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Begen 3 Internet-Draft Cisco 4 Intended status: Informational March 9, 2012 5 Expires: September 10, 2012 7 Considerations for Deploying the Rapid Acquisition of Multicast RTP 8 Sessions (RAMS) Method 9 draft-ietf-avtext-rams-scenarios-03 11 Abstract 13 The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a 14 method based on RTP and RTP Control Protocol (RTCP) that enables an 15 RTP receiver to rapidly acquire and start consuming the RTP multicast 16 data. Upon a request from the RTP receiver, an auxiliary unicast RTP 17 retransmission session is set up between a retransmission server and 18 the RTP receiver, over which the reference information about the new 19 multicast stream the RTP receiver is about to join is transmitted at 20 an accelerated rate. This often precedes, but may also accompany, 21 the multicast stream itself. When there is only one multicast stream 22 to be acquired, the RAMS solution works in a straightforward manner. 23 However, when there are two or more multicast streams to be acquired 24 from the same or different multicast RTP sessions, care should be 25 taken to configure each RAMS session appropriately. This document 26 provides example scenarios and discusses how the RAMS solution could 27 be used in such scenarios. 29 Status of this Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on September 10, 2012. 46 Copyright Notice 48 Copyright (c) 2012 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 3. Example Scenarios . . . . . . . . . . . . . . . . . . . . . . 4 66 3.1. Scenario #1: Two Multicast Groups . . . . . . . . . . . . 4 67 3.2. Scenario #2: One Multicast Group . . . . . . . . . . . . . 5 68 3.3. Scenario #3: SSRC Multiplexing . . . . . . . . . . . . . . 6 69 3.4. Scenario #4: Payload-Type Multiplexing . . . . . . . . . . 7 70 4. Feedback Target and SSRC Signaling Issues . . . . . . . . . . 7 71 5. FEC during RAMS and Bandwidth Issues . . . . . . . . . . . . . 7 72 5.1. Scenario #1 . . . . . . . . . . . . . . . . . . . . . . . 8 73 5.2. Scenario #2 . . . . . . . . . . . . . . . . . . . . . . . 9 74 5.3. Scenario #3 . . . . . . . . . . . . . . . . . . . . . . . 9 75 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 76 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 77 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 78 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 79 9.1. Normative References . . . . . . . . . . . . . . . . . . . 10 80 9.2. Informative References . . . . . . . . . . . . . . . . . . 11 81 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 1. Introduction 85 The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a 86 method based on RTP and RTP Control Protocol (RTCP) that enables an 87 RTP receiver to rapidly acquire and start consuming the RTP multicast 88 data. Through an auxiliary unicast RTP retransmission session 89 [RFC4588], the RTP receiver receives a reference information about 90 the new multicast stream it is about to join. This often precedes, 91 but may also accompany, the multicast stream itself. The RAMS 92 solution is documented in detail in [RFC6285]. 94 The RAMS specification [RFC6285] has provisions for concurrently 95 acquiring multiple streams inside a multicast RTP session. However, 96 the RAMS specification does not discuss scenarios where an RTP 97 receiver makes use of the RAMS method to rapidly acquire multiple and 98 associated multicast streams in parallel, or where different RTP 99 sessions are part of the same source-specific multicast (SSM) 100 session. The example presented in Section 8.3 of [RFC6285] addresses 101 only the simple case of an RTP receiver rapidly acquiring only one 102 multicast stream to explain the protocol details. 104 There are certain deployment models where a multicast RTP session 105 might have two or more multicast streams associated with it. There 106 are also cases, where an RTP receiver might be interested in 107 acquiring one or more multicast streams from several multicast RTP 108 sessions. In scenarios where multiple RAMS sessions, each initiated 109 with an individual RAMS Request message to a different feedback 110 target, will be simultaneously run by an RTP receiver, they need to 111 be coordinated. In this document, we present scenarios from real- 112 life deployments and discuss how the RAMS solution could be used in 113 such scenarios. 115 2. Background 117 In the following discussion, we assume that there are two RTP streams 118 (1 and 2) that are somehow associated with each other. These could 119 be audio and video elementary streams for the same TV channel, or 120 they could be an MPEG2-TS stream (that has audio and video 121 multiplexed together) and its Forward Error Correction (FEC) stream. 123 An SSM session is defined by its (distribution) source address and 124 (destination) multicast group and there can be only one feedback 125 target per SSM session [RFC5760]. So, if the RTP streams are 126 distributed by different sources or over different multicast groups, 127 they are considered different SSM sessions. While different SSM 128 sessions can normally share the same feedback target address and/or 129 port, RAMS requires each unique feedback target (i.e., the 130 combination of address and port) to be associated with at most one 131 RTP session (See Section 6.2 of [RFC6285]). 133 Two or more multicast RTP streams can be transmitted in the same RTP 134 session (e.g., in a single UDP flow). This is called Synchronization 135 Source (SSRC) multiplexing. In this case, (de)multiplexing is done 136 at the SSRC level. Alternatively, the multicast RTP streams can be 137 transmitted in different RTP sessions (e.g., in different UDP flows), 138 which is called session multiplexing. In practice, there are 139 different deployment models for each multiplexing scheme. 141 Generally, two different media streams with different clock rates are 142 suggested to use different SSRCs or to be carried in different RTP 143 sessions to avoid complications in RTCP reports. Some of the fields 144 in RAMS messages might depend on the clock rate. Thus, in a single 145 RTP session, RTP streams carrying payloads with different clock rates 146 need to have different SSRCs. Since RTP streams with different SSRCs 147 do not share the sequence numbering, each stream needs to be acquired 148 individually. 150 In the remaining sections, only the relevant portions of the SDP 151 descriptions [RFC4566] will be provided. For an example of a full 152 SDP description, refer to Section 8.3 of [RFC6285]. 154 3. Example Scenarios 156 3.1. Scenario #1: Two Multicast Groups 158 This is the scenario for session multiplexing where RTP streams 1 and 159 2 are transmitted over different multicast groups. A practical use 160 case is where the first and second SSM sessions carry the primary 161 video stream and its associated FEC stream, respectively. 163 We run an individual RAMS session for each of these RTP streams that 164 we want to rapidly acquire. Each requires a separate RAMS Request 165 message to be sent. These RAMS sessions can be run in parallel. If 166 they are, the RTP receiver needs to pay attention to using the shared 167 bandwidth appropriately among the two unicast bursts. As explained 168 earlier, there has to be a different feedback target for these two 169 SSM sessions. 171 a=group:FEC-FR Channel1_Video Channel1_FEC 172 m=video 40000 RTP/AVPF 96 173 c=IN IP4 233.252.0.1/127 174 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 175 a=rtcp:41000 IN IP4 192.0.2.1 176 a=ssrc:1 cname:ch1_video@example.com 177 a=mid:Channel1_Video 178 m=application 40000 RTP/AVPF 97 179 c=IN IP4 233.252.0.2/127 180 a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 181 a=rtcp:42000 IN IP4 192.0.2.1 182 a=ssrc:2 cname:ch1_fec@example.com 183 a=mid:Channel1_FEC 185 Note that the multicast destination ports in the above SDP do not 186 matter, and they could be the same or different. The "FEC-FR" 187 grouping semantics are defined in [RFC5956]. 189 3.2. Scenario #2: One Multicast Group 191 Here RTP streams 1 and 2 are transmitted over the same multicast 192 group with different destination ports. A practical use case is 193 where the SSM session carries the primary video and audio streams, 194 each destined to a different port. 196 The RAMS Request message sent by an RTP receiver to the feedback 197 target could indicate the desire to acquire all or a subset or one of 198 the available RTP streams. Thus, both the primary video and audio 199 streams can be acquired rapidly in parallel. Or, the RTP receiver 200 can acquire only the primary video or audio stream, if desired, by 201 indicating the specific SSRC in the request. Compared to the 202 previous scenario, the only difference is that in this case the join 203 times for both streams need to be coordinated as they are delivered 204 in the same multicast session. 206 m=video 40000 RTP/AVPF 96 207 c=IN IP4 233.252.0.1/127 208 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 209 a=rtcp:41000 IN IP4 192.0.2.1 210 a=ssrc:1 cname:ch1_video@example.com 211 a=mid:Channel1_Video 212 m=audio 40001 RTP/AVPF 97 213 c=IN IP4 233.252.0.1/127 214 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 215 a=rtcp:41000 IN IP4 192.0.2.1 216 a=ssrc:2 cname:ch1_audio@example.com 217 a=mid:Channel1_Audio 219 Note that the destination ports in "m" lines need to be distinct per 220 [RFC5888]. 222 If RTP streams 1 and 2 share the same distribution source, then there 223 is only one SSM session, which means that there can be only one 224 feedback target (as shown in the SDP description above). This 225 requires RTP streams 1 and 2 to have their own unique SSRC values 226 (also as shown in the SDP description above). If RTP streams 1 and 2 227 do not share the same distribution source, meaning that their 228 respective SSM sessions can use different feedback target transport 229 addresses, then their SSRC values do not have to be different from 230 each other. 232 3.3. Scenario #3: SSRC Multiplexing 234 This is the scenario for SSRC multiplexing where both RTP streams are 235 transmitted over the same multicast group to the same destination 236 port. This is a less practical scenario but it could be used where 237 the SSM session carries both the primary video and audio stream, 238 destined to the same port. 240 Similar to scenario #2, both the primary video and audio streams can 241 be acquired rapidly in parallel. Or, the RTP receiver can acquire 242 only the primary video or audio stream, if desired, by indicating the 243 specific SSRC in the request. In this case, there is only one 244 distribution source and the destination multicast address is shared. 245 Thus, there is always one SSM session and one feedback target. 247 m=video 40000 RTP/AVPF 96 97 248 c=IN IP4 233.252.0.1/127 249 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 250 a=rtcp:41000 IN IP4 192.0.2.1 251 a=ssrc:1 cname:ch1_video@example.com 252 a=ssrc:2 cname:ch1_audio@example.com 253 a=mid:Channel1 255 3.4. Scenario #4: Payload-Type Multiplexing 257 This is the scenario for payload-type multiplexing. 259 In this case, instead of two, we have only one RTP stream (and one 260 RTP session) carrying both payload types (e.g., media payload and its 261 FEC data). While this scheme is permissible per [RFC3550], it has 262 several drawbacks. For example, RTP packets carrying different 263 payload formats will share the same sequence numbering space, and the 264 RAMS operations will not be able to be applied based on the payload 265 type. For other drawbacks and details, see Section 5.2 of [RFC3550]. 267 4. Feedback Target and SSRC Signaling Issues 269 The RAMS protocol uses the common packet format from [RFC4585], which 270 has a field to signal the media sender SSRC. The SSRCs for the RTP 271 streams can be signaled out-of-band in the SDP, or could be learned 272 from the RTP packets once the transmission starts. In RAMS, the 273 latter cannot be used. 275 Signaling the media sender SSRC value helps the feedback target 276 correctly identify the RTP stream to be acquired. If a feedback 277 target is serving multiple SSM sessions on a particular port, all the 278 RTP streams in these SSM sessions are supposed to have a unique SSRC 279 value. However, this is not an easy requirement to satisfy. Thus, 280 RAMS specification forbids to have more than one RTP session to be 281 associated with a specific feedback target on a specific port. 283 5. FEC during RAMS and Bandwidth Issues 285 Suppose that RTP stream 1 denotes the primary video stream that has a 286 bitrate of 10 Mbps and RTP stream 2 denotes the associated FEC stream 287 that has a bitrate of 1 Mbps. Also assume that the RTP receiver 288 knows that it can receive data at a maximum bitrate of 22 Mbps. SDP 289 can specify the bitrate ("b=" line in Kbps) of each media session 290 (per "m" line). 292 5.1. Scenario #1 294 This is the scenario for session multiplexing where RTP streams 1 and 295 2 are transmitted over different multicast groups. 297 This is the preferred deployment model for FEC [RFC6363]. Having FEC 298 in a different multicast group provides two flexibility points: RTP 299 receivers that are not FEC capable can receive the primary video 300 stream without FEC, and RTP receivers that are FEC capable can decide 301 to not receive FEC during the rapid acquisition (but still start 302 receiving the FEC stream after the acquisition of the primary video 303 stream has been completed). 305 a=group:FEC-FR Channel1_Video Channel1_FEC 306 m=video 40000 RTP/AVPF 96 307 c=IN IP4 233.252.0.1/127 308 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 309 a=rtcp:41000 IN IP4 192.0.2.1 310 a=rtpmap:96 MP2T/90000 311 b=TIAS:10000 312 a=ssrc:1 cname:ch1_video@example.com 313 a=mid:Channel1_Video 314 m=application 40000 RTP/AVPF 97 315 c=IN IP4 233.252.0.2/127 316 a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 317 a=rtcp:42000 IN IP4 192.0.2.1 318 a=rtpmap:97 1d-interleaved-parityfec/90000 319 b=TIAS:1000 320 a=ssrc:2 cname:ch1_fec@example.com 321 a=mid:Channel1_FEC 323 If the RTP receiver does not want to receive FEC until the 324 acquisition of the primary video stream is completed, the total 325 available bandwidth can be used for faster acquisition of the primary 326 video stream. In this case, the RTP receiver can request a Max 327 Receive Bitrate of 22 Mbps in the RAMS Request message for the 328 primary video stream. Once RAMS has been completed, the RTP receiver 329 can join the FEC multicast session, if desired. 331 If the RTP receiver wants to rapidly acquire both primary and FEC 332 streams, it needs to allocate the total bandwidth among the two RAMS 333 sessions and indicate individual Max Receive Bitrate values in each 334 respective RAMS Request message. Since less bandwidth will be used 335 to acquire the primary video stream, the acquisition of the primary 336 video session will take a longer time on the average. 338 While the RTP receiver can update the Max Receive Bitrate values 339 during the course of the RAMS session, this approach is more error- 340 prone due to the possibility of losing the update messages. 342 5.2. Scenario #2 344 Here RTP streams 1 (primary video) and 2 (FEC) are transmitted over 345 the same multicast group with different destination ports. This is 346 not a preferred deployment model. 348 a=group:FEC-FR Channel1_Video Channel1_FEC 349 m=video 40000 RTP/AVPF 96 350 c=IN IP4 233.252.0.1/127 351 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 352 a=rtcp:41000 IN IP4 192.0.2.1 353 a=rtpmap:96 MP2T/90000 354 b=TIAS:10000 355 a=ssrc:1 cname:ch1_video@example.com 356 a=mid:Channel1_Video 357 m=application 40001 RTP/AVPF 97 358 c=IN IP4 233.252.0.1/127 359 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 360 a=rtcp:41000 IN IP4 192.0.2.1 361 a=rtpmap:97 1d-interleaved-parityfec/90000 362 b=TIAS:1000 363 a=ssrc:2 cname:ch1_fec@example.com 364 a=mid:Channel1_FEC 366 The RAMS Request message sent by an RTP receiver to the feedback 367 target could indicate the desire to acquire all or a subset or one of 368 the available RTP streams. Thus, both the primary video and FEC 369 streams can be acquired rapidly in parallel sharing the same 370 available bandwidth. Or, the RTP receiver can acquire only the 371 primary video stream by indicating its specific SSRC in the request. 372 In this case, the RTP receiver can first acquire the primary video 373 stream at the full receive bitrate. But, upon the multicast join, 374 the available bandwidth for the burst drops to 11 Mbps instead of 12 375 Mbps. Regardless of whether FEC is desired or not by the RTP 376 receiver, its bitrate needs to be taken into account once the RTP 377 receiver joins the SSM session. 379 5.3. Scenario #3 381 This is the scenario for SSRC multiplexing where both RTP streams are 382 transmitted over the same multicast group to the same destination 383 port. 385 m=video 40000 RTP/AVPF 96 97 386 c=IN IP4 233.252.0.1/127 387 a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 388 a=rtcp:41000 IN IP4 192.0.2.1 389 a=rtpmap:96 MP2T/90000 390 a=rtpmap:97 1d-interleaved-parityfec/90000 391 a=fmtp:97 L=10; D=10; repair-window=200000 392 a=ssrc:1 cname:ch1_video@example.com 393 a=ssrc:2 cname:ch1_fec@example.com 394 b=TIAS:11000 395 a=mid:Channel1 397 Similar to scenario #2, both the primary video and audio streams can 398 be acquired rapidly in parallel. Or, the RTP receiver can acquire 399 only the primary video stream, if desired, by indicating its specific 400 SSRC in the request. 402 Note that based on the "a=fmtp" line for the FEC stream, it could be 403 possible to infer the bitrate of this FEC stream and set the Max 404 Receive Bitrate value accordingly. 406 6. Security Considerations 408 There are no security considerations in this document. 410 7. IANA Considerations 412 There are no IANA considerations in this document. 414 8. Acknowledgments 416 I would like to thank various individuals in the AVTEXT and MMUSIC 417 WGs, and my friends at Cisco for their comments and feedback. 419 9. References 421 9.1. Normative References 423 [RFC6285] Ver Steeg, B., Begen, A., Van Caenegem, T., and Z. Vax, 424 "Unicast-Based Rapid Acquisition of Multicast RTP 425 Sessions", RFC 6285, June 2011. 427 9.2. Informative References 429 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 430 Jacobson, "RTP: A Transport Protocol for Real-Time 431 Applications", STD 64, RFC 3550, July 2003. 433 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 434 Description Protocol", RFC 4566, July 2006. 436 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 437 "Extended RTP Profile for Real-time Transport Control 438 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 439 July 2006. 441 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 442 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 443 July 2006. 445 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 446 Protocol (RTCP) Extensions for Single-Source Multicast 447 Sessions with Unicast Feedback", RFC 5760, February 2010. 449 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 450 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 452 [RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in 453 the Session Description Protocol", RFC 5956, 454 September 2010. 456 [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error 457 Correction (FEC) Framework", RFC 6363, October 2011. 459 Author's Address 461 Ali Begen 462 Cisco 463 181 Bay Street 464 Toronto, ON M5J 2T3 465 Canada 467 Email: abegen@cisco.com