draft-ietf-avt-srtp-not-mandatory-00.txt		draft-ietf-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: January 30, 2009 Ericsson		Expires: May 6, 2009 Ericsson
	July 29, 2008		November 2, 2008
	Why RTP Does Not Mandate a Single Security Mechanism		Why RTP Does Not Mandate a Single Security Mechanism
	draft-ietf-avt-srtp-not-mandatory-00.txt		draft-ietf-avt-srtp-not-mandatory-01.txt
	Status of this Memo		Status of this Memo
	By submitting this Internet-Draft, each author represents that any		By submitting this Internet-Draft, each author represents that any
	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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	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	skipping to change at page 1, line 35		skipping to change at page 1, line 35
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	This Internet-Draft will expire on January 30, 2009.		This Internet-Draft will expire on May 6, 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. On the Requirement for Strong Security in IETF protocols . . . 6		5. On the Requirement for Strong Security in IETF protocols . . . 6
	6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 6		6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
	7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7		7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	9. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7		9. Informative References . . . . . . . . . . . . . . . . . . . . 7
	10. Informative References . . . . . . . . . . . . . . . . . . . . 7		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8		Intellectual Property and Copyright Statements . . . . . . . . . . 10
	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
	of media streams, and to exchange security keys. This memo outlines		of media streams, and to exchange security keys. This memo outlines
	skipping to change at page 4, line 15		skipping to change at page 4, line 15
	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 underlying transport. Some run on
	optical networks, others use low rate unreliable wireless networks.		highly reliable optical networks, others use low rate unreliable
	Some applications of RTP operate entirely within a single trust		wireless networks. Some applications of RTP operate entirely within
	domain, others are inter-domain, with untrusted (and potentially		a single trust domain, others are inter-domain, with untrusted (and
	unknown) users. The range of scenarios is wide, and growing both in		potentially unknown) users. The range of scenarios is wide, and
	number and in heterogeneity.		growing both in 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 widely applicable of these
	is the Secure RTP (SRTP) framework [3]. This is an application-level		solutions is the Secure RTP (SRTP) framework [3]. This is an
	media security solution, encrypting the media payload data (not the		application-level media security solution, encrypting the media
	RTP headers) to provide some degree of confidentiality, and providing		payload data (but not the RTP headers) to provide some degree of
	optional source origin authentication. It was carefully designed to		confidentiality, and providing optional source origin authentication.
	be both low overhead, and to support the group communication features		It was carefully designed to be both low overhead, and to support the
	of RTP, across a range of networks.		group communication features 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 [4]. SRTP is, again, unnecessary in such
			environments, and its use would only introduce overhead for no gain.
			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		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
	More diverse than the different use cases is the different protocols		With such a diverse range of use case come a range of different
	used for RTP session establishment. Providing keying for these		protocols for RTP session establishment. Mechanisms used to provide
	different session establishment can basically be put into two		security keying for these different session establishment protocols
	categories, inband and out-of-band in relation to the session		can basically be put into two categories: inband and out-of-band in
	establishment mechanism. The requirement on these solution are		relation to the session establishment mechanism. The requirements
	highly varying. Thus a wide range of solutions have been developed		for these solutions are highly varying. Thus a wide range of
	in this space:		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 [4] or the MIKEY		In such environments, SDP security descriptions [6] or the MIKEY
	[5] protocol are appropriate keying mechanisms, piggybacked onto		[7] protocol are appropriate keying mechanisms, piggybacked onto
	the SDP exchange.		the SDP exchange. The infrastructure may be secured by protecting
			the SIP exchange using SIPS or S/MIME, for example.
	o SIP/SDP with SIPS
	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.
	Protocols such as ZRTP or DTLS protocols are useful here.		Protocols such as DTLS [8] or ZRTP [9] are useful here.
	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.
	o Email with SDP, secured using X.500 or PGP
	o SDP file retrieved using HTTPS		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.
	o FLUTE using S/MIME to secure SDP		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 SAP with SDP		o A session description may be distributed to a multicast group
			using SAP or FLUTE secured with S/MIME.
	o OMAs DRM keymanagement [6] with pointer from SDP for point to		o A session description may be distributed using OMA's DRM key
	point streamingsetup with RTSP in 3GPP [7].		management [11] with pointer for point to point streaming setup
			with RTSP in 3GPP [12].
	o Usage of HTTP and MIKEY for key management in MBMS [8].		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		A more detailed survey of requirements for media security management
	protocols can be found in [9]. As can be seen, the range of use		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		cases is wide, and there is no single protocol that is appropriate
	for all scenarios. These solutions have be further diversified by		for all scenarios. These solutions have be further diversified by
	the existence of infrastructure elements such as authentication		the existence of infrastructure elements such as authentication
	solutions that is tied into the key manangement.		solutions that are tied into the key manangement.
	5. On the Requirement for Strong Security in IETF protocols		5. On the Requirement for Strong Security in IETF protocols
	BCP 61 [10] puts a requirement on IETF protocols to provide strong,		BCP 61 [15] puts a requirement on IETF protocols to provide strong,
	mandatory to implement, security solutions. This is actually quite a		mandatory to implement, security solutions. This is actually quite a
	difficult requirement for any type of framework protocol, like RTP,		difficult requirement for any type of framework protocol, like RTP,
	since one can never know all the deployement scenarios, and if the		since one can never know all the deployement scenarios, and if they
	security solution provided covers them. It would clearly be		are covered by the security solution. It would clearly be desirable
	desirable if a single media security solution and a single key		if a single media security solution and a single key management
	management solution could be developed, satisfying the range of use		solution could be developed, satisfying the range of use cases for
	cases for RTP. The authors are not aware of any such solution,		RTP. The authors are not aware of any such solution, however, and it
	however, and it is not clear that any single solution can be		is not clear that any single solution can be developed.
	developed.
	For a framework protocol it appears that the only sensible solution		For a framework protocol it appears that the only sensible solution
	to the requirement of BCP 61 is to develop or use security building		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		blocks, like SRTP, SDES, MIKEY, DTLS, or IPsec, to provide the basic
	security services of authorization, data integrity protocetion and		security services of authorization, data integrity protocetion and
	date confidentiality protection. When new usages of the RTP		date confidentiality protection. When new usages of the RTP
	framework arise, one needs to analyze the situation, to determine of		framework arise, one needs to analyze the situation, to determine of
	the existing building blocks satisfy the requirements. If not, it is		the existing building blocks satisfy the requirements. If not, it is
	necessary to develop new security building blocks.		necessary to develop new security building blocks.
	skipping to change at page 7, line 4		skipping to change at page 7, line 13
	mechanisms and key-management should be used. If such agreement is		mechanisms and key-management should be used. If such agreement is
	not possible, there will be increased cost, either in the lack of		not possible, there will be increased cost, either in the lack of
	interoperability, or in the need to implement more solutions.		interoperability, or in the need to implement more solutions.
	Unfortunately this situation, if not handled reasonably well, can		Unfortunately this situation, if not handled reasonably well, can
	result in a failure to satisfy the requirement of providing the users		result in a failure to satisfy the requirement of providing the users
	with an option of turining on strong security when desired.		with an option of turining on strong security when desired.
	6. Conclusions		6. Conclusions
	As discussed earlier it appears that a single solution can't be		As discussed earlier it appears that a single solution can't be
	designed meet the diverse requirements. In the absense of such a		designed to meet the diverse requirements. In the absense of such a
	solution, it is hoped that this memo explains why SRTP is not		solution, it is hoped that this memo explains why SRTP is not
	mandatory as the media security solution for RTP-based systems, and		mandatory as the media security solution for RTP-based systems, and
	why we can expect multiple key management solutions for systems using		why we can expect multiple key management solutions for systems using
	RTP.		RTP.
	In respect to the above it is important for any RTP-based application		It is important for any RTP-based application to consider how it
	to consider how they meet the application's security requirements.		meets the security requirements. This will require some analysis to
	This will requires some analysis to determine these requirements.		determine these requirements, followed by a selection of a mandatory
	Followed by a selection of preferably a single to mandatory to		to implement solution, or in exceptional scenarios several solutions,
	implement solution including the desired RTP traffic protection and		including the desired RTP traffic protection and key-management.
	key-management. As SRTP can be used in a large number of use cases,		SRTP is a preferred solution for the protection of the RTP traffic in
	it is a preferred solution for the protection of the RTP traffic, for		those use cases where it is applicable. It is out of scope for this
	those use cases where it is applicable. Currently it is much harder		memo to recommend a preferred key management solution.
	to point out a preferred key-management solution.
	7. Security Considerations		7. Security Considerations
	This entire memo is about security.		This entire memo is about security.
	8. IANA Considerations		8. IANA Considerations
	No IANA actions are required.		No IANA actions are required.
	9. To Do		9. Informative References
	Update references
	IPsec example
	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] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols		[2] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols
	and codecs TS 26.346".		and codecs TS 26.346".
	[3] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.		[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.
	[4] Andreasen, F., Baugher, M., and D. Wing, "Session Description		[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",		Protocol (SDP) Security Descriptions for Media Streams",
	RFC 4568, July 2006.		RFC 4568, July 2006.
	[5] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.		[7] 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.
	[6] Open Mobile Alliance, "DRM Specification 2.0".		[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.
	[7] 3GPP, "Transparent end-to-end Packet-switched Streaming Service		[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".		(PSS); Protocols and codecs TS 26.234".
	[8] 3GPP, "Security of Multimedia Broadcast/Multicast Service		[13] 3GPP, "Security of Multimedia Broadcast/Multicast Service
	(MBMS) TS 33.246".		(MBMS) TS 33.246".
	[9] Wing, D., Fries, S., Tschofenig, H., and F. Audet,		[14] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
	"Requirements and Analysis of Media Security Management		"Requirements and Analysis of Media Security Management
	Protocols", draft-ietf-sip-media-security-requirements-02 (work		Protocols", draft-ietf-sip-media-security-requirements-08 (work
	in progress), January 2008.		in progress), October 2008.
	[10] Schiller, J., "Strong Security Requirements for Internet		[15] 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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