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
draft-ietf-avt-srtp-not-mandatory-01.txt
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Abstract
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
[2])
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
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 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
RTP.
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
UK
Email: csp@csperkins.org
Magnus Westerlund
Ericsson
Torshamgatan 23
Stockholm SE-164 80
Sweden
Email: magnus.westerlund@ericsson.com
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