Securing the RTP Framework: Why RTP Does Not Mandate a Single Media Security Solution
RFC 7202

Internet Engineering Task Force (IETF)                        C. Perkins
Request for Comments: 7202                         University of Glasgow
Category: Informational                                    M. Westerlund
ISSN: 2070-1721                                                 Ericsson
                                                              April 2014

                      Securing the RTP Framework:
       Why RTP Does Not Mandate a Single Media Security Solution

Abstract

   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; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7202.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  RTP Applications and Deployment Scenarios . . . . . . . . . .   3
   3.  RTP Media Security  . . . . . . . . . . . . . . . . . . . . .   4
   4.  RTP Session Establishment and Key Management  . . . . . . . .   5
   5.  On the Requirement for Strong Security in Framework Protocols   5
   6.  Securing the RTP Framework  . . . . . . . . . . . . . . . . .   6
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. Informative References  . . . . . . . . . . . . . . . . . . .   8

1.  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 requirements

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   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;

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   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.

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   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 in Section 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 single

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   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 in Section 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.

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   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.

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7.  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, RFC
                793, September 1981.

   [RFC3365]    Schiller, J., "Strong Security Requirements for Internet
                Engineering Task Force Standard Protocols", BCP 61, RFC
                3365, August 2002.

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   [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)", RFC
                4585, 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.

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Authors' 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.com

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