Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Intended status: Informational                             M. Westerlund
Expires: January 17, 2013                                       Ericsson
                                                           July 16, 2012

          Why RTP Does Not Mandate a Single Security Mechanism


   This memo discusses the problem of securing real-time multimedia
   sessions, and explains why the Real-time Transport Protocol (RTP),
   and the associated RTP control protocol (RTCP), do not mandate a
   single media security mechanism.  Guidelines for designers and
   reviewers of future RTP extensions are provided, to ensure that
   appropriate security mechanisms are mandated, and that any such
   mechanisms are specified in a manner that conforms with the RTP

Status of this Memo

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   This Internet-Draft will expire on January 17, 2013.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   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.  Security Mechanisms for RTP . . . . . . . . . . . . . . . . . . 6
   7.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   11. Informative References  . . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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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 RTCP control 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.

   The IETF policy on Strong Security Requirements for IETF 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 mechanisms defined for use with RTP
   allow these requirements 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 memo provides information for the community and for reviewers of
   future RTP-related work in the IETF.  It does not specify a standard
   of any kind.

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 (fixed and wireless networks)

   o  Point-to-point voice and video conferencing

   o  Centralised group video conferencing with a multipoint conference
      unit (MCU)

   o  Any Source Multicast video conferencing (light-weight sessions;
      Mbone conferencing)

   o  Point-to-point streaming audio and/or video

   o  Source-specific multicast (SSM) streaming to large group (IPTV and
      3GPP Multimedia Broadcast Multicast Service (MBMS) [MBMS])

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   o  Replicated unicast streaming to a group

   o  Interconnecting components in music production studios and video
      editing suites

   o  Interconnecting components of distributed simulation systems

   o  Streaming real-time sensor data (e.g., e-VLBI radio astronomy)

   As can be seen, these scenarios vary from point-to-point to 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 DCCP [RFC4340] as their underlying transport.  Some run
   on highly reliable optical networks, others use low rate unreliable
   wireless networks.  Some applications of RTP operate entirely within
   a single trust domain, others are inter-domain, with untrusted (and
   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 both low overhead, and to support the group communication and
   third-party performance monitoring features of RTP, across a range of

   SRTP is not the only media security solution in use, however, and
   alternatives are more appropriate for some scenarios, and necessary
   in some cases where SRTP is not suitable.  At present, there is no
   media security protocol that is appropriate for all the environments
   where RTP is used.  Multiple RTP media security protocols can be
   expected to remain in wide use for the forseeable future.

   The range of available RTP security options, and their applicability,
   are described in [I-D.ietf-avtcore-rtp-security-options].

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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 signalling
   channel.  The requirements for these two classes of solution are
   different, and a wide range of solutions have been developed in this

   A more detailed survey of requirements for media security management
   protocols can be found in [RFC5479].  As can be seen, the range of
   use cases is wide, and there is no single key management protocol
   that is appropriate for all scenarios.  These solutions have been
   further diversified by the existence of infrastructure elements such
   as authentication solutions that are tied into the key management.
   Some of the available keying options for RTP sessions are described
   in [I-D.ietf-avtcore-rtp-security-options], although this list is not
   ensured to be exhaustive but include the ones known to the authors at
   the time of publication.

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
   for use by different classes of applications, in 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,
   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 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.

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   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, authorisation, and authentication.  When new
   uses for the framework arise, they need to be studied to check if the
   existing building blocks satisfy the requirements.  A mandatory to
   implement set of security building blocks can then be specified for
   that usage scenario of the framework.

   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 supported.  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
   where IETF has the appropriate knowledge of the class of

6.  Security Mechanisms for RTP

   RTP is a framework protocol, so the arguments in in Section 5 apply.
   The security building blocks available for RTP at the time of this
   writing are described in [I-D.ietf-avtcore-rtp-security-options].
   That memo also gives examples of how those security building blocks
   can be combined to give mandatory to implement security for some RTP
   application scenarios.

   RTP can be extended in different ways.  Two important extension
   points are RTP Payload Formats and RTP Profiles.  An RTP Payload
   Format defines how the output of a new media codec can be used with
   RTP.  It is appropriate for an RTP payload format to discuss specific
   security implications of using that codec with RTP, but it is not
   appropriate for an RTP payload format to mandate the use of SRTP, or
   any other security building blocks, since that payload format might
   be used in a range of different scenarios.

   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.  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.  Any new RTP profile ought to discuss if it
   makes sense to mandate particular security building blocks be used

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   with implementations of that profile, but without the expectation
   that all RTP profiles will mandate particular security solutions.

7.  Conclusions

   RTP is used in a wide range of scenarios, without comon security
   requirements.  Accordingly, a single security solution cannot be
   mandated for all scenarios.  In the absence of such a solution, it is
   hoped that this memo explains why SRTP is not mandatory as the media
   security solution for RTP-based systems, and why we can expect
   multiple key management solutions for systems using RTP.

   It is important consider how strong and interoperable security can be
   offered for every scenario in which RTP applications are used, and
   for every class of RTP applications.  This will require analysis to
   determine the security requirements, followed by the selection of a
   mandatory to implement security building blocks for that class of
   application, including the desired RTP traffic protection and key-
   management.  Commonality of security mechanisms is desirable, where

8.  Security Considerations

   This entire memo is about security.

9.  IANA Considerations


10.  Acknowledgements

   Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes,
   Martin Ellis, Ali Begen, and Keith Drage for their feedback.

11.  Informative References

              Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", draft-ietf-avtcore-rtp-security-options-00
              (work in progress), July 2012.

   [MBMS]     3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
              Protocols and codecs TS 26.346".

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   [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.

   [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.

   [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.

Authors' Addresses

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ


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   Magnus Westerlund
   Farogatan 6
   Kista  SE-164 80


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