idnits 2.17.1 draft-ietf-avtcore-rtp-multi-stream-optimisation-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (February 17, 2015) is 3349 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'TBA' is mentioned on line 300, but not defined == Missing Reference: 'RFCXXXX' is mentioned on line 660, but not defined == Outdated reference: A later version (-11) exists of draft-ietf-avtcore-rtp-multi-stream-06 ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 2326 (Obsoleted by RFC 7826) Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVTCORE WG J. Lennox 3 Internet-Draft Vidyo 4 Intended status: Standards Track M. Westerlund 5 Expires: August 21, 2015 Ericsson 6 Q. Wu 7 Huawei 8 C. Perkins 9 University of Glasgow 10 February 17, 2015 12 Sending Multiple Media Streams in a Single RTP Session: Grouping RTCP 13 Reception Statistics and Other Feedback 14 draft-ietf-avtcore-rtp-multi-stream-optimisation-05 16 Abstract 18 RTP allows multiple media streams to be sent in a single session, but 19 requires each Synchronisation Source (SSRC) to send RTCP reception 20 quality reports for every other SSRC visible in the session. This 21 causes the number of RTCP reception reports to grow with the number 22 of SSRCs, rather than the number of endpoints. In many cases most of 23 these RTCP reception reports are unnecessary, since all SSRCs of an 24 endpoint are co-located and see the same reception quality. This 25 memo defines a Reporting Group extension to RTCP to reduce the 26 reporting overhead in such scenarios. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 21, 2015. 45 Copyright Notice 47 Copyright (c) 2015 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 63 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 3. RTCP Reporting Groups . . . . . . . . . . . . . . . . . . . . 3 65 3.1. Semantics and Behaviour of RTCP Reporting Groups . . . . 4 66 3.2. Identifying Members of an RTCP Reporting Group . . . . . 5 67 3.2.1. Definition and Use of the RTCP RGRP SDES Item . . . . 5 68 3.2.2. Definition and Use of the RTCP RGRS Packet . . . . . 6 69 3.3. Interactions with the RTP/AVPF Feedback Profile . . . . . 8 70 3.4. Interactions with RTCP Extended Report (XR) Packets . . . 9 71 3.5. Middlebox Considerations . . . . . . . . . . . . . . . . 9 72 3.6. SDP Signalling for Reporting Groups . . . . . . . . . . . 10 73 4. Properties of RTCP Reporting Groups . . . . . . . . . . . . . 11 74 4.1. Bandwidth Benefits of RTCP Reporting Groups . . . . . . . 11 75 4.2. Compatibility of RTCP Reporting Groups . . . . . . . . . 11 76 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 77 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 78 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 79 7.1. Normative References . . . . . . . . . . . . . . . . . . 15 80 7.2. Informative References . . . . . . . . . . . . . . . . . 15 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 83 1. Introduction 85 The Real-time Transport Protocol (RTP) [RFC3550] is a protocol for 86 group communication, supporting multiparty multimedia sessions. A 87 single RTP session can support multiple participants sending at once, 88 and can also support participants sending multiple simultaneous media 89 streams. Examples of the latter might include a participant with 90 multiple cameras who chooses to send multiple views of a scene, or a 91 participant that sends audio and video flows multiplexed in a single 92 RTP session. Rules for handling RTP sessions containing multiple 93 media streams are described in [RFC3550] with some clarifications in 94 [I-D.ietf-avtcore-rtp-multi-stream]. 96 An RTP endpoint will have one or more synchronisation sources (SSRCs) 97 that send media streams. It will have at least one SSRC for each 98 media stream it sends, and might use multiple SSRCs when using media 99 scalability features [RFC6190], forward error correction, RTP 100 retransmission [RFC4588], or similar mechanisms. An endpoint that is 101 not sending any media streams, will have at least one SSRC to use for 102 reporting and any feedback messages. Each SSRC has to send RTCP 103 sender reports corresponding to the RTP packets it sends, and 104 receiver reports for traffic it receives. That is, every SSRC will 105 send RTCP packets to report on every other SSRC. This rule is 106 simple, but can be quite inefficient for endpoints that send large 107 numbers of media streams in a single RTP session. Consider a session 108 comprising ten participants, each sending three media streams with 109 their own SSRC. There will be 30 SSRCs in such an RTP session, and 110 30 RTCP reception reports will be sent per reporting interval as each 111 SSRC reports on all the others. However, the three SSRCs comprising 112 each participant will almost certainly see identical reception 113 quality, since they are co-located. If there was a way to indicate 114 that several SSRCs are co-located, and see the same reception 115 quality, then two-thirds of those RTCP reports could be suppressed. 116 This would allow the remaining RTCP reports to be sent more often, 117 while keeping within the same RTCP bandwidth fraction. 119 This memo defines such an RTCP extension, RTCP Reporting Groups. 120 This extension is used to indicate the SSRCs that originate from the 121 same endpoint, and therefore have identical reception quality, hence 122 allowing the endpoints to suppress unnecessary RTCP reception quality 123 reports. 125 2. Terminology 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 129 document are to be interpreted as described in [RFC2119]. 131 3. RTCP Reporting Groups 133 An RTCP Reporting Group is a set of synchronization sources (SSRCs) 134 that are co-located at a single endpoint (which could be an end host 135 or a middlebox) in an RTP session. Since they are co-located, every 136 SSRC in the RTCP reporting group will have an identical view of the 137 network conditions, and see the same lost packets, jitter, etc. This 138 allows a single representative to send RTCP reception quality reports 139 on behalf of the rest of the reporting group, reducing the number of 140 RTCP packets that need to be sent without loss of information. 142 3.1. Semantics and Behaviour of RTCP Reporting Groups 144 A group of co-located SSRCs that see identical network conditions can 145 form an RTCP reporting group. If reporting groups are in use, an RTP 146 endpoint with multiple SSRCs MAY put those SSRCs into a reporting 147 group if their view of the network is identical; i.e., if they report 148 on traffic received at the same interface of an RTP endpoint. SSRCs 149 with different views of the network MUST NOT be put into the same 150 reporting group. 152 An endpoint that has combined its SSRCs into an RTCP reporting group 153 will choose one (or a subset) of those SSRCs as a "reporting source" 154 for that RTCP reporting group. A reporting source will send RTCP SR/ 155 RR reception quality reports on behalf of the other members of the 156 RTCP reporting group. A reporting source MUST suppress the RTCP SR/ 157 RR reports that relate to other members of the reporting group, and 158 only report on remote SSRCs. The other members (non reporting 159 sources) of the RTCP reporting group will suppress their RTCP 160 reception quality reports, and instead send an RTCP RGRS packet (see 161 Section 3.2.2) to indicate that they are part of an RTCP reporting 162 group and give the SSRCs of the reporting sources. 164 If there are large numbers of remote SSRCs in the RTP session, then 165 the reception quality reports generated by the reporting source might 166 grow too large to fit into a single compound RTCP packet, forcing the 167 reporting source to use a round-robin policy to determine what remote 168 SSRCs it includes in each compound RTCP packet, and so reducing the 169 frequency of reports on each SSRC. To avoid this, in sessions with 170 large numbers of remote SSRCs, an RTCP reporting group MAY use more 171 than one reporting source. If several SSRCs are acting as reporting 172 sources for an RTCP reporting group, then each reporting source MUST 173 have non-overlapping sets of remote SSRCs it reports on. 175 An endpoint SHOULD NOT create an RTCP reporting group that comprises 176 only a single local SSRC (i.e., an RTCP reporting group where the 177 reporting source is the only member of the group), unless it is 178 anticipated that the group might have additional SSRCs added to it in 179 the future. 181 If a reporting source leaves the RTP session (i.e., if it sends a 182 RTCP BYE packet, or leaves the session without sending BYE under the 183 rules of [RFC3550] section 6.3.7), the remaining members of the RTCP 184 reporting group MUST either (a) have another reporting source, if 185 existing, report on the remote SSRCs the leaving SSRC reported on, 186 (b) choose a new reporting source, or (c) disband the RTCP reporting 187 group and begin sending reception quality reports following [RFC3550] 188 and [I-D.ietf-avtcore-rtp-multi-stream]. 190 The RTCP timing rules assign different bandwidth fractions to senders 191 and receivers. This lets senders transmit RTCP reception quality 192 reports more often than receivers. If a reporting source in an RTCP 193 reporting group is a receiver, but one or more non-reporting SSRCs in 194 the RTCP reporting group are senders, then the endpoint MAY treat the 195 reporting source as a sender for the purpose of RTCP bandwidth 196 allocation, increasing its RTCP bandwidth allocation, provided it 197 also treats one of the senders as if it were a receiver and makes the 198 corresponding reduction in RTCP bandwidth for that SSRC. However, 199 the application needs to consider the impact on the frequency of 200 transmitting of the synchronization information included in RTCP 201 Sender Reports. 203 3.2. Identifying Members of an RTCP Reporting Group 205 When RTCP Reporting Groups are in use, the other SSRCs in the RTP 206 session need to be able to identify which SSRCs are members of an 207 RTCP reporting group. Two RTCP extensions are defined to support 208 this: the RTCP RGRP SDES item is used by the reporting source(s) to 209 identify an RTCP reporting group, and the RTCP RGRS packet is used by 210 other members of an RTCP reporting group to identify the reporting 211 source(s). 213 3.2.1. Definition and Use of the RTCP RGRP SDES Item 215 A new RTCP SDES item is defined to identify an RTCP reporting group. 216 The motivation for giving a reporting group an identify is to ensure 217 that the RTCP reporting group and its member SSRCs can be correctly 218 associated when there are multiple reporting sources, and to ensure 219 that a reporting SSRC can be associated with the correct reporting 220 group if an SSRC collision occurs. 222 The RTCP Source Description (SDES) RGRP item is defined. The RTCP 223 SDES RGRP item MUST be sent by the reporting sources in a reporting 224 group, and MUST NOT be sent by other members of the reporting group 225 or by SSRCs that are not members of any RTCP reporting group. 226 Specifically, every reporting source in an RTCP reporting group MUST 227 include an RTCP SDES packet containing an RGRP item in every compound 228 RTCP packet in which it sends an RR or SR packet (i.e., in every RTCP 229 packet it sends, unless Reduced-Size RTCP [RFC5506] is in use). 231 Syntactically, the format of the RTCP SDES RGRP item is identical to 232 that of the RTCP SDES CNAME item [RFC7022], except that the SDES item 233 type field MUST have value RGRP=(TBA) instead of CNAME=1. The value 234 of the RTCP SDES RGRP item MUST be chosen with the same concerns 235 about global uniqueness and the same privacy considerations as the 236 RTCP SDES CNAME them. The value of the RTCP SDES RGRP item MUST be 237 stable throughout the lifetime of the reporting group, even if the 238 some or all of the reporting sources change their SSRC due to 239 collisions, or if the set of reporting sources changes. 241 Note to RFC Editor: please replace (TBA) in the above paragraph 242 with the RTCP SDES item type number assigned to the RGRP item, 243 then delete this note. 245 An RTP mixer or translator that forwards RTCP SR or RR packets from 246 members of a reporting group MUST forward the corresponding RTCP SDES 247 RGRP items as well, even if it otherwise strips SDES items other than 248 the CNAME item. 250 3.2.2. Definition and Use of the RTCP RGRS Packet 252 A new RTCP packet type is defined to allow the members of an RTCP 253 reporting group to identify the reporting sources for that group. 254 This allows participants in an RTP session to distinguish an SSRC 255 that is sending empty RTCP reception reports because it is a member 256 of an RTCP reporting group, from an SSRC that is sending empty RTCP 257 reception reports because it is not receiving any traffic. It also 258 explicitly identifies the reporting sources, allowing other members 259 of the RTP session to know which SSRCs are acting as the reporting 260 sources for an RTCP reporting group, and allowing them to detect if 261 RTCP packets from any of the reporting sources are being lost. 263 The format of the RTCP RGRS packet is defined below. It comprises 264 the fixed RTCP header that indicates the packet type and length, the 265 SSRC of the packet sender, and a list of reporting sources for the 266 RTCP reporting group of which the packet sender is a member. 268 0 1 2 3 269 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 1 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 |V=2|P| SC | PT=RGRS(TBA) | length | 272 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 273 | SSRC of packet sender | 274 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 275 : List of SSRCs for the Reporting Source(s) : 276 : ... : 277 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 The fields in the RTCP RGRS packet have the following definition: 281 version (V): This field identifies the RTP version. The current RTP 282 version is 2. 284 padding (P): If set, the padding bit indicates that the RTCP packet 285 contains additional padding octets at the end that are not part of 286 the control information but are included in the length field. See 287 [RFC3550]. 289 Source Count (SC): Indicates the number of reporting source SSRCs 290 that are included in this RTCP packet. As the RTCP RGRS packet 291 MUST NOT be not sent by reporting sources, all the SSRCs in the 292 list of reporting sources will be different from the SSRC of the 293 packet sender. Every RTCP RGRS packet MUST contain at least one 294 reporting source SSRC. 296 Payload type (PT): The RTCP packet type number that identifies the 297 packet as being an RTCP RGRS packet. The RGRS RTCP packet has the 298 value [TBA]. 300 Note to RFC Editor: please replace [TBA] here, and in the 301 packet format diagram above, with the RTCP packet type that 302 IANA assigns to the RTCP RGRS packet. 304 Length: The length of this packet in 32-bit words minus one, 305 including the header and any padding. This is in line with the 306 definition of the length field used in RTCP sender and receiver 307 reports [RFC3550]. Since all RTCP RGRS packets include at least 308 one reporting source SSRC, the length will always be 2 or greater. 310 SSRC of packet sender: The SSRC of the sender of this packet. 312 List of SSRCs for the Reporting Source(s): A variable length size 313 (as indicated by SC header field) of the 32 bit SSRC values of the 314 reporting sources for the RTCP Reporting Group of which the packet 315 sender is a member. 317 Every source that belongs to an RTCP reporting group but is not a 318 reporting source MUST include an RTCP RGRS packet in every compound 319 RTCP packet in which it sends an RR or SR packet (i.e., in every RTCP 320 packet it sends, unless Reduced-Size RTCP [RFC5506] is in use). Each 321 RTCP RGRS packet MUST contain the SSRC identifier of at least one 322 reporting source. If there are more reporting sources in an RTCP 323 reporting group than can fit into an RTCP RGRS packet, the members of 324 that reporting group MUST send the SSRCs of the reporting sources in 325 a round-robin fashion in consecutive RTCP RGRS packets, such that all 326 the SSRCs of the reporting sources are included over the course of 327 several RTCP reporting intervals. 329 An RTP mixer or translator that forwards RTCP SR or RR packets from 330 members of a reporting group MUST also forward the corresponding RGRS 331 RTCP packets. If the RTP mixer or translator rewrites SSRC values of 332 the packets it forwards, it MUST make the corresponding changes to 333 the RTCP RGRS packets. 335 3.3. Interactions with the RTP/AVPF Feedback Profile 337 Use of the RTP/AVPF Feedback Profile [RFC4585] allows SSRCs to send 338 rapid RTCP feedback requests and codec control messages. If use of 339 the RTP/AVPF profile has been negotiated in an RTP session, members 340 of an RTCP reporting group can send rapid RTCP feedback and codec 341 control messages following [RFC4585] and [RFC5104], as updated by 342 Section 5.4 of [I-D.ietf-avtcore-rtp-multi-stream], and by the 343 following considerations. 345 The members of an RTCP reporting group will all see identical network 346 conditions. Accordingly, one might therefore think that it doesn't 347 matter which SSRC in the reporting group sends the RTP/AVPF feedback 348 or codec control messages. There might be, however, cases where the 349 sender of the feedback/codec control message has semantic importance, 350 or when only a subset of the members of an RTCP reporting group might 351 want to send RTP/AVPF feedback or a codec control message in response 352 to a particular event. For example, an RTP video sender might choose 353 to treat packet loss feedback received from SSRCs known to be audio 354 receivers with less urgency than feedback that it receives from video 355 receivers when deciding what packets to retransmit, and a multimedia 356 receiver using reporting groups might want to chose the outgoing SSRC 357 for feedback packets to reflect this. 359 Each member of an RTCP reporting group SHOULD therefore send RTP/AVPF 360 feedback/codec control messages independently of the other members of 361 the reporting group, to respect the semantic meaning of the message 362 sender. The suppression rules of [RFC4585] will ensure that only a 363 single copy of each feedback packet is (typically) generated, even if 364 several members of a reporting group send the same feedback. When an 365 endpoint knows that several members of its RTCP reporting group will 366 be sending identical feedback, and that the sender of the feedback is 367 not semantically important, then that endpoint MAY choose to send all 368 its feedback from the reporting source and deterministically suppress 369 feedback packets generated by the other sources in the reporting 370 group. 372 It is important to note that the RTP/AVPF timing rules operate on a 373 per-SSRC basis. Using a single reporting source to send all feedback 374 for a reporting group will hence limit the amount of feedback that 375 can be sent to that which can be sent by one SSRC. If this limit is 376 a problem, then the reporting group can allow each of its members to 377 send its own feedback, using its own SSRC. 379 If the RTP/AVPF feedback messages or codec control requests are sent 380 as compound RTCP packets, then those compound RTCP packets MUST 381 include either an RTCP RGRS packet or an RTCP SDES RGRP item, 382 depending on whether they are sent by the reporting source or a non- 383 reporting source in the RTCP reporting group respectively. The 384 contents of non-compound RTCP feedback or codec control messages are 385 not affected by the use of RTCP reporting groups. 387 3.4. Interactions with RTCP Extended Report (XR) Packets 389 When using RTCP Extended Reports (XR) [RFC3611] with RTCP reporting 390 groups, it is RECOMMENDED that the reporting source is used to send 391 the RTCP XR packets. If multiple reporting sources are in use, the 392 reporting source that sends the SR/RR packets that relate to a 393 particular remote SSRC SHOULD send the RTCP XR reports about that 394 SSRC. This is motivated as one commonly combine the RTCP XR metrics 395 with the regular report block to more fully understand the situation. 396 Receiving these blocks in different compound packets reduces their 397 value as the measuring intervals are not synchronized in those cases. 399 Some RTCP XR report blocks are specific to particular types of media, 400 and might be relevant to only some members of a reporting group. For 401 example, it would make no sense for an SSRC that is receiving video 402 to send a VoIP metric RTCP XR report block. Such media specific RTCP 403 XR report blocks MUST be sent by the SSRC to which they are relevant, 404 and MUST NOT be included in the common report sent by the reporting 405 source. This might mean that some SSRCs send RTCP XR packets in 406 compound RTCP packets that contain an empty RTCP SR/RR packet, and 407 that the time period covered by the RTCP XR packet is different to 408 that covered by the RTCP SR/RR packet. If it is important that the 409 RTCP XR packet and RTCP SR/RR packet cover the same time period, then 410 that source SHOULD be removed from the RTCP reporting group, and send 411 standard RTCP packets instead. 413 3.5. Middlebox Considerations 415 Many different types of middlebox are used with RTP. RTCP reporting 416 groups are potentially relevant to those types of RTP middlebox that 417 have their own SSRCs and generate RTCP reports for the traffic they 418 receive. RTP middleboxes that do not have their own SSRC, and that 419 don't send RTCP reports on the traffic they receive, cannot use the 420 RTCP reporting groups extension, since they generate no RTCP reports 421 to group. 423 An RTP middlebox that has several SSRCs of its own can use the RTCP 424 reporting groups extension to group the RTCP reports it generates. 425 This can occur, for example, if a middlebox is acting as an RTP mixer 426 for both audio and video flows that are multiplexed onto a single RTP 427 session, where the middlebox has one SSRC for the audio mixer and one 428 for the video mixer part, and when the middlebox wants to avoid cross 429 reporting between audio and video. 431 A middlebox cannot use the RTCP reporting groups extension to group 432 RTCP packets from the SSRCs that it is forwarding. It can, however, 433 group the RTCP packets from the SSRCs it is forwarding into compound 434 RTCP packets following the rules in Section 6.1 of [RFC3550] and 435 Section 5.3 of [I-D.ietf-avtcore-rtp-multi-stream]. If the middlebox 436 is using RTCP reporting groups for its own SSRCs, it MAY include RTCP 437 packets from the SSRCs that it is forwarding as part of the compound 438 RTCP packets its reporting source generates. 440 A middlebox that forwards RTCP SR or RR packets sent by members of a 441 reporting group MUST forward the corresponding RTCP SDES RGRP items, 442 as described in Section 3.2.1. A middlebox that forwards RTCP SR or 443 RR packets sent by member of a reporting group MUST also forward the 444 corresponding RTCP RGRS packets, as described in Section 3.2.2. 445 Failure to forward these packets can cause compatibility problems, as 446 described in Section 4.2. 448 If a middlebox rewrites SSRC values in the RTP and RTCP packets that 449 it is forwarding, then it MUST make the corresponding changes in RTCP 450 SDES packets containing RGRP items and in RTCP RGRS packets, to allow 451 them to be associated with the rewritten SSRCs. 453 3.6. SDP Signalling for Reporting Groups 455 This document defines the "a=rtcp-rgrp" Session Description Protocol 456 (SDP) [RFC4566] attribute to indicate if the session participant is 457 capable of supporting RTCP Reporting Groups for applications that use 458 SDP for configuration of RTP sessions. The attribute takes no value. 459 A participant that proposes the use of RTCP Reporting Groups SHALL 460 itself support the reception of RTCP Reporting Groups. 462 An offering client that wishes to use RTCP Reporting Groups MUST 463 include the attribute "a=rtcp-rgrp" in the SDP offer. If "a=rtcp- 464 rgrp" is present in the offer SDP, the answerer that supports RTCP 465 Reporting Groups and wishes to use it SHALL include the "a=rtcp-rgrp" 466 attribute in the answer. In cases the answer has been excluded, 467 neither agents SHALL use the RTCP Reporting Groups. 469 In declarative usage of SDP, such as the Real Time Streaming Protocol 470 (RTSP) [RFC2326] and the Session Announcement Protocol (SAP) 471 [RFC2974], the presence of the attribute indicates that the session 472 participant MAY use RTCP Reporting Groups in its RTCP transmissions. 473 An implementation that doesn't explicitly support RTCP Reporting 474 Groups MAY join a RTP session as long as it has been verified that 475 the implementation doesn't suffer from the problems discussed in 476 Section 4.2. 478 4. Properties of RTCP Reporting Groups 480 This section provides additional information on what the resulting 481 properties are with the design specified in Section 3. The content 482 of this section is non-normative. 484 4.1. Bandwidth Benefits of RTCP Reporting Groups 486 To understand the benefits of RTCP reporting groups, consider a 487 scenario in which the two endpoints in a session each have a hundred 488 sources, of which eight each are sending within any given reporting 489 interval. 491 For ease of analysis, we can make the simplifying approximation that 492 the duration of the RTCP reporting interval is equal to the total 493 size of the RTCP packets sent during an RTCP interval, divided by the 494 RTCP bandwidth. (This will be approximately true in scenarios where 495 the bandwidth is not so high that the minimum RTCP interval is 496 reached.) For further simplification, we can assume RTCP senders are 497 following the recommendations regarding Compound RTCP Packets in 498 [I-D.ietf-avtcore-rtp-multi-stream]; thus, the per-packet transport- 499 layer overhead will be small relative to the RTCP data. Thus, only 500 the actual RTCP data itself need be considered. 502 In a report interval in this scenario, there will, as a baseline, be 503 200 SDES packets, 184 RR packets, and 16 SR packets. This amounts to 504 approximately 6.5 kB of RTCP per report interval, assuming 16-byte 505 CNAMEs and no other SDES information. 507 Using the original [RFC3550] everyone-reports-on-every-sender 508 feedback rules, each of the 184 receivers will send 16 report blocks, 509 and each of the 16 senders will send 15. This amounts to 510 approximately 76 kB of report block traffic per interval; 92% of RTCP 511 traffic consists of report blocks. 513 If reporting groups are used, however, there is only 0.4 kB of 514 reports per interval, with no loss of useful information. 515 Additionally, there will be (assuming 16-byte RGRPs, and a single 516 reporting source per reporting group) an additional 2.4 kB per cycle 517 of RGRP SDES items and RGRS packets. Put another way, the unmodified 518 [RFC3550] reporting interval is approximately 8.9 times longer than 519 if reporting groups are in use. 521 4.2. Compatibility of RTCP Reporting Groups 523 The RTCP traffic generated by receivers using RTCP Reporting Groups 524 might appear, to observers unaware of these semantics, to be 525 generated by receivers who are experiencing a network disconnection, 526 as the non-reporting sources appear not to be receiving a given 527 sender at all. 529 This could be a potentially critical problem for such a sender using 530 RTCP for congestion control, as such a sender might think that it is 531 sending so much traffic that it is causing complete congestion 532 collapse. 534 However, such an interpretation of the session statistics would 535 require a fairly sophisticated RTCP analysis. Any receiver of RTCP 536 statistics which is just interested in information about itself needs 537 to be prepared that any given reception report might not contain 538 information about a specific media source, because reception reports 539 in large conferences can be round-robined. 541 Thus, it is unclear to what extent such backward compatibility issues 542 would actually cause trouble in practice. 544 5. Security Considerations 546 The security considerations of [RFC3550] and 547 [I-D.ietf-avtcore-rtp-multi-stream] apply. If the RTP/AVPF profile 548 is in use, then the security considerations of [RFC4585] (and 549 [RFC5104], if used) also apply. If RTCP XR is used, the security 550 consideration of [RFC3611] and any XR report blocks used also apply. 552 The RTCP SDES RGRP item is vulnerable to malicious modifications 553 unless integrity protected is used. A modification of this item's 554 length field cause the parsing of the RTCP packet in which it is 555 contained to fail. Depending on the implementation, parsing of the 556 full compound RTCP packet can also fail causing the whole packet to 557 be discarded. A modification to the value of this SDES item would 558 make the receiver of the report think that the sender of the report 559 was a member of a different RTCP reporting group. This will 560 potentially create an inconsistency, when the RGRS reports the source 561 as being in the same reporting group as another source with another 562 reporting group identifier. What impact on a receiver implementation 563 such inconsistencies would have are difficult to fully predict. One 564 case is when congestion control or other adaptation mechanisms are 565 used, an inconsistent report can result in a media sender to reduce 566 its bit-rate. However, a direct modification of the receiver report 567 or a feedback message itself would be a more efficient attack, and 568 equally costly to perform. 570 The new RGRS RTCP Packet type is very simple. The common RTCP packet 571 type header shares the security risks with previous RTCP packet 572 types. Errors or modification of the length field can cause the full 573 compound packet to fail header validation (see Appendix A.2 in 575 [RFC3550]) resulting in the whole compound RTCP packet being 576 discarded. Modification of the SC or P fields would cause 577 inconsistency when processing the RTCP packet, likely resulting it 578 being classified as invalid. A modification of the PT field would 579 cause the packet being interpreted under some other packet type's 580 rules. In such case the result might be more or less predictable but 581 packet type specific. Modification of the SSRC of packet sender 582 would attribute this packet to another sender. Resulting in a 583 receiver believing the reporting group applies also for this SSRC, if 584 it exists. If it doesn't exist, unless also corresponding 585 modifications are done on a SR/RR packet and a SDES packet the RTCP 586 packet SHOULD be discarded. If consistent changes are done, that 587 could be part of a resource exhaustion attack on a receiver 588 implementation. Modification of the "List of SSRCs for the Reporting 589 Source(s)" would change the SSRC the receiver expect to report on 590 behalf of this SSRC. If that SSRC exist, that could potentially 591 change the report group used for this SSRC. A change to another 592 reporting group belonging to another endpoint is likely detectable as 593 there would be a mismatch between the SSRC of the packet sender's 594 endpoint information, transport addresses, SDES CNAME etc and the 595 corresponding information from the reporting group indicated. 597 In general the reporting group is providing limited impacts attacks. 598 The most significant result from an deliberate attack would be to 599 cause the information to be discarded or be inconsistent, including 600 discard of all RTCP packets that are modified. This causes a lack of 601 information at any receiver entity, possibly disregarding the 602 endpoints participation in the session. 604 To protect against this type of attacks from external non trusted 605 entities, integrity and source authentication SHOULD be applied. 606 This can be done, for example, by using SRTP [RFC3711] with 607 appropriate key-management, other options exist as discussed in RTP 608 Security Options [RFC7201]. 610 The Report Group Identifier has a potential privacy impacting 611 properties. If this would be generated by an implementation in such 612 a way that is long term stable or predictable, it could be used for 613 tracking a particular end-point. Therefore it is RECOMMENDED that it 614 be generated as a short-term persistent RGRP, following the rules for 615 short-term persistent CNAMEs in [RFC7022]. The rest of the 616 information revealed, i.e. the SSRCs, the size of reporting group and 617 the number of reporting sources in a reporting group is of less 618 sensitive nature, considering that the SSRCs and the communication 619 would anyway be revealed without this extension. By encrypting the 620 report group extensions the SSRC values would preserved confidential, 621 but can still be revealed if SRTP [RFC3711] is used. The size of the 622 reporting groups and number of reporting sources are likely 623 determinable from analysis of the packet pattern and sizes. However, 624 this information appears to have limited value. 626 6. IANA Considerations 628 (Note to the RFC-Editor: in the following, please replace "TBA" with 629 the IANA-assigned value, and "XXXX" with the number of this document, 630 then delete this note) 632 The IANA is requested to register one new RTCP SDES items in the 633 "RTCP SDES Item Types" registry, as follows: 635 Value Abbrev Name Reference 636 TBA RGRP Reporting Group Identifier [RFCXXXX] 638 The definition of the RTCP SDES RGRP item is given in Section 3.2.1 639 of this memo. 641 The IANA is also requested to register one new RTCP packet type in 642 the "RTCP Control Packet Types (PT)" Registry as follows: 644 Value Abbrev Name Reference 645 TBA RGRS Reporting Group Reporting Sources [RFCXXXX] 647 The definition of the RTCP RGRS packet type is given in Section 3.2.2 648 of this memo. 650 The IANA is also requested to register one new SDP attribute: 652 SDP Attribute ("att-field"): 653 Attribute name: rtcp-rgrp 654 Long form: RTCP Reporting Groups 655 Type of name: att-field 656 Type of attribute: Media or session level 657 Subject to charset: No 658 Purpose: Negotiate or configure the use of the RTCP 659 Reporting Group Extension. 660 Reference: [RFCXXXX] 661 Values: None 663 The definition of the "a=rtcp-rgrp" SDES attribute is given in 664 Section 3.6 of this memo. 666 7. References 667 7.1. Normative References 669 [I-D.ietf-avtcore-rtp-multi-stream] 670 Lennox, J., Westerlund, M., Wu, W., and C. Perkins, 671 "Sending Multiple Media Streams in a Single RTP Session", 672 draft-ietf-avtcore-rtp-multi-stream-06 (work in progress), 673 October 2014. 675 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 676 Requirement Levels", BCP 14, RFC 2119, March 1997. 678 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 679 Jacobson, "RTP: A Transport Protocol for Real-Time 680 Applications", STD 64, RFC 3550, July 2003. 682 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 683 Description Protocol", RFC 4566, July 2006. 685 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 686 "Guidelines for Choosing RTP Control Protocol (RTCP) 687 Canonical Names (CNAMEs)", RFC 7022, September 2013. 689 7.2. Informative References 691 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 692 Streaming Protocol (RTSP)", RFC 2326, April 1998. 694 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 695 Announcement Protocol", RFC 2974, October 2000. 697 [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control 698 Protocol Extended Reports (RTCP XR)", RFC 3611, November 699 2003. 701 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 702 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 703 RFC 3711, March 2004. 705 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 706 "Extended RTP Profile for Real-time Transport Control 707 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 708 2006. 710 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 711 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 712 July 2006. 714 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 715 "Codec Control Messages in the RTP Audio-Visual Profile 716 with Feedback (AVPF)", RFC 5104, February 2008. 718 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 719 Real-Time Transport Control Protocol (RTCP): Opportunities 720 and Consequences", RFC 5506, April 2009. 722 [RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis, 723 "RTP Payload Format for Scalable Video Coding", RFC 6190, 724 May 2011. 726 [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP 727 Sessions", RFC 7201, April 2014. 729 Authors' Addresses 731 Jonathan Lennox 732 Vidyo, Inc. 733 433 Hackensack Avenue 734 Seventh Floor 735 Hackensack, NJ 07601 736 US 738 Email: jonathan@vidyo.com 740 Magnus Westerlund 741 Ericsson 742 Farogatan 6 743 SE-164 80 Kista 744 Sweden 746 Phone: +46 10 714 82 87 747 Email: magnus.westerlund@ericsson.com 749 Qin Wu 750 Huawei 751 101 Software Avenue, Yuhua District 752 Nanjing, Jiangsu 210012 753 China 755 Email: sunseawq@huawei.com 756 Colin Perkins 757 University of Glasgow 758 School of Computing Science 759 Glasgow G12 8QQ 760 United Kingdom 762 Email: csp@csperkins.org