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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT B. VerSteeg 3 Internet-Draft A. Begen 4 Intended status: Standards Track Cisco 5 Expires: February 27, 2011 T. VanCaenegem 6 Alcatel-Lucent 7 Z. Vax 8 Microsoft Corporation 9 August 26, 2010 11 Unicast-Based Rapid Acquisition of Multicast RTP Sessions 12 draft-ietf-avt-rapid-acquisition-for-rtp-13 14 Abstract 16 When an RTP receiver joins a multicast session, it may need to 17 acquire and parse certain Reference Information before it can process 18 any data sent in the multicast session. Depending on the join time, 19 length of the Reference Information repetition (or appearance) 20 interval, size of the Reference Information as well as the 21 application and transport properties, the time lag before an RTP 22 receiver can usefully consume the multicast data, which we refer to 23 as the Acquisition Delay, varies and can be large. This is an 24 undesirable phenomenon for receivers that frequently switch among 25 different multicast sessions, such as video broadcasts. 27 In this document, we describe a method using the existing RTP and 28 RTCP protocol machinery that reduces the acquisition delay. In this 29 method, an auxiliary unicast RTP session carrying the Reference 30 Information to the receiver precedes/accompanies the multicast 31 stream. This unicast RTP flow can be transmitted at a faster than 32 natural bitrate to further accelerate the acquisition. The 33 motivating use case for this capability is multicast applications 34 that carry real-time compressed audio and video. However, the 35 proposed method can also be used in other types of multicast 36 applications where the acquisition delay is long enough to be a 37 problem. 39 Status of this Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at http://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on February 27, 2011. 56 Copyright Notice 58 Copyright (c) 2010 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents 63 (http://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. Code Components extracted from this document must 67 include Simplified BSD License text as described in Section 4.e of 68 the Trust Legal Provisions and are provided without warranty as 69 described in the Simplified BSD License. 71 This document may contain material from IETF Documents or IETF 72 Contributions published or made publicly available before November 73 10, 2008. The person(s) controlling the copyright in some of this 74 material may not have granted the IETF Trust the right to allow 75 modifications of such material outside the IETF Standards Process. 76 Without obtaining an adequate license from the person(s) controlling 77 the copyright in such materials, this document may not be modified 78 outside the IETF Standards Process, and derivative works of it may 79 not be created outside the IETF Standards Process, except to format 80 it for publication as an RFC or to translate it into languages other 81 than English. 83 Table of Contents 85 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 86 1.1. Acquisition of RTP Streams vs. RTP Sessions . . . . . . . 6 87 1.2. Outline . . . . . . . . . . . . . . . . . . . . . . . . . 7 88 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 7 89 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7 90 4. Elements of Delay in Multicast Applications . . . . . . . . . 9 91 5. Protocol Design Considerations and Their Effect on 92 Resource Management for Rapid Acquisition . . . . . . . . . . 10 93 6. Rapid Acquisition of Multicast RTP Sessions (RAMS) . . . . . . 12 94 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12 95 6.2. Message Flows . . . . . . . . . . . . . . . . . . . . . . 13 96 6.3. Synchronization of Primary Multicast Streams . . . . . . . 23 97 6.4. Burst Shaping and Congestion Control in RAMS . . . . . . . 23 98 6.5. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 26 99 7. Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 27 100 7.1. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 28 101 7.1.1. Vendor-Neutral Extensions . . . . . . . . . . . . . . 29 102 7.1.2. Private Extensions . . . . . . . . . . . . . . . . . . 29 103 7.2. RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 30 104 7.3. RAMS Information . . . . . . . . . . . . . . . . . . . . . 32 105 7.4. RAMS Termination . . . . . . . . . . . . . . . . . . . . . 35 106 8. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 36 107 8.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 36 108 8.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 36 109 8.3. Example and Discussion . . . . . . . . . . . . . . . . . . 37 110 9. NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 40 111 10. Security Considerations . . . . . . . . . . . . . . . . . . . 41 112 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43 113 11.1. Registration of SDP Attributes . . . . . . . . . . . . . . 43 114 11.2. Registration of SDP Attribute Values . . . . . . . . . . . 43 115 11.3. Registration of FMT Values . . . . . . . . . . . . . . . . 43 116 11.4. SFMT Values for RAMS Messages Registry . . . . . . . . . . 44 117 11.5. RAMS TLV Space Registry . . . . . . . . . . . . . . . . . 44 118 11.6. RAMS Response Code Space Registry . . . . . . . . . . . . 45 119 11.6.1. Response Code Definitions . . . . . . . . . . . . . . 48 120 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 49 121 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49 122 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49 123 14.1. Normative References . . . . . . . . . . . . . . . . . . . 49 124 14.2. Informative References . . . . . . . . . . . . . . . . . . 51 125 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52 127 1. Introduction 129 Most multicast flows carry a stream of inter-related data. The 130 receivers need to acquire certain information to start processing any 131 data sent in the multicast session. This document refers to this 132 information as Reference Information. The Reference Information is 133 conventionally sent periodically in the multicast session (although 134 its content can change over time) and usually consists of items such 135 as a description of the schema for the rest of the data, references 136 to which data to process, encryption information including keys, as 137 well as any other information required to process the data in the 138 multicast stream [IC2009]. 140 Real-time multicast applications require the receivers to buffer 141 data. The receiver may have to buffer data to smooth out the network 142 jitter, to allow loss-repair methods such as Forward Error Correction 143 and retransmission to recover the missing packets, and to satisfy the 144 data processing requirements of the application layer. 146 When a receiver joins a multicast session, it has no control over 147 what point in the flow is currently being transmitted. Sometimes the 148 receiver might join the session right before the Reference 149 Information is sent in the session. In this case, the required 150 waiting time is usually minimal. Other times, the receiver might 151 join the session right after the Reference Information has been 152 transmitted. In this case, the receiver has to wait for the 153 Reference Information to appear again in the flow before it can start 154 processing any multicast data. In some other cases, the Reference 155 Information is not contiguous in the flow but dispersed over a large 156 period, which forces the receiver to wait for all of the Reference 157 Information to arrive before starting to process the rest of the 158 data. 160 The net effect of waiting for the Reference Information and waiting 161 for various buffers to fill up is that the receivers can experience 162 significantly large delays in data processing. In this document, we 163 refer to the difference between the time an RTP receiver joins the 164 multicast session and the time the RTP receiver acquires all the 165 necessary Reference Information as the Acquisition Delay. The 166 acquisition delay might not be the same for different receivers; it 167 usually varies depending on the join time, length of the Reference 168 Information repetition (or appearance) interval, size of the 169 Reference Information as well as the application and transport 170 properties. 172 The varying nature of the acquisition delay adversely affects the 173 receivers that frequently switch among multicast sessions. While 174 this problem equally applies to both any-source (ASM) and source- 175 specific (SSM) multicast applications, in this specification we 176 address it for the SSM-based multicast applications by describing a 177 method that uses the fundamental tools offered by the existing RTP 178 and RTCP protocols [RFC3550]. In this method, either the multicast 179 source (or the distribution source in an SSM session) retains the 180 Reference Information for a period after its transmission, or an 181 intermediary network element (that we refer to as Retransmission 182 Server) joins the multicast session and continuously caches the 183 Reference Information as it is sent in the session and acts as a 184 feedback target (See [RFC5760]) for the session. When an RTP 185 receiver wishes to join the same multicast session, instead of simply 186 issuing a Source Filtering Group Management Protocol (SFGMP) Join 187 message, it sends a request to the feedback target for the session 188 and asks for the Reference Information. The retransmission server 189 starts a new unicast RTP (retransmission) session and sends the 190 Reference Information to the RTP receiver over that session. If 191 there is spare bandwidth, the retransmission server might burst the 192 Reference Information faster than its natural rate. As soon as the 193 receiver acquires the Reference Information, it can join the 194 multicast session and start processing the multicast data. A 195 simplified network diagram showing this method through an 196 intermediary network element is depicted in Figure 1. 198 This method potentially reduces the acquisition delay. We refer to 199 this method as Unicast-based Rapid Acquisition of Multicast RTP 200 Sessions. A primary use case for this method is to reduce the 201 channel-change times in IPTV networks where compressed video streams 202 are multicast in different SSM sessions and viewers randomly join 203 these sessions. 205 ----------------------- 206 +--->| Intermediary | 207 | | Network Element | 208 | ...|(Retransmission Server)| 209 | : ----------------------- 210 | : 211 | v 212 ----------- ---------- ---------- 213 | Multicast | | |---------->| Joining | 214 | Source |------->| Router |..........>| RTP | 215 | | | | | Receiver | 216 ----------- ---------- ---------- 217 | 218 | ---------- 219 +---------------->| Existing | 220 | RTP | 221 | Receiver | 222 ---------- 224 -------> Multicast RTP Flow 225 .......> Unicast RTP Flow 227 Figure 1: Rapid acquisition through an intermediary network element 229 A principle design goal in this solution is to use the existing tools 230 in the RTP/RTCP protocol family. This improves the versatility of 231 the existing implementations, and promotes faster deployment and 232 better interoperability. To this effect, we use the unicast 233 retransmission support of RTP [RFC4588] and the capabilities of RTCP 234 to handle the signaling needed to accomplish the acquisition. 236 1.1. Acquisition of RTP Streams vs. RTP Sessions 238 In this memo we describe a protocol that handles the rapid 239 acquisition of a single multicast RTP session (called primary 240 multicast RTP session) carrying one or more RTP streams (called 241 primary multicast streams). If desired, multiple instances of this 242 protocol may be run in parallel to acquire multiple RTP sessions 243 simultaneously. 245 When an RTP receiver requests the Reference Information from the 246 retransmission server, it can opt to rapidly acquire a specific 247 subset of the available RTP streams in the primary multicast RTP 248 session. Alternatively, the RTP receiver can request the rapid 249 acquisition of all of the RTP streams in that RTP session. 250 Regardless of how many RTP streams are requested by the RTP receiver 251 or how many will be actually sent by the retransmission server, only 252 one unicast RTP session will be established by the retransmission 253 server. This unicast RTP session is separate from the associated 254 primary multicast RTP session. As a result, there are always two 255 different RTP sessions in a single instance of the rapid acquisition 256 protocol: (i) the primary multicast RTP session with its associated 257 unicast feedback and (ii) the unicast RTP session. 259 If the RTP receiver wants to rapidly acquire multiple RTP sessions 260 simultaneously, separate unicast RTP sessions will be established for 261 each of them. 263 1.2. Outline 265 In the rest of this specification, we have the following outline: In 266 Section 4, we describe the delay components in generic multicast 267 applications. Section 5 presents an overview of the protocol design 268 considerations for rapid acquisition. We provide the protocol 269 details of the rapid acquisition method in Section 6 and Section 7. 270 Section 8 and Section 9 discuss the SDP signaling issues with 271 examples and NAT-related issues, respectively. Finally, Section 10 272 discusses the security considerations. 274 Section 3 provides a list of the definitions frequently used in this 275 document. 277 2. Requirements Notation 279 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 280 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 281 document are to be interpreted as described in [RFC2119]. 283 3. Definitions 285 This document uses the following acronyms and definitions frequently: 287 (Primary) SSM (or Multicast) Session: The multicast session to which 288 RTP receivers can join at a random point in time. A primary SSM 289 session can carry multiple RTP streams. 291 Primary Multicast RTP Session: The multicast RTP session an RTP 292 receiver is interested in acquiring rapidly. From the RTP receiver's 293 viewpoint, the primary multicast RTP session has one associated 294 unicast RTCP feedback stream to a Feedback Target, in addition to the 295 primary multicast RTP stream(s). 297 Primary Multicast (RTP) Streams: The RTP stream(s) carried in the 298 primary multicast RTP session. 300 Source Filtering Group Management Protocol (SFGMP): Following the 301 definition in [RFC4604], SFGMP refers to the Internet Group 302 Management Protocol (IGMP) version 3 [RFC3376] and the Multicast 303 Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and 304 IPv6 networks, respectively. However, the rapid acquisition method 305 introduced in this document does not depend on a specific version of 306 either of these group management protocols. In the remainder of this 307 document, SFGMP will refer to any group management protocol that has 308 Join and Leave functionalities. 310 Feedback Target (FT): Unicast RTCP feedback target as defined in 311 [RFC5760]. FT_Ap denotes a specific feedback target running on a 312 particular address and port. 314 Retransmission (or Burst) Packet: An RTP packet that is formatted as 315 defined in Section 4 of [RFC4588]. The payload of a retransmission 316 or burst packet comprises the retransmission payload header followed 317 by the payload of the original RTP packet. 319 Reference Information: The set of certain media content and metadata 320 information that is sufficient for an RTP receiver to start usefully 321 consuming a media stream. The meaning, format and size of this 322 information are specific to the application and are out of scope of 323 this document. 325 Preamble Information: A more compact form of the whole or a subset 326 of the Reference Information transmitted out-of-band. 328 (Unicast) Burst (or Retransmission) RTP Session: The unicast RTP 329 session used to send one or more unicast burst RTP streams and their 330 associated RTCP messages. The terms "burst RTP session" and 331 "retransmission RTP session" can be used interchangeably. 333 (Unicast) Burst (Stream): A unicast stream of RTP retransmission 334 packets that enable an RTP receiver to rapidly acquire the Reference 335 Information associated with a primary multicast stream. Each burst 336 stream is identified by its Synchronization Source (SSRC) identifier 337 that is unique in the primary multicast RTP session. Following the 338 session-multiplexing guidelines in [RFC4588], each unicast burst 339 stream will use the same SSRC and CNAME as its primary multicast RTP 340 stream. 342 Retransmission Server (RS): The RTP/RTCP endpoint that can generate 343 the retransmission packets and the burst streams. The RS may also 344 generate other non-retransmission packets to aid rapid acquisition. 346 4. Elements of Delay in Multicast Applications 348 In a source-specific (SSM) multicast delivery system, there are three 349 major elements that contribute to the overall acquisition delay when 350 an RTP receiver switches from one multicast session to another one. 351 These are: 353 o Multicast switching delay 355 o Reference Information latency 357 o Buffering delays 359 Multicast switching delay is the delay that is experienced to leave 360 the current multicast session (if any) and join the new multicast 361 session. In typical systems, the multicast join and leave operations 362 are handled by a group management protocol. For example, the 363 receivers and routers participating in a multicast session can use 364 the Internet Group Management Protocol (IGMP) version 3 [RFC3376] or 365 the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810]. 366 In either of these protocols, when a receiver wants to join a 367 multicast session, it sends a message to its upstream router and the 368 routing infrastructure sets up the multicast forwarding state to 369 deliver the packets of the multicast session to the new receiver. 370 Depending on the proximity of the upstream router, the current state 371 of the multicast tree, the load on the system and the protocol 372 implementation, the join times vary. Current systems provide join 373 latencies usually less than 200 milliseconds (ms). If the receiver 374 had been participating in another multicast session before joining 375 the new session, it needs to send a Leave message to its upstream 376 router to leave the session. In common multicast routing protocols, 377 the leave times are usually smaller than the join times, however, it 378 is possible that the Leave and Join messages might get lost, in which 379 case the multicast switching delay inevitably increases. 381 Reference Information latency is the time it takes the receiver to 382 acquire the Reference Information. It is highly dependent on the 383 proximity of the actual time the receiver joined the session to the 384 next time the Reference Information will be sent to the receivers in 385 the session, whether the Reference Information is sent contiguously 386 or not, and the size of the Reference Information. For some 387 multicast flows, there is a little or no interdependency in the data, 388 in which case the Reference Information latency will be nil or 389 negligible. For other multicast flows, there is a high degree of 390 interdependency. One example of interest is the multicast flows that 391 carry compressed audio/video. For these flows, the Reference 392 Information latency can become quite large and be a major contributor 393 to the overall delay. 395 The buffering component of the overall acquisition delay is driven by 396 the way the application layer processes the payload. In many 397 multicast applications, an unreliable transport protocol such as UDP 398 [RFC0768] is often used to transmit the data packets, and the 399 reliability, if needed, is usually addressed through other means such 400 as Forward Error Correction (e.g., 401 [I-D.ietf-fecframe-interleaved-fec-scheme]) and retransmission. 402 These loss-repair methods require buffering at the receiver side to 403 function properly. In many applications, it is also often necessary 404 to de-jitter the incoming data packets before feeding them to the 405 application. The de-jittering process also increases the buffering 406 delays. Besides these network-related buffering delays, there are 407 also specific buffering needs that are required by the individual 408 applications. For example, standard video decoders typically require 409 an amount, sometimes up to a few seconds, of coded video data to be 410 available in the pre-decoding buffers prior to starting to decode the 411 video bitstream. 413 5. Protocol Design Considerations and Their Effect on Resource 414 Management for Rapid Acquisition 416 This section is for informational purposes and does not contain 417 requirements for implementations. 419 Rapid acquisition is an optimization of a system that is expected to 420 continue to work correctly and properly whether or not the 421 optimization is effective, or even fails due to lost control and 422 feedback messages, congestion, or other problems. This is 423 fundamental to the overall design requirements surrounding the 424 protocol definition and to the resource management schemes to be 425 employed together with the protocol (e.g., QoS machinery, server load 426 management, etc). In particular, the system needs to operate within 427 a number of constraints: 429 o First, a rapid acquisition operation must fail gracefully. The 430 user experience must be not significantly worse for trying and 431 failing to complete rapid acquisition compared to simply joining 432 the multicast session. 434 o Second, providing the rapid acquisition optimizations must not 435 cause collateral damage to either the multicast session being 436 joined, or other multicast sessions sharing resources with the 437 rapid acquisition operation. In particular, the rapid acquisition 438 operation must avoid interference with the multicast session that 439 might be simultaneously being received by other hosts. In 440 addition, it must also avoid interference with other multicast 441 sessions sharing the same network resources. These properties are 442 possible, but are usually difficult to achieve. 444 One challenge is the existence of multiple bandwidth bottlenecks 445 between the receiver and the server(s) in the network providing the 446 rapid acquisition service. In commercial IPTV deployments, for 447 example, bottlenecks are often present in the aggregation network 448 connecting the IPTV servers to the network edge, the access links 449 (e.g., DSL, DOCSIS) and in the home network of the subscribers. Some 450 of these links might serve only a single subscriber, limiting 451 congestion impact to the traffic of only that subscriber, but others 452 can be shared links carrying multicast sessions of many subscribers. 453 Also note that the state of these links can vary over time. The 454 receiver might have knowledge of a portion of this network, or might 455 have partial knowledge of the entire network. The methods employed 456 by the devices to acquire this network state information is out of 457 scope for this document. The receiver should be able to signal the 458 server with the bandwidth that it believes it can handle. The server 459 also needs to be able to rate limit the flow in order to stay within 460 the performance envelope that it knows about. Both the server and 461 receiver need to be able to inform the other of changes in the 462 requested and delivered rates. However, the protocol must be robust 463 in the presence of packet loss, so this signaling must include the 464 appropriate default behaviors. 466 A second challenge is that for some uses (e.g., high-bitrate video) 467 the unicast burst bitrate is high while the flow duration of the 468 unicast burst is short. This is because the purpose of the unicast 469 burst is to allow the RTP receiver to join the multicast quickly and 470 thereby limit the overall resources consumed by the burst. Such 471 high-bitrate, short-duration flows are not amenable to conventional 472 admission control techniques. For example, end-to-end per-flow 473 signaled admission control techniques such as RSVP have too much 474 latency and control channel overhead to be a good fit for rapid 475 acquisition. Similarly, using a TCP (or TCP-like) approach with a 476 3-way handshake and slow-start to avoid inducing congestion would 477 defeat the purpose of attempting rapid acquisition in the first place 478 by introducing many round-trip times (RTT) of delay. 480 These observations lead to certain unavoidable requirements and goals 481 for a rapid acquisition protocol. These are: 483 o The protocol must be designed to allow a deterministic upper bound 484 on the extra bandwidth used (compared to just joining the 485 multicast session). A reasonable size bound is e*B, where B is 486 the nominal bandwidth of the primary multicast streams, and e is 487 an excess-bandwidth coefficient. The total duration of the 488 unicast burst must have a reasonable bound; long unicast bursts 489 devolve to the bandwidth profile of multi-unicast for the whole 490 system. 492 o The scheme should minimize (or better eliminate) the overlap of 493 the unicast burst and the primary multicast stream. This 494 minimizes the window during which congestion could be induced on a 495 bottleneck link compared to just carrying the multicast or unicast 496 packets alone. 498 o The scheme must minimize (or better eliminate) any gap between the 499 unicast burst and the primary multicast stream, which has to be 500 repaired later, or in the absence of repair, will result in loss 501 being experienced by the application. 503 In addition to the above, there are some other protocol design issues 504 to be considered. First, there is at least one RTT of "slop" in the 505 control loop. In starting a rapid acquisition burst, this manifests 506 as the time between the client requesting the unicast burst and the 507 burst description and/or the first unicast burst packets arriving at 508 the receiver. For managing and terminating the unicast burst, there 509 are two possible approaches for the control loop: The receiver can 510 adapt to the unicast burst as received, converge based on observation 511 and explicitly terminate the unicast burst with a second control loop 512 exchange (which takes a minimum of one RTT, just as starting the 513 unicast burst does). Alternatively, the server generating the 514 unicast burst can pre-compute the burst parameters based on the 515 information in the initial request and tell the receiver the burst 516 duration. 518 The protocol described in the next section allows either method of 519 controlling the rapid acquisition unicast burst. 521 6. Rapid Acquisition of Multicast RTP Sessions (RAMS) 523 We start this section with an overview of the rapid acquisition of 524 multicast sessions (RAMS) method. 526 6.1. Overview 528 [RFC5760] specifies an extension to the RTP Control Protocol (RTCP) 529 to use unicast feedback in an SSM session. It defines an 530 architecture that introduces the concept of Distribution Source, 531 which - in an SSM context - distributes the RTP data and 532 redistributes RTCP information to all RTP receivers. This RTCP 533 information is retrieved from the Feedback Target, to which RTCP 534 unicast feedback traffic is sent. One or more entities different 535 from the Distribution Source MAY implement the feedback target 536 functionality, and different RTP receivers MAY use different feedback 537 targets. 539 This document builds further on these concepts to reduce the 540 acquisition delay when an RTP receiver joins a multicast session at a 541 random point in time by introducing the concept of the Burst Source 542 and new RTCP feedback messages. The Burst Source has a cache where 543 the most recent packets from the primary multicast RTP session are 544 continuously stored. When an RTP receiver wants to receive a primary 545 multicast stream, it can first request a unicast burst from the Burst 546 Source before it joins the SSM session. In this burst, the packets 547 are formatted as RTP retransmission packets [RFC4588] and carry 548 Reference Information. This information allows the RTP receiver to 549 start usefully consuming the RTP packets sent in the primary 550 multicast RTP session. 552 Using an accelerated bitrate (as compared to the nominal bitrate of 553 the primary multicast stream) for the unicast burst implies that at a 554 certain point in time, the payload transmitted in the unicast burst 555 is going to be the same as the payload in the associated multicast 556 stream, i.e., the unicast burst will catch up with the primary 557 multicast stream. At this point, the RTP receiver no longer needs to 558 receive the unicast burst and can join the SSM session. This method 559 is referred to as the Rapid Acquisition of Multicast Sessions (RAMS). 561 This document defines extensions to [RFC4585] for an RTP receiver to 562 request a unicast burst as well as for additional control messaging 563 that can be leveraged during the acquisition process. 565 6.2. Message Flows 567 Figure 2 shows the main entities involved in rapid acquisition and 568 the message flows. They are 570 o Multicast Source 572 o Feedback Target (FT) 574 o Burst/Retransmission Source (BRS) 576 o RTP Receiver (RTP_Rx) 577 ----------- -------------- 578 | |------------------------------------>| | 579 | |.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->| | 580 | | | | 581 | Multicast | ---------------- | | 582 | Source | | Retransmission | | | 583 | |-------->| Server (RS) | | | 584 | |.-.-.-.->| | | | 585 | | | ------------ | | | 586 ----------- | | Feedback | |<.=.=.=.=.| | 587 | | Target (FT)| |<~~~~~~~~~| RTP Receiver | 588 PRIMARY MULTICAST | ------------ | | (RTP_Rx) | 589 RTP SESSION with | | | | 590 UNICAST FEEDBACK | | | | 591 | | | | 592 - - - - - - - - - - - |- - - - - - - - |- - - - - |- - - - - - - |- - 593 | | | | 594 UNICAST BURST | ------------ | | | 595 (or RETRANSMISSION) | | Burst and | |<~~~~~~~~>| | 596 RTP SESSION | | Retrans. | |.........>| | 597 | |Source (BRS)| |<.=.=.=.=>| | 598 | ------------ | | | 599 | | | | 600 ---------------- -------------- 602 -------> Multicast RTP Flow 603 .-.-.-.> Multicast RTCP Flow 604 .=.=.=.> Unicast RTCP Reports 605 ~~~~~~~> Unicast RTCP Feedback Messages 606 .......> Unicast RTP Flow 608 Figure 2: Flow diagram for unicast-based rapid acquisition 610 The feedback target (FT) is the entity as defined in [RFC5760], to 611 which the RTP_Rx sends its RTCP feedback messages indicating packet 612 loss by means of an RTCP NACK message or indicating RTP_Rx's desire 613 to rapidly acquire the primary multicast RTP session by means of an 614 RTCP feedback message defined in this document. While the Burst/ 615 Retransmission Source (BRS) is responsible for responding to these 616 messages and for further RTCP interaction with the RTP_Rx in the case 617 of a rapid acquisition process, it is assumed in the remainder of the 618 document that these two logical entities (FT and BRS) are combined in 619 a single physical entity and they share state. In the remainder of 620 the text, the term Retransmission Server (RS) is used whenever 621 appropriate, to refer to this single physical entity. 623 The FT is involved in the primary multicast RTP session and receives 624 unicast feedback for that session as in [RFC5760]. The BRS is 625 involved in the unicast burst (or retransmission) RTP session and 626 transmits the unicast burst and retransmission packets formatted as 627 RTP retransmission packets [RFC4588] in a single separate unicast RTP 628 session to each RTP_Rx. In the unicast burst RTP session, as in any 629 other RTP session, the BRS and RTP_Rx regularly send the periodic 630 sender and receiver reports, respectively. 632 The unicast burst is started by an RTCP feedback message that is 633 defined in this document based on the common packet format provided 634 in [RFC4585]. An RTP retransmission is triggered by an RTCP NACK 635 message defined in [RFC4585]. Both of these messages are sent to the 636 FT of the primary multicast RTP session, and can start the unicast 637 burst/retransmission RTP session. 639 In the RTP/AVPF profile, there are certain rules that apply to 640 scheduling of both of these messages sent to the FT in the primary 641 multicast RTP session, and these are detailed in Section 3.5 of 642 [RFC4585]. One of the rules states that in a multi-party session 643 such as the SSM sessions we are considering in this specification, an 644 RTP_Rx cannot send an RTCP feedback message for a minimum of one 645 second period after joining the session (i.e., Tmin=1.0 second). 646 While this rule has the goal of avoiding problems associated with 647 flash crowds in typical multi-party sessions, it defeats the purpose 648 of rapid acquisition. Furthermore, when RTP receivers delay their 649 messages requesting a burst by a deterministic or even a random 650 amount, it still does not make a difference since such messages are 651 not related and their timings are independent from each other. Thus, 652 in this specification we initialize Tmin to zero and allow the RTP 653 receivers to send a burst request message right at the beginning. 654 For the subsequent messages during rapid acquisition, the timing 655 rules of [RFC4585] still apply. 657 Figure 3 depicts an example of messaging flow for rapid acquisition. 658 The RTCP feedback messages are explained below. The optional 659 messages are indicated in parentheses and they might or might not be 660 present during rapid acquisition. In a scenario where rapid 661 acquisition is performed by a feedback target co-located with the 662 media sender, the same method (with the identical message flows) 663 still applies. 665 ------------------------- 666 | Retransmission Server | 667 ----------- | ------ ------------ | -------- ------------ 668 | Multicast | | | FT | | Burst/Ret. | | | | | RTP | 669 | Source | | | | | Source | | | Router | | Receiver | 670 | | | ------ ------------ | | | | (RTP_Rx) | 671 ----------- | | | | -------- ------------ 672 | ------------------------- | | 673 | | | | | 674 |-- RTP Multicast ---------->--------------->| | 675 |-. RTCP Multicast -.-.-.-.->-.-.-.-.-.-.-.->| | 676 | | | | | 677 | | |********************************| 678 | | |* PORT MAPPING SETUP *| 679 | | |********************************| 680 | | | | | 681 | |<~~~~~~~~~~~~~~~~~~~~~~~~~ RTCP RAMS-R ~~~| 682 | | | | | 683 | | |********************************| 684 | | |* UNICAST SESSION ESTABLISHED *| 685 | | |********************************| 686 | | | | | 687 | | |~~~ RTCP RAMS-I ~~~~~~~~~~~~~~~>| 688 | | | | | 689 | | |... Unicast RTP Burst .........>| 690 | | | | | 691 | |<~~~~~~~~~~~~~~~~~~~~~~~~ (RTCP RAMS-R) ~~| 692 | | | | | 693 | | |~~ (RTCP RAMS-I) ~~~~~~~~~~~~~~>| 694 | | | | | 695 | | | | | 696 | | | |<= SFGMP Join ==| 697 | | | | | 698 |-- RTP Multicast ------------------------------------------->| 699 |-. RTCP Multicast -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.>| 700 | | | | | 701 | | | | | 702 | | |<~~~~~~~~~~~~~~~ RTCP RAMS-T ~~~| 703 | | | | | 704 : : : : : 705 | | |<.=.= Unicast RTCP Reports .=.=>| 706 : : : (for the unicast session) : 707 | | | | | 709 -------> Multicast RTP Flow 710 .-.-.-.> Multicast RTCP Flow 711 .=.=.=.> Unicast RTCP Reports 712 ~~~~~~~> Unicast RTCP Feedback Messages 713 =======> SFGMP Messages 714 .......> Unicast RTP Flow 716 Figure 3: Message flows for unicast-based rapid acquisition 718 This document defines the expected behaviors of the RS and RTP_Rx. 720 It is instructive to consider independently operating implementations 721 on the RS and RTP_Rx that request the burst, describe the burst, 722 start the burst, join the multicast session and stop the burst. 723 These implementations send messages to each other, but provisions are 724 needed for the cases where the control messages get lost, or re- 725 ordered, or are not being delivered to their destinations. 727 The following steps describe rapid acquisition in detail: 729 1. Port Mapping Setup: For the primary multicast RTP session, the 730 RTP and RTCP destination ports are declaratively specified 731 (Refer to Section 8 for examples in SDP). However, the RTP_Rx 732 needs to choose its RTP and RTCP receive ports for the unicast 733 burst RTP session. Since this unicast session is established 734 after the RTP_Rx has sent a RAMS-Request (RAMS-R) message as 735 unicast feedback in the primary multicast RTP session, the 736 RTP_Rx MUST first setup the port mappings between the unicast 737 and multicast sessions and send this mapping information to the 738 FT along with the RAMS-R message so that the BRS knows how to 739 communicate with the RTP_Rx. 741 The details of this setup procedure are explained in 742 [I-D.ietf-avt-ports-for-ucast-mcast-rtp]. Other NAT-related 743 issues are left to Section 9 to keep the present discussion 744 focused on the RAMS message flows. 746 2. Request: the RTP_Rx sends a rapid acquisition request (RAMS-R) 747 for the primary multicast RTP session to the unicast feedback 748 target of that session. The request contains the SSRC 749 identifier of the RTP_Rx (aka SSRC of packet sender) and can 750 contain the media sender SSRC identifier(s) of the primary 751 multicast stream(s) of interest (aka SSRC of media source). The 752 RAMS-R message can contain parameters that constrain the burst, 753 such as the buffer and bandwidth limits. 755 Before joining the SSM session, the RTP_Rx learns the addresses 756 for the multicast source, group and RS by out-of-band means. If 757 the RTP_Rx desires to rapidly acquire only a subset of the 758 primary multicast streams available in the primary multicast RTP 759 session, the RTP_Rx MUST also acquire the SSRC identifiers for 760 the desired RTP streams out-of-band. Based on this information, 761 the RTP_Rx populates the desired SSRC(s) in the RAMS-R message. 763 When the FT successfully receives the RAMS-R message, the BRS 764 responds to it by accepting or rejecting the request. 765 Immediately before the BRS sends any RTP or RTCP packet(s) 766 described below, it establishes the unicast burst RTP session. 768 3. Response: The BRS sends RAMS-Information (RAMS-I) message(s) to 769 the RTP_Rx to convey the status for the burst(s) requested by 770 the RTP_Rx. 772 If the primary multicast RTP session associated with the FT_Ap 773 (a specific feedback target running on a particular address and 774 port) on which the RAMS-R message was received contains only a 775 single primary multicast stream, the BRS SHALL always use the 776 SSRC of the RTP stream associated with the FT_Ap in the RAMS-I 777 message(s) regardless of the media sender SSRC requested in the 778 RAMS-R message. In such cases the 'ssrc' attribute MAY be 779 omitted from the media description. If the requested SSRC and 780 the actual media sender SSRC do not match, the BRS MUST 781 explicitly populate the correct media sender SSRC in the initial 782 RAMS-I message (See Section 7.3). 784 The FT_Ap could also be associated with an RTP session that 785 carries two or more primary multicast streams. If the RTP_Rx 786 will issue a collective request to receive the whole primary 787 multicast RTP session, it does not need the 'ssrc' attributes to 788 be described in the media description. 790 If the FT_Ap is associated with two or more RTP sessions, 791 RTP_Rx's request will be ambiguous. To avoid any ambiguity, 792 each FT_Ap MUST be only associated with a single RTP session. 794 If the RTP_Rx is willing to rapidly acquire only a subset of the 795 primary multicast streams, the RTP_Rx MUST list all the media 796 sender SSRC(s) denoting the stream(s) it wishes to acquire in 797 the RAMS-R message. Upon receiving such a message, the BRS MAY 798 accept the request for all or a subset of the media sender 799 SSRC(s) that matched the RTP stream(s) it serves. The BRS MUST 800 reject all other requests with an appropriate response code. 802 * Reject Responses: The BRS MUST take into account any 803 limitations that may have been specified by the RTP_Rx in the 804 RAMS-R message when making a decision regarding the request. 805 If the RTP_Rx has requested to acquire the whole primary 806 multicast RTP session but the BRS cannot provide a rapid 807 acquisition service for any of the primary multicast streams, 808 the BRS MUST reject the request via a single RAMS-I message 809 with a collective reject response code and whose media sender 810 SSRC field is set to one of SSRCs served by this FT_Ap. Upon 811 receiving this RAMS-I message, the RTP_Rx abandons the rapid 812 acquisition attempt and can immediately join the multicast 813 session by sending an SFGMP Join message towards its upstream 814 multicast router. 816 In all other cases, the BRS MUST send a separate RAMS-I 817 message with the appropriate response code for each primary 818 multicast stream that has been requested by the RTP_Rx but 819 cannot be served by the BRS. There could be multiple reasons 820 why the BRS has rejected the request, however, the BRS 821 chooses the most appropriate response code to inform the 822 RTP_Rx. 824 * Accept Responses: The BRS MUST send at least one separate 825 RAMS-I message with the appropriate response code (either 826 zero indicating a private response or a 2xx-level response 827 code indicating success as listed in Section 11.6) for each 828 primary multicast stream that has been requested by the 829 RTP_Rx and will be served by the BRS. Such RAMS-I messages 830 comprise fields that can be used to describe the individual 831 unicast burst streams. When there is a RAMS-R request for 832 multiple primary multicast streams, the BRS MUST include all 833 the individual RAMS-I messages corresponding to that RAMS-R 834 request in the same compound RTCP packet if these messages 835 fit in the same packet. 837 The RAMS-I message carries the RTP sequence number of the 838 first packet transmitted in the respective RTP stream to 839 allow the RTP_Rx to detect any missing initial packet(s). 840 When the BRS accepts a request for a primary multicast 841 stream, this field MUST always be populated in the RAMS-I 842 message(s) sent for this particular primary multicast stream. 843 It is RECOMMENDED that the BRS sends a RAMS-I message at the 844 start of the burst so that the RTP_Rx can quickly detect any 845 missing initial packet(s). 847 It is possible that the RAMS-I message for a primary multicast 848 stream can get delayed or lost, and the RTP_Rx can start 849 receiving RTP packets before receiving a RAMS-I message. An 850 RTP-RX implementation MUST NOT assume it will quickly receive 851 the initial RAMS-I message. For redundancy purposes, it is 852 RECOMMENDED that the BRS repeats the RAMS-I messages multiple 853 times as long as it follows the RTCP timer rules defined in 854 [RFC4585]. 856 4. Unicast Burst: For the primary multicast stream(s) for which 857 the request is accepted, the BRS starts sending the unicast 858 burst(s) that comprises one or more RTP retransmission packets 859 sent in the unicast burst RTP session. In addition, the BRS MAY 860 send preamble information data to the RTP_Rx in addition to the 861 requested burst, to prime the media decoder(s). The format of 862 this preamble data is RTP-payload specific and not specified 863 here. 865 5. Updated Request: The RTP_Rx MAY send an updated RAMS-R message 866 (as unicast feedback in the primary multicast RTP session) with 867 a different value for one or more fields of an earlier RAMS-R 868 message. If there is already a burst planned for or being 869 transmitted to a particular RTP_Rx for a particular stream, the 870 newly arriving RAMS-R is an updated request; otherwise, it is a 871 new request. The BRS determines the identity of the requesting 872 RTP_Rx by looking at its canonical name identifier (CNAME item 873 in the SDES source description). Thus, the RTP_Rx MUST choose a 874 globally unique CNAME identifier. Different such ways are 875 provided in [I-D.ietf-avt-rtp-cnames]. In addition to one or 876 more fields with updated values, an updated RAMS-R message may 877 also include the fields whose values did not change. 879 Upon receiving an updated request, the BRS can use the updated 880 values for sending/shaping the burst, or refine the values and 881 use the refined values for sending/shaping the burst. 882 Subsequently, the BRS MAY send an updated RAMS-I message in the 883 unicast burst RTP session to indicate the changes it made. 885 It is an implementation-dependent decision for an RTP_RX whether 886 and when to send an updated request. 888 6. Updated Response: The BRS can send more than one RAMS-I 889 messages in the unicast burst RTP session, e.g., to update the 890 value of one or more fields in an earlier RAMS-I message. The 891 updated RAMS-I messages might or might not be a direct response 892 to a RAMS-R message. The BRS can also send updated RAMS-I 893 messages to signal the RTP_Rx in real time to join the SSM 894 session, when the BRS had already sent an initial RAMS-I 895 message, e.g., at the start of the burst. The RTP_Rx depends on 896 the BRS to learn the join time, which can be conveyed by the 897 first RAMS-I message, or can be sent/revised in a later RAMS-I 898 message. If the BRS is not capable of determining the join time 899 in the initial RAMS-I message, the BRS MUST send another RAMS-I 900 message (with the join time information) later. 902 7. Multicast Join Signaling: The RAMS-I message allows the BRS to 903 signal explicitly when the RTP_Rx needs to send the SFGMP Join 904 message. The RTP_Rx SHOULD use this information from the most 905 recent RAMS-I message unless it has more accurate information. 906 If the request is accepted, this information MUST be conveyed in 907 at least one RAMS-I message and its value MAY be updated by 908 subsequent RAMS-I messages. 910 There can be missing packets if the RTP_Rx joins the multicast 911 session too early or too late. For example, if the RTP_Rx 912 starts receiving the primary multicast stream while it is still 913 receiving the unicast burst at a high excess bitrate, this can 914 result in an increased risk of packet loss. Or, if the RTP_Rx 915 joins the multicast session some time after the unicast burst is 916 finished, there can be a gap between the burst and multicast 917 data (a number of RTP packets might be missing). In both cases, 918 the RTP_Rx can issue retransmission requests (via RTCP NACK 919 messages sent as unicast feedback in the primary multicast RTP 920 session) [RFC4585] to the FT entity of the RS to fill the gap. 921 The BRS might or might not respond to such requests. When it 922 responds and the response causes significant changes in one or 923 more values reported earlier to the RTP_Rx, an updated RAMS-I 924 SHOULD be sent to the RTP_Rx. 926 8. Multicast Receive: After the join, the RTP_Rx starts receiving 927 the primary multicast stream(s). 929 9. Terminate: The BRS can know when it needs to ultimately stop 930 the unicast burst based on its parameters. However, the RTP_Rx 931 may need to ask the BRS to terminate the burst prematurely or at 932 a specific sequence number. For this purpose, the RTP_Rx uses 933 the RAMS-Termination (RAMS-T) message sent as RTCP feedback in 934 the unicast burst RTP session. A separate RAMS-T message is 935 sent for each primary multicast stream served by the BRS unless 936 an RTCP BYE message has been sent in the unicast burst RTP 937 session as described in Step 10. For the burst requests that 938 were rejected by the BRS, there is no need to send a RAMS-T 939 message. 941 If the RTP_Rx wants to terminate a burst prematurely, it SHALL 942 send a RAMS-T message for the SSRC of the primary multicast 943 stream it wishes to terminate. This message is sent in the 944 unicast burst RTP session. Upon receiving this message, the BRS 945 MUST terminate the unicast burst. If the RTP_Rx requested to 946 acquire the entire primary multicast RTP session but wants to 947 terminate this request before it learns the individual media 948 sender SSRC(s) via RAMS-I message(s) or RTP packets, the RTP_Rx 949 cannot use RAMS-T message(s) and thus MUST send an RTCP BYE 950 message in the unicast burst RTP session to terminate the 951 request. 953 Otherwise, the default behavior for the RTP_Rx is to send a 954 RAMS-T message in the unicast burst RTP session immediately 955 after it joins the multicast session and started receiving 956 multicast packets. In that case, the RTP_Rx SHALL send a RAMS-T 957 message with the sequence number of the first RTP packet 958 received in the primary multicast stream. Then, the BRS MUST 959 terminate the respective burst after it sends the unicast burst 960 packet whose Original Sequence Number (OSN) field in the RTP 961 retransmission payload header matches this number minus one. 963 If an RTCP BYE message has not been issued yet as described in 964 Step 10, the RTP_Rx MUST send at least one RAMS-T message for 965 each primary multicast stream served by the BRS. The RAMS-T 966 message(s) MUST be sent to the BRS in the unicast burst RTP 967 session. Against the possibility of a message loss, it is 968 RECOMMENDED that the RTP_Rx repeats the RAMS-T messages multiple 969 times as long as it follows the RTCP timer rules defined in 970 [RFC4585]. 972 The binding between RAMS-T and ongoing bursts is achieved 973 through RTP_Rx's CNAME identifier, and packet sender and media 974 sender SSRCs. Choosing a globally unique CNAME makes sure that 975 the RAMS-T messages are processed correctly. 977 10. Terminate with RTCP BYE: When the RTP_Rx is receiving one or 978 more burst streams, if the RTP_Rx becomes no longer interested 979 in acquiring any of the primary multicast streams, the RTP_Rx 980 SHALL issue an RTCP BYE message for the unicast burst RTP 981 session and another RTCP BYE message for the primary multicast 982 RTP session. These RTCP BYE messages are sent to the BRS and FT 983 logical entities, respectively. 985 Upon receiving an RTCP BYE message, the BRS MUST terminate the 986 rapid acquisition operation, and cease transmitting any further 987 burst packets and retransmission packets. If support for 988 [RFC5506] has been signaled, the RTCP BYE message MAY be sent in 989 a reduced-size RTCP packet. Otherwise, Section 6.1 of [RFC3550] 990 mandates the RTCP BYE message always to be sent with a sender or 991 receiver report in a compound RTCP packet. If no data has been 992 received, an empty receiver report MUST be still included. With 993 the information contained in the receiver report, the RS can 994 figure out how many duplicate RTP packets have been delivered to 995 the RTP_Rx (Note that this will be an upper-bound estimate as 996 one or more packets might have been lost during the burst 997 transmission). The impact of duplicate packets and measures 998 that can be taken to minimize the impact of receiving duplicate 999 packets will be addressed in Section 6.4. 1001 Since an RTCP BYE message issued for the unicast burst RTP 1002 session terminates that session and ceases transmitting any 1003 further packets in that session, there is no need for sending 1004 explicit RAMS-T messages, which would only terminate their 1005 respective bursts. 1007 For the purpose of gathering detailed information about RTP_Rx's 1008 rapid acquisition experience, [I-D.ietf-avt-multicast-acq-rtcp-xr] 1009 defines an RTCP Extended Report (XR) Block. This report is designed 1010 to be payload-independent, thus, it can be used by any multicast 1011 application that supports rapid acquisition. Support for this XR 1012 report is, however, OPTIONAL. 1014 6.3. Synchronization of Primary Multicast Streams 1016 When an RTP_RX acquires multiple primary multicast streams, it might 1017 need to synchronize them for playout. This synchronization is 1018 achieved by the help of the RTCP sender reports [RFC3550]. If the 1019 playout will start before the RTP_Rx has joined the multicast 1020 session, the RTP_Rx needs to receive the information reflecting the 1021 synchronization among the primary multicast streams early enough so 1022 that it can play out the media in a synchronized fashion. 1024 The suggested approach is to use the RTP header extension mechanism 1025 [RFC5285] and convey the synchronization information in a header 1026 extension as defined in [I-D.ietf-avt-rapid-rtp-sync]. Per [RFC4588] 1027 "if the original RTP header carried an RTP header extension, the 1028 retransmission packet SHOULD carry the same header extension." Thus, 1029 as long as the multicast source emits a header extension with the 1030 synchronization information frequently enough, there is no additional 1031 task that needs to be carried out by the BRS. The synchronization 1032 information will be sent to the RTP_Rx along with the burst packets. 1033 The frequent header extensions sent in the primary multicast RTP 1034 sessions also allow rapid synchronization of the RTP streams for the 1035 RTP receivers that do not support RAMS or that directly join the 1036 multicast session without running RAMS. Thus, in RAMS applications, 1037 it is RECOMMENDED that the multicast sources frequently send 1038 synchronization information by using header extensions following the 1039 rules presented in [I-D.ietf-avt-rapid-rtp-sync]. The regular sender 1040 reports are still sent in the unicast session by following the rules 1041 of [RFC3550]. 1043 6.4. Burst Shaping and Congestion Control in RAMS 1045 This section provides informative guidelines about how the BRS can 1046 shape the transmission of the unicast burst and how congestion can be 1047 dealt within the RAMS process. When the RTP_Rx detects that the 1048 unicast burst is causing severe congestion, it can prefer to send a 1049 RAMS-T message immediately to stop the unicast burst. 1051 A higher bitrate for the unicast burst naturally conveys the 1052 Reference Information and media content to the RTP_Rx faster. This 1053 way, the RTP_Rx can start consuming the data sooner, which results in 1054 a faster acquisition. A higher bitrate also represents a better 1055 utilization of the BRS resources. As the burst may continue until it 1056 catches up with the primary multicast stream, the higher the bursting 1057 bitrate, the less data the BRS needs to transmit. However, a higher 1058 bitrate for the burst also increases the chances for congestion- 1059 caused packet loss. Thus, as discussed in Section 5, there has to be 1060 an upper bound on the bandwidth used by the burst. 1062 When the BRS transmits the unicast burst, it is expected to take into 1063 account all available information to prevent any packet loss that 1064 might take place during the bursting as a result of buffer overflow 1065 on the path between the RS and RTP_Rx and at the RTP_Rx itself. The 1066 bursting bitrate can be determined by taking into account the 1067 following information, when available: 1069 a. Information obtained via the RAMS-R message, such as Max RAMS 1070 Buffer Fill Requirement and/or Max Receive Bitrate (See 1071 Section 7.2). 1073 b. Information obtained via RTCP receiver reports provided by the 1074 RTP_Rx in the retransmission session, allowing in-session bitrate 1075 adaptations for the burst. When these receiver reports indicate 1076 packet loss, this can indicate a certain congestion state in the 1077 path from the RS to the RTP_Rx. 1079 c. Information obtained via RTCP NACKs provided by the RTP_Rx in the 1080 primary multicast RTP session, allowing in-session bitrate 1081 adaptations for the burst. Such RTCP NACKs are transmitted by 1082 the RTP_Rx in response to packet loss detection in the burst. 1083 NACKs can indicate a certain congestion state on the path from 1084 the RS to RTP_Rx. 1086 d. There can be other feedback received from the RTP_Rx, e.g., in 1087 the form of ECN-CE markings [I-D.ietf-avt-ecn-for-rtp] that can 1088 influence in-session bitrate adaptation. 1090 e. Information obtained via updated RAMS-R messages, allowing in- 1091 session bitrate adaptations, if supported by the BRS. 1093 f. Transport protocol-specific information. For example, when DCCP 1094 is used to transport the RTP burst, the ACKs from the DCCP client 1095 can be leveraged by the BRS / DCCP server for burst shaping and 1096 congestion control. 1098 g. Pre-configured settings for each RTP_Rx or a set of RTP_Rxs that 1099 indicate the upper-bound bursting bitrates for which no packet 1100 loss will occur as a result of congestion along the path of the 1101 RS to RTP_Rx. For example, in managed IPTV networks, where the 1102 bottleneck bandwidth along the end-to-end path is known and where 1103 the network between the RS and this link is provisioned and 1104 dimensioned to carry the burst streams, the bursting bitrate does 1105 not exceed the provisioned value. These settings can also be 1106 dynamically adapted using application-aware knowledge. 1108 The BRS chooses the initial burst bitrate as follows: 1110 o When using RAMS in environments as described in (g), the BRS MUST 1111 transmit the burst packets at an initial bitrate higher than the 1112 nominal bitrate, but within the engineered or reserved bandwidth 1113 limit. 1115 o When the BRS cannot determine a reliable bitrate value for the 1116 unicast burst (using a through g), it is desirable that the BRS 1117 chooses an appropriate initial bitrate not above the nominal 1118 bitrate and increases it gradually unless a congestion is 1119 detected. 1121 In both cases, during the burst transmission the BRS MUST 1122 continuously monitor for packet losses as a result of congestion by 1123 means of one or more of the mechanisms described in (b,c,d,e,f). 1124 When the BRS relies on RTCP receiver reports, sufficient bandwidth 1125 needs to be provided to RTP Rx for RTCP transmission in the unicast 1126 burst RTP session. To achieve a reasonable fast adaptation against 1127 congestion, it is recommended that the RTP_Rx sends a receiver report 1128 at least once every two RTTs between the RS and RTP_Rx. Although the 1129 specific heuristics and algorithms that deduce a congestion state and 1130 how subsequently the BRS acts are outside the scope of this 1131 specification, the following two methods are the best practices: 1133 o Upon detection of a significant amount of packet loss, which the 1134 BRS attributes to congestion, the BRS decreases the burst bitrate. 1135 The rate by which the BRS increases and decreases the bitrate for 1136 the burst can be determined by a TCP-friendly bitrate adaptation 1137 algorithm for RTP over UDP , or in the case of (f) by the 1138 congestion control algorithms defined in DCCP [RFC5762]. 1140 o If the congestion is persistent and the BRS has to reduce the 1141 burst bitrate to a point where the RTP Rx buffer might underrun or 1142 the burst will consume too many resources, the BRS terminates the 1143 burst and transmits a RAMS-I message to RTP Rx with the 1144 appropriate response code. It is then up to RTP Rx to decide when 1145 to join the multicast session. 1147 Even though there is no congestion experienced during the burst, 1148 congestion may anyway arise near the end of the burst as the RTP_Rx 1149 eventually needs to join the multicast session. During this brief 1150 period both the burst packets and the multicast packets can be 1151 simultaneously received by the RTP_Rx, thus enhancing the risk of 1152 congestion. 1154 Since the BRS signals the RTP_Rx when the BRS expects the RTP_Rx to 1155 send the SFGMP Join message, the BRS can have a rough estimate of 1156 when the RTP_Rx will start receiving multicast packets in the SSM 1157 session. The BRS can keep on sending burst packets but reduces the 1158 bitrate accordingly at the appropriate instant by taking the bitrate 1159 of the whole SSM session into account. If the BRS ceases 1160 transmitting the burst packets before the burst catches up, any gap 1161 resulting from this imperfect switch-over by the RTP_Rx can be later 1162 repaired by requesting retransmissions for the missing packets from 1163 the RS. The retransmissions can be shaped by the BRS to make sure 1164 that they do not cause collateral loss in the primary multicast RTP 1165 session and the unicast burst RTP session. 1167 6.5. Failure Cases 1169 In the following, we examine the implications of losing the RAMS-R, 1170 RAMS-I or RAMS-T messages and other failure cases. 1172 When the RTP_Rx sends a RAMS-R message to initiate a rapid 1173 acquisition but the message gets lost and the FT does not receive it, 1174 the RTP_Rx will get neither a RAMS-I message, nor a unicast burst. 1175 In this case, the RTP_Rx MAY resend the request when it is eligible 1176 to do so based on the RTCP timer rules defined in [RFC4585]. Or, 1177 after a reasonable amount of time, the RTP_Rx can time out (based on 1178 the previous observed response times) and immediately join the SSM 1179 session. 1181 In the case the RTP_Rx starts receiving a unicast burst but it does 1182 not receive a corresponding RAMS-I message within a reasonable amount 1183 of time, the RTP_Rx can either discard the burst data or decide not 1184 to interrupt the unicast burst, and be prepared to join the SSM 1185 session at an appropriate time it determines or as indicated in a 1186 subsequent RAMS-I message (if available). If the network is subject 1187 to packet loss, it is RECOMMENDED that the BRS repeats the RAMS-I 1188 messages multiple times based on the RTCP timer rules defined in 1189 [RFC4585]. 1191 In the failure cases where the RAMS-R message is lost and the RTP_Rx 1192 gives up, or the RAMS-I message is lost, the RTP_Rx MUST still 1193 terminate the burst(s) it requested by following the rules described 1194 in Section 6.2. 1196 In the case a RAMS-T message sent by the RTP_Rx does not reach its 1197 destination, the BRS can continue sending burst packets even though 1198 the RTP_Rx no longer needs them. In such cases, it is RECOMMENDED 1199 that the RTP_Rx repeats the RAMS-T message multiple times based on 1200 the RTCP timer rules defined in [RFC4585]. The BRS MUST be 1201 provisioned to terminate the burst when it can no longer send the 1202 burst packets faster than it receives the primary multicast stream 1203 packets. 1205 Section 6.3.5 of [RFC3550] explains the rules pertaining to timing 1206 out an SSRC. When the BRS accepts to serve the requested burst(s) 1207 and establishes the retransmission session, the BRS needs to check 1208 the liveness of the RTP_Rx via the RTCP messages and reports the 1209 RTP_Rx sends. The default rules explained in [RFC3550] apply in RAMS 1210 as well. 1212 7. Encoding of the Signaling Protocol in RTCP 1214 This section defines the formats of the RTCP transport-layer feedback 1215 messages that are exchanged between the retransmission server (RS) 1216 and RTP receiver (RTP_Rx) during rapid acquisition. These messages 1217 are referred to as the RAMS Messages. They are payload-independent 1218 and MUST be used by all RTP-based multicast applications that support 1219 rapid acquisition regardless of the payload they carry. 1221 Payload-specific feedback messages are not defined in this document. 1222 However, further optional payload-independent and payload-specific 1223 information can be included in the exchange. 1225 The common packet format for the RTCP feedback messages is defined in 1226 Section 6.1 of [RFC4585] and is also sketched in Figure 4. 1228 0 1 2 3 1229 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 1230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1231 |V=2|P| FMT | PT | length | 1232 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1233 | SSRC of packet sender | 1234 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1235 | SSRC of media source | 1236 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1237 : Feedback Control Information (FCI) : 1238 : : 1240 Figure 4: The common packet format for the RTCP feedback messages 1242 Each feedback message has a fixed-length field for version, padding, 1243 feedback message type (FMT), payload type (PT), length, SSRC of 1244 packet sender, SSRC of media sender as well as a variable-length 1245 field for feedback control information (FCI). 1247 In the RAMS messages, the PT field is set to RTPFB (205) and the FMT 1248 field is set to RAMS (6). Individual RAMS messages are identified by 1249 a sub-field called Sub Feedback Message Type (SFMT). Any Reserved 1250 field SHALL be set to zero and ignored. 1252 Depending on the specific scenario and timeliness/importance of a 1253 RAMS message, it can be desirable to send it in a reduced-size RTCP 1254 packet [RFC5506]. However, unless support for [RFC5506] has been 1255 signaled, compound RTCP packets MUST be used by following [RFC3550] 1256 rules. 1258 Following the rules specified in [RFC3550], all integer fields in the 1259 messages defined below are carried in network-byte order, that is, 1260 most significant byte (octet) first, also known as big-endian. 1261 Unless otherwise stated, numeric constants are in decimal (base 10). 1263 7.1. Extensions 1265 To improve the functionality of the RAMS method in certain 1266 applications, it can be desirable to define new fields in the RAMS 1267 Request, Information and Termination messages. Such fields MUST be 1268 encoded as Type-Length-Value (TLV) elements as described below and 1269 sketched in Figure 5: 1271 o Type: A single-octet identifier that defines the type of the 1272 parameter represented in this TLV element. 1274 o Length: A two-octet field that indicates the length (in octets) 1275 of the TLV element excluding the Type and Length fields, and the 1276 8-bit Reserved field between them. This length does not include 1277 any padding that is required for alignment. 1279 o Value: Variable-size set of octets that contains the specific 1280 value for the parameter. 1282 In the extensions, the Reserved field SHALL be set to zero and 1283 ignored. If a TLV element does not fall on a 32-bit boundary, the 1284 last word MUST be padded to the boundary using further bits set to 1285 zero. 1287 In a RAMS message, any vendor-neutral or private extension MUST be 1288 placed after the mandatory fields and mandatory TLV elements (if 1289 any). The extensions MAY be placed in any order. In a RAMS message, 1290 multiple TLV elements with the same Type value MUST NOT exist. 1292 The support for extensions (unless specified otherwise) is OPTIONAL. 1294 0 1 2 3 1295 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 1296 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1297 | Type | Reserved | Length | 1298 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1299 : Value : 1300 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1302 Figure 5: Structure of a TLV element 1304 7.1.1. Vendor-Neutral Extensions 1306 If the goal in defining new TLV elements is to extend the 1307 functionality in a vendor-neutral manner, they MUST be registered 1308 with IANA through the guidelines provided in Section 11.5. 1310 The current document defines several vendor-neutral extensions in the 1311 subsequent sections. 1313 7.1.2. Private Extensions 1315 It is desirable to allow vendors to use private extensions in a TLV 1316 format. For interoperability, such extensions must not collide with 1317 each other. 1319 A certain range of TLV Types (between - and including - 128 and 254 ) 1320 is reserved for private extensions (Refer to Section 11.5). IANA 1321 management for these extensions is unnecessary and they are the 1322 responsibility of individual vendors. 1324 The structure that MUST be used for the private extensions is 1325 depicted in Figure 6. Here, the enterprise numbers are used from 1326 http://www.iana.org/assignments/enterprise-numbers. This will ensure 1327 the uniqueness of the private extensions and avoid any collision. 1329 0 1 2 3 1330 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 1331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1332 | Type | Reserved | Length | 1333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1334 | Enterprise Number | 1335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1336 : Value : 1337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1339 Figure 6: Structure of a private extension 1341 7.2. RAMS Request 1343 The RAMS Request (RAMS-R) message is identified by SFMT=1. This 1344 message is sent as unicast feedback in the primary multicast RTP 1345 session by the RTP_Rx to request rapid acquisition of a primary 1346 multicast RTP session, or one or more primary multicast streams 1347 belonging to the same primary multicast RTP session. In the RAMS-R 1348 message, the RTP_Rx MUST set both the packet sender SSRC and media 1349 sender SSRC fields to its own SSRC since the media sender SSRC may 1350 not be known. The RTP_Rx MUST provide explicit signaling as 1351 described below to request stream(s). This minimizes the chances of 1352 accidentally requesting a wrong primary multicast stream. 1354 The FCI field MUST contain only one RAMS Request. The FCI field has 1355 the structure depicted in Figure 7. 1357 The semantics of the RAMS-R message is independent of the payload 1358 type carried in the primary multicast RTP session. 1360 0 1 2 3 1361 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 1362 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1363 | SFMT=1 | Reserved | 1364 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1365 : Requested Media Sender SSRC(s) : 1366 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1367 : Optional TLV-encoded Fields (and Padding, if needed) : 1368 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1370 Figure 7: FCI field syntax for the RAMS Request message 1372 o Requested Media Sender SSRC(s): Mandatory TLV element that lists 1373 the media sender SSRC(s) requested by the RTP_Rx. The BRS MUST 1374 ignore the media sender SSRC specified in the header of the RAMS-R 1375 message. 1377 If the Length field is set to zero (i.e., no media sender SSRC is 1378 listed), it means that the RTP_Rx is requesting to rapidly acquire 1379 the entire primary multicast RTP session. Otherwise, the RTP_Rx 1380 lists the individual media sender SSRCs in this TLV element and 1381 sets the Length field of the TLV element to 4*n, where n is the 1382 number of SSRC entries. 1384 Type: 1 1386 o Min RAMS Buffer Fill Requirement (32 bits): Optional TLV element 1387 that denotes the minimum milliseconds of data that the RTP_Rx 1388 desires to have in its buffer before allowing the data to be 1389 consumed by the application. 1391 The RTP_Rx can have knowledge of its buffering requirements. 1392 These requirements can be application and/or device specific. For 1393 instance, the RTP_Rx might need to have a certain amount of data 1394 in its application buffer to handle transmission jitter and/or to 1395 be able to support error-control methods. If the BRS is told the 1396 minimum buffering requirement of the receiver, the BRS can tailor 1397 the burst(s) more precisely, e.g., by choosing an appropriate 1398 starting point. The methods used by the RTP_Rx to determine this 1399 value are application specific, and thus, out of the scope of this 1400 document. 1402 If specified, the amount of backfill that will be provided by the 1403 unicast bursts and any payload-specific information MUST NOT be 1404 smaller than the specified value. Otherwise, the backfill will 1405 not be able to build up the desired level of buffer at the RTP_Rx 1406 and can cause buffer underruns. 1408 Type: 2 1410 o Max RAMS Buffer Fill Requirement (32 bits): Optional TLV element 1411 that denotes the maximum milliseconds of data that the RTP_Rx can 1412 buffer without losing the data due to buffer overflow. 1414 The RTP_Rx can have knowledge of its buffering requirements. 1415 These requirements can be application or device specific. For 1416 instance, one particular RTP_Rx might have more physical memory 1417 than another RTP_Rx, and thus, can buffer more data. If the BRS 1418 knows the buffering ability of the receiver, the BRS can tailor 1419 the burst(s) more precisely. The methods used by the receiver to 1420 determine this value are application specific, and thus, out of 1421 scope. 1423 If specified, the amount of backfill that will be provided by the 1424 unicast bursts and any payload-specific information MUST NOT be 1425 larger than this value. Otherwise, the backfill may cause buffer 1426 overflows at the RTP_Rx. 1428 Type: 3 1430 o Max Receive Bitrate (64 bits): Optional TLV element that denotes 1431 the maximum bitrate (in bits per second) at which the RTP_Rx can 1432 process the aggregation of the unicast burst(s) and any payload- 1433 specific information that will be provided by the BRS. The limits 1434 can include local receiver limits as well as network limits that 1435 are known to the receiver. 1437 If specified, the total bitrate of the unicast burst(s) plus any 1438 payload-specific information MUST NOT be larger than this value. 1439 Otherwise, congestion and packet loss may occur. 1441 Type: 4 1443 o Request for Preamble Only (0 bits): Optional TLV element that 1444 indicates that the RTP_Rx is only requesting the preamble 1445 information for the desired primary multicast stream(s). If this 1446 TLV element exists in the RAMS-R message, the BRS MUST NOT send 1447 any burst packets other than the preamble packets. Since this TLV 1448 element does not carry a Value field, the Length field MUST be set 1449 to zero. 1451 Type: 5 1453 7.3. RAMS Information 1455 The RAMS Information (RAMS-I) message is identified by SFMT=2. This 1456 message is used to describe the unicast burst that will be sent for 1457 rapid acquisition. It also includes other useful information for the 1458 RTP_Rx as described below. 1460 A separate RAMS-I message with the appropriate response code is sent 1461 in the unicast burst RTP session by the BRS for each primary 1462 multicast stream that has been requested by the RTP_Rx. In each of 1463 these RAMS-I messages, the media sender SSRC and packet sender SSRC 1464 fields are both set to the SSRC of the BRS, which equals the SSRC of 1465 the respective primary multicast stream. 1467 The FCI field MUST contain only one RAMS Information message. The 1468 FCI field has the structure depicted in Figure 8. 1470 The semantics of the RAMS-I message is independent of the payload 1471 type carried in the primary multicast RTP session. 1473 0 1 2 3 1474 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 1475 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1476 | SFMT=2 | MSN | Response | 1477 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1478 : Optional TLV-encoded Fields (and Padding, if needed) : 1479 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1481 Figure 8: FCI field syntax for the RAMS Information message 1483 A RAMS-I message has the following fields: 1485 o Message Sequence Number (8 bits) : Mandatory field that denotes 1486 the sequence number of the RAMS-I message for the particular media 1487 sender SSRC specified in the message header. The MSN value SHALL 1488 be set to zero only when a new RAMS request is received. During 1489 rapid acquisition, the same RAMS-I message MAY be repeated for 1490 redundancy purposes without incrementing the MSN value. If an 1491 updated RAMS-I message will be sent (either with a new information 1492 or an updated information), the MSN value SHALL be incremented by 1493 one. In the MSN field, the regular wrapping rules apply. 1495 o Response (16 bits): Mandatory field that denotes the response 1496 code for this RAMS-I message. This document defines several 1497 initial response codes in Section 11.6 and registers them with 1498 IANA. If a new vendor-neutral response code will be defined, it 1499 MUST be registered with IANA through the guidelines specified in 1500 Section 11.6. If the new response code is intended to be used 1501 privately by a vendor, there is no need for IANA management. 1502 Instead, the vendor MUST use the private extension mechanism 1503 (Section 7.1.2) to convey its message and MUST indicate this by 1504 putting zero in the Response field. When the RTP_Rx receives an 1505 RAMS-I message with a private response code that it does not 1506 understand, the RTP_Rx still needs to process the TLV elements it 1507 understands. 1509 The following TLV elements have been defined for the RAMS-I messages: 1511 o Media Sender SSRC (32 bits): Optional TLV element that specifies 1512 the media sender SSRC of the unicast burst stream. While this 1513 information is already available in the message header, it can be 1514 useful to repeat it in an explicit field. If the FT_Ap that 1515 received the RAMS-R message is associated with a single primary 1516 multicast stream but the requested media sender SSRC does not 1517 match the SSRC of the RTP stream associated with this FT_Ap, the 1518 BRS includes this TLV element in the initial RAMS-I message to let 1519 the RTP_Rx know that the media sender SSRC has changed. If the 1520 two SSRCs match, there is no need to include this TLV element. 1522 Type: 31 1524 o RTP Seqnum of the First Packet (16 bits): TLV element that 1525 specifies the RTP sequence number of the first packet that will be 1526 sent in the respective unicast RTP stream. This allows the RTP_Rx 1527 to know whether one or more packets sent by the BRS have been 1528 dropped at the beginning of the stream. If the BRS accepts the 1529 RAMS request, this element exists. If the BRS rejects the RAMS 1530 request, this element SHALL NOT exist. 1532 Type: 32 1534 o Earliest Multicast Join Time (32 bits): TLV element that 1535 specifies the delta time (in ms) between the arrival of the first 1536 RTP packet in the unicast RTP stream (which could be a burst 1537 packet or a payload-specific packet) and the earliest time instant 1538 when an RTP_Rx MAY send an SFGMP Join message to join the 1539 multicast session. A zero value in this field means that the 1540 RTP_Rx MAY send the SFGMP Join message right away. 1542 If the RAMS request has been accepted, the BRS sends this field at 1543 least once, so that the RTP_Rx knows when to join the multicast 1544 session. If the burst request has been rejected as indicated in 1545 the Response field, this field MUST be set to zero. In that case, 1546 it is up to the RTP_Rx when or whether to join the multicast 1547 session. 1549 When the BRS serves two or more bursts and sends a separate RAMS-I 1550 message for each burst, the join times specified in these RAMS-I 1551 messages should correspond to more or less the same time instant, 1552 and the RTP_Rx sends the SFGMP Join message based on the earliest 1553 join time. 1555 Type: 33 1557 o Burst Duration (32 bits): Optional TLV element that denotes the 1558 time the burst will last (in ms), i.e., the delta difference 1559 between the expected transmission times of the first and the last 1560 burst packets that the BRS is planning to send in the respective 1561 unicast RTP stream. In the absence of additional stimulus, the 1562 BRS will send a burst of this duration. However, the burst 1563 duration can be modified by subsequent events, including changes 1564 in the primary multicast stream and reception of RAMS-T messages. 1566 The BRS MUST terminate the flow in the timeframe indicated by this 1567 burst duration or the duration established by those subsequent 1568 events, even if it does not get a RAMS-T or a BYE message from the 1569 RTP_Rx. It is OPTIONAL to send this field in a RAMS-I message 1570 when the burst request is accepted. If the burst request has been 1571 rejected as indicated in the Response field, this field MAY be 1572 omitted or set to zero. 1574 Type: 34 1576 o Max Transmit Bitrate (64 bits): Optional TLV element that denotes 1577 the maximum bitrate (in bits per second) that will be used by the 1578 BRS for the RTP stream associated with this RAMS-I message. 1580 Type: 35 1582 7.4. RAMS Termination 1584 The RAMS Termination (RAMS-T) message is identified by SFMT=3. 1586 The RAMS Termination is used to assist the BRS in determining when to 1587 stop the burst. A separate RAMS-T message is sent in the unicast 1588 burst RTP session by the RTP_Rx for each primary multicast stream 1589 that has been served by the BRS. Each of these RAMS-T messages has 1590 the appropriate media sender SSRC populated in its message header. 1592 If the RTP_Rx wants the BRS to stop a burst prematurely, it sends a 1593 RAMS-T message as described below. Upon receiving this message, the 1594 BRS stops the respective burst immediately. If the RTP_Rx wants the 1595 BRS to terminate all of the bursts, it needs to send all of the 1596 respective RAMS-T messages in a single compound RTCP packet. 1598 The default behavior for the RTP_Rx is to send a RAMS-T message 1599 immediately after it joined the multicast session and started 1600 receiving multicast packets. In that case, the RTP_Rx includes the 1601 sequence number of the first RTP packet received in the primary 1602 multicast stream in the RAMS-T message. With this information, the 1603 BRS can decide when to terminate the unicast burst. 1605 The FCI field MUST contain only one RAMS Termination. The FCI field 1606 has the structure depicted in Figure 9. 1608 The semantics of the RAMS-T message is independent of the payload 1609 type carried in the primary multicast RTP session. 1611 0 1 2 3 1612 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 1613 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1614 | SFMT=3 | Reserved | 1615 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1616 : Optional TLV-encoded Fields (and Padding, if needed) : 1617 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1619 Figure 9: FCI field syntax for the RAMS Termination message 1621 o Extended RTP Seqnum of First Multicast Packet (32 bits): Optional 1622 TLV element that specifies the extended RTP sequence number of the 1623 first packet received from the SSM session for a particular 1624 primary multicast stream. The low 16 bits contain the sequence 1625 number of the first packet received from the SSM session, and the 1626 most significant 16 bits extend that sequence number with the 1627 corresponding count of sequence number cycles, which can be 1628 maintained according to the algorithm in Appendix A.1 of 1630 [RFC3550]. 1632 Type: 61 1634 8. SDP Signaling 1636 8.1. Definitions 1638 The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585]. 1639 Here we add the following syntax to the 'rtcp-fb' attribute (the 1640 feedback type and optional parameters are all case sensitive): 1642 (In the following ABNF [RFC5234], rtcp-fb-nack-param is used as 1643 defined in [RFC4566].) 1645 rtcp-fb-nack-param =/ SP "rai" 1647 The following parameter is defined in this document for use with 1648 'nack': 1650 o 'rai' stands for Rapid Acquisition Indication and indicates the 1651 use of RAMS messages as defined in Section 7. 1653 This document also defines a new media-level SDP attribute ('rams- 1654 updates') that indicates whether the BRS supports updated request 1655 messages or not. This attribute is used in a declarative manner and 1656 no Offer/Answer Model behavior is specified. If the BRS supports 1657 updated request messages and this attribute is included in the SDP 1658 description, the RTP_Rx can send updated requests. The BRS might or 1659 might not be able to accept value changes in every field in an 1660 updated RAMS-R message. However, if the 'rams-updates' attribute is 1661 not included in the SDP description, the RTP_Rx SHALL NOT send 1662 updated requests. The RTP_Rx MAY still repeat its initial request 1663 without changes, though. 1665 8.2. Requirements 1667 The use of SDP to describe the RAMS entities normatively requires the 1668 support for: 1670 o The SDP grouping framework and flow identification (FID) semantics 1671 [RFC5888] 1673 o The RTP/AVPF profile [RFC4585] 1674 o The RTP retransmission payload format [RFC4588] 1676 o The RTCP extensions for SSM sessions with unicast feedback 1677 [RFC5760] 1679 o The multicast RTCP port attribute [I-D.ietf-avt-rtcp-port-for-ssm] 1681 o Multiplexing RTP and RTCP on a single port on both endpoints in 1682 the unicast session[RFC5761] 1684 The support for the source-specific media attributes [RFC5576] can 1685 also be needed. 1687 8.3. Example and Discussion 1689 This section provides a declarative SDP [RFC4566] example for 1690 enabling rapid acquisition of multicast RTP sessions. 1692 v=0 1693 o=ali 1122334455 1122334466 IN IP4 rams.example.com 1694 s=Rapid Acquisition Example 1695 t=0 0 1696 a=group:FID 1 2 1697 a=rtcp-unicast:rsi 1698 m=video 41000 RTP/AVPF 98 1699 i=Primary Multicast Stream 1700 c=IN IP4 233.252.0.2/255 1701 a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 1702 a=rtpmap:98 MP2T/90000 1703 a=multicast-rtcp:42000 1704 a=rtcp:43000 IN IP4 192.0.2.1 1705 a=rtcp-fb:98 nack 1706 a=rtcp-fb:98 nack rai 1707 a=ssrc:123321 cname:iptv-ch32@rams.example.com 1708 a=rams-updates 1709 a=mid:1 1710 m=video 51000 RTP/AVPF 99 1711 i=Unicast Retransmission Stream (Ret. and Rapid Acq. Support) 1712 c=IN IP4 192.0.2.1 1713 a=sendonly 1714 a=rtpmap:99 rtx/90000 1715 a=rtcp-mux 1716 a=fmtp:99 apt=98;rtx-time=5000 1717 a=mid:2 1719 Figure 10: Example SDP for a single-channel RAMS scenario 1721 In this example SDP description, we have a primary multicast (source) 1722 stream and a unicast retransmission stream. The source stream is 1723 multicast from a distribution source (with a source IP address of 1724 198.51.100.1) to the multicast destination address of 233.252.0.2 and 1725 port 41000. The corresponding RTCP traffic is multicast to the same 1726 multicast destination address at port 42000. Here, we are assuming 1727 that the multicast RTP and RTCP ports are carefully chosen so that 1728 different RTP and RTCP streams do not collide with each other. 1730 The feedback target (FT_Ap) residing on the RS (with an address of 1731 192.0.2.1) at port 43000 is declared with the "a=rtcp" line 1732 [RFC3605]. The support for the conventional retransmission is 1733 indicated through the "a=rtcp-fb:98 nack" line. The RTP receiver(s) 1734 can report missing packets on the source stream to the feedback 1735 target and request retransmissions. The SDP above includes the 1736 "a=sendonly" line for the media description of the retransmission 1737 stream since the retransmissions are sent in only one direction. 1739 The support for rapid acquisition is indicated through the "a=rtcp- 1740 fb:98 nack rai" line. The parameter 'rtx-time' applies to both the 1741 conventional retransmissions and rapid acquisition. However, when 1742 rapid acquisition is enabled, the standard definition for the 1743 parameter 'rtx-time' given in [RFC4588] is not followed. Instead, 1744 when rapid acquisition support is enabled, 'rtx-time' specifies the 1745 time in milliseconds that the BRS keeps an RTP packet in its cache 1746 available for retransmission (measured from the time the packet was 1747 received by the BRS, not from the time indicated in the packet 1748 timestamp). 1750 Once an RTP_Rx has acquired an SDP description, it can ask for rapid 1751 acquisition before it joins a primary multicast RTP session. To do 1752 so, it sends a RAMS-R message to the feedback target of that primary 1753 multicast RTP session. If the FT_Ap is associated with only one RTP 1754 stream, the RTP_Rx does not need to learn the SSRC of that stream via 1755 an out-of-band method. If the BRS accepts the rapid acquisition 1756 request, it will send an RAMS-I message with the correct SSRC 1757 identifier. If the FT_Ap is associated with a multi-stream RTP 1758 session and the RTP_Rx is willing to request rapid acquisition for 1759 the entire session, the RTP_Rx again does not need to learn the SSRCs 1760 via an out-of-band method. However, if the RTP_Rx intends to request 1761 a particular subset of the primary multicast streams, it must learn 1762 their SSRC identifiers and list them in the RAMS-R message. Since 1763 this RTP_Rx has not yet received any RTP packets for the primary 1764 multicast stream(s), the RTP_Rx must in this case learn the SSRC 1765 value(s) from the 'ssrc' attribute of the media description 1766 [RFC5576]. In addition to the SSRC value, the 'cname' source 1767 attribute must also be present in the SDP description [RFC5576]. 1769 Listing the SSRC values for the primary multicast streams in the SDP 1770 file does not create a problem in SSM sessions when an SSRC collision 1771 occurs. This is because in SSM sessions, an RTP_Rx that observed an 1772 SSRC collision with a media sender must change its own SSRC [RFC5760] 1773 by following the rules defined in [RFC3550]. 1775 A feedback target that receives a RAMS-R message becomes aware that 1776 the RTP_Rx wants to rapidly catch up with a primary multicast RTP 1777 session. If the necessary conditions are satisfied (as outlined in 1778 Section 7 of [RFC4585]) and available resources exist, the BRS can 1779 react to the RAMS-R message by sending any transport-layer (and 1780 optional payload-specific, when allowed) feedback message(s) and 1781 starting the unicast burst. 1783 In this section, we considered the simplest scenario where the 1784 primary multicast RTP session carried only one stream and the RTP_Rx 1785 wanted to rapidly acquire this stream only. Best practices for 1786 scenarios where the primary multicast RTP session carries two or more 1787 streams or the RTP_Rx wants to acquire one or more streams from 1788 multiple primary multicast RTP sessions at the same time are 1789 presented in [I-D.begen-avt-rams-scenarios]. 1791 9. NAT Considerations 1793 For a variety of reasons, one or more NAPT devices (hereafter simply 1794 called NAT) can exist between the RTP_Rx and RS. NATs have a variety 1795 of operating characteristics for UDP traffic [RFC4787]. For a NAT to 1796 permit traffic from the BRS to arrive at the RTP_Rx, the NAT(s) must 1797 first either: 1799 a. See UDP traffic sent from the RTP_Rx (which is on the 'inside' of 1800 the NAT) to the BRS (which is on the 'outside' of the NAT). This 1801 traffic has the same transport address as the subsequent response 1802 traffic, or; 1804 b. Be configured to forward certain ports (e.g., using HTML 1805 configuration and UPnP IGD [UPnP-IGD]). Details of this are out 1806 of scope of this document. 1808 For both (a) and (b), the RTP_Rx is responsible for maintaining the 1809 NAT's state if it wants to receive traffic from the BRS on that port. 1810 For (a), the RTP_Rx MUST send UDP traffic to keep the NAT binding 1811 alive, at least every 30 seconds [RFC4787]. While (a) is more like 1812 an automatic/dynamic configuration, (b) is more like a manual/static 1813 configuration. 1815 When the RTP_Rx sends a request (RAMS-R) message to the FT as unicast 1816 feedback in the primary multicast RTP session, and the request is 1817 received by the FT, a new unicast burst RTP session will be 1818 established between the BRS and RTP_Rx. 1820 While the FT and BRS ports on the RS are already signaled via out-of- 1821 band means (e.g., SDP), the RTP_Rx needs to convey the RTP and RTCP 1822 ports it wants to use on its side for the new session. Since there 1823 are two RTP sessions (one multicast and one unicast) involved during 1824 this process and one of them is established upon a feedback message 1825 sent in the other one, this requires an explicit port mapping method. 1827 Applications using RAMS MUST support the solution presented in 1828 [I-D.ietf-avt-ports-for-ucast-mcast-rtp] both on the RS and RTP_Rx 1829 side to allow RTP receivers to use their desired ports and to support 1830 RAMS behind NAT devices. The port mapping message exchange needs to 1831 take place whenever the RTP_Rx intends to make use of the RAMS 1832 protocol for rapidly acquiring a specific multicast RTP session, 1833 prior to joining the associated SSM session. 1835 10. Security Considerations 1837 Applications that are using RAMS make heavy use of the unicast 1838 feedback mechanism described in [RFC5760], the payload format defined 1839 in [RFC4588] and the port mapping solution specified in 1840 [I-D.ietf-avt-ports-for-ucast-mcast-rtp]. Thus, these applications 1841 are subject to the general security considerations discussed in 1842 [RFC5760], [RFC4588] and [I-D.ietf-avt-ports-for-ucast-mcast-rtp]. 1843 In this section, we give an overview of the guidelines and 1844 suggestions described in these specifications from a RAMS 1845 perspective. We also discuss the security considerations that 1846 explicitly apply to applications using RAMS. 1848 First of all, much of the session description information is 1849 available in the SDP descriptions that are distributed to the media 1850 senders, retransmission servers and RTP receivers. Adequate security 1851 measures are RECOMMENDED to ensure the integrity and authenticity of 1852 the SDP descriptions so that transport addresses of the media 1853 senders, distribution sources, feedback targets as well as other 1854 session-specific information can be protected. 1856 Compared to an RTCP NACK message that triggers one or more 1857 retransmissions, a RAMS Request (RAMS-R) message can trigger a new 1858 burst stream to be sent by the retransmission server. Depending on 1859 the application-specific requirements and conditions existing at the 1860 time of the RAMS-R reception by the retransmission server, the 1861 resulting burst stream can potentially contain a large number of 1862 retransmission packets. Since these packets are sent at a faster 1863 than the nominal rate, RAMS consumes more resources on the 1864 retransmission servers, RTP receivers and the network. In 1865 particular, this can make denial-of-service attacks more intense, and 1866 hence, more harmful than attacks that target ordinary retransmission 1867 sessions. 1869 Following the suggestions given in [RFC4588], counter-measures SHOULD 1870 be taken to prevent tampered or spoofed RTCP packets. Tampered 1871 RAMS-R messages can trigger inappropriate burst streams or alter the 1872 existing burst streams in an inappropriate way. For example, if the 1873 Max Receive Bitrate field is altered by a tampered RAMS-R message, 1874 the updated burst can overflow the buffer at the receiver side, or 1875 oppositely, can slow down the burst to the point that it becomes 1876 useless. Tampered RAMS Termination (RAMS-T) messages can terminate 1877 valid burst streams prematurely resulting in gaps in the received RTP 1878 packets. RAMS Information (RAMS-I) messages contain fields that are 1879 critical for a successful rapid acquisition. Any tampered 1880 information in the RAMS-I message can easily cause an RTP receiver to 1881 make wrong decisions. Consequently, the RAMS operation can fail. 1883 While most of the denial-of-service attacks can be prevented by the 1884 integrity and authenticity checks enabled by Secure RTP (SRTP) 1885 [RFC3711], an attack can still be started by legitimate endpoints 1886 that send several valid RAMS-R messages to a particular feedback 1887 target in a synchronized fashion and very short amount of time. 1888 Since a RAMS operation can temporarily consume a large amount of 1889 resources, a series of the RAMS-R messages can temporarily overload 1890 the retransmission server. In these circumstances, the 1891 retransmission server can, for example, reject incoming RAMS requests 1892 until its resources become available again. One means to ameliorate 1893 this threat is to apply a per-endpoint policing mechanism on the 1894 incoming RAMS requests. A reasonable policing mechanism should 1895 consider application-specific requirements and minimize false 1896 negatives. 1898 In addition to the denial-of-service attacks, man-in-the-middle and 1899 replay attacks can also be harmful. However, RAMS itself does not 1900 bring any new risks or threats other than the ones discussed in 1901 [RFC5760]. 1903 [RFC4588] RECOMMENDS that the cryptography mechanisms are used for 1904 the retransmission payload format to provide protection against known 1905 plain-text attacks. As discussed in [RFC4588], the retransmission 1906 payload format sets the timestamp field in the RTP header to the 1907 media timestamp of the original packet and this does not compromise 1908 the confidentiality. Furthermore, if cryptography is used to provide 1909 security services on the original stream, then the same services, 1910 with equivalent cryptographic strength, MUST be provided on the 1911 retransmission stream per [RFC4588]. 1913 To protect the RTCP messages from man-in-the-middle and replay 1914 attacks, the RTP receivers and retransmission server SHOULD perform a 1915 DTLS-SRTP handshake [RFC5764] over the RTCP channel. Because there 1916 is no integrity-protected signaling channel between an RTP receiver 1917 and the retransmission server, the retransmission server MUST 1918 maintain a list of certificates owned by legitimate RTP receivers, or 1919 their certificates MUST be signed by a trusted Certificate Authority. 1920 Once the DTLS-SRTP security is established, non-SRTCP-protected 1921 messages received from a particular RTP receiver are ignored by the 1922 retransmission server. To reduce the impact of DTLS-SRTP overhead 1923 when communicating with different feedback targets on the same 1924 retransmission server, it is RECOMMENDED that RTP receivers and the 1925 retransmission server both support TLS Session Resumption without 1926 Server-side State [RFC5077]. To help scale SRTP to handle many RTP 1927 receivers asking for retransmissions of identical data, implementors 1928 may consider using the same SRTP key for SRTP data sent to the 1929 receivers [I-D.ietf-avt-srtp-ekt] and consider the security of such 1930 SRTP key sharing. 1932 11. IANA Considerations 1934 The following contact information shall be used for all registrations 1935 in this document: 1937 Ali Begen 1938 abegen@cisco.com 1940 Note to the RFC Editor: In the following, please replace "XXXX" with 1941 the number of this document prior to publication as an RFC. 1943 11.1. Registration of SDP Attributes 1945 This document registers a new attribute name in SDP. 1947 SDP Attribute ("att-field"): 1948 Attribute name: rams-updates 1949 Long form: Support for Updated RAMS Request Messages 1950 Type of name: att-field 1951 Type of attribute: Media level 1952 Subject to charset: No 1953 Purpose: See this document 1954 Reference: [RFCXXXX] 1955 Values: None 1957 11.2. Registration of SDP Attribute Values 1959 This document registers a new value for the 'nack' attribute to be 1960 used with the 'rtcp-fb' attribute in SDP. For more information about 1961 the 'rtcp-fb' attribute, refer to Sections 4.2 and 6.2 of [RFC4585]. 1963 Value name: rai 1964 Long name: Rapid Acquisition Indication 1965 Usable with: nack 1966 Reference: [RFCXXXX] 1968 11.3. Registration of FMT Values 1970 Within the RTPFB range, the following format (FMT) value is 1971 registered: 1973 Name: RAMS 1974 Long name: Rapid Acquisition of Multicast Sessions 1975 Value: 6 1976 Reference: [RFCXXXX] 1978 11.4. SFMT Values for RAMS Messages Registry 1980 This document creates a new sub-registry for the sub-feedback message 1981 type (SFMT) values to be used with the FMT value registered for RAMS 1982 messages. The registry is called the SFMT Values for RAMS Messages 1983 Registry. This registry is to be managed by the IANA according to 1984 the Specification Required policy of [RFC5226]. 1986 The length of the SFMT field in the RAMS messages is a single octet, 1987 allowing 256 values. The registry is initialized with the following 1988 entries: 1990 Value Name Reference 1991 ----- -------------------------------------------------- ------------- 1992 0 Reserved [RFCXXXX] 1993 1 RAMS Request [RFCXXXX] 1994 2 RAMS Information [RFCXXXX] 1995 3 RAMS Termination [RFCXXXX] 1996 4-254 Assignable - Specification Required 1997 255 Reserved [RFCXXXX] 1999 The SFMT values 0 and 255 are reserved for future use. 2001 Any registration for an unassigned SFMT value needs to contain the 2002 following information: 2004 o Contact information of the one doing the registration, including 2005 at least name, address, and email. 2007 o A detailed description of what the new SFMT represents and how it 2008 shall be interpreted. 2010 New RAMS functionality is intended to be introduced by using the 2011 extension mechanism within the existing RAMS message types not by 2012 introducing new message types unless it is absolutely necessary. 2014 11.5. RAMS TLV Space Registry 2016 This document creates a new IANA TLV space registry for the RAMS 2017 extensions. The registry is called the RAMS TLV Space Registry. 2018 This registry is to be managed by the IANA according to the 2019 Specification Required policy of [RFC5226]. 2021 The length of the Type field in the TLV elements is a single octet, 2022 allowing 256 values. The Type values 0 and 255 are reserved for 2023 future use. The Type values between (and including) 128 and 254 are 2024 reserved for private extensions. 2026 The registry is initialized with the following entries: 2028 Type Description Reference 2029 ---- -------------------------------------------------- ------------- 2030 0 Reserved [RFCXXXX] 2031 1 Requested Media Sender SSRC(s) [RFCXXXX] 2032 2 Min RAMS Buffer Fill Requirement [RFCXXXX] 2033 3 Max RAMS Buffer Fill Requirement [RFCXXXX] 2034 4 Max Receive Bitrate [RFCXXXX] 2035 5 Request for Preamble Only [RFCXXXX] 2036 6-30 Assignable - Specification Required 2037 31 Media Sender SSRC [RFCXXXX] 2038 32 RTP Seqnum of the First Packet [RFCXXXX] 2039 33 Earliest Multicast Join Time [RFCXXXX] 2040 34 Burst Duration [RFCXXXX] 2041 35 Max Transmit Bitrate [RFCXXXX] 2042 36-60 Assignable - Specification Required 2043 61 Extended RTP Seqnum of First Multicast Packet [RFCXXXX] 2044 62-127 Assignable - Specification Required 2045 128-254 No IANA Maintenance 2046 255 Reserved [RFCXXXX] 2048 Any registration for an unassigned Type value needs to contain the 2049 following information: 2051 o Contact information of the one doing the registration, including 2052 at least name, address, and email. 2054 o A detailed description of what the new TLV element represents and 2055 how it shall be interpreted. 2057 11.6. RAMS Response Code Space Registry 2059 This document creates a new IANA TLV space registry for the RAMS 2060 response codes. The registry is called the RAMS Response Code Space 2061 Registry. This registry is to be managed by the IANA according to 2062 the Specification Required policy of [RFC5226]. 2064 The length of the Response field is two octets, allowing 65536 codes. 2066 However, the response codes have been classified and registered 2067 following an HTTP-style code numbering in this document. New 2068 response codes should be classified following the guidelines below: 2070 Code Level Purpose 2071 ---------- --------------- 2072 1xx Informational 2073 2xx Success 2074 3xx Redirection 2075 4xx RTP Receiver Error 2076 5xx Retransmission Server Error 2078 The Response code 65535 is reserved for future use. 2080 The registry is initialized with the following entries: 2082 Code Description Reference 2083 ----- -------------------------------------------------- ------------- 2084 0 A private response code is included in the message [RFCXXXX] 2086 100 Parameter update for RAMS session [RFCXXXX] 2088 200 RAMS request has been accepted [RFCXXXX] 2089 201 Unicast burst has been completed [RFCXXXX] 2091 400 Invalid RAMS-R message syntax 2092 401 Invalid min buffer requirement in RAMS-R message [RFCXXXX] 2093 402 Invalid max buffer requirement in RAMS-R message [RFCXXXX] 2094 403 Invalid max bitrate requirement in RAMS-R message [RFCXXXX] 2096 500 An unspecified BRS internal error has occurred [RFCXXXX] 2097 501 BRS has insufficient bandwidth to start RAMS 2098 session [RFCXXXX] 2099 502 Burst is terminated due to network congestion [RFCXXXX] 2100 503 BRS has insufficient CPU cycles to start RAMS 2101 session [RFCXXXX] 2102 504 RAMS functionality is not available on BRS [RFCXXXX] 2103 505 RAMS functionality is not available for RTP_Rx [RFCXXXX] 2104 506 RAMS functionality is not available for 2105 the requested multicast stream [RFCXXXX] 2106 507 BRS has no valid starting point available for 2107 the requested multicast stream [RFCXXXX] 2108 508 BRS has no reference information available for 2109 the requested multicast stream [RFCXXXX] 2110 509 BRS has no RTP stream matching the requested SSRC [RFCXXXX] 2111 510 RAMS request to acquire the entire session 2112 has been denied [RFCXXXX] 2113 511 Only the preamble information is sent [RFCXXXX] 2114 512 RAMS request has been denied due to a policy [RFCXXXX] 2116 The definitions for these codes are provided in Section 11.6.1. 2118 Any registration for an unassigned Response code needs to contain the 2119 following information: 2121 o Contact information of the one doing the registration, including 2122 at least name, address, and email. 2124 o A detailed description of what the new Response code describes and 2125 how it shall be interpreted. 2127 11.6.1. Response Code Definitions 2129 o 100: This is used when the BRS wants to update a value that was 2130 sent earlier to the RTP_Rx. 2132 o 200: This is used when the server accepts the RAMS request. 2134 o 201: This is used when the unicast burst has been completed and 2135 the BRS wants to notify the RTP_Rx. 2137 o 400: This is used when the RAMS-R message is improperly 2138 formatted. 2140 o 401: This is used when the minimum RAMS buffer fill requirement 2141 value indicated in the RAMS-R message is invalid. 2143 o 402: This is used when the maximum RAMS buffer fill requirement 2144 value indicated in the RAMS-R message is invalid. 2146 o 403: This is used when the maximum receive bitrate value 2147 indicated in the RAMS-R message is invalid. 2149 o 500: This is used when the BRS has experienced an internal error 2150 and cannot accept the RAMS request. 2152 o 501: This is used when the BRS does not have enough bandwidth to 2153 send the unicast burst stream. 2155 o 502: This is used when the BRS terminates the unicast burst 2156 stream due to network congestion. 2158 o 503: This is used when the BRS does not have enough CPU resources 2159 to send the unicast burst stream. 2161 o 504: This is used when the BRS does not support sending a unicast 2162 burst stream. 2164 o 505: This is used when the requesting RTP_Rx is not eligible to 2165 receive a unicast burst stream. 2167 o 506: This is used when RAMS functionality is not enabled for the 2168 requested multicast stream. 2170 o 507: This is used when the BRS cannot find a valid starting point 2171 for the unicast burst stream satisfying the RTP_Rx's requirements. 2173 o 508: This is used when the BRS cannot find the essential 2174 reference information for the requested multicast stream. 2176 o 509: This is used when the BRS cannot match the requested SSRC to 2177 any of the streams it is serving. 2179 o 510: This is used when the BRS cannot serve the requested entire 2180 session. 2182 o 511: This is used when the BRS sends only the preamble 2183 information but not the whole unicast burst stream. 2185 o 512: This is used when the RAMS request is denied due to a policy 2186 specified for the requested multicast stream, requesting RTP_Rx or 2187 this particular BRS. 2189 12. Contributors 2191 Dave Oran, Magnus Westerlund and Colin Perkins have contributed 2192 significantly to this specification by providing text and solutions 2193 to some of the issues raised during the development of this 2194 specification. 2196 13. Acknowledgments 2198 The following individuals have reviewed the earlier versions of this 2199 specification and provided helpful comments: Colin Perkins, Joerg 2200 Ott, Roni Even, Dan Wing, Tony Faustini, Peilin Yang, Jeff Goldberg, 2201 Muriel Deschanel, Orit Levin, Guy Hirson, Tom Taylor, Xavier Marjou, 2202 Ye-Kui Wang, Zixuan Zou, Ingemar Johansson, Haibin Song, Ning Zong, 2203 Jonathan Lennox, Jose Rey, Sean Sheedy and Keith Drage. 2205 14. References 2207 14.1. Normative References 2209 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 2210 Jacobson, "RTP: A Transport Protocol for Real-Time 2211 Applications", STD 64, RFC 3550, July 2003. 2213 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2214 Requirement Levels", BCP 14, RFC 2119, March 1997. 2216 [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. 2217 Thyagarajan, "Internet Group Management Protocol, Version 2218 3", RFC 3376, October 2002. 2220 [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery 2221 Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. 2223 [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet 2224 Group Management Protocol Version 3 (IGMPv3) and Multicast 2225 Listener Discovery Protocol Version 2 (MLDv2) for Source- 2226 Specific Multicast", RFC 4604, August 2006. 2228 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2229 Description Protocol", RFC 4566, July 2006. 2231 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 2232 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 2234 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 2235 "Extended RTP Profile for Real-time Transport Control 2236 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 2237 July 2006. 2239 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 2240 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 2241 July 2006. 2243 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 2244 Protocol (RTCP) Extensions for Single-Source Multicast 2245 Sessions with Unicast Feedback", RFC 5760, February 2010. 2247 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 2248 Media Attributes in the Session Description Protocol 2249 (SDP)", RFC 5576, June 2009. 2251 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 2252 in Session Description Protocol (SDP)", RFC 3605, 2253 October 2003. 2255 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2256 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2258 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 2259 Real-Time Transport Control Protocol (RTCP): Opportunities 2260 and Consequences", RFC 5506, April 2009. 2262 [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP 2263 Header Extensions", RFC 5285, July 2008. 2265 [I-D.ietf-avt-rapid-rtp-sync] 2266 Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP 2267 Flows", draft-ietf-avt-rapid-rtp-sync-12 (work in 2268 progress), July 2010. 2270 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 2271 Control Packets on a Single Port", RFC 5761, April 2010. 2273 [I-D.ietf-avt-rtcp-port-for-ssm] 2274 Begen, A., "RTP Control Protocol (RTCP) Port for Source- 2275 Specific Multicast (SSM) Sessions", 2276 draft-ietf-avt-rtcp-port-for-ssm-02 (work in progress), 2277 August 2010. 2279 [I-D.ietf-avt-ports-for-ucast-mcast-rtp] 2280 Begen, A. and B. Steeg, "Port Mapping Between Unicast and 2281 Multicast RTP Sessions", 2282 draft-ietf-avt-ports-for-ucast-mcast-rtp-02 (work in 2283 progress), May 2010. 2285 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 2286 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 2287 RFC 3711, March 2004. 2289 [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer 2290 Security (DTLS) Extension to Establish Keys for the Secure 2291 Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. 2293 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 2294 "Transport Layer Security (TLS) Session Resumption without 2295 Server-Side State", RFC 5077, January 2008. 2297 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2298 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 2299 May 2008. 2301 14.2. Informative References 2303 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 2304 August 1980. 2306 [I-D.begen-avt-rams-scenarios] 2307 Begen, A., "Considerations for RAMS Scenarios", 2308 draft-begen-avt-rams-scenarios-00 (work in progress), 2309 October 2009. 2311 [I-D.ietf-avt-rtp-cnames] 2312 Begen, A., Perkins, C., and D. Wing, "Guidelines for 2313 Choosing RTP Control Protocol (RTCP) Canonical Names 2314 (CNAMEs)", draft-ietf-avt-rtp-cnames-01 (work in 2315 progress), August 2010. 2317 [I-D.ietf-avt-multicast-acq-rtcp-xr] 2318 Begen, A. and E. Friedrich, "Multicast Acquisition Report 2319 Block Type for RTP Control Protocol (RTCP) Extended 2320 Reports (XRs)", draft-ietf-avt-multicast-acq-rtcp-xr-01 2321 (work in progress), May 2010. 2323 [I-D.ietf-avt-ecn-for-rtp] 2324 Westerlund, M., Johansson, I., Perkins, C., and K. 2325 Carlberg, "Explicit Congestion Notification (ECN) for RTP 2326 over UDP", draft-ietf-avt-ecn-for-rtp-02 (work in 2327 progress), July 2010. 2329 [I-D.ietf-fecframe-interleaved-fec-scheme] 2330 Begen, A., "RTP Payload Format for 1-D Interleaved Parity 2331 FEC", draft-ietf-fecframe-interleaved-fec-scheme-09 (work 2332 in progress), January 2010. 2334 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 2335 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 2336 RFC 4787, January 2007. 2338 [RFC5762] Perkins, C., "RTP and the Datagram Congestion Control 2339 Protocol (DCCP)", RFC 5762, April 2010. 2341 [I-D.ietf-avt-srtp-ekt] 2342 McGrew, D., Andreasen, F., Wing, D., and K. Fischer, 2343 "Encrypted Key Transport for Secure RTP", 2344 draft-ietf-avt-srtp-ekt-01 (work in progress), July 2010. 2346 [UPnP-IGD] 2347 Forum, UPnP., "Universal Plug and Play (UPnP) Internet 2348 Gateway Device (IGD)", November 2001. 2350 [IC2009] Begen, A., Glazebrook, N., and W. VerSteeg, "Reducing 2351 Channel Change Times in IPTV with Real-Time Transport 2352 Protocol (IEEE Internet Computing)", May 2009. 2354 Authors' Addresses 2356 Bill VerSteeg 2357 Cisco 2358 5030 Sugarloaf Parkway 2359 Lawrenceville, GA 30044 2360 USA 2362 Email: billvs@cisco.com 2363 Ali Begen 2364 Cisco 2365 181 Bay Street 2366 Toronto, ON M5J 2T3 2367 Canada 2369 Email: abegen@cisco.com 2371 Tom VanCaenegem 2372 Alcatel-Lucent 2373 Copernicuslaan 50 2374 Antwerpen, 2018 2375 Belgium 2377 Email: Tom.Van_Caenegem@alcatel-lucent.be 2379 Zeev Vax 2380 Microsoft Corporation 2381 1065 La Avenida 2382 Mountain View, CA 94043 2383 USA 2385 Email: zeevvax@microsoft.com