Criteria for Evaluating Reliable Multicast Transport and 896 Application Protocols", RFC 2357, June 1998. 898 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 899 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 900 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 902 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 903 Announcement Protocol", RFC 2974, October 2000. 905 [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media 906 Types", RFC 3023, January 2001. 908 9.2. Informative references 910 [HOL2001] Holbrook, H., "A Channel Model for Multicast", Ph.D. 911 Dissertation, Stanford University, Department of Computer 912 Science, Stanford, CA , August 2001. 914 [PER2001] Perrig, A., Canetti, R., Song, D., and J. Tygar, 915 "Efficient and Secure Source Authentication for 916 Multicast", Network and Distributed System Security 917 Symposium, NDSS 2001, pp. 35-46 , February 2001. 919 [RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M., 920 Floyd, S., and M. Luby, "Reliable Multicast Transport 921 Building Blocks for One-to-Many Bulk-Data Transfer", 922 RFC 3048, January 2001. 924 [RFC3269] Kermode, R. and L. Vicisano, "Author Guidelines for 925 Reliable Multicast Transport (RMT) Building Blocks and 926 Protocol Instantiation documents", RFC 3269, April 2002. 928 [RFC3450] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J. 929 Crowcroft, "Asynchronous Layered Coding (ALC) Protocol 930 Instantiation", RFC 3450, December 2002. 932 [RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, 933 M., and J. Crowcroft, "The Use of Forward Error Correction 934 (FEC) in Reliable Multicast", RFC 3453, December 2002. 936 Authors' Addresses 938 Michael Luby 939 Digital Fountain 940 39141 Civic Center Dr. 941 Suite 300 942 Fremont, CA 94538 943 US 945 Email: luby@digitalfountain.com 947 Mark Watson 948 Digital Fountain 949 39141 Civic Center Dr. 950 Suite 300 951 Fremont, CA 94538 952 US 954 Email: mark@digitalfountain.com 956 Lorenzo Vicisano 957 Cisco Systems, Inc. 958 510 McCarthy Blvd. 959 Milpitas, CA 95035 960 US 962 Email: lorenzo@cisco.com 964 Intellectual Property Statement 966 The IETF takes no position regarding the validity or scope of any 967 Intellectual Property Rights or other rights that might be claimed to 968 pertain to the implementation or use of the technology described in 969 this document or the extent to which any license under such rights 970 might or might not be available; nor does it represent that it has 971 made any independent effort to identify any such rights. Information 972 on the procedures with respect to rights in RFC documents can be 973 found in BCP 78 and BCP 79. 975 Copies of IPR disclosures made to the IETF Secretariat and any 976 assurances of licenses to be made available, or the result of an 977 attempt made to obtain a general license or permission for the use of 978 such proprietary rights by implementers or users of this 979 specification can be obtained from the IETF on-line IPR repository at 980 http://www.ietf.org/ipr. 982 The IETF invites any interested party to bring to its attention any 983 copyrights, patents or patent applications, or other proprietary 984 rights that may cover technology that may be required to implement 985 this standard. Please address the information to the IETF at 986 ietf-ipr@ietf.org. 988 Disclaimer of Validity 990 This document and the information contained herein are provided on an 991 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 992 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 993 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 994 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 995 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 996 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 998 Copyright Statement 1000 Copyright (C) The Internet Society (2006). This document is subject 1001 to the rights, licenses and restrictions contained in BCP 78, and 1002 except as set forth therein, the authors retain all their rights. 1004 Acknowledgment 1006 Funding for the RFC Editor function is currently provided by the 1007 Internet Society.