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Internet Engineering Task Force (IETF)                        C. PerkinsRequest for Comments: 7202                         University of GlasgowCategory: Informational                                    M. WesterlundISSN: 2070-1721                                                 Ericsson                                                              April 2014Securing the RTP Framework:Why RTP Does Not Mandate a Single Media Security SolutionAbstract   This memo discusses the problem of securing real-time multimedia   sessions.  It also explains why the Real-time Transport Protocol   (RTP) and the associated RTP Control Protocol (RTCP) do not mandate a   single media security mechanism.  This is relevant for designers and   reviewers of future RTP extensions to ensure that appropriate   security mechanisms are mandated and that any such mechanisms are   specified in a manner that conforms with the RTP architecture.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7202.Perkins & Westerlund          Informational                     [Page 1]

RFC 7202               Securing the RTP Framework             April 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  RTP Applications and Deployment Scenarios . . . . . . . . . .33.  RTP Media Security  . . . . . . . . . . . . . . . . . . . . .44.  RTP Session Establishment and Key Management  . . . . . . . .5   5.  On the Requirement for Strong Security in Framework Protocols   56.  Securing the RTP Framework  . . . . . . . . . . . . . . . . .67.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .88.  Security Considerations . . . . . . . . . . . . . . . . . . .89.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .810. Informative References  . . . . . . . . . . . . . . . . . . .81.  Introduction   The Real-time Transport Protocol (RTP) [RFC3550] is widely used for   voice over IP, Internet television, video conferencing, and other   real-time and streaming media applications.  Despite this use, the   basic RTP specification provides only limited options for media   security and defines no standard key exchange mechanism.  Rather, a   number of extensions are defined that can provide confidentiality and   authentication of RTP media streams and RTP Control Protocol (RTCP)   messages.  Other mechanisms define key exchange protocols.  This memo   outlines why it is appropriate that multiple extension mechanisms are   defined rather than mandating a single security and keying mechanism   for all users of RTP.   The IETF policy "Strong Security Requirements for Internet   Engineering Task Force Standard Protocols" [RFC3365] (the so-called   "Danvers Doctrine") states that "we MUST implement strong security in   all protocols to provide for the all too frequent day when the   protocol comes into widespread use in the global Internet".  The   security mechanisms defined for use with RTP allow these requirementsPerkins & Westerlund          Informational                     [Page 2]

RFC 7202               Securing the RTP Framework             April 2014   to be met.  However, since RTP is a protocol framework that is   suitable for a wide variety of use cases, there is no single security   mechanism that is suitable for every scenario.  This memo outlines   why this is the case and discusses how users of RTP can meet the   requirement for strong security.   This document provides high-level guidance on how to handle security   issues for the various types of components within the RTP framework   as well as the role of the service or application using RTP to ensure   strong security is implemented.  This document does not provide the   guidance that an individual implementer, or even specifier of an RTP   application, really can use to determine what security mechanism they   need to use; that is not intended with this document.   A non-exhaustive list of the RTP security options available at the   time of this writing is outlined in [RFC7201].  This document gives   an overview of the available RTP solutions and provides guidance on   their applicability for different application domains.  It also   attempts to provide an indication of actual and intended usage at the   time of writing as additional input to help with considerations such   as interoperability, availability of implementations, etc.2.  RTP Applications and Deployment Scenarios   The range of application and deployment scenarios where RTP has been   used includes, but is not limited to, the following:   o  Point-to-point voice telephony;   o  Point-to-point video conferencing and telepresence;   o  Centralized group video conferencing and telepresence, using a      Multipoint Conference Unit (MCU) or similar central middlebox;   o  Any Source Multicast (ASM) video conferencing using the      lightweight sessions model (e.g., the Mbone conferencing tools);   o  Point-to-point streaming audio and/or video (e.g., on-demand TV or      movie streaming);   o  Source-Specific Multicast (SSM) streaming to large receiver groups      (e.g., IPTV streaming by residential ISPs or the Third Generation      Partnership Project (3GPP) Multimedia/Broadcast Multicast Service      [T3GPP.26.346]);   o  Replicated unicast streaming to a group of receivers;Perkins & Westerlund          Informational                     [Page 3]

RFC 7202               Securing the RTP Framework             April 2014   o  Interconnecting components in music production studios and video      editing suites;   o  Interconnecting components of distributed simulation systems; and   o  Streaming real-time sensor data (e.g., electronic Very Long      Baseline Interferometry (e-VLBI) radio astronomy).   As can be seen, these scenarios vary from point-to-point sessions to   very large multicast groups, from interactive to non-interactive, and   from low bandwidth (kilobits per second) telephony to high bandwidth   (multiple gigabits per second) video and data streaming.  While most   of these applications run over UDP [RFC0768], some use TCP [RFC0793]   [RFC4614] or the Datagram Congestion Control Protocol (DCCP)   [RFC4340] as their underlying transport.  Some run on highly reliable   optical networks, while others use low-rate unreliable wireless   networks.  Some applications of RTP operate entirely within a single   trust domain, while others run interdomain with untrusted (and, in   some cases, potentially unknown) users.  The range of scenarios is   wide and growing both in number and in heterogeneity.3.  RTP Media Security   The wide range of application scenarios where RTP is used has led to   the development of multiple solutions for securing RTP media streams   and RTCP control messages, considering different requirements.   Perhaps the most widely applicable of these security options is the   Secure RTP (SRTP) framework [RFC3711].  This is an application-level   media security solution, encrypting the media payload data (but not   the RTP headers) to provide confidentiality and supporting source   origin authentication as an option.  SRTP was carefully designed to   be low overhead, including operating on links subject to RTP header   compression, and to support the group communication and third-party   performance monitoring features of RTP across a range of networks.   SRTP is not the only media security solution for RTP, however, and   alternatives can be more appropriate in some scenarios, perhaps due   to ease of integration with other parts of the complete system.  In   addition, SRTP does not address all possible security requirements,   and other solutions are needed in cases where SRTP is not suitable.   For example, ISMACryp payload-level confidentiality [ISMACryp2] is   appropriate for some types of streaming video application, but is not   suitable for voice telephony, and uses features that are not provided   by SRTP.Perkins & Westerlund          Informational                     [Page 4]

RFC 7202               Securing the RTP Framework             April 2014   The range of available RTP security options, and their applicability   to different scenarios, is outlined in [RFC7201].  At the time of   this writing, there is no media security protocol that is appropriate   for all the environments where RTP is used.  Multiple RTP media   security protocols are expected to remain in wide use for the   foreseeable future.4.  RTP Session Establishment and Key Management   A range of different protocols for RTP session establishment and key   exchange exist, matching the diverse range of use cases for the RTP   framework.  These mechanisms can be split into two categories: those   that operate in band on the media path and those that are out of band   and operate as part of the session establishment signaling channel.   The requirements for these two classes of solutions are different,   and a wide range of solutions have been developed in this space.   A more-detailed survey of requirements for media security management   protocols can be found in [RFC5479].  As can be seen from that memo,   the range of use cases is wide, and there is no single key management   protocol that is appropriate for all scenarios.  The solutions have   been further diversified by the existence of infrastructure elements,   such as authentication systems, that are tied to the key management.   The most important and widely used keying options for RTP sessions at   the time of this writing are described in [RFC7201].5.  On the Requirement for Strong Security in Framework Protocols   The IETF requires that all protocols provide a strong, mandatory-to-   implement security solution [RFC3365].  This is essential for the   overall security of the Internet to ensure that all implementations   of a protocol can interoperate in a secure way.  Framework protocols   offer a challenge for this mandate, however, since they are designed   to be used by different classes of applications in a wide range of   different environments.  The different use cases for the framework   have different security requirements, and implementations designed   for different environments are generally not expected to interwork.   RTP is an example of a framework protocol with wide applicability.   The wide range of scenarios described inSection 2 show the issues   that arise in mandating a single security mechanism for this type of   framework.  It would be desirable if a single media security   solution, and a single key management solution, could be developed   that is suitable for applications across this range of use scenarios.   The authors are not aware of any such solution, however, and believe   it is unlikely that any such solution will be developed.  In part,   this is because applications in the different domains are not   intended to interwork, so there is no incentive to develop a singlePerkins & Westerlund          Informational                     [Page 5]

RFC 7202               Securing the RTP Framework             April 2014   mechanism.  More importantly, though, the security requirements for   the different usage scenarios vary widely, and an appropriate   security mechanism in one scenario simply does not work for some   other scenarios.   For a framework protocol, it appears that the only sensible solution   to the strong security requirement of [RFC3365] is to develop and use   building blocks for the basic security services of confidentiality,   integrity protection, authorization, authentication, and so on.  When   new uses for the framework protocol arise, they need to be studied to   determine if the existing security building blocks can satisfy the   requirements, or if new building blocks need to be developed.   Therefore, when considering the strong and mandatory-to-implement   security mechanism for a specific class of applications, one has to   consider what security building blocks need to be integrated, or if   any new mechanisms need to be defined to address specific issues   relating to this new class of application.  To maximize   interoperability, it is important that common media security and key   management mechanisms are defined for classes of application with   similar requirements.  The IETF needs to participate in this   selection of security building blocks for each class of applications   that use the protocol framework and are expected to interoperate, in   cases where the IETF has the appropriate knowledge of the class of   applications.6.  Securing the RTP Framework   The IETF requires that protocols specify mandatory-to-implement (MTI)   strong security [RFC3365].  This applies to the specification of each   interoperable class of application that makes use of RTP.  However,   RTP is a framework protocol, so the arguments made inSection 5 also   apply.  Given the variability of the classes of application that use   RTP, and the variety of the currently available security mechanisms   described in [RFC7201], no one set of MTI security options can   realistically be specified that apply to all classes of RTP   applications.   Documents that define an interoperable class of applications using   RTP are subject to [RFC3365], and thus need to specify MTI security   mechanisms.  This is because such specifications do fully specify   interoperable applications that use RTP.  Examples of such documents   under development in the IETF at the time of this writing are "WebRTC   Security Architecture" [WebRTC-SEC] and "Real Time Streaming Protocol   2.0 (RTSP)" [RTSP].  It is also expected that a similar document will   be produced for voice-over-IP applications using SIP and RTP.Perkins & Westerlund          Informational                     [Page 6]

RFC 7202               Securing the RTP Framework             April 2014   The RTP framework includes several extension points.  Some extensions   can significantly change the behavior of the protocol to the extent   that applications using the extension form a separate interoperable   class of applications to those that have not been extended.  Other   extension points are defined in such a manner that they can be used   (largely) independently of the class of applications using RTP.  Two   important extension points that are independent of the class of   applications are RTP payload formats and RTP profiles.   An RTP payload format defines how the output of a media codec can be   used with RTP.  At the time of this writing, there are over 70 RTP   payload formats defined in published RFCs, with more in development.   It is appropriate for an RTP payload format to discuss the specific   security implications of using that media codec with RTP.  However,   an RTP payload format does not specify an interoperable class of   applications that use RTP since, in the vast majority of cases, a   media codec and its associated RTP payload format can be used with   many different classes of application.  As such, an RTP payload   format is neither secure in itself nor something to which [RFC3365]   applies.  Future RTP payload format specifications need to explicitly   state this and include a reference to this memo for explanation.  It   is not appropriate for an RTP payload format to mandate the use of   SRTP [RFC3711], or any other security building blocks, since that RTP   payload format might be used by different classes of application that   use RTP and that have different security requirements.   RTP profiles are larger extensions that adapt the RTP framework for   use with particular classes of application.  In some cases, those   classes of application might share common security requirements so   that it could make sense for an RTP profile to mandate particular   security options and building blocks (the RTP/SAVP profile [RFC3711]   is an example of this type of RTP profile).  In other cases, though,   an RTP profile is applicable to such a wide range of applications   that it would not make sense for that profile to mandate particular   security building blocks be used (the RTP/AVPF profile [RFC4585] is   an example of this type of RTP profile, since it provides building   blocks that can be used in different styles of application).  A new   RTP profile specification needs to discuss whether or not it makes   sense to mandate particular security building blocks that need to be   used with all implementations of that profile; however, there is no   expectation that all RTP profiles will mandate particular security   solutions.  RTP profiles that do not specify an interoperable usage   for a particular class of RTP applications are neither secure in   themselves nor something to which [RFC3365] applies; any future RTP   profiles in this category need to explicitly state this with   justification and include a reference to this memo.Perkins & Westerlund          Informational                     [Page 7]

RFC 7202               Securing the RTP Framework             April 20147.  Conclusions   The RTP framework is used in a wide range of different scenarios with   no common security requirements.  Accordingly, neither SRTP [RFC3711]   nor any other single media security solution or keying mechanism can   be mandated for all uses of RTP.  In the absence of a single common   security solution, it is important to consider what mechanisms can be   used to provide strong and interoperable security for each different   scenario where RTP applications are used.  This will require analysis   of each class of application to determine the security requirements   for the scenarios in which they are to be used, followed by the   selection of MTI security building blocks for that class of   application, including the desired RTP traffic protection and key   management.  A non-exhaustive list of the RTP security options   available at the time of this writing is outlined in [RFC7201].  It   is expected that each class of application will be supported by a   memo describing what security options are mandatory to implement for   that usage scenario.8.  Security Considerations   This entire memo is about mandatory-to-implement security.9.  Acknowledgements   Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes,   Martin Ellis, Ali Begen, Keith Drage, Ray van Brandenburg, Stephen   Farrell, Sean Turner, John Mattsson, and Benoit Claise for their   feedback.10.  Informative References   [ISMACryp2]  Internet Streaming Media Alliance (ISMA), "ISMA                Encryption and Authentication Version 2.0", November                2007, <http://www.oipf.tv/images/site/DOCS/mpegif/ISMA/isma_easpec2.0.pdf>.   [RFC0768]    Postel, J., "User Datagram Protocol", STD 6,RFC 768,                August 1980.   [RFC0793]    Postel, J., "Transmission Control Protocol", STD 7,RFC793, September 1981.   [RFC3365]    Schiller, J., "Strong Security Requirements for Internet                Engineering Task Force Standard Protocols",BCP 61,RFC3365, August 2002.Perkins & Westerlund          Informational                     [Page 8]

RFC 7202               Securing the RTP Framework             April 2014   [RFC3550]    Schulzrinne, H., Casner, S., Frederick, R., and V.                Jacobson, "RTP: A Transport Protocol for Real-Time                Applications", STD 64,RFC 3550, July 2003.   [RFC3711]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and                K.  Norrman, "The Secure Real-time Transport Protocol                (SRTP)",RFC 3711, March 2004.   [RFC4340]    Kohler, E., Handley, M., and S. Floyd, "Datagram                Congestion Control Protocol (DCCP)",RFC 4340, March                2006.   [RFC4585]    Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.                Rey, "Extended RTP Profile for Real-time Transport                Control Protocol (RTCP)-Based Feedback (RTP/AVPF)",RFC4585, July 2006.   [RFC4614]    Duke, M., Braden, R., Eddy, W., and E. Blanton, "A                Roadmap for Transmission Control Protocol (TCP)                Specification Documents",RFC 4614, September 2006.   [RFC5479]    Wing, D., Fries, S., Tschofenig, H., and F. Audet,                "Requirements and Analysis of Media Security Management                Protocols",RFC 5479, April 2009.   [RFC7201]    Westerlund, M. and C. Perkins, "Options for Securing RTP                Sessions",RFC 7201, April 2014.   [RTSP]       Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,                and M. Stiemerling, "Real Time Streaming Protocol 2.0                (RTSP)", Work in Progress, February 2014.   [T3GPP.26.346]                3GPP, "Multimedia Broadcast/Multicast Service (MBMS);                Protocols and codecs", 3GPP TS 26.346 10.7.0, March                2013,                <http://www.3gpp.org/ftp/Specs/html-info/26346.htm>.   [WebRTC-SEC] Rescorla, E.,"WebRTC Security Architecture", Work in                Progress, February 2014.Perkins & Westerlund          Informational                     [Page 9]

RFC 7202               Securing the RTP Framework             April 2014Authors' Addresses   Colin Perkins   University of Glasgow   School of Computing Science   Glasgow  G12 8QQ   United Kingdom   EMail: csp@csperkins.org   URI:http://csperkins.org/   Magnus Westerlund   Ericsson   Farogatan 6   Kista  SE-164 80   Sweden   EMail: magnus.westerlund@ericsson.comPerkins & Westerlund          Informational                    [Page 10]