Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Intended status: Informational                             M. Westerlund
Expires: May 3, 2012                                            Ericsson
                                                        October 31, 2011

          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.  It also discusses how applications
   using RTP can meet the goals of BCP 61 to have strong and mandatory
   to implement security.

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
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   This Internet-Draft will expire on May 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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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 RTP media streams and RTCP control messages, and to
   exchange security keys.  This memo outlines why it is appropriate
   that multiple extension mechanisms are defined, rather than mandating
   a single security and keying mechanism.

   The consensus for Strong Security Requirements  for IETF Standard
   Protocols (BCP61) [RFC3365] describes the Danvers Doctrine, which
   states that:

      "The solution is 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."

   BCP 61 also discusses that security must be implemented, and makes
   the following statement:

      "However security must be a MUST IMPLEMENT so that end users will
      have the option of enabling it when the situation calls for it."

   This IETF consensus provides a clear challange for RTP security, due
   to the heterogenous scenarios in which RTP can be used, and the wide
   choice of security mechanisms available.  This memo describes how RTP
   based applications, or classes of applications, can best meet the
   security goals of BCP 61.

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

   The structure of this memo is as follows.  Section 2 describes a
   number of 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 can meet the requirement of BCP 61.  Section 6 then
   concludes and gives some recommendations.

2.  RTP Applications and Deployment Scenarios

   The range of application and deployment scenarios where RTP has been

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   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])

   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

   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 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 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

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   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).  One 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
   unnecessary, 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
   to secure the RTP session [TS-33210].  SRTP is, again, unnecessary in
   such environments, and its use would only introduce overhead for no

   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 ISMA Cryp 2.0
   [ISMA] to encrypt the RTP payload data only.

   All these are application scenarios where RTP has seen commercial
   deployment.  Other use cases exist, with additional requirements.
   For example, if the media transport is done over UDP [RFC0768], DCCP
   [RFC4340] or SCTP [RFC4960], then using DTLS [RFC4347] to protect the
   whole RTP packets is an option.  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 cases 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

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   solutions have been developed in this space:

   o  A 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 [RFC3830] protocol using the Key
      Management Extensions for SDP [RFC4567], are appropriate keying
      mechanisms, where the keying messages/material are embedded in the
      SDP [RFC4566] exchange.  The infrastructure may be secured by
      protecting the SDP exchange in SIP using TLS or S/MIME, for
      example [RFC3261].  Protocols such as DTLS-SRTP [RFC5764] or ZRTP
      [RFC6189] are also appropriate in such environments.

   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
      can be run on the media path, bypassing the untrusted
      infrastructure.  Protocols such as DTLS-SRTP [RFC5764] or ZRTP
      [RFC6189] are useful here, as are inband mechanism that protect
      the keying material such as MIKEY [RFC3830] using the Key
      Management Extensions for SDP [RFC4567].  It should be noted that
      the end-points for all the above mechanisms must prevent total
      downgrade to no security for the RTP media streams.

   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.  But also
      using SRTP with DTLS-SRTP keying or DTLS are appropriate choices.

   o  A session description may be sent by email, secured using S/MIME
      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 [RFC5479].  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

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   diversified by the existence of infrastructure elements such as
   authentication solutions that are tied into the key management.

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 solution.  This is actually
   quite a difficult requirement for any type of framework protocol like
   RTP, or for that matter the Reliable Multicast Transport suite
   [RFC3048], 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 believe it is unlikely 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, MIKEY, DTLS, DTLS-SRTP,
   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
   building blocks.

   When it comes to fulfilling the "MUST Implement" strong security for
   a specific application, or class of applications, 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.

   The IETF needs to perform this selection of security building blocks
   whenever it is possible.  This can be done if the application, or
   class of applications, is being specified within the IETF, or wich a
   scope where the IETF can take the role to provide a security profile.
   However, it is clear that many applications, or classes of
   application, are specified outside the scope and influence of the
   IETF.  In those case we can't do other than strongly recommend these

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   organizations perform a security analysis, taking into account other
   applications, to try to maximize the security and interoperability.

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 very 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.  When defining applications or protocols using RTP within
   the IETF, the responsibility for fulfilling the BCP 61 requirements
   will fall onto the developers of these applications.  IETF also
   should be open to help other standards bodies by defining security
   profiles suitable for classes of applications.

   Anyone defining an RTP based application needs to take care to
   consider how to fulfill its security goals and specify which
   mechanisms that are to be implemented.  In that work interoperability
   with similar applications should be considered, so that when such
   applications becomes desirable to interconnect those applications,
   their security solutions are compatible and will not require
   additional implementation or costly gateways that also reduce
   security by forcing a trusted third party.

   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 in general.
   The authors do note that DTLS-SRTP was developed in the IETF to meet
   the goals of point to point media sessions established by SIP.

7.  Security Considerations

   This entire memo is about security.

8.  IANA Considerations

   No IANA actions are required.

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9.  Acknowledgements

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

10.  Informative References

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

              Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, "Real Time Streaming Protocol 2.0
              (RTSP)", draft-ietf-mmusic-rfc2326bis-28 (work in
              progress), October 2011.

   [ISMA]     Internet Streaming Media Alliance, "Encryption and
              Authentication Version 2.0", November 2007.

   [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".

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC3048]  Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
              Floyd, S., and M. Luby, "Reliable Multicast Transport
              Building Blocks for One-to-Many Bulk-Data Transfer",
              RFC 3048, January 2001.

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

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              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.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 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.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security

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              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5479]  Wing, D., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", RFC 5479, April 2009.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [RFC6189]  Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Unicast Secure RTP", RFC 6189,
              April 2011.

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

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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