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Versions: (draft-perkins-avt-srtp-not-mandatory) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7202

Network Working Group                                       C.S. Perkins
Internet-Draft                                     University of Glasgow
Intended status: Informational                             M. Westerlund
Expires: November 07, 2013                                      Ericsson
                                                            May 06, 2013


 Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single
                        Media Security Solution
                draft-ietf-avt-srtp-not-mandatory-13.txt

Abstract

   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
   architecture.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 07, 2013.

Copyright Notice

   Copyright (c) 2013 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
   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



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   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 Contents

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

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 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 security 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.






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   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;

   o  Point-to-point video conferencing and telepresence

   o  Centralised group video conferencing and telepresence, using a
      multipoint conference unit (MCU) or similar central middlebox

   o  Any Source Multicast video (ASM) conferencing using the light-
      weight 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 3GPP Multimedia
      Broadcast Multicast Service [MBMS])

   o  Replicated unicast streaming to a group of receivers

   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 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 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 run inter-domain, with untrusted (and,
   in some cases, potentially unknown) users.  The range of scenarios is
   wide, and growing both in number and in heterogeneity.





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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 [ISMACrypt2] 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.

   The range of available RTP security options, and their applicability
   to different scenarios, is outlined in
   [I-D.ietf-avtcore-rtp-security-options].  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 signalling
   channel.  The requirements for these two classes of solution 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



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   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
   [I-D.ietf-avtcore-rtp-security-options].

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,
   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.

   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, 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.  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



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   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.  Guidelines for Securing the RTP Protocol 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 [I-D.ietf-avtcore-rtp-security-options], 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 so 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 the
   RTCWEB Security Architecture [I-D.ietf-rtcweb-security-arch] and the
   Real Time Streaming Protocol 2.0 (RTSP) [I-D.ietf-mmusic-rfc2326bis].
   It is also expected that a similar document will be produced for
   voice-over-IP applications using SIP and RTP.

   The RTP framework includes several extension points.  Some extensions
   can significantly change the behaviour 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 can be 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 it's 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



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   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.

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 a mandatory to implement 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 [I-D.ietf-avtcore-rtp-security-options].  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




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   This entire memo is about security.

9.  IANA Considerations

   None.

10.  Acknowledgements

   Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes,
   Martin Ellis, Ali Begen, Keith Drage, Ray van Brandenburg, Stephen
   Farrell, and Sean Turner for their feedback.

11.  Informative References

   [I-D.ietf-avtcore-rtp-security-options]
              Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", draft-ietf-avtcore-rtp-security-options-03
              (work in progress), May 2013.

   [I-D.ietf-mmusic-rfc2326bis]
              Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, "Real Time Streaming Protocol 2.0
              (RTSP)", draft-ietf-mmusic-rfc2326bis-34 (work in
              progress), April 2013.

   [I-D.ietf-rtcweb-security-arch]
              Rescorla, E., "RTCWEB Security Architecture", draft-ietf-
              rtcweb-security-arch-06 (work in progress), January 2013.

   [ISMACrypt2]
              Internet Streaming Media Alliance (ISMA), , "ISMA
              Encryption and Authentication, Version 2.0 release
              version", November 2007.

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

   [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.

Authors' Addresses

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

   Email: csp@csperkins.org


   Magnus Westerlund
   Ericsson
   Farogatan 6
   Kista  SE-164 80
   Sweden

   Email: magnus.westerlund@ericsson.com








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