Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Intended status: Informational                             M. Westerlund
Expires: January 14, 2010                                       Ericsson
                                                           July 13, 2009

          Why RTP Does Not Mandate a Single Security Mechanism

Status of this Memo

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   This memo discusses the problem of securing real-time multimedia
   sessions, and explains why the Real-time Transport Protocol (RTP)

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   does not mandate a single media security mechanism.

Table of Contents

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

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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 various
   other real-time and streaming media applications.  Despite this, the
   base RTP specification provides very limited options for media
   security, and defines no standard key exchange mechanism.  Rather, a
   number of extensions are defined to provide confidentiality and
   authentication of media streams, and to exchange security keys.  This
   memo outlines why it is appropriate that multiple extension
   mechanisms are defined, rather than mandating a single media security
   and keying mechanism.

   This memo provides information for the community; it does not specify
   a standard of any kind.

   The structure of this memo is as follows: we begin, in Section 2 by
   describing the scenarios in which RTP is deployed.  Following this,
   Section 3 outlines the implications of this range of scenarios for
   media confidentially and authentication, and Section 4 outlines the
   implications for key exchange.  Section 5 outlines how the RTP
   framework meets the requirement of BCP 61.  Section 6 then concludes
   and gives some recommendations.  Finally, Section 7 outlines the
   security considerations, and Section 8 outlines IANA considerations.

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 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  Single Source Multicast streaming to large group (IPTV and MBMS

   o  Replicated unicast streaming to a group

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   o  Interconnecting components in music production studios and video
      editing suites

   o  Interconnecting components of distributed simulation systems

   o  Streaming real-time sensor data

   As can be seen, these scenarios vary from point-to-point to very
   large multicast groups, from interactive to non-interactive, and from
   low bandwidth (kilobits per second) to very high bandwidth (multiple
   gigabits per second).  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.  Implications for RTP Media Security

   The wide range of application scenarios where RTP is used has led to
   the development of multiple solutions for media security, considering
   different requirements.  Perhaps the most widely applicable of these
   solutions 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 some degree of
   confidentiality, and providing optional source origin authentication.
   It was carefully designed to be both low overhead, and to support the
   group communication features of RTP, across a range of networks.

   SRTP is not the only media security solution in use, however, and
   alternatives are more appropriate for some scenarios.  For example,
   many client-server streaming media applications can run over a single
   TCP connection, multiplexing media data with control information on
   that connection (RTSP [I-D.ietf-mmusic-rfc2326bis] is a widely used
   example of such a protocol).  The natural way to provide media
   security for such client-server media applications is to use TLS
   [RFC5246] to protect the TCP connection, sending the RTP media data
   over the TLS connection.  Using the SRTP framework in addition to TLS
   is unncessary, and would result in double encryption of the media,
   and SRTP cannot be used instead of TLS since it is RTP-specific, and
   so cannot protect the control traffic.

   Other RTP use cases work over networks which provide security at the
   network layer, using IPsec.  For example, certain 3GPP networks need
   IPsec security associations for other purposes, and can reuse those

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   to secure the RTP session [3GPP.33.210].  SRTP is, again, unnecessary
   in such environments, and its use would only introduce overhead for
   no gain.

   For some applications it is sufficient to protect the RTP payload
   data while leaving RTP, transport, and network layer headers
   unprotected.  An example of this is RTP broadcast over DVB-H
   [ETSI.TS.102.474], where one mode of operation uses ISMAcryp
   ( to protect the media data only.

   Finally, the link layer may be secure, and it may be known that the
   RTP media data is constrained to that single link (for example, when
   operating in a studio environment, with physical link security).  An
   environment like this is inherently constrained, but might avoid the
   need for application, transport, or network layer media security.

   All these are application scenarios where RTP has seen commerical
   deployment.  Other use case also exist, with additional requirements.
   There is no media security protocol that is appropriate for all these
   environments.  Accordingly, multiple RTP media security protocols can
   be expected to remain in wide use.

4.  Implications for Key Management

   With such a diverse range of use case come a range of different
   protocols for RTP session establishment.  Mechanisms used to provide
   security keying for these different session establishment protocols
   can basically be put into two categories: inband and out-of-band in
   relation to the session establishment mechanism.  The requirements
   for these solutions are highly varying.  Thus a wide range of
   solutions have been developed in this space:

   o  The most common use case for RTP is probably point-to-point voice
      calls or centralised group conferences, negotiated using SIP
      [RFC3261] with the SDP offer/answer model [RFC3264], operating on
      a trusted infrastructure.  In such environments, SDP security
      descriptions [RFC4568] or the MIKEY [RFC4567] protocol are
      appropriate keying mechanisms, piggybacked onto the SDP [RFC4566]
      exchange.  The infrastructure may be secured by protecting the SIP
      exchange using TLS or S/MIME, for example [RFC3261].

   o  Point-to-point RTP sessions may be negotiated using SIP with the
      offer/answer model, but operating over a network with untrusted
      infrastructure.  In such environments, the key management protocol
      is run on the media path, bypassing the untrusted infrastructure.
      Protocols such as DTLS [I-D.ietf-avt-dtls-srtp] or ZRTP
      [I-D.zimmermann-avt-zrtp] are useful here.

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   o  For point-to-point client-server streaming of RTP over RTSP, a TLS
      association is appropriate to manage keying material, in much the
      same manner as would be used to secure an HTTP session.

   o  A session description may be sent by email, secured using X.500 or
      PGP, or retrieved from a web page, using HTTP with TLS.

   o  A session description may be distributed to a multicast group
      using SAP or FLUTE secured with S/MIME.

   o  A session description may be distributed using the Open Mobile
      Alliance DRM key management specification [OMA-DRM] when using a
      point-to-point streaming session setup with RTSP in the 3GPP PSS
      environment [PSS].

   o  In the 3GPP Multimedia Broadcast Multicast Service (MBMS) system,
      HTTP and MIKEY are used for key management [MBMS-SEC].

   A more detailed survey of requirements for media security management
   protocols can be found in [I-D.ietf-sip-media-security-requirements].
   As can be seen, the range of use cases is wide, and there is no
   single 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 manangement.

5.  On the Requirement for Strong Security in IETF protocols

   BCP 61 [RFC3365] puts a requirement on IETF protocols to provide
   strong, mandatory to implement, security solutions.  This is actually
   quite a difficult requirement for any type of framework protocol,
   like RTP, since one can never know all the deployment scenarios, and
   if they are covered by the security solution.  It would clearly be
   desirable if a single media security solution and a single key
   management solution could be developed, satisfying the range of use
   cases for RTP.  The authors are not aware of any such solution,
   however, and it is not clear that any single solution can be

   For a framework protocol it appears that the only sensible solution
   to the requirement of BCP 61 is to develop or use security building
   blocks, like SRTP, SDP security descriptions [RFC4568], MIKEY, DTLS,
   or IPsec, to provide the basic security services of authorization,
   data integrity protection and date confidentiality protection.  When
   new usages of the RTP framework arise, one needs to analyze the
   situation, to determine if the existing building blocks satisfy the
   requirements.  If not, it is necessary to develop new security

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   building blocks.

   When it comes to fulfilling the "MUST Implement" strong security for
   a specific application, it will fall on that application to actually
   consider what building blocks it is required to support.  To maximize
   interoperability it is desirable if certain applications, or classes
   of application with similar requirements, agree on what data security
   mechanisms and key-management should be used.  If such agreement is
   not possible, there will be increased cost, either in the lack of
   interoperability, or in the need to implement more solutions.
   Unfortunately this situation, if not handled reasonably well, can
   result in a failure to satisfy the requirement of providing the users
   with an option of turning on strong security when desired.

6.  Conclusions

   As discussed earlier it appears that a single solution can't be
   designed to meet the diverse requirements.  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

   It is important for any RTP-based application to consider how it
   meets the security requirements.  This will require some analysis to
   determine these requirements, followed by the selection of a
   mandatory to implement solution, or in exceptional scenarios several
   solutions, including the desired RTP traffic protection and key-
   management.  SRTP is a preferred solution for the protection of the
   RTP traffic in those use cases where it is applicable.  It is out of
   scope for this memo to recommend a preferred key management solution.

7.  Security Considerations

   This entire memo is about security.

8.  IANA Considerations

   No IANA actions are required.

9.  Acknowledgements

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

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10.  Informative References

              3GPP, "IP network layer security", 3GPP TS 33.210,
              September 2008.

              ETSI, "Digital Video Broadcasting (DVB); IP Datacast over
              DVB-H: Service  Purchase and Protection", ETSI TS 102 474,
              November 2007.

              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-ietf-avt-dtls-srtp-07 (work in progress),
              February 2009.

              Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, "Real Time Streaming Protocol 2.0
              (RTSP)", draft-ietf-mmusic-rfc2326bis-21 (work in
              progress), June 2009.

              Wing, D., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", draft-ietf-sip-media-security-requirements-09
              (work in progress), January 2009.

              Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Secure RTP",
              draft-zimmermann-avt-zrtp-15 (work in progress),
              March 2009.

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

              3GPP, "Security of Multimedia Broadcast/Multicast Service
              (MBMS) TS 33.246".

   [OMA-DRM]  Open Mobile Alliance, "DRM Specification 2.0".

   [PSS]      3GPP, "Transparent end-to-end Packet-switched Streaming
              Service (PSS); Protocols and codecs TS 26.234".

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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.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [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.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, 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.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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Authors' Addresses

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


   Magnus Westerlund
   Farogatan 6
   Kista  SE-164 80


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