draft-perkins-avt-srtp-not-mandatory-00.txt   draft-perkins-avt-srtp-not-mandatory-01.txt 
Network Working Group C. Perkins Network Working Group C. Perkins
Internet-Draft University of Glasgow Internet-Draft University of Glasgow
Intended status: Informational M. Westerlund Intended status: Informational M. Westerlund
Expires: August 20, 2008 Ericsson Expires: January 15, 2009 Ericsson
February 17, 2008 July 14, 2008
Why RTP Does Not Mandate a Single Security Mechanism Why RTP Does Not Mandate a Single Security Mechanism
draft-perkins-avt-srtp-not-mandatory-00.txt draft-perkins-avt-srtp-not-mandatory-01.txt
Status of this Memo Status of this Memo
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applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on August 20, 2008. This Internet-Draft will expire on January 15, 2009.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
Abstract Abstract
This memo discusses the problem of securing real-time multimedia This memo discusses the problem of securing real-time multimedia
sessions, and explains why the Real-time Transport Protocol (RTP) sessions, and explains why the Real-time Transport Protocol (RTP)
does not mandate a single media security mechanism. does not mandate a single media security mechanism.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. RTP Applications and Deployment Scenarios . . . . . . . . . . . 3 2. RTP Applications and Deployment Scenarios . . . . . . . . . . . 3
3. Implications for RTP Media Security . . . . . . . . . . . . . . 4 3. Implications for RTP Media Security . . . . . . . . . . . . . . 4
4. Implications for Key Management . . . . . . . . . . . . . . . . 5 4. Implications for Key Management . . . . . . . . . . . . . . . . 5
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. On the Requirement for Strong Security in IETF protocols . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Informative References . . . . . . . . . . . . . . . . . . . . 7 9. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 10. Informative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 9
1. Introduction 1. Introduction
The Real-time Transport Protocol (RTP) [1] is widely used for voice The Real-time Transport Protocol (RTP) [1] is widely used for voice
over IP, Internet television, video conferencing, and various other over IP, Internet television, video conferencing, and various other
real-time and streaming media applications. Despite this, the base real-time and streaming media applications. Despite this, the base
RTP specification provides very limited options for media security, RTP specification provides very limited options for media security,
and defines no standard key exchange mechanism. Rather, a number of and defines no standard key exchange mechanism. Rather, a number of
extensions are defined to provide confidentiality and authentication extensions are defined to provide confidentiality and authentication
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why it is appropriate that multiple extension mechanisms are defined, why it is appropriate that multiple extension mechanisms are defined,
rather than mandating a single media security and keying mechanism. rather than mandating a single media security and keying mechanism.
This memo provides information for the community; it does not specify This memo provides information for the community; it does not specify
a standard of any kind. a standard of any kind.
The structure of this memo is as follows: we begin, in Section 2 by The structure of this memo is as follows: we begin, in Section 2 by
describing the scenarios in which RTP is deployed. Following this, describing the scenarios in which RTP is deployed. Following this,
Section 3 outlines the implications of this range of scenarios for Section 3 outlines the implications of this range of scenarios for
media confidentially and authentication, and Section 4 outlines the media confidentially and authentication, and Section 4 outlines the
implications for key exchange. Section 5 then concludes and gives implications for key exchange. Section 5 outlines how the RTP
some recommendations. Finally, Section 6 outlines the security framework meets the requirement of BCP 61. Section 6 then concludes
considerations, and Section 7 outlines IANA considerations. and gives some recommendations. Finally, Section 7 outlines the
security considerations, and Section 8 outlines IANA considerations.
2. RTP Applications and Deployment Scenarios 2. RTP Applications and Deployment Scenarios
The range of application and deployment scenarios where RTP has been The range of application and deployment scenarios where RTP has been
used includes, but is not limited to, the following: used includes, but is not limited to, the following:
o Point-to-point voice telephony (fixed and wireless networks) o Point-to-point voice telephony (fixed and wireless networks)
o Point-to-point video conferencing o Point-to-point video conferencing
o Centralised group video conferencing with a multipoint conference o Centralised group video conferencing with a multipoint conference
unit (MCU) unit (MCU)
o Any Source Multicast video conferencing (light-weight sessions; o Any Source Multicast video conferencing (light-weight sessions;
Mbone conferencing) Mbone conferencing)
o Point-to-point streaming audio and/or video o Point-to-point streaming audio and/or video
o Single Source Multicast streaming to large group (IPTV) o Single Source Multicast streaming to large group (IPTV and MBMS
[2])
o Replicated unicast streaming to a group o Replicated unicast streaming to a group
o Interconnecting components in music production studios and video o Interconnecting components in music production studios and video
editing suites editing suites
o Interconnecting components of distributed simulation systems o Interconnecting components of distributed simulation systems
o Streaming real-time sensor data o Streaming real-time sensor data
As can be seen, these scenarios vary from point-to-point to very As can be seen, these scenarios vary from point-to-point to very
large multicast groups, from interactive to non-interactive, and from large multicast groups, from interactive to non-interactive, and from
low bandwidth (kilobits per second) to very high bandwidth (multiple low bandwidth (kilobits per second) to very high bandwidth (multiple
gigabits per second). While most of these applications run over UDP, gigabits per second). While most of these applications run over UDP,
some use TCP or DCCP as their transport. Some run on highly reliable some use TCP or DCCP as their transport. Some run on highly reliable
optical networks, others use low rate unreliable wireless networks. optical networks, others use low rate unreliable wireless networks.
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Some applications of RTP operate entirely within a single trust Some applications of RTP operate entirely within a single trust
domain, others are inter-domain, with untrusted (and potentially domain, others are inter-domain, with untrusted (and potentially
unknown) users. The range of scenarios is wide, and growing both in unknown) users. The range of scenarios is wide, and growing both in
number and in heterogeneity. number and in heterogeneity.
3. Implications for RTP Media Security 3. Implications for RTP Media Security
The wide range of application scenarios where RTP is used has led to The wide range of application scenarios where RTP is used has led to
the development of multiple solutions for media security, considering the development of multiple solutions for media security, considering
different requirements. Perhaps the most general of these solutions different requirements. Perhaps the most general of these solutions
is the Secure RTP (SRTP) framework [2]. This is an application-level is the Secure RTP (SRTP) framework [3]. This is an application-level
media security solution, encrypting the media payload data (not the media security solution, encrypting the media payload data (not the
RTP headers) to provide some degree of confidentiality, and providing RTP headers) to provide some degree of confidentiality, and providing
optional source origin authentication. It was carefully designed to optional source origin authentication. It was carefully designed to
be both low overhead, and to support the group communication features be both low overhead, and to support the group communication features
of RTP, across a range of networks. of RTP, across a range of networks.
SRTP is not the only media security solution in use, however, and SRTP is not the only media security solution in use, however, and
alternatives are more appropriate for some scenarios. For example, alternatives are more appropriate for some scenarios. For example,
many client-server streaming media applications run over a single TCP many client-server streaming media applications run over a single TCP
connection, multiplexing media data with control information on that connection, multiplexing media data with control information on that
connection (for example, on an RTSP connection). The natural way to connection (for example, on an RTSP connection). The natural way to
provide media security for such client-server media applications is provide media security for such client-server media applications is
to use TLS to protect the TCP connection, sending the RTP media data 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 over the TLS connection. Using the SRTP framework in addition to TLS
is unncessary, and would result in double encryption of the media, 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 and SRTP cannot be used instead of TLS since it is RTP-specific, and
so cannot protect the control traffic. 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. SRTP is, once again, unnecessary in such
environments, and its use would only introduce overhead for no gain.
Finally, the link layer may be secure, and it may be known that the 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 RTP media data is constrained to that single link (for example, when
operating in a studio environment, with physical link security). An operating in a studio environment, with physical link security). An
environment like this is inherently constrained, but might avoid the environment like this is inherently constrained, but might avoid the
need for application, transport, or network layer media security. need for application, transport, or network layer media security.
All these are application scenarios where RTP has seen commerical All these are application scenarios where RTP has seen commerical
deployment. Other use case also exist, with additional requirements. deployment. Other use case also exist, with additional requirements.
There is no media security protocol that is appropriate for all these There is no media security protocol that is appropriate for all these
environments. Accordingly, multiple RTP media security protocols can environments. Accordingly, multiple RTP media security protocols can
be expected to remain in wide use. be expected to remain in wide use.
4. Implications for Key Management 4. Implications for Key Management
Similar issues apply when considering key management protocols for More diverse than the different use cases is the different protocols
RTP sessions, and a wide range of solutions have been developed in used for RTP session establishment. Providing keying for these
this space: different session establishment can basically be put into two
categories, inband and out-of-band in relation to the session
establishment mechanism. The requirement on these solution 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 o The most common use case for RTP is probably point-to-point voice
calls or centralised group conferences, negotiated using SIP with calls or centralised group conferences, negotiated using SIP with
the SDP offer/answer model, operating on a trusted infrastructure. the SDP offer/answer model, operating on a trusted infrastructure.
In such environments, SDP security descriptions [3] or the MIKEY In such environments, SDP security descriptions [4] or the MIKEY
[4] protocol are appropriate keying mechanisms, piggybacked onto [5] protocol are appropriate keying mechanisms, piggybacked onto
the SDP exchange. the SDP exchange.
o SIP/SDP with SIPS o SIP/SDP with SIPS
o SIP/SDP with S/MIME o SIP/SDP with S/MIME
o Point-to-point RTP sessions may be negotiated using SIP with the o Point-to-point RTP sessions may be negotiated using SIP with the
offer/answer model, but operating over a network with untrusted offer/answer model, but operating over a network with untrusted
infrastructure. In such environments, the key management protocol infrastructure. In such environments, the key management protocol
is run on the media path, bypassing the untrusted infrastructure. is run on the media path, bypassing the untrusted infrastructure.
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same manner as would be used to secure an HTTP session. same manner as would be used to secure an HTTP session.
o Email with SDP, secured using X.500 or PGP o Email with SDP, secured using X.500 or PGP
o SDP file retrieved using HTTPS o SDP file retrieved using HTTPS
o FLUTE using S/MIME to secure SDP o FLUTE using S/MIME to secure SDP
o SAP with SDP o SAP with SDP
o (TBD: complete this list) o OMAs DRM keymanagement [6] with pointer from SDP for point to
point streamingsetup with RTSP in 3GPP [7].
o Usage of HTTP and MIKEY for key management in MBMS [8].
A more detailed survey of requirements for media security management A more detailed survey of requirements for media security management
protocols can be found in [5]. As can be seen, the range of use protocols can be found in [9]. As can be seen, the range of use
cases is wide, and there is no single protocol that is appropriate cases is wide, and there is no single protocol that is appropriate
for all scenarios. for all scenarios. These solutions have be further diversified by
the existence of infrastructure elements such as authentication
solutions that is tied into the key manangement.
5. Conclusions 5. On the Requirement for Strong Security in IETF protocols
It would clearly be desirable if a single media security solution and BCP 61 [10] puts a requirement on IETF protocols to provide strong,
a single key management solution could be developed, satisfying the mandatory to implement, security solutions. This is actually quite a
range of use cases for RTP. The authors are not aware of any such difficult requirement for any type of framework protocol, like RTP,
solution, however, and it is not clear that any single solution can since one can never know all the deployement scenarios, and if the
be developed. In the absense of such a solution, it is hoped that security solution provided covers them. It would clearly be
this memo explains why SRTP is not mandatory as the media security desirable if a single media security solution and a single key
solution for RTP-based systems, and why we can expect multiple key management solution could be developed, satisfying the range of use
management solutions for systems using RTP. 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
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 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.
In respect to the above it is important for any RTP-based application In respect to the above it is important for any RTP-based application
to consider how they meet the application's security requirements. to consider how they meet the application's security requirements.
This will requires some analysis to determine these requirements. This will requires some analysis to determine these requirements.
Followed by a selection of preferably a single to mandatory to Followed by a selection of preferably a single to mandatory to
implement solution including the desired RTP traffic protection and implement solution including the desired RTP traffic protection and
key-management. As SRTP can be used in a large number of use cases, key-management. As SRTP can be used in a large number of use cases,
it is a preferred solution for the protection of the RTP traffic, for it is a preferred solution for the protection of the RTP traffic, for
those use cases where it is applicable. Currently it is much harder those use cases where it is applicable. Currently it is much harder
to point out a preferred key-management solution. to point out a preferred key-management solution.
6. Security Considerations 7. Security Considerations
This entire memo is about security. This entire memo is about security.
7. IANA Considerations 8. IANA Considerations
No IANA actions are required. No IANA actions are required.
8. To Do 9. To Do
Complete Section 4 Update references
Incorporate discussion of RFC 3365 [6] into this document, to explain IPsec example
how those requirements are satisfied.
9. Informative References 10. Informative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64, "RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003. RFC 3550, July 2003.
[2] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [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)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[3] Andreasen, F., Baugher, M., and D. Wing, "Session Description [4] Andreasen, F., Baugher, M., and D. Wing, "Session Description
Protocol (SDP) Security Descriptions for Media Streams", Protocol (SDP) Security Descriptions for Media Streams",
RFC 4568, July 2006. RFC 4568, July 2006.
[4] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. [5] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session Description Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)", Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006. RFC 4567, July 2006.
[5] Wing, D., Fries, S., Tschofenig, H., and F. Audet, "Requirements [6] Open Mobile Alliance, "DRM Specification 2.0".
and Analysis of Media Security Management Protocols",
draft-ietf-sip-media-security-requirements-02 (work in
progress), January 2008.
[6] Schiller, J., "Strong Security Requirements for Internet [7] 3GPP, "Transparent end-to-end Packet-switched Streaming Service
(PSS); Protocols and codecs TS 26.234".
[8] 3GPP, "Security of Multimedia Broadcast/Multicast Service
(MBMS) TS 33.246".
[9] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
"Requirements and Analysis of Media Security Management
Protocols", draft-ietf-sip-media-security-requirements-02 (work
in progress), January 2008.
[10] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61, RFC 3365, Engineering Task Force Standard Protocols", BCP 61, RFC 3365,
August 2002. August 2002.
Authors' Addresses Authors' Addresses
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
Department of Computing Science Department of Computing Science
Sir Alwyn Williams Building Sir Alwyn Williams Building
Lilybank Gardens Lilybank Gardens
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