Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Intended status: Informational                             M. Westerlund
Expires: May 6, 2009                                            Ericsson
                                                        November 2, 2008

          Why RTP Does Not Mandate a Single Security Mechanism

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This memo discusses the problem of securing real-time multimedia
   sessions, and explains why the Real-time Transport Protocol (RTP)
   does not mandate a single media security mechanism.

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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.  Informative References . . . . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   The Real-time Transport Protocol (RTP) [1] 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,
   some use TCP or DCCP 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 [3].  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 run over a single TCP
   connection, multiplexing media data with control information on that
   connection (for example, on an RTSP connection).  The natural way to
   provide media security for such client-server media applications is
   to use TLS 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
   to secure the RTP session [4].  SRTP is, again, unnecessary in such
   environments, and its use would only introduce overhead for no gain.

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   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 [5],
   where one mode of operation uses ISMAcryp to protect the media data

   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 with
      the SDP offer/answer model, operating on a trusted infrastructure.
      In such environments, SDP security descriptions [6] or the MIKEY
      [7] protocol are appropriate keying mechanisms, piggybacked onto
      the SDP exchange.  The infrastructure may be secured by protecting
      the SIP exchange using SIPS or S/MIME, for example.

   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 [8] or ZRTP [9] are useful here.

   o  For point-to-point client-server streaming of RTP over RTSP [10],
      a TLS association is appropriate to manage keying material, in
      much the same manner as would be used to secure an HTTP session.

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   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 OMA's DRM key
      management [11] with pointer for point to point streaming setup
      with RTSP in 3GPP [12].

   o  In the 3GPP MBMS system, HTTP and MIKEY are used for key
      management [13].

   A more detailed survey of requirements for media security management
   protocols can be found in [14].  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 be 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 [15] 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 deployement 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 developed.

   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, SDES, MIKEY, DTLS, or IPsec, to provide the basic
   security services of authorization, data integrity protocetion and
   date confidentiality protection.  When new usages of the RTP
   framework arise, one needs to analyze the situation, to determine of
   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, 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

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   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 turining 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 absense 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 a 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.  Informative References

   [1]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [2]   3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols
         and codecs TS 26.346".

   [3]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)",
         RFC 3711, March 2004.

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   [4]   3GPP, "IP network layer security", 3GPP TS 33.210,
         September 2008.

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

   [6]   Andreasen, F., Baugher, M., and D. Wing, "Session Description
         Protocol (SDP) Security Descriptions for Media Streams",
         RFC 4568, July 2006.

   [7]   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.

   [8]   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-06 (work
         in progress), October 2008.

   [9]   Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media Path
         Key Agreement for Secure RTP", draft-zimmermann-avt-zrtp-10
         (work in progress), October 2008.

   [10]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M.
         Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)",
         draft-ietf-mmusic-rfc2326bis-18 (work in progress), May 2008.

   [11]  Open Mobile Alliance, "DRM Specification 2.0".

   [12]  3GPP, "Transparent end-to-end Packet-switched Streaming Service
         (PSS); Protocols and codecs TS 26.234".

   [13]  3GPP, "Security of Multimedia Broadcast/Multicast Service
         (MBMS) TS 33.246".

   [14]  Wing, D., Fries, S., Tschofenig, H., and F. Audet,
         "Requirements and Analysis of Media Security Management
         Protocols", draft-ietf-sip-media-security-requirements-08 (work
         in progress), October 2008.

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

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Authors' Addresses

   Colin Perkins
   University of Glasgow
   Department of Computing Science
   Sir Alwyn Williams Building
   Lilybank Gardens
   Glasgow  G12 8QQ


   Magnus Westerlund
   Torshamgatan 23
   Stockholm  SE-164 80


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