idnits 2.17.1 draft-ietf-avtcore-rtp-multi-stream-optimisation-08.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 (October 2, 2015) is 3128 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is mentioned on line 301, but not defined == Missing Reference: 'RFCXXXX' is mentioned on line 672, but not defined == Outdated reference: A later version (-11) exists of draft-ietf-avtcore-rtp-multi-stream-09 ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) -- Obsolete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: April 4, 2016 Ericsson 6 Q. Wu 7 Huawei 8 C. Perkins 9 University of Glasgow 10 October 2, 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-08 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 April 4, 2016. 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 . . . . . . . . . 12 76 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 77 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 78 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 79 7.1. Normative References . . . . . . . . . . . . . . . . . . 15 80 7.2. Informative References . . . . . . . . . . . . . . . . . 15 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 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 each of those 30 SSRCs will send an RTCP Sender Report/Receiver 111 Report packet (containing several report blocks) per reporting 112 interval as each SSRC reports on all the others. However, the three 113 SSRCs comprising each participant will almost certainly see identical 114 reception quality, since they are co-located. If there was a way to 115 indicate that several SSRCs are co-located, and see the same 116 reception quality, then two-thirds of those RTCP reports could be 117 suppressed. This would allow the remaining RTCP reports to be sent 118 more often, while keeping within the same RTCP bandwidth fraction. 120 This memo defines such an RTCP extension, RTCP Reporting Groups. 121 This extension is used to indicate the SSRCs that originate from the 122 same endpoint, and therefore have identical reception quality, hence 123 allowing the endpoints to suppress unnecessary RTCP reception quality 124 reports. 126 2. Terminology 128 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 129 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 130 document are to be interpreted as described in [RFC2119]. 132 3. RTCP Reporting Groups 134 An RTCP Reporting Group is a set of synchronization sources (SSRCs) 135 that are co-located at a single endpoint (which could be an end host 136 or a middlebox) in an RTP session. Since they are co-located, every 137 SSRC in the RTCP reporting group will have an identical view of the 138 network conditions, and see the same lost packets, jitter, etc. This 139 allows a single representative to send RTCP reception quality reports 140 on behalf of the rest of the reporting group, reducing the number of 141 RTCP packets that need to be sent without loss of information. 143 3.1. Semantics and Behaviour of RTCP Reporting Groups 145 A group of co-located SSRCs that see identical network conditions can 146 form an RTCP reporting group. If reporting groups are in use, an RTP 147 endpoint with multiple SSRCs MAY put those SSRCs into a reporting 148 group if their view of the network is identical; i.e., if they report 149 on traffic received at the same interface of an RTP endpoint. SSRCs 150 with different views of the network MUST NOT be put into the same 151 reporting group. 153 An endpoint that has combined its SSRCs into an RTCP reporting group 154 will choose one (or a subset) of those SSRCs as a "reporting source" 155 for that RTCP reporting group. A reporting source will send RTCP SR/ 156 RR reception quality reports on behalf of the other members of the 157 RTCP reporting group. A reporting source MUST suppress the RTCP SR/ 158 RR reports that relate to other members of the reporting group, and 159 only report on remote SSRCs. The other members (non reporting 160 sources) of the RTCP reporting group will suppress their RTCP 161 reception quality reports, and instead send an RTCP RGRS packet (see 162 Section 3.2.2) to indicate that they are part of an RTCP reporting 163 group and give the SSRCs of the reporting sources. 165 If there are large numbers of remote SSRCs in the RTP session, then 166 the reception quality reports generated by the reporting source might 167 grow too large to fit into a single compound RTCP packet, forcing the 168 reporting source to use a round-robin policy to determine what remote 169 SSRCs it includes in each compound RTCP packet, and so reducing the 170 frequency of reports on each SSRC. To avoid this, in sessions with 171 large numbers of remote SSRCs, an RTCP reporting group MAY use more 172 than one reporting source. If several SSRCs are acting as reporting 173 sources for an RTCP reporting group, then each reporting source MUST 174 have non-overlapping sets of remote SSRCs it reports on. 176 An endpoint SHOULD NOT create an RTCP reporting group that comprises 177 only a single local SSRC (i.e., an RTCP reporting group where the 178 reporting source is the only member of the group), unless it is 179 anticipated that the group might have additional SSRCs added to it in 180 the future. 182 If a reporting source leaves the RTP session (i.e., if it sends a 183 RTCP BYE packet, or leaves the session without sending BYE under the 184 rules of [RFC3550] section 6.3.7), the remaining members of the RTCP 185 reporting group MUST either (a) have another reporting source, if 186 existing, report on the remote SSRCs the leaving SSRC reported on, 187 (b) choose a new reporting source, or (c) disband the RTCP reporting 188 group and begin sending reception quality reports following [RFC3550] 189 and [I-D.ietf-avtcore-rtp-multi-stream]. 191 The RTCP timing rules assign different bandwidth fractions to senders 192 and receivers. This lets senders transmit RTCP reception quality 193 reports more often than receivers. If a reporting source in an RTCP 194 reporting group is a receiver, but one or more non-reporting SSRCs in 195 the RTCP reporting group are senders, then the endpoint MAY treat the 196 reporting source as a sender for the purpose of RTCP bandwidth 197 allocation, increasing its RTCP bandwidth allocation, provided it 198 also treats one of the senders as if it were a receiver and makes the 199 corresponding reduction in RTCP bandwidth for that SSRC. However, 200 the application needs to consider the impact on the frequency of 201 transmitting of the synchronization information included in RTCP 202 Sender Reports. 204 3.2. Identifying Members of an RTCP Reporting Group 206 When RTCP Reporting Groups are in use, the other SSRCs in the RTP 207 session need to be able to identify which SSRCs are members of an 208 RTCP reporting group. Two RTCP extensions are defined to support 209 this: the RTCP RGRP SDES item is used by the reporting source(s) to 210 identify an RTCP reporting group, and the RTCP RGRS packet is used by 211 other members of an RTCP reporting group to identify the reporting 212 source(s). 214 3.2.1. Definition and Use of the RTCP RGRP SDES Item 216 A new RTCP SDES item is defined to identify an RTCP reporting group. 217 The motivation for giving a reporting group an identify is to ensure 218 that the RTCP reporting group and its member SSRCs can be correctly 219 associated when there are multiple reporting sources, and to ensure 220 that a reporting SSRC can be associated with the correct reporting 221 group if an SSRC collision occurs. 223 The RTCP Source Description (SDES) RGRP item is defined. The RTCP 224 SDES RGRP item MUST be sent by the reporting sources in a reporting 225 group, and MUST NOT be sent by other members of the reporting group 226 or by SSRCs that are not members of any RTCP reporting group. 227 Specifically, every reporting source in an RTCP reporting group MUST 228 include an RTCP SDES packet containing an RGRP item in every compound 229 RTCP packet in which it sends an RR or SR packet (i.e., in every RTCP 230 packet it sends, unless Reduced-Size RTCP [RFC5506] is in use). 232 Syntactically, the format of the RTCP SDES RGRP item is identical to 233 that of the RTCP SDES CNAME item [RFC7022], except that the SDES item 234 type field MUST have value RGRP=(TBA) instead of CNAME=1. The value 235 of the RTCP SDES RGRP item MUST be chosen with the same concerns 236 about global uniqueness and the same privacy considerations as the 237 RTCP SDES CNAME. The value of the RTCP SDES RGRP item MUST be stable 238 throughout the lifetime of the reporting group, even if some or all 239 of the reporting sources change their SSRC due to collisions, or if 240 the set of reporting sources changes. 242 Note to RFC Editor: please replace (TBA) in the above paragraph 243 with the RTCP SDES item type number assigned to the RGRP item, 244 then delete this note. 246 An RTP mixer or translator that forwards RTCP SR or RR packets from 247 members of a reporting group MUST forward the corresponding RTCP SDES 248 RGRP items as well, even if it otherwise strips SDES items other than 249 the CNAME item. 251 3.2.2. Definition and Use of the RTCP RGRS Packet 253 A new RTCP packet type is defined to allow the members of an RTCP 254 reporting group to identify the reporting sources for that group. 255 This allows participants in an RTP session to distinguish an SSRC 256 that is sending empty RTCP reception reports because it is a member 257 of an RTCP reporting group, from an SSRC that is sending empty RTCP 258 reception reports because it is not receiving any traffic. It also 259 explicitly identifies the reporting sources, allowing other members 260 of the RTP session to know which SSRCs are acting as the reporting 261 sources for an RTCP reporting group, and allowing them to detect if 262 RTCP packets from any of the reporting sources are being lost. 264 The format of the RTCP RGRS packet is defined below. It comprises 265 the fixed RTCP header that indicates the packet type and length, the 266 SSRC of the packet sender, and a list of reporting sources for the 267 RTCP reporting group of which the packet sender is a member. 269 0 1 2 3 270 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 1 271 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 272 |V=2|P| SC | PT=RGRS(TBA) | length | 273 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 274 | SSRC of packet sender | 275 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 276 : List of SSRCs for the Reporting Source(s) : 277 : ... : 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 280 The fields in the RTCP RGRS packet have the following definition: 282 version (V): 2 bits unsigned integer. This field identifies the RTP 283 version. The current RTP version is 2. 285 padding (P): 1 bit. If set, the padding bit indicates that the RTCP 286 packet contains additional padding octets at the end that are not 287 part of the control information but are included in the length 288 field. See [RFC3550]. 290 Source Count (SC): 5 bits unsigned integer. Indicates the number of 291 reporting source SSRCs that are included in this RTCP packet. As 292 the RTCP RGRS packet MUST NOT be not sent by reporting sources, 293 all the SSRCs in the list of reporting sources will be different 294 from the SSRC of the packet sender. Every RTCP RGRS packet MUST 295 contain at least one reporting source SSRC. 297 Payload type (PT): 8 bits unsigned integer. The RTCP packet type 298 number that identifies the packet as being an RTCP RGRS packet. 299 The RGRS RTCP packet has the value [TBA]. 301 Note to RFC Editor: please replace [TBA] here, and in the 302 packet format diagram above, with the RTCP packet type that 303 IANA assigns to the RTCP RGRS packet. 305 Length: 16 bits unsigned integer. The length of this packet in 306 32-bit words minus one, including the header and any padding. 307 This is in line with the definition of the length field used in 308 RTCP sender and receiver reports [RFC3550]. Since all RTCP RGRS 309 packets include at least one reporting source SSRC, the length 310 will always be 2 or greater. 312 SSRC of packet sender: 32 bits. The SSRC of the sender of this 313 packet. 315 List of SSRCs for the Reporting Source(s): A variable length size 316 (as indicated by SC header field) of the 32 bit SSRC values of the 317 reporting sources for the RTCP Reporting Group of which the packet 318 sender is a member. 320 Every source that belongs to an RTCP reporting group but is not a 321 reporting source MUST include an RTCP RGRS packet in every compound 322 RTCP packet in which it sends an RR or SR packet (i.e., in every RTCP 323 packet it sends, unless Reduced-Size RTCP [RFC5506] is in use). Each 324 RTCP RGRS packet MUST contain the SSRC identifier of at least one 325 reporting source. If there are more reporting sources in an RTCP 326 reporting group than can fit into an RTCP RGRS packet, the members of 327 that reporting group MUST send the SSRCs of the reporting sources in 328 a round-robin fashion in consecutive RTCP RGRS packets, such that all 329 the SSRCs of the reporting sources are included over the course of 330 several RTCP reporting intervals. 332 An RTP mixer or translator that forwards RTCP SR or RR packets from 333 members of a reporting group MUST also forward the corresponding RGRS 334 RTCP packets. If the RTP mixer or translator rewrites SSRC values of 335 the packets it forwards, it MUST make the corresponding changes to 336 the RTCP RGRS packets. 338 3.3. Interactions with the RTP/AVPF Feedback Profile 340 Use of the RTP/AVPF Feedback Profile [RFC4585] allows SSRCs to send 341 rapid RTCP feedback requests and codec control messages. If use of 342 the RTP/AVPF profile has been negotiated in an RTP session, members 343 of an RTCP reporting group can send rapid RTCP feedback and codec 344 control messages following [RFC4585] and [RFC5104], as updated by 345 Section 5.4 of [I-D.ietf-avtcore-rtp-multi-stream], and by the 346 following considerations. 348 The members of an RTCP reporting group will all see identical network 349 conditions. Accordingly, one might therefore think that it doesn't 350 matter which SSRC in the reporting group sends the RTP/AVPF feedback 351 or codec control messages. There might be, however, cases where the 352 sender of the feedback/codec control message has semantic importance, 353 or when only a subset of the members of an RTCP reporting group might 354 want to send RTP/AVPF feedback or a codec control message in response 355 to a particular event. For example, an RTP video sender might choose 356 to treat packet loss feedback received from SSRCs known to be audio 357 receivers with less urgency than feedback that it receives from video 358 receivers when deciding what packets to retransmit, and a multimedia 359 receiver using reporting groups might want to choose the outgoing 360 SSRC for feedback packets to reflect this. 362 Each member of an RTCP reporting group SHOULD therefore send RTP/AVPF 363 feedback/codec control messages independently of the other members of 364 the reporting group, to respect the semantic meaning of the message 365 sender. The suppression rules of [RFC4585] will ensure that only a 366 single copy of each feedback packet is (typically) generated, even if 367 several members of a reporting group send the same feedback. When an 368 endpoint knows that several members of its RTCP reporting group will 369 be sending identical feedback, and that the sender of the feedback is 370 not semantically important, then that endpoint MAY choose to send all 371 its feedback from the reporting source and deterministically suppress 372 feedback packets generated by the other sources in the reporting 373 group. 375 It is important to note that the RTP/AVPF timing rules operate on a 376 per-SSRC basis. Using a single reporting source to send all feedback 377 for a reporting group will hence limit the amount of feedback that 378 can be sent to that which can be sent by one SSRC. If this limit is 379 a problem, then the reporting group can allow each of its members to 380 send its own feedback, using its own SSRC. 382 If the RTP/AVPF feedback messages or codec control requests are sent 383 as compound RTCP packets, then those compound RTCP packets MUST 384 include either an RTCP RGRS packet or an RTCP SDES RGRP item, 385 depending on whether they are sent by the reporting source or a non- 386 reporting source in the RTCP reporting group respectively. The 387 contents of non-compound RTCP feedback or codec control messages are 388 not affected by the use of RTCP reporting groups. 390 3.4. Interactions with RTCP Extended Report (XR) Packets 392 When using RTCP Extended Reports (XR) [RFC3611] with RTCP reporting 393 groups, it is RECOMMENDED that the reporting source is used to send 394 the RTCP XR packets. If multiple reporting sources are in use, the 395 reporting source that sends the SR/RR packets that relate to a 396 particular remote SSRC SHOULD send the RTCP XR reports about that 397 SSRC. This is motivated as one commonly combine the RTCP XR metrics 398 with the regular report block to more fully understand the situation. 399 Receiving these blocks in different compound packets reduces their 400 value as the measuring intervals are not synchronized in those cases. 402 Some RTCP XR report blocks are specific to particular types of media, 403 and might be relevant to only some members of a reporting group. For 404 example, it would make no sense for an SSRC that is receiving video 405 to send a VoIP metric RTCP XR report block. Such media specific RTCP 406 XR report blocks MUST be sent by the SSRC to which they are relevant, 407 and MUST NOT be included in the common report sent by the reporting 408 source. This might mean that some SSRCs send RTCP XR packets in 409 compound RTCP packets that contain an empty RTCP SR/RR packet, and 410 that the time period covered by the RTCP XR packet is different to 411 that covered by the RTCP SR/RR packet. If it is important that the 412 RTCP XR packet and RTCP SR/RR packet cover the same time period, then 413 that source SHOULD be removed from the RTCP reporting group, and send 414 standard RTCP packets instead. 416 3.5. Middlebox Considerations 418 Many different types of middlebox are used with RTP. RTCP reporting 419 groups are potentially relevant to those types of RTP middlebox that 420 have their own SSRCs and generate RTCP reports for the traffic they 421 receive. RTP middleboxes that do not have their own SSRC, and that 422 don't send RTCP reports on the traffic they receive, cannot use the 423 RTCP reporting groups extension, since they generate no RTCP reports 424 to group. 426 An RTP middlebox that has several SSRCs of its own can use the RTCP 427 reporting groups extension to group the RTCP reports it generates. 428 This can occur, for example, if a middlebox is acting as an RTP mixer 429 for both audio and video flows that are multiplexed onto a single RTP 430 session, where the middlebox has one SSRC for the audio mixer and one 431 for the video mixer part, and when the middlebox wants to avoid cross 432 reporting between audio and video. 434 A middlebox cannot use the RTCP reporting groups extension to group 435 RTCP packets from the SSRCs that it is forwarding. It can, however, 436 group the RTCP packets from the SSRCs it is forwarding into compound 437 RTCP packets following the rules in Section 6.1 of [RFC3550] and 438 Section 5.3 of [I-D.ietf-avtcore-rtp-multi-stream]. If the middlebox 439 is using RTCP reporting groups for its own SSRCs, it MAY include RTCP 440 packets from the SSRCs that it is forwarding as part of the compound 441 RTCP packets its reporting source generates. 443 A middlebox that forwards RTCP SR or RR packets sent by members of a 444 reporting group MUST forward the corresponding RTCP SDES RGRP items, 445 as described in Section 3.2.1. A middlebox that forwards RTCP SR or 446 RR packets sent by member of a reporting group MUST also forward the 447 corresponding RTCP RGRS packets, as described in Section 3.2.2. 448 Failure to forward these packets can cause compatibility problems, as 449 described in Section 4.2. 451 If a middlebox rewrites SSRC values in the RTP and RTCP packets that 452 it is forwarding, then it MUST make the corresponding changes in RTCP 453 SDES packets containing RGRP items and in RTCP RGRS packets, to allow 454 them to be associated with the rewritten SSRCs. 456 3.6. SDP Signalling for Reporting Groups 458 This document defines the "a=rtcp-rgrp" Session Description Protocol 459 (SDP) [RFC4566] attribute to indicate if the session participant is 460 capable of supporting RTCP Reporting Groups for applications that use 461 SDP for configuration of RTP sessions. The attribute takes no value. 462 A participant that proposes the use of RTCP Reporting Groups SHALL 463 itself support the reception of RTCP Reporting Groups. The formal 464 definition of this attribute is: 466 Name: rtcp-rgrp 467 Value: 468 Usage Level: session, media 469 Charset Dependent: no 470 Example: 471 a=rtcp-rgrp 473 When doing SDP Offer/Answer [RFC3264] an offering client that wishes 474 to use RTCP Reporting Groups MUST include the attribute "a=rtcp-rgrp" 475 in the SDP offer. If "a=rtcp-rgrp" is present in the offer SDP, the 476 answerer that supports RTCP Reporting Groups and wishes to use it 477 SHALL include the "a=rtcp-rgrp" attribute in the answer. In cases 478 the answer has been excluded, neither agents SHALL use the RTCP 479 Reporting Groups. 481 In declarative usage of SDP, such as the Real Time Streaming Protocol 482 (RTSP) [RFC2326] and the Session Announcement Protocol (SAP) 483 [RFC2974], the presence of the attribute indicates that the session 484 participant MAY use RTCP Reporting Groups in its RTCP transmissions. 485 An implementation that doesn't explicitly support RTCP Reporting 486 Groups MAY join a RTP session as long as it has been verified that 487 the implementation doesn't suffer from the problems discussed in 488 Section 4.2. 490 4. Properties of RTCP Reporting Groups 492 This section provides additional information on what the resulting 493 properties are with the design specified in Section 3. The content 494 of this section is non-normative. 496 4.1. Bandwidth Benefits of RTCP Reporting Groups 498 To understand the benefits of RTCP reporting groups, consider a 499 scenario in which the two endpoints in a session each have a hundred 500 sources, of which eight each are sending within any given reporting 501 interval. 503 For ease of analysis, we can make the simplifying approximation that 504 the duration of the RTCP reporting interval is equal to the total 505 size of the RTCP packets sent during an RTCP interval, divided by the 506 RTCP bandwidth. (This will be approximately true in scenarios where 507 the bandwidth is not so high that the minimum RTCP interval is 508 reached.) For further simplification, we can assume RTCP senders are 509 following the recommendations regarding Compound RTCP Packets in 510 [I-D.ietf-avtcore-rtp-multi-stream]; thus, the per-packet transport- 511 layer overhead will be small relative to the RTCP data. Thus, only 512 the actual RTCP data itself need be considered. 514 In a report interval in this scenario, there will, as a baseline, be 515 200 SDES packets, 184 RR packets, and 16 SR packets. This amounts to 516 approximately 6.5 kB of RTCP per report interval, assuming 16-byte 517 CNAMEs and no other SDES information. 519 Using the original [RFC3550] everyone-reports-on-every-sender 520 feedback rules, each of the 184 receivers will send 16 report blocks, 521 and each of the 16 senders will send 15. This amounts to 522 approximately 76 kB of report block traffic per interval; 92% of RTCP 523 traffic consists of report blocks. 525 If reporting groups are used, however, there is only 0.4 kB of 526 reports per interval, with no loss of useful information. 527 Additionally, there will be (assuming 16-byte RGRPs, and a single 528 reporting source per reporting group) an additional 2.4 kB per cycle 529 of RGRP SDES items and RGRS packets. Put another way, the unmodified 530 [RFC3550] reporting interval is approximately 8.9 times longer than 531 if reporting groups are in use. 533 4.2. Compatibility of RTCP Reporting Groups 535 The RTCP traffic generated by receivers using RTCP Reporting Groups 536 might appear, to observers unaware of these semantics, to be 537 generated by receivers who are experiencing a network disconnection, 538 as the non-reporting sources appear not to be receiving a given 539 sender at all. 541 This could be a potentially critical problem for such a sender using 542 RTCP for congestion control, as such a sender might think that it is 543 sending so much traffic that it is causing complete congestion 544 collapse. 546 However, such an interpretation of the session statistics would 547 require a fairly sophisticated RTCP analysis. Any receiver of RTCP 548 statistics which is just interested in information about itself needs 549 to be prepared that any given reception report might not contain 550 information about a specific media source, because reception reports 551 in large conferences can be round-robined. 553 Thus, it is unclear to what extent such backward compatibility issues 554 would actually cause trouble in practice. 556 5. Security Considerations 558 The security considerations of [RFC3550] and 559 [I-D.ietf-avtcore-rtp-multi-stream] apply. If the RTP/AVPF profile 560 is in use, then the security considerations of [RFC4585] (and 561 [RFC5104], if used) also apply. If RTCP XR is used, the security 562 consideration of [RFC3611] and any XR report blocks used also apply. 564 The RTCP SDES RGRP item is vulnerable to malicious modifications 565 unless integrity protected is used. A modification of this item's 566 length field cause the parsing of the RTCP packet in which it is 567 contained to fail. Depending on the implementation, parsing of the 568 full compound RTCP packet can also fail causing the whole packet to 569 be discarded. A modification to the value of this SDES item would 570 make the receiver of the report think that the sender of the report 571 was a member of a different RTCP reporting group. This will 572 potentially create an inconsistency, when the RGRS reports the source 573 as being in the same reporting group as another source with another 574 reporting group identifier. What impact on a receiver implementation 575 such inconsistencies would have are difficult to fully predict. One 576 case is when congestion control or other adaptation mechanisms are 577 used, an inconsistent report can result in a media sender to reduce 578 its bit-rate. However, a direct modification of the receiver report 579 or a feedback message itself would be a more efficient attack, and 580 equally costly to perform. 582 The new RGRS RTCP Packet type is very simple. The common RTCP packet 583 type header shares the security risks with previous RTCP packet 584 types. Errors or modification of the length field can cause the full 585 compound packet to fail header validation (see Appendix A.2 in 586 [RFC3550]) resulting in the whole compound RTCP packet being 587 discarded. Modification of the SC or P fields would cause 588 inconsistency when processing the RTCP packet, likely resulting it 589 being classified as invalid. A modification of the PT field would 590 cause the packet being interpreted under some other packet type's 591 rules. In such case the result might be more or less predictable but 592 packet type specific. Modification of the SSRC of packet sender 593 would attribute this packet to another sender. Resulting in a 594 receiver believing the reporting group applies also for this SSRC, if 595 it exists. If it doesn't exist, unless also corresponding 596 modifications are done on a SR/RR packet and a SDES packet the RTCP 597 packet SHOULD be discarded. If consistent changes are done, that 598 could be part of a resource exhaustion attack on a receiver 599 implementation. Modification of the "List of SSRCs for the Reporting 600 Source(s)" would change the SSRC the receiver expect to report on 601 behalf of this SSRC. If that SSRC exist, that could potentially 602 change the report group used for this SSRC. A change to another 603 reporting group belonging to another endpoint is likely detectable as 604 there would be a mismatch between the SSRC of the packet sender's 605 endpoint information, transport addresses, SDES CNAME etc and the 606 corresponding information from the reporting group indicated. 608 In general the reporting group is providing limited impacts attacks. 609 The most significant result from an deliberate attack would be to 610 cause the information to be discarded or be inconsistent, including 611 discard of all RTCP packets that are modified. This causes a lack of 612 information at any receiver entity, possibly disregarding the 613 endpoints participation in the session. 615 To protect against this type of attacks from external non trusted 616 entities, integrity and source authentication SHOULD be applied. 618 This can be done, for example, by using SRTP [RFC3711] with 619 appropriate key-management, other options exist as discussed in RTP 620 Security Options [RFC7201]. 622 The Report Group Identifier has a potential privacy impacting 623 properties. If this would be generated by an implementation in such 624 a way that is long term stable or predictable, it could be used for 625 tracking a particular end-point. Therefore it is RECOMMENDED that it 626 be generated as a short-term persistent RGRP, following the rules for 627 short-term persistent CNAMEs in [RFC7022]. The rest of the 628 information revealed, i.e. the SSRCs, the size of reporting group and 629 the number of reporting sources in a reporting group is of less 630 sensitive nature, considering that the SSRCs and the communication 631 would anyway be revealed without this extension. By encrypting the 632 report group extensions the SSRC values would preserved confidential, 633 but can still be revealed if SRTP [RFC3711] is used. The size of the 634 reporting groups and number of reporting sources are likely 635 determinable from analysis of the packet pattern and sizes. However, 636 this information appears to have limited value. 638 6. IANA Considerations 640 (Note to the RFC-Editor: in the following, please replace "TBA" with 641 the IANA-assigned value, and "XXXX" with the number of this document, 642 then delete this note) 644 The IANA is requested to register one new RTCP SDES items in the 645 "RTCP SDES Item Types" registry, as follows: 647 Value Abbrev Name Reference 648 TBA RGRP Reporting Group Identifier [RFCXXXX] 650 The definition of the RTCP SDES RGRP item is given in Section 3.2.1 651 of this memo. 653 The IANA is also requested to register one new RTCP packet type in 654 the "RTCP Control Packet Types (PT)" Registry as follows: 656 Value Abbrev Name Reference 657 TBA RGRS Reporting Group Reporting Sources [RFCXXXX] 659 The definition of the RTCP RGRS packet type is given in Section 3.2.2 660 of this memo. 662 The IANA is also requested to register one new SDP attribute: 664 SDP Attribute ("att-field"): 665 Attribute name: rtcp-rgrp 666 Long form: RTCP Reporting Groups 667 Type of name: att-field 668 Type of attribute: Media or session level 669 Subject to charset: No 670 Purpose: Negotiate or configure the use of the RTCP 671 Reporting Group Extension. 672 Reference: [RFCXXXX] 673 Values: None 675 The definition of the "a=rtcp-rgrp" SDES attribute is given in 676 Section 3.6 of this memo. 678 7. References 680 7.1. Normative References 682 [I-D.ietf-avtcore-rtp-multi-stream] 683 Lennox, J., Westerlund, M., Wu, W., and C. Perkins, 684 "Sending Multiple Media Streams in a Single RTP Session", 685 draft-ietf-avtcore-rtp-multi-stream-09 (work in progress), 686 September 2015. 688 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 689 Requirement Levels", BCP 14, RFC 2119, 690 DOI 10.17487/RFC2119, March 1997, 691 . 693 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 694 Jacobson, "RTP: A Transport Protocol for Real-Time 695 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 696 July 2003, . 698 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 699 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 700 July 2006, . 702 [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, 703 "Guidelines for Choosing RTP Control Protocol (RTCP) 704 Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, 705 September 2013, . 707 7.2. Informative References 709 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 710 Streaming Protocol (RTSP)", RFC 2326, 711 DOI 10.17487/RFC2326, April 1998, 712 . 714 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 715 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 716 October 2000, . 718 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 719 with Session Description Protocol (SDP)", RFC 3264, 720 DOI 10.17487/RFC3264, June 2002, 721 . 723 [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., 724 "RTP Control Protocol Extended Reports (RTCP XR)", 725 RFC 3611, DOI 10.17487/RFC3611, November 2003, 726 . 728 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 729 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 730 RFC 3711, DOI 10.17487/RFC3711, March 2004, 731 . 733 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 734 "Extended RTP Profile for Real-time Transport Control 735 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 736 DOI 10.17487/RFC4585, July 2006, 737 . 739 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 740 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 741 DOI 10.17487/RFC4588, July 2006, 742 . 744 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 745 "Codec Control Messages in the RTP Audio-Visual Profile 746 with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, 747 February 2008, . 749 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 750 Real-Time Transport Control Protocol (RTCP): Opportunities 751 and Consequences", RFC 5506, DOI 10.17487/RFC5506, April 752 2009, . 754 [RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis, 755 "RTP Payload Format for Scalable Video Coding", RFC 6190, 756 DOI 10.17487/RFC6190, May 2011, 757 . 759 [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP 760 Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, 761 . 763 Authors' Addresses 765 Jonathan Lennox 766 Vidyo, Inc. 767 433 Hackensack Avenue 768 Seventh Floor 769 Hackensack, NJ 07601 770 US 772 Email: jonathan@vidyo.com 774 Magnus Westerlund 775 Ericsson 776 Farogatan 2 777 SE-164 80 Kista 778 Sweden 780 Phone: +46 10 714 82 87 781 Email: magnus.westerlund@ericsson.com 783 Qin Wu 784 Huawei 785 101 Software Avenue, Yuhua District 786 Nanjing, Jiangsu 210012 787 China 789 Email: sunseawq@huawei.com 791 Colin Perkins 792 University of Glasgow 793 School of Computing Science 794 Glasgow G12 8QQ 795 United Kingdom 797 Email: csp@csperkins.org