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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(Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 5117 (Obsoleted by RFC 7667) == Outdated reference: A later version (-10) exists of draft-ietf-avtcore-rtp-topologies-update-07 == Outdated reference: A later version (-08) exists of draft-ietf-avtext-rtp-grouping-taxonomy-06 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 2 comments (--).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: October 18, 2015 Medooze 6 V. Pascual 7 Quobis 8 April 16, 2015 10 Guidelines to support RTCP end-to-end in Back-to-Back User Agents 11 (B2BUAs) 12 draft-ietf-straw-b2bua-rtcp-06 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 October 18, 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. In case the SSRC in any of the 319 chunks is changed, the related CNAME item SHOULD be updated as 320 well in order to avoid potential CNAME collisions, especially if 321 the RFC 3550 CNAME algorithm is used. 323 BYE: [RFC3550] 324 If the B2BUA has changed any SSRC in any direction, it MUST update 325 the SSRC in the BYE message. 327 APP: [RFC3550] 328 If the B2BUA has changed any SSRC in any direction, it MUST update 329 the SSRC in the APP message. Should the B2BUA be aware of any 330 specific APP message format that contains additional information 331 related to SSRCs, it SHOULD update them as well. 333 Extended Reports (XR): [RFC3611] 334 If the B2BUA has changed any SSRC in any direction, it MUST update 335 the SSRC-related information in the incoming XR message header 336 before forwarding it. This includes the source SSRC, which MUST 337 be rewritten with the one the B2BUA uses to send RTP packets to 338 each sender participant, and the SSRC information in all the block 339 types that include it, which MUST be rewritten using the related 340 sender participant(s) SSRC. If the B2BUA has also changed the 341 base RTP sequence number when forwarding RTP packets, then this 342 change needs to be properly addressed in the 'begin_seq' and 343 'end_seq' fields that are available in most of the Report Block 344 types that are part of the XR specification. 346 Receiver Summary Information (RSI): [RFC5760] 347 If the B2BUA has changed any SSRC in any direction, it MUST update 348 the SSRC-related information in the incoming RSI message header 349 before forwarding it. This includes the distribution source SSRC, 350 which MUST be rewritten with the one the B2BUA uses to send RTP 351 packets to each sender participant, the summarized SSRC and, in 352 case a Collision Sub-Report Block is available, the SSRCs in the 353 related list. 355 Port Mapping (TOKEN): [RFC6284] 356 If the B2BUA has changed any SSRC in any direction, it MUST update 357 the SSRC-related information in the incoming TOKEN message before 358 forwarding it. This includes the Packet Sender SSRC, which MUST 359 be rewritten with the one the B2BUA uses to send RTP packets to 360 each sender participant, and the Requesting Client SSRC in case 361 the message is a response, which MUST be rewritten using the 362 related sender participant(s) SSRC. 364 Feedback messages: [RFC4585] 365 All Feedback messages have a common packet format, which includes 366 the SSRC of the packet sender and the one of the media source the 367 feedack is related to. Just as described for the previous 368 messages, these SSRC identifiers MUST be updated if the B2BUA has 369 changed any SSRC in any direction. It MUST NOT, though, change a 370 media source SSRC that was originally set to zero, unless zero is 371 actually the SSRC that was chosen by one of the involved 372 endpoints, in which case the above mentioned rules as to SSRC 373 rewriting apply. Besides, considering that many feedback messages 374 also include additional data as part of their specific Feedback 375 Control Information (FCI), a media-aware B2BUA MUST take care of 376 them accordingly, if it can parse and regenerate them, according 377 to the following guidelines. 379 NACK: [RFC4585] 380 Besides the common packet format management for feedback messages, 381 a media-aware B2BUA MUST also properly rewrite the Packet ID (PID) 382 of all addressed lost packets in the NACK FCI if it changed the 383 RTP sequence numbers before forwarding a packet. 385 TMMBR/TMMBN/FIR/TSTR/TSTN/VBCM: [RFC5104] 386 Besides the common packet format management for feedback messages, 387 a media-aware B2BUA MUST also properly rewrite the additional SSRC 388 identifier all those messages envisage as part of their specific 389 FCI if it changed the related RTP SSRC of the media sender. 391 REMB: [I-D.alvestrand-rmcat-remb] 392 Besides the common packet format management for feedback messages, 393 a media-aware B2BUA MUST also properly rewrite the additional SSRC 394 identifier(s) REMB packets envisage as part of their specific FCI 395 if it changed the related RTP SSRC of the media sender. 397 Explicit Congestion Notification (ECN): [RFC6679] 398 Besides the common packet format management for feedback messages, 399 the same guidelines given for SR/RR management apply as well, 400 considering the presence of sequence numbers in the ECN Feedback 401 Report format. For what concerns the management of RTCP XR ECN 402 Summary Report messages, the same guidelines given for generic XR 403 messages apply. 405 Apart from the generic guidelines related to Feedback messages, no 406 additional modifications are needed for PLI, SLI and RPSI feedback 407 messages instead. 409 Of course, the same considerations about the need for SDP and RTP/ 410 RTCP information to be coherent also applies to media-aware B2BUAs. 411 This means that, if a B2BUA is going to change any SSRC, it SHOULD 412 update the related 'ssrc' attributes if they were present in the 413 original description before sending it to the recipient, just as it 414 MUST rewrite the 'rtcp' attribute if provided. At the same time, the 415 ability for a media-aware B2BUA to inspect/modify RTCP packets may 416 also mean such a B2BUA may choose to drop RTCP packets it can't 417 parse: in that case, a media-aware B2BUA MUST also advertize its RTCP 418 level of support in the SDP in a coherent way, in order to prevent, 419 for instance, a UAC to make use of NACK messages that would never 420 reach the intended recipients. It's important to point out that, in 421 case any RTCP packet needs to be dropped, then only the offending 422 RTCP packet needs to be dropped, and not the whole compound RTCP 423 packet it may belong to. 425 A different set of considerations, instead, is worthwhile for what 426 concerns RTP/RTCP multiplexing [RFC5761] and Reduced-Size RTCP 428 [RFC5506]. While the former allows for a better management of 429 network resources by multiplexing RTP packets and RTCP messages over 430 the same transport, the latter allows for a compression of RTCP 431 messages, thus leading to less network traffic. For what concerns 432 RTP/RTCP multiplexing, a B2BUA acting as a Media Relay can use it on 433 either RTP session independently: this means that, for instance, a 434 Media Relay B2BUA may use RTP/RTCP multiplexing on one side of the 435 communication, and not use it on the other side, if it's not 436 supported. This allows for a better management of network resources 437 on the side that does support it. In case any of the parties in the 438 communications supports it and the B2BUA does too, the related 'rtcp- 439 mux' SDP attribute MUST be forwarded on the other side(s); if the 440 B2BUA detects that any of the parties in the communication does not 441 support the feature, it may decide to either disable it entirely or 442 still advertize it for the RTP sessions with parties that do support 443 it. In case the B2BUA decides to involve RTP/RTCP multiplexing, it 444 MUST ensure that there are no conflicting RTP payload type numbers on 445 both sides, and in case there are, it MUST rewrite RTP payload type 446 numbers to ensure no conflict in the domain where the RTP/RTCP 447 multiplexing is applied. Should RTP payload types be rewritten, the 448 related information in the SDP MUST be updated accordingly. 450 For what concerns Reduced-Size RTCP, instead, the considerations are 451 a bit different. In fact, while a Media Relay B2BUA may choose to 452 use it on the side that supports it and not on the side that doesn't, 453 there are other aspects to take into account before doing so. While 454 Reduced-Size allows indeed for less network traffic related to RTCP 455 messaging in general, this gain may lead a Reduced-Size RTCP 456 implementation to also issue a higher rate of RTCP feedback messages. 457 This would result in an increased RTCP traffic on the side that does 458 not support Reduced-Size, and could as a consequence be actually 459 counterproductive if the bandwidth is different on each side. That 460 said, the B2BUA can choose whether or not to advertize support for 461 Reduced-Size RTCP on either side by means of the 'rtcp-rsize' SDP 462 attribute. Should a B2BUA decide to allow the sides to independently 463 use or not Reduced-Size, then the B2BUA MUST advertize support for 464 the feature on the sides that support it, and MUST NOT advertize it 465 on the sides that don't, by removing the related attribute from the 466 SDP before forwarding it. Should the B2BUA decide to disable the 467 feature on all sides, instead, it MUST NOT advertize support for the 468 Reduced-Size RTCP functionality on any side, by removing the 'rtcp- 469 rsize' attribute from the SDP. 471 3.3. Media Terminator 473 A Media Terminator B2BUA, unlike simple relays and media-aware ones, 474 is also able to terminate media itself, that is taking care of RTP 475 payloads as well and not only headers. This means that such 476 components, for instance, can act as media transcoders and/or 477 originate specific RTP media. Using the RTP Topologies terminology, 478 this can be seen as a RTP Media Translator. Such a topology can also 479 be seen as a Back-to-back RTP sessions through a Middlebox, as 480 described in Section 3.2.2 of 481 [I-D.ietf-avtcore-rtp-topologies-update]. Such a capability makes 482 them quite different from the previously introduced B2BUA typologies, 483 as this means they are going to terminate RTCP as well: in fact, 484 since the media is terminated by themselves, the related statistics 485 and feedback functionality can be taken care directly by the B2BUA, 486 and does not need to be relayed to the other participants in the 487 multimedia session. 489 For this reason, no specific guideline is needed to ensure a proper 490 end-to-end RTCP behaviour in such scenarios, mostly because most of 491 the times there would be no end-to-end RTCP interaction among the 492 involved participants at all, as the B2BUA would terminate them all 493 and take care of them accordingly. Nevertheless, should any RTCP 494 packet actually need to be forwarded to another participant in the 495 multimedia session, the same guidelines provided for the media-aware 496 B2BUA case apply. 498 For what concerns RTP/RTCP multiplexing support, the same 499 considerations already given for the Media Relay management basically 500 apply for a Media Terminator as well. Some different considerations 501 might be given as to the Reduced-Size RTCP functionality, instead: in 502 fact, in the Media Terminator case it is safe to use the feature 503 independently on each leg. In that case, the same considerations 504 about advertizing the support, or lack of support, of the feature on 505 either side as described for the Media Relay case apply here as well. 507 4. Media Path Security 509 The discussion made in the previous sections on the management of 510 RTCP messages by a B2BUA has so far mostly worked under the 511 assumption that the B2BUA has actually access to the RTP/RTCP 512 information itself. This is indeed true if we assume that plain RTP 513 and RTCP is being handled, but this may not be true once any security 514 is enforced on RTP packets and RTCP messages by means of SRTP 515 [RFC3711], whether the keying is done using Secure Descriptions 516 [RFC4568] or DTLS-SRTP [RFC5764]. 518 While typically not an issue in the Media Relay case, where RTP and 519 RTCP packets are forwarded without any modification no matter whether 520 security is involved or not, this could definitely have an impact on 521 Media-aware Relays and Media Terminator B2BUAs. To make a simple 522 example, if we think of a SRTP/SRTCP session across a B2BUA where the 523 B2BUA itself has no access to the keys used to secure the session, 524 there would be no way to manipulate SRTP headers without violating 525 the hashing on the packet; at the same time, there would be no way to 526 rewrite the RTCP information accordingly either, as most of the 527 packet (especially when RTCP compound packets are involved) would be 528 encrypted. 530 For this reason, it is important to point out that the operations 531 described in the previous sections are only possible if the B2BUA has 532 a way to effectively manipulate the packets and messages flowing by. 533 This means that, in case media security is involved, only the Media- 534 unaware Relay scenario can be properly addressed. Attempting to 535 cover Media-aware Relay and Media Terminarion scenarios when 536 involving secure sessions will inevitably lead to the B2BUA acting as 537 a man-in-the-middle, and as such its behaviour is unspecified and 538 discouraged. 540 5. IANA Considerations 542 This document makes no request of IANA. 544 6. Security Considerations 546 This document, being a summary and vest common practice overview that 547 covers different standards, does not introduce any additional 548 consideration to those described by the aforementioned standard 549 documents themselves. 551 It is worth pointing out, though, that there are scenarios where an 552 improper management of RTCP messaging across a B2BUA may lead, 553 willingly or not, to situations not unlike an attack. To make a 554 simple example, an improper management of a REMB feedback message 555 containing, e.g., information on the limited bandwidth availability 556 for a user, may lead to missing information to its peer, who may end 557 up increasing the encoder bitrate up to a point where the user with 558 poor connectivity will inevitably be choked by an amount of data it 559 cannot process. This scenario may as such result in what looks like 560 a Denial of Service (DOS) attack towards the user. 562 7. Change Summary 564 Note to RFC Editor: Please remove this whole section. 566 The following are the major changes between the 05 and the 06 567 versions of the draft: 569 o Addressed comment by Colin Perkins on the management of CNAME 570 items in SDES. 572 The following are the major changes between the 04 and the 05 573 versions of the draft: 575 o Clarified behaviour when SSRC is zero. 577 o Fixed a couple of nits found by the Idnits tool. 579 The following are the major changes between the 03 and the 04 580 versions of the draft: 582 o Addressed review by Magnus Westerlund. 584 o Added guidelines for ECN RTCP messages. 586 o Clarified that if an RTCP packet is dropped because unsupported, 587 only the unsupported packet is dropped and not the compound packet 588 that contains it. 590 o Added reference to Section 3.2.2 of 591 [I-D.ietf-avtcore-rtp-topologies-update] to Section 3.3. 593 o Added considerations on RTP/RTCP multiplexing and Reduced-Size 594 RTCP. 596 The following are the major changes between the 02 and the 03 597 versions of the draft: 599 o Rephrased the Media Path Security section to take into account the 600 MITM-related discussion in Honolulu. 602 o Added some Security Considerations. 604 The following are the major changes between the 01 and the 02 605 versions of the draft: 607 o Updated terminology to better adhere to 608 [I-D.ietf-avtext-rtp-grouping-taxonomy]. 610 o Rephrased the Media Path Security section to take into account the 611 MITM-related discussion in Toronto. 613 o Clarified that NACK management might be trickier when SRTP is 614 involved. 616 The following are the major changes between the 00 and the 01 617 versions of the draft: 619 o Updated references and mapping per taxonomy RFC (7092). 621 o Added a reference to RTP topologies, and tried a mapping as per- 622 discussion in London. 624 o Added more RTCP packet types to the Media-Aware section. 626 o Clarified that fixing the 'rtcp' SDP attribute is important. 628 o Added a new section on the impact of media security. 630 8. Acknowledgements 632 The authors would like to thank Flavio Battimo and Pierluigi Palma 633 for their invaluable feedback in the early stages of the document. 634 The authors would also like to thank Colin Perkins, Bernard Aboba, 635 Albrecht Schwarz, Hadriel Kaplan, Keith Drage, Jonathan Lennox, 636 Stephen Farrell and Magnus Westerlund for their constructive 637 comments, suggestions, and reviews that were critical to the 638 formulation and refinement of this document. 640 9. References 642 9.1. Normative References 644 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 645 Requirement Levels", BCP 14, RFC 2119, March 1997. 647 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 648 A., Peterson, J., Sparks, R., Handley, M., and E. 649 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 650 June 2002. 652 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 653 Description Protocol", RFC 4566, July 2006. 655 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 656 with Session Description Protocol (SDP)", RFC 3264, June 657 2002. 659 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 660 Jacobson, "RTP: A Transport Protocol for Real-Time 661 Applications", STD 64, RFC 3550, July 2003. 663 9.2. Informative References 665 [RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session 666 Initiation Protocol (SIP) Back-to-Back User Agents", RFC 667 7092, December 2013. 669 [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, 670 January 2008. 672 [I-D.ietf-avtcore-rtp-topologies-update] 673 Westerlund, M. and S. Wenger, "RTP Topologies", draft- 674 ietf-avtcore-rtp-topologies-update-07 (work in progress), 675 April 2015. 677 [I-D.ietf-avtext-rtp-grouping-taxonomy] 678 Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro, 679 "A Taxonomy of Grouping Semantics and Mechanisms for Real- 680 Time Transport Protocol (RTP) Sources", draft-ietf-avtext- 681 rtp-grouping-taxonomy-06 (work in progress), March 2015. 683 [I-D.alvestrand-rmcat-remb] 684 Alvestrand, H., "RTCP message for Receiver Estimated 685 Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in 686 progress), October 2013. 688 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 689 "Extended RTP Profile for Real-time Transport Control 690 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 691 2006. 693 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 694 "Codec Control Messages in the RTP Audio-Visual Profile 695 with Feedback (AVPF)", RFC 5104, February 2008. 697 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 698 Media Attributes in the Session Description Protocol 699 (SDP)", RFC 5576, June 2009. 701 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 702 in Session Description Protocol (SDP)", RFC 3605, October 703 2003. 705 [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control 706 Protocol Extended Reports (RTCP XR)", RFC 3611, November 707 2003. 709 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 710 Protocol (RTCP) Extensions for Single-Source Multicast 711 Sessions with Unicast Feedback", RFC 5760, February 2010. 713 [RFC6284] Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping 714 between Unicast and Multicast RTP Sessions", RFC 6284, 715 June 2011. 717 [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., 718 and K. Carlberg, "Explicit Congestion Notification (ECN) 719 for RTP over UDP", RFC 6679, August 2012. 721 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 722 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 723 RFC 3711, March 2004. 725 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 726 Description Protocol (SDP) Security Descriptions for Media 727 Streams", RFC 4568, July 2006. 729 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 730 Control Packets on a Single Port", RFC 5761, April 2010. 732 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 733 Real-Time Transport Control Protocol (RTCP): Opportunities 734 and Consequences", RFC 5506, April 2009. 736 [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer 737 Security (DTLS) Extension to Establish Keys for the Secure 738 Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. 740 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 741 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 742 July 2006. 744 Authors' Addresses 746 Lorenzo Miniero 747 Meetecho 749 Email: lorenzo@meetecho.com 751 Sergio Garcia Murillo 752 Medooze 754 Email: sergio.garcia.murillo@gmail.com 756 Victor Pascual 757 Quobis 759 Email: victor.pascual@quobis.com