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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 5117 (Obsoleted by RFC 7667) == Outdated reference: A later version (-10) exists of draft-ietf-avtcore-rtp-topologies-update-06 == Outdated reference: A later version (-08) exists of draft-ietf-avtext-rtp-grouping-taxonomy-06 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 STRAW Working Group L. Miniero 3 Internet-Draft Meetecho 4 Intended status: Standards Track S. Garcia Murillo 5 Expires: September 25, 2015 Medooze 6 V. Pascual 7 Quobis 8 March 24, 2015 10 Guidelines to support RTCP end-to-end in Back-to-Back User Agents 11 (B2BUAs) 12 draft-ietf-straw-b2bua-rtcp-05 14 Abstract 16 SIP Back-to-Back User Agents (B2BUAs) are often envisaged to also be 17 on the media path, rather than just intercepting signalling. This 18 means that B2BUAs often implement an RTP/RTCP stack as well, whether 19 to act as media transcoders or to just passthrough the media 20 themselves, thus leading to separate multimedia sessions that the 21 B2BUA correlates and bridges together. If not disciplined, though, 22 this behaviour can severely impact the communication experience, 23 especially when statistics and feedback information contained in RTCP 24 packets get lost because of mismatches in the reported data. 26 This document defines the proper behaviour B2BUAs should follow when 27 also acting on the signalling/media plane in order to preserve the 28 end-to-end functionality of RTCP. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on September 25, 2015. 47 Copyright Notice 49 Copyright (c) 2015 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 65 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 3. Signalling/Media Plane B2BUAs . . . . . . . . . . . . . . . . 5 67 3.1. Media Relay . . . . . . . . . . . . . . . . . . . . . . . 5 68 3.2. Media-aware Relay . . . . . . . . . . . . . . . . . . . . 6 69 3.3. Media Terminator . . . . . . . . . . . . . . . . . . . . 10 70 4. Media Path Security . . . . . . . . . . . . . . . . . . . . . 11 71 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 73 7. Change Summary . . . . . . . . . . . . . . . . . . . . . . . 12 74 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 75 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 76 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 77 9.2. Informative References . . . . . . . . . . . . . . . . . 14 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 80 1. Introduction 82 Session Initiation Protocol [RFC3261] Back-to-Back User Agents 83 (B2BUAs) are SIP entities that can act as a logical combination of 84 both a User Agent Server (UAS) and a User Agent Client (UAC). As 85 such, their behaviour is not always completelely adherent to the 86 standards, and can lead to unexpected situations the IETF is trying 87 to address. [RFC7092] presents a taxonomy of the most deployed B2BUA 88 implementations, describing how they differ in terms of the 89 functionality and features they provide. 91 Such components often do not only act on the signalling plane, that 92 is intercepting and possibly modifying SIP messages, but also on the 93 media plane. This means that, when on the signalling path between 94 two or more participants willing to communicate, such components also 95 manipulate the session description [RFC4566] in order to have all RTP 96 and RTCP [RFC3550] pass through it as well within the context of an 97 SDP offer/answer [RFC3264]. The reasons for such a behaviour can be 98 different: the B2BUA may want, for instance, to provide transcoding 99 functionality for participants with incompatible codecs, or it may 100 need the traffic to be directly handled for different reasons like 101 billing, lawful interception, session recording and so on. This can 102 lead to several different topologies for RTP-based communication, as 103 documented in [RFC5117]. These topologies are currently being 104 updated to address new commonly encountered scenarios as well 105 [I-D.ietf-avtcore-rtp-topologies-update]. 107 Whatever the reason, such a behaviour does not come without a cost. 108 In fact, whenever a media-aware component is placed on the path 109 between two or more participants that want to communicate by means of 110 RTP/RTCP, the end-to-end nature of such protocols is broken, and 111 their effectiveness may be affected as a consequence. While this may 112 not be a problem for RTP packets, which from a protocol point of view 113 just contain opaque media packets and as such can be quite easily 114 relayed, it definitely can cause serious issue for RTCP packets, 115 which carry important information and feedback on the communication 116 quality the participants are experiencing. In fact, RTCP packets 117 make use of specific ways to address the media they are referring to. 118 Consider, for instance, the simple scenario only involving two 119 participants and a single RTP session depicted in Figure 1: 121 +--------+ +---------+ +---------+ 122 | |=== SSRC1 ===>| |=== SSRC3 ===>| | 123 | Alice | | B2BUA | | Bob | 124 | |<=== SSRC2 ===| |<=== SSRC4 ===| | 125 +--------+ +---------+ +---------+ 127 Figure 1: B2BUA modifying RTP headers 129 In this common scenario, a participant (Alice) is communicating with 130 another participant (Bob) as a result of a signalling session managed 131 by a B2BUA: this B2BUA is also on the media path between the two, and 132 is acting as a media relay. This means that two separate RTP 133 sessions are involved (one per side), each carrying two RTP streams 134 (one per media direction). As part of this process, though, it is 135 also rewriting some of the RTP header information on the way, for 136 instance because that's how its RTP relaying stack works: in this 137 example, just the SSRC of the incoming RTP audio streams is changed, 138 but more information may be changed as well (e.g., sequence numbers, 139 timestamps, etc.). In particular, whenever Alice sends an audio RTP 140 packet, she sets her SSRC (SSRC1) to the RTP header of her RTP source 141 stream; the B2BUA rewrites the SSRC (SSRC3) before relaying the 142 packet to Bob. At the same time, RTP packets sent by Bob (SSRC4) get 143 their SSRC rewritten as well (SSRC2) before being relayed to Alice. 145 Assuming now that Alice needs to inform Bob she has lost several 146 audio packets in the last few seconds, maybe because of a network 147 congestion, she would of course place the related received RTP stream 148 SSRC she is aware of (SSRC2), together with her own (SSRC1), in RTCP 149 Reports and/or NACKS to do so, hoping for a retransmission [RFC4588] 150 or for Bob to slow down. Since the B2BUA is making use of different 151 SSRCs for the RTP streams in the RTP session it established with each 152 participant, a blind relaying of the RTCP packets to Bob would in 153 this case result, from Bob's perspective, in unknown SSRCs being 154 addressed, thus resulting in the precious information being dropped. 155 In fact, Bob is only aware of SSRCs SSRC4 (the one his source RTP 156 stream uses) and SSRC3 (the one he's receiving from the B2BUA in the 157 received RTP stream), and knows nothing about SSRCs SSRC1 and SSRC2 158 in the RTCP packets he would receive instead. As a consequence of 159 the feedback being dropped, unaware of the issue Bob may continue to 160 flood Alice with even more media packets and/or not retransmit Alice 161 the packets she missed, which may easily lead to a very bad 162 communication experience, if not eventually to an unwanted 163 termination of the communication itself. 165 This is just a trivial example that, together with additional 166 scenarios, will be addressed in the following sections. 167 Nevertheless, it is a valid example of how such a trivial mishandling 168 of precious information may lead to serious consequences, especially 169 considering that more complex scenarios may involve several 170 participants at the same time, multiple RTP sessions (e.g., a video 171 stream along audio) rather than a single one, redundancy RTP streams, 172 SSRC multiplexing and so on. Considering how common B2BUA 173 deployments are, it is very important for them to properly address 174 such feedback, in order to be sure that their activities on the media 175 plane do not break anything they're not supposed to. 177 2. Terminology 179 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 180 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 181 document are to be interpreted as described in [RFC2119]. 183 Besides, this document addresses, where relevant, the RTP-related 184 terminology as disciplined in 185 [I-D.ietf-avtext-rtp-grouping-taxonomy]. 187 3. Signalling/Media Plane B2BUAs 189 As anticipated in the introductory section, it's very common for 190 B2BUA deployments to also act on the media plane, rather than just 191 signalling alone. In particular, [RFC7092] describes three different 192 categories of such B2BUAs, according to the level of activities 193 performed on the media plane: a B2BUA, in fact, may act as a simple 194 media relay (1), effectively unaware of anything that is transported; 195 it may be a media-aware relay (2), also inspecting and/or modifying 196 RTP and RTCP packets as they flow by; or it may be a full-fledged 197 media termination entity, terminating and generating RTP and RTCP 198 packets as needed. 200 While [RFC3550] and [RFC5117] already mandate some specific 201 behaviours when specific topologies are deployed, not all deployments 202 strictly adhere to the specifications and as such it's not rare to 203 encounter issues that may be avoided with a more disciplined 204 behaviour in that regard. For this reason, the following subsections 205 will describe the proper behaviour B2BUAs, whatever above category 206 they fall in, should follow in order to avoid, or at least minimize, 207 any impact on end-to-end RTCP effectiveness. 209 3.1. Media Relay 211 A media relay as identified in [RFC7092] basically just forwards, 212 from an application level point of view, all RTP and RTP packets it 213 receives, without either inspecting or modifying them. Using the RTP 214 Topologies terminology, this can be seen as a RTP Transport 215 Translator. As such, B2BUA acting as media relays are not aware of 216 what traffic they're handling, meaning that not only the packet 217 payloads are opaque to them, but headers as well. Many Session 218 Border Controllers (SBC) implement this kind of behaviour, e.g., when 219 acting as a bridge between an inner and outer network. 221 Considering all headers and identifiers in both RTP and RTCP are left 222 untouched, issues like the SSRC mismatch described in the previous 223 section would not occur. Similar problems could occur, though, 224 should the session description end up providing incorrect information 225 about the media flowing (e.g., if the SDP on either side contain 226 'ssrc' [RFC5576] attributes that don't match the actual SSRC being 227 advertized on the media plane) or about the supported RTCP mechanisms 228 (e.g., in case the B2BUA advertized support for NACK because it 229 implements it, but the original INVITE didn't). Such an issue might 230 occur, for instance, in case the B2BUA acting as a media relay is 231 generating a new session description when bridging an incoming call, 232 rather than taking into account the original session description in 233 the first place. This may cause the participants to find a mismatch 234 between the SSRCs advertized in SDP and the ones actually observed in 235 RTP and RTCP packets (which may indeed change during a multimedia 236 session anyway, but having them synced during setup would help 237 nonetheless), or having them either ignore or generate RTCP feedback 238 packets that were not explicitly advertized as supported. 240 In order to prevent such an issue, a media-relay B2BUA SHOULD forward 241 all the SSRC- and RTCP-related SDP attributes when handling a 242 multimedia session setup between interested participants: this 243 includes attributes like 'ssrc' [RFC3261], 'rtcp-fb' [RFC4585], 244 'rtcp-xr-attrib' [RFC3611] and others. It SHOULD NOT, though, 245 blindly forward all SDP attributes, as some of them (e.g., 246 candidates, fingerprints, crypto, etc.) may lead to call failures for 247 different reasons out of scope to this document. One notable example 248 is the 'rtcp' [RFC3605] attribute that UAC may make use of to 249 explicitly state the port they're willing to use for RTCP: 250 considering the B2BUA would relay RTCP packets, the port as seen by 251 the other UAC involved in the communication would differ from the one 252 negotiated originally, and as such it MUST be rewritten accordingly. 254 Besides, it is worth mentioning that, leaving RTCP packets untouched, 255 a media relay may also let through information that, according to 256 policies, may be best left hidden or masqueraded, e.g., domain names 257 in CNAME items. Nevertheless, that information cannot break the end- 258 to-end RTCP behaviour. 260 3.2. Media-aware Relay 262 A Media-aware relay, unlike the the Media Relay addressed in the 263 previous section, is actually aware of the media traffic it is 264 handling. As such, it is able to inspect RTP and RTCP packets 265 flowing by, and may even be able to modify the headers in any of them 266 before forwarding them. Using the RFC3550 terminology, this can be 267 seen as a RTP Translator. A B2BUA implementing this role would 268 typically not, though, inspect the RTP payloads as well, which would 269 be opaque to them: this means that the actual media would not be 270 manipulated (e.g, transcoded). 272 This makes them quite different from the Media Relay previously 273 discussed, especially in terms of the potential issues that may occur 274 at the RTCP level. In fact, being able to modify the RTP and RTCP 275 headers, such B2BUAs may end up modifying RTP related information 276 like SSRC (and hence CSRC lists, that must of course be updated 277 accordingly), sequence numbers, timestamps and the like in an RTP 278 stream, before forwarding the modified packets to the other 279 interested participants in the multimedia sessions on the RTP streams 280 they're using to receive the media. This means that, if not properly 281 disciplined, such a behaviour may easily lead to issues like the one 282 described in the introductory section. As such, it is very important 283 for a B2BUA modifying RTP-related information across two related RTP 284 streams to also modify the same information in RTCP packets as well, 285 and in a coherent way, so that not to confuse any of the participants 286 involved in a communication. 288 It is worthwile to point out that such a B2BUA would not necessarily 289 forward all the packets it is receiving, though: Selective Forwarding 290 Units (SFU) [I-D.ietf-avtcore-rtp-topologies-update], for instance, 291 could aggregate or drop incoming RTCP messages, while at the same 292 time originating new ones on their own. For the messages that are 293 forwarded and/or aggregated, though, it's important to make sure the 294 information is coherent. 296 Besides the behaviour already mandated for RTCP translators in 297 Section 7.2 of [RFC3550], a media-aware B2BUA MUST also handle 298 incoming RTCP messages to forward following this guideline: 300 SR: [RFC3550] 301 If the B2BUA has changed any SSRC in any RTP streams relation, it 302 MUST update the SSRC-related information in the incoming SR packet 303 before forwarding it. This includes the sender SSRC, which MUST 304 be rewritten with the one the B2BUA uses in the RTP stream used to 305 receive RTP packets from each participant, and the SSRC 306 information in all the blocks, which MUST be rewritten using the 307 related sender participant(s) SSRC. If the B2BUA has also changed 308 the base RTP sequence number when forwarding RTP packets, then 309 this change needs to be properly addressed in the 'extended 310 highest sequence number received' field in the Report Blocks. 312 RR: [RFC3550] 313 The same guidelines given for SR apply for RR as well. 315 SDES: [RFC3550] 316 If the B2BUA has changed any SSRC in any direction, it MUST update 317 the SSRC-related information in all the chunks in the incoming 318 SDES packet before forwarding it. 320 BYE: [RFC3550] 321 If the B2BUA has changed any SSRC in any direction, it MUST update 322 the SSRC in the BYE message. 324 APP: [RFC3550] 325 If the B2BUA has changed any SSRC in any direction, it MUST update 326 the SSRC in the APP message. Should the B2BUA be aware of any 327 specific APP message format that contains additional information 328 related to SSRCs, it SHOULD update them as well. 330 Extended Reports (XR): [RFC3611] 331 If the B2BUA has changed any SSRC in any direction, it MUST update 332 the SSRC-related information in the incoming XR message header 333 before forwarding it. This includes the source SSRC, which MUST 334 be rewritten with the one the B2BUA uses to send RTP packets to 335 each sender participant, and the SSRC information in all the block 336 types that include it, which MUST be rewritten using the related 337 sender participant(s) SSRC. If the B2BUA has also changed the 338 base RTP sequence number when forwarding RTP packets, then this 339 change needs to be properly addressed in the 'begin_seq' and 340 'end_seq' fields that are available in most of the Report Block 341 types that are part of the XR specification. 343 Receiver Summary Information (RSI): [RFC5760] 344 If the B2BUA has changed any SSRC in any direction, it MUST update 345 the SSRC-related information in the incoming RSI message header 346 before forwarding it. This includes the distribution source SSRC, 347 which MUST be rewritten with the one the B2BUA uses to send RTP 348 packets to each sender participant, the summarized SSRC and, in 349 case a Collision Sub-Report Block is available, the SSRCs in the 350 related list. 352 Port Mapping (TOKEN): [RFC6284] 353 If the B2BUA has changed any SSRC in any direction, it MUST update 354 the SSRC-related information in the incoming TOKEN message before 355 forwarding it. This includes the Packet Sender SSRC, which MUST 356 be rewritten with the one the B2BUA uses to send RTP packets to 357 each sender participant, and the Requesting Client SSRC in case 358 the message is a response, which MUST be rewritten using the 359 related sender participant(s) SSRC. 361 Feedback messages: [RFC4585] 362 All Feedback messages have a common packet format, which includes 363 the SSRC of the packet sender and the one of the media source the 364 feedack is related to. Just as described for the previous 365 messages, these SSRC identifiers MUST be updated if the B2BUA has 366 changed any SSRC in any direction. It MUST NOT, though, change a 367 media source SSRC that was originally set to zero, unless zero is 368 actually the SSRC that was chosen by one of the involved 369 endpoints, in which case the above mentioned rules as to SSRC 370 rewriting apply. Besides, considering that many feedback messages 371 also include additional data as part of their specific Feedback 372 Control Information (FCI), a media-aware B2BUA MUST take care of 373 them accordingly, if it can parse and regenerate them, according 374 to the following guidelines. 376 NACK: [RFC4585] 377 Besides the common packet format management for feedback messages, 378 a media-aware B2BUA MUST also properly rewrite the Packet ID (PID) 379 of all addressed lost packets in the NACK FCI if it changed the 380 RTP sequence numbers before forwarding a packet. 382 TMMBR/TMMBN/FIR/TSTR/TSTN/VBCM: [RFC5104] 383 Besides the common packet format management for feedback messages, 384 a media-aware B2BUA MUST also properly rewrite the additional SSRC 385 identifier all those messages envisage as part of their specific 386 FCI if it changed the related RTP SSRC of the media sender. 388 REMB: [I-D.alvestrand-rmcat-remb] 389 Besides the common packet format management for feedback messages, 390 a media-aware B2BUA MUST also properly rewrite the additional SSRC 391 identifier(s) REMB packets envisage as part of their specific FCI 392 if it changed the related RTP SSRC of the media sender. 394 Explicit Congestion Notification (ECN): [RFC6679] 395 Besides the common packet format management for feedback messages, 396 the same guidelines given for SR/RR management apply as well, 397 considering the presence of sequence numbers in the ECN Feedback 398 Report format. For what concerns the management of RTCP XR ECN 399 Summary Report messages, the same guidelines given for generic XR 400 messages apply. 402 Apart from the generic guidelines related to Feedback messages, no 403 additional modifications are needed for PLI, SLI and RPSI feedback 404 messages instead. 406 Of course, the same considerations about the need for SDP and RTP/ 407 RTCP information to be coherent also applies to media-aware B2BUAs. 408 This means that, if a B2BUA is going to change any SSRC, it SHOULD 409 update the related 'ssrc' attributes if they were present in the 410 original description before sending it to the recipient, just as it 411 MUST rewrite the 'rtcp' attribute if provided. At the same time, the 412 ability for a media-aware B2BUA to inspect/modify RTCP packets may 413 also mean such a B2BUA may choose to drop RTCP packets it can't 414 parse: in that case, a media-aware B2BUA MUST also advertize its RTCP 415 level of support in the SDP in a coherent way, in order to prevent, 416 for instance, a UAC to make use of NACK messages that would never 417 reach the intended recipients. It's important to point out that, in 418 case any RTCP packet needs to be dropped, then only the offending 419 RTCP packet needs to be dropped, and not the whole compound RTCP 420 packet it may belong to. 422 A different set of considerations, instead, is worthwhile for what 423 concerns RTP/RTCP multiplexing [RFC5761] and Reduced-Size RTCP 424 [RFC5506]. While the former allows for a better management of 425 network resources by multiplexing RTP packets and RTCP messages over 426 the same transport, the latter allows for a compression of RTCP 427 messages, thus leading to less network traffic. For what concerns 428 RTP/RTCP multiplexing, a B2BUA acting as a Media Relay can use it on 429 either RTP session independently: this means that, for instance, a 430 Media Relay B2BUA may use RTP/RTCP multiplexing on one side of the 431 communication, and not use it on the other side, if it's not 432 supported. This allows for a better management of network resources 433 on the side that does support it. In case any of the parties in the 434 communications supports it and the B2BUA does too, the related 'rtcp- 435 mux' SDP attribute MUST be forwarded on the other side(s); if the 436 B2BUA detects that any of the parties in the communication does not 437 support the feature, it may decide to either disable it entirely or 438 still advertize it for the RTP sessions with parties that do support 439 it. In case the B2BUA decides to involve RTP/RTCP multiplexing, it 440 MUST ensure that there are no conflicting RTP payload type numbers on 441 both sides, and in case there are, it MUST rewrite RTP payload type 442 numbers to ensure no conflict in the domain where the RTP/RTCP 443 multiplexing is applied. Should RTP payload types be rewritten, the 444 related information in the SDP MUST be updated accordingly. 446 For what concerns Reduced-Size RTCP, instead, the considerations are 447 a bit different. In fact, while a Media Relay B2BUA may choose to 448 use it on the side that supports it and not on the side that doesn't, 449 there are other aspects to take into account before doing so. While 450 Reduced-Size allows indeed for less network traffic related to RTCP 451 messaging in general, this gain may lead a Reduced-Size RTCP 452 implementation to also issue a higher rate of RTCP feedback messages. 453 This would result in an increased RTCP traffic on the side that does 454 not support Reduced-Size, and could as a consequence be actually 455 counterproductive if the bandwidth is different on each side. That 456 said, the B2BUA can choose whether or not to advertize support for 457 Reduced-Size RTCP on either side by means of the 'rtcp-rsize' SDP 458 attribute. Should a B2BUA decide to allow the sides to independently 459 use or not Reduced-Size, then the B2BUA MUST advertize support for 460 the feature on the sides that support it, and MUST NOT advertize it 461 on the sides that don't, by removing the related attribute from the 462 SDP before forwarding it. Should the B2BUA decide to disable the 463 feature on all sides, instead, it MUST NOT advertize support for the 464 Reduced-Size RTCP functionality on any side, by removing the 'rtcp- 465 rsize' attribute from the SDP. 467 3.3. Media Terminator 469 A Media Terminator B2BUA, unlike simple relays and media-aware ones, 470 is also able to terminate media itself, that is taking care of RTP 471 payloads as well and not only headers. This means that such 472 components, for instance, can act as media transcoders and/or 473 originate specific RTP media. Using the RTP Topologies terminology, 474 this can be seen as a RTP Media Translator. Such a topology can also 475 be seen as a Back-to-back RTP sessions through a Middlebox, as 476 described in Section 3.2.2 of 477 [I-D.ietf-avtcore-rtp-topologies-update]. Such a capability makes 478 them quite different from the previously introduced B2BUA typologies, 479 as this means they are going to terminate RTCP as well: in fact, 480 since the media is terminated by themselves, the related statistics 481 and feedback functionality can be taken care directly by the B2BUA, 482 and does not need to be relayed to the other participants in the 483 multimedia session. 485 For this reason, no specific guideline is needed to ensure a proper 486 end-to-end RTCP behaviour in such scenarios, mostly because most of 487 the times there would be no end-to-end RTCP interaction among the 488 involved participants at all, as the B2BUA would terminate them all 489 and take care of them accordingly. Nevertheless, should any RTCP 490 packet actually need to be forwarded to another participant in the 491 multimedia session, the same guidelines provided for the media-aware 492 B2BUA case apply. 494 For what concerns RTP/RTCP multiplexing support, the same 495 considerations already given for the Media Relay management basically 496 apply for a Media Terminator as well. Some different considerations 497 might be given as to the Reduced-Size RTCP functionality, instead: in 498 fact, in the Media Terminator case it is safe to use the feature 499 independently on each leg. In that case, the same considerations 500 about advertizing the support, or lack of support, of the feature on 501 either side as described for the Media Relay case apply here as well. 503 4. Media Path Security 505 The discussion made in the previous sections on the management of 506 RTCP messages by a B2BUA has so far mostly worked under the 507 assumption that the B2BUA has actually access to the RTP/RTCP 508 information itself. This is indeed true if we assume that plain RTP 509 and RTCP is being handled, but this may not be true once any security 510 is enforced on RTP packets and RTCP messages by means of SRTP 511 [RFC3711], whether the keying is done using Secure Descriptions 512 [RFC4568] or DTLS-SRTP [RFC5764]. 514 While typically not an issue in the Media Relay case, where RTP and 515 RTCP packets are forwarded without any modification no matter whether 516 security is involved or not, this could definitely have an impact on 517 Media-aware Relays and Media Terminator B2BUAs. To make a simple 518 example, if we think of a SRTP/SRTCP session across a B2BUA where the 519 B2BUA itself has no access to the keys used to secure the session, 520 there would be no way to manipulate SRTP headers without violating 521 the hashing on the packet; at the same time, there would be no way to 522 rewrite the RTCP information accordingly either, as most of the 523 packet (especially when RTCP compound packets are involved) would be 524 encrypted. 526 For this reason, it is important to point out that the operations 527 described in the previous sections are only possible if the B2BUA has 528 a way to effectively manipulate the packets and messages flowing by. 529 This means that, in case media security is involved, only the Media- 530 unaware Relay scenario can be properly addressed. Attempting to 531 cover Media-aware Relay and Media Terminarion scenarios when 532 involving secure sessions will inevitably lead to the B2BUA acting as 533 a man-in-the-middle, and as such its behaviour is unspecified and 534 discouraged. 536 5. IANA Considerations 538 This document makes no request of IANA. 540 6. Security Considerations 542 This document, being a summary and vest common practice overview that 543 covers different standards, does not introduce any additional 544 consideration to those described by the aforementioned standard 545 documents themselves. 547 It is worth pointing out, though, that there are scenarios where an 548 improper management of RTCP messaging across a B2BUA may lead, 549 willingly or not, to situations not unlike an attack. To make a 550 simple example, an improper management of a REMB feedback message 551 containing, e.g., information on the limited bandwidth availability 552 for a user, may lead to missing information to its peer, who may end 553 up increasing the encoder bitrate up to a point where the user with 554 poor connectivity will inevitably be choked by an amount of data it 555 cannot process. This scenario may as such result in what looks like 556 a Denial of Service (DOS) attack towards the user. 558 7. Change Summary 560 Note to RFC Editor: Please remove this whole section. 562 The following are the major changes between the 04 and the 05 563 versions of the draft: 565 o Clarified behaviour when SSRC is zero. 567 o Fixed a couple of nits found by the Idnits tool. 569 The following are the major changes between the 03 and the 04 570 versions of the draft: 572 o Addressed review by Magnus Westerlund. 574 o Added guidelines for ECN RTCP messages. 576 o Clarified that if an RTCP packet is dropped because unsupported, 577 only the unsupported packet is dropped and not the compound packet 578 that contains it. 580 o Added reference to Section 3.2.2 of 581 [I-D.ietf-avtcore-rtp-topologies-update] to Section 3.3. 583 o Added considerations on RTP/RTCP multiplexing and Reduced-Size 584 RTCP. 586 The following are the major changes between the 02 and the 03 587 versions of the draft: 589 o Rephrased the Media Path Security section to take into account the 590 MITM-related discussion in Honolulu. 592 o Added some Security Considerations. 594 The following are the major changes between the 01 and the 02 595 versions of the draft: 597 o Updated terminology to better adhere to 598 [I-D.ietf-avtext-rtp-grouping-taxonomy]. 600 o Rephrased the Media Path Security section to take into account the 601 MITM-related discussion in Toronto. 603 o Clarified that NACK management might be trickier when SRTP is 604 involved. 606 The following are the major changes between the 00 and the 01 607 versions of the draft: 609 o Updated references and mapping per taxonomy RFC (7092). 611 o Added a reference to RTP topologies, and tried a mapping as per- 612 discussion in London. 614 o Added more RTCP packet types to the Media-Aware section. 616 o Clarified that fixing the 'rtcp' SDP attribute is important. 618 o Added a new section on the impact of media security. 620 8. Acknowledgements 622 The authors would like to thank Flavio Battimo and Pierluigi Palma 623 for their invaluable feedback in the early stages of the document. 624 The authors would also like to thank Colin Perkins, Bernard Aboba, 625 Albrecht Schwarz, Hadriel Kaplan, Keith Drage, Jonathan Lennox, 626 Stephen Farrell and Magnus Westerlund for their constructive 627 comments, suggestions, and reviews that were critical to the 628 formulation and refinement of this document. 630 9. References 632 9.1. Normative References 634 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 635 Requirement Levels", BCP 14, RFC 2119, March 1997. 637 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 638 A., Peterson, J., Sparks, R., Handley, M., and E. 639 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 640 June 2002. 642 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 643 Description Protocol", RFC 4566, July 2006. 645 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 646 with Session Description Protocol (SDP)", RFC 3264, June 647 2002. 649 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 650 Jacobson, "RTP: A Transport Protocol for Real-Time 651 Applications", STD 64, RFC 3550, July 2003. 653 9.2. Informative References 655 [RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session 656 Initiation Protocol (SIP) Back-to-Back User Agents", RFC 657 7092, December 2013. 659 [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, 660 January 2008. 662 [I-D.ietf-avtcore-rtp-topologies-update] 663 Westerlund, M. and S. Wenger, "RTP Topologies", draft- 664 ietf-avtcore-rtp-topologies-update-06 (work in progress), 665 March 2015. 667 [I-D.ietf-avtext-rtp-grouping-taxonomy] 668 Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro, 669 "A Taxonomy of Grouping Semantics and Mechanisms for Real- 670 Time Transport Protocol (RTP) Sources", draft-ietf-avtext- 671 rtp-grouping-taxonomy-06 (work in progress), March 2015. 673 [I-D.alvestrand-rmcat-remb] 674 Alvestrand, H., "RTCP message for Receiver Estimated 675 Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in 676 progress), October 2013. 678 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 679 "Extended RTP Profile for Real-time Transport Control 680 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 681 2006. 683 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 684 "Codec Control Messages in the RTP Audio-Visual Profile 685 with Feedback (AVPF)", RFC 5104, February 2008. 687 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 688 Media Attributes in the Session Description Protocol 689 (SDP)", RFC 5576, June 2009. 691 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 692 in Session Description Protocol (SDP)", RFC 3605, October 693 2003. 695 [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control 696 Protocol Extended Reports (RTCP XR)", RFC 3611, November 697 2003. 699 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 700 Protocol (RTCP) Extensions for Single-Source Multicast 701 Sessions with Unicast Feedback", RFC 5760, February 2010. 703 [RFC6284] Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping 704 between Unicast and Multicast RTP Sessions", RFC 6284, 705 June 2011. 707 [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., 708 and K. Carlberg, "Explicit Congestion Notification (ECN) 709 for RTP over UDP", RFC 6679, August 2012. 711 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 712 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 713 RFC 3711, March 2004. 715 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 716 Description Protocol (SDP) Security Descriptions for Media 717 Streams", RFC 4568, July 2006. 719 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 720 Control Packets on a Single Port", RFC 5761, April 2010. 722 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 723 Real-Time Transport Control Protocol (RTCP): Opportunities 724 and Consequences", RFC 5506, April 2009. 726 [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer 727 Security (DTLS) Extension to Establish Keys for the Secure 728 Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. 730 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 731 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 732 July 2006. 734 Authors' Addresses 736 Lorenzo Miniero 737 Meetecho 739 Email: lorenzo@meetecho.com 741 Sergio Garcia Murillo 742 Medooze 744 Email: sergio.garcia.murillo@gmail.com 746 Victor Pascual 747 Quobis 749 Email: victor.pascual@quobis.com