Internet Engineering Task Force (IETF) C. Perkins
Request for Comments: 7202 University of Glasgow
Category: Informational M. Westerlund
ISSN: 2070-1721 Ericsson
April 2014
Securing the RTP Protocol Framework:
Why RTP Does Not Mandate a Single Media Security Solution
Abstract
This memo discusses the problem of securing real-time multimedia
sessions. It also explains why the Real-time Transport Protocol
(RTP) and the associated RTP Control Protocol (RTCP) do not mandate a
single media security mechanism. This is relevant for designers and
reviewers of future RTP extensions 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 document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7202.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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. Securing the RTP Protocol Framework . . . . . . . . . . . . . . . . . 6
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. Informative References . . . . . . . . . . . . . . . . . . . 8
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 "Strong Security Requirements for Internet
Engineering Task Force 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.
This document provides high-level guidance on how to handle security
issues for the various types of components within the RTP framework
as well as the role of the service or application using RTP to ensure
strong security is implemented. This document does not provide the
guidance that an individual implementer, or even specifier of an RTP
application, really can use to determine what security mechanism they
need to use; that is not intended with this document.
A non-exhaustive list of the RTP security options available at the
time of this writing is outlined in [RFC7201]. This document gives
an overview of the available RTP solutions and provides guidance on
their applicability for different application domains. It also
attempts to provide an indication of actual and intended usage at the
time of writing as additional input to help with considerations such
as interoperability, availability of implementations, etc.
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 Centralized group video conferencing and telepresence, using a
Multipoint Conference Unit (MCU) or similar central middlebox;
o Any Source Multicast (ASM) video conferencing using the
lightweight 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 Third Generation
Partnership Project (3GPP) Multimedia/Broadcast Multicast Service
[T3GPP.26.346]);
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; and
o Streaming real-time sensor data (e.g., electronic Very Long
Baseline Interferometry (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 the Datagram Congestion Control Protocol (DCCP)
[RFC4340] as their underlying transport. Some run on highly reliable
optical networks, while others use low-rate unreliable wireless
networks. Some applications of RTP operate entirely within a single
trust domain, while others run interdomain with untrusted (and, in
some cases, potentially unknown) users. The range of scenarios is
wide and growing both in number and in heterogeneity.
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 [ISMACryp2] 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 [RFC7201]. 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 signaling channel.
The requirements for these two classes of solutions 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
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 [RFC7201].
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
that is 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, authorization, 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.
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 integrated, or if
any new mechanisms need to be defined to address specific issues
relating to this new class of application. 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
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. 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 [RFC7201], 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 thus 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 "WebRTC
Security Architecture" [WebRTC-SEC] and "Real Time Streaming Protocol
2.0 (RTSP)" [RTSP]. 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 behavior 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 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 its 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
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 MTI 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 [RFC7201]. 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
This entire memo is about mandatory-to-implement security.
9. Acknowledgements
Thanks to Ralph Blom, Hannes Tschofenig, Dan York, Alfred Hoenes,
Martin Ellis, Ali Begen, Keith Drage, Ray van Brandenburg, Stephen
Farrell, Sean Turner, John Mattsson, and Benoit Claise for their
feedback.
10. Informative References
[ISMACryp2]
Internet Streaming Media Alliance (ISMA), "ISMA Encryption
and Authentication Version 2.0", November 2007,
<http://www.oipf.tv/images/site/DOCS/mpegif/ISMA/
isma_easpec2.0.pdf>.
[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.
[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.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, April 2014.
[RTSP] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, "Real Time Streaming Protocol 2.0
(RTSP)", Work in Progress, February 2014.
[T3GPP.26.346]
3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs", 3GPP TS 26.346 10.7.0, March 2013,
<http://www.3gpp.org/ftp/Specs/html-info/26346.htm>.
[WebRTC-SEC]
Rescorla, E., "WebRTC Security Architecture", Work in
Progress, February 2014.
Authors' Addresses
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
EMail: csp@csperkins.org
URI: http://csperkins.org/
Magnus Westerlund
Ericsson
Farogatan 6
Kista SE-164 80
Sweden
EMail: magnus.westerlund@ericsson.com