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Perkins 3 Internet-Draft University of Glasgow 4 Intended status: Informational M. Westerlund 5 Expires: July 26, 2010 Ericsson 6 January 22, 2010 8 Why RTP Does Not Mandate a Single Security Mechanism 9 draft-ietf-avt-srtp-not-mandatory-05.txt 11 Abstract 13 This memo discusses the problem of securing real-time multimedia 14 sessions, and explains why the Real-time Transport Protocol (RTP), 15 and the associated RTP control protocol (RTCP), do not mandate a 16 single media security mechanism. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF), its areas, and its working groups. Note that 25 other groups may also distribute working documents as Internet- 26 Drafts. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 The list of current Internet-Drafts can be accessed at 34 http://www.ietf.org/ietf/1id-abstracts.txt. 36 The list of Internet-Draft Shadow Directories can be accessed at 37 http://www.ietf.org/shadow.html. 39 This Internet-Draft will expire on July 26, 2010. 41 Copyright Notice 43 Copyright (c) 2010 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. RTP Applications and Deployment Scenarios . . . . . . . . . . 3 60 3. Implications for RTP Security . . . . . . . . . . . . . . . . 4 61 4. Implications for Key Management . . . . . . . . . . . . . . . 5 62 5. On the Requirement for Strong Security in IETF protocols . . . 6 63 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 65 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 66 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 67 10. Informative References . . . . . . . . . . . . . . . . . . . . 8 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 70 1. Introduction 72 The Real-time Transport Protocol (RTP) [RFC3550] is widely used for 73 voice over IP, Internet television, video conferencing, and various 74 other real-time and streaming media applications. Despite this, the 75 base RTP specification provides very limited options for media 76 security, and defines no standard key exchange mechanism. Rather, a 77 number of extensions are defined to provide confidentiality and 78 authentication of RTP media streams and RTCP control messages, and to 79 exchange security keys. This memo outlines why it is appropriate 80 that multiple extension mechanisms are defined, rather than mandating 81 a single security and keying mechanism. 83 This memo provides information for the community; it does not specify 84 a standard of any kind. 86 The structure of this memo is as follows. Section 2 describes the 87 scenarios in which RTP is deployed. Following this, Section 3 88 outlines the implications of this range of scenarios for media 89 confidentially and authentication, and Section 4 outlines the 90 implications for key exchange. Section 5 outlines how the RTP 91 framework meets the requirement of BCP 61. Section 6 then concludes 92 and gives some recommendations. 94 2. RTP Applications and Deployment Scenarios 96 The range of application and deployment scenarios where RTP has been 97 used includes, but is not limited to, the following: 99 o Point-to-point voice telephony (fixed and wireless networks) 101 o Point-to-point video conferencing 103 o Centralised group video conferencing with a multipoint conference 104 unit (MCU) 106 o Any Source Multicast video conferencing (light-weight sessions; 107 Mbone conferencing) 109 o Point-to-point streaming audio and/or video 111 o Source-specific multicast (SSM) streaming to large group (IPTV and 112 3GPP Multimedia Broadcast Multicast Service (MBMS) [MBMS]) 114 o Replicated unicast streaming to a group 115 o Interconnecting components in music production studios and video 116 editing suites 118 o Interconnecting components of distributed simulation systems 120 o Streaming real-time sensor data 122 As can be seen, these scenarios vary from point-to-point to very 123 large multicast groups, from interactive to non-interactive, and from 124 low bandwidth (kilobits per second) to very high bandwidth (multiple 125 gigabits per second). While most of these applications run over UDP 126 [RFC0768], some use TCP [RFC0793], [RFC4614] or DCCP [RFC4340] as 127 their underlying transport. Some run on highly reliable optical 128 networks, others use low rate unreliable wireless networks. Some 129 applications of RTP operate entirely within a single trust domain, 130 others are inter-domain, with untrusted (and potentially unknown) 131 users. The range of scenarios is wide, and growing both in number 132 and in heterogeneity. 134 3. Implications for RTP Security 136 The wide range of application scenarios where RTP is used has led to 137 the development of multiple solutions for securing RTP media streams 138 and RTCP control messages, considering different requirements. 139 Perhaps the most widely applicable of these solutions is the Secure 140 RTP (SRTP) framework [RFC3711]. This is an application-level media 141 security solution, encrypting the media payload data (but not the RTP 142 headers) to provide some degree of confidentiality, and providing 143 optional source origin authentication. It was carefully designed to 144 be both low overhead, and to support the group communication features 145 of RTP, across a range of networks. 147 SRTP is not the only media security solution in use, however, and 148 alternatives are more appropriate for some scenarios. For example, 149 many client-server streaming media applications can run over a single 150 TCP connection, multiplexing media data with control information on 151 that connection (RTSP [I-D.ietf-mmusic-rfc2326bis] is a widely used 152 example of such a protocol). The natural way to provide media 153 security for such client-server media applications is to use TLS 154 [RFC5246] to protect the TCP connection, sending the RTP media data 155 over the TLS connection. Using the SRTP framework in addition to TLS 156 is unncessary, and would result in double encryption of the media, 157 and SRTP cannot be used instead of TLS since it is RTP-specific, and 158 so cannot protect the control traffic. 160 Other RTP use cases work over networks which provide security at the 161 network layer, using IPsec. For example, certain 3GPP networks need 162 IPsec security associations for other purposes, and can reuse those 163 to secure the RTP session [3GPP.33.210]. SRTP is, again, unnecessary 164 in such environments, and its use would only introduce overhead for 165 no gain. 167 For some applications it is sufficient to protect the RTP payload 168 data while leaving RTP, transport, and network layer headers 169 unprotected. An example of this is RTP broadcast over DVB-H 170 [ETSI.TS.102.474], where one mode of operation uses ISMAcryp 171 (http://www.isma.tv/specs/ISMA_E&Aspec2.0.pdf) to encrypt the RTP 172 payload data only. 174 Finally, the link layer may be secure, and it may be known that the 175 RTP media data is constrained to that single link (for example, when 176 operating in a studio environment, with physical link security). An 177 environment like this is inherently constrained, but might avoid the 178 need for application, transport, or network layer media security. 180 All these are application scenarios where RTP has seen commercial 181 deployment. Other use case also exist, with additional requirements. 182 There is no media security protocol that is appropriate for all these 183 environments. Accordingly, multiple RTP media security protocols can 184 be expected to remain in wide use. 186 4. Implications for Key Management 188 With such a diverse range of use cases come a range of different 189 protocols for RTP session establishment. Mechanisms used to provide 190 security keying for these different session establishment protocols 191 can basically be put into two categories: inband and out-of-band in 192 relation to the session establishment mechanism. The requirements 193 for these solutions are highly varying. Thus a wide range of 194 solutions have been developed in this space: 196 o The most common use case for RTP is probably point-to-point voice 197 calls or centralised group conferences, negotiated using SIP 198 [RFC3261] with the SDP offer/answer model [RFC3264], operating on 199 a trusted infrastructure. In such environments, SDP security 200 descriptions [RFC4568] or the MIKEY [RFC4567] protocol are 201 appropriate keying mechanisms, piggybacked onto the SDP [RFC4566] 202 exchange. The infrastructure may be secured by protecting the SIP 203 exchange using TLS or S/MIME, for example [RFC3261]. 205 o Point-to-point RTP sessions may be negotiated using SIP with the 206 offer/answer model, but operating over a network with untrusted 207 infrastructure. In such environments, the key management protocol 208 is run on the media path, bypassing the untrusted infrastructure. 210 Protocols such as DTLS [I-D.ietf-avt-dtls-srtp] or ZRTP 211 [I-D.zimmermann-avt-zrtp] are useful here. 213 o For point-to-point client-server streaming of RTP over RTSP, a TLS 214 association is appropriate to manage keying material, in much the 215 same manner as would be used to secure an HTTP session. 217 o A session description may be sent by email, secured using S/MIME 218 or PGP, or retrieved from a web page, using HTTP with TLS. 220 o A session description may be distributed to a multicast group 221 using SAP or FLUTE secured with S/MIME. 223 o A session description may be distributed using the Open Mobile 224 Alliance DRM key management specification [OMA-DRM] when using a 225 point-to-point streaming session setup with RTSP in the 3GPP PSS 226 environment [PSS]. 228 o In the 3GPP Multimedia Broadcast Multicast Service (MBMS) system, 229 HTTP and MIKEY are used for key management [MBMS-SEC]. 231 A more detailed survey of requirements for media security management 232 protocols can be found in [I-D.ietf-sip-media-security-requirements]. 233 As can be seen, the range of use cases is wide, and there is no 234 single protocol that is appropriate for all scenarios. These 235 solutions have been further diversified by the existence of 236 infrastructure elements such as authentication solutions that are 237 tied into the key manangement. 239 5. On the Requirement for Strong Security in IETF protocols 241 BCP 61 [RFC3365] puts a requirement on IETF protocols to provide 242 strong, mandatory to implement, security solutions. This is actually 243 quite a difficult requirement for any type of framework protocol, 244 like RTP, since one can never know all the deployment scenarios, and 245 if they are covered by the security solution. It would clearly be 246 desirable if a single media security solution and a single key 247 management solution could be developed, satisfying the range of use 248 cases for RTP. The authors are not aware of any such solution, 249 however, and it is not clear that any single solution can be 250 developed. 252 For a framework protocol it appears that the only sensible solution 253 to the requirement of BCP 61 is to develop or use security building 254 blocks, like SRTP, SDP security descriptions [RFC4568], MIKEY, DTLS, 255 or IPsec, to provide the basic security services of authorization, 256 data integrity protection and date confidentiality protection. When 257 new usages of the RTP framework arise, one needs to analyze the 258 situation, to determine if the existing building blocks satisfy the 259 requirements. If not, it is necessary to develop new security 260 building blocks. 262 When it comes to fulfilling the "MUST Implement" strong security for 263 a specific application, it will fall on that application to actually 264 consider what building blocks it is required to support. To maximize 265 interoperability it is desirable if certain applications, or classes 266 of application with similar requirements, agree on what data security 267 mechanisms and key-management should be used. If such agreement is 268 not possible, there will be increased cost, either in the lack of 269 interoperability, or in the need to implement more solutions. 270 Unfortunately this situation, if not handled reasonably well, can 271 result in a failure to satisfy the requirement of providing the users 272 with an option of turning on strong security when desired. 274 6. Conclusions 276 As discussed earlier it appears that a single solution can't be 277 designed to meet the diverse requirements. In the absence of such a 278 solution, it is hoped that this memo explains why SRTP is not 279 mandatory as the media security solution for RTP-based systems, and 280 why we can expect multiple key management solutions for systems using 281 RTP. 283 It is important for any RTP-based application to consider how it 284 meets the security requirements. This will require some analysis to 285 determine these requirements, followed by the selection of a 286 mandatory to implement solution, or in exceptional scenarios several 287 solutions, including the desired RTP traffic protection and key- 288 management. SRTP is a preferred solution for the protection of the 289 RTP traffic in those use cases where it is applicable. It is out of 290 scope for this memo to recommend a preferred key management solution. 292 7. Security Considerations 294 This entire memo is about security. 296 8. IANA Considerations 298 No IANA actions are required. 300 9. Acknowledgements 302 Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes, 303 Martin Ellis, and Ali Begen for their feedback. 305 10. Informative References 307 [3GPP.33.210] 308 3GPP, "IP network layer security", 3GPP TS 33.210, 309 September 2008. 311 [ETSI.TS.102.474] 312 ETSI, "Digital Video Broadcasting (DVB); IP Datacast over 313 DVB-H: Service Purchase and Protection", ETSI TS 102 474, 314 November 2007. 316 [I-D.ietf-avt-dtls-srtp] 317 McGrew, D. and E. Rescorla, "Datagram Transport Layer 318 Security (DTLS) Extension to Establish Keys for Secure 319 Real-time Transport Protocol (SRTP)", 320 draft-ietf-avt-dtls-srtp-07 (work in progress), 321 February 2009. 323 [I-D.ietf-mmusic-rfc2326bis] 324 Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 325 and M. Stiemerling, "Real Time Streaming Protocol 2.0 326 (RTSP)", draft-ietf-mmusic-rfc2326bis-22 (work in 327 progress), July 2009. 329 [I-D.ietf-sip-media-security-requirements] 330 Wing, D., Fries, S., Tschofenig, H., and F. Audet, 331 "Requirements and Analysis of Media Security Management 332 Protocols", draft-ietf-sip-media-security-requirements-09 333 (work in progress), January 2009. 335 [I-D.zimmermann-avt-zrtp] 336 Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media 337 Path Key Agreement for Secure RTP", 338 draft-zimmermann-avt-zrtp-17 (work in progress), 339 January 2010. 341 [MBMS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 342 Protocols and codecs TS 26.346". 344 [MBMS-SEC] 345 3GPP, "Security of Multimedia Broadcast/Multicast Service 346 (MBMS) TS 33.246". 348 [OMA-DRM] Open Mobile Alliance, "DRM Specification 2.0". 350 [PSS] 3GPP, "Transparent end-to-end Packet-switched Streaming 351 Service (PSS); Protocols and codecs TS 26.234". 353 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 354 August 1980. 356 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 357 RFC 793, September 1981. 359 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 360 A., Peterson, J., Sparks, R., Handley, M., and E. 361 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 362 June 2002. 364 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 365 with Session Description Protocol (SDP)", RFC 3264, 366 June 2002. 368 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 369 Engineering Task Force Standard Protocols", BCP 61, 370 RFC 3365, August 2002. 372 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 373 Jacobson, "RTP: A Transport Protocol for Real-Time 374 Applications", STD 64, RFC 3550, July 2003. 376 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 377 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 378 RFC 3711, March 2004. 380 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram 381 Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 383 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 384 Description Protocol", RFC 4566, July 2006. 386 [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. 387 Carrara, "Key Management Extensions for Session 388 Description Protocol (SDP) and Real Time Streaming 389 Protocol (RTSP)", RFC 4567, July 2006. 391 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 392 Description Protocol (SDP) Security Descriptions for Media 393 Streams", RFC 4568, July 2006. 395 [RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap 396 for Transmission Control Protocol (TCP) Specification 397 Documents", RFC 4614, September 2006. 399 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 400 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 402 Authors' Addresses 404 Colin Perkins 405 University of Glasgow 406 Department of Computing Science 407 Glasgow G12 8QQ 408 UK 410 Email: csp@csperkins.org 412 Magnus Westerlund 413 Ericsson 414 Farogatan 6 415 Kista SE-164 80 416 Sweden 418 Email: magnus.westerlund@ericsson.com