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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: draft-ietf-avtcore-rtp-security-options has been published as RFC 7201 == Outdated reference: draft-ietf-mmusic-rfc2326bis has been published as RFC 7826 == Outdated reference: draft-ietf-rtcweb-security-arch has been published as RFC 8827 -- Obsolete informational reference (is this intentional?): RFC 4614 (Obsoleted by RFC 7414) Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C.S. Perkins 3 Internet-Draft University of Glasgow 4 Intended status: Informational M. Westerlund 5 Expires: April 18, 2014 Ericsson 6 October 15, 2013 8 Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single 9 Media Security Solution 10 draft-ietf-avt-srtp-not-mandatory-14.txt 12 Abstract 14 This memo discusses the problem of securing real-time multimedia 15 sessions, and explains why the Real-time Transport Protocol (RTP), 16 and the associated RTP Control Protocol (RTCP), do not mandate a 17 single media security mechanism. Guidelines for designers and 18 reviewers of future RTP extensions are provided, to ensure that 19 appropriate security mechanisms are mandated, and that any such 20 mechanisms are specified in a manner that conforms with the RTP 21 architecture. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on April 18, 2014. 40 Copyright Notice 42 Copyright (c) 2013 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 2. RTP Applications and Deployment Scenarios . . . . . . . . . . 3 59 3. RTP Media Security . . . . . . . . . . . . . . . . . . . . . 4 60 4. RTP Session Establishment and Key Management . . . . . . . . 4 61 5. On the Requirement for Strong Security in Framework protocols 5 62 6. Guidelines for Securing the RTP Protocol Framework . . . . . 6 63 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 65 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 66 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 67 11. Informative References . . . . . . . . . . . . . . . . . . . 8 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 70 1. Introduction 72 The Real-time Transport Protocol (RTP) [RFC3550] is widely used for 73 voice over IP, Internet television, video conferencing, and other 74 real-time and streaming media applications. Despite this use, the 75 basic RTP specification provides only limited options for media 76 security, and defines no standard key exchange mechanism. Rather, a 77 number of extensions are defined that can provide confidentiality and 78 authentication of RTP media streams and RTP Control Protocol (RTCP) 79 messages. Other mechanisms define key exchange protocols. This memo 80 outlines why it is appropriate that multiple extension mechanisms are 81 defined rather than mandating a single security and keying mechanism 82 for all users of RTP. 84 The IETF policy on Strong Security Requirements for IETF Standard 85 Protocols [RFC3365] (the so-called "Danvers Doctrine") states that 86 "we MUST implement strong security in all protocols to provide for 87 the all too frequent day when the protocol comes into widespread use 88 in the global Internet". The security mechanisms defined for use 89 with RTP allow these requirements to be met. However, since RTP is a 90 protocol framework that is suitable for a wide variety of use cases, 91 there is no single security mechanism that is suitable for every 92 scenario. This memo outlines why this is the case, and discusses how 93 users of RTP can meet the requirement for strong security. 95 This memo provides information for the community and for reviewers of 96 future RTP-related work in the IETF. It does not specify a standard 97 of any kind. 99 2. RTP Applications and Deployment Scenarios 101 The range of application and deployment scenarios where RTP has been 102 used includes, but is not limited to, the following: 104 o Point-to-point voice telephony; 106 o Point-to-point video conferencing and telepresence; 108 o Centralised group video conferencing and telepresence, using a 109 Multipoint Conference Unit (MCU) or similar central middlebox; 111 o Any Source Multicast (ASM) video conferencing using the light- 112 weight sessions model (e.g., the Mbone conferencing tools); 114 o Point-to-point streaming audio and/or video (e.g., on-demand TV or 115 movie streaming); 117 o Source-Specific Multicast (SSM) streaming to large receiver groups 118 (e.g., IPTV streaming by residential ISPs, or the 3GPP Multimedia 119 Broadcast Multicast Service [MBMS]); 121 o Replicated unicast streaming to a group of receivers; 123 o Interconnecting components in music production studios and video 124 editing suites; 126 o Interconnecting components of distributed simulation systems; and 128 o Streaming real-time sensor data (e.g., e-VLBI radio astronomy). 130 As can be seen, these scenarios vary from point-to-point sessions to 131 very large multicast groups, from interactive to non-interactive, and 132 from low bandwidth (kilobits per second) telephony to high bandwidth 133 (multiple gigabits per second) video and data streaming. While most 134 of these applications run over UDP [RFC0768], some use TCP [RFC0793], 135 [RFC4614] or DCCP [RFC4340] as their underlying transport. Some run 136 on highly reliable optical networks, others use low rate unreliable 137 wireless networks. Some applications of RTP operate entirely within 138 a single trust domain, others run inter-domain, with untrusted (and, 139 in some cases, potentially unknown) users. The range of scenarios is 140 wide, and growing both in number and in heterogeneity. 142 3. RTP Media Security 144 The wide range of application scenarios where RTP is used has led to 145 the development of multiple solutions for securing RTP media streams 146 and RTCP control messages, considering different requirements. 148 Perhaps the most widely applicable of these security options is the 149 Secure RTP (SRTP) framework [RFC3711]. This is an application-level 150 media security solution, encrypting the media payload data (but not 151 the RTP headers) to provide confidentiality, and supporting source 152 origin authentication as an option. SRTP was carefully designed to 153 be low overhead, including operating on links subject to RTP header 154 compression, and to support the group communication and third-party 155 performance monitoring features of RTP, across a range of networks. 157 SRTP is not the only media security solution for RTP, however, and 158 alternatives can be more appropriate in some scenarios, perhaps due 159 to ease of integration with other parts of the complete system. In 160 addition, SRTP does not address all possible security requirements, 161 and other solutions are needed in cases where SRTP is not suitable. 162 For example, ISMAcryp payload-level confidentiality [ISMACrypt2] is 163 appropriate for some types of streaming video application, but is not 164 suitable for voice telephony, and uses features that are not provided 165 by SRTP. 167 The range of available RTP security options, and their applicability 168 to different scenarios, is outlined in 169 [I-D.ietf-avtcore-rtp-security-options]. At the time of this 170 writing, there is no media security protocol that is appropriate for 171 all the environments where RTP is used. Multiple RTP media security 172 protocols are expected to remain in wide use for the foreseeable 173 future. 175 4. RTP Session Establishment and Key Management 177 A range of different protocols for RTP session establishment and key 178 exchange exist, matching the diverse range of use cases for the RTP 179 framework. These mechanisms can be split into two categories: those 180 that operate in-band on the media path, and those that are out-of- 181 band and operate as part of the session establishment signalling 182 channel. The requirements for these two classes of solution are 183 different, and a wide range of solutions have been developed in this 184 space. 186 A more detailed survey of requirements for media security management 187 protocols can be found in [RFC5479]. As can be seen from that memo, 188 the range of use cases is wide, and there is no single key management 189 protocol that is appropriate for all scenarios. The solutions have 190 been further diversified by the existence of infrastructure elements, 191 such as authentication systems, that are tied to the key management. 192 The most important and widely used keying options for RTP sessions at 193 the time of this writing are described in 194 [I-D.ietf-avtcore-rtp-security-options]. 196 5. On the Requirement for Strong Security in Framework protocols 198 The IETF requires that all protocols provide a strong, mandatory to 199 implement, security solution [RFC3365]. This is essential for the 200 overall security of the Internet, to ensure that all implementations 201 of a protocol can interoperate in a secure way. Framework protocols 202 offer a challenge for this mandate, however, since they are designed 203 to be used by different classes of applications, in a wide range of 204 different environments. The different use cases for the framework 205 have different security requirements, and implementations designed 206 for different environments are generally not expected to interwork. 208 RTP is an example of a framework protocol with wide applicability. 209 The wide range of scenarios described in Section 2 show the issues 210 that arise in mandating a single security mechanism for this type of 211 framework. It would be desirable if a single media security 212 solution, and a single key management solution, could be developed, 213 suitable for applications across this range of use scenarios. The 214 authors are not aware of any such solution, however, and believe it 215 is unlikely that any such solution will be developed. In part, this 216 is because applications in the different domains are not intended to 217 interwork, so there is no incentive to develop a single mechanism. 218 More importantly, though, the security requirements for the different 219 usage scenarios vary widely, and an appropriate security mechanism in 220 one scenario simply does not work for some other scenarios. 222 For a framework protocol, it appears that the only sensible solution 223 to the strong security requirement of [RFC3365] is to develop and use 224 building blocks for the basic security services of confidentiality, 225 integrity protection, authorisation, authentication, and so on. When 226 new uses for the framework protocol arise, they need to be studied to 227 determine if the existing security building blocks can satisfy the 228 requirements, or if new building blocks need to be developed. A 229 mandatory to implement set of security building blocks can then be 230 specified for that usage scenario of the framework. 232 Therefore, when considering the strong and mandatory to implement 233 security mechanism for a specific class of applications, one has to 234 consider what security building blocks need to be supported. To 235 maximize interoperability it is important that common media security 236 and key management mechanisms are defined for classes of application 237 with similar requirements. The IETF needs to participate in this 238 selection of security building blocks for each class of applications 239 that use the protocol framework and are expected to interoperate, in 240 cases where the IETF has the appropriate knowledge of the class of 241 applications. 243 6. Guidelines for Securing the RTP Protocol Framework 245 The IETF requires that protocols specify mandatory to implement (MTI) 246 strong security [RFC3365]. This applies to the specification of each 247 interoperable class of application that makes use of RTP. However, 248 RTP is a framework protocol, so the arguments made in Section 5 also 249 apply. Given the variability of the classes of application that use 250 RTP, and the variety of the currently available security mechanisms 251 described in [I-D.ietf-avtcore-rtp-security-options], no one set of 252 MTI security options can realistically be specified that apply to all 253 classes of RTP applications. 255 Documents that define an interoperable class of applications using 256 RTP are subject to [RFC3365], and so need to specify MTI security 257 mechanisms. This is because such specifications do fully specify 258 interoperable applications that use RTP. Examples of such documents 259 under development in the IETF at the time of this writing are the 260 RTCWEB Security Architecture [I-D.ietf-rtcweb-security-arch] and the 261 Real Time Streaming Protocol 2.0 (RTSP) [I-D.ietf-mmusic-rfc2326bis]. 262 It is also expected that a similar document will be produced for 263 voice-over-IP applications using SIP and RTP. 265 The RTP framework includes several extension points. Some extensions 266 can significantly change the behaviour of the protocol, to the extent 267 that applications using the extension form a separate interoperable 268 class of applications to those that have not been extended. Other 269 extension points are defined in such a manner that they can be used 270 (largely) independently of the class of applications using RTP. Two 271 important extension points that are independent of the class of 272 applications are RTP Payload Formats and RTP Profiles. 274 An RTP Payload Format defines how the output of a media codec can be 275 used with RTP. At the time of this writing, there are over 70 RTP 276 Payload Formats defined in published RFCs, with more in development. 277 It is appropriate for an RTP Payload Format to discuss the specific 278 security implications of using that media codec with RTP. However, 279 an RTP Payload Format does not specify an interoperable class of 280 applications that use RTP since, in the vast majority of cases, a 281 media codec and its associated RTP Payload Format can be used with 282 many different classes of application. As such, an RTP Payload 283 Format is neither secure in itself, nor something to which [RFC3365] 284 applies. Future RTP Payload Format specifications need to explicitly 285 state this, and include a reference to this memo for explanation. It 286 is not appropriate for an RTP Payload Format to mandate the use of 287 SRTP [RFC3711], or any other security building blocks, since that RTP 288 Payload Format might be used by different classes of application that 289 use RTP, and that have different security requirements. 291 RTP Profiles are larger extensions that adapt the RTP framework for 292 use with particular classes of application. In some cases, those 293 classes of application might share common security requirements so 294 that it could make sense for an RTP Profile to mandate particular 295 security options and building blocks (the RTP/SAVP profile [RFC3711] 296 is an example of this type of RTP Profile). In other cases, though, 297 an RTP profile is applicable to such a wide range of applications 298 that it would not make sense for that profile to mandate particular 299 security building blocks be used (the RTP/AVPF profile [RFC4585] is 300 an example of this type of RTP Profile, since it provides building 301 blocks that can be used in different styles of application). A new 302 RTP Profile specification needs to discuss whether, or not, it makes 303 sense to mandate particular security building blocks that need to be 304 used with all implementations of that profile; however, there is no 305 expectation that all RTP Profiles will mandate particular security 306 solutions. RTP Profiles that do not specify an interoperable usage 307 for a particular class of RTP applications are neither secure in 308 themselves, nor something to which [RFC3365] applies; any future RTP 309 Profiles in this category need to explicitly state this with 310 justification, and include a reference to this memo. 312 7. Conclusions 314 The RTP framework is used in a wide range of different scenarios, 315 with no common security requirements. Accordingly, neither SRTP 316 [RFC3711], nor any other single media security solution or keying 317 mechanism, can be mandated for all uses of RTP. In the absence of a 318 single common security solution, it is important to consider what 319 mechanisms can be used to provide strong and interoperable security 320 for each different scenario where RTP applications are used. This 321 will require analysis of each class of application to determine the 322 security requirements for the scenarios in which they are to be used, 323 followed by the selection of a mandatory to implement security 324 building blocks for that class of application, including the desired 325 RTP traffic protection and key-management. A non-exhaustive list of 326 the RTP security options available at the time of this writing is 327 outlined in [I-D.ietf-avtcore-rtp-security-options]. It is expected 328 that each class of application will be supported by a memo describing 329 what security options are mandatory to implement for that usage 330 scenario. 332 8. Security Considerations 333 This entire memo is about security. 335 9. IANA Considerations 337 None. 339 10. Acknowledgements 341 Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes, 342 Martin Ellis, Ali Begen, Keith Drage, Ray van Brandenburg, Stephen 343 Farrell, Sean Turner, and John Mattsson for their feedback. 345 11. Informative References 347 [I-D.ietf-avtcore-rtp-security-options] 348 Westerlund, M. and C. Perkins, "Options for Securing RTP 349 Sessions", draft-ietf-avtcore-rtp-security-options-07 350 (work in progress), October 2013. 352 [I-D.ietf-mmusic-rfc2326bis] 353 Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 354 and M. Stiemerling, "Real Time Streaming Protocol 2.0 355 (RTSP)", draft-ietf-mmusic-rfc2326bis-38 (work in 356 progress), October 2013. 358 [I-D.ietf-rtcweb-security-arch] 359 Rescorla, E., "WebRTC Security Architecture", draft-ietf- 360 rtcweb-security-arch-07 (work in progress), July 2013. 362 [ISMACrypt2] 363 Internet Streaming Media Alliance (ISMA), , "ISMA 364 Encryption and Authentication, Version 2.0 release 365 version", November 2007. 367 [MBMS] 3GPP, , "Multimedia Broadcast/Multicast Service (MBMS); 368 Protocols and codecs TS 26.346", . 370 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 371 August 1980. 373 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 374 793, September 1981. 376 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 377 Engineering Task Force Standard Protocols", BCP 61, RFC 378 3365, August 2002. 380 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 381 Jacobson, "RTP: A Transport Protocol for Real-Time 382 Applications", STD 64, RFC 3550, July 2003. 384 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 385 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 386 RFC 3711, March 2004. 388 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram 389 Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 391 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 392 "Extended RTP Profile for Real-time Transport Control 393 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 394 2006. 396 [RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap 397 for Transmission Control Protocol (TCP) Specification 398 Documents", RFC 4614, September 2006. 400 [RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet, 401 "Requirements and Analysis of Media Security Management 402 Protocols", RFC 5479, April 2009. 404 Authors' Addresses 406 Colin Perkins 407 University of Glasgow 408 School of Computing Science 409 Glasgow G12 8QQ 410 UK 412 Email: csp@csperkins.org 413 URI: http://csperkins.org/ 415 Magnus Westerlund 416 Ericsson 417 Farogatan 6 418 Kista SE-164 80 419 Sweden 421 Email: magnus.westerlund@ericsson.com