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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-40) exists of draft-ietf-mmusic-rfc2326bis-21 == Outdated reference: A later version (-22) exists of draft-zimmermann-avt-zrtp-15 -- Obsolete informational reference (is this intentional?): RFC 793 (Obsoleted by RFC 9293) -- Obsolete informational reference (is this intentional?): RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 4614 (Obsoleted by RFC 7414) -- Obsolete informational reference (is this intentional?): RFC 5246 (Obsoleted by RFC 8446) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Perkins 3 Internet-Draft University of Glasgow 4 Intended status: Informational M. Westerlund 5 Expires: June 25, 2010 Ericsson 6 December 22, 2009 8 Why RTP Does Not Mandate a Single Security Mechanism 9 draft-ietf-avt-srtp-not-mandatory-04.txt 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on June 25, 2010. 34 Copyright Notice 36 Copyright (c) 2009 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents in effect on the date of 41 publication of this document (http://trustee.ietf.org/license-info). 42 Please review these documents carefully, as they describe your rights 43 and restrictions with respect to this document. 45 Abstract 47 This memo discusses the problem of securing real-time multimedia 48 sessions, and explains why the Real-time Transport Protocol (RTP), 49 and the associated RTP control protocol (RTCP), do not mandate a 50 single media security mechanism. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. RTP Applications and Deployment Scenarios . . . . . . . . . . 3 56 3. Implications for RTP Security . . . . . . . . . . . . . . . . 4 57 4. Implications for Key Management . . . . . . . . . . . . . . . 5 58 5. On the Requirement for Strong Security in IETF protocols . . . 6 59 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7 60 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 61 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 62 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 63 10. Informative References . . . . . . . . . . . . . . . . . . . . 8 64 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 66 1. Introduction 68 The Real-time Transport Protocol (RTP) [RFC3550] is widely used for 69 voice over IP, Internet television, video conferencing, and various 70 other real-time and streaming media applications. Despite this, the 71 base RTP specification provides very limited options for media 72 security, and defines no standard key exchange mechanism. Rather, a 73 number of extensions are defined to provide confidentiality and 74 authentication of RTP media streams and RTCP control messages, and to 75 exchange security keys. This memo outlines why it is appropriate 76 that multiple extension mechanisms are defined, rather than mandating 77 a single security and keying mechanism. 79 This memo provides information for the community; it does not specify 80 a standard of any kind. 82 The structure of this memo is as follows: we begin, in Section 2 by 83 describing the scenarios in which RTP is deployed. Following this, 84 Section 3 outlines the implications of this range of scenarios for 85 media confidentially and authentication, and Section 4 outlines the 86 implications for key exchange. Section 5 outlines how the RTP 87 framework meets the requirement of BCP 61. Section 6 then concludes 88 and gives some recommendations. Finally, Section 7 outlines the 89 security considerations, and Section 8 outlines IANA considerations. 91 2. RTP Applications and Deployment Scenarios 93 The range of application and deployment scenarios where RTP has been 94 used includes, but is not limited to, the following: 96 o Point-to-point voice telephony (fixed and wireless networks) 98 o Point-to-point video conferencing 100 o Centralised group video conferencing with a multipoint conference 101 unit (MCU) 103 o Any Source Multicast video conferencing (light-weight sessions; 104 Mbone conferencing) 106 o Point-to-point streaming audio and/or video 108 o Source-specific multicast (SSM) streaming to large group (IPTV and 109 3GPP Multimedia Broadcast Multicast Service (MBMS) [MBMS]) 111 o Replicated unicast streaming to a group 112 o Interconnecting components in music production studios and video 113 editing suites 115 o Interconnecting components of distributed simulation systems 117 o Streaming real-time sensor data 119 As can be seen, these scenarios vary from point-to-point to very 120 large multicast groups, from interactive to non-interactive, and from 121 low bandwidth (kilobits per second) to very high bandwidth (multiple 122 gigabits per second). While most of these applications run over UDP 123 [RFC0768], some use TCP [RFC0793], [RFC4614] or DCCP [RFC4340] as 124 their underlying transport. Some run on highly reliable optical 125 networks, others use low rate unreliable wireless networks. Some 126 applications of RTP operate entirely within a single trust domain, 127 others are inter-domain, with untrusted (and potentially unknown) 128 users. The range of scenarios is wide, and growing both in number 129 and in heterogeneity. 131 3. Implications for RTP Security 133 The wide range of application scenarios where RTP is used has led to 134 the development of multiple solutions for securing RTP media streams 135 and RTCP control messages, considering different requirements. 136 Perhaps the most widely applicable of these solutions is the Secure 137 RTP (SRTP) framework [RFC3711]. This is an application-level media 138 security solution, encrypting the media payload data (but not the RTP 139 headers) to provide some degree of confidentiality, and providing 140 optional source origin authentication. It was carefully designed to 141 be both low overhead, and to support the group communication features 142 of RTP, across a range of networks. 144 SRTP is not the only media security solution in use, however, and 145 alternatives are more appropriate for some scenarios. For example, 146 many client-server streaming media applications can run over a single 147 TCP connection, multiplexing media data with control information on 148 that connection (RTSP [I-D.ietf-mmusic-rfc2326bis] is a widely used 149 example of such a protocol). The natural way to provide media 150 security for such client-server media applications is to use TLS 151 [RFC5246] to protect the TCP connection, sending the RTP media data 152 over the TLS connection. Using the SRTP framework in addition to TLS 153 is unncessary, and would result in double encryption of the media, 154 and SRTP cannot be used instead of TLS since it is RTP-specific, and 155 so cannot protect the control traffic. 157 Other RTP use cases work over networks which provide security at the 158 network layer, using IPsec. For example, certain 3GPP networks need 159 IPsec security associations for other purposes, and can reuse those 160 to secure the RTP session [3GPP.33.210]. SRTP is, again, unnecessary 161 in such environments, and its use would only introduce overhead for 162 no gain. 164 For some applications it is sufficient to protect the RTP payload 165 data while leaving RTP, transport, and network layer headers 166 unprotected. An example of this is RTP broadcast over DVB-H 167 [ETSI.TS.102.474], where one mode of operation uses ISMAcryp 168 (http://www.isma.tv/specs/ISMA_E&Aspec2.0.pdf) to encrypt the RTP 169 payload data only. 171 Finally, the link layer may be secure, and it may be known that the 172 RTP media data is constrained to that single link (for example, when 173 operating in a studio environment, with physical link security). An 174 environment like this is inherently constrained, but might avoid the 175 need for application, transport, or network layer media security. 177 All these are application scenarios where RTP has seen commerical 178 deployment. Other use case also exist, with additional requirements. 179 There is no media security protocol that is appropriate for all these 180 environments. Accordingly, multiple RTP media security protocols can 181 be expected to remain in wide use. 183 4. Implications for Key Management 185 With such a diverse range of use case come a range of different 186 protocols for RTP session establishment. Mechanisms used to provide 187 security keying for these different session establishment protocols 188 can basically be put into two categories: inband and out-of-band in 189 relation to the session establishment mechanism. The requirements 190 for these solutions are highly varying. Thus a wide range of 191 solutions have been developed in this space: 193 o The most common use case for RTP is probably point-to-point voice 194 calls or centralised group conferences, negotiated using SIP 195 [RFC3261] with the SDP offer/answer model [RFC3264], operating on 196 a trusted infrastructure. In such environments, SDP security 197 descriptions [RFC4568] or the MIKEY [RFC4567] protocol are 198 appropriate keying mechanisms, piggybacked onto the SDP [RFC4566] 199 exchange. The infrastructure may be secured by protecting the SIP 200 exchange using TLS or S/MIME, for example [RFC3261]. 202 o Point-to-point RTP sessions may be negotiated using SIP with the 203 offer/answer model, but operating over a network with untrusted 204 infrastructure. In such environments, the key management protocol 205 is run on the media path, bypassing the untrusted infrastructure. 207 Protocols such as DTLS [I-D.ietf-avt-dtls-srtp] or ZRTP 208 [I-D.zimmermann-avt-zrtp] are useful here. 210 o For point-to-point client-server streaming of RTP over RTSP, a TLS 211 association is appropriate to manage keying material, in much the 212 same manner as would be used to secure an HTTP session. 214 o A session description may be sent by email, secured using X.500 or 215 PGP, or retrieved from a web page, using HTTP with TLS. 217 o A session description may be distributed to a multicast group 218 using SAP or FLUTE secured with S/MIME. 220 o A session description may be distributed using the Open Mobile 221 Alliance DRM key management specification [OMA-DRM] when using a 222 point-to-point streaming session setup with RTSP in the 3GPP PSS 223 environment [PSS]. 225 o In the 3GPP Multimedia Broadcast Multicast Service (MBMS) system, 226 HTTP and MIKEY are used for key management [MBMS-SEC]. 228 A more detailed survey of requirements for media security management 229 protocols can be found in [I-D.ietf-sip-media-security-requirements]. 230 As can be seen, the range of use cases is wide, and there is no 231 single protocol that is appropriate for all scenarios. These 232 solutions have been further diversified by the existence of 233 infrastructure elements such as authentication solutions that are 234 tied into the key manangement. 236 5. On the Requirement for Strong Security in IETF protocols 238 BCP 61 [RFC3365] puts a requirement on IETF protocols to provide 239 strong, mandatory to implement, security solutions. This is actually 240 quite a difficult requirement for any type of framework protocol, 241 like RTP, since one can never know all the deployment scenarios, and 242 if they are covered by the security solution. It would clearly be 243 desirable if a single media security solution and a single key 244 management solution could be developed, satisfying the range of use 245 cases for RTP. The authors are not aware of any such solution, 246 however, and it is not clear that any single solution can be 247 developed. 249 For a framework protocol it appears that the only sensible solution 250 to the requirement of BCP 61 is to develop or use security building 251 blocks, like SRTP, SDP security descriptions [RFC4568], MIKEY, DTLS, 252 or IPsec, to provide the basic security services of authorization, 253 data integrity protection and date confidentiality protection. When 254 new usages of the RTP framework arise, one needs to analyze the 255 situation, to determine if the existing building blocks satisfy the 256 requirements. If not, it is necessary to develop new security 257 building blocks. 259 When it comes to fulfilling the "MUST Implement" strong security for 260 a specific application, it will fall on that application to actually 261 consider what building blocks it is required to support. To maximize 262 interoperability it is desirable if certain applications, or classes 263 of application with similar requirements, agree on what data security 264 mechanisms and key-management should be used. If such agreement is 265 not possible, there will be increased cost, either in the lack of 266 interoperability, or in the need to implement more solutions. 267 Unfortunately this situation, if not handled reasonably well, can 268 result in a failure to satisfy the requirement of providing the users 269 with an option of turning on strong security when desired. 271 6. Conclusions 273 As discussed earlier it appears that a single solution can't be 274 designed to meet the diverse requirements. In the absence of such a 275 solution, it is hoped that this memo explains why SRTP is not 276 mandatory as the media security solution for RTP-based systems, and 277 why we can expect multiple key management solutions for systems using 278 RTP. 280 It is important for any RTP-based application to consider how it 281 meets the security requirements. This will require some analysis to 282 determine these requirements, followed by the selection of a 283 mandatory to implement solution, or in exceptional scenarios several 284 solutions, including the desired RTP traffic protection and key- 285 management. SRTP is a preferred solution for the protection of the 286 RTP traffic in those use cases where it is applicable. It is out of 287 scope for this memo to recommend a preferred key management solution. 289 7. Security Considerations 291 This entire memo is about security. 293 8. IANA Considerations 295 No IANA actions are required. 297 9. Acknowledgements 299 Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes, 300 Martin Ellis, and Ali Begen for their feedback. 302 10. Informative References 304 [3GPP.33.210] 305 3GPP, "IP network layer security", 3GPP TS 33.210, 306 September 2008. 308 [ETSI.TS.102.474] 309 ETSI, "Digital Video Broadcasting (DVB); IP Datacast over 310 DVB-H: Service Purchase and Protection", ETSI TS 102 474, 311 November 2007. 313 [I-D.ietf-avt-dtls-srtp] 314 McGrew, D. and E. Rescorla, "Datagram Transport Layer 315 Security (DTLS) Extension to Establish Keys for Secure 316 Real-time Transport Protocol (SRTP)", 317 draft-ietf-avt-dtls-srtp-07 (work in progress), 318 February 2009. 320 [I-D.ietf-mmusic-rfc2326bis] 321 Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 322 and M. Stiemerling, "Real Time Streaming Protocol 2.0 323 (RTSP)", draft-ietf-mmusic-rfc2326bis-21 (work in 324 progress), June 2009. 326 [I-D.ietf-sip-media-security-requirements] 327 Wing, D., Fries, S., Tschofenig, H., and F. Audet, 328 "Requirements and Analysis of Media Security Management 329 Protocols", draft-ietf-sip-media-security-requirements-09 330 (work in progress), January 2009. 332 [I-D.zimmermann-avt-zrtp] 333 Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media 334 Path Key Agreement for Secure RTP", 335 draft-zimmermann-avt-zrtp-15 (work in progress), 336 March 2009. 338 [MBMS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 339 Protocols and codecs TS 26.346". 341 [MBMS-SEC] 342 3GPP, "Security of Multimedia Broadcast/Multicast Service 343 (MBMS) TS 33.246". 345 [OMA-DRM] Open Mobile Alliance, "DRM Specification 2.0". 347 [PSS] 3GPP, "Transparent end-to-end Packet-switched Streaming 348 Service (PSS); Protocols and codecs TS 26.234". 350 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 351 August 1980. 353 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 354 RFC 793, September 1981. 356 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 357 A., Peterson, J., Sparks, R., Handley, M., and E. 358 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 359 June 2002. 361 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 362 with Session Description Protocol (SDP)", RFC 3264, 363 June 2002. 365 [RFC3365] Schiller, J., "Strong Security Requirements for Internet 366 Engineering Task Force Standard Protocols", BCP 61, 367 RFC 3365, August 2002. 369 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 370 Jacobson, "RTP: A Transport Protocol for Real-Time 371 Applications", STD 64, RFC 3550, July 2003. 373 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 374 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 375 RFC 3711, March 2004. 377 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram 378 Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 380 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 381 Description Protocol", RFC 4566, July 2006. 383 [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. 384 Carrara, "Key Management Extensions for Session 385 Description Protocol (SDP) and Real Time Streaming 386 Protocol (RTSP)", RFC 4567, July 2006. 388 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 389 Description Protocol (SDP) Security Descriptions for Media 390 Streams", RFC 4568, July 2006. 392 [RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap 393 for Transmission Control Protocol (TCP) Specification 394 Documents", RFC 4614, September 2006. 396 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 397 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 399 Authors' Addresses 401 Colin Perkins 402 University of Glasgow 403 Department of Computing Science 404 Glasgow G12 8QQ 405 UK 407 Email: csp@csperkins.org 409 Magnus Westerlund 410 Ericsson 411 Farogatan 6 412 Kista SE-164 80 413 Sweden 415 Email: magnus.westerlund@ericsson.com